So far we have dealt with a special class of scatterers that are static in nature. In living systems, tissues and organs are in constant motion to accomplish their biologic functions. Change in normal blood flow or the movements associated with heart, lung, and gut borders are often indicative of the physical state of the organ. As the tissue scatterers move during ultrasonic irradiation, one observes a scattering behavior that is typical of motion. For example, the frequency of reflected sound is shifted as the reflector moves with respect to the transducer; the displacement of a reflecting surface like that of a heart valve modulates the echo signal about a mean position; or if a scattering medium is interrogated with two pulses separated by an interval, the pulses are temporally shifted closer or away due to the movement of scatterers. Any of these phenomena like frequency shift, echo modulation, or temporal shift between the two pulses during round-trip travel to a scatterer can, in principle, be used to obtain information on motion. Techniques based on these phenomena are discussed below. Each of the techniques compensates for the weakness of the other and in turn suffers from some limitation of its own.
KeywordsDoppler Signal Ultrasonic Irradiation Blood Velocity Doppler Method Measure Frequency Shift
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