Single-coil arterial spin-tagging for estimating cerebral blood flow as viewed from the capillary: Relative contributions of intra- and extravascularsignal

The single-capillary model was applied to the exchange microvessels for wat er in the cerebral parenchyma and used to calculate blood-to-brain flux of water; the theory of the steady-state arterial spin-tagging (AST) technique for estimating cerebral blood flow (CBF) was revised to incorporate the pr esence of both extravascular (tissue) and capillary signal. A crucial eleme nt of the single-coil AST experiment is that magnetization transfer (MT) sh ortens the effective T-1 of the extravascular water, making it one-quarter that of the T-1 of capillary blood. Furthermore, the mean capillary transit time is on the order of the T-1 of the extravascular water. The single-coi l AST experiment is distinguished from other methods which use water as an indicator for measurement of CBF in that the (flow-dependent) populations o f inverted protons in the intra- and extravascular compartments can be near ly equal for normal physiological conditions. The following questions are c onsidered: Is single-coil AST contrast linear in resting CBF? Is contrast i n the single-coil AST technique likely to be linear under changes in CBF in normal tissue? Is the contrast likely to be linear in such common patholog ies as stroke and cerebral tumor? We demonstrate that, if the population of inverted protons in the microvessels is included in the experiment, the vo xel population of inverted protons will be approximately linear with flow a cross a broad range of flow values. We predict that the single-coil AST exp eriment will systematically overestimate resting CBF for flows in the norma l range, that changes in CBF in normal tissue will produce an approximately linear response in AST measurement, and, finally, we predict the operating characteristics of the measurement in common cerebral pathologies. (C) 200 1 Wiley-Liss, Inc.