Development of standard measurement methods for essential properties of ultrasound therapy equipment

In a European collaborative project, partly funded by the EC Community Bureau of Reference (BCR), reliable methods of measurement for characterising the output and performance of ultrasound physiotherapy equipment have been developed. Experimental investigations using miniature hydrophones to scan the distribution of pressure in therapeutic fields have been undertaken in combination with theoretical simulations of the sound fields. Important parameters such as Beam Cross Sectional Area (BCSA), Effective Radiating Area (ERA) and Beam Nonuniformity Ratio (BNR) (characterising "Hot-spots": potentially harmful to patients) have been redefined, and these new definitions have been incorporated in a revision of IEC 150:1963. The reproducibility and accuracy of measurements of ERA based on these procedures are presented in detail for a variety of therapy fields. Furthermore, it is shown that the value of the BNR for any treatment head should not exceed 8. Values of effective intensity derived using the new procedures are shown to be significantly higher than those obtained using FDA (USA) definitions, a conclusion in agreement with the theoretical expectations. Measurements on four treatment heads were used to validate the procedures of the proposed revised standard. Values of ERA derived by the two laboratories were in agreement to within 2.5%.