Internal Carotid Artery Stenosis

Stroke is a leading cause of death worldwide and 20% to 30% of stroke results from emboli dislodged from atherosclerotic disease at the carotid bifurcation [1]. The goal of carotid imaging is to detect hemodynamically significant stenosis of the internal carotid artery (ICA) where carotid endarterectomy significantly reduces the risk of stroke and death. Ultrasound is an accurate and inexpensive screening method for assessing carotid vessels and it provide morphological and functional details [2]. B mode Grey scale sonography allows measurement of Intima-Media thickness and characterization of the plaque. Normal Intima- Media thickness is measured by the distance between the two echogenic lines represented by the lumen intima interface and the media adventitia interface. This is called the Intima Media complex (I-M Complex). The anechoic to Hypoechoic zone in between these echogenic lines is the media. The normal I-M complex measures less than .8mm in thickness [2]. Plaque characterization is an important part of carotid Doppler evaluation. This is done by documenting the location of the plaque, plaque extent, surface contour, texture and the assessment of luminal stenosis [3]. According to the hemodynamic states plaque is classified from H1 to H5. H1 is mild, less than 50% diameter reduction. H2, moderate 50% to 69% diameter stenosis. H3, severe, 70% to 90% occlusion. H4, critical near occlusion. H5, total occlusion [2]. Plaque texture is classified as homogenous plaques which has a uniform echogenicity (P1) and heterogenous plaques that may contain intraplaque hemorrhages or lipid deposition (P2). Surface characteristics are categorized from S1 to S3. S1 where the surface of the plaque is smooth S2 plaque which is ulcerated with a surface defect less than 2mm and S3 an ulcer defect more than 2mm [2]. Plaques that are predominantly echolucent and ulcerated are more prone for sudden increase in size due to hemorrhage and thomboembolism. These plaques are considered unstable and commonly found in symptomatic patients. Echogenic plaques with smooth outline generally considered benign or stable and common in asymptomatic individuals [2]. Plaque characterization is an independent parameter from plaque size and diameter stenosis which should be taken into consideration in management of patients when the individual is symptomatic with an area reduction of less than 70% [3]. Color Doppler and power Doppler ultrasonograghy improves the detection of plaque ulceration and area stenosis. In plaque ulceration color or power Doppler images would show, slow moving eddies of color within an anechoic plaque [2]. Area stenosis is measured in transverse plane, perpendicular to longitudinal axis of the vessel, using Grey scale scan with Color or Power Doppler or B-flow imaging [2]. On color flow imaging the area of stenosis demonstrates aliasing. In a nearly occluded lumen a string of color can be demonstrated through the patent lumen by Color or Power Doppler which is called the “string sign.” [3] Each vessel has its characteristic spectral waveform that depicts the resistance of its distal vascular bed. The normal internal carotid artery has low resistance wave with a tall and sharp upstroke and continuous forward flow throughout cardiac cycle [4]. External carotid artery has a high resistance flow. It has a tall and sharp upstroke in systole and returns to baseline quickly with little flow during diastole [4]. The normal common carotid artery waveform is a combination of distal high resistance waveform of external carotid artery and low resistance pattern of internal carotid artery [4]. Plaques cause disturbances to the normal smooth laminar flow and cause spectral broadening which is seen as fill in of spectral window [1]. Flow velocity analysis is considered the main parameter for assessing the severity of stenosis. The Doppler parameters usually considered are internal carotid artery peak systolic velocity, ICA end diastolic velocity, CCA peak systolic velocity, CCA end diastolic velocity, peak systolic ICA/CCA ratios and peak end diastolic ICA/CCA [2] .Peak systolic velocity of ICA has proved accurate for quantifying high degree stenosis and it is easily measured.   Velocities between 125 cm/s to 210 cm/s correspond with stenosis of 50% to 69%. Velocities higher than 230 cm/s correspond with > 70% stenosis.  Near occlusion velocities may be low or undetectable [2, 5]. Usually Peak systolic velocity of ICA above 230cm/s, ICA/CCA peak systolic velocity ratio more than 4 and End diastolic velocity of ICA above 100cm/s indicate hemodynamically significant stenosis [6].  Tortuosity of the ICA due to kinking can cause errousouly high velocities. Also heavily calcified plaques may cause barrier to ultrasound waves due to posterior shadowing.PSV values vary and become less reliable when variations of cardiovascular physiologic states occur such as hypertension, high or low cardiac output states, aortic diseases and bilateral carotid occlusions. Velocity ratios of ICA/CCA would be helpful in these situations [2, 3]. Contrast enhanced MRA has shown better sensitivity and specificity compared to color Doppler ultrasound in high grade stenosis and complete occlusions [3]. Differentiation of near total and total occlusion is critical for treatment decision and color Doppler is less reliable as the ICA peak systolic velocity decreases in near total occlusion. Contrast enhanced CT and MRA may compliment when the color Doppler results are indeterminate and not compatible with the clinical findings [2].