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Figure 2. Mechanisms of radiopharmaceutical accumulation at sites of inflammation. In passive diffusion (left), a metal radionuclide (circle) bound to a circulating plasma carrier protein (rectangle) extravasates through permeable capillary endothelium. A leukocyte-derived protein (triangle) may bind the radionuclide in the interstitium. In active migration (right), leukocytes labelled with a metal radionuclide (circle) are induced to adhere and transmigrate by the presence of endothelial adhesion molecules (rectangles).

Figure 2. Mechanisms of radiopharmaceutical accumulation at sites of inflammation. In passive diffusion (left), a metal radionuclide (circle) bound to a circulating plasma carrier protein (rectangle) extravasates through permeable capillary endothelium. A leukocyte-derived protein (triangle) may bind the radionuclide in the interstitium. In active migration (right), leukocytes labelled with a metal radionuclide (circle) are induced to adhere and transmigrate by the presence of endothelial adhesion molecules (rectangles).

Preparation

In order to cross the leukocyte's cell membrane, indium must be chelated to a lipophilic carrier molecule such as oxine or tropolone. The labeling protocol is outlined in Table 1. Not all departments have the ability to perform cell labeling as this requires experienced personnel, laminar flow hoods and impeccable quality control to ensure a safe and satisfactory product. Although there is no specific patient preparation for the test, it is important to ensure that the patient has an adequate number of circulating leukocytes (at least 3x109/L). A large quantity of anticoagulated blood (40-50 ml) must be withdrawn from the patient to obtain sufficient numbers of leukocytes. Autologous leukocyte scanning is not feasible in infants due to the large volume of blood required. Similarly, neutropenic patients are unsuitable subjects. In rare circumstances, donor leukocytes can be used.

Leukocytes must be handled gently to avoid activation as this compromises their viability and ability to migrate to sites of inflammation. In general, the period of time during which leukocytes remain ex vivo should be as short as possible and ideally less than 2 hours.

Imaging Considerations

Imaging typically involves both early and late scans. Early imaging is performed between one and four hours post-injection and the delayed images are performed at 24 hours. Due to the long half-life of mIn (approximately 3 days), further images up to 48 hours are also possible. With inflammatory bowel disease leukocyte

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