Pseudo continuous arterial spin labeling quantification in anemic subjects with hyperemic cerebral blood flow

Purpose: To investigate possible sources of quantification errors in global cerebral blood flow (CBF) measurements by comparing pseudo continuous arterial spin labeling (PCASL) and phase contrast (PC) MRI in anemic, hyperemic subjects.

Methods: All studies were performed on a Philips 3T Achieva MRI scanner. PC and PCASL CBF examinations were performed in 10 healthy, young adult subjects and 18 young adults with chronic anemia syndromes including sickle cell disease and thalassemia. CBF estimates from single and two compartment ASL kinetic models were compared. Numerical simulation and flow phantom experiments were used to explore the effects of blood velocity and B1⁺ on CBF quantification and labeling efficiency.

Results: PCASL CBF underestimated PC in both populations using a single compartment model (30.1±9.2% control, 45.2±17.2% anemia). Agreement substantially improved using a two-compartment model (-8.0±6.0% control, 11.7±12.3% anemia). Four of the anemic subjects exhibited venous outflow of ASL signal, suggestive of cerebrovascular shunt, possibly confounding PC-PCASL comparisons. Additionally, sub-study experiments demonstrated that B1⁺ was diminished at the labeling plane (82.9±5.1%), resulting in suboptimal labeling efficiency. Correcting labeling efficiency for diminished B1⁺, PCASL slightly overestimated PC CBF in controls (-15.4±6.8%) and resulted in better matching of CBF estimates in anemic subjects (0.7±10.0% without outflow, 10.5±9.4% with outflow).

Conclusions: This work demonstrates that a two-compartment model is critical for PCASL quantification in hyperemic subjects. Venous outflow and B1⁺ under-excitation may also contribute to flow underestimation, but further study of these effects is required.