finding the center of the pixel

i have a medical image (attached). I want to find the patches from two images(original and the deformed version of original) without the black background.

startCoord = [30,40];
 centerCoord = [70,75];
 patchSize = 32;

patchSourceImage = im(centerCoord(1)-ceil(patchSize/2):centerCoord(1)+ceil(patchSize/2)-1,centerCoord(2)-ceil(patchSize/2):centerCoord(2)+ceil(patchSize/2)-1);

patchTargetImage = locally_deformed_image(centerCoord(1)-ceil(patchSize/2):centerCoord(1)+ceil(patchSize/2)-1,centerCoord(2)-ceil(patchSize/2):centerCoord(2)+ceil(patchSize/2)-1);

Using this i want to find the displacment values from original and deformed image. MY question how should i determine my starting coordinate andcenter coordinate? My image if size 256*150.

[Robert Fennis]

I'm sorry, I really don't get what you are trying to do. You already have a center and start coordinate. Do you have displaced images that you are trying to align?

[prashant singh]

no, the center and starting coordinate values inside the code are just randomly taken for explanation purpose . i want to know how should i find those values for a given image. i have figured it out for the centre values . Now can you help me, to find the starting coordinate value of a given image. The starting coordinate should not be the top of the frame but it should be the image inside the frame i.e no black background. I have to do this on multiple images so i need more generalised solution.