Determination of R2* Across Multiple Postlabeling Delays in ASL & Comparison with Flow, Arterial Volume & Transit Times in Physiological Challenges

Yi-Ching Lynn Ho^1,2, Esben Thade Petersen³, Xavier Golay⁴

In Look-Locker-based ASL sequences, the transverse relaxation time R2* is assumed to be constant across the multiple postlabeling delay times, permitting accurate cerebral blood flow (CBF) quantification. Using dual echoes, R2* was found to be consistent across the multiple time points, validating the assumption. Secondly, the R2* results allowed a regional determination of oxygenation levels for arterial input function sampling. Finally, it is seen that R2* changes are tightly coupled with CBF changes, rather than with arterial blood volume or transit time changes and this during neuronal activity alone. The presence of hypercapnia increases the complexity of the relationships.