Ultrasonic Evaluation of the Carotid Bifurcation

Noninvasive tests for the detection of carotid occlusive disease have focused primarily on the indirect assessment of the internal carotid artery by the analysis of arterial pressure and flow. These tests can only detect the presence of flow-reducing lesions and occlusions; they cannot differentiate between these two entities. Also, the assessment of bilateral disease is difficult using these techniques. The introduction of ultrasonic methods has widened the diagnostic capabilities of noninvasive testing. Analysis of the velocity waveform envelope using zero-crossing detectors allows highly accurate identification of normal and severely diseased arteries. Used alone, however, this technique is limited in its ability to detect occlusions and provides no information on the energy content of the velocity spectrum. Furthermore, because it is often difficult to identify the sampling site with precision, the method does have limitations. The development of imaging techniques has helped to overcome this difficulty. While B-mode scanning appears promising, reliance on the image alone may be misleading. By producing an acoustic shadow, calcification may in fact hide a lesion of interest. Also, the similar acoustic characteristics shared by soft plaques, thrombi, and blood make it difficult to identify accurately soft plaques and occlusions. The addition of Doppler devices to ultrasonic imaging techniques has provided instruments that produce an image of the artery from which the flow information is collected. Methods involving the combination of a pulsed Doppler unit and an imaging system are particularly useful in detecting flow from discrete points within the vessel and prevent errors associated with flow disturbances near the wall. Errors associated with these techniques are usually the result of misidentification of vessels and tend to decrease as the examiner gains experience. Although auditory interpretation of the Doppler signal has produced good results, the quantitative analysis of the spectra represents a considerable improvement in diagnostic accuracy. Correct vessel identification and more precise quantification of the degree of narrowing can be achieved with this technique. In a research setting, the duplex technique provides a useful means of assessing the natural history of carotid artery disease before and after endarterectomy. Further improvements, such as the computer-based velocity waveform pattern recognition technique, have shown great potential in the objective evaluation of carotid disease. Although a great deal of work remains to be done, such techniques should provide us with a better understanding of and a more rational approach to the management of atherosclerotic disease at the carotid bifurcation.