Ultrasound

SummaryUltrasonic imaging provides a non-invasive assessment of the arterial and venous circulation in the lower limb and is accepted as a valuable diagnostic technique. Grey-scale images identify plaque and thrombus, duplex assessment provides a measurement of blood velocity through a vessel, and colour Doppler imaging enables the rapid localization of arterial stenoses and occlusions and the identification of incompetent veins. This article outlines the principles of the different techniques and presents normal images. Procedures for investigating arterial stenoses, superficial venous incompetence and deep venous thrombosis are described, abnormal images presented and the limitations discussed. It is hoped to provide an insight into the strengths and limitations of ultrasonic vascular investigations for those involved in tissue viability and ulcer management.

IntroductionUltrasound equipment has developed rapidly over the past 30 years and is now used routinely for assessment of arterial stenosis, venous incompetence and venous thrombosis. This article provides practical details of the main lower-limb vascular examinations currently undertaken in vascular laboratories. It is intended to give an insight into the strengths and limitations of the techniques for those involved in tissue viability and ulcer management. For those interested in obtaining more detail, a comprehensive review of the developments has been carried out by Meire and Farrant [1] . A general introduction to the impact of ultrasonic scanning on vascular assessment is given by Androulakis et al [2] , and Polak [3] gives practical detail on performing the basic examinations.
Ultrasound images are obtained by holding a probe on the skin surface. An ultrasonic scanner usually has a range of probes with different characteristics, and for lower-limb vascular scanning a linear array probe is normally used. This produces a rectangular image which is displayed with the skin surface at the top, the vertical axis showing depth into the body and the horizontal axis showing position along the probe. When imaging blood vessels, the probe can either be placed along the vessel to produce a longitudinal scan, or across the vessel to produce a transverse scan

To produce the images, the probe emits short pulses of ultra-sound, and these travel into the body from the probe. Within the soft tissues or at boundaries between them, a small proportion of the ultrasound is scattered or reflected and arrives back at the probe as an echo . The speed of ultrasound in the body is constant (1540 m/s), so the depth of any scatterer or reflector can be found from the time delay from emitting the pulse to receiving the echo. The main pulse continues deeper into the body to be scattered or reflected from deeper structures. When the echoes from one pulse have died down, the next pulse is emitted from a slightly different position along the probe. In this way, it is possible to build up an image of a plane in the body, with depth into the body as the vertical axis and position along the probe as the horizontal axis.

The probe determines the frequency of the ultrasound within the pulses. Frequencies between 3 and 7 MHz are generally used for peripheral vascular imaging. Higher frequencies give better resolution and more detailed images, but the higher frequency sound loses energy more quickly as it travels through the body so the depth of penetration is less. The operator usually uses as high a frequency as possible. Ultrasound of these frequencies does not travel through air, so a layer of water-based coupling medium is used between probe and skin.