A comparison of FAIR and CASL perfusion imaging in mice

We present a quantitative comparison of continuous and pulsed spin labelling (CASL and FAIR respectively) in the mouse brain. The quantification of perfusion measurements is complicated by several physiological parameters such as the arterial transit time; also these parameters may change during abnormal conditions. The aim of this study is to compare the performance of the two sequences in mice, both for normal flow rates and during ischemia, when perfusion is low. Eight mice were anaesthetised and maintained on 1.3–1.7% isoflurane with 100% oxygen. The middle cerebral artery was occluded, and animals were scanned at 2 and 24 hours after occlusion. Coronal EPI images were obtained on a 2.35 T SMIS MRI scanner. FAIR: Short repetition time FAIR sequence ¹ , non-selective inversion or a selective FOCI inversion pulse, delay time 1300 ms, TR 1.5 s, and 22 averages. CASL: transit time insensitive CASL ² , interleaved adiabatic inversion labelling and control measurements, post-labelling delay time 500 ms, TR 1 s, and 22 averages. T1sat and T1 were also measured. Data processing: 4 ROIs were selected in each animal for further analysis. FAIR: the ssIR data were used to fit M0, T1app and a. These values were then used to fit the magnetisation difference for CBF ¹ . CASL: T1, T1sat and M0 were fitted using the IR data; subsequently these values were used to fit the magnetisation difference according to Alsop et al. ² for CBF. FAIR was assumed to be transit time insensitive. For CASL: arterial transit time cortex 440 ms, basal ganglia 290 ms (previous results; data not shown). Transit time correction (see results): CASL: cortex 800 ms, basal ganglia 600 ms, based on previous rat studies ³ . FAIR: cortex 500 ms, basal ganglia 500 ms. For normal perfusion rates (>100 ml/100g/min) there was an excellent correlation between the CASL and FAIR data (r = 0.96). The mean difference between the techniques was negligible, with a 95% confidence interval of −30 to +31 ml/100 g/min between the two methods for any given measurement. In the case of compromised CBF however, there was a marked discrepancy between the techniques, with FAIR showing little sensitivity to different flow rates, in agreement with earlier reports ¹ . This is most likely due to increased arterial transit times during occlusion, which would render the assumption that the technique is transit-time insensitive invalid. A transit time correction in the FAIR and CASL calculations for the ROIs with low CBF improved the correspondence between the techniques, though there was still a difference, indicating that either the FAIR transit time during occlusion is even larger than estimated, or there are other confounding factors influencing the measurement. Further studies using transit-time insensitive sequences such as multi-TI FAIR or QUIPSS II ⁴ are needed to investigate this phenomenon.