Ultra-high-field Clinical Magnetic Resonance Imaging—Challenges and Excitement
is one of the main forces making UHF MRI one of the fastest growing sectors in the MR marketplace.
The first UHF images of the human brain came from the work of Robitaille at Ohio State University in 1998.12,13
With the initial
demonstration of the feasibility and preliminary clinical applications of UHF MRI at a few academic institutions, including Ohio State University and the University of Minnesota, major MR vendors throughout the medical imaging industry rushed to develop and commercialize UHF whole-body systems, mostly with field strengths of 7 T.10,11
However,
many of the inherent challenges and potential advantages of the UHF MRI, as well as the optimal field strength for a cost-effective clinical system, remain to be determined. Considering that the increased cost of doubling the field strength from the currently popular 1.5 T MRI to the 3 T system is estimated to be about 20–25 %, at present, the cost of UHF MRI is prohibitive for routine clinical applications. For UHF MRI, the cost is estimated at about one million dollars per Tesla, resulting in the cost of a 7 T unit being approximately seven million dollars. This article focuses on the challenges, opportunities, and initial clinical applications of UHF MRI, based partly on our experience with the development of both the currently popular 7 T system and the previous 8 T UHF human whole-body MRI.10,12,13,31–38
Challenges and Opportunities Challenges
The challenges associated with UHF MRI include the inhomogeneity of the main magnetic field (B0) and the transmit field (B1), coil design, increased chemical-shift artifacts, susceptibility artifacts, radiofrequency (RF) energy deposition, and relaxation times.31,39
However, some of these challenges are being overcome, making UHF MRI feasible for many clinical applications.40–46
The superior signal-to-noise ratio (SNR) with UHF MRI is offset by major technical and clinical challenges. Susceptibility artifacts occur near air–tissue interfaces owing to B0 field inhomogeneity, resulting in geometric distortions of the image at those boundaries. Strategies to reduce susceptibility artifacts such as 3D z-shim have been implemented. Susceptibility artifacts are much more significant with UHF MRI and are exponentially increased compared wth conventional MRI near the interface between the soft tissue and the air-containing paranasal sinuses and at the bone–soft tissue interfaces near the brain cortex and in the spinal canal. For example, visualization of the posterior fossa and medial temporal lobe region has been challenging, owing to the exaggerated susceptibility artifacts on UHF MRI, although preliminary reports suggest it is possible to improve imaging in these areas.21,47
Chemical-shift artifacts result from a shift in the spatial location of voxels with different chemical compounds. The spatial location in the frequency-encode direction is assumed to be the consequence of the frequency-encode gradient. Owing to the chemical shift, voxels containing chemical compounds other than water will not have the expected resonance frequency and will be spatially misregistered, causing a shift in the spatial location in the frequency-encode direction. These artifacts appear as a dark (or white) band at the interfaces between water and fat. Chemical-shift artifacts can be reduced by fat-suppression techniques, by swapping the frequency-encode and phase-encode directions, and by using a wider receiver bandwidth.
US RADIOLOGY
Conversely, the increased chemical shift in UHF MRI enhances the ability to resolve chemical peaks in MR spectroscopy.
The specific absorption rate (SAR) is another challenge that UHF MRI has to address. SAR is higher for UHF imaging (proportional to the square of the B0 field) and is particularly limiting for pulse sequences that require multiple RF pulses including fast-spin echo (FSE), fat saturation, and magnetization transfer imaging.7,8,29
New pulse designs, such as
frequency-modulated pulses, have reduced some of these limitations. A challenge of UHF imaging is the development of transmit-and-receive RF coils. B1 inhomogeneity is a limiting factor in coil design and therefore only a few specialized coils are currently commercially available for UHF systems. However, demand for UHF imaging in clinical research has stimulated the development of new coils, and the availability of dedicated coil systems continues to improve.48,49
The much stronger magnetic field and RF power of UHF MRI means that the potential for injury is also increased compared with conventional MRI. Possible injuries include burns from excess heat generation in embedded metallic devices owing to RF exposure and mechanical injury from ferromagnetic implants or metallic foreign bodies. Based on our limited investigations, non-ferromagnetic body piercings, tattoo ink, and silver burn dressings have shown no evidence of harmful heating or mechanical injury during UHF MRI.50
However, further testing is needed
with larger sample sizes, more comprehensive configurations, and more varied metallic contents. In addition, the magnetohydrodynamic effect and human cognition response increases alongside field strength.29,34,50–54 Nausea, vomiting, audio and visual disturbances, and discomfort in the patient’s teeth fillings have been reported. These appear to be transient and no serious or permanent damage has been reported.
The other potential challenge is the prolonged T1 relaxation time in UHF MRI. Although the change in T2 relaxation time with UHF MRI is not great, there is significant prolongation of T1 relaxation time. Therefore, our current understanding and interpretation of T1-weighted images, including the characteristics of normal and pathologic tissue, will have to be redefined. While the increase in the T1 values makes UHF MRI ideal for MR angiography (MRA), it makes the differentiation of gray and white matter in the brain more difficult. In addition, the prolonged T1 values require the adjustment of timing parameters in sequences such as contrast-enhanced MR, inversion recovery, and fat saturation.55
Opportunities
The improved SNR with UHF MRI provides the opportunity to increase spatial or temporal resolution compared with conventional MRI. UHF MRI provides a unique opportunity to non-invasively assess the anatomic and functional properties within the microenvironment of the underlying pathophysiology. Smaller lesions can be detected and functional capabilities within the small areas of the lesions improved, including MR spectroscopy (MRS) and diffusion tensor imaging (DTI). Magnetic susceptibility is much higher with UHF MRI, which can be either disadvantageous or advantageous for clinical applications depending on the desired contrast. For instance, magnetic susceptibility can be exploited to improve lesion detection or delineation by exaggerating or taking advantage of the ‘worsening’ susceptibility artifact associated with pathology such as microhemorrhage or increased deoxyhemoglobin content.
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