Protons in -CH2- groups, e.g., contained in fatty emulsions, mineral or vegetable oil or sucrose polyester, have a fast relaxation and short T1 time. These agents with short T1-relaxation, if used in gastrointestinal imaging, produce bright signal intensities in the intestine on T1 weighted sequences. Palatable oil emulsions can produce appropriate contrast opacification of the stomach as well as the small bowel, but caused by absorption in the distal small bowel these materials are not suitable for use in MRI colonoscopy.

See also Positive Oral Contrast Agents and Gastrointestinal Paramagnetic Contrast Agents.

Types of oral contrast agents with positive signal enhancement:
Ideal oral contrast agents are immiscible with water, biologically inert, have a low viscosity and surface tension. Oral positive contrast agents may improve the separation of bowel loops, the detection of polyps in colon MRI or the assessment of inflammatory bowel in the small intestine. Several positive oral contrast agents are available and they are safe to use, for example gadolinium solution, ferric ammonium citrate, different oil emulsions and pediatric formula.
Unfavorably motion artifacts caused by respiration and peristalsis may be increased in MR imaging. In addition, the signal of the positive contrast medium may decrease caused by dilution in gastrointestinal (GI) secretions. With the use of contrast agents that are immiscible with water, no dilution and accompanying signal loss occur even when the contrast agent is in contact with the intraluminal contents of the GI tract. Another disadvantage may be residual substances in the bowel, resembling masses when enclosed by bright signal. In addition, positive contrast agents may have a similar signal as bright masses, which make their (e.g. lipoma) detection difficult.

See also Gastrointestinal Paramagnetic Contrast Agents, Combination Oral Contrast Agents, Gastrointestinal Diamagnetic Contrast Agents.