Positioning method and apparatus for x-ray tomography

A method and apparatus accurately positions a selected cranial implant site for x-ray tomography. The method and apparatus comprise making a stent of the mandible and maxilla of a patient and making a mark on the stent at a position which corresponds to the selected cranial implant site. The stent is then positioned with regard to a tomographic x-ray imaging apparatus so that, (a) the mark on the stent is aligned with a light beam, preferably generated by a laser source, projected downwardly to intersect the central axis of the tomographic apparatus and, (b) the selected plane through the mark is perpendicular to the x-ray beam of the tomographic apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to a method and an apparatus for positioning 
the head of a patient accurately for tomographic x-ray imaging of a 
selected cross-sectional plane through the mandible or maxilla of the 
patient. 
The method and the apparatus of the present invention are designed 
particularly, although by no means exclusively, for use with existing 
tomographic apparatus installed, for example, in dental professional 
offices and used for site evaluation of the mandible and the maxilla prior 
to installing osseointegrated implant posts to support various dental 
prosthesis. More particularly, the present invention is concerned with a 
method and an apparatus for precisely locating the patient in relation to 
a tomographic apparatus such that the site of the anatomy of interest is 
at the correct position to provide an accurate tomographic x-ray of the 
site. 
2. Description of the Prior Art 
By way of background, the dental profession has been involved to an 
increasing degree with the osseointegrated implant system to facilitate 
dental prosthesis. 
In order to properly install an implant a tomographic x-ray of the proposed 
site should be taken so that the dentist can investigate the adequacy of 
the osseous tissue to support an implant post at that site. The x-ray of 
the mandible or maxilla should employ tomography in order to project the 
cross-sectional area of the mandible or maxilla at the site. 
X-ray tomography involves rotating an x-ray source and a photographic plate 
in an arc relative to a patient during exposure of the x-ray image. 
Specifically, the x-ray source and the photographic plate are rotated 
through an arc about a central axis that is perpendicular to the x-ray 
beam. The result of such movement is that all bone structure in front and 
behind a vertical plane through the central axis are blurred and 
essentially do not appear in the image. The vertical plane through the 
central axis is hereinafter referred to as the "tomographic plane". 
Thus, only that structure located at the tomographic plane appears in the 
x-ray image so that the image is of a cross-section of the bone tissue. 
Therefore, by positioning the head of a patient so that a cross-sectional 
area which includes a particular site of interest of the mandible or 
maxilla of a patient coincides with the tomographic plane it is possible 
to obtain an x-ray image of the particular site of interest. 
Clearly, an important consideration in the use of x-ray tomography is the 
accurate positioning of the anatomy of interest in the tomographic plane. 
U.S. Pat. No. 4,974,243 entitled "Positioning System for a X-Ray 
Tomography" assigned to AxialTome Corporation discloses a method and an 
apparatus for such accurate positioning of the head of a patient in 
relation to a tomographic apparatus which works effectively in many 
situations. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved method and 
apparatus for positioning the head of a patient in relation to a 
tomographic apparatus. 
According to the present invention there is provided a method of 
positioning the head of a patient accurately for tomographic x-ray imaging 
of a selected plane through a particular site of interest on the mandible 
or maxilla of the patient by means of a tomographic x-ray imaging 
apparatus, the x-ray tomographic imaging apparatus comprising an x-ray 
source that is rotatable in an arc around a central axis, the method 
comprising: 
(a) forming a stent of the mandible or maxilla of the patient; 
(b) marking the stent at a position on the stent which corresponds with the 
site of interest; and 
(c) positioning the stent with regard to the tomographic x-ray imaging 
apparatus so that: 
(i) the mark on the stent is aligned with a light beam projected vertically 
downwardly to intersect the central axis of the tomographic x-ray imaging 
apparatus, the light beam thereby providing an indication of the 
tomographic plane, as hereinbefore described, of the tomographic x-ray 
imaging apparatus; and 
(ii) the selected plane through the particular site of interest is 
perpendicular in use to an x-ray beam from the x-ray source. 
It can readily be appreciated that once step (c) above has been completed, 
and the light source is turned off, the patient may position 
himself/herself to engage the stent and the tomographic x-ray imaging 
apparatus may be operated to produce an image of the selected plane 
through the particular site of interest on the mandible or maxilla of the 
patient. In this connection it can also readily be appreciated that the 
use of the stent and the light beam enables timely and accurate 
positioning of the patient with respect to the tomographic x-ray imaging 
apparatus. 
It is preferred that the mark on the stent be a radio opaque marker. 
It is preferred particularly that the marker be a metallic ball or other 
plug inserted into the stent at the site of interest. 
It is preferred that the light beam be a laser beam. 
It is preferred that the light beam project a cross and that the cross be 
oriented so that a first arm of the cross is coincident with a central 
axis of the x-ray beam which in use is projected from the x-ray source and 
a second arm of the cross is co-planar with the tomographic plane of the 
tomographic x-ray imaging apparatus. 
It is preferred that the step of positioning the stent comprises 
positioning the long axis of the side of the stent that includes the mark 
in the stent to be parallel to the first arm of the cross. 
According to the present invention there is also provided an apparatus for 
positioning the head of a patient accurately for tomographic x-ray imaging 
by means of a x-ray imaging apparatus of a selected tomographic plane 
through a particular site of interest on the mandible or maxilla of the 
patient, the tomographic x-ray imaging apparatus comprising an x-ray 
source that is rotatable in an arc around a central axis, the positioning 
apparatus comprising: 
(a) a light source for projecting a light beam vertically downwardly to 
intersect the central axis of the tomographic x-ray imaging apparatus, the 
light beam thereby providing an indication of the tomographic plane, as 
hereinbefore described, of the tomographic x-ray imaging apparatus; and 
(b) a means for holding and positioning the stent so that a mark on the 
stent that indicates the particular site of interest on the mandible or 
maxilla is aligned with the light beam and so that in use the selected 
plane through the particular site of interest is perpendicular to an x-ray 
beam from the x-ray source. 
It is preferred that the mark on the stent be a radio opaque marker. 
It is preferred particularly that the marker be a metallic ball or other 
plug inserted into the stent at the site of interest. 
It is preferred that the light source be a laser source. 
It is preferred particularly that the light source be adapted to project a 
cross and that the cross be oriented so that a first arm of the cross is 
coincident with a central axis of the x-ray beam which in use is projected 
from the x-ray source and a second arm of the cross is co-planar with the 
tomographic plane of the tomographic x-ray imaging apparatus. 
It is preferred that the holding and positioning means be adapted to 
position the stent so that the long axis of the side of the stent that 
includes the mark is parallel to the first arm of the cross. 
According to the present invention there is also provided a tomographic 
x-ray imaging apparatus for producing a tomographic x-ray image of a 
selected plane through a particular site of interest on the mandible or 
maxilla of a patient comprising, the apparatus for positioning the head of 
the patient accurately for tomographic x-ray imaging by means of the 
tomographic x-ray imaging apparatus as described in the preceding 
paragraphs.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The tomographic x-ray imaging apparatus shown in the figures comprises a 
column 5 with a base or floor stand 7, an arm or spar 9 (only partially 
shown) which is rotatable about a horizontal axis 11 (the "central axis"), 
an x-ray source 13 at one end of the spar 9 and an x-ray film cassette 
holder (not shown) at the other end of the spar 9. 
In use, the tomographic x-ray imaging apparatus is operable to rotate the 
x-ray source 13 in an arc (typically 60.degree.) about the central axis to 
produce an x-ray image of a tomographic plane extending vertically through 
the central axis 11. Therefore, in use, by positioning the head of a 
patient so that a particular site of interest on the mandible or maxilla 
of the patient is positioned in the tomographic plane it is possible to 
produce a tomographic image through that site of interest. 
In this connection, the positioning apparatus in accordance with the 
preferred embodiment of the present invention is operable in relation to 
the tomographic x-ray imaging apparatus shown in the figures to so 
position the head of the patient. The positioning apparatus is based on 
the use of a stent 41 (FIGS. 3 and 4) of the mandible or maxilla of the 
patient which is marked in an appropriate way, typically by means of a 
radio opaque marker (not shown) such as a metallic ball, to indicate the 
particular site of interest. 
The positioning apparatus comprises, a laser beam projection assembly, 
generally identified by the numeral 19, a holder 21 for securely holding a 
stent 41 (FIGS. 3 and 4) of the mandible or maxilla of the patient, and a 
positioning assembly, generally identified by the numeral 23, for 
effecting the necessary x-, y-, z-axis and rotational movement of the 
stent holder 21 and the stent 41 held at that time by the stent holder 21 
to locate the stent 41 so that the area of interest of the mandible or 
maxilla of the patient, as represented by the stent 41, is positioned to 
coincide with the tomographic plane extending vertically through the 
central axis 11. 
The laser beam projection assembly 19 comprises a laser source (not shown) 
and a support arm 25 arranged so that, in use, the laser source projects a 
laser beam 43 (FIGS. 2 and 3) vertically downwardly in the tomographic 
plane to intersect a central axis 45 (FIG. 3) of the x-ray beam from the 
x-ray source 13. It can readily be appreciated that the laser beam 43, so 
projected, provides a readily identifiable visual indication of the 
location of the tomographic plane. 
In the preferred embodiment of the apparatus of the present invention, the 
laser beam is projected in the form of a cross which is oriented so that a 
first arm is parallel to the central x-ray axis 45 and a second arm is 
co-planar with the tomographic plane. In this connection, it can readily 
be appreciated that the laser beam, when projected onto the surface of the 
stent 41 provides a readily identifiable means of assessing the position 
of the stent 41 with respect to the marking on the stent 41 which 
indicates the particular site of interest of the mandible or maxilla of 
the patient. 
With particular reference to FIG. 4, the stent holder 21 comprises a 
clamping member 29 for clamping securely the stent 41 of a patient and a 
vertically extending support arm 31 which, at an upper end, is detachably 
mounted to the positioning assembly 23. The stent holder 21 is formed to 
allow selected vertical adjustment of the stent 41 with respect to the 
positioning assembly 23. 
The positioning assembly 23, in the preferred embodiment shown in the 
figures, comprises a Wehmer Cephalostat, of generally conventional 
configuration, which is mounted to the column 5 and supports the stent 
holder 21 and enables positioning of the stent 41 by movement in the x-, 
y-, and z-axes and rotation about a vertical axis 49. 
in use of the apparatus described above, a clinician prepares a stent 41 of 
the mandible or maxilla of a patient and marks the stent 41 in an 
appropriate manner to indicate the location of a particular site of 
interest on the mandible or maxilla, for example for an osseointegrated 
implant post, and then locates the stent 41 in the stent holder 21. The 
clinician thereafter manipulates the positioning assembly 23 to position 
the stent 41 so that the metallic ball or other marking in the stent 41 is 
aligned with the downwardly projected laser cross. The laser beam is then 
turned off and the patient positions himself/herself to engage the stent 
41 and by so doing positions the particular site of interest on the 
mandible or maxilla in the tomographic plane. The clinician then operates 
the tomographic apparatus so that a x-ray image of the tomographic plane, 
and thereby the particular site of interest, is produced. By inspection of 
the x-ray image the clinician can determine whether the particular site of 
interest is acceptable for receiving an osseointegrated implant post. 
Many modifications may be made to the preferred embodiment of the method 
and apparatus of the present invention as described above without 
departing from the spirit and scope of the present invention. 
For example, whilst the preferred embodiment of the apparatus includes the 
use of a Wehmer Cephalostat it can readily be appreciated that the present 
invention is not so limited and any suitable means for positioning a stent 
may be used.