Ultrasound power measurement by pulsed radiation pressure

Abstract

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 measurements 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 10-3 (1), 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.

Acoustic radiation pressure in laterally unconfined plane wave beams

“…Beissner [21] points out that if acoustic radiation pressure is used to calibrate acoustic sources the 'measured radiation force must be converted to the ultrasonic power value and this is carried out with the help of theory.' The measured radiation force on a target is generally assumed to result from the Langevin relation, equation (47), between the radiation pressure generated by the acoustic source and the energy density of the wave [18][19][20][21][22]. It is appropriate to consider the implications of this assumption.…”

“…Indeed, few absolute measurements obtained independently in the same experiment of the acoustic power generated by an acoustic source and the radiation force incident on a target to assess the radiation pressure-energy density relationship have been reported. Typically, experimental assessments of the radiation pressure have either relied on the assumption of the Langevin relation a priori in evaluating the transducer power output [18][19][20][21][22] or have considered relative measurements without directly evaluating the transducer power output (cf, [8,9,38,39]). Beissner [21] points out that if acoustic radiation pressure is used to calibrate acoustic sources the 'measured radiation force must be converted to the ultrasonic power value and this is carried out with the help of theory.'…”

“…The radiation pressure generated by an acoustic wave is used in a variety of applications such as acoustic radiation force-based elasticity imaging [1][2][3][4][5][6][7][8][9], acoustophoretic drug delivery [10], acoustic tweezers [11][12][13][14][15], the characterization of atomic force microscope cantilevers [16,17], and the calibration of ultrasonic transducers [18][19][20][21][22][23][24]. The search for the proper understanding of radiation pressure in acoustic fields has been controversial and elusive since the pioneering efforts of Lord Rayleigh [25], Brillouin [26,27], Hertz and Mende [28], and Langevin [29,30] in the early twentieth century and has continued to the present time .…”

Progress in medical ultrasound exposimetry

“…One RFB approach to minimize these confounding forces involves modulating the ultrasound and measuring the resultant radiation force at a frequency removed from their influence (20 Hz) [185], [186]. Errors of less than a few percent are reported for powers from 0.1 mW-30 W, and frequencies from 0.5-30 MHz.…”

In-situ Attenuation Corrections for Radiation Force Measurements of High Frequency Ultrasound With a Conical Target

“…Radiation pressure [ 1 – 13 ] has been employed in a wide variety [ 14 – 27 ] of ultrasound power meter designs. In these instruments, an appropriately constructed target properly aligned in a steady-state underwater ultrasound field is subjected to a radiation force F given by where e T is a correction factor determined by various properties of the target, W is the time-averaged spatially integrated power intercepted by the target, and c is the speed of sound in the water.…”