Purpose: To develop an image receptor capable of forming high-quality megavoltage CT images using modest radiation doses.
Methods and materials: A flat-panel imaging system consisting of a conventional flat-panel sensor attached to a thick CsI scintillator has been fabricated. The scintillator consists of individual CsI crystals 8 mm thick by 0.38 mm x 0.38-mm pitch. Five sides of each crystal are coated with a reflecting powder/epoxy mixture, and the sixth side is in contact with the flat-panel sensor. A timing interface coordinates acquisition by the imaging system and pulsing of the linear accelerator. With this interface, as little as one accelerator pulse (0.023 cGy at the isocenter) can be used to form projection images. Different CT phantoms irradiated by a 6-MV X-ray beam have been imaged to evaluate the performance of the imaging system. The phantoms have been mounted on a rotating stage and rotated while 360 projection images are acquired in 48 s. These projections have been reconstructed using the Feldkamp cone-beam CT reconstruction algorithm.
Results and discussion: Using an irradiation of 16 cGy (360 projections x 0.046 cGy/projection), the contrast resolution is approximately 1% for large objects. High-contrast structures as small as 1.2 mm are clearly visible. The reconstructed CT values are linear (R(2) = 0.98) for electron densities between 0.001 and 2.16 g/cm(3), and the reconstruction time for a 512 x 512 x 512 data set is 6 min. Images of an anthropomorphic phantom show that soft-tissue structures such as the heart, lung, kidneys, and liver are visible in the reconstructed images (16 cGy, 5-mm-thick slices).
Conclusions: The acquisition of megavoltage CT images with soft-tissue contrast is possible with irradiations as small as 16 cGy.