This educational reviews the system requirements of a modern ultra high field MR system. It describes magnetic and gradient specifications and discusses in more depth RF challenges and solutions for successful UHF MR imaging.
Magnet specifications
A modern 7T MRI magnetic is an actively shielded superconducting (NbTi) magnet with a total footprint of approximately 4.8m radially and 7.8m axially. The weight has been significantly reduced compared to the first generation passively shielded 7T magnets. The weight drop is important as it allows these modern magnets to be “cold shipped” by airfreight. Modern 7T magnets are equipped with ’zero-boil-off’ technology to reduce helium consumption and life cycle costs. At the moment at various centres worldwide, whole body MR systems with field strengths higher than 7T are explored ranging from 9.4, 10.5 and 11.7 T making use of several magnet technologies [1-4]. Magnetic field distortions, most notably patient induced distortions, will be inherently higher at UHF compared to lower fields. For mitigation 2nd and 3rd order active shimming coils are typically available on 7T systems.
Gradient specifications
Maximum gradient strength at 7T are between 40 and 70 T/m combined with a maximum slew rate of 200 T/m/s. This is significantly lower than maximum gradient strengths of the gradient systems of the newest 3T MR systems such as used in the Human Connectome Project (100 to even 300 T/m) [5-7]. The key reason for the lower gradient strengths at 7T compared to 3T, is that Lorentz forces on the current carrying conductors in the gradient coils become greater at 7T. However, with the application of more sophisticated electro-mechanical design, 7T gradient coils with strengths above 100 T/m should be possible in the near future [8]. Furthermore, head insert gradient coils might be another attractive alternative for brain applications.
RF challenges of UHF.
The shorter RF wavelength at UHF (@ 7T 12 cm in muscle) results leads to constructive and destructive B1+ interferences in the human body. This results in image brightening and shading effects respectively. RF safety at UHF requires special attention as RF induced tissue heating scales approximately quadratically with RF frequency making careful local SAR assessment at UHF pertinent. For 7T body imaging, the B1+ interferences are pronounced. Interestingly, for field strengths beyond 7T, similar RF challenges start to arise in the brain.
A clinical 7T imaging setup
As stated above, the brain and the knee can be imaged with conventional quadrature volume coils albeit with lower radial dimensions. This quadrature transmit coil is for 7T brain imaging combined with a separate 32 channel receiver head array. The two channels are driven by one (sometimes two) RF amplifiers of 4 -7 kW power.
Parallel imaging performance is augmented by the high channel count and the increased RF encoding arising from the shorter RF wavelength. Modern image acceleration techniques such as Simultaneously MultiSlice (SMS) imaging, WAVE CAIPI, 2D CAIPIRINHA exploit this even more by “controlled aliasing” allowing higher image acceleration at a mild signal-to-noise penalty [9]. Consequently, for a modern 7T MR system a dense (≥ 32) multi-element receiver array is essential to attain 3D high resolution brain imaging within acceptable acquisition times. For knee imaging a quadrature transmission and 28 channel receiver array is available. Implants are generally seen as a contra-indication at 7T due to concerns of focused RF heating in the proximity of implant although this might be manageable [10].
Parallel transmission technology
Parallel transmission can address B1+ interferences but increases operational complexity of an UHF experiment and requires dedicated transmit array. Eight channel transmit arrays for head and body applications are available from commercials coil vendors and supported by several 7T MR vendors. In parallel transmission each channel is connected to a separately controlled RF amplifier (1-2 kW). By modulating its phase and amplitude, the excited spatial magnetization pattern can be controlled. For 7T body imaging on-body transmit arrays are widely due to their increased RF efficiency. For imaging deeply situated body region electric dipoles have emerged as the optimal choice as transmit element at 7T [11,12], while reception for the body can best be performed with a receiver consisting of loops and dipoles [13]. Studies have shown that for even higher field strengths, a mix of loops and dipole will also be favorable for the brain [14,15].
RF safety
For parallel transmission technology monitoring of forward and reflected power per channel in the transmit chain is necessary and available on most 7T systems. In addition, local SAR monitoring is pertinent requiring pre-knowledge of the electrical fields per channels by means of electromagnetic simulations for the employed transmit array loaded with a realistic human dielectric model. This increases considerably complexity and requires sufficient MR physics support, e.g. to demonstrate RF safety of these non-certified transmit arrays to obtain IRB approval of in-vivo use in patient studies.
7T technology has matured greatly over the last decade. The smaller footprint allows 7T MR systems to be placed in a regular hospital setting improving clinical adoption. 7T MR vendor should take up the challenge to translate the new generation 3T gradient coils to 7T. RF technology for 7T has been a topic of intense research and consequently has greatly advanced. RF safety for parallel transmit setups and metal implants needs further research. Nevertheless, today we are in the fortunate position that one vendor has released its 7T system (Magnetom Terra, Siemens) for clinical use in brain and knee imaging. This is a great step forward since the introduction of the first 7T system almost twenty years ago.
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