Newer Methods

Small-Bowel Magnetic Resonance Imaging

A magnetic resonance image (MRI)-based method to determine the small-intestine transit time has been described in the literature in a small number of healthy individuals.25,26

Different contrast agents, gadolinium and perfluorononane, have been used to track the flow of these agents through the gastrointestinal (GI) tract noninvasively. Gadolinium is a highly stable nontoxic aqueous-phase marker, and the safety of the agent following oral administration

Figure 7.4. Patterns of colonic filling in health and disease. Note the delayed but normal bolus transfer in a neuropathic disorder; in contrast, the patient with systemic sclerosis transfers small amounts of radiolabel from ileum to colon over several hours.

has been documented extensively in gastric emptying studies.7,27

Magnetic resonance imaging is performed immediately before and for up to 96 hours after oral contrast ingestion. Each imaging session lasts about 5 minutes, and contrast is easily followed. The three-dimensional (3D) images using this method ensure sufficient image quality to permit delineation of individual bowel loops and to characterize any particular small- or large-bowel segment.

This method seems attractive in that there is no ionizing radiation, images can be collected as often as necessary, and other disease processes such as neoplasia can be detected; however, cost may represent a major limitation. Furthermore, the clinical utility of the proposed method remains to be proven with large number of subjects. This method has not been either standardized or validated for research and clinical practice.

Magnetic Telemetry

A method using a special magnetic marker to measure gastrointestinal transit in healthy subjects has been documented.28 This method requires continuous monitoring of an ingested, magnetically marked capsule with biomagnetic equipment. After ingestion of a magnetic

Figure 7.4. Patterns of colonic filling in health and disease. Note the delayed but normal bolus transfer in a neuropathic disorder; in contrast, the patient with systemic sclerosis transfers small amounts of radiolabel from ileum to colon over several hours.

capsule, its magnetic field distribution over the abdomen is recorded with a magnetometer. Using the magnetic field distribution, the position of the capsule within the gastrointestinal tract is calculated.

This is an attractive, noninvasive method to measure gastrointestinal transit without the risk of radiation exposure. However, it requires a stable magnetic environment, and any heavy metal can disturb the magnetic field. This method requires presence in the lab for the entire duration of the test, and any movement, even breathing, of the subject can interfere with the trajectory of the magnet and cause artifacts. This method is at a preliminary stage and has not been standardized or validated.

Stable Isotope, [13C] Ureide, Breath Test

The stable isotope, [13C] ureide, breath test is an indirect method to measure orocecal transit time of solids or liquids that produces results that are comparable with those of radioscintig-raphy.29 This test measures the changes in the expired air of the 13C to 12C ratio measured by mass spectrometry. Breath samples are collected over 10 hours after administration of a stable isotope (13C)-labeled test substrate, lactose-ureide. The orocecal transit time is then determined semiquantitatively from the kinetics of the breath 13CO2 excretion. Lactose-ureide is not absorbed in the human small intestine, but colonic bacteria readily metabolize lactose-ureide, producing 13C-labeled CO2. The time at which 13CO2 appears in the breath corresponds to the orocecal transit time. Breath samples can be mailed to the analytic laboratory and can be stored for months without a change in their isotopic enrichment.

The advantages of the breath test are that it is noninvasive, there is no use of ionizing radiation, and it is convenient. The limitations are the lack of information on the possible influence of noncolonic bacterial flora in the upper GI tract, such as in the oral cavity and small intestine. Furthermore, the test does not provide information about gastric emptying or large-bowel transit studies that are possible with scinti-graphic technique.

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Constipation Prescription

Constipation Prescription

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