When Perfusion Meets Diffusion: in vivo Measurement of Water Permeability in Human Brain

Abstract

Quantification of water permeability can improve the accuracy of perfusion measurements obtained with arterial spin labeling (ASL) methods, and may provide clinically relevant information regarding the functional status of the microvasculature. The amount of labeled water in the vascular and tissue compartments in an ASL experiment can be estimated based on their distinct diffusion characteristics, and in turn, water permeability determined from the relative vascular and tissue contributions. In the present study, a hybrid magnetic resonance imaging technique was introduced by marrying a continuous ASL method with a twice-refocused spin-echo diffusion sequence. Series of diffusion-weighted ASL signals were acquired with systematically varied b values. The signals were modeled with fast and slow decaying components that were associated with the vascular and tissue compartments, respectively. The relative amount of labeled water in the tissue compartment increased from 61% to 74% and to 86% when the postlabeling delay time was increased from 0.8 to 1.2 and to 1.5 secs. With a b value of 50 secs/mm 2 , the capillary contribution (fast component) of the ASL signal could be effectively minimized. Using the single-pass approximation model, the water permeability of gray matter in the human brain was estimated based on the derived relative water fractions in the tissue and microvasculature. The potential for in vivo magnetic resonance mapping of water permeability was showed using two diffusion weighted ASL measurements with b = 0 and 50 secs/mm 2 in both healthy subjects and a case of brain tumor.

Comparison of arterial transit times estimated using arterial spin labeling

“…Importantly, neither LL-ASL nor FEAST is capable of isolating transit to the tissue compartment. Other techniques such as detecting changes in T 2 of the spin label [16,18] or using stronger diffusion gradients to separate the capillary and tissue compartments [19] are necessary for accurate tissue transit time mapping.…”

Characterizing the Origin of the Arterial Spin Labelling Signal in MRI Using a Multiecho Acquisition Approach

“…However, this result may simply indicate rapid exchange of labelled water into the tissue after entering the capillary bed. In humans, a greater proportion of labelled water reside in the intravascular compartment during image acquisition (Wang et al, 2007). This discordance principally reflects the marked difference in CBF (B50 mL/ mins/100 g) and transit time (1 secs) to the human brain in comparison to the rat brain (where CBF B200 mL/mins/100 g and transit time B 0.2 secs).…”

“…For example, perfusion may be overestimated by standard quantification methods if a significant proportion of the tagged spins reside in the intravascular compartment, as this blood may still be in transit to its eventual location for oxygen and nutrient delivery and exchange (Silva et al, 1997a). Several studies have attempted to address this uncertainty by estimating the proportions of labelled spins in the vascular space relative to the tissue space (IV relative to (EC + IC)) using a variety of methods that take advantage of differences in apparent diffusion, the effect of contrast agents, or magnetization transfer in the two compartments (Silva et al, 1997a, b;Wang et al, 2003;Wang et al, 2007;Zaharchuk et al, 1998;Kim and Kim, 2006). Recently, initial data have been presented investigating the potential use of T2 differences to observe compartmentation of the ASL signal in the human brain using an FAIR-CPMG approach (He and Yablonskiy, 2007).…”

Variability of Renal Apparent Diffusion Coefficients: Limitations of the Monoexponential Model for Diffusion Quantification

“…When analyzed with a biexponential model, IVIM effects provide useful sensitivity to perfusion that is modifi ed in disease states. Extensive human ( 27,28 ) and animal ( 29,30 ) IVIM imaging has been devoted to assessing cerebral microcirculation, and animal model tumors, with pathologically abundant vascularity, have also provided an ideal target for the technique ( 31 ). However, if ignored because of the use of a monoexponential model, IVIM effects can distort the measurement of the ADC.…”