Evaluation of 3D stack-of-spiral turbo FLASH acquisitions for pseudo-continuous and velocity-selective ASL-derived brain perfusion mapping

Abstract

Purpose: The most-used 3D acquisitions for ASL are gradient and spin echo (GRASE)- and stack-of-spiral (SOS)-based fast spin echo, which require multiple shots. Alternatively, turbo FLASH (TFL) allows longer echo trains, and SOS-TFL has the potential to reduce the number of shots to even single-shot, thus improving the temporal resolution. Here we compare the performance of 3D SOS-TFL and 3D GRASE for ASL at 3T.

Methods: The 3D SOS-TFL readout was optimized with respect to fat suppression and excitation flip angles for pseudo-continuous ASL- and velocity-selective (VS)ASL-derived cerebral blood flow (CBF) mapping as well as for VSASL-derived cerebral blood volume (CBV) mapping. Results were compared with 3D GRASE readout on healthy volunteers in terms of perfusion quantification and temporal SNR (tSNR) efficiency. CBF and CBV mapping derived from 3D SOS-TFL-based ASL was demonstrated on one stroke patient, and the potential for single-shot acquisitions was exemplified.

Results: SOS-TFL with a 15° flip angle resulted in adequate tSNR efficiency with negligible image blurring. Selective water excitation was necessary to eliminate fat-induced artifacts. For pseudo-continuous ASL- and VSASL-based CBF and CBV mapping, compared to the employed four-shot 3D GRASE with an acceleration factor of 2, the fully sampled 3D SOS-TFL delivered comparable performance (with a similar scan time) using three shots, which could be further undersampled to achieve single-shot acquisition with higher tSNR efficiency. SOS-TFL had reduced CSF contamination for VSASL-CBF.

Conclusion: 3D SOS-TFL acquisition was found to be a viable substitute for 3D GRASE for ASL with sufficient tSNR efficiency, minimal relaxation-induced blurring, reduced CSF contamination, and the potential of single-shot, especially for VSASL.

Keywords: cerebral blood flow; cerebral blood volume; pseudo-continuous ASL; stack-of-spiral turbo FLASH; velocity-selective arterial spin labeling.

Figure 1:

A sagittal slice of the double inversion recovery (DIR), perfusion-weighted signal (PWS) and temporal SNR (tSNR) efficiency images of 3D stack-of-spiral turbo FLASH (SOS-TFL) of one representative subject with FAs of 9°, 12°, 15° and 18° (subject 5). Gray matter (GM) ROI border is drawn on a DIR image (red line). Different FAs generated similar PWS for PCASL-CBF and VSASL-CBF, -CBV methods, with larger FAs resulting in higher tSNR efficiency but more image blurring.

Figure 2:

Scatter plot of PWS (top row), tSNR efficiency (center row), and the corresponding total variation (TV) representing image sharpness of the perfusion maps (bottom row) derived from SOS-TFL based PCASL-CBF (left), VSASL-CBF (middle), and VSASL-CBV (right) as a function of flip angle (FA) of Subjects 4 (FAs=5°/15°/25°) and 5 to 7 (FAs=9°/12°/15°/18°), with each subject as individual solid lines, showing constant PWS between FAs while increased tSNR efficiency and decreased TV (more blurring) for larger FA.

Figure 3:

Double inversion recovery (DIR), perfusion and temporal SNR (tSNR) efficiency maps of Subject 8, showing close associations between 3D GRASE and stack-of-spiral turbo FLASH (SOS-TFL) (water excitation, flip angle = 15°) for PCASL-CBF, VSASL-CBF, and VSASL-CBV methods, with gray matter (GM) ROI borders drawn on DIR images (red lines). Numbers in each subplot indicate perfusion/tSNR efficiency values averaged within the GM ROI. Small dark holes were observed on superior slices of GRASE-based PCASL- and VSASL-CBF (white arrows) but not visible on SOS-TFL-based images. SOS-TFL-based VSASL-CBF maps have reduced CSF artifacts compared to GRASE (red circle).

Figure 4:

Correlation of whole-brain gray matter (GM) perfusion (a–c), box plot of whole-brain GM perfusion (d–f), and temporal SNR (tSNR) efficiency (g–i) of 12 subjects using 3D GRASE and stack-of-spiral turbo FLASH (SOS-TFL) (water excitation, flip angle = 15°) readouts, derived from PCASL-CBF, VSASL-CBF, and VSASL-CBV methods. Their correlation coefficients (r) are listed for each plot (a–c). */**/**** indicates significant difference in t-test with p <= 0.05/0.01/0.0001, respectively.

Figure 5:

Double inversion recovery (DIR) and perfusion images of a patient three months following a hemorrhagic stroke involving the right dorsal insula and deep gray matter. Decreased CBF was observed in the right parietal lobe region distal to the site of hemorrhage (purple circles).

Figure 6:

Off-line image reconstruction of the representative raw datasets with 3 shots (all 3 spiral arms, fully sampled) and with a single shot (one spiral arm, acceleration factor R = 3) using SENSE and CS-SENSE image reconstruction for double inversion recovery (DIR) image, as well as perfusion and tSNR efficiency maps. The gray matter (GM) ROI border is drawn on the 3-shot DIR image (red lines) and averaged perfusion or tSNR efficiency values within the GM ROIs are labeled on the top left corner of each quantitative map. Pearson correlation coefficient (PCC) of the single-shot images with the 3-shot as the reference are labeled at the bottom right corner of each image.