The Accurate Electron Density Image Containing Metal Prosthesises Through the Hybrid Sinogram for Tomotherapy: A Phantom Study
H Jeon1*, H Youn2, D Park3, Y Kim3, H Kim2, H Park3, J Nam1, W Kim3, Y Ki3, J Lee3, (1) Pusan National University Yangsan Hospital, Yangsan, South Korea,(2) Pusan National University, Busan, South Korea,(3) Pusan National University Hospital, Busan, South KoreaSU-E-J-179 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To obtain the hybrid CT images, which have both the high contrast of kV-CT and very low metal artifact of MV-CT, for tomotherapy patients with metal prosthesises
Methods: We used a standard cheese phantom containing four metal inserts (copper, silver, tin, tungsten) to acquire a kV-sinogram by a CT simulator and a MV-CT image by Tomotherapy. The MV-sinogram was acquired by the forward projection of the MV-CT image using the geometry of kV-CT system. Both sinograms were normalized with their total length of x-ray attenuation and then were converted to the mean electron densities using corresponding calibration curves. The metal data in the MV-sinogram were segmented using a given density threshold and transplanted to the kV-sinogram row by row to acquire the normalized hybrid sinogram. The misalignment between metal data of two sinograms was corrected by mutual information technique. The hybrid CT image was finally obtained through de-normalization and back-projection procedures. For the experimental verification, film measurements were performed and analyzed using three head and neck treatment plans.
Results: We found that the hybrid CT image of the cheese phantom containing four metal inserts has similar contrast and very low metal artifact compared to the kV-CT image. Moreover, the metal densities on the hybrid CT-image were accurate with the mean deviation of 3.3%. As the result of film measurements, the average gamma pass rate of kV-CT plan and hybrid-CT plan were 93.7% and 96.5% with the criteria of 2%/2mm, and 67.1% and 81.4% with the criteria of 1%/1mm.
Conclusions: This work indicated that the hybrid CT image can improve the accuracy of tomotherapy treatment plan with metal prosthesises without additional imaging dose. Our further study is to acquire and evaluate hybrid CT images from clinical kV-sinograms and MV-CT images.