Labelled leucocyte scintigraphy in inflammatory bowel disease: clinical applications.

Labelled leucocyte imaging is an established technique for the detection of inflammation and sepsis. Of the many radioisotopes used to label leucocytes, Indiuml1 tropolonate (11 1In) and Technetium-99m hexamethyl propylene amine oxime (99mTc HMPAO) are the most widely used. Leucocytes are obtained from heparinised, peripheral venous blood (40-100 ml) and then separated from erythrocytes and platelets by sedimentation with hydroxyethyl starch. Cell labelling is achieved by incubation with the radioisotope in plasma (or saline). After washing, the labelled leucocytes are injected intravenously and serial images are obtained using a gamma camera. 'In labels and remains firmly bound to all cells in mixed leucocyte suspensions. The use of pure granulocyte preparations probably reduces background activity and improves the image quality.' 99mTc HMPAO initially binds to all blood cells but subsequently elutes from platelets and red cells leaving labelled granulocytes behind. 99mTc HMPAO is therefore considered a selective granulocyte label even when mixed leucocyte preparations are used.2 3 After injection, the labelled granulocytes marginate and then migrate into the inflamed lesion before being shed into the bowel lumen. Serial imaging is important to distinguish luminal from bowel wall activity. With "'In images are most often obtained at three to four hours.4 Forward migration of luminal radioactivity5 makes late (24 hours) scans of little value, except when the technique is used to detect sepsis, for which late imaging is required. After 99mTc HMPAO imaging at 30-60 minutes and at two hours is preferable.3 The technique of labelled leucocyte imaging has been used to detect abscesses and other inflammatory lesions in which there is heavy granulocyte accumulation. Mucosal infiltration with leucocytes is a characteristic histological feature in inflammatory bowel disease particularly during periods when the disease is active. Labelled leucocyte scanning has been used in the global assessment of patients with IBD, with encouraging results. In this article we examine the role of this non-invasive technique in the management of patients with ulcerative colitis and Crohn's disease.