Evaluation of indium-111 chloride as a radiopharmaceutical for joint imaging in a rabbit model of arthritis.

The potential use of 111In-chloride as a radiopharmaceutical for quantitating the extent of synovitis was investigated in a rabbit model of antigen-induced arthritis. Rabbits with monoarticular disease were injected with 111In-chloride and imaged at 30 minutes and 24, 48, 72, and 96 hours. In control knee joints, the ratio of activity in the knee to a soft-tissue region in the thigh remained constant (1.5-1.85:1); in arthritic knees, the 111In uptake ratio increased from 2:1 to 3.2, 3.5, 3.9, and 4.6:1 at the later time points. The difference between the knee-to-soft-tissue ratios observed in the control and involved joints was significant (P less than .01, 48-96 hr). Tissue distribution measurements demonstrated that the synovium and intra-articular structures covered by synovium in arthritic joints had the highest concentration of 111In. In control joints, these tissues were found to take up tenfold less activity. Good correlation was observed between the 111In knee-to-soft-tissue ratios calculated from the images and histologic assessment of the severity of synovitis.