X-ray or gamma ray systems or devices – Specific application – Dental panoramic
Reexamination Certificate
2000-06-26
2002-10-15
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Dental panoramic
Reexamination Certificate
active
06466641
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cranial radiography apparatus, particularly intended for dental radiography, said apparatus comprising a first body part to which is connected a second body part, the other end of which being connected to a third body part, whose other end carries a fourth body part having an x-ray source mounted at its one end and an x-ray detector at its other end and in which apparatus said second body part and said third body part are connected to each other via a first pivot shaft and, further, said third body part and said fourth body part are connected to each other via a second pivot shaft, both of said pivot shafts being essentially parallel to each other.
BACKGROUND OF THE INVENTION
The most important tasks of radiography in regard to cranial imaging of the human skull include panoramic tomography of the dental arch, radiography of the dental arch in a transverse plane, radiography of the temporo-mandibular joints and cephalometric radiography of the whole skull. These imaging tasks can be accomplished using a variety of different radiographic techniques: narrow-beam tomography, linear tomography, full-field fluoroscopy, slit fluoroscopy, tomosynthesis radiography and computerized tomography, for instance. In some radiographic apparatus configurations, these different imaging modes can be carried out in the one and the same apparatus by making certain arrangements in the apparatus prior to commencing the radiographic operation and by selecting a suitable control program for the imaging operation. One of the most important and also most demanding radiographic imaging modes in the use of radiographic equipment is panoramic tomography whose basics are next described in greater detail.
Conventional panoramic radiography apparatuses are characterized in that the x-ray source is arranged to orbit about the patient's skull, whereby the dental arch can be imaged by means of an x-ray detector orbiting on the opposite side of the scull. The image can be formed either directly on a film or stored by means of various types of solid-state detectors such as a CCD array sensor in digital format and then displayed on a screen.
In order to obtain a sharp image from a desired object, e.g., the dental arch, the sweep velocity of the x-ray detector must be equal to the sweep velocity of the x-ray beam over the object multiplied by the magnification. Then, the undesirable structures of the object being imaged are blurred invisible. The magnification is determined by the distance between the x-ray tube focus and the film plane to the distance between the x-ray tube focus and the object.
The thickness of the sharply imaged tissue layer is linearly proportional to the distance of the center of rotation to the x-ray detector plane and inversely proportional to the magnification and the beam width. Hence, the imaging process is only related to the mutual dispositions of the focus, the object and the x-ray detector plane.
The basic equation of panoramic imaging is expressed as follows:
v
1
/v
0
=L
1
/L
0
v
O
=&ohgr;r, where
L
O
=distance from tube focus F to the point of the object being imaged
L
1
=distance from tube focus F to the x-ray detector plane
&ohgr;=angular velocity of rotational movement about the center of rotation
r=distance of the object point being imaged from the center of rotation
v
1
=velocity of the image point over the film or x-ray detector plane.
The function of the rotating mechanism of a panoramic tomography apparatus is to direct the x-ray beam through the patient's jaw at a desired angle and to keep the x-ray detector at a given distance from the object being imaged. During the rotational movement, the center of rotation is moved in order to fulfill the following criteria:
orthogonality: the x-ray beam shall be incident on the object as orthogonally as possible in order to prevent adjacent teeth from overlapping at any point of the recorded image;
constant magnification: the magnification shall be maintained constant over the entire dental arch, which requirement can be fulfilled by keeping the distance between the layer being imaged and the image plane of the x-ray detector constant over the entire swept sector of the orbital movement;
motion continuity: the center of rotation shall move monotonously without discontinuities that could cause excessive forces of acceleration and thereby problems in the image quality, and
minimization of radiation dose imposed on the patient: the projection image shall be recorded so that the patient will not be exposed to an unnecessarily large dose of radiation.
The x-ray source of the panoramic tomography apparatus and the rotating mechanism of its x-ray detector must be capable of forming a projection image that satisfies the above-stated requirements set for the recording of the projection radiograph. Furthermore, the apparatus must have a design that can be manufactured at a reasonable cost to a precision free from slack causing disturbing inaccuracies in the recording of a radiograph, such as excessive play between the different elements of the rotating/translating mechanism. Hence, said mechanism must be able to accomplish the desired orbital movement of the center of rotation in a horizontal plane and, additionally, provide vertical support to the entire apparatus so that the desired orbit can be implemented with a good accuracy.
Such an orbital movement can be achieved by virtue of different conventional rotating mechanisms. In the embodiment disclosed in applicant's patent FI 73091 (corresponding U.S. Pat. No. 4,741,007), the orbital movement is accomplished by means of two pivot shafts placed at a constant distance from each other. This construction forms the orbital geometry of the imaging process with the help of a guide groove and an active actuator.
Another prior-art technique for producing the orbital movement is the method disclosed in patent FI 87135 (filed by Instrumentarium Oy), in which the mutual distance between the pivot shafts is made variable. Hence, the orbital movement can be generated as a combination of the rotational movement and the linear movement, thus offering a freely variable orbital geometry of the imaging process.
However, the combination of a linear movement with a rotational movement has been found problematic to implement due to the demanding accuracy requirements of the orbital movement. This is because more difficult to obtain the same accuracy for the mechanism of the orbital movement when a linear movement is involved than when a rotational movement is employed alone.
In prior-art constructions, the generation of the orbital geometry has been optimized particularly for panoramic tomography. Simultaneously, the implementation of other radiographic imaging modes has generally become clumsy if not even impossible.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of accomplishing a rotational movement in a manner capable of providing accurate orbital geometries at more reasonable equipment costs than those of conventional constructions and simultaneously offering a selection of orbital geometries facilitating flexible switching between different cranial imaging modes (including, e.g., panoramic tomography of the dental arch, radiography of the dental arch in transverse projections commonly known as transverse radiography, imaging of the temporomandibular joints and the entire skull) using at all times a radiographic technique most appropriate for the need (e.g., narrow-beam tomography, slice tomography, full-field fluoroscopy, slit fluoroscopy, tomosynthesis radiography or computer tomography).
It is a further nonlimiting object of the invention to provide an apparatus capable of fulfilling the above-described orthogonality requirement with a good accuracy.
It is a still further nonlimiting object of the invention to provide an apparatus capable of keeping constant magnification over the entire imaged area.
It is still another further nonlim
Müller Timo
Nyholm Liina
Virta Arto
Church Craig E.
Planmeca Oy
Steinberg & Raskin
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