Cerebral blood volume (CBV) is related with cerebral blood flow (CBF); an increase in CBF accompanies an increase in CBV and vise versa. If CBV is quantitatively related to CBF, knowing this relationship may have significant implications for investigating the quantitative relationship between neuronal activity and blood oxygenation level-dependent (BOLD) fMRI response measurements. A recent MRI study reports a varied CBV-CBF relationship both spatially and with sex [1]. This study investigates the spatial homogeneity of the CBV-CBF relationship across the visual cortex at both rest and activation states.Absolute CBV and CBF were quantified noninvasively at both rest and activation states for 5 female and 5 male subjects [1]. Areas with significant activation in the primary visual cortex were identified using t tests of task-induced changes between rest and activation in CBV and CBF at four levels of significance (P<0.01, P<0.05, P<0.1, and P<0.2), and nine region of interest (ROI) masks in the primary visual cortex covering a range of sizes were identified (Table 1, the first two columns) [1]. Mean CBV and CBF values were calculated within each ROI mask, and this resulted in one data point for each subject at rest and activation states in each ROI. For each sex, group-mean CBV and CBF values at rest and during activation are shown in Table 1 (columns from 3rd to 6th). The spatial homogeneity of the CBV-CBF relationship across the visual cortex can be investigated by analyzing the behaviors of CBV and CBF as a function of ROI size at both rest and activation states.Females and Males showed a very similar CBV behavior at rest with a relatively very small difference in CBV across the whole range of ROI size from 4 cm³ to 89 cm³ (Fig. 1, top left). During activation, they also showed a similar CBV behavior with a slightly larger difference in CBV compared to that at rest. For both rest and activation states, CBV showed a similar decreasing behavior with increasing ROI size, though the decreasing rate was slightly bigger during activation. Overall, CBV showed a very similar behavior for both state and sex. This is also reflected in the very similar, activation-induced relative CBV changes across the whole range of ROI size for both sex (Fig. 1, top right). The bottom left plot in Fig. 1 shows the behavior of CBF as a function of ROI size for Females and Males at rest and during activation. At rest, CBF showed an almost ROI size-independent behavior for both Females and Males, but the difference in CBF was very large between sex. During activation, CBF showed a very similar decreasing behavior with increasing ROI size for both Females and Males, and the difference in CBF was at a similar level as that at rest. The activation-induced relative CBF change also showed a very similar behavior across the whole size range for both sex (Fig. 1, bottom right). Thus, like CBV, CBF also behaved similarly across the whole size range for both state and sex except the very large difference in CBF between sex.During activation, both CBV and CBF varied with ROI size in the same way: the smaller the ROI size, the larger the CBV and CBF (Fig. 1, left). As these ROIs were determined using t tests of task-induced changes between rest and activation in CBV and CBF at the four levels of significance, the smaller the ROI size, the smaller the P-value and, roughly speaking, the larger the activation within the ROI. A larger activation in a ROI should induce a larger CBV and CBF, reflected in these observed behaviors of CBV and CBF during activation. At rest, the ROI size-independent behavior of CBF showed a homogeneous CBF across the visual cortex (Fig. 1, bottom left). The size-dependent behavior of CBV at rest, however, showed an inhomogeneous CBV across the visual cortex; the smaller the ROI size, the larger the CBV (Fig. 1, top left). The homogeneous CBF but inhomogeneous CBV imply a spatially inhomogeneous CBV-CBF relationship across the visual cortex.