In this study we investigated whether velocity-selective arterial spin labeling (VS-ASL) and acceleration-selective spin labeling (Acc-ASL) can be used to measure pulsatility in the microvascular system of the brain, since the amount of generated label will depend on the velocity or acceleration of the spins at the moment the VS-ASL (Acc-ASL) module is applied. Results showed no significant variation in the amount of label when applying these modules at different cardiac phases. However, the generated label did show a pattern which coincides with the flow territories, suggesting a underlying physiological cause.
Measurement of the influence of the cardiac cycle on ASL is always challenging, since the cardiac cycle can affect the labeling process, as well as the transport of the label during the post-labeling-delay (PLD). To isolate the effect on the labeling, pCASL with a long labeling-duration of approximately 7 seconds was used to fill up the vasculature with label, followed by crushing with a VS- or Acc-ASL saturation module applied at different cardiac phases (see Figure 1). The scans combining pCASL with a VS-ASL or Acc-ASL module had a total scan time of 35mins, with ten averages for each label/control combination, a scan resolution of 3x3x7mm, seventeen slices, TR/TE of 10000ms/14ms and a 2D-EPI single-shot readout. For the VS-ASL module a gradient strength of 13mT/m was used and for Acc-ASL 30mT/m, both used a gradient duration of 1ms and total duration of 37ms, thereby encoding a cut-off velocity of 2cm/s and cut-off acceleration of 1.3m/s2. The end of pCASL-labeling and therefore the start of the VS- and Acc-ASL module was triggered using a PPU-device on the left index finger. Five trigger-delays at equal interval were calculated based on a phase-contrast scan of the labeling location (Figure 3). Furthermore, a flow-territory scan3 and 3DT1 was acquired.
Eight volunteers (mean age 26 years) were scanned with VS-ASL and nine (32 years) with Acc-ASL, who all gave written informed consent prior to inclusion. All volunteers were scanned on a 3T Achieva (Philips) using a 32-channel head coil. The acquired data allowed reconstruction of a short PLD VS- or Acc-ASL image, a pCASL image, a crushed pCASL image (pCASL followed by VS (or Acc) crushing) and an image, named VSlabel (Acclabel), that aims to isolate the effect of VS- or Acc-ASL crushing on the pCASL label (Figure 2). To test whether VS (Acc) encoding at a certain cardiac phase is more stable than untriggered encoding, signal variation between different cardiac phases was calculated from the median of the gray matter signal (averaged per flow-territory) over all dynamics, and standardized with respect to the median signal over all cardiac phases. Finally the standardized deviations were averaged over all volunteers. In addition, a one-way ANOVA was performed for each flow-territory to test for significant differences in VSlabel (Acclabel) between the cardiac phases. All analyses were performed in MATLAB and the SPM12 toolbox.
1. Wong, E. C., Cronin, M., Wu, W.C., Inglis, B., Frank, L.R., Liu, T.T. Velocity-selective arterial spin labeling. Magn. Reson. Med. 55, 1334–41 (2006).
2. Schmid, S., Ghariq, E., Teeuwisse, W. M., Webb, A. & van Osch, M. J. P. Acceleration-selective arterial spin labeling. Magn. Reson. Med. 71, 191–199 (2014).
3. Wong, E. C. Vessel-encoded Arterial Spin-Labeling Using Pseudocontinuous Tagging. Magn. Reson. Med. 1091, 1086–1091 (2007).