Author: Shusen Jing, Qihui Lyu, Dan Ruan, Ke Sheng, Qifan Xu 👨🔬
Affiliation: Department of Radiation Oncology, University of California, Los Angeles, University of California San Francisco, Department of Radiation Oncology, University of California, San Francisco 🌍
Purpose: Conventional computed tomography (CT) measures the collective attenuation of primary photon fluence and then reconstructs the 3D images without using the timing information of individual photons. With the recent introduction of time-of-flight (ToF) detection, the timing, energy, and location of primary and scatter X-ray photons can be individually tracked. However, a reconstruction method to fully utilize such information is unavailable. The study develops the first pipeline for time and energy-encoded (TEE) CT reconstruction.
Methods: The photon transportation of a 100keV monchromatic source through the phantom was simulated using Geant4. The phantom was placed at the center of a cubic detector volume enclosed by six square ToF flat panel detectors. We additionally reconstructed Compton and Rayleigh scatter images based on the initial photon direction, the detection location, and ToF data. Assuming single-scatter events for each detected scattered photon, we determined the scatter location along the initial beam direction such that the sum of the path lengths, i.e., source-to-scatter and scatter-to-detector, matches the ToF-derived path length. The scatter events within each voxel were then computed to generate the scatter image.
Results: Different from conventional CT, TEE-CT acquires necessary data for reconstruction in a single projection without any mechanical movement. TEE-CT differentiates Compton and Rayleigh scattered photons and reconstructs both scatter images. Two phantoms were used to the evaluate the proposed TEE reconstruction. The first phantom is a spheric phantom with 0.1 mm checkerboard pattern. TEE-CT reconstructed the high-resolution pattern with an imaging dose < 10mGy, while attenuation-based CT shows severe artifacts and distortion even with dose hundreds times of TEE-CT.
Conclusion: We demonstrate a theoretical framework for TEE-CT, which allows for the reconstruction of Rayleigh and Compton scatter images from a single projection, achieving significantly lower dose levels than conventional CT methods.