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PERIPHERAL VASCULAR ULTRASOUND
this chapter is to introduce the sonographer to the practical aspects of scanning, covering the basic use of scanner controls and reiterating some of the principles discussed in the preceding chapters. Imaging artifacts will also be discussed to assist the sonographer in the interpretation of images. It is likewise essential that the sonographer has a good understanding of the principles relating to ultra- sound safety in order to minimize any exposure risks to the patient.
THE PATIENT
It is an ironic fact that a sonographer may use a state of the art duplex scanner but fail to obtain any useful diagnostic information because of an inadequate approach to the examination and the patient. For example, an introduction and simple explanation of the test may put patients more at ease and willing to cooperate, especially if they are nervous or in some discomfort. Local protocols that are rigid and do not allow any flexibility can also lead to problems. For example, a protocol that requires that patients always be completely flat with the head fully extended during carotid scans may lead to severe discomfort for patients with breathing difficulties, dizziness, angina or spondy- losis of the neck. It is possible they will not be able to tolerate the examination at all. An alternative would be to perform the scan with the patient sit- ting up on a low chair. It is still possible to obtain good images from this scanning position. Most problems can be solved with a little careful thought and the occasional inventive approach.
STARTING THE SCAN
The scan should be carried out in a dimly lit room to optimize visualization of the black and white image. The transducer selected should be of the highest frequency that allows adequate penetration to the area to be examined. Most duplex systems use a broad-band transducer, and some allow the sono- grapher to alter the imaging, color and spectral Doppler transmit frequencies to optimize the image or enable better penetration. When scanning, it is important to adopt a logical approach. Using a sys- tematic technique cuts down on examination time and ensures that pathology is less likely to be missed. The scan is best started by examining the region of
interest with B-mode imaging alone, to identify rel- evant structures. In general, a cross-sectional survey should be performed before a longitudinal scan as this helps to relate structures to each other. Avoid switching on the color flow or spectral Doppler straight away, unless they are essential for identifying vessels, as the imaging frame rate will be reduced and the display may be confusing if anatomy has not been clearly identified.
B-MODE CONTROLS
Always set the focal zones at the depth of interest on the scan image. B-mode frame rates of modern scanners are generally high even when multiple focus zones have been selected, but reducing the number of focal zones will increase the frame rate even further. If the region of interest in the B-mode image is very small, or very deep, consider using the zoom control to magnify the area. This will improve the frame rate. Scanners usually allow the operator to adjust the size and position of the area to be magnified. Most systems have a write zoom facility that will improve image resolution. Many vascular sonographers prefer B-mode images with a reasonable degree of contrast using a lower dynamic range. Duplex systems have examination- specific presets that are optimized to produce the best images of vascular structures. It is also worth- while experimenting with different pre- and post- processing controls in order to understand the function of these controls. Try this when imaging a carotid plaque and note the difference in the appearance of the image. Optimize the total gain and depth gain compensation sliders so that the returning echoes are of relatively uniform intensity throughout the image. In general, the gain should be set so that the lumen of any large nondiseased vessel appears clear or black but any further increase in gain would introduce noise or speckle. Harmonic imaging can be especially useful in the abdomen and may produce clearer and less noisy images. The use of compound imaging, if available, may also improve the overall image.
IMAGING ARTIFACTS
An imaging artifact is a feature on the image that does not relate exactly to a structure within the