Single‐scan MRI with exceptional resilience to field heterogeneities

Abstract

An approach delivering single-scan MRI with unprecedented resilience to field inhomogeneities, is proposed and illustrated. The method departs from conventional k-based scanning methods, and relies instead on spatiotemporally encoding the image being sought. Unlike hitherto proposed methods, however, this MRI image readout does not take place utilizing a magnetic field gradient along the direction being probed, but rather with the aid of an ancillary source of inhomogeneous frequency broadening. This ancillary dimension can arise from an orthogonal field gradient, from susceptibility-or shift-imposed frequency distributions, from intrinsic spin anisotropies, or from a combination of these all. By allowing such broadenings to act as the agents that spatiotemporally encode and readout the desired imaging information, the ensuing MR images become insensitive to the presence of field inhomogeneities or internal shifts. Even when dealing with notably distorted spin-echo multi-scan images acquired in low-homogeneity magnets or next to metallic objects, the new approach delivers unbiased single-shot images. The principles and characteristics of this new approach -compatible with existing scanners and free from the need to collect auxiliary information such as field maps-are presented and discussed, together with single-and multi-slice in vitro, ex vivo and in vivo MRI comparisons.

Phase‐encoded xSPEN: A novel high‐resolution volumetric alternative to RARE MRI

“…The data were collected without downsampling, using a full k x / k y / k z data set and processed according to the recipe in Figure . These images are dominated by a T 2 contrast and lack any noticeable susceptibility or fat–water shift in‐plane distortions despite the absence of multiple spin echoes, owing to xSPEN's built‐in compensation characteristics . Nearly absent as well are “stitching effects” along the SS axis, owing to the cleanness with which the SLR slab‐selective excitation and the subsequent inversion pulses addressed each of the “diamonds” introduced in Figure .…”

“…To consider all this, we neglect, as before, the conventional RO domain and focus on events defining images in the y / z plane. As discussed in Figure of Zhang et al, the initial SS plus the two ensuing swept pulses will define, in unison with the various G y / G z gradients, a diamond‐like targeted shape in this plane. These are shown as red polygons in Figure under idealized conditions that disregard field inhomogeneity or chemical/susceptibility shifts, as well as non‐idealities of the adiabatic sweeps that are considered in more detail in the Supporting Information Figure S4.…”

“…The saddle‐shaped phase profile associated to this encoding leaves a sensitive region fulfilling , where substantial cancelation among neighboring spins does not occur; over the course of the signal acquisition the continued action of G z displaces this saddle‐shaped profile, progressively probing ρ( y ). The mechanism by which the constant application of a G z gradient delivers an image free from inhomogeneities along an orthogonal axis, has been discussed in detail elsewhere . In brief, this behavior follows from calculating the time‐domain signal that will be originated in such experiment, which for a thin enough slice can be written as …”

“…Imaging methods can be further accelerated by echo planar strategies, yet the relatively low bandwidth(s) of the phase‐encoded dimension(s) that these experiments involve, makes them susceptible to field distortions. Recent years have seen the introduction of methods that, based on a spatiotemporal encoding (SPEN), can deliver single‐scan single‐slice images with higher robustness to field inhomogeneities than their EPI counterparts . Unlike what happens in conventional k ‐based scans, SPEN experiments read out their images in direct, physical space.…”

“…To do so, swept pulses and gradients are used to imprint a non‐linear phase during an initial encoding process, which then provides a focal point for a subsequent, gradient‐driven image probing. In their original SPEN implementation this phase involved a quadratic, ϕ = C y modulation, for which the signal from anywhere but the center of the parabola, is dephased; more recently, an alternative cross‐term (xSPEN) procedure was introduced whereby the phase modulation takes a hyperbolic, ϕ = C y × z profile . As a result of such pre‐acquisition phase the signal received will be dominated by the saddle point at the center of an hyperbola.…”