What is the role of nuclear imaging in the workup of ulcerative colitis (UC)?

Updated: Feb 23, 2019
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Eugene C Lin, MD  more...
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Answer

Gallium-67 (67Ga) was once the only radionuclide used in the scanning of inflammatory bowel disease (IBD); however, leukocyte labeling has largely replaced the use of gallium because of better dosimetry and resolution and a lower radiation burden to the patient. Jones and associates performed 67Ga citrate scanning in 9 patients with ulcerative colitis, [30] and in all patients, there was good correlation between the regional uptake of gallium and the extent and activity of disease. In 2 patients, scans were positive during an acute exacerbation and reverted to normal or near normal during clinical remission. In 1 patient in whom the colon was resected because of toxic dilatation, good correlation was found between pathologic and scintigraphic results.

During an acute attack of ulcerative colitis, when colonoscopy or barium enema may be contraindicated, gallium scanning may provide a noninvasive means of assessing the extent of colonic involvement. It may also be an alternative means of following the clinical course of the disease.

With the availability of indium-labeled WBCs, radionuclide-imaging studies have a definite role in the diagnosis and staging of IBD. The indium-111 (111In) WBC study is particularly helpful in evaluating recurrent disease in patients with severe intercurrent diseases and in screening patients, without the need for barium examinations. [31]

Indium-111 oxine and 111In tropolone are neutral, nonpolar, lipophilic chelates that penetrate the cell membrane of leukocytes. Once inside the leukocytes, the 111In ion dissociates from the chelate and forms a relatively stable bond with cytoplasmic and nuclear proteins. There are a number of differences in the 2 chelating agents; one of the most important is that 111In oxine cannot be used to label leukocytes in the presence of plasma because of the higher affinity of 111In for transferrin than for oxine. Indium-111 tropolone is a stronger chelating agent; its use does not require the removal of plasma before cell labeling.

Both types of 111In label all types of blood cells indiscriminately; therefore, the leukocytes must first be separated from the other cells. The result is a mixed population of labeled leukocytes, including neutrophils, monocytes, and lymphocytes; this is acceptable for most imaging. Labeling efficiencies on the order of 80-90% may be achieved with either agent. If the labeling efficiency is less than 40%, the cells should not be reinjected.

Indium-111–labeled leukocytes are normally distributed in the spleen, liver, and bone marrow and transiently in the lungs. The accumulation of activity at sites other than these sites is suggestive of infection or inflammation.

WBCs may be labeled with 99mTc. Technetium-99m HMPAO is a neutral, nonpolar, lipophilic agent that was originally developed for cerebral perfusion imaging (see the image below). The compound is unstable and breaks down into a hydrophilic secondary complex. In its lipophilic form, 99mTc HMPAO can cross the cell membrane of leukocytes. Once within the cell, its structure is altered to the hydrophilic form, and 99mTc is trapped inside. [11]

Scan obtained with technetium-99m hexamethylpropyl Scan obtained with technetium-99m hexamethylpropylamine oxime (HMPAO)–labeled WBCs in a patient with active colitis involving the transverse and descending colon.

Technetium-99m HMPAO leukocyte labeling may be performed in the presence of plasma and appears to have significant selectivity for granulocytes. As with 111In-labeled leukocytes, 99mTc-labeled leukocytes are localized in the spleen, liver, and bone marrow and transiently in the lungs.

False-positive diagnosis of bowel inflammation is possible with 99mTc-labeled leukocytes. In adults, normal localization occurs in the bowel about 3-4 hours after administration; in children, normal localization occurs earlier. This localization is associated with sloughing of leukocytes into the lumen of the GI tract. In addition, activity is seen in the urinary tract and occasionally within the gallbladder. Fasting for 2-4 hours before imaging may reduce hepatobiliary excretion and subsequent bowel activity, especially in children. The 1-hour images demonstrate lung activity but no activity in the GI system.

Fluorodeoxyglucose positron tomography has been used in the diagnosis of pediatric IBD. This technique appears to provide adequate information for patients suspected of having IBD. [32]


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