RF safety

Research participants:
A.L.H.M.W. van Lier, B. van den Bergen, C.A.T. van den Berg, J.J.W. Lagendijk


The electromagnetic interaction of radiofrequency (RF) field with the human body is a complex process governed by the Maxwell equations and the dielectric properties of the human anatomy. This interaction leads to a heterogeneous distribution of electromagnetic heat dissipation. An example is given in figure 1. This phenomenon poses limits to the safe use of RF fields to humans.


Figure 1, local foci in the energy deposition due to the electromagnetic field around the human symphyse.











To study the interaction of RF fields with human subjects we apply a dual approach. At the one hand we are using computational electromagnetic field solvers to obtain fundametal knowlegde on the electromagnetics of RF exposure to humans. On the other hand we are following a more a experimental route to be able to perform in-vivo measurements. For this purpose we are applying MRI.

In MRI the RF transmit fields used to excite hydrogen spins result in an intense RF exposure. As such, MRI is a very relevant study case for RF safety in general. However, the advantage of MRI is that it is able to image the electromagnetic field and the local temperature in the body. See for an example figure 2. As such, MRI is a powerful instrument to study in-vivo the response of the human body to RF exposure.
 
Figure 2. Simulated and measured RF field in the human pelvis. The global pattern in the simulation is also clearly visible in the measurements. However, the fine details visible in the measurements are absent in the simulations due to a too coarse simulation grid.


An example of the electromagnetic impact of fine structures in the anatomy is shown in figure 3, where high resolution simulations of the MRI RF field around the hip joint are compared with RF field measurements. The heterogeneity of the local anatomy is reflected in the electromagnetic field. Sharp gradients in induced current generate local variations in the RF excitation (B1+) field, which can also be measured. This opens up possibilities to quantitatively measure with MRI these foci of electromagnetic energy in real patients.


Figure 3. Using the socalled quasi-stationary zooming technique high resolution computations of the electromagnetic field in sub-volumes are possible. These simulations show that fine structures in the antomy result in local variations of the RF excitation field.