Design of control electronics of CCD and EMCCD sensors for digital X-ray imaging
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Currently, the main imaging modalities for acquiring images of the human body are X-ray imaging, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission tomography (SPECT), ultrasound, and optical imaging. Of these, X-rays offer significant anatomical information and are able to easily pick out bones, tumors, surgical implants and so on. X-ray beams result from the conversion of the kinetic energy when electrons accelerated by high voltage collide with a target material. An X-ray imager or radiographic detector senses X-ray photons and converts them into light and subsequently into electrical signals. In this thesis, we investigate the use of electron multiplying charge coupled devices (EMCCD) as potential replacements for high-resolution high-speed detectors for X-ray imaging. Frame transfer type CCD sensors, which are used to convert a photon image into electrical signals, consists of a large number of light sensing elements, a storage area, and readout electronics. However, EMCCD detectors have higher quantum efficiency (QE), higher sensitivity, and lower noise characteristics than a typical CCD detector since they have added electron multiplying gain registers which are located ahead of the output amplifier. In this work, we selected the low cost TC237B CCD sensor by Texas Instruments as a demonstration vehicle because it has very similar architecture to the significantly more costly EMCCD. To implement the light sensing part of a high spatial resolution and highly sensitive imaging system for radiography and fluoroscopy, we have designed a complete system that includes the CCD sensor, FPGA board, ADC evaluation board, and TI 1GHz DSP starter kit as well as a clock driver circuit. We used Verilog(TM) source code to program a Xilinx SpartanII(TM) XC2S200 FPGA to generate clock pulses, and C source code to program a TMS320C6416T1000 Digital Signal Processor to serve as a data acquisition system. For high frame rate operation, noise problems were alleviated by adding decoupling capacitors, digital isolators and damping resistors. We have successfully completed building a single module based on the CCD sensor and we have demonstrated acquisition of images. Finally, to achieve a large field of view image in the future detectors, we propose to introduce detector modules with fiber optic tapers in each sensor where the CCDs are replaced by EMCCDs. Such new radiographic detector designs will enable not only high resolution vascular angiography but also low dose fluoroscopy, and will allow for a single detector to serve as the primary image acquisition system for image guided endovascular interventions.