Non-invasive methods of arterial and venous assessmentBMJ 2000; 320 doi: https://doi.org/10.1136/bmj.320.7236.698 (Published 11 March 2000) Cite this as: BMJ 2000;320:698
- Richard Donnelly,
- David Hinwood,
- Nick J M London
Although diagnostic and therapeutic decisions in patients with vascular disease are guided primarily by the history and physical examination, the use of non-invasive investigations has increased significantly in recent years, mainly as a result of technological advances in ultrasonography. This article describes the main investigative techniques.
Principles of vascular ultrasonography
In the simplest form of ultrasonography, ultrasound is transmitted as a continuous beam from a probe that contains two piezoelectric crystals. The transmitting crystal produces ultrasound at a fixed frequency (set by the operator according to the depth of the vessel being examined), and the receiving crystal vibrates in response to reflected waves and produces an output voltage. Conventional B mode (brightness mode) ultrasonography records the ultrasound waves reflected from tissue interfaces, and a two dimensional picture is built up according to the reflective properties of the tissues.
Ultrasound signals reflected off stationary surfaces retain the same frequency with which they were transmitted, but the principle underlying Doppler ultrasonography is that the frequency of signals reflected from moving objects such as red blood cells shifts in proportion to the velocity of the target. The output from a continuous wave Doppler ultrasonograph is usually presented as an audible signal, so that a sound is heard whenever there is movement of blood in the vessel being examined.
Continuous wave ultrasonography provides little scope for restricting the area of …