Performance of a novel high-resolution infrared thermography marker in detecting and assessing joint inflammation: a comparison with joint ultrasound.

Objectives: To examine the value of a novel high-resolution thermographic marker in the detection of joint inflammation compared to joint ultrasound (US) and to suggest thermographic cut-off values of joint inflammatory activity.

Methods: Infrared thermographies were performed in patients with inflammatory arthritides and healthy controls. Patients were moreover examined clinically and by joint-US [Power-Doppler-(PDUS), Greyscale-US (GSUS)]. Regions of interest (ROIs) were defined for every joint and absolute temperature values within the ROIs were documented. The hottest areas ("hotspots") were identified by a clustering algorithm and the Hotspot/ROI-Ratio (HRR)-values were calculated. Subsequently, the HRR of patient-joints with different grades of hypervascularity (PDUS I°-III°) were compared among each other and with PDUS 0° control-joints. Diagnostic HRR-performance was tested by receiver-operating-characteristics.

Results: 360 joints of 75 arthritis-patients and 1,808 joints of 70 controls were thermographically examined. HRR-values were statistically different between PDUS I-III vs. PDUS 0 and vs. healthy subjects for all four joint groups as well as in the majority of cases between patient-joints with different grades of hypervascularity (PDUS I°-III°; p<0.05). Taking joint-US as a reference, the best performance of HRR was found at the level of the wrist-joints by an area under the curve (AUC) of 0.91 (95%CI 0.84-0.98) with a sensitivity of 0.83 and specificity of 0.88.

Conclusions: HRR showed an excellent performance in the differentiation of joints with US inflammatory activity from non-inflamed joints. Moreover, HRR was able to differentiate between joints with different grades of hypervascularity, making HRR a promising tool to assist disease activity monitoring.