Arterial spin labeling-fast imaging with steady-state free precession (ASL-FISP): a rapid and quantitative perfusion technique for high-field MRI

Abstract

Arterial spin labeling (ASL) is a valuable non-contrast perfusion MRI technique with numerous clinical applications. Many previous ASL MRI studies have utilized either echo-planar imaging (EPI) or true fast imaging with steady-state free precession (true FISP) readouts, which are prone to off-resonance artifacts on high-field MRI scanners. We have developed a rapid ASL-FISP MRI acquisition for high-field preclinical MRI scanners providing perfusion-weighted images with little or no artifacts in less than 2 s. In this initial implementation, a flow-sensitive alternating inversion recovery (FAIR) ASL preparation was combined with a rapid, centrically encoded FISP readout. Validation studies on healthy C57/BL6 mice provided consistent estimation of in vivo mouse brain perfusion at 7 and 9.4 T (249 ± 38 and 241 ± 17 mL/min/100 g, respectively). The utility of this method was further demonstrated in the detection of significant perfusion deficits in a C57/BL6 mouse model of ischemic stroke. Reasonable kidney perfusion estimates were also obtained for a healthy C57/BL6 mouse exhibiting differential perfusion in the renal cortex and medulla. Overall, the ASL-FISP technique provides a rapid and quantitative in vivo assessment of tissue perfusion for high-field MRI scanners with minimal image artifacts.

Keywords: arterial spin labeling; fast imaging with steady-state free precession (FISP); high-field MRI; perfusion.

Copyright © 2014 John Wiley & Sons, Ltd.

Figures

Figure 1

Schematic of ASL-FISP acquisition. A FAIR ASL preparation is combined with a centrically-encoded FISP acquisition. This acquisition is repeated for both a slice-selective inversion (with shaded gradient) and a non-selective inversion (without the shaded gradient) to generate the perfusion contrast. Note that all lines of k-space are acquired following a single ASL preparation.

Figure 2

Representative axial mouse brain images at 7 T using (a) spin echo, (b) FISP, (c) True FISP, and (d) EPI acquisitions. Note the similar lack of artifacts in the spin echo and FISP images in comparison to the True FISP (banding) and EPI (ghosting / distortion) images.

Figure 3

Representative ASL-FISP images of a healthy C57/BL6 mouse brain at 7 T. (a) Slice-selective (bright-blood); (b) non-selective (dark blood) ASL-FISP images (40 averages); (c) M0 image (no inversion); (d) Brain T1 map from Look-Locker acquisition; perfusion map from (e) ASL-FISP and (f) ASL-GRE in ml/min/100g of tissue. Note that the ASL-FISP and ASL-GRE perfusion maps are from different mice.

Figure 4

(a) Mean C57/BL6 mouse brain perfusion from the ASL-FISP method at 7 T (gray) and 9.4 T (white), respectively. Results are plotted as a function of the number of ASL-FISP averages. No significant differences in mean perfusion were observed for different number of averages (p > 0.6) or field strength (p > 0.2). (b) Mean brain perfusion from a single C57/BL6 mouse at 7 T as a function of the slice thickness ratio (inversion slab thickness / imaging slice thickness) and the number of ASL-FISP averages. Note the large decrease in mean brain perfusion as the slice thickness ratio is increased from 1 to 3. The mean perfusion also appears to be more sensitive to the inversion slab thickness than the number of ASL-FISP averages.

Figure 5

ASL-FISP images of an MCAo mouse model of stroke at 7 T. (a) Slice-selective (bright-blood) and (b) non-selective (dark blood) ASL-FISP images; (c) M0 FISP image (no inversion); (d) diffusion-weighted image (b=500 s/mm2) showing right brain infarct; (e) Look-Locker T1 map; and (f) perfusion map. The primary infarct is visible in the right brain in all images. A potential contralateral perfusion deficit is also observed in the perfusion map, but less evident in T1 and diffusion weighted images.

Figure 6

Kidney ASL-FISP images from a healthy C57/BL6 mouse at 7 T. (a) Slice-selective (bright-blood) and (b) non-selective (dark blood) ASL-FISP images; (c) M0 FISP image (no inversion); (d) Look-Locker T1 map; and (e) perfusion map. Renal arteries (high perfusion) and renal medulla (low perfusion) are clearly visible in the perfusion map.