(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: To assess the energy independence of a novel CT image reconstruction algorithm (DirectDensity, Siemens AG), specifically related to the determination of mass density for treatment planning dose calculation.
Methods: The DirectDensity algorithm converts pixel values in CT-scans of different energies to DirectDensity CT (DDCT) values, which are HU-like numbers designed to be energy independent. Siemens Somatom Confidence RT Pro was used to scan a phantom with calibrated Gammex inserts representing 11 different mass densities, ranging from 0.32 g/cm³ to 1.824 g/cm³. Five scans were collected using tube potentials of 70, 80, 100, 120, and 140kVp. Scans were reconstructed using the DirectDensity reconstruction algorithm. The mean DDCT value over all five scans was determined for each of the Gammex inserts. These average values were used to generate an “energy-independent� DDCT vs mass density curve. Next, DDCT values for each individual scan were used to create five separate, energy-dependent DDCT vs mass density curves. Finally, mass density values for each Gammex insert, as determined by each of the energy-dependent curves, were compared to mass density values determined by the “energy-independent� curve.
Results: Mass density inaccuracies introduced by the use of a single energy-independent mass density conversion curve vs separate energy-dependent curves were determined for a wide range of scan energies and mass densities. The maximum and mean error of derived mass density values were found to be 3.6% and 0.8%, respectively. The lung tissue insert demonstrated the maximum relative error, but the largest absolute error was observed in the adipose tissue range.
Conclusion: The implementation of a single energy-independent DDCT vs mass-density curve results in minor errors in mass density determination, potentially allowing for the use of a wider variety of CT-scanning energies in radiation oncology. However, the dosimetric impact of these errors requires further investigation.
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