Fagan, Andrew

Objectives The aim of this work was to devise mitigation strategies for addressing a range of image artifacts on a clinical 7 T magnetic resonance imaging scanner using the regulatory-approved single-channel radiofrequency transmit mode and vendor-supplied radiofrequency coils to facilitate clinical scanning within reasonable scan times. Materials and Methods Optimized imaging sequence protocols were developed for routine musculoskeletal knee and neurological imaging. Sources of severe image nonuniformities were identified, and mitigation strategies were devised. A range of custom-made high permittivity dielectric pads were used to compensate for B1 + and B1 − inhomogeneities, and also for magnetic susceptibility-induced signal dropouts particularly in the basal regions of the temporal lobes and in the cerebellum. Results Significant improvements in image uniformity were obtained using dielectric pads in the knee and brain. A combination of small voxels, reduced field of view B0 shimming, and high in-plane parallel imaging factors helped to minimize signal loss in areas of high susceptibility-induced field distortions. The high inherent signal-to-noise ratio at 7 T allowed for high receiver bandwidths and thin slices to minimize chemical shift artifacts. Intermittent artifacts due to radiofrequency inversion pulse limitations (power, bandwidth) were minimized with dielectric pads. A patient with 2 implanted metallic cranial fixation devices located within the radiofrequency transmit field was successfully imaged, with minimal image geometric distortions. Conclusions Challenges relating to severe image artifacts at 7 T using single-channel radiofrequency transmit functionality in the knee and brain were overcome using the approaches described in this article. The resultant high diagnostic image quality paves the way for incorporation of this technology into the routine clinical workflow. Further developmental efforts are required to expand the range of applications to other anatomical areas, and to expand the evidence- and knowledge-base relating to the safety of scanning patients with implanted metallic devices.

PurposeTo quantify the effects of DCE acquisition and pharmacokinetic modeling processing methodologies on the absolute accuracy and precision of derived pharmacokinetic (PK) parameter values using a novel anthropomorphic phantom test device in which “ground truth” values were known a priori.MethodsGround truth arterial input function (AIF), tumor, and healthy tissue contrast agent concentration‐time curves (CTCs) were established within the phantom and repeatedly measured on a 3T MRI scanner with varying temporal resolution (Tres = 1.22–30.6 s). Ground truth CTCs, Ktrans, ve, and kep values were directly compared to measured values as a function of Tres, with and without the application of voxel‐wise flip‐angle corrections applied to the data and PK modeling performed using linear and nonlinear forms of the standard Tofts model.ResultsMeasurement of the AIF was strongly affected by the Tres used (AIF curve‐shape feature errors: 3%–222% for Tres: 1.22–30.6 s), which directly translated to errors in the derived Ktrans, ve, and kep values of 1%–24%, 2%–5%, and 1%–26% respectively across this Tres range (flip‐angle correction applied). Further appreciable improvements in accuracy and precision arising from the use of flip angle corrections and nonlinear least squares fitting were quantified and used to identify optimal acquisition and analysis methodologies for which measurement errors could be constrained below threshold levels.ConclusionThis quantitative study provides insight into how errors in AIF measurement propagate to errors in PK parameter outputs. Absolute quantification of the accuracy and precision of MR‐measured CTCs, and resultant PK parameter values, allowed for an optimal temporal resolution to be defined commensurate with maintaining Ktrans, ve, and kep measurement errors below 5% and 10% levels. An appreciable gain in PK parameter estimation accuracy at the analysis stage was also demonstrated using flip‐angle corrections and a linear approach to PK model fitting.

BackgroundMR 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.PurposeTo 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 TypeRetrospective image analysis study.SubjectsMRI 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.AssessmentThree 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 TestsIntraclass 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.ResultsUNICORN 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 ConclusionWithout 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: 4Technical Efficacy: Stage 1J. Magn. Reson. Imaging 2019;50:1534–1544.