Temperature measurements during ultrasound hyperthermia

In this study the ultrasonic field distortions and the temperature measurement errors caused by temperature sensing probes were investigated. It was found that probes with diameters equal to or greater than 1/2 of the square root of the wavelength (lambda) scatter and reflect the waves, and thus distort the field significantly. Smaller probes down to lambda 1/2/5 in diameter had a detectable effect which was very local and, therefore, will probably not have a significant effect on the overall temperature distribution. When the temperature measurement errors were studied, even the smallest probes showed some self-heating artifact and its magnitude depended on the probe size, material, structure, orientation, and the operating frequency. This error was small with most of the nonultrasound absorbing probes (such as bare wires or stainless steel needle probes), but significant with plastic shielded thermocouples. The energy absorption associated with plastic coated probes can be reduced by orienting the probe parallel to the beam, by scanning the focus in such a manner that the focus is not passing on the sensor, and by inserting the probe in a stainless steel catheter. The temperature measurement error can also be reduced by covering the plastic probe with a high- (or low-) acoustical impedance material around the sensor. This would scatter the sound around the probe, thus preventing energy absorption in the plastic. This arrangement can preserve the desirable properties of plastic probes (mainly flexibility, strength, and electrical isolation from the patient) while at the same time allowing fairly absorption artifact free measurements.