Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Patent
1997-03-05
1998-04-28
Wong, Don
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
378 19, 378 40, H05G 164, H01L 2500
Patent
active
057448060
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to sensors for x-ray imaging and more particularly to a multi sensor arrangement for a scanning digital radiographic system.
PRIOR ART
Since the middle of 1950 a common method to produce overview x-ray images of teeth and jaws for use in dentistry is the so called panoramic x-ray method. FIG. 1. demonstrates a typical prior art set up. Assuming the head of the patient is oriented in an upright position, the method uses a movable x-ray source 1 with the, beam collimated to a narrow (3-6 mm) width in the horizontal dimension, and elongated at least 12 cm in the vertical dimension. The collimation is done with a narrow slit in a piece of sheet metal made of an x-ray absorbing material placed at a suitable distance from the focal spot of the x-ray tube, but before the radiation reaches the patient. On the opposing side of the patient, relative to the x-ray source, a second metal plate 3 with a narrow second slit, corresponding to the fan shaped beam is placed. The slits and the x-ray source with it's collimator are rigidly attached to each other. A film 5 is placed further away from the x-ray source, in the direction of the x-ray beam, at a position behind the second slit. During the exposure, usually taking 15-20 seconds for an ordinary panoramic x-ray, 30 cm wide, the arrangement of x-ray source and slit is rotated in a direction 3 around the head of the patient in a controlled manner so that a rotational center 7 of the imaging system will be situated within the head of the patient. At the same time, the imaging system will move relative to the film which will be exposed by radiation through the patients head and the second slit, piece by piece, until all of the film has been exposed during the time the imaging system was rotated around its rotational center 7.
By controlling the film speed relative to the object, the x-ray beam and the projection geometry with the rotational center assumed as a "virtual focus" that is moved during the exposure, it can be demonstrated that only a predetermined layer in the object is sharply depicted. In order to bring this sharp layer to coincide with the dental arches and to compensate for magnification variations in the vertical and horizontal dimensions, a rather complex motion sequence is required, where the vertically oriented rotational axis 7 is not fixed within the patients head but will be moving along a continuous path in the horizontal plane approximately parallel to the occlusal plane according to FIG. 2.
A good reference for a popular technical description of conventional panoramic radiography is found in chapter 2, "Theory of Rotational Panoramic Radiography", second edition of "Panoramic radiology" by Olaf Langland, Robert Langlais, Doss McDavid, Angelo DelBalso, printed by Lea & Febiger, Philadelphia 1989.
The traditional method can be viewed, as indicated in FIGS. 3 and 4, as a method to add an indefinite number of images, each of them with indefinitely low exposure, in size corresponding to the slit, directly on to the analog film medium, simultaneously moving the film past the slit as described above. However, the method would work equally well using a large but finite number of discrete steps both in the x-ray source/slit assembly motion and in the motion of the film.
It is obvious that the described method could be modified to utilize for instance semiconductor detectors replacing the film. Three examples of relevant patents are the U. S. Pat. Nos. 4,878,234, 4,823,369 and 5,018,177.
The first patent document U.S. Pat. No. 4,878,234 deals with a method and an apparatus to use one or several CCD (Charge Coupled Device) detectors for both detecting the image information and for simulation of the film motion directly in the imaging area by, since the early development of CCD:s, the well known method named TDI (Time Delayed Integration) wherein the individual pixel charges can be moved to simulate film motion by the clocking sequence of the detector. This method has the same limitations as the film method re
REFERENCES:
patent: 4070578 (1978-01-01), Timothy et al.
patent: 4823369 (1989-04-01), Guenther et al.
patent: 4878234 (1989-10-01), Pfeiffer et al.
patent: 5018177 (1991-05-01), McDavid et al.
Chapter 2, "Theory of Rotational Panoramic Radiography", second edition of Panoramic radiology, Olaf Langland, Robert Langlais, Doss McDavid and Angelo DelBalso, Printed by Lea & Febiger, Philadelphia 1989, pp. 38-75.
AFP Imaging Corporation
Wong Don
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