U.S. patent application number 11/562129 was filed with the patent office on 2007-05-03 for edge effects treatment for crystals.
Invention is credited to Jack E. Juni.
Application Number | 20070096034 11/562129 |
Document ID | / |
Family ID | 34636485 |
Filed Date | 2007-05-03 |
United States Patent
Application |
20070096034 |
Kind Code |
A1 |
Juni; Jack E. |
May 3, 2007 |
EDGE EFFECTS TREATMENT FOR CRYSTALS
Abstract
A scintillator, for use in a radiation imaging device, has a
light-emitting face, a radiation receiving face, and a perimeter
extending between the light-emitting face and the radiation
receiving face, the perimeter including an edge, the edge having an
edge thickness. The scintillator emits scintillation light from the
light emitting face in response to radiation incident on the
radiation receiving face. The scintillator has one or more light
guides formed therein with guides within a peripheral region being
deeper than guides in a non-peripheral region.
Inventors: |
Juni; Jack E.; (Royal Oak,
MI) |
Correspondence
Address: |
GIFFORD, KRASS, SPRINKLE,ANDERSON & CITKOWSKI, P.C
PO BOX 7021
TROY
MI
48007-7021
US
|
Family ID: |
34636485 |
Appl. No.: |
11/562129 |
Filed: |
November 21, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10993012 |
Nov 19, 2004 |
7138638 |
|
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11562129 |
Nov 21, 2006 |
|
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60523765 |
Nov 20, 2003 |
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Current U.S.
Class: |
250/370.11 |
Current CPC
Class: |
G01T 1/20 20130101 |
Class at
Publication: |
250/370.11 |
International
Class: |
G01T 1/20 20060101
G01T001/20 |
Claims
1. A radiation detector comprising: a continuous body of
scintillation material, the body-having a light-emitting face, a
radiation receiving face, and a perimeter extending between the
light-emitting face and the radiation receiving face, the perimeter
including an edge, the edge having an edge thickness; the body of
scintillation material emitting scintillation light from the light
emitting face in response to radiation incident on the radiation
receiving face; the body of scintillation material having a
peripheral region proximate to the edge and a non-peripheral region
spaced from the edge; and the body of scintillation material
including a plurality of spaced apart light guides formed in the
peripheral region and in the non-peripheral region, each of the
light guides being operable to redirect a portion of scintillation
light before the portion of scintillation light emerges from the
light emitting face; wherein each of the light guides has a depth,
the depth of each light guide in the peripheral region being
greater than the depth of the light guides in the non-peripheral
region.
2. The radiation detector of claim 1, wherein the depth of the
light guides is inversely correlated with the distance of the light
guide from the edge.
3. The radiation detector of claim 2, wherein the inverse
correlation is a linear correlation.
4. The radiation detector of claim 1, wherein at least two of the
light guides in the non-peripheral region having the same
depth.
5. The radiation detector of claim 4, wherein each of the light
guides in the non-peripheral region have the same depth.
6. The radiation detector of claim 1, further comprising a
plurality of light sensors, the sensors receiving scintillation
light, the sensors each having a sensor diameter, the peripheral
region being a region within a distance approximately equal to a
sensor diameter from the edge.
7. The radiation detector of claim 1, wherein the non-peripheral
region is larger than the peripheral region.
8. The radiation detector of claim 1, wherein the peripheral region
lies within a distance approximately equal to eight times the edge
thickness from the edge.
9. The radiation detector of claim 1, wherein the light guides
comprise grooves formed in the light emitting face or the radiation
receiving face.
10. The radiation detector of claim 1, wherein the light guides
comprise reflecting films.
11. The radiation detector of claim 1, wherein the light guides
each comprise an interface between two regions of different
refractive indices.
12. The radiation detector of claim 1, wherein each light guide
provides an internal reflection of scintillation light within the
body of scintillation material.
13. The radiation detector of claim 1, wherein the light guides are
each substantially parallel to the edge.
14. The radiation detector of claim 1, wherein each of the light
guides is disposed generally in a plane defined perpendicular to
one of the faces of the body.
15. The radiation detector of claim 1, wherein the perimeter
includes a first pair of opposed edges and a second pair of opposed
edges each extending between the first pair of edges, the
peripheral region including regions adjacent each of the first pair
of edges.
16. The radiation detector of claim 1, wherein the plurality of
spaced apart light guides are generally evenly spaced.
17. The radiation detector of claim 1, wherein some of the
plurality of light guides extend from the light-emitting face part
way to the radiation receiving face.
18. The radiation detector of claim 1, wherein some of the
plurality of light guides extend from the radiation receiving face
part way to the light-emitting face.
19. The radiation detector of claim 1, further comprising a window
disposed adjacent the light-emitting face of the body of
scintillation material, the window being formed of a material
substantially transparent to scintillation light, the window having
at least one light guide formed therein.
20. The radiation detector of claim 1, further comprising an
optical transmission element disposed adjacent the radiation
receiving face of the body of scintillation material, the optical
transmission element having at least one light guide formed
therein.
21. The radiation detector of claim 1, wherein the perimeter
includes a first pair of opposed edges and a second pair of opposed
edges each extending between the first pair of edges, the
peripheral region including regions adjacent each of the first pair
of edges.
22. The radiation detector of claim 21, wherein the body of
scintillation material is generally rectangular and the edges are
each generally straight edges.
23. The radiation detector of claim 21, wherein the body of
scintillation material is generally curved, the first pair of
opposed edges being parallel generally straight edges and the
second pair of opposed edges being curved edges.
24. A radiation detector, comprising: a scintillator producing
scintillation light in response to incident radiation, the
scintillator having a radiation receiving face, and a light
emitting face, and a perimeter edge; an array of sensors, each
sensor in optical communication with the light emitting face of the
scintillator, each light sensor having a light sensor diameter; a
window between the scintillator and the array of light sensors, the
window having a first face, and a second face, and a perimeter
edge; a perimeter region being defined adjacent the perimeter edges
of the scintillator and window and a central region being defined
inboard of the perimeter region; the scintillator or the window
having a plurality of grooves formed in one face thereof, the
grooves each having a depth and being formed in the perimeter
region and in the central region, the depth of the grooves in the
perimeter region being greater than the depth of the grooves in the
central region
25. A method of treating an optical material so as to modify the
effect of internal edge reflections, the optical material having a
face bounded by a perimeter, the perimeter including an edge, the
method comprising: forming a plurality of grooves spaced apart
across the face, each of the grooves having a depth; the face
having a peripheral region proximate to the edge and a
non-peripheral region spaced from the edge; the depth of the
grooves in the peripheral region being greater than the depth of
the grooves in the non-peripheral region.
26. The method of claim 25, wherein the grooves are formed by
cutting the optical material.
27. The method of claim 25, wherein the optical material is a
scintillator.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/993,012, filed Nov. 19, 2004, which claims
priority of U.S. Provisional Patent Application Ser. No. 60/523,765
filed Nov. 20, 2003, the entire content of both of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to improved apparatus and methods
relating to reduction or elimination of edge effects in optical
elements, for example, scintillators.
BACKGROUND OF THE INVENTION
[0003] A scintillation material, or scintillator, produces light in
response to incident radiation, typically ionizing radiation such
as gamma rays or x-rays. Such a material can be a crystal such as
thallium-doped sodium iodide, NaI(T1), or a non-crystalline
material such as a plastic.
[0004] Radiation detectors using scintillators include gamma
cameras, x-ray detectors, and other radiation imaging or sensing
devices. The scintillator generally has a radiation receiving face
on which radiation such as gamma rays are incident. The
scintillator has a light output face from which light emerges in
response to incident radiation, and also an outside edge. For
example, a disk-shaped scintillator would have a cylindrical edge
surface. An optical window, such as glass, is typically bonded to
the light output face of the scintillator. An array of light
sensors, such as photomultiplier tubes, receive scintillation light
emitted from the light output face of the scintillator, and
transmitted through the window to the detectors. A plastic "light
pipe" is sometimes interposed between the window and the light
sensors.
[0005] The light is generated within the scintillator as pulses
(also termed scintillation events). The positions of light pulses
generated within the scintillator are used in imaging applications,
such as SPECT (single photon emission tomography) and PET (positron
emission tomography) imaging. Such uses may be generally referred
to as scintillation imaging.
[0006] A problem in scintillation imaging is that reflections from
the edge of the scintillator will reduce the measured positional
accuracy of imaging data, particularly for scintillation events
occurring close to the edge of the scintillator. Light from
scintillation events occurring near an edge will be reflected from
that edge, causing a serious reduction in ability to accurately
determine the position of such events. In practice, a dead zone may
exist around a peripheral region, proximate to the edge of the
scintillator or window, from which meaningful positional data
cannot be collected. This dead zone or "edge effect" reduces the
effective usable portion of the scintillator. In addition, the
presence of this unusable region at the periphery of the detector
prevents the detector from being positioned optimally in many
medical applications, for example breast and brain scintigraphy.
The increase in detector bulk caused by the wasted portion of the
detector may also make it difficult to position the detector
sufficiently close to the patient in applications such as heart
scintigraphy and many forms of SPECT.
[0007] In some detector designs, it is desirable to have more than
one scintillator element in close proximity to another. In this
situation, the junctions between elements tend to act as reflecting
edges causing an unusable dead zone of edge effect on each side of
the junctions.
[0008] This problem is well recognized in the field. For example, a
previous attempt to solve this problem is described in U.S. Patent
Application Publication 2003/0034455 to Schreiner et al., which
suggests segmenting the scintillator into a number of triangular
segments. However, such segmentation adds to the cost of a device,
is difficult to fabricate and may cause problems if the
scintillator absorbs moisture from the air. For example, it is well
known that sodium iodide should be protected from atmospheric
moisture.
[0009] Another attempted solution is described by U.S. Pat. No.
4,284,891 to Pergale et al., which suggests providing a diffused
light reflector around the periphery of the optical window.
However, it can be difficult to provide a true diffused reflector,
as reflection properties of many materials and the crystal edge
will change with time and environmental conditions. In addition,
such diffused edge treatments have been found in practice to
provide an unsatisfactory degree of improvement to the problem.
[0010] Hence, there is a need for improved scintillators and
radiation detectors which reduce or eliminate the undesirable
effects of edge reflection.
SUMMARY OF THE INVENTION
[0011] Methods and apparatus are provided to reduce edge effects,
such as loss of positional accuracy due to edge reflection, in
optical elements such as scintillators. For example, one or more
light guides can be provided in a peripheral region of a
scintillator or optical window close to an edge.
[0012] A radiation detector according to an example of the present
invention comprises a scintillator having a light-emitting face, a
radiation receiving face, and a perimeter extending between the
light-emitting face and the radiation receiving face, the perimeter
including an edge. The scintillator emits scintillation light from
the light emitting face in response to radiation incident on the
radiation receiving face. One or more light guides are formed
within a peripheral region proximate to the edge, a light guide
redirecting a portion of scintillation light before it emerges from
the light emitting face. The radiation detector may further
comprise a number of light sensors receiving scintillation light
from the scintillator.
[0013] Light guides provided within the peripheral region can
improve the positional accuracy of the radiation detector. There
may be a plurality of spaced apart light guides formed only within
the peripheral region. The light guides may include grooves having
a groove depth which decreases as the distance of the groove from
the edge increases.
[0014] The light emitting face of the scintillator may have a
non-peripheral region, such as a central region, in which there are
no light guides. Depending on the application, the non-peripheral
region may be larger, sometimes substantially larger, than the
peripheral region.
[0015] The peripheral region is a region within a certain distance
of the edge. The distance may be some multiple of an edge
thickness, such as less than ten times, for example within eight
times the edge thickness of the edge. The distance may also be the
approximate diameter of a light sensor.
[0016] A light guide may comprise a groove formed in the light
emitting face and/or the radiation receiving face of a
scintillator. In other examples, light guides may be provided in a
window between a scintillator and a detector or detectors. A light
guide may comprise a groove, reflecting film, an interface or other
boundary between two regions of different refractive indices, or
other structure providing an internal reflection or refraction of
light within the scintillator. The light guide can be substantially
parallel to the edge.
[0017] Another example of an improved radiation detector comprises
a scintillator, an array of sensors in optical communication with a
light emitting face of the scintillator, a window between the
scintillator and the array of light sensors, the scintillator
and/or the window having one or more grooves formed in a face
thereof. The grooves may be formed only within a peripheral region
of the scintillator and/or window.
[0018] In examples discussed below, the term "crystal" is often
used for convenience to refer to a scintillation material. However,
any example discussed here equally applies to non-crystalline
scintillators. The methods and apparatus described can also be
adapted for use in other optical elements, as will be clear to
those skilled in the optical arts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows a side view of a pair of crystals, each having
a number of grooves cut in respective peripheral regions proximate
to crystal edges;
[0020] FIG. 2 shows a side view of a crystal, showing grooves cut
in the light output face of the crystal within a peripheral region
proximate to the crystal edge;
[0021] FIG. 3 is a side view of a pair of crystals, each having
grooves cut in a peripheral region of the radiation receiving
face;
[0022] FIGS. 4A and 4B show top views of a crystal;
[0023] FIG. 5 shows a side view of a crystal, having both a window
and an optical transmission element, the latter having grooves cut
in the lower face;
[0024] FIG. 6 is a side view of a radiation detector, including a
crystal and a window having grooves cut in the lower face of the
window;
[0025] FIG. 7 is a side view of a radiation detector, having both a
window and an optical transmission element, each having
grooves;
[0026] FIG. 8 is a side view of a radiation detector, having
grooves in both the crystal and the window;
[0027] FIG. 9 is a side view of a radiation detector having grooves
in the crystal and the window, the grooves not being in
register;
[0028] FIG. 10 shows a radiation detector, having grooves in the
upper and lower faces of both the crystal and window;
[0029] FIG. 11 shows a radiation detector, the window/light pipe
having an upper surface shaped so as to direct light to a plurality
of sensors, the window also having grooves in a peripheral
region;
[0030] FIG. 12 shows a curved crystal having grooves in a
peripheral region;
[0031] FIG. 13 shows a circular crystal having circular
grooves;
[0032] FIG. 14 shows a radiation detector configuration including a
computer and display;
[0033] FIG. 15 shows a radiation detector having two crystals and a
metal housing;
[0034] FIG. 16 shows a cross section of a scintillation crystal
with light guides formed in the peripheral and non-peripheral
regions; and
[0035] FIG. 17 shows a cross section of another scintillation
crystal with light guides formed in the peripheral and
non-peripheral regions.
DETAILED DESCRIPTION OF THE INVENTION
[0036] In examples discussed below, the term "crystal" is used for
convenience to refer to a scintillation material. However, examples
discussed here apply equally to non-crystalline scintillators.
Also, in examples discussed below, light guides are provided by
grooves cut into one or more surface of the crystal (or associated
window). As discussed in more detail below, other forms of light
guide can also be used, for example, reflective films.
[0037] FIG. 1 shows a portion of a first crystal 10, a portion of a
second crystal 12, a crystal gap 14 between the two crystals, a
plurality of sensors such as sensor 16, a plurality of grooves in
the light emitting face of each crystal, such as grooves 18a-18d, a
first crystal edge 20, and a second crystal edge 22.
[0038] As drawn, the lower surface of the crystal 10 is the
radiation receiving face, and the upper surface of the crystal is
the light emitting face. This convention will be followed (for
convenience only) through the various figure descriptions.
[0039] The light emitting face and radiation receiving face can be
generally parallel and coextensive, and interconnected by a
perimeter, the perimeter defining an edge. The crystal can be in
the form of a cuboid, having a length, width, and thickness, the
thickness being the edge thickness. The thickness may be
substantially less than the length and/or width of the crystal.
[0040] Sensors may be in different positions from those shown. A
sensor may, for example, span the gap between two crystals. Sensors
may be provided in a linear or two-dimensional array, or other
configuration. Scanning imaging methods may also be used.
[0041] FIG. 2 shows another view of crystal 10. The star symbol at
28 represents a pulse of scintillation light (a scintillation
event) produced by radiation interacting with the crystal. For
convenience, the term "scintillation light" will often be
abbreviated to "tight". The zigzag arrow labeled L represents a
possible path of light out of the crystal. For convenience, not all
grooves are shown.
[0042] Light will tend to propagate in all directions within the
crystal 10 from the scintillation event (the production of a pulse
of light). However, the arrows show the groove 18a and crystal edge
20 operating cooperatively to provide a light guiding effect,
whereby light is reflected from the crystal edge and from the inner
surface of the groove so as to take a path remaining proximate to
the edge, and to emerge from the light emitting face within a
portion of the light emitting face between the groove and the
crystal edge. Hence, the groove 18a and the crystal edge 20 provide
partial optical conferment, or a waveguide effect.
[0043] FIG. 3 shows a first crystal 40, a second crystal 42, a
separation gap 44, grooves such as groove 46 in the radiation
receiving face of each crystal, and a plurality of sensors such as
sensor 48. The sensors are in optical communication with the upper
face, or light emitting face, of each crystal. This figure
illustrates that grooves may be provided in the radiation receiving
face of the crystal, as well as or instead of grooves in the light
emitting face.
[0044] FIG. 4A shows a top view of a crystal, showing a rectangular
light emitting face, having a surface (such as the light emitting
face) 50, and a plurality of grooves such as 56 and 58 around the
periphery of the crystal, proximate to the edge. The edge is shown
as rectangular, including sides 52 and 54. The grooves may have
equal depth, or in other examples the groove depth is greatest for
the groove closest to the edge (groove 58), and shallowest for the
groove furthest for the edge (groove 56). The depth of grooves can
be correlated with the distance from the edge, for example
inversely proportional to the distance, as discussed in more detail
below.
[0045] The crystal may also have a rectangular cross section, so as
to have a cuboid form. The orthogonal edge regions of the crystal,
as illustrated in FIG. 4A, may be designated as sides or edges, as
preferred. Analogous groove patterns may be formed in one or more
faces of the crystal and/or window.
[0046] FIG. 4B shows a top view of another example, in which
grooves formed in the surface 59 form a cross-hatched pattern in
the comers. Other details may be the same as discussed above in
relation to FIG. 4A.
[0047] FIG. 5 shows a side view of a radiation detector comprising
a radiation-side optical transmission element 60, a crystal 62, a
window 64, an optical matching medium 66, a plurality of sensors
such as sensor 68, and a plurality of grooves such as 70 and 72. In
this example the grooves are formed in the upper side of the
optical transmission element 60, proximate to the radiation
receiving face of the crystal 62.
[0048] In another example which may appear similar to that
illustrated in FIG. 5, the radiation detector comprises a crystal
and lower and upper optical transmission elements or window layers
supported by the light emitting face of the crystal. The grooves
can be formed in the light emitting face of the crystal.
[0049] FIG. 6 represents a side view of a radiation detector,
comprising a crystal 80, a window 82, a plurality of sensors such
as sensor 84, and a plurality of grooves such as grooves 86 and 88.
In this example, the grooves are formed in the lower side of the
window, the side of the window proximate to the light emitting face
of the crystal. It should be recalled that the crystal and the
window, both typically being transparent, may be considered as a
single optical element., i.e. together they form a continuous
medium for transmission of light. Thus, placement of groves in the
window provides a reduction in edge effect in the same manner,
although to a lesser extent, as do grooves in the crystal itself.
It should also be appreciated that light reflections occur from
both the edge of the crystal and from the edge of the window.
Reflections from the edge of the window may further degrade spatial
accuracy beyond the effects of edge reflections in the crystal
itself. An existing radiation detector may be improved by the
simple process of replacing the existing window with a window
having a plurality of grooves within the side of the window
proximate to the crystal. In another example, the grooves may be
formed in the upper face of the window, the face most distant from
the crystal.
[0050] FIG. 7 shows a radiation detector comprising a radiation
side window or optical transmission element 100, a crystal 102, a
window 104, a sensor 106, grooves such as 108 and 110 in the
radiation-side window proximate to the radiation receiving face of
the crystal, and grooves such as 112 and 114 in the lower surface
of the window proximate to the light emitting face of the
crystal.
[0051] In another example, which may appear similar to that
illustrated in FIG. 7, a radiation detector comprises a crystal, a
lower window, and an upper window. In this example, grooves are
provided within the crystal and within one or two window layers
transmitting light from the crystal to the sensor,
[0052] FIG. 8 illustrates a radiation detector comprising a crystal
120, a window 122, and a sensor 124, with grooves such as 128
provided in the lower surface of the window. Grooves such as 126
are provided in the upper surface of the crystal. In this example
the grooves in the crystal and the grooves in the window are
substantially in register.
[0053] FIG. 9 shows a radiation detector comprising a crystal 140,
a window 142, a sensor 144, grooves such as 146 in the upper
surface of the crystal, and grooves such as 148 in the lower
surface of the window. In this example the grooves in the crystal
and grooves in the window are substantially out of register.
[0054] FIG. 10 shows a radiation detector comprising a crystal 160,
a window 162, a sensor 164, grooves such as 166 in the lower face
of the crystal, grooves such as 168 in the upper surface of the
crystal, grooves such as 170 in the lower surface of the window,
and grooves such as 172 in the upper surface of the window.
[0055] FIG. 11 shows a radiation detector comprising a crystal 180,
a window 182, and a sensor 184, the window having an upper surface
topography 186 designed so as to direct light towards the light
sensitive regions of the sensors. Grooves such as 188 are shown
provided in the lower surface of the window, but alternatively or
additionally could be in either surface of the crystal or the upper
surface of the window.
[0056] A series of triangular indentations 190 are provided in the
upper surface of the window so as to prevent light being lost to
dead spaces within or between the sensors, which would otherwise
not be detected Alternatively, structure 182 may be composed of two
elements, a window proximate to the crystal and a "light pipe"
interposed between the window and the light sensors. Other surface
topographies can be used, as discussed further below.
[0057] FIG. 12 shows a curved (arcuate) crystal 200 having grooves
202 within a peripheral region, proximate to one edge of the
crystal. In this example the grooves are substantially radial
extending from the lower surface of the crystal towards the
interior.
[0058] FIG. 13 is a top view of a circular crystal 220, having a
circular edge 222, and grooves 224 and 226 within a peripheral
region proximate to the edge.
[0059] FIG. 14 shows a radiation detector comprising a radiation
source 240, a crystal 242, a window 244, a light sensor array 246,
a computer 248, a display device 250, a data port (for example, a
data input device) 252, and an analysis circuit 254. A computer
program running on the computer 248 can be used to extract position
information provided by the light sensor array. Algorithms may be
provided to provide edge corrections, depth corrections and other
corrections as well known in the art. The analysis circuit may
comprise noise reduction circuitry, and the like, and may be
integrated with the computer into a single device. The radiation
source may be a mammal under diagnosis.
[0060] FIG. 15 shows a side view cross section of a radiation
detector comprising a housing 260, a first crystal 262, a second
crystal 264, substantially in abutment to the first crystal, and a
window 266. The housing 260 can be a metal such as aluminum, and
may provide protection of the crystal from atmospheric moisture or
other sources of degradation. A detector array 270 includes a
plurality of light sensors such as 272 as well as associated
positioning circuitry. Grooves such as 268 are shown within
peripheral regions of each crystal so as to reduce edge effects
from reflections, and improve the accuracy of positional data
provided by the sensor array 272 provided above the window.
[0061] In other examples, two windows can be provided in register
with the crystals, and grooves provided in the windows close to the
ends.
Light Guides (Grooves)
[0062] The term "light guide" can be used to refer to any
structures that may be provided within a crystal to provide
internal redirection of light. The light guide may be a groove
(such as a cut in the surface of the crystal), and the term
"groove" is used elsewhere for convenience to represent light
guides. The term groove includes structures such as cuts, slots,
and the like.
[0063] A light guide may include a groove, an interface between
media of substantially different refractive indices, a reflective
film, bubbles, defects, crystal defects such as crystal grain
boundaries, fracture films, or other structure or components that
provide redirection of light within the crystal before the light
emerges from the light emitting surface. Light guides may also
comprise embedded fibers, plastic or metal films, or other
materials.
[0064] A groove can be air filled, or filled with fill material
such as a liquid, plastic, glass, reflective film (such as a
plastic or metal film), multilayer reflective film, fibers,
spheroids (for example, forming a photonic band-gap reflector),
interferometric structure, inert gas, vacuum (if the scintillator
is in a sealed housing), or other material.
[0065] A light guide can be substantially parallel to a proximate
edge region, and/or substantially normal to a surface in which it
is formed, or nearby surface. A plurality of spaced apart light
guides can be formed within a peripheral region. The depth or other
extent of each light guide can be inversely correlated with the
distance of the light guide from the edge (the distance being
measured between the light guide and the most proximate region of
the edge). The light guides may not extend entirely to any surface
of the crystal or window, but may instead be disposed inside the
volume of the crystal or window.
[0066] In other examples, light guides can be provided across the
fill extent of a surface, not just in a peripheral region. As for
peripheral light guides, the depth (or analogous extent) of the
light guide can be inversely correlated with the distance from the
nearest edge (less when further from the edge, the relationship can
be linear or nonlinear). In one example, peripheral light guides
have a depth that is inversely correlated with the distance from
the nearest edge, and light guides in a middle region of the
surface can all have an equal depth. This example is illustrated in
FIG. 16, where the body of scintillation material is illustrated at
300. A plurality of light guides 302 are formed in the body 300.
Peripheral region 304 is illustrated between outboard edge 306 and
line P-P. An opposing peripheral region is illustrated at 308. The
light guides in the peripheral region all have depths greater than
the depths of the guides in the non-peripheral region 310. In this
example, the light guides in the non-peripheral region all have the
same depth. In another example, illustrated in FIG. 17, the light
guide depths are inversely correlated with the distance from an
edge, across the entire surface.
[0067] A light guide can provide partial optical confinement of
scintillation light between the light guide and either another
light guide or an edge. The partial optical confinement can improve
the positional accuracy of a radiation detector using the
scintillator.
[0068] If the scintillator has an elongated form having a uniform
cross-section, having a first end and a second end, light guides
can be formed in peripheral regions proximate to one or both
ends.
[0069] The number of light guides proximate to an edge may be a
number within the range 1-20 (inclusive), such as in the range 1-10
(inclusive), for example, one, two, three, four, five, six, seven,
eight, nine, or ten. Example scintillators were made with 5-7
grooves, which were found to Improve positional accuracy and
dramatically reduce the edge effect dead zone near crystal
edges.
[0070] Groove spacing may be regular (equal spacing), or
non-equally spaced. Graduations in groove depth can be linear or
non-linear with distance from the edge, or all grooves can be the
same depth.
[0071] It is preferred that the light guide depth near the edges be
graduated. Equal-depth grooves or light guides in a periphery
region may be beneficial for some applications, but they tend to
produce a zone of edge effect inside the innermost groove, i.e. the
innermost groove acts like an edge. A groove extending only partway
through the crystal does produce less of an edge effect than a full
edge however, and is therefore somewhat useful.
[0072] The preferred embodiment, however, is the progressively
graduated grooves or light guides becoming shallower as one moves
inward from the edge. Since the grooves are deeper on one side of
the "waveguide" than the other, they limit light spread more in one
direction than the other. This produces a gradual effect rather
than a sharp edge, thus eliminating all or more of the dead zone.
This is due to a "one-way" diffusion aspect of the graduated depth
arrangement. This can be most easily explained by reference to one
of the simplest arrangements of the present invention, where
grooves or light guides are provided in the light emission side of
the crystal only. Light can diffuse beyond the confines of the
"waveguide" at the bottom. Light that exits a waveguide at the
bottom can go either toward the edge or away from the edge. Since
the open path (space between bottom of light guide and bottom
surface of crystal) towards the edge is smaller than the open path
away from the edge, light is more likely to diffuse away from the
edge. This "pushes" the emitted light away from the edge. The
intensity of light reaching the light sensors, instead of being a
bell shaped curve is now skewed, with a wider spread away from the
edge. This "pushing" of the light emission profile away from the
edge also happens for light reaching the bottom of the next
waveguide further from the edge. This includes both light that
traveled down that waveguide plus light that traveled to that point
from the adjacent waveguides. This causes the emission profile to
be skewed even further away from the edge. This skewing effect
becomes less and less, however, as one looks at waveguides further
and further from the edge. This is because, proportionally, the
relative openings toward and away from the edge become more and
more equal. For the innermost waveguide, the chance of a light
photon going to the right is almost as great as its chance of going
to the left. Thus, the emitted light is pushed away from the edge
effect dead zone, but the degree to which it is pushed away gets
less and less, the further one moves from the edge. This causes the
edge effect to be blurred out and spread over a wide region.
[0073] In addition to the above, the resolution enhancing effects
of the light grooves both improve resolution and mitigate the
resolution reducing effects of "smearing" the edge reflections over
the whole peripheral region.
[0074] The positional accuracy of an imaging device can be
increased by providing more closely spaced grooves. The groove
spacing may be, for example, a fraction of a sensor diameter, such
as a spacing within the range 0.01-1 times the sensor diameter,
such as in the range 0.05-0.5 of the sensor diameter. The groove
spacing may also be a fraction of the edge thickness, such as in
the range 0.01-0.5 times the edge thickness.
[0075] If the grooves have variable groove depth, such as groove
depths inversely correlated with distance from the edge, in some
examples the shallowest groove may be approximately 1 mm, and the
deepest groove approximately equal to half the edge thickness. In
some examples, the grooves may be curved.
Peripheral Region
[0076] The peripheral region can be defined in terms of the sensor
width, for a radiation detector including a plurality of sensors.
The sensor width may be for example, the outer diameter of a
sensor, or the average spacing distance of sensors. The peripheral
region can be defined as a region proximate to the edge of the
scintillator, and not more than a distance approximately equal to
the sensor width from the edge. Alternatively, the peripheral
region can be defined as a region proximate to the edge of the
scintillator, and not more than a distance approximately equal to
the half the sensor width from the edge. If the sensor is a
photomultiplier tube, the sensor width may be termed the tube
width.
[0077] The peripheral region can also be defined in terms of a
fraction of the overall dimensions of a surface. For example, the
peripheral region of a crystal or window may be a region proximate
to the edge, and not more than a certain fraction of the distance
from the center to that edge. The certain fraction may be, for
example, 5 percent, 10 percent, 15 percent, or 20 percent.
[0078] The peripheral region can also be defined as a region
proximate to the edge, and not more than six to eight times the
thickness of the slab from the edge. Alternatively, the region may
be less, such as 3-4 times the thickness. The peripheral region can
also be defined as a region proximate to the edge, and not more
than approximately the thickness from the edge. In one example, the
crystal has a thickness of 1/4'' to 3/8'', the phototubes have a
diameter of 2''-3'', and the peripheral region is 1''-1.5''.
[0079] In one example, a radiation detector can include a
scintillator in the form of a cuboid slab having a slab length, a
slab width, and a slab thickness (equal to an edge thickness), with
the slab length and slab width both being substantially greater
than the slab thickness. The peripheral region, for some examples,
may be an outer region of the slab within approximately three to
eight times or approximately equal to the edge thickness from the
edge.
Manufacture of Light Guides
[0080] Light guides (such as grooves) may be formed by a variety of
mechanical, chemical, optical, ultrasonic, or other means. For
example, a saw can be used to cut grooves in one or more surfaces
of a crystal. Grooves may also be formed by a high pressure jet of
fluid. It is known that sodium iodide, a common crystal material,
is soluble in water. In this case a non-aqueous fluid could be
used, such as an oil, supercritical carbon dioxide, or other fluid.
Water can also be used, and the surface dried quickly after jet
cutting. A high pressure gas jet such as carbon dioxide or nitrogen
can also be used to provide grooves in a surface.
[0081] A laser can be used to ablate or otherwise provide grooves
or cuts in the surface of a crystal. For laser cutting, it is
advantageous to use a different wavelength from that of the light
produced by radiation within the crystal, as the crystal will
presumably be substantially transparent to that wavelength; for
example, x-ray, U-V or IR wavelengths may be used if the
scintillation light is in the visible region. The crystal may
include additives so as to absorb laser radiation within a
predetermined range of wavelengths to facilitate crystal
processing.
[0082] Inclusions, bubbles, or defects may also be used to provide
redirection of light within the crystal. For example a laser
focused within the bulk of the crystal may be used to vaporize part
of the crystal so as to provide an air bubble. Such defects may be
provided in a regular array or pattern so as to provide the desired
light guiding effect.
[0083] Bubbles may also be injected into the molten medium from
which the crystal is formed. The crystal may also include other
materials having a substantially different refractive index so as
to provide refractive light guiding. Light guides, such as grooves,
may also be formed by molding (for example of plastic or molten
materials, for example during crystal growth), stamping, drilling,
other mechanical processes, chemical etching, ion bombardment,
electron beams, atomic beams, lithographic processes, and the
like.
Crystal Geometry
[0084] A crystal may have a cuboid shape, having a light emitting
face, an opposed radiation receiving face, and a rectangular edge
(including first and second opposed ends, and first and second
opposed sides). Any pair of opposed ends may be alternatively
designated as sides, or vice versa.
[0085] The crystal has a crystal thickness, defined as the distance
between the light emitting face and opposed radiation receiving
face. The two faces may be parallel, providing a uniform
thickness.
[0086] The light emitting face extends between the first and second
ends, and between the first and second sides. Similarly, the
radiation receiving face, opposed to the light emitting face, can
be substantially parallel to the light emitting face and also
extending between the first and second ends.
[0087] In other examples the crystal may be curved (actuate), for
example either being formed initially in a curved or actuate shape,
bent after crystal formation, or bent after heating or other
softening process. The bending process may take place during
heating of a crystal. The curve may be in a single plane, or may be
in three dimensions so as to provide, for example, a spherical
section.
[0088] Grooves may be formed generally normal to either the light
emitting face or radiation receiving face. Grooves may be parallel
to the portion of the edge to which they are proximate, for example
parallel to the first or second end.
[0089] The grooves may be equally spaced, or provided at irregular
intervals. The depth of the groove within the crystal, the depth
being defined as the distance between the face into which the
groove is cut and the distal end of the groove, can be correlated
from the distance of the groove from the nearest end. For example,
the correlation may be linear, exponential, quadratic, or other
mathematical form.
[0090] Hence, an improved radiation detector includes a
scintillator having the form of a slab, the radiation receiving
face and the light emitting face being generally parallel, the slab
having a slab width and a slab thickness, the slab thickness being
equal to the edge thickness, the slab width being substantially
greater than the edge thickness, the scintillator having a
peripheral region, the peripheral region being proximate to the
edge, the scintillator including one or more light guides formed
only within the peripheral region. A plurality of grooves are
formed in the peripheral region, the grooves acting as light
guides, the depth of each groove being inversely correlated (such
as inversely proportional) to a distance of the groove from the
edge.
[0091] The crystal and window thicknesses can be approximately the
same. The crystal and window thicknesses can be different relative
thicknesses than shown in the Figures. A typical crystal (or edge)
thickness may be in the range 0.125-3 inches, such as in the range
0.25 inches-0.5 inches, such as in the range of 0.25 to 0.375
inches. In some examples, the deepest groove can less than or equal
to one half the edge thickness.
[0092] Some crystals, especially for PET work, may be substantially
thicker, such as having a thickness greater than a width. The
present invention may be used with these thicker crystals as
well.
Scintillation Materials
[0093] Scintillation materials may comprise halides (such as sodium
iodide, cesium iodide), oxides (such as bismuth germinate (BGO),
cadmium tungstate, gadolinium orthosilicate (GSO), cerium doped
yttrium orthosilicate (YSO), cerium doped lutetium orthosilicate
(LSO), and the like), other inorganic materials (for example, as
inorganic crystals), organic crystals, other organic materials, and
other materials. Scintillation materials may include an activator
and a host material, in which the activator is dispersed or
otherwise disposed. The activator may be a transition metal, such
as a rare earth metal. Scintillation materials can be crystalline
or non-crystalline. Non-crystalline scintillation materials may
comprise, for example, polymers, glasses, and other materials
providing light in response to incident radiation.
[0094] In this specification, examples are provided referring to
crystals, where the term crystal is used to refer to the
scintillation material, such as a scintillation crystal. However,
the methods and apparatus described herein can be used with any
scintillation material, such as crystal or non-crystal
scintillators, and also with other materials that produce light in
response to non-ionizing radiation, such as fluorescent materials,
or other optical elements in which edge reflections are a
problem.
Windows
[0095] A window generally comprises a material substantially
transparent to scintillation light. For example, a window may be
bonded to, abutting, or proximate to the light output face of a
crystal. The window can provide protection of the crystal from
degradation, for example by protecting from scratches, moisture,
fracture, and the like.
[0096] The window may be formed from any material substantially
transparent to the scintillation light. Examples include glass,
polymers (such as acrylic polymers, for example PMMA), transparent
oxides, or other materials.
[0097] The topography of the upper surface of the window may
include triangular indentations, pyramids, truncated pyramids,
cones of conic sections such as frustoconical shapes, lenses,
microlens arrays, Fresnel lens patterns, or other surface features
operable to guide light towards light sensitive regions of the
sensor. Equivalently, a window may be slab shaped, with a separate
layer in optical communication with the upper surface providing
light guiding. In the field of nuclear medicine, this separate
layer is often referred to as the "light pipe".
[0098] The window may have a thickness in the range 0.1-0.375
inches, though this is not limiting. If the window has a surface
topography within the peripheral region, for example to direct
light to sensors, grooves can be formed through such features. The
grooves may be normal to the average plane of the upper surface,
may be parallel to a proximate edge, or otherwise provided.
[0099] Light guides (such as grooves) can be provided in the window
material, so as to reduce edge effects due to reflections from the
edge of the window. The grooves in the window can be in addition
to, or instead of, grooves in the crystal.
[0100] Analogous geometries can be used to the crystal examples
described previously, and vice versa. Grooves in the crystal can be
combined with grooves in the window.
[0101] The window may overhang the crystal edges, which may reduce
edge effects due to the crystal, and the addition of grooves in the
window may further reduce edge effects due to reflections from the
window edges.
[0102] Provision of grooves in the window can advantageously
increase positional accuracy of an imaging device. The improvement
may not be as great as grooves formed in a crystal. However, even
if it is not possible to replace the crystal of an imaging device,
the positional accuracy of the device can be improved by replacing
a non-grooved window with a grooved window.
Housing
[0103] If a humidity sensitive scintillator crystal such as sodium
iodide is used, a housing may be necessary to protect the crystal
from the effects of atmospheric moisture. A housing may also be
provided to increase robustness, hold together multiple crystals,
or other purpose. The housing preferably does not substantially
attenuate the incident radiation, but need not be transparent to
scintillation light. Example housing materials include aluminum,
other metals, plastics and the like.
Reflectors
[0104] To prevent light escaping from the radiation receiving face
of the crystal, and therefore not being detected by the sensor
array, reflectors may be provided. For example, a reflector may be
provided by the inner reflective surface of an aluminum housing. A
reflective film, such as a separate metal film or photonic band gap
layer, may also be provided. For example, a reflective metal film
may be evaporated onto the radiation receiving surface of a
crystal. Interferometric reflectors and diffuse reflectors may also
be used. Diffuse reflectors include polymer films (such as Teflon,
porous Teflon, polyethylene, and the like), inorganic materials
such as substantially white crystal layers, inorganic material
doped polymers, and the like.
[0105] Reflectors may also be provided at the edges of the crystal
and/or windows. The edge of the crystal will typically reflect
scintillation light. However, additional reflector(s) can be
provided.
[0106] Generally, the groove spacing will be much greater than the
wavelength of scintillation light, so as not to provide a photonic
band gap (PBG) effect or other interference effect. However, PBG
regions or other reflective films can be provided at the edges or
radiation receiving face of the crystal, if required. This can be
in addition to any grooves or other light guides provided.
[0107] Absorbers may also be provided to reduce unwanted stray
reflection from any surface, for example around the apertures of
the sensors.
Sensors and Circuitry
[0108] Sensors which may be used include photomultiplier tubes
CPMT), solid state sensors such as avalanche photodiodes, or other
light sensitive devices. The sensors can be provided in a linear or
two-dimensional array. Positional information of scintillation
events can be determined from sensor signals.
[0109] After grooves are cut in a crystal, there may be a
sensitivity reduction, due to the reduced local volume of crystal,
requiring an attenuation correction. A linearity correction can
also be provided to correct for any warping or spatial distortion
of the image.
[0110] Amplifiers may be provided to improve the signal-to-noise
ratio of the sensor array. The incident radiation may have phase
information that can be used by lock-in amplifiers, an optical
modulator through which scintillation light passes, or similar to
reduce signal noise. Analysis circuitry can include a computer,
algorithms to correct for edge effects and the like, noise reducing
circuitry, and the like.
[0111] An optical coupling material, such as a liquid, gel, grease,
polymer, or the like, can be provided between components so as to
reduce scattering from refractive index discontinuities. For
example, an optical coupling material can be provided between
sensors and a window (or crystal, if no window is used) to cut down
light losses.
Applications
[0112] Applications include gamma ray cameras, and other radiation
detectors and imaging devices, such as nuclear medical devices.
Applications include positron emission tomography (PET), single
photon emission computed tomography (SPECT), combined PET/SPECT,
x-ray imaging, UV imaging, cosmic ray detection, and other imaging
and detection applications.
[0113] The improved radiation detection efficiency, compared with
provision of uniform grooves across an entire light emitting face,
is advantageous for all applications, particularly where
sensitivity is an issue (such as combined PET/SPECT devices).
[0114] The methods and apparatus described herein may also be
adapted to other applications, such as reducing edge effects in
other materials, such as lenses, fluorescent materials, light
emitting materials, light guiding materials (such as light pipes)
and the like. Other examples are discussed in more detail
below.
General Removal of Edge Effects from Optical Elements
[0115] Approaches described herein can be used generally to remove
edge effects from optical elements. For example, a lens may be
provided with one or more grooves around the periphery of one or
both surfaces of the lens. Grooves (the term is used generally to
refer to any light guide) may also be provided around the
peripheral edges of corneal implants, spectacle lenses, and other
lenses and/or lens arrays. For example, in a spectacle application,
one or more grooves could be partially or completely covered by a
frame element.
[0116] A lasing material may be provided with one or more grooves
in the peripheral region of the light emitting face of the laser
material. This may be used to reduce stray light emerging from the
laser material.
[0117] Grooves may also be provided in the peripheral regions of
other optical components and systems, such as along the peripheral
region of waveguides, or other components of integrated optical
systems. For example, they may help reduce edge effects within a
waveguide.
[0118] Hence, an example improved radiation detector includes a
scintillator, the scintillator having a light-emitting face, a
radiation receiving face, and a periphery between the
light-emitting face and the radiation receiving face, the periphery
including an edge having an edge thickness. The scintillator emits
scintillation light from the light emitting face in response to
radiation incident on the radiation receiving face. The
scintillator has a peripheral region proximate to the edge, the
scintillator including one or more light guides formed only within
the peripheral region. This is in contrast to other designs where
light guides are formed uniformly across the surface of the
scintillator. The peripheral region can be a region within
approximately three to eight times the edge thickness from the
edge, or within a distance approximately equal to a sensor spacing
or sensor diameter if an array of sensors is used, or within half a
sensor diameter (or sensor spacing). The area of the peripheral
region can be less than the area of a non-peripheral region (such
as a central region) not proximate to the edge. The light guide
provides an internal reflection or redirection of scintillation
light within the scintillator, before the scintillation emerges
from the light emitting face.
[0119] If the radiation detector comprises a scintillator, a
window, and an array of sensors, each sensor in optical
communication with the light emitting face of the scintillator
through the window, the light sensors having a light sensor
diameter, the scintillator or the window can be provided with one
or more grooves formed in one or both faces thereof, the one or
more grooves being formed within a distance less than a light
sensor diameter from an edge thereof.
[0120] The invention is not restricted to the illustrative examples
described above. Examples are not intended as limitations on the
scope of the invention. Methods, apparatus, compositions, and the
like described herein are exemplary and not intended as limitations
on the scope of the invention. Changes therein and other uses will
occur to those skilled in the art. The scope of the invention is
defined by the scope of the claims. Subheadings in the
specification are provided for convenience only. Examples,
alternatives, and the like should be sought within the entire
specification.
[0121] Patents or publications mentioned in this specification are
incorporated herein by reference to the same extent as if each
individual publication was specifically and individually indicated
to be incorporated by reference. In particular, U.S. Prov. Pat.
App. Ser. No. 60/523,765 filed Nov. 20, 2003, is incorporated
herein in its entirety. Additional information concerning imaging
systems can be found in Applicant's issued patents U.S. Pat. Nos.
6,525,320; 6.525,321; and 6,504,157; and Pub. App. No.
2003/0136912, the entire content of all of which are incorporated
herein by reference.
* * * * *