The higher signal-to-noise-ratio (SNR) offered at ultra-high magnetic field strengths, such as 7 Tesla (7T), has been shown to increase the resolution of diffusion MRI (dMRI) as well as the precision and directional certainty of diffusion-based parameters [1,2] However, two major drawbacks of 7T dMRI include lengthy acquisition times and signal loss due to increased inhomogeneity in the applied radiofrequency (RF) field (B₁).

Simultaneous multi-slice (SMS) imaging using multiband excitation and parallel imaging is becoming more common for accelerating full brain acquisitions [3]. While SMS reduces the duration of the acquisition, the refocusing pulses typically used in dMRI are particularly sensitive to B₁ non-uniformities leading to a loss in signal, or even complete signal dropout in parts of the image. SEmi-Adiabatic Matched-phase Spin-echo (SEAMS) Power Independent of the Number of Slices (PINS) is an SMS technique that can accelerate imaging while reducing a spin-echo based sequence's sensitivity to B₁ non-uniformities [4] . We have created a dMRI sequence with SEAMS PINS excitation and refocusing and an EPI readout that provides improved immunity to B₁-inhomogeneity with RF power deposition, as measured by the specific absorption rate (SAR), comparable to existing sequences.

We created a SEAMS PINS pulse-pair with an excitation pulse duration of 21 ms and a refocusing pulse duration of 14 ms (Figure 1). The pulse-pair was created to match a refocusing pulse with a bandwidth of 1.11 kHz sampled by 94 lobes, creating a slice separation/slice thickness ratio of 12. The pulse-pair was inserted into a dMRI sequence with an EPI readout (Figure 2), and EPI navigators acquired pre-scan [2]. Diffusion-weighted images were acquired using 2 diffusion sequences 1) the SEAMS PINS pulse-pair and 2) a standard non-adiabatic single-refocused SMS diffusion pulse-pair with a time-shifted RF pulse multiband factor of 2 [5]. Imaging was performed in a spherical water phantom on a 7T whole body MRI system (Siemens, Erlangen). The acquisition was made with: b = 1500 s/mm²; TE = 73 ms; FOV = 21 cm x 21 cm; matrix = 210 x 210; Time = 9 minutes. Each sequence was acquired at four power settings, at adiabatic threshold or with the power calibrated to produce the expected flip angle at the center of the magnet (P_thresh), at 1.2 x P_thresh, 1.4 x P_thresh, and 1.6 x P_thresh.

The SAR ratio for each sequence was noted, although none of the sequences exceeded the SAR limits for the scanner. The SEAMS PINS diffusion pulse deposited 80% of the power of the time-shifted sequence, for the same TR and scan times. The B₁-insensitivity of the sequence is shown in Figure 3. For a central slice of the phantom (Figure 3A), b = 1500 s/mm², the signal received on a projection through the phantom (Fig. 3A white line) is plotted for each power setting for both sequences. The signal produced by the non-adiabatic SMS diffusion sequence shows high sensitivity to B₁ across the initial projection through the phantom. As the transmit B₁ increases by 20%, most of the signal in the center of the phantom is destroyed, and as the B₁ continues to increase, a larger region of the phantom experiences complete dropout. In contrast to this, the signal produced by the SEAMS PINS pulse-pair, is more consistent across the body of the phantom. It shows much less variation over the phantom cross-section as the power is increased by 20%, and even at 1.6 x adiabatic threshold, no portion of the phantom experiences complete signal dropout.

At ultra-high fields, dMRI using SEAMS PINS can achieve accelerated simultaneous multi-slice imaging while restoring diffusion-weighted signal to regions that would experience excessive signal dropout using non-adiabatic pulse-pairs. The sequence is substantially more robust to B₁ inhomogeneity, while maintaining SAR deposition levels similar to those of existing non-adiabatic sequences. Future work involves further reduction of minimum achievable echo time and validation of the sequence in vivo.