Ultrasound sensor for biomedical applications

Abstract

Vesicoureteral reflux (VUR) is an abnormality that causes urine to flow back from the bladder to the kidneys, which may cause renal scarring and kidney damage. A non-ionizing and non-invasive VUR diagnostic technique, based on microwave hyperthermia, is under development. The possibility for improved hyperthermia efficiency and patient safety through implementation of ultrasound transducers is the motivation behind this thesis. The transducers are intended to measure the thickness of the water bolus, which is a cooling devise placed on the patient to avoid skin damage during the heating process.

The main focus of this thesis is on Polyvinylidene fluoride (PVDF) films, which are thin ultrasonic transducers. Basic principles of ultrasound theory and the current microwave antenna are presented. Pressure wave theory and experimental results show that the size and shape of these transducers influence both the transmitted pressure field and the received pulse detectability. Reducing the size of the transducer gives shorter near-field and increased beam width, but these positive properties comes with a trade-off as a smaller size also reduces the sent/received acoustic power. Rectangular transducers are considered for array configurations with transducers dedicated to transmitting or receiving pulses, where the effect of decreased beam spread at corners are negligible. When a transducer operates as both transmitter and receiver, a circular shape is preferable as it is more robust with regard to variations of the angle between the antenna and bolus/skin interface.

The results from experimental work shows that a circular transducer with diameter 2.5 mm fulfills the requirements on beam width, near field length and pulse detectability. Array configuration can not be used as the required beam width is not achievable without compromising pulse detectability. We propose placing three of these circular transducers at the corners of the antenna PCB to reduce their influence on the microwave radiometric measurements. The acoustical backing offered by the microwave antenna is adequate, and the accuracy in the thickness measurements are expected to be $<0.1$ mm with the applied detection method.