Ultra-High-Field Musculoskeletal MRI
Stefan Zbyn1
1Program of Advanced Musculoskeletal Imaging (PAMI), Department of Biomedical Engineering, Cleveland Clinic, Cleveland, OH, United States

Synopsis

Keywords: Musculoskeletal: Joints, Physics & Engineering: High-Field MRI

MRI at ultra-high-field (7T and above) offers unique opportunities for improved resolution, contrast, and more accurate biochemical characterization of Musculoskeletal tissues compared to lower field strengths. While some challenges remain to be addressed for more impactful clinical workflows, recent hardware and software developments, as well as advancements in morphological and quantitative MRI techniques for ultra-high-field MR systems have great potential to further increase the relevance of 7T MRI in clinical musculoskeletal research and advance the translation of 7T MRI to clinical arena.

Abstract

The approval of ultra-high-field (UHF) MR systems for clinical use by regulatory authorities1 and the release of 7T MR scanners and RF coils by major MR vendors has increased the number of clinical research studies exploring the potential of musculoskeletal (MSK) MR applications at UHF. Although MSK MRI at 7T has made a great progress in hardware and software development over the last two decades, UHF systems provide a unique set of challenges, but also opportunities, when compared to 1.5T and 3T scanners.2,3 In general, 7T offers an increase in signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), susceptibility effects and spectral resolution. On the other hand, 7T faces challenges such as specific absorption rate (SAR) limitations and non-uniform B1+ transmit fields. It is important to note that different MR applications have unique sets of advantages but also current challenges at 7T. The decision on whether 7T is beneficial or not for a particular clinical research study should therefore be determined by the intended application. Additionally, MSK MRI at 7T is continuously evolving and more applications are becoming feasible as technical challenges are resolved. This presentation will review concepts behind advantages and disadvantages of individual MSK MRI applications at 7T and recent advancements in techniques addressing challenges at UHF.
Morphologic MRI at 7T allows increased spatial resolution for more accurate measurements of cartilage thickness and volume.4,5 Increased susceptibility and SNR at 7T also enables enhanced visualization of bone trabeculae in the wrist joint.6 Adding dielectric pads inside the RF knee coil has shown promising improvement of B1+ homogeneity in the knee at 7T.7
T2 Mapping provides non-invasive information about the biochemical composition of articular cartilage and has been used to assess the risk for the development of osteoarthritis.8 Various techniques designed to overcome SAR limitations and unlock increased SNR at UHF were successfully used for T2 mapping of cartilage at 7T.9,10
T1ρ Mapping has been used for the evaluation of changes in cartilage composition, especially for the detection of early degeneration. Development of methods for reduction of sequence sensitivity to SAR limits, B0 and B1+ inhomogeneities resulted in successful implementation of T1ρ mapping of cartilage at 7T.10,11
T2* Mapping with ultra-short echo time sequences provides insight into biochemical composition of fast relaxing tissues such as tendons, ligaments, bone and menisci. Although T2* values at 7T are shorter compared to 3T, UHF offers increased SNR and improved contrast related to susceptibility differences.
Glycosaminoglycan Chemical Exchange Saturation Transfer (gagCEST) allows quantitative evaluation of proteoglycan content in cartilage and greatly benefits from increased spectral resolution at 7T.12
Sodium Imaging has been used for quantification of proteoglycan content in cartilage13 and for the evaluation of metabolic activity in skeletal muscles14. Sodium (23Na) imaging greatly benefits from increased SNR without experiencing B1+ inhomogeneities at 7T.15
Phosphorus Spectroscopy provides unique information about resting and post-exercise metabolism in skeletal muscles.16 Phosphorus (31P) spectroscopy takes advantage of reduced T1 relaxation times along with increased SNR and spectral resolution at 7T.17
In conclusion, MRI at UHF (7T) offers unique opportunities for improved resolution, contrast, and more accurate biochemical characterization of MSK tissues compared to lower field strengths. While some challenges remain to be addressed for more impactful clinical workflows at UHF, recent and future technical improvements will further increase the relevance of 7T MRI in clinical MSK research and advance the translation of 7T MRI to clinical arena.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)