Gerhard Mönich

Simulation studies promote technological development of radiofrequency phased array hyperthermia

A treatment planning program package for radiofrequency hyperthermia has been developed. It consists of software modules for processing three-dimensional computerized tomography (CT) data sets, manual segmentation, generation of tetrahedral grids, numerical calculation and optimisation of three-dimensional Ε field distributions using a volume surface integral equation algorithm as well as temperature distributions using an adaptive multilevel finite-elements code, and graphical tools for simultaneous representation of CT data and simulation results. Heat treatments are limited by hot spots in healthy tissues caused by Ε field maxima at electrical interfaces (bone/muscle). In order to reduce or avoid hot spots suitable objective functions are derived from power deposition patterns and temperature distributions, and are utilised to optimise antenna parameters (phases, amplitudes). The simulation and optimisation tools have been applied to estimate the improvements that could be reached by upgrades of the clinically used SIGMA-60 applicator (consisting of a single ring of four antenna pairs). The investigated upgrades are increased number of antennas and channels (triple-ring of 3 × 8 antennas and variation of antenna inclination. Significant improvement of index temperatures (1–2°C) is achieved by upgrading the single ring to a triple ring with free phase selection for every antenna or antenna pair. Antenna amplitudes and inclinations proved as less important parameters.

Design and test of a new multi-amplifier system with phase and amplitude control

The clinical relevance of the radiofrequency regional hyperthermia (RF-RHT) as an adjuvant cancer therapy grows continuously. Simulation studies for optimization of RF-RHT based on the annular phased array systems have shown a significant improvement of power deposition patterns with increasing number of channels. However, this probably requires higher phase accuracy and amplitude stability than are provided by presently used clinical systems, e.g. BSD-2000. Measurements performed on the BSD-200 electronic revealed phase inaccuracies up to +/- 20 degrees and errors in the power registration of +/- 20 W (up to +/- 50 W in the low power range). These errors are further enhanced by the mismatching of the external load (antenna applicator) and thermal instabilities. To achieve the required phase accuracy and long-term stability in the prototype of a new amplifier system, single-sideband (SSB) mixing in combination with direct digital synthesizers (DDS), in-phase and quadrature-phase (IQ) processing and phase-lock loop (PLL) were used. In the DDSs the actual phase of the output signal of each channel is calculated in real-time. No analogue control loop is involved that may cause thermal offset or drift problems. Each DDS operates at a low intermediate frequency (IF) of 1 MHz. To transform the phase information of this IF signal into the desired RF band, SSB mixing-up is performed. A second frequency source, operating as a local oscillator (LO) in the RF band, is required for this technique. Also, the frequency adjustment of the desired RF signal is performed in the LO. These phase and frequency adjustment units are followed by the high efficiency AB-class solid state amplifier unit. The phase and power level stability of the amplifier are controlled by means of digital PLL structures in conjunction with look-up tables. For this control test signals are coupled out by means of directional couplers. The phase control is based on very sensitive phase comparison. These digital control loops are programmable and allow the implementation of different control algorithms. The achieved long-term accuracy (95% confidence interval) is +/- 1-3 W for output power levels ranging from 10-100 W, and +/- 1 degree for phase differences between each channel and a reference signal at a constant power level, and +/- 1.5 degrees for phase difference values at variable power levels between 10-100 W. In conclusion, the new amplifier system is smaller and more efficient than presently available commercial systems.

Mode suppression in TEM cells

TEM cells are cost efficient and well defined EMC measuring devices. But their bandwidth is limited due to resonances. This article describes methods of mode suppression, especially a new type of absorber-wall absorbers. For the first time to our knowledge it is shown that also already existing cells can be upgraded by simple modification up to the double bandwidth.

Development and evaluation of a three-dimensional hyperthermia applicator with water-coated antennas (WACOA)

A novel twelve-channel three-dimensional (3-D) hyperthermia applicator has been developed and evaluated, which consists of twelve separate WAter COated Antenna (WACOA) modules. The modules are arranged in three transversal antenna rings (sub-arrays) and are placed into an acrylic applicator frame as cartridge-like elements in a staggered arrangement. The operating frequency is 100 MHz. For the design of the applicator, the finite-difference time-domain (FDTD) method was used. The applicators dimensions allow its placement into the gantry of a magnetic resonance (MR) tomograph. The WACOA modules are designed as MR-compatible specially shaped metallic cylindrical dipole structures that are placed into hermetically closed water-filled cassettes. Due to the design of the dipole structures, only a conventional coaxial feed circuitry is needed, and no external impedance matching networks are necessary. Instead, fine on-line impedance matching is realized using adjustable tuning rods and matching rings, both elements being parts of the radiating antenna structure. Experimental and numerical evaluations demonstrate a good stability of impedance matching, a low inter-channel coupling of less than -20 dB, and a good ability of field pattern steering.

Solid materials with high dielectric constants for hyperthermia applications

The manufacture of solid components with high permittivities epsilon r of 1-100 and differing conductivities sigma of 0-1.0 S/m has practical significance for fabricating applicators and phantoms in radiofrequency hyperthermia. For this purpose, various plastics (resins, polyurethane and silicone) were combined with additives (graphite and metal powder) and tested to assess their radiofrequency and mechanical characteristics and to identify manufacturing problems. Most of the plastics could be made highly dielectric and conductive by adding graphite in the range of muscle tissue (i.e. epsilon r approximately 80, sigma approximately 0.8 S/m). However, there are major differences between the materials with respect to mechanical behaviour, durability, feasibility of manufacture, and reproducibility. Manufacturing water-equivalent plastics (low conductivity sigma < 0.05 S/m and epsilon r value of 70-80) is particularly difficult. A less filled polyester resin in which concentration of brass powder can achieve an epsilon r value of up to 100 at low conductivity proved to be the only suitable medium. Such a plastic can be used for future applicator designs. Other materials of interest include plastics equivalent to lossy media (e.g. sigma = 0.45-0.55 S/m, epsilon r = 70-80), fat-equivalent plastics (polyurethane with graphite) and higher dielectric flexible plastics (silicone with brass powder).

On the Interplay Between the Equipment Under Test and TEM Cells

In all applications of the gigahertz transverse electromagnetic (GTEM) cell, there is an interplay between the equipment under test (EUT) and the cell reflecting metal walls, as represented by images of the EUT. Moreover, owing to the multiple reflections within the cell and to its tapering structure, the phenomenon of illumination and reillumination of the EUT can hardly be avoided. These problems, known as reactive effects of the cell, are investigated in this paper. A simple new approach for an efficient investigation of the interactions between the EUT and the cell is provided. Closed-form expressions for the reflection coefficient, the relative deviation in field, and the relative error in the induced current caused by the reactive effect of the cell are derived. Each of these expressions encompasses both the mutual influence of the EUTs radar cross section (RCS) and the reactive effects of the cell. It is shown that depending on the frequency-dependent phase factor ejpsi, strong and weak test conditions may occur since the overall incident field will exhibit maxima and minima. Hence, evidence showing that the total incident field impinging on the EUT generally deviates from the primary excited TEM field is provided. Moreover, the relative deviation in the field and the relative error in the induced current are quantified. Finally, the frequency dependence of the well-known one-third-rule of thumb is demonstrated.

On the role of essential higher order modes in a GTEM cell

The GTEM cell essentially combines the elements of a transmission line and anechoic chamber. Power absorption is therefore a mutual phenomenon between the lumped element resistors and the high frequency absorbers. The different locations of the two absorbers require change in the energy flow. Investigation of this energy flow shows that essential higher order modes play a significant role in the transition between the two regime. In fact, once above the cutoff, higher order TM modes generate eddy regions within the cell. These eddies act as a barrier in the regions where they are located. As a consequence, most of the high frequency energy is linked to the lumped elements which can only absorbs low frequency energy. This results in a poor performance of the hybrid termination of the cell.

Optimization of the GTEM cell wideband termination

The suppression of non-essential modes is investigated in this work upon applying the network analysis. A simplified equivalent circuit model of the cell allowing separation of TEM and TM11 modes is developed. This approach enables the determination of the reflected power, power absorbed in the pyramid absorbers and dissipated in the lumped elements as well as separate evaluation of the attenuation for each mode although there is an energy transfer from one mode to another. The expected poor absorption character of the lumped elements with respect to essential higher order mode and the postulated high midway frequency of cells lined with pyramid absorbers are confirmed. Further, determination of the performance of each element of the circuit permits optimization of the cells parameters, thus enabling the enlargement of the overall usable test volume.

Figure of merit for determining the usable test volume of the GTEM cell

A simple closed-form solution for local ratios between the TEM mode and the higher order TM11 mode at different frequencies is given. This formula can be used as a figure of merit to estimate and/or define the maximum usable test volume during the design phase of the GTEM cell and measurements within the Cell but may also be applied as enlargement strategy when applying performance optimization approaches in the GTEM cell. This is an important tool for both the EMC engineer seeking to use the GTEM cell for testing purposes as well as the cells designers who would like to design a GTEM cell with a maximum possible working volume.

Realization of All-Pass-Networks for Linearizing Log.-Periodic Dipole Antennas

Due to its the logarithmic phase response log.-periodic dipole antennas Are not able radiating short impulses. An all-pass model of the antenna proves this thesis and offers a strategy how to compensate this behaviour at the same time. The compensatory all-passes, which are gained by a mirroring principle, are realized as asymmetric second-order all-pass-networks. The coupling, which is needed by principle for asymmetric structures, is realized by coupled transmission lines. An optimization procedure finally leads to the desired results.