Errors resulting from finite beamwidth and sample dimensions in the determination of the ultrasonic absorption coefficient

The error resulting from heat conduction i the measurement ofthe ultrasonic absorption coeffcient using the transient thermoelectric method is studied analytically. An expression for the temperature increase in a tissue specimen of finite dimensions, irradiated by a focused ultrasonic transducer, isgiven as a function of spatial coordinates, time, radial and axial beam dimensions, and absorption. An error is defined, and results are presented for various values of beamwidth, tissue dimensions, absorption, and time for the purpose of quantifying the experimental error due to heat conduction, and to provide guidance for minimizing this error in experimental procedures. For example, it is shown that the effect of heat conduction on the measured rate of temperature increase is less than 7% when using a transducer with a 5-ram half-power beamwidth at depths greater than 1.5 mm in the tissue.