Defecography is a functional study of the distal colon, which includes fluoroscopic or radiographic imaging during defecation and is used to evaluate fecal incontinence and other functional problems of the lower GI.
If you can't tell, this chapter in positioning has to do with imaging the upper and lower GI (Gastrointestinal) system and there are several new procedures we have to learn, one of them being defecography.
Defecography is not an uncommon procedure and it can be performed at most hospitals and imaging clinics. However, in my opinion, the way they go about getting the images is a bit.....ummm odd.
In order to visualize soft tissue (e.g. parts of the digestive tract), a contrast material must be used. A contrast material, often just called contrast, is a substance that is radio-opaque, or does not allow x-rays to pass through it, and shows up as white on the radiograph (aka x-ray image). The contrast material most often used for contrast studies of the GI system is barium sulfate. Barium usually comes in liquid or semi-liquid form and generally has the consistency of pepto bismol, so that it coats the walls of your GI tract and also does not pass through too quickly. It's administered either orally or via a barium enema.
Defecographs are different in that they are a functional study and the actual movement of the colon is observed using fluoroscopy, a sort of live, moving x-ray. But for the colon to be able to act on the barium contrast in order for the radiologist to observe function, the contrast must have a thicker consistency than a pepto bismol-like liquid.
Here is where it gets interesting. A common practice among radiographers is to mix the regular barium liquid with freeze-dried instant potato flakes in order to obtain the proper consistency. Of course, other substances have been tried, but most don't offer the proper consistency, even dispersion of barium and "cost effectiveness" as simple, over-the-counter instant potatoes.
So if you ever have the need for a defecography study, just know that somewhere in Idaho a farmer worked very hard on his harvest just so the doctor could get a good look at your large intestine in action.