How to measure the pixels of the central column

Hi

See attached image please.

I want somehow, Matlab, identify the central column of pixels that start from the top (red pixel, contour) of the mask, until down, last pixel (red pixel, contour) of the mask. I have annotated the central column I need to estimate the pixels, with a yellow line. So, Matlab needs to identify the first red pixel and the last one (at the bottom), of this column.

Any help? Any ideas?

Thanks in advance