Foundamentals and Applications of Abdominal Doppler

Abstract

Since the Doppler effect was firstly described by Christian Doppler in 1842, it has been applied in many different fields. In human medicine has been extremely helpful in monitoring the fetal viability or assessing the carotid flow, and it is currently being used in most of the disciplines. In veterinary medicine, the Doppler effect is a helpful tool in abdominal ultrasound, and essential in the echocardiography exam. Its principle can be defined as the apparent shift in transmitted frequency, reflected back to the source off a target, which occurs as a result of the movement of this target. When this effect is applied in ultrasonography, the red blood cells (RBC’s) are the moving targets, and the apparent shift in the frequency of the sound reflected back to the transducer is proportional to their velocity and direction of the movement. The software of the ultrasound machine displays this values in a color code (Color Doppler) or in a graphic, (spectral trace of the Pulsed wave Doppler (PW) or Continuous wave Doppler (CW)). In Color Doppler, a given color is usually assigned to the direction of flow; red is flow toward, and blue is flow away from the transducer (Figure 1). The center of the color bar, displayed in the screen, is black and represents zero flow. In addition to simple direction, velocity information is also displayed. Progressively increasing velocities are encoded in varying ranges of either red or blue. The more dull the hue, the slower the velocity. The brighter the hue, the faster the relative velocity. Color Doppler is also used to display turbulent flow (showing a mosaic of many colors) and allows an operator to discriminate between normal and abnormal flow states. Color Doppler is useful for assessing relatively big areas, whilst PW (Pulsed Wave) and CW (Continous Wave) Doppler are used for assessing smaller areas of interest. Since Color Doppler is a type of pulsed wave Doppler, it suffers from the same limitations. Before explaining the difference between CW and PW, explaining the concept of spectral trace is required. This is the graphic representation of velocity flow profile against time. Depending of the number of cells crossing the amount of signal increases (Figure 2).