U.S. patent application number 14/932493 was filed with the patent office on 2017-05-04 for apparatus including scintillation crystal array with different reflector layers and associated methods.
The applicant listed for this patent is CRYSTAL PHOTONICS, INCORPORATED. Invention is credited to Bruce CHAI, Xiao-mei PAN, Bao-hui ZHOU.
Application Number | 20170123080 14/932493 |
Document ID | / |
Family ID | 58635447 |
Filed Date | 2017-05-04 |
United States Patent
Application |
20170123080 |
Kind Code |
A1 |
CHAI; Bruce ; et
al. |
May 4, 2017 |
APPARATUS INCLUDING SCINTILLATION CRYSTAL ARRAY WITH DIFFERENT
REFLECTOR LAYERS AND ASSOCIATED METHODS
Abstract
A radiation detector, such as for a PET scanner, may include an
array of scintillator crystals, with each scintillator crystal
having a polished end, a roughened end opposite the polished end,
and polished sides extending between the polished end and the
roughened end. The detector may also include a specular reflector
layer between adjacent polished sides of adjacent ones of the array
of scintillator crystals, and a diffusive reflector layer adjacent
the roughened ends of the array of scintillator crystals. The
detector may further include at least one photodetector adjacent
the polished ends of the array of scintillator crystals.
Inventors: |
CHAI; Bruce; (Oviedo,
FL) ; PAN; Xiao-mei; (Nan-jing City, CN) ;
ZHOU; Bao-hui; (Nan-jing City, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CRYSTAL PHOTONICS, INCORPORATED |
Sanford |
FL |
US |
|
|
Family ID: |
58635447 |
Appl. No.: |
14/932493 |
Filed: |
November 4, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01T 1/1641 20130101;
G01T 1/2023 20130101; G01T 1/202 20130101; G01T 1/2018 20130101;
G01T 1/2002 20130101 |
International
Class: |
G01T 1/202 20060101
G01T001/202; G01T 1/20 20060101 G01T001/20; G01T 1/164 20060101
G01T001/164 |
Claims
1. An apparatus comprising: an array of scintillator crystals, each
scintillator crystal having a polished end, a roughened end
opposite the polished end, and a plurality of polished sides
extending between the polished end and the roughened end; a
specular reflector layer between adjacent polished sides of
adjacent ones of said array of scintillator crystals; and a
diffusive reflector layer adjacent the roughened ends of said array
of scintillator crystals.
2. The apparatus according to claim 1 wherein said specular
reflector layer comprises a plurality of stacked plastic
layers.
3. The apparatus according to claim 1 wherein said diffusive
reflector layer comprises polytetrafluoroethelene (PTFE).
4. The apparatus according to claim 1 wherein said diffusive
reflector layer comprises at least one of MgO, TiO.sub.2, and
BaSO.sub.4.
5. The apparatus according to claim 1 wherein each scintillator
crystal comprises one of LYSO, LSO, BGO, NaI(Tl), LaBr.sub.3, GSO,
LGSO and GAGG.
6. The apparatus according to claim 1 wherein said roughened end is
based upon lapping with a 600 mesh grit abrasive.
7. The apparatus according to claim 1 wherein each scintillator
crystal has respective x and y dimensions in a range of 0.4 to 6.3
mm.
8. The apparatus according to claim 1 wherein each scintillator
crystal has a z dimension in a range of 5 to 30 mm.
9. The apparatus according to claim 1 wherein each of said
roughened end and said polished end has a square shape.
10. The apparatus according to claim 1 further comprising at least
one photodetector adjacent the polished ends of said array of
scintillator crystals.
11. The apparatus according to claim 10 further comprising a
processor and a memory associated therewith coupled to said at
least one photodetector.
12. The apparatus according to claim 10 further comprising: at
least one other imaging scanner; and a processor and an associated
memory coupled to said at least one other imaging scanner and said
array of scintillator crystals.
13. The apparatus according to claim 10 wherein said at least one
photodetector comprises a plurality of photomultiplier tubes.
14. The apparatus according to claim 10 wherein said at least one
photodetector comprises a plurality of solid state
photodetectors.
15. An apparatus comprising: an array of scintillator crystals,
each scintillator crystal having a polished end, a roughened end
opposite the polished end, and a plurality of polished sides
extending between the polished end and the roughened end, said
roughened end being based upon lapping with a 600 mesh grit
abrasive; a specular reflector layer between adjacent polished
sides of adjacent ones of said array of scintillator crystals; a
diffusive reflector layer adjacent the roughened ends of said array
of scintillator crystals; at least one photodetector adjacent the
polished ends of said array of scintillator crystals; and a
processor and a memory associated therewith and coupled to said at
least one photodetector.
16. The apparatus according to claim 15 wherein said specular
reflector layer comprises a plurality of stacked plastic
layers.
17. The apparatus according to claim 15 wherein said diffusive
reflector layer comprises polytetrafluoroethelene (PTFE).
18. The apparatus according to claim 15 wherein said diffusive
reflector layer comprises at least one of MgO, TiO.sub.2, and
BaSO.sub.4.
19. The apparatus according to claim 15 wherein each scintillator
crystal comprises one of LYSO, LSO, BGO, NaI(Tl), LaBr.sub.3, GSO,
LGSO and GAGG.
20. The apparatus according to claim 15 wherein each scintillator
crystal has respective x and y dimensions in a range of 1 to 4 mm,
and a z dimension in a range of 5 to 30 mm.
21. (canceled)
22. A method for making a radiation detector comprising: forming a
plurality of scintillator crystals so that each scintillator
crystal has a polished end, a roughened end opposite the polished
end, and a plurality of polished sides extending between the
polished end and the roughened end; arranging the plurality of
scintillator crystals into an array with a specular reflector layer
between adjacent polished sides of adjacent ones of the array of
scintillator crystals; and providing a diffusive reflector layer
adjacent the roughened ends of the array of scintillator
crystals.
23. The method according to claim 22 wherein the specular reflector
layer comprises a plurality of stacked plastic layers.
24. The method according to claim 22 wherein the diffusive
reflector layer comprises polytetrafluoroethelene (PTFE).
25. The method according to claim 22 wherein the diffusive
reflector layer comprises at least one of MgO, TiO.sub.2, and
BaSO.sub.4.
26. The method according to claim 22 wherein each scintillator
crystal comprises one of LYSO, LSO, BGO, NaI(Tl), LaBr.sub.3, GSO,
LGSO and GAGG.
27. The method according to claim 22 wherein the roughened end is
based upon lapping with a 600 mesh grit abrasive.
28. The method according to claim 22 wherein each scintillator
crystal has respective x and y dimensions in a range of 1 to 4 mm,
and a z dimension in a range of 5 to 30 mm.
29. The method according to claim 22 further comprising positioning
at least one photodetector adjacent the polished ends of the array
of scintillator crystals.
Description
FIELD
[0001] The present invention relates to the field of radiation
detectors, and, more particularly, to such detectors based on
scintillation crystals and related methods.
BACKGROUND
[0002] Positron Emission Tomography (PET) is one of the three major
modern medical diagnostic approaches used as a non-evasive tool to
detect tumors or other abnormal conditions of the body. A modern
PET scanner may typically use a very large number of scintillating
crystals as gamma-ray detectors with a relatively small x-y
dimension, and a long z dimension. Typically on the order of 20,000
to 30,000 such scintillation crystals are used in a PET scanner
detector ring to capture the two emitted gamma rays from each
positron-electron annihilation event to thereby locate the location
with high precision. It is possible to trace back the locations of
emission and thus reconstruct the tumor image accordingly.
[0003] Scintillation is a process to capture the gamma-ray and
convert it into visible light that, in turn, can be detected with a
photodetector, such as photo-multiplier tube (PMT), photo diode, or
more advanced silicon photo-multiplier (SiPM). Of course, to have
effective detection, it is desired to have more visible photons
generated by each gamma-ray capture event.
[0004] Because of the desire in high energy physics to detect
various high energy particles, there have been extensive searches
in the past century for more efficient scintillator crystals that
generate more photons per gamma-ray capture. For PET scanners, it
is desired not only to capture the gamma-ray, but also to know the
position of capture to make the accurate image reconstruction. To
do so, the scintillator crystals are normally cut into thin long
rectangular rods packed into two dimensional array blocks. These
array blocks are installed in the PET scanner to form a detector
ring of different sizes. Depending on the specific application,
patient or animals will be scanned through to detect the tumors
inside their body.
[0005] The construction of the detector array block is important to
the performance of the PET scanner. It is desired to select have
scintillating crystals with high light yield and good stopping
power to have efficient capture of the gamma-ray. Since there are a
large number of crystals packed together, it is desired that the
crystals are mechanically strong so that they can be cut and
polished into such small thin rods. The typical physical size of
these crystals varies from 6 mm down to 0.5 mm in cross-dimension
and 30 mm down to 5 mm in length depending on the specific
application. The size of the array block will also vary depending
on the kind of photodetector used, the specific dimension and
geometry of the photodetector and finally the specific scheme of
detection.
[0006] In the traditional way of gamma-ray detection using PMT
detectors, the scintillating crystal blocks are each typically
built in a 12.times.12, 13.times.13, 14.times.14 or even larger
array depending on the PMT arrangement. Each of the individual
crystals are optically isolated to each other with high reflection
films that covers five sides of the crystal surfaces except one end
where the scintillating light will be emitted to reach to the PMTs.
For example, a 14.times.14 array block will contain 196 crystal
pixels which share only four PMTs. To have the emitted
scintillating light reach to all four PMTs, a special designed
light guide may be used between the array block and the PMTs. The
principle used to locate the exact position of the emitting crystal
is based on the calculation of the distributed light sharing ratio
from these four PMTs. Since the PMT is a relatively expensive
detector, to control the total cost of a typical PET scanner, it is
desirable to use as few PMTs as possible, but at the same time it
is also desirable to be able to accurately locate the position of
the scintillating light source. At the present time, the
14.times.14 array has reached just about the detecting limit for
the PMTs. Each PMT in this case will share forty-nine crystals. So
for a full size scanner ring, a manufacturer may limit the total
number of PMTs used to below 600 units to control the cost, but
still have high enough image resolution.
[0007] To be able to have an accurate calculation of the emitting
pixel position, all the four PMTs should be able to detect adequate
amount emitting photons at the same time frame. This means that the
scintillating crystal should desirably emit as many visible photons
as possible with each capture of the incoming gamma ray. This is
the very reason that there are extensive research efforts to find
the best scintillating crystals. However, there is a limit on how
one can find such a crystal with so much light emission. Even with
good light emission, it may also be equally important to be able to
channel the scintillating light to the end of the crystal so that
it can reach the PMTs. Accordingly, there are also efforts to
select the best reflecting film so that sufficient light can be
reflected out at one end of a crystal.
[0008] To be able to capture as much of the emitting gamma rays
from a patient's body, it is desired to have the detector ring
packed with a maximum volume of the scintillating crystals. This
means that one would to reduce the amount of volume for the
reflecting material. So the reflecting film should be thin and
effective. Over the years there are a number of materials that have
been selected as reflectors. Here are listed of some of the
well-known reflectors that have been used: liquid white paints made
of MgO, TiO.sub.2 or BaSO.sub.4; solid powders of MgO, or
TiO.sub.2; reflecting films, such as Teflon tape, Lumirror film or
3M Vikuiti Enhanced Specular Reflector (ESR) film. Such materials
should also be compatible for ease of manufacture, especially for
mass production. In the traditional PMT detector based array
blocks, the choice of reflecting film is relatively forgiving,
since the scintillating crystal size is usually not so small.
[0009] A number of patents disclose approaches to preparing the
scintillator crystals and the various reflective materials used to
make an array of such crystals. For example, U.S. Pat. Nos.
5,610,401; 8,481,952; 9,012,854, and 8,426,823 each discloses
various crystal and packaging configurations.
[0010] There still exists a desire for better scintillation crystal
performance, especially for radiation detectors, such as PET
scanners.
SUMMARY
[0011] An apparatus, such as a PET scanner, may include an array of
scintillator crystals, each scintillator crystal having a polished
end, a roughened end opposite the polished end, and a plurality of
polished sides extending between the polished end and the roughened
end. A specular reflector layer may be between adjacent polished
sides of adjacent ones of the array of scintillator crystals, and a
diffusive reflector layer adjacent the roughened ends of the array
of scintillator crystals.
[0012] The embodiments use different types of reflecting materials
for the array of scintillator crystals to increase light output at
one end of the array, reduce the volume fraction of the reflecting
materials between adjacent crystals, and/or simplify the assembling
procedure for mass production.
[0013] For example, the specular reflector layer may comprise a
Vikuiti Enhanced Specular Reflector (ESR) layer. Also, the
diffusive reflective layer may comprises polytetrafluoroethelene
(PTFE), or at least one of MgO, TiO.sub.2, and BaSO.sub.4, for
example. Each scintillator crystal may comprise one of LYSO, LSO,
BGO, NaI(Tl), LaBr.sub.3, GSO, LGSO and GAGG.
[0014] The roughened end of each scintillator crystal may be based
upon lapping with a 600 mesh grit abrasive, although other
approaches are also contemplated.
[0015] In some embodiments, scintillation crystal may have
respective x and y dimensions in a range of 0.4 to 6.3 mm. And each
scintillation crystal may have a z dimension in a range of 5 to 30
mm. Further, each of the roughened end and the polished end may
have a square shape.
[0016] The apparatus may further comprise at least one
photodetector adjacent the polished ends of the array of
scintillator crystals. The apparatus may also include a processor
and a memory associated therewith coupled to the at least one
photodetector for processing signals to generate images, for
example, such as for PET scanning. In some embodiments, the
apparatus may further comprise at least one other imaging scanner
coupled to the processor.
[0017] The at least one photodetector may comprise a plurality of
photomultiplier tubes. In other embodiments, the at least one
photodetector may comprise a plurality of solid state
photodetectors.
[0018] A method aspect is for making a radiation detector. The
method may include forming a plurality of scintillator crystals so
that each scintillator crystal has a polished end, a roughened end
opposite the polished end, and a plurality of polished sides
extending between the polished end and the roughened end. The
method may further include arranging the plurality of scintillator
crystals into an array with a specular reflector layer between
adjacent polished sides of adjacent ones of the array of
scintillator crystals, and providing a diffusive reflector layer
adjacent the roughened ends of the array of scintillator
crystals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic diagram of a PET/CT scanner in
accordance with the present invention.
[0020] FIG. 2 is an exploded perspective view of a radiation
detector as in the scanner of FIG. 1.
[0021] FIG. 3 is a schematic side view of a portion of a radiation
detector that may be used in the scanner of FIG. 1.
[0022] FIG. 4 is a top plan view of the 14.times.14 scintillation
crystal array of FIG. 2.
[0023] FIG. 5 is a cross-sectional view of FIG. 4 taken along line
5-5.
[0024] FIG. 6 is a photograph of the 14.times.14 array in
accordance with FIGS. 4 and 5.
[0025] FIG. 7 is a perspective view of another embodiment of a
16.times.16 array of scintillation crystals according to the
invention.
[0026] FIG. 8 is a top plan view of the 14.times.14 scintillation
crystal array of FIG. 7.
[0027] FIG. 9 is a cross-sectional view of FIG. 8 taken along line
9-9.
[0028] FIG. 10 is a photograph of the 14.times.14 array in
accordance with FIGS. 7-9.
[0029] FIG. 11 is a graph of light output data of Example 1.
[0030] FIG. 12 is a graph of light output data of Example 2.
[0031] FIG. 13 is a graph of light output data of Example 4.
[0032] FIG. 14 is a graph of light output data of Example 6.
[0033] FIG. 15 is a graph of light output data of Example 7.
[0034] FIG. 16 is a graph of light output data of Example 8.
[0035] FIG. 17 is a graph of light output data of Example 9.
DETAILED DESCRIPTION
[0036] The present invention will now be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein. Rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout, and prime notation is used to indicate similar
elements in alternative embodiments.
[0037] Referring initially to FIGS. 1 and 2 an apparatus in the
form of a combined PET/CT scanner 20 is now described including the
scintillation crystals 30 having improved light output as described
in greater detail below. The PET/CT scanner 20 includes a
cylindrical sensor ring 21 and a patient carrying platform 22
associated therewith. The PET/CT scanner 20 may include
conventional CT components from which a CT image may be generated
as will be appreciated by those skilled in the art. As will also be
appreciated by those skilled in the art, the scintillation crystals
30 may be used in a PET scanner, or a SPECT scanner. In addition,
the scintillation crystals 30 may be used with a multimodal PET/MRI
scanner in other embodiments.
[0038] The PET/CT scanner 20 may also include a processor and
associated memory 23 that generate, on a display 24, a combined PET
and CT image as will be appreciated by those skilled in the art.
The scintillation crystals 30 may be used in a PET scanner for an
animal or human body part, which is typically smaller that the full
sized PET/CT scanner 20 illustrated in FIG. 1. The scintillation
crystals 30 may be used in other radiation detecting applications,
such as, for example, for the inspection of goods/containers, as
will be appreciated by those skilled in the art.
[0039] The PET/CT scanner 20 includes an array of scintillator
crystals 30, made from a plurality of array blocks 35, with a
single 14.times.14 array block shown in FIG. 2. Each scintillator
crystal 30 has a polished end 30a, a roughened end 30b opposite the
polished end, and a plurality of polished sides 30c extending
between the polished end and the roughened end. As shown in the
exploded enlarged portion of FIG. 1, the specular reflector layer
31 is illustratively positioned between adjacent polished sides of
adjacent ones of the array of scintillator crystals 30 (only one of
which is shown for clarity), and a diffusive reflector layer 32 is
positioned adjacent the roughened ends of the array of scintillator
crystals.
[0040] The embodiments use different types of reflecting materials
for the array of scintillator crystals 30 to increase light output
at one end of the array 30a, reduce the volume fraction of the
reflecting materials between adjacent crystals, and simplify the
assembling procedure for mass production.
[0041] For example, the specular reflector layer may comprise a
Vikuiti.TM. Enhanced Specular Reflector (ESR) layer available from
3M Electronic Display Lighting Optical Division of St. Paul,
Minn.
[0042] Also, the diffusive reflective layer 32 may comprise
polytetrafluoroethelene (PTFE). Alternatively, the diffusive
reflector layer 32 may comprise at least one of MgO, TiO.sub.2, and
BaSO.sub.4, for example. Each scintillator crystal may comprise one
of LYSO, LSO, BGO, NaI(Tl), LaBr.sub.3, GSO, LGSO and GAGG as will
be appreciated by those skilled in the art.
[0043] The roughened end 30b of each scintillator crystal 30 may be
based upon lapping with a 600 mesh grit abrasive, although other
approaches are also contemplated. For example, the roughened end
30b may be formed by other abrasive or surface treatments, and may
be formed by initially cutting of the crystal with a saw.
[0044] In some embodiments, scintillation crystal 30 may have
respective x and y dimensions in a range of 0.4 to 6.3 mm. And each
scintillation crystal may have a z dimension in a range of 5 to 30
mm. Further, each of the roughened end 30b and the polished end 30a
may have a square shape. Of course, in other embodiments the ends
30a, 30b may be rectangular with different x and y dimensions. For
example, some crystals may have a rectangular shape of
6.28.times.4.18 mm, and a length of 25 mm. In addition, smaller
crystals may have a 0.5 mm pitch and an actual width of 0.43
mm.
[0045] The PET/CT scanner 20 includes at least one photodetector
adjacent the polished ends 30a of the array of scintillator
crystals 30. More particularly as shown in FIG. 2 four
photomultiplier tubes 36 and an intervening light guide 37 is
provided to detect light from the array block 35. In another
embodiment described with additional reference to FIG. 3, the
photodetectors may be provided by a plurality of solid state
photodetectors, such as SiPM's as will be appreciated by those
skilled in the art.
[0046] A method aspect is for making a radiation detector. The
method may include forming a plurality of scintillator crystals 30
so that each scintillator crystal has a polished end 30a, a
roughened end 30b opposite the polished end, and a plurality of
polished sides 30c extending between the polished end and the
roughened end. The method may further include arranging the
plurality of scintillator crystals into an array with a specular
reflector layer 31 between adjacent polished sides of adjacent ones
of the array of scintillator crystals 30, and providing a diffusive
reflector layer 32 adjacent the roughened ends of the array of
scintillator crystals.
[0047] The conventional approach has been to use the highest
reflecting material for the thin film between the crystals and also
to use the same material to cover the outside face of an array
block. This will leave one end face open to let the scintillating
light out to reach the photodetectors.
[0048] As disclosed herein a combination of different reflecting
materials and different crystal surfaces are used to increase the
fraction of the scintillating light that can be reflected out from
the open end of a long thin crystal. Both specular and diffusive
reflector materials work together with the matching crystal surface
finishing to guide the light out. The actual efficiency will also
depend upon the physical size and the length to width aspect ratio
of the scintillating crystal. A specular reflector material, such
as ESR is used for the highly polished sides 30c of the long
crystals 30 and a diffused reflector material, such as Teflon tape
(PTFE) or BaSO.sub.4 paste at one end face 30b opposite the light
exiting face 30a. This diffusivity is further enhanced with the
fine ground diffusive end 30b. The diffused surface at the
roughened end 30b will disperse the scintillating light into many
directions to achieve low incident angle to the sides 30c so that
light can be reflected out with a reduced or minimum number of
reflections and thus a reduced or minimum reflection loss. It is
theorized without Applicant wishing to be bound thereto that
reducing the total number of internal reflections inside the
crystal 30 is an important aspect, because no reflection is 100%
without loss. Even with a few percent loss per reflection, a large
number of reflections may quickly absorb most of the light by the
sides 30c leave little to go out at the polished end 30a. This may
be particularly true for thin and long crystals with a high length
to width aspect ratio.
[0049] Unfortunately, with the push to higher image resolution and
the adaptation of all solid state photodetectors, the general trend
is to reduce the width of the crystals pixels without a reduction
of the crystal length since the crystal length is needed to have
sufficient stopping power to capture the incoming gamma-rays. The
embodiments disclosed herein work particular well for the arrays
made with thin long crystals 30 of high length to width ratios.
[0050] In the past, almost all the efforts have been focused on the
search for the best reflecting materials. Specular type reflectors,
such as metallic foil and thin metallic coatings, have been tried
with very poor result. Diffusive type reflectors, such as white
paint or paste materials have shown much better results. White
pigments such as MgO, TiO.sub.2 and BaSO.sub.4 have been used. At
the present time, BaSO.sub.4 paint or paste seems to give the
better results. However, to apply a thin and uniform thickness
layer of BaSO.sub.4 paint or paste under 0.1 mm thick between the
crystals in an array block is a very slow and difficult process. So
for practical purposes, the paint thickness has been limited to
about 0.2 mm or thicker. This may be satisfactory, if the crystal
width is relatively large (>3 mm). But it will become a more
important issue for arrays with crystals of smaller width (<2
mm) since the volume fraction of the reflector material will become
significant.
[0051] For thinner diffusive reflector material, Teflon tape has
been the most commonly used material with good results. By far,
Teflon tape seems to be the best and most versatile reflector
material for all types of scintillation applications whether it is
a single piece of crystal or an array block. Unfortunately, it also
has its limitation. First, installing a Teflon tape reflector is
always a very labor intensive process since it is only done by
hand. Second, even though the Teflon tape thickness can be as thin
as 0.07 mm, it seems still to work well only with large (>3 mm)
size crystals. The performance will become worse with thinner
crystals and it will also become unpractical to wrap crystals with
a width less than 2 mm. The reason for the degraded performance is
also due to excessive reflection and absorption of diffused light
at each reflection.
[0052] Teflon wrapped crystals may work well if the crystal pixel
length to width ratio is less than 6. For example, a 4.times.4 mm
crystal should not have a length greater than 24 mm, preferably
down to 20 mm. This is because the total number of the internal
reflections will be less. For thinner crystals of similar length,
such as 2.times.2.times.20 mm size with an aspect ratio of 10, the
light output will become much worse due to excessive internal
reflections.
[0053] The new ESR film is a totally different class of reflecting
material. It is a sheet made with multiple thin layers of plastic
film with the thickness that will satisfy the equation of total
reflection, d=m.lamda./2n, in the visible region of the light
spectrum. The ESR film is also very thin with an average thickness
of 0.065 mm. Indeed, array blocks have been made with ESR film
reflector with relatively good results. The advantage of the ESR
film is the low reflection loss at each reflection. However, the
ESR film does suffer with the problem of being too specular and not
diffusive enough. It has good performance in many other
applications, but for scintillator array blocks, it is not perfect.
The ESR film seems work reasonably well with smaller dimension
pixels (<2 mm) and not so well with larger dimension pixels
(>3 mm) as compared with the diffusive type of reflectors. This
is based on array blocks made with crystals with all surfaces
polished. The result seems to be the reverse with respect to that
of BaSO.sub.4 paste and Teflon tape.
[0054] The embodiments disclosed herein are directed to an array
block 35 with hybrid reflectors that will take advantage of the
merits of each type of reflector material to match with proper
crystal surfaces in order to increase or maximize the light output.
The embodiments may also be suitable for many size array blocks
with both large and small widths of the crystals.
[0055] The hybrid reflector array block 35 is based on the
following design features. The specular ESR reflector layer 31 is
used for the four sides 30c of the crystals 30. The crystal 30 will
have the four sides being polished. It is desirable to have a
reduced or minimum loss on each of the reflections by the four
sides 30c. On the other hand, it may not be desirable to have the
light inside the crystal 30 with only specular reflection. The high
angle specular reflection will trap the light inside the crystal 30
by the four sides 30c and never be able to get out at the polished
ends 30a. It may be desirable to have the light generated by the
scintillation process break into diffusive reflections with many
directions. Accordingly, it may be desirable to have the diffusive
reflecting surface, and the fine-ground or roughened end 30b in
combination with a diffusive reflector 32 to help get the light out
of the open polished end 30a of a long crystal 30.
[0056] Since the ESR film or specular reflector layer 31 is very
thin (0.065 mm), this reduces the volume fraction of the reflecting
material and yields a relatively high packing density of
scintillator crystals 30 to improve the chance of gamma ray
capture. This is particularly true for crystals of small width
dimension.
[0057] The thickness of the reflecting film is important since the
fraction of the scintillating material volume is reduced if the
reflecting film becomes too thick. For example, for a 4.times.4 mm
cross section crystal with 0.1 mm thick reflector, the volume
fraction of the reflector film will be 4.8%. When the crystal cross
section is reduced down to 1.times.1 mm, then with the same 0.1 mm
thick reflector film, the reflector volume fraction will increase
to 17.4%. If the reflector film thickness goes higher than 0.1 mm,
then its volume fraction will be even higher. For 4.times.4 mm
crystals with 0.2 mm thick reflector, the volume fraction of the
reflector will be increased to 9.3%. This kind of high volume
fraction may not be acceptable for modern high-end PET scanners,
for example.
[0058] A PET detector ring typically contains a large number of
array blocks 35 with many crystals 30. It is desirable to simplify
the assembly process for mass production. With a solid ESR film,
one is able to pack the crystals 30 and specular layer 31 together
easily with high precision. This is particularly true with smaller
dimensions of the crystals 30. With the modern array block 35'
using all solid state photodetectors 40' (FIG. 3) the high
precision pitch of the array block may be important for the design,
and the embodiments described herein are especially helpful in
achieving this goal.
[0059] FIGS. 4-6 illustrate a configuration of the large
14.times.14 array block 35. It typically matches with PMTs 36 as
the photodetectors as shown in FIG. 2. For modern PET scanners, for
example, the most common scintillator crystals used are BGO, LSO
and LYSO. The embodiments disclosed herein will work well for all
these crystals and also other possibilities as will be appreciated
by those skilled in the art.
[0060] FIGS. 7-10 illustrate a configuration of a small 16.times.16
array block 35'. Small size array blocks 35' are generally matched
with the SiPM type, solid-state photodetectors 40'(FIG. 3).
Example 1
[0061] A 14.times.14 array block was built with
3.86.times.3.86.times.19 mm size LYSO crystals with all surfaces
polished. The LYSO crystal has an aspect ratio of 19/3.86=5.0. An
ESR film was used as the specular reflector layer between the
crystals and also all around the five outer surfaces of the array
block except the bottom face where scintillating light exits. The
block was facing down with the open face placed on top of a
Hamamatsu R877 PMT in an enclosed box without any external light
leaking into it. A Na-22 radiation source was placed at a small
distance above the array block. This was used to generate the 511
KeV gamma ray radiation based on the positron-electron annihilation
process. Scintillating light was produced by the LYSO crystals
after capturing the 511 KeV gamma rays. The intensity of the
scintillating radiation was recorded by a Canberra Genie 2000
Spectrometer. The Spectrometer was pre-calibrated with a BGO
crystal standard which sets the intensity at 100, so all the
measured scintillating light intensities can be compared directly.
With this block built with all ESR reflector layers, we recorded a
light intensity of 414. The light output data is shown in FIG. 11,
in which plot 51 is the data of the actual counts in each channel,
plot 52 is the smoothed curve based upon the actual counts, plot 53
is the line segment showing the base line of the light peak, plot
54 is the line segment showing the FWHM of the light peak, and plot
55 is the slope of the light peak spectra.
Example 2
[0062] The exact same set up as described in Example 1 was used to
measure the scintillating light intensity in all the subsequent
examples. Again, the spectrometer was pre-calibrated so that the
recorded scintillating light intensity can be compared directly
among all the examples. Here the same 14.times.14 array block as in
Example 1 was used with top ESR reflector layer removed and
replaced with a Teflon tape reflector layer. We measured a light
intensity of 440, a slight improvement of light output. The light
output data is shown in FIG. 12, in which plot 61 is the data of
the actual counts in each channel, plot 62 is the smoothed curve
based upon the actual counts, plot 63 is the line segment showing
the base line of the light peak, plot 64 is the line segment
showing the FWHM of the light peak, and plot 65 is the slope of the
light peak spectra. This example demonstrates the improvement of
light exit with a diffusive end layer as compared to a specular
reflecting layer.
Example 3
[0063] Using the same block described in Example 2, we removed the
top Teflon tape reflector layer. We then roughened the top end of
the array block with a 600 mesh abrasive. The block was thoroughly
cleaned and dried afterward. We then attached to the top
fine-ground surface an ESR reflector cover. With the same set up,
we measured a light intensity of 534. This is a significant
improvement for the light output. Clearly, even with a ESR specular
end face reflector layer, the fine ground end faces of the crystals
were able to break the light into diffusive reflections and thus
facilitate the exit at the open end of the array block.
Example 4
[0064] We took the same block as in Example 3 and removed the ESR
reflector layer on the top. We then replaced it with a Teflon tape
reflector layer. With the same set up, we measured a light
intensity of 595. The light output data is shown in FIG. 13, in
which plot 71 is the data of the actual counts in each channel,
plot 72 is the smoothed curve based upon the actual counts, plot 73
is the line segment showing the base line of the light peak, plot
74 is the line segment showing the FWHM of the light peak, and plot
75 is the slope of the light peak spectra. This is the best result
among the four similar examples. Comparing with Example 1, we
observed that the light output intensity has increased by 44%. The
combination of the diffusive Teflon tape and the fine ground
(roughened) diffusive end of the crystal as well as the specular
ESR reflector film with fully polished sides gave the most
efficient way to guide the scintillating light out of the
crystals.
[0065] These four prior examples demonstrated that the hybrid array
block works well with array blocks made of relatively large size
crystals. On the other hand, for array blocks with large crystals
and a small length to width aspect ratio, one can find that a pure
diffusive reflector design can also achieve very good light exiting
at the end of the array block.
Example 5
[0066] A different 14.times.14 array block was built with
3.61.times.3.61.times.19 mm size LYSO crystals with five surfaces
polished and one end 3.61 mm.times.3.61 mm face fine lapped with
600 mesh abrasive. The LYSO pixel has an aspect ratio of
19/3.61=5.26. Teflon tape was used as reflector between the
crystals and also all around the five outer surfaces of the array
block except the bottom end where scintillating light exits. With
the same set up, a light intensity of 570 was measured. The result
is close to the Example 4 hybrid embodiment.
[0067] Again the diffusive reflection facilitates the light exit at
the end of the array block. The small aspect ratio of the crystal
pixels may be important to achieve this high performance. So even
with total diffusive reflections, the short aspect ratio of the
crystals may minimize the total number of internal reflections and
maximize the light exit.
[0068] However, Teflon tape may only be suitable for large array
blocks with large and wide crystals. This is because of the
extensive labor work needed to wrap the crystals with the Teflon
tape. Moreover, the volume fraction of the Teflon tape is also high
as compared with ESR film. So for the small size array blocks with
smaller size and width crystals, its performance drops quickly and
the difficulty in making the array block increases rapidly.
Example 6
[0069] A small 16.times.16 array block was built with
1.52.times.1.52.times.12 mm size LYSO crystals with all surfaces
polished. The LYSO crystal has an aspect ratio of 12/1.52=7.9. The
ESR film was used as reflector between the crystals and also all
around the five outer sides of the array block except the bottom
end where the scintillating light exits. With the same set up of
the scintillating detection system, a light intensity of 399 was
measured. The light output data is shown in FIG. 14, in which plot
81 is the data of the actual counts in each channel, plot 82 is the
smoothed curve based upon the actual counts, plot 83 is the line
segment showing the base line of the light peak, plot 84 is the
line segment showing the FWHM of the light peak, and plot 85 is the
slope of the light peak spectra.
Example 7
[0070] Here the same 16.times.16 array block as in Example 6 is
used with the top ESR reflector cover replaced with a Teflon tape
diffusive reflector layer. So with the same measurement set up, a
light intensity of 459 was measured. The light output data is shown
in FIG. 15, in which plot 91 is the data of the actual counts in
each channel, plot 92 is the smoothed curve based upon the actual
counts, plot 93 is the line segment showing the base line of the
light peak, plot 94 is the line segment showing the FWHM of the
light peak, and plot 95 is the slope of the light peak spectra.
[0071] This example again demonstrates the improvement of light
exit with a diffusive end compared to the specular end in a small
size array block. Comparing this example with the relative results
of Examples 1 and 2 with a light improvement from 414 to 440, we
can draw the following conclusion: with the polished end of the
crystals, the diffusive reflector layer works better for smaller
width crystal blocks than the larger width crystal blocks.
Example 8
[0072] This example is a small 16.times.16 array block from the
same batch of 1.52.times.1.52.times.12 mm size LYSO crystals with
five surfaces polished and one end face fine ground with 600 mesh
abrasive (i.e. roughened). The LYSO crystals also have an aspect
ratio of 12/1.52=7.9. The ESR film was used as the specular
reflector between the crystals and also all around the five outer
sides of the array block except the bottom end where scintillating
light exits. With the same set up of the scintillating detection
system, a light intensity of 518 was measured. The light output
data is shown in FIG. 16, in which plot 101 is the data of the
actual counts in each channel, plot 102 is the smoothed curve based
upon the actual counts, plot 103 is the line segment showing the
base line of the light peak, plot 104 is the line segment showing
the FWHM of the light peak, and plot 105 is the slope of the light
peak spectra. Comparing this with the result of Example 6, the
improvement is significant from 399 to 518, a 30% increase.
Example 9
[0073] This is the same 16.times.16 array block in Example 8 with
the top ESR reflector layer replaced with a Teflon tape reflector
layer. So with the same measurement set up, a light intensity of
521 was measured. The light output data is shown in FIG. 17, in
which plot 111 is the data of the actual counts in each channel,
plot 112 is the smoothed curve based upon the actual counts, plot
113 is the line segment showing the base line of the light peak,
plot 114 is the line segment showing the FWHM of the light peak,
and plot 115 is the slope of the light peak spectra. Again, this is
the best result among the four examples of small crystal array
blocks. Comparing this to the result of Example 6, the improvement
is also significant from 399 to 521, a 30.6% increase.
[0074] The Examples are summarized in Table 1 below.
TABLE-US-00001 TABLE 1 Input Size (mm) Side End Light Ex. (LYSO
crystal) Input End Film Film Intensity 1 3.86 .times. 3.86 .times.
19 polished ESR ESR 414 (14 .times. 14 array) 2 3.86 .times. 3.86
.times. 19 polished ESR Teflon 440 (14 .times. 14 array) 3 3.86
.times. 3.86 .times. 19 roughened ESR ESR 534 (14 .times. 14 array)
4 3.86 .times. 3.86 .times. 19 roughened ESR Teflon 595 (14 .times.
14 array) 5 3.61 .times. 3.61 .times. 19 roughened Teflon Teflon
570 (14 .times. 14 array) 6 1.52 .times. 1.52 .times. 12 polished
ESR ESR 399 (16 .times. 16 array) 7 1.52 .times. 1.52 .times. 12
polished ESR Teflon 459 (16 .times. 16 array) 8 1.52 .times. 1.52
.times. 12 roughened ESR ESR 518 (16 .times. 16 array) 9 1.52
.times. 1.52 .times. 12 roughened ESR Teflon 521 (16 .times. 16
array)
[0075] Based on a comparison of Examples 8 and 9, one can see that
for small size arrays with small width crystals with a large length
to width aspect ratio, grinding the end face with the 600 mesh
abrasives will give more light output improvement (30 and 30.6%,
respectively). But the replacement of the ESR reflector layer with
Teflon tape showed a small improvement (0.6% only).
[0076] Applicants theorize without wishing to be bound thereto that
the diffusivity created by the roughening the end with 600 mesh
abrasive may be sufficient to reach near the maximum of light exit.
For larger width crystal array blocks as shown in Examples 3 and 4,
just roughening the end face with 600 mesh abrasive with the ESR
reflector layer may not be sufficient to reach the maximum of the
desired light diffusivity--replacing the ESR layer with Teflon tape
reflector gave another 11.5% improvement.
[0077] Comparing the light output of Examples 3 and 4 of 534 and
595, respectively, and that of Examples 8 and 9 of 518 and 521,
respectively, one can see the effect of crystal pixels width (3.86
mm vs. 1.52 mm) and also the aspect ratio (5.0 vs. 7.9) on the
light output of the array blocks. All these blocks are made with
LYSO crystals that have an average of light output around 650.
[0078] These examples show that the hybrid embodiments of Examples
4 and 9 can significantly improve the light output from the open
end as compared with the conventionally made detector arrays. The
embodiments will work for both arrays made with large width
crystals and arrays made with small width crystals. There may be a
desire for further optimization depending on the width as well as
the aspect ratio of the crystals. The examples demonstrated that it
is possible to get 79% and 90% of the scintillating light exiting
out of the open end of a small and a large size array block,
respectively.
[0079] All these examples demonstrate that specular reflector with
a high reflectivity is really not ideal to guide the light out at
the end of a long crystal. The reason is because there may be too
many high angle internal reflections. These multiple reflections
will cause most of the light to be absorbed by the walls without
any chance to get out. The diffusive reflector at the end will
break the light into many directions. The low incident angle of the
light will be reflected by the specular side surfaces of the
crystals. It is much easier for the light to get out of the crystal
end with a minimum number of reflections and thus a minimum loss.
This may be more true when the pixel length to width aspect ratio
becomes higher. With modern all solid state SiPM detectors with
thinner and thinner crystals, efficient light exit from the end of
the crystal has become an important issue to determine the
performance of a scanner. The hybrid embodiments disclosed herein
offer efficient approaches to increase the light exit from the end
face of an array block.
[0080] Many modifications and other embodiments of the invention
will come to the mind of one skilled in the art having the benefit
of the teachings presented in the foregoing descriptions and the
associated drawings. Therefore, it is understood that the invention
is not to be limited to the specific embodiments disclosed, and
that modifications and embodiments are intended to be included
within the scope of the appended claims.
* * * * *