See capistep.xml of the Step by Step Chapter, for a quick tutorial on making velocity measurements.
The velcoity calculation requires a measurement line. Most of the time, CapiScope can determine which line to use, but in the case of several dimension lines, or several video sequences open at the same time, you will need to specify which line to use.
Right click on the first node of the line. This should open a small menu. Click on "measurement line" to make this line the current measurement line.
| Warning |
All data in the linescan window will be lost without warning when switching to another line. |
There are a few points to consider when creating measurement lines.
Maximum velocity is half the line length multiplied by number of fields per second (i.e. 50 or 60). This is reduced if the sample rate or number of linescan samples between correlations are not both 1.
A straight line avoids the distortions which occurs at bends.
Horizontal lines have twice the resolution as vertical lines.
Using a line which is wider than the vessel, might help when there are movement artifacts, but the signal, and hence correlation will be poorer, since it will be averaged with useless background information.
If the vessel is large (> 30 um) then make the line cover only the central third of the vessel, otherwise the correlation will tend to pick out the slower rolling leukocytes along the vessel wall.
The line needs to be at least twice as long as typical pattern (erythrocyte gap or leukocyte). Note that the resolution of the cbv velocity measurement is half the length of the measurement line.
It is important to have no movement in the video sequence. If the subject moves, it is possible to reposition the line by dragging the first node of the measurement line using the mouse and left mouse button. This can be done whilst the sequence is running, or by stopping the video sequence, repositioning the measurement line, then resuming the sequence by clicking on the play button.