Abstract
ObjectiveTo compare integrated slice-specific dynamic shim (iShim) with distortion correction post-processing to conventional 3D volume shim for the reduction of artefacts and signal loss in 1.5 T whole-body diffusion-weighted imaging (WB-DWI).MethodsTen volunteers underwent WB-DWI using conventional 3D volume shim and iShim. Forty-eight consecutive patients underwent WB-DWI with either volume shim (n = 24) or iShim (n = 24) only. For all subjects, displacement of the spinal cord at imaging station interfaces was measured on composed b = 900 s/mm2 images. The signal intensity ratios, computed as the average signal intensity in a region of high susceptibility gradient (sternum) divided by the average signal intensity in a region of low susceptibility gradient (vertebral body), were compared in volunteers. For patients, image quality was graded from 1 to 5 (1 = Poor, 5 = Excellent). Signal intensity discontinuity scores were recorded from 1 to 4 (1 = 2 + steps, 4 = 0 steps). A p value of < 0.05 was considered significant.ResultsSpinal cord displacement artefacts were lower with iShim (p < 0.05) at the thoracic junction in volunteers and at the cervical and thoracic junctions in patients (p < 0.05). The sternum/vertebra signal intensity ratio in healthy volunteers was higher with iShim compared with the volume shim sequence (p < 0.05). There were no significant differences between the volume shim and iShim patient groups in terms of image quality and signal intensity discontinuity scores.ConclusioniShim reduced the degree of spinal cord displacement artefact between imaging stations and susceptibility-gradient-induced signal loss.
Highlights
Whole-body MRI is accepted as the gold-standard imaging modality for suspected and newly diagnosed myeloma and plasmacytoma, and is recommended by the International Myeloma Working Group and the National Institute for Health and Care Excellence [1, 2]
Twenty-four consecutive patients scanned with the conventional 3D volume shim Whole-body diffusion-weighted imaging (WB-DWI) sequence (13 male, 11 female, median age 72 years, age range 49–87) and 24 consecutive patients scanned with the integrated slice-specific dynamic shimming method (iShim) WB-DWI sequence (11 male, 13 female, median age 62 years, age range 28–87) were included
The sternum-to-vertebra signal intensity ratio (SIR) was significantly increased in the iShim images compared with matched volume shim images (Fig. 4, p < 0.05, paired t test)
Summary
EPI is, prone to geometric distortion and signal loss, which are the result of phase accumulation in regions of high susceptibility gradient. Magnetic Resonance Materials in Physics, Biology and Medicine (2021) 34:513–521 head (e.g. base of skull, sinuses), neck, and thorax (e.g. chest wall) [7,8,9]. Another drawback of conventional WB-DWI techniques is the spinal cord displacement artefact which is often apparent when WB-DWI is viewed as a sagittally reformatted image (as is common in clinical practice) [10]. The different shim settings which are applied for the acquisition of each station result in an apparent spatial displacement at the junction of adjacent stations when reformatted in the sagittal plane
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