Inflow-based vascular-space-occupancy (iVASO) MRI.

Vascular-space-occupancy (VASO) MRI, a blood nulling approach for assessing changes in cerebral blood volume (CBV), is hampered by low signal-to-noise ratio (SNR) because only 10-20% of tissue signal is recovered when using nonselective inversion for blood nulling. A new approach, called inflow-VASO (iVASO), is introduced in which only blood flowing into the slice has experienced inversion, thereby keeping tissue and cerebrospinal fluid (CSF) signal in the slice maximal and reducing CSF partial volume effects. SNR increases of 198% ± 12% and 334% ± 9% (mean ± SD, n = 7) with respect to VASO were found at TR values of 5 s and 2 s, respectively. When using inflow approaches, data interpretation is complicated by the fact that signal changes are affected by vascular transit times. An optimal TR-range (1.5-2.5 s) was derived in which the iVASO response during activation predominantly reflects arterial/arteriolar CBV (CBV(a)) changes. In this TR-range, perfusion contributions to the signal change are negligible because arterial label has not yet undergone capillary exchange, and arterial and precapillary blood signals are nulled. For TR = 2 s, the iVASO signal change upon visual stimulation corresponded to a CBV(a) increase of 58% ± 7%, in agreement with arteriolar CBV changes previously reported. The onset of the hemodynamic response for iVASO occurred 1.2 ± 0.5 s (n = 7) faster than for conventional VASO.