J NASA CASE NO. LAR_15_ PRINT FIG. NOTICE The invention disclosed in this document resulted from research in aeronautical and space activities performed under programs of the National Aeronautics and Space Administration. The invention is owned by NASA and is, therefore, available for licensing in accordance with the patent licensing regulations applicable to U.S. Government-owned inventions (37 CFR 404.1 st seq.). To encourage commercial utilization of NASA-owned inventions, it is NASA policy to grant licenses to commercial concerns. Although NASA encourages nonexclusive licensing to promote competition and achieve the widest possible utilization, NASA will provide the necessary incentive to the licensee to achieve early practical application of the invention. Address informational requests for this invention to the Technology Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia 23681-0001. Address applications for license to the Patent Counsel Office, Mail Stop 212, 3 Langley Boulevard, Hampton, Virginia 23681-0001. Serial No.: 08/601,143 Filed" 01/26/96 LaRC LAR 15059-1 -1- PATENT APPLICATION DIGITAL MAMMOGRAPHY WITH A MOSAIC OF CCD-ARRAYS Origin of the Invention The invention described herein was made by an employee of the U.S. Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. Background of the Invention 10 1. Technical Field of the invention The present invention relates generally to a mammography device and method and more particularly to a novel digital mammography device and method to detect 15 microcalcifications of precancerous tissue. 2. Description of the related art Diagnostic criteria require that mammograms exhibit excellent spatial 20 resolution and contrast sensitivity. X-ray mammography is currently performed by using a conventional phosphor screen film combination as the image receptor. Properly exposed film mammograms can reveal dense precancerous tissue. However, the size of the smallest detectable calcifications, which are indicative of malignancy, is typically about 0.2 mm. 25 An imaging system which offers wider dynamic range, higher contrast sensitivity, higher spatial resolution, and the ability to manipulate and archive the image is desirable. Digital x-ray mammography can provide the solution to this problem by providing an advanced method and device for diagnosing minimal breast cancers. This digital system can allow for precise identification of microcalcifications, the tiny 30 hardenings typically 0.1 to 0.2 mm in diameter found in precancerous breast tissue. LAR 15059-1 -2- PATENT APPLICATION The large image size (typically 18 x 24 cm for clinical mammography) could, in principle, be achieved with grazing-incidence-reflection imaging systems in conjunction with existing small-area imaging array detectors. However their bulk and enormous cost make them unsuitable for clinical mammography applications. 5 Two approaches are currently under investigation for digital mammography. One is the secondary digitization technique, in which conventional film mammograms are digitized. The other approach is the acquisition of primary digital images, the "electronic imaging technique". There are numerous studies addressing the technical challenges of digital radiography which may be adapted for mammography, examples 10 are: scanning laser stimulated luminescence system, i.e., computed radiography (J.W. Oestmann, D. Kopans, and D.A. Hall, "A comparison of digitized storage phosphors and conventional mammography in the detection of malignant microcalcifications," Invest. Radiol., vol. 23, no. 725, 1988); slot scanning digital imaging, including the TDI technique (Maidment et al., "Scanned-slot mammography," SPIE Proc., vol. 15 1231, Medical Imaging IV: Image Formation, p. 316, 1990); large size flat panel detectors such as amorphous silicon and selenium detectors (Rowlands, et al., "X-Ray imaging using amorphous selenium: A photoinduced discharge readout method for digital mammography," Med. Phys., vol. 18, pp. 421-431, 1991); and optically coupled electronic imager (e.g., CCD) techniques (H. Liu, A. Karellas, S.C. Moore, and L.J. 20 Harris, "Lesion detectability considerations for an optically coupled CCD x-ray imaging system," IEEE Trans. Nucl. Sci., vol. 41, no. 4, pp. 1506-1509, August 1994). In U.S. Pat. No. 5,105,087, issued to Jagielinski, incorporated by reference herein, multiple detector arrays are used to image over the large area needed in clinical 25 mammography applications. This invention relies on multiple layers of detector elements, one above the other, to provide a complete image with no gaps. One disadvantage with this system is that enough photo detectors must be used to cover the active area which increases the cost of the device. Another disadvantage is the effect of the edges of the detector arrays in one layer on the x-ray image seen by the 30 detectors below these edges. The present invention is able to use a fewer number of LAR 15059-1 -3- PATENT APPLICATION detectors by repositioning the detectors several times in order to cover the entire active area. Thus, the cost of the system is greatly reduced. In U.S. Pat. No. 5,043,582 issued to Cox et al., incorporated by reference herein, the photo sensitive properties of transistors found in dynamic random access 5 memory (DRAM) integrated circuits are used to detect photons emitted from x-ray sensitive phosphors. The use of DRAM cells as photo sensitive pixels results in less optical sensitivity, because the entire active area of each pixel is not photo sensitive, due to the requirements for addressing the DRAM cells. Furthermore, the detection scheme described by Cox et al. is binary in nature. Therefore, substantial effort would 10 be required to obtain gray scales. The present invention uses CCD detectors, which are optimized for use as photo detectors. This results in greater system sensitivity and image quality. Also, CCDs produce gray scales naturally and with high sensitivity. Optically coupled CCD techniques are described in two U.S. Patents. In U.S. 15 Pat. Nos. 5,142,557 issued to Toker et al., and 5,216,250 issued to Pellegrino et al., incorporated by reference herein, an optical lens is used to image the visible photons emitted by an x-ray sensitive phosphor screen onto a single CCD detector. Because the CCD detector is smaller than the phosphor screen, the image from the screen must be reduced, or demagnified, in order for the detector to record the entire image. This 20 means that each pixel on the CCD detector corresponds to a larger equivalent area on the phosphor screen. Therefore, the spatial resolution of this system is less than the spatial resolution of the CCD detector. Also, using an optical lens to couple the image on the phosphor screen to the CCD detector is inefficient. The optical lens cannot collect all of the light that is emitted by the phosphor screen. This results in a 25 reduction in signal-to-noise performance. The present invention is able to achieve an increased spatial resolution because each region on the phosphor screen corresponds to a pixel area on the CCD. In addition, the present invention does not use an optical lens. Therefore, the coupling losses associated with imaging optics are eliminated. This results in higher quality 30 images and less patient dose of x-rays compared to alternate approaches. LAR 15059-1 -4- PATENT APPLICATION Summary of the Invention An object of this invention is to provide a digital mammography device with large field coverage. Another object of this invention is to provide a digital mammography device with high spatial resolution. Another object of this invention is to provide a digital mammography device with scatter rejection. Another object of this invention is to provide a digital mammography device 10 with excellent contrast characteristics and lesion detectability under clinical conditions. Another object of this invention is to provide a mammography device which shields the patient from excessive radiation. Another object of this invention is to provide a mammography device which can detect extremely small calcifications. 15 Another object of this invention is to provide a mammography device which can manipulate and archive the image. These and other objects of the invention are met by providing an apparatus and method for large field digital mammography. The invention uses a mosaic of electronic digital imaging arrays to scan an image. The imaging arrays are mounted on 20 a carrier platform to form a pattern. The arrays are then exposed to a portion of a radiated image, and convert this radiation into digital data. The platform is subsequently repositioned and the arrays are exposed to another portion of the image. While the arrays are being repositioned, the digital data in the arrays is transferred to a computer memory. This process is repeated until the entire image has been exposed to 25 the arrays. The stored multiple image data is combined by a data processor to form data which corresponds to the original radiated image. This digital x-ray image can then be viewed on a computer display. To reduce exposure and x-ray scatter, a metallic aperture plate is interposed between the x-ray source and the patient. The aperture plate has a mosaic of square holes in alignment with the imaging array pattern. LAR 15059-I -5- PATENT APPLICATION The plate is repositioned in synchronism with the carrier platform. The device is suitable for incorporation into standard mammography units. Brief Description of the Drawings Fig. 1shows a schematic description of the mammographic system; Fig. 2a shows a CCD mosaic; Fig. 2b shows a CCD imager with phosphor and fiber bundle; Fig. 3 shows arrangement of CCD arrays on the platform where i', j' denote I0 CCD arrays and i", j" denote sub-images acquired by CCD arrays in various positions; and Figs. 4 through 7 show the positions of the arrays which are needed to convert the entire x-ray into digital data. 15 Detailed Description of the Invention Referring now to Fig. 1, the mammography system is shown generally by number 1. X-ray tube 2 emits x-rays 3 through the aperture plate 4 then through the patient 5. Aperture plate 4 serves to decrease patient x-ray dose and to reduce 20 scattering of the x-ray beam. A phosphorescent screen 6 converts the x-ray image into a visible light image. Optical fibers 7, which attach the phosphorescent screen 6 to the CCD arrays, then transmit the visible light image to a mosaic of CCD arrays 8, which converts the light image into digital data. The CCD/readout electronics subsystem 10, some of which are located directly on the platform 16 and some of which are located 25 externally, are used to transfer this data into a personal computer 11 for storage. A mechanical repositioning stage 9 moves the mosaic of CCD arrays to a new position, and this process is repeated until the entire image is exposed. The personal computer 11 combines this data to produce data which corresponds to the entire x-ray image. This x-ray image is displayed on an image display 12. Repositioning stage 9 is driven 30 by the CCD mosaic repositioning stage electronics 13, under the control of the LAR 15059-1 -6- PATENT APPLICATION personal computer 1I. The aperture plate repositioning stage electronics 14 moves the aperture plate 4 in synchronism with the CCD repositioning stage 9, also under control of the personal computer ] 1. Sensor means 5 Referring now to Figs. 2a and 2b, CCD mosaic 8 consists of CCD arrays 15 mounted onto a carrier platform 16. Fig. 3 shows that the length 22 between neighboring arrays is equal to the length 23 of one side of a square array. However, the arrays can have any shape and length so long as they are separated by a distance equal to the dimension of the arrays along each axis of motion minus an allowance for 10 overlap of approximately 10 pixels between sub-images generated by each CCD in both directions. With this detector geometry, a single x-ray exposure will result in an image with gaps. These gaps in the image are removed by using multiple x-ray exposures. After each exposure, the platform 16 which carries the mosaic 8 of CCD arrays 15 is rapidly and accurately repositioned with respect to the patient 5 along two 15 orthogonal axes. The repositioning can be accomplished with commercial mechanical stages. The mosaic 8 is repositioned rapidly in order to minimize the effects of patient movement between exposures. The movement of the mosaic 8 is facilitated by the presence of an x-ray transparent plastic spacer plate 17 located between the patient 5 20 and the surface of the CCD mosaic 8. Forty-eight 1024 x 1024 pixel CCD arrays, each measuring 15 mm x 15 mm, are fixed to a 24 cm wide by 18 cm high carrier platform. Fig. 3 shows that the length 22 between neighboring arrays is equal to the length 23 of one side of a square array. However, the arrays can have any shape and length, so long as they are separated by a 25 distance equal to the dimension &the arrays along each axis of motion minus an allowance for overlap of approximately 10 pixels between sub-images generated by each CCD in both directions. In order to provide a complete and contiguous image, the mosaic 8 is repositioned three times, as shown in Figs 4 through 7. Four x-ray exposures are 30 made. Aider the first exposure, the mosaic 8 is moved along the x axis a length 22, LAR 15059-1 -7- PATENT APPLICATION then asecond exposure is made. The mosaic 8 is then moved along the y axis by a length 22, followed by a third exposure. The mosaic is then moved along x axis in a direction opposite the first motion. A final exposure then completes the data acquisition sequence. 5 In an alternate embodiment, the detector mosaic has 48 individual CCD arrays are assembled into a 6 x 8 mosaic with less than 5 mm wide gaps (where the gap width is W) between the individual CCD arrays. The repositioning takes place along a diagonal direction of the array. After a first x-ray exposure, the entire mosaic of CCD arrays is mechanically repositioned with respect to the human subject to be imaged. 10 The mosaic is first moved in a diagonal direction by v/2 W mm (simultaneously, W mm upward and W mm sideways). Then a second exposure is made followed by a second movement of the detectors along the same diagonal direction. A third exposure completes the data acquisition sequence. As in the preferred embodiment, the aperture shield is moved in synchronism with the mosaic of CCD arrays. 15 Shielding means These multiple x-ray exposures can result in a high dose of radiation to the patient. In order to reduce the amount of x-ray exposure and scatter, a metal aperture plate 18 as shown in Fig. 2a is interposed between the x-ray source 2 and the patient 5. The plate 18 has a mosaic of apertures 19 which are in exact alignment with the mosaic 20 8 of detector arrays 15. This aperture plate 18 is moved by a second repositioning device in synchronism with the mosaic 8 detector arrays 15. The patient 5 receives a small amount of additional exposure from x-rays in a narrow borderline area that surrounds each array. However, appropriate configuring of the instrument allows this area to be kept to less than 7.5 percent. Thus, patient 25 dosage is increased by this percentage. Scatter Reduction The aperture plate provides a significant reduction in x-ray scattering, which results in improved image contrast. Table 1 shows scatter as a function of compression and Bucky grid use based on 15 - 20 keV x-ray properties. The LAR 15059-1 -8- PATENT APPLICATION instrument configuration for the CCD mosaic technique allows easy adaptation of a screen film type Bucky grid subsystem for scatter control. Table 1 S/P values 3.0cm Compression 4.5cm Compression 5 With Bucky Grid Screen Film: 0.14 Screen Film: 0.26 This device: 0.05 This device: 0.09 W/O Bucky Grid Screen Film: 0.40 Screen Film: 0.75 This device: 0.14 This device: 0.27 10 Scatter reduction yields better contrast ratios and thus enhanced lesion detectability. For a performance comparison consider Bucky grid equipped film systems. With 3 cm breast compression they typically are capable of a Scatter to 15 Primary (S/P) ratio in the range from 0.10 to 0.15. For the present invention, the Table shows a comparable S/P ratio of 0.14 for 3 cm compression without a grid. However, this system has a dose penalty of just under 7.5 percent, as a result of border line area considerations discussed above, compared with an approximately 100 percent dose increase required for the Bucky grid which is generally used by film 20 systems. A trade off analysis which considers contrast shows that the present invention can achieve identical lesion detectability with 45 to 50 percent less radiation dosage, depending on compression. This result clearly shows the advantage of the present invention, which can operate without a Bucky grid because of the inherent scatter rejection of the aperture plate. 25 Repositioning means The synchronous repositioning of the mosaic 8 of detectors 15 and aperture plate 18 is accomplished with two separate 2 axis mechanical repositioning devices with electronic coupling, such as the 800000 series precision positioning stage from Parker Hannifin. Stage movement of the repositioning devices is controlled by a 30 Computermotor Plus closed loop brushless servo-motor system from Parker Hannifin.