Measuring Biexponential Transverse Relaxation of the ASL Signal at 9.4 T to Estimate Arterial Oxygen Saturation and the Time of Exchange of Labeled Blood Water into Cortical Brain Tissue

Abstract

The transverse decay of the arterial spin labeling (ASL) signal was measured at four inflow times in the rat brain cortex at 9.4 T. Biexponential T2 decay was observed that appears to derive from different T2 values associated with labeled water in the intravasculature (IV) and extravascular (EV) compartments. A two compartment biexponential model was used to assess the relative contribution of the IV and EV compartments to the ASL signal, without assuming a value for T2 of labeled blood water in the vessels. This novel methodology was applied to estimate the exchange time of blood water into EV tissue space and the oxygen saturation of blood on the arterial side of the vasculature. The mean exchange time of labeled blood water was estimated to be 370 ± 40 ms. The oxygen saturation of the arterial side of the vasculature was significantly less than 100% (B85%), which may have implications for quantitative functional magnetic resonance imaging studies where the arterial oxygen saturation is frequently assumed to be 100%.

Transit time mapping in the mouse brain using time‐encoded pCASL

“…However, this approach further lowers SNR, as part of the perfusion signal is also attenuated because of the diffusion weighting of the crushers. Another method exploits the measurement of T 2 of the ASL signal, as T 2 of the label depends on whether it is intra‐ or extravascular . This approach has the potential to distinguish between labeled spins in vascular and extravascular spaces, but has to deal with the difficulties inherent in separating the components in biexponential transverse relaxation signals.…”

Non‐contrast MR imaging of blood‐brain barrier permeability to water

“…However, this method relies on the T 1 relaxation difference before and after the exchange of water molecules, therefore the intrinsic SNR of this method is limited . Other studies aimed to separate blood and tissue ASL spins based on their T 2 relaxation time . However, the T 2 difference can also be because of oxygenation changes as the spin flows from artery to capillary, to veins.…”

Impact of tissue T1on perfusion measurement with arterial spin labeling

“…The two‐compartment model led to a different distribution of the errors between the striatum and the cortex but did not improve the accuracy (ipsilateral/contralateral CBF difference) of the CBF map overall. This two‐compartment approach could be refined by mapping the arterial transit time [e.g., using a dynamic ASL approach such as DASL or Hadamard ASL ] and the permeability to water [e.g., using an additional diffusion or T 2 encoding or using a contrast agent ]. Mapping these parameters might reveal regional differences that could explain the difference in CBF quantification observed between cortex and striatum in this study.…”