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

The transient relationship between arterial cerebral blood flow (CBF A ) and total cerebral blood volume (CBV T ) 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% CO 2 ventilation). The blood oxygenation level‐dependent (BOLD) contrast was determined by a gradient‐echo echo‐planar imaging (GE‐EPI) pulse sequence, and CBV T 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 CBV T and CBF A under transient conditions is similar to the power law under steady‐state conditions. In addition, the transient relationship between CBV T and CBF A is region‐specific. Voxels with ≥15% BOLD signal changes from hypercapnia (7.5% CO 2 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. Magn Reson Med 48:987–993, 2002. © 2002 Wiley‐Liss, Inc.