An x-ray diagnostics installation includes a device for positioning a radiation emitter such that the reference ray of an emitted ray beam always and automatically intersects a reference axis that is stationary with reference to said device at an acute angle .alpha. when the radiation emitter is coupled to the device. X-ray exposures for tomosynthesis can thus be produced in a simple way.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to an x-ray diagnostics installation of 
the type for producing medical x-ray images, such as dental x-ray images. 
2. Description of the Prior Art 
A conventional x-ray installation has a radiation emitter that is 
adjustably held at a wall of a room or at a stand. In order to produce an 
x-ray exposure, the radiation emitter is aligned with reference to an 
examination subject and an x-rays are generated which penetrate the 
examination subject and are then incident on a radiation receiver. The 
radiation receiver can be implemented as x-ray film, an image intensifier 
video chain or a CCD radiation converter. 
In a known way, such a radiation receiver converts the x-ray shadow 
proceeding from the examination subject into state changes in the case of 
film, or signals in the case of electronic detection, so that a 
two-dimensional image is obtained, or can be calculated on the basis of 
the signals. The x-ray shadow arises from the absorption sum of all 
penetrated layers of the subject. The spatial allocation of the image 
content of the two-dimensional image is made by the viewer on the basis of 
his or her experience. When, however, diagnostically relevant details are 
occluded by more highly absorbent structures or when pseudo-structures 
arise, then these can remain hidden from even the most experienced viewer 
or can lead to misdiagnoses. This is true, of course, because of the 
complex bone structures, especially when producing x-ray exposures in the 
tooth or jaw region in dental diagnostics. 
An improvement in the ability to diagnose the contents of images of a 
subject to be examined is achieved on the basis of a three-dimensional or 
a slice-by-slice image presentation. Such three-dimensional or 
slice-by-slice presentations can be obtained, for example, by computed 
tomography or nuclear magnetic resonance tomography. The technical outlay 
and thus the costs, however, are thereby substantial. 
Tomosynthesis comes into consideration as an alternative. In tomosynthesis, 
an examination subject is irradiated from different projection directions 
and the x-ray shadow is imaged on a radiation receiver. Particularly in 
the case of radiation receivers which generate electrical signals 
dependent on incident x-ray shadows, tomograms or three-dimensional images 
can be calculated with known methods. The number of necessary exposures 
and the solid angle of the fluoroscopy are set dependent on the desired 
depth resolution of the slice thickness of an image. 
In conventional tomograms (tomography), a radiation emitter and a radiation 
receiver are coupled and adjusted oppositely relative to one another. 
Subjects that lie in the focal plane are sharply imaged since they are 
always projected onto the same location of the radiation receiver during 
the opposed adjustment. Subjects that lie outside the focal plane are 
imaged unsharp since they are projected onto different locations of the 
radiation receiver during the opposed adjustment. For producing an 
interpretable exposure, the subject in the focal plane is imaged onto the 
radiation receiver with a number of individual projections at projection 
angles .alpha.. By direct superimposition of the radiation images acquired 
by the individual projections, a tomographic image of the subject in the 
focal plane can be produced. A tomographic image of a subject that is 
arranged in a plane parallel to the focal plane can be produced by 
shifting the radiation images acquired by the individual projections by 
distances .DELTA.S relative to one another before the superimposition. The 
size and the direction of the shift .DELTA.S is dependent on the position 
of the radiation emitter and on the location (attitude) of the plane to be 
reconstructed. 
The shift .DELTA.S for what is referred to as linear tomography, wherein 
the radiation emitter is adjusted in one dimension, is determined from the 
equation Bildgebende Systeme fur die medizinische Diagnostik, Krestel, 
(Siemens) pages 380 and 381) 
##EQU1## 
wherein: x=distance of the focus of the radiation emitter from the 
radiation receiver, 
h=distance of the focal plane from the plane in which a subject is to be 
reconstructed, 
y=distance of the focal plane from the plane of the radiation receiver, and 
.alpha.=projection angle, i.e. the angle that a reference beam of the ray 
beam assumes relative to a reference axis, whereby the reference axis is 
aligned perpendicularly relative to the focal plane. 
When the radiation receiver converts the received x-ray shadow of the 
subject into electrical signals, digital tomosynthesis enables the 
reconstruction of tomographic images in a number of planes from the 
signals of the individual projections of the subject that were produced 
given different projection angles: Known, digital image generating and 
processing systems can be employed in digital tomosynthesis for producing 
a visible image from the signals of the radiation receiver. 
PCT Application WO 93 22 893 discloses a method with which it is possible 
to reconstruct an exposure of a subject without the projection angles 
.alpha. and the geometrical arrangement of radiation emitter, radiation 
receiver and focal plane being known. According to this method, a 
reference of radiation-absorbent material having a known size and known 
distance from the radiation receiver is provided in the region of the 
radiation receiver, this reference being projected onto the radiation 
receiver in every individual projection. The geometrical arrangement and 
the two-dimensional projection angle .alpha. can be identified on the 
basis of the spatial imaging of the reference on the radiation receiver 
for each individual projection. This reconstruction is time-intensive and 
complex due to the extensive calculations. 
The radiation emitter must assume predetermined positions and alignments 
relative to the examination subject for obtaining an image sequence that 
can be interpreted tomosynthetically. The alignment can be set, for 
example, by an operator of the x-ray diagnostics installation or by 
employing and driving a radiation emitter that has multiple foci. Manual 
alignment requires several operating steps and is thus time-consuming and 
susceptible to error. The employment of a radiation emitter having 
multiple foci is technologically complicated, and thus expensive. 
SUMMARY OF THE INVENTION 
An object of the present invention is to implement an x-ray diagnostics 
installation of the type initially described which can produce x-ray 
images for tomosynthesis in a simple way. Another object is to provide an 
x-ray diagnostics installation fashioned for that purpose which is less 
technologically complicated than known systems but nonetheless is reliable 
in employment. 
These objects are inventively achieved in an x-ray diagnostics installation 
having a device for positioning a radiation emitter so as to always and 
automatically cause a reference ray of an x-ray beam emitted by the 
radiation emitter to intersect a reference axis that is stationary with 
respect to the device at an acute angle .alpha. when the radiation emitter 
is coupled to the device. An advantage of the invention is that the angle 
between the reference ray and the reference axis, and thus the projection 
angle .alpha., is predetermined, so that a calculation of an x-ray image 
from the signals of the radiation converter is possible in a simple way. 
The description of the positioning device as "always and automatically" 
causing the reference ray to intersect the reference axis at an acute 
angle .alpha. does not preclude the possibility of the device being 
adjustable so as to permit the acute angle .alpha. to be change from 
examination-to-examination. For a series of exposures for producing a 
tomographic image, however, the angle .alpha., once set, will not vary 
from projection-to-projection, and the positioning device of the invention 
permits the radiation emitter to automatically and always be positioned so 
as to produce the same acute angle for each exposure position. 
In an embodiment, the device includes a disk having a recess for the 
acceptance of a part of the radiation emitter, particularly for the tube 
or barrel of the radiation emitter, and a drive for the disk, the latter 
being adjustable around its longitudinal axis. For producing a series of 
x-ray exposures for tomosynthesis, the radiation emitter merely has to 
have its tube or barrel introduced into the recess, as a result of which 
the angle of the reference ray relative to the reference axis is 
automatically prescribed. The direction of the irradiation is prescribed 
positionally dependent by triggering x-rays at predetermined positions 
during an adjustment of the disk. A better mixing of subject details lying 
outside the reconstructed plane is achieved if the adjustment of the 
radiation emitter takes place in two dimensions, for example on a circular 
orbit; the size and direction of the shift .DELTA.S then also arise 
analogously to the aforementioned equation. 
In another embodiment of the invention, the device is annularly fashioned 
and has at least two recesses for the acceptance of the radiation emitter, 
preferably the tube or barrel of the radiation emitter. Given a 
predetermined angle, the reference ray intersects the reference axis in a 
first predetermined direction when the radiation emitter is accepted in 
the first recess and intersects it in a second predetermined direction 
when the radiation emitter is accepted in the second recess. The 
irradiation direction and the angle .alpha. of the reference ray relative 
to the reference axis are predetermined and defined by the recesses, 
particularly by aligning their respective central longitudinal axes. The 
recesses are thereby preferably implemented as circular segments, so that 
the tube or barrel can be placed against them.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a dental x-ray diagnostics installation that includes a device 
1 for positioning a radiation emitter 2 that is seated at an adjustable 
column 3. The device 1 is adjustable along the column 3 via an articulated 
arm 4, is pivotable around the longitudinal axis of the column and is 
seated so as to be rotatable around an axis 5 of the articulated arm 4. 
The radiation emitter 2 is adjustably held at the column 3 via a retainer 
arm 6, so that it can be positioned with reference to an exposure subject 
for producing an x-ray exposure. 
A seat 7 for an examination subject is mounted on the column 3 so as to be 
adjustable along the column 3 and pivotable around the longitudinal axis 
thereof. A head support 8 is likewise adjustably mounted on the column 3. 
For producing an intraoral x-ray exposure for tomosynthesis, it is 
necessary to irradiate an exposure subject, for example a tooth, from 
different directions. A radiation receiver converts the X-radiation 
incident from the different directions into signals that are supplied to 
an evaluation unit for producing an x-ray image of the exposure subject. 
Knowledge of the angle .alpha. of the reference ray of a ray beam emitted 
from the radiation emitter 2 relative to a reference axis is required for 
the calculation of such an x-ray image. According to the invention, the 
dental x-ray diagnostics installation therefore employs the device 1 for 
positioning the radiation emitter 2 such that the reference ray of an 
emitted ray beam always and automatically intersects a reference axis of 
the device 1 at an acute angle in a reference point when the radiation 
emitter 2 is coupled to the device 1. 
The device 1 of FIG. 1 is shown in detail in FIG. 2. The device 1 has a 
disk 9 in the form of a ring having an inside edge 10 provided with a 
number of recesses 11. These recesses 11 serve the purpose of coupling the 
radiation emitter 2, particularly the tube or barrel 12 thereof, to the 
device 1. As a result of the recesses 11 that, for example, extend along a 
circular arc whose center lies on a reference axis 13 of the device 1, a 
number of positions of the radiation emitter 2 and--due to the alignment 
of the central longitudinal axis thereof--a number of irradiation 
directions are automatically prescribed. The angle .alpha. at which a 
reference ray, for example the central ray of an x-ray beam emitted by the 
radiation emitter 2, intersects the reference axis 13 of the device 1 in a 
reference point is thus always predetermined by inserting the tube or 
barrel 12 into a recess 11. According to the invention, the device 1 has 
at least two and preferably eight recesses 11 formed as circular segments 
for the positioning of the radiation emitter 2. It has proven advantageous 
for the calculation of an x-ray image and in order to produce a device 1 
when the angle .alpha. lies in the range below 45.degree., preferably 
below 15.degree.. 
In the exemplary embodiment, a bracket 15 for a radiation converter 16 that 
prescribes the distance of a schematically indicated tooth 17 from the 
device 1, and thus from the focus of the radiation emitter 2, can be 
coupled to the device 1. The distance is preferably selected such that the 
reference ray 14 of the x-ray beam is incident at the center of the 
radiation converter 16. When the radiation converter 16 is arranged in the 
focal plane and at the reference point, then a calculation of an image 
from the signals of the radiation converter 16 is possible in a desired 
plane in a simple way. The shift .DELTA.S is then directly dependent on 
the distance between the reconstructed plane and the focal plane (distance 
from the radiation converter 16), and a constant factor. 
In order to avoid exposures from being made when the radiation emitter 2 is 
not exactly accepted by the device 1, so that the angle .alpha. is 
different and/or the distance between the focus of the radiation emitter 2 
and the radiation receiver 16 is not exactly observed, or if these 
particulars are not known or are not known with the required precision, it 
is advantageous to provide a reference 18 of radiation-absorbent material 
and which, for example, is spherically fashioned. The reference 18 is held 
in front of the tooth 17 and the radiation converter 16 as seen in the 
radiation propagation direction. As a consequence of the projection of the 
reference 18 onto the radiation receiver 16 (which is dependent on the 
projection angle a and on the distance between the focus of the radiation 
emitter 2 and the radiation receiver 16) and as a consequence of the 
signals thereby generated, a subsequent calculation of the protection 
angle .alpha. and of the distance is possible. Given employment of such a 
reference 18, the bracket 15 can be composed of elastic material, so that 
it allows the patient whose tooth is to be examined a limited freedom of 
movement. 
It is also within the context of the invention, to provide the recesses 11 
at the outside edge 19 of the disk 9, for example as circular segments. 
Further, the distance of the device 1, or of the radiation emitter 2 from 
the exposure subject can be prescribed or acquired in a non-contacting, 
wireless manner (for example, with optical or acoustic means). 
In the further exemplary embodiment of the device 1 shown in FIG. 3, a 
circular disk 20 is mounted in a housing 21 so as to be rotatable around 
its central axis 22. It is possible to manually rotatably adjust the 
circular disk 20; however, it is advantageous to provide a drive 23 
therefor that engages the circular disk 20, such as by meshing. The 
circular disk 20 has a recess 11 for coupling the radiation emitter 2, 
preferably the tube or barrel 12 thereof. The annular position of the 
recess 11 is adjusted by operating the drive 23 so as to move the recess 
11 along a circular arc whose center lies on the reference axis. The 
alignment of the central longitudinal axis of the recess 11 thereby also 
automatically prescribes the projection angle .alpha. that the reference 
ray 14 assumes relative to the reference axis, namely the central axis 22. 
When the radiation emitter 2 is coupled thereto, an adjustment of the 
circular disk 20 around its central axis 22 changes the direction of the 
irradiation. The drive of the radiation emitter 2 for emitting a ray beam 
can ensue automatically when the circular disk 2, especially the recess 11 
thereof, assumes predetermined positions. 
In the exemplary embodiment shown in FIG. 4, the radiation emitter 2 and 
the device 4 are arranged in a common housing. The device 1 for 
positioning the radiation emitter 2 is implemented such that the radiation 
emitter 2 is adjustable at guides in a direction perpendicular to the 
reference axis 11. This adjustment takes place by means of a toothed rack 
26 that is rotatable via a first drive 25. A second drive 27 is provided 
for swivelling of the radiation emitter 2, so that the reference ray 14 
either describes an acute angle .alpha. with the reference axis 13 or can 
be aligned parallel thereto. A third drive 28 engages the device 1 for the 
adjustment thereof around the reference axis 13. As a result of the 
compact unit composed of the radiation emitter 2 and the device 1, 
exposures of the tooth 17 can be produced from different directions. 
Swivelling of the radiation emitter 2 is preferably accomplished such that 
the reference ray 14, given a prescribed distance of the radiation 
converter 16, is always incident at the center of the converter 16. 
A gating mechanism 29 for the x-ray beam as well as the bracket 15, can be 
mounted on the housing 24. 
According to another exemplary embodiment of the invention shown in FIG. 5, 
the radiation emitter 2 is held by a wobble bearing in a housing 30. The 
wobble bearing has a swash plate 31 which holds a sliding, frictional 
bearing 36 in which a recess 32 accepting the radiation emitter 2 is 
provided. Bearings 33 engage the edge of the swash plate 31, so that the 
swash plate 31 is rotatable around the reference axis 22. A drive 34 is 
provided that engages the swash plate 31 for rotation thereof. A tube or 
barrel 35 is provided at the housing 30 so that the radiation emitter 2 
can rotate through the entire solid angle that is prescribed by the recess 
32. The angle .alpha. that the reference ray 14 assumes relative to the 
reference axis 22 also lies in a range from 2.degree. through 12.degree., 
preferably at 8.degree.. This embodiment also makes it possible to 
irradiate an examination subject from either permanently prescribed or 
freely selectable positions and directions by slightably adjusting the 
bearing 36 in the swash plate 31. The friction between the bearing 36 and 
the swash plate 31 holds the bearing 36, and thus the radiation emitter 2, 
at whatever angle at which the bearing 36 is set. As in the previous 
embodiments wherein the angle .alpha. can be freely selected, this only 
means that the angle .alpha. can be changed, as needed, from 
examination-to-examination, but for producing a given series of exposures 
for generating a tomogram, the angle .alpha. will be set at the beginning 
of the exposure sequence, and will be automatically and always retained at 
each exposure position, as the swash plate 31 is rotated by the drive 34. 
A predetermined connection between the radiation emitter 2, the subject to 
be examined and the radiation converter 16 can also be produced for 
preparing an x-ray exposure. 
The column 3 on which the device 1, the radiation emitter 2 and the seat 7 
are mounted can be a part of a movable (mobile) or a stationary stand. It 
is also possible to mount the device 1 and/or the radiation emitter 2 
and/or the seat 7 at such a stand and/or at the wall of an examination 
room. 
A digital, intraoral x-ray camera, for example a CCD converter, having a 
phosphor layer can be used as the radiation converter 16 and a 
conventional x-ray source for intraoral radiography that is suitable for 
digital exposures can be used as the radiation emitter. The x-ray source 
can be operated to produce a dose that can be set lower by an order of 
magnitude than is standard for individual digital exposures. The 
tomosynthetic multiple exposures thus do not require a significantly 
higher overall dose than is required for a single "normal" exposure. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventor to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of his contribution to the art.