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An Experimental Evaluation of Mega-Voltage Fan-Beam CT Using a Linear Accelerator

H Gong1*, S Tao1 , J Gagneur2 , W Liu2 , C McCollough1 , Y Hu2 , S Leng1 , (1) Mayo Clinic, Rochester, MN, (2) Mayo Clinic Arizona, Phoenix, AZ


(Sunday, 7/14/2019) 4:00 PM - 4:30 PM

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 512×512 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 Pearson’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 5±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 Pearson’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.


CT, Linear Accelerator, Megavoltage Imaging


IM- CT: Quantitative imaging/analysis

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