New spatiotemporal approaches for fully refocused, multislice ultrafast 2D MRI

Abstract

Purpose-Single-scan multi-slice acquisition schemes play key roles in MRI. Central among these "ultrafast" experiments stands EPI, a technique that although of optimal sampling is challenged by T 2 * artifacts. Recent studies described alternatives based on spatiotemporal encoding (SPEN), which are particularly robust if implemented in a "full-refocusing" mode. The present work extends this modality from the single-slice acquisitions in which it has hitherto been implemented, by introducing a variety of multi-slice schemes scanning 3D volumes.Methods-Multi-slice SPEN employing either inversion or stimulated echo pulses and timed to fulfill the demands of full-refocusing, are demonstrated. The performance of the ensuing methods was examined in "Hybrid" modalities encoding data in k-and direct-space, in low-SAR stimulated-echo approaches, and in direct-space SPEN approaches.Results-When applied in phantoms and in in vivo systems, the ensuing single-shot sequences evidenced similar robustness, sensitivity and resolution qualities as previously discussed 2D single-slice schemes -while enabling a rapid sampling of the third dimension via multi-slicing. Conclusion-The unique benefits deriving from fully-refocused, multi-slice, single-scan SPEN sequences were corroborated by phantom tests, as well as by in vivo scans at 3T and 7T. Low SAR multi-slice SPEN variants compatible with human studies were demonstrated.

In vivo single-shot 13C spectroscopic imaging of hyperpolarized metabolites by spatiotemporal encoding

“…Although further optimizations could yield improved spectral and spatial resolution as well as added sensitivity, the current results already show that SPEN-based sequences can be utilized as a modality for fast dynamic imaging. Further developments are in progress to exploit additional benefits of spatiotemporal encoding, such as a built-in restricted FOV capability [24,26], multi-echo experiments and parallel imaging acquisition [34]. Representative metabolic images from a rat following injection of hyperpolarized [1][2][3][4][5][6][7][8][9][10][11][12][13] …”

“…Hybrid SPEN has a higher robustness against magnetic field inhomogeneities than EPI, reflecting the larger bandwidths it can access for the spatially sequential excitation and detection. This is aided by the possibility of refocusing all of the spin packets throughout acquisition [25,26]. Equally important is the fact that SPEN allows spectral-spatial imaging, at no additional expense over the basic imaging pulse sequence [27].…”

Ultrafast multi-slice spatiotemporally encoded MRI with slice-selective dimension segmented

“…As discussed in the previous report [32], multi-slice acquisition can be benefited from the quadratic phase produced by a 180°chirp pulse rather than usual 90°chirp pulse. The ensuing pulse will progressively refocus the magnetizations of the excited slices, and inverse the remaining longitudinal magnetization in the sample.…”

“…The 90 ss divides the targeted segment into several slices without inter-slice gap. For full decoding, the acquisition gradient G acq applied over a time T acq should meet the following condition [32]:…”

“…However, for the multi-slice SPEN MRI realized by the repetition of single-slice SPEN MRI, the SAR problem is exacerbated, which greatly limits its value in clinical application [31]. To relieve the SAR limitation, a novel multi-slice SPEN method was proposed by Frydman and coauthors in 2013 [32]. This method is based on a full volume of interest (VOI) excitation and encoding, which is realized by the combination use of a 90°hard pulse and a 180°chirp pulse in the phase encoding dimension.…”