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Evaluation of 3D Stack-of-Spiral Turbo FLASH Acquisitions for PCASL- and VSASL-Derived Brain Perfusion Mapping
Dan Zhu1,2, Feng Xu1,2, Dapeng Liu1,2, Doris Lin2, Peter van Zijl1,2, and Qin Qin1,2
1F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States, 2The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins School of Medicine, Baltimore, MD, United States

Synopsis

Keywords: Arterial spin labelling, Arterial spin labelling

The most-used 3D acquisition for ASL at 3T is GRASE or stack-of-spiral (SOS) based FSE, which requires multiple shots to cover the full k-space. Alternatively, turbo FLASH (TFL) acquisition allows longer echo trains with slower T1 (than T2) relaxation, and 3D SOS-TFL has the potential to reduce the number of shots to even single-shot, thus improving the temporal resolution for ASL. Here we demonstrated comparable performance of 3D SOS-TFL with 3D GRASE on PCASL- and VSASL-derived CBF mapping and VSASL-derived CBV mapping at 3T on 12 healthy subjects, and the utility of 3D SOS-TFL on a stroke patient.

Introduction

The most-used 3D acquisition for ASL-derived brain perfusion mapping at 3T is EPI-based GRASE or stack-of-spiral (SOS)-based FSE1–3. Because fast T2 decay during the long echo train leads to through-slice blurring, conventional GRASE or SOS-FSE requires multiple shots to cover the whole brain. An alternative method, turbo FLASH (TFL) acquisition, allows longer echo trains (due to the slower T1 relaxation4) and 3D SOS-TFL has the potential to reduce the number of shots to as low as single-shot, thus improving the temporal resolution for ASL5,6. In this work, we compared the performance of 3D SOS-TFL with 3D GRASE on PCASL7- and VSASL8,9-derived cerebral blood flow (CBF) mapping as well as VSASL-derived cerebral blood volume (CBV) mapping10,11 at 3T, and tested the utility of 3D SOS-TFL on a stroke patient.

Methods

Experiments were conducted on 3T Philips using a 32-channel head-only receive coil. 12 healthy subjects (age 41.6±13.6, 6F) and one stroke patient three-months post-onset were enrolled with written informed consent.

The labeling preparations for PCASL- and VSASL-derived CBF mapping and VSASL-derived CBV mapping used the following parameters. PCASL-CBF7: labeling duration=1800ms, PLD=2000ms, 4 background suppression (BS) pulses. VSASL-CBF2,9: global pre-saturation with a 2s delay, Fourier-transform based (FT-) VS inversion Vcut=2cm/s (duration=64ms), bolus duration=1400ms, 3 BS pulses. For CBF mapping, both PCASL and VSASL applied vascular crushing modules with Vcut=2cm/s. VSASL-CBV11: global pre-saturation with a 2s delay, FT-VS saturation Vcut=0.5cm/s (duration=96ms) right before acquisition. All VS gradients were in FH direction.

The axial acquisition of 3D SOS-TFL and 3D GRASE both had FOV=220×220×120mm3 and resolution=3.4×3.4×5mm3 (AP×RL×FH). SOS-TFL: turbo factor=24 in the slice direction, centric ordering, TR/TE=18/2.6ms, readout time=11ms, echo-train duration=432ms, 3-shot per image, label/control pairs=8, shot interval=4.3/3.9/2.6s and scan time=3.4/3.1/2.2min (PCASL-CBF/VSASL-CBF/VSASL-CBV). No acceleration was applied for SOS-TFL. SPIR and selective water excitation (WE) fat suppression methods were compared on 3 subjects. FA=[5°,15°, 25°] were compared on one subject and FA=[9°,12°,15°,18°] were compared on 3 subjects. WE and FA=15° were by default when not specified. GRASE: turbo factor=12, echo spacing=13ms, EPI factor=15, SENSE factor=2, readout time=9ms, echo-train duration=156ms, 4-shot per image, 6 label/control pairs, TR=4.1/3.6/2.3s and scan time=3.7/3.3/2.1min (PCASL-CBF/VSASL-CBF/VSASL-CBV).

For each acquisition, a proton-density weighted image (SIPD, TR=10s), a double inversion recovery (DIR, visualize gray matter, TR/TI1/TI2 = 10/3.58/0.48s), and an image acquired immediately after global saturation (TFL only, for T1-induced bias correction4,12) were acquired.

The images using the fully-sampled 3D SOS-TFL (including deblurring) and the 3D GRASE with a SNESE factor of 2 were reconstructed automatically by the vendor. CBF and CBV perfusion maps were quantified from the perfusion-weighted signal (PWS=(control-label)/SIPD) with formula in publications1,8,11. We also evaluated the temporal signal-to-noise-ratio (tSNR) efficiency as tSNR divided by the square root of the time per label/control pair13,14. Gray matter (GM) ROIs were generated from DIR images with threshold=(max intensity+min intensity)/2. Whole-brain GM perfusion and tSNR efficiency were averaged within the GM ROIs.

Results

Fig.1 displays the control and PWS images of SOS-TFL scans with SPIR and WE fat suppression of two subjects. Fat-induced artifacts were visible in SPIR-based scans, but not in WE scans, because fast-recovering fat signals are suppressed in each water excitation repeatedly but only before the first excitation in SPIR. Artifacts were less visible on the PWS of VSASL-CBV, possibly due to higher CBV-weighted PWS than CBF-weighted PWS.

Fig.2 arrays the DIR, PWS and tSNR efficiency of SOS-TFL with different FAs. Larger FAs resulted in higher tSNR efficiency but more image blurring.

Fig.3a demonstrates that SOS-TFL-based CBF and CBV maps and tSNR efficiency images are comparable to GRASE. Note that SOS-TFL-based VSASL-CBF reduces CSF artifacts compared to GRASE (purple arrow). A possible reason is that the T1-weighted TFL has lower CSF signals. The correlations of the whole-brain GM perfusion between SOS-TFL and GRASE for 12 subjects were plotted in Fig.3b, all showing high correlation coefficients (r≥0.95).

Table 1 summarizes the average and SD of perfusion and tSNR efficiency from whole-brain GM ROIs of 12 subjects. Compared to GRASE, SOS-TFL showed no significant difference in GM perfusions for PCASL-CBF (P=0.48), was 7.1% lower for VSASL-CBF (P<0.05) and 15.7% higher for VSASL-CBV (P<0.05). A possible explanation for lower VSASL-CBF is that the T1-weighting reduced CSF partial volume effects. For tSNR efficiency, SOS-TFL was 29.2% lower for PCASL-CBF (P<0.05), comparable for VSASL-CBF (P=0.75), and 14.2% higher for VSASL-CBV (P<0.05).

Perfusion images of a patient 3-months following a hemorrhagic stroke involving the right dorsal insula and deep GM are shown in Fig.4. Decreased CBF, increased CBV, and prolonged mean transient time (MTT=CBV/CBF) were observed in the right parietal lobe region distal to the site of hemorrhage (purple circles).

Discussion and Conclusion

We evaluated different SOS-TFL acquisitions for PCASL and VSASL sequences for brain perfusion mapping. Water excitation is necessary to eliminate fat-induced artifacts. The choice of flip angle needs to consider balancing image blurring and tSNR efficiency. SOS-TFL reduced CSF artifacts for VSASL-CBF and increased tSNR for VSASL-CBV. For PCASL- and VSASL-based CBF and CBV mapping, compared to the employed 4-shot 3D GRASE with an acceleration factor of 2, the fully-sampled 3D SOS-TFL delivered comparable performance (with a similar scan time) using 3 shots, which can potentially be undersampled with SENSE or compressed sensing to further achieve single-shot acquisition.

Acknowledgements

No acknowledgement found.

References

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Figures

Figures 1: Control and perfusion-weighted signal (PWS) images of stack-of-spiral turbo FLASH (SOS-TFL) scans with SPIR and water-excitation (WE) fat suppression on two subjects. SPIR scans show severe fat-related artifacts on all control images, PWS images of both subjects for PCASL-CBF, PWS image of subject 1 for VSASL-CBF, and small artifacts on PWS images of subject 2 for VSASL-CBF. Artifacts were not visible on PWS images of both subjects for VSASL-CBV, possibly due to the higher CBV-weighted PWS than CBF-weighted PWS. No artifacts were observed on any WE scans.

Figure 2: 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) with different flip angles (FAs) on two subjects. 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 (most visible with 25°).

Figure 3: a) double inversion recovery (DIR), perfusion and temporal SNR (tSNR) efficiency maps of a representative subject, 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 on DIR images (red contour). SOS-TFL based VSASL-CBF maps have reduced CSF artifacts compared to GRASE (purple arrow). b) Scatter plots of whole-brain GM perfusion SOS-TFL and GRASE for 12 subjects, showing high correlation coefficients (r).

Table 1: Averaged results of whole-brain gray matter (GM) perfusion and temporal SNR (tSNR) efficiency 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.

Figure 4: double inversion recovery (DIR) and perfusion images of a patient 3-months following a hemorrhagic stroke involving the right dorsal insula and deep gray matter are shown in Fig.4. Decreased CBF, increased CBV, and prolonged mean transient time (MTT=CBV/CBF) were observed in the right parietal lobe region distal to the site of hemorrhage (purple circles).

Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)
2584
DOI: https://doi.org/10.58530/2023/2584