Edited by: Zhendong Liu, Shandong First Medical University, China
Reviewed by: Rahul Pratap Kotian, Gulf Medical University, United Arab Emirates; Xiaoniu Liang, Fudan University, China; Antonio Coca, University of Barcelona, Spain
This article was submitted to Neuroinflammation and Neuropathy, a section of the journal Frontiers in Aging Neuroscience
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In this pilot study, we investigated microvascular impairment in patients with cerebral small vessel disease (CSVD) using non-invasive arterial spin labeling (ASL) magnetic resonance imaging (MRI). This method enabled us to measure the perfusion parameters, cerebral blood flow (CBF), and arterial transit time (ATT), and the effective T1-relaxation time (T1eff) to research a novel approach of assessing perivascular clearance. CSVD severity was characterized using the Standards for Reporting Vascular Changes on Neuroimaging (STRIVE) and included a rating of white matter hyperintensities (WMHs), lacunes, enlarged perivascular spaces (EPVSs), and cerebral microbleeds (CMBs). Here, we found that CBF decreases and ATT increases with increasing CSVD severity in patients, most prominent for a white matter (WM) region-of-interest, whereas this relation was almost equally driven by WMHs, lacunes, EPVSs, and CMBs. Additionally, we observed a longer mean T1eff of gray matter and WM in patients with CSVD compared to elderly controls, providing an indication of impaired clearance in patients. Mainly T1eff of WM was associated with CSVD burden, whereas lobar lacunes and CMBs contributed primary to this relation compared to EPVSs of the centrum semiovale. Our results complement previous findings of CSVD-related hypoperfusion by the observation of retarded arterial blood arrival times in brain tissue and by an increased T1eff as potential indication of impaired clearance rates using ASL.
In cerebral small vessel disease (CSVD), the physiology of small arteries (arterioles), capillaries, and venules is pathologically affected (Pantoni and Garcia,
Perivascular spaces (PVSs) surrounding the perforating vessels are part of the brain's outflow routes, in which fluid transport takes place through arterial pulsation to remove metabolic by-products and proteins aiming to maintain tissue homeostasis (Rasmussen et al.,
It is possible to assess (perivascular) clearance function in humans by applying MRI, but thus far several suitable approaches demand the
In the study published by Joseph et al., the ASL signal decay over time was fitted linearly as an approximation of the exponential decay described by the Bloch equations (Bloch,
There is overall growing evidence, that CSVD can be considered as a model disease of perivascular clearance failure, an assumption that is also based on the frequent detection of PVS enlargement in different regions of the microvascular diseased brain (Carare et al.,
We here thus present a pilot study in patients with CSVD, showing, that noninvasive ASL MRI has the potential to assess impaired tissue clearance in patients with small vessel disease by fitting the ASL signal decay rate over time.
In total, 14 elderly participants (10 patients with CSVD and four controls) participated in this study approved by the Ethics Committee of the Otto-von-Guericke University Magdeburg, Germany (93/17). Signed informed consent was provided by each participant. CSVD diagnosis was based on the existence of cerebral microbleeds (CMBs), either in lobar or in deep and mixed regions, detected on prior 1.5 Tesla (T) iron/blood-sensitive MRI conducted for diagnostic work-up (Mugler and Brookeman,
Demographical information of the study groups.
Females, |
2 (50%) | 4 (40%) |
Median age (P25, P75) in [years] | 71.0 (68.3, 76.8) | 70.0 (62.5, 77.0) |
Median education (P25, P75) in [years] | 15.5 (13.5, 16.0) | 13.0 (11.8, 13.8) |
Median MMSE-score |
30.0 (30.0, 30.0) | 27 (25, 28) |
Median CSVD-score (P25, P75) | 8.0 (7.9, 9.5) | 23.4 (16.3, 28.7) |
Hypertension yes/ no | 2/ 2 | 10/ 0 |
Median systolic blood pressure (P25, P75) in [mmHg] | 131.5 (130.0, 151.8) | 130.0 (120.0, 157.5) |
Median diastolic blood pressure (P25, P75) in [mmHg] | 80.0 (77.0, 89.8) | 80.0 (78.0, 80.5) |
All participants underwent an MRI acquisition at a 3-T Siemens MAGNETOM Skyra MRI Scanner (Siemens, Erlangen, Germany) with a 32-channel head coil (Nova Medical, Wilmington, Massachusetts, USA). The MRI protocol included a structural whole-brain T1-weighted MPRAGE sequence (Mugler and Brookeman,
Additionally, a multi-inversion time (mTI) PASL scan with FAIR labeling, 3D-GRASE (Günther et al.,
Cerebral small vessel disease markers according to STRIVE were evaluated visually based on a fluid-attenuated inversion recovery (FLAIR) sequence with 1 mm3 isotropic resolution, a T2-weighted TSE sequence with 0.5 × 0.5 mm2 in-plane resolution and 2 mm slice thickness, and a susceptibility weighted imaging (SWI) sequence with 0.6 × 0.6 × 2 mm3 voxel size (see “Visual MRI Analysis According to STRIVE” section).
All MRI measurements were conducted before noon to minimize perfusion signal variations caused by diurnal fluctuations (Ssali et al.,
Individual structural T1-weighted scans were pre-processed using tools of the Oxford Centre for functional MRI of the BRAIN (FMRIB) Software Library (FSL; Jenkinson et al.,
Arterial spin labeling control and tag images, and related reference scans (M0 scans), were registered (Jenkinson et al.,
Arterial spin labeling MR images were used to evaluate a novel approach of assessing differences in the clearance rates between patients with CSVD and healthy elderly controls.
Therefore, individual ASL control and tag images were subtracted to obtain perfusion-weighted difference images for every TI. Only the ASL difference signal of the last three TIs (TI = 2,800/3,050/3,300 ms) was considered for this analysis, to reduce the impact of the bolus decay, or rather the arterial input function as much as possible. Mean signal intensities of the difference images of those TIs were calculated using individual masks for the BG, and for GM and WM.
The signal decay of the ASL difference signal over time for the last three TIs (
An MRI analysis of all subjects was performed in a semiquantitative manner according to STRIVE by a trained investigator (SS), blinded to all demographics and clinical information (Wardlaw et al.,
In short, CMBs were defined as small (2–10 mm), round, or oval hypointense lesions visible on SWI, categorized into lobar (frontal, temporal, parietal, occipital, and insula), deep (BG, thalamus, internal capsule, external capsule, corpus callosum, deep and periventricular WM), and infratentorial (cerebellum and brainstem) applying the Microbleed Anatomical Rating Scale (MARS; Gregoire et al.,
The CSVD sum score was modified from Charidimou et al. [CAA score, (Charidimou et al.,
IBM SPSS Statistics 23 was used for statistical analysis. Because of the small sample size, Kruskal–Wallis tests were chosen to obtain group differences between patients with CSVD and healthy controls. Pearson's correlations were used for a comparison of continuous data, and Spearman's correlation coefficients were calculated for ordinal data. CBF, ATT, and T1eff values, which differed for more than 1.5 times the interquartile range from the median, were removed from further analysis.
As expected, overall CSVD burden, e.g., the CSVD-score, was significantly higher in the CSVD group compared to controls [H(1) = 8.0,
We first compared CBF and ATT between CSVD and controls. Two outliers from the CSVD group were excluded for statistical analysis of GM, WM, and HCr CBF, and one additional CSVD outlier was excluded for BG CBF analysis (see Section Statistical Analysis for exclusion criteria).
Our data showed a significant CBF decrease in the CSVD group compared to controls affecting the GM, WM, BG, and the hippocampus (
Mean cerebral blood flow (CBF) differences between patients with cerebral small vessel disease (CSVD) and controls (CON) for several regions-of-interest. Significant group differences (
Group comparison between patients with cerebral small vessel disease and controls of the mean cerebral blood flow (CBF) and mean arterial transit time (ATT) in several regions-of-interest (ROIs).
Gray matter | H(1) = 4.875, |
H(1) = 5.780, |
White matter | H(1) = 7.385, |
H(1) = 5.780, p = 0.016 |
Basal ganglia | H(1) = 4.024, |
H(1) = 0, |
Left Hippocampus | H(1) = 4.5, |
H(1) = 0.720, |
Right Hippocampus | H(1) = 6.49, |
H(1) = 5.120, |
Mean arterial transit time (ATT) differences between patients with cerebral small vessel disease (CSVD) and controls (CON) for several regions-of-interest. Significant group differences (
Relating perfusion results to CSVD markers, we found moderate-to-large effect size correlations between widespread hypoperfusion and greater CSVD burden (
When considering lobar lacunes, lobar CMBs and CSO EPVSs separately, lacunes and CMB, but not EPVSs were driving WM/GM hypoperfusion. Conversely, for deep lacunes and CMBs, and BG EPVSs, CMBs and EPVSs were explaining BG CBF reduction (
Arterial spin labeling data of one participant from the CSVD group was excluded from this analysis of the signal decay rate, because of unphysiologically estimated, negative effective T1 values (probably induced by participant's motion during ASL imaging). Additionally, for the analysis of the mean signal decay rate of WM, one, and for the mean BG signal decay rate, two outliers from the CSVD group were excluded, as related values differed for more than 1.5 times the interquartile range from the median. The mean goodness (
We found significant group differences of the estimated T1eff in GM [H(1) = 4.024,
Mean arterial spin labeling (ASL) signal decay of white matter over time for patients with cerebral small vessel disease (CSVD, red) and controls (CON, black). Shown is the scaled ASL difference signal for the last three inversion times (TI). Each dot represents the measured value of each participant at TI = 2,800/3,050/3,300 ms. Thin lines show the results for the individual signal fits. Bold lines are the mean signal decays for both study groups. The small image in the upper right corner shows the mean-scaled ASL difference signal of all participants for the whole ASL time series.
Mean effective T1-relaxation time (T1eff) differences between patients with cerebral small vessel disease (CSVD) and controls (CON) for several regions-of-interest. Significant group differences (
Relating T1eff results to CSVD markers, we found moderate-to-large effect size correlations between greater T1eff in the WM and GM and higher CSVD burden (
When considering lobar lacunes, lobar CMBs and CSO EPVSs separately, lacunes and CMBs, but not EPVSs were driving T1eff in the WM. Additionally, there was a moderate effect size correlation between T1eff and perfusion in the WM and GM ROI (
In this pilot study, we investigated a noninvasive ASL MRI technique to simultaneously assess brain perfusion and T1eff in patients with CSVD.
Our main findings comprise widespread hypoperfusion in the GM, WM, and subcortical regions (BG, hippocampus) in CSVD, which was related to greater small vessel disease burden, especially in the WM, where it was rather equally driven by several CSVD features. Considering EPVS region-wise, in the BG, they were related to local flow reduction, whereas CSO EPVSs did not account for GM/WM hypoperfusion. T1eff, as a potential measure of clearance, was increased in the GM and WM of patients with CSVD, but not in their BG, which was—again—related to several CSVD markers, but not necessarily to CSO EPVS burden.
Most previous studies focused on the relation of greater WMH severity and CBF decrease in CSVD (Shi et al.,
Additionally, we observed increased mean ATT in GM and WM in the CSVD group compared to controls. These longer ATTs may not only reflect micro- but also macrovascular changes (Dai et al.,
The quantification of CBF was based on the general kinetic model. The ASL difference signal of an imaged voxel is related to the amount of labeled arterial blood water delivered to this voxel by venous outflow (although usually negligible) and by T1-relaxation, whereas the latter is affected by the blood water extraction within the capillaries and the exchange with tissue water (Buxton et al.,
The single-compartment model assumes that there is an instantaneous exchange of labeled blood water and tissue in the capillaries and that the signal of labeled blood water therefore decays with T1 of tissue. The mean WM T1eff of the control group in this study confirmed reported values for WM T1-relaxation times at 3T (Bojorquez et al.,
For the estimation of the effective T1-relaxation, we only considered the last three TIs (TI = 2,800/3,050/3,300 ms), as a trade-off between acquisition time and an accurate perfusion estimation, to omit signal contributions of the arterial input function including bolus dispersion effects and variances of the estimated bolus arrival in the voxel. Although the T1-relaxation time of blood is shorter in men compared to women (Zhang et al.,
Here, we found a relation of T1eff (perivascular clearance) and overall CSVD severity. But, there might even be regional differences of the clearance function between CAA and HA, which could be potentially resolved in future studies with larger sample sizes. In relation to the literature, could comparable studies on a larger size of patients with CAA possibly even show an association between clearance failure and CSO EPVS. Another approach of restricting T1eff analysis to the location of PVSs could be evaluated using high-resolution T2-weighted and high-resolution ASL images obtained with high-field 7T MRI.
Benefits of this ASL approach to assess impaired clearance in patients with CSVD are its noninvasiveness, the short acquisition time, the possibility to estimate yet cerebral perfusion, which might be of high interest in CSVD examinations, and the relatively simple procedure of fitting the ASL signal decay. If just clearance rates are of interest, the acquisition time could even be reduced to the later inversions times, making an application in a hospital environment for patients even more suitable.
Limitations of this pilot study are its small sample size. With a larger sample size and increased statistical power, we will be able to draw more precise and reliable conclusions, which will deepen our knowledge of microvascular impairments in patients with CSVD. In addition, the individual caffeine uptake before the MRI examinations was not controlled for, although it is well-known that caffeine alters cerebral perfusion (Mutsaerts et al.,
This pilot study was aimed to investigate the feasibility to use a noninvasive ASL MRI approach to assess T1-relaxation times of labeled blood water, which possibly reflect clearance dysfunction in CSVD. Here, we found indications that the presented ASL method is indeed able to depict perfusion and T1eff differences in patients compared to elderly controls, and that T1eff is highly associated with perfusion and CSVD severity.
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
The studies involving human participants were reviewed and approved by Ethics Committee of the Otto-von-Guericke University Magdeburg, Germany. The patients/participants provided their written informed consent to participate in this study.
KN analyzed the data and wrote this article. MG generated the used PASL sequence. ED supervised this work. SS performed clinical examinations (inclusive visual MRI analysis according to STRIVE) and supervised this study. All authors discussed the results and proofread this article. All authors contributed to the article and approved the submitted version.
Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 425899996, and the Federal Ministry of Education and Research (BMBF).
MG receives royalties from Siemens Healthineers for technology using ASL and was employed by mediri GmbH. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
The Supplementary Material for this article can be found online at:
Correlation matrix of the CSVD sum score (CSVD-score), white matter hyperintensities (WMHs) related to FAZEKAS scale (WMH-Faz) and Charidimou (WMH-Cha), lacunes, lobar and deep lacunes, total number of enlarged perivascular spaces (EPVSs) in the basal ganglia, centrum semiovale, and hippocampus (EPVS), EPVSs in the centrum semiovale and basal ganglia (EPVS-CSO and EPVS-BG), microbleeds (CMBs), lobar and deep CMBs, mean cerebral blood flow (CBF), mean arterial transit time (ATT) and mean effective T1-relaxation time (T1eff) in gray matter (GM), WM, basal ganglia (BG), and for the perfusion results additionally for the left hippocampus (HCl) and right hippocampus (HCr). Shown are positive (red) and negative (blue) correlation coefficients between all parameters.
Axial slice of the MPRAGE
Demographical information and individual study results used for statistical analysis.