Effect of inflow of fresh blood on vascular-space-occupancy (VASO) contrast

In vascular-space-occupancy (VASO)-MRI, cerebral blood volume (CBV)-weighted contrast is generated by applying a non-selective 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 (δ_tot < 10 mm), VASO signal changes were positive (ΔS/S = 2.1-2.6%). Signal changes were negative and varied in magnitude for intermediate inversion volumes (δ_tot = 100-300 mm), yet did not differ significantly (P > 0.05) for δ_tot > 300 mm. These data suggest that blood water is in steady state for δ_tot > 300 mm. In this appropriate range, long-TR VASO data converged to a less negative value (ΔS/S = -1.4% ± 0.2%) than short-TR data (ΔS/S = -2.2% ± 0.2%), implying that cerebral blood flow or transit-state effects may influence VASO contrast at short TR.