X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Patent
1993-08-20
1995-09-05
Church, Craig E.
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
378 4, H05G 160
Patent
active
054486092
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates generally to computed tomography X-ray systems, and more particularly to methods and apparatus for reducing the complexity of the analog-to-digital processors and the number of downstream channels required to process x-ray detector output signals in such systems.
BACKGROUND OF THE INVENTION
Early in this century, the Austrian mathematician J. Radon demonstrated that a two-dimensional slice of a three-dimensional object may be reproduced from an infinite set of all of its projections. Computed tomography ("CT") X-ray systems generate an infinite set of X-ray beam projections through an object to be examined. The resultant detected X-ray data are computer processed to reconstruct a tomographic image-slice of the object.
More specifically, CT systems subject the object under examination to one or more pencil-like X-ray beams from all possible directions. X-ray beams passing through the object are attenuated by various amounts, depending upon the nature of the object traversed (e.g., bone, tissue, metal). One or more X-ray detectors, disposed on the far side of the object, receive these beams and provide analog output signals proportional to the strength of the incoming X-rays. Each detector output is then digitized and computer processed to help produce an image of a slice of the object.
For an ideally reconstructed image, a CT system would have an infinitely large number of extremely tiny closely-spaced detectors (or fewer detectors repositionable to an infinite number of very closely spaced locations). Because of limitations imposed by economics and hardware, practical CT systems process X-ray beam data from hundreds or thousands of detectors (or detector positions). However, the cost of analog-to-digital ("A/D") conversion of the output of hundreds or thousands of detectors can be prohibitive. The present invention discloses an apparatus and method for reducing the complexity of A/D converters required by CT systems, as well as the number of digitized information channels to be computer processed, without substantial degradation of the image produced.
At this juncture it is helpful to briefly overview the various types of CT X-ray scanner systems that have been developed, to appreciate better the problems associated with obtaining sufficient data to provide a meaningful output image.
A "first generation" CT scanner configuration is schematically depicted in FIG. 1A, and includes a single X-ray source (S), and a single detector (D), both of which move in a translate-rotate fashion (indicated by arrows T, R) relative to the object under examination (O). The translate-rotate movement causes the X-ray beam B to pass through the object O from all directions, with the moving detector D providing an analog output signal at many detector positions responsive to the various beam paths. A signal processing mechanism (SP) digitizes the output of D and ultimately reconstructs an image of a slice of the object O.
For ease of illustration, FIG. 1A (and indeed FIGS. 1B, 1C, 1D) depicts detector D and source S rotating with an equal radius about the object O, but in practice these radii need not be equal. Further, as is also the case with FIGS. 1B, 1C and 1D, relative motion between the object and the source and detector is all that is required, and in some applications the detector D and source S are stationary and the object O is rotated. For example, some industrial CT scanners that image large objects (an airplane wing, for example) require a source S which is too large to move, and therefore instead move the object relative to a stationary source and detector.
FIG. 1B depicts a second generation scanner wherein a source S emits an X-ray fan beam B, portions of which pass through the object O and are detected by a single detector D, where both source and detector move in a translate-rotate fashion. Because a fan beam of X-rays is used, the system of FIG. 1B can complete a scan more quickly than the system of FIG. 1A. For example, if the fan beam B comprises N ra
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patent: 4066900 (1978-01-01), LeMay
patent: 5046158 (1991-09-01), Goern
patent: 5142286 (1992-08-01), Ribner et al.
patent: 5203335 (1993-04-01), Noujaim et al.
patent: 5287107 (1994-02-01), Gampell et al.
Bruening Horst
Couch John L.
Hahn Guenter
Church Craig E.
Edelman Lawrence C.
Imatron Inc.
Siemens Aktiengesellschaft
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