How to find and segment circular object, starting with a center point of circle

Hi all, i have an CT image which I've done enhanced and extracted a circular object (lumen of the aorta) from it. I got its coordinates of x and y center points. How can i utilize this point (create a mask?) to extract the same circular object (lumen) for all remaining images?

Starting from the second image, by using thersholding, i get the objects which have intensity within 230-580. Then i labelled the candidates of lumen in binary image. I have centerx and centery as the pixel coordinate passed from the lumen image i found in the first image. code as below:

[nx ny] = size(HU);    
for i = 1:nx 
      for j = 1:ny 
          if (HU(i,j) < 230) 
              canlumen(i,j) = 0; 
          end 
          if ((HU(i,j) >= 230) & (HU(i,j) <= 580)) 
              canlumen(i,j) = 1; 
          end
           if (HU(i,j) >580) 
              canlumen(i,j) = 0; 
          end 
      end
  end
  figure;imshow(canlumen,[]);

    L = bwconncomp(canlumen);
    previousx = centerx;
    previousy = centery;

How can i proceed from here? Appreciate if you can help to provide me guidance how to proceed from here. Thanks.