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.