ZERO-CROSSING RATEMETER

Typical Number in Hospital:

Cost Bands:

References: 1,9

The velocity of blood flowing in a vessel can be calculated from the doppler-shifted reflections of a continuous-wave ultrasound beam, transmitted through it from a point outside the body. A major problem arises because in addition to the pulsatile nature of the blood flow, many different doppler-shifted frequencies are received due to the differing velocities flowing at each point across the vessel (flow at the centre is normally faster than near the walls).

There are two main methods of dealing with this problem. One is to apply a complete frequency analysis to the signals (using a spectrograph), and the second is to derive an approximate figure for average blood velocity using a zero- crossing ratemeter. This counts the rate at which the signal passes through the zero level and the result is a mean velocity which can be recorded on a strip chart, but does not exactly reflect the flow pattern.

Such devices are used in the more basic types of doppler blood flowmeters for estimating the quality of flow through peripheral blood vessels during the assessment of vascular disease, and in more complex types, including pulsed-wave doppler velocity meters, where accuracy is greater in view of the smaller sampling volume.