I've had 3 such scans. Its not all that uncommon to have Crohn's only in the small intestine. More common than to have it in the small intestine AND elsewhere. Mine had been confined to my small intestine since its inception in 1975.
Depends on the radiologist's particular protocol whether you will have to be NPO for a particular amount of time which can vary. You arrive at the imagining facility, they draw 50 ccs of blood, they use a butterfly type and cap it off. The blood is sent to the lab where it is spun down, the white blood cells extraced and then a "marker dye" added to them. This can take a couple of hours, depends on how busy the lab is. When the marked WBCs are received in the imaging facility they are injected into the uncapped butterfly (the draw and infusion are usually done in the inner elbow area as for an IV, etc.). Then scans of the abdomen are done on a time schedule. Just before each scan they will ask that you empty your bladder so the full bladder doesn't interfere w/scans of the intesting. You may or may NOT be asked to return to the imaging facility 24 hours later for another scan. Time spent w/the scanner is minimal but a lot of time is spent in the waiting room or wherever between scans. Plan on at least 4 hours.
WBCs rush to the scene of infection or inflammation and cluster there to fight the infection or inflammation. By marking the WBCs the radiologist can see where in the intestine there is inflammation or infection by the white clumps of WBCs that show up on the films. Now days most imaging is done digitally so you might want to ask for a CD of your scans. You can view them at home on your 'puter and have them to loan to any new doctor you might opt to see.
In our area the WBC intestinal scan is cheaper than an upper GI w/SBFT or a colonoscopy if copays are an issure for you.
My computer says I need to upgrade my brain to be compatible with its new software.