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Working Group 1.62

Metrology of High Intensity Therapeutic Ultrasound (HITU) fields

The abbreviation HITU stands for 'High Intensity Therapeutic Ultrasound'. The terms HIFU ('High Intensity Focused Ultrasound'), FUS ('Focused Ultrasound Surgery') and 'Ultrasound ablation' are also well known. The principle of therapy with the high intensity ultrasound method is that a strongly focused ultrasound field is generated by means of a special transducer and this focus is exactly directed to the region to be treated inside the human body. During treatment it is possible to reach temperatures over 65 °C inside the spatially limited focal spot in just a few seconds, which lead to the destruction (necrosis) of the tissue.

The treated, necrotic tissue region is separated by a sharp and clear border from the surrounding, healthy tissue and is shaped like an ellipsoid of approx. 1 cm length and 1 to 2 mm diameter. Usually the tissue region to be treated is larger than the size of the focal spot, so that - for a complete treatment - the HITU transducer is moved gradually (see figure).

HITU is nowadays applied for the treatment of different tumors in organs like the prostate, the liver and the uterus. Additional applications in treating mamma carcinomas and brain tumors are under research. It is basically true for all HITU procedures, that the exact planning and monitoring of the treatment is strongly required, as local intensities of more than 2000 W/cm² can easily be reached, which is far beyond the intensities used in diagnostic ultrasound. A precondition for appropriate therapy planning and monitoring is the reliable measurement of the total acoustic output power and the sound field distribution of the HITU transducer used.

Although HITU is already in clinical use, the conditions for the application of existing measurement devices in high intensity ultrasound fields have not been completely investigated up to now. This involves especially the durability of the sensor elements and the measurement uncertainty. Thus, Working Group 1.62 is developing methods for the metrology of high intensity ultrasound fields. The starting point is the extension of the measurement range of established methods like the determination of the total acoustic output power by means of the radiation force balance, the measurement of the sound pressure distribution using hydrophones or fibre optic sensors and the spatially resolved measurement of the acoustic intensity with thermo-acoustic sensors.

The research in our department is connected to the EURAMET Joint Research Project (JRP) entitled 'External Beam Cancer Therapy' (EBCT), which is supported by the European Community in the frame of the iMERA programme.

The goals of the research project comprise the measurement of the total acoustic output power in the frequency range from 0.5 MHz to 3.0 MHz up to a power of 500 W with an uncertainty of ± 5 %. An inter-laboratory comparison with European partner laboratories concerning power measurements up to 150 W has already started. Another topic is the development of phantoms for the measurement of temperature distributions caused by HITU applications under clinical equivalent conditions, which serves as a basis for the further development and improvement of models for sound and heat propagation in tissue-mimicking material.

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