With time, the market will be able to determine whether this benefit is sufficient to justify the widespread installation of 7T scanners in hospitals for clinical usage. If this is the case, a new MRI scale will be developed, starting with neuroimaging.
From the installation of the first 7T magnetic resonance imaging system at the University of Minnesota more than 20 years ago, ultra-high field (UHF) MRI has progressively affirmed its role as a powerful technology for human neuroimaging in several research centers worldwide. As of today, there are approximately 90 scanners for human imaging operating at UHF, with growing efforts in the development of next-generation UHF magnets for human imaging, operating at up to 20T.
With the United States’ FDA 510(k) clearance to a 7T clinical system to one manufacturer way back in 2017, it was only in 2020 that another vendor received FDA clearance for a 7T scanner. These approvals, however, set the ground for a transition from pure research applications to research and clinical use of 7T MRI systems.
Moving toward UHF brings not only opportunities, but also considerable challenges for clinical imaging. With the increase of the static (B0) magnetic field strength, MRI physics determine pros and cons for diagnostic applications.
The major benefits of 7T imaging are observed in brain studies, especially for those techniques exploiting magnetic susceptibility phenomena, such as susceptibility-weighted imaging (SWI) and functional MRI (fMRI). Since the magnetic susceptibility affects scale linearly with B0, 7T MRI enables new types of contrasts based on the small difference in susceptibility that are not unveiled at the conventional magnetic field strength. The higher sensitivity to susceptibility has introduced a new frontier for SWI, which is able to create contrasts between tissues containing different amount of paramagnetic or diamagnetic substances, such as iron, deoxyhemoglobin, hemosiderin, myelin, or calcium. SWI at UHF enables high-resolution imaging with unprecedented anatomical detail and related techniques, such as quantitative susceptibility mapping – QSM can supply information about microstructure and composition of brain tissue. Additionally, the higher sensitivity to deoxyhemoglobin, and thus to the blood oxygenation level-dependent (BOLD) effect, improves the detection of the neurovascular coupling at the basis of fMRI, opening the perspective to boost the exploration of functional activation even at the level of single subjects with higher spatiotemporal resolution and sensitivity.
Increased spatial/anatomical resolution fosters new possibilities for the functional study of small structures, including cortical columns and laminae and subnuclei of the brainstem.
A further diagnostic gain could derive from the changes in relaxation times. An example is the improvement in magnetic resonance angiography (MRA) with time-of-flight (TOF) techniques favored by the increased longitudinal relaxation time T1 at 7T promoting the background suppression and enhancing the vessel flow-related signal. In fact, at 7T, the background stationary spins have a longer T1 and they better saturate in the time between the radiofrequency (RF) pulses of the TOF sequence, increasing the contrast of the flowing spins within the peripheral small vessels.
Research and technological advancements are now focused on supporting the gains of UHF and solving potential issues related to the application of UHF MRI to humans. The realization of more powerful gradient coils with greater amplitudes and slew rates is a prerequisite for whole-brain imaging with further increased spatial resolution and reduced geometric distortions. Amelioration of gradients performance would favor, for example, the application of echo-planar-EPI for DWI and fMRI. Higher-order shimming gradients aim to improve the homogeneity of the static magnetic field to fully exploit the UHF potential in improving the spectral resolution of magnetic resonance spectroscopy (MRS).
The translation of UHF to clinical applications implies the absence of dangerous side effects and the demonstration of a diagnostic gain. Since the introduction of UHF in the research environment, tens of thousands of MRI examinations have been performed on humans without reporting additional serious adverse side effects with respect to conventional clinical systems.
Concerning the diagnostic gain, numerous research articles demonstrate the clinical advantage of using 7T systems in diagnosing central nervous system diseases, also in comparative studies with conventional MRI systems. There are several areas of interest in which 7T imaging has been tested and some might be particularly promising in the clinical arena.
The use of UHF MRI has become an important technology to investigate central nervous system involvement in MS and has narrowed the gap between the macroscopic view of the radiologist and the microscopic view of the pathologist, for instance, identifying in vivo the paramagnetic rim sign that may be a marker of compartmentalized inflammation at the lesion edge. The use of 7T MRI also improves the detection of cortical plaques in MS, in particular of those in subpial location. These lesions are not usually evaluated when imaging patients at 1.5T and 3T MRI, but it is well known that this type of cortical pathology contributes to neurological disability and, although more prominent in secondary progressive MS, it is present from the earliest disease phases. Thus, cortical lesion’s detection might have a role in patient monitoring with appropriate protocols. Furthermore, 7T MRI has been demonstrated to be specific in revealing the perivenular distribution of MS plaques and the identification of a venule at the center of MS lesions, the so-called central vein sign. This sign has received great interest for its clinical contribution in the differential diagnosis of white matter pathologies and for its potential in increasing confidence in the radiological diagnosis of MS.
Another area of interest of 7T MRI research is epilepsy. In studying drug-resistant epilepsy, 7T MRI has demonstrated added diagnostic value in revealing epileptogenic lesions. At 1.5T or 3T magnetic field strengths, approximately 60–85 percent of MRI examinations reveal such lesions. Studies have demonstrated that in patients with drug-resistant epilepsy, review of 7T MR images can unveil lesions, which are undetected on images obtained at lower fields.
In the evaluation of neurodegenerative disorders, UHF MRI provides new radiological markers of disease in pathologies with unremarkable conventional MRI examinations at lower field strength.
In Parkinson disease, 7T MR has been used to visualize alterations within the substantia nigra, resulting in loss of the normal appearance of its dorsolateral area, which is accepted as a radiological sign of nigral pathology, useful to increase the diagnostic accuracy compared to conventional MRI systems.
In cerebrovascular diseases, UHF MRI demonstrates its value in improving the identification and characterization of different types of pathology, including microbleeds, leveraging on its superiority in SWI and ischemic lesions, which are often invisible at lower MRI fields, such as cortical microinfarcts. Another emerging field of application of UHF MRI is the imaging of intracranial arteries anatomy and pathology – high-resolution (MRA) allows improved detection of small arterial vessels, such as the lenticulostriate arteries. The assessment of intracranial atherosclerosis favors 7T MRI with respect to 3T MRI, with greater vessel-wall visibility and more lesions detected; UHF MRI opens new frontiers in the imaging of intracranial aneurysms, thanks to the identification of aneurysm wall microstructures not depictable at lower spatial resolutions. The combination of MRI and MRA techniques thus enables the assessment of various aspects of cerebrovascular disease at the level of both brain parenchyma and cerebral vasculature, allowing the visualization of pathological features that are often unrecognized at lower MRI fields.
Even if 1.5T systems are still the most-used scanners, 3T clinical systems are today commonly present in neuroradiology departments. However, in early 2000, some radiologists had opposed the clinical use of 3T, given the burden of artefacts that were complex to overcome and often required the shift toward unfamiliar pulse sequences to achieve better image quality. On the other hand, other radiologists were already appreciating strengths over limitations when using second-generation 3T scanners, equipped with multichannel receiver coils and with parallel imaging capabilities.
The advantages of UHF imaging in neuroscience research are clear and enormous. Without any doubt, there are great potentials for clinical imaging, with applications that can benefit from 7T MRI that are becoming well defined. Nevertheless, the high costs and the complexity in operating these systems might impede their fast widespread installation, and one possible realistic scenario for the next future is that their installation will be confined to a relatively limited number of radiological hubs where UHF studies would be obtained as a complementary examination to 1.5T and 3T studies to provide additional information not achievable at lower fields.
Dr Sudarshan Rawat
HOD & Consultant – Radiology,
Manipal Hospital, Bangalore
“MRI has become increasingly popular in recent times due to its ability to capture detailed pictures of the internal body organs, without using ionizing radiation unlike X-ray or CT scans. In terms of patient comfort, the new MRI includes wide bore space in the front and back that eliminates claustrophobia. Nowadays, MRI scanners use noise-reduction technology that helps people feel more at ease. Advanced MRI scanners allow patients with any painful condition who find it difficult to move their body to be positioned at up to a 90-degree tilt, and capture images as opposed to down with conventional MRI scanners. Recent advances in technology have enabled faster scans, improved image quality, and simplified cardiac imaging workflows. Intraoperative magnetic resonance imaging allows a surgeon to operate with precision and accuracy. It can be used for scanning the smallest part of the body in the finest details. It is very useful for any internal human body-part evaluation, especially in soft tissues, joints, and the nervous system-related pathologies. MRI plays a major role in detection of early cancer and post-treatment follow-up.”
Today, there are mixed expectations with enthusiastic proponents of the clinical use of 7T MRI on the one side and clinicians showing a more conservative approach on the other. The cautious arguments derive from considerations on high costs for installation, more complex maintenance and calibration, need for revision and fine tuning of acquisition protocols, and completion of the technical refinements for whole-brain imaging acquisitions. Moreover, the clinicians reading UHF MRI studies will have to become acquainted with unprecedented contrasts and higher anatomical resolution, and they will have to acquire new skills for the correct interpretation of these outstanding images.
UHF MRI can significantly improve the clinical diagnostic process in selected pathologies. With time, the market will be able to determine whether this benefit is sufficient to justify the widespread installation of 7T scanners in hospitals for clinical usage. If this is the case, a new MRI scale will be developed, starting with neuroimaging.