H Fähling — Page 2 | PubMed Health

PubMedDate not supplied

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).

Electric Conductivity↗

PubMedDate not supplied

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 DDS's 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.

Electric Power Supplies↗