In this paper, we present a CMOS digital intra-oral sensor for x-ray radiography. The sensor system consists of a custom
CMOS imager, custom scintillator/fiber optics plate, camera timing and digital control electronics, and direct USB
communication. The CMOS imager contains 1700 x 1346 pixels. The pixel size is 19.5um x 19.5um. The imager was
fabricated with a 0.18um CMOS imaging process. The sensor and CMOS imager design features chamfered corners for
patient comfort. All camera functions were integrated within the sensor housing and a standard USB cable was used to
directly connect the intra-oral sensor to the host computer. The sensor demonstrated wide dynamic range from 5uGy to
1300uGy and high image quality with a SNR of greater than 160 at 400uGy dose. The sensor has a spatial resolution
more than 20 lp/mm.
We evaluate the effects of 10 MeV proton irradiation on the performance of a 5.5 Mpixel scientific grade CMOS image
sensor based on a 5T pixel architecture with pinned photodiode and transfer gate. The sensor has on-chip dual column
level amplifiers and 11-bit single slope analog to digital converters (ADC) for high speed readout and wide dynamic
range. The operation of the sensor is programmable and controlled by on-chip digital control modules. Since the image
sensor features two identical halves capable of operating independently, we used a mask to expose only one half of the
sensor to the proton beam, leaving the other half intact to serve as a reference. In addition, the pixel array and the digital
logic control section were irradiated separately, at dose rates varying from 4 rad/s to 367 rad/s, for a total accumulated
dose of 146 krad(Si) to assess the radiation effects on these key components of the image sensor. We report the resulting
damage effects on the performance of the sensor including increase in dark current, temporal noise, dark spikes,
transient effects and latch-up. The dark signal increased by about 55 e-/pixel after exposure to 14 krad (Si) and the dark
noise increased from about 2.75e- to 6.5e-. While the number of hot pixels increased by 6 percent and the dark signal
non uniformity degraded, no catastrophic failure mechanisms were observed during the tests, and the sensor did not
suffer from functional failures.
In this paper we present radiation studies performed on a low-noise, high-speed, largearea
CMOS image sensor (CIS) based on the 0.18 μm CMOS process. The sensor has
2560(H) x 2160(V) pixels with a readout speed of 100 frames/sec and a readout noise of
less than 2 e- rms. The sensor features 5T pinned photodiode pixels on a 6.5 μm pitch. In
order to measure the impact of radiation exposure on the sensor performance, the device
was subjected to x-ray exposure of 50 kRads of incident radiation using a broad band 50
KVP x-ray source to assess Total Ionizing Dose (TID) sensitivity. The active area and the
digital control block and amplification circuitry were separately irradiated to evaluate the
damage to each. Dark data was captured as a function of radiation dose in order to
measure dark current and offset changes in the signal.
In this paper we present radiation studies performed on a low-noise, high-speed, large-area CMOS
image sensor (CIS) based on the 0.18 μm CMOS process. The sensor has 2560(H) × 2160(V) pixels
with a readout speed of 100 frames/sec and a readout noise of less than 2 e- rms. The sensor features
5T pinned photodiode pixels on a 6.5 um pitch. In order to measure the impact of radiation exposure
on the sensor performance, the device was subjected to x-ray exposure of 50 kRads of incident
radiation using a broad band 50 KVP x-ray source to assess Total Ionizing Dose (TID) sensitivity.
The active area and the digital control block and amplification circuitry were separately irradiated to
evaluate the damage to each. Dark data was captured as a function of radiation dose in order to
measure dark current and offset changes in the signal.
The field of ultrafast x-ray science is flourishing, driven by emerging synchrotron sources (e.g., time-slice storage rings, energy recovery linacs, free electron lasers) capable of fine time resolution. New hybrid x-ray detectors are under development in order to exploit these new capabilities.
This paper describes the development of a 2160 x 2560 CMOS image sensor (CIS) system with a 6.5 µm pitch optimized for time-resolved x-ray scattering studies. The system is single photon quantum limited from 8 keV to 20 keV. It has a wide dynamic range and can operate at 100 Hz full-frame and at higher frequencies using a region-of-interest (ROI) readout. Fundamental metrics of linearity, dynamic range, spatial resolution, conversion gain, sensitivity and Detective Quantum Efficiency are estimated. Experimental time-resolved data are also presented.
CCDs have been the primary sensor in imaging systems for x-ray diffraction and imaging
applications in recent years. CCDs have met the fundamental requirements of low noise,
high-sensitivity, high dynamic range and spatial resolution necessary for these scientific
applications. State-of-the-art CMOS image sensor (CIS) technology has experienced
dramatic improvements recently and their performance is rivaling or surpassing that of
most CCDs. The advancement of CIS technology is at an ever-accelerating pace and is
driven by the multi-billion dollar consumer market. There are several advantages of CIS
over traditional CCDs and other solid-state imaging devices; they include low power,
high-speed operation, system-on-chip integration and lower manufacturing costs. The
combination of superior imaging performance and system advantages makes CIS a good
candidate for high-sensitivity imaging system development.
This paper will describe a 1344 x 1212 CIS imaging system with a 19.5μm pitch
optimized for x-ray scattering studies at high-energies. Fundamental metrics of linearity,
dynamic range, spatial resolution, conversion gain, sensitivity are estimated. The
Detective Quantum Efficiency (DQE) is also estimated. Representative x-ray diffraction
images are presented. Diffraction images are compared against a CCD-based imaging
system.
Precise simulation of digital camera architectures requires an accurate description of how the radiance image is transformed by optics and sampled by the image sensor array. Both for diffraction-limited imaging and for all practical lenses, the width of the optical-point-spread function differs at each wavelength. These differences are relatively small compared to coarse pixel sizes (6μm-8μm). But as pixel size decreases, to say 1.5μm-3μm, wavelength-dependent point-spread functions have a significant impact on the sensor response. We provide a theoretical treatment of how the interaction of spatial and wavelength properties influences the response of high-resolution color imagers. We then describe a model of these factors and an experimental evaluation of the model's computational accuracy.
The presampling modulation transfer function (MTF) of a digital imaging system is commonly determined by measuring the system’s line spread function (LSF) using a narrow slit or differentiating the detector’s edge spread function (ESF) with an edge device. The slit method requires precise fabrication and alignment of a slit as well as a high radiation exposure. The edge method [3] is a complicated image processing procedure, requiring determination of the edge angle, reprojection, sub-binning, smoothing and differentiating the ESF, and spectral estimation. In this paper, a simple method is employed to evaluate the MTF using an edge device. The image processing procedures required by this method involve simply the determination of the over-sampling rate and the Fourier transform of the modified ESF. Differentiation and signal to noise ratio (SNR) improvement are jointly applied in the Fourier domain. The MTFs obtained by this simple method are compared to the theoretical MTF and the previously proposed more complicated edge method. The experimental results show that the proposed method provides a simple, accurate and convenient measurement of the presampling MTF for digital imaging systems.
The new generation of Digital Still Cameras (DSCs) provide a capability of capturing raw data that make it possible to measure the fundamental metrics of the camera. Although CCDs are used in a majority of DSCs, the number of cameras with CMOS-based sensors are increasing. Using first principles, the performance of comparable CCD and CMOS- based DSCs are measured. The performance metrics measured are electronic noise, signal-to-noise ratio, linearity, dynamic range, resolution, and sensitivity. The dark noise and dark current are measured as a function of exposure time and ISO speed. The signal response and signal-to-noise response are measured as a function of intensity and ISO speed. The resolution is measured in terms of the Modulation Transfer Function (MTF) using both raw and rendered data. The spectral sensitivity is measured in terms of camera constants at several wavelengths. Subjective image quality is also measured using scenes that exhibit limiting performance. The ISO speed performance is compared against a film camera.
Direct conversion of x-ray energy into electrical charge has been extensively developed into imaging products in the past few years. Applications include general radiography, mammography, x-ray crystallography, portal imaging, and non-destructive testing. Direct methods avoid intermediate conversion of x-rays into light prior to generating a measurable electrical charge. This eliminates light scattering effects on image sharpness, allowing detectors to be designed to the limit of the theoretical modulation transfer function for a discrete-pixel sensor. Working exposure range can be customized by adjusting bias and thickness of sensor layers in coordination with readout-electronics specifications. Mature amorphous selenium technology and recent progress on high-quality Thin-Film Transistor (TFT) arrays for computer displays have allowed development of practical large-area high-resolution flat-panel x-ray imaging systems. A variety of design optimizations enable direct-conversion technology to satisfy a wide range of applications.
This paper will describe details of and results for a frequency-dependent filtered gain calibration technique that optimizes DQE, yet does not reduce MTF performance which is important to both systems.
Professional and consumer digital photography cameras use either CCD or CMOS sensors. Both of these sensors are fabricated using crystalline silicon technology. The advantage of this technology is that the pixel sizes can be made relatively small with resolutions approaching that of conventional photographic film. The disadvantage is that the active area is limited by the size of the silicon wafers, thereby making large format photography difficult. A new class of sensor using amorphous silicon on glass has bene developed for the medical field of radiography, fluoroscopy, and mammography. These pixilated devices have a thin-film- transistor (TFT) switch coupled to a photodiode or storage capacitor located at each pixel. Devices with 70 micrometers pixel pitch and nominally 10 inch by 12 inch active area are under development. Results are presented on a 14 inch by 17 inch TFT-based large area sensor with a pixel pitch of 139 micrometers and a prototype 512 by 512 pixel device with a 70 micrometers pitch. Characterizations include linearity, dynamic range, input-output transfer characteristics and resolution. Advantages and limitations of this technology for large format photography will be discussed.
Digital imaging systems require offset and gain calibration to normalize the behavior of individual pixels. This normalization corrects for imperfections in the system and also external variables that have effects on uniformity. Imaging metrics like Detective Quantum Efficiency (DQE) and Modulation Transfer Function (MTF) define how sensitivity and resolution are transferred through the system. Gain calibration can result in a loss of DQE due to the noise associated with its application. The typical technique to minimize this noise is to average several gain calibration procedures so that the introduced noise is minimized. This paper discuses the effects of gain calibration on DQE. It measures DQE as a function of the number of gain calibration procedures averaged and contrasts it with a novel technique that uses a single filtered gain calibration. It demonstrates that noise filter techniques, applied to a single gain calibration, regains the loss in DQE without any degradation in resolution. This paper also compares imaging performance of a system using a filtered gain map against a system that has many gain calibrations averaged. The technique is demonstrated using a Thin-Film-Transistor (TFT)-based large area medical imaging system.
The development of a new high spatial resolution x-ray detector system is described. The prototype detector is based on a patented detector technology that utilizes selenium for the x-ray conversion material, charge storage capacitors, and a thin film transistor (TFT) array for reading out the charge image. This experimental detector consists of a 512 X 512 matrix with a pixel pitch of 70 microns. The selenium layer deposited on the TFT array is 250 microns thick. With a low absorption entrance window the system is optimized for an energy range of 10 - 30 keV, and is designed for applications that require high spatial resolution and low noise. This presentation describes the imaging performance of the detector using the DQE and MTF metrics. Example images of phantoms are shown. Previously, we demonstrated a practical flat-panel self-scanned digital radiography system based on amorphous selenium and TFT technology. This system is being used clinically for chest radiography and general musculoskeletal imaging, and in industrial applications. The current work demonstrates the feasibility of adapting this technology for applications requiring higher spatial resolution.
Progress is discussed on the improvement of a Direct RadiographyTM solid state, flat panel, digital detector designed for use in general radiographic applications. This detector, now known as DirectRayTM, operates on the principle of direct detection of X-ray photons with a selenium photoconductor and consists of 500 micrometer thick amorphous selenium coupled to an amorphous silicon thin-film-transistor (TFT) readout array. This device is fabricated with a 14 X 17-inch (35 X 43-cm) active imaging area, corresponding to 2560 X 3072 pixels having dimensions of 139 micrometer X 139 micrometer and a geometrical fill factor of 86%. Improvements include a TFT array design upgrade with reduced noise characteristic, lower-noise readout electronics, and improved interfaces. Clinical radiographic images are currently being generated with the DirectRay detector using an X-ray exposure level equivalent to that of a 400 speed screen- film combination while maintaining the superior spatial resolution that is inherent in the direct conversion method. An effective sensor restoration technique has been implemented that eliminates the potential for selenium memory artifacts after a high dose. New results on NPS, MTF, DQE and signal linearity are presented. Detectability of low contrast objects using FAXiL test objects as well as the results of clinical studies are discussed.
Progress on the development of a semiconductor-based, direct-detection, flat-panel digital radiographic imaging device will be discussed. The device consists of a 500 micrometers thick amorphous selenium sensor coupled to an amorphous silicon thin-film-transistor (TFT) readout matrix. This detector has an active imaging area of 14 inches X 17 inches, 3072 X 2560 pixels with dimensions 139 micrometers X 139 micrometers and a geometrical fill factor of 86 percent. Charges generated primarily as a consequence of photoelectric interaction between the incoming x-rays and Se are integrated on storage capacitors that are located at each pixel. The high electric field applied across the Se minimizes the lateral spreading of the signal resulting in a significantly higher spatial resolution when compared to conventional film/screen systems used for general radiography. The sensor array is read out one pixel line at a time by manipulating the source and gate lines of the TFT matrix. Data are digitized to 14 bits. This paper will discuss the statistical photon counting analysis performed on an early prototype device. Measurements will include modulation transfer function, detector quantum efficiency, linearity, and noise analysis. Image analysis will include small contrast object visibility studies using a Faxil x-ray test object T016. Advantages of this flat-panel electronic sensor over conventional systems are discussed.
High-pressure-Bridgman grown CdZnTe x-ray detectors 1.25 approximately 1.7 mm thick were tested using monochromatic x-rays of 30 to 100 keV generated by a high energy x-ray generator. The results were compared with a commercially available 5 cm thick Nal detector. A linear dependence of the counting rate versus the x-ray generator tube current was observed at 58.9 keV. The measured pulse height of the photopeaks shows a linear dependence on energy. Electron and hole mobility-lifetime products were deduced by fitting bias dependent photopeak channel numbers at 30 keV x-ray energy. Values of 2 X 10-3 cm2/V and 2 X 10-4 cm2/V were obtained for (mu) (tau) e and (mu) (tau) p, respectively. The detector efficiency of CdZnTe at a 100 V bias was as high as, or higher than 90 percent compared to a Nal detector. At x-ray energies higher than 70 keV, the detection efficiency becomes a dominant factor and decreases to 75 percent at 100 keV.
A large-area pixel x-ray detector is being developed to collect eight successive frames of wide dynamic 2D images at 200kHz rates. Such a detector, to conjunction with a synchrotron radiation x-ray source, will enable time-resolved x-ray studies of proteins and other materials on time scales which have previously been inaccessible. The detector will consist of an array of fully-depleted 150 micron square diodes connected to a CMOS integrated electronics layer with solder bump-bonding. During each framing period, the current resulting from the x-rays stopped in the diodes is integrated in the electronics layer, and then strored in one of eight storage capacitors underneath the pixel. After the last frame, the capacitors are read out at standard data transmission rates. The detector has been designed for well-depth of at least 10,000 x-rays (at 12 keV), and a noise level of one x-ray. Ultimately, we intend to construct a detector with over one million pixels (1024 by 1024). We present the result of our development effort and various features of the design. The electronics design is discussed, with special attention to the performance requirements. The choice and design of the detective diodes, as they relate to x-ray stopping power and charge collection, are presented. An analysis of various methods of bump bonding is also presented. Finally, we discuss the possible need for a radiation-blocking layer, to be placed between the electronics and the detective layer, and various methods we have pursued in the construction of such a layer.
Photoconductor array devices were fabricated using molecular beam epitaxially (MBE) grown CdTe. The detectors are stable in the presence of hard x-rays, and they have been tested at room temperature for over a year without any noticeable degradation. The performance of the photoconductor was greatly improved when the detector was cooled using the Peltier effect. The uniformity of the 64 element linear array device was measured at various temperatures. We observed an exponential decrease of the photoconductor dark current with temperatures down to 200 degrees K. The dark current and noise of the array detector decreased by more than 3 orders of magnitude from 300 degrees K to 200 degrees K. As a result, the minimum sensitivity to x-ray photons was increased by nearly 3 orders of magnitude. Finally an x-ray transmission image was obtained using a single element MBE CdTe photoconductor at 230 degrees K.
MBE (molecular beam epitaxy) grown CdTe layers were processed to fabricate a photoconductor array for the diagnosis of short x-ray pulses from synchrotron radiation sources. The MBE (111)B CdTe layers were grown on (100)Si substrates. Photoconductor arrays were fabricated with gaps of 5 - 50 micrometers using conventional photolithography. Electroless Au or sputtered Au/Ni was used as a contact metal. The temporal response of the resulting CdTe photoconductor was measured with mode-locked 100 fsec Ti:Sapphire laser pulses. The FWHM of single crystalline CdTe photoconductor response pulse is as short as 37 psec with a 20 psec risetime. The photoconductor responds linearly to the x-ray tube photon flux with fixed accelerating voltage up to 40 kV. A significant response increase to the x-ray beam is observed for a layer with good crystalline quality. Spatial response of the CdTe photoconductor array was measured using rotating anode and synchrotron x rays for different beam sizes. Excellent spatial resolution was obtained from narrow angular radiation synchrotron x rays. The CdTe photoconductor was exposed to synchrotron x rays for 60 hours without any noticeable degradation.
Robert Altkorn, Rudy Haidle, Jon Chang, Melville Ulmer, Gregory Dace, Peter Teague, D. Upham, Peter Takacs, Brian Rodricks, P. Georgopoulos, N. Viles, K. Butler, M. DeBooth, D. Wiewel
In this paper we describe the fabrication of replica Wolter I optics from gold-coated lacquer- polished mandrels and the effect of plating bath temperature on the surface quality of electroforms produced from lacquer-polished substrates. We also discuss the use of ceramic masters to electroform replicas having high-frequency surface roughness as low as 3 angstroms.
In this paper we discuss the fabrication and testing of electroformed replica Wolter I optics made from gold-coated lacquered mandrels. We also discuss testing of gold- and palladium- coated lacquered test flats. X ray and (5 keV for Wolter I mirror and 8 - 40 keV for test flats) and optical (Wyko NCP-1000 profiler measurements were used to evaluate the mirrors.
The development of ultrahigh-brightness x-ray sources makes time-resolved x-ray studies more and more feasible. Improvements in x-ray optics components are also critical for obtaining the appropriate beam for a particular type of experiment. Moreover, fast parallel detectors will be essential in order to exploit the combination of high intensity x-ray sources and novel optics for time-resolved experiments. A CCD detector with a time resolution of microseconds has been developed at the Advanced Photon Source (APS). This detector is fully programmable using CAMAC electronics and a MicroVax computer. The techniques of time- resolved x-ray studies, which include scattering, microradiography, microtomography, stroboscopy, etc., can be applied to a range of phenomena (including rapid thermal annealing, surface ordering, crystallization, and the kinetics of phase transition) in order to understand these time-dependent microscopic processes. Some of these applications are illustrated by recent results performed at synchrotrons. New powerful x-ray sources now under construction offer the opportunity to apply innovative approaches in time-resolved work.
We discuss the fabrication and testing of electroformed replica Wolter I optics made from gold-coated lacquered mandrels. We also discuss testing of gold- and palladium-coated lacquered test flats. X-ray (5 keV for Wolter I mirror and 8 to 40 keV for test flats) and optical (Wyko NCP-1 000 profiler) measurements were used to evaluate the mirrors.
We discuss the application of high-resolution x-ray diffractometry to studies of semiconductor heterostructures. A new technique has been devised which extends structural measurements into the time domain. Using x-ray synchrotron radiation in conjunction with dispersive optics and fast x-ray area detectors we have been able to study for the first time the structure of heterointerfaces undergoing thermal processing. The techniques are illustrated with results on the strain kinetics of SQW''s and ion-implanted InAli_As layers.
An X-ray photoelectron microscope capable of providing both spatial and chemical information on the nature of a sample has been developed. Photons from the Aladdin Synchrotron are monochromatized by an extended-range Grasshopper monochromator covering the range from 40 to 1500 eV with an energy resolution between 10 and 200 meV. The monochromatized radiation generates photoelectrons in the sample, which is energy-analyzed with a resolving power E/delta E greater than 50,000 and imaged by a multichannel plate array. The visible image is transferred to a computer by a virtual-phase charge-coupled device camera with a dynamic range of 4096:1. Preliminary coarse measurements indicate a spatial resolution of the instrument better than 1 micron, although a limit of 600 A is possible. The instrument provides chemical shift-resolved images of low-lying core levels in a variety of samples.
The advent of extremely bright x-ray beams from new low-emittance sources such as ESRF and APS offers new opportunities for materials research. One of the most exciting and technologically demanding areas is likely to be time-resolved x-ray studies. Our recent experiments at NSLS Brookhaven (Beamline X-16B) explore some of the challenges for time-resolved x-ray scattering combining developments in x-ray optics (dispersive geometry) area detectors (CCD''s) and fast data acquisition. The techniques are illustrated with results on the rapid thermal annealing of electronic materials including strained-layer InGai_As quantum-well structures. We describe the application of a new virtual-phase CCD detector for real-time diffraction studies at the microsecond time scale. 1.
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