find distance from center pixel to next white pixel

You can generate a distance grid with the code below. Using find, you can find what you define as a high intensity pixel.

x_center=320;
y_center=240;
[X,Y]=meshgrid((1:640)-x_center,(1:480)-y_center);
distance_to_center=hypot(X,Y);

You can find the center point of a blob with regionprops:

stats = regionprops(your_binary_image);
centroid = stats.centroid;

Then you should be able to use find on the relevant row and column to find the outer edges of your shape.