X-ray or gamma ray systems or devices – Beam control – Collimator
Reexamination Certificate
1999-02-18
2001-08-07
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Collimator
C378S154000, C378S098800
Reexamination Certificate
active
06272207
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus employing detector pixels for obtaining an image having a resolution which is not directly related to the sizes of the detector pixels. More particularly, the present invention relates to a method and apparatus which obtains a series of spatially filtered high-resolution digital x-ray or gamma ray images of portions of an object or objects while minimizing image degradation due to conversion blurring and radiation scattering, and which arranges the spatially modulated images into a larger complete image of the object or objects.
2. Description of the Related Art
Various techniques currently exist and many are under development for obtaining digital x-ray and gamma ray images of an object for purposes such as x-ray diagnostics, medical radiology, non-destructive testing, and so on. Known devices include line digital detectors, which obtain images along essentially one direction, and therefore must be scanned across an object to obtain sectional images of the object which can be arranged into an image of the entire object. Also known are two-dimensional digital detectors which can obtain an image of the entire object at one time, and thus can operate faster than an apparatus which includes a line detector.
A digital x-ray imager creates a digital image by converting received x-rays, which are used to form the image, into electrical charges, and displaying the charge as a function of position. Digital x-ray detectors typically have the potential of high sensitivity and large dynamic range. Therefore, when used in medical applications, a digital x-ray detector will generally be capable of obtaining a suitable image of the patient without requiring the patient to receive a large dose of x-ray radiation.
Digital image data is also much easier to store, retrieve and transmit over communication networks, and is better suited for computer-aided diagnostics, than conventional film x-rays. Digital x-ray images can also be displayed more easily than conventional film x-rays, and provide greater image enhancement capabilities, a faster data acquisition rate, and simplified data archival over conventional film x-rays. These advantages make digital x-ray imaging apparatus more desirable than film x-ray apparatus for use in many diagnostic radiology applications, such as mammography.
The general construction and operation of digital x-ray detectors will now be described. As discussed briefly above, digital x-ray detectors collect electrical charges produced by x-rays as a function of position, where the amount of charge is directly proportional to the x-ray intensity. Two general approaches for x-ray conversion are currently under investigation for flat-panel digital x-ray detectors. These approaches are generally referred to as the indirect method and the direct method.
In the indirect method, x-rays are converted to low-energy photons by a scintillator, and the low-energy photons are then converted to electrical charges by solid-state detectors. This method is described in a publication by L. E. Antonuk et al., “Signal, Noise, and Readout Considerations in the Development of Amorphous Silicon Photodiode Arrays for Radiotheraphy and Diagnostic Imaging”
Proc. SPIE
1443:108 (1991), the entire contents of which is incorporated by reference herein.
In the direct method, x-rays are converted to electron-hole pairs by photoconductors. An electric field applied to the photoconductor separates the electrons from the holes. This method is described in a publication by J. A. Rowlands et al. entitled “Flat Panel Detector for Digital Radiology Using Active Matrix Readout of Amorphous Selenium,”
Physics of Medical Imaging SPIE
3032: 97-108(1997), and in an article by R. Street, K. Shah, S. Ready, R. Apte, P. Bennett, M. Klugerman and Y. Dmitriyev, entitled “Large Area X-Ray Image Sensing Using a PbI
hd 2
Photoconductor,”
Proc. SPIE
3336: 24-32 (1998). The entire contents of both of these papers are incorporated by reference herein. Many types of photoconductors are under development by medical imaging community.
A type of flat-panel, two-dimensional, digital x-ray, imager comprises a plurality of charge-coupled devices (CCDs) on a silicon substrate. The CCDs can be easily made on the silicon substrate to have a pixel pitch smaller than 10 &mgr;m×10 &mgr;m. However, because the maximum size of silicon substrates is limited, to achieve the dimensions needed for a large-area flat-panel x-ray detector, multiple wafers have to be patched together. Some of the CCD x-ray detectors are described in the following publications: F. Takasashi, et al., “Development of a High Definition Real-Time Digital Radiography System Using a 4 Million Pixels CCD Camera”,
Physics of Medical Imaging SPIE
3032: 364-375 (1997); J. M. Henry, Martin J. Yaffe and T. O. Tumer, “Noise in Hybrid Photodiode Array—CCD X-ray Image Detectors for Digital Mammography,”
Proc. SPIE
2708: 106 (1996); and M. P. Andre, B. A. Spivey, J. Tran, P. J. Martin and C. M. Kimme-Smith, “Small-Field Image-Stitching Approach to Full-View Digital Mammography,” Radiology 193, Suppl. Nov.-Dec., 253-253 (1994), the entire contents of each being incorporated by reference herein.
Alternatively, a flat-panel imager can include active matrix arrays of thin film transistors (TFTs) on a glass substrate. Because glass substrates can be large, the digital x-ray imager can, in principle, be made of a single substrate. However, it is very difficult to make a digital detector with a pixel pitch much smaller than 100 &mgr;m using substrates other than silicon wafers, as described in the following publications: L. E. Antonuk et al., “Development of Thin-Film, Flat-Panel Arrays for Diagnostic and Radiotherapy Imaging”,
Proc. SPIE
1651: 94 (1992); L. E. Antonuk et al., “Large Area, Flat-Panel, Amorphous Silicon Imagers”,
Proc SPIE
2432: 216 (1995); and L. E. Antonuk et al., “A Large-Area, 97 &mgr;m Pitch, Indirect-Detection, Active Matrix Flat-Panel Imager (AMFPI)”,
SPIE Medical Imaging
1998
Technical Abstracts,
San Diego, 83 (1998), the entire contents of each being incorporated by reference herein.
As discussed above, digital x-ray imaging techniques represent a vast improvement over conventional film x-ray apparatus. However, digital x-ray imaging systems experience certain drawbacks with regard to image resolution.
It has been a common belief that the resolution of the digital image can be no better than the pixel pitch (pixel periodicity) of the imaging apparatus, and is rather often much worse due to various types of blurring phenomena which occur during image acquisition. However, as can be appreciated from the description of the operation of digital x-ray detectors set forth below, pixel pitch is only one of the many factors that influence the resolution of a digital image obtainable by a digital imaging apparatus.
Detectors for digital radiography are composed of discrete pixels which generally have a uniform size, shape and spacing. The “fill factor” is defined as the active portion of each detector pixel that is used for charge collection relative to pixel pitch or, in other words, the fraction of the pixel area occupied by the sensor for x-ray detection. A flat-panel imager having thin-film transistors (TFTs), for example, has a fill factor which decreases dramatically as the pixel pitch decreases. The TFTs are large compared to transistors on silicon substrates, and the various electrode lines occupy much surface area of the glass substrate. Hence, the fill factor decreases greatly as the pixel pitch decreases.
For example, the fill factor is 57% for a 127 &mgr;m pixel pitch array, and is 45% for a 97 &mgr;m pixel pitch array which performs indirect x-ray conversion and has been aggressively designed, as described in the article entitled “A Large-Area, 97 &mgr;m Pitch, Indirect-Detection, Active Matrix Flat-Panel Imager (AMFPI)” cited above.
The fill factor approaches zero as the pixel pitch decreases toward 50 &mgr;m in a detector
Bruce David V.
Creatv MicroTech, Inc.
Dunn Drew A.
Roylance Abrams Berdo & Goodman L.L.P.
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