19.03.2012
High-intensity therapeutic ultrasound (HITU) is a non-invasive and non-ionizing therapy method in which locally very high ultrasonic intensities (> 10 kW/cm²) can be produced in the body by focusing an ultrasonic field. These high intensities then induce, for example, necrotization and thus the destruction of tumor tissue. Although the number of applications is constantly increasing, there are hardly any reliable measurement methods for the characterization and evaluation of HITU devices yet.
In one work package of the iMERA+ project “External Beam Cancer Therapy” therefore improved measurement methods for high-intensity ultrasonic fields were developed in a collaboration between four national metrology institutes (PTB, Germany; NPL, UK; INRIM, Italy; UME, Turkey).
As a first step, methods for determining the acoustic output power up to several hundred watts were accomplished in the different laboratories and then successfully validated in an intercomparison. Another focus of the project was concerned with the development of sensors for sound field measurements. A membrane hydrophone with a special protective coating developed at NPL was successfully used to measure peak pressures up to 30 MPa and a fiber-optic displacement sensor developed at PTB was used up to about 50 MPa. Comparative measurements of these two sensors and one other commercially available HITU needle hydrophone, however, showed that further improvement is needed concerning both the durability and the accuracy of the sensors.
A third part of the project concerned the measurement of temperatures in HITU fields that are necessary for reliable therapy monitoring. Measurements using well-known methods (magnetic resonance thermometry, thin-film thermocouples, an infrared camera) showed significant differences among each other. Therefore, a special MR-compatible phantom was developed for the calibration of magnetic resonance thermometry setups that is capable of applying reliable and defined temperature distributions with spatial dimensions (2 mm diameter) and heating rates (about 10 K/s) similar to those occurring in HITU fields. This phantom allows the quick and simple calibration of temperature measurement methods for HITU fields as well as a determination of the respective uncertainties.
In another part of the project a phantom for the determination of the cavitation threshold in HITU fields was developed as well as a prototype sensor for the spatially resolved detection of cavitation events. While the former has already been successfully used for measurements, further improvements of the latter are needed.
Figure 1: Top left: Radiation force balance (RFB) setup at PTB for the measurement of acoustical output power up to 400 W (1: HITU transducer, 2: absorbing target, 3: RFB). Top right: Exemplary results for the measurement of peak pressure in a HITU field with a fiber-optic displacement sensor (red), a membrane hydrophone with protective coating (blue) and a commercial needle hydrophone (green) as well as results from a numerical simulation (black). Bottom left: Prototype MR-compatible phantom for producing defined temperature distributions, photograph overlaid with a color representation of a temperature measurement. Bottom right: Prototype sensor for spatially resolved detection of cavitation events.
Contact person:
Julian Haller, Dept. 1.6, WG 1.62, E-Mail: julian.haller@ptb.de