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Color Maps: Variance Maps

Variance maps indicate the presence of turbulent flow, as well as the degree of turbulence. A green overlay indicates turbulence across the standard red/blue velocity color bar, from left to right. Greater turbulence results in the addition of more green. In a variance map, normal, laminar blood flow appears as either red or blue, depending on its direction. Non-laminar blood flow is indicated by any color other than red or blue.

You read a variance map both vertically and horizontally. The vertical axis indicates the relative mean velocity; colors close to the color bar baseline indicate low velocity blood flow, and colors toward the edge of the color bar indicate laminar blood flow, while colors along the right edge of the color bar indicate turbulent blood flow.

To estimate the degree of turbulence, the color algorithm first calculates the mean velocity at each sample site. It then plots this velocity on a standard red/blue map. For example, if a sample site has a mean velocity of 30 cm/sec., and blood flow is moving toward the transducer, the color displayed at this site will be medium red (halfway up the color bar).

The algorithm then calculates the amount of difference, or variance, between the individual data points at each sample site. In laminar blood flow, the data points measured at a specific sample site are close in value, while non-laminar blood flow has a wide range of data point values at each sample site. If the data points are all very similar, the variance is low; if they are greatly different, the variance is high. The greater the variance in the data point values, the higher the degree of turbulence in the blood flow.

NOTE Variances are statistical expressions of a distribution of frequencies. As is the case with statistics, there is no one correct answer. The color algorithm used to estimate variances differs among manufacturers.

Because variance maps and enhanced velocity maps provide different information, the same color means something different, depending on which map is used.