The single-scan cross spatiotemporal encoding (xSPEN) technique delivers in-plane distortion-free 2D images under highly inhomogeneous fields, such as those found near metal implants. This study introduces an xSPEN variant incorporating a t₁ evolution encoding time, which together allows one to solve the in-slice residual distortions of the third dimension. By removing “signal voids” and “pile-up” effects the ensuing slice-encoded xSPEN (SexSPEN) method delivers fast, quality 3D images.SEMAC [1] and MAVRIC [2] are ingenious imaging techniques for handling distortions arising near metal implants. Both of these are multi-scan-based sequences with relative long acquisition times, leading to complications in their use for advanced MRI applications such as diffusion-weighted imaging (DWI). We have recently introduced a single-shot method, xSPEN, with exceptional resilience to in-plane field heterogeneities. The present work proposes an xSPEN-based t₁ encoding scheme the deals with residual in-plane distortions that still arise in xSPEN’s slice-selection, based on SEMAC/MAVRIC-related ideas. The SexSPEN method can thus reinstate all spins in a 3D region-of-interest to their actual spatial locations, in a highly time-efficient fashion.SexSPEN’s sequence is shown in Fig.1. In-plane field inhomogeneities are dealt with by xSPEN using a G_z-gradient that cross spatiotemporally encodes and reads out the desired imaging information along y axis, coupled with a strong oscillating readout G_x gradient sampling this dimension’s k_x-space. xSPEN then obtains a 2D MR image by applying a Fourier transform only on k_x, in a single-shot fashion that is remarkably insensitive to in-plane heterogeneities. Field inhomogeneities will, however, still distort xSPEN’s images due to inhomogeneity-imposed distortions in the slice excitation process –as they do in all slice-selective MR experiments. SexSPEN solves this by adding a t₁-encoding that maps the frequency information within each slice. By allowing a repositioning of all spins in a targeted region-of-interest back to their actual spatial locations, through-plane distortions are corrected within a small number of scans.

Figure 2 demonstrates the xSPEN’s exceptional resilience to in-plane field heterogeneities, with a 7T scan targeting a phantom incorporating two features that are notoriously challenging to single-shot acquisitions: a titanium screw of the kind used in orthopedic implants, and a sample composed of several chemically-shifted sites. Figures 2c-2f illustrate images arising from these six slices, using a variety of pulse sequences. Notice that –apart from sensitivity considerations dictated by their single-shot scan– xSPEN-derived images are virtually indistinguishable from those arising from their multiscan counterparts. This, despite the fact that the entire xSPEN set was acquired within 360 ms (60 ms per slice, no relaxation delays within slices, no need for navigator scans or field maps). Notice, however, the residual influence of “pile-up” effects and slice-plane displacements in both xSPEN and multi-scan images, reflecting through-plane distortions affecting the initial slice selection. These artifacts can be solved by the SexSPEN procedure.

Figure 3 illustrates this with 3T scans on a gel phantom containing a hip implant (panel a). A 20-step t₁-encoded SexSPEN experiment with TR/TE=10000/61 ms, 40 slices, 3 mm isotropic resolution, and a 3 min 20 sec total scan time, leads to 3D images that are free from all through-plane and in-plane distortions. Images arising after Fourier transforming the xSPEN data along k_x (Fig. 3b) still show in-plane distortions; after FT processing the t₁ dimension, frequency maps can be obtained that resolve the through-plane distortions (Fig. 3c). Slice-corrected, repositioned images inside a region-of-interest (Fig. 3d) clearly depict the metal position and the details of the phantom. Parallel imaging and partial Fourier reconstruction along the readout dimension may be used to further improve the in-plane resolution and image quality.

A t₁-encoding SexSPEN sequence that can deliver both in-plane and through-plane distortion-free images in a fast manner is introduced and demonstrated. This could enable advanced MRI applications near metal implants, including diffusion, flow, real-time and functional studies.