Dual energy CT for the diagnosis of gout: Evaluating the optimal Hounsfield unit setting for dual energy processing

Rationale and objectives

Dual-energy CT (DECT) plays an important role in the evaluation of gout based on its ability to identify monosodium urate (MSU) crystals. Post-processing of DECT includes selection of a minimum Hounsfield unit (HU) threshold, currently set at 150 HU. Adjustments to this setting will affect the amount of MSU that is identified, with lower thresholds identifying more true and false positive MSU. The purpose of this study is to assess the performance of several HU thresholds.

Materials and methods

134 DECTs were reprocessed with minimum HU thresholds set to 150, 140, 130, and 120 HU. Volume of MSU detected was compared across thresholds. Subsequently, readers reviewed 3D axial images from 109 cases at 150, 130, and 120 HU thresholds. For each case, the reader made a diagnosis of gout, rated their confidence, and reported the location of MSU deposits.

Results

Mean MSU volume detected was as follows: 0.160 at 150, 0.205 at 140, 0.276 at 130, and 0.444 at 120 HU. Based on reader diagnosis, there was no significant difference in sensitivity for the diagnosis of gout between the 150 (68.8%), 130 (71.8%), and 120 (69.7%) HU thresholds (p = 0.653). Specificity was higher at 150 HU (65.4%) than at 130 (58.4%) and 120 (52.2%), (p < 0.001). Mean reader confidence was 90.1 for 150 HU, 79.9 for 130 HU, and 74.0 for 120 HU (p < 0.001).

Conclusion

Between the 150, 130, and 120 HU thresholds, 150 HU performed best overall for making the diagnosis of gout on DECT. Further research is needed to determine if there is utility for utilizing lower threshold settings for certain clinical scenarios.