U.S. patent application number 10/687685 was filed with the patent office on 2005-04-21 for collimator fabrication.
This patent application is currently assigned to JMP Industries, Inc., an Ohio corporation. Invention is credited to Pinchot, James M..
Application Number | 20050084072 10/687685 |
Document ID | / |
Family ID | 34521025 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050084072 |
Kind Code |
A1 |
Pinchot, James M. |
April 21, 2005 |
Collimator fabrication
Abstract
A collimator that formed from a plurality of metal foil layers
that are shaped by use of lithographic techniques in specific
shapes. The formed metal foil layers are stacked and aligned
together and then connected together to form the collimator.
Inventors: |
Pinchot, James M.; (Parma,
OH) |
Correspondence
Address: |
Fay, Sharpe, Fagan,
Minnich & McKee, LLP
7th Floor
1100 Superior Avenue
Cleveland
OH
44114-2579
US
|
Assignee: |
JMP Industries, Inc., an Ohio
corporation
|
Family ID: |
34521025 |
Appl. No.: |
10/687685 |
Filed: |
October 17, 2003 |
Current U.S.
Class: |
378/154 |
Current CPC
Class: |
G01N 23/201 20130101;
Y02P 10/25 20151101; G21K 1/00 20130101; B22F 2998/00 20130101;
G21K 2201/00 20130101; G21K 1/025 20130101; G21F 5/00 20130101;
G21K 1/02 20130101; B22F 2998/00 20130101; B22F 10/10 20210101;
B22F 2998/00 20130101; B22F 10/10 20210101 |
Class at
Publication: |
378/154 |
International
Class: |
G21K 001/00 |
Claims
I claim:
1. A method of manufacturing a collimator comprising: providing a
plurality of metal foil layers; forming a plurality of metal foil
layers into specific shapes by use of at least one lithographic
technique; stacking and aligning said plurality of formed metal
layers; connecting together said plurality of formed metal layers
to form said collimator.
2. The method as defined in claim 1, wherein a plurality of said
metal foil layers each have an average density of at least about
8.5 g/cm.sup.3.
3. The method as defined in claim 1, wherein a plurality of said
metal foil layers each have average thickness of less than about
400 microns.
4. The method as defined in claim 1, wherein said at least one
lithographic technique includes photo-etching.
5. The method as defined in claim 1, wherein said step of forming
includes the formation of at least one alignment opening in at
least one metal foil layer.
6. The method as defined in claim 5, wherein said step of stacking
and aligning includes the use of at least one alignment opening
formed in a plurality of metal foil layers.
7. The method as defined in claim 1, wherein said step of
connecting together includes brazing together a plurality of metal
foil layers.
8. The method as defined in claim 7, including the step of coating
at least one side of a plurality of metal foil layers with a
brazing metal.
9. The method as defined in claim 7, wherein said brazing metal has
an average density of at least about 8.5 g/cm.sup.3.
10. The method as defined in claim 7, wherein said brazing metal
has an average coating thickness of less than about 10 microns.
11. The method as defined in claim 7, wherein said step of brazing
includes vacuum brazing.
12. The method as defined in claim 1, including the step of
generating a computer image of a plurality of said formed metal
foil layers.
13. The method as defined in claim 1, including the step of
generating a computer image of said collimator and then sectioning
said computer image of said collimator into a plurality of
sectional images that correspond to a plurality of said formed
metal foil layers.
14. The method as defined in claim 12, including the step of
forming at least one mask from at least one of said computer images
and at least partially forming at least one of said formed metal
foil layers using said mask.
15. The method as defined in claim 13, including the step of
forming at least one mask from at least one of said sectional
images and at least partially forming at least one of said formed
metal foil layers using said mask.
16. A collimator formed of a plurality of metal layers, each of
said metal layers connected together by a brazing metal having a
different composition than said metal of said metal layers.
17. The collimator as defined in claim 16, wherein a plurality of
said metal layers each have an average density of at least about
8.5 g/cm.sup.3.
18. The collimator as defined in claim 16, wherein a plurality of
said metal layers each have average thickness of less than about
400 microns.
19. The collimator as defined in claim 16, wherein said brazing
metal has an average density of at least about 8.5 g/cm.sup.3.
20. The collimator as defined in claim 16, wherein said brazing
metal has an average coating thickness of less than about 10
microns.
Description
[0001] This invention relates in general to grid-like structures of
the type suitable for use as collimators. In particular, the
invention relates to a method and an apparatus for forming
collimator strips which can be assembled to form a collimator that
can be used in imaging, diagnosing and/or treatment apparatuses
that take images and/or effect treatment by use of gamma rays,
electron beams, photon (X-ray) beams, or similar penetrating
rays.
BACKGROUND OF INVENTION
[0002] Radiation emitting devices are generally known and used as
imaging and as radiation therapy devices for the treatment of
patients.
[0003] 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.
[0004] Particularly in radiation imagers used for medical
diagnostic analyses 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 contrast resolution that result from the detection of
the scattered or secondary radiation rays. Examples of imagers and
collimators for such imagers are disclosed in U.S. Pat. Nos.
6,556,657; 6,507,642; 6,505,966; 6,396,902; 6,388,816; 6,377,661;
and 6,271,524, all of which are incorporated herein by
reference.
[0005] Collimators are positioned to substantially absorb the
undesired radiation before it reaches the detector. Collimators are
traditionally made of a material that has a relatively high atomic
number, such as tungsten, 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.
[0006] Some radiation imaging systems, such as computed tomography
(CT) systems used in medical diagnostic work, or such as industrial
imaging devices, use a point (i.e. a relatively small, such as 1 mm
in diameter or smaller) source of x-ray radiation to illuminate 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 one-dimensional array of
detector elements. Each detector element is disposed on a module,
and the modules are typically arranged end to end along a curved
surface to form a radiation detector arm. The distance to the
center of the module, on any one of the separate modules is the
same, i.e., each panel is at substantially the same radius from the
radiation source. On any given module there is a difference from
one end of the module to the other in the angle of incidence of the
radiation beams arriving from the point source.
[0007] For example, in a common medical CT device, the detector is
made up of a number of x-ray detector modules, 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 detector module 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 toward
the ends of the detector module. 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 non-uniformity
in the x-ray intensity and spectral distribution across the
detector module. In the one-dimensional 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.
[0008] Advanced CT technology (e.g., volumetric CT), however, makes
use of two-dimensional arrays, i.e., arrays of detector elements
that are arranged in rows and columns. The same is true of the
precision required for industrial imagers. 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. Additionally, detector 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 become more
important. In any system using a "point source" of radiation and
flat panels, 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.
[0009] Gamma ray imaging is currently used in medicine to obtain 3D
images of patients' internal organs. One such gamma ray imaging
device is disclosed in U.S. Pat. No. 6,271,524, which is
incorporated herein by reference. Positron Emission Tomography
(PET) is a medical gamma ray imaging technique frequently used for
this purpose. Prior to conducting the imaging procedure, a patient
is given a radio-pharmaceutical, which contains a positron emitting
substance and which is selectively accumulated in a region of
interest. When a positron emitted by the radio-pharmaceutical
encounters an electron, the electron-positron pair annihilates,
emitting two gamma photons of 511 keV each, flying in opposite
directions. The simultaneous detection of these gamma photons by
two gamma detectors positioned opposite to each other, indicates
that a positron has been emitted and annihilated inside an organ of
a patient. The simultaneous attribution of 2D coordinates to each
one of the photons allows for the determination of the photon's
line of flight. The position of the annihilation is along this
line. When a multitude of gamma photon pairs are detected and the
information is processed using appropriate algorithms, electronic
circuitry, software, etc., a 3D image of the organ under
examination can be reconstructed.
[0010] In radiation therapy, the device generally includes a gantry
which can be swivelled around a horizontal axis of rotation in the
course of a therapeutic treatment. Two such devices are disclosed
in U.S. Pat. Nos. 6,526,123 6,240,161, both of which are
incorporated herein by reference. A linear accelerator is located
in the gantry for generating a high energy radiation beam for
therapy. This high energy radiation beam can be an electron beam or
photon (X-ray) beam. During treatment, this radiation beam is
trained on one zone of a patient lying in the isocenter of the
gantry rotation. To control the radiation emitted toward an object,
a beam shielding device, such as a plate arrangement or a
collimator, is typically provided in the trajectory of the
radiation beam between the radiation source and the object.
[0011] A collimator is a beam shielding device which can include
multiple leaves, for example, a plurality of relatively thin plates
or rods, typically arranged as opposing leaf pairs. The plates
themselves are formed of a relatively dense and radiation
impervious material and are generally independently positionable to
delimit the radiation beam. The beam shielding device defines a
field on the object to which a prescribed amount of radiation is to
be delivered. The usual treatment field shape results in a
three-dimensional treatment volume which includes segments of
normal tissue, thereby limiting the dose that can be given to the
tumor. The dose delivered to the tumor can be increased if the
amount of normal tissue being irradiated is decreased and the dose
delivered to the normal tissue is decreased. Avoidance of delivery
of radiation to the organs surrounding and overlying the tumor
determines the dosage that can be delivered to the tumor. Once an
analysis is completed as to the intensity level of radiation at a
particular region on the body, the beam shielding device settings
must be chosen according to the output number of fields. Often, the
application of a particular sequence of radiation requires a
prohibitive amount of time to deliver, or which is physically
impossible for the beam shielding device to achieve. As a result,
to provide a realizable dosage, fewer intensity levels of radiation
must be provided, and/or fewer radiation fields are used, thus the
dose volume histograms are thereby degraded. While methods are
known to address deliver dosage demands according to the intensity
maps (See U.S. Pat. No. 5,663,999), such systems still cause a
degradation of the dose volume histogram.
[0012] Various methods have been used to manufacture thicker
collimators. One method is to cast the collimator. Several methods
of casting are disclosed in U.S. Pat. No. 3,988,589, which is
incorporated herein by reference. One casting method is to cast the
collimator as a single unit using removable pins in the mold to
provide holes in the collimator. This method of manufacture, while
producing an operational collimator, is impractical since, due to
high friction between the cast lead and the pins and the fact that
some collimators are convergent or divergent (to allow enlarged or
miniaturized image formation) relative to the radiation source,
each of the pins used to create the holes must be removed
individually. This process is time consuming and costly, especially
when one realizes that some such collimators have 1000 or more such
holes. Another casting method is to cast thick corrugated lead
sheets and assemble them. This alternative also is unsatisfactory
due to joint leakage (i.e. the epoxied joints are permeable to high
energy radiation) and to too much distorting radiation reaching the
receiver of the medical device. Still another casting method is to
cast a plurality of modules that are press fitted or cemented
together to form the collimator.
[0013] Several other methods for forming collimators are disclosed
in U.S. Pat. No. 4,450,706, which is incorporated herein by
reference. One method includes the dissolving metal by a chemical
reagent to form a specific collimator shape. Another method
includes wrapping radiation-absorbing foils around a large number
of mandrels. Another method involves the formation of a plurality
of collimator strips which are folded transversely to their
longitudinal extension such that the flat portions of two adjacent
strips engage each other, whereby the outwardly extending portions
of these two adjacent strips extend in opposite directions to form
a series of parallel channels. Still another method involves the
use of strips that have been stamped into a shape and subsequently
bonded together.
[0014] The casting methods described above for manufacturing a
collimator can only be used to fabricate relatively simple
collimators having high error tolerances in design. As technology
has advanced, a need for more complex collimators has arisen
wherein such collimators have very low error tolerances. One
manufacturing method to address this problem is disclosed in U.S.
Pat. No. 6,377,661, which is incorporated herein by reference. This
patent discloses a collimator manufacturing process which includes
the steps of generating a computer-aided-drawing (AutoCAD) drawing
of a two-dimensional (2D) collimator based upon overall imager
system parameters, generating a stereo-lithographic (STL) file or
files corresponding to the AutoCAD drawing and to the chosen size,
position and orientation of the focally aligned channels to be
formed in the collimator, and interfacing the STL files with
machining equipment to machine out the material to be removed from
a solid slab (workpiece) of radiation-absorbing material, to form
the plurality of focally aligned channels extending through the
workpiece.
[0015] Another method for manufacturing a collimator is disclosed
in United States Patent Publication No. 2003/0128813 published on
Jul. 10, 2003 entitled "Devices, methods, and systems involving
cast computed tomography collimators" and 2003/0128812 published on
Jul. 10, 2003 entitled "Devices, methods, and systems involving
cast collimators", both of which are incorporated herein by
reference. In this patent publication, a cast computed-tomography
collimator is formed from a lithographically-derived micro-machined
metallic foil stack lamination mold. The mold has a stacked
plurality of micro-machined metallic foil layers. The mold is
filled with a first casting material to form a collimator.
[0016] Although these casting techniques have improved the quality
of collimator production, the casting process still cannot meet
certain tolerances that are needed for highly sensitive medical
devices. In view of the prior art, there is a need for a
manufacturing process for a collimator that is cost effective, not
overly time consuming to manufacture, and which can produce a very
precise collimator in a variety of shapes and sizes.
SUMMARY OF THE INVENTION
[0017] The present invention pertains to a method for manufacturing
a collimator for use in medical devices and will be described with
particular reference thereto; however, the invention has much
broader applications and can be used to form a collimator for
applications in devices other than medical devices. In additional,
the invention can be expanded beyond collimators and can be used to
form a variety of metallic and non-metallic materials that require
very low error tolerances. The novel method of manufacturing the
collimator includes 1) generating a computer image of the
collimator, 2) sectioning the computer generated image, 3) forming
sections of the collimator from a metal material based on each of
the drawing sections, and 4) connecting the individual sections to
form a collimator that substantially matches the computer generated
drawing of the collimator. By using this novel manufacturing
technique, collimators having very precise dimensions can be
manufactured having very low error tolerances.
[0018] In one aspect of the invention, the computer drawing of the
collimator can be generated by commercially available or
proprietary software. One common commercial software package is
AutoCAD. Many other software packages can be used. The computer
drawing is at least a two dimensional drawing and typically a three
dimensional drawing of the collimator. Once the computer generated
drawing matches the shape of the collimator, the drawing is then
sectioned to emulate layers of the collimator. Typically, the
layers are divided or sectioned along the longitudinal axis or
vertical axis of the collimator; however, layers of the collimator
can be divided along other axes of the collimator. The divided or
sectioned layers typically have the same thickness, however, this
is not required. The computer generated images for the collimator
can be saved, used in other processes (e.g., lithography process,
etc.) or the like.
[0019] In still another and/or alternative embodiment of the
invention, one or more sections of the collimator are formed from a
metal material that matches low error tolerances. Various
techniques can be used to produce the one or more sections of the
collimator. In one embodiment of the invention, lithography is used
to at least partially form one or more sections of the collimator.
When using a lithography process, a photo-sensitive resist material
coating is applied to one or more of the surfaces (i.e., either of
the relatively large planar "top" or "bottom" surfaces) of a blank
of metal material (e.g. metal foil, etc.). After the blank has been
provided with the photo-resist material coating, "mask tools" or
"negatives" or "negative masks", containing a positive or negative
image of the desired section of the collimator are etched in the
blank. The mask tools can be made from glass or other materials,
which has a relatively low thermal expansion coefficient and
transmits radiation such as ultraviolet light. The blank is then
exposed to radiation, typically in the form of ultraviolet light,
to expose the photo-resist coatings to the radiation. The masks are
then removed and a developer solution is applied to the surfaces of
the blank to develop the exposed (sensitized) photo-resist
material. Once the photo-resist is developed, the blanks are etched
or micro-machined. Once etching or machining is complete, the
remaining unsensitized photo-resist material can be removed such as
by, but not limited to, a chemical stripping solution. When using
lithography as a basis for layer fabrication of the collimator
sections, parts and/or features can be designed as diameters,
squares, rectangles, hexagons, and/or any other shape and/or
combination of shapes. The combinations of any number of shapes can
result in non-redundant design arrays (i.e. patterns in which not
all shapes, sizes, and/or spacings are identical). Lithographic
features can represent solid or through aspects of the final
collimator. Such feature designs can be useful for fabricating
micro-structures, surfaces, and/or any other structure that can
employ a redundant and/or non-redundant design for certain
micro-structural aspects. Large area, dense arrays can be produced
through the lithographic process, thereby enabling creation of
devices with sub-features or the production of multiple devices in
a batch format. Lithography can also allow the creation of very
accurate feature tolerances since those features can be derived
from a potentially high-resolution photographic mask. The tolerance
accuracy can include line-width resolution and/or positional
accuracy of the plotted features over the desired area.
Photographic masks can assist with achieving high accuracy when
chemical or ion-etched, or deposition-processed layers are being
used to form a collimator from the stack of sections. Because
dimensional changes can occur during the final formation of the
collimator, compensation factors can be engineered at the
photo-mask stage, which can be transferred into the fabrication of
the collimator. For instance, when the full collimator or a portion
of the collimator needs to be angled for radial designs or other
designs, the photo-mask typically needs to be applied to both sides
of the metal foil layer with a slight offset to allow for the
angle. This offset will eliminate a stack-up look even though the
steps will be very thin. When the brazing material is coated on
both sides of every other metal foil layer, the etching solution
typically performs a better job to form a better angled stack. In
another and/or alterative embodiment, fabricating the sections of
the collimator can be formed by one or more micro-machining
techniques. Some of the micromachining techniques that can be used
include, but are not limited to, photo-etching, laser machining,
reactive ion etching, electroplating, vapor deposition, bulk
micro-machining, surface micro-machining, and/or conventional
machining. Ion etching techniques can form sections of the
collimator that have tolerances of less than about 1.25 microns.
Photochemical-machining techniques can etched a section of the
collimator to tolerances of less than about 2.5 microns or about
10% of the metal thickness. Laser micromachining techniques can
produce sections of the collimator to a tolerance of less than
about 0.3 micron. Electro-forming techniques can produce sections
of the collimator to a tolerance of less than about 0.1 micron.
[0020] In yet another and/or alternative embodiment of the
invention, one or more sections of the collimator are connected
together by a lamination process. Once the multiple sections of the
collimator are formed in the metal material, the sections are
placed together to define the desired collimator. The total number
(and thickness) of the collimator sections define the overall
height and aspect ratio of the collimator. In one embodiment, a
metal-to-metal brazing technique is used to connect together one or
more sections of the collimator. Prior to the assembly of the
collimator, one or more sections of the collimator can have one or
both surfaces coated with a thin metal layer. In one non-limiting
example, the metal foil layers are coated on one side of each foil
layer. In another non-limiting example, the both sides of "every
other" metal foil layer are coated with the brazing metal. Such
coating techniques can include, but are not limited to, thermal
spraying and electroplating. Generally the thickness of the metal
coating is less than about 10 microns and typically about 0.1-10
microns, and more typically about 0.5-4 microns. The coated metal
should have a relatively high density (e.g. 8.5 g/cm.sup.3 or
greater) and a melting temperature that is less than the metal used
to form the sections of the collimator. Typically the average
density of the coating metal is at least about 8.8 g/cm.sup.3 and
has an average metaling point that is at least about 100.degree. C.
less than the average melting point of the metal used to form the
sections of the collimator, and typically is at least about
500.degree. C. less than the average melting point of the metal
used to form the sections of the collimator. Examples of coating
metal materials include, but are not limited to, copper, gold,
lead, nickel, platinum and silver. As can be appreciated, alloys of
these metals and/or other high density metals can be used. During
the brazing process, the sectioned assembly can be heated in an
inert atmosphere to an elevated temperature to cause the metal
coating to flow. The heating of the brazing metal can be achieved
by use of induction heating, radiant heating, lasers, furnaces,
ovens, etc. Typically the brazing temperature is at least about
10.degree. C. higher than the average melting point of the brazing
metal and at least 100.degree. C. less than the average melting
point of the metal foil. The atmosphere about the collimator
sections can be held under vacuum to result in a vacuum brazing
process. The atmosphere is typically an inert atmosphere. Gas
atmospheres that include hydrogen, nitrogen or noble gases can be
used. The time of brazing is typically about 0.1-4 hours. The
elevated temperature during brazing causes the brazing metal to
flow between the metal foil layers. The brazing procedure is
completed by cooling the layered collimator. The atmosphere during
cooling is typically inert. The cooling times are typically 0.1-5
hours. As the temperatures elevate, the sections of the collimator
can expand. Various types of alignment structures (e.g., pins,
etc.) can be used to maintain the sections of the collimator in the
proper position during the heating process. In one non-limiting
embodiment, construction holes or slots are formed in each foil
layer which are used to align the foil layers. The construction
holes or slots can be sized and shaped to account for expansion
and/or contraction of the foil layers when exposed to heat.
Typically, each foil layer includes a plurality of construction
holes or slots to facilitate in the proper orientation of the layer
layers when forming the collimator. The pins can be made of the
same or similar expanding and contracting material as the foil
layers so that the pins expand and contact at the same rate as the
foil layers when exposed to heating and cooling. As such, the
brazing fixtures (e.g., pins) typically are made of a material that
has a coefficient of linear expansion close to that of the metal
leaves so that the fixtures grow in the furnace at substantially
the same rate as the collimator assembly grows and shrinks at
substantially the same rate when the collimator is cooled.
Alternatively, the pins can be formed of carbon material (e.g.
graphite) or other type of material that has little or no expansion
during heating and cooling. The carbon material has a very low
expansion rate and can take the heat during the brazing process.
The difference in expansion rates using carbon pins can be easily
incorporated in the design of the slots in the metal foil layers.
In addition, the carbon pins are less apt to "stick" to any brazing
material that may seep from the stacked metal foil layers thus
improving the quality of the final formed product. The layers of
metal foil can also be clamped together or otherwise placed under
pressure to limit movement of the foil layers during the brazing
process. In addition to using alignment structures, positional
errors of the collimator sections (stacking errors) and tolerances
can be controlled by the photographic masks used to produce the
layers. The geometric size and tolerance of the sections can be
partially controlled by the layer thickness and/or micromachining
methods used to produce the sections. When producing a laminated
collimator, numerous factors can be an influence on the overall
tolerances of the sections of the collimator. For example, when
using a stacking fixture, the flatness of the laminating surface of
the collimator sections and the perpendicularity of the sides of
the collimator sections can be controlled. In addition, the
dimensional tolerance of the alignment features of a collimator
section and/or the positional tolerance of a collimator section can
be an influence. In another and/or alterative embodiment of the
invention, one or more layers of metal foil can be laminated
together by use of an adhesive. Such adhesives can include, but are
not limited to, thermo-cured epoxy, non-thermo-cured epoxy,
silicone rubber products, urethanes, etc. When using lamination
techniques other than brazing, the layers of the collimator are
typically clamped together or otherwise placed under pressure until
the adhesive has at least partially dried and/or cured.
[0021] In still yet another and/or alternative embodiment of the
invention, the metal sections of the collimator are formed from
high density metal foil. The metal foil can be made of a single
metal or be a metal alloy. The average density of the metal forming
the metal foil is greater than about 8.5 g/cm.sup.3, and typically
greater than about 9 g/cm.sup.3. In addition, the average melting
point of the metal forming the metal foil is generally greater than
about 1000.degree. C., and typically greater than about
1500.degree. C. The metal forming the metal foil is also
non-radioactive or substantially non-radioactive (i.e. stable).
Non-limiting examples of the metals that can be used individually
or in combination with other metals to form the metal foil include
bismuth, cadmium, cobalt, copper, erbium, gold, hafnium, iridium,
lead, nickel, niobium, osmium, palladium, platinum, rhenium,
rhodium, ruthenium, silver, tantalum, technetium, terbium,
thallium, thulium and/or tungsten. The metal foil is selected to
have a thin thickness. The thin thickness facilitates in the ease
of processing the metal foil during the lithography process and
also results in a higher quality final product. Generally the foil
thickness is about 10-400 microns, and more typically about 40-150
microns.
[0022] A primary object of the present invention is a manufacturing
process for a collimator that forms the collimator with high
precision.
[0023] Another and/or alternative object of the present invention
is a manufacturing process for a collimator that includes the use
of computer generated images and lithographic techniques to
manufacture a manufacturing process for a collimator.
[0024] Still another and/or alternative object of the present
invention is a manufacturing process for a collimator that includes
the connecting of thin layers of dense metal to form the
collimator.
[0025] Yet another and/or alternative object of the present
invention is a manufacturing process for a collimator that includes
vacuum brazing to connect together one or more layers of a
collimator.
[0026] Still yet another and/or alternative object of the present
invention is a manufacturing process for a collimator that includes
a lithographic technique to form distinct shapes in a metal foil
that is representative of a section of the collimator.
[0027] A further and/or alternative object of the present invention
is a manufacturing process for a collimator that utilizes guide
structures and holes or slots to properly align the foil layers to
facilitate in the proper formation of the collimator.
[0028] Still a further and/or alternative object of the present
invention is a manufacturing process for a collimator that includes
coating one or more sides of a metal foil with a thin metal layer
for use in brazing one or more metal foil layers together to form a
collimator.
[0029] Yet a further and/or alternative object of the present
invention is a manufacturing process for a collimator that can form
a collimator having a planar shape, a curvilinear shape or any
other desired simple or complex shape.
[0030] Still yet a further and/or alternative object of the present
invention is a manufacturing process for a collimator that can form
a collimator having a simple or complex face surface.
[0031] These and other objects and advantages will become apparent
from the discussion of the distinction between the invention and
the prior art and when considering the preferred embodiment as
shown in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The foregoing objects, and others, will in part be obvious
and in part pointed out more fully hereinafter in conjunction with
the written description of preferred embodiments of the invention
illustrated in the accompanying drawings in which:
[0033] FIG. 1 is a general block diagram of a prior art gamma ray
imaging device that can be used with the collimator of the present
invention;
[0034] FIG. 2 is a general diagram of a radiation treatment device
and treatment console that can be used with the collimator of the
present invention;
[0035] FIG. 3 is a flowchart of an exemplary embodiment of a method
of the present invention;
[0036] FIG. 4 is a side elevation view of a collimator made by the
process illustrated in FIG. 4;
[0037] FIG. 5A is a exemplary collimator that can be formed by the
method of the present invention; and,
[0038] FIG. 5B is another exemplary collimator that can be formed
by the method of the present invention.
DETAILED DESCRIPTION OF ONE PREFERRED EMBODIMENT
[0039] Referring now in greater detail to the drawings, wherein the
showings are for the purpose of illustrating preferred embodiments
of the invention only and not for the purpose of limiting the
invention, FIG. 1 shows a block diagram of a prior art gamma ray
detector system used for diagnostic purposes. A pair of gamma
detectors 10, each optically coupled to a scintillation crystal 12,
are disposed parallel to each other. Detector pair 10 is mounted on
a gantry that can rotate about a patient P resting on a table 20.
Additionally, either detector pair 10 or patient P can be
transversely displaced in the direction perpendicular to the plane
of the figure. This configuration allows for total body scanning
and/or static imaging, both well-known techniques in NM coincidence
measurements. System hardware and software, schematically described
in FIG. 1 by blocks 30, 40, 50 and 60, allows for coincidence
measurements in accordance with technology well known in the
art.
[0040] Thus, no further details on system operation will be given
in the description of preferred embodiments in accordance with the
present invention, except for distinctive features of the
invention. This hardware generally includes an energy discriminator
that rejects events having a low energy. Such events are presumed
to be caused by scatter.
[0041] Prior to an imaging procedure, patient P is given a
radiopharmaceutical, which contains a positron emitting substance
and which is selectively accumulated in a region of interest. When
a positron emitted by the radiopharmaceutical encounters an
electron, the electron-positron pair annihilates, emitting two
gamma photons of 511 keV each, flying in opposite directions. The
simultaneous detection of these two 511 keV gamma photons by the
two gamma detectors 10 positioned opposite to each other, indicates
that a positron has been emitted and annihilated inside an organ of
a patient P. The simultaneous attribution of 2D coordinates to each
one of the photons allows for the determination of the photon's
line of flight. The position of the annihilation is along this
line. When a multitude of gamma photon pairs are detected and the
information processed using appropriate algorithms, electronic
circuitry, software, etc., a 3D image of the organ under
examination is reconstructed. A collimator is used to detect gamma
photons along a particular path. The detected gamma photos are then
used to image a particular portion of the patient's body for
diagnostic purposes. It is desirable, in PET, to improve the
efficiency of gamma detectors by reducing the number of stray
photons detected relative to the number of non-stray photons
detected and to improve the depth discrimination in coincidence
measurements. It is also desirable to perform attenuation and
coincidence measurements in sequence without moving or replacing
parts of the imaging system and, in attenuation measurements, to
reduce radioactivity losses due to line source diameter while using
a large diameter source to improve statistics by increasing the
total radiation while keeping the source strictly collimated. To
achieve these results, collimators having specific designs are
used.
[0042] Referring now to FIG. 2, there is illustrated a prior art
radiation treatment apparatus 100. The radiation treatment
apparatus 100 includes a beam shielding device (not shown) within a
treatment head 110, a control unit in a housing 120 and a treatment
unit 130. The radiation treatment device 110 includes a gantry 140
which can be swivelled around a horizontal axis of rotation A in
the course of a therapeutic treatment. The treatment head 110 is
fastened to projection of the gantry 140. A linear accelerator is
located in the gantry 140 to generate the high powered radiation
required for the therapy. The axis of the radiation bundle emitted
from the linear accelerator and the gantry 140 is designated by R.
Electron, photon or any other detectable radiation can be used for
the therapy. During the treatment, the radiation beam is trained on
a zone Z on a patient P who is to be treated and who lies at the
isocenter of the gantry rotation. The rotational axis A of the
gantry 110, the rotational axis T of a treatment table 150, and the
beam axis R intersect in the isocenter. The plates or leaves of the
beam shielding device within the treatment head 110 are
substantially impervious to the emitted radiation. The collimator
is mounted between the radiation source and the patient in order to
delimit the field. Areas of the body, for example, healthy tissue,
are therefore subject to as little radiation as possible and
preferably to none at all. The collimator can be a single piece or
be made of multiple pieces that are movable such that the
distribution of radiation over the field need not be uniform (one
region can be given a higher dose than another). The gantry can be
rotated so as to allow different beam angles and radiation
distributions without having to move the patient. The central
treatment processing or control unit 130 is typically located apart
from the radiation treatment device 100. The treatment unit 130
includes output devices such as at least one visual display unit or
monitor 160 and an input device such as a keyboard 170. Data can be
input also through data carriers such as data storage devices or a
verification and recording or automatic setup system. The treatment
processing unit 180 is typically operated by the therapist who
administers actual delivery of radiation treatment as prescribed by
an oncologist by using the keyboard 170 or other input device. The
therapist enters into the control unit of the treatment unit 130
the data that defines the radiation dose to be delivered to the
patient, for example, according to the prescription of the
oncologist. The program can also be input via another input device,
such as a data storage device. Various data can be displayed before
and during the treatment on the screen of the monitor 160. Similar
to the gamma ray imager described in FIG. 1, the desired results
from the radiation treatment apparatus is in part accomplished by
the use of a collimator having a specific design.
[0043] FIGS. 1 and 2 merely are two examples of medical
applications that utilize collimators. Many other medical devices
incorporate the use of collimators. Collimators in these other
types of medical devices can be formed by the method of the present
invention. As medical technology has progressed, the sophistication
of design for the components of these medical devices has
significantly increased. With respect to collimators, the specific
configurations of the collimators being used in various medical
devices has become much more complex in order to achieve more
accurate results. In addition, the acceptable error tolerances of
manufacture for these collimators has significantly decreased. The
present invention addresses the latest technology demands for the
manufacture of collimators.
[0044] Reference is now made to FIG. 3 which illustrates a
flowchart of for manufacturing a collimator in accordance with the
present invention. The first step of the manufacturing process 200
is to determine the desired shape of the collimator. Typically the
medical device manufacturer will have or provide the particular
specifications for the collimator to be used in the medical device.
The drawing of the device may be a mechanically drawn device and/or
may be an electronically generated device.
[0045] Once the desired shape of the collimator is determined, the
shape of the collimator needs to be electronically entered 210 so
as to form a three dimensional computer generated image of the
collimator. One software package that can be used to generate the
three dimensional computer generated collimator is AutoCAD. Many
other CAD software programs or other types of drawing programs can
be used.
[0046] After the collimator is electronically entered, the drawing
is electronically sectioned or sliced into a plurally of
cross-sections 220. The sections or slices of the collimator are
taken along a single axis (e.g., longitudinal, vertical,
horizontal, etc.). The thickness of each section or slice of the
collimator is representative of the thickness of the metal foil to
be used to form the collimator. The thickness of the metal foil is
typically very thin, thus many sections or slices of the collimator
need to be electronically generated. Each of the sections also
includes one or more holes or slots that will be used to orient the
formed foil layers and also be used to maintain the position of the
formed foil layers during heating and cooling of the foil layers.
Typically these holes or slots are positioned about the periphery
of the each section; however, the holes or slots can be positioned
in other locations.
[0047] Once the sections or slices of the collimator are generated,
a lithographic mask is produced 230 for each section of the
collimator. Each lithographic mask defines the features of each
unique section of the collimator. The process for producing
lithographic masks is well known in the art, thus will not be
further described herein.
[0048] After the lithographic masks are produced for each section
of the collimator, metal foil that is coated with a brazing metal
is obtained 240. As can be appreciated, the coated metal foil can
be obtained prior to the formation of the lithographic masks. The
metal foil that is used to form the collimator typically is a high
density material having a specific gravity of at least about 8.5
g/cm.sup.3. One metal that can be used is tungsten having a
specific gravity of about 19.3 g/cm.sup.3. Materials formed of
tungsten or other high density materials are typically very
difficult to form. Tungsten is a very hard substance and has a
extremely high melting point. Consequently, past collimators made
of tungsten or other high density metals were very difficult and
expensive to manufacture and further resulted in an end product
that often did not meet the tolerance requirement necessary for the
collimator, thereby resulting in expensive waste. The present
invention overcomes this problem. Thin metal foils of tungsten and
other high density metals are commonly available. The thickness of
the metal foil used in the present invention is generally about
40-150 microns. As can be appreciated, other metal thicknesses can
be used. The metal foil is also coated on one or both sides by a
brazing metal. The coating of the brazing metal is typically by an
eletroplating process; however, other coating processes can be
used. The coating thickness of the brazing metal is typically about
0.2-1.5 microns; however, other thicknesses can be used. The
brazing metal is typically a high density metal (e.g., at least
about 8 g/cm.sup.3) having a melting point that is less than the
metal that forms the metal foil. When tungsten is used for the
metal foil, nickel is typically used as the brazing metal; however,
other brazing metals can be used.
[0049] Once the coated metal foil is obtained, the metal foil is
subjected to lithographic micro-machining techniques and/or
micro-machining techniques 250 to produce patterned metal foil
layers that are ultimately used to form the collimator. Some of the
micromachining techniques that can be used include photo-etching
and reactive ion etching.
[0050] After the foil layers have been formed, the foil layers are
aligned and stacked 250 to form the desired 3-D1 shape of the
collimator. The foil layers should be stacked so that a brazing
metal exists between each foil layer. This arrangement can be
achieved in a number of different ways. One non-limiting way is to
have one side of each of the foil layers coated with the brazing
metal. The alignment of the foil layers can also be accomplished in
a variety of ways. Typically alignment pins or other fixed
structures are used to align the multiple layers of metal foil. The
holes or slots in the metal foil are inserted onto the alignment
pins thereby properly orienting the foil layers with respect to one
another.
[0051] The aligned and stacked metal foil layers are then subjected
to heat 270 so as to braze together the metal foil layers. The
heating of the coated metal foil layer at a proper elevated
temperature for a sufficient time will result in the metal coating
to melt and flow between the metal layers. Typically, the brazing
process is conducted under a vacuum; however, this is not required.
The heating of the metal foil layers typically occurs in an inert
atmosphere; however, this is not required. During the heating
process, the metal foil layers expand. The alignment holes or slots
maintain the foil layers in alignment during this heating process.
Typically the alignment holes or slots in each foil layer is sized
and shaped to account for the expansion of the foil layers during
heating. As such, when the foil layers are heated at or near their
maximum temperature, wherein the brazing material is partially or
fully liquified, the holes or slots line up relative to the
alignment pins so as to form the desired shaped of the
collimator.
[0052] Once the metal foil layers are heated for a sufficient time,
the formed collimator is cooled 280. When the foil layers are
cooled, the brazing material solidifies thereby locking the foil
layers in position relative to one another. The alignment holes or
slots in the foil layers are sized and shaped so as to allow the
locked together foil layers to contract during cooling. Typically,
the cooling occurs in an inert atmosphere; however, this is not
required. The use of the above method to manufacture a collimator
results in a cost effective process to manufacture high density
materials into a variety of shapes within very low error
tolerances. FIG. 4 illustrates a section of a collimator that has
been formed by the above-described process. Collimator 300 includes
a plurality of metal foil layers 340 that are connected together by
brazing metal 350. The bottom surface of the collimator has a
non-planar surface 360. As can be appreciated, many bottom surface
configurations can be formed to be used in a particular
application. FIG. 5A illustrates a collimator 300 that can be
formed by the present invention. Collimator 300 can be formed in a
variety of other shapes and sizes, depending on the desired end use
and configuration of the collimator. Collimator 300 is illustrated
as being a single component of the whole collimator. Mount holes
302, 304 are used to mount or secure the collimator component on a
frame or other structure. Four guide holes 320, 322, 324, 326 are
located on the face of the collimator. These guide holes are used
to align the foil layers during the formation of the collimator. As
can be appreciated, the holes can be used to align and/or mount or
secure the collimator to a medical device. The top face of the
collimator 330 has a non-flat surface that has been selected for a
particular application. As can be appreciated, other surface
configurations can be formed. In addition, the surface of the
collimator can include slots, grooves, channels, holes, etc. to
achieve the desired results from the collimator. As stated above,
the collimator 300 is represented as one section of a larger
collimator. As can be appreciated, collimator can be formed from a
single piece instead of from a plurality of sections. Such a
collimator is illustrated in FIG. 5B. In several medical devices
the length of the collimator is about 0.5-2 meters long. The method
of forming a collimator in accordance with the present invention
can be used to form a one piece collimator that has a length of
0.5-2 meters. Heretofore, it is believed that a one piece metal
collimator having low error tolerances has not been made. Single
piece collimators have advantages over segmented collimators,
especially in newer scanner designs. In the newer collimators, the
collimator spins around the patient and results in increased
vibrations and forces on the collimator. When sectioned collimators
are used in such systems, the vibrations and forces on the
collimator act on the interlocking slits and cause the slits to
wear and break. Single piece collimators overcome this problem. The
method of the present invention can form metal foil layers that can
be used to form a one piece collimator or a collimator formed from
a plurality of sections. Collimator 300 is also illustrated as
having a generally planar surface 330. As can be appreciated, the
one piece collimator or one or more sections of a collimator can
have an arcuate profile or a number of other profiles.
[0053] The following example illustrates the manufacture of a
collimator in accordance with the present invention. The
manufacturing process of the present invention can provide methods
for fabricating grid structures having high-resolution and
high-aspect ratio, which can be used for radiation collimators,
scatter reduction grids, and/or detector array grids. Such devices
can be used in the field of radiography to, for example, enhance
image contrast and quality by filtering out and absorbing scattered
radiation (sometimes referred to as "off-axis" radiation and/or
"secondary" radiation). These devices can be used in nearly every
type of imaging, including astronomy, land imaging, medical
imaging, magnetic resonance imaging, tomography, fluoroscopy,
non-destructive inspection, non-destructive testing, optical
scanning (e.g., scanning, digital copying, optical printing,
optical plate-making, faxing, and so forth), photography,
ultra-violet imaging, etc. Thus, these devices can be used in
telescopes, satellites, imaging machines, inspection machines,
testing machines, scanners, copiers, printers, facsimile machines,
cameras, etc. The term "collimator" is used generally to describe a
radiation collimator, x-ray grid, scatter reduction grid, detector
array grid, or any other grid used in an imaging apparatus and/or
process. Certain collimators can be placed between the object and
the image receptor to absorb and reduce the effects of scattered
x-rays. Moreover, such collimators can be used in a stationary
fashion, like those used in SPECT (Single Photon Emission Computed
Tomography) imaging, or can be moved in a reciprocating or
oscillating motion during the exposure cycle to obscure the grid
lines from the image, as is usually done in x-ray imaging systems.
Grids that are moved are known as Potter-Bucky grids. X-ray grid
configurations can be specified by grid ratio, which can be defined
as the ratio of the height of the grid to the distance between the
septa. The density, grid ratio, cell configuration, and/or
thickness of the structure can have a direct impact on the grid's
ability to absorb off-axis radiation and/or on the energy level of
the x-rays that the grid can block. The open cells of the ceramic
grid structure can be filled with detector materials that can be
accurately registered to a collimator. The grids can be fabricated
to have high-resolution grid geometries that can be made in
parallel or focused configurations. The grid can have very fine
septal walls, or can have an air-cell grid structure. The
manufacturing process of the present invention can be used to
manufacture any collimator configuration desired for a particular
application.
[0054] The first part of the manufacturing process involves the
generation of a three-dimensional computer model of the collimator.
The computer generated model of the collimator is divided into a
plurality of thin sections that are cut parallel to the
longitudinal axis of the collimator. The thickness of the sections
is substantially uniform and reflects the thickness of the metal
foil to be used to make the collimator. Guide holes or slots are
also inserted for each section. The number, size and shape of the
guide holes or slots are selected to achieve the proper orientation
of the foil layers during the heating and cooling of the foil
layers.
[0055] The metal foil used to form the collimator has a specific
gravity of at least about 10.2 g/cm.sup.3, a melting point of at
least about 1600.degree. C., and a thickness of about 30-150
microns. One non-limiting metal foil is a metal foil formed of
molybdenum, niobium, platinum, tantalum and/or tungsten. The metal
foil is coated on one side with a thin metal electroplated layer of
a metal having a specific gravity of at least 8.5 g/cm.sup.3, a
metaling point of less than 1470.degree. C., and a thickness of
about 0.1-10 microns. Non-limiting examples of metals for the metal
coating include copper, gold, lead, nickel and/or silver. A
specific example of a coated metal foil for use in manufacturing a
collimator is a tungsten metal foil coated on one side with a
electroplated nickel layer wherein the thickness of the tungsten
foil is about 77 microns, the thickness of the nickel coating is
about 1 micron and the total thickness of the coated metal foil is
about 78 microns. In this example, the sliced sections of the
computer generated collimator would represent sections having a
thickness of about 78 microns. The collimator would thus be formed
from about 50-600 layers of metal foil. As can be appreciated, the
back or rear portion of the collimator may have a uniform thickness
and shape and only the front portion of the collimator has a
non-uniform shape. In such circumstances, the metal foil layers can
be used to only form the non-uniform portion of the collimator and
the uniformly shaped portion of the collimator can be manufactured
by a different process (e.g., machining, stamping, molding, etc.).
In such an arrangement, the layers of metal foil are later
connected to the uniformly shaped portion of the collimator to form
the full collimator.
[0056] Each of the coated metal foil sheets of nickel coated
tungsten were chemically etched to match a specific section of a
computer generated section of the collimator. Photo-masks were
produced for etching each of the metal foil layers. Each metal foil
layer was processed using standard photo-etching techniques and
were etched in such a way that the cross-sectional shape of the
etched walls for each layer are perpendicular to the top and bottom
surfaces of the layer (commonly referred to as straight
sidewalls).
[0057] Once all the metal foil layers were etched, the metal foil
layers were stacked together in order to form the collimator. The
guide holes or slots were used to orient the foil layers on
graphite guide pins. The metal foil was specifically coated such
that a nickel layer existed between each metal foil layer. The
stacked metal foil layers were then bonded together by a vacuum
brazing process. During the brazing process, the layered assembly
was heated in a hydrogen atmosphere to a temperature of
1500-1700.degree. C. for about 20-75 minutes, which caused the
coated nickel layer to flow, thereby wetting the surfaces of the
tungsten foil layers. The temperature and time of heating was
sufficient to allow the nickel to uniformly flow and connect the
layers of tungsten foil together at all contact points. The brazed
layers of tungsten foil were then cooled in a hydrogen atmosphere
for about 1-3 hours and then removed. The formed collimator was
removed from the guide pins and then inspected for quality control
purposes to determine if the formed collimator fell within accepted
tolerances.
[0058] While considerable emphasis has been placed herein on
preferred embodiments of the invention, it will be appreciated that
other embodiments can be devised and that many changes can be made
in the preferred embodiments without departing from the principles
of the invention. Accordingly, it is to be distinctly understood
that the foregoing descriptive matter is to be interpreted merely
as illustrative of the invention and not as a limitation.
* * * * *