Over the past few decades, amorphous selenium (a-Se) X-ray detectors have gained widespread use in mammography due to their remarkable spatial resolution capabilities. However, these devices encounter challenges in applications requiring lower radiation exposures and dynamic imaging, such as dynamic mammography tomosynthesis. The potential enhancement of sensitivity and temporal performance by increasing the applied voltage across the selenium layer is counteracted by concerns about dark current. Furthermore, certain applications necessitate the placement of a low-temperature hole-blocking layer on the top surface of a-Se to enable high-voltage mode in hole collecting mode. Although the use of SU-8 as a top layer has demonstrated satisfactory temporal performance, there remains room for improvement. Additionally, the use of SU-8 at the bottom has revealed interface compatibility issues. In this research, we address the compatibility challenges associated with the positioning of the SU-8 layer at the bottom by introducing a novel bilayer configuration. This bilayer setup was evaluated in both top and bottom positions. Our results indicate that when the bilayer configuration is located at the top, it combines the strengths of both layers, merging the high signal level in a Cs-doped a-Se device with the low dark current characteristics of the SU-8 layer. Notably, among the samples incorporating hole-blocking layers, the bilayer positioned at the top exhibits the most favorable lag performance, measuring below 0.5% after the 7th exposure, and a sensitivity reduction of 14% after 20 exposures (equivalent of 0.175 Gy). Moreover, employing this bilayer arrangement at the bottom enhances sensitivity by 16.1% compared to devices utilizing only the SU-8 hole-blocking layer. This improvement underscores the effective mitigation of interface challenges through the utilization of Cs-doped a-Se when SU-8 is placed at the bottom.
The prevalence of kidney stones has significantly risen among the elderly population in recent decades, with some countries experiencing rates approaching 15%. Due to high recurrence rates, analyzing stone composition is essential for recurrence prevention. Fourier transform infrared spectroscopy (FT-IR) and conventional x-ray diffractometry (XRD) are used for this purpose, however, both methods are labor-intensive and require skilled operators, as stones need careful dissection and grinding. Many kidney stones have heterogeneous compositions, and even with spectroscopic analyses, error rates remain high. Despite improvements, challenges in stone sampling and tiny material amounts can still lead to inaccuracies. This prompts the question of how to better manage patients and potentially enhance recurrence prevention strategies despite the shift toward spectroscopic techniques in analytical labs. In this work, we demonstrate the potential of an in-house grating-based high-resolution x-ray phase-contrast μ-CT imaging system in characterizing kidney stones and their compositional analysis. Employing the reported compact benchtop x-ray phase-contrast μ-CT imaging system facilitates stone identification and analysis using additional quantitative data—phase and scattering—obtained from the emerging x-ray phase-contrast imaging system. Having the three sets of images—transmission, phase, and scattering—could potentially pave the way for an accurate and faster stone compositional analysis with clinical values in studying the pathophysiological mechanisms of kidney stone disease that could help improve the recurrence prevention rates.
Grating-based X-ray phase-contrast imaging (XPCi) systems offer higher sensitivity compared to other XPCi methods; however, realizing a high-resolution, compact, dose-efficient imaging system has been a significant challenge from technological and practical points of view until now. X-ray gratings quality and characteristics directly determine the final imaging quality, where a proper grating fabrication process can potentially minimize image artifacts and increase the system visibility. To achieve a high-resolution, compact, dose-efficient XPCi system, high-resolution detectors and high-resolution X-ray absorption gratings are a must. Moreover, an efficient image processing method is required to retrieve multimodal XPCi information—transmission, refraction (phase-contrast), and dark-field—efficiently and simultaneously. In this work, we report on a compact XPCi system that enables multimodal information retrieval through single-shot imaging with two-directional sensitivity. We first present an elegant cost-effective fabrication method to make high-resolution micropillar-based X-ray absorption gratings. A prototype 2D grating is fabricated with micropillars with 4 μm in diameter, periodicity of 16.3 μm, and aspect ratio of more than 40. This grating is then employed along with a prototype hybrid a-Se/CMOS direct conversion high-resolution X-ray detector with a pixel pitch of 8 μm in a compact system with a polychromatic microfocus source to perform X-ray phase-contrast imaging of various samples. We successfully demonstrate a single-shot XPCi and retrieve multimodal XPCi data with transmission and dark-field metrics. The final system is a promising candidate for XPCi applications as it facilitates single-shot imaging, which reduces the exposure dose on samples and yields multimodal XPCi images efficiently, all in a compact bench-top setup. The delivered X-ray dose at the sample, resolution of the system, and compactness of the reported imaging setup are potentially beneficial for ex vivo, in vivo, and computed tomography (CT) imaging applications.
A high spatial resolution and cost-effective amorphous selenium multi-layer detector architecture was previously proposed for high-energy applications including single-photon emission computed tomography (SPECT). To overcome the complexity of reading out signal from the multi-layer detector, we propose a resistive microstrip detector readout using charge division. The charge induced on the readout electrode undergoes charge division along the electrode and is proportional to the physical location of absorbed photons, enabling position sensitivity. A 1 mm collimated blue LED is employed to illuminate the electrode, which has a resistance of 242 kΩ. To improve the linearity of the position sensing, the microstrips were deposited underneath a-Se in order to move bonding pads far away from the active area. A low deposition temperature evaporated Cs-doped and As-stabilized a-Se layer is used as a hole blocking layer to ensure that the resistivity of microstrips is not affected and to prevent crystallization of the temperature sensitive a-Se layer. The detector structure was fabricated on a flexible substrate that can potentially be incorporated, in the future, into a curved detector structure for large area SPECT systems. The results presented demonstrate that the detected position is linearly proportional to the location of absorbed photons with a spatial resolution of 1 mm.
Among different techniques developed for x-ray phase-contrast imaging (XPCi) coded-aperture and Talbot-Lau can potentially provide higher contrast and higher sensitivity in imaging soft and hard tissues. Absorption grating is the central component that makes these techniques different from other XPCi techniques. For hard xray imaging, compatible x-ray absorption gratings are a must so x-ray can be stopped at these absorption gratings to create the desired wave-fronts. For hard x-ray imaging or soft x-ray imaging with high-resolution detectors, the aspect-ratio of gratings feature size goes beyond 10:1 (height:width) – in other words thickness of absorbing part increases – which is technologically challenging through conventional UV lithography techniques. Although SU-8 photoresist along with x-ray lithography is an alternative to fabricate such a high-aspect ratio grating, a compatible UV lithograph-based fabrication process is more desirable as it reduces the cost and increases the throughput. In this work, we have broken down a high-aspect ratio x-ray grating design into multiple layers of lower-aspect ratio structures and employed only the conventional UV lithography. SU-8 photoresists are known for their multi-layer coating specification, which is used in this study. The new fabrication process proposed in this work results in a final high-aspect ratio x-ray absorption grating through accessible UV lithography with lower cost and scaling-up compatibility, thus every research group and industry can benefit from it. To the best of our knowledge, this is the first time a multi-layer x-ray grating design is proposed and reported. The output of this work can be used to perform large field-of-view high-energy coded-aperture x-ray phase-contrast imaging.
X-ray absorption gratings are the heart of coded-aperture and Talbot-Lau x-ray phase-contrast imaging (XPCi) techniques. The quality of imaging – visibility – in the aforementioned techniques is highly dependent on the quality of gratings. SU-8, an epoxy-based photoresist, is valued in Micro-Electro-Mechanical-Systems (MEMS) fabrication due to its excellent mechanical and optical properties. For fabricating x-ray absorption grating following the MEMS process, gold is the material most widely used to stop incident x-rays. One of the main challenges in fabricating a quality x-ray absorption grating is the adhesion between SU-8 molds and gold thin film, where due to poor adhesion of SU-8 to gold thin films and SU-8’s internal high level of stress, SU-8 molds delaminate, crack, or peel off during developing or electroplating steps. This problem becomes more challenging when a greater field of view gratings are required. Employing SU-8 on a gold substrate requires an adhesion layer to assure SU-8’s mechanical stability during fabrication steps. Without this layer, realizing a high-quality grating is challenging. In this work, we have investigated the adhesion quality of SU-8 to gold thin films for different adhesion promoter layers. We have employed a combination of a SU-8 thin film and a metallic-silane-based nanometer-thin film to improve the adhesion quality between SU-8 photoresist and a gold thin film substrate. We have tested the quality of adhesion using tape test and through a harsh (highly acidic) electroplating solution.
High-aspect ratio x-ray absorption gratings are required for hard x-ray or high-resolution soft x-ray phasecontrast imaging (XPCi) applications. X-ray gratings play a central role in grating-based XPCi techniques, where visibility – image quality – is highly dependent on the quality of gratings. Although the fabrication process of x-ray gratings for soft x-ray imaging is well-developed, there are technological challenges in highresolution hard x-ray imaging that impede us from making high-aspect ratio large field-of-view fine x-ray gratings through conventional fabrication processes. Here we present a design and a fabrication process to fabricate highaspect ratio gratings that benefit from a self-aligned hard-mask – a patterned chromium-gold-chromium thin film deposited on a transparent ITO-on-glass substrate – which facilitates both lithography and electroplating processes. The repeatability of the proposed method makes it suitable for achieving high-aspect ratio fine structures as thick as desired through a multi-layer structure without any restraint or limitation on the aspect ratio of features. The multi-layer structure design overcomes the aspect ratio limitation associated with UVlithography. Although this method works well with X-ray lithography, the key advantage of this design is that it enables UV lithography for high-aspect ratio grating fabrication through a reliable yet simple process. To the best of our knowledge, self-aligned multi-layer SU-8 based grating design has not been previously reported. The proposed design and fabrication process help researchers further develop x-ray gratings performance to facilitate high-resolution coded-aperture and Talbot-Lau high-energy x-ray phase-contrast imaging.
The aim of this study is to investigate and reveal the potential of employing a direct conversion amorphous selenium (a-Se) CMOS based high resolution x-ray detector in both propagation-based (PB) and edge illumination (EI) x-ray phase contrast imaging (XPCi) techniques. Both PB-XPCi and EI-XPCi modalities are evaluated through a numerical model and are compared based on their contrast, edge-enhancement, visibility, and dose efficiency characteristics. It is demonstrated how EI-XPCi configuration outperforms the PB-XPCi one considering using the same x-ray source and detector. After highlighting the strength of EI-XPCi system and reviewing today’s XPCi technologies, absorption mask grating fabrication is addressed as the main challenge to upgrade and improve EI-XPCi setups to higher resolution detectors. Mammography is considered as a case study to elucidate the importance of employing a high resolution EI-XPCi technique for microcalcification detection through numerical simulation of a breast phantom.
In this work, we have evaluated the performance of a coded-aperture (edge-illumination) x-ray phase contrast imaging (CA-XPCi) system employing a high resolution x-ray detector with pixel pitch and size equal to 7.8 µm. Two of the main challenges concerning a high resolution x-ray detector employed in a CA-XPCi system that are fabrication of high resolution x-ray absorption masks and environmental vibration are addressed in this paper. We have investigated both the effect of absorption mask thicknesses and mechanical vibration on the performance of a high resolution CA-XPCi system employing a simulation tool based on a wave-optics model. It is demonstrated how the thickness of absorption mask affects the behavior of the CA-XPCi system when more than 30% of the incident x-ray is transmitted through the absorption masks. The behavior of the CA-XPCi system will change to propagation-based (PB) XPCi one when the transmitted portion of x-rays through absorption masks exceeds 60% of the incident beam. It is also highlighted how mechanical (environmental) vibration has an almost minor effect on CA-XPCi systems with big pixel sizes, however, it has a considerable impact on a CA-XPCi system when a high resolution detector is employed. Albeit a high resolution CA-XPCi system has not been yet realized in reality due to technological bottlenecks related to high resolution mask fabrication, this study provides a comprehensive analysis on the challenges we will face to use a high resolution x-ray detector with the current technology, thus they can be considered in future designs.
KEYWORDS: Sensors, Single photon emission computed tomography, Monte Carlo methods, Photon transport, Photodetectors, Modulation transfer functions, Spatial resolution, Point spread functions, Energy efficiency, Compton scattering
In this work, we have used Geant4 Monte Carlo simulations to evaluate the spatial resolution and detection efficiency metrics of an amorphous selenium (a-Se) photon-counting detector for high energy gamma-ray detection with 1mm and 50µm pixel pitches and a varying number of detector layers. A noiseless monochromatic particle gun with an energy of 140 keV is used to resemble the typical energy of single-photon emission computed tomography (SPECT). At high energies like 140 keV, a-Se detection efficiency significantly drops due to its absorption coefficient. By using this novel multilayer a-Se detector, the drop in detection efficiency can be compensated. The point spread function (PSF) is obtained by illuminating the detector with 106 photons. The modulation transfer function (MTF) is calculated from the one-dimensional integral of the PSF, known as line spread function (LSF), and is compared to the ideal pixel MTF. Spatial resolution is considered as the spatial frequency at which the MTF is equal to 0.5. The simulation results indicate that by increasing the number of layers, the MTF was degraded slightly due to Compton scattering, however, it did not degrade spatial resolution for the 1mm pixel size. At the same time, by using more layers, the detection efficiency is increased to 80% for 10 layers. This detection efficiency includes the noise counts (error counts) caused by Compton scattering. Leveraging the photon-counting energy threshold enables partial compensation for noise counts. Using an energy threshold of 110 keV results in 52% efficiency for 10 layers and reduces the noise counts significantly. This increase in detection efficiency along with the high intrinsic spatial resolution makes a-Se a cost-effective candidate for large area SPECT applications.
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