Whole brain snapshot CEST at 3T using 3D-EPI: Aiming for speed, volume, and homogeneity.

Sebastian Mueller,Tony Stöcker,Philipp Ehses,Rüdiger Stirnberg,Moritz Zaiss,Suzan Akbey,Klaus Scheffler

doi:10.1002/mrm.28298

Abstract

CEST MRI enables imaging of distributions of low-concentrated metabolites as well as proteins and peptides and their alterations in diseases. CEST examinations often suffer from low spatial resolution, long acquisition times, and concomitant motion artifacts. This work aims to maximize both resolution and volume coverage with a 3D-EPI snapshot CEST approach at 3T, allowing for fast and robust whole-brain CEST MRI. Resolution and temporal SNR of 3D-EPI examinations with nonselective excitation were optimized at a clinical 3T MR scanner in five healthy subjects using a clinical head/neck coil. A CEST presaturation module for low power relayed nuclear Overhauser enhancement and amide proton transfer contrast was applied as an example. The suggested postprocessing included motion correction, dynamic B0 correction, denoising, and B1 correction and was compared to an established 3D-gradient echo-based sequence. CEST examinations were performed at 1.8mm nominal isotropic resolution in 4.3s per presaturation offset. In contrast to slab-selective 3D or multislice approaches, the whole brain was covered. Repeated examinations at three different B1 values took 13 minutes for 58 presaturation offsets with temporal SNR around 75. The resulting CEST effects revealed significant gray and white matter contrast and were of similar quality across the whole brain. Coefficient of variation across three healthy subjects was below 9%. The suggested protocol enables whole brain coverage at 1.8mm isotropic resolution and fast acquisition of 4.3s per presaturation offset. For the fitted CEST amplitudes, high reproducibility was proven, increasing the opportunities of quantitative CEST investigations at 3T significantly.

Highlights

CEST MRI allows indirect detection of low concentrated solutes
The intrasubject coefficients of variation (COV) in fitted amplitudes were below 8.5% for examinations in the same session and below 7% for examinations in different sessions for amide proton transfer (APT), ssMT, and relayed nuclear Overhauser enhancement (rNOE) both in GM and WM (Figure 7A,B)
The coefficient of variation (COV) across three subjects and sessions was below 4% for APT, rNOE, and ssMT

Summary

Introduction

CEST MRI allows indirect detection of low concentrated solutes. CEST uses two properties of these low concentrated solutes: their chemical exchange with the bulk water proton pool, and their chemical shift relative to water by a certain frequency offset δω. A snapshot CEST approach with centric reordered 3D-gradient echo (GRE) readout scheme was proposed that was optimized for maximized number of k-space lines being acquired following a single presaturation.[2] Still, this GRE-based method was limited to 16 slices and a resolution of 1.7 × 1.7 × 5 mm[3]. To correct for spatial inhomogeneity in B1, repeated measurements at different nominal B1 values are needed.[3] In this work, encouraged by recent findings at 7T,4 we extended a 3D-EPI readout to allow semielliptical scanning along with CAIPIRINHA5 undersampling. It was optimized with centric reordering and nonselective excitation for whole-brain snapshot CEST MRI at 3T. Together with the proposed postprocessing, this enables multipool CEST MRI with whole brain coverage and isotropic resolution of 1.8 mm in less than 4:30 minutes for a fully sampled Z-spectrum at a clinical MR system

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Conclusion