Effect of machine parameters on variance display in Doppler color flow mapping.

In color Doppler flow studies, "variance" is an important display modality for diagnosing stenotic, regurgitant, and shunt lesions. Variance, a mathematical calculation based on the variation in the Doppler signal frequencies, has been reported to reflect the degree of flow disturbance. A wide-band pulsed Doppler spectrum results in a larger degree of variance. It has been suggested that variance area (green color or mosaic area) might provide useful quantitative information regarding the severity of stenotic, regurgitant, and shunt lesions. Since ultrasound machine settings may affect the color Doppler variance image, we evaluated in 101 free jet experiments the effect of packet size (eight versus four samples per line), pulse repetition frequency (3.9 versus 5.2 kHz), frame rate (11 versus 22 frames per second), system gain (+15 dB versus -15 dB), transmit power (high versus low), and moving target indicator (MTI) filter setting (high versus low) on variance display. The variance area was planimetered using an image analysis computer. The following machine parameters were inversely correlated with variance area: (1) packet size (p less than 0.01), (2) pulse repetition frequency (p less than 0.001), and (3) frame rate (p less than 0.05). Both system gain (p less than 0.001) and wall filter setting (p less than 0.01) showed a direct correlation with variance area. We conclude that machine factors must be standardized in evaluating stenotic, regurgitant, and shunt lesions by color Doppler variance display imaging.