Image intensifier

Abstract: We investigate a concept for making a large area, flat-panel detector for digital radiology. It employs an x-ray sensitive photoconductor to convert incident x-radiation to a charge image which is then electronically read out with a large area integrated circuit. The large area integrated circuit, also called an active matrix, consists of a two-dimensional array of thin film transistors (TFTs). The potential advantages of the flat-panel detector for digital radiography include: instantaneous digital radiographs without operator intervention; compact size approaching that of a screen-film cassette and thus compatibility with existing x-ray equipment; high quantum efficiency combined with high resolution. Its potential advantages over the x-ray image intensifier (XRII)/video systems for fluoroscopy include: compactness; geometric accuracy; high resolution, and absence of veiling glare. The feasibility of the detector for digital radiology was investigated using the properties of a particular photoconductor (amorphous selenium) and active matrix array (with cadmium selenide TFTs). The results showed that it can potentially satisfy the detector design requirements for radiography (e.g., chest radiography and mammography). For fluoroscopy, the images can be obtained in real-time but the detector is not quantum noise limited below the mean exposure rate typically used in fluoroscopy. Possible improvements in x-ray sensitivity and noise performance for the application in fluoroscopy are discussed.

Abstract: Noise properties of active matrix, flat-panel imagers under conditions relevant to diagnostic radiology are investigated. These studies focus on imagers based upon arrays with pixels incorporating a discrete photodiode coupled to a thin-film transistor, both fabricated from hydrogenated amorphous silicon. These optically sensitive arrays are operated with an overlying x-ray converter to allow indirect detection of incident x rays. External electronics, including gate driver circuits and preamplification circuits, are also required to operate the arrays. A theoretical model describing the signal and noise transfer properties of the imagers under conditions relevant to diagnostic radiography, fluoroscopy, and mammography is developed. This frequency-dependent model is based upon a cascaded systems analysis wherein the imager is conceptually divided into a series of stages having intrinsic gain and spreading properties. Predictions from the model are compared with x-ray sensitivity and noise measurements obtained from individual pixels from an imager with a pixel format of 153631920 pixels at a pixel pitch of 127 mm. The model is shown to be in excellent agreement with measurements obtained with diagnostic x rays using various phosphor screens. The model is used to explore the potential performance of existing and hypothetical imagers for application in radiography, fluoroscopy, and mammography as a function of exposure, additive noise, and fill factor. These theoretical predictions suggest that imagers of this general design incorporating a CsI:Tl intensifying screen can be optimized to provide detective quantum efficiency ~DQE! superior to existing screen-film and storage phosphor systems for general radiography and mammography. For fluoroscopy, the model predicts that with further optimization of a-Si:H imagers, DQE performance approaching that of the best x-ray image intensifier systems may be possible. The results of this analysis suggest strategies for future improvements of this imaging technology. © 1997 American Association of Physicists in Medicine.@S0094-2405~97!01401-6#

Abstract: PURPOSE To evaluate the potential use of a C-arm mounted X-ray image intensifier (XRII) system to generate three-dimensional computed rotational angiograms during interventional neuroradiologic procedures. METHODS A clinical angiographic system was modified to allow collection of sufficient views during selective intraarterial contrast injections for CT reconstruction of a 15 x 15 x 15-cm3 volume. Image intensifier distortion and C-arm instabilities were corrected by using image-based techniques. The impact of the pulsatile nature of the vessels during image data acquisition and of the presence of bone on the 3-D reconstructions was investigated by generating 3-D reconstructions of an anesthetized 20-kg pig and of a human skull phantom. RESULTS A sequence of images sufficient for 3-D reconstruction was acquired in less than 5 seconds. Image intensifier distortion and C-arm instabilities were corrected to subpixel accuracy (0.035 mm and 0.07 mm, respectively). Both the intracranial vessels of the pig and the small, high-contrast structures in the skull were reconstructed with negligible artifacts. CONCLUSIONS Using a C-arm mounted XRII system, computed rotational angiography can provide true 3-D images of diagnostic quality.

Abstract: Objective: The purpose of this paper is to present the system configuration and physical properties of a new dentomaxillofacial X-ray cone beam CT system (CB MercuRay™) being developed. Methods: The system consists of an image intensifier and a cone beam X-ray source. There are two different models of this system, each with a different size image intensifier, 9″ or 12″. Each system has three field of view (FOV) modes. The 12″ system has facial (F), panoramic (P) and implant (I) FOV modes. The 9″ system has P, I and dental (D) modes. Images produced by these systems consist of 512×512×512 isotropic voxels. Physical properties such as resolution, noise and distortion of the images were evaluated in this study. Modulation transfer function (MTF) was measured using Boone's method. Image noise was measured as the standard deviation of the CT value in water. Circularity of the axial images yielded by the two models was measured using an 8 mm diameter acrylic pipe phantom. Results: The resolving power at a MTF o...