Best Practice Guide to Measurement of Acoustic Output Power (Additional Information)
Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading
Other measurement methods
Although the radiation force method has been described in depth there are several more cost-effective methods available that can give a user an indication of the output of a transducer with varying levels of accuracy.
Power meter
The power meter, developed at NPL, utilises the pyro-electric effect of a thin layer of piezo-electric polymer membrane such as polyvinylidene fluoride (pvdf) to determine the acoustic output power.The membrane is backed with a polymeric material which is highly absorbing to ultrasound. Ultrasonic energy absorbed by the backing material generates a rapid temperature rise when the transducer is first switched on, leading to a pyro-electric voltage across electrodes of the pvdf film.
The initial peak in the voltage generated when the transducer is switched on is proportional to power applied. This method has been developed initially for the therapy level measurement but has potential for use for a wider range of ultrasound applications from diagnostic through to HIFU. Studies into the potential of this technique are ongoing at NPL.
Calorimetry
Calorimetry is the science of measuring heat in either a chemical reaction or a physically changing system. When a transducer is energised an amount of energy will be deposited in the water from the ultrasound wave and from self-heating of the transducer itself. The resultant temperature rise in the water can be an indicator as to the level of acoustic power emitted.
A simple method, devised by Dr. Stephen Pye at Western General Hospital, Edinburgh, compares the temperature in beakers of water with nominally the same volume. A standard measurement procedure is followed where the transducer is insonated for a fixed period, for example 60 seconds, and at a fixed intensity, typically 1.0-1.5 W cm2. At the end of the insonation period the treatment head is removed and a digital thermometer is used to measure the difference in temperature between the water in the two containers. The temperature difference is noted, and used as a test of consistency at that output setting. Temperature differences are in the range 1.5-4.0 °C and, typically, the reproducibility of the test is better than ±10%.
Note: this method does not measure power but can be applied as a consistency check for transducers.
EMS budget indicator
The budget indicator operates using a vain that deflects a distance proportional to the force applied to it. The indicator is supplied with a graduated scale on the side which is labelled in units of Watts or Watts per square cm. For the purpose of using as a monitor, the units should be disregarded and results recorded just as a number. Close attention should be paid to the instructions supplied with the unit, especially in terms of positioning the treatment head, degassing the water and filling the balance to let the absorbers soak for several minutes before use. The most satisfactory results are obtained with large treatment heads running at frequencies of 3 MHz and above. These can generate sufficient power to produce a large vane deflection without inducing significant cavitation in the water. When the balance has been prepared and set up for use, the lowest frequency heads should be tested first before the water begins to absorb appreciable amounts of gas from the atmosphere. The velocity of sound in water (and the force exerted on the balance target) varies by approximately 0.2% °C-1: water temperatures in the range 15 - 25° C are thus quite acceptable. Each treatment frequency needs to be tested separately, and the reading on the balance at a fixed CW output power should be noted. (Appropriate settings may be 0.5 or 1.0 W cm-2 for large treatment heads, and 1.5 to 2.0 W cm-2 for small treatment heads.) Any variation in the reading for a particular head of more than ± 20-25% should trigger the user to request a more detailed calibration so as to establish whether a fault is present.
References:
- Pye, SD and Milford, C (1994). The performance of ultrasound therapy machines in Lothian region, 1992. Ultrasound in Medicine and Biology, 20, 4, 347-359.