The CBV weighting in VASO is based on T1-selective nulling of the blood signal, while keeping the extravascular signal with its different T1 preserved for image creation (Hua et al. 2013; Lu et al. 2003). Thus, VASO is inherently a single-slice method, as multi-slice acquisitions would suffer from inconsistent or insufficient blood nulling across slices. With 3D-readout methodologies, the readout window can be substantially increased up to a time window of 0.5 – 1 seconds (≪ T1) without compromising blood nulling across the entire imaging volume (Hua et al. 2013; Poser and Norris 2009; 2011). Extending the readout acquisition window even further by another 1 – 4 seconds (≈ T1) is technically possible, however, it comes along with some challenges. Namely, the longer TRs mean that the timing of the inversion-recovery relaxation is faster than the k-space acquisition. Compared to previous VASO sequences, these limitations have been recently mitigated with a series of new approaches:
