Jan Vosshenrich1
1Department of Radiology, University Hospital Basel, Basel, Switzerland
Synopsis
Keywords: Musculoskeletal: Skeletal, Musculoskeletal: Joints, Physics & Engineering: Low-Field MRI
Although the vast majority of musculoskeletal MRI examinations is performed at 1.5 and 3.0T, low-field MRI currently experience a renaissance and offers new opportunities for affordable imaging given lower installation and operational cost. Despite inherently lower signal-to-noise ratios, developments in coil design and image reconstruction techniques improve signal yield and allow to assess musculoskeletal pathologies with high diagnostic confidence. Additionally, imaging of patients with high-susceptibility metallic implants benefits from lower field strengths in terms of reduced metal artifacts. This lecture reviews the technical and economic aspects of low-field musculoskeletal MRI and discusses its applications, challenges and opportunities.
Target Audience
This lecture is designated for both radiologists involved or interested in musculoskeletal imaging and MRI physicists working in MSK sequence development and optimization.Objectives
As a result of attending this lecture, participants should be able to summarize both the most important technical/physical and economical aspects of 0.55T MRI compared to imaging at higher field strengths, and know about the potential applications and challenges of low-field musculoskeletal MRI in clinical practice.Purpose
To provide an overview on the latest developments in low-field MRI, to address its applications in musculoskeletal imaging, and to outline potential challenges compared with imaging at higher field strengths which represents the current standard of practice.Technical and Economic Considerations
The main drawback of MRI at lower field-strengths such as 0.55T is the inherently lower signal-to-noise ratio (SNR). By allowing the bandwidth to vary with field strength, SNR scales with √B0 and is thus about 40% lower at 0.55T compared with 1.5T. Given developments in gradient systems, signal processing and images reconstruction algorithms, SNRs of images acquired on modern low-field scanners systems are however substantially higher than those of early generation low-field MRI systems from the 1980s and 1990s, while simultaneously providing opportunities for relatively fast image acquisition times.
Due to lower total cost of ownership, low-field MRI scanners may allow to expand imaging to geographic regions and patient populations currently lacking access to this technique. Radiology practices considering to purchase a low-field MRI unit may benefit from 40-50% lower acquisition cost, up to 70% lower installation cost and around 45% lower maintenance cost compared with 1.5T or 3.0T scanner systems. Also, as awareness for sustainability in radiology is currently increasing, the lower energy consumption and carbon footprint of low-field MRI may be a factor in the decision process when acquiring a new MRI unit.Applications and Opportunities
The most promising application of low-field musculoskeletal MRI is imaging of orthopedic hardware. Given lower susceptibility artifacts, imaging of metallic implants may benefit substantially from lower field strengths. Several studies have already provided evidence of higher artifact reduction at 0.55T compared with 1.5T, e.g. in hip arthroplasty implants. In terms of image quality, results from recently published literature indicate good to very good image quality of the lumbar spine and diagnostic image quality of the knee at 0.55T in healthy volunteers, especially when deep learning-based image reconstruction algorithms are used for denoising. Despite large comparative studies assessing the diagnostic accuracy of new generation low-field MRI scanners are still pending, the detectability of incidental pathologies in volunteer studies was not different between 0.55T and 1.5T MRI.
Modern low-field MRI systems enable larger bore widths of up to 80-cm diameter, providing an opportunity for imaging of obese and claustrophobic patients and potentially reducing the need for sedation or anesthesia. Finally, these larger bore sizes might also facilitate MRI-guided interventions.Challenges
Even though deep learning-based image reconstruction algorithms and the use of newly developed coil systems improve SNRs of modern low-field MRI systems compared with previous scanner generations, image acquisition times often remain longer than at 1.5T. As a result, imaging studies might be prone to a higher number of motion artifacts and an increased need for repeat imaging. This translates into longer slot times and reduced patient throughput per scanner with potential negative effects on the anticipated economic advantages. From a technical perspective, especially chemical shift selective fat suppression techniques are more challenging at low-field MRI due to smaller differences between fat and water spectral peaks. However, recent experiences provide first evidence that robust and relatively homogenous fat suppression can be achieved with modern low-field MRI systems.Conclusions
Modern low-field MRI currently experiences a renaissance and promises to broaden the access to this technique given substantially lower operational cost. Advances in hardware technology as well as image acquisition and reconstruction techniques render low-field MRI an alternative to imaging at 1.5T or 3.0T for specific musculoskeletal applications. Despite, interesting opportunities of 0.55T MRI for example in metal artifact reduction, there remain challenges, especially with regard to longer acquisition times.Acknowledgements
No acknowledgement found.References
1.) Khodarahmi I, Brinkmann IM, Lin DJ, et al (2022) New-Generation Low-Field Magnetic Resonance Imaging of Hip Arthroplasty Implants Using Slice Encoding for Metal Artifact Correction: First in Vitro Experience at 0.55 T and Comparison with 1.5 T. Invest Radiol 57:517–526. https://doi.org/10.1097/RLI.0000000000000866
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