While VASO was originally proposed as a blood-nulling method (Lu et al. 2003), it has in the last 15 years been generalized to a general T1-contrast without specific blood-nulling requirements. Early VASO versions without blood nulling used a T1-selective GM-nulling procedure to estimate an inverse VASO contrast (Shen et al. 2009; Wu et al. 2008). Later on, the VASO formalism was further generalized to extract CBV changes at any inversion time (Ciris et al. 2014; W.C. et al. 2007). This literature has shown that the experimental trick of blood-nulling is not the only way of obtaining a CBV-weighting. In fact, as long as there is a different T1-weighting between the extravascular signal and intravascular signal, any volume redistribution between these pools of z-magnetization, will result in a VASO signal change. Thus, instead of using an inversion pulse, T1-weighting can also be introduced by variable flip angles that create a dynamic steady-state across k-space segments along the 3D-EPI trajectory. This approach has the advantage that the T1-weighting can be maintained in a dynamic equilibrium for as long as needed.
