Colonic Scintigraphy

More recently, radioscintigraphy has been used to provide a more detailed assessment of overall and regional colonic transit with an acceptable radiation exposure. Two approaches have been used.

Orocecal Intubation

To avoid dispersion of radiolabel during its passage through the stomach and small bowel, orocecal intubation was utilized to instill liquid radioisotopes, such as mIn-DTPA, directly into the large bowel.42 This method has been adapted by simply following an mIn-liquid meal emptied from the stomach.43 Thus, there is a definite starting time for colonic transit, and liquid transit through the different colonic regions could be assessed. Using this method, Krevsky et al42 suggested that the ascending colon empties content rapidly, in an exponential manner; in contrast, the transverse colon was the major site of colonic storage. This method was a significant advance that allowed for a more detailed and dynamic assessment of colonic transit, previously impossible using radiopaque markers; because the radiation exposure does not increase with the number of gamma camera scans taken, multiple images can be taken at defined time intervals. However, the need for orocecal intubation renders such a method much less applicable in clinical practice. An alternative approach involves cecal intubation via colonoscopy; however, the latter requires emptying of the colon and, hence, transit tests obtained after colonoscopic intubation may not mimic physiologic conditions.

Delayed-Release Capsule

In an attempt to avoid orocecal intubation, a new technique has been developed that exploits the pH gradient in the small intestine (mean duodenal pH, 6.5; ileal pH, 7.4; cecal pH, 6.8.44-46 The distal ileum empties solid residue in boluses, and this ensures that isotope in the distal ileum is transferred to the colon as a bolus.20 Radiolabeled polystyrene pellets (99mTc or mIn) or activated charcoal was placed in a medication capsule covered with a single coating of the pH-sensitive polymer, methacrylate (Fig. 7.7). In vitro studies showed that this capsule dissolved at a pH of between 7.2 and 7.4 within approximately 1 hour of exposure, which was the average residence time of radiolabeled pellets in the distal small bowel,20 where the intraluminal pH is typically alkaline. However, if the pH was

Radioscintigraphy

Figure 7.7. Scintigraphic method for combined gastric, small-bowel, and colonic transit measurement. Note the use of different isotopes to label solids or activated charcoal that are incorporated in the meal and the methacrylate-coated capsule. Adapted from Burton DD, Camillieri M, Mullan BP et al. Colonic transit scintigraphy labeled activated charcoal compared with ion exchange pellets. J. Nucl. Med. 1997;38:1807-1810. Reprinted by permission of the Royal Society of Nuclear Medicine.

Figure 7.7. Scintigraphic method for combined gastric, small-bowel, and colonic transit measurement. Note the use of different isotopes to label solids or activated charcoal that are incorporated in the meal and the methacrylate-coated capsule. Adapted from Burton DD, Camillieri M, Mullan BP et al. Colonic transit scintigraphy labeled activated charcoal compared with ion exchange pellets. J. Nucl. Med. 1997;38:1807-1810. Reprinted by permission of the Royal Society of Nuclear Medicine.

Figure 7.8. Entry of '"In pellets from distal ileum into ascending colon following release from a methacrylate-coated capsule.

below 7, dissolution of the polymer did not occur for several hours.47 Thus by using the natural pH gradient in the small bowel, radiolabeled solid pellets can be reliably delivered directly into the right colon (Fig. 7.8), permitting noninvasive evaluation of colonic transit.47 Application of this novel technique to the unprepared GI tract showed the ascending colon to be a reservoir for solid residue (Fig. 7.9). Emptying of the ascending colon occurs in two phases: an initial lag phase, followed by an emptying phase, which is often linear.47 By simultaneous study of the transit of radiolabeled solid particles with liquid delivered into the cecum by means of an oroce-cal tube, it was found that, in healthy subjects, a small volume of liquid isotope traverses the ascending colon with the solid pellets. This finding suggested that there is no discrimination between solid and liquid transit in the ascending colon.48 Instead of polystyrene pellets, the mIn can be adsorbed on activated charcoal.49

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Constipation Prescription

Constipation Prescription

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