It's a well researched problem for many decades. Find algorithms here: http://www.visionbib.com/bibliography/contentsmedical.html#Medical%20Applications,%20CAT,%20MRI,%20Ultrasound,%20Heart%20Models,%20Brain%20Models. Pick on and code it up.
Try using
imshow(doubleImage, []);
to display your image. The [] lets you display the image without being in the 0-1 range that imshow() expects floating point images to be in. Otherwise you'll get all white like you did.
