Modeling the Effect of Changes in Arterial Blood Volume on the BOLD Signal

Purpose: The BOLD effect is modeled as primarily due to a mismatch of cerebral blood flow (CBF) and cerebral metabolic rate of O2 (CMRO2), but it is still not clear how we should include the effects of changes in cerebral blood volume (CBV) in the model. Having an accurate model is important because it serves as the framework for interpreting combined CBF and BOLD measurements in the calibrated-BOLD approach, providing quantitative assessments of n, an index of CBF/CMRO2 coupling defined as the ratio of the two fractional changes. The original Davis model [1] only took into account CBV changes on the venous side, and yet a number of recent studies have found significant CBV changes on the arterial side [2]. While CBV increases on the venous side increase the local deoxyhemoglobin content and reduce the BOLD signal, arterial CBV changes can increase the BOLD signal by exchanging blood water, with an intrinsically higher MR signal, for tissue water. Here the BOLD signal model is extended to include arterial CBV effects, and the new model was used to test the range of error that occurs when data is analyzed with the simpler Davis model to estimate n.