A Theoretical and Experimental Investigation of Vascular-Space-Occupancy (VASO) Blood Nulling Times: Influence of Hematocrit and Oxygenation on Null Times and CBV Quantification

Manus Joseph Donahue¹, Stefan K. Piechnik¹, Rob Tijssen¹, Daniel Gallichan¹, Karla L. Miller¹, Peter Jezzard¹

Traditionally, macrovascular blood T1 is nulled in CBV-weighted VASO experiments, although CBV changes occur in microvasculature. VASO fMRI (n=8) is investigated for varying blood-nulling inversion times corresponding to a range of blood oxygenation and hematocrit. Microvascular null times produce similar CBV changes (DCBV=18.92.7%) to macrovascular blood null times (DCBV=23.23.1%) at long TR=5s. In agreement with previous reports, we show that the choice of TI predictably influences the observed sensitivity of VASO to functional stimulation but that this influence is small at long TR over a blood T1 range corresponding to typical variation in average blood oxygen saturation and hematocrit.