LED-based photoacoustic imaging for early detection of joint inflammation in rodents: towards achieving 3Rs in rheumatoid arthritis research

Synovial angiogenesis and hypoxia in the joints are biomarkers of Rheumatoid Arthritis (RA). The ability to probe blood and accurately estimate the oxygen concentration make multiwavelength Photoacoustic (PA) imaging a potential tool for early detection of RA. In this work, a multiwavelength LED-based PA imaging system was characterized based on its imaging depth, resolution and accuracy of oxygen saturation estimation. A multicenter 3R (Replace, Refine and Reduce) focused small animal study was conducted. The 3R strategy was devised by reusing RA animal models, in vivo imaging of healthy animals and ex vivo studies with human blood. RA animal cadaver models with different levels of synovial angiogenesis (control, positive RA and treated) were imaged and compared against results from a previous study using the same samples. An ex vivo PA oxygen saturation imaging using human blood was validated against oximeter readings and further verified it with in vivo animal studies. An imaging depth of 8 mm with an SNR of 10 dB was achieved for RA samples. A difference in PA intensity was observed for RA models compared to control and treated group. The PA oxygen saturation estimation correlates with oximeter readings, which is confirmed with in vivo studies. The results show the efficacy of the LED-based PA imaging system in RA diagnosis based on synovial angiogenesis and hypoxia. The imaging depth, resolution and oxygen saturation estimate are sufficient to differentiate RA samples from control. Our future work will focus on validating the method using arthritis animal models and demonstrating the 3R potential.