Diffusion kurtosis MRI tracks gray matter myelin content in the primate cerebral cortex

Diffusion magnetic resonance imaging (dMRI) has been widely employed to model the trajectory of myelinated fiber bundles in white matter. Increasingly, dMRI is also used to assess local tissue properties throughout the brain. In the cerebral cortex, myelin content is a critical indicator of the maturation, regional variation, and disease related degeneration of gray matter tissue. Gray matter myelination can be measured and mapped using several non-diffusion MRI strategies; however, first order diffusion statistics such as fractional anisotropy (FA) show only weak spatial correlation with cortical myelin content. Here we show that a simple higher order diffusion parameter, the mean diffusion kurtosis (MK), is strongly correlated with the laminar and regional variation of myelin in the primate cerebral cortex. We carried out ultra-high resolution, multi-shelled dMRI in ex vivo marmoset monkey brains and compared dMRI parameters from a number of higher order models (diffusion kurtosis, NODDI and MAP MRI) to the distribution of myelin obtained using histological staining, and via Magnetization Transfer Ratio MRI (MTR), a non-diffusion MRI method. In contrast to FA, MK closely matched the myelin content assessed by histology and by MTR in the same sample. The parameter maps from MAP-MRI and NODDI also showed good correspondence with cortical myelin content. The results demonstrate that dMRI can be used to assess the variation of local myelin content in the primate cortical cortex, which may be of great value for assessing tissue integrity and tracking disease in living human patients.

High-resolution cortical MAP-MRI reveals areal borders and laminar substructures observed with histological staining

The variations in cellular composition and tissue architecture measured with histology provide the biological basis for partitioning the brain into distinct cytoarchitectonic areas and for characterizing neuropathological tissue alterations. Clearly, there is an urgent need to develop whole-brain neuroradiological methods that can assess cortical cyto- and myeloarchitectonic features non-invasively. Mean apparent propagator (MAP) MRI is a clinically feasible diffusion MRI method that quantifies efficiently and comprehensively the net microscopic displacements of water molecules diffusing in tissues. We investigate the sensitivity of high-resolution MAP-MRI to detecting areal and laminar variations in cortical cytoarchitecture and compare our results with observations from corresponding histological sections in the entire brain of a rhesus macaque monkey. High-resolution images of MAP-derived parameters, in particular the propagator anisotropy (PA), non-gaussianity (NG), and the return-to-axis probability (RTAP) reveal cortical area-specific lamination patterns in good agreement with the corresponding histological stained sections. In a few regions, the MAP parameters provide superior contrast to the five histological stains used in this study, delineating more clearly boundaries and transition regions between cortical areas and laminar substructures. Throughout the cortex, various MAP parameters can be used to delineate transition regions between specific cortical areas observed with histology and to refine areal boundaries estimated using atlas registration-based cortical parcellation. Using surface-based analysis of MAP parameters we quantify the cortical depth dependence of diffusion propagators in multiple regions-of-interest in a consistent and rigorous manner that is largely independent of the cortical folding geometry. The ability to assess cortical cytoarchitectonic features efficiently and non-invasively, its clinical feasibility, and translatability make high-resolution MAP-MRI a promising 3D imaging tool for studying whole-brain cortical organization, characterizing abnormal cortical development, improving early diagnosis of neurodegenerative diseases, identifying targets for biopsies, and complementing neuropathological investigations.

Diffusion MRI anisotropy in the cerebral cortex is determined by unmyelinated tissue features

Diffusion magnetic resonance imaging (dMRI) is commonly used to assess the tissue and cellular substructure of the human brain. In the white matter, myelinated axons are the principal neural elements that shape dMRI through the restriction of water diffusion; however, in the gray matter the relative contributions of myelinated axons and other tissue features to dMRI are poorly understood. Here we investigate the determinants of diffusion in the cerebral cortex. Specifically, we ask whether myelinated axons significantly shape dMRI fractional anisotropy (dMRI-FA), a measure commonly used to characterize tissue properties in humans. We compared ultra-high resolution ex vivo dMRI data from the brain of a marmoset monkey with both myelin- and Nissl-stained histological sections obtained from the same brain after scanning. We found that the dMRI-FA did not match the spatial distribution of myelin in the gray matter. Instead dMRI-FA was more closely related to the anisotropy of stained tissue features, most prominently those revealed by Nissl staining and to a lesser extent those revealed by myelin staining. Our results suggest that unmyelinated neurites such as large caliber apical dendrites are the primary features shaping dMRI measures in the cerebral cortex.

Sensitive detection of extremely small iron oxide nanoparticles in living mice using MP2RAGE with advanced image co-registration

Magnetic resonance imaging (MRI) is a widely used non-invasive methodology for both preclinical and clinical studies. However, MRI lacks molecular specificity. Molecular contrast agents for MRI would be highly beneficial for detecting specific pathological lesions and quantitatively evaluating therapeutic efficacy in vivo. In this study, an optimized Magnetization Prepared—RApid Gradient Echo (MP-RAGE) with 2 inversion times called MP2RAGE combined with advanced image co-registration is presented as an effective non-invasive methodology to quantitatively detect T1 MR contrast agents. The optimized MP2RAGE produced high quality in vivo mouse brain T1 (or R1 = 1/T1) map with high spatial resolution, 160 × 160 × 160 µm³ voxel at 9.4 T. Test–retest signal to noise was > 20 for most voxels. Extremely small iron oxide nanoparticles (ESIONPs) having 3 nm core size and 11 nm hydrodynamic radius after polyethylene glycol (PEG) coating were intracranially injected into mouse brain and detected as a proof-of-concept. Two independent MP2RAGE MR scans were performed pre- and post-injection of ESIONPs followed by advanced image co-registration. The comparison of two T1 (or R1) maps after image co-registration provided precise and quantitative assessment of the effects of the injected ESIONPs at each voxel. The proposed MR protocol has potential for future use in the detection of T1 molecular contrast agents.

Ultrahigh-resolution MRI Reveals Extensive Cortical Demyelination in a Nonhuman Primate Model of Multiple Sclerosis

Cortical lesions are a primary driver of disability in multiple sclerosis (MS). However, noninvasive detection of cortical lesions with in vivo magnetic resonance imaging (MRI) remains challenging. Experimental autoimmune encephalomyelitis (EAE) in the common marmoset is a relevant animal model of MS for investigating the pathophysiological mechanisms leading to brain damage. This study aimed to characterize cortical lesions in marmosets with EAE using ultrahigh-field (7 T) MRI and histological analysis. Tissue preparation was optimized to enable the acquisition of high-spatial resolution (50-μm isotropic) T2*-weighted images. A total of 14 animals were scanned in this study, and 70% of the diseased animals presented at least one cortical lesion on postmortem imaging. Cortical lesions identified on MRI were verified with myelin proteolipid protein immunostaining. An optimized T2*-weighted sequence was developed for in vivo imaging and shown to capture 65% of cortical lesions detected postmortem. Immunostaining confirmed extensive demyelination with preserved neuronal somata in several cortical areas of EAE animals. Overall, this study demonstrates the relevance and feasibility of the marmoset EAE model to study cortical lesions, among the most important yet least understood features of MS.

Spatial organization of occipital white matter tracts in the common marmoset

The primate brain contains a large number of interconnected visual areas, whose spatial organization and intracortical projections show a high level of conservation across species. One fiber pathway of recent interest is the vertical occipital fasciculus (VOF), which is thought to support communication between dorsal and ventral visual areas in the occipital lobe. A recent comparative diffusion MRI (dMRI) study reported that the VOF in the macaque brain bears a similar topology to that of the human, running superficial and roughly perpendicular to the optic radiation. The present study reports a comparative investigation of the VOF in the common marmoset, a small New World monkey whose lissencephalic brain is approximately tenfold smaller than the macaque and 150-fold smaller than the human. High-resolution ex vivo dMRI of two marmoset brains revealed an occipital white matter structure that closely resembles that of the larger primate species, with one notable difference. Namely, unlike in the macaque and the human, the VOF in the marmoset is spatially fused with other, more anterior vertical tracts, extending anteriorly between the parietal and temporal cortices. We compare several aspects of this continuous structure, which we term the VOF complex (VOF +), and neighboring fasciculi to those of macaques and humans. We hypothesize that the essential topology of the VOF+ is a conserved feature of the posterior cortex in anthropoid primates, with a clearer fragmentation into multiple named fasciculi in larger, more gyrified brains.

Accelerating the Evolution of Nonhuman Primate Neuroimaging

Nonhuman primate neuroimaging is on the cusp of a transformation, much in the same way its human counterpart was in 2010, when the Human Connectome Project was launched to accelerate progress. Inspired by an open data-sharing initiative, the global community recently met and, in this article, breaks through obstacles to define its ambitions.

A resource for the detailed 3D mapping of white matter pathways in the marmoset brain

While the fundamental importance of the white matter in supporting neuronal communication is well known, existing publications of primate brains do not feature a detailed description of its complex anatomy. The main barrier to achieving this is that existing primate neuroimaging data have insufficient spatial resolution to resolve white matter pathways fully. Here we present a resource that allows detailed descriptions of white matter structures and trajectories of fiber pathways in the marmoset brain. The resource includes: (1) the highest-resolution diffusion-weighted MRI data available to date, which reveal white matter features not previously described; (2) a comprehensive three-dimensional white matter atlas depicting fiber pathways that were either omitted or misidentified in previous atlases; and (3) comprehensive fiber pathway maps of cortical connections combining diffusion-weighted MRI tractography and neuronal tracing data. The resource, which can be downloaded from marmosetbrainmapping.org, will facilitate studies of brain connectivity and the development of tractography algorithms in the primate brain.

Anatomical and functional investigation of the marmoset default mode network

The default mode network (DMN) is associated with a wide range of brain functions. In humans, the DMN is marked by strong functional connectivity among three core regions: medial prefrontal cortex (mPFC), posterior parietal cortex (PPC), and the medial parietal and posterior cingulate cortex (PCC). Neuroimaging studies have shown that the DMN also exists in non-human primates, suggesting that it may be a conserved feature of the primate brain. Here, we found that, in common marmosets, the dorsolateral prefrontal cortex (dlPFC; peak at A8aD) has robust fMRI functional connectivity and reciprocal anatomical connections with the posterior DMN core regions (PPC and PCC), while the mPFC has weak connections with the posterior DMN core regions. This strong dlPFC but weak mPFC connectivity in marmoset differs markedly from the stereotypical DMN in humans. The mPFC may be involved in brain functions that are further developed in humans than in other primates.

An Open Resource for Non-human Primate Imaging

Non-human primate neuroimaging is a rapidly growing area of research that promises to transform and scale translational and cross-species comparative neuroscience. Unfortunately, the technological and methodological advances of the past two decades have outpaced the accrual of data, which is particularly challenging given the relatively few centers that have the necessary facilities and capabilities. The PRIMatE Data Exchange (PRIME-DE) addresses this challenge by aggregating independently acquired non-human primate magnetic resonance imaging (MRI) datasets and openly sharing them via the International Neuroimaging Data-sharing Initiative (INDI). Here, we present the rationale, design, and procedures for the PRIME-DE consortium, as well as the initial release, consisting of 25 independent data collections aggregated across 22 sites (total = 217 non-human primates). We also outline the unique pitfalls and challenges that should be considered in the analysis of non-human primate MRI datasets, including providing automated quality assessment of the contributed datasets.

Three-Dimensional Digital Template Atlas of the Macaque Brain

We present a new 3D template atlas of the anatomical subdivisions of the macaque brain, which is based on and aligned to the magnetic resonance imaging (MRI) data set and histological sections of the Saleem and Logothetis atlas. We describe the creation and validation of the atlas that, when registered with macaque structural or functional MRI scans, provides a straightforward means to estimate the boundaries between architectonic areas, either in a 3D volume with different planes of sections, or on an inflated brain surface (cortical flat map). As such, this new template atlas is intended for use as a reference standard for macaque brain research. Atlases and templates are available as both volumes and surfaces in standard NIFTI and GIFTI formats.

A digital 3D atlas of the marmoset brain based on multi-modal MRI

The common marmoset (Callithrix jacchus) is a New-World monkey of growing interest in neuroscience. Magnetic resonance imaging (MRI) is an essential tool to unveil the anatomical and functional organization of the marmoset brain. To facilitate identification of regions of interest, it is desirable to register MR images to an atlas of the brain. However, currently available atlases of the marmoset brain are mainly based on 2D histological data, which are difficult to apply to 3D imaging techniques. Here, we constructed a 3D digital atlas based on high-resolution ex-vivo MRI images, including magnetization transfer ratio (a T1-like contrast), T2w images, and multi-shell diffusion MRI. Based on the multi-modal MRI images, we manually delineated 54 cortical areas and 16 subcortical regions on one hemisphere of the brain (the core version). The 54 cortical areas were merged into 13 larger cortical regions according to their locations to yield a coarse version of the atlas, and also parcellated into 106 sub-regions using a connectivity-based parcellation method to produce a refined atlas. Finally, we compared the new atlas set with existing histology atlases and demonstrated its applications in connectome studies, and in resting state and stimulus-based fMRI. The atlas set has been integrated into the widely-distributed neuroimaging data analysis software AFNI and SUMA, providing a readily usable multi-modal template space with multi-level anatomical labels (including labels from the Paxinos atlas) that can facilitate various neuroimaging studies of marmosets.

Occipital White Matter Tracts in Human and Macaque

We compare several major white-matter tracts in human and macaque occipital lobe using diffusion magnetic resonance imaging. The comparison suggests similarities but also significant differences in the tracts. There are several apparently homologous tracts in the 2 species, including the vertical occipital fasciculus (VOF), optic radiation, forceps major, and inferior longitudinal fasciculus (ILF). There is one large human tract, the inferior fronto-occipital fasciculus, with no corresponding fasciculus in macaque. We could identify the macaque VOF (mVOF), which has been little studied. Its position is consistent with classical invasive anatomical studies by Wernicke. VOF homology is supported by similarity of the endpoints in V3A and ventral V4 across species. The mVOF fibers intertwine with the dorsal segment of the ILF, but the human VOF appears to be lateral to the ILF. These similarities and differences between the occipital lobe tracts will be useful in establishing which circuitry in the macaque can serve as an accurate model for human visual cortex.

A population MRI brain template and analysis tools for the macaque

The use of standard anatomical templates is common in human neuroimaging, as it facilitates data analysis and comparison across subjects and studies. For non-human primates, previous in vivo templates have lacked sufficient contrast to reliably validate known anatomical brain regions and have not provided tools for automated single-subject processing. Here we present the "National Institute of Mental Health Macaque Template", or NMT for short. The NMT is a high-resolution in vivo MRI template of the average macaque brain generated from 31 subjects, as well as a neuroimaging tool for improved data analysis and visualization. From the NMT volume, we generated maps of tissue segmentation and cortical thickness. Surface reconstructions and transformations to previously published digital brain atlases are also provided. We further provide an analysis pipeline using the NMT that automates and standardizes the time-consuming processes of brain extraction, tissue segmentation, and morphometric feature estimation for anatomical scans of individual subjects. The NMT and associated tools thus provide a common platform for precise single-subject data analysis and for characterizations of neuroimaging results across subjects and studies.

Blindsight depends on the lateral geniculate nucleus

Injury to the primary visual cortex (V1) leads to the loss of visual experience. Nonetheless, careful testing shows that certain visually guided behaviours can persist even in the absence of visual awareness. The neural circuits supporting this phenomenon, which is often termed blindsight, remain uncertain. Here we demonstrate that the thalamic lateral geniculate nucleus (LGN) has a causal role in V1-independent processing of visual information. By comparing functional magnetic resonance imaging (fMRI) and behavioural measures with and without temporary LGN inactivation, we assessed the contribution of the LGN to visual functions of macaque monkeys (Macaca mulatta) with chronic V1 lesions. Before LGN inactivation, high-contrast stimuli presented to the lesion-affected visual field (scotoma) produced significant V1-independent fMRI activation in the extrastriate cortical areas V2, V3, V4, V5/middle temporal (MT), fundus of the superior temporal sulcus (FST) and lateral intraparietal area (LIP) and the animals correctly located the stimuli in a detection task. However, following reversible inactivation of the LGN in the V1-lesioned hemisphere, fMRI responses and behavioural detection were abolished. These results demonstrate that direct LGN projections to the extrastriate cortex have a critical functional contribution to blindsight. They suggest a viable pathway to mediate fast detection during normal vision.

Effect of sound intensity on tonotopic fMRI maps in the unanesthetized monkey

The monkey's auditory cortex includes a core region on the supratemporal plane (STP) made up of the tonotopically organized areas A1, R, and RT, together with a surrounding belt and a lateral parabelt region. The functional studies that yielded the tonotopic maps and corroborated the anatomical division into core, belt, and parabelt typically used low-amplitude pure tones that were often restricted to threshold-level intensities. Here we used functional magnetic resonance imaging in awake rhesus monkeys to determine whether, and if so how, the tonotopic maps and the pattern of activation in core, belt, and parabelt are affected by systematic changes in sound intensity. Blood oxygenation level-dependent (BOLD) responses to groups of low- and high-frequency pure tones 3-4 octaves apart were measured at multiple sound intensity levels. The results revealed tonotopic maps in the auditory core that reversed at the putative areal boundaries between A1 and R and between R and RT. Although these reversals of the tonotopic representations were present at all intensity levels, the lateral spread of activation depended on sound amplitude, with increasing recruitment of the adjacent belt areas as the intensities increased. Tonotopic organization along the STP was also evident in frequency-specific deactivation (i.e. "negative BOLD"), an effect that was intensity-specific as well. Regions of positive and negative BOLD were spatially interleaved, possibly reflecting lateral inhibition of high-frequency areas during activation of adjacent low-frequency areas, and vice versa. These results, which demonstrate the strong influence of tonal amplitude on activation levels, identify sound intensity as an important adjunct parameter for mapping the functional architecture of auditory cortex.

Correction for geometric distortion and N/2 ghosting in EPI by phase labeling for additional coordinate encoding (PLACE)

Echo-planar imaging (EPI) is vulnerable to geometric distortion and N/2 ghosting. These artifacts can be analyzed with an intuitive k-t space tool, and here we propose a simple method for their correction. In a slightly modified additional EPI acquisition, we sample the k-t space with a shift in k(y) by adding a small area to the phase-encoding (PE) gradient. Physically, the added gradient area creates a relative phase ramp across the object and directly encodes the undistorted original y-coordinate of each voxel into a phase difference between two distorted complex images, in a method called "phase labeling for additional coordinate encoding" (PLACE). The phase information is then used to map the mismapped signals back to their original locations for geometric and intensity correction. Smoothing of expanded complex data matrix effectively reduces noise in the differential phase map and allows subpixel warping. The two acquired images can also be averaged to effectively suppress the N/2 ghost. Efficient correction for both artifacts can be achieved with three acquisitions. These acquisitions can also serve as reference scans to correct for geometric distortion and/or N/2 ghost artifacts on all images in a time series. The technique was successfully demonstrated in phantom and animal studies.

Noninvasive quantification of cerebral blood volume in humans during functional activation

Like cerebral blood flow (CBF), cerebral blood volume (CBV) is an important physiological parameter closely associated with brain activity and thus, noninvasive quantification of CBV during brain activation provides another opportunity to investigate the relationship between neuronal activity and hemodynamic changes. In this paper, a new method is presented that is able to quantify CBV at rest and during activation. Specifically, using an inversion recovery pulse sequence, a set of brain images was collected at various inversion times (TIs). At each TI, functional images were acquired with a block-design visual stimulation paradigm. A biophysical model comprised of multiple tissue components was developed and was utilized for the determination of CBV using the visual stimulation data. MRI experiments on five healthy volunteers showed that CBV was 5.0 +/- 1.5 ml blood/100 ml brain during rest and increased to 6.6 +/- 1.8 ml blood/100 ml brain following visual stimulation. Furthermore, experiments with visual stimulation at two frequencies (2 and 8 Hz) showed that the increases in CBV correlated with the strength of stimulation. This technique, with its ability to measure quantitative CBV values noninvasively, provides a valuable tool for quantifying hemodynamic signals associated with brain activation.

Comparison of continuous overt speech fMRI using BOLD and arterial spin labeling

Overt speech production in functional magnetic resonance imaging (fMRI) studies is often associated with imaging artifacts, attributable to both movement and susceptibility. Various image-processing methods have been proposed to remove these artifacts from the data but none of these methods has been shown to work with continuous overt speech, at least over periods greater than 3 s. In this study natural, continuous, overt sentence production was evaluated in normal volunteers using both arterial spin labeling (ASL) and conventional echoplanar blood oxygenation level-dependent (BOLD) imaging sequences on the same 1.5-T scanner. We found a high congruency between activation results obtained with ASL and the de facto gold standard in overt language production imaging, positron emission tomography (PET). No task-related artifacts were found in the ASL study. However, the BOLD data showed artifacts that appeared as large bilateral false-positive temporopolar activations; percent signal change estimated in these regions showed signal increases and temporal dynamics that were incongruent with typical BOLD activations. These artifacts were not distributed uniformly, but were aligned at the frontotemporal base, close to the oropharynx. The calculated head movement parameters for overt speech blocks were within the range of the rest blocks, indicating that head movement is unlikely the reason for the artifact. We conclude that ASL is not influenced by overt speech artifacts, whereas BOLD showed significant susceptibility artifacts, especially in the opercular and insular regions, where activation would be expected. ASL may prove to be the method of choice for fMRI investigations of continuous overt speech.

Noise reduction in multi-slice arterial spin tagging imaging

Attenuating the static signal in arterial spin tagging (ASSIST) was initially developed for 3D imaging of cerebral blood flow. To enable the simultaneous collection of cerebral blood flow and BOLD data, a multi-slice version of ASSIST is proposed. As with the 3D version, this sequence uses multiple inversion pulses during the tagging period to suppress the static signal. To maintain background suppression in all slices, the multi-slice sequence applies additional inversion pulses between slice acquisitions. The utility of the sequence was demonstrated by simultaneously acquiring ASSIST and BOLD data during a functional task and by collecting resting-state ASSIST data over a large number of slices. In addition, the temporal stability of the perfusion signal was found to be 60% greater at 3 T compared to 1.5 T, which was attributed to the insensitivity of ASSIST to physiologic noise.

Whole-brain 3D perfusion MRI at 3.0 T using CASL with a separate labeling coil

A variety of continuous and pulsed arterial spin labeling (ASL) perfusion MRI techniques have been demonstrated in recent years. One of the reasons these methods are still not routinely used is the limited extent of the imaging region. Of the ASL methods proposed to date, continuous ASL (CASL) with a separate labeling coil is particularly attractive for whole-brain studies at high fields. This approach can provide an increased signal-to-noise ratio (SNR) in perfusion images because there are no magnetization transfer (MT) effects, and lessen concerns regarding RF power deposition at high field because it uses a local labeling coil. In this work, we demonstrate CASL whole-brain quantitative perfusion imaging at 3.0 T using a combination of strategies: 3D volume acquisition, background tissue signal suppression, and a separate labeling coil. The results show that this approach can be used to acquire perfusion images in all brain regions with good sensitivity. Further, it is shown that the method can be performed safely on humans without exceeding the current RF power deposition limits. The current method can be extended to higher fields, and further improved by the use of multiple receiver coils and parallel imaging techniques to reduce scan time or provide increased resolution.

Measuring the effects of indomethacin on changes in cerebral oxidative metabolism and cerebral blood flow during sensorimotor activation

The work presented here uses combined blood oxygenation level-dependent (BOLD) and arterial spin tagging (AST) approaches to study the effect of indomethacin on cerebral blood flow (CBF) and oxygen consumption (CMRO(2)) increases during motor activation. While indomethacin reduced the CBF increase during activation, it did not significantly affect the CMRO(2) increase during activation. The ratio of the activation-induced CBF increase in the presence and absence of indomethacin was 0.54 +/- 0.08 (+/-SEM, n = 8, P < 0.001), while the ratio of the CMRO(2) increase in the presence and absence of the drug was 1.02 +/- 0.08 (+/-SEM, N = 8, ns). Potential difficulties in estimating CMRO(2) changes from combined BOLD/AST data are discussed.

Effects of indomethacin on cerebral blood flow at rest and during hypercapnia: an arterial spin tagging study in humans

PURPOSE

To investigate using an arterial spin tagging (AST) approach the effect of indomethacin on the cerebral blood flow (CBF) response to hypercapnia.

MATERIALS AND METHODS

Subjects inhaled a gas mixture containing 6% CO(2) for two 5-minute periods, which were separated by a 10-minute interval, in which subjects inhaled room air. In six subjects, indomethacin (i.v., 0.2 mg/kg) was infused in the normocapnic interval between the two hypercapnic periods.

RESULTS

Indomethacin reduced normocapnic gray matter CBF by 36 +/- 5% and reduced the CBF increase during hypercapnia from 43 +/- 9% to 16 +/- 5% in gray matter (P < 0.001) and from 48 +/- 11% to 35 +/- 9% in white matter (P < 0.025).

CONCLUSION

The results demonstrate that an AST approach can measure the effects of indomethacin on global CBF increases during hypercapnia and suggest that an AST approach could be used to investigate pharmacological effects on focal CBF increases during functional activation.

Supracondylar extension fracture of the humerus in children. Manipulative reduction, immobilisation and fixation using a U-shaped plaster slab with the elbow in full extension

We present a method of manipulative reduction, immobilisation and fixation using a U-shaped plaster with the elbow in extension for extension-type supracondylar fractures of the humerus in children. When the elbow is in full extension, both the extensor and the flexor muscles are neutralised during manipulative reduction and the carrying angle can be easily assessed thus preventing cubitus varus, the most common complication. In order to evaluate the efficiency of this method, we compared the clinical results of the new method with those of conventional treatment. In a group of 95 children who sustained an extension-type supracondylar fracture of the humerus, 49 were treated by the new method and 46 by the conventional method, reduction and immobilisation in a plaster slab with the elbow in flexion. Reduction and immobilisation were easily achieved and reliably maintained by one manipulation for all the children treated by the new method. In 12 children treated by the conventional method, the initial reduction failed and in seven secondary displacement of the distal fragment occurred during the period of immobilisation in plaster. All required a second or third manipulation. Of the 46 children, 28 (60.9%) had developed cubitus varus at a mean follow-up of 4.6 years when treated by the conventional method. None of the children treated by the new method developed cubitus varus. The mean score, according to the Hospital for Special Surgery (HSS) elbow scoring system, was 91 points using the new method and 78 with the conventional method. The results were statistically significant with regard to the incidence of cubitus varus and the elbow score (p < 0.01) suggesting that the new method is reliable and gives a satisfactory outcome.

H(2)(15)O PET validation of steady-state arterial spin tagging cerebral blood flow measurements in humans

Steady-state arterial spin tagging approaches can provide quantitative images of CBF, but have not been validated in humans. The work presented here compared CBF values measured using steady-state arterial spin tagging with CBF values measured in the same group of human subjects using the H(2)(15)O IV bolus PET method. Blood flow values determined by H(2)(15)O PET were corrected for the known effects of incomplete extraction of water across the blood brain barrier. For a cortical strip ROI, blood flow values determined using arterial spin tagging (64+/-12 cc/100 g/min) were not statistically different from corrected blood flow values determined using H(2)(15)O PET (67+/-13 cc/100 g/min). However, for a central white matter ROI, blood flow values determined using arterial spin tagging were significantly underestimated compared to corrected blood flow values determined using H(2)(15)O PET. This underestimation could be caused by an underestimation of the arterial transit time for white matter regions.

Noise reduction in 3D perfusion imaging by attenuating the static signal in arterial spin tagging (ASSIST)

Phase-encoded multishot SPIRAL approaches were used to acquire true 3D cerebral blood flow images of the human head using arterial spin tagging approaches. Multiple-inversion background suppression techniques, which suppress phase noise due to interacquisition fluctuations in the static magnetic field, reduced the temporal standard deviation of true 3D delta M images acquired using arterial spin tagging approaches by approximately 50%. Background suppressed arterial spin tagging (ASSIST) approaches were used to obtain high-resolution isotropic true 3D cerebral blood flow images, and to obtain true 3D activation images during cognitive (working memory) tasks. Magn Reson Med 44:92-100, 2000. Published 2000 Wiley-Liss, Inc.

Quantitation of regional cerebral blood flow increases during motor activation: A multislice, steady-state, arterial spin tagging study

Steady-state arterial spin tagging approaches were used to construct multislice images of relative cerebral blood flow changes during finger-tapping tasks. Statistically significant increases in cerebral blood flow were observed in primary sensorimotor cortex in all seven subjects. The mean volume of the activated region in the contralateral primary sensorimotor cortex was 0.9 cm(3), and the mean increase in cerebral blood flow in the activated area was 54% +/- 11%. Although the extended spatial coverage is advantageous for activation studies, the intrinsic sensitivity of the multislice approach is smaller than the intrinsic sensitivity of the single-slice, arterial spin tagging approach. Magn Reson Med 42:404-407, 1999. Published 1999 Wiley-Liss, Inc.

Investigation of low frequency drift in fMRI signal

Low frequency drift (0.0-0.015 Hz) has often been reported in time series fMRI data. This drift has often been attributed to physiological noise or subject motion, but no studies have been done to test this assumption. Time series T*2-weighted volumes were acquired on two clinical 1.5 T MRI systems using spiral and EPI readout gradients from cadavers, a normal volunteer, and nonhomogeneous and homogeneous phantoms. The data were tested for significant differences (P = 0.001) from Gaussian noise in the frequency range 0.0-0.015 Hz. The percentage of voxels that were significant in data from the cadaver, normal volunteer, nonhomogeneous and homogeneous phantoms were 13.7-49.0%, 22.1-61.9%, 46.4-68.0%, and 1.10%, respectively. Low frequency drift was more pronounced in regions with high spatial intensity gradients. Significant drifting was present in data acquired from cadavers and nonhomogeneous phantoms and all pulse sequences tested, implying that scanner instabilities and not motion or physiological noise may be the major cause of the drift.

Quantitation of regional cerebral blood flow increases in prefrontal cortex during a working memory task: a steady-state arterial spin-tagging study

Steady-state arterial spin-tagging MRI approaches were used to quantitate regional cerebral blood flow increases in prefrontal cortex during a working memory ("two-back") task in six normal subjects. Statistically significant increases in cerebral blood flow in prefrontal cortex were observed in all six subjects: the average increase in cerebral blood flow in activated prefrontal cortex regions was 22 +/- 5 cc/100 g/min (23 +/- 7%). The results demonstrate that spin-tagging approaches can be used to follow focal activation in prefrontal cortex during cognitive tasks.

Multislice imaging of quantitative cerebral perfusion with pulsed arterial spin labeling

A method is presented for multislice measurements of quantitative cerebral perfusion based on magnetic labeling of arterial spins. The method combines a pulsed arterial inversion, known as the FAIR (Flow-sensitive Alternating Inversion Recovery) experiment, with a fast spiral scan image acquisition. The short duration (22 ms) of the spiral data collection allows simultaneous measurement of up to 10 slices per labeling period, thus dramatically increasing efficiency compared to current single slice acquisition protocols. Investigation of labeling efficiency, suppression of unwanted signals from stationary as well as intraarterial spins, and the FAIR signal change as a function of inversion delay are presented. The assessment of quantitative cerebral blood flow (CBF) with the new technique is demonstrated and shown to require measurement of arterial transit time as well as suppression of intraarterial spin signals. CBF values measured on normal volunteers are consistent with results obtained from H2O15 positron emission tomography (PET) studies and other radioactive tracer approaches. In addition, the new method allows detection of activation-related perfusion changes in a finger-tapping experiment, with locations of activation corresponding well to those observed with blood oxygen level dependent (BOLD) fMRI.

Increasing striatal iron content associated with normal aging

Free-radical-mediated mechanisms may contribute to neuronal damage in Parkinson's disease (PD), other neurodegenerative conditions also associated with aging, and the aging process itself. Cytotoxic free radicals are generated in the brain by oxidation/reduction reactions that are catalyzed by transition metals such as iron. Any regional increase in brain iron concentration may increase the potential for local free-radical formation. The purpose of this study was to determine the relationship between age and basal ganglia iron content in 20 normal individuals ranging from 24 to 79 years of age. We used an in vivo magnetic resonance method to quantify the effects of paramagnetic centers sequestered inside cellular membranes, thereby enabling the determination of a quantitative index of local brain iron content. We observed a strong direct relationship between age and regional iron content in the putamen (r = 0.76, p < 0.0001) and caudate (r = 0.69, p < 0.001), but not in the globus pallidus (r = 0.32, p = 0.17) or thalamus (r = 0.13, p = 0.58). In conclusion, striatal iron content increases with advancing age. This increase may increase the probability of free-radical formation in the striatum, therefore representing a risk factor for the development of neurodegenerative disorders such as PD in which nigrostriatal neurons may be affected by increased oxidant stress.

Increased basal ganglia iron in striatonigral degeneration: in vivo estimation with magnetic resonance

BACKGROUND

As many as 20% of individuals with the clinical diagnosis of Parkinson's disease (PD) do not have the characteristic neuropathologic features of PD at post mortem. The striatonigral degeneration (SND) subtype of multiple system atrophy is one of the categories of pathology which may be incorrectly diagnosed as PD on the basis of clinical presentation. SND may be associated with increased iron deposition in the putamen which can be detected with magnetic resonance imaging.

METHODS

We have estimated regional brain iron content in a patient with probable SND, using a novel imaging method developed in our laboratory, and have compared the results in this patient to those which we have previously reported in patients with PD and in age-matched controls.

RESULTS

We observed that putamenal iron content was increased in our SND patient, beyond the 95% confidence limit for inclusion in the PD group, even when considering clinical severity. In contrast, pallidal and thalamic iron were within the PD range.

CONCLUSIONS

The demonstration of increased putamenal iron content may be a useful adjunctive investigative procedure in patients with suspected SND.

Quantitation of regional cerebral blood flow increases during motor activation: a steady-state arterial spin tagging study

Steady-state arterial spin tagging MRI approaches were used to quantitate regional cerebral blood flow increases during finger tapping tasks in seven normal subjects. Statistically significant increases in cerebral blood flow were observed in the contralateral primary sensorimotor cortex in all seven subjects and in the supplementary motor area in five subjects. The intrinsic spatial resolution of the cerebral blood flow images was approximately 4 mm. If no spatial filtering was applied, the average increase in cerebral blood flow in the activated primary sensorimotor cortex was 60 +/- 10 cc/100 g/min (91 +/- 32%). If the images were filtered to a spatial resolution of 15 mm, the average increase in cerebral blood flow in the activated primary sensorimotor cortex was 23 +/- 7 cc/100 g/min (42 +/- 15%), in agreement with previously reported 133Xe and PET results.

Effect of magnetization transfer on the measurement of cerebral blood flow using steady-state arterial spin tagging approaches: a theoretical investigation

A simple four-compartment model for magnetization transfer was used to obtain theoretical expressions for the relationship between regional cerebral blood flow and delta M, the change in longitudinal magnetization of brain water spins when arterial water spins are perturbed. The theoretical relationship can be written in two forms, depending on the approach used to normalize delta M. Using the first approach, the calculation of cerebral blood flow requires a knowledge of R1(omega 1, delta omega), the longitudinal relaxation rate observed in the presence of continuous off-resonance RF irradiation. Using the second approach, the calculation of cerebral blood flow requires a knowledge of R1(omega 1, delta omega), where R1(omega 1, delta omega) is given by the product of R1(omega 1, delta omega) and the fractional steady-state longitudinal water magnetization in the presence of off-resonance RF irradiation. If the off-resonance RF irradiation used for arterial tagging does not produce appreciable magnetization transfer effects, R1(omega 1, delta omega) can be approximated by the longitudinal relaxation rate measured in the absence of off-resonance RF irradiation, R1obs. Theoretical expressions obtained by using the four-component model for magnetization transfer are compared with equivalent expressions obtained by using two-compartment models.

Correction for vascular artifacts in cerebral blood flow values measured by using arterial spin tagging techniques

"Vascular" artifacts can have substantial effects on human cerebral blood flow values calculated by using arterial spin tagging approaches. One vascular artifact arises from the contribution of "tagged" arterial water spins to the observed change in brain water MR signal. This artifact can be reduced if large bipolar gradients are used to "crush" the MR signal from moving arterial water spins. A second vascular artifact arises from relaxation of "tagged" arterial blood during transit from the tagging plane to the capillary exchange site in the imaging slice. This artifact can be corrected if the arterial transit times are measured by using "dynamic" spin tagging approaches. The mean transit time from the tagging plane to capillary exchange sites in a gray matter region of interest was calculated to be approximately 0.94 s. Cerebral blood flow values calculated for seven normal volunteers agree reasonably well with values calculated by using radioactive tracer approaches.

Perfusion imaging of the human brain at 1.5 T using a single-shot EPI spin tagging approach

Single-shot echo planar imaging (EPI) techniques have been applied, in conjunction with arterial spin tagging approaches, to obtain images of cerebral blood flow in a single axial slice in the human brain. Serial studies demonstrate that cerebral blood flow images acquired in 8 min are reproducible, with a statistical precision of approximately +/-10 cc/100 g/min. The average value of cerebral blood flow in the slice is 51 +/- 11 cc/100 g/min for six normal subjects. The cerebral blood flow images contain two types of artifact, probably due to arterial and venous blood volume contributions, which must be overcome before the arterial spin tagging approach can be used for routine clinical studies.

Estimation of brain iron in vivo by means of the interecho time dependence of image contrast

An imaging protocol for a quantitative estimation of disease-induced variations in brain iron is proposed and then validated, first, on a phantom and second, on a group of 11 healthy volunteers. The relative estimate of brain iron is achieved from a rate difference image that measures the enhancement, delta R2app, of the transverse relaxation rate of water protons brought about by the heterogeneous accumulation of iron in the glial cells. At 1.5 T, the phantom study demonstrates, over the range 0-6 A/m, a linear dependence of delta R2app on the magnetization difference between microspheres and a paramagnetic gel, with a sensitivity of approximately 2 s-1 A-1 m. In the group of healthy volunteers (mean age 33 +/- 7 years) devoid of disease-related or appreciable age-related accumulations of iron, the precision of delta R2app was still sufficient to distinguish the globus pallidus and the putamen from all of the other iron-containing brain structures in a manner that was significant at the 99% confidence level.

Basal ganglia iron content in Parkinson's disease measured with magnetic resonance

The possibility of using magnetic resonance (MR) to evaluate the severity of the pathological changes of Parkinson's disease (PD) is suggested by the known accumulation of iron in the basal ganglia in PD and the reduced signal evident from this area with conventional T2-weighted MR imaging. To improve the specificity of MR for the measurement of tissue iron content, we have developed a method that quantifies the effects of paramagnetic centers sequestered inside cellular membranes, based on the echo time dependence of the decay of transverse magnetization caused by the local field inhomogeneities which are due to intracellular iron. This method enables an index of local tissue iron content to be calculated for structures of the basal ganglia. We report here the application of this method to a series of patients with PD (n = 12) and of normal, age-matched controls (n = 13). Our objective was to determine whether this measurement of basal ganglia iron concentration correlates with the presence and severity of PD. We observed a significant increase in iron content in both the putamen and pallidum in PD as well as a correlation with the severity of clinical symptomatology. More severely affected patients had a higher iron content in both of these structures. Our results suggest that this MR measurement may provide a noninvasive method of measuring the severity of the pathological changes underlying PD.

Estimation of the iron concentration in excised gray matter by means of proton relaxation measurements

To validate their correlation with tissue iron concentration, proton transverse relaxation measurements have been made at 2.35 T (100 MHz) in 25 samples of excised, frozen, but unfixed human gray matter tissue obtained from the globus pallidus, putamen, caudate, thalamus, and cortex of five postmortem brains free of neurological disease. The iron concentration was independently measured, using atomic absorption spectroscopy. The proton transverse relaxation measurements exploited the interecho time dependence of the apparent transverse relaxation rate, R2app, obtained from a Carr-Purcell-Meiboom-Gill (CPMG) sequence. An empirical semilogarithmic relationship between R2app and the interecho time provided a measure of the relaxation enhancement due to iron, namely, a slope p, which demonstrated a significant correlation (r = 0.78, P < 0.001) with tissue iron concentration. Moreover, a simple rate difference, delta R2app, determined between interecho time values of 6 and 60 ms, was also found to correlate significantly with iron concentration (r = 0.81, P < 0.001). Both of the foregoing correlations were better than that of R2app itself. When the tissue samples were subdivided into brain structure groups, the intergroup differences in rho reflected their known differences in iron accumulation and correlated with those of the mean group iron content, determined by atomic absorption spectrometry.

Relaxation enhancement of the transverse magnetization of water protons in paramagnetic suspensions of red blood cells

The enhancement of the water proton transverse relaxation, delta R2, brought about by a difference between intra and extracellular paramagnetic susceptibilities in a suspension of red blood cells (RBC) has been evaluated both experimentally and theoretically in terms of (i) the refocusing interval, delta 180, of a CPMG pulse sequence, (ii) the difference in paramagnetic susceptibility, and (iii) the shape of the cell surface. At a hematocrit of 45, the increase in the relaxation enhancement, delta R2, with increasing delta 180, was a factor of two greater for the naturally biconcave RBC, than for the quasi-spherical RBC in hypotonic suspensions. This difference could be modeled in terms of a transmembrane correlation time, tau = 5.5 ms, across an RBC surface characterized by a demagnetizing factor which differs by 0.13 from that of a sphere. The increase in delta R2 with increasing magnetization difference between the RBC and its surroundings was found to be marginally less than quadratic, both experimentally and from the model.