The dominant powder scintillator in most medical imaging modalities for decades is Gd2O2S:Tb due to the very good intrinsic properties and overall efficiency. Except for Gd2O2S:Tb there are alternative powder phosphor scintillators like Lu2SiO5:Ce and Gd2O2S:Eu that has been suggested for use in various medical imaging modalities. Gd2O2S:Eu emits red light and can be combined mainly with digital imaging devices like CCDs and CMOS based detectors. The purposes of the present thesis, is to investigate the fundamental imaging performance of a high resolution CMOS based imaging sensor combined with custom made Europium (Eu3+) activated Gd2O2S screens in terms of Modulation Transfer Function (MTF), Normalized Noise Power Spectrum (NNPS), Detective Quantum Efficiency (DQE), Noise Equivalent Quanta (NEQ) and Information Capacity (IC) covering the mammography and general radiography energy ranges. The CMOS sensor was coupled to two Gd2O2S:Eu scintillator screens with coating thicknesses of 33.3 and 65.1 mg/cm2, respectively, which were placed in direct contact with the photodiode array. The CMOS photodiode array, featuring 1200x1600 pixels with a pixel pitch of 22.5 m , was used as an optical photon detector. In addition to frequency dependent parameters (MTF, NPS, DQE) characterizing image quality, image information content was assessed through the application of information capacity (IC). The MTF was measured using the slanted-edge method to avoid aliasing while the Normalized NPS (NNPS) was determined by two-dimensional (2D) Fourier transforming of uniformly exposed images. Both parameters were assessed by irradiation under the RQA-5 protocol (70kVp digital-radiography) recommended by the International Electrotechnical Commission Reports 62220-1 and the W/Rh, W/Ag beam qualities (28kVp digital-mammography). The DQE was assessed from the measured MTF, NNPS and the direct entrance surface air-Kerma (ESAK) obtained from X-ray spectra measurement with a portable cadmium telluride (CdTe) detector. The spectral matching factor between the optical spectra emitted by the Gd2O2S:Eu and the Gd2O2S:Tb screens and the CMOS optical sensor, evaluated in the present study, was 1 and 0.95 respectively. The ESAK values ranged between 11.2-87.5 Gy , for RQA-5, and between 65.8-334 Gy , for W/Rh, W/Ag beam qualities. It was found that the detector response function was linear for the exposure ranges under investigation. Under radiographic conditions the MTF of the present system was found higher than previously published MTF data for a 48 m CMOS sensor, in the low up to medium frequency ranges. DQE was found comparable, while the NNPS appeared to be higher in the frequency range under investigation (0–10 cycles/mm). NEQ reached a maximum (73563 mm-2) in the low frequency range (1.8 cycles/mm), under the RQA 5 (ESAK: 11.2 Gy ) conditions. IC values were found to range between 1730-1851 bits/mm2. Under mammographic conditions MTF, NNPS and NEQ were found comparable to data previously published for the 48 m CMOS sensor while the DQE was found lower. The corresponding IC values were found ranging between 2475 and 2821 bits/mm2. The imaging performance of europium (Eu3+) activated Gd2O2S screens in combination to the CMOS sensor, investigated in the present study, was found comparable to those of Terbium (Tb) activated Gd2O2S screens (combined with the CMOS sensor). It can be thus claimed that red emitting phosphors could be suitably used in digital imaging systems, where the Silicon (Si) based photodetectors are more sensitive to longer wavelength ranges, and particularly in the red wavelength range. / -

Bone age assessment is a method of assigning a level of biological maturity to a child. It is usually performed either by comparing an x-ray of a child's left hand and wrist with an atlas of known bones, or by analysing specific features of bones such as ratios of width to height, or the degree of overlap with other bones. Both methods of assessment are labour intensive and prone to both inter- and intra-observer variability. This is motivation for developing a computerised method of bone age assessment. The majority of research and development on computerised bone age assessment has focussed on analysing the bones of the hand. The wrist bones, especially the carpal bones, have received far less attention and have only been analysed in young children in which there is clear separation of the bones. An argument is presented that the evidence for excluding the carpal bones from computerised bone age assessment is weak and that research is required to identify the role of carpal bones in the computerised assessment of bone age for children over eight years of age. Computerised analysis of the carpal bones in older children is a difficult computer vision problem plagued by radiographic noise, poor image contrast, and especially poor definition of bone contours. Traditional image processing methods such as region growing fail and even the very successful Canny linear edge detector can only find the simplest of bone edges in these images. The field of partial differential equation-based image processing provides some possible solutions to this problem, such as the use of active contour models to impose constraints upon the contour continuity. However, many of these methods require regularisation to achieve unique and stable solutions. An important part of this regularisation is image denoising. Image denoising was approached through development of a noise model for the Kodak computed radiography system, estimation of noise parameters using a robust estimator of noise per pixel intensity bin, and incorporation of the noise model into a denoising method based on oriented Laplacians. The results for this approach only showed a marginal improvement when using the signal-dependent noise model, although this likely reflects how the noise characteristics were incorporated into the anisotropic diffusion method, rather than the principle of this approach. Even without the signal-dependent noise term the oriented Laplacians denoising of the hand-wrist radiographs was very effective at removing noise and preserving edges.

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The information transfer efficiency of any quantum detection imaging system can be described by a unique measure: the detective quantum efficiency {DQE(f)}, which is a function of the statistically independent frequency channels. The DQE(f) is a combined descriptor which takes into account the signal transfer as well as noise transfer properties of a complete system. For a complicated multistage imaging system, each system component contributes noise. In this dissertation, physical and mathematical models for noise analysis are developed and verified experimentally with an x-ray image intensifier (XRII)-based imaging system. In such a system, the DQE at low frequency range is primarily determined by the x-ray detection and scintillation processes at the CsI layer of the XRII. The effects of x-ray photon energy and sensor layer thickness on DQE are measured in detail. Numerical calculations based on a physical model of x-ray interactions show a general agreement with the experimental data. At higher frequencies, the DQE behavior becomes more complicated. A mathematical model which combines the micro-image properties and noise statistics is formulated to analyze the noise power spectrum (NPS) of a linear n-stage imaging system. Measurement of NPS components of an XRII system verifies the validity of this analytical prediction. The associated image transfer properties are also measured with emphasis on the effect of signal-induced background on the image information transfer. The low frequency data derived from these image property measurements show further agreement with the numerical calculations based on the physical model. As a result of this predictability of information transfer efficiency, system gain and recording capacity are emphasized in the design consideration of a projected high performance XRII radiographic system.

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This investigation dealt with locating and measuring x-ray absorption of radiographic images. The methods developed provide a fast, accurate, minicomputer control, for analysis of embedded objects. A PDP/8 computer system was interfaced with a Joyce Loebl 3CS Microdensitometer and a Leeds & Northrup Recorder. Proposed algorithms for bone location and data smoothing work on a twelve-bit minicomputer. Designs of a software control program and operational procedure are presented. The filter made wedge and limb scans monotonic from minima to maxima. It was tested for various convoluted intervals. Ability to resmooth the same data in multiple passes was tested. An interval size of fifteen works well in one pass.

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