Virtual noncontrast images reveal gouty tophi in contrast-enhanced dual-energy CT: a phantom study.

BACKGROUND: Dual-energy computed tomography (DECT) is useful for detecting gouty tophi. While iodinated contrast media (ICM) might enhance the detection of monosodium urate crystals (MSU), higher iodine concentrations hamper their detection. Calculating virtual noncontrast (VNC) images might improve the detection of enhancing tophi. The aim of this study was to evaluate MSU detection with VNC images from DECT acquisitions in phantoms, compared against the results with standard DECT reconstructions. METHODS: A grid-like and a biophantom with 25 suspensions containing different concentrations of ICM (0 to 2%) and MSU (0 to 50%) were scanned with sequential single-source DECT using an ascending order of tube current time product at 80 kVp (16.5-220 mAs) and 135 kVp (2.75-19.25 mAs). VNC images were equivalently reconstructed at 80 and 135 kVp. Two-material decomposition analysis for MSU detection was applied for the VNC and conventional CT images. MSU detection and attenuation values were compared in both modalities. RESULTS: For 0, 0.25, 0.5, 1, and 2% ICM, the average detection indices (DIs) for all MSU concentrations (35-50%) with VNC postprocessing were respectively 25.2, 36.6, 30.9, 38.9, and 45.8% for the grid phantom scans and 11.7, 9.4, 5.5, 24.0, and 25.0% for the porcine phantom scans. In the conventional CT image group, the average DIs were respectively 35.4, 54.3, 45.4, 1.0, and 0.0% for the grid phantom and 19.4, 17.9, 3.0, 0.0, and 0.0% for the porcine phantom scans. CONCLUSIONS: VNC effectively reduces the suppression of information caused by high concentrations of ICM, thereby improving the detection of MSU. RELEVANCE STATEMENT: Contrast-enhanced DECT alone may suffice for diagnosing gout without a native acquisition. KEY POINTS: • Highly concentrated contrast media hinders monosodium urate crystal detection in CT imaging • Virtual noncontrast imaging redetects monosodium urate crystals in high-iodinated contrast media concentrations. • Contrast-enhanced DECT alone may suffice for diagnosing gout without a native acquisition.

Influence of contrast medium on tophus detection using dual-energy CT: phantom study and clinical illustration.

BACKGROUND: To investigate the influence of iodinated contrast medium (ICM) on detection of monosodium urate (MSU) with dual-energy computed tomography (DECT) in two types of phantoms and demonstrate an example patient for clinical illustration. METHODS: Approval is by the institutional review board, and written informed consent was obtained. A grid-like and a biophantom with 25 suspensions containing different concentrations of ICM (0 to 2%) and MSU (0 to 50%) were prepared and scanned with sequential single-source DECT using established methodology. Ascending orders of tube currents were applied at 80 kVp (16.5 to 220.0 mAs) and 135 kVp (2.75 to 19.25 mAs). Volume and mass measurements were performed using clinical gout software (dual-energy decomposition analysis). Numbers of true-positive and false-positive MSU detections were recorded and compared for different ICM concentrations. We demonstrate a patient with gouty arthritis for clinical illustration. RESULTS: Effects of ICM on MSU detection varied with the amount of iodine. Lower ICM concentrations (0.25 and 0.50%) improved detection of small uric acid concentrations of 35 to 45% in comparison to scans without ICM. However, high ICM concentrations (1 and 2%) almost completely precluded MSU detection for all MSU concentrations investigated. In a patient with gouty arthritis, tophi in the wrist were only detected after intravenous ICM administration. CONCLUSIONS: Exploring multimodal DECT for arthritis imaging, enhancement of ICM influences tophus detection. It can help in visualizing previously undetected MSU depositions but, with too strong enhancement, also obscure tophi. RELEVANCE STATEMENT: Use of iodinated contrast media in dual-energy CT might help in visualizing previously undetected uric acid depositions but, with too strong enhancement, obscure gouty tophi. KEY POINTS: • Iodine significantly influences the uric acid crystal detection in systematic phantom studies. • Lower iodine concentrations improved detection of low and medium uric acid concentrations. • High concentrations of iodine hampered detection of all uric acid concentrations.

Dual-energy computed tomography: Tube current settings and detection of uric acid tophi.

PURPOSE: To derive optimal scanning parameters for single-source dual-energy computed tomography (DECT) in the detection of urate by analyzing influence of tube current ratio (TCR) and total radiation exposure in a phantom. METHOD: Specimens with different urate concentrations in a realistic porcine bio-phantom were repeatedly imaged with sequential single-source DECT scans at 80 kVp (16.5-220 mA s) and 135 kVp (2.75-19.25 mA s). Detection index (DI - true positive minus false positive urate volume) was calculated for every possible tube current combination. Optimal tube current combinations reaching at least 85 % of the highest measured DI of all combinations without exceeding 150 % of equivalent single-energy radiation dose were identified. TCR, DLP and DI were plotted and compared. RESULTS: Cubic regression analysis showed a flattening increase in the DI with increasing tube currents. Five out of the 100 tube current combinations analyzed achieved the detection target: the lowest DLP of 53.9 mGy*cm at 19.25/16.5 mAs (135/80 kVp) achieved a DI of 2.07 mL and the highest DI of 2.11 mL at a dose of 65.3 mGy*cm and 8.25/79.75 mAs. The optimal TCR is between two and four, while both, higher and lower ratios decreased DI. CONCLUSIONS: A minimum tube current of the high-energy scans is needed before an acceptable overall sensitivity is achieved and before increases in low-energy exposure result in more urate detection. High TCRs above 10 are not beneficial while the optimal TCR ranges between two and four, indicating that special care has to be taken in designing a suitable DECT protocol.

Development and validation of a quantitative method for estimation of the urate burden in patients with gouty arthritis using dual-energy computed tomography.

OBJECTIVES: To develop a method that allows approximating the mass of monosodium uric acid (MSU) in a gouty tophus using phantom measurements and including tophus density into the calculation of the dual-energy computed tomography (DECT) tophus volumetry. METHODS: We prepared specimens of different concentrations of MSU placed in an epoxy-based phantom and an excised porcine foreleg. Density and volumetric measurements were performed in sequential single-source DECT scans acquired with increasingly higher tube currents. We developed a method for estimating the tophus mass by multiplying the detected tophus volume with its mean density and adding a specific gravimetric coefficient k. k was derived from the DECT scans by comparing the approximated MSU masses in the epoxy phantom with the known true MSU masses of the specimens. RESULTS: Comparison of the approximated MSU masses in the porcine foreleg scans with the true MSU masses of the syringe contents showed similar performance to sole volume measurement while providing additional information on the true uric acid burden: Over 70% of the true urate masses have been detected in MSU concentrations ≥ 45%, while the detection rate was much lower for MSU concentrations ≤ 40%. Retrospective analysis of patients with proven gouty arthritis confirmed the diagnostic potential of the mass approximation technique. CONCLUSIONS: We successfully established a method to include tophus density measurement for estimation of the uric acid burden in milligrams (instead of ml) in a phantom setting for MSU concentrations above 40%. Future studies should validate its use for follow-up in clinical practice. KEY POINTS: • Including tophus density measurement in dual-energy computed tomography scans in a phantom setting can be used for estimating the urate burden in milligrams, which might be useful for imaging follow-up. • The mass [mg] of the uric acid burden in a patient with gouty arthritis can be calculated by multiplying volume [ml] with mean density [HU] using a specific gravimetric coefficient. • Retrospective analysis of two patients with gouty tophi showed the relevance of measuring urate mass in addition to urate volume alone.

Iterative Reconstruction May Improve Diagnosis of Gout: An Ex Vivo (Bio)Phantom Dual-Energy Computed Tomography Study.

OBJECTIVES: We hypothesize that, compared with filtered back projection (FBP), iterative reconstruction (IR) increases the detected volume of uric acid, which serves as a measure of the sensitivity of the method, and therefore allows examinations with lower radiation exposure. Thus, the aim of our study was to evaluate the role of IR in dual-energy computed tomography (DECT) for gout assessment and volumetry in an ex vivo gout model using an epoxy phantom and a biophantom. MATERIALS AND METHODS: We prepared specimens with different concentrations of monosodium uric acid in ultrasound gel (30%-50%) and inserted them into an epoxy-based phantom and a porcine foreleg. Computed tomography scanning was performed on a 320-row single-source DECT scanner in volume mode with an ascending order of tube currents. Image data sets were reconstructed using FBP and IR with 2 iteration levels (IR1 and IR2). Measurements in regions of interest (ROIs) and volumetric measurements were performed using either fixed parameters for the analysis software (epoxy phantom) or parameters specifically adapted for each data set (porcine phantom). Seven retrospectively identified clinical data sets were used for validation in patients. Rm-ANOVA was used for statistical analysis. No institutional review board approval was required. RESULTS: We found no differences in Hounsfield units between the different reconstruction algorithms (P = 0.09 and 0.05 for 80 and 135 kVp, respectively) and the calculated dual-energy gradient (P = 0.27). Filtered back projection detected less uric acid compared with IR (with fixed parameters: 6.0 ± 0.3 cm for FBP and 6.1 ± 0.2 for IR1 and IR2; with adapted thresholds: 1.2 ± 0.3 cm for FBP and 1.8 ± 0.1 and 2.0 ± 0.1 for IR1 and IR2, respectively). The differences were significant for all measurements (P = 0.0003). Also in the test cases, FBP detected significantly fewer tophi (mean volume, 1.38 ± 2.1 cm) compared with IR1 (1.86 ± 2.9) and IR2 (2.07 ± 3.1) (P = 0.04). CONCLUSIONS: Iterative reconstruction has the potential to improve the sensitivity of a DECT scan for gouty tophi, to decrease radiation exposure, or to combine both options.