how to get only the central part of the dicom frame using adaptive thresholding or any onther segmentation tool

sairam to every one i have a myocardial dicomfile containg 72 frames of 36 short axis images where i need to do get the myocardium volume from those images where i need to do segment the central part using adaptive thresholding or canny edge detection or any other segmentation technique please can any one help me iam getting like this when i apply adaptive thresholding

for i=1:18 v(:,:,i)=X(:,:,:,i); s(:,:,i)=edge(v(:,:,i),'canny','both'); y(:,:,i)=bwmorph(s(:,:,i),'skel',inf); z{i}=bwarea(y(:,:,i)); sum=sum+z{i}; figure() imshow(v(:,:,i),[]) end first image original image 2nd image is after applying adaptive thresholding but i need only central part