Uniform combined reconstruction of multichannel 7T knee MRI receive coil data without the use of a reference scan

Abstract

Background MR image intensity nonuniformity is often observed at 7T. Reference scans from the body coil used for uniformity correction at lower field strengths are typically not available at 7T. Purpose To evaluate the efficacy of a novel algorithm, Uniform Combined Reconstruction (UNICORN), to correct receive coil‐induced nonuniformity in musculoskeletal 7T MRI without the use of a reference scan. Study Type Retrospective image analysis study. Subjects MRI data of 20 subjects was retrospectively processed offline. Field Strength/Sequence: Knees of 20 subjects were imaged at 7T with a single‐channel transmit, 28‐channel phased‐array receive knee coil. A turbo‐spin‐echo sequence was used to acquire 33 series of images. Assessment Three fellowship‐trained musculoskeletal radiologists with cumulative experience of 42 years reviewed the images. The uniformity, contrast, signal‐to‐noise ratio (SNR), and overall image quality were evaluated for images with no postprocessing, images processed with N4 bias field correction algorithm, and the UNICORN algorithm. Statistical Tests Intraclass correlation coefficient (ICC) was used for measuring the interrater reliability. ICC and 95% confidence intervals (CIs) were calculated using the R statistical package employing a two‐way mixed‐effects model based on a mean rating (k = 3) for absolute agreement. The Wilcoxon signed‐rank test with continuity correction was used for analyzing the overall image quality scores. Results UNICORN was preferred among the three methods evaluated for uniformity in 97.9% of the pooled ratings, with excellent interrater agreement (ICC of 0.98, CI 0.97–0.99). UNICORN was also rated better than N4 for contrast and equivalent to N4 in SNR with ICCs of 0.80 (CI 0.72–0.86) and 0.67 (CI 0.54–0.77), respectively. The overall image quality scores for UNICORN were significantly higher than N4 (P < 6 × 10‐13), with good to excellent interrater agreement (ICC 0.90, CI 0.86–0.93). Data Conclusion Without the use of a reference scan, UNICORN provides better image uniformity, contrast, and overall image quality at 7T compared with the N4 bias field‐correction algorithm. Level of Evidence: 4 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:1534–1544.

7 T Musculoskeletal MRI

“…Promising steps toward a 7 T parallel-transmit body coil were recently reported 68 . Post–data acquisition attempts to homogenize signal intensity seem moderately successful 69 …”

Improved TSE imaging at ultrahigh field using nonlocalized efficiency RF shimming and acquisition modes optimized for refocused echoes (AMORE)

“…Given that neither LS-TIAMO nor AMORE is explicitly optimized for the uniformity of this term, the resulting images may still manifest a subtle spatially nonuniform weighting, which may be ameliorated by using N4 ITK 39 and UNICORN. 40 Although body imaging at UHF, especially TSE imaging, is challenging due to inhomogeneities and limited peak B 1 + , our results demonstrate that the B 1 + shading and nulls in the torso can be effectively reduced by designing 2 phase-only modes with AMORE. The concept is extended in a straightforward manner to more modes as might be beneficial with increasing field strength such as 10.5 T, where the benefits and limitations of static RF shimming have been previously demonstrated.…”

“…As demonstrated in the , the combined TIAMO images remain weighted by a B 1 + ‐dominated term in addition to the receive sensitivity. Given that neither LS‐TIAMO nor AMORE is explicitly optimized for the uniformity of this term, the resulting images may still manifest a subtle spatially nonuniform weighting, which may be ameliorated by using N4 ITK 39 and UNICORN 40 …”

Whole‐liver flip‐angle shimming at 7 T using parallel‐transmit kT‐point pulses and Fourier phase‐encoded DREAMB1+ mapping

“…Figure S3 shows that this brightening can be greatly reduced by applying bias field correction 60 following reconstruction to a GRE image acquired with k T ‐points, providing a highly homogeneous appearance across the liver. Future work will investigate the improvement of correction methods using receive coil sensitivity maps acquired without homogeneous reference scan (e.g., uniform combined reconstruction 61 ) and its application alongside retrospective bias correction methods like N4 to deliver clinically attractive liver images.…”