Combination contrast agentsĬombinations of oil emulsion and paramagnetic substances may be used as bowel contrast agents. This is circumvented by using a contrast enema when the colon must be better visualized.Ī novel approach to retrograde opacification of the colon has been shown in rats with sucrose polyester, a non-absorbable fat substitute, but no human trials for this use have been done 2. 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. Of these materials only oil emulsions have been used successfully in humans. This gives a bright signal in the bowel on T1-weighted sequences. In these materials, protons contained in -CH2- groups relax at a faster rate than those in water resulting in a short T1 time. Short T1 relaxation time GI contrast agents include mineral oil, oil emulsions, and sucrose polyester. It also needs to be buffered when used orally since this chelate is not very stable at the low pH found in the stomach 1. Gd-DTPA without mannitol is well tolerated but usually fails to opacify the entire small bowel. Mannitol can cause nausea, vomiting, and diarrhea. Ferric iron can cause teeth staining, gastric irritation, nausea, diarrhea, and constipation. 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.įerric ammonium citrate and Gd-DTPA with mannitol are safe and effective in humans, but both have minor side effects. This resembles the effect seen with superparamagnetic iron oxide (see negative GI contrast agents). 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. Paramagnetic materials cause both T1 and T2 shortening.Īt low concentrations used for bowel opacification, the T1 shortening dominates the signal intensity. Proposed paramagnetic, positive GI contrast agents include ferric chloride, ferric ammonium citrate, and gadolinium-DTPA (with and without mannitol). combination agents containing both "1." and "2.".short T1-relaxation agents (e.g. mineral oil).paramagnetic agents (e.g. Gd-DTPA solutions).Positive GI contrast agents can be divided into three categories:
0 Comments
Leave a Reply. |