Design and production of benchtop x-ray imaging system

A low cost X-ray imaging device based on BPW-34 Si-PIN photodiode was designed and produced. X-rays were generated from a CEI OX/70-P dental X-ray tube using a custom made ±20 kV power supply. A charge sensitive preamplifier and a shaping amplifier were built for the amplification of small signals produced by photons in the BPW-34 Si-PIN photodiode. A two dimensional position control unit was used for moving the detector in small steps to measure the intensity of X-rays absorbed in the object to be imaged. An Arduino Mega 2560 micro controller and Aessent AES220B FPGA module was used for transferring the image data to a computer via USB. Images of various samples were obtained with acceptable image quality despite of the low cost of the device. Designing a device for fast imaging time was not the main motivation in this thesis. However, having an acceptable image quality together with reduced cost and simple design were the main considerations. Total cost of the imaging device presented in this thesis is under 1000 USD. Filtering applications based on known energy spectrum are helpful tools to obtain images with better quality. For that reason, energy spectra of the CEI OX/70-P tube for different tube voltages were determined. Additionaly, energy spectra of CEI OX/70-P dental tube were determined experimentally and were compared with Geant 4 simulation results. Beside the experimental and simulation comparison studies, energy spectra were taken with BPW-34 Si-PIN photodiode and a commercial spectrometer Amptek X123 to ensure the consistency within the experimental spectra. In these studies, the geometries of BPW-34 Si-PIN photodiode, CEI OX/70-P tube and Amptek X123 detector, provided by the manufacturers, were embedded in Geant 4 simulations. A power supply was necessary to provide high tension between anode and cathode of the tube. It was produced as three stages. The first stage was a Pulse Width Modulator (PWM) circuit with adjustable frequency and duty. PWM was used for generating high frequency signal for feeding up two identical step-up transformers. Two identical step-up transformers were driven by a MOSFET circuitry after the PWM in the second stage. Instead of using a high voltage transformer, driving simple transformers with an optimum frequency PWM signal has provided the high voltage we need. Finally, a well-known Villard-Cascade voltage multiplication topology was used to multiply the initial stepped-up potential to the desired value. The multiplier units are simple circuits made from diodes and capacitors. In this method, an n-stage voltage multiplier unit produce 2n folded output voltage. The multiplier unit was designed to give 10-fold increase in the output voltage. We have also designed a negative 10-stage voltage multiplier unit by simply reversing the polarities of all the multiplier diodes. Instead of running the tube for our measurements with a single supply (+40 kV), it was preferred to generate a symmetrical high voltage (±20 kV) with two independent cascade units. This helps to overcome unwanted discharges and to ease electrical isolation problems. A custom made filament transformer was wounded as an isolation transformer and it was embedded in epoxy-resin to provide electrical. The isolation transformer was used between the mains and the filament power supply running at 2 VAC and 1.5 A current in regular operating conditions. Drifts in the filament current or tube voltage cause X-ray intensity to change and this can degrade the image quality. Most industrial type power supplies used for X-ray tubes are designed with feedback circuitry to stabilize the voltage/current output. However, our design does not include such a feedback control for simplicity. Instead of this, drifts problem was solved by correcting the image when necessary by using the data obtained from an X-ray intensity monitor. Since there were no feedback controls for precise regulation of the filament current and the high voltage of the tube, X-ray intensity was continuously monitored with another BPW-34 based detector designed. In order to investigate the magnitude of X-ray intensity fluctuations which may deteriorate the image quality, we have recorded the intensity of incident X-rays during imaging time. To minimize the image reconstruction artifacts, the object-detector distance was chosen to be smallest possible and single hole aluminum collimator was used to minimize the scattered X-rays from large angles. The image data containing the position information and the count rate were constructed as gray-scale images. Scintillator crystals have an important role in nuclear imaging to convert ionizing radiation to visible or ultraviolet photons. They permit to increase speed of response, the accuracy and the sensitivity of the radiation sensors. The CsI crystals are widely used scintillators in the nuclear detection systems due to its relatively low hygroscopicity, cost and high atomic number. Vertical Bridgman crystal growth technique was studied to produce single CsI scintillator using three temperature zone Crystalox vertical Bridgman Single Crystal Growth System equipment. The system designed includes single Si-PIN photodiode as a detector. When this is the case, it is required to move either the object or the detector through the region of interest. In order to ensure the possibility of working with liquid containing samples, moving the detector within the defined limits was preferred. For that reason, a 2D scanning bench consisting of two linear trays positioned perpendicular was used. The one on top has a load-bearing panel and the detector box was fixed on it. Linear axis trays were moved by two step motors placed on each side of the trays. Since parallel port is the easiest way to communicate with the peripherals, the motor drivers were connected to the parallel port of the computer using a communication board. Mach3 CNC Software package program was used for controlling the software of the X-Y scanning bench. Coding language for Mach3 is special and called as G-code programming language. Loop properties of the G programming language was used in the automatized movements of the motors. Writing the G-code using loop structure reduced the time spent for coding and removed the motor fails. One can adjust the speed of acceleration, step size, start-stop positions with a graphical user interface (GUI). Synchronization between transferring of the data and motor motions should be established for error free data taking. A pilot signal was produced in the motor driver unit when motors were active. This pilot signal was processed to be able to transfer the image data to the computer properly. A delay circuitry was built using NE555 integrated circuit to have an interrupt during the motion of the detector between the scan steps. Thereby, we could track the position of the motors when they were moving. Image data was transferred to computer via an Arduino unit when a single Si-PIN photodiode used as a detector. Combination of hardware units enabled us to obtain X-ray images of various samples with accepted image quality. Depending on the desired resolution, images may take up to several hours even for small sized objects. However, when time is not number one priority, this device can provide a good quality X-ray images at low cost. The time requirement for imaging with only one pixel detector is relatively long. Thus, a multidetector array, which compromises 24 BPW-34 detectors was designed and produced. Imaging with this detector array required using an FPGA to transfer the data from detectors to the computer simultaneously. For test purposes, an FPGA code for four detectors was written and it was tested during real data taking. There are some improvements can be made in the future with the cost of extra labor. One of them is using a detector array to reduce the scanning time. Additionally using scintillators in front of photodiodes will dramatically increase the efficiency. Collimator studies are also worth obtaining scatter free images. These additions in hardware will not change the overall cost dramatically. However, they will shorten the scanning time.