[Doppler diagnosis in peripheral arterial occlusive disease].

PHYSICAL AND TECHNICAL FUNDAMENTALS OF DOPPLER ULTRASONOGRAPHIC METHODS

In addition to units recording both velocity and direction of blood flow, mostly using two ultrasonic frequencies and phase-out technique, there are small non-directional units available which provide useful diagnostic information from the acoustic Doppler signal derived. Doppler ultrasonic techniques utilize two physical phenomena: a) High-frequency ultrasonic energy penetrates biologic tissue and is partially reflected at borders between tissues of differing density. b) If the border area is in motion, due to the Doppler effect, there is a change in the reflected ultrasonic frequency with respect to the frequency emitted. In blood vessels the ultrasonic beam is primarily reflected from the flowing red blood cells where the change in frequency is a function of the velocity of flow (Doppler effect). From the Doppler transducer, the continuously-emitted ultrasonic beam is also received after being reflected. The frequency of the reflected beam is directly proportional to the velocity of the flowing blood. If flow is directed toward the transducer, the frequency of the reflected beam increases and if the flow is away from the transducer, the converse is true. The best Doppler signals can be received when the angle beta of the transducer to the studied vessel is about 45 degrees. The unprocessed Doppler signal represents a frequency spectrum corresponding to the various velocities of the individual lamina of the blood stream from which the prevailing velocity is integrated and registered. The penetration depth is dependent on the frequency emitted. Doppler units are preferred with working frequencies of 8 to 10 MHz and 3 to 5 MHz. With 8 MHz, the maximal depth of penetration is 3.5 cm, with 4 MHz, 8 cm. The lowest detectable velocity is also dependent on the frequency emitted: with 8 MHz, minimum is 3 cm/s. Since flow toward the transducer results in a positive Doppler shift and flow away in a negative shift, with the Doppler signal, the direction of flow can also be determined. The recorded Doppler curves enable a qualitative and, to some degree, quantitative assessment. Phase-out and frequency analysis systems enable differentiation of forward and backward flow components. From separate forward and backward flow curves, the instantaneous summation curve (integrated instantaneous hemotachygram) as well as a trend curve over 5 to 7 seconds can be constructed and the mean flow velocity displayed.