Digital X-ray Imaging Applications Research Articles

Design and image-quality performance of high resolution CMOS-based X-ray imaging detectors for digital mammography

In digital X-ray imaging systems, X-ray imaging detectors based on scintillating screens with electronic devices such as charge-coupled devices (CCDs), thin-film transistors (TFT), complementary metal oxide semiconductor (CMOS) flat panel imagers have been introduced for general radiography, dental, mammography and non-destructive testing (NDT) applications. Recently, a large-area CMOS active-pixel sensor (APS) in combination with scintillation films has been widely used in a variety of digital X-ray imaging applications. We employed a scintillator-based CMOS APS image sensor for high-resolution mammography. In this work, both powder-type Gd2O2S:Tb and a columnar structured CsI:Tl scintillation screens with various thicknesses were fabricated and used as materials to convert X-ray into visible light. These scintillating screens were directly coupled to a CMOS flat panel imager with a 25 × 50 mm2 active area and a 48 μm pixel pitch for high spatial resolution acquisition. We used a W/Al mammographic X-ray source with a 30 kVp energy condition. The imaging characterization of the X-ray detector was measured and analyzed in terms of linearity in incident X-ray dose, modulation transfer function (MTF), noise-power spectrum (NPS) and detective quantum efficiency (DQE).

Studying the luminescence efficiency of Lu2O3:Eu nanophosphor material for digital X-ray imaging applications

Scintillator materials are widely used in X-ray medical imaging detector applications, coupled with available photoreceptors like radiographic film or photoreceptors suitable for digital imaging like a-Si, charge-coupled devises (CCD), complementary metal-oxide-semiconductors (CMOS) and GaAs). In addition, scintillators can be utilized in non-medical imaging detectors such as industrial detectors for non-destructive testing (NDT) and detectors used for security purposes (i.e. airport luggage control). Image quality and dose burden in the above applications is associated with the amount of optical photons escaping the scintillator as well as the amount of optical photons captured by the photoreceptor. The former is characterized by the scintillator efficiency and the latter by the spectral matching between the emission spectrum of the scintillator and the spectral response of the photoreceptor. Recently, a scintillator material, europium-activated lutetium oxide (Lu2O3:Eu), has shown improved scintillating properties. Lu2O3:Eu samples of compact nanocrystalline non-agglomerated powder were developed in our laboratory using homogeneous precipitation from a water-toluene solution in the presence of polyvinyl alcohol as a surfactant. In order to test their light-emission properties, experimental measurements under the excitation of X-ray spectra with X-ray tube voltages between 50 kVp and 140 kVp were performed. This range of applied voltages is appropriate for X-ray radiology, NDT and security applications. Lu2O3:Eu was evaluated with respect to output yield and spectral compatibility of digital imaging photoreceptors (CCD-based, CMOS-based, amorphous silicon a:Si flat panels, ES20 and GaAs). High light yield and spectral compatibility increase the performance of the medical detector and reduce the dose burden to the personnel involved. In addition a theoretical model was used to determine the values for the Lu2O3:Eu optical photon light propagation parameters. The inverse diffusion length was found to be equal to 33 cm2/g. In addition Lu2O3:Eu was found to match well with several photoreceptors capable of digital imaging (i.e. GaAs).

Low Dark-Current Lateral Amorphous-Selenium Metal–Semiconductor–Metal Photodetector

We report a lateral amorphous-selenium (a-Se) metal-semiconductor-metal detector with a blocking contact. The blocking contact, a polyimide layer, is shown to significantly reduce the dark current even at high applied biases that result in high photo-current-to-dark-current ratios, thus leading to wide dynamic range and high signal-to-noise ratio. The use of the polyimide blocking contact prevents the injection of both holes and electrons and improves considerably upon the high dark current of previously reported lateral a-Se detectors. The proposed detector demonstrates the feasibility of low-cost lateral a-Se devices for indirect conversion digital X-ray imaging applications such as chest radiography, fluoroscopy, and computed tomography.

Use and imaging performance of CMOS flat panel imager with LiF/ZnS(Ag) and Gadox scintillation screens for neutron radiography

In digital neutron radiography system, a thermal neutron imaging detector based on neutron-sensitive scintillating screens with CMOS(complementary metal oxide semiconductor) flat panel imager is introduced for non-destructive testing (NDT) application. Recently, large area CMOS APS (active-pixel sensor) in conjunction with scintillation films has been widely used in many digital X-ray imaging applications. Instead of typical imaging detectors such as image plates, cooled-CCD cameras and amorphous silicon flat panel detectors in combination with scintillation screens, we tried to apply a scintillator-based CMOS APS to neutron imaging detection systems for high resolution neutron radiography. In this work, two major Gd2O2S:Tb and 6LiF/ZnS:Ag scintillation screens with various thickness were fabricated by a screen printing method. These neutron converter screens consist of a dispersion of Gd2O2S:Tb and 6LiF/ZnS:Ag scintillating particles in acrylic binder. These scintillating screens coupled-CMOS flat panel imager with 25x50mm2 active area and 48μm pixel pitch was used for neutron radiography. Thermal neutron flux with 6x106n/cm2/s was utilized at the NRF facility of HANARO in KAERI. The neutron imaging characterization of the used detector was investigated in terms of relative light output, linearity and spatial resolution in detail. The experimental results of scintillating screen-based CMOS flat panel detectors demonstrate possibility of high sensitive and high spatial resolution imaging in neutron radiography system.

Characterization of imaging performance of a large-area CMOS active-pixel detector for low-energy X-ray imaging

Abstract We report the imaging characteristics of the recently developed large-area complementary metal-oxide-semiconductor (CMOS) active-pixel detector for low-energy digital X-ray imaging applications. The detector consists of a scintillator to convert X-ray into light and a photodiode pixel array made by the CMOS fabrication process to convert light into charge signals. Between two layers, we introduce a fiber-optic faceplate (FOP) to avoid direct absorption of X-ray photons in the photodiode array. A single pixel is composed of a photodiode and three transistors, and the pixel pitch is 96 μm. The imaging characteristics of the detector have been investigated in terms of modulation-transfer function (MTF), noise-power spectrum (NPS), and detective quantum efficiency (DQE). From the measured results, the MTF at the Nyquist frequency is about 20% and the DQE around zero-spatial frequency is about 40%. Simple cascaded linear-systems analysis has showed that the FOP prevents direct absorption of X-ray photons within the CMOS photodiode array, leading to a lower NPS and consequently improved DQE especially at high spatial frequencies.

Fabrication and characterization of pixelated Gd 2O 2S:Tb scintillator screens for digital X-ray imaging applications

X-ray imaging detectors in combination with scintillator screens have been widely used in digital X-ray imaging applications. Gd 2O 2S:Tb was used as scintillation material for pixelated scintillator screens based on silicon substrates (wafer) with a micropore array of various dimensions fabricated using the photolithography and deep reactive ion etching (DRIE) process. The relative light output and the modulation transfer function (MTF) of each fabricated scintillator screen were measured by a cooled CCD and compared with those of Lanex screens. The spatial resolution of our scintillator screens was higher but their light outputs were lower than those of Lanex screen probably due to the loss of light at the wall surfaces. Therefore further treatment of the wall surface, such as reflective coating, seems necessary to compensate the light loss.

Lateral metal-semiconductor-metal photodetectors based on amorphous selenium

We report a lateral metal-semiconductor-metal (MSM) photodetector (PD) based on an amorphous selenium (a-Se). The PD exhibits a dark current below 200 fA under electric fields ranging from 6 to 12 V/μm, a responsivity of up to 0.45 A/W, a photogain of 1.2 near short wavelengths of 468 nm, and a high-speed photoresponse with a rise time of 50 μs, fall time of 60 μs, and time constant of 30 μs, respectively. The lateral MSM PD based on a-Se has great potential for use in digital x-ray imaging applications.

Comparison of Mercuric Iodide and Lead Iodide X-Ray Detectors for X-Ray Imaging Applications

Mercuric iodide (HgI2) and lead iodide (PbI2) materials have been investigated for several years as direct converter layers for digital x-ray imaging applications. A difficult challenge of both lead iodide and mercuric iodide is the higher than desired leakage currents. These currents are influenced by different factors such as applied electrical field, layer thickness, layer density, electrode structure, material purity and by the deposition parameters. Minimizing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is influenced by these parameters. Other challenges relate to increasing film thickness without degrading electrical properties. This paper compares some imagers as the result of optimization process. We deposited the above materials on flat panel thin film transistor (TFT) arrays with 127 um pixel pitch. The imagers were evaluated for both radiographic and fluoroscopic imaging. Modulation Transfer Function (MTF) was measured as a function of the spatial frequency. The MTF data were compared to values published in the literature for indirect detector (CsI). Image lag characteristics of mercuric iodide appear adequate for fluoroscopic rates. The structure and x-ray diffraction data of the two materials were compared to explain the difference in image lag between them

Fabrication of Gd2O2S:Tb based phosphor films coupled with photodetectors for x-ray imaging applications

The Gd2O2S:Tb based phosphor coupled with photodetectors has been widely used in digital x-ray imaging applications. The key issues associated with phosphor film are x-ray absorption and conversion efficiency, spatial resolution, deposition uniformity, and integration with the imaging array. In this article we report on experimental studies of the phosphor film synthesis, deposition, and characterization. A composite material, consisting of Gd2O2S:Tb, polyvinyl alcohol, and water coupled with a small amount of additives, is synthesized as a phosphor paste. The parameters controlling film quality include individual component concentrations, grain sizes of phosphor, and viscosity of the solution. A sedimentation technique is used to deposit the phosphor directly on the imaging array. A number of phosphor films have been synthesized with the particle sizes ranging from 2.5 to 25 μm and film thickness ranging from 85 to 1100 μm, and measurement results of x-ray conversion efficiency and spatial resolution in terms of modulation transfer function are presented. A new technology for phosphor patterning is proposed to seamlessly integrate the phosphor with the photodetectors by using negative photoresist SU-8 and preliminary results are presented.

In application specific integrated circuit and data acquisition system for digital X-ray imaging

We have developed an Application Specific Integrated Circuit (ASIC) and data acquisition system for digital X-ray imaging. The chip consists of 16 parallel channels, each containing preamplifier, shaper, comparator and a 16 bit counter. We have demonstrated noiseless single-photon counting over a threshold of 7.2 keV using Silicon detectors and are presently capable of maximum counting rates of 2 MHz per channel. The ASIC is controlled by a personal computer through a commercial PCI card, which is also used for data acquisition. The content of the 16 bit counters are loaded into a shift register and transferred to the PC at any time at a rate of 20 MHz. The system is non-complicated, low cost and high performance and is optimised for digital X-ray imaging applications.