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Purpose: Megavoltage computed tomography (MVCT) and megavoltage cone beam computed tomography (MVCBCT) can be used for visualizing anatomical structures prior to radiation therapy treatments to assist in patient setup and target localization. These systems are less susceptible to metal artifacts and provide better CT number linearity than conventional CT scanners. However, their contrast is limited by the properties of the megavoltage photons and the low detective quantum efficiency (DQE) of flat panel detector systems currently available. By using higher DQE, thick, segmented cadmium tungstate detectors, the authors can improve the low contrast detectability of a MVCT system. This in turn would permit greater soft tissue visualization for a given radiation dose, allowing MVCT to be used in more clinical situations. Methods: This article describes the evaluation of our prototype system that uses thick, segmented detectors. In order to create images using a dose that would be acceptable for day to day patient imaging, the authors evaluated their system using the low intensity bremsstrahlung component of a 6 MeV electron beam. The system was evaluated for its uniformity, high contrast resolution, low contrast detectability, signal to noise ratio, contrast to noise ratio, and CT number linearity. Results: The prototype system wasmore » found to have a high contrast spatial resolution of about 5 line pairs per cm, and to be able to visualize a 15 mm 1.5% contrast target with 2 cGy of radiation dose delivered. SNR{sup 2} vs radiation dose and mean pixel value vs electron density curves were linear. Conclusions: This prototype system shows a large improvement in low contrast detectability over current MVCBCT systems.« less

Megavoltage computed tomography (MVCT) and megavoltage cone beam computed tomography (MVCBCT) can be used for visualizing anatomical structures prior to radiation therapy treatments to assist in patient setup and target localization. These systems are less susceptible to metal artifacts and provide better CT number linearity than conventional CT scanners. However, their contrast is limited by the properties of the megavoltage photons and the low detective quantum efficiency (DQE) of flat panel detector systems currently available. By using higher DQE, thick, segmented cadmium tungstate detectors, the authors can improve the low contrast detectability of a MVCT system. This in turn would permit greater soft tissue visualization for a given radiation dose, allowing MVCT to be used in more clinical situations.This article describes the evaluation of our prototype system that uses thick, segmented detectors. In order to create images using a dose that would be acceptable for day to day patient imaging, the authors evaluated their system using the low intensity bremsstrahlung component of a 6 MeV electron beam. The system was evaluated for its uniformity, high contrast resolution, low contrast detectability, signal to noise ratio, contrast to noise ratio, and CT number linearity.The prototype system was found to have a high contrast spatial resolution of about 5 line pairs per cm, and to be able to visualize a 15 mm 1.5% contrast target with 2 cGy of radiation dose delivered. SNR2 vs radiation dose and mean pixel value vs electron density curves were linear.This prototype system shows a large improvement in low contrast detectability over current MVCBCT systems.

Purpose: To evaluate the performance of a model based image reconstruction in reducing metal artifacts in MVCT system in the image of a phantom representing bilateral hip prostheses, and to compare with traditionally reconstructed image.Method and Materials: A cylindrical plexiglass (19 cm diameter) phantom containing two steel inserts (2 cm diameter) was scanned using bremsstrahlung radiation from 6MeV electron beam passing through 4 cm thick solid water in Varian Clinac 2300C with bench‐top MVCT system. Iterative maximum‐likelihood polychromatic algorithm for CT (IMPACT) modified to include pair and triplet production for the MV spectrum was used with air, plexiglass, bone, and iron as base substances. Detector calibrated signal without beam‐hardening correction was used to get filtered back‐projected (FBP) image and used as an initial image in IMPACT. The final image at 1.25 MeV was obtained after 130 iterations. The second image was reconstructed using FBP with traditional signal and beam‐hardening corrections. The quantitative analysis included calculation of average attenuation coefficient in various regions of interest and their comparisons with theoretical value. Results: Visual inspection of the traditional image shows that the region between two steel inserts is dark with additional white streaks. Attenuation coefficients in ROIs fluctuate largely and deviate from theoretical values. In the iteratively reconstructed image, metal artifacts are remarkably reduced leaving behind only faint shadings in between the inserts. The average attenuation coefficients in ROIs were close to theoretical values. Image profile through the steel inserts shows restored uniform background between them. Conclusion: This experiment emphasizes the importance of model based image reconstruction and MVCT system for the metal artifact reduction. The beam‐hardening correction applied in conventional image reconstruction creates severe darkening and white streaks in the image. The complete removal of the metal artifacts requires further modeling and strategies which will be employed in future studies.

Purpose: To evaluate the performance of a MVCT system based on multi‐slice, thick, and segmented cadmium tungstate‐photodiode detectors.Method and Materials: Our MVCT's detector is a 2D array of 1mm × 1mm (pitch) × 10mm (thickness) cadmium tungstate crystals separated by a septa paint of reflectivity higher than 0.975. The scintillators are mounted on ten 16 × 16 element photodiode arrays (SCA‐CA256ES, Semicoa, Costa Mesa, CA). The total array size is 320 detectors along the arc by 16 detectors along the slice thickness direction. The radius of curvature of detector arc is 92.5cm. In our system, the source and detectors remain stationary while the object being imaged is placed on a precision rotating stage. Due to the high dose per pulse provided by our clinical linac, it is not possible to image a test object with a dose of < 50cGy in a 6MV beam. In order to obtain low dose images, we used the small amount of Bremsstrahlung radiation produced in the scattering foils in 6 MeV electron beam after removing the electrons from the beam by placing 4cm of solid water in the beam.Results: Our system demonstrates a uniformity index of 0.4% at 1.9cGy. The noise standard deviation is around 2% at 1.9cGy. CT number linearity (R2= 0.9982) and low contrast resolution (15 mm object with 1.5% contrast at 2 cGy) are superior to published evaluations of commercially available tomotherapy MVCT. The spatial resolution is about 4lp/cm which is limited mainly by the diffused Bremsstrahlung source. Conclusion: Thick segmented cadmium tungstate detectors offer significantly better low contrast resolution per unit dose at MV energy than commercially available gas filled or flat panel detectors. This work demonstrates the feasibility of creating a fully functional MVCT system using this technology.