Magnetic resonance imaging (MRI) at fields strengths ranging up to 3T is a widely diffused imaging exam with a well-recognized clinical value. Compared to computed tomography or ultrasound, MRI is a multiparametric imaging technique and provides higher contrast, particularly useful when evaluating soft tissues in the abdomen or in the musculoskeletal imaging.
The push to explore increasing static field is the quest for a higher signal-to-noise ratio, contrast-to-noise ratio (CNR), spectral resolution, and time-resolution [1]. An increased relative contrast between tissues and higher CNR at 3T can improve lesion conspicuity and contrast-enhanced evaluation of solid organs, MR angiography, MR cholangiopancreatography, diffusion-weighted imaging and MR spectroscopy.
However, it is not always clear whether advanced image quality with higher contrast leads to higher diagnostic accuracy. Moreover, many of these pros are frequently challenging to achieve and do not come without cons. Indeed, the higher static field leads to MR signal changes in the different tissues as well as technical issues such as magnetic fields inhomogeneity, specific absorption rate issues, and artifacts. These difficulties have been overcome by adjusting the sequence parameters and through the continuing technologic developments, such as multi-channel transmit/receive radiofrequency body coil or B₁ shimming. However, MRI of the abdomen imaging is still challenging, and this is particularly true at higher field [2]. When investigating the applications of MRI for body imaging, we have to deal with many challenges such as respiratory and bowel motion. Brain MRI employs long sequences with high CNR, which are problematic for body acquisitions with suspended respiration.
Another relevant issue of body MRI is the heightened artifacts due to the increased susceptibility at the interfaces between soft tissues and air in the bowel or the lungs. Musculoskeletal MRI lacks many of limitations of body MRI, providing important diagnostic improvements in musculoskeletal imaging. Indeed, the higher SNR and CNR allow achieving improved anatomic and pathologic details of musculoskeletal structures together with notable advancements in biochemical and metabolic tissues characterization.
All the above-mentioned challenges are furtherly augmented with ultra-high field (UHF) MRI with static field higher than 3T.
To make UHF-MRI valuable from a clinical point of view, the benefits must outweigh the risk and disadvantages.
When addressing risk and disadvantages, we must mention the reported transient side effects due to exposure to UHF, including vertigo, nausea, headache, sleepiness, numbness, muscle twitches [3].
However, even at 10.5T human exposure, side effects compromising patients' safety were not observed [4].
UHF-MRI needs a different approach compared to 1.5T and 3T, as conventional sequences may result inadequate at UHF due to challenging technical issues [5].
For, instance, on UHF the precessional frequencies of protons belonging to water and fat tissues increase and the time interval between in-phase and out-of-phase gets increasingly shorter, making it extremely difficult to implement dual-echo sequences on UHF-MRI. Alternatively, UHF-MRI may evaluate fat tissue through spectroscopic sequences, allowing its quantification and more precise differential diagnosis in the oncologic field.
Given the not entirely overcome difficulties of UHF-MRI in the study of the abdomen, the major results to date have been obtained in the MSK field.
The main application is the cartilage study, not only concerning its morphologic study but also compositional imaging ranging from T2 mapping, T2* mapping, T1 rho mapping to sodium imaging [6]. Despite the achieved results, all of the authors pretty much highlight the need for sequences optimization and dedicated coils.
It is important to associate an objective evaluation of the clinical implication of adopting UHF-MRI when comparing it to traditional 1.5T and 3T. For instance, Springer et al. demonstrated an improvement in diagnostic confidence at 7T compared to 3T [7].
Other UHF MRI applications in MSK imaging regarded the bone studies, focusing on trabecular bone quantitative assessment in osteoporotic patients, ligaments, and tendons, better depicted at UHF, and menisci and intervertebral discs.

Acknowledgements

No acknowledgement found.

References

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