Cerebral Vascular Mean Transit Time in Healthy Humans: A Comparative Study with PET and Dynamic Susceptibility Contrast-Enhanced MRI

Abstract

Cerebral vascular mean transit time (MTT), defined as the ratio of cerebral blood volume to cerebral blood flow (CBV/CBF), is a valuable indicator of the cerebral circulation. Positron emission tomography (PET) and dynamic susceptibility contrast-enhanced magnetic resonance imaging (DSC-MRI) are useful for the quantitative determination of MTT in the clinical setting. The aim of this study was to establish a normal value set of MTT as determined by PET and by DSC-MRI and to identify differences between these methods. Seven healthy volunteers were studied with 15 O-PET (H 2 15 O and C 15 O) and gradient-echo echo-planar DSC-MRI at 1.5 T. In the DSC-MRI study with bolus injection of contrast agent, deconvolution analysis was performed. Comparison of gray-to-white matter ratios showed fairly good agreement between PET and DSC-MRI for all parameters (relative CBV, relative CBF, and relative MTT), confirming the validity of relative measurements with DSC-MRI. However, quantitative MTT measured by DSC-MRI was significantly shorter than that measured by PET in cerebral cortical regions (2.8 to 3.0 secs for DSC-MRI versus 3.9 to 4.3 secs for PET) and the centrum semiovale (3.5 secs for DSC-MRI versus 4.8 secs for PET). These discrepancies may be because of the differences in the intrinsic sensitivity of each imaging modality to vascular components; whereas PET measurement of CBV is equally sensitive to all vascular components, measurement with DSC-MRI originates from the microvasculature in the vicinity of the brain parenchyma. This underlying difference may influence interpretation of MTT determined by PET or by DSC-MRI for patients with cerebrovascular disease.

The Performance of MRI-Based Cerebral Blood Flow Measurements in Acute and Subacute Stroke Compared With 15O-Water Positron Emission Tomography

“…This result supports the general concept of PWI-based CBF measurement as previously indicated in animal studies, 40,41 in healthy volunteers 17,38,42,43 as well as in patients with chronic ischemia 42 and chronic carotid occlusive disease. 42,44,45 However, the aforementioned clinical studies did not include patients with acute stroke and used ROI-based values instead of a volumetric analysis.…”

Cerebrovascular Occlusive Disease: Quantitative Cerebral Blood Flow Using Dynamic Susceptibility Contrast MR Imaging Correlates with Quantitative H2[15O] PET

“…In a study by Tanaka et al (28) MR and PET CBF values were calculated relative to an internal control and compared between the two modalities; however no significant correlation was found. Ibaraki et al (29) found significant correlations when MR and PET CBF were expressed as relative to white matter; however quantitation was not successful owing to difficulties in obtaining a reliable scaling for the AIF.…”

Cerebral blood flow estimation in vivo using local tissue reference functions

“…DSC-MR acquisition involves the injection of a bolus of paramagnetic tracer followed by rapid image acquisition to measure the bolus as it passes through the brain. Postprocessing of DSC-MR data can yield relative estimates of cerebral blood flow (CBF) that agree well with other perfusion imaging techniques (5)(6)(7)(8)(9).…”

“…However, acquisition limitations in the measurement of the tracer concentration from arterial blood (called the arterial input function [AIF]) is a cause of large errors in absolute DSC-MR CBF estimates derived by deconvolution (10). If left uncorrected, these AIF CBF errors generally cause large, physiologically unreasonable CBF estimates relative to gold standard techniques (e.g., positron emission tomography [PET]) (3,7,9,(11)(12)(13). As a result, DSC-MR CBF maps are often scaled to correspond to PET using either a constant conversion factor derived from normal patients (14) or are cross-calibrated by setting a reference region (or cross-calibration region [CCR]) to a known population-based CBF average assumed to have low variation (15)(16)(17) in order to provide CBF estimates suitable for between-subject comparisons.…”