This invention relates generally to imaging of radiation such as x-rays and the like, and more particularly relates to digital imaging systems adapted to image objects having spatial frequency components above the imaging system's Nyquist frequency.
Solid state radiation imagers are used for imaging non-optical radiation such as x-rays and high energy nuclear radiation such as gamma rays. Incident radiation is typically detected in solid state imagers through a process in which the incident radiation is absorbed in a scintillator, resulting in the generation of optical photons. Photosensors, such as photodiodes or the like, disposed in an array adjacent to the scintillator detect the optical photons. The location in the array of the photosensor detecting the light and the intensity of the signal generated by the photosensor are processed for display and analysis of the incident radiation. Alternatively, the incident radiation may be absorbed directly in photosensitive elements which convert the energy of the incident radiation into mobile charge particles.
One typical use of solid state radiation imaging systems is medical imaging, in which radiation passing through or emanating from a patient's body is used to visualize objects or materials within the body. Medical imaging devices preferably exhibit high spatial frequency response, short image acquisition time, and high detective quantum efficiency (DQE, a measure of how efficiently an image recorder uses the radiation to which it is exposed). Efforts to improve spatial resolution (that is spatial frequency response, described by an imaging system's modulation transfer function and corresponding to the preservation of the object contrast of an imaged object in the output image) typically include manufacturing photosensor pixels of smaller sizes. The pixel pitch effectively determines the smallest resolvable dimension because a point source of light incident on a pixel is integrated over the area of the pixel. Imager arrays of smaller size pixels, however, require more technically demanding and costly manufacturing methods.
In the film scanning art, a number of imaging methods for solid state imagers have been developed. For example, increased spatial resolution has been obtained through the use of a so-called tiled area scanner, in which the scanned area is divided into slightly overlapping areas, with the imager having only sufficient pixels to cover one area at a time. The images developed by one imager sequentially passing over the areas or by multiple imagers imaging different regions can be combined to generate a complete image of the object. Problems with this approach include a device that is relatively temporally slow (low temporal resolution) and subject to image degradation from seaming, that is the mismatch of signals generated in the overlap region of two adjoining regions imaged by the moving imager. Another method used with solid state imagers is called fine-scan sampling, in which an imager with non-contiguous pixels is placed in a particular position for a time period to allow the photosites to integrate a portion of the film to be imaged, then the imager is shifted to a second position and a new integration cycle is commenced in which another set of pixels is imaged. Disadvantages of the fine scan method include the additional digital image processing required and the sensitivity of the display to artifacts introduced in the processing electronics. See generally "Image Scanning and Digitization" by J. Milch, Ch. 10, pp. 314-318, in Imaging Processes and Materials (Neblette's Eight Edition), ed. J. Sturge, V. Walworth, and A. Shepp (1989), which is incorporated herein by reference.
It is therefore an object of this invention to provide a high resolution radiation imaging system having a non-aliased spatial frequency response at frequencies greater than the imaging system's Nyquist frequency.
A further object of this invention is to provide a radiation imaging system providing high spatial resolution with deblurred images without requiring the need to increase irradiation of the object to be imaged.
A still further object of this invention is to provide a radiation imaging system that generates a high resolution image through oversampling.