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). 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.

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.

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.