Lawrence K. Cheung

New digital detector for projection radiography

The operational principle of a new, patented digital radiographic system using a multi-layer structure consisting of a thin-film pixel array, selenium x-ray photoconductor, dielectric layer and top electrode is described. Under an applied electric field, a diagnostic x-ray signal is obtained by the direct conversion of x-ray energy to electron-hole pairs which are collected as electrical charges by individual storage capacitor associated with each pixel element. The electronic readout sequence is initiated immediately after the x-ray exposure, and in several seconds, the image data is available for display on a video monitor, for data storage, data transmission, and hard copy generation. Signal strength of this direct conversion method is estimated to be significantly higher than that of other indirect conversion methods where light is first generated using a scintillator or phosphor and then detected by charge-coupled devices (CCDs) or thin-film-transistor (TFT) arrays in conjunction with photodiodes. In addition, since charges generated by x-ray photons move mostly along the direction of the bias electric field, images of very high spatial resolution can be obtained. The resolution limits are principally defined by the smallest pixel that can be manufactured. Recent x-ray images obtained from experimental detector panels are presented. X-ray sensitivity, dynamic range, signal-to-noise ratio, and spatial resolution are discussed.

Improved imaging performance of a 14"x17" direct radiography system using a Se/TFT detector

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.

Discussion on resolution and dynamic range of Se-TFT direct digital radiographic detector

The imaging performance of a new direct digital radiographic detector based on amorphous selenium and amorphous silicon TFT array which is under development is discussed. Progress has been made on the development of a multilayer digital x-ray detector panel with a structure consisting of a thin-film transistor pixel array, selenium x-ray photoconductor, dielectric layer and top electrode. An electronic system allows the rapid readout of image data which produces high resolution and wide dynamic range images. Using a straight edge, small wires and low contrast small holes targets, we have studied the spatial resolution, contrast detectability, and dynamic range of this new detector. Digital signals obtained from each pixel of this detector are almost linear with the total x-ray energy absorbed within the pixel area over a wide range of x-ray exposures. The resultant wide dynamic range allows extended latitude of exposure conditions and the enhancement or emphasis of different gray level regions from a single set of image data. For example, from one single exposure of the head, the soft tissue of the nose, detail structure of the teeth, as well as the bone structure of the neck can be examined by displaying and emphasizing selective gray levels of the image data. Image information obtained from this detector appears to be more evenly distributed over a wide dynamic range which is different from digital data obtained from other digital modalities such as the electrometer sensing of discharged potentials on photoconductors or from film digitization. Examples of images are shown. The discrete pixel structure of this detector and the higher intrinsic spatial resolution of selenium combine to produce image sharpness greater than those produced from digital detectors of similar pixel pitch using indirect conversion method or from digitizing film-screen images. The applicability of mathematical tools, such as the MTF which was developed primarily for analog images on a continuous imaging medium, is discussed with respect to our new discrete element detector.

Radiographic imaging characteristics of a direct conversion detector using selenium and thin film transistor array

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.

Direct-conversion x-ray imaging detectors for medical nondestructive testing (NDT) and other applications

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.