An account of the discrepancy between MRI and PET cerebral blood flow measures. A high-field MRI investigation
ABSTRACT There is controversy concerning the discrepancy between absolute cerebral blood flow (CBF) values measured using positron emission tomography (PET) and magnetic resonance imaging (MRI). To gain insight into this problem, the increased signal-to-noise ratio (SNR) and extended T(1) relaxation times of blood and tissue at 3.0 T were exploited to perform pulsed arterial spin labeling (PASL) MRI measurements as a function of spatial resolution and post-labeling delay. The results indicate that, when using post-labeling delays shorter than 1500 ms, MRI gray matter flow values may become as high as several times the correct CBF values owing to tissue signal contamination by remaining arterial blood water label. For delays above 1500 ms, regional PASL-based CBF values (n = 5; frontal gray matter: 48.8 +/- 3.3(SD) ml/100 g/min; occipital gray matter: 49.3 +/- 4.5 ml/100 g/min) comparable with PET-based measurements can be obtained by using spatial resolutions comparable with PET (5-7.5 mm in-plane). At very high resolution (2.5 x 2.5 x 3 mm(3)), gray matter CBF values were found to increase by 10-20%, a consequence attributed to reduction in partial volume effects with cerebrospinal fluid and white matter. The recent availability of MRI field strengths of 3.0 T and higher will facilitate the use of MRI-based CBF measurements in the clinic.
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ABSTRACT: Arterial spin labeling (ASL) is an emerging MRI technique for non-invasive measurement of cerebral blood flow (CBF). Compared to invasive perfusion imaging modalities, ASL suffers from low sensitivity due to poor signal-to-noise ratio (SNR), susceptibility to motion artifacts and low spatial resolution, all of which limit its reliability. In this work, the effects of various state of the art image processing techniques for addressing these ASL limitations are investigated. A processing pipeline consisting of motion correction, ASL motion correction imprecision removal, temporal and spatial filtering, partial volume effect correction, and CBF quantification was developed and assessed. To further improve the SNR for pseudo-continuous ASL (PCASL) by accounting for errors in tagging efficiency, the data from multiphase (MP) acquisitions were analyzed using a novel weighted-averaging scheme. The performances of each step in terms of SNR and reproducibility were evaluated using test-retest ASL data acquired from 12 young healthy subjects. The proposed processing pipeline was shown to improve the within-subject coefficient of variation and regional reproducibility by 17% and 16%, respectively, compared to CBF maps computed following motion correction but without the other processing steps. The CBF measurements of MP-PCASL compared to PCASL had on average 23% and 10% higher SNR and reproducibility, respectively. Copyright © 2015. Published by Elsevier Inc.NeuroImage 05/2015; 117. DOI:10.1016/j.neuroimage.2015.05.048 · 6.13 Impact Factor
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ABSTRACT: Caffeine, as the most commonly used stimulant drug, improves vigilance and, in some cases, cognition. However, the exact effect of caffeine on brain activity has not been fully elucidated. Because caffeine has a pronounced vascular effect which is independent of any neural effects, many hemodynamics-based methods such as fMRI cannot be readily applied without a proper calibration. The scope of the present work is two-fold. In Study 1, we used a recently developed MRI technique to examine the time-dependent changes in whole-brain cerebral metabolic rate of oxygen (CMRO2) following the ingestion of 200mg caffeine. It was found that, despite a pronounced decrease in CBF (p<0.001), global CMRO2 did not change significantly. Instead, the oxygen extraction fraction (OEF) was significantly elevated (p=0.002) to fully compensate for the reduced blood supply. Using the whole-brain finding as a reference, we aim to investigate whether there are any regional differences in the brain's response to caffeine. Therefore, in Study 2, we examined regional heterogeneities in CBF changes following the same amount of caffeine ingestion. We found that posterior brain regions such as posterior cingulate cortex and superior temporal regions manifested a slower CBF reduction, whereas anterior brain regions including dorsolateral prefrontal cortex and medial frontal cortex showed a faster rate of decline. These findings have a few possible explanations. One is that caffeine may result in a region-dependent increase or decrease in brain activity, resulting in an unaltered average brain metabolic rate. The other is that caffeine's effect on vasculature may be region-specific. Plausibility of these explanations is discussed in the context of spatial distribution of the adenosine receptors. Copyright © 2014. Published by Elsevier Inc.NeuroImage 01/2015; 110. DOI:10.1016/j.neuroimage.2015.01.046 · 6.13 Impact Factor
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ABSTRACT: Short- and long-term effects of transjugular intrahepatic portosystemic shunt (TIPS) on cerebral blood flow (CBF) in patients with cirrhosis are still unclear. The purpose of this longitudinal study was to explore CBF alteration patterns in cirrhotic patients after TIPS. Thirteen cirrhotic patients (7 male, 6 female, mean age 50.0 ± 9.3 years) underwent arterial-spin labeling (ASL) MRI 1-9 days (median 1 days) before TIPS. Follow-up MR examinations were performed about 1 week (median 6 days), 3 months (n = 6), 6-9 months (n = 5) and 12-18 months (n = 5) after TIPS. CBF, ammonia level, Child-Pugh score, number connection test type A (NCT-A) and digit symbol test (DST) scores were converted into relative values by dividing by his/her pre-TIPS values, and then, compared via one-way analysis of variance (ANOVA). Correlations between the pre- and post-TIPS changes of relative CBF (rCBF) and the changes of relative ammonia (rAmmonia), Child-Pugh (rChild-Pugh), and NCT-A/DST (rNCT-A/rDST) scores were calculated by crossing subjects. Compared with the pre-TIPS level, the global rCBF slightly increased by 10.9 % about 1 week later, then rapidly decreased by 14.2 % 3 months later, and flatly decreased by 17.2 % in 6-9 months and 18.0 % in 12-18 months following TIPS. The changes of 3-month rDST score were slightly correlated with 3-month rCBF rather than 1-week rCBF, (P < 0.1, FDR-corrected) No difference was found between the pre- and post-TIPS rAmmonia levels, rChild-Pugh and rNCT-A/rDST scores (Post-hoc P > 0.05). CBF measured at different time points after TIPS insertion showed different patterns, indicating varying longitudinal effects of TIPS on CBF. A sharp decline of rCBF was found in the 1 week to 3 months period after insertion, indicating that high event rate of hepatic encephalopathy might relate with the unadaptable CBF in patients undergoing TIPS insertion.Metabolic Brain Disease 04/2013; 28(3). DOI:10.1007/s11011-013-9400-8 · 2.40 Impact Factor