Support assembly for scintillating crystal

A gamma camera includes at least one detector assembly 10. The detector assembly includes an array of photomultiplier tubes, a sheet of scintillating crystal material, a sheet of optical glass, and a matrix of mu-metal material. The mu-metal matrix defines an array of apertures corresponding to the array of photomultiplier tubes into which apertures the photomultiplier tubes are inserted. The scintillating crystal is bonded to the first surface of the optical glass. The mu-metal matrix is connected to the second surface of the sheet of optical glass using an adhesive such as an epoxy cement. By integrating the metal matrix into the structure formed by the glass sheet and the crystal, the glass sheet may be made thinner than before while the crystal is supported against possible bending and consequent fracture.

BACKGROUND 
The present invention relates to the art of nuclear imaging. It finds 
particular application in relation to gamma cameras having a detector 
which utilizes a scintillating crystal. 
In nuclear imaging, a radiopharmaceutical containing a radionuclide such as 
.sup.99m Tc or .sup.201 Tl is introduced into the body of a patient. As 
the radiopharmaceutical decays, gamma rays are generated. These gamma rays 
are detected and used to construct a clinically useful image. 
The gamma rays are detected using one or more detectors. These detectors 
ordinarily include a scintillator crystal which emits photons or light 
energy in response to incident radiation such as a gamma ray or other high 
energy photon. An array of photomultiplier tubes (PMTs) is used to detect 
the light emitted by the scintillator crystal. The signals generated by 
the PMTs are in turn used to determine the location and energy of the 
detected event. This information is used to produce an image indicative of 
the patient's anatomy. 
A sheet of optical glass has been placed adjacent the scintillator crystal 
on the side facing the source of radiation (i.e. on the side nearer the 
imaging region of the gamma camera). The sheet of optical glass has been 
bonded to the scintillator using a silicon optical adhesive in a procedure 
performed by the Bicron Technology business unit of St. Gobain/Norton 
Industrial Ceramics Corporation, located in Newbury, Ohio. The bond allows 
the crystal and the optical glass to be mechanically joined without 
adversely affecting the path of photons through the interface between the 
glass and crystal. Hence, one function of the optical glass is to provide 
structural support to the scintillator crystal. 
Conventionally, the scintillating crystals used in gamma cameras have had a 
thickness of 0.375 inches (0.9525 cm), and the optical glass has had a 
thickness of 0.675 inches (1.7145 cm). The bonding material has a nominal 
thickness of approximately 0.030 inches (0.0762 cm) such that the total 
thickness of the crystal-glass structure is approximately 1.030 inches 
(2.62 cm). 
A honeycomb of a nickel alloy commonly known as mu-metal has, together with 
the PMTs, been pressed against the rear surface of the optical glass. The 
mu-metal structure defines a plurality of hexagonal apertures into which 
the PMTs have been placed. The mu-metal structure surrounding each of the 
PMTs reduces the effects of the earth's magnetic field as the detector is 
moved about the patient. 
A number of factors make it desirable to vary the relative thicknesses of 
the optical glass and the scintillator crystal. One example arises in 
positron emission tomography (PET), a branch of nuclear medicine in which 
a positron emitting radiopharmaceutical such as .sup.18 
F-fluorodeoxyglucose (FDG) is introduced into the body of a patient. Each 
emitted positron reacts with an electron in what is known as an 
annihilation event, thereby generating a pair of gamma rays which are 
emitted in directions approximately 180 degrees apart, i.e. in opposite 
directions. The gamma rays produced by a positron annihilation are 
characterized by a photopeak at 511 keV, as compared to a 140 keV 
photopeak for .sup.99m Tc. 
A pair of detectors registers the positions and energy of the respective 
gamma rays, thereby providing information as to the position of the 
annihilation event and hence the positron source. Because the gamma rays 
travel in opposite directions, the positron annihilation is said to have 
occurred along a line of coincident connecting the detected gamma rays. A 
number of such events are collected and used to reconstruct a clinically 
useful image. 
Sensitivity and resolution are important gamma camera characteristics. A 
higher sensitivity permits the use of smaller doses of 
radiopharmaceutical. For a given amount of incident radiation, the gamma 
camera detects a larger number of events and thereby produces images 
having greater diagnostic utility. 
One factor which affects the sensitivity and resolution of a gamma camera 
is the efficiency of its scintillating crystal. In fact, many of the 
incident gamma rays pass through the crystal without any interaction and 
are thus not detected by the gamma camera. The efficiency of the crystal 
is also function of the energy of the gamma radiation. For example, 
conventional NaI(Tl) crystals have a lower efficiency at energies 
associated with PET imaging than at energies associated with more 
conventional nuclear imaging. 
In order to improve gamma camera performance, it is becoming increasingly 
important to increase the efficiency of the scintillating crystal. For 
example, nuclear cameras are increasingly being used to perform positron 
annihilation imaging, with its relatively higher energies. It is also 
desirable to increase the efficiency of the scintillating crystal at lower 
energies. 
One technique for improving the efficiency of the scintillator crystal is 
to increase its thickness. Gamma rays passing through the crystal are thus 
more likely to interact with the crystal and thus produce a flash of light 
detectable by the PMTs. There are, however, a variety of practical 
considerations which make it difficult to simply increase the thickness of 
the crystal. 
Among these considerations is the fact that optical design constraints 
limit the overall thickness of the detector. Moreover, retrofitting 
thicker crystals on existing cameras and camera designs is facilitated if 
the improved crystal structure fits in the pre-existing support structure. 
Further, adequate structural support for the relatively fragile crystal 
must still be provided. Thus, a technique which facilitates the use of a 
thicker scintillating crystal while providing adequate structural support 
in a size efficient package is needed. 
SUMMARY 
The present invention addresses these matters, and others. 
According to a first aspect of the present invention, an apparatus for use 
with a gamma camera detector assembly has an array of photodetectors in 
optical communication with a layer of scintillating material. The 
apparatus also includes a sheet of light transmissive material having 
first and second opposed surfaces. The first surface of the light 
transmissive material is adapted to support the scintillating material. 
The apparatus also includes a member which defines an array of apertures 
adapted to allow light emitted by the scintillating material to be 
received by the array of photodetectors. The member is rigidly attached to 
the second surface of the sheet of light transmissive material, and its 
array of apertures corresponds to the array of photomultiplier tubes. 
According to a more limited aspect of the present invention, the member is 
attached to the sheet of light transmissive material using an adhesive. 
According to a still more limited aspect of the present invention, the 
member is in physical contact with the second surface of the sheet of 
light transmissive material and the adhesive forms a bead along an 
interface thereof. 
According to a still more limited aspect of the present invention, the 
sheet of light transmissive material is made of glass, and the member 
includes a web of material extending in a direction normal to the second 
surface of the sheet of light transmissive material. According to still 
more limited aspects of the invention, the member is fabricated from 
mu-metal, and the photodetectors are photomultiplier tubes, and the 
photomultiplier tubes are inserted in the apertures. 
According to another aspect of the present invention, a gamma camera 
detector assembly includes a scintillator crystal having a first generally 
planar surface, an array of photomultiplier tubes in optical communication 
with the scintillator crystal, and a sheet of light transmissive material 
disposed between the scintillator crystal and the array of photomultiplier 
tubes. The sheet has first and second opposed, generally planar surfaces, 
and the first surface of the sheet supports the first surface of the 
crystal. The assembly also includes a mu-metal matrix which defines an 
array of apertures corresponding to the array of photomultiplier tubes, 
and each of the tubes is inserted in one of the apertures. The improvement 
is characterized in that the mu-metal matrix resists flexure of the sheet 
of light transmissive material. 
According to a more limited aspect, the mu-metal matrix is rigidly affixed 
to the second surface of the light transmissive material using an epoxy. 
According to yet another aspect of the present invention, a gamma camera 
includes first and second detectors mounted about an examination region 
for detecting gamma radiation emanating therefrom. The first and second 
detectors each comprise a scintillator crystal having opposed first and 
second surfaces, with the first surface facing the examination region. The 
detector also includes a member defining a plurality of apertures having 
axes normal to the second surface of the sheet of light transmissive 
material, a plurality of photodetectors in optical communication with the 
crystal, the apertures allowing light emitted by the scintillator crystal 
to be received by the photodetectors, and means for rigidly affixing the 
member to the second surface of the sheet of light transmissive material. 
According to a more limited aspect of the present invention, the gamma 
camera includes means for determining whether events detected by the 
respective detectors are characteristic of a positron annihilation event. 
According to another more limited aspect, the crystal has a thickness 
greater than or equal to 0.750 inches, and the sheet of light transmissive 
material has a thickness less than 0.375 inches. 
A first advantage of the present invention is that the efficiency of a 
gamma camera may be increased without increasing the thickness of the 
detector. 
Another advantage of the present invention is that a relatively thicker 
scintillator crystal may be provided with adequate structural support. 
Still another advantage of the present invention is that a scintillator 
crystal adapted for positron coincidence imaging may be readily installed 
in a conventional gamma camera. 
Still another advantage of the present invention is that additional 
structural support is provided without adding an existing member to the 
gamma camera. 
Still other advantages of the present invention will be appreciated by 
those skilled in the art upon reading and understanding the appended 
description.

DESCRIPTION 
With reference to FIG. 1, a gamma camera 12 includes a pair of radiation 
sensitive detectors 10a, 10b disposed in an opposed relationship about an 
examination region 11 in to which a patient (not shown) may be placed. The 
detectors 10 a, 10b include radiation sensitive faces 13 which detect 
gamma radiation emanating from within the examination region, for example 
caused by the decay of radionuclides such as .sup.99m Tc, .sup.201 Tl, 
FDG, or the like introduced into the anatomy of the patient. While two 
detectors are shown, it will be appreciated that a greater or lesser 
number of detectors may be used. Similarly, the detectors may be in other 
than an opposed relationship, for example at approximately right angles or 
spaced about the examination region 11 at equal angular intervals. As is 
known in the art, the detectors 10a, 10b are preferably rotatable about 
the examination region 11. 
Each detector 10 includes a crystal assembly 14 which is mounted in an 
enclosure 16 such as a generally rectangular lead box. The enclosure 16 is 
conventional and is preferably the same size and configuration as that 
used by Picker International, Inc., the assignee of this invention, in its 
currently marketed Prism 2000 cameras. 
With reference to FIGS. 2 and 3, the enclosure 16 includes a peripheral lip 
20 which projects inward around the bottom of the enclosure 16 (as viewed 
in FIGS. 2 and 3) and defines an opening in which the crystal assembly 14 
is received. The lip 20 has a vertical dimension of 0.375 inches (0.9525 
cm) as indicated by the dimension H in FIG. 3. 
The crystal assembly 14 is mounted in the enclosure 16. The crystal 
assembly includes a scintillating crystal 24, and a sheet of optical glass 
26. The crystal 24 is bonded to the glass 26 by Bicron Technology of 
Newbury, Ohio using a process which utilizes a silicon optical adhesive. A 
covering 32 of aluminum 0.050 inches (0.127 cm) thick protects the 
otherwise exposed face 34 of the crystal. The covering 32 is hermetically 
sealed along the perimeter of the optical glass sheet 26 using an epoxy. 
In a preferred embodiment, the crystal 24 has a thickness of approximately 
0.750 inches (1.905 cm), while the optical glass 26 has a thickness of 
approximately 0.375 inches (0.9525 cm). The optical adhesive layer 
introduces a nominal thickness of 0.030 inches (0.0762 cm). Thus, crystal 
and optical glass have a combined thickness of approximately 1.155 inches 
(2.9337 cm). The glass sheet has dimensions of 24.375.times.19.5 inches 
(61.9.times.49.5 cm). The crystal 24 is approximately one inch smaller in 
both dimensions, resulting in an overhang of glass 26 about 0.5 inches 
(1.27 cm) wide around the perimeter of the crystal 24. 
A preferred scintillator crystal 24 is NaI(Tl) (sodium iodide doped with 
thallium). However, the crystal 24 may be any other known scintillating 
material such as CsI(Na) (cesium iodide doped with sodium) or CSI(Th) 
(cesium iodide doped with thallium). 
The crystal assembly 14 rests on an aluminum shim 34, which in turn rests 
on the lip 20 of the enclosure 16. The shim 34 is dimensioned so that when 
the crystal assembly 14 is installed, the aluminum covering 32 is flush 
with the bottom 36 of the enclosure 16. Accordingly, the shim 34 has a 
thickness of approximately 0.375 inches (0.9575 cm). 
With continuing to FIGS. 2 and 3 and further reference to FIG. 4, a 
mu-metal honeycomb or matrix 28 defines an array of hexagonal apertures 
configured to accept a corresponding array of light sensitive devices such 
as conventional PMTs 29 in a conventional close-packed arrangement. The 
apertures have axes which extend in a direction normal to the glass sheet 
26. The matrix 28 has generally planar top 44 and bottom 46 surfaces which 
are orthogonal to the longitudinal axes of the tubes 42. The matrix 28 has 
a height of approximately 5.63 to 5.69 inches (14.30 to 14.45 cm). 
The matrix 28 is formed from a plurality of hexagonal tubes 42. Each of the 
hexagonal tubes 42 is formed from a sheet of Mu-metal having a thickness 
of 0.014 inch (0.356 mm). The tubes 42 are spot-welded together to form 
the matrix 28. 
While the majority of apertures are sized to accept PMTs 29 having a 
nominal diameter of 3 inches (7.62 cm), several PMTs near the edge of the 
detector have a nominal diameter of 2 inches (5.08 cm). Of course, the 
tubes 42 can be configured to receive PMTs having other form factors or 
having a different arrangement. Similarly, PMTs having hexagonal or square 
faces can readily be accommodated. 
A PMT is inserted into each aperture from the direction of the top surface 
44 such that the PMTs are constrained against the top surface 46 of the 
glass sheet 26 with their light sensitive faces facing the glass sheet 26. 
Thus, light emitted by the scintillator crystal 24 is detectable by the 
PMTs 29. A layer 51 of foam contains an array of die cut apertures which 
corresponds to the array of apertures in the matrix 28 and the array of 
PMTs 29. The apertures are slightly smaller than the diameter of the PMTs. 
The layer of foam 51 is held in place by a compression plate 50 such the 
PMTs 29 and the matrix 28 are pressed against the top surface 46 of the 
glass sheet 26. 
With reference to FIGS. 3, 4, 5 and 6, the bottom surface 46 of the matrix 
28 is rigidly attached to the top surface 40 of the glass sheet 26 using 
an epoxy adhesive. The preferred adhesive is marketed under the trade name 
EpoTek 310 by Epoxy Technology, Inc. of 14 Fortune Drive, Billerica, Mass. 
01821. The epoxy is mixed at a ratio of 10 parts resin to 5.5 parts 
hardener. After curing, the epoxy has a transmission of greater than 96% 
for light with wavelengths between 3400 and 9000 .ANG. and an index of 
refraction of 1.5071 and a lap shear strength of 570 PSI. 
The surfaces which are to be glued or bonded are first cleaned with a 
suitable cleaner such as glass cleaner or alcohol, and the matrix 28 is 
placed in position on top of the glass sheet 26. After mixing, the epoxy 
is cured for two (2) hours at a temperature of 65 degrees Fahrenheit (room 
temperature). A syringe having a needle with an inner diameter of 0.070 
inches (0.1778 cm) is then used to apply a bead 48 of epoxy having a width 
of approximately 0.125 inches (0.3175 cm) to the interface between the 
matrix 28 and the glass sheet 26. 
With reference to FIG. 5, the bead 48 of epoxy is applied along the 
interface in the areas marked with heavy lines. Although the lines are 
shown to be spaced apart from the matrix 28 for ease of illustration, it 
will be appreciated that the epoxy is applied at the interface between the 
matrix 28 an the glass sheet, taking care to apply the glue to the two 
surfaces evenly. The epoxy is allowed to cure at room temperature for at 
least twenty four hours before handling. The crystal assembly may then be 
installed in the detector assembly 10. 
The honeycomb 28 shown is formed of sheets or webs of mu-metal bent into 
hexagonal tubes and connected to each other. Other configurations are also 
possible. For example, the metal could be arranged to form square or 
diamond shaped tubes, or the honeycomb could be formed of cylinders 
connected at their tangent points. Alternately, the honeycomb may be 
formed from a single member with material removed to form aperture or or 
apertures, or it could be cast, molded, or the like in a desired 
configuration. Of course, in practicing the invention, the designer must 
consider the size and shape of the radiation detecting device, such as a 
PMT, that will be installed in the honeycomb. The primary consideration is 
that the matrix 28 should, when bonded to the optical glass, resist 
flexure of the glass sheet 26. 
The matrix 28 has been described as being rigidly affixed to the optical 
glass using an epoxy. The matrix 28 may be bonded to the glass using other 
suitable adhesives, for example a pressure sensitive adhesive die cut to 
conform to the web of material which forms the matrix. The adhesive thus 
adheres the matrix 28 to the glass 26 without interfering with light 
transmission to the PMTs. Similarly, other mechanical fasteners may also 
be used, provided that they provide a suitable interconnection without 
unduly interfering with transmission of light from the crystal 24 to the 
PMTs. 
With reference to FIG. 7, the outputs of the PMTs 29 are fed to coincidence 
logic circuitry 52 and position and energy determining circuitry 54a, 54b. 
The coincidence logic 52 determines if events are detected substantially 
simultaneously by detectors 10a, 10b and are thus characteristic of a 
positron annihilation event. If so, the position and energy determining 
circuitry 54a, 54b determines the positions and energies of the detected 
events. The information is fed to a processing computer 56 which uses the 
uses conventional techniques to reconstruct a clinically useful image of 
the patient. The image is displayed on an operator interface 58 which 
includes a computer monitor, printer, or the like. In conventional nuclear 
imaging, the coincidence logic circuitry 52 may be omitted or disabled. 
In operation, the detectors 10a, 10b are installed on the gamma camera 12 
with their radiation sensitive faces 13 facing the examination region 11. 
Gamma rays emitted from a patient inserted in the examination region 
interact with the scintillating crystals 24, with these interactions 
producing a flash of light. Light traveling in the direction of the PMTs 
passes through the optical glass 26 and is received by one or more of the 
PMTs 29. 
In positron coincidence imaging, the coincidence detection circuitry 52 
determines if the detected events are valid coincidence events. If so, the 
locations and energies of the events are appropriately logged and used to 
produce an image indicative of the internal anatomy of the patient. In 
conventional nuclear imaging, the locations and energies of the detected 
events are logged and used to produce an image indicative of the patient's 
internal anatomy. 
The detectors 10a, 10b and the scintillating crystals 24 are subject to 
varying mechanical stresses, for example caused by the compression plate 
50 and foam 51, during shipment and handling of the gamma camera 12 or as 
the detectors 10a, 10b are rotated about the examination region. The 
mu-metal matrix 28 imparts structural rigidity to the optical glass 26 and 
hence provides additional support to the scintillating crystals 24. In 
particular, the matrix 28 inhibits flexure of the glass sheet 26 in the 
directions normal to the plane defined by the sheet 28. 
It should be noted that the present invention is not limited to use in 
gamma cameras for use in positron coincidence imaging. It finds particular 
application in systems having detectors where additional structural 
rigidity is desired. Thus, for example, the invention is applicable where 
the optical material has undesirable physical properties, the optical 
material is relatively thin, where the scintillator material is relatively 
fragile, or in wide field of view or other large detectors. 
It should also be noted that the matrix 28 may be fabricated from materials 
other than mu-metal. It may be manufactured, for example, from other 
metals, polymers, or other materials possessing a desired structural 
rigidity. It is also not necessary that the height of the matrix 28 be 
sufficient to extend the length of the PMTs 29, provided that the 
configuration of the matrix 28 imparts the desired rigidity. It is 
particularly advantageous, however, if the height of the matrix 28 is 
sufficient to locate the PMT's in their respective positions in the x,y 
matrix. 
The invention has been described with reference to the preferred 
embodiment. Obviously, modifications and alterations will occur to others 
upon reading an understanding the preceding description. It is intended 
that the invention be construed as including all such modifications and 
alterations insofar as they come within the scope of the appended claims 
or the equivalents thereof.