Focal Increased Tc-99m MDP Uptake in the Nutrient Foramen of the Femoral Diaphysis on Bone SPECT/CT

An 83-year-old female with osteoporosis visited our hospital for examination of left knee pain from 2 weeks prior, without a previous history of trauma or excessive exercise. She had undergone bilateral total knee replacement arthroplasty surgery at another institute 3 years ago. Bone scintigraphy and single-photon emission computed tomography/computed tomography (SPECT/CT) were performed on a hybrid SPECT/CT scanner (Symbia Intevo, Siemens Medical Solutions, IL), 3 h after injection of 925 MBq Tc-99m methylene diphosphonate (MDP). CT was performed using a 16-slice helical CT scanner with 110 KV and 30 mA in the AutomA mode. Attenuation-corrected SPECT images were reconstructed by Flash 3D algorithms (8 subsets and 3 iterations; Gaussian filter of 8.4). Bone scintigraphy and SPECT/CT revealed focally increased MDP uptake in the left femoral lateral condyle, which was possibly due to prosthesis loosening. Furthermore, there was a focally increased uptake in the medial cortex of the mid-shaft of left femur with matched linear radiolucency on CT images of SPECT/CT (arrow) (a: bone scan; b: axial CT, SPECT, and fusion SPECT/CT; c: coronal CT, SPECT, and fusion SPECT/CT). Based on the typical location, the classical bone scan findings of stress fracture (e.g., focal intense and fusiform cortical uptake), the concurrent CT abnormality on SPECT/CT, and the patient’s history of osteoporosis, atypical left femoral stress fracture was suspected (1–4). Plain radiography, thin-section non-enhanced CT, and contrast-enhanced magnetic resonance imaging (MRI) of the left femur were requested for confirmatio