ARRS 2022 Abstracts

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E1442. Twin Robotic X-Ray System for Extremity Cone-Beam CT: Comparison of Tin and Copper Prefiltration
Authors
  1. Andreas Kunz; University Hospital Würzburg
  2. Jan-Peter Grunz; University Hospital Würzburg
Objective:
Spectral shaping is an established approach to achieve dose reduction in various CT applications, whereas in cone-beam CT, the benefits of tin prefiltration compared to optimized spectra have not been thoroughly assessed thus far. The aim of this study was to investigate the effect of tin filer application on radiation exposure and image quality of extremity cone-beam CT scans with a twin-robotic x-ray system.

Materials and Methods:
Elbow, wrist, and ankle joints of two cadaveric specimens were examined in a laboratory setup using the high-resolution scan mode of a twin robotic x-ray system with various combinations of prefiltration (standard copper, experimental tin filter), tube voltage, and current-time product. Volume CT dose indices were computed for a 16-cm polymethylmethacrylate phantom. Image quality of protocols was assessed subjectively by five radiologists and compared with Wilcoxon signed rank tests. Fleiss’ kappa was calculated to measure interrater agreement. To provide an additional semiquantitative criterion for image quality, contrast-to-noise ratios (CNR) were computed in standardized ROIs.

Results:
Radiation dose ranged from 17.4 mGy in the clinical standard protocol without tin filter to as low as 0.7 mGy with tin prefiltration. Image quality ratings and CNR for tin-filtered scans with 100 kV were lower than for 80 kV studies with copper filtration despite higher dose (11.2 and 5.6 vs. 4.5 mGy; p < 0.001). Observer evaluation and CNR showed no difference between 100 kV scans with tin filtration and 60 kV copper-filtered scans with 75% dose reduction (subjective: p = 0.101; CNR: p = 0.706). Moderate to substantial interrater agreement was indicated by Fleiss’ kappa of 0.597 (95% confidence interval 0.567–0.626; p < 0.001).

Conclusion:
Considerable dose reduction could be achieved with tin filter application, the twin-robotic x-ray system’s potential for low-dose extremity joint imaging was maximized with a dedicated low-kilovolt scan protocol in patients without extensive beam hardening artifacts, e.g., due to metal implants.