X-ray Luminescence Computed Tomography for Small Animal Imaging

High-resolution imaging modalities play a critical role for advancing biomedical sciences. Within the last decade, x-ray induced luminescence imaging has emerged and demonstrated great potentials for the molecular imaging of small animals by combining the high-spatial resolution of conventional x-ray imaging and the high measurement sensitivity of optical imaging. Specifically, x-ray luminescence computed tomography (XLCT) imaging has been introduced as a powerful new hybrid molecular imaging modality capable of the high-resolution imaging of deeply embedded x-ray excitable contrast agents in three-dimensions (3D). In principle for XLCT, x-ray photons are used to penetrate samples deeply, with negligible scattering, and contrast agents within the path of the excitation beam will absorb the x-ray energy and generate many optical photons, some of which pass through tissue and escape from the skin. Then by using highly sensitive optical detectors, the emitted optical photons can be measured for XLCT image reconstruction. We have developed several prototype XLCT imaging scanners. First, we developed a collimated superfine x-ray beam based XLCT imaging and validated the system using both numerical simulations and physical experiments. We also systematically investigated the effects of the scanning x-ray beam size and number of angular projections on the spatial resolution of XLCT imaging. We found that the obtainable spatial resolution had a high dependency on the scanning x-ray beam size and that more angular projections improved the imaging quality. Particularly, the obtainable spatial resolution was found to be double the scanning x-ray beam diameter. To address the long data acquisition time with this method, we then proposed a multiple x-ray beam scanning strategy and were able to reduce the scan time dramatically. Then, due to the low x-ray photon utilization efficiency of the collimated based XLCT imaging system, we developed a focused x-ray beam based XLCT imaging system by using a polycapillary lens to focus the x-ray photons to a fine spot. In addition, we improved the measurement sensitivity by using an optical fiber and photomultiplier tube set-up instead of the conventional electron-multiplying charged coupled device (EMCCD) camera measurement set-up. This new set-up was validated systematically using both numerical simulations and phantom experiments. Due to the increased x-ray photon flux as well as higher-sensitivity set-up, the imaging time was further reduced. We have also measured the radiation dose in both the collimated and focused x-ray beam-based set-ups and found that the radiation dose was within the range of a typical CT scan. To improve upon the spatial resolution of XLCT imaging, we then proposed a new scanning scheme for XLCT imaging which was accomplished by simply reducing the x-ray beam scanning step size. We found that the spatial resolution limit could be further improved and with a four-times reduction in the step size, we could improve the spatial resolution by 1.6 times. We then validated the high-resolution capabilities of our focused x-ray beam based XLCT imaging system by performing high-resolution phantom studies and demonstrated that cylindrical targets with edge-to-edge distances of 150 µm could be successfully imaged in our scanner. We also have validated the feasibility of our imaging system for small animal imaging with a euthanized mouse study. We embedded a single capillary tube target filled with phosphor particles and were successfully able to reconstruct the distribution of the particles with high precision and accuracy. Finally, based on our previous work, we have proposed and are currently building the next generation XLCT scanner, dedicated for small animal molecular imaging. The proposed focused x-ray luminescence tomography (FXLT) scanner incorporates both a microCT and XLCT scanner in a single imaging system for easy registration of the anatomical and optical imaging. We first designed the scanner with computer-aided design (CAD), then the parts were purchased or fabricated according to our model. We also performed numerical simulations to verify the feasibility of the scanner. We anticipate the completed scanner will become a powerful tool for the molecular imaging community