An ultrasonic imaging catheter is disclosed that includes a thermistor mounted on the catheter in the vicinity of the ultrasound transducer for monitoring the temperature in the vicinity thereof. The electrical signal from the thermistor can be used to control the output of the ultrasound transducer to thereby control the temperature of the same. In a further aspect of the invention, an isolation box is provided that is external to the ultrasound machine itself and is connected to the same by a cable. Ideally, the isolation box which houses a sufficient number of isolation transformers, is small, so it can be placed easily on or near the patient's bed.