VARIANCE MAPPING IN COLOR-FLOW IMAGING - WHAT DOES IT MEASURE

Flow disturbance is known to occur in association with high grade carotid stenosis and cause spectral broadening and fluttering in the Doppler spectral waveform distal to the stenosis. Colour flow variance mapping has been proposed as a means of documenting its presence. We prospectively evaluated this hypothesis by studying 15 patients with high grade (> 50%) stenotic lesions in the internal carotid artery. In all 15 cases, the site of increased variance on the colour Doppler map corresponded to the point of maximum velocity. In 10 cases of greater than 75% stenosis, the point of aliasing on the traditional colour Doppler map and the site of maximal variance occurred at the stenotic jet (length 7.3 +/- 2.3 mm) and not along the full length of disturbed flow (19.3 +/- 5.5 mm). In five patients with stenoses between 50-75% diameter narrowing, decreasing the colour Doppler pulse repetition frequency (PRF) created a region of abnormal variance corresponding to the site of aliasing. In all 15 stenoses, the site of maximum flow disturbance, defined as fluttering in the Doppler spectral waveform, was not apparent on the variance map. We conclude that the variance map does not reflect flow disturbance and is most likely to occur at sites of aliasing.