Spatial dependency and the role of local susceptibility for velocity selective arterial spin labeling (VS‐ASL) relative tagging efficiency using accelerated 3D radial sampling with a BIR‐8 preparation

Purpose Velocity selective arterial spin labeling (VS‐ASL) is a promising approach for non‐contrast perfusion imaging that provides robustness to vascular geometry and transit times; however, VS‐ASL assumes spatially uniform tagging efficiency. This work presents a mapping approach to investigate VS‐ASL relative tagging efficiency including the impact of local susceptibility effects on a BIR‐8 preparation. Methods Numerical simulations of tagging efficiency were performed to evaluate sensitivity to regionally varying local susceptibility gradients and blood velocity. Tagging efficiency mapping was performed in susceptibility phantoms and healthy human subjects (N = 7) using a VS‐ASL preparation module followed by a short, high spatial resolution 3D radial‐based image acquisition. Tagging efficiency maps were compared to 4D‐flow, B1, and B0 maps acquired in the same imaging session for six of the seven subjects. Results Numerical simulations were found to predict reduced tagging efficiency with the combination of high blood velocity and local gradient fields. Phantom experiments corroborated numerical results. Relative efficiency mapping in normal volunteers showed unique efficiency patterns depending on individual subject anatomy and physiology. Uniform tagging efficiency was generally observed in vivo, but reduced efficiency was noted in regions of high blood velocity and local susceptibility gradients. Conclusion We demonstrate an approach to map the relative tagging efficiency and show application of this methodology to a novel BIR‐8 preparation recently proposed in the literature. We present results showing rapid flow in the presence of local susceptibility gradients can lead to complicated signal modulations in both tag and control images and reduced tagging efficiency.