Dynamic glucose‐enhanced (DGE) MRI in the human brain at 7 T with reduced motion‐induced artifacts based on quantitative R_1ρ mapping

Abstract

Purpose Dynamic glucose‐enhanced (DGE)‐MRI based on chemical exchange‐sensitive MRI, that is, glucoCEST and gluco‐chemical exchange‐sensitive spin‐lock (glucoCESL), is intrinsically prone to motion‐induced artifacts because the final DGE contrast relies on the difference of images, which were acquired with a time gap of several mins. In this study, identification of different types of motion‐induced artifacts led to the development of a 3D acquisition protocol for DGE examinations in the human brain at 7 T with improved robustness in the presence of subject motion. Methods DGE‐MRI was realized by the chemical exchange‐sensitive spin‐lock approach based either on relaxation rate in the rotating frame (R1ρ)‐weighted or quantitative R1ρ imaging. A 3D image readout was implemented at 7 T, enabling retrospective volumetric coregistration of the image series and quantification of subject motion. An examination of a healthy volunteer without administration of glucose allowed for the identification of isolated motion‐induced artifacts. Results Even after coregistration, significant motion‐induced artifacts remained in the DGE contrast based on R1ρ‐weighted images. This is due to the spatially varying sensitivity of the coil and was found to be compensated by a quantitative R1ρ approach. The coregistered quantitative approach allowed the observation of a clear increase of the DGE contrast in a patient with glioblastoma, which did not correlate with subject motion. Conclusion The presented 3D acquisition protocol enables DGE‐MRI examinations in the human brain with improved robustness against motion‐induced artifacts. Correction of motion‐induced artifacts is of high importance for DGE‐MRI in clinical studies where an unambiguous assignment of contrast changes due to an actual change in local glucose concentration is a prerequisite.

Real‐time motion and retrospective coil sensitivity correction for CEST using volumetric navigators (vNavs) at 7T

“…43 Studies combining the correction of these 3 types of artifacts have only recently emerged. 5,8 Herz et al 8 managed them retrospectively in the following way:…”

“…There may be residual out‐of‐slice misalignment of consecutive CEST measurements because of motion in the time frame between the 2nd vNav and CEST acquisition (~1.3 s from each other). This can be resolved in the future using 3D image encoding for CEST, such as the Snapshot‐CEST sequence proposed by Zaiss et al, 5,58 which will allow better retrospective co‐registration.…”

“…Previous vNav studies tackled the mitigation of subject motion simultaneously with a third type of motion‐induced artifacts (ie, B 0 fluctuations), 27‐29,33,54 whereas we opted for a self‐corrected dynamic ΔB 0 method directly from the phase of the pre‐saturated GRE images 43 . Studies combining the correction of these 3 types of artifacts have only recently emerged 5,8 . Herz et al 8 managed them retrospectively in the following way:…”

“…Chemical exchange-sensitive MRI techniques such as CEST [1][2][3] and chemical exchange based on T 1ρ or T 2 relaxation enhancement (ie, chemical exchange sensitive spin-lock, CESL) provide an alternative chemically specific MRI contrast to MR spectroscopy, based on indirect detection of endogenous and exogenous molecules. [4][5][6][7][8][9][10] In contrast to MRSI, the encoding of the spectral (chemically specific) dimension is slow. CEST and similar techniques rely on the comparison between images (ie, representing different spectral points) acquired at different time points.…”

“…To overcome this effect several strategies have been proposed, including adjustments of the receiver coil sensitivity maps in response to head motion for functional MRI or by a more robust definition of parameter estimation metrics for quantitative MRI (eg, in which the signal acquired by the coil is corrected by dividing it by a factor proportional to the coil receive sensitivity field) 44,45 . In agreement with this, Boyd et al 5 and Herz et al 8 defined a chemical exchange‐sensitive DGE metric, which included a normalization parameter to account for the time‐varying coil sensitivities.…”