Findings: A three-phase bone scan was performed after administration of approximately 30 mCi of Tc-99m-MDP intravenously. Dynamic images demonstrate hyperemia and increased uptake of radiopharmaceutical activity in the patient's left knee joint. Blood pool and delayed images both demonstrate abnormally increased uptake in the same knee. Additionally, there is increased uptake in the patient's left shoulder, to a lesser degree in his right shoulder. All of the foci of increased activity could be explained based on the clinical history of arthritis. However, intense uptake in the distal left femur and the tibial plateau were suspicious for septic arthritis. The clinical examination also pointed towards infection. Decision was made to perform imaging with Tc-99m-HMPAO-labeled leukocytes. Intense uptake was seen in only the left knee joint and proximal tibial shaft.

Diagnosis: Septic arthritis.

Discussion: Bone scans with Tc-99m-MDP is commonly performed for evaluation of osteomyelitis. There is increased concentration in areas of increased blood flow and osteoblastic activity. This increased uptake is nonspecific in areas such as the metabolically active epiphyses in children, in tissues involved with infection, inflammation, or other reparative processes such as Paget's disease, or neoplasm. When bone scans are positive but nonspecific, correlation with clinical information and with other modalities is important. Ga-67-citrate, In-111-WBC, or Tc-99-HMPAO-WBC imaging may be used in conjunction with bone imaging to study osteomyelitis, septic arthritis and similar conditions. Disadvantages of In-111 leukocyte imaging predominantly stem from the cyclotron production of the radionuclide and unfavorable dosimetry. The major advantage of its widespread availability in the kit form make Tc-99m-hexamethylpropyleneamineoxime (HMPAO) makes it a suitable agent for this purpose. Physical characteristics of Tc-99m as a radionuclide are also favorable. Schauwecker found that in many clinical trials, the specificity and sensitivity of this agent in comparison with others was found as follows:

	Radiopharmaceutical	Sensitivity (%)	Specificity (%)
	Tc-99m-MDP		94-95			33-95
	Ga-67			81			69
	In-111-WBC		88			85
	Tc-99m-HMPAO WBC	87			81

References:
Schauwecker DS. The Scintigraphic Diagnosis of Osteomyelitis. AJR 1991;158:9-18.

Lantto T, Kaukonen JP, et al. Tc-99m HMPAO Labeled Leucocytes Superior to Bone
Scan in the Detection of Osteomyelitis in Children. Clin Nucl Med. 1991;17:7-10.

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