Assessment of CT-to-physical density table for multiple image reconstruction functions with a large-bore scanner for radiotherapy treatment planning

Abstract

Purpose

To evaluate the performance of the Aquilion Exceed LB computed tomography (CT) scanner for radiotherapy treatment planning, this study examined the effect of different combinations of the image reconstruction function (IRF) (AiCE and AIDR) and scan parameters on the CT-to-physical density (CT-PD) table and radiation dose in the phantom, and the effect of different object positions on CT values.

Methods

To investigate IRF’s influence on each material, we calculated CT values by varying tube current, pitch, field of view (FOV), and phantom position for each IRF, comparing them with reference values using filtered back projection (FBP). Furthermore, we evaluated changes in depth dose values due to IRF differences using a solid phantom.

Results

In the combinations of changes in IRF and scan parameters the change in CT value ($\Delta HU$) of each material was within $\pm$10 HU, except for most conditions. The change in physical density ($\Delta PD$) was within $\pm$0.02 g/cm³ for all combinations. For changes in phantom position, $\Delta HU$ was within $\pm$25 HU for changes within the scan FOV, except for Bone 200 mg/cc and 1250 mg/cc. In areas outside the scan FOV with an expanded FOV, $\Delta HU$ was significantly larger than within the scan FOV. Variations in depth dose for different IRFs using solid phantoms were within $\pm$0.5 %, except at material boundaries.

Conclusion

Our evaluations of the CT values and dose calculations suggested no need to change the CT-PD table, even with multiple IRFs.