Our study compared the evaluation of RA activity by thermal imaging, through miniaturized thermal cameras, with clinical and ultrasound evaluation. Using an automatic detection of the ROI temperature, thermal imaging was correlated to tender joint count and US mode B results, but was not able to discriminate RA activity at the joint level for more relevant parameters of inflammation markers, i.e. swollen joints and PDUS.
Using direct visual identification of synovitis, thermal imaging showed only limited concordance with US measurements (mode B and PDUS).
There are few studies comparing clinical evaluation with thermal imaging from miniaturized thermal cameras, and their results are discordant.
In 2018, Jones et al. (24) compared thermal imaging of the hand joints of patients with RA to those of healthy volunteers. Their patients’ characteristics were similar to ours. Thermographic analysis of joint temperature was not associated with clinical measures of disease activity. The calibration of their camera, the computer software, and the temperature assessment were not specified (24). The authors suggest that the clinical examination may lack sensitivity and that it would be interesting to compare it with US assessment.
Tan et al. (27) recently compared thermal imaging with US and clinical evaluation. Their sample was smaller (n = 37) with patients whose RA was more recent, but with the same activity as our population. They reported a significant association between thermal imaging and US data, but not with clinical examination. However, they performed assessment on all MCP and PIP joints, including thumbs. In our study, we made the choice to exclude thumbs as we wanted to avoid the bias of spotting thermal arthritis that could be either attributed to RA flare or to trapezo-metacarpal osteoarthritis which is common on this joint. Neither the thermal camera used in Tan’s study, nor the way of spotting joints and measuring their temperature, were described thus preventing detailed comparison with our study.
Despite the validity of our results, our study had several limitations. First, there was a lack of statistical power. Although including a large number of patients, the number of joints with the highest markers of RA activity was small compared to the number of joints with low activity (173 swollen joints vs 748 non-swollen, and 42 PDUS grade 3 joints vs 782 PDUS grade 0 joints). To increase the power of the study, it would require a greater number of active joints (swollen joints and PDUS grade 2–3 joints).
Another concern was the potential bias in PDUS evaluation. Five evaluators performed the ultrasound evaluation as our work was a daily practice observational study. We did not assess the intra and inter-reproducibility of the ultrasound evaluation.
The sensitivity of the thermal camera was also a potential limit. Indeed, the model used had a sensitivity of 0.1 °C (23). More recent cameras have been marketed since the start of our study, with a finer sensitivity (< 0.06 °C) (23). A new study with the use of a more sensitive thermal camera would be needed to better assess thermal arthritis, particularly those clinically swollen or presenting PDUS grade 2–3.
The methods used in the different studies to assess the presence of thermal synovitis from temperature data were heterogeneous, as HDI was used for Salisbury (18), ΔT in our study and Tan’s (27), or not mentioned for Jones (24). A consensus on standardized measures would be required in future works.
Although our results did not show significant results on inflammation markers used for RA activity’s evaluation (swollen-joint and PDUS-3), we observed a numerical trend for these parameters. Indeed, the temperature difference of joints with PDUS-3 was higher, meaning clinically “hotter”, than those without doppler signal (PDUS-0). This trend was also found between the group with doppler signal (PDUS 2–3) and those without PD (PDUS 0–1). This was also the case for clinically swollen joints versus non-swollen joints.
Finally, thermal images were analyzed through automatic and direct visual detection. The results of both analyses were consistent, indicating that direct visual reading could be used without needing the software. This strengthens the thermal camera as a quick and easy tool to be used in ambulatory practice.