Metaphase capture scan is done at 100X magnification. Stage is moved to the location of the metaphases found in pre scan. Since the operating distances are in units of microns, the location of the metaphase when seen at 100X magnification might not be at the center of the field of view, for this a metaphase centering algorithm is used so that the final image of the metaphase is at the center.
The center of the metaphase in the image is calculated. Nucleus and other debris if present in the image are removed and center of only metaphase is found. Sometimes entire metaphase may not be in the field of view; in that case center of the part of the metaphase in the field of view is found. If this center is equal to the center of the image then metaphase can be assumed to be at center, otherwise stage is moved to the calculated center of metaphase and the process is repeated again. Since this process may get stuck in an infinite loop, it is executed for not more than 3 times on an image. The performance of the algorithm has been satisfactory.
It takes around 1 to 2 seconds to calculate the center of a metaphase using Metcenter.m Matlab script. Time taken for metaphase capture scan depends on the number of metaphases found on slide. For example, if there are 100 metaphases on a slide it takes around 10 minutes to complete the metaphase capture scan.