Basic Ultrasound Principles

Michael A. Ricci, MD, RVT

An ultrasound transducer transmits ultrasound waves that are reflected by an interface (ie, moving red blood cells) back to a receiver. The originally transmitted waves undergo a detectable change in frequency caused by the moving blood cells (Doppler effect). Likewise, the Doppler effect occurs if the interface is stationary (ie, tissue) and the transducer is moving. In the first instance, the change in frequency (Df) is transformed by the scanner's computer to produce a spectrum of all the frequencies present at a given point in the vessel (spectral analysis) useful in diagnosing stenosis. In the second instance, different tissue interfaces produce an anatomic ultrasonic picture (gray scale image). Combining these two components produces the conventional "duplex" scanner. Some equipment adds a third component. Simultaneous receiving from a multitude of sites records Df for essentially every moving point in the field. A color code is assigned to each point (blue=vein, red=artery, white=stenosis) and the image is continuously updated so that a dynamic real-time ultrasonic arteriogram-like image is created.

Because stenosis produces an increased velocity (v) of flow, the reflected frequency (Df) detected is related to velocity by the following equation:

v (cm/sec) = (Df · c) (2F0 · cosq)

where c=1540 m/sec, F0=transmitted frequency, and 0 - 60° (incident angle). Convention now usually expresses Df after it is converted (by the scanner) to velocity, a more clinically useful and understandable number.