For a better interpretation of ultrasound images and an accurate measurement of the structure of a tissue (e.g., size or volume of a tumor), ultrasonic properties of biological tissues such as velocity, absorption, attenuation, and scattering must be known. A variety of methods can be used in vitro to measure these properties. Performing these measurements in vivo, however, is a totally different matter. A number of methods have been developed for in vivo measurements with little success (Greenleaf, 1986; Shung and Thieme, 1993).
Ultrasound velocity can be measured with a continuous wave excitation or with a 8pulsed excitation (Schwan, 1969).This method, uses continuous wave excitation and has been used to measure the velocity in a liquid sample with negligible attenuation to an accuracy of 0.1%. A standing wave is set up between the reflector and the transducer. The wavelength can be determined by adjusting the position of the reflector. If the frequency is known, sound velocity can be easily calculated from Equation (2 Pulse–Echo Method.2).
The same arrangement can also be used for pulsed excitation. A measurement of the time of flight t=2x/cof the pulse, where xis the distance between the reflector and the transducer, would yield c. The accuracy of this method relies on the sharpness of the pulse and is affected by the attenuation of the sample, which can cause a change in the pulse shape. An improvement in accuracy can be made by substituting the sample with a liquid, such as water, with a velocity that is precisely known. From measurements of two time of flights,ts and tr, in the sample
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