07.04.2010
Using a modified setup of the radiation force balance, the primary standard for the realization of the unit W in ultrasound, the output power of High Intensity Therapeutic Ultrasound (HITU) transducers could be measured reliably. In the process, a strongly focusing high power transducer was excited with electrical powers up to 500 W.
Innovative ultrasound therapy procedures for the destruction of tumorous tissue or other surgical applications are continuously spreading in clinical use and have already become a standard procedure included in the catalogue of benefits of statutory health insurance, such as e. g. the treatment of prostate carcinoma. Their application in neurology (for the non-invasive treatment of stroke and brain tumours) is being researched and is currently at the experimental stage. In any event, both now and in the future – in relation to therapy planning and the safety aspect - the measurement of acoustical parameters and the characterization of HITU fields are strongly required. Due to the high intensity levels and peak pressures occurring, the established measurement procedures are not meeting all demands at present.
Consequently the metrology of HITU fields is part of a project supported by the European Union (iMERA-plus programme) entitled External beam cancer therapy (EBCT), in which nine European National Metrological Institutes (NMIs) are involved.
By means of curved ultrasound transducers, the ultrasound waves are strongly focused – like sunlight by a burning glass – leading to intensities up to 10 000 W/cm² in the focal spot. Hence, the tissue in the narrow and spatially limited focal region is heated up to temperatures over 85 °C and necrotized (Fig. 1). The lesions induced in the tissue are shaped like ellipsoids with diameters of 1 to 2 mm and lengths of 10 to 20 mm.
Figure 1: Principle of tumour therapy by means of high intensity, focused ultrasound fields. During treatment the focal spot of the HITU transducer will be – guided by medical imaging devices (ultrasound or MRI) - adjusted to the desired tissue region, thereafter the therapy is undertaken and the result verified. For larger tumours the treatment is accomplished by inducing multiple, overlapping lesions.
In multiple, independent measurements the efficiency of a commercial HITU transducer excited with electrical powers in the range from 100 to 500 W was investigated by means of the modified radiation force balance equipped with an absorbing target. The suspension of the target was adapted to the expected force variations (approx. 0.25 N) and the time course of the on-off cycles was optimized concerning the inevitable target heating. The challenge of power measurements at this level became apparent, considering that when the ultrasound transducer is switched on, the target jumps up visibly and the change of the balance read out is in the gram range, whereas during measurements of diagnostic sound fields, read outs in the micro gram range occur.
As a result of the measurements, a linear relationship between the applied electrical power and the determined acoustical power could be established for all power levels (Fig. 2). The measurement uncertainty was below 5 % (k = 2) and so only slightly higher in comparison to achievable uncertainties for power measurements in diagnostic sound fields in the frequency range from 1 to 3 MHz. Overall it was confirmed that a reliable measurement of acoustical output powers up to 500 W is possible.
Figure 2: Time-averaged total acoustical output power of an HITU transducer (working frequency 1.5 MHz, aperture 100 mm, radius of curvature 100 mm) plotted against the applied electrical power. The inserted picture shows the transducer with the target at the top (distance 5 mm).
Contact person:
Klaus-Vitold Jenderka, FB 1.6, AG 1.62, E-Mail: klaus-vitold.jenderka@ptb.de