Ultrasound Power Measurement by Pulsed Radiation Pressure
Steven E. Fick
In a specially designed radiation force balance (RFB), low-frequency pulse modulation of the incident ultrasound allows high-accuracy measurement of time-averaged spatially-integrated ultrasound power radiated into a reflectionless water load. Errors characteristic of force sensors are precluded by operating the RFB as a force comparator, without directly measuring force. Equipped with purpose-built transducers and electronics, the RFB is adjusted to equate the radiation force and a counterforce generated by an actuator calibrated against reference masses using direct current as the transfer variable. Special techniques enable RFB easurements of the output of continuous-wave and pulsed diagnostic medical systems. Nearly twenty years ago, customer feedback indicated that the accuracy of power levels replicated using radiation conductance was seriously compromised by limitations of equipment typically available. Accordingly, an alternative scheme, based on a stable radio frequency (rf) voltage sensor treated as part of the transfer standard, was developed which allows ultrasound power-transfer measurements to be achieved with relative standard uncertainty components as low as 2 x IO-? (1 a), using equipment already available to most customers. A detailed description is given of the dc-level method (DCLM), which since 1985 has been, by a wide margin, the method most frequently requested.