Quantification of Atheromatous Stenosis in the Extracranial Internal Carotid Artery

An international consensus meeting to determine criteria for the quantification of stenosis of the extracranial internal carotid artery was held in Paris on December 2 and 3, 1994. A review of the literature and expert analysis of validity and reproducibility led to the following recommendations: Intra-arterial X-ray angiography: At present, two main methods of quantification are being evaluated, the distal and the local method, the former being the better validated. The latter method, on the other hand, is better able to represent moderate lesions at the level of the carotid bulb. Therefore, it is suggested that a ratio between the stenosis and the common carotid artery should also be established. X-ray angiography prior to carotid endarterectomy can be avoided if ultrasound and MR angiography concur in identifying severe stenosis provided that intracranial vessels are without relevant disease. Ultrasound Doppler duplex methods can quantify the degree of extracranial carotid artery stenosis in terms of both diameter reduction according to the criteria established by recent surgical studies (NASCET – distal degree of stenosis – and ECST – local degree of stenosis – as well as residual area in cross-sections. The latter is more suitable since hemodynamic effect, local increase of velocity, and pressure drop are taken into consideration. Technical requirements (carrier frequency 4–5 MHz, Doppler angle inferior to 60°, sample volume ≧5 mm) call for a combination of three validated criteria. Maximum Doppler shift/flow velocities measured at the narrowest point of the stenosis and the degree of poststenotic flow disturbances should be examined. In systole, a value of 4 kHz (120 cm/s) (f0 = 4 MHz), identifies most stenoses >50% in local diameter reduction and as in end-diastole, a value of 4.5 kHz (135 cm/s) identifies stenoses of >80%. Carotid ratio: The systolic velocity ratio should be recorded between the site of the stenosis and the common carotid artery obtained 3 cm below the bifurcation. This limits the influence of general hemodynamic factors unrelated to the stenosis such as the cardiac output. A threshold value of >1.5 determines stenoses of >50% and a threshold value of >4, stenoses of >70%. Area ratio: The ratio between the total arterial lumen in cross-section and the minimal residual lumen should be determined by echotomography and additional color Doppler flow imaging. In addition there are indirect criteria, such as asymmetry of pulsatility of the common carotid artery and middle cerebral artery signals as well as inverted flow of the ophthalmic artery which distinguish moderate from high-degree stenoses (>80% diameter reduction). Magnetic resonance angiography (MRA): With this method, a diameter ratio using the distal degree of stenosis is recommended with data obtained from transverse source images in addition to transverse T1 sequences. MRA is a complement to ultrasonic methods, particularly in cases of calcified stenoses and for the analysis of intracranial vessels.