Doppler Ultrasound Velocity Detection

TYPES OF DIAGNOSTIC ULTRASOUND

Most diagnostic ultrasound today is based on the reflectivity of sound waves within tissue. There are two principal types: Doppler ultrasound velocity detection and imaging reflective ultrasound.

Doppler Ultrasound Velocity Detection

Ultrasound may be used to sense motion within tissue by application of the Doppler effect. The Doppler effect is a change in the apparent frequency of a sound when either the source or the receiver is in motion relative to the other. If the distance is decreasing, the detected frequency is increased. If the distance is increasing, the frequency is decreased. A reflector placed in the sound beam between the source and the receiver, also will produce a frequency shift if it shortens or lengthens the sound-path distance between the sound source and the receiver. If both sound source and receiver are stationary, the velocity of the reflector will determine the magnitude of the frequency shift.

The cardiovascular system produces motion changes that can be measured by Doppler ultrasound velocity detectors. Two piezoelectric transducers are mounted in close proximity to each other. One transducer acts as the sound source or transmitter, and the other transducer functions as the receiver. The transducers are angled slightly to detect motion in tissue deep to and between both transducers. The reflection from moving objects is due either to the pulsation of the vessel walls or to backscattering from the blood cells. The backscattering from the blood cells is the more important reflection. The detector discerns the frequency difference between transmitted and received signals (frequency shift) and provides this information as either an audible signal or. an analog display on a monitor or recording paper. The greater the frequency shift, the greater the velocity of the blood with respect to the transmitting and receiving transducers.

FIG. 1-7. Imaging reflective ultrasound. In A, a short ultrasound pulse is transmitted by the transducer (T) toward a reflector (R). In B, a weaker amplitude echo is reflected back to the transducer, which is now in a receive mode.

a receiver mode for a relatively long period. The long reception time permits a sound pulse to range out and be reflected back to the transducer. The cycle of pulse transmissions and receptions is called the repetition rate and varies from 1,000 to 8,000 pulses per second. The higher repetition rates signifi­cantly limit the depth of penetration in tissue. To detect specular reflection using the same transducer for transmission and reception, the angle of beam incidence and reflection must be at or near 90°.

Дата добавления: 2015-11-14