Fig. 1

Six sets of six sterile pipes were prepared at different concentrations, 36 pipes overall (a). For all sets, Gd-BOPTA and saline solution (NaCl 0.9%) were mixed into five pipes in order to achieve a concentration of 4 mmol/l, 2 mmol/l, 1 mmol/l, 0.67 mmol/l, and 0.5 mmol/l; the sixth pipe was filled with 5 ml of 2 mmol/l pre-diluted Gd-DTPA and served as a reference (b). To simulate the environment of a joint, five sets of pipes were added with 0.5 ml of fresh synovial fluid withdrawn from the knees of five patients presenting to our department for intra-articular injections (c)