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The advent of computers has greatly affected the course of medical ultrasound. Unlike the conventional ultrasound discussed in a previous section, Doppler Ultrasound would not be possible without the technologically advanced computers that are available today. Doppler ultrasound is used primarily for the hemodynamic assessment of blood vessels, arteries and veins. The Doppler technology is used in combination with Duplex imaging. Doppler provides the quantitative data while Duplex color flow delivers quick detection of motion and its direction.

The Doppler shift is an apparent change in the frequency of the reflected ultrasound wave. It is thought to be due to the relative motion between the reflector and transducer (Krebs et al., 1993). The equation below describes this relationship (where D is the Doppler Frequency, V - the velocity, @ - the angle of incidence between the beam and interface, c - the velocity of sound in the medium). Even though ultrasound gives only one dimensional velocity information, volumetric flow can be accurately extrapolated with no angle corrections (Poulsen and Kim, 1996).

Although the Doppler principle is used to detect motion in all Doppler Ultrasound systems, the remainder of the sonographic unit differs from system to system. The two most common types of Doppler Ultrasound transducers are Continuous Wave Doppler and Pulsed-Wave Doppler. Continuous Wave Doppler Units use two crystals, one to send and one to receive the echoes. The transmitter inputs a continuous sinusoidal wave. The receiver detects the shift. An audible sound is created and recorded by either an analog recorder or spectral analyzer. Spectral analysis separates the signal into individual components and assigns a relative importance. Normally, Fast Fourier Transform is the preferred method of analyis (Krebs et al. , 1993). The benefits of CW Doppler include high sensitivity to low velocities and detection of high velocities without aliasing. On the other hand, CW Doppler cannot distinguish between the sending and receiving signals or extraneous echoes. Nor does CW Doppler produce a precise image like Pulsed Wave Doppler.

The second Doppler Ultrasound Transducer, Pulsed-Wave, is combined with Duplex to produce images of a certain structure while eliminating unwanted structures. PW uses a pulsed echo system that is discriminating in its range. The PW transducer both sends and receives the signal. It sends in short bursts and receives in the time when it is not sending. The returned signal is gated so that only information about the desired depth within the body is transmitted. The choice of the gate is determined by Nyquist's Theorem. The Doppler shift cannot exceed one half the Pulse Repetition Frequency, or aliasing will occur.

Duplex is a term used to descsribe the scanning device in most PW Doppler systems. There are various types of scanners including mechanical sector, electronically steered scanner, electronically steered linear array, and linear array with offset. Duplex can be used with both PW and CW if PW Doppler is first used to find the shift location and CW is used as a check to guarantee that the shift detected with PW is the highest over the entire anatomy.

With Color Flow Duplex, evaluating blood flow has been made much easier. These systems combine conventional B-imaging with Doppler and color to show motion, direction, and velocity. The color relates to direction of flow while the color intensity shows the velocity strength. Linear or phased array transducers analyze the signal by amplitude, phase, and frequency. Phase changes describe direction and presence of motion, so only structures that move produce a phase shift. Frequency shift describes the velocity of motion and is depicted with varying color intensities. Objects that do not move are gray with the shade depending on the signal strength. One drawback to this recent technology deserves further study in the future. It has been suggested that measurements made with these systems are particularly influenced by the concentration of red blood cells and backscattering. Since few samples are taken at each location, information about signal power is questionable (Bascom and Cobbold, 1996).

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