Babeh Edi

Positive Contrast Agents

Positive GI contrast agents can be divided into three categories: Paragmagnetic agents (e.g., Gd-DTPA solutions), short T1-relaxation agents (e.g., mineral oil), and combinations of these.

Paramagnetic Agents

Proposed paramagnetic, positive GI contrast agents include ferric chloride, ferric ammonium citrate, and gadolinium-DTPA (with and without mannitol). Paramagnetic materials cause both T1 and T2 shortening. At low concentrations used for bowel opacification, the T1 shortening dominates the signal intensity. This results in high intensity on T1-weighted, T2-weighted and gradient echo images. At high concentrations, T2 shortening causes decreased signal in all but very short echo sequences. This resembles the effect seen with superparamagnetic iron oxide (see Negative GI Contrast Agents). At intermediate concentrations, a mixture of T1 and T2 shortening results in increased signal on T1-weighted images and decreased signal on T2-weighted images. Ferric ammonium citrate and Gd-DTPA with mannitol are safe and effective in humans, but both have minor side effects. Ferric iron can cause teeth staining, gastric irritation, nausea, diarrhea, and constipation. Mannitol can nausea, vomiting, and diarrhea. Gd-DTPA without mannitol is well tolerated but usually fails in opacify the entire small bowel. It also needs to be buffered when used orally since this chelate is not very stable at the low pH found in the stomach.

Short T1-relaxation Agents

Short T1 relaxation time GI contrast agents include mineral oil, oil emulsions, and sucrose polyester. In these materials, protons contained in -CH2- groups relax at a faster rate than those in water resulting in a short T1 time. This gives a bright signal in the bowel on T1-weighted sequences. Of these materials only oil emulsions have been used successfully in humans. These are palatable and produce homogeneous opacification of the stomach and small bowel, but are absorbed in the distal small bowel and fail to fill the colon. This is circumvented by using a contrast enema when the colon must be better visualized. A novel approach to retrograde opacification of the colon has been shown in rats with a nonabsorbable fat substitute, sucrose polyester but no human trials for this use have been done.

Combination Contrast Agents

Combinations of oil emulsion and paramagnetic substances may be used as bowel contrast agents. These include an emulsion containing corn oil and ferric ammonium citrate, and an emusion containing baby formula with ferrous sulfate. These are palatable mixtures that distribute uniformly in the bowel, however signal is lost in the distal small bowel in adults, because of absorption of both the oil and the iron. Unlike in adults, the faster transit through the small bowel in infants delivers bright contrast to the colon. The advantage of this combination over oil emulsions alone is the enhancement of signal on T1-weighted and especially T2-weighted images.