How to correct susceptibility distortions in spin-echo echo-planar images: application to diffusion tensor imaging

In vivo cortical microstructure mapping using high-gradient diffusion MRI accounting for intercompartmental water exchange effects

In recent years, mapping tissue microstructure in the cortex using high gradient diffusion MRI has received growing attention. The Soma And Neurite Density Imaging (SANDI) explicitly models the soma compartment in the cortex assuming impermeable membranes. As such, it does not account for diffusion time dependence due to water exchange in the estimated microstructural properties, as neurites in gray matter are much less myelinated than in white matter. In this work, we performed a systematic evaluation of an extended SANDI model for in vivo human cortical microstructural mapping that accounts for water exchange effects between the neurite and extracellular compartments using the anisotropic Kärger model. We refer to this model as in vivo SANDIX, adapting the nomenclature from previous publications. As in the original SANDI model, the soma compartment is modeled as an impermeable sphere due to the much smaller surface-to-volume ratio compared to the neurite compartment. A Monte Carlo simulation study was performed to examine the sensitivity of the in vivo SANDIX model to sphere radii, compartment fractions, and water exchange times. The simulation results indicate that the proposed in vivo SANDIX framework can account for the water exchange effect and provide measures of intra-soma and intra-neurite signal fractions without spurious time-dependence in estimated parameters, whereas the measured water exchange times need to be interpreted with caution. The model was then applied to in vivo diffusion MRI data acquired in 13 healthy adults on the 3-Tesla Connectome MRI scanner equipped with 300 mT/m gradients. The in vivo results exhibited patterns that were consistent with corresponding anatomical characteristics in both cortex and white matter. In particular, the estimated water exchange times in gray and white matter were distinct and differentiated between the two tissue types. Our results show the SANDIX approach applied to high-gradient diffusion MRI data achieves cortical microstructure mapping of the in vivo human brain with the evaluation of water exchange effects. This approach potentially provides a more appropriate description of in vivo cortical microstructure for improving data interpretation in future neurobiological studies.

Longitudinal trajectories of brain development from infancy to school age and their relationship with literacy development

Reading is one of the most complex skills that we utilize daily, and it involves the early development and interaction of various lower-level subskills, including phonological processing and oral language. These subskills recruit brain structures, which begin to develop long before the skill manifests and exhibit rapid development during infancy. However, how longitudinal trajectories of early brain development in these structures support long-term acquisition of literacy subskills and subsequent reading is unclear. Children underwent structural and diffusion MRI scanning at multiple timepoints between infancy and second grade and were tested for literacy subskills in preschool and decoding and word reading in early elementary school. We developed and implemented a reproducible pipeline to generate longitudinal trajectories of early brain development. We then examined whether these trajectories were associated with literacy (sub)skills or influenced by familial risk of reading difficulty and children's home literacy environments, two common literacy-related covariates. Results showed that individual differences in curve features (e.g., intercepts and slopes) for longitudinal trajectories of volumetric, surface-based, and white matter organization measures were linked directly to phonological processing and indirectly to early elementary school decoding and word reading skills via phonological processing. Altogether, these findings suggest that the brain bases of phonological processing, previously identified as one of the strongest behavioral predictors of decoding and word reading skills, may already begin to develop by birth but undergo further refinement between infancy and preschool. The present study underscores the importance of considering academic skill acquisition from the very beginning of life.

Top-down attention and Alzheimer's pathology affect cortical selectivity during learning, influencing episodic memory in older adults

Effective memory formation declines in human aging. Diminished neural selectivity-reduced differential responses to preferred versus nonpreferred stimuli-may contribute to memory decline, but its drivers remain unclear. We investigated the effects of top-down attention and preclinical Alzheimer's disease (AD) pathology on neural selectivity in 166 cognitively unimpaired older participants using functional magnetic resonance imaging during a word-face/word-place associative memory task. During learning, neural selectivity in place- and, to a lesser extent, face-selective regions was greater for subsequently remembered than forgotten events; positively scaled with variability in dorsal attention network activity, within and across individuals; and negatively related to AD pathology, evidenced by elevated plasma phosphorylated Tau181 (pTau181). Path analysis revealed that neural selectivity mediated the effects of age, attention, and pTau181 on memory. These data reveal multiple pathways that contribute to memory differences among older adults-AD-independent reductions in top-down attention and AD-related pathology alter the precision of cortical representations of events during experience, with consequences for remembering.

Diffusion basis spectrum imaging of white matter in schizophrenia and bipolar disorder

Multiple studies point to the role of neuroinflammation in the pathophysiology of schizophrenia (SCZ), however, there have been few in vivo tools for imaging brain inflammation. Diffusion basis spectrum imaging (DBSI) is an advanced diffusion-based MRI method developed to quantitatively assess microstructural alternations relating to neuroinflammation, axonal fiber, and other white matter (WM) pathologies. We acquired one-hour-long high-directional diffusion MRI data from young control (CON, n = 27), schizophrenia (SCZ, n = 21), and bipolar disorder (BPD, n = 21) participants aged 18-30. We applied Tract-based Spatial Statistics (TBSS) to allow whole-brain WM analyses and compare DBSI-derived isotropic and anisotropic diffusion measures between groups. Clinical relationships of DBSI metrics with clinical symptoms were assessed across SCZ and control participants. In SCZ participants, we found a generalized increase in DBSI-derived cellularity (a putative marker of neuroinflammation), a decrease in restricted fiber fraction (a putative marker of apparent axonal density), and an increase in extra-axonal water (a putative marker of vasogenic edema) across several WM tracts. There were only minimal WM abnormalities noted in BPD, mainly in regions of the corpus callosum (increase in DTI-derived RD and extra-axonal water). DBSI metrics showed significant partial correlations with psychosis and mood symptoms across groups. Our findings suggest that SCZ involves generalized white matter neuroinflammation, decreased fiber density, and demyelination, which is not seen in bipolar disorder. Larger studies are needed to identify medication-related effects. DBSI metrics could help identify high-risk groups requiring early interventions to prevent the onset of psychosis and improve outcomes.

Distinct white matter fiber density patterns in bipolar and depressive disorders: Insights from fixel-based analysis

BACKGROUND

Differentiating Bipolar (BD) and depressive (DD) disorders remains challenging in clinical practice due to overlapping symptoms. Our study employs fixel-based analysis (FBA) to examine fiber-specific white matter differences in BD and DD and gain insights into the ability of FBA metrics to predict future spectrum mood symptoms.

METHODS

163 individuals between 18 and 45 years with BD, DD, and healthy controls (HC) underwent Diffusion Magnetic Resonance Imaging. FBA was used to assess fiber density (FD), fiber cross-section (FC), and fiber density cross-section (FDC) in major white matter tracts. A longitudinal follow-up evaluated whether FBA measures predicted future spectrum depressive and hypomanic symptom trajectories over six months.

RESULTS

Direct comparisons between BD and DD indicated lower FD in the right superior longitudinal and uncinate fasciculi and left thalamo-occipital tract in BD versus DD. Individuals with DD exhibited lower FD in the left arcuate fasciculus than those with BD. Compared to HC, both groups showed lower FD in the splenium of the corpus callosum and left striato-occipital and optic radiation tracts. FD in these tracts predicted future spectrum symptom severity. Exploratory analyses revealed associations between FD, medication use, and marijuana exposure.

CONCLUSIONS

Our findings highlight distinct and overlapping white matter alterations in BD and DD. Furthermore, FD in key tracts may serve as a predictor of future symptom trajectories, supporting the potential clinical utility of FD as a biomarker for mood disorder prognosis. Future longitudinal studies are needed to explore the impact of treatment and disease progression on white matter microstructure.

Unbiased population-based statistics to obtain pathologic burden of injury after experimental TBI

Reproducibility of scientific data is a current concern throughout the neuroscience field. There are multiple on-going efforts to help resolve this problem. Within the preclinical neuroimaging field, the continued use of a region-of interest (ROI) type approaches combined with the well-known spatial heterogeneity of traumatic brain injury pathology is a barrier to the replicability and repeatability of data. Here we propose the conjoint use of an unbiased analysis of the whole brain after injury together with a population-based statistical analysis of sham-control brains as one approach that has been used in clinical research to help resolve this issue. The approach produces two volumes of pathology that are outside the normal range of sham brains, and can be interpreted as whole brain burden of injury. Using diffusion weighted imaging derived scalars from a tensor analysis of data acquired from adult, male rats at 2, 9 days, 1 and 5 months after lateral fluid percussion injury (LFPI) and in shams (n = 73 and 12, respectively), we compared a data-driven, z-score mapping method to a whole brain and white matter-specific analysis, as well as an ROI-based analysis with brain regions preselected by virtue of their large group effect sizes. We show that the data-driven approach is statistically robust, providing the advantage of a large group effect size typical of a ROI analysis of mean scalar values derived from the tensor in regions of gross injury, but without the large multi-region statistical correction required for interrogating multiple brain areas, and without the potential bias inherent with using preselected ROIs. We show that the technique correctly captures the expected longitudinal time-course of the diffusion scalar volumes based on the spatial extent of the pathology and the known temporal changes in scalar values in the LFPI model.

The Transdiagnostic Connectome Project: an open dataset for studying brain-behavior relationships in psychiatry

An important aim in psychiatry is to establish valid and reliable associations linking profiles of brain functioning to clinically relevant symptoms and behaviors across patient populations. To advance progress in this area, we introduce an open dataset containing behavioral and neuroimaging data from 241 individuals aged 18 to 70, comprising 148 individuals meeting diagnostic criteria for a broad range of psychiatric illnesses and a healthy comparison group of 93 individuals. These data include high-resolution anatomical scans, multiple resting-state, and task-based functional MRI runs. Additionally, participants completed over 50 psychological and cognitive assessments. Here, we detail available behavioral data as well as raw and processed MRI derivatives. Associations between data processing and quality metrics, such as head motion, are reported. Processed data exhibit classic task activation effects and canonical functional network organization. Overall, we provide a comprehensive and analysis-ready transdiagnostic dataset to accelerate the identification of illness-relevant features of brain functioning, enabling future discoveries in basic and clinical neuroscience.

White Matter Microstructure Alterations in Older Adults With Dyslipidemia Associated With Cognitive and Locomotor Dysfunction Evaluated Using Neurite Orientation Dispersion and Density Imaging

INTRODUCTION

Diffusion tensor imaging (DTI) studies have shown white matter (WM) microstructural alterations in individuals with dyslipidemia; however, DTI indices are not specific to WM pathology. However, neurite orientation dispersion and density imaging (NODDI) provides more specific measurements of WM microstructure. This study aimed to evaluate dyslipidemia-related WM microstructure alterations and their association with cognitive and motor functions using NODDI.

METHODS

The DTI and NODDI metrics were analyzed through tract-based spatial statistics between 24 older adults with dyslipidemia (low-density lipoprotein ≥140 mg/dL, high-density lipoprotein <40 mg/dL, and triglyceride ≥150 mg/dL, or under treatment) and 18 healthy control participants (HCs). Partial correlation tests were performed between diffusion magnetic resonance imaging measures and lipid profiles, cognitive, or locomotor scores in the dyslipidemia and HC groups separately. WM volumetry between HCs and dyslipidemia groups was also assessed. Age, gender, intracranial volume, and years of education were included as covariates in all analyses. A false discovery rate-corrected P value of <0.05 was considered statistically significant.

RESULTS

Individuals with dyslipidemia exhibited a notably reduced neurite density index (NDI) in several WM areas, including the posterior and superior corona radiata, the body, the genu, and the splenium of the corpus callosum, as well as the bilateral anterior and posterior internal capsule, compared with HCs. In the dyslipidemia group, lower NDI was significantly correlated with lower scores on the stand-up test and the Japanese version of the Montreal Cognitive Assessment. No significant differences were found in DTI metrics or WM volumes between dyslipidemia individuals and HCs.

CONCLUSION

Our findings suggest that NODDI can serve as a biomarker for assessing WM microstructural alterations in older adults with dyslipidemia. Particularly, NODDI indicates a lower intra-axonal volume, which may suggest axonal loss associated with dyslipidemia, and correlates with cognitive and locomotor function decline.

Prediction Model and Nomogram for Amyloid Positivity Using Clinical and MRI Features in Individuals With Subjective Cognitive Decline

There is an urgent need for the precise prediction of cerebral amyloidosis using noninvasive and accessible indicators to facilitate the early diagnosis of individuals with the preclinical stage of Alzheimer's disease (AD). Two hundred and four individuals with subjective cognitive decline (SCD) were enrolled in this study. All subjects completed neuropsychological assessments and underwent 18F-florbetapir PET, structural MRI, and functional MRI. A total of 315 features were extracted from the MRI, demographics, and neuropsychological scales and selected using the least absolute shrinkage and selection operator (LASSO). The logistic regression (LR) model, based on machine learning, was trained to classify SCD as either β-amyloid (Aβ) positive or negative. A nomogram was established using a multivariate LR model to predict the risk of Aβ+. The performance of the prediction model and nomogram was assessed with area under the curve (AUC) and calibration. The final model was based on the right rostral anterior cingulate thickness, the grey matter volume of the right inferior temporal, the ReHo of the left posterior cingulate gyrus and right superior temporal gyrus, as well as MoCA-B and AVLT-R. In the training set, the model achieved a good AUC of 0.78 for predicting Aβ+, with an accuracy of 0.72. The validation of the model also yielded a favorable discriminatory ability with an AUC of 0.88 and an accuracy of 0.83. We have established and validated a model based on cognitive, sMRI, and fMRI data that exhibits adequate discrimination. This model has the potential to predict amyloid status in the SCD group and provide a noninvasive, cost-effective way that might facilitate early screening, clinical diagnosis, and drug clinical trials.

White matter trajectories in Down syndrome and Alzheimer's disease: Insights from diffusion tensor-based morphometry

INTRODUCTION

Diffusion magnetic resonance imaging studies investigating Down syndrome (DS) and Alzheimer's disease (AD) have mainly relied on white matter (WM) skeleton-based techniques, potentially overlooking broader WM architecture.

METHOD

We applied diffusion tensor-based morphometry (D-TBM), a novel whole-volume WM registration technique, to characterize WM properties in DS. Between- and within-group analyses were conducted in 51 adults with DS and 35 controls, divided into two age groups, using diffusion tensor imaging (DTI)-derived metrics and local volumetric changes.

RESULTS

DS participants exhibited extensive volumetric and DTI-based differences affecting association fibers and commissures. Within-group comparisons revealed further changes in older DS participants in these fibers. Reduced axial diffusivity (AxD) in temporal and commissural WM was reported for the first time in DS.

DISCUSSION

DTI changes in the older DS cohort affect WM structures supporting language, memory, and executive functions and may be due to AD-related atrophy. Reduced AxD may reflect neuroinflammation or atypical WM development.

HIGHLIGHTS

Diffusion tensor-based morphometry (D-TBM) was applied for the first time in Down syndrome (DS) adults. Diffusion tensor imaging alterations in DS affect structures for language, memory, and executive functions. Increased radial diffusivity and mean diffusivity in older DS adults highlight Alzheimer's disease (AD)-related neurodegeneration in key tracts. AD in DS affects commissural structures, including the genu of the corpus callosum. Axial diffusivity reductions in DS may indicate neuroinflammation.

How does SES influence the brain circuitry for literacy? Modeling the association between SES, oral language, white matter integrity, and reading

Reading is pivotal for educational and occupational success, hence, understanding the factors contributing to reading skill variation is a major educational objective. Although cognitive and neurobiological factors that influence reading are well documented, the contributions of environmental factors, such as socioeconomic status (SES), fiv to reading-related neurobiology are relatively understudied. Studies have shown that SES predicts reading and the integrity of reading-related white matter tracts; however, the direct and indirect contributions of SES to reading via white matter integrity remain undifferentiated. Further, while oral language (both phonological awareness [PA] and vocabulary) has been positively associated with both SES and reading, only a few studies have attempted to model the SES-reading association via oral language, and none of them included white matter integrity. The current study closes these gaps by using Structural Equation Modeling in a large sample of children from the Healthy Brain Network biobank, testing the (in)direct paths by which SES (parental education) influences reading through oral language and white matter integrity. Results reveal an effect of SES on reading that is indirectly affected by oral language, though not by white matter integrity. These findings reinforce the role of oral language skills as a key pathway linking SES and reading.

3D MERMAID: 3D Multi-shot enhanced recovery motion artifact insensitive diffusion for submillimeter, multi-shell, and SNR-efficient diffusion imaging

PURPOSE

To enhance SNR per unit time of diffusion MRI to enable high spatial resolution and extensive q-sampling in a feasible scan time on clinical scanners.

METHODS

3D multi-shot enhanced recovery motion-insensitive diffusion (MERMAID) consists of a whole brain nonselective 3D multi-shot spin-echo sequence with an inversion pulse immediately before the excitation pulse to enhance the recovery of longitudinal magnetization. The excitation flip angle is reduced to the Ernst angle. The sequence includes a trajectory using radially batched internal navigator echoes (TURBINE) readout, where a 3D projection of the FOV is acquired at a different radial angle in every shot. An image-based phase-correction method combined with compressed sensing image reconstruction was developed to correct phase errors between shots. The performance of the 3D MERMAID sequence was investigated using Bloch simulations as well as phantom and human scans at 3 T and then compared to a typical multi-slice 2D spin-echo sequence.

RESULTS

Improvements in SNR per unit time of 70%-240% were observed in phantom and human scans when using 3D MERMAID compared to a single-slice 2D spin-echo sequence. This SNR per unit time improvement allowed scans to be acquired at a nominal isotropic resolution of 0.74 mm and a total of 112 directions across four shells (b = 150, 300, 1000, 2000 s/mm2) in 37 min on a clinical scanner.

CONCLUSION

The 3D MERMAID sequence was shown to significantly improve SNR per unit time compared to multi-slice 2D and 3D diffusion sequences. This SNR improvement allows for shorter scan times and higher spatial and angular resolutions on clinical scanners.

Glymphatic Impairment Associated with Neurocognitive Dysfunction in Moyamoya Disease

Glymphatic system alterations have been proved to be associated with cognitive dysfunction in neurodegenerative diseases. The glymphatic pathway has not been elucidated in moyamoya disease (MMD), which was recognized as a chronic hypoperfusion model for neurodegenerative disease. Here, we aimed to investigate the glymphatic system activity and its relation with neurocognition, and associated hallmarks in MMD. We prospectively recruited 30 MMD patients and 30 matched healthy controls (HC). Participants underwent MRI and neurocognition evaluation. The glymphatic function was assessed by diffusion tensor image analysis along perivascular space (DTI-ALPS) index. Gray matter volume (GMV) and microstructural alterations were calculated. Neurodegenerative-related serum biomarkers were examined. The mediation effect of ALPS index in the associations between variables and neurocognition were further explored. A lower ALPS index was identified in patients with MMD (P < 0.001). The decreased ALPS index was significantly correlated with declined neurocognitive performance. Moreover, the reduced ALPS index was notably linked with lower total GMV% and deep GMV% (P < 0.01). Microstructural changes in the periventricular areas were detected and associated with ALPS index in MMD. Serum neurodegenerative biomarkers (ApoE, Aβ40, Aβ42, and Aβ42/Aβ40) were significantly elevated and related to ALPS index. Additionally, the ALPS index significantly mediated the associations of microstructural alterations and ApoE level with neurocognitive dysfunction. The ALPS index was notably lower MMD in patients, suggesting the utility as a marker of potential glymphatic dysfunction. The index acted as a significant mediator in neurocognitive dysfunction. These findings indicated that glymphatic impairment may interact with MMD-related pathophysiological processes.

Quantitative magnetization transfer and g-ratio imaging of white matter myelin in early psychotic spectrum disorders

Myelin abnormalities in white matter have been implicated in the pathophysiology of psychotic spectrum disorders (PSD), which are characterized by brain dysconnectivity as a core feature. Among evidence from in vivo MRI studies, diffusion imaging findings have largely supported disrupted white matter integrity in PSD; however, they are not specific to myelin changes. Using a multimodal imaging approach, the current study aimed to further delineate myelin and microstructural changes in the white matter of a young PSD cohort. We utilized quantitative magnetization transfer (qMT) imaging combined with advanced diffusion imaging to estimate specific myelin-related biophysical properties in 51 young adult PSD patients compared with 38 age-matched healthy controls. The macromolecular proton fraction (MPF) obtained from qMT was used as a specific marker of myelin content. Additionally, MPF was employed along with diffusion metrics of axonal density (vic) and extra-cellular volume fraction to derive the g-ratio, a measure of relative myelin sheath thickness defined as the ratio of inner to outer axonal diameter. Compared to controls, we observed a widespread MPF reduction and localized g-ratio increase in patients, primarily those with a diagnosis of schizophrenia or depressive schizoaffective disorder. No between-group differences were noted in vic, suggesting similar axonal densities across groups. Correlation analysis revealed that lower MPF was significantly related to poorer working memory performance in PSD, while the HC group showed a positive association for working memory with both g-ratio and vic. The pattern of changes observed in our multimodal imaging markers suggests that PSD, depending on symptomatology, is characterized by specific alterations in white matter integrity and myelin-axonal geometry of major white matter tracts, which may impact working memory function. These findings provide a more detailed view of myelin-related white matter changes in early stages of PSD.

Denoising complex-valued diffusion MR images using a two-step, nonlocal principal component analysis approach

PURPOSE

To propose a two-step, nonlocal principal component analysis (PCA) method and demonstrate its utility for denoising complex diffusion MR images with a few diffusion directions.

METHODS

A two-step denoising pipeline was implemented to ensure accurate patch selection even with high noise levels and was coupled with data preprocessing for g-factor normalization and phase stabilization before data denoising with a nonlocal PCA algorithm. At the heart of our proposed pipeline was the use of a data-driven optimal shrinkage algorithm to manipulate the singular values in a way that would optimally estimate the noise-free signal. Our approach's denoising performances were evaluated using simulation and in vivo human data experiments. The results were compared with those obtained with existing local PCA-based methods.

RESULTS

In both simulation and human data experiments, our approach substantially enhanced image quality relative to the noisy counterpart, yielding improved performances for estimation of relevant diffusion tensor imaging metrics. It also outperformed existing local PCA-based methods in reducing noise while preserving anatomic details. It also led to improved whole-brain tractography relative to the noisy counterpart.

CONCLUSION

The proposed denoising method has the utility for improving image quality for diffusion MRI with a few diffusion directions and is believed to benefit many applications, especially those aiming to achieve high-quality parametric mapping using only a few image volumes.

Multimodal MEG and Microstructure-MRI Investigations of the Human Hippocampal Scene Network

Although several studies have demonstrated that perceptual discrimination of complex scenes relies on an extended hippocampal posteromedial system, we currently have limited insight into the specific functional and structural properties of this system in humans. Here, combining electrophysiological (magnetoencephalography) and advanced microstructural (multishell diffusion magnetic resonance imaging; quantitative magnetization transfer) imaging in healthy human adults (30 females/10 males), we show that both theta power modulation of the hippocampus and fiber restriction/hindrance (reflecting axon packing/myelination) of the fornix (a major input/output pathway of the hippocampus) were independently related to scene, but not face, perceptual discrimination accuracy. Conversely, microstructural features of the inferior longitudinal fasciculus (a long-range occipitoanterotemporal tract) correlated with face, but not scene, perceptual discrimination accuracy. Our results provide new mechanistic insight into the neurocognitive systems underpinning complex scene discrimination, providing novel support for the idea of multiple processing streams within the human medial temporal lobe.

The relationship of white matter tract orientation to vascular geometry in the human brain

The white matter of the human brain exhibits highly ordered anisotropic structures of both axonal nerve fibers and cerebral vasculature. Separately, the anisotropic nature of white matter axons and white matter vasculature have been shown to cause an orientation dependence on various MRI contrasts used to study the structure and function of the brain; however, little is known of the relationship between axonal and vascular orientations. Thus, the aim of this study is to compare the orientation between nerve fibers and vasculature within the white matter. To do this, we use diffusion MRI and susceptibility weighted imaging acquired in the same healthy young adult volunteers and analyze the alignment between white matter fibers and blood vessels in different brain regions, and along different pathways, to determine the degree of alignment between these structures. We first describe vascular orientation throughout the brain and note several regions with consistent orientations across individuals. Next, we find that vasculature does not necessarily align with the dominant direction of white matter in many regions, but, due to the presence of crossing fiber populations, does align with at least some white matter within each MRI voxel. Even though the spatial patterns of blood vessels run in parallel to several white matter tracts, they do not do so along the entire pathway, nor for all pathways, suggesting that vasculature does not supply/drain blood in a tract-specific manner. Overall, these findings suggest that the vascular architecture within the white matter is closely related to, but not the same as, the organization of neural pathways. This study contributes to a better understanding of the microstructural arrangement of the brain and may have implications for interpreting neuroimaging data in health and disease.

Impact of Rod-Dominant Mesopic Conditions on Spatial Summation and Surround Suppression in Early Visual Cortex

Mesopic (dim light) conditions are prevalent in everyday environments, yet most human vision research is conducted under idealized, photopic (bright) conditions. Electrophysiological studies suggest that under mesopic conditions, contrast-encoding retinal ganglion cell receptive fields expand their center width while diminishing surround inhibition. These retinal modifications enhance light capture by increasing the summation area but they limit spatial resolution. However, the impact of mesopic conditions on human cortical spatial integration mechanisms remains unclear. To address this, we investigate how mesopic conditions affect early visuocortical processing, specifically spatial summation and surround suppression. Across two experiments, we acquired fMRI BOLD responses from 11 normally sighted participants of both sexes under photopic and mesopic conditions in visual areas V1-V3. The first experiment estimated population receptive field (pRF) properties while the second experiment assessed cortical surround suppression. Photopic and mesopic psychophysical surround suppression, contrast sensitivity function (CSF), and visual acuity were also measured. At the cortical level, mesopic conditions were associated with smaller pRF sizes, while surround suppression remained robust. At the perceptual level, mesopic conditions led to reduced acuity, lower CSF, and weaker suppression, diverging from the observed cortical effects. Importantly, individual differences linked these findings: participants who exhibited greater mesopic reductions in visual acuity also showed larger decreases in early visuocortical surround suppression, underscoring its role in contrast coding and spatial resolution. Altogether, our fMRI findings contrast with retinal electrophysiology and suggest that early visual cortex may employ distinct, perhaps compensatory, mechanisms in response to reduced retinal input under mesopic conditions.

gammaSTAR: A framework for the development of dynamic, real-time capable MR sequences

PURPOSE

To present the real-time capability and advanced MR sequence library of the MR sequence development framework gammaSTAR.

METHODS

The presented platform consists of four different components: (1) a frontend for sequence development combined with a Python backend for sequence generation; (2) a Lua backend for the creation of hardware instructions; (3) a vendor-specific driver for translation of these instructions into scanner-specific objects; and (4) an interface for real-time feedback capability. In vivo measurements of the same volunteer were performed for comparison of imaging and spectroscopy sequences implemented in this framework with those of one main vendor (Siemens Healthineers) at magnetic field strengths of 3 T and 1.5 T. Prospective motion correction was integrated into a spin echo EPI sequence to demonstrate the real-time feedback capability.

RESULTS

The imaging and spectroscopy results of the gammaSTAR sequences show very similar image contrasts and qualities compared to those by the vendor. ADC maps were calculated and show values of (0.80 ± 0.14)10-3 mm2/s in white matter. Results of pseudo-continuous spin-echo (pCASL GRASE) and 3D radial UTE imaging demonstrate the ability to run complex sequences without long sequence preparation times. Prospective motion correction is possible by means of real-time feedback and shows much fewer movement artifacts with mean voxel displacement of 1.63 mm (uncorrected) versus 0.37 mm (corrected). All images were reconstructed using the vendor's reconstruction pipeline.

CONCLUSION

The platform gammaSTAR allows for MR sequence development with real-time feedback capability demonstrated by a large number of MR sequences and applications.

Structural brain correlates of balance control in children with cerebral palsy: baseline correlations and effects of training

Structural brain abnormalities likely underlie impaired balance control in cerebral palsy (CP). This study investigated whether balance measures were associated with measures derived from conventional MRI and diffusion tensor imaging (DTI), and whether an X-Box One Kinect balance training (6 weeks, 5 days/week, 30 min/session) could induce neuroplastic changes in CP. Twelve children with spastic CP (age:11.3 ± 2.3y) underwent balance evaluation and MRI examination, at baseline and after training. Nine age-matched typically developing (TD) children underwent baseline measurements. Balance control was evaluated testing advanced motor skills (Challenge score) and during gait (medio-lateral Margin of Stability, MoS). With conventional MRI, but especially with DTBM (DTI-based VBM), we found smaller volumes of several deep grey matter structures and within the right inferior parietal cortex, right supramarginal cortex, and left postcentral cortex, and lower fractional anisotropy (FA) and smaller volumes of various white matter regions in CP compared to TD. Within the CP group alone, no correlations within brain tissue were found. After training, Challenge scores of children with CP improved. In an exploratory analysis DTBM showed a trend for volume increase within the right inferior parietal cortex, volume decrease within the right retrolenticular limb of the internal capsule, and an increase of FA within the right corticospinal tract. This indicates that a 6-week balance intervention may induce neuroplastic changes in children with CP. CP-RehOP (trial registration number: NTR6034/NL5854, date of registration: August 26th 2016).

Overlaps of fMRI activation patterns of the anxiety-emotional and the vestibular-sensory networks

Clinical and meta-analytic imaging data suggest a considerable overlap between vestibular-sensory and anxiety-emotional processing networks. We therefore examined functional MRI activation using galvanic vestibular stimulation (GVS) and a fear conditioning paradigm in the same 28 healthy individuals. This study was to proof the effects of both stimulations in the same individual whereas our earlier meta-analytical analysis compared groups of participants who had received only one or the other stimulation. In the actual study we further assessed subjective experience (expectancy ratings, questionnaires) and autonomic arousal (skin conductance response; SCR). Activation patterns during vestibular stimulation confirmed previous findings showing highest fMRI-activation in the parieto-insular vestibular cortex. Fear conditioning activated the anterior insula, secondary somatosensory cortex (S2) and thalamus. A conjunction of fMRI-activation maps for both stimulation paradigms revealed bilateral anterior and posterior insula, dorsolateral prefrontal cortex and S2 as well as cerebellar hemisphere fMRI-activation. Regression analyses showed a high positive association of left anterior insular activation during the fear extinction period with trait anxiety. The vestibular intensity during GVS was positively associated with right ventro-lateral prefrontal cortex (PFC) fMRI-activation. This is compatible with the earlier hypothesized top-down regulation of vestibular perception which involves the PFC beneficial for suppression of unusual vestibular excitation or vertigo related to vestibular disorders.

Functional connectivity of thalamic nuclei during sensorimotor task-based fMRI at 9.4 Tesla

The thalamus is the brain's central communication hub, playing a key role in processing and relaying sensorimotor and cognitive information between the cerebral cortex and other brain regions. It consists of specific and non-specific nuclei, each with a different role. Specific thalamic nuclei relay sensory and motor information to specific cortical and subcortical regions to ensure precise communication. In contrast, non-specific thalamic nuclei are involved in general functions such as attention or consciousness through broader and less targeted connections. In the present study, we aimed to investigate the functional connectivity patterns of the thalamic nuclei identified in our previous study as being involved in motor (finger-tapping) and sensory (finger-touch) tasks. The results of this study show that thalamic nuclei are not static hubs with a predefined role in neural signal processing, as they show different task-specific functional connectivity patterns in the anterior, middle, lateral, and posterior thalamic nuclei. Instead, they are all functional hubs that can flexibly change their connections to other brain regions in response to task demands. This work has important implications for understanding task-dependent functional connectivity between thalamic nuclei and different brain regions using task-based fMRI at 9.4 Tesla.

Longitudinal analysis highlights structural changes in grey- and white-matter within military personnel exposed to blast

OBJECTIVE

The purpose of this study was to determine whether gray matter volume and diffusion-based metrics in associated white matter changed in breachers who had neuroimaging performed at two timepoints. A secondary purpose was to compare these changes in a group who had a one-year interval between their imaging timepoints to a group that had a two-year interval between imaging.

METHODS

Between timepoints, clusters with significantly different gray matter volume were used as seeds for reconstruction of associated structural networks using diffusion metrics.

RESULTS

Of 92 eligible participants, 62 had imaging at two timepoints, 36 with a one-year interval between scans and 26 with a two-year interval between scans. A significant effect of time was documented in the midcingulate cortex, but there was no effect of timepoint (1 versus 2 years). The associated white matter in this cluster had three regions with differences in fractional anisotropy compared to baseline, while there was no effect of timepoint (1 versus 2 years).

CONCLUSIONS

This study provides preliminary evidence that military personnel involved in repetitive exposure to sub-concussive blast overpressures may experience changes to both gray matter and white matter structures.

Insight predicts subsequent memory via cortical representational change and hippocampal activity

The neural mechanisms driving creative problem-solving, including representational change and its relation to memory, still remain largely unknown. We focus on the creative process of insight, wherein rapid knowledge reorganization and integration-termed representational change-yield solutions that evoke suddenness, certainty, positive emotion, and enduring memory. We posit that this process is associated with stronger shifts in activation patterns within brain regions housing solution-relevant information, including the visual cortex for visual problems, alongside regions linked to feelings of emotion, suddenness and subsequent memory. To test this, we collect participants' brain activity while they solve visual insight problems in the MRI. Our findings substantiate these hypotheses, revealing stronger representational changes in visual cortex, coupled with activations in the amygdala and hippocampus-forming an interconnected network. Importantly, representational change and hippocampal effects are positively associated with subsequent memory. This study provides evidence of an integrated insight mechanism influencing memory.

Unbiased Population-Based Statistics to Obtain Pathologic Burden of Injury after Experimental TBI

Reproducibility of scientific data is a current concern throughout the neuroscience field. There are multiple on-going efforts to help resolve this problem. Within the preclinical neuroimaging field, the continued use of a region-of interest (ROI) type approaches combined with the well-known spatial heterogeneity of traumatic brain injury pathology is a barrier to the replicability and repeatability of data. Here we propose the conjoint use of an unbiased analysis of the whole brain after injury together with a population-based statistical analysis of sham-control brains as one approach that has been used in clinical research to help resolve this issue. The approach produces two volumes of pathology that are outside the normal range of sham brains, and can be interpreted as whole brain burden of injury. Using diffusion weighted imaging derived scalars from a tensor analysis of data acquired from adult, male rats at 2, 9 days, 1 and 5 months after lateral fluid percussion injury (LFPI) and in shams (n=73 and 12, respectively), we compared a data-driven, z-score mapping method to a whole brain and white matter-specific analysis, as well as an ROI-based analysis with brain regions preselected by virtue of their large group effect sizes. We show that the data-driven approach is statistically robust, providing the advantage of a large group effect size typical of a ROI analysis of mean scalar values derived from the tensor in regions of gross injury, but without the large multi-region statistical correction required for interrogating multiple brain areas, and without the potential bias inherent with using preselected ROIs. We show that the technique correctly captures the expected longitudinal time-course of the diffusion scalar volumes based on the spatial extent of the pathology and the known temporal changes in scalar values in the LFPI model.

Alterations of diffusion kurtosis measures in gait-related white matter in the "ON-OFF state" of Parkinson's disease

BACKGROUND

Gait impairment is closely related to quality of life in patients with Parkinson's disease (PD). This study aimed to explore alterations in brain microstructure in PD patients and healthy controls (HCs) and to identify the correlation of gait impairment in the ON and OFF states of patients with PD, respectively.

METHODS

We enrolled 24 PD patients and 29 HCs from the Movement Disorders Program at Beijing Friendship Hospital Capital Medical University between 2019 and 2020. We acquired magnetic resonance imaging (MRI) scans and processed the diffusion kurtosis imaging (DKI) images. Preprocessing of diffusion-weighted data was performed with Mrtrix3 software, using a directional distribution function to track participants' main white matter fiber bundles. Demographic and clinical characteristics were recorded. Quantitative gait and clinical scales were used to assess the status of medication ON and OFF in PD patients.

RESULTS

The axial kurtosis (AK), mean kurtosis (MK), and radial kurtosis (RK) of five specific white matter fiber tracts, the bilateral corticospinal tract, left superior longitudinal fasciculus, left anterior thalamic radiation, forceps minor, and forceps major were significantly higher in PD patients compared to HCs. Additionally, the MK values were negatively correlated with Timed Up and Go Test (TUG) scores in both the ON and OFF in PD patients. Within the PD group, higher AK, MK, and RK values, whether the patients were ON or OFF, were associated with better gait performance (i.e., higher velocity and stride length).

CONCLUSIONS

PD exhibits characteristic regional patterns of white matter microstructural degradation. Correlations between objective gait parameters and DKI values suggest that dopamine-responsive gait function depends on preserved white matter microstructure. DKI-based Tract-Based Spatial Statistics (TBSS) analysis may serve as a tool for evaluating PD-related motor impairments (e.g., gait impairment) and could yield potential neuroimaging biomarkers.

Development of a non-invasive novel individual marmoset holder for evaluation by awake functional magnetic resonance brain imaging

BACKGROUND

Although functional MRI (fMRI) in awake marmosets (Callithrix jacchus) is fascinating for functional brain mapping and evaluation of brain disease models, it is difficult to launch awake fMRI on scanners with bore sizes of less than 16 cm. A universal marmoset holder for the small-bore size MRI was designed, and it was evaluated whether this holder could conduct auditory stimulation fMRI in the awake state using 16 cm bore size MRI scanner.

NEW METHOD

The marmoset holder was designed with an outer diameter of 71.9 mm. A holder was designed to allow adjustment according to the individual head shape, enabling the use of the holder universally. An awake fMRI study of auditory response was conducted to evaluate the practicality of the new holder. Whole-brain activation was investigated when marmosets heard the marmoset social communication "phee call" an artificial tone sound and reversed of those.

RESULTS

The prefrontal cortex was significantly activated in response to phee calls, whereas only the auditory cortex was activated in response to pure tones. In contrast, the auditory response was decreased when marmosets heard phee call. Their stimulus-specific responses indicated they perceived and differentiated sound characteristics in the fMRI environment.

COMPARISON WITH EXISTING METHODS

A holder does not require surgical intervention or a custom-made helmet to minimize head movement in a small space.

CONCLUSION

Our newly developed holder made it possible to perform longitudinal fMRI experiments on multiple marmosets in a less invasive manner.

Stable White Matter Structure in the First Three Years After Psychosis Onset

Background

White matter alterations observed using diffusion weighted imaging have become a hallmark of chronic schizophrenia, but it is unclear when these changes arise over the course of the disease. Nearly all studies reported to date have been cross-sectional, so despite their large sample sizes, they cannot determine whether changes accumulate as a degenerative process or patients with preexisting white matter damage are predisposed to more chronic forms of schizophrenia.

Methods

We examined 160 scans comprising 2 years of annual follow-up data from 42 control participants and 28 patients with schizophrenia recruited in the first 2 years since their diagnosis, totaling 2 to 3 scans per participant. We also examined 6-month follow-up data obtained from an ultra-high field (7T) scanner (68 scans; n = 19 patients with first-episode schizophrenia, n = 15 control participants) as a validation dataset. A longitudinal model was used to compare the trajectory of diffusion tensor parameters in patients and control participants.

Results

Positive and negative symptom scores were correlated with diffusion parameters using region of interest-based approaches. No longitudinal differences between patients and control participants were observed for any diffusion tensor imaging parameter in either dataset. However, we did observe consistent associations between white matter alterations and negative symptoms in both datasets.

Conclusions

White matter does not appear to be susceptible to schizophrenia-linked degeneration in the early stages of disease, but preexisting pathology may be linked to disease severity.

Aging-dependent loss of functional connectivity in a mouse model of Alzheimer's disease and reversal by mGluR5 modulator

Amyloid accumulation in Alzheimer's disease (AD) is associated with synaptic damage and altered connectivity in brain networks. While measures of amyloid accumulation and biochemical changes in mouse models have utility for translational studies of certain therapeutics, preclinical analysis of altered brain connectivity using clinically relevant fMRI measures has not been well developed for agents intended to improve neural networks. Here, we conduct a longitudinal study in a double knock-in mouse model for AD (AppNL-G-F/hMapt), monitoring brain connectivity by means of resting-state fMRI. While the 4-month-old AD mice are indistinguishable from wild-type controls (WT), decreased connectivity in the default-mode network is significant for the AD mice relative to WT mice by 6 months of age and is pronounced by 9 months of age. In a second cohort of 20-month-old mice with persistent functional connectivity deficits for AD relative to WT, we assess the impact of two-months of oral treatment with a silent allosteric modulator of mGluR5 (BMS-984923/ALX001) known to rescue synaptic density. Functional connectivity deficits in the aged AD mice are reversed by the mGluR5-directed treatment. The longitudinal application of fMRI has enabled us to define the preclinical time trajectory of AD-related changes in functional connectivity, and to demonstrate a translatable metric for monitoring disease emergence, progression, and response to synapse-rescuing treatment.

Sex-differences in brain multimodal estimates of white matter microstructure during early adolescence: Sex-specific associations with biological factors

Adolescence is marked by significant maturation of brain white matter microstructure, with evidence for sex-specific maturational trajectory. Most studies have examined conventional diffusion tensor imaging (DTI) metrics, which lack specificity to the underlying tissue modifications. In this study, we characterized sex-differences in white matter microstructure cross-sectionally using DTI, advanced diffusion spectrum imaging (DSI) and diffusion kurtosis imaging (DKI), as well as the white matter tract integrity-Watson (WMTI-W) biophysical model. We also aimed to explore the effect of age and biological systems undergoing sex-specific changes during adolescence, namely pubertal hormones, hypothalamic-pituitary-adrenal (HPA)-axis function, and glutathione-redox cycle homeostasis. The results indicate widespread sex-differences in all the white matter derived metrics, suggesting more advanced maturation in females compared to males as well as distinct tissue modifications underlying white matter maturation between males and females during this narrow developmental period. Additionally, the three biological factors explored appeared to be associated with indices of white matter maturation in females specifically, emphasizing this period as critical in female white matter development and sensitivity to environmental factors.

Reduced myelin contributes to cognitive impairment in patients with monogenic small vessel disease

INTRODUCTION

Myelin is pivotal for signal transfer and thus cognition. Cerebral small vessel disease (cSVD) is primarily associated with white matter (WM) lesions and diffusion changes; however, myelin alterations and related cognitive impairments in cSVD remain unclear.

METHODS

We included 64 patients with familial cSVD (i.e., cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy [CADASIL]) and 20 cognitively unimpaired individuals. χ separation applied to susceptibility weighted imaging was used to assess myelin and iron within WM hyperintensities, normal appearing WM, and two strategic fiber tracts. Diffusion-based mean diffusivity and free water were analyzed for comparisons. Cognitive impairment was assessed by the Trail Making Test.

RESULTS

CADASIL patients showed reduced myelin within WM hyperintensities and its penumbra in the normal appearing WM. Myelin was moderately correlated with diffusion and iron changes and associated with slower processing speed controlled for diffusion and iron alterations.

DISCUSSION

Myelin constitutes WM alterations distinct from diffusion changes and substantially contributes to explaining cognitive impairment in cSVD.

HIGHLIGHTS

χ-negative magnetic resonance signal was reduced within white matter hyperintensities and normal appearing white matter in patients with cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, suggesting widespread myelin decreases due to cerebral small vessel disease (cSVD). χ-negative values were only moderately associated with diffusion tensor imaging derived indices including free water and mean diffusivity, suggesting that χ separation depicts distinct microstructural changes in cSVD. Alterations in χ-negative values made a unique contribution to explain processing speed impairment, even when controlled for diffusion and iron changes.

Investigation of electrical conductivity changes during brain functional activity in 3T MRI

Blood oxygenation level-dependent functional magnetic resonance imaging (fMRI) is widely used to visualize brain activation regions by detecting hemodynamic responses associated with increased metabolic demand. Although alternative MRI methods have been employed to monitor functional activities, the investigation of in-vivo electrical property changes during brain function remains limited. In this study, the relationship between fMRI signals and electrical conductivity (measured at the Larmor frequency) changes was explored using phase-based electrical property tomography. Results revealed consistent patterns: conductivity changes showed negative correlations, with conductivity decreasing in functionally active regions whereas B1 phase mapping exhibited positive correlations around the activation regions. These observations were consistent across the motor and visual cortex activations To further substantiate these findings, electromagnetic radio-frequency simulations that modeled activation states with varying conductivities were conducted, demonstrating trends similar to in-vivo results for B1 phase and conductivity. Notably, we observed that false-positive activation signals could occur depending on the level of noise and the reconstruction method applied. These findings suggested that in-vivo electrical conductivity changes can indeed be measured during brain activity. However, further investigation is needed to fully understand the underlying mechanisms driving these measurements.

CLIMATE BRAIN - Questionnaires, Tasks and the Neuroimaging Dataset

Climate change threatens human populations and ecosystems worldwide. Neuroscience research on this topic is emerging, but validated questionnaires, stimuli, and fMRI tasks remain scarce. Here, we present the CLIMATE BRAIN dataset, a multimodal collection of questionnaire, behavioral, and neuroimaging data from 160 young, healthy Polish individuals. Designed to advance research on climate emotions and pro-environmental behavior, the dataset includes individuals with moderate climate change concern. Participants read anger and hope-evoking stories about climate change and made pro-environmental decisions. The dataset includes data from (1) various questionnaire measures, including the Inventory of Climate Emotions (ICE); (2) a neuroimaging task for measuring emotional reactions to standardized Emotional Climate Change Stories (ECCS); and (3) a neuroimaging task based on Carbon Emission Task (CET) to measure climate action-taking. For technical validation, we provide image quality metrics and show the evidence for the effectiveness of the tasks consistent with prior studies. To our knowledge, the proposed multimodal dataset is currently the only publicly available resource specifically designed to investigate human brain responses to climate change.

SpinFlowSim: A blood flow simulation framework for histology-informed diffusion MRI microvasculature mapping in cancer

Diffusion Magnetic Resonance Imaging (dMRI) sensitises the MRI signal to spin motion. This includes Brownian diffusion, but also flow across intricate networks of capillaries. This effect, the intra-voxel incoherent motion (IVIM), enables microvasculature characterisation with dMRI, through metrics such as the vascular signal fraction fV or the vascular Apparent Diffusion Coefficient (ADC) D∗. The IVIM metrics, while sensitive to perfusion, are protocol-dependent, and their interpretation can change depending on the flow regime spins experience during the dMRI measurements (e.g., diffusive vs ballistic), which is in general not known for a given voxel. These facts hamper their practical clinical utility, and innovative vascular dMRI models are needed to enable the in vivo calculation of biologically meaningful markers of capillary flow. These could have relevant applications in cancer, as in the assessment of the response to anti-angiogenic therapies targeting tumour vessels. This paper tackles this need by introducing SpinFlowSim, an open-source simulator of dMRI signals arising from blood flow within pipe networks. SpinFlowSim, tailored for the laminar flow patterns within capillaries, enables the synthesis of highly-realistic microvascular dMRI signals, given networks reconstructed from histology. We showcase the simulator by generating synthetic signals for 15 networks, reconstructed from liver biopsies, and containing cancerous and non-cancerous tissue. Signals exhibit complex, non-mono-exponential behaviours, consistent with in vivo signal patterns, and pointing towards the co-existence of different flow regimes within the same network, as well as diffusion time dependence. We also demonstrate the potential utility of SpinFlowSim by devising a strategy for microvascular property mapping informed by the synthetic signals, and focussing on the quantification of blood velocity distribution moments and of an apparent network branching index. These were estimated in silico and in vivo, in healthy volunteers scanned at 1.5T and 3T and in 13 cancer patients, scanned at 1.5T. In conclusion, realistic flow simulations, as those enabled by SpinFlowSim, may play a key role in the development of the next-generation of dMRI methods for microvascular mapping, with immediate applications in oncology.

Aging of gray matter microstructure: A brain-wide characterization of age group differences using NODDI

This study aimed to provide a complete characterization of age group differences in cortical lobar, hippocampal, and subcortical gray matter microstructure using a multi-compartment diffusion-weighted MRI (DWI) approach with parameters optimized for gray matter (Neurite Orientation Dispersion and Density Imaging, NODDI). 76 younger (undergraduate students) and 64 older (surrounding communities) adults underwent diffusion-, T1-, and susceptibility-weighted MRI. Results revealed eight unique patterns across the 12 regions of interest in the relative direction and magnitude of age effects across NODDI metrics, which were grouped into three prominent patterns: cortical gray matter had predominantly higher free diffusion in older than younger adults, the hippocampus and amygdala had predominantly higher dispersion of diffusion and intracellular diffusion in older than younger adults, and the putamen and globus pallidus had lower dispersion of diffusion in older than younger adults. Results remained largely unchanged after controlling for normalized regional volume, suggesting that higher free diffusion in older than younger adults in cortical gray matter was not driven by macrostructural atrophy. Results also remained largely unchanged after controlling for iron content (QSM, R2*), even in iron-rich subcortical regions. Taken together, these patterns of age effects across NODDI metrics provide evidence of region-specific neurobiological substrates of aging of gray matter microstructure.

Versatile MRI acquisition and processing protocol for population-based neuroimaging

Neuroimaging has an essential role in studies of brain health and of cerebrovascular and neurodegenerative diseases, requiring the availability of versatile magnetic resonance imaging (MRI) acquisition and processing protocols. We designed and developed a multipurpose high-resolution MRI protocol for large-scale and long-term population neuroimaging studies that includes structural, diffusion-weighted and functional MRI modalities. This modular protocol takes almost 1 h of scan time and is, apart from a concluding abdominal scan, entirely dedicated to the brain. The protocol links the acquisition of an extensive set of MRI contrasts directly to the corresponding fully automated data processing pipelines and to the required quality assurance of the MRI data and of the image-derived phenotypes. Since its successful implementation in the population-based Rhineland Study (ongoing, currently more than 11,000 participants, target participant number of 20,000), the proposed MRI protocol has proved suitable for epidemiological and clinical cross-sectional and longitudinal studies, including multisite studies. The approach requires expertise in magnetic resonance image acquisition, in computer science for the data management and the execution of processing pipelines, and in brain anatomy for the quality assessment of the MRI data. The protocol takes ~1 h of MRI acquisition and ~20 h of data processing to complete for a single dataset, but parallelization over multiple datasets using high-performance computing resources reduces the processing time. By making the protocol, MRI sequences and pipelines available, we aim to contribute to better comparability, interoperability and reusability of large-scale neuroimaging data.

Brain and cardiovascular responses to acute stress in remitted and recurrent late-life depression

In individuals with remitted late-life depression (LLD), stress exposure can increase the likelihood of a new, recurrent depressive episode. Variability in the effect of stress on recurrence risk may reflect underlying brain and physiological processes mediating the stress response. We examined how subjective, physiological, and brain responses to an experimental stressor differs in older adults with and without remitted depression, and how these stress responses relate to future relapse. Participants were recruited through 3 sites and included 76 older adults with remitted LLD and 36 age-matched healthy comparison (HC) adults. Participants completed an acute stressor task during functional brain imaging with behavioral and cardiovascular monitoring. Remitted LLD participants were followed longitudinally to evaluate depression recurrence. Compared to HC, the remitted LLD group exhibited reduced stressor-evoked systolic blood pressure and heart rate responses, as well as reduced stressor-evoked posterior insula activity. This blunted stress response phenotype appeared more specific to the stable remitter group than the relapsing LLD group. Survival analyses demonstrated that greater stressor-evoked bed nucleus of the stria terminalis (BNST) activity was associated with faster time to recurrence. These findings add to a growing literature reporting so-called "blunted" stressor-evoked cardiovascular and brain reactivity in remitted depression. Moreover, they link the stress response in visceral interoceptive brain circuits with relapse vulnerability. Future work involving longer follow-up periods may reveal additional stress-related brain and behavioral predictors of recurrence in remitted LLD.

Quantitative T1 mapping indicates elevated white matter myelin in children with RASopathies

BACKGROUND

Evidence suggests a pathological role of myelination in neurodevelopmental disorders with links to cognitive difficulties, but in vivo assessment remains challenging. Quantitative T1 mapping (QT1) has been used in prior clinical studies (e.g., of multiple sclerosis) and shows promise for reliable measurement of myelin alterations. We investigated QT1 for measuring myelination in children with neurodevelopmental disorders of the RAS-MAPK signaling pathway (RASopathies).

METHODS

We collected QT1, diffusion-weighted, and structural MRI scans from 72 children (49 RASopathies, 23 typical developing (TD)). QT1 myelin content measures included white matter macromolecular tissue volume (MTV) and cortical R1 (1/T1 relaxation). Group differences were assessed across 39 white matter tracts. Principal components analysis captured cortical myelination patterns across 360 regions, followed by a MANOVA. A support vector machine (SVM) identified the most discriminative features between-groups.

RESULTS

Thirty-four of 39 tracts were higher in MTV in RASopathies relative to TD (pFDR<.05), indicating widespread elevation in myelination. MANOVA revealed a group effect on cortical R1 (p=.002, η2=.028), suggesting cortical myelination differences between-groups. The SVM yielded an accuracy of 87% and identified cognitive and cortical R1 features as the most discriminant between-groups.

CONCLUSIONS

We found widespread elevated white matter tract myelin and region-dependent cortical myelination patterns in children with RASopathies. Leveraging preclinical models showing oligodendrocyte dysfunction, QT1 revealed precocious myelination. Further work is needed to explore relationships with cognition. QT1 is a promising tool for identification and monitoring of myelin as a treatment target in neurodevelopmental disorders, offering significant potential for advancing current therapeutic strategies.

Functional connectivity between the visual and salience networks and autistic social features at school-age

BACKGROUND

Autism spectrum disorder (ASD) is highly heritable and phenotypically variable. Neuroimaging markers reflecting variation in behavior will provide insights into circuitry subserving core features. We examined functional correlates of ASD symptomology at school-age, while accounting for associated behavioral and cognitive domains, in a longitudinal sample followed from infancy and enriched for those with a genetic liability for ASD.

METHODS

Resting state functional connectivity MRIs (fcMRI) and behavioral data were analyzed from 97 school-age children (8.1-12.0 years, 55 males, 15 ASD) with (n = 63) or without (n = 34) a family history of ASD. fcMRI enrichment analysis (EA) was used to screen for associations between network-level functional connectivity and six behaviors of interest in a data-driven manner: social affect, restricted and repetitive behavior (RRB), generalized anxiety, inattention, motor coordination, and matrix reasoning.

RESULTS

Functional connectivity between the visual and salience networks was significantly associated with social affect symptoms at school-age after accounting for all other behaviors. Results indicated that stronger connectivity was associated with higher social affect scores. No other behaviors were robustly associated with functional connectivity, though trends were observed between visual-salience connectivity and RRBs.

CONCLUSIONS

Connectivity between the visual and salience networks may play an important role in social affect symptom variability among children with ASD and those with genetic liability for ASD. These findings align with and extend earlier reports in this sample of the central role of the visual system during infancy in ASD.

Ultrafast and robust T 2 mapping using optimized single-shot multi-echo planar imaging with alternating blips

PURPOSE

To develop a rapid, motion-robustT 2 $$ {\mathrm{T}}_2 $$ mapping technique suitable for clinical use across the body, including traditionally challenging, motion-prone patient populations or body parts.

METHODS

A novel single-shot multi-echo spin-echo EPI sequence with alternating phase encoding direction on each echo was implemented. This sequence acquires multiple echoes to measureT 2 $$ {\mathrm{T}}_2 $$ from a single RF excitation. The alternating phase encoding gradient polarity enables the correction of geometric distortions in EPI using post-processing software. Stimulated echoes were removed by optimizing spoiler gradients. Diffusion MRI can also be achieved by incorporating diffusion-encoding gradients.

RESULTS

Phantom experiments showed no significant difference between measured and referenceT 2 $$ {\mathrm{T}}_2 $$ values, indicating high precision and repeatability. In vivo, brainT 2 $$ {\mathrm{T}}_2 $$ maps exhibited similar anatomical detail and tissue contrast as a reference sequence, withT 2 $$ {\mathrm{T}}_2 $$ values of 70.0 ± $$ \kern0.5em \pm \kern0.5em $$  4.0 ms for gray matter, 56.8 ± $$ \kern0.5em \pm \kern0.5em $$  3.4 ms for the white matter at a magnetic field strength of 3 Tesla. High-quality diffusion-weighted images with minimal distortion were generated, even at high b-values.T 2 $$ {\mathrm{T}}_2 $$ mapping results from the kidney and fetal brain showcased the method's applicability across different anatomical regions and patient populations.

CONCLUSION

The single-shot multi-echo EPI sequence provided a basis for rapid, accurateT 2 $$ {\mathrm{T}}_2 $$ relaxation mapping by correcting distortion and mitigating motion artifacts. This sequence enhances the clinical feasibility of quantitativeT 2 $$ {\mathrm{T}}_2 $$ mapping across diverse patient populations and body areas.

Asymmetric white matter degeneration in amyotrophic lateral sclerosis: a diffusion kurtosis imaging study of motor and extra-motor pathways

Background

Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disease that lacks effective early biomarkers. This study investigated the potential of diffusion kurtosis imaging (DKI) as a non-invasive biomarker for detecting and monitoring ALS progression through a comprehensive analysis of white matter alterations.

Methods

We performed a cross-sectional analysis of magnetic resonance images with advanced diffusion imaging techniques in ALS patients recruited from a neurodegenerative consultation service over a 3-year period and healthy controls. Our methodology employed multi-shell multi-tissue constrained spherical deconvolution (MSMT-CSD) for tract reconstruction and diffusion kurtosis imaging for microstructural analysis. The study focused particularly on the corticospinal tract and associated pathways, utilizing both tract-specific Bundle Analytics (BUAN) and whole-brain Tract-Based Spatial Statistics (TBSS) approaches.

Results

The study included 33 ALS patients and 37 controls with no significant differences in age or gender. ALS patients predominantly presented with spinal onset and exhibited moderate functional impairment (ALSFRS-R: 39.09 ± 5). Whole-brain TBSS revealed widespread white matter alterations, with increased MD, RD, and AD, and decreased FA notably in the corona radiata, internal capsule, and corticospinal tracts. Detailed fiber tracking of the corticospinal tracts showed significant microstructural changes, with the left CST displaying pronounced increases in MD and AD alongside reduced FA, while the right CST exhibited distinctive regional variations. Additionally, analyses of the frontopontine and parietopontine tracts uncovered further alterations in diffusion metrics. Despite imaging findings, clinical-radiological correlations with functional scores and disease progression were not statistically significant.

Conclusions

This study explores DKI as a potential biomarker for ALS pathology, revealing microstructural changes in both motor and extra-motor pathways. Using whole-brain TBSS analysis and tractography with DIPY, we identified an asymmetric pattern of degeneration and involvement of integrative neural networks, providing new insights into ALS pathophysiology. These findings contribute to our understanding of the complex structural alterations in ALS and suggest that DKI-derived metrics may have utility in characterizing the disease process.

Low-intensity transcranial focused ultrasound amygdala neuromodulation: a double-blind sham-controlled target engagement study and unblinded single-arm clinical trial

Mood, anxiety, and trauma-related disorders (MATRDs) are highly prevalent and comorbid. A sizable number of patients do not respond to first-line treatments. Non-invasive neuromodulation is a second-line treatment approach, but current methods rely on cortical targets to indirectly modulate subcortical structures, e.g., the amygdala, implicated in MATRDs. Low-intensity transcranial focused ultrasound (tFUS) is a non-invasive technique for direct subcortical neuromodulation, but its safety, feasibility, and promise as a potential treatment is largely unknown. In a target engagement study, magnetic resonance imaging (MRI)-guided tFUS to the left amygdala was administered during functional MRI (tFUS/fMRI) to test for acute modulation of blood oxygenation level dependent (BOLD) signal in a double-blind, within-subject, sham-controlled design in patients with MATRDs (N = 29) and healthy comparison subjects (N = 23). In an unblinded treatment trial, the same patients then underwent 3-week daily (15 sessions) MRI-guided repetitive tFUS (rtFUS) to the left amygdala to examine safety, feasibility, symptom change, and change in amygdala reactivity to emotional faces. Active vs. sham tFUS/fMRI reduced, on average, left amygdala BOLD signal and produced patient-related differences in hippocampal and insular responses. rtFUS was well-tolerated with no serious adverse events. There were significant reductions on the primary outcome (Mood and Anxiety Symptom Questionnaire General Distress subscale; p = 0.001, Cohen's d = 0.77), secondary outcomes (Cohen's d of 0.43-1.50), and amygdala activation to emotional stimuli. Findings provide initial evidence of tFUS capability to modulate amygdala function, rtFUS safety and feasibility in MATRDs, and motivate double-blind randomized controlled trials to examine efficacy.ClinicalTrials.gov registration: NCT05228964.

Associations among white matter microstructural changes and the development of emotional reactivity and regulation in infancy

Deficits in emotional reactivity and regulation assessed in infancy, including high levels of negative emotionality (NE), low positive emotionality (PE) and low soothability, can predict future affective and behavioral disorders. White matter (WM) tracts develop rapidly in the first postnatal year, paralleling the development of emotional regulation. During this period, examining the development of white matter microstructure in tracts connecting cortical and/or subcortical regions supporting emotional regulation, including the cingulum bundle (CB), uncinate fasciculus (UF), and forceps minor (FM), can provide neural markers reflecting pathophysiological processes underlying early emotional regulation development. The Neurite Orientation Dispersion and Density Imaging (NODDI) model can be used to estimate with high intercellular specificity microstructural integrity and myelination using the neurite density index (NDI), and dispersion, using the orientation dispersion index (ODI). Examining relationships among changes in WM tract NODDI measures and changes in emotional reactivity and regulation during the first 3-to-9-months of age (n = 39), we showed that larger 3-to-9-month increases in right UF, FM, and left CB ODI were associated with larger decreases or smaller increases in soothability during this period, while a larger increase in right UF NDI was associated with a smaller increase in PE. These findings suggest that in infancy, larger microstructural changes in major WM tracts interconnecting neural networks supporting emotional regulation are associated with disrupted development of PE and soothability. These findings could provide early neural markers of child emotional dysregulation and may have implications for future affective or behavioral trajectories.

Late-onset GM2 gangliosidosis: magnetic resonance imaging, diffusion tensor imaging, and correlational fiber tractography differentiate Tay-Sachs and Sandhoff diseases

GM2 gangliosidosis is lysosomal storage disorder caused by deficiency of the heterodimeric enzyme β-hexosaminidase A. Tay-Sachs disease is caused by variants in HEXA encoding the α-subunit and Sandhoff disease is caused by variants in HEXB encoding the β-subunit. Due to shared clinical and biochemical findings, the two have been considered indistinguishable. We applied T1-weighted volumetric analysis, diffusion tensor imaging (DTI), and correlational fiber tractography to assess phenotypic differences in these two diseases. 51 T1-weighted and 40 DTI scans from 19 Late-Onset GM2 patients with either late-onset Sandhoff disease (LOSD), or late-onset Tay-Sachs (LOTS) were included and compared to 1033 neurotypical control volumetric MRI scans. LOTS patients had significantly smaller cerebellum volume compared to neurotypical controls (p < 0.0001) and LOSD patients (p < 0.0001). There was no statistical difference for the volume of any structure between LOSD and neurotypical controls. DTI analysis showed LOTS patients had higher mean diffusivity (MD) in the left cerebellum (p = 0.003703), right cerebellum (p = 0.003435), superior cerebellar peduncle (p = 0.007332), and vermis (p = 0.01007) compared to LOSD. LOTS patients had lower fractional anisotropy (FA) in the left cerebellum (p = 0.005537), right cerebellum (p = 0.01905), SCP (p = 0.02844), and vermis (p = 0.02469) when compared to LOSD. Correlational fiber tractography identified fiber tracts in cerebellar pathways with higher FA and lower MD in LOSD patients compared to LOTS patients. Our study shows neurobiologic differences between these two related disorders. To our knowledge, this is the first study using correlational tractography in a lysosomal storage disorder. This result indicates a greater burden of cerebellar pathology in LOTS patients compared with LOSD patients.

Molecular-Informed Network Analysis Unveils Fatigue-Related Functional Connectivity in Parkinson's Disease

BACKGROUND

Fatigue in Parkinson's disease (PD) is a prevalent and debilitating non-motor symptom. Despite its significant impact on quality of life, the underlying neurochemical and network-based mechanisms remain poorly understood.

OBJECTIVES

This observational study applied a multimodal imaging approach to explore potential links between the functional connectivity of neurotransmitter-specific circuits and fatigue in a sample of patients with PD.

METHODS

We acquired resting-state functional magnetic resonance imaging data in 35 patients with PD including 18 with clinically significant fatigue and 17 without. We applied the receptor-enriched analysis of functional connectivity by targets (REACT) pipeline to derive patients' specific molecularly enriched networks informed by the spatial distribution of the dopamine, noradrenaline, serotonin transporters, and metabotropic glutamate 5 receptors as assessed using molecular imaging data in independent samples of healthy controls. We then conducted whole-brain analyses inspecting both categorical differences between groups of patients with and without clinically significant fatigue, and associations exploring the full within-sample variation in symptom ratings.

RESULTS

We found a significant decrease in noradrenaline-enriched and glutamate-enriched functional connectivity in key regions, belonging to the sensorimotor, salience, and default mode network, with increasing fatigue severity. Notably, noradrenaline-enriched functional connectivity reductions were widespread, while glutamate-enriched functional connectivity reductions were more restricted to the supplementary motor area. No significant relationships between fatigue and dopamine or serotonin-enriched functional connectivity were found.

CONCLUSIONS

These findings offer supportive evidence for the putative involvement of the noradrenaline and glutamate systems in the genesis of fatigue in PD, opening new directions for treatment development exploring these neurochemical systems. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Individual variation in the chimpanzee arcuate fasciculus predicts vocal and gestural communication

Whether language has its evolutionary origins in vocal or gestural communication has long been a matter of debate. In humans, the arcuate fasciculus, a major fronto-temporal white matter tract, is left-lateralized, is larger than in nonhuman apes, and is linked to language. However, the extent to which the arcuate fasciculus of nonhuman apes is linked to vocal and/or manual communication is currently unknown. Here, using probabilistic tractography in 67 chimpanzees (45 female, 22 male), we report that the chimpanzee arcuate fasciculus is not left-lateralized at the population level, in marked contrast with humans. However, individual variation in the anatomy and leftward asymmetry of the chimpanzee arcuate fasciculus is associated with individual variation in the use of both communicative gestures and communicative sounds under volitional orofacial motor control. This indicates that the arcuate fasciculus likely supported both vocal and gestural communication in the chimpanzee/human last common ancestor, 6-7 million years ago.

White-Matter fiber tract and resting-state functional connectivity abnormalities in young children with autism spectrum disorder

Autism spectrum disorder (ASD) is a complex developmental disorder characterized by difficulties in social interaction and communication and repetitive behaviors. Although abnormal brain development has been shown to exist in children with ASD, the link between structural brain abnormalities and resting-state functional connectivity (rsFC) disruptions in children with ASD remains understudied. To address this limitation, we utilized the population-based bundle-to-region connectome, providing a detailed understanding of the connectivity between cortical regions and white matter (WM) tracts. By precisely indexing WM-Gray Matter (GM) interactions, we investigated the rsFC of the cortex-associated ROIs to explore the association between structural and rsFC abnormalities and clinical symptoms in young children with ASD. This MRI study identified significant differences in WM structure and rsFC between children with ASD (n = 34) and typically developing children (TD, n = 43). Our results showed that decreased fractional anisotropy (FA) and increased mean diffusivity (MD) and radial diffusivity (RD) in ASD WM tracts compared to TD, particularly in left hemisphere tracts (anterior thalamic radiation [ATR], cingulum, inferior fronto-occipital fasciculus [IFOF], inferior longitudinal fasciculus [ILF], superior longitudinal fasciculus [SLF], and uncinate fasciculus [UF]). Abnormal rsFC was observed in GM areas connected by ATR, cingulum, IFOF, ILF, and SLF. Furthermore, abnormalities in the structural and functional connectivity index (SFCI) within the SLF and cingulum were identified. An association has been observed between these abnormalities and clinical symptoms. Specifically, SLF structural and functional connectivity appear to be associated with repetitive and restrictive behavior (RRB), while cingulum connectivity is associated with communication abilities. In conclusion, young children with ASD exhibit abnormal WM tract structures and associated rsFC abnormalities. These differences highlight significant disruptions in rsFC mapped from WM tracts to cortical areas in ASD, correlating with the severity of ASD symptoms, and suggest the importance of multi-modal imaging in capturing these variations.

Exploring the causal involvement of the rIPL and white matter interindividual variability in spatial orienting and consciousness

BACKGROUND

Spatial attention enables the selection of relevant over irrelevant stimuli through dorsal and ventral fronto-parietal networks. These networks are connected through long white matter tracts, such as the superior longitudinal fasciculus (SLF) and the Inferior Fronto-Occipital fasciculus (IFOF).

OBJECTIVE/HYPOTHESIS

The main purpose of this study was to explore, in healthy participants, the causal role of the right Inferior Parietal Lobe (rIPL) in spatial orienting and conscious perception. We also explored how interindividual differences in the microstructural properties of white matter were related to the effects of transcranial magnetic stimulation (TMS) and, secondarily, to attentional orienting effects in the control stimulation condition.

METHODS

Participants (n=51) performed a behavioural task involving the detection of a visual stimulus at the threshold of consciousness, preceded by either central (endogenous) or peripheral (exogenous) cues. After cue onset, a burst of TMS pulses was applied over the rIPL or a control active region (vertex). White matter properties were explored through diffusion-weighted imaging tractography and whole-brain NODDI analysis.

RESULTS

TMS over the rIPL (compared to the control condition) did not modulate spatial attention nor conscious perception, but it decreased accuracy when attention was endogenously oriented (compared to the exogenous condition) and speeded up reaction times when targets were presented in the attended right hemifield (compared to the left hemifield). Part of the variability in the TMS and attentional orienting effects were explained by the integrity of the SLF and the IFOF.

CONCLUSIONS

Individual variability in attentional orienting effects was associated with the anatomical links between attentional networks. Negative correlations between TMS effects and relevant white matter tracts were interpreted as compensatory mechanisms, while positive correlations with tracts innervating the stimulated area could reflect a TMS signal propagation effect. These results will contribute to the understanding of the role of white matter variability in the susceptibility to neuromodulation, with potential implications for research and clinical treatment.

Latent preference representation in the human brain for scented products: Effects of novelty and familiarity

Decoding latent preferences for novel products is crucial for understanding decision-making processes, especially when subjective evaluations are unclear. Brain activity in regions like the medial orbitofrontal cortex and nucleus accumbens (NAcc) correlates with subjective preferences. However, whether these regions represent preferences toward novel products and whether coding persists after familiarity remain unclear. We examined the brain coding of latent preferences for novel scented products and how they evolve with familiarity. We measured functional magnetic resonance imaging (fMRI) signals evoked by three fabric softener odors, both when novel and when familiar, in 25 previously unexposed females. To obtain reliable preferences, participants chose one softener after using all three twice at home after the first fMRI measurement (Day 1) and continued using it at home for four months until the second day of the fMRI measurement (Day 2). Subjective ratings were also obtained after each fMRI run. On Day 1, no significant differences in subjective ratings between selected and non-selected odors were found. However, the decoding analysis revealed that future odor preferences for novel products were coded in several regions, including the left superior frontal lobe (SF), right NAcc, and left piriform cortex. On Day 2, the left SF continued to encode preferences after familiarity. These results suggest that odor preferences for novel products are coded in the brain even without conscious awareness, and that the coding in the SF is robust against familiarity. These findings provide insights into a more comprehensive understanding of the brain coding of latent preferences.

Ex vivo brain MRI to assess conventional and FLASH brain irradiation effects

BACKGROUND AND PURPOSE

The FLASH effect expands the therapeutic ratio of tumor control to normal tissue toxicity observed after delivery of ultra-high (>100 Gy/s FLASH-RT) vs. conventional dose rate radiation (CONV-RT). In this first exploratory study, we assessed whether ex vivo Magnetic Resonance Imaging (MRI) could reveal long-term differences after FLASH-RT and CONV-RT whole-brain irradiation.

MATERIALS AND METHODS

Female C57BL/6 mice were divided into three groups: control (non-irradiated), conventional (CONV-RT 0.1 Gy/s), and ultra-high dose rates (FLASH-RT 1 pulse, 5.5 x 10^6 Gy/s), and received 10 Gy of whole-brain irradiation in a single fraction at 10 weeks of age. Mice were evaluated by Novel Object Recognition cognitive testing at 10 months post-irradiation and were sampled at 13 months post-irradiation. Ex vivo brains were imaged with a 14.1 Tesla/26 cm magnet with a multimodal MRI protocol, including T2-weighted TurboRare (T2W) and diffusion-weighted imaging (DWI) sequences.

RESULTS

In accordance with previous results, cognitive tests indicated that animals receiving CONV-RT exhibited a decline in cognitive function, while FLASH-RT performed similarly to the controls. Ex vivo MRI showed decreased hippocampal mean intensity in the CONV-RT mice compared to controls, but not in the FLASH-RT group. Comparing CONV-RT to control, we found significant changes in multiple whole-brain diffusion metrics, including the mean Apparent Diffusion Coefficient (ADC) and Mean Apparent Propagator (MAP) metrics. By contrast, no significant diffusion changes were found between the FLASH-RT and control groups. In an exploratory analysis, compared to controls, regional diffusion metrics were primarily altered in the basal forebrain and the insular cortex after conventional radiation therapy (CONV-RT), and to a lesser extent after flash radiation therapy (FLASH-RT).

CONCLUSION

This study presents initial evidence that ex vivo MRI uncovered changes in the brain after CONV-RT but not after FLASH-RT. The study indicates the potential use of ex vivo MRI to analyze the brain radiation responses at different dose rates.