High intensity therapeutic ultrasound transducer performance and characterisation

Potential applications of High Intensity Therapeutic Ultrasound (HITU) are numerous. Some commercial devices already exist to treat for instance prostate cancers or uterine fibroids. Strong technical and clinical research effort is also being made to validate systems and treatment procedures for various organs: for instance, breast, liver, kidney, thyroid and even brain. Many other applications are under investigation or to be explored in the future. This diversity of applications leads to requirements for the transducer which are specific to each application and significantly different from one case to another. In this paper, specifications of HITU transducers are discussed in relation to different conditions of use. The first criterion that determines the design of the transducer is the way chosen to reach the organ to be treated: treatment can be performed from outside the body, through a natural cavity or by interstitial means. The size and geometry of the radiating surface, the acoustic power level and the operation frequency are then specified for each configuration. Starting from these requirements, a dedicated transducer is designed. Different technological solutions are available including piezocomposite technologies widely used in current HITU transducer design. An important point to take into account at this stage is the imaging method used for locating the area to be treated, positioning the transducer and finally monitoring the treatment. Ultrasound Imaging or Magnetic Resonance Imaging is generally chosen in HITU procedures. The HITU transducer has to be compatible with the specified imaging environment. After manufacture, the performance of the transducer is evaluated with respect to expected characteristics. Specific methods have been developed for this purpose such as, for instance, the radiation force balance with a liquid target to evaluate the total output power or acoustic holography to estimate acoustic field parameters. All recent developments in HITU transducer design and characterisation are driven by the needs of HITU procedures to provide a better medical service: quicker treatment, less expensive total procedure including hospitalisation time, safety for the patient, and efficiency evaluated with respect to alternative methods. These developments will be reinforced by the availability of dedicated imaging methods allowing better monitoring of the treatment, but also by the elaboration of standards currently under preparation concerning the characterisation of HITU systems from the acoustic and safety point of view.