Excitation of slice profiles with a limited FOV was shown to be advantageous for distortion reduction in zoomed EPI applications such as DWI [1-4]. Profile selection is accomplished normally by 2D pulses based on EPI excitation trajectories [3,4]. However, due to long durations, 2D pulses are sensitive to B₀ deviations, manifesting themselves as distortions in the excited profile [5]. In this study, we analyze off-resonance effects observed in SE-EPI combined with 2D pulses. The main focus is placed on new opportunities brought by parallel excitation (PEX, [5-7]) for shortening 2D pulses and further minimizing the distortion.

The thickness of a designed slice is usually encoded either along the phase encoding (PE) or frequency encoding (FE) directions of the excitation trajectory (Fig. 1). In PEX, the trajectories are undersampled by skipping lines and the resulting excitation replicates are suppressed by using the B₁ sensitivities during pulse design [6,7]. When FE is along the slice selection direction (Fig. 1A), the PE gradients of excitation and acquisition are applied in the same directions in SE-EPI (Fig. 2A). Therefore, excitation and acquisition distortions are observed along the same spatial dimension and combined with each other in the final image. In another encoding scheme (Fig. 1B), the excitation and acquisition PE gradients are in orthogonal dimensions (Fig. 2B). Thus, strong field deviations might shift the profile outside the refocusing position during excitation, resulting in an SNR decrease during the EPI readout. PEX allows shortening of the 2D pulses and thus reduces these effects in both techniques in Fig. 2.

Experiments were carried out on a 3T MR scanner (Siemens Magnetom Trio) with an 8-channel TxArray extension. The scanned object was a container filled with water and plastic tubes. 2D PEX was exploited for both techniques in Fig. 1 and 2. The RF pulses were designed using the small-tip-angle algorithm combined with conjugate gradient optimization to incorporate B₁ data [8]. The excitation trajectories with slice selection in the PE direction were defined over an FOE of 38.4×24 cm² with a grid size of 192×16. The trajectories with FE along the slice selection direction had an FOE of 38.4×9.6 cm² and 32×32 samples. Both trajectories were undersampled by skipping every second k-space line.

The selected profiles were imaged using GRE, which reflected the excitation distortions. Afterwards, SE-EPI methods (Fig. 2) were tested for varying shim settings. For the sequence in Fig. 2A, imaging was in a sagittal orientation for PE along the "anterior-posterior" and "posterior-anterior" directions, which corresponded to opposite polarities of the PE acquisition gradients. The imaging in Fig. 2B was in a coronal orientation.

For default shim settings (Fig. 3A), the profiles exhibited no visible distortions, when FE of the pulses was along the slice selection direction. After the shim currents were modified and scanning was repeated, stronger B₀ deviations led to shearing of the profiles (Fig. 3B, C). The distortion during excitation for the accelerated pulse was weaker compared to the fully-sampled counterpart as shown in the GRE images. The profiles underwent further distortions in EPI and appeared to be sheared dramatically when PE was "anterior-posterior". For PE along the "posterior-anterior" direction, the observed distortion was minimal.

For excitation of profiles with slice selection in the PE direction and default shim settings, the observed distortions were also minimal (Fig. 4A). After the resonance frequency was changed by 100 Hz, the selected slices were shifted from the original position, resulting in a signal decrease in EPI (Fig. 4B,C). However, the signal was partially restored for the accelerated pulse due to its shorter duration.

PEX allows for a more robust selection of limited profiles against off-resonance effects. For the imaging scheme with slice selection along the FE dimension, PEX offers additional degrees of freedom, in order to better confine the FOV to the targeted organ in SE-EPI and DWI. For slice selection in the PE direction, PEX makes DWI more robust again signal loss.