A 77-year-old male with history of right knee joint replacement 4 years ago, presented with swelling and pain following a fall. He soon developed fever with elevated WBC count, ESR and CRP. Blood cultures were negative. Whole body images were acquired in anterior and posterior projections, after the intravenous administration of 543 μCi of In-111 labeled autologous WBCs. They show a large area of abnormal accumulation of labeled WBCs in right thigh.

CT and fused SPECT/CT images in the coronal plane localize the uptake to the antero-lateral aspect of the right femoral prosthesis, extending into the knee.

Transaxial CT and fused SPECT/CT images localize the uptake to an abnormal density with an air fluid level, characteristic of an abscess. Patient underwent drainage, irrigation and debridement.

In-111 labeled leukocyte scintigraphy is the imaging modality of choice for evaluation of patients with suspected joint replacement infections. As compared to CT and MRI, scintigraphy is not limited by orthopedic hardware, and in combination with bone marrow scan (using Tc-99m sulfur colloid), provides excellent sensitivity and specificity in the diagnosis of prosthesis infection.

SPECT/CT is gaining popularity as a hybrid imaging modality where the functional (scintigraphic) and anatomic (low dose CT) information is acquired together to generate coregistered images. SPECT/CT imaging has been shown to improve sensitivity and specificity of labeled leukocyte imaging by providing precise localization and extent of abnormalities. This case highlights the importance of SPECT/CT hybrid imaging in In-111 leukocyte scintigraphy.

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This case was compiled by Dr. Farbod Nasseri, BCM