Modeling and optimization of look-locker spin labeling for measuring perfusion and transit time changes in activation studies taking into account arterial blood volume
This work describes a new compartmental model with step-wise temporal analysis for a Look-Locker (LL)-flow-sensitive alternating inversion-recovery (FAIR) sequence, which combines the FAIR arterial spin labeling (ASL) scheme with a LL echo planar imaging (EPI) measurement, using a multireadout EPI sequence for simultaneous perfusion and T *2 measurements. The new model highlights the importance of accounting for the transit time of blood through the arteriolar compartment, δ, in the quantification of perfusion. The signal expected is calculated in a step-wise manner to avoid discontinuities between different compartments. The optimal LL-FAIR pulse sequence timings for the measurement of perfusion with high signal-to-noise ratio (SNR), and high temporal resolution at 1.5, 3, and 7T are presented. LL-FAIR is shown to provide better SNR per unit time compared to standard FAIR. The sequence has been used experimentally for simultaneous monitoring of perfusion, transit time, and T *2 changes in response to a visual stimulus in four subjects. It was found that perfusion increased by 83 ± 4% on brain activation from a resting state value of 94 ± 13 ml/100 g/min, while T *2 increased by 3.5 ± 0.5%. Magn Reson Med 2007. © 2007 Wiley-Liss, Inc.