Eugene F. Palecki

Characterization of a full-field digital mammography detector based on direct x-ray conversion in selenium

We describe a high-resolution digital x-ray detector suitable for producing high quality mammographic images. The detector consists of an array of 3584 by 4096 pixels on 70 micrometer centers covering an area of 25 cm by 29 cm. The conversion layer of the detector is 250 micrometer thick amorphous selenium. Each pixel of the array consists of a storage capacitor for collecting x-ray signals and an amorphous silicon switching transistor. The signal is read out by custom high-speed, low-noise electronics. The integration of this detector with a mammographic x-ray system and acquisition console is described, as well as algorithms for calibration of the full system. We review characterization of the imaging performance of our system based on quantitative analyses of MTF and DQE data, and compare experimental results with theoretical calculations. We compare the performance of our direct conversion system with that of screen/film analog systems and indirect conversion digital detectors, such as LORADs CsI/CCD detector, operated under similar conditions. MTF degradation mechanisms and system noise sources and their effect on DQE are discussed. We review qualitative aspects of image quality from our detector and present preliminary observer performance characteristics on clinical studies run with our system.

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.

Productivity improvements with a direct digital radiography system integrated with PACS

The productivity-improving features of a direct digital radiography system for projection radiography are introduced and the integration of the system with PACS is discussed. A flat panel digital array, an array controller, and a system controller with video monitor and standard interface to a local or wide area communications network are the main components of the direct digital radiography system of which prototypes have been built and tested in laboratory settings. When used in radiology room for projection radiography, the digital array converts x-ray photons into digital image data and makes the data available immediately for display on a video monitor for the technologists review. Upon the technologists acceptance of the image, an industry-standard network allows the transmission of the image to a workstation where additional image processing can be performed and where the image can be viewed on a high resolution display by the radiologist. If so desired, the image may be routed to a laser printer, digital mass storage medium, or to a PACS communication interface. By connecting the direct digital radiography system with a PACS network, radiographs may be shared electronically like images of other digital modalities. In this scenario, productivity improvements would come mainly from shorter patient exams as a result of the immediate availability of the review images, wider exposure latitude and ease of handling of the digital array, and from the electronic transport, storage and retrieval of image files and patient data in a PACS environment.