Influence of patient models and numerical methods on predicted power deposition patterns

A hyperthermia planning system has been developed for generating patient and applicator models as well as calculating and visualizing E-field and temperature distributions. Significant dependencies on models and algorithms have been found.

Computerized tomography (CT) data sets are first transformed into so called 'labelled CT-volume'-data sets of equal resolution, which are used for segmentation. The first type of patient model obtained subsequently is based on regions with specified electrical properties representing tissues or organs (so called 'region-based model'). The second patient model renders a direct transformation of Hounsfield Units (HU) to electrical constants (so called 'HU-based model'). The FDTD-method (finite difference time domain) is then applied on a cubic lattice employing either an auxiliary 'sub-cubic lattice' (for HU-based segmentation) or a tetrahedron grid (for region-based segmentation) to assign the electrical properties, both representing the anatomy of the patient. E-field distributions are corrected by a post-processing procedure with respect to the geometry of interfaces defined by the tetrahedron grid. For comparison, the VSIE method (volume surface integral equation) is performed on the same tetrahedron grid. The applicator model assumes eight half-wavelength dipole antennas fed with constant voltages with water as background medium.

For both numerical methods (FDTD, VSIE) the resulting antenna input impedances as well as the current distributions along the antennas were quite similar and almost insensitive to the particular geometry model (region-based, HU-based). In contrast to that, the power deposition patterns in the interior of the patient depended strongly on those models. Major differences can be related to different labels of the tissue type bone in the HU-based model in comparison to the definition via regions. Conversely, comparable results were obtained using the VSIE method and the FDTD method on the region-based patient model with a posteriori correction at the tetrahedron grid points. SAR (specific absorption rate) elevations up to a factor of 10 were predicted when employing region-based models. Those peaks might correspond to specific toxicity of electromagnetic radiation clinically known as hot spot phenomena or musculo-skeletal syndromes. Conversely, HU-based models generated quite homogeneous power deposition patterns with fluctuations of at most factor 2.

The methods employing region-based geometry models such as the VSIE method and FDTD method in conjunction with a posteriori correction at tissue interfaces result in comparable E-field distributions for regional hyperthermia. Due to its shorter calculation time, the FDTD method is currently used in the clinic. Predictions derived from HU-based models without prior corrections of tissue specifications are not always supported by clinical experience.