Comparison of CT Number to Mass Density Curves for Multiple Scanners with Universal Protocols for Photon Treatment Planning
Abstract
Purpose
Universal CT number to mass density (CTN-to-MD) curves in large hospital networks with multiple CT simulators and treatment planning systems reduces the potential for error during photon treatment planning. The goal of this study was to evaluate the effects of peak kilovoltage (kVp) and scatter conditions of clinical protocols and to develop universal curves to be used across the hospital network.
Methods
We acquired scans of the Advanced Electron Density phantom (Sun Nuclear Corporation, Melbourne, FL) with 12 matched clinical pediatric and adult protocols on four SOMATOM CT simulators: Drive, Edge+, go.Open, Definition (Siemens Heathineers, Erlangen, Germany). ). The protocols include scans at 100kVp, 120kVp, and 140kVp. The phantom has a large (40x30cm2 elliptic cylinder) and small (20cm diameter cylinder) configuration. Three high-density inserts (MD=2.71-8.0g/cc) and four bone inserts (MD=1.21-1.93g/cc) were placed sequentially in the central insert. Low-density, soft-tissue, and solid-water inserts (MD=0.29-1.08g/cc) were placed in the remaining spaces. Extended dynamic range was configured for all protocols. Scans with high-density inserts were also reconstructed with iterative metal artifact reduction (IMAR) algorithms.
Results
The phantom configuration demonstrated effects on CTN for 140kVp scans. The average HU in the Cortical Bone material was 1134HU and 1295HU with the small and large configurations, respectively. The effect was less pronounced for 120kVp scans with average values of 1219HU and 1258HU, respectively. The variation in CTN between three scanners was <1% for all inserts and protocols. The go.Open CTN varied by up to 5%, attributed to a different collimator size. IMAR reconstructions of scans with high-density inserts changed the CTN of soft-tissue inserts by <32HU, regardless of phantom configuration, kVp, and IMAR algorithm.
Conclusion
Five universal CTN-to-MD curves for 12 matched scan protocols were created for clinical use of three scanners across the hospital network based on phantom size and kVp.