High resolution solid state X-ray image intensifier (SSXII) using a modular array of the impactron EMCCD sensors
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X-ray imaging is widely used as a powerful tool for diagnosis and noninvasive treatment in the medical field. The increasing demand for the imaging of higher resolution and better quality with the ability to visualize fine anatomical detail and accurate image guidance places an ever increasing demand on detector technologies. Current state-of-the-art medical x-ray detector technologies, including x-ray image intensifiers (XIIs) and flat panel detectors (FPDs), have inherent limitations. To overcome these limitations, our group has been developing a high resolution, high sensitivity Solid State X-ray Image Intensifier (SSXII) from the component level. The SSXII can provide real-time, high spatial-resolution, and low noise (hence, high dynamic range) images even at very low exposures because of the high variable on-chip gain of the Electron Multiplying CCDs (EMCCDs). To provide extended field-of-view (FOV) with higher resolution images, a modular design is proposed and implemented for the SSXII. This design covers all aspects from concept, circuit design, assembly with fiber optics, into a prototype product ready for medical application. The schematic for the SSXII module consists of a CsI (Tl) phosphor plate converting x-ray photon into light photon, a fiber optic taper (FOT) as a lens, and a custom-built EMCCD camera with a preinstalled fiber-optic input window. For a modular array, FOTs are glued together as an array and each of them focuses light into an EMCCD sensor. A geometric correction algorithm to account for the misalignment of the EMCCDs and FOTs is included in the customized software that enables acquisition, correction, and display of the frames for the array in real-time. Artifacts due to the chamfer edge on the FOTs are reduced by grinding off the chamfers on the tapers. The images of various objects acquired using a 1 by 2 modular array demonstrates that the SSXII is capable of providing seamless images with larger FOV. To quantitatively assess various characteristics of the SSXII enabling an optimization analysis and overall performance evaluation, a unified framework for characterizing the detector using the slope of the linear fits is developed. The flat-field images over a range of exposures for each electronic gain are acquired and flat-field corrected. The average signal, in the form of flat-field mean value, and noise, in the form of variance or standard deviation, are calculated for each exposure and each multiplication gain voltage. Then, average signal digital number (DN) and average variance in DN were linearly fitted and plotted as a function of exposure (μR) or signal mean value. The multiplication gain can be obtained from the plots of the signal versus exposure, read-out noise in the absolute units of e − rms can be evaluated from the plots of variance versus signal, and the instrumentation noise equivalent exposure (INEE) can be calculated from the plots of variance versus exposure. Other results including the excess noise factor, conversion gain, full-well capacity, and x-ray sensitivity are computed similarly. The effect of cooling to the EMCCD sensor is also characterized. To better assess detector performance in areas such as spatial resolution, noise performance, and overall performance, the modulation transfer function (MTF), INEE, and detective quantum efficiency (DQE) measurements in frequency domain are utilized. A theoretical linear cascade model was developed for the SSXII. The good agreement of the theoretical and actual measurement in MTF, INEE, and DQE demonstrates the usefulness of the linear cascade model, which can be used to analyze the signal and noise propagation in the system and optimize the detector design. Furthermore, a completed comparison of the SSXII and FPD detector for different exposures was conducted and the results demonstrate that the SSXII has less instrumentation noise, higher resolution, and higher DQE than the FPD. Additional x-ray images for the neurovascular devices and the stent deployment process demonstrate the real-time imaging capabilities and large dynamic range of the SSXII. Further discussion regarding the different types and thickness of CsI phosphor and fiber optic taper ratios is also introduced. In the future, the SSXII detector with the larger array, which provides extended FOV and better performance, should enable the replacement of conventional lower-resolution XIIs or FPDs.