Device for aligning an x-ray source with an image receptor

A device for aligning the central ray of an x-ray beam with the center of an image receptor comprises a metal tube in a mounting which is provided with means for establishing the axis of the tube perpendicular to the plane of the image receptor or other plane which is perpendicular to the central ray. The tube is mounted in such manner that the region around it is x-ray permeable. The mounting is adapted for being moved in parallelism with the plane such that alignment of the central ray with a point on the receptor may be determined when the circular shadow cast by the outside of the tube is concentric with the circular image of the tube bore.

BACKGROUND OF THE INVENTION 
This invention pertains to a beam alignment device for use with diagnostic 
x-ray apparatus. 
In one type of diagnostic x-ray apparatus the x-ray tube is mounted inside 
of the body of the patient's supporting x-ray table. An image receptor 
such as an image intensifier or a radiographic film is mounted above the 
table top in such manner that the image receptor may be moved toward and 
away from the x-ray tube. In this arrangement, it is necessary to have the 
central ray of the beam, which diverges from the focal spot of the tube, 
perpendicular to and centered on the plane of the image receptor. As is 
well known, if the central ray is not perpendicular and centered, the 
central ray will shift laterally along the receptor plane as the plane is 
raised and lowered relative to the focal spot. A consequence is that the 
edges of the collimated x-ray beam will not coincide with the edges of the 
receptor plane at all levels. Thus, the beam may fall short of the edge of 
the receptor plane on one side and overlap on the other side, which in the 
latter case, results in part of the x-ray image being cut off and not 
recorded. This is undesirable, not only because of the loss of information 
in the image, but also the patient is exposed to radiation dosage which 
produces no information. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a device for aligning an 
x-ray source with an image receptor such that the aforementioned problems 
resulting from misalignment may be overcome. 
Other objects are to provide an aligning device which is rugged, 
inexpensive and easy to use. 
In general terms, the aligning device comprises a metal tube in a mounting 
which allows the x-ray beam to pass generally coaxially along the inside 
and the outside of the tube. When the metal tube is positioned 
perpendicular to the x-ray tube horizontal reference plane and in vertical 
alignment with the tube focal spot, the projected image of the tube 
outside diameter and inside diameter are concentric. If the metal tube 
bore is not aligned precisely with the central ray of the beam emanating 
from the focal spot, the outside diameter and inside diameter images which 
are projected on the receptor plane will not be concentric. The device is 
then moved until concentricity is obtained on the receptor plane and then 
the plane is shifted as required until the concentric image coincides with 
the center of the plane. The center of the receptor plane will then be on 
the vertical line of the focal spot regardless of the elevation to which 
the receptor plane is set.

DESCRIPTION OF A PREFERRED EMBODIMENT 
A typical x-ray diagnostic system wherein the new device may be used to 
align the central ray or ray bundle of an x-ray beam with the image 
receptor center is shown diagrammatically in FIG. 1. The system comprises 
an x-ray table having an enclosed metal body 10 supported on a stand 11. 
Usually the body 10 is tiltable on stand 11 about a laterally extending 
axis which, in this case, means about an axis that is perpendicular to the 
plane of the drawing. The table has a patient supporting top 12. An x-ray 
tube casing 13 is mounted in any well-known manner within body 10. The 
details of the x-ray tube within the casing are not shown but it will be 
understood that the tube has the usual target 14 on which an electron beam 
impinges to produce a focal spot from which an x-ray beam emanates. An 
x-ray beam collimator 15 is coupled with the x-ray casing and performs its 
usual function of defining the boundaries of the diverging x-ray beam 
which is projected through the table top 13 and a patient reposed thereon. 
Mounted above the table top 12, in this example, is a casing 16 which, 
except for its bottom, is x-ray impervious. Inside of the casing there is 
a conventional x-ray image-to-optical-image converter tube, commonly known 
as an x-ray tube 17 which is shown in dashed lines. The x-ray image 
impinges on a receptor plane 18 and it is converted to an optical image 
within tube 17 which image may be viewed with a television camera 19 and 
displayed on a television montior 20. In this example, the image receptor 
18 is the input fluorescent screen of image tube 17 but it should be 
understood that the image receptor can be a radiographic film or a direct 
viewing fluorscopic screen for other recording medium. 
In FIG. 1, the new beam aligning device is generally designated by the 
reference numeral 25. It is shown positioned on table top 12 approximately 
as it would be during the aligning procedure but it will be understood 
that the device 25 is removed when central ray alignment has been 
accomplished. X-ray tube casings such as 13 are usually provided with a 
horizontal reference plane and with means for adjusting the position of 
the casing until the reference plane is parallel with the plane of table 
top 12 in which case the central ray of the x-ray beam emanating from 
target 14 will be perpendicular to the table top 12. 
As indicated earlier, during diagnostic procedures, it is necessary to 
position the image intensifier tube 17 and its receptor plane 18 at 
various distances from the x-ray tube focal spot on target 14 and it is 
necessary for the center of the image receptor plane 18 to remain on the 
same vertical line as the focal spot if image distortion and displacement 
is to be avoided. 
Geometrical considerations of alignment and misalignment will now be 
discussed in reference to FIG. 4 which shows an exaggerated case of 
misalignment for the sake of illustration. Assume that the image receptor 
plane 18, illustrated by a solid line, is at a particular elevation 
relative to focal spot 14' such that the central x-ray 26 or small central 
ray bundle is coincident with the center 27 of the receptor plane. The 
boundary rays 28 and 29 are so defined by collimator 15 that the edges 30 
and 31 of the receptor field coincident with the boundary rays. In other 
words, there is edge-to-edge coincidence and center coincidence when the 
plane of the image receptor 18 is at the illustrated level. The image 
formed on the receptor will be that which is within the limits of boundary 
rays 28 and 29 as they pass through a body 35. In this diagram it will be 
obvious that the center 27 of the receptor is not on the same vertical 
line as focal spot 14. Now, it will be observed that if the receptor plane 
18 is moved to its dashed line position closer to focal spot 14, the 
central x-ray 26 will no longer pass through center point 27 and there 
will be an edge area 32 on the receptor plane on which no image will be 
recorded. On the other edge, a portion 33 of the image will be cut off. If 
the receptor plane 18 is moved farther away from focal spot 14 it will be 
obvious that there will be a reversal between the cut off and unexposed 
edges. 
The new device for aligning the center point 27 of the receptor 18 on the 
same vertical line as focal spot 14 will now be described in greater 
detail in reference to FIGS. 2 and 3. The device comprises a metal tube 36 
which may be stainless steel or other metal which has high x-ray 
absorption. In a commercial embodiment, the tube is about 2 inches long 
and has as outside diameter of about 0.073 inch and a wall thickness of 
about 0.016 inch. Thus, the bore of the tube is about 0.041 inch. These 
dimensions are given for the purpose of illustration and should not be 
construed as limiting for the tube may be a little smaller or larger and 
longer but preferably, it should not be substantially shorter. The metal 
tube 36 should have a length which is much longer than its outside 
diameter. 
Mounting means are provided for the tube 36 in the form of a plastic block 
37 which, in a commercial embodiment, is methyl methacrylate such as a 
material identified by either of the trademarks "Lucite" or "Plexiglass." 
Any suitable material which is transparent to x-radiation may be used but 
transparency to light is not essential. The tube 36 may be mounted in 
other ways as long as passage of radiation coaxially therewith and without 
significant absorption is permitted. 
Tube 36 is cemented in a machined hole in block 37 perpendicularly to the 
upper surface 38 of the block and to its preferably planar lower surface 
39. The block has a circular recess 40 in its bottom. The block also has 
three leveling screws 41, 42 and 43 threaded into it. The tips of the 
screws 41-43 extend out from the bottom of the block. The screws are 
accessible through counterbored holes 44, 45 and 46 for being turned with 
a screwdriver or Allen wrench. By proper adjustment of the screws, the 
axis of tube 36 may be placed in precise perpendicularity with the x-ray 
table top 12 and coincident with the central x-ray beam when the device is 
set on the top. 
Block 37 has a recess 47 in which there is a bubble level device 48 of a 
readily commercially available type. The bottom 49 of recess 47 is made 
parallel to the transverse top and bottom planes 38 and 39 so it is in 
true perpendicularity with the axis of tube 36. When the device is placed 
on the x-ray table top, leveling screws 41-43 are adjusted until the 
bubble 50 in the level is centered in which case the axis of tube 36 is 
truly perpendicular. 
Use of the device to align a selected ray in a diverging ray beam is 
illustrated in FIGS. 5 and 6 where the tube 36 is shown diagrammatically 
but the plastic mounting block and leveling elements are omitted. Assume 
in reference to FIGS. 5 and 6 that the alignment procedure is in progress. 
The device 25 will be set over the x-ray tube as closely as possible to 
the central ray of the beam as illustrated in FIG. 1. The image receptor 
plane 18 will be in spaced relationship to the focal spot 14 and the 
location of tube 36 relative to the receptor plane and focal spot will be 
such, preferably, that the image of the tube appearing on the receptor 
will be magnified by at least a factor of two. The leveling screws 41-43 
are then adjusted until the bubble in level 48 is centered at which time 
the axis of metal tube 36 will be perpendicular to the horizontal 
reference plane of the x-ray tube casing 13 and to the table top and the 
axis will be parallel with and possibly coincident with the central ray 
path. When the set up is made, the x-ray source is turned on and the image 
of the tube 36 will be cast on the receptor 18. In the FIG. 1 apparatus, 
the image on the receptor may be viewed on television monitor 20. If, as 
is the case in FIG. 5, the axis of the tube 36 is laterally displaced from 
the vertical central ray of the x-ray beam, the circular pattern 50 of the 
x-ray image resulting from the radiation passing through the bore of tube 
36 will be eccentric with respect to the generally circular pattern of the 
shadow 51 cast by part of the x-ray beam being intercepted by the leading 
edge 52 of the outside diameter of tube 36. As long as the axis of tube 36 
is misaligned with the central x-ray, the image defining circles 50 and 51 
will be eccentric. The device is shifted in a sequence of steps until 
circles 50 and 51 become concentric as shown in FIG. 6 in which case it is 
known that the central x-ray is precisely perpendicular to the plane of 
image receptor 18. If, when perpendicularity is known, the central ray 
does not coincide with the center of the receptor, measures are taken to 
establish the center of the receptor on the same vertical line as the 
central ray and focal spot. 
In systems that use radiographic film instead of an image intensifier, the 
test procedure can be carried out using a fluorescent sheet as the image 
receptor plane in which case the circular shadow patterns 50 and 51 may be 
visualized and the tube 36 repositioned until concentricity is obtained. 
If, in this case, as in the preceding case, the aperture provided by the 
tube bore is grossly misaligned, there will be no image of the inside 
diameter of the aperture but, instead, a foreshortened image of the length 
of the tube 36. 
Although use of the new alignment device has been described in reference to 
a rather conventional x-ray table which has the x-ray tube mounted inside 
of the table body, it should be appreciated that the device may be used to 
align the central x-ray beam and image receptor in diagnostic systems 
where the x-ray source is not even mounted in a table such as when the 
source and image receptor are mounted on a C-arm or U-arm or other 
support.