In vascular-space-occupancy (VASO)-MRI, cerebral blood volume (CBV)-weighted contrast is generated by applying a nonselective inversion pulse followed by imaging when blood water magnetization is zero. An uncertainty in VASO relates to the completeness of blood water nulling. Specifically, radio frequency (RF) coils produce a finite inversion volume, rendering the possibility of fresh, non-nulled blood. Here, VASO-functional MRI (fMRI) was performed for varying inversion volume and TR using body coil RF transmission. For thin inversion volume thickness (delta(tot) < 10 mm), VASO signal changes were positive (DeltaS/S = 2.1-2.6%). Signal changes were negative and varied in magnitude for intermediate inversion volumes (delta(tot) = 100-300 mm), yet did not differ significantly (P > 0.05) for delta(tot) > 300 mm. These data suggest that blood water is in steady state for delta(tot) > 300 mm. In this appropriate range, long-TR VASO data converged to a less negative value (DeltaS/S = -1.4% +/- 0.2%) than short-TR data (DeltaS/S = -2.2% +/- 0.2%), implying that cerebral blood flow or transit-state effects may influence VASO contrast at short TR.
Copyright 2009 Wiley-Liss, Inc.