Purpose Diffusion encoding gradients are known to yield vibrations of the typical clinical MR scanner hardware with a frequency of 20 to 30 Hz, which may lead to signal loss in diffusion‐weighted MR measurements. This work proposes to mitigate vibration‐induced signal loss by introducing a vibration‐matching gradient (VMG) to match vibrational states during the 2 diffusion gradient pulses. Theory and Methods A theoretical description of displacements induced by gradient switching was introduced and modeled by a 2‐mass‐spring‐damper system. An additional preceding VMG mimicking timing and properties of the diffusion encoding gradients was added to a high b‐value diffusion‐weighted MR spectroscopy sequence. Laser interferometry was employed to measure 3D displacements of a phantom surface. Lipid ADC was assessed in water–fat phantoms and in vivo in the tibial bone marrow of 3 volunteers. Results The modeling and the laser interferometer measurements revealed that the displacement curves are more similar during the 2 diffusion gradients with the VMG compared to the standard sequence, resulting in less signal loss of the diffusion‐weighted signal. Phantom results showed lipid ADC overestimation up to 119% with the standard sequence and an error of 5.5% with the VMG. An 18% to 35% lower coefficient of variation was obtained for in vivo lipid ADC measurement when employing the VMG. Conclusion The application of the VMG reduces the signal loss introduced by hardware vibrations in a high b‐value diffusion‐weighted MRS sequence in phantoms and in vivo. Reference measurements based on laser interferometry and mechanical modelling confirmed the findings.
To study the effect of field inhomogeneity distributions in trabecularized bone regions on the gradient echo (GRE) signal with short TEs and to characterize quantification errors on R * 2 and proton density fat fraction (PDFF) maps when using a water-fat model with an exponential R * 2 decay model at short TEs. Methods: Field distortions were simulated based on a trabecular bone micro CT dataset. Simulations were performed for different bone volume fractions (BV/TV) and for different bone-fat composition values. A multi-TE UTE acquisition was developed to acquire multiple UTEs with random order to minimize eddy currents. The acquisition was validated in phantoms and applied in vivo in a volunteer's ankle and knee. Chemical shift encoded MRI (CSE-MRI) based on a Cartesian multi-TE GRE scan was acquired in the spine of patients with metastatic bone disease.Results: Simulations showed that signal deviations from the exponential signal decay at short TEs were more prominent for a higher BV/TV. UTE multi-TE measurements reproduced in vivo the simulation-based predicted behavior. In regions with high BV/TV, the presence of field inhomogeneities induced an R * 2 underestimation in trabecularized bone marrow when using CSE-MRI at 3T with a short TE. Conclusion: R *2 can be underestimated when using short TEs (<2 ms at 3 T) and a water-fat model with an exponential R * 2 decay model in multi-echo GRE acquisitions of trabecularized bone marrow. K E Y W O R D Schemical shift encoding (CSE), Gaussian decay, magnetically inhomogeneous tissues, PDFF mapping, R * 2 mapping, signal decay, static dephasing regime, trabecularized bone, ultra-short echo time (UTE)This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.
Volumetric liver T2-mapping is of interest in the characterization of focal lesions and diffuse disease but remains technically challenging due to respiratory motion. The present work introduces a novel T2-prepared (T2prep) radial stack-of-stars two-point Dixon gradient echo sequence (T2prep-SoS-Dixon-TFE) for achieving motion-robust 3D isotropic T2-mapping in navigator-gated scans using a B0/B1-insusceptible modified adiabatic BIR-4 RF-pulse for T2prep. T2-mapping is performed via dictionary matching to Bloch simulations for the Dixon-decomposed water signal. The proposed method enables motion-robust liver fat confounder-free volumetric T2 quantification in better agreement with MRS compared to GraSE and may improve the clinical applicability of free-breathing abdominal T2-mapping.
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