The new dynamic capillaroscopy option for the CapiScope software now enables capillary blood cell velocity to measured from live or recorded video using a spatial correlation technique.

Measurements can be made in real time, directly from the subject. However, it is usually more convenient to record onto good quality video tape or, for best quality, directly to the computer hard disk. This also allows more than one vessel to be measured from the same image sequence.

Velocity is measured by using a mouse to draw a line along the vessel. The vessel can have any orientation, and does not even need to be straight. The line thickness can be adjusted so that it covers the width of the capillary. This gives an average of the pattern over the whole width.

The grey level profile along the line is taken for each field every 1/50th of a second (1/60th second for NTSC based systems).

The grey level pattern along each line is compared to the pattern from the next field (or several fields later for very low velocities). The comparison is performed by calculating the correlation coefficient for every possible shift of the previous grey level profile relative to the new profile. The shift which produces the highest correlation, which can be seen as the peak in the correlation (x1000) against relative shift, indicates the distance travelled between the two grey level profile measurements, and hence the velocity. CapiScope displays the correlation, along with the velocity trace (cbv) as a colour map, showing red for a high correlation through to blue for low correlation.