7T 3D-EPI PCASL With High SNR Efficiency and Robustness to Through-Plane B₀ Field Gradients

7T pseudo-continuous arterial spin labeling (PCASL) can benefit from combining 3D-EPI with high-performance background suppression (BS) pulses. However, the perfusion signal can be compromised by increased B₁ and $∆$ B₀ inhomogeneity, and high SAR demands limiting the labeling duration (LD), labeling efficiency and BS inversion efficiency.

7T PCASL SNR efficiency (SNReff) was quantified using a range of LD (500-4000 ms) with optimized BS inversion pulses and 3D-EPI acquisitions. PCASL sensitivity to $∆$ B₀ was also investigated by measuring through-plane field variations along the feeding arteries and acquiring perfusion data across a range of PCASL mean gradients (G_mean, -0.8 to +0.8 mT/m) with in-plane $∆$ B₀ corrections.

Optimized WURST BS pulses achieved ∼0.93 inversion efficiency. PCASL SNReff was maximized when employing a LD of ∼3 s (B_1pcasl = 1 $\mu$ T, duty cycle = 60%), providing ∼25% higher SNReff than 1 s LD typically used at 7T. Arteries with strong negative though-plane $∆$ B₀ gradients ( $G_{∆B_0}$ ) at the labeling plane generated low perfusion signal in the corresponding arterial territories when using a G_mean of 0.4 mT/m. In these cases, the perfusion signal increased ∼128% by using a higher G_mean of 0.8 mT/m.

7T PCASL SNReff can be maximized with LD of ∼3 s when using 3D-EPI and optimized WURST BS pulses. A G_mean of 0.8 mT/m improved PCASL robustness to $∆$ B₀ field gradients along the feeding arteries without the need for subject specific corrections. Use of higher G_mean also offers benefits in cases with multiple $G_{∆B_0}$ polarities and non-linear field variations.