Quantification and assessment of carotid artery lesions: degree of stenosis and plaque volume.

Various methods were developed for the quantification of the degree of stenosis: B-mode imaging, CW Doppler with spectral analysis, PW-duplex, and color-flow imaging. The degree of stenosis can be evaluated using transverse views of the carotid in conventional B-mode imaging. The diameters of the residual lumen (Ds) and the external diameter (De) of the artery at the same level are measured and the degree of stenosis (in area) is calculated. Two different Doppler methods have been designed and validated for the quantification of the stenosis degree. The first one is based on the determination of the maximal velocity inside the stenosis and requires the use of a duplex device. The second one, based on the quantification of the hemodynamic disturbances at the outlet of the stenosis, can be used either with a duplex system or with a continuous-wave Doppler system (pencil probe). The color display mode facilitates the determination of the residual lumen and therefore contributes to making the B-mode method more accurate. Moreover, it provides a visualization of the blood flow which avoids most of the misinterpretations of B-mode or Doppler data. Several classification systems based on subjective or semi-quantitative criteria (morphological or hemodynamic) were used to identify different grades of stenosis (< 40%, 40% to 60%, 60% to 90%, etc.) These classifications provide an evaluation of the degree of stenosis accurate enough to discuss the possibility of performing an endarterectomy on a symptomatic patient. When the surgical treatment seems to be inappropriate, the patient is treated medically; the follow-up of the lesion (ie, every 6 months) requires a more precise evaluation of the plaque changes than does stenosis classification. Several parameters have been designed and validated for the quantification of the degree of stenosis by ultrasound. These parameters, whether measured with the B-mode and color image or with the Doppler spectrum, allow quantification of the stenosis degree in percentage of lumen reduction with a precision of approximately +/- 10%. Such a quantitative assessment of carotid lesions for a long period of time may be very helpful in evaluating the beneficial effects of medical treatment or in detecting any significant increase of the stenosis that could lead to surgical treatment. A new method for the plaque volume assessment has been recently validated. The plaque volume index expressed in mm3 is calculated from longitudinal and transversal B mode views of the bifurcation.