An Ultrasonic Radiation Calorimeter

Abstract

This paper describes a constant temperature environment calorimeter for the absolute measurement of ultrasonic power. This design uses a variation of the classical calorimetric procedure of power measurement as applied specifically to ultrasonic transducers. This measurement technique has advantages over other methods in that it is the least affected by beam shape and pulse duration. The calorimeter consists of a conical shaped calorimeter cup mounted in an outer brass cylindrical container. The calorimeter is placed in a water bath, which provides both a constant temperature environment and an effective medium for the propagation of ultrasound. The calorimeter cup is filled with Dow Corning 710 silicon fluid, which is an effective absorption medium. It has a 112-in. entrance window of gold coated nylon separating the silicon fluid from the water bath. The gold film is used as a resistance heater to maintain quasi-adiabatic conditions between the silicon absorbing fluid and the water environment at the entrance membrane. The temperature difference between the silicon absorbing fluid and the outer brass cylinder is measured with a bank of chromel-constantan thermocouples. This measured temperature difference is a measure of the ultrasonic power. Calibration of the system is accomplished with a constant resistance heating coil immersed in the Dow Corning 710 absorbing fluid.

A Buoyancy Method for the Measurement of Total Ultrasound Power Generated by HIFU Transducers

“…The use of multiple thermal sensors can cater better for nonuniform heating distributions provided that the number of sensors is adequate and that their presence does not affect the absorption process. It has been proposed previously for the measurement of ultrasound power or intensity (Wells 1963;Mikhailov 1964;Lloyd 1967;van den Ende 1969;Herman and Stewart, 1973;Zapf et al 1976;Torr and Watmough 1977;Delchar and Melvin, 1994). To do this in a HIFU field where the Ϫ6 dB focal diameter may be 1 mm or less would require an extremely high sensor density.…”

Calibration and measurement issues for therapeutic ultrasound

“…Calorimetry. The absorption of ultrasound in water allows the classical power determination technique of calorimetry to be used [45][46][47]. Provided that care is taken to minimise ancillary heat sources and losses, and to ensure that appropriate acoustical and thermal coupling is achieved between the source and the measurement device, measurements can be obtained at therapy-level powers that show good agreement with absorbing and reflecting target radiation force balance data: see Table 2 (after [48]).…”

Quality assurance for clinical high intensity focused ultrasound fields

“…If this momentum is absorbed or reflected, a net force will be applied on a target in the beam's path [30,32,[45][46][47][48]. Calorimetric methods require the complete absorption of the energy carried by an ultrasound beam and its conversion to heat [49][50], but these have traditionally been considered difficult, mainly due to the challenge of accurately measuring the extent of heating, but also in terms of limiting thermal losses. A recent development is the buoyancy approach [51] which combines radiation force and calorimetry methods into one system.…”