Synopsis

Ultra-High Field MRI has developed from a plain research instrument for mastering the required technology to a clinically recognised MR imaging field strength. Besides stronger magnets, the entire RF transmit and receive chain needed to be updated to the higher MR frequencies. The mitigation of the transmit B1+ inhomogeneity by means of parallel transmission is now fully understood and utilized in almost every UHF site in some sort or another. Beside the significantly higher resolution achieved with UHF, unique contrast and imaging possibilities opened up: better susceptibility weighting, quantitative susceptibility mapping, CEST imaging and sodium imaging, aiming for better diagnosis in neuro degenerative diseases, tumour imaging, musculoskeletal imaging and much more. All these aspects of UHF imaging will be considered in this educational talk.

Abstract

General

To strive for higher and higher magnetic fields is in the gene pool of MR development. In the mid 1970ties the early MRI systems utilized magnetic fields 0.01 T and lower. In the mid-eighties, when commercial MRI development started, the main field strength ranged from 0.35T - 1.5T, settling in at 1.5T as the field strength mostly used so far. That remained there until the end of the 1990ties. Then 3T started to take off and showed its potential first in neuro imaging in particular fMRI. This MRI field strength matured in the first decade of 2000 and is now a very important MRI field strength with a strong focus in neuro, spine and MSK imaging. All other body part can be imaged also, but some, such as abdominal, MR breast and cardiac imaging, remain mostly at 1.5T. Although the first Ultra High Field (UHF) system had a field strength of 8 Tesla (Ohio State University, Columbus Ohio installed in 1998) later the community settled to 7T (/37/) with CMRR’s first 7T system (University of Minnesota, Minneapolis) in 2000 and remains a strong hold since then. About 60 such human 7T systems are currently running world-wide, with the trend to increase over time and getting frecognized as a tool for clinical and research applications (/44/). A few of these systems (three in total) are head-only systems, however, the vast majority are whole body systems. Figure 1 shows the distribution of MRI field strength from the year 2000 until 2014, indicating that 1.5T stays flat, 3T is increasing and 7T is very small in number of sold units but receiving increased recognition in the field.

Already in the early 2004 the first 9.4T magnet was installed at CMRR, followed by 4 more magnets of that field strength until now. A 10.5T system is operating now since 2013 at CMRR in Minneapolis and produces images of large animals. Human permission is still pending but expected to arrive soon. Eventually higher field strength project evolved with two 11.7T head magnet systems (NIH Bethesda and Seoul, Korea) and one whole body system (Neurospin in Saclay, France). All of them are planned to be installed by end of 2017 or 2018.

In 2016 two independent extreme Ultra-High Field (xUHF) human MRI projects have been initiated aiming for 20 Tesla with an intermediate step at 14 T before reaching out to 20T (/38-40/). Quite impressively over 30 years of serious MRI development the field strength has increased by 4 orders of magnitude. After the FDA in 2008 and the IEC in 2014 classified up to 8T as nonsignificant risk, it is now obvious that 7T will become the first clinically approved UHF system field strength.

Technical challenges

Besides the stronger magnet (which is not part of my considerations here; see also /1/), the move from 3T to 7T involves some serious HW reengineering especially on the RF chain and RF coils. First of all, the MR (Larmor) frequency increases from 128MHz to 300 MHz. That involves a new RF transmit high power chain, including powerful RF Amplifiers. While in conventional MRI at 1.5T and 3T the RF power level is at 10 /35 kW, respectively, at 7T it has reached 8 kW only with a move toward 16 kW for head and joint imaging. However, general body imaging would require significantly more power of about 32 or 64 kW. It has become clear that higher power is desired but at current time not yet easily affordable. The increasing 7T market may change that.

The receive signal chain has to be modified to accept the new 1H MR Frequency and the X-Nuclei MR frequencies (Table 1 shows all relevant nucleus MR frequencies ranging from 7T to 20T). New RF coils need to be designed and build to match that new frequency range (/2/).

Due to dielectric effects B1 homogeneity is no longer ideal, as it is very subject dependent (/3-5/). This is because RF eddy currents in human tissue (having varying dielectric properties and anatomy) are resulting in variations of B1+ in tissue.

In order to correct for it a new concept of parallel/simultaneous RF transmission (pTx) (/ 6-10/) has been introduced (See figure 3 for the general topology). In principle, what has been used for RF transmit in the lower fields, i.e. 1 channel transmit at 1.5T and 2 channel transmit at 3T, is now extended to 8 or more transmit channel (with a clear trend to go higher in channel number, /11-12, 41, 42/) to allow B1 shimming or other higher sophisticated excitation schemes. With it comes the need to supervise and monitor the local SAR (/13-15, 45/) the human subject is exposed to, which was not necessary for the lower MR fields. These technical hurdles are more or less solved now and UHF MRI can achieve similar homogenous B1 distribution as 1.5T and 3T gets. RF coils therefore have to consider multiple transmit channels (typical for 7T is 8 channel, some sites have 16 channel TX. On the receive side 32 RX channels (/43/) are now common for head coils and 28 channels for knee coil for example (similar to lower fields), more RF receive channel will be coming also as UHF utilizes these pRX techniques better than lower fields.

Potential Clinical application at 7T

It has been obvious from the beginning, that 7T allows higher resolution imaging due to the increased SNR (/34/). Up until the early days of 7T imaging the general believe was that SNR increases linearly with B0. In 2015 it was reported that SNR increases supra linear with B0 (/16/, see also Figure 2)). This increased resolution is utilized to perform morphometric measurements in the Hippocampus to explore effects of Alzheimer Disease (AD) inpatients (/46/, /48/).

Particularly for MR Angiography where the prolonged T1 relaxation time (/17/) and hence the better background noise suppression allowes to view vasculature with MR of before unseen resolution (/ 18-19 /,/47/). An example is shown in Figure 4 A showing fine vessel in the lenticular striate vasculature feeding the basal ganglia.

Also, it became clear that the increased susceptibility contrast at 7T (and higher) is creating new contrast images, unseen at lower fields, as the susceptibility effect scales linearly with B0. In principle possible at lower fields too, It opens up new ways for the diagnosis of Multiple Sclerosis, MS, Parkinson’s disease, PD, micro bleeds, etc (/50-52/). Disease progression using susceptibility weighted imaging in MS has been reported (/20/, /49/). When imaging PD patients the 7.0 T images clearly shows the cell groups of the Substantia Nigra and Red Nucleus and therefore the possible damage as a result of PD (/21/). Figure 4 B-C show comparison of 1.5T, 3T and 7T acquisitions of the hippocampus, vain vasculature and lenticulostriate arteries.

T2* related imaging at UHF also spurred the development of Quantitative Susceptibility Mapping, QSM, with the potential for using it in clinical and research applications ranging from iron distributions and metabolic consumption of oxygen to viewing white matter tracts (/22- 23/).

In MR Spectroscopy, besides SNR increase, the sensitivity is also increasing due to the linearly with Bo increasing line spread. In the same category falls Chemical Exchange Saturation Transfer (CEST) (/24-30/). The Z-spectrum can be acquired on a much better detailed basis. Every step above in fields brings its own improvement (see Figure 5, a comparison of Z-Spectrum at 3T, 7T and 9.4T). All these lines in the z-spectrum correspond to metabolic processes of some kind, therefore CEST would allow for metabolic weighted imaging, such as Glucose-CEST (/30/, /35/) which is vital to visualizing tumour metabolism and may add additional information to the well-established FDG PET scanning.

Beside 1H other nuclei can be detected in MR experiments and being utilized for imaging (besides MR spectroscopy). Due to the very week concentration and sensitivity the x-nuclei signal is many orders of magnitude smaller when compared to 1H. The “best” nuclei in this sense is 23Na which was already been imaged at 1.5T in mid-eighties but suffered serious noise. This changed with the move towards 7T and higher (see Figure 5: 23NA SNR comparison 1.5T, 3T, 7T). Reasonable image quality can be achieved in a still long but clinically acceptable scan time of about 20 minutes. However, 23Na provides additional information for research and clinical applications opening up a new use (/31/). For example, in cartilage imaging, tumour imaging and in imaging of diabetes and MS. In MS it has been shown that total Sodium concentration can reflect the state of physical inability (/32/). Other nuclei are also of interest, such as 39K, and have been imaged recently (/33/).

Conclusion

Acknowledgements

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