Time Efficient ASL Imaging with Segmented Multiband-acquisition (TEAISM)
Xiufeng Li1, Dingxin Wang1,2, Edward J. Auerbach1, Steen Moeller1, Gregory J. Metzger1, and Kamil Ugurbil1

1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, 2Siemens Medical Solutions USA Inc., Minneapolis, MN, United States

Synopsis

Recent studies have demonstrated the benefits of multi-band (MB) EPI for high-resolution whole brain PCASL imaging. However, the intrinsic nature of spatially interleaved simultaneous slice acquisition of MB-EPI requires a post-labeling delay larger than the longest arterial transit time in the brain, resulting in SNR efficiencies in the inferior or middle-inferior slices only comparable to or lower than those in SB-EPI PCASL imaging. To overcome the limitations of traditional MB-EPI and further improve SNR efficiencies of whole-brain high-resolution PCASL imaging, an alternative imaging acquisition strategy has been proposed and demonstrated: Time Efficient ASL Imaging with Segmented Multiband-acquisition (TEAISM).

PURPOSE

Multi-band (MB) echo planar imaging (EPI) 1,2 has been applied for pseudo-continuous arterial spin labeling (PCASL) imaging 3,4 on both 3T and 7T 5,6. Particularly, recent high-resolution whole brain MB-EPI PCASL imaging study indicates that 1) MB-EPI significantly increases spatial and temporal perfusion signal-to-noise ratio (SNR) efficiencies and 2) improves the accuracy of cerebral blood flow (CBF) quantification, and that 3) confounding factors induced by MB-EPI, such as leakage contaminations, have negligible effects on cerebral blood flow (CBF) quantification 7. However, in contrast to single-band (SB) EPI PCASL imaging, MB-EPI PCASL imaging requires a much longer post-labeling delay (PLD) to allow labeled blood to reach arterioles or capillary bed in order to avoid intravascular artifacts in brain regions with the longest arterial transit time because the slices covering the whole brain are acquired in a spatially interleaved fashion in MB-EPI acquisition. Unfortunately, the use of a long PLD causes perfusion SNR efficiencies in the inferior brain region comparable not better than those of standard SB-EPI PCASL imaging even when a MB factor as high as 6 is employed 7. Furthermore, using a high MB factor (e.g. 6) for the whole brain also results in perfusion SNR efficiencies in some middle-inferior slices lower than those in standard SB-EPI PCASL imaging due to elevated thermal noise or g-factor penalty resulting from slice-GRAPPA. To overcome these limitations and further improve SNR efficiencies for whole-brain high-resolution MB-EPI PCASL imaging, an alternative imaging acquisition strategy has been proposed and demonstrated: Time Efficient ASL Imaging with Segmented Multiband-acquisition (TEAISM).

METHODS

Studies with healthy volunteers were performed on a Siemens 3T MRI scanner using a 32-channel head coil under an IRB approved protocol with informed written consent. TEAISM divides imaging slices into two groups: one consisting of inferior slices and another consisting of the rest, and applies different MB factors, smaller than that needed for the imaging with a single MB-EPI acquisition, for each slice group (Figure 1). The high-resolution whole brain MRI parameters for both SB- and MB-EPI acquisitions were the same as those used in previous studies (e.g. 2.5 x 2.5 x 3.0 mm3 resolution and 36 slices) 7. In TEAISM, the 12 most inferior slices were acquired with a MB factor of 1 (or SB-EPI) using the same PLD (PLD 1 =1.1 s) as in standard SB-EPI PCASL acquisition, and the superior 24 slices were acquired with a MB factor 4 using the same PLD (PLD 2 = 1.6 s) as in traditional MB-EPI PCASL acquisition with a MB factor 6 (Figure 1). The same image processing and analysis methods as previously defined were applied 7.

RESULTS AND DISCUSSION

Study results suggest that TEAISM can provide extra benefits for whole brain high-resolution PCASL imaging, not only further increasing both spatial and temporal perfusion SNR efficiencies overall, but more importantly, providing superior SNR efficiencies for all slices (Figure 2) compared to SB-EPI PCASL imaging; this confirmed our theoretical simulation results (not shown). The CBF maps from one subject are presented in Figure 3.

In SB-EPI PCASL imaging, by taking advantage of the small arterial transit time (ATT) in the inferior brain region, a short PLD (e.g. 1.1 s) can be applied with ascending slice acquisition order while avoiding intravascular artifacts. In traditional MB-EPI PCASL imaging, the longest ATT in specific brain regions (e.g. visual cortex) must be considered, leading to a long PLD (e.g. 1.6 s), which reduces perfusion SNR efficiencies for the inferior or middle-inferior slices. In TEAISM, by combining SB-EPI acquisition for inferior slices with MB-EPI acquisition for the rest of the slices, the following extra benefits have been successfully realized: 1) reducing g-factor penalty over the whole brain; 2) maintaining or further decreasing (e.g. using MB-EPI with a factor 6 for the rest 24 slice) imaging repetition time; 3) avoiding or minimizing the level of total leakage contamination, which increases with increasing MB factor, across the brain and therefore further increasing perfusion signal stability and temporal SNR efficiency. The overall net effect of these benefits will be higher spatial and temporal perfusion SNR efficiencies across the whole brain compared to a single MB-EPI or SB-EPI acquisition. TEAISM may also be a good alternative for PCASL imaging with higher imaging resolution (e.g. 2 x 2 x 2 mm3) 8 where two different small MB factors can be applied for the inferior and other brain regions; this is under current investigation.

CONCLUSION

Compared to imaging acquisitions using traditional MB-EPI, TEAISM is a better alternative approach to further boost perfusion SNR efficiencies for whole brain high-resolution PCASL imaging.

Acknowledgements

P41 EB015894, P30 NS076408 and Human Connectome Project (1U54 MH091657), and UL1TR000114. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

References

1. Moeller S, Yacoub E, Olman CA, et al. Multiband multislice GE-EPI at 7 tesla, with 16-fold acceleration using partial parallel imaging with application to high spatial and temporal whole-brain fMRI. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 2010;63(5):1144-1153.

2. Setsompop K, Gagoski BA, Polimeni JR, Witzel T, Wedeen VJ, Wald LL. Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g-factor penalty. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 2012;67(5):1210-1224.

3. Dai W, Garcia D, de Bazelaire C, Alsop DC. Continuous flow-driven inversion for arterial spin labeling using pulsed radio frequency and gradient fields. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 2008;60(6):1488-1497.

4. Wu WC, Fernandez-Seara M, Detre JA, Wehrli FW, Wang J. A theoretical and experimental investigation of the tagging efficiency of pseudocontinuous arterial spin labeling. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 2007;58(5):1020-1027.

5. Li X, Wang D, Moeller S, Ugurbil K, Metzger GJ. Theoretical and Experimental Benefits of Multi-Band (MB) EPI for PCASL Brain Imaging. In: Proceedings of the 21st Annual Meeting of ISMRM, Milano, Italy, 2014:Abstract 4557.

6. Li X, Wang D, Moeller S, Ugurbil K, Metzger GJ. Feasibility of Applying MB-EPI pCASL for High-Resolution Whole Brain Perfusion Imaging at 7T. In: Proceedings of the 21st Annual Meeting of ISMRM, Milano, Italy, 2014:Abstract 0995.

7. Li X, Wang D, Auerbach EJ, Moeller S, Ugurbil K, Metzger GJ. Theoretical and experimental evaluation of multi-band EPI for high-resolution whole brain pCASL Imaging. NeuroImage. 2015;106:170-181.

8. Li X, Wang D, Ugurbil K, Metzger GJ. MB-EPI PCASL and T1 Imaging for Ultra High-Resolution Whole Brain Cerebral Blood Flow Quantification, OHBM 2015, Honolulu, Hawaii, USA, #1866.

Figures

Figure 1. Sequence diagrams for SB-EPI (SB), MB-EPI with a MB factor 6 (MB6) , and TEAISM using SB-EPI for inferior slices and MB-EPI with a MB factor 4 (MB4) for other slices (A), and the illustrations of imaging slice groups for acquisitions using MB6 and TEAISM (B). PLD represents post-labeling delay.

Figure 2. Normalized spatial and temporal SNR efficiencies of PCASL imaging studies from four healthy volunteers using single-band EPI (SB), multi-band EPI with a MB factor 6 (MB6) and TEAISM for data acquisitions. sSNR represents spatial SNR, and tSNR temporal SNR. Error bars represent one standard error of the mean.

Figure 3. One subject’s CBF maps from PCASL imaging studies using SB-EPI (SB), MB-EPI with a MB factor 6 (MB6) and TEAISM (SB for 12 inferior slices and MB4 for the rest 24 slices). Vertical yellow lines indicate slice groups.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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