Multi-echo-based fat artifact correction for CEST MRI at 7 T

Purpose: CEST MRI is influenced by fat signal, which can reduce the apparent CEST contrast or lead to pseudo-CEST effects. Our goal was to develop a fat artifact correction based on multi-echo fat-water separation that functions stably for 7 T knee MRI data.Methods: Our proposed algorithm utilizes the full complex data and a phase demodulation with an off-resonance map estimation based on the Z-spectra prior to fat-water separation to achieve stable fat artifact correction. Our method was validated and compared to multi-echo-based methods originally proposed for 3 T by Bloch-McConnell simulations and phantom measurements. Moreover, the method was applied to in vivo 7 T knee MRI examinations and compared to Gaussian fat saturation and a published single-echo Z-spectrum-based fat artifact correction method.Results: Phase demodulation prior to fat-water separation reduced the occurrence of fat-water swaps. Utilizing the complex signal data led to more stable correction results than working with magnitude data, as was proposed for 3 T. Our approach reduced pseudo-nuclear Overhauser effects compared to the other correction methods. Thus, the mean asymmetry contrast at 3.5 ppm in cartilage over five volunteers increased from -9.2% (uncorrected) and -10.6% (Z-spectrum-based) to -1.5%. Results showed higher spatial stability than with the fat saturation pulse.Conclusion: Our work demonstrates the feasibility of multi-echo-based fat-water separation with an adaptive fat model for fat artifact correction for CEST MRI at 7 T. Our approach provided better fat artifact correction throughout the entire spectrum and image than the fat saturation pulse or Z-spectrum-based correction method for both phantom and knee imaging results

Investigation of a parallel transmission (pTx) GRE-readout with customized pulses for CEST MRI at 7 Tesla

In Chemical Exchange Saturation Transfer MRI B1+-inhomogeneity influences both saturation and acquisition of the signal. Typically only a correction of the inhomogeneity of the CEST saturation is performed while the inhomogeneity of the readout is neglected. The influence of the readout was investigated in measurements using standard inhomogeneous 1Tx readout and a homogenized readout with customized pTx pulses. Compared to the 1Tx readout the readout with pTx pulses shows an increase of the homogeneity of the CEST contrast for CEST agents with low SNR like the amides and in regions with low 1Tx flip angle like the cerebellum

Multiple interleaved mode saturation (MIMOSA) for B1 + inhomogeneity mitigation in chemical exchange saturation transfer

PURPOSE: To mitigate B+1 inhomogeneity in quantitative CEST MRI at ultra-high magnetic field strengths (B0 ≥ 7 Tesla) using a parallel transmit system. METHODS: Multiple interleaved mode saturation employs interleaving of 2 complementary phase sets during the saturation pulse train. Phase differences of 45° (first mode) and 90° (second mode) between 2 adjacent transmitter coil channels are used. The influence of the new saturation scheme on the CEST contrast was analyzed using Bloch-McConnell simulations. The presented method was verified in phantom and in vivo measurements of the healthy human brain. The relayed nuclear Overhauser effect was evaluated, and the inverse magnetic transfer ratio metric was calculated. Results were compared to a published B+1 correction method. All measurements were conducted on a whole-body 7 Tesla MRI system using an 8 transmitter and 32 receiver channel head coil. RESULTS: Simulations showed that the inverse magnetic transfer ratio metric contrast of relayed nuclear Overhauser effect shows a smaller dependency on the relative amplitudes of the 2 different modes than the contrasts of Cr and amide proton transfer. Measurements of an egg white phantom showed markedly improved homogeneity compared to the uncorrected inverse magnetic transfer ratio metric (relayed nuclear Overhauser effect) images and slightly improved results compared to B+1 corrected images. In vivo multiple interleaved mode saturation images showed similar contrast compared to B+1 corrected images. CONCLUSION: Multiple interleaved mode saturation can be used as a simple method to mitigate B+1 inhomogeneity effects in CEST MRI at ultra-high magnetic field strengths. Compared to previous B+1 correction methods, acquisition time can be reduced because an additional scan, usually required for B+1 correction, can be omitted

Towards Super-resolution CEST MRI for Visualization of Small Structures

The onset of rheumatic diseases such as rheumatoid arthritis is typically subclinical, which results in challenging early detection of the disease. However, characteristic changes in the anatomy can be detected using imaging techniques such as MRI or CT. Modern imaging techniques such as chemical exchange saturation transfer (CEST) MRI drive the hope to improve early detection even further through the imaging of metabolites in the body. To image small structures in the joints of patients, typically one of the first regions where changes due to the disease occur, a high resolution for the CEST MR imaging is necessary. Currently, however, CEST MR suffers from an inherently low resolution due to the underlying physical constraints of the acquisition. In this work we compared established up-sampling techniques to neural network-based super-resolution approaches. We could show, that neural networks are able to learn the mapping from low-resolution to high-resolution unsaturated CEST images considerably better than present methods. On the test set a PSNR of 32.29 dB (+10%), a NRMSE of 0.14 (+28%), and a SSIM of 0.85 (+15%) could be achieved using a ResNet neural network, improving the baseline considerably. This work paves the way for the prospective investigation of neural networks for super-resolution CEST MRI and, followingly, might lead to a earlier detection of the onset of rheumatic diseases

On the feasibility of a single scan B1+ correction scheme in combination with Multiple Interleaved Mode Saturation for quantitative CEST at 7 Tesla

In Chemical Exchange Saturation Transfer MR both B1+-inhomogeneity correction and mitigation have their limitations, in particular if large field-of views shall be covered. To overcome these limitations a Multiple Interleaved Mode Saturation scheme was applied together with a linear B1+ inhomogeneity correction method. Repeatability and reproducibility of the MTRRex metric for rNOE and APT were investigated. A combination of MIMOSA and a simple single point correction allows achieving repeatable and reproducible CEST contrast in whole brain with an acquisition time of 4min 54s