Editorial for special issue on ultra-high field MRI

Abstract

Ultra-high field (UHF) MRI has become a main trend of MR research in the past few decades, which is driven by the human ambition to explore the frontier of in vivo imaging of human body with ever greater spatial and temporal resolutions. The signal-to-noise ratio (SNR) has a superlinear relationship with the main magnetic field strength characterized as SNR ∝ B0^1.65 [1]. In addition, the increased sensitivity to susceptibility effects and other contrasts at UHF makes it appealing to perform functional MRI as well as other MRI modalities to reveal mesoscopic structures and functions of human brain and body organs. Traditionally, UHF refers to a main magnetic field equal to or greater than 7T. Currently, there are approximately 130 7T MRI systems in the world, some of them have received US FDA and EU regulatory approval since 2017 and are being used clinically for neuroimaging and musculoskeletal imaging. There are several research UHF MR systems beyond 7T such as the 9.4T system at Max Planck Institute, 10.5T at the Center for MR Research, University of Minnesota, 11.7T at Neurospin (CEA Paris-Saclay), and the 14T whole body system being developed in the Netherlands. A second 10.5T whole body MR system is slated to be installed in Hefei, China. This global booming trend of UHF systems echoes the slogan for the Olympic Games—“Faster, Higher, Stronger—Together”.

However, with increasing field strength the frequency of radiofrequency (RF) pulses or B1 field also increases proportionally, resulting in shortened RF wavelength (52 cm at 1.5T, 26 cm at 3T, and 11 cm at 7T) [2]. This will lead to image inhomogeneities when the size of the imaged object is comparable to or greater than the wavelength (e.g., abdominal imaging at 3T and above, brain imaging at 7T, and beyond). In addition, the specific absorption rate (SAR) of RF power also increases with higher RF frequencies or shortened RF wavelengths at UHF. Furthermore, local SAR need to be estimated based on the accurate geometry of imaged object at UHF, which remains challenging especially with parallel RF transmission (pTx) to improve the B1 field homogeneity. To date, 7T MR systems were only approved for clinical neuro and musculoskeletal imaging, while imaging of other body organs remains for research purpose.

During the past 5 years, the 5T whole body MR system has been introduced and received US FDA approval for clinical use in 2024. 5T fills in the gap between the clinical field strength of 3T and UHF of 7T. It is equipped with a parallel RF transmission body coil that allows whole body clinical MRI with adequate image homogeneity and quality within the SAR limit of RF power. A few clinical evaluation studies have shown comparable MRA and MRI image quality and clinical value between 5 and 7T [3]. It is expected that 5T and UHF of 7T and beyond will continue to grow worldwide in the coming decade (Figure 1).

This special issue of high and UHF MRI includes 5 latest studies, of which 3 were performed at 7T and the rest 2 at 5T. Gokyar et al. [4] presented a novel three-dimensional surface coil (3D Coil) architecture that offers increased depth penetration and SNR compared to the single channel surface coil for parotid gland imaging at 7T. They further developed a deep learning based noise reduction method that receives inputs from three elements of the 3D coil to improve SNR. Nie et al. [5] provided an overview of advancements in diffusion imaging at 7T by investigating whether 7T diffusion imaging offers significant benefits over lower field strengths. A comparative analysis between 3 and 7T systems demonstrates significant improvements in SNR and spatial resolution at 7T with a powerful gradient system, facilitating enhanced visualization of microstructural changes. Despite greater geometric distortions and signal inhomogeneity at 7T, the system shows clear advantages in high b-value imaging and high-resolution diffusion tensor imaging with promising applications of 7T diffusion imaging in structural analysis and disease characterization.

In summary, this special issue of 5 studies demonstrated excellent potential for brain MRI at high and UHF strengths of 5 and 7T, especially in conjunction with deep learning based methods for image reconstruction, denoising, and classification. It remains to be seen more developed and clinical translation of body MRI at 5 and 7T.

Danny J. J. Wang draft the manuscript.

The authors declare that they have no conflicts of interest. If authors are from the editorial board of iRADIOLOGY, they will be excluded from the peer-review process and all editorial decisions related to the publication of this article.

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