The measurement of fringing fields in a radio-frequency hyperthermia array with emphasis on bolus size

Abstract

The limited aperture size through which the em-field of the applicator is emanated and the constraining of this em-field near the bolus' edge is related to the appearance of superficial 'hot spot' phenomena in radiative hyperthermia. Regarding systems based on the concept of the annular phased array two questions arise: (1) what is the relative strength of the radial component present in the incident field of the radiators, and (2) in what way are fringing fields related to the bolus size? To address both of the above questions, the spatial distribution of the em-field emanated through the aperture of an applicator of the Amsterdam four waveguide-array system has been investigated for a long bolus and a short bolus. The em vector field emanated by the applicator has been characterized in two perpendicular planes, i.e. the aperture midplane and the sagittal midplane. It should be noted that this distribution depends on the propagation conditions throughout the coupling bolus, the phantom and other volumes attached, such as other applicators. Therefore two sets of propagation conditions have been measured: (1) the minimum number of parameters determining the propagation of the em-field namely one single waveguide, one bolus and a homogeneous phantom, and (2) the propagation conditions as for the clinical setting. It is stressed that the study concerns one specific radiative hyperthermia system, namely the AMC four-waveguide array, but that, based on the similarities discussed above, results may be extrapolated towards other radiative hyperthermia systems. According to the current study, bolus prolongation might lead to a clear clinical improvement, which is due to a decrease of the fringing field amplitude compared to the field amplitude in the centre of the aperture midplane. Bolus prolongation will lead to an extended heating area, the field lines being more aligned to the patient's main axis.

A flexible optimization tool for hyperthermia treatments with RF phased array systems

“…The remaining space between the patient and the applicators is ®lled by the waterboli. Wiersma et al 4 give a schematic illustration of this phased array. The heating distribution in the patient, given in speci®c absorption rate (SAR), i.e.…”

RF hyperthermia array modelling; validation by means of measured EM-field distributions

“…The EM-® eld measurements described by Schneider et al (1995) and Wiersma et al (1998) result in a relative amplitude and a phase diOE erence distribution; the amplitude is given as a fraction of the EM-® eld present in the centre of the system' s aperture mid-plane; the phase is given as a diOE erence to the phase at this location. The centre of the systems aperture mid-plane has been selected by Schneider et al (1995) because of the symmetry present in the`clinical' set-up relative to this point.…”

“…In order to analyse the fringing ® eld eOE ects present in hyperthermia systems, Wiersma et al (1998) characterizes the three dimensional propagation of the EM-® eld in a rectangular phantom (length: 78.5 cm, square cross-section 30 £ 30 cm 2 ) by scanning the EM-® eld, both amplitude and phase, in two planes, i.e. the aperture mid-plane and the sagittal mid-plane.…”

“…Proceeding on this line, the current study presents both a qualitative and quantitative comparison of simulation data and the data measured by Wiersma et al (1998). The simulation data is obtained by the Weak Formulation of the CGFFTmethod.…”

“…In contrast to the more commonplace SAR measurements in heterogeneous phantoms, Schneider et al (1995) presented simultaneous amplitude and phase measurements in the aperture midplane of homogeneous phantoms with a rectangular and elliptical cross section. Wiersma et al (1998) analysed the role of the waterbolus in relation to fringing ® eld eOE ects and, in order to do so amplitude and phase data of the EM-® eld components in both the aperture mid-plane and the sagittal mid-plane of a rectangular phantom (length: 78.5 cm, square cross section of 30 £ 30 cm 2 ) were collected.…”