Implications of using thermocouple thermometry in 27 MHz capacitively coupled interstitial hyperthermia

Abstract

The 27 MHz Multi Electrode Current Source (MECS) interstitial hyperthermia system uses segmented electrodes, 10-20 mm long, to steer the 3D power deposition. This power control at a scale of 1-2 cm requires detailed and accurate temperature feedback data. To this end seven-point thermocouples are integrated into the probes. The aim of this work was to evaluate the feasibility and reliability of integrated thermometry in the 27 MHz MECS system, with special attention to the interference between electrode and thermometry and its effect on system performance. We investigated the impact of a seven-sensor thermocouple probe (outer diameter 150 microns) on the apparent impedance and power output of a 20 mm dual electrode (O.D. 1.5 mm) in a polyethylene catheter in a muscle equivalent medium (sigma 1 = 0.6 S m-1). The cross coupling between electrode and thermocouple was found to be small (1-2 pF) and to cause no problems in the dual-electrode mode, and only minimal problems in the single-electrode mode. Power loss into the thermometry system can be prevented using simple filters. The temperature readings are reliable and representative of the actual tissue temperature around the electrode. Self-heating effects, occurring in some catheter materials, are eliminated by sampling the temperature after a short power-off interval. We conclude that integrated thermocouple thermometry is compatible with 27 MHz capacitively coupled interstitial hyperthermia. The performance of the system is not affected and the temperatures measured are a reliable indication of the maximum tissue temperatures.

Comparison of temperature distributions in interstitial hyperthermia: experiments in bovine tongues versus generic simulations

“…In figure 1 schematically. A seven-point thermocouple string (constantan manganin) is included in the electrode (Crezee et al 1997). The duty cycle steering of each electrode is controlled by a reference temperature of the corresponding thermocouple.…”

Quality assurance guidelines for interstitial hyperthermia

“…The impedance associated with the catheter wall far exceeds the tissue impedance, so RF-CC electrodes are less sensitive to heterogeneous tissue properties and parallel alignment, and multielectrode applicators were developed to achieve 3-D control over the spatial power deposition [33,34,42]. Multi-junction thermocouple probes are integrated into the electrodes and are used for treatment monitoring and feedback [72]. Interstitial hot source techniques have not been used extensively, but good temperatures have been reported by several groups [36,41,73].…”

Determination and validation of the actual 3D temperature distribution during interstitial hyperthermia of prostate carcinoma

“…Dual electrodes per catheter and multiple catheters ensure 3D controlled power deposition in the target volume . Online temperature measurements from within the applicators are used to control the 3D temperature distribution (Crezee et al 1997, Kaatee et al 1999.…”