Small bowel followthrough should be able to detect abnormalities in the structure of the bowel wall, indicative of crohn's -- ulcerations, granulomous tissue etc., gas patterns, not just the string segments that indicate narrowing. The problem with SBFT is with how they are performed. Timing and patience are required to get clear, unobstructed images. busy radiology departments often cut costs by hurrying the process and/or waiting too long in between images. Once the contrast medium has emptied into the large bowel, much of the small bowel is obscured. If the images haven't been taken prior to the emptying, it can be difficult to visualize the last bit of small bowel. Some parts of the bowel are more difficult to visualize than others. Also, after multiple surgeries and scarring that comes along with that, the difficulty of visualizing the entire small bowel can increase.
Also, it is important to remember that inflammation will show up as narrowing too and that is often the first thing that happens, long before the real nasty damage takes place.
30+ years living with Crohn's.