Note: Descriptions are shown in the official language in which they were submitted.
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CROSS REFERENCE TO RELATED APPLICATION
This application is related to our copending application
serial no. 323,047, entitled GAMMA RAY C~RA, filed 9th March,
1979 and is related to U~S. Patent No. 4,140,900, issued
20 February 1979, and entitled PANEL TYPE X-RAY II~AGE
INTENSIFIER TUBE AND RADIOGRAPHIC CAMERA SYSTEM.
BACKGROUND OF THE INVENTION
This invention pertains to medical x-ray apparatus,
and more particularly to an x-ray image intensifier tube of
the proximity type for medical x-ray diagnostic use.
In U.S. Patent No. 4,140,900 a proximity type image
intensifier tube is described. The device uses all linear
components and has a high brightness gain. It also has several
constructional advantages which contribute to its safety in
use, as explained in greater detail in the patent. One
disadvantage, however, of the device is that its gain is
limited to about 5,000 cd - sec/m2 - R for high resolution,
high contrast applications,
The present applicants have found that many factors
contribute to this limitation. Brightness gain in the single
stage tube of the type described in Patent No. 4,140,900 is
proportional to the spacing between the scintillator-photocathode
screen and the output phosphor display screen and to the
accelerating electrostatic potential applied between them.
Wider spacing and higher potential, although producing a
higher gain, also reduce the contrast ratio and the resolution.
The reduction in contrast ratio is believed to be due
to certain feedback mechanisms operating within the tube. One
of these feedback mechanisms is that electrons which strike
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the output phosphor display s~reen are, in some cases,
reflected back and then are r~,accelerated back to the phosphor
display screen to strike it again at a different location,
thereby reducing both the contrast and the resolution. Also
light which is transmitted through the aluminum backing layer
on the display screen strikes the photocathode, which produces
corresponding electrons, which are then accelerated to strike
the phosphor display screen and again reduce the contrast as
well as the resolution. Still another feedback mechanism is
that because of the high acceleration applied to the electrons
traveling from the scintillator-photocathode screen, when the
electrons strike the phosphor display screen they produce ions
and x-rays which can find their way back to the scintillator and
produce unwanted "noise" in the image signal.
~ art of the resolution problem is that the scintillator-
photocathode surface is relatively rough due to the method
(vapor deposition) by which the scintillator material is applied
to the support surface. This produces a rough photocathode
surface which emits electrons in a relatively wide dispersion.
This dispersion is aggravated as the distance between the
scintillator-photoCathode screen and the output phosphor display
screen is increased.
Originally it was thought that simply increasing the
gain would not solve these problems but, on the contrary, would
merely aggravate the problem.
:~ SUMMARY OF THE INVENTION
The above disadvantages of a single stage proximity
type image intensifier tube were overcome by the applicant~s
invention which yields a much higher gain with even better
contrast ratio and resolution than a single stage type tube.
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The multi-stage image intensifier tube according to
applicant's invention comprises a flat scintillator screen,
an output display screen and multi-stage light amplification
means intermediate the scintillator screen and the output
display screen. The multi-stage light amplification means
include at least a first flat photocathode exposed with its
flat surfaces parallel to and adjacent to the scintillator
screen, and an intermediate flat phosphor display screen,
the display screen having its flat surfaces parallel to and
spaced apart from the flat surfaces of the photocathode and
on its side opposite from the scintillator screen. This
constitutes a first light amplification stage of the image
intensifier tube. A second light amplification stage of the
tube includes a fiberoptic plate, a second photocathode, and
the output phosphor display screen. me intermediate display screen,
that is the display screen of the first stage, is mounted on
one side of the fiberoptic plate and the second photocathode,
which together with the output phosphor display screen
constitutes the second stage, is mounted on the other side
of the fiberoptic plate. The output display screen is spaced
apart from the second photocathode and plane parallel to it. Ileans are
provided for applying an accelerating electrostatic potential
between the first display screen and the first photocathode
and for applying an accelerating electrostatic potential
between the intermediate display screen and the output display
screen. An open ended, hollow, evacuated envelope surrounds
the scintillator screen, the fiberoptic plate, the first and
second photocathodes, intermediate and output display screens,
and is closed at one end by a glass output window and at the
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opposite end by a concave metallic input window.
In the preferred embodiment the tube envelope i5
metal and the electrostatic potential means supply high
negative potentials to the scintillator screen, the first
and second photocathodes, and a ground potential to the
second display screen and the envelope.
The scintillation screen, the first and second
photocathodes and the first and second display screens have
substantially the same diagonal dimensions so that full size
x-ray images may be intensified, as opposed to the minified
images of some prior art non-proximity type image intensifier
; tubes.
In the preferred embodiment the scintillator screen
is a furnace grown scintillator crystal or vapor deposited
polycrystalline screen selected from the group consisting
essentially of CsI(Na) or NaI(Tl). Some embodiments further
include a barrier layer interposed between the scintillator
crystal and the photocathode. The barrier layer is transparent
and has an index of refraction which matches the index of
refraction of the scintillator crystal. The barrier layer is
made of a material selected from the group consisting essentially
of CsI(Na), CsI, Bismuth Germavate or Al203.
In the preferred embodiment the spacing between the
first photocathode and the first display screen is preferably
- 10 mm although in some embodiments the spacing may range
between 5 mm to 15 mm. The spacing between the second
photocathode and the second display screen is preferably 15 mm
in the preferred embodiment however in other embodiments the
spacing may vary between 10 mm and 25 mm. The electrostatic
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potential which is applied between the first photocathode and
the first display screen is preferably 20,000 volts however
in other embodiments it could range from 10,000 to 30,000 volts.
The potential which is applied between the second photocathode
and the second display screen is preferably 30,000 volts
although in other embodiments it could range between 20,000
to 40,000 volts.
In the preferred embodiment the envelope is metal
and the electrostatic potential means supply high negative
potentials to the scintillator screen and the first and second
photocathodes and a ground potential to the second display
screen and the tube envelope. An intermediate potential is,
of course, supplied to the first display screen.
It is therefore an object of the present invention
to provide an improved proximity type x-ray image intensifier
tube having both high gain and good resolution.
It is still another object of the invention to
provide an improved panel type x-ray image intensifier tube
having high gain and a high contrast ratio.
It is a further object of the invention to provide
a high gain x-ray image intensifier tube which is safe in its
operation.
The foregoing and other objectives, features and
advantages of the invention will be more readily understood
upon consideration of the following detailed description of
certain preferred embodiments of the invention, taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a diagrammatic illustration of the two
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stage proximity image intensifier tube according to the
invention;
Figure 2 is a vertical, sectional view of the image
intensifier tube of the invention;
Figure 3 is an enlarged, vertical, sectional view
of a portion of the image intensifier tube depicted in
Figure 2; and
Figure 4 is a vertical, sectional view, taken
generally along the lines 4-4 in Figure 2.
Figure 5 is a graph showing the total tube gain as
a function of total voltage across tube.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIME~T
Referring to Figures l and 2, a pane~shaped proximity
- type x-ray image intensifier tube 10 according to the invention
- is illustrated. The image intensifier tube 10 comprises a
-~ metallic, typically type 304 stainless steel, vacuum tube
envelope 12 and a metallic, inwardly concave input window 14.
The window 14 is made of a specially chosen metal foil or
alloy metal foil in the family of iron, chromium, and nickel
and in some embodiments, additionally combinations of iron
or nickel together with cobalt or vanadium. It is important
to note that these elements are not customarily recognized in
the field as a good x-ray window material in the diagnostic
region of the x-ray spectrum. By making the window thin, down
to 0.1 mm in thickness, the applicant was able to achieve high
x-ray transmission with these materials and at the same time
obtain the desired tensile strength. In particular, a foil
made of 17-7 PH type of precipitation hardened chromium-nickel
stainless steel is utilized in the preferred embodiment. This
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alloy is vacuum tight, high in tensile strength and has very
attractive x-ray properties: high transmission to primary
x-rays, low self-scattering, and reasonably absorbing with
respect to patient scattered x-rays. The window 14 is
concaved into the tube like a drum head.
The use of materials which are known for high
x-ray transmission such as beryllium, aluminum and titanium
for example cause the undesirable scattering which is present
in some prior art proximity type, x-ray image intensifier
devices.
One purpose of having a metallic window 14 is that
it can be quite large in diameter with respect to the prior
art type of convex, glass window without affecting the x-ray
image quality. In one embodiment, the window measures 0.1 mm
thick, 25 cm by 25 cm and withstood over 100 pounds per square
inch of pressure. The input window can be square, rectangular
or circular in shape, since it is a high tensile strength
material and is under tension rather than compression.
In operation, an x-ray source 16 generates a beam
of x-rays 18 which passes through a patient's body 20 and
casts a shadow onto the face of the tube 10. The x-ray image
passes through the window 14 and impinges upon a flat
scintillation screen 22 which converts the image into a light
image. This light image is contact transformed directly to an
immediately adjacent, first flat photocathode screen 24 which
converts the light image into a pattern of electrons. The
scintillator and photocathode screens 22 and 24 comprise a
complete assembly 23.
A first or intermediate phosphor display screen 26
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is mounted on one face of a fiberoptic plate 28 which issuspended from the tube envelope 12 by means of insulators
30. On the opposite face of the fiberoptic plate 28 a second
photocathode 32 is deposited. The fiberoptic plate 28 is
oriented in a plane parallel to the plane of the first
scintillation screen 22.
A second or output phosphor display screen 34 is
deposited on an output window 36. A high voltage power supply
38 is connected between the first phosphor display screen 26
and the first photocathode 24 as well as between the second
photocathode 32 and the second phosphor display screen 34.
The power supply is biased through a resistance divider 40
such that the potential between the first photocathode screen
24 and the first display screen 26 is -20Kv or approximately
80~ of the potential (-30Kv) between the second photocathode
32 and the second display screen 34. The first display screen
and the second photocathode are connected together to have the
same potential with respect to the second display screen 34.
In operation, the electron pattern on the negatively
charged first photocathode screen 24 is accelerated towards the
first, positively charged (relative to the photocathode screen
24), display screen 26 by means of the electrostatic potential
supplied by the high voltage source 38 connected between the
display screen 26 and the photocathode screen 24, The electrons
striking the display screen 26 produce a corresponding light
image (i.e. photons are emitted in a corresponding pattern)
which passes through the fiberoptic plate 28 to impinge on
the second photocathode 32. The photocathode 32 then re-emits
a corresponding pattern of electrons which are accelerated
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toward the display screen 34 to produce an output light image
which is viewable through the window 36.
Although the display screen 34 is positive with
respect to the photocathode screen 32, it is at a neutral
potential with respect to the remaining elements of the tube,
including the metallic envelope 12, to thereby reduce
distortion due to field emission
It should be noted that substantially no focusing
takes place in the tube as opposed to prior art, non-proximity
type tubes. The scintillator screen 22, the photocathode
screens 24, 32 and the display screens 26 and 34 are parallel
to each other. In contrast to the applicant's single stage
proximity image intensifier tube described in U. S. Patent No~
4,140,900, the gap spacing between the photocathodes and the
phosphor screens are relatively short. The spacing between
- the first photocathode screen 24 and the first display screen
26 is preferably 10 mm and the spacing between the second
photocathode 32 and the second display screen 34 is preferably
15 mm. In other embodiments these spacings could range between
5 to 15 mm and 10 to 25 mm, respectively. In the single stage
tube described in the above mentioned patent, the photocathode
to display screen spacing is much larger (20 mm) for high gain
3,000 - 5,000 cd-sec/~2-R tubes.
Furthermore, the applied voltages across the first
and second stage gaps between photocathode layers and the
display screens are 20,000 and 30,000 volts, respectively,
which are each lower than in the single stage tube described
in the patent. The voltage applied in high gain single stage
tubes is between 30-40 Kv. Thus, the voltage per unit of
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distance, ie., the field strengths of the two stage tube
according to the invention are 2 Kv/mm (first stage) and
2 Kv/mm (second stage).
By keeping the photocathode to phosphor screen
spacing and the field strength within the above mentioned
limits the two stage image intensifier tube is not only able
to achieve high gain at the same over-all operating voltage
(see Figure 5), on the order of 30,000 - 50,000 cd-sec/M2-R,
but is also able to do this with a higher resolution and
contrast ratio than the highest gain (3,000 - 5,000 cd-sec/M2-R)
single stage proximity type tubes. This is because the effect
of the dispersion of the electrons at the first photocathode
(due to the uneven, scintillator undersurface) are minimized
by the shorter photocathode to phosphor screen gap.
Also the various feedback mechanisms, such as ions
and x-rays generated at the output display screen are either
eliminated or greatly diminished in their effect, The lower
stage and shorter gap reduces the velocity and dispersion of
the electrons striking the display screen and therefore reduces
or eliminates the number of ions and x-rays which would be
generated by higher velocity electrons striking the display
screen. Also the fiberoptic plate 28, the photocathode 32 and
the phosphor screen 26 help prevent such spurious x-rays and
ions from reaching the scintillation screen 22 where they
would otherwise produce signal "noise".
The scintillation screen 22 can be calcium tungstate
(CaWO4) or sodium activated cesium iodide (CsI(Na)) or any
other type of suitable scintillator material such as NaI(Tl~.
However, vapor deposited, mosaic grown scintillator layers are
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preferred for the highly desired smoothness and cleanliness.
Since such materials and their methods of application are
well known to those skilled in the art, see for example,
U.S. Patent No. 3,~25,763 issued July 23, 1974 to Ligtenberg
et al, they will not be described in greater detail.
The overall thickness of the scintillator screen 22 is
chosen to be 50 to 600 microns thick to give a higher x-ray
photon utilization ability than prior art devices, thereby
allowing overall lower patient x-ray dosage levels without
a noticeable loss of quality as compared to prior art devices.
This is because the format of the tube and the absence of
several sources of "unsharpness" gives an extra margin of
sharpness to the image which can be traded off in favour of
lower patient dosage levels with greater x-ray stopping power
at the scintillator screen 22.
Similarly, the first and second photocathode layers
24 and 32 are also of a material well known to those skilled
in the art, being cesium and antimony (Cs3Sb) (industry
photocathode types S-9 or S-ll) or multi-alkali metal
,.,
(combinations of cesium, potassium and sodium) and antimony.
The image produced on the output phosHor screen 34
is the same size as the input x-ray image. Both of the phosphor
screens 26 and 34 can be of the well known zinc-cadmium sulfide
type (ZnCds(Ag)) or zinc sulfide type (ZnS(Ag)) or a rare earth
material like yttrium oxsulfide type (Y2O2S(Tb)) or any other
suitable high efficiency blue and/or green emitting phosphor
material.
Referring to Figure 3, the interiorly facing surfaces
of the display screens 26 and 34 are cover-ed with a metallic
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alumlnum film 40 and 40' in the standard manner. The phosphor
layer constituting the screen 34 is deposited on a high Z
glass output window 36. By high Z is meant that the window
glass has a high concentration of barium or lead to reduce
x-ray back scatter inside and outside the tube and to shield
the radiologist from both primary and scattered radiation.
Referring again more particularly to Figure 3, in
an enlarged cross-sectional view, the details of the scintillation
and photocathode screen assembly 23 and the fiberoptic plate
28 are illustrated. The screen assembly 23 comprises the
scintillator layer 22 of very smooth calcium tungstate or
sodium activated cesium iodide which is vapor deposited on a
smoothly polished nickel plated aluminum substrate or an
anodized aluminum substrate 42 which faces the input window
14. The techniques of such vapor deposition processes are
known to those skilled in the art, see for example, U. S.
Patent No. 3,~25,763. For direct viewing purposes, the layer
22 is between 200 to 600 microns thick. For radiographic
purposes, the layer 22 could be thinner (50 - 200~), i.e., the
image could be less bright.
As mentioned above, the purpose of the scintillator
screen 22 is to convert the x-ray image into a light image. On
the surface of the scintillation layer 72 which faces away from
the substrate 42, a thin, conductive, transparent electrode
layer 44 such as a vapor deposited metallic foil, ie., titanium
or nickel, is deposited and on top of this is deposited the
first photocathode 24. The first photocathode layer 24 converts
the light image from the scintillator layer 22 into an electron
pattern image and the free electrons from the first photocathode
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24 are accelerated by means of the high voltage potential
38 toward the first display screen 26, all as mentioned
above. The planar surface of the fiberoptic plate 28 which
faces toward the output window 36 is covered with a thin,
conductive transparent electrode layer 48 such as vapor
deposited metallic foil, ie. titanium or nickel. The second
photocathode layer 32 is then deposited on top of this layer.
The scintillator photocathode screen 23 in this invention is
suspended from the tube envelope 12 between the input window
14 and the fiberoptic plate 28 by several insulating posts 31.
One or more of these posts may be hollow in the center to allow
a high voltage cable 47 from the source 38 to be inserted to
provide the scintillator photocathode screen 23 at the layer
44 with a negative high potential. Similarly the electrodes 30
contain a high voltage cable 46 to connect the display screen
26 and the electrode 48 to the high voltage supply 38.
The remaining parts of the inten~ification tube
including the metallic envelope 12, are all operated at ground
potential. This concept of minimizing the surface area which
is negative with respect to the output screen results in reduced
field emission rate inside the tube and allows the tube to be
operable at higher voltages and thus higher brightness gain.
It also minimizes the danger of electrical shock to the patient
or workers if one should somehow come in contact with the
exterior envelope of the tube.
To reduce charges accumulated on the insulating posts
30, 31 they are coated with a slightly conductive material such
as chrome oxide which bleeds off the accumulated charge by
providing a leakage path.
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The essentially all metallic and rugged construction
of the tube minimizes the danger of implosion. The small vacuum
space enclosed by the tube represents much smaller stored
potential energy as compared with a conventional tube which
further minimizes implosion danger. Furthermore, if punctured,
the metal behaves differently from glass and the air supply
leaks in without fracturing or imploding.
The terms and expressions which have been employed
here are used as terms of description and not of limitations,
and there is no intention, in the use of such terms and
expressions, of excluding equivalents of the features shown
and described, or portions thereof, it being recognized that
various modifications are possible within the scope of the
invention claimed.
Further advantage could be obtained with 3-stage
in cases where both high gain and higher resolution is neededO
Additional stages, such as 3 or more are obvious extensions
of this invention.
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