THREE-DIMENSIONAL PANORAMIC DENTAL
RADIOGRAPHY METHOD AND APPARATUS
WHICH AVOIDS THE SUBJECT'S SPINE
FIELD OF THE INVENTION
This invention relates to radiographic imaging and more particularly to dental radiographic imaging.
BACKGROUND OF THE INVENTION 10
Tomography is an analog imaging process which is widely used for radiographic imaging. In tomography, an x-ray beam source and an x-ray film are moved in predetermined directions relative to one another, such that the fulcrum of relative movement between the 15 source and the film defines a region of interest. Since relative movement at the fulcrum is zero, this region is projected unchanged onto the film. The radiographic projections of all tissues lying outside this region move during exposure, so that they appear more or less 20 blurred depending on their distance from the fulcrum. The amount of blur is directly proportional to their distance from the fulcrum. Accordingly, the region or fulcrum is often referred to as the "focal plane".
In summary, the tomographic process capitalizes on 25 the angular disparity produced by relative motion between x-ray source and x-ray detector to selectively isolate a region, the location of which can be varied by controlling motion relative to the tissues of interest. In tomographic imaging, linear motion of the source and 30 detectors may be used ("linear tomography") or circular motion of the source and detector may be used ("circular tomography"). Depending upon the diagnostic task, other more complex motions may be used, such as hypercycloidal motion ("hypercycloidal tomogra- 35 phy").
A related but computationally distinct imaging process for obtaining tomographic "slices" is computed tomography. In computed tomography, the projection geometry is characterized by a fan-shaped x-ray beam 40 which lies in the same plane as a detector. This geometry renders details in one focal plane independent from those in another focal plane, but at the expense of having the plane of the source and detector motion coincident with the focal plane. 45
Linear tomography has been used in dental radiography to obtain images of individual teeth or groups of teeth. Computed tomography also has been used in dental radiography to obtain a slice through a subject's head, parallel to the plane of the subject's teeth. 50
In the above description, and in the description to follow, reference is made to the "plane of the teeth", which is defined as the plane formed by the intersection of a subject's upper and lower teeth. This plane, which is parallel to the plane of the jaw, is typically a horizon- 55 tal plane when the subject is in a normal upright (standing or sitting) position. Accordingly, the term "horizontal" will be used to refer to a direction parallel to the plane of the teeth, which is typically a horizontal plane. The term "vertical" will be used to refer to a direction 60 perpendicular to the plane of the teeth, which is generally parallel to a subject's spine, and is vertical when the subject is in the typical upright position.
The process of tomography has been extended to digital tomosynthesis, which produces a series of dis- 65 crete images taken from different positions as the x-ray source and detector move about a fixed fulcrum. Each discrete image corresponds to a different relative posi
tion of the source and film plane, rather than a single image produced continuously. In linear tomography and linear tomosynthesis the source and the detector move in a straight line about a fulcrum lying in the tissues which define the plane to be seen sharply in the reconstruction.
It has been shown that the tomosynthetic process is theoretically indistinguishable from the tomographic process, provided that the size of the smallest detail of interest can be specified and the number of discrete projections is sufficiently large. These processes are indistinguishable because they both operate by isolating a focal plane through a controlled blur as described above. However, tomosynthesis has an advantage over tomography, because simple manipulation of the radiographic intensity data obtained from the multiple images permits the position of the focal plane to be adjusted after the fact. Accordingly, a computer can be used to selectively search through the multiple images and to render the image of a particular structure of interest in proper focus, irrespective of its location.
The theoretical and practical designs of a tomosynthetic x-ray system are well known to those having skill in the art, and are described, for example, in an article entitled Tomosynthesis: Three-Dimensional Radiographic Imaging Technique by D. G. Grant, published in the IEEE Transactions on Bio-Medical Engineering, Vol. BME-19, No. 1, Jan., 1972, pp. 20-28. The present inventor likewise has coauthored a number of articles describing computer tomosynthesis. See the articles entitled Computerized Tomosynthesis of Dental Tissues, coauthored with Groenhuis and Ruttimann, published in Oral Surgery, Vol. 56, No. 2, pp. 206-214, Aug., 1983; A Prototype Digital Tomographic X-Ray System For Dental Applications coauthored with Groenhuis and Ruttimann, published in the IEEE International Symposium on Medical Images and Icons, Jul. 24-27, 1984, pp. 218-221; Restoration of Digital Multiplane Tomosynthesis By a Constrained Iteration Method, coauthored with Ruttimann and Groenhuis, published in the IEEE Transactions on Medical Imaging, Vol. MI-3 Sep., 1984, pp. 141-148; and Synthesis of Arbitrary X-ray Projections From a Finite Number of Existing Projections coauthored with Ruttimann, Groenhuis and Edholm, published in Society of Photo-optical Instrumentation of Engineers, Application of Optical Instrumentation in Medicine XIII, Vol. 535, pp. 84-90, 1985.
Panoramic dental radiography is also widely used for dental imaging. In contrast with conventional single tooth exposures, panoramic dental radiography attempts to "unwrap" the curved jaw and teeth into a flat panoramic image. In other words, panoramic dental radiography is a unique imaging technique for showing all of the teeth and related dental tissues on a single exposed film in one "panoramic sweep". It results from a system that dynamically alters the swept projection using an exposure geometry that establishes foci that are different in the horizontal and vertical directions.
Panoramic radiography resembles linear tomography to the extent that it produces images characterized by a region of sharp focus that is controlled by the motion of the x-ray beam and the x-ray film relative to the irradiated tissues. However, unlike tomography the image is produced from a fan-shaped x-ray beam which scans across a moving film as the source of radiation moves in a complicated path around the patient's head. The result is an image which shows relatively sharp images of the