Best Practice Guide to Measurement of Acoustic Output Power (Introduction)
Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading
Introduction
These pages are designed for hospital physicists and industrial manufacturers of medical ultrasonic equipment and aim to provide practical guidance on all aspects of measuring the output power of such equipment using a radiation force balance.
What is Acoustic Power?
Power is a convenient parameter to measure because many measurement systems are, more or less, portable. The powers from medical ultrasonic equipment range from microwatts, for diagnostic equipment, to several watts for physiotherapy equipment, up to hundreds of watts for surgical or therapeutic applications. There are various reasons for making power measurements but in general they are related to assessment of performance or safety of equipment. For therapy equipment, where continuous-wave or long toneburst transducer excitation is normally used, it is important to know the total power produced by a system for several reasons:
- To ensure the most effective exposure levels are used during patient treatment
- To ensure exposure does not take place at levels which can be harmful to healthy tissue
- To allow consistent application and intercomparison of treatment regimes
- To ascertain whether the equipment is performing satisfactorily
For diagnostic equipment, where short time-duration pulses of ultrasound are used, power levels are usually low (except for Doppler equipment) which means it is often more important to know the spatial distribution of intensity and pressure. Nevertheless, power remains a useful characterising parameter even at these lower levels of exposure as it can serve as a means of checking consistency when combined with a range of other field quantities such as temporal-average intensity and peak-negative acoustic pressure.
Power information is also required when deriving values for thermal indices, which estimate the likely elevation of tissue temperature during imaging, and which are commonly displayed on-screen on modern diagnostic scanners. Being a simple parameter to measure, power can therefore be a useful means of periodic performance evaluation.