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Imaging Agents

Technetium-99m MDP/HDP

Bone scintigraphy is most often performed utilizing technetium-99m (Tc-99m) methylene diphosphonate (MDP), or Tc-99m hydroxymethylene diphosphonate (HDP). Both compounds are diphosphonates (bisphosphonates) that show chemisorption to hydroxyapatite and chelate the radionuclide Tc-99m for imaging purposes.1 Tc-99m has ideal imaging properties secondary to its relatively low energy and abundant gamma ray at 140 kilo electron volts (keV), its short 6 hour half-life, and its ready availability in generator form. The resulting chelates show an in vivo distribution of radioactivity that reflects the osteoblastic activity of bone with relatively high sensitivity but low specificity. This low specificity often requires correlation with other imaging studies and patient history to better define lesion etiology.2 The usual administered adult dose is approximately 740-1100 megabecquerels (MBq), [20–30 millicuries (mCi)], resulting in an effective total dose of approximately 5.9-8.8 millisieverts (mSv).3

Indium-111-Labeled White Blood Cells

An extremely useful method for imaging inflammation/infection is the utilization of radiolabeled white blood cells (WBC). The vast majority of white cells extracted from peripheral blood are polymorphonuclear leukocytes (PMNL) that are chemotactically attracted to foci of most acute inflammatory/infectious processes. Both Tc-99m hexamethylpropyleneamine oxime (HMPAO) and indium-111 (In-111) oxyquinoline are lipophilic compounds that may be used to label isolated peripheral WBCs. For imaging musculoskeletal infections, In-111 is felt to be the optimal radionuclide because of its longer half-life of 67 hours and its medium energy gamma rays at 174 and 247 keV.1 The longer half-life allows more time for labeled leukocytes to localize in regions of osteomyelitis that may have overall lower numbers of associated WBCs compared with acute pyogenic abscesses. The medium energy photopeaks allow for simultaneous image acquisition with Tc-99m-labeled bone marrow for improving the specificity of osteomyelitis detection. This technique poses moderate risk for the patient. Proper labeling of peripheral WBCs is technically complex and is usually performed at experienced tertiary commercial radiopharmacies. This requires collection and centralization of blood samples from multiple, usually highly infectious, patients daily with the associated risk of cross-contamination. Radiation-sensitive white cells, including a small number of lymphocytes, are internally exposed to ionizing radiation resulting in an unknown but assumable significant risk. The usual dose of In-111 for this procedure is 10-20 MBq (0.3–0.5 mCi), resulting in an effective total dose of approximately 6-12 mSv.4

Gallium-67 Citrate

Gallium-67 (Ga-67) citrate is a radionuclide that has been used for over 30 years for tumor and inflammatory imaging. Its localization in infectious or inflammatory lesions is felt to be secondary to multiple factors including binding to transferrin and lactoferrin, both found in high concentration in inflammatory foci. Direct uptake by iron binding bacteria as well as by activated phagocytic cells has also been seen. The imaging properties of Ga-67 are somewhat less than ideal with multiple low-to-medium energy gamma rays at 93 keV, 184 keV, 296 ...

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