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Tissue-Mimicking Liquid for Use in Exposimetry

Medical Physics (T.A.S., E.L.M., G.R.F.) and Food Science (J.A.L.) Departments, University of Wisconsin-Madison, Madison, Wisconsin USA; and Diehl Technical, Inc, Defiance, Ohio USA.

Address correspondence and reprint requests to Timothy A. Stiles, MS, Medical Physics Department, University of Wisconsin-Madison, 1300 University Ave, Room 1530, Madison, WI 53706 USA. E-mail: tastiles{at}wisc.edu

Objective. Current determinations of diagnostic ultrasound exposure parameters (eg, peak rarefactional pressure and pulse intensity integral) are intended to correspond to propagation through soft tissue with a propagation speed of 1540 m/s and attenuation of 0.3 dB • cm^–1 • MHz^–1. These current measurements are made in water, which has very little attenuation, and a linear derating factor is applied to approximate 0.3 dB • cm^–1 • MHz^–1 attenuation. The fact that propagation through water as well as through soft tissue involves nonlinear propagation is not directly addressed. A better way to determine exposure parameters would be to use a liquid that has the desired tissue-mimicking properties, including a value of the nonlinearity parameter B/A representative of soft tissue. To be of practical use in the laboratory, the ultrasonic properties of this liquid must remain stable and spatially uniform for many months or years without need for periodic mixing by the user. Methods. Fifty-two samples of fat-free milk that was concentrated to one third of its original volume by ultrafiltration were created. Each sample was preserved by a different method. The speed of sound, attenuation, and nonlinearity parameter B/A of each sample were periodically monitored by narrowband through-transmission techniques. Results. Six of the 52 samples remained liquid and retained acceptably stable acoustic properties over 22 months of storage at room temperature. Conclusions. Fat-free milk, concentrated via ultrafiltration and preserved in 1 of 6 different methods, has been found to be a stable tissue-mimicking liquid with acoustic properties appropriate for use in exposimetry.

Key Words: acoustic output • acoustic properties • bovine milk • exposimetry • liquid • tissue mimic

Abbreviations: PII, pulse intensity integral • p_r, peak rarefactional pressure • TM, tissue-mimicking