Transient relationships among BOLD, CBV, and CBF changes in rat brain as detected by functional MRI

Abstract

The transient relationship between arterial cerebral blood flow (CBFA) and total cerebral blood volume (CBVT) was determined in the rat brain. Five rats anesthetized with urethane (1.2 g/kg) were examined under graded hypercapnia conditions (7.5% and 10% CO2 ventilation). The blood oxygenation level-dependent (BOLD) contrast was determined by a gradient-echo echo-planar imaging (GE-EPI) pulse sequence, and CBVT changes were determined after injection of a monocrystalline iron oxide nanocolloid (MION) contrast agent using an iron dose of 12 mg/kg. The relationship between CBVT and CBFA under transient conditions is similar to the power law under steady-state conditions. In addition, the transient relationship between CBVT and CBFA is region-specific. Voxels with ≥15% BOLD signal changes from hypercapnia (7.5% CO2 ventilation) have a larger power index (α = 3.26), a larger maximum possible BOLD response (M = 0.85), and shorter T*2 (32 ms) caused by deoxyhemoglobin, compared to voxels with <15% BOLD signal changes (α = 1.82, M = 0.16, and T*2 = 169 ms). It is suggested that the biophysical model of the BOLD signal can be extended under the transient state, with a caution that α and M values are region-specific. To avoid overestimation of the cerebral metabolic rate of oxygen changes seen using fMRI, caution should be taken to not include voxels with large veins and a large BOLD signal. © 2002 Wiley-Liss, Inc.