Collimator for an X-ray examination apparatus

A collimator for an X-ray examination apparatus has a plurality of diaphragm plates adjustable relative to each other in pairs for defining a cone of radiation having a variable rectangular cross-section, and has a plurality of additional diaphragm plates mounted for pivotal movement into the four corner regions of the radiation cone for defining an approximately circular radiation cone of variable diameter. Each additional diaphragm plate has a substantially triangular plan shape and are movable by respective parallelogram linkages with one edge facing the radiation cone, the edge facing the radiation cone being alignable perpendicularly to the angle bisector of the respective corner in which the additional diaphragm plate is located. The additional diaphragms are respectively pivotable substantially radially in the direction of the axis of symmetry of the collimator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a collimator for an X-ray examination 
apparatus having several diaphragm plates adjustable in pairs relative to 
one another for defining a radiation cone having a rectangular 
cross-section of variable size, and in particular to such a collimator 
having an additional set of diaphragm plates pivotable at the four corner 
regions of the radiation cone for defining an approximately circular 
radiation cone of variable diameter. 
2. Description of the Prior Art 
A collimator for X-rays is disclosed in U.S. Pat. No. 2,675,486 wherein 
four diaphragm plates are mounted in a displaceable manner in a plane 
disposed perpendicularly to the direction of radiation propagation. This 
conventional collimator generates a rectangular radiation cone of variable 
size. If used in combination with conventional X-ray image intensifiers, 
which have a round inlet fluorescent screen, a 30% overshoot of radiation 
at the corners of the rectangular radiation cone generated by such 
conventional collimators is necessary in order to obtain full illumination 
of the field of the inlet fluorescent screen. Such radiation overshoot 
decreases the contrast of the resulting radiograph and increases the 
radiaton exposure of the patient. 
Another collimator is disclosed in German As No. 1037035 for use in 
radiation therapy wherein four diaphragm plates are also displaceably 
mounted in a plane perpendicular to the direction of radiation 
propagation, each of the diaphragm plates being obliquely subdivided. Each 
diaphragm plate thus consists of two approximately triangular halves which 
are displaceable relative to each other by means of a spindle. This 
collimator which permits definition of tetragonal (square) to maximally 
octagonal fields, is quite costly to manufacture and moreover requires 
independent adjustment of five control grips in order to operate. 
A collimator is described in German Pat. No. 2053089 for an X-ray 
examination apparatus which again has a set of diaphragm plates for 
defining a rectangular radiation cone and has an additional set of a 
diaphragm plates disposed in a different plane, those plates being 
triangular and having edges which slide against each other. The additional 
plates are adjustable along a curved guidance means about the axis of 
symmetry of the collimator. Depending upon the number of additional 
triangular diaphragm plates which are utilized, this collimator permits 
cross sections which approximate a circle to be defined. This collimator, 
however, has the disadvantage that the large number of edges of the plates 
sliding against each other, with substantially no gap therebetween, 
requires a considerable amount of friction to be overcome during 
adjustment. This disadvantage makes this collimator generally unsuitable 
for remote control. Another disadvantage is that the collimator must be 
regularly serviced in order to avoid jamming of the triangular diaphragm 
plates. 
Another collimator which permits definition of a rectangular radiation cone 
is distributed by the Philips Company. This collimator reduces radiation 
overshoot of the circular inlet fluorescent screen by the use of 
additional diaphragm plates disposed in at least two different planes 
which can be pivoted into the radiation cone. The additional diaphragm 
plates must be arranged in at least two different planes so as to avoid 
obstructing each other in the case of small radiation fields. Moreover, 
the radiation overshoot of the round inlet fluorescent screen is dependent 
upon the diameter of the defined radiation cone. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a collimator for an 
X-ray examination apparatus which permits rectangular as well as circular 
radiation fields to be defined. 
Another object of the present invention is to provide such a collimator 
wherein the radiation overshoot of a circular inlet fluorescent screen for 
an X-ray image intensifier is as small as possible, and is equal for all 
diameters of the radiation cone. 
Another object of the present invention is to provide such a collimator 
which exhibits low friction during displacement of the diaphragm plates 
and is thus suitable for remote control operation. 
A further object of the present invention is to provide such a collimator 
which occupies as small a volume as possible in the radiation direction, 
has a low weight, and is substantially maintenance-free. 
The above objects are inventively achieved in a collimator for an X-ray 
examination apparatus having a set of additional diaphragm plates of 
substantially triangular shape which are pivotable by means of 
parallelogram linkages from a standby position situated laterally of the 
maximally definable cross-section of the radiation cone at the corners 
thereof, with one edge facing the cone of rays and aligned substantially 
perpendicularly to the angle bisector of the respective corner of the 
radiation cone, to a position within the radiation cone for defining the 
cross-sectional shape thereof. Pivoting or displacement of the respective 
additional diaphragm plates from the standby position into a 
radiation-blocking position occurs radially in the direction of the axis 
of symmetry of the collimator. This manner of operation and structure has 
the advantage that the additional diaphragm plates, in changing the shape 
of the radiation cone from a rectangular field to a generally circular 
field, do not interfer with each other even for radiating very small 
diameters. Because of such lack of interference between the plates, 
substantially no friction forces need be overcome to displace the plates. 
This permits all of the additional plates to be disposed in the same 
plane. Moreover, radiation overshoot of the circular inlet fluorescent 
screen for an X-ray image intensifier is independent of the diameter of 
the inlet fluorescent screen because of the use of parallelogram linkages 
for displacing the additional diaphragm plates which always maintains the 
same straight edge of each plate (the hypotenuse thereof) bounding the 
radiation cone. 
The variability of the collimator can be substantially expanded in a 
further embodiment of the invention wherein all additional diaphragm 
plates are displaceable along tracks in the direction of the axis of 
symmetry of the collimator so that the respectively same corners of each 
of the plates move toward that axis of symmetry. This arrangement even 
further reduces the interaction between the plates disposed in the same 
plane even in the case of a very small radiation field. Thus randomly 
small approximately circular radiation fields can be defined with the same 
facility as larger radiation fields.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A collimator 5 constructed in accordance with the principles of the present 
invention is shown in FIG. 1 for shaping the cross-sectional area of a 
radiation cone 3 issuing from a focus 1 of an X-ray tube 2 having an exit 
flange 4. The collimator 5 has a housing 6. The radiation cone 3 is 
propagated into the interior of the collimator 5 wherein it is acted upon 
from four sides by each of four focus-proximate diaphragm plates, three of 
which are visible in FIG. 1 referenced at 7, 8 and 9, as well as by four 
focus-remote diaphragm plates, three of which are also visible in FIG. 1 
referenced at 10, 11 and 12. The plates comprising the focus-remote and 
focus-proximate sets of diaphragm plates are respectively adjustable in 
pairs perpendicularly to the axis of symmetry 13 of the collimator 5 to 
define a rectangular radiation cone of variable size. The superimposed 
focus-proximate and focus-remote diaphragm plates which bound the same 
side of the radiation cone 3 are coupled with each other in a manner known 
to those skilled in the art and not further illustrated in FIG. 1. A 
mirror 14 is obliquely positioned in the housing 6 of the collimator 5 in 
the radiation cone 3 between the focus-proximate and the focus-remote 
diaphragm plates. A light source (not illustrated) is aligned in the 
housing 6 of the collimator 5 relative to the mirror 14 such that a light 
cone 15, reflected by the mirror 14, is defined congruently with the X-ray 
cone 3 by the focus-remote diaphragm plates 10, 11 and 12. 
The housing 6 of the collimator 5 also has an internal frame 16 for 
supporting eight cantilever rods or arms 17, 18, 19, 20, 21, 22, 23 and 24 
in a single plane disposed in the direction of radiation propagation 
immediately behind the focus-remote diaphragm plates 10, 11 and 12. The 
rods 17 are pivotally mounted in pairs about respective axes, 25, 26, 27, 
28, 29, 30, 31 and 32 which extend parallel to the axis of symmetry 13 of 
the collimator 5. The rods 17 through 24 are disposed in pairs, the free 
ends of each pair of the arms 17 through 24 carrying a respective 
additional triangular diaphragm plate 33, 34, 35 or 36 pivotally mounted 
thereon. The two rods comprising each pair, in combination with the 
additional diaphragm plate mounted thereon and the internal frame 16, form 
a parallelogram linkage system by means of which the additional diaphragm 
plates 33, 34, 35 and 36 can be pivoted into the radiation cone 3 so as to 
block portions thereof for further defining the cross-section thereof. The 
additional diaphragm plates 33, 34, 35 and 36 are thus respectively 
pivotable, along a quarter circle curve, from a standby position at one 
corner of the interior frame 16 toward the axis of symmetry 13 of the 
collimator 5. 
Alignment of the additional diaphragm plates 33, 34, 35 and 36 is selected 
so that each plate bounds or meets the radiation cone 3 with respective 
edges 37, 38, 39 and 40, which form the respective hypotenuses of the 
triangular plates. In the standby position, the edges 37, 38, 39 and 40 
are disposed perpendicularly to the angle bisector for the respective 
corner of the rectangular radiation cone 3 at which the plate is located. 
The two other edges of each additional triangular diaphragm plate are 
matched to the corner region of the interior frame 16. As a consequence of 
the pivoting movement of the respective parallelogram system associated 
therewith, each of the four additional collimator plates 33, 34, 35 and 36 
reaches the axis of symmetry 13 of the collimator 5 with the same 
respective corner. In the embodiment shown in FIG. 2, viewed from the axis 
of symmetry 13, this is the left corner. 
As shown in FIGS. 1 and 2, one of the two arms in each pair of arms forming 
a parallelogram linkage system is coupled with respective crown gears 41, 
42, 43 and 44. A continuous toothed belt 45 is trained about all four 
crown gears. At one corner of the interior frame 16 the continuous belt 45 
is guided by a crown gear 46 which is driven by a motor 47 by means of a 
gear 48. One of the focus-remote plates, as shown in FIG. 1, is coupled 
with a spring-loaded cable line 49 by means of which a path-dependent 
analog-to-digital converter 50 is adjusted. The converter 50 supplies 
coded signals to one input of a difference-forming stage 51. The motor 47 
is also coupled to a path-dependent analog-to-digital converter 52, which 
supplies signals to another input of the difference-forming stage 51. 
During conventional fluoroscopy and radiography operations the collimator 5 
may be utilized as any other collimator. The size of the radiation cone 3 
is variable, in a manner known to those skilled in the art, by synchronous 
adjustment of the focus-proximate and focus-remote diaphragm plates 7 
through 12, disposed in sequence in the direction of radiation 
propagation. The focus-proximate and focus-remote diaphragm plates thus 
define or collimate a rectangular radiation field of variable length and 
width. The additional diaphragm plates 33 through 36 are during this 
operation in the standby position indicated by the dashed lines in FIG. 2 
at the extreme periphery of the interior frame 16. The plates 33 through 
36 are in this position disposed beneath the interior frame 16, and 
protrude only a few millimeters beyond the interior corners of the frame 
16, however, do not project into the radiation cone 3 maximally defined by 
the focus-proximate and focus-remote diaphragm plates 7 through 12. The 
size of the rectangular defined radiation cone 3 can thus be matched in 
width and length as desired to the dimensions of the examination region or 
to a selected cassette or film format. 
If the X-ray examination apparatus is to be utilized in conjunction with an 
X-ray image intensifier, the size of the cone of rays is defined, by 
adjustment of the focus-proximate and focus-remote diaphragm plates 7 
through 12 to the diameter of the inlet fluorescent screen of the image 
intensifier (not illustrated). In this case, however, the thus-adjusted 
quadratic radition field, in the region of its four corners, radiates 
beyond the round inlet fluorescent screen of the image intensifier. In 
order to reduce such radiation overshoot, the motor 47 is actuated, for 
example, by means of a relay (not illustrated) which responds during 
switch over to operation with the image intensifier. The motor 47 drives 
the toothed belt 45 which in turn rotates the crown gears 41 through 44 
for moving the parallelogram linkages for each of the additional diaphragm 
plates 33 through 36 until the signals generated by the path-dependent 
converter 42 of the motor 47 are determined by the difference forming 
stage 51 to be equal to those which are supplied to the difference-forming 
stage 51 by the path-dependent converter 50 associated with the 
focus-remote diaphragm plate 11. When brought to rest, the edges 37, 38, 
39 and 40 bounding the radiation cone 3 will define a radiation cone 3 
which is equally as wide as that defined by the other diaphragm plates 7 
through 12. The parallax difference resulting from the different focus 
distance of the additional diaphragm plates 33 through 36 with respect to 
the diaphragm plate 11 coupled to the cable line 49 is taken into account 
by suitable design of the gearing (or transmission) ratio in the 
regulating drive system. 
It is also possible to adjust the additional diaphragm plates 33 through 36 
with the aid of the light cone 15 cast by the mirror 14 onto the 
examination subject manually or with the aid of the motor 47, which may 
also be switched on manually. Instead of a motor having a path-dependent 
converter as described above, a stepping motor may be utilized, in which 
case a pulse counter may be employed in place of the difference-forming 
stage 52, the counter counting backwards by one unit with each step of a 
stepping motor and the motor being disconnected when the counter reaches 
zero. 
Although other 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.