Radiation imager collimator

A collimator for use in an imaging system with a radiation point source has a plurality of channels formed therein along longitudinal axes aligned with selected orientation angles that correspond to the direct beam path from the radiation source to the radiation detectors. The collimator comprises a photosensitive material coated with a radiation absorbent material. The cross-sectional shape of the channels corresponds to the cross-sectional shape of the radiation detecting area of the detector element adjoining the channel, and the sidewalls of the channel are smooth along their length. The collimator may be fabricated by forming a mask on a photosensitive collimator substrate, exposing the photosensitive substrate to light beams traveling along a path corresponding to a direct path of radiation from the radiation source to the detector elements in the assembled array, etching the collimator substrate to form channels therein along the exposed area of the substrate, and coating the substrate with a radiation absorbent material.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to the application of C. Y. Wei, R. F. 
Kwasnick, and G. E. Possin entitled "X-ray Collimator," Ser. No. 
07/802,789, filed concurrently with this application, and assigned to the 
assignee of the present application. 
FIELD OF THE INVENTION 
This invention relates generally to radiation imagers, and in particular to 
focused collimators used in conjunction with radiation detection 
equipment. 
BACKGROUND OF THE INVENTION 
Collimators are used in a wide variety of equipment in which it is desired 
to permit only beams of radiation emanating along a particular path to 
pass beyond a selected point or plane. Collimators are frequently used in 
radiation imagers to ensure that only radiation beams emanating along a 
direct path from the known radiation source strike the detector, thereby 
minimizing detection of beams of scattered or secondary radiation. 
Collimator design affects the field-of-view, spatial resolution, and 
sensitivity of the imaging system. 
Particularly in radiation imagers used for medical diagnostic analysis or 
for non-destructive evaluation procedures, it is important that only 
radiation emitted from a known source and passing along a direct path from 
that source through the subject under examination be detected and 
processed by the imaging equipment. If the detector is struck by undesired 
radiation, i.e., radiation passing along non-direct paths to the detector, 
such as rays that have been scattered or generated in secondary reactions 
in the object under examination, performance of the imaging system is 
degraded. Performance is degraded by lessened spatial resolution and 
lessened energy resolution that result from noise in the signal processing 
circuits generated by the detection of the scattered or secondary 
radiation rays. 
Collimators are positioned to substantially absorb the undesired radiation 
before it reaches the detector. The collimator comprises a relatively high 
atomic number material placed so that radiation approaching the detector 
along a path other than one directly from the known radiation source 
strikes the body of the collimator and is absorbed before being able to 
strike the detector. In a typical detector system, the collimator includes 
barriers extending outwardly from the detector surface in the direction of 
the radiation source so as to form channels through which the radiation 
must pass in order to strike the detector surface. 
Some radiation imaging systems, such as computerized tomography (CT) 
systems used in medical diagnostic work, use a point (i.e. a relatively 
small, such as 1 mm in diameter or smaller) source of x-ray radiation to 
expose the subject under examination. The radiation passes through the 
subject and strikes a radiation detector positioned on the side of the 
subject opposite the radiation source. In a CT system the radiation 
detector typically comprises a number of one-dimensional arrays of 
detector elements. Each array is disposed on a flat panel or module, and 
the flat panels are typically arranged end to end along a curved surface 
to form a radiation detector arm. The distance to a given position on any 
of the separate panels, typically the center of the panel, on any one of 
the separate panels is the same, i.e., each panel is at substantially the 
same radius from the radiation source. On any given panel there is a 
difference from one end of the panel to the other in the angle of 
incidence of the radiation beams arriving from the point source. In any 
system using a "point source" of radiation and flat panels or modules of 
detector elements, some of the radiation beams that are desired to be 
detected, i.e., ones emanating directly from the radiation source to the 
detector surface, strike the detector surface at some angle offset from 
vertical. 
For example, in a common medical CT device, the detector is made up of a 
number of panels, each of which has dimensions of about 32 mm by 16 mm, 
positioned along a curved surface having a radius of about 1 meter from 
the radiation point source. Each panel has about 16 separate detector 
elements about 32 mm long by 1 mm wide arranged in a one-dimensional 
array, with collimator plates situated between the elements and extending 
outwardly from the panel to a height above the surface of the panel of 
about 8 mm. As the conventional CT device uses only a one-dimensional 
array (i.e., the detector elements are aligned along only one row or 
axis), the collimator plates need only be placed along one axis, between 
each adjoining detector element. Even in an arrangement with a panel of 
sixteen 1 mm-wide detector elements adjoining one another (making the 
panel about 16 mm across), if the collimator plates extend perpendicularly 
to the detector surface, there can be significant "shadowing" of the 
detector element by the collimator plates towards the ends of the panel. 
This shadowing results from some of the beams of incident radiation 
arriving along a path such that they strike the collimator before reaching 
the detector surface. Even in small arrays as mentioned above (i.e. 
detector panels about 16 mm across), when the source is about 1 meter from 
the panel with the panel positioned with respect to the point source so 
that a ray from the source strikes the middle of the panel at right 
angles, over 7.5% of the area of the end detector elements is shadowed by 
collimator plates that extend 8 mm vertically from the detector surface. 
Even shadowing of this extent can cause significant degradation in imager 
performance as it results in nonuniformity in the x-ray intensity and 
spectral distribution across the detector module. In the onedimensional 
array, the collimator plates can be adjusted slightly from the vertical to 
compensate for this variance in the angle of incidence of the radiation 
from the point source. 
Advanced CT technology, however, requires use of two-dimensional arrays, 
i.e., arrays of detector elements on each panel that are arranged in rows 
and columns. In such an array, a collimator must separate each detector 
element along both axes of the array. The radiation vectors from the point 
source to each detector on the array have different orientations, varying 
both in magnitude of the angle and direction of offset from the center of 
the array. Setting up collimator plates along two axes between each of the 
detector elements in two dimensional arrays would be extremely time 
consuming and difficult. Additionally, arrays larger than the one 
dimensional array discussed above may be advantageously used in imaging 
applications. As the length of any one panel supporting detector elements 
increases, the problem of the collimator structure shadowing large areas 
of the detector surface becomes more important. 
Accordingly, one object of the present invention is to provide a highly 
focused collimator for use in imagers having point radiation sources and 
an efficient method to readily fabricate such a collimator. 
Another object is to provide a readily-fabricated collimator for use with 
two-dimensional detector arrays in conjunction with a point radiation 
source. 
SUMMARY OF THE INVENTION 
In a radiation detecting system in which the radiation desired to be 
detected is emitted from a single point source, a collimator is provided 
which has channels that allow radiation emanating along a direct path from 
the point source to pass through to underlying radiation detectors while 
substantially all other radiation beams striking the collimator are 
absorbed. The axis of each channel has a selected orientation angle so 
that it is substantially aligned with the direct beam path between the 
radiation point source and the underlying radiation detector element. The 
sidewalls of the collimator are substantially smoothly shaped with a 
uniform slope and the channels preferably have a cross-sectional shape 
that corresponds to the shape of the adjoining detector element. The 
collimator body comprises at least one substrate made of a photosensitive 
material, the surfaces of which are coated with a radiation absorbent 
material. The radiation absorbent material is selected to absorb radiation 
of the energy level and wavelength emitted by the radiation source and 
typically comprises a material having a relatively large atomic number 
(i.e., about 72 or larger). The collimator body may be formed from two or 
more collimator substrates joined together so that the passages in each 
substrate are aligned to form channels through the assembled device that 
have the desired selected orientation angle. Such a collimator is 
advantageously used in an x-ray imager having a two-dimensional radiation 
detector array. 
A method of forming a collimator is also provided, including the steps of 
forming a mask corresponding to the pattern of radiation detector 
elements; exposing the photosensitive substrate through the mask to light 
beams passing along paths corresponding to those taken by light emitted 
from a point source, the light beams exposing the photosensitive substrate 
at respective selected orientation angles; etching the photosensitive 
material to form channels having the selected orientation angle; and 
coating the photosensitive collimator substrate with a radiation absorbent 
material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A radiation imager system 10, such as a medical computed tomography (CT) 
system, incorporating the device of the present invention is shown in 
schematic form in FIG. 1. CT system 10 comprises a radiation point source 
20 and a radiation detector 30 comprising a plurality of radiation 
detector panels 40 and a plurality of collimators 50 disposed between 
radiation source 20, typically an x-ray source, and detector panels 40. 
Each detector plate comprises a plurality of detector elements (not shown) 
that convert incident radiation into electrical signals. The detector 
elements are typically arranged in a one- or two-dimensional array. The 
radiation detector elements are coupled to a signal processing circuit 60 
and thence to an image analysis and display circuit 70. Detector panels 40 
are mounted on a curved supporting surface 80 which is positioned at a 
substantially constant radius from radiation point source 20. 
This arrangement allows an object or subject 90 to be placed at a position 
between the radiation source and and the radiation detector for 
examination. Collimators 50 are positioned over radiation detector panels 
40 to allow passage of radiation beams that emanate directly from 
radiation source 20, through exam subject 90, to radiation detector panels 
40, while absorbing substantially all other beams of radiation that strike 
the collimator. The details of steps in the fabrication, and the resulting 
structure, of collimators 50 in accordance with this invention are set out 
below. 
FIG. 2 is a cross-sectional view of a representative portion of a 
collimator substrate 110. Substrate 110 comprises photosensitive material, 
i.e., a material that will react to exposure to light in a manner similar 
to photoresist, to allow etching of a pattern in the material. Such 
photosensitive material may lose its photosensitive characteristics after 
it has been exposed to light and processed. One example of this type of 
substrate material is the Corning, Inc. product known as Fotoform.RTM. 
glass. An optically opaque mask 112 is formed by conventional methods on a 
first surface 110a of collimator substrate 110. The pattern of openings in 
mask 112 corresponds to the pattern of detector elements in each radiation 
detector panel 40 (FIG. 1). For example, mask 112 would have a pattern 
generally mimicking the arrangement, e.g., rows and columns in a 
two-dimensional array, as well as the cross-sectional shapes of detector 
elements at the interface between radiation detector panel 40 and 
collimator 50 (FIG. 1). Alternatively, mask 112 need not be on the surface 
of the collimator substrate but can be positioned with respect to the 
substrate in accordance with known photolithographic techniques to provide 
the desired exposure of the photosensitive material in substrate 110. In 
any event, the pattern of the mask is selected to expose areas of 
photosensitive collimator substrate 110 of sufficient size and orientation 
so that, upon completion of the fabrication of collimator 50, the surface 
of each radiation detector element for receiving the radiation is exposed 
to radiation passing along the desired paths from the radiation source. 
In accordance with the present invention, collimator substrate 110 and mask 
112 are exposed to light from light source 114. Light source 114 is 
preferably a laser, an ultraviolet light source, or the like, and is 
positioned with respect to collimator substrate 110 so that light beams 
pass through the openings in mask 112 and strike collimator substrate 110 
along paths corresponding to direct paths between radiation point source 
20 and radiation detector 30 (FIG. 1). As illustrated in FIG. 2, exemplar 
pairs of light beams 116a-b, 116c-d, and 116e-f define the boundaries of 
exposed photosensitive material shown in cross section. The light beams 
exposing the photosensitive material under each respective opening in mask 
112 strike the collimator substrate at slightly different angles, the 
magnitude and orientation of which depend on the position along the length 
of the collimator substrate where the light strikes. For example, light 
beams 116a and 116b strike the collimator substrate at angles which differ 
in magnitude and orientation (i.e. left or right with respect to a 
perpendicular between the substrate and the light source) from light beams 
116c-d and 116e-f. The light beams falling on photosensitive collimator 
substrate 110 define a plurality of respective exposed volumes 118 in the 
photosensitive material under each opening in the mask through which the 
light beams pass. Each exposed volume 118 has a longitudinal axis at a 
selected orientation angle corresponding to the angle at which the light 
beams emanating along a direct path from light source 114 strike the 
collimator substrate. Thus light beams 116a-b expose a volume that has a 
selected orientation angle .beta., whereas light beams 116e-f expose a 
volume having a different selected orientation angle, .differential.. The 
position of the collimator substrate with respect to light source 114 is 
selected to correspond with the distance that the collimator substrate 
will be from the radiation source in the assembled imager. Further, to 
ensure that the exposed volumes have the correct selected orientation 
angles required for collimating radiation in the assembled device, the 
plane of the collimator substrate is oriented at a "planar angle" so that 
the plane of the substrate has the same orientation with respect to the 
light source as the radiation detector panel with respect to the radiation 
source in the assembled device. 
Collimator substrate 110 is then etched using conventional techniques 
appropriate for the photosensitive material used in the substrate to 
remove the exposed volumes 118 of photosensitive material and create a 
pluraltiy of channels or passages 120 through the substrate, as 
illustrated in FIG. 3. Each of these channels has a longitudinal axis 122 
aligned with the selected orientation angle defined when the 
photosensitive material was exposed to light source 114 (FIG. 2). 
Typically the selected orientation angles of the longitudinal axes of the 
channels range between about 0.degree. and 10.degree.. Each channel has a 
channel sidewall 124 which is substantially smooth along its length and 
has a substantially uniform slope formed when the photosensitive material 
exposed by the light beams in the previous step is removed in the etching 
process. The slope of the sidewalls is typically substantially aligned 
with the selected orientation angle of the channel defined by those 
sidewalls. The remaining portions of mask 112 may next removed to prepare 
the collimator substrate for the next step in the process of forming the 
collimator. 
A radiation absorbent material layer 130 (FIG. 3) is then applied on 
collimator substrate 110 so as to cover at least the surfaces of the 
substrate which will be exposed to incident radiation when assembled in an 
imager device. The radiation absorbent material at least covers all of the 
sidewalls defining the channel. The cross-sectional portion of the 
radiation absorbent material on the sidewalls and the top and bottom of 
substrate 110 is illustrated in FIG. 3 in cross-hatch, while the radiation 
absorbent material on the "back" sidewall of the channel is illustrated in 
alternating cross-hatch and dashed lines. The radiation absorbent material 
can be applied through known techniques, such as vapor deposition 
techniques. Radiation absorbent material 130 is selected to absorb 
radiation of the wavelength distribution emitted by radiation source 20 
(FIG. 1) in the imager device. The radiation absorbent material typically 
has a relatively high atomic number, e.g., greater than about 72, and 
advantageously comprises tungsten, lead, or gold when the radiation used 
in the imager device is x-ray. The thickness of the radiation absorbent 
material layer is selected to provide efficient absorption of the incident 
radiation and depends on the type of incident radiation and the energy 
level of the radiation when it strikes the collimator. For example, in a 
typical CT system using an x-ray point radiation source of about 100 KeV 
positioned approximately one meter from the detector array, a total 
thickness in the range of about 30 to 40 mils of tungsten in one or more 
layers disposed along the path of the radiation will substantially absorb 
the x-rays emitted by the source. After application of the radiation 
absorbent material, the cross-sectional area of the opening or void space 
in the channel is substantially the same as the area for receiving 
radiation on the detector element which it adjoins so as to allow 
substantially all radiation rays emanating along direct paths from the 
radiation source to strike the detector element. 
Collimator 50 of FIG. 1, shown in an enlarged and simplified view in FIG. 
4, comprises a collimator body 55 including at least one substrate 110 
coated with radiation absorbent material 130. Collimator body 55 may 
comprise a plurality of substrates joined together as illustrated in FIG. 
4. When two or more substrates are joined together to form the collimator 
body, the openings of the channels in the respective surfaces of the 
collimator substrates are aligned to form continuous channels through the 
collimator body The channel sidewalls are advantageously aligned so that 
the sidewalls of the respective channels in the adjoining substrates are 
contiguous. Dependent on the energy level and wavelength of the radiation 
to be collimated, different thicknesses of collimator bodies may be 
required. Once the necessary thickness has been determined, an appropriate 
thickness of collimator substrate, or plurality of substrates, can be 
selected and fabricated in accordance with this invention. For example, 
the thickness of a collimator for an imager system using x-rays, such as a 
CT system, may be only about 8 mm, but for an imager using gamma rays, the 
collimator preferably would be three to five times thicker than that used 
for x-ray radiation. 
In the assembled device, collimator body 55 is disposed to adjoin radiation 
detector panel 40, as illustrated in FIG. 4. Radiation detector elements 
42 are positioned along detector panel 40 and typically comprise a 
scintillator coupled to a photodetector. Collimator body 55 is positioned 
to allow incident radiation on a direct path between the radiation source 
and one of the radiation detector elements 42 to pass through the channels 
in the collimator. Beams of radiation that are not aligned with such a 
direct path strike the collimator body and are absorbed. 
The collimator of the present invention is readily used with either a 
one-dimensional or a two-dimensional array of radiation detector elements. 
A plan view of a collimator fabricated in accordance with the present 
invention and showing a representative number of channels 120 appears in 
FIG. 5. The figure has been marked to show left, right, upper and lower 
edges solely to provide a reference for ease of discussion, and the 
selection and positioning of such references is not meant to constitute 
any limitation on the structure or positioning of the device of the 
invention. Openings 122 of channels 120 on the opposite surface of 
collimator body 55 are shown in phantom. In the two-dimensional array the 
center channel is in substantial vertical alignment with the radiation 
source, and the opening 122 of the channel on the side of the collimator 
body opposite the radiation source is aligned with the opening in the 
surface closest to the radiation source. As the radiation beams spread out 
as they emanate from the point source, each of openings 122 has a slightly 
larger cross-sectional area than the respective opening of the channel 120 
in the surface of the collimator closest to the radiation source. Openings 
122 for channels on the left, right, top, or bottom are slightly offset 
from being in vertical alignment with their respective openings in the 
upper surface of the substrate. The direct path from the radiation source 
to a radiation detector in the upper left hand corner, for example, is 
offset both to the left and the upper side of the array. The selected 
orientation angle of the axis of the channel is substantially aligned with 
this direct path, and the channel thus extends through the collimator body 
at this angle. The selected orientation angle for each channel is 
different from any other channel in the collimator. Such a structure, 
which would be extremely difficult and time consuming to construct with 
conventional collimator fabrication techniques, is readily produced in 
accordance with this invention. 
While only certain features of the invention have been illustrated and 
described herein, many modifications and changes will occur to those 
skilled in the art. It is, therefore, to be understood that the appended 
claims are intended to cover all such modifications and changes as fall 
within the true spirit of the invention.