7 T Musculoskeletal MRI: Fundamentals and Clinical Implementation

High-Resolution 3D Turbo Spin-Echo Wrist MRI at 7T Accelerated by Compressed Sensing

This study aimed to obtain high-resolution 3D isotropic turbo spin-echo (TSE) wrist MRI acquisitions at 7T, with and without fat suppression, facilitated by compressed-sensing (CS) acceleration. In 16 healthy subjects, fat-suppressed (FS) and nonfat-suppressed (NFS) TSE wrist images were obtained. The protocol consisted of a SENSE-accelerated scan, with an isotropic voxel size of 0.45 mm and acquisition time of 7 min ("SENSE45"), a 0.45-mm, 4-min CS-accelerated scan ("CS45"), and a 0.35-mm, 7-min CS-accelerated scan ("CS35"). For two subjects, additional 0.45-mm, 4-min SENSE-accelerated scans were acquired ("High-SENSE"). For the NFS scans, refocusing pulses were optimized to mitigate water-fat chemical-shift artifacts in the slab-selection direction. Anatomical visibility of wrist structures and image quality were assessed qualitatively and through musculoskeletal radiologist grading. The use of nonselective hard refocusing pulses with optimized bandwidths and a center frequency in between water and fat enabled NFS imaging. The image quality of the faster CS45 scans was lower than for SENSE45, with statistically significantly different grading in 9/16 (FS) and 2/6 (NFS) grading parameters. Nonetheless, a similar scan time reduction could not be achieved using High-SENSE. No distinct benefit of CS35 compared to SENSE45 was evident in either the FS or NFS scans. NFS CS35 exhibited enhanced bone sharpness compared to SENSE45 for some subjects, yet on a group level, the difference was not statistically significant. In conclusion, for maintained voxel size, CS presents the opportunity to achieve shorter scan times than possible with SENSE alone, but with reduced image quality. For maintained scan time, although higher resolution CS incidentally showed a promising increase in NFS bone sharpness compared to SENSE, it does not present an unequivocal advantage for 3D 7-T TSE wrist MRI at this stage. Further optimization of the acquisition and reconstruction process is recommended.

Evaluating Undersampling Schemes and Deep Learning Reconstructions for High-Resolution 3D Double Echo Steady State Knee Imaging at 7 T: A Comparison Between GRAPPA, CAIPIRINHA, and Compressed Sensing

OBJECTIVE

The 3-dimensional (3D) double echo steady state (DESS) magnetic resonance imaging sequence can image knee cartilage with high, isotropic resolution, particularly at high and ultra-high field strengths. Advanced undersampling techniques with high acceleration factors can provide the short acquisition times required for clinical use. However, the optimal undersampling scheme and its limits are unknown.

MATERIALS AND METHODS

High-resolution isotropic (reconstructed voxel size: 0.3 × 0.3 × 0.3 mm3) 3D DESS images of 40 knees in 20 volunteers were acquired at 7 T with varying undersampling factors (R = 4-30) and schemes (regular: GRAPPA, CAIPIRINHA; incoherent: compressed sensing [CS]), whereas the remaining imaging parameters were kept constant. All imaging data were reconstructed with deep learning (DL) algorithms. Three readers rated image quality on a 4-point Likert scale. Four-fold accelerated GRAPPA was used as reference standard. Incidental cartilage lesions were graded on a modified Whole-Organ Magnetic Resonance Imaging Score (WORMS). Friedman's analysis of variance characterized rating differences. The interreader agreement was assessed using κ statistics.

RESULTS

The quality of 16-fold accelerated CS images was not rated significantly different from that of 4-fold accelerated GRAPPA and 8-fold accelerated CAIPIRINHA images, whereas the corresponding data were acquired 4.5 and 2 times faster (01:12 min:s) than in 4-fold accelerated GRAPPA (5:22 min:s) and 8-fold accelerated CAIPIRINHA (2:22 min:s) acquisitions, respectively. Interreader agreement for incidental cartilage lesions was almost perfect for 4-fold accelerated GRAPPA (κ = 0.91), 8-fold accelerated CAIPIRINHA (κ = 0.86), and 8- to 16-fold accelerated CS (κ = 0.91).

CONCLUSIONS

Our results suggest significant advantages of incoherent versus regular undersampling patterns for high-resolution 3D DESS cartilage imaging with high acceleration factors. The combination of CS undersampling with DL reconstruction enables fast, isotropic, high-resolution acquisitions without apparent impairment of image quality. Since DESS specific absorption rate values tend to be moderate, CS DESS with DL reconstruction promises potential for high-resolution assessment of cartilage morphology and other musculoskeletal anatomies at 7 T.

7 T Lumbosacral Plexus Neurography: Feasibility and Comparison of Spinal Nerve Visualization With 3 T MRI

OBJECTIVES

7 T magnetic resonance (MR) imaging can offer superior spatial resolution compared with lower field strengths. However, its use for imaging of the lumbosacral plexus has been constrained by technical challenges and therefore remained relatively unexplored. Therefore, this study investigated the feasibility of 7 T MR neurography by means of comparing the visibility of the spinal nerves and image quality to 3 T MR neurography.

MATERIALS AND METHODS

In this monocentric, institutional review board-approved, prospective study, 30 healthy subjects underwent acquisition time-matched 7 T MR neurography and 3 T MR neurography of the lumbar spine using a 3-dimensional dual-echo steady-state sequence. Visibility of the nerve root, dorsal root ganglia, and spinal nerve fascicles of L1-S1, along with image artifacts and overall image quality, were compared between the different field strengths by 2 radiologists using 4-point Likert scales (1 = poor, 4 = excellent). Comparisons between field strengths were made using the Wilcoxon signed rank test, and interobserver agreement was assessed.

RESULTS

7 T MR neurography enabled significantly improved visualization of the lumbar nerve roots, dorsal root ganglia, and spinal nerve fascicles ( P ≤ 0.002). Compared with 3 T MR neurography, no difference in overall image quality was observed ( P = 0.211), although 7 T MR imaging exhibited significantly increased image artifacts ( P < 0.001). Interobserver agreement (κ) for qualitative measures ranged from 0.71 to 0.88 for 7 T, and from 0.75 to 0.91 for 3 T.

CONCLUSIONS

7 T MR neurography allowed for improved visualization of lumbar spinal nerves, whereas overall image quality was comparable to 3 T MR neurography. This supports the feasibility of 7 T MR neurography of the lumbosacral plexus, even though image artifacts at 7 T were significantly increased.

Morphology of the popliteomeniscal fascicles around the popliteal hiatus on three-dimensional images reconstructed from 7 T magnetic resonance imaging: A cadaveric study

BACKGROUND

This study aimed to clarify the characteristic features of the anteroinferior and posterosuperior popliteomeniscal fascicles (aiPMF and psPMF, respectively) and popliteal hiatus using three-dimensional (3D) reconstructions of 7 T magnetic resonance imaging (MRI) arthrography.

METHODS

Six knees from human cadavers fixed using the Thiel embalming method were examined using 7 T MRI arthrography. 3D Images of the structures around the popliteal hiatus were reconstructed. Morphologies of the psPMF, aiPMF, and popliteal hiatus were investigated and their positional relationships analyzed.

RESULTS

The PMFs attached to the periphery of the lateral meniscus (LM) to form the popliteal hiatus. Each coursed in an oblique direction. The mean length of the psPMF and aiPMF attachments to the LM were 6.8 and 21.6 mm, respectively; mean popliteal hiatus length was 12.8 mm. These lengths corresponded to 7.5%, 24.3%, and 14.5% of the total length of the LM, respectively. The aiPMF was thick near the lateral aspect of the articular capsule and became thinner towards the posteromedial aspect of the LM. The psPMF was thick near the posterior aspect of the articular capsule and became thinner towards the posterolateral aspect of the LM.

CONCLUSION

Morphological properties of the aiPMF, psPMF, their attachments to the LM, and the popliteal hiatus were consistent across the cadaver specimens examined. Each PMF was thin near the popliteal hiatus and became thicker towards its attachment to the articular capsule. These findings may be useful for anatomical repair for the LM hypermobility.

Accelerated High-Resolution Deep Learning Reconstruction Turbo Spin Echo MRI of the Knee at 7 T

OBJECTIVES

The aim of this study was to compare the image quality of 7 T turbo spin echo (TSE) knee images acquired with varying factors of parallel-imaging acceleration reconstructed with deep learning (DL)-based and conventional algorithms.

MATERIALS AND METHODS

This was a prospective single-center study. Twenty-three healthy volunteers underwent 7 T knee magnetic resonance imaging. Two-, 3-, and 4-fold accelerated high-resolution fat-signal-suppressing proton density (PD-fs) and T1-weighted coronal 2D TSE acquisitions with an encoded voxel volume of 0.31 × 0.31 × 1.5 mm 3 were acquired. Each set of raw data was reconstructed with a DL-based and a conventional Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA) algorithm. Three readers rated image contrast, sharpness, artifacts, noise, and overall quality. Friedman analysis of variance and the Wilcoxon signed rank test were used for comparison of image quality criteria.

RESULTS

The mean age of the participants was 32.0 ± 8.1 years (15 male, 8 female). Acquisition times at 4-fold acceleration were 4 minutes 15 seconds (PD-fs, Supplemental Video is available at http://links.lww.com/RLI/A938 ) and 3 minutes 9 seconds (T1, Supplemental Video available at http://links.lww.com/RLI/A939 ). At 4-fold acceleration, image contrast, sharpness, noise, and overall quality of images reconstructed with the DL-based algorithm were significantly better rated than the corresponding GRAPPA reconstructions ( P < 0.001). Four-fold accelerated DL-reconstructed images scored significantly better than 2- to 3-fold GRAPPA-reconstructed images with regards to image contrast, sharpness, noise, and overall quality ( P ≤ 0.031). Image contrast of PD-fs images at 2-fold acceleration ( P = 0.087), image noise of T1-weighted images at 2-fold acceleration ( P = 0.180), and image artifacts for both sequences at 2- and 3-fold acceleration ( P ≥ 0.102) of GRAPPA reconstructions were not rated differently than those of 4-fold accelerated DL-reconstructed images. Furthermore, no significant difference was observed for all image quality measures among 2-fold, 3-fold, and 4-fold accelerated DL reconstructions ( P ≥ 0.082).

CONCLUSIONS

This study explored the technical potential of DL-based image reconstruction in accelerated 2D TSE acquisitions of the knee at 7 T. DL reconstruction significantly improved a variety of image quality measures of high-resolution TSE images acquired with a 4-fold parallel-imaging acceleration compared with a conventional reconstruction algorithm.

7 T MRI of the Cervical Neuroforamen: Assessment of Nerve Root Compression and Dorsal Root Ganglia in Patients With Radiculopathy

OBJECTIVES

The aim of this study was to assess the diagnostic value of 3-dimensional dual-echo steady-state (DESS) magnetic resonance imaging (MRI) of the cervical spine at 7 T compared with 3 T in patients with cervical radiculopathy.

MATERIALS AND METHODS

Patients diagnosed with cervical radiculopathy were prospectively recruited between March 2020 and January 2023 before undergoing surgical decompression and received 3-dimensional DESS imaging at 3 T and 7 T MRI. Cervical nerve root compression and the dimensions of the dorsal root ganglia were assessed by 2 radiologists independently. Signal intensity, visibility of nerve anatomy, diagnostic confidence, and image artifacts were evaluated with Likert scales. The degree of neuroforaminal stenosis was assessed on standard clinical 3 T scans. Statistics included the analysis of the diagnostic accuracy and interreader reliability. The Wilcoxon signed rank test was used to assess differences between the groups.

RESULTS

Forty-eight patients (mean age, 57 ± 12 years; 22 women) were included in the study with the highest prevalence of severe neuroforaminal stenosis observed at C6 (n = 68) followed by C7 (n = 43). Direct evaluation of nerve root compression showed significantly higher diagnostic confidence and visibility of cervical nerve rootlets, roots, and dorsal root ganglia on 7 T DESS than on 3 T DESS (diagnostic confidence: P = 0.01, visibility: P < 0.01). Assessment of nerve root compression using 7 T DESS allowed more sensitive grading than standard clinical MRI ( P < 0.01) and improved the performance in predicting sensory or motor dysfunction (area under the curve combined: 0.87).

CONCLUSIONS

7 T DESS imaging allows direct assessment of cervical nerve root compression in patients with radiculopathy, with a better prediction of sensory or motor dysfunction than standard clinical MRI. Diagnostic confidence and image quality of 7 T DESS were superior to 3 T DESS.

New-Generation 0.55 T MRI of the Knee-Initial Clinical Experience and Comparison With 3 T MRI

OBJECTIVES

The aim of this study was to compare the detection rate of and reader confidence in 0.55 T knee magnetic resonance imaging (MRI) findings with 3 T knee MRI in patients with acute trauma and knee pain.

MATERIALS AND METHODS

In this prospective study, 0.55 T and 3 T knee MRI of 25 symptomatic patients (11 women; median age, 38 years) with suspected internal derangement of the knee was obtained in 1 setting. On the 0.55 T system, a commercially available deep learning image reconstruction algorithm was used (Deep Resolve Gain and Deep Resolve Sharp; Siemens Healthineers), which was not available on the 3 T system. Two board-certified radiologists reviewed all images independently and graded image quality parameters, noted MRI findings and their respective reporting confidence level for the presence or absence, as well as graded the bone, cartilage, meniscus, ligament, and tendon lesions. Image quality and reader confidence levels were compared ( P < 0.05 = significant), and clinical findings were correlated between 0.55 T and 3 T MRI by calculation of the intraclass correlation coefficient (ICC).

RESULTS

Image quality was rated higher at 3 T compared with 0.55 T studies (each P ≤ 0.017). Agreement between 0.55 T and 3 T MRI for the detection and grading of bone marrow edema and fractures, ligament and tendon lesions, high-grade meniscus and cartilage lesions, Baker cysts, and joint effusions was perfect for both readers. Overall identification and grading of cartilage and meniscal lesions showed good agreement between high- and low-field MRI (each ICC > 0.76), with lower agreement for low-grade cartilage (ICC = 0.77) and meniscus lesions (ICC = 0.49). There was no difference in readers' confidence levels for reporting lesions of bone, ligaments, tendons, Baker cysts, and joint effusions between 0.55 T and 3 T (each P > 0.157). Reader reporting confidence was higher for cartilage and meniscal lesions at 3 T (each P < 0.041).

CONCLUSIONS

New-generation 0.55 T knee MRI, with deep learning-aided image reconstruction, allows for reliable detection and grading of joint lesions in symptomatic patients, but it showed limited accuracy and reader confidence for low-grade cartilage and meniscal lesions in comparison with 3 T MRI.

Quantitative Skeletal Imaging and Image-Based Modeling in Pediatric Orthopaedics

PURPOSE OF REVIEW

Musculoskeletal imaging serves a critical role in clinical care and orthopaedic research. Image-based modeling is also gaining traction as a useful tool in understanding skeletal morphology and mechanics. However, there are fewer studies on advanced imaging and modeling in pediatric populations. The purpose of this review is to provide an overview of recent literature on skeletal imaging modalities and modeling techniques with a special emphasis on current and future uses in pediatric research and clinical care.

RECENT FINDINGS

While many principles of imaging and 3D modeling are relevant across the lifespan, there are special considerations for pediatric musculoskeletal imaging and fewer studies of 3D skeletal modeling in pediatric populations. Improved understanding of bone morphology and growth during childhood in healthy and pathologic patients may provide new insight into the pathophysiology of pediatric-onset skeletal diseases and the biomechanics of bone development. Clinical translation of 3D modeling tools developed in orthopaedic research is limited by the requirement for manual image segmentation and the resources needed for segmentation, modeling, and analysis. This paper highlights the current and future uses of common musculoskeletal imaging modalities and 3D modeling techniques in pediatric orthopaedic clinical care and research.

Morphological assessment of cartilage and osteoarthritis in clinical practice and research: Intermediate-weighted fat-suppressed sequences and beyond

Magnetic resonance imaging (MRI) is widely regarded as the primary modality for the morphological assessment of cartilage and all other joint tissues involved in osteoarthritis. 2D fast spin echo fat-suppressed intermediate-weighted (FSE FS IW) sequences with a TE between 30 and 40ms have stood the test of time and are considered the cornerstone of MRI protocols for clinical practice and trials. These sequences offer a good balance between sensitivity and specificity and provide appropriate contrast and signal within the cartilage as well as between cartilage, articular fluid, and subchondral bone. Additionally, FS IW sequences enable the evaluation of menisci, ligaments, synovitis/effusion, and bone marrow edema-like signal changes. This review article provides a rationale for the use of FSE FS IW sequences in the morphological assessment of cartilage and osteoarthritis, along with a brief overview of other clinically available sequences for this indication. Additionally, the article highlights ongoing research efforts aimed at improving FSE FS IW sequences through 3D acquisitions with enhanced resolution, shortened examination times, and exploring the potential benefits of different magnetic field strengths. While most of the literature on cartilage imaging focuses on the knee, the concepts presented here are applicable to all joints. KEY POINTS: 1. MRI is currently considered the modality of reference for a "whole-joint" morphological assessment of osteoarthritis. 2. Fat-suppressed intermediate-weighted sequences remain the keystone of MRI protocols for the assessment of cartilage morphology, as well as other structures involved in osteoarthritis. 3. Trends for further development in the field of cartilage and joint imaging include 3D FSE imaging, faster acquisition including AI-based acceleration, and synthetic imaging providing multi-contrast sequences.

Radiofrequency antenna concepts for human cardiac MR at 14.0 T

OBJECTIVE

To examine the feasibility of human cardiac MR (CMR) at 14.0 T using high-density radiofrequency (RF) dipole transceiver arrays in conjunction with static and dynamic parallel transmission (pTx).

MATERIALS AND METHODS

RF arrays comprised of self-grounded bow-tie (SGBT) antennas, bow-tie (BT) antennas, or fractionated dipole (FD) antennas were used in this simulation study. Static and dynamic pTx were applied to enhance transmission field (B₁⁺) uniformity and efficiency in the heart of the human voxel model. B₁⁺ distribution and maximum specific absorption rate averaged over 10 g tissue (SAR_10g) were examined at 7.0 T and 14.0 T.

RESULTS

At 14.0 T static pTx revealed a minimum B₁⁺_ROI efficiency of 0.91 μT/√kW (SGBT), 0.73 μT/√kW (BT), and 0.56 μT/√kW (FD) and maximum SAR_10g of 4.24 W/kg, 1.45 W/kg, and 2.04 W/kg. Dynamic pTx with 8 kT points indicate a balance between B₁⁺_ROI homogeneity (coefficient of variation < 14%) and efficiency (minimum B₁⁺_ROI > 1.11 µT/√kW) at 14.0 T with a maximum SAR_10g < 5.25 W/kg.

DISCUSSION

MRI of the human heart at 14.0 T is feasible from an electrodynamic and theoretical standpoint, provided that multi-channel high-density antennas are arranged accordingly. These findings provide a technical foundation for further explorations into CMR at 14.0 T.

Scientific Advances and Technical Innovations in Musculoskeletal Radiology

ABSTRACT

Decades of technical innovations have propelled musculoskeletal radiology through an astonishing evolution. New artificial intelligence and deep learning methods capitalize on many past innovations in magnetic resonance imaging (MRI) to reach unprecedented speed, image quality, and new contrasts. Similarly exciting developments in computed tomography (CT) include clinically applicable molecular specificity and substantially improved spatial resolution of musculoskeletal structures and diseases. This special issue of Investigative Radiology comprises a collection of expert summaries and reviews on the most impactful innovations and cutting-edge topics in musculoskeletal radiology, including radiomics and deep learning methods for musculoskeletal disease detection, high-resolution MR neurography, deep learning-driven ultra-fast musculoskeletal MRI, MRI-based synthetic CT, quantitative MRI, modern low-field MRI, 7.0 T MRI, dual-energy CT, cone beam CT, kinematic CT, and synthetic contrast generation in musculoskeletal MRI.