U.S. patent application number 16/405310 was filed with the patent office on 2019-11-07 for multi-pixel x-ray source with tungsten-diamond transmission target.
This patent application is currently assigned to Washington University. The applicant listed for this patent is Praneeth Kandlakunta, Tiezhi Zhang. Invention is credited to Praneeth Kandlakunta, Tiezhi Zhang.
Application Number | 20190341219 16/405310 |
Document ID | / |
Family ID | 68385189 |
Filed Date | 2019-11-07 |
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United States Patent
Application |
20190341219 |
Kind Code |
A1 |
Zhang; Tiezhi ; et
al. |
November 7, 2019 |
MULTI-PIXEL X-RAY SOURCE WITH TUNGSTEN-DIAMOND TRANSMISSION
TARGET
Abstract
A multi-pixel x-ray source is provided. The x-ray source
includes a plurality of transmission target assemblies. The
transmission target assembly includes a tungsten target and a
diamond substrate. The substrate includes a first transmission
surface and a second transmission surface opposite first
transmission surface. The substrate further includes a first side
surface and a second side surface disposed between the first and
second transmission surfaces. The target covers the first
transmission surface of the substrate. The transmission target
assembly further includes a base. The base surrounds the first and
second side surfaces of substrate, exposing a collimator surface of
the second transmission surface and the target. The transmission
target assembly is configured to transmit x-ray generated by the
target through the target and the substrate.
Inventors: |
Zhang; Tiezhi; (St. Louis,
MO) ; Kandlakunta; Praneeth; (St. Louis, MO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zhang; Tiezhi
Kandlakunta; Praneeth |
St. Louis
St. Louis |
MO
MO |
US
US |
|
|
Assignee: |
Washington University
St. Louis
MO
|
Family ID: |
68385189 |
Appl. No.: |
16/405310 |
Filed: |
May 7, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62667929 |
May 7, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 35/116 20190501;
G06F 2111/08 20200101; H01J 2235/1291 20130101; G06F 30/23
20200101; H01J 2235/068 20130101; H01J 2235/088 20130101; G06F
2119/08 20200101; G06F 30/20 20200101; H01J 2235/081 20130101; H05G
1/22 20130101; H01J 35/12 20130101; H01J 2235/083 20130101 |
International
Class: |
H01J 35/08 20060101
H01J035/08; G06F 17/50 20060101 G06F017/50 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &
DEVELOPMENT
[0002] This invention was made with government support under Grant
No. 1R03EB024952-01 awarded by National Institutes of Health. The
government has certain rights in the invention.
Claims
1. (canceled)
2. A transmission target assembly, comprising: a substrate, wherein
the substrate comprises: a first transmission surface; a second
transmission surface opposite the first transmission surface, the
second transmission surface including a collimator surface; a first
side surface; and a second side surface, the first and second side
surfaces being disposed between and connecting the first and second
transmission surfaces; a target covering at least a portion of the
first transmission surface of the substrate; and a base, wherein
the base surrounds the first and second side surfaces and a portion
of the second transmission surface such that the collimator surface
of the second transmission surface and the target are exposed.
3. The transmission target assembly of claim 2, wherein the
substrate comprises diamond.
4. The transmission target assembly of claim 2, wherein the target
comprises tungsten.
5. The transmission target assembly of claim 2, wherein the
transmission target assembly is configured to operate in a pulse
mode.
6. The transmission target assembly of claim 2, wherein the
transmission target assembly is optimized by a Monte Carlo
simulation.
7. The transmission target assembly of claim 6, wherein the Monte
Carlo simulation comprises a finite element thermal simulation of a
temperature distribution on the transmission target assembly.
8. The transmission target assembly of claim 2, wherein the target
comprises a thickness in a range from approximately 5 .mu.m to
approximately 6 .mu.m, and the transmission target assembly is
configured to receive electronic beams having an energy in a range
from approximately 60 kVp to approximately 140 kVp.
9. The transmission target assembly of claim 2, wherein the
transmission target assembly is configured to transmit x-rays
through the target and through the substrate at the collimator
surface of the substrate.
10. The transmission target assembly of claim 9, wherein the base
of the transmission target assembly comprising a collimator edge,
the collimator edge forming a collimator and configured to guide
the x-rays toward a fluence detector.
11. A multi-pixel x-ray source, comprising: an anode including a
target assembly, the target assembly comprising: a substrate; and a
target disposed on the substrate, the target comprising a plurality
of focal spots, wherein the anode is configured to emit x-rays from
the plurality of focal spots; and a housing enclosing the anode,
the housing further comprising a viewport disposed on the housing,
the viewport configured to estimate a temperature of the plurality
of focal spots based on a color of the plurality of focal
spots.
12. The multi-pixel x-ray source of claim 11, wherein the target
assembly is a transmission target assembly.
13. The multi-pixel x-ray source of claim 11, wherein the target
assembly is a reflection target assembly.
14. The multi-pixel x-ray source of claim 13, wherein the substrate
of the target assembly comprises annealed pyrolytic graphite
(APG).
15. The multi-pixel x-ray source of claim 13, wherein the substrate
of the target assembly comprises pyrolytic graphite (PG), and an
a-b plane of the PG of the substrate is aligned with a
cross-section of the housing.
16. A multi-pixel x-ray source having a plurality of target
assemblies, each target assembly comprising: a target comprising a
plurality of tungsten layers laminated in pyrolytic graphite (PG);
and a base comprising PG, the base including a lateral surface,
wherein the target is disposed on the lateral surface of the base,
wherein heat is conducted in a direction of starting from the
target toward the base.
17. The multi-pixel x-ray source of claim 16, wherein the PG
comprises annealed pyrolytic graphite (APG).
18. The multi-pixel x-ray source of claim 16, the PG including an
a-b plane, wherein the a-b plane of the PG is aligned with the heat
conduction direction of each target assembly.
19. The multi-pixel x-ray source of claim 18, wherein each target
assembly comprises an APG plate having its a-b plane perpendicular
to a direction of its thickness, and the plurality of target
assemblies are stacked together along the direction of the
thickness of the APG plate.
20. The multi-pixel x-ray source of claim 16, wherein each tungsten
layer of the target is deposited in the PG by chemical vapor
deposition (CVD).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 62/667,929, filed May 7, 2018, entitled
"MULTI-PIXEL X-RAY SOURCE WITH W-DIAMOND TRANSMISSION TARGET,"
which is hereby incorporated in its entirety herein.
BACKGROUND
[0003] Kilovoltage X-ray imaging is one of the most common
diagnostic imaging modality in radiology as well as image guided
intervention and radiotherapy. In x-ray generation, a high voltage
is used to accelerate electrons released by a cathode to a high
velocity and the high-velocity electrons collide with a target on
an anode, creating x-rays. One of the predominant x-ray production
processes is Bremsstrahlung interaction process, where radiation is
given off by electrons as they are scattered by the strong electric
field near high-Z (proton number) nuclei. This process is highly
inefficient, where only 1% of the energy is converted to x-ray
photons. The rest of the electrons' kinetic energy is converted to
heat, deposited on the target, and eventually dissipated to the
environment.
BRIEF DESCRIPTION
[0004] In one aspect, a multi-pixel x-ray source is provided. The
x-ray source includes a plurality of transmission target
assemblies. The transmission target assembly includes a tungsten
target and a diamond substrate. The substrate includes a first
transmission surface and a second transmission surface opposite
first transmission surface. The substrate further includes a first
side surface and a second side surface disposed between the first
and second transmission surfaces. The target covers the first
transmission surface of the substrate. The transmission target
assembly further includes a base. The base surrounds the first and
second side surfaces of substrate, exposing a collimator surface of
the second transmission surface and the target. The transmission
target assembly is configured to transmit x-ray generated by the
target through the target and the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The drawings described below illustrate various aspects of
the disclosure.
[0006] FIG. 1 shows the three key heat transfer steps in an x-ray
tube.
[0007] FIG. 2A shows a schematic diagram of the cross-section of a
transmission target.
[0008] FIG. 2B shows a schematic diagram of the cross-section of a
reflection target.
[0009] FIG. 3A shows a simulation model of x-ray generation using a
transmission target.
[0010] FIG. 3B shows a simulation model of x-ray generation using a
reflection target.
[0011] FIG. 4A shows simulated trajectories of 120 keV electrons in
a transmission target.
[0012] FIG. 4B shows simulation results of energy deposition as a
function of depth for 80 keV, 100 keV and 120 keV electrons.
[0013] FIG. 5A shows transmission x-ray fluence from a transmission
target as a function of tungsten thickness with different electron
beam energies.
[0014] FIG. 5B shows energy fluence of transmission x-rays from a
transmission target as a function of tungsten thickness with
electron beam energy at 120 keV.
[0015] FIG. 6 shows optimal thickness of the tungsten (W) layer in
a transmission target as a function of electron energy.
[0016] FIG. 7 shows fluence efficiency of transmission targets
having tungsten thicknesses from 1 .mu.m to 11 .mu.m as a function
of electron beam energies.
[0017] FIG. 8 shows x-ray spectra of a 5 .mu.m thick transmission
target and a 5 mm thick reflection target as a function of
energy.
[0018] FIG. 9 shows x-ray fluence profiles from transmission
targets having different thicknesses and x-ray fluence profile of a
reflection target, where the electron beam energy is at 120
keV.
[0019] FIG. 10A is a schematic diagram of the transmission target
model used in the simulations.
[0020] FIG. 10B shows simulated temperature distribution near the
focal spot on the W-diamond transmission target shown in FIG. 10A,
where a 3 ms, 11 kW beam is used in the simulations.
[0021] FIG. 10C shows simulated temperature distribution in the x-y
plane of the target shown in FIG. 10A with a 3 ms, 11 kW beam.
[0022] FIG. 11A shows variation of the maximum focal spot
temperatures versus time for a transmission target.
[0023] FIG. 11B shows variation of the maximum focal spot
temperatures versus time for a reflection target.
[0024] FIG. 12 shows maximum electron beam power as a function of
pulse width for a transmission target and a reflection target.
[0025] FIG. 13A shows a schematic diagram of a tungsten-annealed
pyrolytic graphite (W-APG) laminate target on an annealed pyrolytic
graphite (APG) anode base.
[0026] FIG. 13B shows the thermal conductivity of APG compared to
other materials.
[0027] FIG. 14A shows an exemplary multi-pixel thermionic emission
x-ray (MPTEX) prototype.
[0028] FIG. 14B shows a functional diagram of the prototype shown
in FIG. 14A.
[0029] FIG. 15A shows the temperature distribution of a tungsten
target, generated by a simulation.
[0030] FIG. 15B shows temperature distributions of a dual layer
W-PG target with the pyrolytic graphite (PG) having a low a-b plane
thermal conductivity.
[0031] FIG. 15C shows temperature distributions of a dual layer
W-PG target with the PG having a high thermal conductivity.
[0032] FIG. 15D shows maximum power allowed for the three types of
anodes illustrated in FIGS. 15A-15C, while keeping focal spot
temperature under 3000.degree. C.
[0033] FIG. 16A shows a cross section of the MPTEX tube shown in
FIG. 14A.
[0034] FIG. 16B shows the APG anode plate of the tube shown in FIG.
16A.
[0035] FIG. 17 shows CVD apertures for W and PG deposition.
[0036] FIG. 18 shows a block diagram illustrating a computing
device in accordance with an aspect of the disclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The present disclosure is directed to a high brightness and
high-efficiency x-ray source. The x-ray source may be used for
tetrahedron beam computed tomography (TBCT). The present disclosure
is based, at least in part on, a tungsten diamond or pyrolytic
graphite laminated target. As shown herein, a W-diamond target
configuration can be used to improve the tube power, such as by
improving the focal spot power density. Further, a multiple-pixel
x-ray source using a transmission target eases the geometry design
of an x-ray tube and increases the tube power with multiple
pixels.
[0038] Because of the inefficiency in the x-ray generation
processes, a majority of electrons' kinetic energy is converted to
heat. Heat management is therefore important in x-ray tube design
to protect the x-ray generating target.
[0039] FIG. 1 illustrates the three major steps of heat transfer in
an x-ray tube. Electrons' kinetic energy is converted to heat at
the focal spot, the location on the target where the electrons hit,
when the high velocity electrons hit the target. The heat is
conducted to the anode body that surrounds the target. The heat is
then radiated if a rotating anode is used, or conducted if a fixed
anode is used, to the housing of the x-ray generation tube or tube
housing. The heat is then dissipated to the atmosphere through the
tube housing.
[0040] With large amount of heat deposited on a small spot size,
the maximum focal spot power density of x-ray tubes is about 0.5-1
kW/mm2, limited by the thermal conductivity of the target material,
before the target melts. Modern x-ray tubes may have a peak power
as high as 100 kW, thus thermal management is very challenging in
the design of x-ray sources. Most x-ray tubes use a rotating anode
to spread the heat from the small focal spot to a larger area. Even
so, high power x-ray tubes still need a relatively large focal spot
size to prevent the tungsten target from being damaged by the heat.
A rotating anode used to manage heat does not work well for
multi-pixel x-ray sources, because of the sources' elongated
geometries. With the heat distributed to a plurality of focal spot
positions, multi-pixel x-ray sources may use a fixed anode that may
conduct the heat to outside the tube at a fast speed, however, the
total area of focal spots is still limited.
[0041] Further, focal spot power density also affects image
resolution. The image resolution of x-ray systems is determined
jointly by focal spot size and detector pixel size. While current
x-ray detector technology can fabricate detectors with very small
pixel size, the focal spot size of x-ray sources remains at
.about.1 mm due to the limitation of focal spot power density, as
described above. Image resolution is also important for CT, as well
as for other x-ray imaging systems.
[0042] Besides image resolutions, x-ray tube focal spot power
density also limits imaging speed, especially for x-ray scanners
with fixed anode tubes, such as micro-focus and multi-pixel x-ray
sources. Focal spot power density also limits CT imaging speed,
especially for inverse-geometry CT15 and TBCT, where the
multi-pixel x-ray sources use a fixed anode. Due to limited
multi-pixel thermionic emission x-ray (MPTEX) tube output, slow
rotating gantry machines, such as c-arm Linacs, are used in
TBCT.
[0043] Limited focal spot power density imposes an even more
significant constraint on phase-contrast CT and micro-focus x-ray
sources that are used in small animal imaging platforms. In
phase-contrast CT, a grating interferometer is often used to
produce coherent x-ray beams. X-ray photons are largely blocked by
the grating interferometer, thus a much higher flux is needed from
x-ray sources in phase-contrast imaging. In micro-focus x-ray
sources, the electron beam is focused to a tiny spot smaller than
100 .mu.m for small animal imaging. Micro-focus x-ray sources also
have to use a fixed anode due to the long scanning time.
[0044] Accordingly, limited focal spot power density of x-ray
sources becomes a major roadblock in the further advancement of
x-ray imaging technologies. The high output of MPTEX source with
the anodes disclosed herein allows faster imaging speed and higher
image resolution.
[0045] Due to engineering difficulties, multi-pixel x-ray sources
have multiple targets that are spatially distributed. A multi-pixel
x-ray source can also be referred to as a distributed x-ray source.
A multi-pixel x-ray source uses a stationary reflecting anode.
Unlike conventional x-ray tubes with a single focal spot, the heat
of distributed x-ray source is distributed to a plurality of focal
spot positions. The total area of focal spots is, however, still
limited. A multi-pixel x-ray source operates in a pulse mode. The
x-ray pixels are activated sequentially with a short dwell
duration. Oosterkamp described the power limitation of the x-ray
target as,
P = .DELTA. TA 2 .pi..lamda..rho. c t , ( 1 ) ##EQU00001##
where P is the electron beam power deposited in the focal spot,
.DELTA.T is the temperature rise, A is the focal spot area, t is
the dwell duration, and .lamda., .rho. and c are respectively the
thermal conductivity, density, and specific heat of the target
material. The maximum power allowed is inversely proportional to
the square root of dwell duration.
[0046] By rotating the anode, the dwell duration of electrons
reduces with increase in rotation speed of the anode. Development
of multi-pixel x-ray sources with a rotating anode is challenging
due to its elongated anode geometry. Multi-pixel x-ray sources can
reduce dwell duration by increasing the scanning speed. The
scanning speed, however, is limited by the imaging detector readout
speed. For rotating anode x-ray sources, the anode rotates around
the x-ray focal spot while the x-ray focal spot remains at the same
position, and thus the detector integration time is independent of
the anode rotation speed. For multi-pixel x-ray sources, the data
of each x-ray pulse need to be differentiated as they represent
sampling at different locations. The x-ray detectors used by CT
scanners can be read out at about 10,000-20,000 samples per second,
which limits the minimum dwell duration to about 50 .mu.s for
multi-pixel x-ray sources.
[0047] Enhanced thermal performance of the x-ray target material
described herein is used to increase the focal spot power density.
According to the Equation 1, the power of an x-ray source is
proportional to the square root of the thermal conductivity of the
target material, such as tungsten (W). Diamond has the highest
thermal conductivity (2200 W/mK) among all known materials. But,
because of the low-Z number of carbon atoms, diamond is inefficient
in x-ray production. The two materials are combined to improve
x-ray production. In one embodiment, a thin layer of tungsten is
deposited or grown on the diamond substrate to improve x-ray
production. Alternatively, diamond is grown on tungsten substrate.
In some embodiments, chemical vapor deposition (CVD) techniques are
used to achieve diamond thickness on the order of few mm with a
high growth rate. The temperature of the substrate for growing
diamond is kept above 700.degree. C. to enhance the growth of
diamond crystals and also suppress the growth of graphite. Tungsten
is used as substrate materials where a localized carbide layer of a
few nm is formed. Diamond crystals can be grown on diamond and
non-diamond substrate like copper, gold, silicon or tungsten by
chemical transport in a closed system. Substrates made of carbides
such as SiC, WC and TiC are particularly suitable for diamond
deposition. A pressure vapor deposition technique may be used to
fabricate a W-diamond target.
[0048] FIG. 2A illustrates a cross-sectional view of a transmission
target assembly 200. Transmission target assembly 200 may
correspond to a single x-ray source pixel in a multi-pixel x-ray
source array. For comparison, a reflection target assembly 202 is
shown in FIG. 2B. In transmission target assembly 200, x-rays pass
through the target. In reflection target assembly 202, x-rays are
reflected by the target.
[0049] In the exemplary embodiment, a transmission target assembly
200 comprises a target or transmission target 204 and a substrate
206. Target 204 may be made of tungsten. Substrate 206 may be made
of diamond. Substrate 206 comprises a first transmission surface
208 and a second transmission surface 210 opposite first
transmission surface 208. Substrate 206 further comprises a first
side surface 214 and a second side surface 216. First and second
side surfaces 214, 216 are disposed between first and second
transmission surfaces 208, 210. Target 204 covers first
transmission surface 208 of substrate 206. Transmission target
assembly 200 further comprises a base 212. Base 212 may be made of
copper. Base 212 surrounds first and second side surfaces 214, 216
of substrate 206, exposing a collimator surface 218 of second
transmission surface 210 and target 204. Base 212 may form a
collimator edge 220. Target 204 may be made of a thin tungsten
layer deposited on a diamond substrate 206 brazed on a copper base
212. Generated x-rays are transmitted through target 204 and
substrate 206, and further through collimator surface 218.
Collimator edge 220 of base 212 forms a collimator and guide the
generated x-ray into a fan or cone shape.
[0050] In comparison, reflection target assembly 202 comprises a
reflection target or target 222 and a base 224. Base 224 surrounds
all outer surfaces of target 222, except a reflection surface 226.
Target 222 may be made of a thick tungsten slab embedded in copper
base 224. The generated x-rays are reflected from target 226 and
received by a detector.
[0051] A transmission x-ray target produces uniform x-ray beam
intensity without producing "heel-effect" characteristic of a
reflective target, where only fluence at a large angle can be used.
It also allows more compact tube geometry, which is important for
multi-pixel x-ray sources. The tungsten layer is thin enough such
that the diamond target is used for transmission.
[0052] In one embodiment, transmission target assembly 200
comprises a thin tungsten film deposited on a .about.2 mm thick
diamond substrate brazed on a copper or graphite base. Diamond is
stable at high temperature in vacuum environment. Its high thermal
conductivity allows fast heat removal from target 204 and its low
atomic number results in low x-ray attenuation and low
Bremsstrahlung yield.
[0053] The copper base 212 of the transmission target 204 not only
allows for fast heat removal from the target 204, but also
collimates the beams into cone- or fan-shaped beams. Primary
collimation close to the target allows a multi-pixel x-ray source
with finer pixel spacing.
[0054] In the exemplary embodiment, the diamond transmission target
is evaluated and optimized using Monte Carlo and finite element
thermal simulations.
Monte Carlo (MC) Simulations of Energy Deposition
[0055] In the exemplary embodiment, transmission target assembly
200 comprises a thin layer of tungsten (W) deposited on a diamond
substrate. The thickness of W in the transmission target was
optimized using Geant4 Monte Carlo (MC) simulations. A transient
thermal model was built in a finite element analysis software.
Finite element thermal simulations were performed to evaluate
temperature distributions in the target under different power
loadings. The maximum allowed power while keeping the target
temperature below 3000.degree. C. was determined for different
pulse widths. The x-ray fluence and thermal performance of the
transmission target were compared to that of a reflection
target.
[0056] Electrons impinging on the target deposit their energy at
various depths in the target. To model the energy deposition for
the purpose of thermal analysis, MC simulations were performed
using Geant4 simulation toolkit. Energy deposition of 80 keV, 100
keV and 120 keV electrons were obtained as a function of depth in
tungsten.
Monte Carlo Simulation of X-Ray Fluence and Spectrum
[0057] The geometrical model of a transmission target for the
Geant4 MC simulation is shown in FIG. 3A. The MC model of x-ray
source with a transmission target comprising a monoenergetic
electron beam striking the tungsten-diamond composite target in a
1.times.1 mm.sup.2 focal spot area perpendicularly. The x-ray
fluence was recorded in a detector positioned at a distance of 3 cm
from the focal spot. MC simulations were performed for different
thicknesses of the tungsten target, while the thickness of diamond
was kept at 2 mm.
[0058] For comparison, a reflection target was also modeled by MC
simulation. FIG. 3B shows the model of the reflection target used
in the Geant4 MC simulation. The x-ray fluence is scored for a
15.degree. anode angle 3 cm away from the focal spot. Due to the
anode angle, focal spot area is changed to 1.times.2 mm.sup.2, for
the reflection target model. For both reflection and transmission
target models, a 3 mm aluminum (Al) layer was used as low-energy
x-ray absorber to filter out the low energy photons that do not
come out of the vacuum chamber of x-ray tube, to mimic filtration
by vacuum envelope of an x-ray tube.
[0059] The x-ray fluence generated by the transmission target is
expected to increase with the thickness of tungsten until all
electrons are stopped. However, the self-absorption of tungsten
target also increases with the thickness of tungsten. Thus, the
x-ray fluence of the transmission target reaches a maximum at a
particular thickness for a given electron beam energy. MC
simulations were performed for electron energies in the range
40-140 keV and x-ray fluence for different thicknesses of tungsten
target were calculated. On the other hand, the thickness of
reflection target has no effect on x-ray fluence, therefore
thickness of the reflection target was not changed, and only a 5 mm
thick W target was modelled.
Finite Element Transient Thermal Simulation of Target
Temperature
[0060] To evaluate the focal spot power density limitation, finite
element thermal simulations were performed to study the focal spot
temperature and heat dissipation rate. Finite element models of the
W-diamond transmission target and W reflection target were built
using a Multiphysics Finite Element Analysis (FEA) software. The
FEA model of W-diamond target comprises a 5 .mu.m tungsten target
and 2 mm thick diamond substrate on a copper base as shown in FIG.
2A. The focal spot was modeled as a multilayer heating element with
the power as a function of depth generated by the MC simulation. A
focal spot area of 1.times.1 mm.sup.2 was used in all the
simulations of the transmission target. Only one-fourth of the
actual volume was modeled because of the symmetry in the target
geometry.
[0061] Temperature dependence of tungsten thermal conductivity and
specific heat were included in the model. The temperature of the
top and bottom surfaces were kept constant at 373 K as the boundary
condition, assuming the tube is water cooled.
[0062] Transient thermal simulations were performed with different
incident electron beam energies as the pulse-width varied from 50
.mu.s to 3 ms. The resulting transient temperature distributions of
the x-ray focal spot at different pulse widths and powers were
calculated. The maximum allowed tube power while keeping the target
temperature below 3000.degree. C. for a given pulse duration were
determined.
[0063] A 5-6 .mu.m W layer of the transmission target is suitable
for x-ray systems having peak kilovoltages (kVps) in the ranges of
60-140, which is commonly used for human imaging. Results indicated
that the x-ray fluence of the transmission target can be 20-30%
greater than that of reflected x-rays with electron beams at the
same energy deposited onto the target. The W-diamond transmission
target is able to achieve high power operation under short pulse
loadings. The W-diamond target enables as much as a four-fold
higher power or 8 times higher power density than the reflection
target for the same temperature threshold.
Energy Deposition in the Target
[0064] The penetration of 80 keV, 100 keV and 120 keV electrons and
their energy deposition as a function of depth in tungsten were
modeled using the MC simulations described above. The results are
shown in FIGS. 4A and 4B. FIG. 4A shows the trajectories of 120 keV
electrons in a 10 .mu.m tungsten target. FIG. 4B shows Geant4 MC
simulation results of energy deposition as a function of depth for
80 keV, 100 keV and 120 keV electrons. The maximum depth of 120 keV
electrons deposited in tungsten is less than 8 .mu.m, which is
about half of the continuous slow down approximation (CSDA) range
(described in tungsten-pyrolytic graphite (W-PG) laminate target).
Therefore, a thinner tungsten layer can be used for the target.
Furthermore, as shown in FIG. 4B, most of the electron energy is
deposited within the first few microns of the tungsten target
material. The thickness of the tungsten layer can be further
reduced.
Characteristics of X-Ray Beam Produced by W-Diamond Transmission
Target
[0065] FIGS. 5A and 5B plot x-ray fluence and energy fluence
produced by W-diamond transmission target as a function of tungsten
thickness. FIG. 5A shows transmission x-ray fluence from the
W-diamond target as a function of tungsten thickness recorded for
different electron beam energies. FIG. 5B shows energy fluence of
transmission x-rays from the W-diamond target as a function of
tungsten thickness recorded for 120 keV electron beam energy. The
Aluminum filtration removes low energy x-ray photons that would not
come out of the vacuum envelope. x-ray fluence first increases to a
maximum point and then decreases due to self-absorption by the
tungsten target material. The energy fluence follows the same
trend.
[0066] FIG. 6 shows the thicknesses of tungsten layer that produces
maximum x-ray fluence for different beam energies. The optimal
thickness increases approximately linearly with electron
energy.
[0067] An x-ray system may use different kVp settings in clinical
imaging based on the size of the subjects. The kVp setting used for
imaging humans usually ranges from 60 kVp to 140 kVp. The
efficiencies of 1-11 .mu.m tungsten targets for transmission
fluence were evaluated at different electron energies and the
results are shown in FIG. 7. 100% efficiency is defined as when the
x-ray fluence is maximized for a given energy. The tungsten
thickness of about 5-6 .mu.m appears to be acceptable for the beam
energy in the range of 60-140 keV, where the x-ray fluence remains
above 80% of its maximum value.
[0068] FIG. 8 shows the comparison of the photon spectra produced
by bombarding 120 keV electrons on a 5 .mu.m W-diamond transmission
and on a 5 mm thick reflection targets. Both x-ray beams are
filtered by a 3 mm aluminum filter. The results indicate that the
Bremsstrahlung component of the transmission targets is about 20%
higher than the reflection target. However, the characteristic
x-ray spikes of the transmission target are significantly lower
than that of the reflection target. The lower characteristic x-ray
component can be attributed to the energy threshold of
characteristic x-ray generation. Characteristic x-rays are
generated only in the first few microns of tungsten, after which
the electrons lose their kinetic energy to produce characteristic
x-rays. Thus, although the total numbers of characteristic x-ray
photons are the same in transmission and reflection targets, the
characteristic x-ray photons are absorbed more in transmission
target as they need to pass through more tungsten layers.
Nevertheless, the total integral fluence of high energy x-rays is
still higher for the transmission target despite the additional 2
mm diamond filter.
[0069] FIG. 9 shows x-ray fluence profiles as a function of the
angle between the x-ray and the target surface. The fluence is more
uniform for reflection target than transmission target. However,
the fluence of the reflection target at the central axis cannot be
utilized. X-ray tubes with a reflection target usually have a small
anode angle. Thus only the fluence at large angle in FIG. 9 is
used, which is called a heel effect. Transmission target, on the
other hand, can utilize the photons in central axis where the
fluence is maximal. Although the flat region of the fluence for a
transmission target is smaller compared with a reflection target,
this would not pose as a problem for x-ray imaging as only a small
angular window is used in x-ray systems using a reflection target.
The results indicate that, for a 120 kVp beam, the W-diamond
transmission target with a 5-6 .mu.m W target can produce
approximately 20% higher fluence than reflection target of the same
tube power.
Transient Thermal Simulations
[0070] In transient thermal analysis, the focal spot was modeled as
laminated heating elements. The power of each heating element layer
was assigned as a function of depths based on the MC results above.
The finite element contains only 1/4 of the anode to take advantage
of the symmetry of the geometry. FIGS. 10A-10C show the temperature
distribution of the transmission target caused by a 3 ms pulse of
11 kW power (120 kV and 91.7 mA). Enlarged views of the plots
(pointed by the arrows) are also included FIGS. 10A-10C. FIG. 10A
shows a schematic diagram of the transmission target model used in
FEA simulations. The FEA model includes 1/4 of the transmission
target, taking advantage of geometric symmetry of the target. FIG.
10B shows temperature distribution near the focal spot on the
W-diamond transmission target surface caused by a 3 ms, 11 kW beam
calculated using a FEA simulation. FIG. 10C shows temperature
distribution in the x-y plane of the target caused by a 3 ms, 11 kW
beam. The maximum temperature is observed at the center of the
focal spot as expected. The temperature decreases quickly outside
the focal spot and the gradient is very slow in the copper
base.
[0071] FIGS. 11A and 11B show the temperature history of the focal
spots for W-diamond transmission and W reflection targets,
respectively. FIGS. 11A and 11B shows variation of the maximum
focal spot temperatures with time with a 3 ms pulse (2% duty cycle)
of (a) 11 kW electron beam power for the transmission target (FIG.
11A) and (b) 1.9 kW electron beam power for the reflection target
(FIG. 11B). The inserts in FIGS. 11A and 11B show enlarged plots of
focal spot temperatures during the time interval of 0-5 ms. The
focal spot size of the transmission target and reflection target
are 1.times.1 mm.sup.2 and 1.times.2 mm.sup.2 respectively. For the
transmission target, the focal spot temperature rises very fast
from 20.degree. C. to 2500.degree. C. during the first 0.5 ms, and
to 3000.degree. C. in 3 ms. When the electron beam is turned off,
the temperature drops rapidly to 280.degree. C. in 5 ms. At the end
of the 150 ms pulse cycle (assuming a pulse repetition rate of 6.67
Hz), i.e., before the start of next pulse, the temperature drops to
67.degree. C. The results indicate that pulse mode operation of the
tube enables faster dissipation of heat with low duty cycle.
Therefore, the beam power allowable during the pulse duration may
be kept significantly high for short pulse widths. For the
reflection target, the temperature curves rise continuously and do
not result in a plateau compared to the W-diamond transmission
target. Focal spot temperature also decreases rapidly within 150
ms. But note the power of the electron beam is at 1.9 kW, much
lower than the power for the transmission target.
[0072] To keep the W-diamond target temperature spike below
3000.degree. C., the maximum power allowed for different pulse
widths were obtained and shown in FIG. 12. The maximum power is
greatly affected by the pulse width. When the pulse width increases
from 50 .mu.s to 3 ms, the maximum power is reduced from 22 kW to
11 kW. Accordingly, in order to achieve high tube output, x-ray
sources with stationary anode should operate with short pulse
widths. The simulation also shows more than 22 kW peak power may be
achievable for pulse widths smaller than 50 .mu.s.
[0073] For comparison, the same study was also performed for a 5 mm
thick reflection W target with focal spot size of 1.times.2
mm.sup.2 (the blue line in FIG. 12). The reflection target exhibits
a similar trend, where the maximum power reduced as the pulse width
increases. However, the maximum beam power loading is only 14 kW at
50 .mu.s, compared to 22 kW for a W-diamond target. The difference
in maximum power is even larger for longer pulses. When the pulse
width is longer than 1 ms, the maximum power of W-diamond
transmission target can be four times higher than the reflection
target. FIG. 12 also shows the maximum power calculated using Eq.
(1) (the green line), which is valid for a thick target. The
simulation and analytical calculation results for reflection target
are consistent with each other. Slight deviation may be due to the
nonlinear thermal conductivity and specific heat used in the
simulation model.
[0074] The focal spot power density of multi-pixel x-ray sources
was simulated by using a W-diamond transmission target. The
transmission target design results in advantage over a reflection
target. A transmission target can simplify the geometry of an x-ray
tube as the x-ray beams are generated on the opposite side of the
cathodes. On the other hand, the x-ray beams from the reflection
targets come out between the cathode and the anode, where the space
is usually very limited, thereby limiting the maximum field size.
There are significant amount of electrons scattered back to the
vacuum after bombarding the target. Back-scatter electrons carry
large amount of energy and may add a long tail to the focal spot
when they return to strike the target again. In transmission
target, the x-ray generated by back-scattered electrons are largely
absorbed by the target and blocked by the anode body.
[0075] The optimal thickness of the tungsten layer of W-diamond
target is linearly proportional to the electron energy. A
transmission target having approximately 5-6 .mu.m thick tungsten
may have the best x-ray production efficiency for the energy range
of 60-140 kVp. Based on the simulation calculation results, the
W-diamond transmission target may produce about 20% more x-ray
fluence for the same power compared with a reflection target. The
maximum power that keeps focal spot temperature under melting point
is strongly dependent on the pulse duration. For a pulse of a few
ms, the power allowed by a W-diamond transmission target can be
four-fold higher than a reflection target. Thus, it may allow
significant improvement on the output of multi-pixel x-ray sources.
For example, the power density limit of the 1 mm.times.1 mm focal
spot when the source operates with 50 .mu.s pulses is as high as 22
kW/mm.sup.2. Even though the physical focal spot size of a
reflection target is larger than the projected focal spot due to
the anode angle, a transmission target can still achieve up to four
times higher power despite its focal spot area is only half of that
of the reflection target.
[0076] In another embodiment, a tungsten pyrolytic graphite (W-PG)
laminated target is provided. Due to its high melting point
(3422.degree. C.) and high atomic number (74), tungsten is a choice
for x-ray source target material. But its relatively low thermal
conductivity (173Wm.sup.-1K.sup.-1) significantly limits focal spot
power density of x-ray sources. The maximum depth of 120 keV
electrons deposited in tungsten is .about.10 .mu.m, thus a large
amount of heat is deposited to a very thin layer of tungsten.
Because of limited heat removal rates, the heat is built up quickly
in the target and may melt the tungsten if the tube power is too
high. To improve the tube output, the target needs to have a higher
thermal conductivity. Pyrolytic graphite (PG) is multiple layers of
graphene sheets bonded together by covalent bonding. PG, especially
annealed pyrolytic graphite (APG), has an exceptional high thermal
conductivity of up to 1700Wm.sup.-1K.sup.-1 along its a-b plane,
nearly 10 times higher than that of tungsten at room temperature.
Similar to graphite, APG is also very refractory and can withstand
up to 4000.degree. C. in vacuum before melting, exceeding
tungsten's melting point. Bremsstrahlung x-ray production
efficiency is proportional to target atomic number. The low Z
number of carbon makes APG unsuitable as x-ray target material by
itself. Nevertheless, by laminating W on APG, the high z number of
W and high thermal conductivity of APG can be used to develop a
novel composite anode that overcomes limitation of focal spot power
density of tungsten targets.
[0077] A W-PG laminate target for MPTEX source as shown in FIG. 13A
is in accordance with one embodiment of the disclosure. FIG. 13A
shows a W-APG laminated target positioned on an APG anode base
(FIG. 13A is not in scale). FIG. 13B shows thermal conductivity of
APG compared to other target materials. a, b, and c are the
crystallographic axes of the material. The thermal conductivity of
APG in the a-b plane is more than ten times higher than W and, as a
result, APG rapidly removes head inside the target. The APG anode
body further conducts heat to the tube housing. The thicknesses of
APG and W layers are controlled such that the heat is evenly
divided by multiple W layers. The APG layers embedded between W
targets quickly remove the heat due to their outstanding thermal
conductivity.
[0078] An APG anode body with its thermal conductive a-b plane
aligned with the heat conduction direction is also disclosed
herein. Finite element simulation suggests that a 2-5 times
increase of the focal spot power density is possible with the new
composite anode (see Example 1 below). The new anode fabrication
technique disclosed herein can be used in multi-pixel sources as
well as single focal spot x-ray sources.
[0079] A novel target fabrication technique is disclosed herein to
overcome the limitations described above, and to enhance the
performances of x-ray sources. The described target fabrication
technique includes lamination of W-PG targets and production of an
APG anode base.
[0080] Lamination of W-PG Target:
[0081] Electron kinetic energy is deposited to a very thin layer
(.about.10 .mu.m) of a tungsten target. PG and W layers are
laminated such that the total heat is divided to multiple targets
at different depths. The thermal conductive APG layers embedded
between the targets remove the heat rapidly due to its exceptional
high thermal conductivity. This innovative W-PG laminate target
dramatically increases the focal spot power density allowed.
[0082] APG Anode Base:
[0083] Graphite brazed with a tungsten layer is a common
configuration for x-ray tube anodes. As shown in FIG. 13B, the
thermal conductivity in the a-b plane of annealed pyrolytic
graphite (APG) is 3-4 times higher than regular graphite. APG has
been employed in high-end electronics and aerospace that requires
extreme cooling performance, but has never been employed in x-ray
tube anodes. In multi-pixel x-ray sources, the heat is conducted
primarily within the cross-section plane. When aligning APG's a-b
plane with the tube cross section, the extreme cooling performance
of APG can significantly improve the cooling rate of multi-pixel
x-ray sources. A rotating anode may also employ this technique with
embedded thermal vias to pass the heat to different APG layers.
[0084] Methods disclosed herein may be implemented on a computing
device. FIG. 18 is a block diagram of a computing device 1800. In
the exemplary embodiment, computing device 1800 includes a user
interface 1802 that receives at least one input from a user. User
interface 1802 may include a keyboard 1804 that enables the user to
input pertinent information. User interface 1802 may also include,
for example, a pointing device, a mouse, a stylus, a touch
sensitive panel (e.g., a touch pad, a touch screen), a gyroscope,
an accelerometer, a position detector, and/or an audio input
interface (e.g., including a microphone).
[0085] Moreover, in the exemplary embodiment, computing device 1800
includes a presentation interface 1806 that presents information,
such as input events and/or validation results, to the user.
Presentation interface 1806 may also include a display adapter 1808
that is coupled to at least one display device 1810. More
specifically, in the exemplary embodiment, display device 1810 may
be a visual display device, such as a cathode ray tube (CRT), a
liquid crystal display (LCD), an organic LED (OLED) display, and/or
an "electronic ink" display. Alternatively, presentation interface
1806 may include an audio output device (e.g., an audio adapter
and/or a speaker) and/or a printer.
[0086] Computing device 1800 also includes a processor 1811 and a
memory device 1812. Processor 1811 is coupled to user interface
1802, presentation interface 1806, and to memory device 1812 via a
system bus 1814. In the exemplary embodiment, processor 1811
communicates with the user, such as by prompting the user via
presentation interface 1806 and/or by receiving user inputs via
user interface 1802. The term "processor" refers generally to any
programmable system including systems and microcontrollers, reduced
instruction set circuits (RISC), application specific integrated
circuits (ASIC), programmable logic circuits (PLC), and any other
circuit or processor capable of executing the functions described
herein. The above examples are exemplary only, and thus are not
intended to limit in any way the definition and/or meaning of the
term "processor."
[0087] In the exemplary embodiment, memory device 1812 includes one
or more devices that enable information, such as executable
instructions and/or other data, to be stored and retrieved.
Moreover, memory device 1812 includes one or more computer readable
media, such as, without limitation, dynamic random access memory
(DRAM), static random access memory (SRAM), a solid state disk,
and/or a hard disk. In the exemplary embodiment, memory device 1812
stores, without limitation, application source code, application
object code, configuration data, additional input events,
application states, assertion statements, validation results,
and/or any other type of data. Computing device 1800, in the
exemplary embodiment, may also include a communication interface
1816 that is coupled to processor 1811 via system bus 1814.
[0088] In the exemplary embodiment, processor 1811 may be
programmed by encoding an operation using one or more executable
instructions and providing the executable instructions in memory
device 1812. In the exemplary embodiment, processor 1811 is
programmed to select a model provided by a user.
[0089] In operation, a computer executes computer-executable
instructions embodied in one or more computer-executable components
stored on one or more computer-readable media to implement aspects
of the invention described and/or illustrated herein.
[0090] The order of execution or performance of the operations in
embodiments of the invention illustrated and described herein is
not essential, unless otherwise specified. That is, the operations
may be performed in any order, unless otherwise specified, and
embodiments of the invention may include additional or fewer
operations than those disclosed herein. For example, it is
contemplated that executing or performing a particular operation
before, contemporaneously with, or after another operation is
within the scope of aspects of the invention.
[0091] When introducing elements of aspects of the invention or the
embodiments thereof, the articles "a," "an," "the," and "said" are
intended to mean that there are one or more of the elements. The
terms "comprising," "including," and "having" are intended to be
inclusive and mean that there may be additional elements other than
the listed elements.
[0092] Although described in connection with an exemplary computing
system environment, embodiments of the invention are operational
with numerous other general purpose or special purpose computing
system environments or configurations. The computing system
environment is not intended to suggest any limitation as to the
scope of use or functionality of any aspect of the invention.
[0093] Embodiments of the invention may be described in the general
context of computer-executable instructions, such as program
modules, executed by one or more computers or other devices. The
computer-executable instructions may be organized into one or more
computer-executable components or modules. Generally, program
modules include, but are not limited to, routines, programs,
objects, components, and data structures that perform particular
tasks or implement particular abstract data types. Aspects of the
invention may be implemented with any number and organization of
such components or modules. For example, aspects of the invention
are not limited to the specific computer-executable instructions or
the specific components or modules illustrated in the figures and
described herein. Other embodiments of the invention may include
different computer-executable instructions or components having
more or less functionality than illustrated and described herein.
Aspects of the invention may also be practiced in distributed
computing environments where tasks are performed by remote
processing devices that are linked through a communications
network. In a distributed computing environment, program modules
may be located in both local and remote computer storage media
including memory storage devices.
[0094] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to practice the invention, including making and
using any devices or systems and performing any incorporated
methods. The patentable scope of the invention is defined by the
claims, and may include other examples that occur to those skilled
in the art. Such other examples are intended to be within the scope
of the claims if they have structural elements that do not differ
from the literal language of the claims, or if they include
equivalent structural elements with insubstantial differences from
the literal languages of the claims.
EXAMPLES
Example 1: Evaluation of the Multi-Pixel Thermionic Emission X-Ray
Source (MPTEX)
[0095] This Example describes the development of a multi-pixel
thermionic emission x-ray source (MPTEX) prototype, and the
evaluation of the performance of the prototype.
Methods and Materials
[0096] Multi-Pixel Thermionic Emission X-Ray Source (MPTEX):
[0097] An MPTEX source prototype for TBCT applications is
developed. The tube is made of aluminum body with ConFlat flanges.
Water cooling pipes are embedded in the aluminum tube body for fast
heat removal. The anode is made from a graphite bar brazed with a 5
mm thick tungsten target. FIGS. 14A and 14B show the MPTEX
prototype and its functional diagram. The tube can contain up to 48
thermionic cathodes in a 4 mm spatial spacing and each cathode may
produce the same numbers of focal spots. Both oxide coated and
dispenser cathodes that can produce 100 mA and 500 mA cathode
current respectively are evaluated. The tube is able to operate at
100 kVp limited by the anode high voltage vacuum feedthrough. X-ray
measurement shows the target physical focal spot size is about 2
mm.sup.2 and projected focal spot is under 1 mm.sup.2 with anode
angle. In some embodiments, the tube current is limited under 100
mA and the dwell duration under 50 .mu.s due to the limitation of
focal spot power density.
[0098] Finite Element Analysis (FEA) Thermal Simulation:
[0099] To evaluate the performance of the anode, the heat
distributions were simulated using a thermal finite element
simulation software. Additionally, the maximum power allowed was
estimated by increasing the focal spot power until the maximum
temperature reaches about 3000.degree. C.
Results
[0100] FIGS. 15A-15D show the results of thermal simulations. FIGS.
15A-15C show temperature distributions of tungsten target (15A),
dual layer w-PG target with a low a-b thermal conductivity (15B),
and dual layer w-PG with a high a-b thermal conductivity (15C). 1
kW power was provided to the targets and the right surface was kept
at 500.degree. C. FIG. 15D shows the maximum power allowed for the
anodes while keeping focal spot temperature below 3000.degree. C.
Because thermal conductivity of PG varies greatly depending on the
synthesis and annealing processes, thus low (400 Wm.sup.-1K.sup.-1)
and high (1700 Wm.sup.-1K.sup.-1) thermal conductivity values were
used in the simulations. The W-PG laminate targets have two layers
of tungsten that share the total power. The right surface is kept
at 500.degree. C. as boundary condition. The thermal conductive a-b
planes of PG in the target are perpendicular to anode surface,
whereas in the body they are within the cross sectional plane. With
1 kW power deposited on the same 2 mm.sup.2 focal spots, the
maximum temperature reaches about 2900.degree. C. for conventional
tungsten anode (FIG. 15A), 1725.degree. C. for the composite anodes
with a low PG thermal conductivity value (FIG. 15B), and
993.degree. C. for the anode with a high PG thermal conductivity
value (FIG. 15C) respectively.
[0101] As shown in FIG. 15D, the W-PG laminate targets can sustain
a power as high as 5 kW, five times higher than a traditional
tungsten target. In the simulation, the W-PG target is modeled as
two 5 .mu.m thick W layers that equally share the power load. In
reality, as many W layers as needed can be synthesized using the
chemical vapor deposition method described below in Example 2. The
thickness of W layers can be as thin as the atomic level if needed.
The result indicates that 2-5 times larger focal spot power density
can be achieved with the new anode technique.
Example 2: Fabrication of a W-PG Laminate Anode on an APG Base
[0102] This Example describes the fabrication of APG anode modules
with W-APG laminate targets.
[0103] APG Base:
[0104] FIG. 16A shows the cross section of an MPTEX source and FIG.
16B shows the anode module of the MPTEX source. The anode is about
25 cm long made from a graphite bar brazed with a 5 mm thick
tungsten target. Because of the symmetric temperature distribution
in tube length (z) direction, the heat flows primarily in the cross
sectional plane. Thus, the a-b plane of APG with large thermal
conductivity needs to be aligned parallel to the tube cross
section. The a-b plane of commercial APG plates synthesized with
chemical vapor deposition (CVD) techniques is perpendicular to its
thickness. Commercial APG plates can be purchased and machined into
the dog-bone shape as shown in FIG. 16B. The APG plates may be
coated with W-PG laminate target and then stacked together with the
x-y planes of APG plates along the cross sectional plane (the
dog-bone shaped plane). Each anode module plate can be 4 mm
thickness, matching the cathode pixel spacing of MPTEX source.
After W-PG laminate target is deposited on the lateral surface by
the CVD technique (see the following paragraphs), the anode module
plates can be stacked together to form an elongated anode. As the
heat only transfers in the a-b plane, the imperfect contact between
APG plates would not affect the heat dissipation. High temperature
thermal adhesive can be applied to the top and bottom surfaces to
improve the contact with the ceramic insulators (see FIG. 16A).
Similar thermal paste has been applied between the aluminum housing
and ceramic insulator interfaces.
[0105] CVD Deposition of W-PG Laminate Target:
[0106] Chemical vapor deposition (CVD) is a widely used process for
forming solid materials, such as coatings, from reactants in the
vapor phase. CVD deposition of tungsten on a graphite base is a
technique used to fabricate x-ray tube anodes. PG or graphene is
also produced in a process similar to tungsten CVD deposition,
where hydrocarbon gas is heated until it breaks down into carbon. A
CVD aperture for W and PG synthesis is set up in the arrangement as
shown in FIG. 17. W and PG deposition can be switched by
controlling the gas supplies.
[0107] The target deposited on anode with a CVD method can have a
perfect or near perfect contact with the anode base material. CVD
of tungsten is usually carried out using tungsten hexafluoride,
WF.sub.6, which may be deposited in two ways:
WF.sub.6.fwdarw.W+3F.sub.2, (2)
WF.sub.6.fwdarw.3H.sub.2.fwdarw.W+6HF. (3)
[0108] The byproduct HF is very corrosive, but is tolerable for PG,
which is very inert even in high temperature. The CVD reactions
used to deposit GP are based on the thermal decomposition of
hydrocarbons. An exemplary precursor is methane (CH4), which is
generally pyrolyzed at 1100.degree. C. or above, over a wide range
of pressure from 100 Pa to 10.sup.5 Pa (1 atm). The reaction in a
simplified form is as follows:
CH.sub.4.fwdarw.C+2H.sub.2. (4)
[0109] The thickness of tungsten or PG can be controlled by the
deposition time. The 4V-PG target may only cover the focal spot
instead of the entire lateral surface of APG plates. An enclosure
with only focal spot area exposed is developed during CVD
deposition of W. Once finished, the anode plate with laminate
target may be examined with optical and scanning electron
microscopes.
[0110] The graphene layers between W targets may have defect or
mismatch after CVD. Annealing at a high temperature (e.g. up to
3000.degree. C.), produces more planar and more uninform carbon
structures that a low temperature, thus improving the thermal
conductivity. The APG plate material is already annealed at the
factory, but the PG layers deposited on the target by CVD method
may be annealed, which improves thermal conductivity from
.about.400 to up to 1700 Wm.sup.-1K.sup.-1. Although very thin, the
APG layers, embedded between tungsten targets, are important in
improving focal spot power density. After the laminate target is
deposited with CVD, the anode module is placed in an induction oven
for annealing. The APG plate is held by a tungsten frame and placed
in a quartz tube. The tube is sealed and vacuumed during annealing
to prevent oxidation. A disappearing-filament pyrometer is used to
measure temperature during annealing.
Example 3: Evaluation and Optimization of W-PG Laminate Target
[0111] This Example describes the development of a technique to
evaluate W-PG target and optimize its performance.
[0112] Various parameters, such as number of W-PG layers, thickness
of each PG and W layers, annealing temperature and time need to be
determined. The design is first guided by numerical simulation and
then optimized through experimental studies.
[0113] Optimization of W-PG Laminate Target Via Monte Carlo
Simulation and FEA Method:
[0114] The Continuous Slow Down Approximation (CSDA) range of a 120
keV beam is about 15 .mu.m. Thus, a total thickness of 10-15 .mu.m
for a W target is sufficient. The heat should be evenly divided to
different W layers to achieve the best result. Monte Carlo (MC)
simulation is used to determine the thicknesses of PG and W layers
such that the power transferred to each W layer is approximate the
same. The energy lost to the APG layers should be as low as
possible. On the other hand, thinner APG conducts heat slower. Thus
there is a balance between x-ray production efficiency and cooling
performance. Thus MC simulation and FEA thermal simulation are used
jointly to optimize the design. The actual thermal conductivity of
thin PG layer can vary greatly. Numerical simulations are therefore
used as a rough prediction, and the design is verified via
experimental studies.
[0115] Experimental Studies:
[0116] After anode plates with W-PG laminated targets are
fabricated, they are installed in the MPTEX tube to evaluate its
performance. With a 100 kV anode voltage, the oxide cathodes can
generate up to 100 mA current, enough to melt the tungsten target
with a physical focal spot area of about 2 mm.sup.2 (projected to 1
mm.sup.2 by anode angle). To measure focal spot temperature
directly, a viewport is installed on the MPTEX prototype as shown
in FIG. 16A. Disappearing-filament pyrometer is not suitable for
measuring the temperature of a small spot. A pyrometer camera can
be mounted on the viewport and used to measure temperature. The
focal spot temperature can be estimated from the color of focal
spots, which changes with temperature.
[0117] The focal spot temperature can be measured with a DC load
first. A small 10 mA current generates 1 kW focal spot power with a
100 kV anode voltage. Cooling water flow is kept constant during
the measurement. The advantage of using an MPTEX tube for
evaluation is that multiple anode samples can be tested in one
setup. All anode modular pieces are 4 mm thick when stacked
together, matching the cathode spacing. The focal spot temperature
is compared with that of a standard 5 mm tungsten target.
[0118] Once the optimized W-PG lamination configuration is
obtained, damaging tests are performed to evaluate its extreme
performance. The cathode current is gradually increased while the
water temperature is closely watched. Residual gas analyzer (RGA)
is used to monitor partial vapor pressure in the vacuum chamber. A
sudden increase of W or carbon (C) vapor pressure indicates the
breakdown of a focal spot. A molten focal spot can also be
visualized through the viewport.
[0119] Assuming the new anode has a power density twice as much as
a solid tungsten target and each focal spot can sustain 4 kW power
(P), with a maximum water temperature rise of 80.degree. C., the
water flow rate F is:
F = P C .DELTA. T = 4000 J S - 1 4200 J kg - 1 .times. 80 K = 0.012
kg S - 1 . ( 5 ) ##EQU00002##
[0120] For multi-pixel x-ray sources, the total power is nP, where
n is the number of pixels. If n=50, a water flow rate of 0.6 kg/s
or 9.5 gallons per minute is needed to allow the MPTEX source to
operate continuously. This is manageable with the MPTEX tube design
described herein. As shown in FIGS. 14A and 16A, the MPTEX tube has
four 8 mm diameter water channels run through the aluminum tube
housing, which allows a water flow up to 20 gallon per min.
* * * * *