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

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Abstract
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 (δtot < 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 (δtot = 100–300 mm), yet did not differ significantly (P > 0.05) for δtot > 300 mm. These data suggest that blood water is in steady state for δtot > 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. Magn Reson Med 61:473–480, 2009.