Room: Exhibit Hall | Forum 6
Purpose: To demonstrate the feasibility of Mega-voltage fan-beam CT (MV-FBCT) imaging using the TrueBeam linear accelerator (Varian, Inc.), and to assess its capability in accurate electron density measurement.
Methods: MV-FBCT images of a head-sized electron density phantom (Model 062M, CIRS, Inc.) and a Catphan 604 phantom (The Phantom Laboratory, Inc.) were acquired using the portal imaging capability of a TrueBeam system. The peak x-ray energy was 2.5 MV and the delivered monitor unit (MU) was 30 MU. At isocenter, the irradiated region had an in-plane field-of-view (FOV) of 400 mm and a cross-plane FOV of 20 mm. The CT images were reconstructed using filtered-back-projection, with 3 mm slice thickness, 400 mm FOV and 512Ã—512 image matrix. In the CIRS phantom, we measured mean CT number for various tissue-mimicking inserts (e.g. adipose, breast, lung, muscle and bone). The strength of the linear correlation between the measured CT number and the nominal relative electron density was quantified using Pearsonâ€™s correlation analysis. In the Catphan phantom, we qualitatively evaluated the spatial resolution and the contrast resolution.
Results: No significant artifacts were observed in the reconstructed MV-FBCT images. The high-contrast inserts (e.g. Teflon, Delrin, and Hydroxyapatite) were still observable, while the low-contrast inserts (e.g. PMP and LDPE) were largely corrupted by image noise. The maximal high-contrast spatial resolution was approximately 5Â±0.5 line-pairs / cm. The measured CT number of soft-tissue and bone demonstrated statistically significant linear relationship with the nominal relative electron density. The corresponding Pearsonâ€™s Ï? was 0.9995 (95% confidence interval [0.9981, 0.9999]), respectively.
Conclusion: MV-FBCT images can be obtained from the TrueBeam system. A strong correlation was showed between CT number and electron density. This could potentially provide an accurate estimation for electron density and achieve accurate dose calculation for photon and proton therapy.