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RF fields and Safety in UHF body MRI

Ultra-High Field MR Imaging (UHF-MRI) operating at field strengths of 7 Tesla and beyond offers various advantages such as a higher signal-to-noise ratio (SNR), higher spectral resolution, or in many cases a stronger contrast between different tissues or between tissues and vessels. At the same time, various problems occur in UHF-MRI, which are mostly linked to the higher frequency of the electromagnetic (EM) radio frequency (RF) pulses.

The frequency of the RF pulses increases linearly with the field strength, which shortens the wavelength to about 11 cm in human tissue at 7 Tesla, which is comparable to dimensions of the human head. This leads to spatially varying amplitudes and phases of the transverse magnetic (B1+) and electrical (E) components of the RF excitation field. The inhomogeneous B1+ field generates spatially varying flip angles and thus unwanted inhomogeneous image contrast. Complete cancelation of the signal may occur, particularly in the human body, as shown in the Figure 1. At the same time, the heterogeneous E field may lead to a locally varying specific absorption rate (SAR), which is a safety-relevant parameter.

Fig. 1: Cardiac gradient echo cine images obtained at 7 Tesla before (left) and after (right) B1+ shimming.

To avoid a heterogeneous flip angle distribution and high SAR values, multi-channel transmit (Tx) coils can be used in combination with dedicated parallel transmission (pTx) methods such as static and dynamic pTx. However, when such pTx methods are applied to the human body, the physiological variations by respiratory and cardiac motion need to be considered, which affect not only the resulting flip angle patterns but also the underlying E and B1+ field. Therefore, one focus of the group lies on measuring the B1+ fields within the human body and estimating the corresponding E-fields using EM simulations. In both cases respiratory motion is considered.

Measuring the B1+ fields in the body at 7 Tesla

The magnetic field component B1+ of the RF pulse is responsible for exciting the nuclear spins and, thus, its spatial variations directly result in variations of the signal amplitude and contrast generation in the MRI. While measuring or mapping the B1+ field in the brain at 7 Tesla is already established, the same techniques applied to the body often fail due to high power requirements, sensitivity to blood flow, and particularly due to long scan times that prohibit breath-hold scans. Thus, mapping the B1+ field in the body is highly challenging and the group investigates different methods to measure the B1+ field in vivo within the human body at 7 Tesla. A recent work by the group made it possible to estimate the three-dimensional B1+ field maps of individual elements of a multi-channel Tx coil as a function of the respiratory cycle as shown in Figure 2. This allows generating RF pulses either for one respiratory state (inhale or exhale) or RF pulses that are insensitive to respiratory motion. In addition, the technique enabled the investigation of calibration-free pulses that are outlined in the RF pulse design section (here link). The group furthermore investigates methods for improving the precision and accuracy of B1+ maps.

Fig. 2: 3D B1+ distribution as a function of the respiratory cycle for the 8 different Tx channels of the applied pTx body coil.

Estimating the E-Field and SAR

The E field provides insight into safety relevant quantities, such as the SAR and the temperature increase caused by RF application. The SAR quantifies the amount of energy deposited within the body per time and thus is linked to the temperature increase of the body. Higher fields lead to shorter RF wavelengths, which typically leads to a more localized deposition of the RF energy which generates a more localized heating that is often termed as "hot spot". Unlike the B1+ field, the E field is not accessible via direct measurements in-vivo, therefore the resulting E-field distributions are assessed via EM simulations. Such simulations include a model of the human body as well as an EM model of the RF coil.

Fig. 3: EM simulation setup including a body model and an RF coil model using the software package Sim4Life (ZMT Zurich MedTech AG, Zurich, Switzerland).

Recent work within the group [4,5] investigates the SAR in EM simulations for different respiratory positions within the respiratory cycle showing fluctuation of the hot spot locations and variations of the SAR amplitude over the respiratory cycle. Such simulations are beneficial for estimating the risks and setting the RF power limits for in-vivo body applications at UHF.


  1. Dietrich S, Aigner CS, Kolbitsch C, Mayer J, Ludwig J, Schmidt S, Schaeffter T, Schmitter S. 3D Free-breathing multichannel absolute B1+ Mapping in the human body at 7T. Magnetic resonance in medicine 2021;85:2552–67 doi: 10.1002/mrm.28602.
  2. Ladd ME, Bachert P, Meyerspeer M, Moser M, Nagel AM, Norris, DG, Schmitter S, Speck O, Straub S, Zaiss M. Pros and cons of ultra-high-field MRI/MRS for human application. Progress in nuclear magnetic resonance spectroscopy 2018;109:1–50 doi: 10.1016/j.pnmrs.2018.06.001.
  3. Schmitter S, Wu X, Ugurbil K, van de Moortele P-F. Design of parallel transmission radiofrequency pulses robust against respiration in cardiac MRI at 7 Tesla. Magn Reson Med 2015;74:1291–1305 doi: 10.1002/mrm.25512.
  4. Schön N, Petzold J, Seifert F, Aigner CS, Metzger G, Ittermann B, Schmitter S. Impact of respiration on B1+ field and SAR distribution at 7 T using a novel EM simulation setup. In: Proc. Intl. Soc. for Magn. Reson. Med. 29; Abstract 1120: 2020.
  5. Schön N, Seifert F, Metzger G, Ittermann B, Schmitter S. Investigation of respiration-induced changes of the scattering matrix by EM simulations and a breathing body model. In: Proc. Intl. Soc. for Magn. Reson. Med. 30; Abstract 3342: 2021.