(1/10/03 8:35 am)
Color Doppler, carotid occlusive lesions, stenosis|
Pictorial Essay: Color Duplex Evaluation of Carotid Occlusive Lesions
Rajagopal KV, Bhushan N Lakhkar, Shekhar Banavali, Nitin Kumar Singh
Ind J Radiol Imag 2000; 10:
Key Words: Color Doppler, carotid occlusive lesions, stenosis
Flow limiting stenosis of the carotid artery is a common predisposing cause of stroke. The major role of Doppler examination is the detection of stenotic lesions in the vicinity of the carotid bifurcation. Atherosclerotic plaque with resultant stenosis in the carotid arteries usually involves the internal carotid artery (ICA) within 2 cm of the carotid bifurcation. Accurate diagnosis of significant stenosis is important to identify those patients who would benefit from surgical intervention. Carotid endarterectomy is more beneficial than medical therapy in symptomatic patients with greater than 70% carotid stenosis . Ultrasound can also assess plaque morphology such as hemorrhagic or ulcerative plaque, which increases the risk of thromboembolic events.
Carotid doppler - technique and instrumentation:
Carotid examination is performed with the patient supine, the neck slightly extended and head turned away from the side being examined. Examination can be done facing the patient or sitting behind the patient. Only linear transducers are recommended for the examination of cervical arteries. An acceptable Doppler angle can be achieved with beam steering. A 5-Mhz transducer is essential and will adequately demonstrate cervical arteries even in obese patients.
Gray-scale examination begins in the transverse projection. The transducer may be applied more from the anteromedial or lateral side of the sternocleidomastoid muscle. Scans are obtained along the entire course of the cervical carotid artery from the supraclavicular notch cephalad to the angle of the mandible. Inferior angulation of the transducer in the supraclavicular area images the common carotid artery (CCA) origin. The left CCA origin is deeper and more difficult to image consistently than the right. The carotid bulb is identified as mild widening of the CCA at the bifurcation. The examination plane necessary for optimal longitudinal scans of the carotid artery to perform Doppler spectral analysis is determined by the course of the vessels demonstrated on the transverse study. Images are obtained to display the relationship of both branches of the carotid bifurcation to the visualized plaque disease and the extent of the plaque is measured.
The normal carotid arteries:
The longitudinal view of the normal carotid wall demonstrates two nearly parallel echogenic lines; the inner line is the lumen-intima interface and the outer line is the media-adventitia interface. The distance between these lines is the combined thickness of intima and media (I-M complex)
Thickening of I-M complex more than 0.8 mm represents early changes of atherosclerosis. The intimal reflection should be straight, thin and parallel to the adventitial layer (Fig. 1). Undulations and thickening indicate plaque deposition or more
Longitudinal and transverse scan of CCA. The innerline (arrowhead) indicates lumen-intima interphase. The outerline (arrow) indicates media-adventitia interphase.
rarely fibromuscular hyperplasia. The CCA lies immediately adjacent to the jugular vein but the two vessels are easily differentiated. The carotid artery exhibits pulsatile flow pattern whereas the jugular vein shows continuous low velocity signal. Several anatomic features differentiate ICA from external carotid artery (ECA). In about 95% of the patients, the ICA is posterior and lateral to the ECA. The ICA is usually larger than the ECA and has no branches in the neck whereas the ECA possess branching vessels.
Flow characteristics on color doppler image:
Laminar flow is apparent in normal CCA and ICA as manifested by gradations of shades of color from the periphery to the center of the vessel (Fig. 2). This can be appreciated
Colour Doppler image of CCA showing laminar flow.
on longitudinal as well as transverse images. A tortuous vessel or bifurcation of the vessel may produce flow disturbances that vary in severity in proportion to the curvature or angular measurements of the vessel. Flow disturbances may be manifested by mixtures of shades of color, all flowing cephalad or mixtures of colors representing forward or reversed flow. Normal flow disturbances occur at the carotid bulb.
Aspects of carotid pulsatility that assist with the identification of ECA and ICA are also manifested by the Doppler image. The CCA and ICA exhibit a continuous flow pattern with antegrade flow in diastole indicated by persistence of color throughout the entire cardiac cycle. ECA shows cessation or marked diminution of diastolic flow and this is indicated by the disappearance of color during the diastolic portion of the cardiac cycle.
The ICA and ECA have distinct spectral waveforms. The ECA shows a sharp velocity rise during systole and a rapid fall during diastole, approaching zero or transient reverse direction. This flow pattern is due to the high resistance vascular bed of the facial musculature supplied by the ECA (Fig. 3). The ICA supplies the low resistance circulation