Dual-Energy Computed Tomography for Detection and Characterization of Monosodium Urate, Calcium Pyrophosphate, and Hydroxyapatite

Abstract

Objectives: The aim of this study was to determine the diagnostic performance of dual-energy computed tomography (DECT) to detect and distinguish crystal deposits in a phantom. The primary objective was to determine the cutoff DECT ratio and the cross-sectional area (CSA) of a crystal deposit necessary to differentiate monosodium urate (MSU), calcium pyrophosphate (CPP), and calcium hydroxyapatite (HA) using DECT. Our secondary objective was to determine the concentration for limit of detection for MSU, CPP, and HA crystal deposits. Exploratory objectives included the comparison between 2 generations of DECT scanners from the same manufacturer as well as different scanner settings. Materials and Methods: We used a cylindrical soft tissue phantom with synthetic MSU, CPP, and HA crystals suspended in resin. Crystal suspension concentration increased with similar attenuation between MSU, CPP, and HA in conventional CT. The phantom was scanned on 2 dual-source DECT scanners, at 2 dose levels and all available tube voltage combinations. Both scanners had a tin (Sn) filter at the high-energy spectra. Dual-energy CT ratios were calculated for a given tube voltage combination by dividing linear regression lines of CT numbers against concentration. Dual-energy CT ratios were compared using an analysis of covariance. Receiver operating characteristic curves and corresponding areas under the curve (AUCs) were calculated for individual crystal suspension comparisons (HA vs CPP, MSU vs CPP, and MSU vs HA). Results: At standard clinical scan settings with 8 mGy and 80/Sn150 kV, the DECT ratios were as follows: CPP, 2.02 (95% confidence interval [CI], 1.98-2.07); HA, 2.00 (95% CI, 1.96-2.05); and MSU, 1.09 (95% CI, 1.06-1.11). Ratios varied numerically depending on the scanner and tube voltage combination. Monosodium urate crystal DECT ratios were significantly different from HA and CPP (P < 0.001), whereas DECT ratios for HA and CPP crystals did not differ significantly (P = 0.99). The differentiation of MSU crystals from both calcium crystals (HA and CPP) was excellent with an AUC of 1.00 (95% CI, 1.00-1.00) and an optimal cutoff DECT ratio of 1.43:1.40 depending on the scanner. In addition, differentiation of MSU and calcium-containing crystals (HA and CPP) required a CSA of minimum 4 pixels of crystal at standard clinical scan conditions. In contrast, differentiation between CPP and HA crystals was moderate with AUCs ranging from 0.66 (95% CI, 0.52-0.80) to 0.80 (95% CI, 0.69-0.91) and an optimal cutoff DECT ratio of 2.02:2.06 depending on the scanner. Furthermore, differentiation between CPP and HA crystals required a CSA of minimum 87 pixels of crystal at standard clinical scan conditions, corresponding to a region of interest of 3.7 mm diameter. When scanning at highest possible spectral separation and maximum dose of 50 mGy, the limit of detection for crystals within a region of interest of 50 pixels was 14 mg/cm 3 for MSU and 2 mg/cm 3 for both CPP and HA. Conclusions: This phantom study shows that DECT can be used to ...

Imaging Features of Calcium Pyrophosphate Deposition Disease: Consensus Definitions From an International Multidisciplinary Working Group

“…Switching from DEI to DER would provide similar results with similar caveats, except for adapted reference ranges. Importantly, the reference ranges for DER and DEI must take into account both the DECT systems and acquisition protocols used (i.e., radiograph beam spectral separation), in addition to relying on crystal calibration phantoms, as recently demonstrated in a phantom study (23). The absolute number of HUs was removed and the density of cortical bone was added as a reference point, as for conventional CT.…”

Computed Tomography 2.0

“…Minimum cross-sectional diameter of MSU crystal deposits of approximately 0.8 mm was reported for 2 different dual-source DECT systems and different tube voltage combinations. 124 Comparable performance of crystal differentiation (MSU and HA) was reported for DE-EICT and PCCT. 125 Tse et al 126 reported 3-material decomposition with virtual monochromatic images that may be superior to commercially available 2-material decomposition and dual-thresholding in differentiating MSU and HA at low concentrations (lower limits for MSU detection 3.4 mg/dL, 13.3 mg/dL, and 65.2 mg/dL; rapid kV switching DECT).…”

“…Only few studies focused on cross-platform conformity. Minimum cross-sectional diameter of MSU crystal deposits of approximately 0.8 mm was reported for 2 different dual-source DECT systems and different tube voltage combinations 124 . Comparable performance of crystal differentiation (MSU and HA) was reported for DE-EICT and PCCT 125 …”

Dual-energy CT assessment of rapid monosodium urate depletion and bone erosion remodelling during pegloticase plus methotrexate co-therapy

“…Using dual thresholds (i.e. dual-energy ratios) during DECT post-processing, targeted at the crystal of interest, would have allowed a more accurate quantification of MSU deposition volumes, as recently demonstrated in a phantom study [ 14 ]. However, the default settings were considered acceptable here for measuring the percentage change in MSU volumes over time.…”