Multiphase pseudocontinuous arterial spin labeling (MP-PCASL) for robust quantification of cerebral blood flow

ASL lexicon and reporting recommendations: A consensus report from the ISMRM Open Science Initiative for Perfusion Imaging (OSIPI)

The 2015 consensus statement published by the International Society for Magnetic Resonance in Medicine (ISMRM) Perfusion Study Group and the European Cooperation in Science and Technology ( COST) Action ASL in Dementia aimed to encourage the implementation of robust arterial spin labeling (ASL) perfusion MRI for clinical applications and promote consistency across scanner types, sites, and studies. Subsequently, the recommended 3D pseudo-continuous ASL sequence has been implemented by most major MRI manufacturers. However, ASL remains a rapidly and widely developing field, leading inevitably to further divergence of the technique and its associated terminology, which could cause confusion and hamper research reproducibility. On behalf of the ISMRM Perfusion Study Group, and as part of the ISMRM Open Science Initiative for Perfusion Imaging (OSIPI), the ASL Lexicon Task Force has been working on the development of an ASL Lexicon and Reporting Recommendations for perfusion imaging and analysis, aiming to (1) develop standardized, consensus nomenclature and terminology for the broad range of ASL imaging techniques and parameters, as well as for the physiological constants required for quantitative analysis; and (2) provide a community-endorsed recommendation of the imaging parameters that we encourage authors to include when describing ASL methods in scientific reports/papers. In this paper, the sequences and parameters in (pseudo-)continuous ASL, pulsed ASL, velocity-selective ASL, and multi-timepoint ASL for brain perfusion imaging are included. However, the content of the lexicon is not intended to be limited to these techniques, and this paper provides the foundation for a growing online inventory that will be extended by the community as further methods and improvements are developed and established.

Robust dual-module velocity-selective arterial spin labeling (dm-VSASL) with velocity-selective saturation and inversion

PURPOSE

Compared to conventional arterial spin labeling (ASL) methods, velocity-selective ASL (VSASL) is more sensitive to artifacts from eddy currents, diffusion attenuation, and motion. Background suppression is typically suboptimal in VSASL, especially of CSF. As a result, the temporal SNR and quantification accuracy of VSASL are compromised, hindering its application despite its advantage of being delay-insensitive.

METHODS

A novel dual-module VSASL (dm-VSASL) strategy is developed to improve the SNR efficiency and the temporal SNR with a more balanced gradient configuration in the label/control image acquisition. This strategy applies for both VS saturation (VSS) and VS inversion (VSI) labeling. The dm-VSASL schemes were compared with single-module labeling and a previously developed multi-module schemes for the SNR performance, background suppression efficacy, and sensitivity to artifacts in simulation and in vivo experiments, using pulsed ASL as the reference.

RESULTS

Dm-VSASL enabled more robust labeling and efficient backgroud suppre across brain tissues, especially of CSF, resulting in significantly reduced artifacts and improved temporal SNR. Compared to single-module labeling, dm-VSASL significantly improved the temporal SNR in gray (by 90.8% and 94.9% for dm-VSS and dm-VSI, respectively; P < 0.001) and white (by 41.5% and 55.1% for dm-VSS and dm-VSI, respectively; P < 0.002) matter. Dm-VSI also improved the SNR of VSI by 5.4% (P = 0.018).

CONCLUSION

Dm-VSASL can significantly improve the robustness of VS labeling, reduce artifacts, and allow efficient background suppression. When implemented with VSI, it provides the highest SNR efficiency among VSASL methods. Dm-VSASL is a powerful ASL method for robust, accurate, and delay-insensitive perfusion mapping.

Rapid-onset dystonia-parkinsonism is associated with reduced cerebral blood flow without gray matter changes

Purpose

Previous research showed discrete neuropathological changes associated with rapid-onset dystonia-parkinsonism (RDP) in brains from patients with an ATP1A3 variant, specifically in areas that mediate motor function. The purpose of this study was to determine if magnetic resonance imaging methodologies could identify differences between RDP patients and variant-negative controls in areas of the brain that mediate motor function in order to provide biomarkers for future treatment or prevention trials.

Methods

Magnetic resonance imaging voxel-based morphometry and arterial spin labeling were used to measure gray matter volume and cerebral blood flow, respectively, in cortical motor areas, basal ganglia, thalamus, and cerebellum, in RDP patients with ATP1A3 variants (n = 19; mean age = 37 ± 14 years; 47% female) and variant-negative healthy controls (n = 11; mean age = 34 ± 19 years; 36% female).

Results

We report age and sex-adjusted between group differences, with decreased cerebral blood flow among patients with ATP1A3 variants compared to variant-negative controls in the thalamus (p = 0.005, Bonferroni alpha level < 0.007 adjusted for regions). There were no statistically significant between-group differences for measures of gray matter volume.

Conclusions

There is reduced cerebral blood flow within brain regions in patients with ATP1A3 variants within the thalamus. Additionally, the lack of corresponding gray matter volume differences may suggest an underlying functional etiology rather than structural abnormality.

Evidence of cerebral hypoperfusion consecutive to ultrasound-mediated blood-brain barrier opening in rats

PURPOSE

This work aims to explore the effect of Blood Brain Barrier (BBB) opening using ultrasound combined with microbubbles injection on cerebral blood flow in rats.

METHODS

Two groups of n = 5 rats were included in this study. The first group was used to investigate the impact of BBB opening on the Arterial Spin Labeling (ASL) signal, in particular on the arterial transit time (ATT). The second group was used to analyze the spatiotemporal evolution of the change in cerebral blood flow (CBF) over time following BBB opening and validate these results using DSC-MRI.

RESULTS

Using pCASL, a decrease in CBF of up to 29 . 6 ± 15 . 1 % $$ 29.6\pm 15.1\% $$ was observed in the target hemisphere, associated with an increase in arterial transit time. The latter was estimated to be 533 ± 121ms $$ 533\pm 12\mathrm{1ms} $$ in the BBB opening impacted regions against 409 ± 93ms $$ 409\pm 93\mathrm{ms} $$ in the contralateral hemisphere. The spatio-temporal analysis of CBF maps indicated a nonlocal hypoperfusion. DSC-MRI measurements were consistent with the obtained results.

CONCLUSION

This study provided strong evidence that BBB opening using microbubble intravenous injection induces a transient hypoperfusion. A spatiotemporal analysis of the hypoperfusion changes allows to establish some points of similarity with the cortical spreading depression phenomenon.

Reduced brain activity during a working memory task in middle-aged apolipoprotein E ε4 carriers with overweight/obesity

Objective

The apolipoprotein E ε4 (APOE ε4) allele and midlife obesity are independent risk factors for Alzheimer's disease (AD). Both of these risk factors are also associated with differences in brain activation, as measured by blood oxygenation level-dependent (BOLD) responses, in the absence of detectable cognitive deficits. Although the presence of these risk factors may influence brain activity during working memory tasks, no study to date has examined whether the presence of the ε4 allele explains variation in working memory brain activity while matching for levels of overweight/obesity. The primary aim of this study was to determine whether the presence of the ε4 allele is associated with differences in task-functional magnetic resonance imaging (fMRI) brain activation in adults with overweight/obesity. We predicted that ε4 carriers would have greater brain activation in regions that support working memory.

Methods

This ancillary study included 48 (n = 24 APOE ε4 carriers; n = 24 APOE ε4 non-carriers), sedentary middle-aged adults (Mean age = 44.63 ± 8.36 years) with overweight/obesity (Mean BMI = 32.43 ± 4.12 kg/m²) who were matched on demographic characteristics. Participants were a subsample enrolled in 12-month randomized clinical trial examining the impact of energy-restricted diet and exercise on cardiovascular health outcomes. Participants completed a n-back working memory task with fMRI, which were completed within one month of the start of the intervention. Participants also underwent pseudo-continuous arterial spin labeling scans, a MRI measure of cerebral blood flow (CBF).

Results

Compared to non-ε4 carriers with overweight/obesity, ε4 carriers with overweight/obesity had lower fMRI brain activity in the middle frontal gyrus, pre and post central gyrus, supramarginal gyrus, superior temporal gyrus, lateral occipital cortex, and angular gyrus (z range = 2.52-3.56) during the n-back working memory task. Differences persisted even when controlling for CBF in these brain regions.

Conclusion

These results indicate that presence of the APOE ε4 allele in middle-aged adults with overweight/obesity is related to altered brain activity during a working memory paradigm, which may confer risk for accelerated neurocognitive decline in late adulthood. Future research is needed to clarify the clinical implications of these findings in the context of risk for AD.

The relationship between fat mass and obesity associated gene polymorphism rs9939609 and resting cerebral blood flow in a midlife sample with overweight and obesity

Background

The single nucleotide polymorphism (SNP) rs9939609 in the fat mass and obesity associated fat mass and obesity associated gene (FTO) gene has been linked with increased BMI in adults. Higher BMI has been associated with poor brain health and may exert deleterious effects on neurocognitive health through cerebral hypoperfusion. However, it is unclear if there is a relationship between the FTO genotype and cerebral perfusion, or whether FTO genotype moderates the effects of weight loss on cerebral perfusion. Using data from a randomized controlled behavioral weight loss trial in adults with overweight and obesity, we tested (1) whether carriers of the A allele for FTO rs9939609 demonstrate different patterns of resting cerebral blood flow (rCBF) compared to T carriers, and (2) whether the FTO genotype moderates the effects of weight loss on rCBF. We hypothesized that carriers of the A allele would exhibit lower resting CBF in frontal brain areas compared to T/T homozygotes at baseline, and that intervention-induced weight loss may partially remediate these differences.

Methods and results

One hundred and five adults (75.2% female, mean age 44.9 years) with overweight or obesity were included in the analyses. These participants represent a subsample of participants in a larger randomized controlled trial (NCT01500356). A resting pseudo-continuous arterial spin labeling (pCASL) scan was acquired to examine rCBF. Age, sex, and BMI were included as covariates. At baseline, A carriers had greater rCBF in a diffuse cluster extending into the brainstem, motor cortex, and occipital lobe, but lower perfusion in the temporal lobe. We found no evidence that FTO moderated the effect of the intervention group assignment on rCBF changes.

Conclusion

Overall, these results indicate that (a) individual variation in rCBF within a sample with overweight and obesity may be attributed to a common FTO variant, but (b) a weight loss intervention is effective at increasing rCBF, regardless of FTO genotype.

Vascular and microstructural markers of cognitive pathology

Introduction

Arterial stiffness may play a role in the development of dementia through poorly understood effects on brain microstructural integrity and perfusion.

Methods

We examined markers of arterial stiffness (carotid-femoral pulse wave velocity [cfPWV]) and elevated systolic blood pressure (SBP) in relation to cognitive function and brain magnetic resonance imaging macrostructure (gray matter [GM] and white matter [WM] volumes), microstructure (diffusion based free water [FW] and fractional anisotropy [FA]), and cerebral blood flow (CBF) in WM and GM in models adjusted for age, race, sex, education, and apolipoprotein E ε4 status.

Results

Among 460 participants (70 ± 8 years; 44 dementia, 158 mild cognitive impairment, 258 normal cognition), higher cfPWV and SBP were independently associated with higher FW, higher WM hyperintensity volume, and worse cognition (global and executive function). Higher SBP alone was significantly associated with lower WM and GM CBF.

Discussion

Arterial stiffness is associated with impaired WM microstructure and global and executive cognitive function.

Hypertensive Aspects of Cardiometabolic Disorders Are Associated with Lower Brain Microstructure, Perfusion, and Cognition

BACKGROUND

Cardiometabolic disorders (hypertension, diabetes) are key modifiable risk factors for Alzheimer's disease and related disorders. They often co-occur; yet, the extent to which they independently affect brain structure and function is unclear.

OBJECTIVE

We hypothesized their combined effect is greater in associations with cognitive function and neuroimaging biomarkers of white matter (WM) health and cerebral perfusion in a diverse older adult cohort.

METHODS

Participants aged 50-85 years received: clinical evaluation, oral glucose tolerance testing, neuroimaging, cognitive testing, and adjudication. Neuroimaging included: T1 (gray [GM]/WM segmentation, regional volumes/thicknesses); FLAIR (WM hyperintensity volume [WMHv]; arterial spin labeling (cerebral blood flow); diffusion tensor imaging (fractional anisotropy [FA]); and neurite orientation dispersion and density imaging (Free Water). Hypertension (HTN) and impaired glucose tolerance (IGT) were staged and cardiometabolic status was categorized (HTN only, IGT only, IGT+HTN, neither). Multivariable linear regression modeled associations with cognitive and neuroimaging measures (covariates: age, gender, race).

RESULTS

MRI was available for 478 participants (35% mild cognitive impairment, 10% dementia) with mean age 70±8 years, 74% with HTN, 61% with IGT, and 15% self-identified as Black/African-American. IGT+HTN was significantly associated with cognitive impairment, higher WM Free Water and WMHv, lower FA, and lower GM perfusion compared to neither factor. HTN alone was associated with poorer cognition and lower GM perfusion. Cardiometabolic factors were not associated with GM macrostructure (volumes, temporal lobe cortical thickness) or cognitive status.

CONCLUSION

HTN and its co-occurrence with IGT (HTN+IGT) were associated with lower global cognitive performance and reduced GM perfusion and impaired WM microstructure.

Optimization of pseudo-continuous arterial spin labeling using off-resonance compensation strategies at 7T

PURPOSE

The sensitivity of pseudo-continuous arterial spin labeling (PCASL) to off-resonance effects (ΔB₀ ) is a major limitation at ultra-high field (≥7T). The aim of this study was to assess the effectiveness of different PCASL ΔB₀ compensation methods at 7T and measure the labeling efficiency with off-resonance correction.

THEORY AND METHODS

Phase offset errors induced by ΔB₀ at the feeding arteries can be compensated by adding an extra radiofrequency (RF) phase increment and transverse gradient blips into the PCASL RF pulse train. The effectiveness of an average field correction (AVGcor), a vessel-specific field-map-based correction (FMcor) and a vessel-specific prescan-based correction (PScor) were compared at 7T. After correction, the PCASL labeling efficiency was directly measured in feeding arteries downstream from the labeling location.

RESULTS

The perfusion signal was more uniform throughout the brain after off-resonance correction. Whole-brain average perfusion signal increased by a factor of 2.4, 2.5, and 2.1, respectively, with AVGcor, FMcor and PScor compared to acquisitions without correction. With off-resonance correction, the maximum labeling efficiency was ~0.68 at mean B₁ (B_1mean ) of 0.70 µT when using a mean gradient (G_mean ) of 0.25 mT/m.

CONCLUSION

Either a prescan or a field map can be used to correct for off-resonance effects and retrieve a good brain perfusion signal at 7T. Although the three methods performed well in this study, FMcor may be better suited for patient studies because it accounted for vessel-specific ΔB₀ variations. Further improvements in image quality will be possible by optimizing the labeling efficiency with advanced hardware and software while satisfying specific absorption rate constraints.

Mediterranean and Western diet effects on Alzheimer's disease biomarkers, cerebral perfusion, and cognition in mid-life: A randomized trial

Introduction

Mid‐life dietary patterns are associated with Alzheimer's disease (AD) risk, although few controlled trials have been conducted.

Methods

Eighty‐seven participants (age range: 45 to 65) with normal cognition (NC, n = 56) or mild cognitive impairment (MCI, n = 31) received isocaloric diets high or low in saturated fat, glycemic index, and sodium (Western‐like/West‐diet vs. Mediterranean‐like/Med‐diet) for 4 weeks. Diet effects on cerebrospinal fluid (CSF) biomarkers, cognition, and cerebral perfusion were assessed to determine whether responses differed by cognitive status.

Results

CSF amyloid beta (Aβ)_42/40 ratios increased following the Med‐diet, and decreased after West‐diet for NC adults, whereas the MCI group showed the reverse pattern. For the MCI group, the West‐diet reduced and the Med‐diet increased total tau (t‐tau), whereas CSF Aβ₄₂/t‐tau ratios increased following the West‐diet and decreased following the Med‐diet. For NC participants, the Med‐diet increased and the West‐diet decreased cerebral perfusion.

Discussion

Diet response during middle age may highlight early pathophysiological processes that increase AD risk.

Relationship Between Cerebrovascular Reactivity and Cognition Among People With Risk of Cognitive Decline

Vascular risk factors (e.g., obesity and hypertension) are associated with cerebral small vessel disease, Alzheimer's disease (AD) pathology, and dementia. Reduced perfusion may reflect the impaired ability of blood vessels to regulate blood flow in reaction to varying circumstances such as hypercapnia (increased end-tidal partial pressures of CO₂). It has been shown that cerebrovascular reactivity (CVR) measured with blood-oxygen-level-dependent (BOLD) MRI is correlated with cognitive performance and alterations of CVR may be an indicator of vascular disfunction leading to cognitive decline. However, the underlying mechanism of CVR alterations in BOLD signal may not be straight-forward because BOLD signal is affected by multiple physiological parameters, such as cerebral blood flow (CBF), cerebral blood volume, and oxygen metabolism. Arterial spin labeling (ASL) MRI quantitatively measures blood flow in the brain providing images of local CBF. Therefore, in this study, we measured CBF and its changes using a dynamic ASL technique during a hypercapnia challenge and tested if CBF or CVR was related to cognitive performance using the Mini-mental state examination (MMSE) score. Seventy-eight participants underwent cognitive testing and MRI including ASL during a hypercapnia challenge with a RespirAct computer-controlled gas blender, targeting 10 mmHg higher end-tidal CO₂ level than the baseline while end-tidal O₂ level was maintained. Pseudo-continuous ASL (PCASL) was collected during a 2-min baseline and a 2-min hypercapnic period. CVR was obtained by calculating a percent change of CBF per the end-tidal CO₂ elevation in mmHg between the baseline and the hypercapnic challenge. Multivariate regression analyses demonstrated that baseline resting CBF has no significant relationship with MMSE, while lower CVR in the whole brain gray matter (β = 0.689, p = 0.005) and white matter (β = 0.578, p = 0.016) are related to lower MMSE score. In addition, region of interest (ROI) based analysis showed positive relationships between MMSE score and CVR in 26 out of 122 gray matter ROIs.

Anterior Cingulate Structure and Perfusion is Associated with Cerebrospinal Fluid Tau among Cognitively Normal Older Adult APOEɛ4 Carriers

Evidence suggests the ɛ4 allele of the apolipoprotein E (APOE) gene may accelerate an age-related process of cortical thickening and cerebral blood flow (CBF) reduction in the anterior cingulate cortex (ACC). Although the neural basis of this association remains unclear, evidence suggests it might reflect early neurodegenerative processes. However, to date, associations between cerebrospinal fluid (CSF) biomarkers of neurodegeneration, such as CSF tau, and APOE-related alterations in ACC cortical thickness (CTH) and CBF have yet to be explored. The current study explored the interaction of CSF tau and APOE genotype (ɛ4+, ɛ4-) on FreeSurfer-derived CTH and arterial spin labeling MRI-measured resting CBF in the ACC (caudal ACC [cACC] and rostral ACC [rACC]) among a sample of 45 cognitively normal older adults. Secondary analyses also examined associations between APOE, CTH/CBF, and cognitive performance. In the cACC, higher CSF tau was associated with higher CTH and lower CBF in ɛ4+, whereas these relationships were not evident in ɛ4-. In the rACC, higher CSF tau was associated with higher CTH for both ɛ4+ and ɛ4-, and with lower CBF only in ɛ4+. Significant interactions of CSF tau and APOE on CTH/CBF were not observed in two posterior reference regions implicated in Alzheimer's disease. Secondary analyses revealed a negative relationship between cACC CTH and executive functioning in ɛ4+ and a positive relationship in ɛ4-. Findings suggest the presence of an ɛ4-related pattern of increased CTH and reduced CBF in the ACC that is associated with biomarkers of neurodegeneration and subtle decrements in cognition.

Interaction of APOE, cerebral blood flow, and cortical thickness in the entorhinal cortex predicts memory decline

The ε4 allele of the apolipoprotein E (APOE) gene, a risk factor for cognitive decline, is associated with alterations in medial temporal lobe (MTL) structure and function, yet little research has been dedicated to understanding how these alterations might interact to negatively impact cognition. To bridge this gap, the present study employed linear regression models to determine the extent to which APOE genotype (ε4+, ε4-) modifies interactive effects of baseline arterial spin labeling MRI-measured cerebral blood flow (CBF) and FreeSurfer-derived cortical thickness/volume (CT/Vo) in two MTL regions of interest (entorhinal cortex, hippocampus) on memory change in 98 older adults who were cognitively normal at baseline. Baseline entorhinal CBF was positively associated with memory change, but only among ε4 carriers with lower entorhinal CT. Similarly, baseline entorhinal CT was positively associated with memory change, but only among ε4 carriers with lower entorhinal CBF. Findings suggest that APOE ε4 carriers may experience concomitant alterations in neurovascular function and morphology in the MTL that interact to negatively affect cognition prior to the onset of overt clinical symptoms. Results also suggest the presence of distinct multimodal neural signatures in the entorhinal cortex that may signal relative risk for cognitive decline among this group, perhaps reflecting different stages of cerebrovascular compensation (early effective vs. later ineffective).

Optimization of pseudo-continuous arterial spin labeling for renal perfusion imaging

PURPOSE

To evaluate labeling efficiency of pseudo-continuous arterial spin labeling (PCASL) and to find the gradient parameters that increase PCASL robustness for renal perfusion measurements.

METHODS

Aortic blood flow was characterized in 3 groups: young healthy volunteers (YHV1), chronic kidney disease (CKD) patients (CKDP), and healthy controls (HCO). PCASL inversion efficiency was evaluated through numeric simulations considering the measured pulsatile flow velocity profiles and off-resonance effects for a wide range of gradient parameters, and the results were assessed in vivo. The most robust PCASL implementation was used to measure renal blood flow (RBF) in CKDP and HCO.

RESULTS

Aortic blood velocities reached peak values of 120 cm/s in YHV1, whereas for elderly subjects values were lower by approximately a factor of 2. Simulations and experiments showed that by reducing the gradient average (G_ave ) and the selective to average gradient ratio (G_max /G_ave ), labeling efficiency was maximized and PCASL robustness to off-resonance was improved. The study in CKDP and HCO showed significant differences in RBF between groups.

CONCLUSION

An efficient and robust PCASL scheme for renal applications requires a G_max /G_ave ratio of 6-7 and a G_ave value that depends on the aortic blood flow velocities (0.5 mT/m being appropriate for CKDP and HCO).

Supporting measurements or more averages? How to quantify cerebral blood flow most reliably in 5 minutes by arterial spin labeling

Purpose

To determine whether sacrificing part of the scan time of pseudo‐continuous arterial spin labeling (PCASL) for measurement of the labeling efficiency and blood T1 is beneficial in terms of CBF quantification reliability.

Methods

In a simulation framework, 5‐minute scan protocols with different scan time divisions between PCASL data acquisition and supporting measurements were evaluated in terms of CBF estimation variability across both noise and ground truth parameter realizations taken from the general population distribution. The entire simulation experiment was repeated for a single‐post‐labeling delay (PLD), multi‐PLD, and free‐lunch time‐encoded (te‐FL) PCASL acquisition strategy. Furthermore, a real data study was designed for preliminary validation.

Results

For the considered population statistics, measuring the labeling efficiency and the blood T1 proved beneficial in terms of CBF estimation variability for any distribution of the 5‐minute scan time compared to only acquiring ASL data. Compared to single‐PLD PCASL without support measurements as recommended in the consensus statement, a 26%, 33%, and 42% reduction in relative CBF estimation variability was found for optimal combinations of supporting measurements with single‐PLD, free‐lunch, and multi‐PLD PCASL data acquisition, respectively. The benefit of taking the individual variation of blood T1 into account was also demonstrated in the real data experiment.

Conclusions

Spending time to measure the labeling efficiency and the blood T1 instead of acquiring more averages of the PCASL data proves to be advisable for robust CBF quantification in the general population.

Improved velocity-selective-inversion arterial spin labeling for cerebral blood flow mapping with 3D acquisition

PURPOSE

To further optimize the velocity-selective arterial spin labeling (VSASL) sequence utilizing a Fourier-transform based velocity-selective inversion (FT-VSI) pulse train, and to evaluate its utility for 3D mapping of cerebral blood flow (CBF) with a gradient- and spin-echo (GRASE) readout.

METHODS

First, numerical simulations and phantom experiments were done to test the susceptibility to eddy currents and B₁ field inhomogeneities for FT-VSI pulse trains with block and composite refocusing pulses. Second, the choices of the post-labeling delay (PLD) for FT-VSI prepared 3D VSASL were evaluated for the sensitivity to perfusion signal. The study was conducted among a young-age and a middle-age group at 3T. Both signal-to-noise ratio (SNR) and CBF were quantitatively compared with pseudo-continuous ASL (PCASL). The optimized 3D VSI-ASL was also qualitatively compared with PCASL in a whole-brain coverage among two healthy volunteers and a brain tumor patient.

RESULTS

The simulations and phantom test showed that composite refocusing pulses are more robust to both eddy-currents and B₁ field inhomogeneities than block pulses. 3D VSASL images with FT-VSI preparation were acquired over a range of PLDs and PLD = 1.2 s was selected for its higher perfusion signal. FT-VSI labeling produced quantitative CBF maps with 27% higher SNR in gray matter compared to PCASL. 3D whole-brain CBF mapping using VSI-ASL were comparable to the corresponding PCASL results.

CONCLUSION

FT-VSI with 3D-GRASE readout was successfully implemented and showed higher sensitivity to perfusion signal than PCASL for both young and middle-aged healthy volunteers.

Changes in cerebral perfusion following a 12-month exercise and diet intervention

Overweight and obesity may damage the cerebrovascular architecture, resulting in a significant reduction in cerebral blood flow. To date, there have been few randomized clinical trials (RCT) examining whether obesity-related reductions in cerebral blood flow could be modified by weight loss. Further, it is unknown whether the behavioral intervention strategy for weight loss (i.e., diet alone or diet combined with exercise) differentially influences cerebral blood flow in adults with overweight or obesity. The primary aim of this study was to determine whether a 12-month RCT of exercise and diet increases cerebral blood flow in 125 midlife (Mean age ± SD = 44.63 ± 8.36 years) adults with overweight and obesity. Further, we evaluated whether weight loss via diet combined with aerobic exercise has an added effect on changes in cerebral blood flow compared to weight loss via diet alone and whether there were regionally specific effects of the type of behavioral intervention on cerebral blood flow patterns. Consistent with our predictions, a 12-month diet and exercise program resulting in 10% weight loss increased cerebral blood flow. These effects were widespread and extended throughout frontal, parietal, and subcortical regions. Further, there was some regional specificity of effects for both diet-only and diet combined with exercise. Our results demonstrate that weight-related reductions in cerebral blood flow can be modified by 10% weight loss over the course of 12 months and that interventions involving exercise exposure may provide unique effects on cerebral blood flow compared to interventions involving only diet.

Sex-Related Differences in Brain Volumes and Cerebral Blood Flow Among Overweight and Obese Adults With Type 2 Diabetes: Exploratory Analyses From the Action for Health in Diabetes Brain Magnetic Resonance Imaging Study

BACKGROUND

Sex may be an important modifier of brain health in response to risk factors. We compared brain structure and function of older overweight and obese women and men with type 2 diabetes mellitus.

METHODS

Cross-sectional cognitive assessments and magnetic resonance images were obtained in 224 women and 95 men (mean age 69 years) with histories of type 2 diabetes mellitus and overweight or obesity. Prior to magnetic resonance images, participants had completed an average of 10 years of random assignment to either multidomain intervention targeting weight loss or a control condition of diabetes support and education. Total (summed gray and white) matter volumes, white matter hyperintensity volumes, and cerebral blood flow across five brain regions of interest were analyzed using mixed-effects models.

RESULTS

After covariate adjustment, women, compared with men, averaged 10.9 [95% confidence interval 3.3, 18.5; ≈1%] cc greater summed region of interest volumes and 1.39 [0.00002, 2.78; ≈54%] cc greater summed white matter hyperintensity volumes. Sex differences could not be attributed to risk factor profiles or intervention response. Their magnitude did not vary significantly with respect to age, body mass index, intervention assignment, or APOE-ε4 genotype. Sex differences in brain magnetic resonance images outcomes did not account for the better levels of cognitive functioning in women than men.

CONCLUSIONS

In a large cohort of older overweight or obese adults with type 2 diabetes mellitus, differences in brain volumes and white matter disease were apparent between women and men, but these did not account for a lower prevalence of cognitive impairment in women compared with men in this cohort.

TRIAL REGISTRATION

NCT00017953.

Robust estimation of quantitative perfusion from multi-phase pseudo-continuous arterial spin labeling

PURPOSE

Multi-phase PCASL has been proposed as a means to achieve accurate perfusion quantification that is robust to imperfect shim in the labeling plane. However, there exists a bias in the estimation process that is a function of noise in the data. In this work, this bias is characterized and then addressed in animal and human data.

METHODS

The proposed algorithm to overcome bias uses the initial biased voxel-wise estimate of phase tracking error to cluster regions with different off-resonance phase shifts, from which a high-SNR estimate of regional phase offset is derived. Simulations were used to predict the bias expected at typical SNR. Multi-phase PCASL in 3 rat strains (n = 21) at 9.4 T was considered, along with 20 human subjects previously imaged using ASL at 3 T. The algorithm was extended to include estimation of arterial blood flow velocity.

RESULTS

Based on simulations, a perfusion estimation bias of 6-8% was expected using 8-phase data at typical SNR. This bias was eliminated when a high-precision estimate of phase error was available. In the preclinical data, the bias-corrected measure of perfusion (107 ± 14 mL/100g/min) was lower than the standard analysis (116 ± 14 mL/100g/min), corresponding to a mean observed bias across strains of 8.0%. In the human data, bias correction resulted in a 15% decrease in the estimate of perfusion.

CONCLUSIONS

Using a retrospective algorithmic approach, it was possible to exploit common information found in multiple voxels within a whole region of the brain, offering superior SNR and thus overcoming the bias in perfusion quantification from multi-phase PCASL.

Selection of preclinical models to evaluate intranasal brain cooling for acute ischemic stroke

Stroke accounts for a large proportion of global mortality and morbidity. Selective hypothermia, via intranasal cooling devices, is a promising intervention in acute ischemic stroke. However, prior to large clinical trials, preclinical studies in large animal models of ischemic stroke are needed to assess the efficacy, safety, and feasibility of intranasal cooling for selective hypothermia as a neuroprotective strategy. Here, we review the available scientific literature for evidence supporting selective hypothermia and make recommendations of a preclinical, large, animal-based, ischemic stroke model that has the greatest potential for evaluating intranasal cooling for selective hypothermia and neuroprotection. We conclude that among large animal models of focal ischemic stroke including pigs, sheep, dogs, and nonhuman primates (NHPs), cynomolgus macaques have nasal anatomy, nasal vasculature, neuroanatomy, and cerebrovasculature that are most similar to those of humans. Moreover, middle cerebral artery stroke in cynomolgus macaques produces functional and behavioral deficits that are quantifiable to a greater degree of precision and detail than those that can be revealed through available assessments for other large animals. These NHPs are also amenable to extensive neuroimaging studies as a means of monitoring stroke evolution and evaluating infarct size. Hence, we suggest that cynomolgus macaques are best suited to assess the safety and efficacy of intranasal selective hypothermia through an evaluation of hyperacute diffusion-weighted imaging and subsequent investigation of chronic functional recovery, prior to randomized clinical trials in humans.

Dose-dependent association of accelerometer-measured physical activity and sedentary time with brain perfusion in aging

INTRODUCTION

Age-related decreases in cerebral blood flow (CBF) may lead to cognitive decline, while physical activity (PA) can maintain CBF and cognition in aging. The intensity of PA needed to affect CBF in aging, and the independent effects of sedentary time on CBF are currently unknown. Moreover, research conducted in free-living environments with objective measures of PA (e.g., accelerometry) is lacking.

METHODS

This cross-sectional study used accelerometry to objectively measure sedentary time, all light PA [AllLightPA], moderate-to-vigorous PA [MVPA], and total activity counts [TAC] in 52 cognitively healthy older adults. Robust linear regressions investigated the association of CBF (using arterial spin labeling magnetic resonance imaging) in frontal and medial temporal regions, with each PA intensity and sedentary time.

RESULTS

Greater sedentary time was significantly associated with lower CBF in lateral and medial frontal regions after adjusting for MVPA, while higher AllLightPA (adjusted for MVPA), MVPA (adjusted for AllLightPA), and TAC were associated with greater CBF in lateral and medial frontal regions.

DISCUSSION

Lighter activities, as well as MVPA, are beneficial to CBF in brain regions typically affected by the aging process and malleable to exercise interventions (i.e., the frontal lobes), whereas sedentary time is an independent risk factor for neurovascular dysregulation in normal aging.

Modified ketogenic diet is associated with improved cerebrospinal fluid biomarker profile, cerebral perfusion, and cerebral ketone body uptake in older adults at risk for Alzheimer's disease: a pilot study

There is currently no established therapy to treat or prevent Alzheimer's disease. The ketogenic diet supplies an alternative cerebral metabolic fuel, with potential neuroprotective effects. Our goal was to compare the effects of a modified Mediterranean-ketogenic diet (MMKD) and an American Heart Association Diet (AHAD) on cerebrospinal fluid Alzheimer's biomarkers, neuroimaging measures, peripheral metabolism, and cognition in older adults at risk for Alzheimer's. Twenty participants with subjective memory complaints (n = 11) or mild cognitive impairment (n = 9) completed both diets, with 3 participants discontinuing early. Mean compliance rates were 90% for MMKD and 95% for AHAD. All participants had improved metabolic indices following MMKD. MMKD was associated with increased cerebrospinal fluid Aβ42 and decreased tau. There was increased cerebral perfusion and increased cerebral ketone body uptake (¹¹C-acetoacetate PET, in subsample) following MMKD. Memory performance improved after both diets, which may be due to practice effects. Our results suggest that a ketogenic intervention targeted toward adults at risk for Alzheimer's may prove beneficial in the prevention of cognitive decline.

APOE modifies the interaction of entorhinal cerebral blood flow and cortical thickness on memory function in cognitively normal older adults

OBJECTIVE

The ε4 allele of the apolipoprotein E (APOE) gene increases risk for cognitive decline in normal and pathologic aging. However, precisely how APOE ε4 exerts its negative impact on cognition is poorly understood. The present study aimed to determine whether APOE genotype (ε4+ vs. ε4-) modifies the interaction of medial temporal lobe (MTL) resting cerebral blood flow (CBF) and brain structure (cortical thickness [CT], volume [Vo]) on verbal memory performance.

METHODS

Multiple linear regression models were employed to investigate relationships between APOE genotype, arterial spin labeling MRI-measured CBF and FreeSurfer-based CT and Vo in four MTL regions of interest (left and right entorhinal cortex and hippocampus), and verbal memory performance among a sample of 117 cognitively normal older adults (41 ε4+, 76 ε4-) between the ages of 64 and 89 (mean age ​= ​73).

RESULTS

Results indicated that APOE genotype modified the interaction of CBF and CT on memory in the left entorhinal cortex, such that the relationship between entorhinal CBF and memory was negative (lower CBF was associated with better memory) in non-carriers with higher entorhinal CT, positive (higher CBF was associated with better memory) in non-carriers with lower entorhinal CT, and negative (higher CBF was associated with worse memory) in ε4 carriers with lower entorhinal CT.

CONCLUSIONS

Findings suggest that older adult APOE ε4 carriers may experience vascular dysregulation and concomitant morphological alterations in the MTL that interact to negatively affect memory even in the absence overt clinical symptoms, providing potential insight into the mechanistic link between APOE ε4 and detriments in cognition. Moreover, findings suggest a distinct multimodal neural signature in ε4 carriers (higher CBF and lower CT in the entorhinal cortex) that could aid in the identification of candidates for future clinical trials aimed at preventing or slowing cognitive decline. Differential findings with respect to ε4 carriers and non-carriers are discussed in the context of neurovascular compensation.

Practical considerations for territorial perfusion mapping in the cerebral circulation using super-selective pseudo-continuous arterial spin labeling

PURPOSE

This paper discusses several challenges faced by super-selective pseudo-continuous arterial spin labeling, which is used to quantify territorial perfusion in the cerebral circulation. The effects of off-resonance, pulsatility, vessel movement, and label rotation scheme are investigated, and methods to maximize labeling efficiency and overall image quality are evaluated. A strategy to calculate the territorial perfusion fractions of individual vessels is proposed.

METHODS

The effects of off-resonance, label rotation scheme, and vessel movement on labeling efficiency were simulated. Two off-resonance compensation strategies (multiphase prescan, field map), cardiac triggering, and vessel movement were studied in vivo in a group of 10 subjects. Subsequently, a territorial perfusion fraction map was acquired in 2 subjects based on the mean vessel labeling efficiency.

RESULTS

Multiphase calibration provided the highest labeling efficiency (P = .002) followed by the field map compensation (P = .037) compared with the uncompensated acquisition. Cardiac triggering resulted in a qualitative improvement of the image and an increase in signal contrast between the perfusion territory and the surrounding tissue (P = .010) but failed to show a significant change in temporal and spatial SNR. The constant clockwise label rotation scheme yielded the highest labeling efficiency. Significant vessel movement (>2 mm according to simulations) was observed in 50% of subjects. The measured territorial perfusion fractions showed good agreement with anatomical data.

CONCLUSION

Optimized labeling efficiency resulted in increased image quality and accuracy of territorial perfusion fraction maps. Labeling efficiency depends critically on off-resonance calibration, cardiac triggering, optimal label rotation scheme, and vessel location tracking.

Quantitative blood flow measurement in rat brain with multiphase arterial spin labelling magnetic resonance imaging

Cerebral blood flow is an important parameter in many diseases and functional studies that can be accurately measured in humans using arterial spin labelling (ASL) MRI. However, although rat models are frequently used for preclinical studies of both human disease and brain function, rat CBF measurements show poor consistency between studies. This lack of reproducibility is due, partly, to the smaller size and differing head geometry of rats compared to humans, as well as the differing analysis methodologies employed and higher field strengths used for preclinical MRI. To address these issues, we have implemented, optimised and validated a multiphase pseudo-continuous ASL technique, which overcomes many of the limitations of rat CBF measurement. Three rat strains (Wistar, Sprague Dawley and Berlin Druckrey IX) were used, and CBF values validated against gold-standard autoradiography measurements. Label positioning was found to be optimal at 45°, while post-label delay was optimised to 0.55 s. Whole brain CBF measures were 109 ± 22, 111 ± 18 and 100 ± 15 mL/100 g/min by multiphase pCASL, and 108 ± 12, 116 ± 14 and 122 ± 16 mL/100 g/min by autoradiography in Wistar, SD and BDIX cohorts, respectively. Tumour model analysis shows that the developed methods also apply in disease states. Thus, optimised multiphase pCASL provides robust, reproducible and non-invasive measurement of CBF in rats.

Off-resonance correction for pseudo-continuous arterial spin labeling using the optimized encoding scheme

Pseudo-continuous arterial spin labeling (PCASL) MRI has become a popular tool for non-invasive perfusion imaging and angiography. However, it suffers from sensitivity to off-resonance effects within the labeling plane, which can be exacerbated at high field or in the presence of metallic implants, leading to spatially varying signal loss and cerebral blood flow underestimation. In this work we propose a prospective correction technique based on the optimized encoding scheme, which allows the rapid calculation of transverse gradient blips and RF phase modulations that best cancel phase offsets due to off-resonance at the locations of the feeding arteries within the labeling plane. This calculation is based upon a rapidly acquired single-slice fieldmap and is applicable to any number and arrangement of arteries. In addition, this approach is applicable to both conventional PCASL and a vessel-selective variant known as vessel-encoded PCASL (VEPCASL). Through simulations and experiments in healthy volunteers it was shown that in the presence of off-resonance effects a strong bias in the strength of the perfusion signal across vascular territories can be introduced, the signal-to-noise ratio (SNR) efficiency of PCASL and VEPCASL can be severely compromised (∼40% reduction in vivo), and that vessel-selective signal in VEPCASL can be incorrectly assigned. Distortion of the spatial regions placed in the label or control conditions in the presence of off-resonance effects was confirmed in phantom experiments. The application of the proposed correction restored SNR efficiency to levels present in the absence of off-resonance effects and corrected errors in the vascular territory maps derived from VEPCASL. Due to the rapid nature of the required calculations and fieldmap acquisition, this approach could be inserted into protocols with minimal effect on the total scan time.

Total cerebrovascular blood flow and whole brain perfusion in children sedated using propofol with or without ketamine at induction: An investigation with 2D-Cine PC and ASL

BACKGROUND

Multiple sedation regimes may be used to facilitate pediatric MRI scans. These regimes might affect cerebral blood flow and hemodynamics to varying degrees, particularly in children who may be vulnerable to anesthetic side effects.

PURPOSE

To compare the effects of propofol monosedation solely (Pm group) vs. a combination of propofol and ketamine (KP group) on brain hemodynamics and perfusion.

STUDY TYPE

Prospective double-blind randomized trial.

FIELD STRENGTH/SEQUENCES

1.5T and 3T. 2D-Cine phase contrast (2D-Cine PC) and pseudocontinuous arterial spin labeling (ASL).

POPULATION

Children aged from 3 months to 10 years referred for MRI with deep sedation were randomized into either the KP or the Pm group. Perfusion images were acquired with ASL followed by single-slice 2D-Cine PC acquired between the cervical vertebra C2 and C3.

ASSESSMENT

Average whole-brain perfusion (WBP ml.min^-1 .100 ml^-1 ) was extracted from the ASL perfusion maps and total cerebrovascular blood flow (CVF) was quantified by bilaterally summing the flow in the vertebral and the internal carotid arteries. The CVF values were converted to units of ml.min^-1 .100 g^-1 to calculate the tissue CVF_100g (ml.min^-1 .100 g^-1 ). Images were assessed by a neuroradiologist and data from n = 81 (ASL) and n = 55 (PC) cases with no apparent pathology were entered into the analysis.

STATISTICAL TESTS

Multivariate analysis of covariance was performed to compare drug sedation effects on WBP, CVF, and CVF_100g .

RESULTS

No significant difference in arterial flow was observed (P = 0.57), but the KP group showed significantly higher WBP than the Pm group, covarying for scanner and age (P = 0.003). A correlation analysis showed a significant positive correlation between mean WBP (ml.min^-1 .100 g^-1 ) and mean CVF_100g .

DATA CONCLUSION

The KP group showed higher perfusion but no significant difference in vascular flow compared with the Pm group. WBP and CVF_100g correlated significantly, but ASL appeared to have more susceptibility to perfusion differences arising from the different sedation regimes.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 4 J. Magn. Reson. Imaging 2019;50:1433-1440.

Sensitivity of Multiphase Pseudocontinuous Arterial Spin Labelling (MP pCASL) Magnetic Resonance Imaging for Measuring Brain and Tumour Blood Flow in Mice

Brain and tumour blood flow can be measured noninvasively using arterial spin labelling (ASL) magnetic resonance imaging (MRI), but reliable quantification in mouse models remains difficult. Pseudocontinuous ASL (pCASL) is recommended as the clinical standard for ASL and can be improved using multiphase labelling (MP pCASL). The aim of this study was to optimise and validate MP pCASL MRI for cerebral blood flow (CBF) measurement in mice and to assess its sensitivity to tumour perfusion. Following optimization of the MP pCASL sequence, CBF data were compared with gold-standard autoradiography, showing close agreement. Subsequently, MP pCASL data were acquired at weekly intervals in models of primary and secondary brain tumours, and tumour microvessel density was determined histologically. MP pCASL measurements in a secondary brain tumour model revealed a significant reduction in blood flow at day 35 after induction, despite a higher density of blood vessels. Tumour core regions also showed reduced blood flow compared with the tumour rim. Similarly, significant reductions in CBF were found in a model of glioma 28 days after tumour induction, together with an increased density of blood vessels. These findings indicate that MP pCASL MRI provides accurate and robust measurements of cerebral blood flow in naïve mice and is sensitive to changes in tumour perfusion.

CEST, ASL, and magnetization transfer contrast: How similar pulse sequences detect different phenomena

Chemical exchange saturation transfer (CEST), arterial spin labeling (ASL), and magnetization transfer contrast (MTC) methods generate different contrasts for MRI. However, they share many similarities in terms of pulse sequences and mechanistic principles. They all use RF pulse preparation schemes to label the longitudinal magnetization of certain proton pools and follow the delivery and transfer of this magnetic label to a water proton pool in a tissue region of interest, where it accumulates and can be detected using any imaging sequence. Due to the versatility of MRI, differences in spectral, spatial or motional selectivity of these schemes can be exploited to achieve pool specificity, such as for arterial water protons in ASL, protons on solute molecules in CEST, and protons on semi-solid cell structures in MTC. Timing of these sequences can be used to optimize for the rate of a particular delivery and/or exchange transfer process, for instance, between different tissue compartments (ASL) or between tissue molecules (CEST/MTC). In this review, magnetic labeling strategies for ASL and the corresponding CEST and MTC pulse sequences are compared, including continuous labeling, single-pulse labeling, and multi-pulse labeling. Insight into the similarities and differences among these techniques is important not only to comprehend the mechanisms and confounds of the contrasts they generate, but also to stimulate the development of new MRI techniques to improve these contrasts or to reduce their interference. This, in turn, should benefit many possible applications in the fields of physiological and molecular imaging and spectroscopy.

Repeatability of Arterial Spin Labeling MRI in Measuring Blood Perfusion in the Human Eye

BACKGROUND

Quantifying blood perfusion in ocular tissues is challenging, partly because the majority of the blood is carried by the choroid, which is difficult to visualize because it is located between the retina and sclera.

PURPOSE/HYPOTHESIS

To evaluate the intra- and interday repeatability of MRI measures of chorio-retinal blood perfusion.

STUDY TYPE

Prospective, cross-sectional, observational study.

POPULATION

Twenty young healthy adults (six male, age: 25 ± 5 years) scanned twice within a single session repeated at the same time of day on 2 days.

FIELD STRENGTH/SEQUENCE

Arterial spin labeling (ASL) MRI at 3.0T using pseudocontinuous ASL (PCASL) labeling scheme and a 3D turbo-gradient-spin-echo (TGSE) acquisition, including axial T₂ -weighted structural images using a 2D turbo-spin-echo (TSE) sequence.

ASSESSMENTS

Region-of-interest analysis for assessment of chorio-retinal blood perfusion.

STATISTICAL TESTS

Intra- and interday repeatability of measures analyzed using intraclass correlation coefficients (ICC), Pearson's correlation analysis, paired t-tests, and Bland-Altman plots.

RESULTS

The mean chorio-retinal perfusion was 77.86 (standard deviation [SD] = 29.80) ml/100ml/min. Perfusion measurements correlated strongly within a single session (r = 0.95, 95% confidence interval [CI] [0.880-0.980], P < 0.001) and between the two sessions based on a single run (r = 0.80 [0.582-0.913], P < 0.001), and two runs (r = 0.80 [0.479-0.918], P < 0.001). There were mean differences of 2.69 [16.85 to -22.23] ml/100ml/min for intraday measures, -7.44 [27.45 to -42.32] ml/100ml/min for single-run interday measures, and 5.73 [28.71 to -40.17] ml/100ml/min for two-run interday measures, but none were significant (all P > 0.05).

DATA CONCLUSION

Quantitative ASL-MRI measurements of chorio-retinal blood perfusion showed high intra- and interday repeatability. The ASL-MRI technique provides reliable measures of chorio-retinal perfusion in vivo.

LEVEL OF EVIDENCE

1 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2019;49:966-974.

Advances in arterial spin labelling MRI methods for measuring perfusion and collateral flow

With the publication in 2015 of the consensus statement by the perfusion study group of the International Society for Magnetic Resonance in Medicine (ISMRM) and the EU-COST action 'ASL in dementia' on the implementation of arterial spin labelling MRI (ASL) in a clinical setting, the development of ASL can be considered to have become mature and ready for clinical prime-time. In this review article new developments and remaining issues will be discussed, especially focusing on quantification of ASL as well as on new technological developments of ASL for perfusion imaging and flow territory mapping. Uncertainty of the achieved labelling efficiency in pseudo-continuous ASL (pCASL) as well as the presence of arterial transit time artefacts, can be considered the main remaining challenges for the use of quantitative cerebral blood flow (CBF) values. New developments in ASL centre around time-efficient acquisition of dynamic ASL-images by means of time-encoded pCASL and diversification of information content, for example by combined 4D-angiography with perfusion imaging. Current vessel-encoded and super-selective pCASL-methodology have developed into easily applied flow-territory mapping methods providing relevant clinical information with highly similar information content as digital subtraction angiography (DSA), the current clinical standard. Both approaches seem therefore to be ready for clinical use.

Functional MRI of brain physiology in aging and neurodegenerative diseases

Brain aging and associated neurodegeneration constitute a major societal challenge as well as one for the neuroimaging community. A full understanding of the physiological mechanisms underlying neurodegeneration still eludes medical researchers, fuelling the development of in vivo neuroimaging markers. Hence it is increasingly recognized that our understanding of neurodegenerative processes likely will depend upon the available information provided by imaging techniques. At the same time, the imaging techniques are often developed in response to the desire to observe certain physiological processes. In this context, functional MRI (fMRI), which has for decades provided information on neuronal activity, has evolved into a large family of techniques well suited for in vivo observations of brain physiology. Given the rapid technical advances in fMRI in recent years, this review aims to summarize the physiological basis of fMRI observations in healthy aging as well as in age-related neurodegeneration. This review focuses on in-vivo human brain imaging studies in this review and on disease features that can be imaged using fMRI methods. In addition to providing detailed literature summaries, this review also discusses future directions in the study of brain physiology using fMRI in the clinical setting.

Dural Arterivenous Fistula at the Cavernous Sinus Diagnosed by Arterial Spin-labeled Imaging

We herein report a case of dural arteriovenous fistula (DAVF) at the cavernous sinus that was diagnosed by arterial spin-labeled imaging (ASL). A 67-year-old woman was referred to our hospital due to double vision and bilateral conjunctival injection. Conventional magnetic resonance imaging findings were normal. However, abnormal hyperintense signals on ASL were detected. Furthermore, the abnormality disappeared after successful endovascular embolization. Although conventional digital subtraction angiography is the standard tool for diagnosing DAVF, we speculated that ASL might be useful to this end as well.

Arterial spin labeling for the measurement of cerebral perfusion and angiography

Arterial spin labeling (ASL) is an MRI technique that was first proposed a quarter of a century ago. It offers the prospect of non-invasive quantitative measurement of cerebral perfusion, making it potentially very useful for research and clinical studies, particularly where multiple longitudinal measurements are required. However, it has suffered from a number of challenges, including a relatively low signal-to-noise ratio, and a confusing number of sequence variants, thus hindering its clinical uptake. Recently, however, there has been a consensus adoption of an accepted acquisition and analysis framework for ASL, and thus a better penetration onto clinical MRI scanners. Here, we review the basic concepts in ASL and describe the current state-of-the-art acquisition and analysis approaches, and the versatility of the method to perform both quantitative cerebral perfusion measurement, along with quantitative cerebral angiographic measurement.

Subjective Cognitive Decline Modifies the Relationship Between Cerebral Blood Flow and Memory Function in Cognitively Normal Older Adults

OBJECTIVES

Subjective cognitive decline (SCD), or self-reported cognitive decline despite normal neuropsychological test performance, is a risk factor for objective cognitive decline and Alzheimer's disease (AD). While brain mechanisms contributing to SCD are not well defined, studies show associations with vascular risk factors and altered cerebral blood flow (CBF), raising the hypothesis that those with SCD might be experiencing vascular dysregulation, or a disruption in the normal relationship between CBF and cognition. We examined whether the association between CBF and verbal memory performance differs between those with SCD (SCD+) and those without SCD (SCD-).

METHODS

Linear mixed-effects models were used to investigate whether the voxel-wise relationship between arterial spin labeling (ASL) MRI-measured CBF and verbal memory performance was modified by SCD among a group of 70 cognitively normal older adults (35 SCD+, 35 SCD-; mean age=72) matched on age, gender, and symptoms of depression.

RESULTS

Results indicated that the SCD- group exhibited positive associations between verbal memory and CBF within the posterior cingulate cortex, middle temporal gyrus, and inferior frontal gyrus, whereas the SCD+ group displayed negative associations between verbal memory and CBF within the posterior cingulate cortex, middle temporal gyrus, hippocampus, fusiform gyrus, and inferior frontal gyrus.

CONCLUSIONS

Findings suggest that, while higher CBF is supportive of memory function in those without SCD, higher CBF may no longer support memory function in those presenting with SCD, perhaps reflecting neurovascular dysregulation. (JINS, 2018, 24, 213-223).

Long Term Effect of Intensive Lifestyle Intervention on Cerebral Blood Flow

OBJECTIVES

To determine whether long-term behavioral intervention targeting weight loss through increased physical activity and reduced caloric intake would alter cerebral blood flow (CBF) in individuals with type 2 diabetes mellitus.

DESIGN

Postrandomization assessment of CBF.

SETTING

Action for Health in Diabetes multicenter randomized controlled clinical trial.

PARTICIPANTS

Individuals with type 2 diabetes mellitus who were overweight or obese and aged 45 to 76 (N = 310).

INTERVENTIONS

A multidomain intensive lifestyle intervention (ILI) to induce weight loss and increase physical activity for 8 to 11 years or diabetes support and education (DSE), a control condition.

MEASUREMENTS

Participants underwent cognitive assessment and standardized brain magnetic resonance imaging (MRI) (3.0 Tesla) to assess CBF an average of 10.4 years after randomization.

RESULTS

Weight changes from baseline to time of MRI averaged -6.2% for ILI and -2.8% for DSE (P < .001), and increases in self-reported moderate or intense physical activity averaged 444.3 kcal/wk for ILI and 114.8 kcal/wk for DSE (P = .03). Overall mean CBF was 6% greater for ILI than DSE (P = .04), with the largest mean differences between ILI and DSE in the limbic region (3.39 mL/100 g per minute, 95% confidence interval (CI) = 0.07-6.70 mL/100 g per minute) and occipital lobes (3.52 mL/100 g per minute, 95% CI = 0.20-6.84 mL/100 g per minute). In ILI, greater CBF was associated with greater decreases in weight and greater increases in physical activity. The relationship between CBF and scores on a composite measure of cognitive function varied between intervention groups (P = .02).

CONCLUSIONS

Long-term weight loss intervention in overweight and obese adults with type 2 diabetes mellitus is associated with greater CBF.

Pseudo continuous arterial spin labeling quantification in anemic subjects with hyperemic cerebral blood flow

PURPOSE

To investigate possible sources of quantification errors in global cerebral blood flow (CBF) measurements by comparing pseudo continuous arterial spin labeling (PCASL) and phase contrast (PC) MRI in anemic, hyperemic subjects.

METHODS

All studies were performed on a Philips 3T Achieva MRI scanner. PC and PCASL CBF examinations were performed in 10 healthy, young adult subjects and 18 young adults with chronic anemia syndromes including sickle cell disease and thalassemia. CBF estimates from single and two compartment ASL kinetic models were compared. Numerical simulation and flow phantom experiments were used to explore the effects of blood velocity and B1⁺ on CBF quantification and labeling efficiency.

RESULTS

PCASL CBF underestimated PC in both populations using a single compartment model (30.1±9.2% control, 45.2±17.2% anemia). Agreement substantially improved using a two-compartment model (-8.0±6.0% control, 11.7±12.3% anemia). Four of the anemic subjects exhibited venous outflow of ASL signal, suggestive of cerebrovascular shunt, possibly confounding PC-PCASL comparisons. Additionally, sub-study experiments demonstrated that B1⁺ was diminished at the labeling plane (82.9±5.1%), resulting in suboptimal labeling efficiency. Correcting labeling efficiency for diminished B1⁺, PCASL slightly overestimated PC CBF in controls (-15.4±6.8%) and resulted in better matching of CBF estimates in anemic subjects (0.7±10.0% without outflow, 10.5±9.4% with outflow).

CONCLUSIONS

This work demonstrates that a two-compartment model is critical for PCASL quantification in hyperemic subjects. Venous outflow and B1⁺ under-excitation may also contribute to flow underestimation, but further study of these effects is required.

Temporal gradient during famous face naming is associated with lower cerebral blood flow and gray matter volume in aging

OBJECTIVE

Evidence suggests that famous face naming may be a cognitive ability especially sensitive to the early pathological processes of Alzheimer's disease (AD) and that those at risk for AD may demonstrate a Ribot temporal gradient (RTG), characterized by better performance for naming remote famous faces than for naming recent famous faces. Reductions in cerebral blood flow (CBF) and gray matter volume (GMV) have been implicated in the neuropathological cascade of AD and show utility as biomarkers of AD risk. We examined whether a RTG during famous face naming was associated with lower CBF and/or GMV among a group of cognitively normal older adults.

METHODS

Voxel-wise independent samples t-tests were employed to contrast resting CBF values between those who exhibited a RTG (RTG+) during a famous face naming task and those who did not (RTG-) among a sample of 52 cognitively normal older adults (25 RTG-, 27 RTG+; mean age = 73). Groups were also compared on GMV using a voxel-wise general linear model.

RESULTS

Significant group differences in CBF and GMV were found, whereby the RTG+ group demonstrated reduced CBF and GMV within medial temporal lobe regions (hippocampus, parahippocampal gyrus), relative to the RTG- group.

CONCLUSIONS

This represents the first study to show that cognitively intact older adults who demonstrate a RTG during famous face naming exhibit vascular dysregulation and structural changes similar to that seen in AD risk. Findings suggest that famous face naming ability may be particularly sensitive to the very early brain changes associated with AD.

Improving the robustness of pseudo-continuous arterial spin labeling to off-resonance and pulsatile flow velocity

PURPOSE

To improve pseudo-continuous arterial spin labeling (PCASL) robustness to off-resonance and pulsatile blood flow velocity.

METHODS

Bloch equations were solved to evaluate the effect of labeling parameters in a pulsatile flow model for a range of off-resonance. Experimental confirmation was achieved in volunteers using linear phase increase between labeling pulses to approximate off-resonance errors. We first assessed the location of the labeling plane in four volunteers. Next, we explored a range of parameters-including balanced and unbalanced gradients-in five more volunteers at an optimal labeling plane location.

RESULTS

Simulations demonstrated that 1) high velocities are vulnerable to off-resonance, 2) unbalanced PCASL outperforms balanced PCASL, 3) increased B1 and low average gradient improve the labeling efficiency for high-velocity flow, and 4) a low ratio of selective to average gradient improves off-resonance robustness. A good agreement between theory and experiment was observed.

CONCLUSION

The robustness of PCASL can be increased by selecting an unbalanced scheme with a low average gradient (0.5 mT/m), a low ratio (7×) of selective to average gradients, and the highest feasible B1 (1.8 μT). Placing the labeling plane above the carotid bifurcation and below the V3 segment, usually between the second and third vertebrae, yielded robust results. Magn Reson Med 78:1342-1351, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Changes in Cerebral Blood Flow during an Alteration in Glycemic State in a Large Non-human Primate (Papio hamadryas sp.)

Changes in cerebral blood flow (CBF) during a hyperglycemic challenge were mapped, using perfusion-weighted MRI, in a group of non-human primates. Seven female baboons were fasted for 16 h prior to 1-h imaging experiment, performed under general anesthesia, that consisted of a 20-min baseline, followed by a bolus infusion of glucose (500 mg/kg). CBF maps were collected every 7 s and blood glucose and insulin levels were sampled at regular intervals. Blood glucose levels rose from 51.3 ± 10.9 to 203.9 ± 38.9 mg/dL and declined to 133.4 ± 22.0 mg/dL, at the end of the experiment. Regional CBF changes consisted of four clusters: cerebral cortex, thalamus, hypothalamus, and mesencephalon. Increases in the hypothalamic blood flow occurred concurrently with the regulatory response to systemic glucose change, whereas CBF declined for other clusters. The return to baseline of hypothalamic blood flow was observed while CBF was still increasing in other brain regions. The spatial pattern of extra-hypothalamic CBF changes was correlated with the patterns of several cerebral networks including the default mode network. These findings suggest that hypothalamic blood flow response to systemic glucose levels can potentially be explained by regulatory activity. The response of extra-hypothalamic clusters followed a different time course and its spatial pattern resembled that of the default-mode network.

Cerebral hemodynamics and pseudo-continuous arterial spin labeling considerations in adults with sickle cell anemia

Sickle cell anemia (SCA) is a genetic disorder resulting in reduced oxygen carrying capacity and elevated stroke risk. Pseudo-continuous arterial spin labeling (pCASL) measures of cerebral blood flow (CBF) may have relevance for stroke risk assessment; however, the effects of elevated flow velocity and reduced bolus arrival time (BAT) on CBF quantification in SCA patients have not been thoroughly characterized, and pCASL model parameters used in healthy adults are often applied to patients with SCA. Here, cervical arterial flow velocities and pCASL labeling efficiencies were computed in adults with SCA (n = 19) and age- and race-matched controls without sickle trait (n = 7) using pCASL in sequence with phase contrast MR angiography (MRA). Controls (n = 7) and a subgroup of patients (n = 8) also underwent multi-post-labeling-delay pCASL for BAT assessment. Mean flow velocities were elevated in SCA adults (velocity = 28.3 ± 4.1 cm/s) compared with controls (velocity = 24.5 ± 3.8 cm/s), and mean pCASL labeling efficiency (α) was reduced in SCA adults (α = 0.72) relative to controls (α = 0.91). In patients, mean whole-brain CBF from phase contrast MRA was 91.8 ± 18.1 ml/100 g/min, while mean pCASL CBF when assuming a constant labeling efficiency of 0.86 was 75.2 ± 17.3 ml/100 g/min (p < 0.01), resulting in a mean absolute quantification error of 23% when a labeling efficiency appropriate for controls was assumed. This difference cannot be accounted for by BAT (whole-brain BAT: control, 1.13 ± 0.06 s; SCA, 1.02 ± 0.09 s) or tissue T₁ variation. In conclusion, BAT variation influences pCASL quantification less than elevated cervical arterial velocity and labeling efficiency variation in SCA adults; thus, a lower labeling efficiency (α = 0.72) or subject-specific labeling efficiency should be incorporated for SCA patients.

Biomarkers, designs, and interpretations of resting-state fMRI in translational pharmacological research: A review of state-of-the-Art, challenges, and opportunities for studying brain chemistry

A decade of research and development in resting-state functional MRI (RSfMRI) has opened new translational and clinical research frontiers. This review aims to bridge between technical and clinical researchers who seek reliable neuroimaging biomarkers for studying drug interactions with the brain. About 85 pharma-RSfMRI studies using BOLD signal (75% of all) or arterial spin labeling (ASL) were surveyed to investigate the acute effects of psychoactive drugs. Experimental designs and objectives include drug fingerprinting dose-response evaluation, biomarker validation and calibration, and translational studies. Common biomarkers in these studies include functional connectivity, graph metrics, cerebral blood flow and the amplitude and spectrum of BOLD fluctuations. Overall, RSfMRI-derived biomarkers seem to be sensitive to spatiotemporal dynamics of drug interactions with the brain. However, drugs cause both central and peripheral effects, thus exacerbate difficulties related to biological confounds, structured noise from motion and physiological confounds, as well as modeling and inference testing. Currently, these issues are not well explored, and heterogeneities in experimental design, data acquisition and preprocessing make comparative or meta-analysis of existing reports impossible. A unifying collaborative framework for data-sharing and data-mining is thus necessary for investigating the commonalities and differences in biomarker sensitivity and specificity, and establishing guidelines. Multimodal datasets including sham-placebo or active control sessions and repeated measurements of various psychometric, physiological, metabolic and neuroimaging phenotypes are essential for pharmacokinetic/pharmacodynamic modeling and interpretation of the findings. We provide a list of basic minimum and advanced options that can be considered in design and analyses of future pharma-RSfMRI studies. Hum Brain Mapp 38:2276-2325, 2017. © 2017 Wiley Periodicals, Inc.

Dynamic association between perfusion and white matter integrity across time since injury in Veterans with history of TBI

OBJECTIVE

Cerebral blood flow (CBF) plays a critical role in the maintenance of neuronal integrity, and CBF alterations have been linked to deleterious white matter changes. Although both CBF and white matter microstructural alterations have been observed within the context of traumatic brain injury (TBI), the degree to which these pathological changes relate to one another and whether this association is altered by time since injury have not been examined. The current study therefore sought to clarify associations between resting CBF and white matter microstructure post-TBI.

METHODS

37 veterans with history of mild or moderate TBI (mmTBI) underwent neuroimaging and completed health and psychiatric symptom questionnaires. Resting CBF was measured with multiphase pseudocontinuous arterial spin labeling (MPPCASL), and white matter microstructural integrity was measured with diffusion tensor imaging (DTI). The cingulate cortex and cingulum bundle were selected as a priori regions of interest for the ASL and DTI data, respectively, given the known vulnerability of these regions to TBI.

RESULTS

Regression analyses controlling for age, sex, and posttraumatic stress disorder (PTSD) symptoms revealed a significant time since injury × resting CBF interaction for the left cingulum (p < 0.005). Decreased CBF was significantly associated with reduced cingulum fractional anisotropy (FA) in the chronic phase; however, no such association was observed for participants with less remote TBI.

CONCLUSIONS

Our results showed that reduced CBF was associated with poorer white matter integrity in those who were further removed from their brain injury. Findings provide preliminary evidence of a possible dynamic association between CBF and white matter microstructure that warrants additional consideration within the context of the negative long-term clinical outcomes frequently observed in those with history of TBI. Additional cross-disciplinary studies integrating multiple imaging modalities (e.g., DTI, ASL) and refined neuropsychiatric assessment are needed to better understand the nature, temporal course, and dynamic association between brain changes and clinical outcomes post-injury.

Velocity-selective-inversion prepared arterial spin labeling

PURPOSE

To develop a Fourier-transform based velocity-selective inversion (FT-VSI) pulse train for velocity-selective arterial spin labeling (VSASL).

METHODS

This new pulse contains paired and phase cycled refocusing pulses. Its sensitivities to B0/B1 inhomogeneity and gradient imperfections such as eddy currents were evaluated through simulation and phantom studies. Cerebral blood flow (CBF) quantification using FT-VSI prepared VSASL was compared with conventional VSASL and pseudocontinuous ASL (PCASL) at 3 Tesla.

RESULTS

Simulation and phantom results of the proposed FT-VSI pulse train demonstrated excellent robustness to B0/B1 field inhomogeneity and eddy currents. The estimated CBF of gray matter and white matter for the FT-VSI prepared VSASL, averaged among eight healthy volunteers, were 49.5 ± 7.5 mL/100 g/min and 14.8 ± 2.4 mL/100 g/min, respectively. Excellent correlation and agreement between the FT-VSI method and conventional VSASL and PCASL were found. The averaged signal-to-noise ratio (SNR) value in gray matter of the FT-VSI method was 39% higher than VSASL using conventional double refocused hyperbolic tangent pulses and 9% lower than PCASL.

CONCLUSION

A novel FT-VSI pulse train was demonstrated to be a suitable labeling module for VSASL with robustness of velocity-selective profile to B0/B1 field inhomogeneity and gradient imperfections. Compared with conventional VSASL, FT-VSI prepared VSASL produced consistent CBF maps with higher SNR values. Magn Reson Med 76:1136-1148, 2016. © 2015 Wiley Periodicals, Inc.