Evaluation of the Micro-Angiographic Fluoroscope as a High-Resolution, Single-Photon Counting and Energy-Integrating Imager for Transmission and Emission Imaging

Abstract

X-ray and radionuclide imaging are widely popular medical imaging modalities. The requirements for each imaging modality are different, and different detectors are normally used. For this paper, we demonstrate our new detector capable of both fluoroscopy and angiography, to be used as an imager for both single-photon and integral-energy imaging and applied to the dual modalities of radionuclide imaging and X-ray imaging. This newly developed micro-angiographic fluoroscope (MAF) has 1024 × 1024 pixels of 35 μm effective size and is capable of real-time imaging at 30 fps. The large variable gain of its light image intensifier (LII) provides quantum-limited operation with essentially no additive instrumentation noise. We demonstrate that the MAF can be operated in single-photon counting (SPC) mode for X-ray imaging with substantially better resolution than in energy integration (EI) mode. We may use high LII gain with very low exposure (less than 1 X-ray photon/pixel) per frame for SPC mode (with X-ray and radionuclide) and higher exposure with lower gain for EI mode (transmission imaging with X-rays). For a demonstration of the operation in both EI and SPC mode, a heavily K-edge filtered X-ray beam (average energy of 31 keV) was used to provide a nearly monochromatic spectrum. The MTF measured using a standard slit method showed a dramatic improvement for the SPC mode over the EI mode at all spatial frequencies. Images of a line-pair phantom also showed improved spatial resolution in SPC mode compared to EI mode. In SPC mode, images of human distal and middle phalanges showed the trabecular structures of the bone with far better contrast and detail. We also show MAF operation in SPC mode for radionuclide imaging using a custom-built phantom filled with I-125. A 1-mm-diameter parallel hole, medium-energy gamma camera collimator was placed between the phantom, and the MAF and was moved multiple times at equal intervals in random directions to eliminate the pattern corresponding to the collimator septa. Data was acquired at 20 fps, and multiple signal-thresholded frames were summed in SPC mode to provide an integrated frame. The sharpness of the emission image is limited by the collimator resolution and could be improved by optimized collimator design. We demonstrate that the same MAF is capable of operating in both SPC and EI modes and can be used in both X-ray transmission imaging and radionuclide emission imaging.