AN APPROACH FOR MEASURING ULTRASONIC BACKSCATTERING FROM BIOLOGICAL TISSUES WITH FOCUSED TRANSDUCERS

When the standard substitution method is used with a focused transduce r to measure the backscattering coefficient from biological tissues in cluding blood, it yields erroneous results, Extending the backscatteri ng measurements to frequencies beyond 15 MHz necessitates the use of f ocused transducers because of the worsened signal-to-noise ratio - cau sed by the increased attenuation and the smaller transducer aperture s ize - in order to make the measurements close to the transducer, An ap proach which allows the use of focused transducers in backscattering m easurements has been developed, It has been used to measure the backsc attering coefficient of red cell suspensions of hematocrit ranging fro m a few percent to 30% in the frequency range from 5MHz to 30MHz, The results at hematocrits below 20% agree well with those obtained with t he standard substitution method, although they differ as the hematocri t is increased beyond 20%, The experimental results also show that the fourth-power dependence of backscatter on frequency is in general app roximately valid for suspended erythrocytes of hematocrit between 6% a nd 30%.