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Purpose: We present a new framework for theoretical analysis of the noise power spectrum (NPS) of photon‐counting x‐ray detectors, including simple photon‐counting detectors (SPCDs) and spectroscopic x‐ray detectors (SXDs), the latter of which use multiple energy thresholds to discriminate photon energies. Methods: We show that the NPS of SPCDs and SXDs, including spatio‐energetic noise correlations, is determined by the joint probability density function (PDF) of deposited photon energies, which describes the probability of recording two photons of two different energies in two different elements following a single‐photon interaction. We present an analytic expression for this joint PDF and calculate the presampling and digital NPS of CdTe SPCDs and SXDs. We calibrate our charge sharing model using the energy response of a cadmium zinc telluride (CZT) spectroscopic x‐ray detector and compare theoretical results with Monte Carlo simulations. Results: Our analysis shows that charge sharing increases pixel signal‐to‐noise ratio (SNR), but degrades the zero‐frequency signal‐to‐noise performance of SPCDs and SXDs. In all cases considered, this degradation was greater than 10%. Comparing the presampling NPS with the sampled NPS showed that degradation in zero‐frequency performance is due to zero‐frequency noise aliasing induced by charge sharing. Conclusions: Noise performance, including spatial and energy correlations between elements and energy bins, are described by the joint PDF of deposited energies which provides a method of determining the photon‐counting NPS, including noise‐aliasing effects and spatio‐energetic effects in spectral imaging. Our approach enables separating noise due to x‐ray interactions from that associated with sampling, consistent with cascaded systems analysis of energy‐integrating systems. Our methods can be incorporated into task‐based assessment of image quality for the design and optimization of spectroscopic x‐ray detectors. [ABSTRACT FROM AUTHOR]

Digital tomosynthesis (DTS) is an X-ray imaging technique that produces cross-sectional images with a scanning motion in narrow angular ranges. This method is promising for the inspection of internal defects in thin slab objects, such as printed circuit boards. However, this limited angular scan approach can have limited resolution in the depth direction. In addition, it can cause ghosting artifacts that originate from other planes in the reconstructed plane. In this paper, we characterized the imaging performance of a DTS method that uses the filtered backprojection for reconstruction. Various imaging parameters were examined, such as the total scan angle and the number of projections used for the reconstruction. We analyzed the signal and noise characteristics of the reconstructed images obtained for a thin tungsten wire and a thin aluminum disc. We obtained in-plane and in-depth modulation transfer functions (MTFs) from the reconstructed wire response images. From the reconstructed disc response images, we calculated the signal difference between the disc region and the adjacent background region along the depth direction, which is called the artifact spread function (ASF). A narrow angular scan enhances the in-plane MTF performance compared to that obtained from the conventional computed tomography, but it degrades the in-depth MTF performance (i.e., it decreases the depth resolution). In addition, the narrow angular scan degrades the ASF performance and causes severe out-of-plane blur artifacts. The optimal scanning angular range is, therefore, required to obtain tomographic images with high spatial resolution and less blur artifacts. The analysis method performed in this paper could be useful for this optimization. [ABSTRACT FROM PUBLISHER]

Purpose Acquisition of high-quality x-ray images using low patient exposures requires detectors with high detective quantum efficiency ( DQE). We describe a novel apodized-aperture pixel ( AAP) design that increases high-frequency modulation transfer function ( MTF) and DQE values. The AAP design makes a separation of physical sensor elements from image pixels by using very small sensor elements (e.g., 0.010-0.025 mm) to synthesize desired larger image pixels (e.g., 0.1-0.2 mm). Methods A cascaded systems model of signal and noise propagation is developed to describe the benefits of the AAP approach in terms of the MTF, Wiener noise power spectrum ( NPS), and DQE. The theoretical model was validated experimentally using a CMOS/CsI detector with 0.05 mm sensor elements to synthesize 0.20 mm image pixels and a clinical Se detector with 0.07 mm sensor elements to synthesize 0.28 mm pixels. A Monte Carlo study and x-ray images of a star-pattern and rat leg are used to visually compare AAP images. Results When used with a high-resolution converter layer and sensor elements one quarter the size of image pixels, the MTF is increased by 53% and the DQE by a factor of 2.3× at the image sampling cut-off frequency. Both simulated and demonstration images show improved detectability of high-frequency content and removal of aliasing artifacts. Evidence of Gibbs ringing is sometimes seen near high-contrast edges. Conclusions It is shown that the AAP approach preserves the MTF of the small sensor elements and attenuates frequencies above the image sampling cut-off frequency. This has the double benefit of improving the MTF while reducing both signal and noise aliasing, resulting in an increase of the DQE at high spatial frequencies. For optimal implementation, the converter layer must have very high spatial resolution and the detector must have low readout noise. [ABSTRACT FROM AUTHOR]

Goal: We investigate the signal and noise performance of an x-ray microtomography system that incorporates a complementary metal–oxide–semiconductor flat-panel detector as a projection image receptor. Methods: Signal and noise performance is analyzed in the Fourier domain using modulation-transfer function (MTF), noise-power spectrum (NPS), and noise-equivalent number of quanta (NEQ) with respect to magnification and different convolution kernels for image reconstruction. Results: Higher magnification provides lower NPS, and thus, higher NEQ performance in the transaxial planes from microtomography. A window function capable of smoothing the ramp filter edge to below one-half of the Nyquist limit results in better performance in terms of NPS and NEQ. The characteristics of convolution kernels do not affect signal and noise performance in longitudinal planes; hence, MTF performance mainly dominates the NEQ performance. The signal and noise performances investigated in this study are demonstrated with images obtained from the contrast phantom and postmortem mouse. Conclusion: The results of our study could be helpful in developing x-ray microtomography systems based on flat-panel detectors. [ABSTRACT FROM PUBLISHER]

We describe energy-dependent incomplete signal generation in semiconductor detectors with a simple planar geometry using deep charge trapping and depth-of-interaction models. Based on this formalism, we have characterized mercuric iodide (HgI_2) photoconductors by extracting performance parameters from x-ray-induced signals under mammographic imaging conditions. The HgI_2 sample photoconductors were prepared using a simple screen-printing method. For an x-ray tube output from tungsten target and 30-kV setting, the quantum absorption efficiency of samples with a thickness of approximately 0.1 mm was measured to be \sim 70\%. X-ray sensitivity was measured to be 0.4~\nC cm^ - 2 mR^ - 1. Mobility-lifetime products for electrons and holes were measured as \sim 2 \times 10^ - 6~\cm^2~\V^ - 1 and \sim 0.9 \times 10^ - 6~\cm^2~\V^ - 1, respectively. Considering incomplete charge collection, the W-value (or w) was estimated to be \sim 27 ~\eV. Since the characterization method described in this study is simple and does not require sophisticated equipment, it can be useful for the characterization of photoconductor materials. [ABSTRACT FROM AUTHOR]

We describe a multi-layer (“sandwich”) configuration detector consisting of two x-ray imaging flat-panel detectors for single-shot (single-kV) dual-energy imaging. An intermediate copper filter is used to increase spectral separation between the two detectors to improve contrast at the expense of image noise. Monte Carlo and cascaded-systems analyses of the signal and noise performance are described that quantify performance characteristics. Image quality of dual-energy images obtained from a prototype sandwich-detector system is evaluated using a figure of merit (FOM), defined as the squared contrast-to-noise ratio normalized by x-ray exposure for a mouse phantom for preclinical imaging. Demonstration dual-energy bone and soft-tissues images of a postmortem mouse are obtained using the prototype system. While the FOM with the single-shot detector is lower than that achieved using a conventional dual-shot (dual-kV) method, the single-shot approach may be preferable when imaging speed or insensitivity to motion artifacts is a primary concern. [ABSTRACT FROM AUTHOR]

The absorbed energy distribution (AED) in X-ray imaging detectors is an important factor that affects both energy resolution and image quality through the Swank factor and detective quantum efficiency. In the diagnostic energy range (20–140 keV), escape of characteristic photons following photoelectric absorption and Compton scatter photons are primary sources of absorbed-energy dispersion in X-ray detectors. In this paper, we describe the development of an analytic model of the AED in compound X-ray detector materials, based on the cascaded-systems approach, that includes the effects of escape and reabsorption of characteristic and Compton-scatter photons. We derive analytic expressions for both semi-infinite slab and pixel geometries and validate our approach by Monte Carlo simulations. The analytic model provides the energy-dependent X-ray response function of arbitrary compound materials without time-consuming Monte Carlo simulations. We believe this model will be useful for correcting spectral distortion artifacts commonly observed in photon-counting applications and optimal design and development of novel X-ray detectors. [ABSTRACT FROM AUTHOR]

Abstract: A digital X-ray imaging detector based on a complementary metal-oxide-semiconductor (CMOS) image sensor has been developed for X-ray non-destructive inspection applications. This is a cost-effective solution because of the availability of cheap commercial standard CMOS image sensors. The detector configuration adopts an indirect X-ray detection method by using scintillation material and lens assembly. As a feasibility test of the developed lens-coupled CMOS detector as an X-ray inspection system, we have acquired X-ray projection images under a variety of imaging conditions. The results show that the projected image is reasonably acceptable in typical non-destructive testing (NDT). However, the developed detector may not be appropriate for laminography due to a low light-collection efficiency of lens assembly. In this paper, construction of the lens-coupled CMOS detector and its specifications are described, and the experimental results are presented. Using the analysis of quantum accounting diagram, inefficiency of the lens-coupling method is discussed. [Copyright &y& Elsevier]

To realize high-resolution X-ray imaging, this study trains deep neural networks comprising multi-scale encoder/decoder architectures with actual data measured using a low-resolution imaging detector. The network performance is evaluated in the Fourier domain through measurements of the modulation-transfer function (MTF) and noise-power spectrum (NPS), which decompose the contrast and noise power (or variance) in terms of the spatial-frequency components. The designed networks are found to successfully deblur blurry images. The MTF analysis reflects the conventional mean-squared errors and structural similarities within it. Moreover, it indicates the over-regression property of the networks that could not be addressed via the conventional metrics. The NPS analysis reveals that the networks suppressed high-frequency noise. The Fourier analysis shows that the designed networks attempted to match the fidelity between the input and label, while suppressing noise. • Symmetric multi-scale architecture is useful for image deblur. • Supervised U-Net increases data fidelity and reduces noise. • Fourier analysis shows a neural net performance unobserved by SSIM. [ABSTRACT FROM AUTHOR]

The X-ray imaging performance of a phosphor-coupled photodiode array detector was assessed for use in a cone-beam computed tomography (CBCT) system for printed-circuit board inspection. The radiographic image quality of a detector is mainly responsible for the tomographic image quality. The radiographic image quality can be improved by increasing the X-ray exposure level, but CT scans at higher exposure can impose an enormous heat load on the X-ray tube. In this study, the detective quantum efficiency (DQE) was used to describe the image quality per air kerma as a metric to assess the detector performance. Measurement results were obtained for 40-kVp and 50-kVp tungsten-target X-ray beams, which show that the DQE of the detector is tolerant of air kerma levels as low as 0.8 μ Gy. Below this level, the electronic noise of the detector system could degrade the DQE performance. The concept of DQE and the measurement procedure are described, including the modulation-transfer function and the noise-power spectrum. [ABSTRACT FROM AUTHOR]