Contributions of dynamic venous blood volume versus oxygenation level changes to BOLD fMRI

Blood-oxygenation-level-dependent (BOLD) fMRI has contributions from venous oxygenation and venous cerebral blood volume (CBV) changes. To examine the relative contribution of venous CBV change (Delta CBVV) to BOLD fMRI, BOLD and arterial CBV changes (Delta CBVa) to a 40-s forepaw stimulation in six a-chloralose anesthetized rats were measured using a magnetization transfer-varied fMRI technique, while total CBV change (Delta CBVt) was measured with injection of iron oxide nanoparticles. Delta CBVv was obtained by subtracting Delta CBVa from Delta CBVt. We observed a fast Delta CBVa response with a time constant of 2.9 +/- 2.3 s and a slower Delta CBVv response with a time constant of 13.5 +/- 5.7 s and an onset delay of 6.1 +/- 3.3 s. These results are consistent with earlier studies under different anesthesia and stimulus, supporting that fast CBVa and slow CBVv responses are generalizable. Assuming the observed post-stimulus BOLD undershoot is at least partly explained by the Delta CBVv contribution, the relative contribution of the Delta CBVv- and oxygenation-change-related components to the BOLD response was estimated. The relative Delta CBVv contribution increases with time during stimulation; whereby it is <0.14 during initial 10 s and reaches a maximum possible value of similar to 0.45 relative to the oxygenation contribution during the 30-40 s period after stimulus onset. Our data indicates that the contribution of venous oxygenation change to BOLD fMRI is dominant for short stimulations. (C) 2012 Elsevier Inc. All rights reserved.