RF pulses in MRI
In magnetic resonance imaging (MRI) a strong magnetic field (~ 105 times stronger than the Earth's magnetic field) is used to polarize spins; these spins are resonant in the radio frequency (RF) range. By using an RF pulse at the resonance or Larmor frequency it is possible to excite the spins. After the RF pulse the spins will precess around the main magnetic field. This can be measured using the principle of induction. By spatially encoding the spins it is possible to obtain a high-resolution 3D image (< 1 mm3), which can be used for diagnostics and treatment planning.
Absorption of RF power results in local tissue heating The RF pulses pose a safety issue, because their energy will be dissipated (absorbed) in the human body. The dissipated power will lead to tissue heating, and can possibly damage the tissue. The dissipated power is expressed by the SAR (specific absorption rate, W/kg) which is a function of the electric field. The SAR distribution is very heterogeneous, especially at ultra high fields (7 Tesla), which leads to so-called 'hotspots'; small regions with an elevated SAR.
Directly measuring the absorbed power is not possible Ideally, one would like to measure the electric field produced by the RF pulse, which is directly related to the SAR, but unfortunately it is not possible to measure the electric field. Only the magnetic field can be measured. Therefore we use electromagnetic (EM) simulations to determine the electric field and the associated SAR distribution.
There is a need for case-specific safety assessment Nowadays, often only a single position of the human body is considered in the determination of the SAR distribution. In reality however, the position of the body with respect to the RF coil (the antenna which sends the RF pulses) is not fixed. This will lead to another distribution of the 'hot spots'. Therefore we strive for case-specific safety assurance. Which led us to state the following two goals for the project:
- Investigate how position of the human head in a 7 Tesla RF coil influences the SAR distribution.
- Correlate changes in the SAR distribution to the measurable magnetic field and see if it is possible to predict the location (and amplitude) of the SAR peaks based on the measured magnetic field.
Work plan
The project will start with a short training in MR physics and RF safety. The next step will be the implementation of our 7 Tesla RF coil in the EM simulation environment. Then the implementation will be validated by comparing it with measurements of the magnetic field. Hereafter the influence of the head position on the SAR distribution will be investigated in a simulation study. Simultaneously a phantom will be designed, based on simulations, which can be used for validation measurements. Then it will be investigated how we can detect changes in the SAR distribution in the magnetic field. Finally it will be tested if these changes can be seen in MRI measurements.
Work environment The student will be embedded in the MRI research group of the department of Radiotherapy, which works closely together with the high-field MRI group in the department of Radiology. Both groups are internationally oriented.
For this master project we are looking for a motivated student, who has:
- a nearly completed master in (applied) physics, electrical engineering or biomedical engineering
- affinity with performing simulations and experiments and their comparison
- a good knowledge of Matlab
- basic knowledge of MRI (pre)
For more information please contact:
Astrid van Lier (
a.l.h.m.w.vanlier[at]umcutrecht.nl) +31 88 7756037
Dr. Nico van den Berg (
c.a.t.vandenberg[at]umcutrecht.nl) +31 88 7753136