A man with three knees? False “hot spot” with dual isotope imaging

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Abstract
Radionuclide imaging is subject to a wide range of potential artefacts. We have recently come across an unusual example which we have not seen previously described (Wells & Bernier, 1988; Ryo et al, 1985). A 66-year-old man presented with a pyrexia, 3 months after undergoing bilateral upper tibial osteotomies. The patient was referred for a 99Tcm methylene diphosphonate (MDP) bone scan followed by 111 In labelled white blood cell (WBC) scanning to determine whether one of the osteotomy sites was infected. Three hours before imaging, 400 MBq of 99Tcm> MDP was administered. Blood was taken for white cell labelling 90 min prior to MDP imaging. The MDP images showed intense uptake at the osteotomy sites, but were otherwise normal (Fig. la). Immediately after the MDP bone scan images had been obtained, 15.4 MBq of 111 In WBC were injected. Three hours later, the knees were imaged with the same gamma camera (IGE400A) fitted with a high energy collimator. Energy windows were reset to include the two 111 In peaks at 170 and 245 keV. An area of activity was seen at the site of each osteotomy with an additional area located between the knees (Fig. lb). This additional area of activity proved not to be contamination as it could be eliminated by moving one or other knee out of the field of view.

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