Linear thermocouple arrays for in vivo observation of ultrasonic hyperthermia fields

RTOG quality assurance guidelines for clinical trials using hyperthermia administered by ultrasound

Clinical quality assurance guidelines are established for RTOG hyperthermia protocols in which unfocused planar ultrasound may be used to administer hyperthermia. Measurement of temperature at a few fixed points is no longer considered to be adequate. Thermal mapping is required to obtain profiles of the temperature across the tumor dimensions, including margins of normal tissue. The thermometry strategies established for microwaves are to be adhered to with oblique insertion of the probes recommended. Two types of errors arise which are generally not present with microwaves. A measurement error, commonly referred to as a temperature artifact, arises because of absorption and/or viscous heating of the probe. Another error arises when thermocouples are used due to the conduction of heat along the wire leads, especially the copper wire. Several thermometry systems are evaluated with regard to the expected artifact and conduction errors. Acceptable systems include: a) indexing a polyurethane sheathed single sensor thermocouple in a polyurethane catheter, b) indexing a fiberoptic probe in a steel needle, c) indexing a single sensor thermocouple in a steel needle, and d) use of manganin-constantan multisensor thermocouples. Unacceptable systems include: a) fixed or static probes that do not provide profiles of the temperature across the tumor dimensions, b) copper-constantan multisensor thermocouples, and c) teflon sheathed thermocouples inserted into a teflon catheter.

Catheter induced temperature artifacts in ultrasound hyperthermia

Temperature artifacts were evaluated at 72 different sensor locations in 10 different tumour sites heated by use of planar ultrasound transducers operated at 1 and 3 MHz. Thermometry was carried out by single- and multisensor thermocouple probes inserted into 19- and 16-gauge polyurethane catheters, respectively. Nearly all catheters were oriented approximately perpendicular to the ultrasound beam. The artifacts were determined by backward extrapolation of the thermal decay 30-60s after the power was turned off. The effective blood flow and specific absorption rate (SAR) at the sensor locations were determined from the rate of decay and the steady-state temperature. The sample mean steady-state temperature, effective blood flow, and SAR were 41.4 degrees C, 17.5 ml/100 g/min, and 46.3 W/kg, respectively. The most frequent artifact was in the range 0-0.2 degrees C and the mean artifact was 0.6 degrees C. Less than 15% of the artifacts were above 1 degree C. The magnitude of the artifact correlates with the SAR of ultrasonic power, the effective blood flow rate, and the steady-state temperature. These results indicate that the artifact produced at 1 MHz by a multisensor, Teflon-sheathed thermocouple inserted into a 16-gauge polyurethane catheter is 1.7 +/- 0.4 degrees at an SAR of 100 W/kg.

Temperature artifacts produced by thermocouples used in conjunction with 1 and 3 MHz ultrasound

The relative temperature artifacts produced by a selection of commercially available thermocouples and catheters were evaluated in radiation bolus and beef phantoms heated by 1 and 3 MHz continuous ultrasound. The thermocouples included a 23 gauge needle microprobe inserted directly into the phantoms, polyurethane-sheathed, Teflon-sheathed, and exposed-tip thermocouples, each inserted into a 19 gauge polyurethane closed-end catheter, a multisensor Teflon-sheathed probe inserted into a 16 gauge polyurethane catheter and a Teflon-sheathed single-sensor probe without a catheter. The needle microprobe and the polyurethane-sheathed thermocouple produce equivalent artifacts. The artifacts produced by the Teflon and exposed-tip thermocouples are 1.4 +/- 0.3 times greater, that produced by the multisensor Teflon-sheathed probe is 2.1 +/- 0.3 times greater, and that produced by the Teflon-sheathed thermocouple without a catheter is 2.3 +/- 0.4 times greater. The results in the beef phantom indicate that the needle microprobe and polyurethane-sheathed thermocouple both produce artifacts of 0.7 +/- 0.1 degree in tissue at an SAR of 100 W/kg.

Heat delivery and thermometry in clinical hyperthermia

This chapter has discussed some recent technical developments and trends in clinical hyperthermia. Several techniques for the treatment of tumours within 3-4 cm of the body surfaces were described. Each technique has its minor advantages and disadvantages; all techniques employing a single applicator produce temperature distributions with considerable gradients. The introduction of microwave and ultrasound techniques using multiple applicators in which there is some control of the pattern of the energy deposition within the treatment area should improve superficial treatments in this respect. A number of electromagnetic devices for regional hyperthermia are being developed and evaluated. The theoretical predictions of their performances are beginning to suggest restrictions to their use; the limited clinical experience is in general agreement with these predictions. Scanned and focussed ultrasound beams may offer the unique possibility of non-invasive, deep, yet localised hyperthermia in some locations. Such systems are at an early stage of their development; if they prove successful, their controlled and safe use will require detailed information of the temperature distributions produced. Invasive methods for inducing hyperthermia can produce relatively good temperature distributions. The development of 'constant temperature seeds' is promising. Both RF and microwave interstitial systems offering individual control of power to several channels should lead to improved temperature distributions. In general, non-invasive thermometry in clinical hyperthermia remains a distant goal, although developments in microwave radiometry may lead to systems with suitable spatial, temporal and temperature resolutions for use in superficial treatments. Invasive thermometry techniques can provide temperature measurements from several points or from along tracks within the treatment volume. The development of computer models to infer temperature distributions from the limited information available will be a major step in quantifying hyperthermal treatments.

A multiple RF heating system for experimental hyperthermia in small animals

A versatile system is described for locally heating a number of mouse tumours simultaneously using RF capacitive coupling. The system is designed around a single RF power amplifier supplying a number of heating jigs via an efficient isolated power splitter. It is primarily intended for use at 13.56 and 27.12 MHz, but can operate from 2-30 MHz and could be modified for use at other frequencies. Microthermocouples are used for monitoring the intra-tissue temperature quasi-continuously, by making temperature measurements every 220 ms within 20 ms periods during which the RF power is turned off to all the jigs. This method avoids any artefacts in the temperature measurement which are associated with electromagnetic interference. Thermostatic regulation of tissue temperature is provided by on-off control of the average power supplied independently to each heating jig.