Gamma-ray CT system assembly design using novel detector calibration method and minimization of Compton scattering.

Abstract

Computed Tomography (CT) has emerged as a robust, non-destructive evaluation technique capable of generating both 2D and 3D cross-sectional images of scanned object. The CT images provide valuable insights into an object's internal characteristics, including its dimensions, shape, internal defects, and density. The presence of noise in the projection data or CT data corrupts the accuracy of the data. It introduces the artifacts in CT images and, consequently, degrades the quality of the CT image. Artifactfree CT images are desirable for obtaining accurate information about the scanning object. Ongoing developments in CT aim to enhance image quality by addressing and investigating numerous factors that affect spatial resolution and overall CT image quality. The CT image noise is associated with both hardware and software. The noise due to hardware (part of the CT system) is added while performing the CT experiment and collecting data. However, the software adds noise mainly in the mathematical procedure to reconstruct CT images. It might be due to improper inputs and the selection of reconstruction algorithms. Numerous research studies have been carried out to reduce the noise emerging due to software improving the spatial resolution of CT images. This thesis is focused on finding the factors that add noise in projection data due to hardware associated with CT systems and finding a tool, methodology, or solution to estimate, suppress, or reduce the impact of these noise components respectively. The CT image noise due to hardware can be encountered during measurement procedure due to statistics fluctuations in the emission of incident rays, producing signal by the detectors corresponding to these continuous random incident rays, variation in geometry, scanning protocol, alignment of scanning object, selection of geometry parameters and selection of radiation detectors. The detector is one of the main components of the CT system, and the major components of detectors are crystal/sensitive material and hardware. It is one of the critical elements that substantially influences the quality of the CT image by incorporating the primary sources of noise, including attenuation, scattering, and detector response. These components add electronic noise and scattering noise to the response function generated by the detector. Therefore, selecting an optimal detector for CT is crucial for obtaining high-quality CT images. The radiation detector serves as a fundamental tool for studying the interaction of radiation with matter. It comprises sensitive material and associated electronics that require an operator to set several parameters. These electronic parameters encompass high voltage (HV) power supply, gain/amplifier value, energy window, and time to detect radiations or measure counts. Generally, the calibration of electronic settings is performed onsite in a given prior range of these parameters. The initial calibrated electronics settings of the radiation detector are provided by the manufacturer. These settings are considered widely stable; however, they may require re-calibration as the equipment completes its operational life, is employed in different applications, or is exposed to several conditions. The pulse width saturation/plateau method and gross method are popular in the calibration of electronic settings. The electronic settings strongly influence the pulse shape and resolution of the detector. Therefore, it is very important to find the optimal calibration method to obtain the accurate measurement of counts. Along with obtaining optimal values of electronic settings using optimal calibration method, one should require the knowledge of which electronic settings entail to pay more attention to before starting the counts measurement. In addition to the electronic settings, there are some post-processing questions that arise prior to radiation measurement. These involve determining the number of repetitions of a single experiment (sample size) and, if the experiment is repeated, deciding whether to calculate mean, median, or mode. The choice of these statistical parameters significantly impacts the accuracy and precision of count measurement. Three novel calibration methods are proposed in this work and compared with the existing calibration methods to find the optimal calibration method. The quantitative and qualitative analysis of calibration methods and choice of statistical parameters is investigated. The qualitative analysis is conducted by utilizing one of the CT modalities named Gamma-ray computed tomography (Gamma-ray CT), which is basically the real-life application of radiation detectors. The primary noise factor due to radiation detectors is electronic noise. It is associated with hardware/electronic components equipped with the crystal material of the detector and does not convey any relevant information about the scanned object. Analog electronic circuits in the detection system are the primary source of electronic noise. Incorrect selection of electronic settings may also introduce electronic noise into measurement data. The electronic noise becomes comparable to the signal if a relatively weak radiation source is used. Moreover, weak radiation sources are required to reduce the probability of diseases due to radiation exposure. Electronic noise degrades CT data and results in artifacts in CT images. De-noising filters and noise reduction methods are proposed by various groups of researchers to minimize electronic noise. The complete removal of electronic noise is not feasible, but it can be mitigated to a certain extent by calibrating electronic settings and the optimal selection of hardware. Detectors associated with conventional distributed electronics, semi-distributed electronics, and integrated circuit (IC) electronics are compared in terms of electronic noise. The latter component of noise associated with the detector is scattering noise. The transmission CT should encounter the counts from the photopeak region only, but it is very difficult to separate the photopeak region from the compton continuum, and scattering counts are present at low levels in the photopeak region. The scattering counts add scattering noise due to the inability to model scattering phenomena correctly in the reconstruction algorithm of transmission CT. The scattering noise depends on the type and size of the detector's crystal material or sensitive material. A variety of detectors are available for radiation measurement. Scintillation and semiconductor detectors are preferred over gas-filled detectors for X-ray and gamma-ray detection. Multiple types of crystal materials meet the requirements for standard performance matrices/properties of detector crystal material. However, relying solely on comparing the performance matrices is inadequate to determine which detector will provide the optimal CT results. Other factors, such as the type and size of the crystal material, also play a critical role in the quality of the resulting CT images. Unfortunately, a lack of studies explored in the existing literature to compare the radiation detectors based on these critical parameters to identify the optimal radiation detector for CT. Therefore, different types of scintillation and semiconductor detectors are compared in terms of scattering noise. In this investigation, we selected a range of detectors with different material types, diameters, and thicknesses. We then examined the impact of these factors on CT image quality, focusing on the level of scattering noise present in the CT images. The comparison of electronic circuits and crystal material requires a tool that can estimate the level of electronic and scattering noise. Before comparison, tools and methodology are proposed for their individual estimation and decoupling of these noise factors. After the optimum selection of the CT detector and its associated electronics, the user is ready to perform the CT experiment. The CT system comprises multiple detectors in a scanning array, as the resolution of CT images is determined by the number of detectors. However, the presence of multiple detectors introduces a component of scattering noise in CT projection data. A novel CT scanning scheme is proposed to evaluate the scattering noise due to the presence of neighboring detectors in the scanning array. The quantitative estimation of scattering noise with the increase of neighboring detectors is calculated using KT-1. Its impact on CT image quality is also illustrated. The above study explores various factors to improve CT image quality. One such influential factor is CT geometry parameters. The CT geometry information is used to generate the weight matrix. This weight matrix and the projection data acquired by the detector undergo the back-projection step of the tomographic reconstruction algorithm to obtain the scanning object's attenuation coefficient values. It is important to note that the CT geometry parameters significantly impact the projection data. Therefore, selecting an appropriate CT geometry is essential to ensure the best transmission from the radiation source to the detector through the scanning object. Users of commercial CT instruments typically rely on manufacturer-provided CT geometry parameters and assume these settings are optimal. The optimal selection of CT geometry parameters is very important because the non-optimality may produce artifacts and reduce the spatial resolution of reconstructed CT images. Simulation and experimental studies are conducted to optimize and analyze the influence of the position of the scanning object on CT image quality. In summary, the work presented in this thesis is motivated by recognizing the existing research gap in the enhancement of CT image quality by studying the noise factors that arise due to hardware, such as radiation measurement, instrumentation, scattering noise, CT geometry parameters, and CT assembly configuration. By addressing these challenges, the research aims to make substantial contributions to the advancement of CT scanners and improvements in scattering corrections. Keywords: calibration of detectors, scintillation detectors, semiconductor detectors, central limit theorem, Kanpur Theorem-1, gamma-ray spectroscopy, nuclear instrumentation, gamma-ray computed tomography, electronic noise, scattering noise, distributed electronics, IC electronics, standard deviation, RMSE, dice similarity coefficient, signal-to-noise ratio, CT geometry optimization, CT scanning schemes, source-to-object distance, object-to-detector distance.