Optimizing quantum efficiency of image sensors, whether CCD or CMOS, has usually required backside thinning to
bring the photon receiving surface close to the charge generation elements. A new CMOS sensor architecture has been
developed that permits high-fill-factor photodiodes to be placed at the silicon surface without the need for backside
thinning. The photodiode access provided by this architecture permits application of highly-effective anti-reflection
coatings on the input surface and construction of a mirror inside the silicon below the photodiodes to effectively double
the optical thickness of the silicon charge generation volume. Secondary benefits of this architecture include prevention
of light from reaching the CMOS circuitry under the photodiodes, improvement of near-infrared quantum efficiency, and
reduction in optical artifacts caused by reflections from the sensor surface.
Utilizing these techniques, a sensor is being constructed with 4096 x 4096 pixels 4.8 μm square with 95% fill factor
backed by a mirror tuned to the 400-700 nm visible band and a front-surface anti-reflectance coating. The quantum
efficiency is expected to exceed 80% through the visible and the global shutter extinction ratio should exceed 106:1. The
sensors have been fabricated and first test data is due in February 2011.
Recently introduced CMOS sensors using three layers of photodiodes for color separation1 can also function well in the
near ultraviolet and infrared bands. Ultraviolet sensitivity results from the close proximity of the top (blue) photodiode
junctions to the surface of the silicon and the lack of any significant UV-absorbing materials above them. Infrared
sensitivity extending nearly to the silicon band-gap cutoff results from depletion of the bottom (red) phodiodes into the
substrate. Preliminary measurements indicate that the layered structure has high quantum efficiency over most of the
200-1100nm band covered by silicon photodiodes. Uniquely, these devices can be switched rapidly between
narrowband monochrome imaging and full-color imaging in the visible band by the introduction of a visible pass filter.
The response of the three photodiode layers is broad enough to permit stable false color encoding using two or three
channels in conjunction with a redefinable 3 x 3 color transform matrix. Images have been acquired in the 300-400 nm
UV band and for broad and narrow infrared bands out to 1064 nm. Thermal images of objects in the range of 600C have
also been acquired demonstrating color-encoding of various UV, visible and IR bands and applications for particular
High-performance color image acquisition has heretofore relied on color video cameras using multiple image sensors mounted on spectral separation prisms to provide geometrically accurate color data free of reconstruction artifacts. Recently, a CMOS image sensor has been developed that incorporates three complete planes of photodiodes in a single device to provide color separation without the need for external optical elements. The first commercial device based on this technology has 1512 x 2268 three-color photosites on 9.12 micron centers and includes provisions for combining pixels in X and Y, region-of interest selection and sparse scanning. The camera described in this paper operates the sensor in a variety of scan modes offering tradeoffs between resolution, coverage and speed. In this camera, a 128x128 raster of either a matrix of this size or binned from a large area can be scanned at nearly 150 frames per second and a single 2048-element line can be scanned at 7 KHz. At full resolution, the image sensor will acquire four frames per
second. The scan configuration can be reloaded in less than 50 microseconds permitting mod e changes on the fly.
A silicon image sensor has been developed and placed in production using standard 0.18 μm CMOS process having three stacked photodiodes per pixel location to provide full-color imaging without external color filters. With a fill factor exceeding 50%, this image sensor achieves approximately 45% peak quantum efficiency in the mid range visible and provides usable response extending from the near-ultraviolet to the near-infrared. Initial results from a commercial digital still camera indicate that this device can produce excellent color reproduction with equivalent ISO film speeds from 100 to 400 and that it produces images free from color artifacts common in images made with sensors incorporating color filter arrays. Development is now underway on camera equipment designed to operate this image sensor in a variety of scan modes.
This paper describes a third-generation multi-mode x-ray imager whose applications include low-dose fluoroscopy, cine, spot films, and radiography. In addition, volumetric CT and applications whose environment includes a 2 tesla magnetic field are also in development. The VIP-9 is based on an amorphous silicon TFT/Photodiode array and x-ray conversion screen, which is optionally a deposited CsI(Tl) film or a removable Gd2O2S screen. There are three primary modes of operation: RAD for high resolution radiographs and spot films; Fluoro for video rate, low dose fluoroscopy as well as cine; Zoom for high resolution, limited field of view (FOV) fluoroscopy. Through improved electronics, the imager has greater sensitivity at low doses and far better rejection of correlated line noise than its predecessors. In addition, the VIP-9 incorporates many ease-of-use features absent from earlier prototype imagers. While previous reports have primarily focused on the imager construction and noise issues in large area sensing technology, in this paper the emphasis is on features which facilitate integration into a complete imaging system and measures of image quality.
An amorphous silicon medical imaging system designed to operate in both radiographic and fluoroscopic modes is described. Images of medical phantoms are presented for both modes of operation. MTF and DQE measurements are also presented. The effect of recursive filtering on the DQE performance of the system operating in fluoroscopic mode is discussed.
This paper describes a multi-mode, digital imager for real- time x-ray applications. The imager has three modes of operation: low dose fluoroscopy, zoom fluoroscopy, and high resolution radiography. These modes trade-off resolution or field-of-view for frame rate and additionally optimize the sensitivity of the imager to match the x-ray dose used in each mode. This large area sensing technology has a form factor similar to that of a film cassette, no geometric image distortion, no sensitivity to magnetic fields, a very large dynamic range which eliminates repeat shots due to over or under exposure, 12 bit digital output and the ability to switch between operating modes in real-time. The imager, which consists of three modules: the Receptor, the Power Supply and the Command Processor, is intended as a component in a larger imaging system. Preliminary characterization of the prototype imager in fluoroscopic mode at entrance exposure rates down to 2.5 (mu) R/frame, indicates that the DQE(f), MTF and low contrast resolution are comparable to that obtained with an image intensifier tube (IIT) coupled to a video camera.
To provide real-time imaging, x-ray diagnostic imaging relies entirely on the combination of the x-ray image intensifier and the high-performance television camera. Although these devices have been pushed to remarkable degrees of performance, they remain complex electro-optical assemblies with significant built-in errors, instabilities and degradation mechanisms. We describe a replacement for these systems utilizing as a sensor a large array of amorphous silicon photodiodes and thin-film switching transistors. Specially, the equipment described is a replacement for a 9-inch dual-mode x-ray image intensifier with a high-performance 2000-line digital tv camera capable of operating in both real-time video and high-performance spotfilm modes.
To provide real-time x-ray imaging, industry relies almost entirely on the combination of the x-ray image intensifier and the high-performance television camera. Although these devices have ben pushed to remarkable degrees of performance, they remain complex electro-optical assemblies with significant built-in errors, instabilities and degradation mechanisms. We describe a replacement for these system utilizing as a sensor a large array of amorphous silicon photodiodes and thin-film switching transistors. Specifically, the equipment described is a replacement for a 9-inch dual-mode x-ray image intensifier with a high performance 2000-line digital tv camera capable of operating in both real-time video radioscopic and high-performance radiographic modes.
This paper describes a real-time image processing system for correction and enhancement of fluoroscopic (video X-ray) image data obtained from a large area, flat-panel, solid- state medical image sensor. The amorphous silicon sensor is 1536 X 1920 pixels, measuring 20 X 25 cm; for operation at 30 frames per second, the real pixel data rate is approximately 45 MB/sec.
This paper describes a dual-mode, flat panel imaging system capable of both fluoroscopy and radiography. Two generations of large area sensing technology are described. The general system architecture incorporates both the high sensitivity and data throughput required for fluoroscopy with the large signal capacity, spatial resolution and form factor necessary for radiography.
Driven by the rapid growth of computer multimedia and digital broadcast television production and facilitated by the ever-increasing speed of digital video processing circuits, the development and issuance of standards addressing the transfer of images continues to accelerate. While this proliferation appears to widen the options available to the systems designer, it also tends to obscure the basic similarities among the various standards without revealing their key differences. Understanding the implications of these differences, especially as they involve critical timing and channel configuration issues, is important to the user who must assure successful video data transfer. Unfortunately, and despite the many projects underway, the available standards defining the interfaces to image input devices remain inadequate to fully support the designer tasked with producing and maintaining high quality video data acquisition.
In this review. we w ill consider each of the key interfaces in turn, describe and compare the standardized choices available, point up shortcomings in the application of these choices, describe ongoing work intended to address these shortcomings, and suggest additional standards development which might benefit the user of image input devices. We also hope to help the user to ask the right questions and develop the right answers.
In this paper, a summary of existing standards relevant to the interconnection of real-time imaging devices is reviewed, standards in preparation are outlined and directions standards activities may take in the future are projected. Information complied from extensive discussions with users of cameras reveals two key concerns which can be addressed by the development of camera standards: interfacing and presentation of performance data.
Terms such as resolution, sensitivity and signal-to-noise ratio are often discussed by vendors and users without clear understanding by either party of just what they mean. These terms were already difficult enough to understand and apply when only RS-170 scanning was in broad use and when video was obviously an analog signal produced by a vidicon image tube.
Now, with a multiplicity of scanning modes in use, digital video transmission of many different formats and rates and an enormous variety of image sensing devices, both tube and monolithic, available, the need for a common vocabulary, universal test methods and a set of broadly-applicable interface specifications in the optical, mechanical and electronic domains is badly needed.
In this review, we will summarize current efforts to produce appropriate standards documents, assess the progress of these efforts, recommend priorities which might be followed to maximize early utility and identify additional standards efforts which should be undertaken.