B factor dependence of the temporal characteristics of brain activation using dynamic apparent diffusion coefficient contrast

Functional MRI studies to date have been generally performed using the blood oxygenation level dependent (BOLD) contrast mechanism. Recently, it has been proposed that dynamic change in the apparent diffusion coefficient (ADC), measured using intravoxel incoherent motion (IVIM) weighting, can be used as a robust functional contrast mechanism. Based on the differences in the timing characteristics compared to the BOLD activation, the ADC contrast can be selectively sensitized to upstream vascular pools (e.g., arterial networks). In this study we further investigated the timing characteristics of the functional ADC contrast using multiple degrees of IVIM weighting. It was found that the time course in the high b factor range lagged behind that of the low b factor range, indicating that the low b factor ADC contrast included contributions from the larger and faster moving arteries, and the smaller arterioles and capillaries downstream were reflected in the high b factor changes. These changes help confirm the arterial origin of the ADC contrast and offer a direction to improve the localization of activity to small vessel networks. Magn Reson Med 52:1432–1437, 2004. © 2004 Wiley‐Liss, Inc.