An experimental and theoretical study of ultrasound fields with reference to their use in physiotherapy and hyperthermia
Chapter 1, an introduction to the use of ultrasound in physiotherapy and in hyperthermia is given. Previous studies in ultrasound physiotherapy and hyperthermia are reviewed but the emphasis is given to those which investigated the mechanisms responsible for the beneficial effects of therapeutic ultrasound and for the efficacy of cell killing of ultrasound hyperthermia. A review of the studies of the non-thermal effects of ultrasound is the subject of Chapter 2. Some recent studies carried out in this field in our laboratory are included. Knowledge of acoustical power, spatial peak and spatial average intensities are central to useful studies of bioeffects and to patient safety in the cases of therapy and clinical hyperthermia. Some techniques for measurements of ultrasound dosimetry which have been employed (and studied) are outlined in Chapter 3. At present the total power output from a therapeutic transducer is often derived by measuring the radiation force of the ultrasonic beam exerting on either a total absorber or a total reflector suspended at 45 to vertical. In Chapter 4 and Chapter 5, I discuss some new problems of these techniques and the magnitude of errors which might be caused. Chapter 6 and Chapter 7 present the results of theoretical studies of the field distri-butions from a plane circular piston transducer under both progressive and standing wave conditions. The effect of wave diffraction on the field distribution, bulk streaming and mea-surement of radiation force using a plane reflector is discussed. The ratio of the spatial peak to spatial average intensity for plane circular transducers in a progressive field is theoretic-cally calculated and compared with experimental results. The question of how to define the standing wave ratio is considered by taking into account of the effect of wave diffraction. Chapter 8 and Chapter 9 concern the thermal distributions generated by ultrasound in tissues. An experimental study of the temperature distributions in tissue/bone phantoms induced by therapeutic ultrasound using infra-red technique is described in Chapter 8. In Chapter 9 the thermal patterns generated by a plane circular transducer in a 3-D tissue model were calculated and they were compared with the experimental results described in the previous chapter. The thermal distributions generated by an applicator consisting of five divergent transducers were simulated in the same tissue model. Possible advantages of using such an applicator for ultrasound therapy and for treating large superficial tumours are discussed.