U.S. patent application number 10/970384 was filed with the patent office on 2006-01-26 for large-area individually addressable multi-beam x-ray system and method of forming same.
This patent application is currently assigned to The University of North Carolina at Chapel Hill. Invention is credited to Jianping Lu, Qi Qiu, Otto Z. Zhou.
Application Number | 20060018432 10/970384 |
Document ID | / |
Family ID | 27609080 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060018432 |
Kind Code |
A1 |
Zhou; Otto Z. ; et
al. |
January 26, 2006 |
Large-area individually addressable multi-beam x-ray system and
method of forming same
Abstract
A structure to generate x-rays has a plurality of stationary and
individually electrically addressable field emissive electron
sources with a substrate composed of a field emissive material,
such as carbon nanotubes. Electrically switching the field emissive
electron sources at a predetermined frequency field emits electrons
in a programmable sequence toward an incidence point on a target.
The generated x-rays correspond in frequency and in position to
that of the field emissive electron source. The large-area target
and array or matrix of emitters can image objects from different
positions and/or angles without moving the object or the structure
and can produce a three dimensional image. The x-ray system is
suitable for a variety of applications including industrial
inspection/quality control, analytical instrumentation, security
systems such as airport security inspection systems, and medical
imaging, such as computed tomography.
Inventors: |
Zhou; Otto Z.; (Chapel Hill,
NC) ; Lu; Jianping; (Chapel Hill, NC) ; Qiu;
Qi; (Chapel Hill, NC) |
Correspondence
Address: |
BUCHANAN INGERSOLL PC;(INCLUDING BURNS, DOANE, SWECKER & MATHIS)
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
The University of North Carolina at
Chapel Hill
Chapel Hill
NC
|
Family ID: |
27609080 |
Appl. No.: |
10/970384 |
Filed: |
October 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10051183 |
Jan 22, 2002 |
6876724 |
|
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10970384 |
Oct 22, 2004 |
|
|
|
09679303 |
Oct 6, 2000 |
6553096 |
|
|
10051183 |
Jan 22, 2002 |
|
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Current U.S.
Class: |
378/122 |
Current CPC
Class: |
H01J 35/22 20130101;
H05G 1/34 20130101; H01J 35/065 20130101; B82Y 10/00 20130101; A61B
6/4028 20130101; H01J 2201/30469 20130101; A61B 6/032 20130101;
H01J 2235/068 20130101; H01J 2235/064 20130101 |
Class at
Publication: |
378/122 |
International
Class: |
H01J 35/22 20060101
H01J035/22; H01J 35/00 20060101 H01J035/00; H01J 35/32 20060101
H01J035/32 |
Goverment Interests
STATEMENT REGARDING GOVERNMENT SUPPORT
[0002] At least some aspects of this invention were made with
Government support under contract no. N00014-98-1-05907. The
Government may have certain rights in this invention.
Claims
1-59. (canceled)
60. An X-ray source comprising: a cold cathode, the cold cathode
having a curved emission surface capable of emitting electrons; and
an anode, the anode being spaced apart from the cathode, the anode
being capable of emitting X-rays in response to being bombarded
with electrons emitted from the curved emission surface; wherein
the cold cathode comprises a plurality of emitters disposed on a
substrate and a gate conductor disposed adjacent the plurality of
emitters, and wherein the plurality of emitters are operative to
emit electrons when a bias voltage is applied to the gate
conductor; wherein the electrons bombard the anode at a focal spot
of the anode, wherein the plurality of emitters comprises a first
set of emitters, the first set of emitters being operative to emit
a first electron beam having a first focal spot with a first shape,
and a second set of emitters, the second set of emitters being
operative to emit a second electron beam having a second focal spot
with a second shape, the second shape being different than the
first shape, and wherein the first set of emitters and the second
set of emitters are located on the same curved emission surface and
are separately energizable.
61. An X-ray source according to claim 60, wherein the electrons
bombard the anode at a focal spot of the anode, and wherein a size
and shape of the focal spot is determined at least in part by a
curvature of the curved emission surface.
62. An X-ray source according to claim 60, wherein the electrons
bombard the anode at a focal spot of the anode, and wherein the
plurality of emitters are addressable thereby permitting the size
and shape of the focal spot to be controlled.
63. An X-ray source according to claim 60, wherein the electrons
bombard the anode at a focal spot of the anode, the focal spot
being characterized by an intensity distribution which describes
intensity of electron bombardment as a function of position, and
wherein the plurality of emitters are addressable thereby
permitting the intensity distribution of the focal spot to be
controlled.
64. An X-ray source according to claim 60, further comprising a
vacuum housing and an X-ray transmissive window, wherein the
cathode and the anode are disposed within the housing, and wherein
the X-rays exit the X-ray source by way of the transmissive
window.
65. An X-ray source according to claim 60, wherein the curved
emission surface is fabricated so as to be curved along a first
axis and straight along a second axis which is orthogonal to the
first axis.
66. An imaging system for imaging an object of interest, the
imaging system comprising: (A) an X-ray source, the X-ray source
including (1) a cold cathode disposed within a housing, the cold
cathode having a curved emission surface, the cold cathode
comprising a plurality of emitters disposed on a substrate, and (2)
an anode, the anode being disposed within the housing and spaced
apart from the cathode, the anode emitting X-rays in response to
being bombarded with electrons emitted from the curved emission
surface wherein the electrons bombard the anode at a focal spot of
the anode; (B) a detector array, the detector array comprising a
plurality of detector elements, the plurality of detector elements
receiving the X-rays after the X-rays pass through the object of
interest and generating signals in response thereto; (C) an image
reconstructor, the image reconstructor being coupled to receive the
signals from the detector elements, and the image reconstructor
constructing an image of the object of interest based on the
signals from the detector elements; (D) a display, the display
being coupled to the image reconstructor, and the display
displaying the image of the object of interest; and (E) an X-ray
controller, the X-ray controller being coupled to the cold cathode
to provide control signals to control the emission of electrons
from the plurality of emitters, the X-ray controller being coupled
to receive feedback information pertaining to the operation of the
imaging system, and wherein the X-ray controller adjusts the
control signals for the plurality of emitters as a function of the
feedback information.
67. An imaging system according to claim 66, wherein the plurality
of emitters are addressable, such that the X-ray controller
provides different control signals that control different ones of
the plurality of emitters.
68. An imaging system according to claim 67, wherein the X-ray
controller adjusts the control signals to control a size and shape
of the focal spot.
69. An imaging system according to claim 67, wherein the electrons
bombard the anode at a focal spot of the anode, wherein the X-ray
controller adjusts the control signals to control a current density
distribution of an electron beam formed by the electrons bombarding
the focal spot.
70. An imaging system according to claim 66, wherein the cold
cathode further comprises an insulative layer, the insulative layer
being disposed on the substrate and being located between the
plurality of emitters; a gate conductor, the gate conductor being
disposed on the insulative layer; and wherein the plurality of
emitters are operative to emit electrons when a bias voltage is
applied to the gate conductor.
71. An imaging system according to claim 66, wherein the imaging
system is a computed tomography imaging system, wherein the system
further comprises a plurality of additional X-ray sources, the
plurality of additional X-ray sources each comprising a respective
additional cold cathode and a respective additional anode, wherein
the X-ray source and the plurality of additional X-ray sources are
disposed in a ring so as to permit the object of interest to be
imaged without gantry rotation.
72. An imaging system according to claim 71, wherein the system
further comprises an X-ray controller, and wherein the X-ray
controller sequentially activates the X-ray source and the
plurality of additional X-ray sources in a manner that simulates
rotation of a single X-ray source about the object of interest.
73. An imaging system according to claim 66, wherein the imaging
system is a medical imaging system.
74. An imaging system according to claim 66, wherein the imaging
system is a security checkpoint imaging system.
75. An imaging system for imaging an object of interest, the
imaging system comprising: (A) an X-ray source, the X-ray source
including (1) a cold cathode disposed within a housing, the cold
cathode having a curved emission surface, the cold cathode
comprising a plurality of emitters disposed on a substrate, and (2)
an anode, the anode being disposed within the housing and spaced
apart from the cathode, the anode emitting X-rays in response to
being bombarded with electrons emitted from the curved emission
surface; (B) a detector array, the detector array comprising a
plurality of detector elements, the plurality of detector elements
receiving the X-rays after the X-rays pass through the object of
interest and generating signals in response thereto; (C) an image
reconstructor, the image reconstructor being coupled to receive the
signals from the detector elements and the image reconstructor
constructing an image of the object of interest based on the
signals from the detector elements; and (D) a display, the display
being coupled to the image reconstructor, and the display
displaying the image of the object of interest (E) an X-ray
controller, the X-ray controller being coupled to the cold cathode
to provide control signals to control the emission of electrons
from the plurality of emitters, wherein the electrons bombard the
anode at a focal spot of the anode and wherein the X-ray controller
adjusts the control signals for the plurality of emitters to
control a size and shape of the focal spot.
76. An imaging system according to claim 75, wherein the X-ray
controller pulses the control signals for the plurality of emitters
so as to cause the X-rays emitter from the anode to form an X-ray
beam that pulsates.
77. An imaging system according to claim 75, wherein the cold
cathode further comprises an insulative layer, the insulative layer
being disposed on the substrate and being located between the
plurality of emitters; a gate conductor, the gate conductor being
disposed on the insulative layer; and wherein the plurality of
emitters are operative to emit electrons when a bias voltage is
applied to the gate conductor.
78. An imaging system according to claim 75, wherein the imaging
system is a computed tomography imaging system, wherein the system
further comprises a plurality of additional X-ray sources, the
plurality of additional X-ray sources each comprising a respective
additional cold cathode and a respective additional anode, wherein
the X-ray source and the plurality of additional X-ray sources are
disposed in a ring so as to permit the object of interest to be
imaged without gantry rotation.
79. An imaging system according to claim 75, wherein the imaging
system is a medical imaging system.
80. A medical imaging method comprising: generating an X-ray beam
at an X-ray source comprising a cathode having a curved emission
surface, the cathode comprising a plurality of emitter cones and a
thin film gate, the electron beam being emitted towards an anode so
as to cause the anode to be bombarded with electrons, wherein the
X-ray beam is produced in response to being bombarded by the
electrons, wherein the electrons bombard the anode at a focal spot
of the anode, wherein a size and shape of the focal spot is defined
at least in part by a curvature of the curved emission surface, the
generating step including emitting an electron beam from the
cathode, wherein the X-ray source directs the X-ray beam through a
patient, and wherein the emitting step further includes applying a
first electric field between the thin film gate and the plurality
of emitter cones, the first electric field causing the electrons to
be emitted from the plurality of emitter cones, and applying a
second electric field between the anode and the cathode, the second
electric field causing the electrons to accelerate towards the
anode; detecting the X-ray beam after the X-ray beam passes through
at least a portion of the patient; constructing an image of a
portion of the patient based on data collected during the detecting
step; and displaying the image of the portion of the patient.
Description
RELATED APPLICATION DATA
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/051,183, filed on Jan. 22, 2002, which is a
continuation-in-part of U.S. patent application Ser. No.
09/679,303, filed on Oct. 6, 2000, now U.S. Pat. No. 6,553,096. The
entire disclosures of both referenced applications are incorporated
herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to a method and device for
generating and controlling x-ray radiation. For example, the
present invention relates to a method and device that generates
electron emission from an array or matrix of emitters. Emitted
electrons directly impact different locations on a large-area
target to produce multiple x-ray beams from different origins and
thereby providing improved imaging capabilities, including dynamic
imaging.
BACKGROUND OF THE INVENTION
[0004] In the description of the background of the present
invention that follows reference is made to certain structures and
methods, however, such references should not necessarily be
construed as an admission that these structures and methods qualify
as prior art under the applicable statutory provisions. Applicants
reserve the right to demonstrate that any of the referenced subject
matter does not constitute prior art with regard to the present
invention.
[0005] The term "nano-structured" or "nanostructure" material is
used by those familiar with the art to designate materials
including nanoparticles with a particle size or less than 100 nm,
such as nanotubes (e.g.--carbon nanotubes). These types of
materials have been shown to exhibit certain properties that have
raised interest in a variety of applications.
[0006] Current x-ray tubes comprise a target with a single (or
dual) focal point(s) (where electrons bombard a target and x-rays
are emitted), and either a single cathode or dual cathodes.
Electrons are generally thermionically emitted from the cathode(s)
and are focused to impact a small area on the target. For imaging
or diagnostic purposes over a large area, either the object or the
x-ray tube (or both) has to be moved over the area.
[0007] One example of such a use for x-rays includes computed
tomography (CT). FIGS. 1 (a-d) depict the general arrangement of a
CT scanner 100 in which an x-ray source 102 (typically an x-ray
tube) is rotated (direction indicated by the arrow) about the
object to be imaged 104. The data can be collected by either a
single detector 106 coordinately moving with the x-ray source 102
(see FIG. 1a), multiple detectors 106 coordinately moving with the
x-ray source 102 (see FIGS. 1b and 1c), or multiple stationary
detectors 106 (see FIG. 1d). As a consequence of the current CT
designs, the scanners are usually large with complicated mechanical
systems. Scanners can be limited by the rotation speed of the x-ray
tube and by the size and weight of the apparatus. This last
limitation can adversely affect the capture of clear images of
moving parts, such as a beating heart.
[0008] Dynamic three-dimensions (3D) images can be acquired by
so-called "fifth generation CT scanners." An example of a 5.sup.th
generation CT scanner 200 is shown in FIG. 2. An electron beam 202
is produced by a stationary electron gun 204 in a vacuum system
206. The electron beam 202 is focused by a focus coil 208 and
scanned by a deflection coil 210 such that the applied magnetic
field positions the electron beam 202 across any one of several
stationary metal target rings 212 from which an x-ray is emitted.
The emitted x-rays 214 project toward the object 216 to be imaged
and are detected by a stationary detector 218 and processed by a
data acquisition system 220. Although such a system is capable of
producing dynamic 3D images, the scanner 200 is very large and
costly to fabricate.
[0009] In addition to the design limitations of current CT
scanners, current x-ray sources use heated metal filaments as the
sources of electrons. Because of the thermionic emission mechanism,
a very high operating temperature is required, typically in the
order of 1000-1500.degree. C. The high operating temperatures
results in problems such as short lifetime of the filament, slow
response time (i.e., time to warm up the filament before emission),
high-energy consumption, and large device size.
[0010] Further, because of the fundamental physics of the
thermionic emission mechanism, electrons are emitted in all
directions. In x-ray tube applications, an additional electrical
field can be applied to bring the electrons into focus at the
target, but with attendant complexity and cost. Additionally, such
conventional techniques of focusing electron beams have certain
disadvantages, such as limits on the uniformity and size of the
focusing spots.
[0011] Carbon nanotube materials have been shown to be excellent
electron field emission materials. In this regard, carbon nanotube
materials have been shown to possess low electron emission
threshold applied field values, as well as high emitted electron
current density capabilities, especially when compared with other
conventional electron emission materials.
[0012] Commonly owned U.S. Pat. No. ______ (Ser. No. 09/296,572
entitled "Device Comprising Carbon Nanotube Field Emitter Structure
and Process for Forming Device") the disclosure of which is
incorporated herein by reference, in its entirety, discloses a
carbon nanotube-based electron emitter structure.
[0013] Commonly owned U.S. Pat. No. ______ (Ser. No. 09/351,537
entitled "Device Comprising Thin Film Carbon Nanotube Electron
Field Emitter Structure"), the disclosure of which is incorporated
herein by reference, in its entirety, discloses a carbon-nanotube
field emitter structure having a high emitted current density.
[0014] Commonly owned U.S. Pat. No. 6,277,318 entitled "Method for
Fabrication of Patterned Carbon Nanotube Films," the disclosure of
which is incorporated herein by reference, in its entirety,
discloses a method of fabricating adherent, patterned carbon
nanotube films onto a substrate.
[0015] Commonly owned U.S. Pat. No. ______ (Ser. No. 09/679,303
entitled "X-Ray Generating Mechanism Using Electron Field Emission
Cathode"), the disclosure of which is incorporated herein by
reference, in its entirety, discloses an X-ray generating device
incorporating a nanostructure-containing material.
[0016] Thus, there is a need in the art to address the
above-mentioned disadvantages associated with methods and devices
for generating x-rays and for applying x-rays in applications in
imaging environments, such as computed tomography. Further, there
is a need in the art for imaging solutions in both static and
dynamic environments.
SUMMARY OF THE INVENTION
[0017] A structure to generate x-rays with a target and an opposing
surface has a plurality of stationary and individually electrically
addressable electron sources defining a plurality of cathodes. The
structure to generate x-rays can be an x-ray source in a device to
record x-ray images. The electron source is preferably a field
emission electron source and can be a triode-type having a
substrate composed of a field emissive material and a gate
electrode positioned parallel to and insulated from the substrate.
The field emissive material is preferably a material that has a low
threshold field for emission and is stable at high emission
currents. For example, the field emissive material can be carbon
nanotubes such as single wall, double wall, or multi-wall carbon
nanotube, nanowires or nanorods of metal, metal oxide, silicon,
silicon oxide, carbide, boride, nitride, or chalcogenide.
Additionally, nanostructure containing materials formed from
silicon, germanium, aluminum, silicon oxide, germanium oxide,
silicon carbide, boron, boron nitride, and boron carbide can be the
field emissive material. The cathodes and the target are housed in
an evacuated chamber with multiple windows that are transparent to
x-rays.
[0018] The electrons emitted from the stationary cathodes are
accelerated to and impact on different points of the target by the
voltage applied between the target and the cathodes. The target
area or a particular target from amongst multiple targets where an
electron impacts can be controlled or selected by a suitable design
of cathodes and target geometry, or by applying additional focusing
electric fields, or other suitable techniques. Upon impact by the
electrons, x-ray radiation is emitted from the impact surface of
the target materials. Direction and divergence of the x-ray
radiation can be controlled by the location of the windows and
collimators with respect to the chamber.
[0019] The intensity of the x-ray radiation depends on the voltage
applied between the target and the cathode, and the emitted
electron current. The emitted electron current from the emissive
materials is controlled by the electrical field between the gate
electrode and the field emissive material in the triode
configuration or between the target and the field emissive material
in the diode case. The field emitted electrons are essentially
non-divergent and travel toward the gate electrode and anode. The
emission can occur at 300K and can be rapidly switched on and off
by controlling the electric field applied.
[0020] In one embodiment, a method of generating an x-ray image
positions an object within an imaging zone, switches each of a
plurality of cathodes on a stationary cathode structure at a
predetermined frequency to field emit an electron, emits an x-ray
from a target of the stationary target structure at the
predetermined frequency, images the object, and detects the emitted
x-ray. Each of the plurality of cathodes comprises a field emissive
electron source is individually addressable and electrically
switched in a programmable sequence to field emit electrons toward
an incidence point on a stationary target structure.
[0021] In another embodiment, a method for obtaining an x-ray image
places an object in an x-ray source, applies power to at least one
of the plurality of cathodes to generate x-ray radiation for a
pre-set or variably-set exposure time, exposes the object to the
x-ray radiation, and captures an x-ray image corresponding to the
object by either an x-ray detector or an x-ray sensitive film.
[0022] In one aspect, the x-ray detector or x-ray sensitive film
moves at a corresponding frequency to the pre-set or variably-set
frequency applied to the cathode to capture an image and/or detect
an x-ray. Alternatively, the x-ray detector or x-ray sensitive film
is stationary and the individual, group, or portions of the
individual or group of x-ray detectors or x-ray sensitive films are
activated (e.g., enabled to detect and/or capture) at a
corresponding frequency to the pre-set or variably-set frequency
applied to the cathode to capture an image and/or detect an
x-ray.
[0023] In a further aspect, the position on the stationary target
structure from which the x-ray emits corresponds spatially and
temporally to a position on the cathode structure from which the
electron emits and at least one of a circumferential position and
an elevation angle of the emitted x-ray is sufficiently
discriminated with respect to the object to produce a three
dimensional image.
[0024] In an additional aspect, power is applied to all of the
plurality of cathodes simultaneously or to a subset of the
plurality of cathodes sequentially at a pre-set or variably-set
frequency. In the latter, the x-ray detector or the x-ray sensitive
film is moved or activated at a corresponding frequency to the
pre-set or variably-set frequency to capture the x-ray image. The
frequency can be sufficiently rapid to dynamically image a
physiological function.
[0025] The x-ray system can have a large-area target and an array
or matrix of emitters. Electrons are emitted directly towards
different incidence points on the target to produce multiple x-ray
beams from different origins. The result is that images of a large
object can be taken from different areas and/or angles without
moving the object or the x-ray tube. The x-ray system is suitable
for a variety of applications including industrial
inspection/quality control, analytical instrumentation, security
systems such as airport security inspection systems, and medical
imaging, such as computed tomography.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0026] Other objects and advantages of the invention will become
apparent from the following detailed description of preferred
embodiments in connection with the accompanying drawings in which
like numerals designate like elements and in which:
[0027] FIGS. 1(a)-(d) represent computed tomography systems in
which the x-ray source and/or the detector are rotated about an
object to be imaged.
[0028] FIG. 2 is a representative example of a fifth generation
computed tomography system.
[0029] FIG. 3 is an embodiment of a structure to generate
x-rays.
[0030] FIG. 4 a cross sectional view of an embodiment of a device
to record x-ray images.
[0031] FIG. 5 is an embodiment of a cathode structure.
[0032] FIG. 6 is a schematic of the details of the target
structure.
[0033] FIG. 7 is a first schematic representation of generated
x-rays imaging an object.
[0034] FIG. 8 is second schematic representation of generated
x-rays imaging an object
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] A structure to generate x-rays comprises a plurality of
stationary and individually electrically addressable electron
sources defining a plurality of cathodes, at least one target
placed opposing the cathodes, and an evacuated chamber that houses
the plurality of cathodes and the at least one target. The electron
sources can be filed emission electron sources and can be
triode-type or diode-type structures.
[0036] FIG. 3 is an embodiment of a structure 300 to generate
x-rays. A cathode structure 302 and a target structure 304 are
disposed within a chamber 306, which is substantially in the form
of a hollow cylinder having an outer wall 308 and an inner wall
310. The cathode structure 302 and the anode structure 304 are
positioned within the chamber 306 between the inner wall 308 and
the outer wall 310. The chamber 306 is operationally maintained at
a vacuum of at least 1 Torr by a suitable vacuum system operatively
connected to the chamber 306. Feedthroughs (not shown), as known in
the art, can provide services to components within the chamber
306.
[0037] A device to record x-ray images comprises an x-ray source
having a cathode structure and a target structure, an object
positioning system and a detector system. In one aspect the x-ray
source can be a structure to generate x-rays as shown and described
herein.
[0038] On example of an embodiment of a device to record x-ray
images is shown in FIG. 4 in cross sectional view. The device 400
to record x-ray images has a cathode structure 402, a target
structure 404, an object positioner 406, and a detector 408. The
cathode structure 402 can be in the form of a single cathode or a
plurality of cathodes. In the embodiment shown, the cathode
structure 402 is arranged as a ring to correspond to the overall
shape of the chamber 410. Similar to the cathode structure 402, the
target structure 404 can be in the form of a single target or a
plurality of targets. In the embodiment shown, the target structure
404 is arranged as a ring to correspond to the overall shape of the
chamber 410. The target structure 404 can produce an x-ray 412 of a
desired wavelength upon bombardment by electrons 414 emitted from
the cathode structure 402. The x-ray 412 propagates to an imaging
zone 416 containing the object 418 to be imaged and is detected by
the detector 408. The detector 408 can be an array or matrix of
x-ray detectors or x-ray sensitive films opposing the x-ray source,
the array or matrix substantially parallel to and at equal distance
to the x-ray source (e.g. the position from which x-rays propagate
to the object). In one aspect, the detector 408 can be located on
the circumference of the imaging zone 416 adjacent to the inner
wall 418 and outside of the chamber 410. In another aspect, the
x-ray sources and detectors, can be on opposing parallel planes.
The device 400 to record x-ray images is suitable for a variety of
applications including industrial inspection/quality control,
analytical instrumentation, security systems such as airport
security inspection systems, and medical imaging, such as computed
tomography.
[0039] The cathode structure 402 can be any suitable geometric form
having a surface 420 facing the target structure 404. For example,
the cathode structure 402 can be substantially an annulus, a torus,
or a plate. One or more, field emissive electron sources can be
affixed to the cathode structure 402.
[0040] Where the cathode structure is a plane or is planar-like,
the plurality of cathodes and the at least one target can be each
on an opposing plane and he target has a deflection surface that
can be oriented toward a surface of the plurality of cathodes that
emits electrons. In one aspect the deflection surface is oriented
nonparallel to the surface of the plurality of cathodes. Further,
each of the plurality of cathodes can be individually positioned on
one of the opposing planes at a pre-determined spatial
interval.
[0041] Where the cathode structure is a ring or is ring-like, the
plurality of cathodes can be disposed on a first ring and the at
least one target can be disposed on a second ring, the first and
second rings concentric, and the at least one target has a
deflection surface that can be oriented toward a surface of the
plurality of cathodes that emits electrons. In one aspect, the
deflection surface can be oriented non-parallel to the surface of
the plurality of cathodes. Further, each of the plurality of
cathodes can be individually positioned on one of the first or
second rings at a pre-determined spatial interval.
[0042] FIG. 5 shows an exemplary embodiment of a cathode structure
500 as substantially an annulus 502 with multiple discreet field
emissive electron sources 504 disposed on a broad facing surface
506. Each field emissive electron source 504 defines a cathode in
an x-ray generating apparatus. The field emissive electron sources
504 are each individually electrically addressable and stationarily
affixed to the cathode structure 500. In the FIG. 5 embodiment, the
field emissive electron sources 502 are arranged at intervals along
the cathode structure 500. The interval (.alpha.) can be any
suitable interval to obtain the desired imaging results. Both fixed
intervals and random intervals are envisioned. In at least one
exemplary embodiment, the interval between successive field
emissive electron sources 504 is approximately 10.degree..
[0043] In the FIG. 5 embodiment, a recess 508 in the cathode
structure 500 can house a planar circular field emissive material
510. Disposed parallel to the planar surface 512 of the field
emissive material 510 and insulated from it is a gate electrode
514. Further, the gate electrode 514 can be disposed across a
surface of the field emissive material 510 substantially
equidistant from the field emissive material 510. The arrangement
of field emissive material and gate electrode can be, for example,
a triode-type or diode-type structure. When above a threshold
value, a bias (e.g., electric field) between the field emissive
material 510 and the gate electrode 514 extracts and accelerates
electrons from the cathode substantially perpendicularly to the
planar surface 512 of the field emissive material 510. Accordingly,
the field emitted electrons are non-divergent. The plurality of
field emitted electrons pass the gate electrode and are further
directed to a predetermined position and accelerated to impact on
the at least one target by the electric field applied between the
gate electrode and the at least one target. Upon impact at an
incidence point, at least one x-ray having a characteristic
wavelength corresponding to a material of the at least one target
and at least one x-ray having a continuous wavelength are
generated.
[0044] The field emissive material 510 can be formed from any one
of numerous nanostructure materials that possess high emission
current densities. For example, the field emissive material can be
carbon nanotubes (e.g., single walled carbon nanotubes, double
walled carbon nanotubes, or multi-wall carbon nanotubes).
Alternatively, the field emissive material can be a nanotube
comprising at least one non-carbon element. Further, the field
emissive material can be a nanorod/nanowire comprising at least one
of a metal, a metal oxide, silicon, silicon carbide, silicon oxide,
carbon nitride, boron nitride, boron carbide, or a chalcogenide or
can be other nanowires such as SiC or SiC nanowires. Additionally,
nanostructures containing materials formed from silicon, germanium,
aluminum, silicon oxide, germanium oxide, silicon carbide, boron,
boron nitride, and boron carbide are contemplated. More specific
details of the above-mentioned materials are disclosed in commonly
owned U.S. Pat. No. ______; (Ser. No. 09/594,844; Attorney Docket
No. 032566-003), the disclosure of which is incorporated herein by
reference in its entirety.
[0045] The field emissive material can be applied by any suitable
technique and in any suitable geometric form. Examples of suitable
techniques and forms are given in commonly owned U.S. Pat. No.
6,277,318, entitled "Method for Fabrication of Patterned Carbon
Nanotube Films," the disclosure of which is incorporated herein by
reference, in its entirety, in which a method of fabricating
adherent, patterned carbon nanotube films onto a substrate is
disclosed. Further, it is known to form field emitting cathodes
with various geometrical configurations, such as one or more sharp
points or ridges which act to focus the beam of emitted electrons.
See, for example, U.S. Pat. Nos. 3,921,022; 4,253,221; and
5,773,921, the disclosures of which are incorporated herein by
reference. Additional suitable techniques include coating the field
emissive material on the substrate as a film, embedding the field
emissive material in a matrix of the substrate, or forming the
field emissive material as a free-standing substrate structure.
[0046] In an additional embodiment, the cathode structure can be a
suitable structural substrate material coated with a field emissive
material and formed into the field emissive electron source. In
this embodiment, the cathode structure can be formed as a
continuous array or a matrix of field emissive materials and can be
individually electrically addressable. For example, the structural
material can have an interdigitated array (IDA) of conductors with
discrete surface areas exposed on the surface of the upper ring.
Interdigitated arrays can be fabricated for specific applications
by photolithographic techniques and using a lift-off process.
Additionally, IDAs are commercially available in standard forms
from, for example, ABTECH Scientific, Inc., Richmond Va. A layer of
field emissive material, such as single wall, double wall
nanotubes, or multi-wall nanotubes, can be disposed on the
structural material and can cover the discrete areas corresponding
to exposed electrical leads of the IDA. By electrically addressing
the discrete areas individually or in programmed sequence, the
field emissive material can be pulsed advantageously to produce
field emission electrons that can be directed to particular areas
on the target. An insulating layer and a gate electrode can be
coated on the field emissive material by a physical vapor
deposition technique or other suitable technique. Alternatively,
the gate electrode can be a separate structure, such as a screen,
that can be positioned adjacent the insulating layer either
abutting the insulting layer or with a separation distance.
[0047] The field emissive electron source or cathode may be formed
in any suitable manner. For example, a carbon nanotube film can be
formed by depositing carbon nanotubes onto a substrate. The carbon
nanotubes can be deposited onto the substrate by any suitable
means. An example of a deposition technique includes the use of an
adhesion promoting layer. An example of an adhesion promoting layer
is disclosed in commonly owned U.S. Pat. No. ______ (Ser. No.
09/817,164), the disclosure of which is incorporated herein by
reference in its entirety.
[0048] Numerous single wall nanotube fabrication techniques are
envisioned. For example, the single wall nanotubes can be
fabricated using a laser ablation process. This technique is
generally described, for example, in A. Thess et al., Science; 273,
483-487 (1996); C. Bower et al., Applied Physics, Vol. A67, 47
(1998); X. P. Tang et al. Science, Vol. 288, 492 (2000), the
disclosures of which are hereby incorporated by reference. Single
wall carbon nanotubes can also be fabricated by arc-discharge (See,
for example, C. Journet et al., Nature, Vol. 388, 756 (1997)) and
chemical vapor deposition techniques (See, for example, A. M.
Cassell et al. J. of Physical Chemistry B, 103, 6484 (1999)).
[0049] One particularly suitable technique of forming single wall
nanotubes according to the present invention includes a technique
as described in commonly owned U.S. Pat. No. ______ (Ser. No.
09/259,307; Attorney Docket No. 032566-001) the disclosure of which
is incorporated herein by reference, in its entirety.
[0050] According to this technique, a target made from graphite and
a suitable catalyst, such as nickel and/or cobalt is placed within
a quartz tube. The target is formed from a graphite powder mixed
with 0.6 atomic percent nickel and 0.6 atomic percent cobalt, and
graphite cement. The target is then heated within the tube to a
temperature of approximately 1150.degree. C. The tube is evacuated
by means of a vacuum pump and a flow of inert gas, such as argon,
is introduced into the tube by a suitable source. Various control
devices may be attached to the system for controlling and
monitoring the flow of inert gas into the tube, as well as the
vacuum within the tube. The pressure of the inert gas as is
maintained at a suitable level, such as approximately 800 torr.
[0051] An energy source, such as a pulsed Nd:YAG laser, is used to
ablate the target at the above-described temperature. Preferably,
the first and/or second harmonic beam of the laser, i.e. 1064 nm
and 532 nm, respectively, are used to ablate the target.
[0052] As the target is ablated, nanotube-containing material is
transported downstream by the inert gas flow, and forms deposits on
the inner wall of the tube. These deposits are then removed to
recover the nanotube-containing material. This material, as
recovered, has been analyzed and found to contain 50-70 volume % of
SWNTs with individual tube diameters of 1.3-1.6 nm and bundle
diameters of 10-40 nm. The bundles are randomly oriented.
[0053] The as-recovered materials are then purified by a suitable
purification process. In a preferred embodiment, the nanotube
material is placed in a suitable liquid medium, such as an organic
solvent, preferably an alcohol such as methanol. The nanotubes are
kept in suspension within the liquid medium for several hours using
a high powered ultrasonic horn, while the suspension is passed
through a micro porous membrane. In another embodiment, the carbon
nanotube containing material is first purified by reflux in a
suitable solvent, preferably 20% H.sub.2O.sub.2 with subsequent
rinsing in CS.sub.2 and then in methanol, followed by filtration,
as described in Tang et al., Science, Vol. 288, 492 (2000).
[0054] In addition to the above described processing steps, the
purified materials can be further processed by milling, such as
ball-milling or oxidation. The optional step of milling will of
course act to create even more broken nanotubes and theoretically
at least even further increase the number of ends of the nanotubes
which are capable of forming sites for the emission of electrons
toward an anode target. The carbon nanotubes can also be shortened
by oxidation in strong acid. The length of the nanotubes can be
controlled by the acid concentration and reaction time. In a
preferred embodiment, the purified single wall carbon nanotubes are
sonicated in a solution of 3:1 volume ratio of H.sub.2SO.sub.4 and
HNO.sub.3. The average nanotube length is reduced to 0.5 micron
(from 10-30 micron in the as purified case) after 24 hours of
sonication. More emission tips per unit area can be obtained using
the short nanotubes.
[0055] Processing methods and considerations in forming an x-ray
generating mechanism using field emission cathodes are contained in
commonly owned U.S. Pat. No. ______ (U.S. patent application Ser.
No. 09/679,303, Attorney Docket No. 032566-005), the disclosure of
which is herein incorporated by reference.
[0056] The target structure can have one or more targets, each
corresponding to a field emissive electron source. FIG. 6 shows an
exemplary target structure 600 in the form of the target structure
cross section of FIG. 4. In the exemplary embodiment shown, the
target structure 600 is substantially triangular in form with a
deflection surface 602 that generates x-rays from an incidence
point 604. The deflection surface 602 is oriented substantially
non-perpendicularly to the propagation direction of the accelerated
electron beam incident to the deflection surface 602. Accordingly,
a non-divergent and accelerated emitted electron from the field
emissive electron sources impacts the deflection surface 602 at an
incidence point 604 at an angle .beta. such that the emitted x-ray
is propagated toward the imaging zone at a corresponding angle
.gamma. to the deflection surface 602.
[0057] The cathode structure and the target structure can be
mounted within the chamber by suitable means. In one embodiment,
the cathode structure and target structure are mounted directly to
the interior surface of the chamber and are provided with suitable
electrical insulation. In another embodiment, ceramic screws
connect the upper ring to the lower ring. An additional ceramic
screw can be used to affix the lower ring to an interior surface of
the chamber. One particularly suitable ceramic screw is
approximately 5 cm in length and has insulating properties.
[0058] The detector can be any suitable x-ray detecting means. For
example, the detector can be a rotatable detector that can be
positioned circumferentially to the object to be imaged and
opposite to the incidence point of the electron beam on the target
structure. In another example, the detector can be an array or
matrix of x-ray detectors or x-ray sensitive films stationarily
positioned opposite the x-ray source and in line with the image to
be detected. The array or matrix can be substantially parallel to
and at equal distance to the x-ray source. In a still further
example, the detector can be a matrix of detectors stationarily
positioned above the plane of the target structure. In this
embodiment, the deflection surface of the target is appropriately
pitched to generate an x-ray beam that propagates non-planar to the
target structure and in a direction that intersects with the matrix
of detectors. Further, the array or matrix of x-ray detectors or
x-ray sensitive films can detect an x-ray and produce an x-ray
image by the interaction of the x-ray with an individual, group, or
portions of the individual or group of x-ray detectors or x-ray
sensitive films.
[0059] In embodiments in which the detector is stationarily
mounted, the detector can detect or capture an x-ray image or
signal from an emitted x-ray and transfer the image or signal to a
computer storage device. Subsequently, the detector is refreshed,
thereby prepared to detect/capture the next image. In one example,
a suitable detector used in the step of detecting is a
charge-coupled device (CCD).
[0060] Referring to FIG. 4, a plurality of x-ray transparent
windows 422 can be disposed in the inner wall 418 of the chamber
410. The windows 422 can be made of suitable materials, as is known
in the art. Additionally, the windows 422 can be operatively
positioned with respect to the imaging zone 416 corresponding to
the path of the x-rays 412 to allow the passage of at least one
x-ray beam generated by the plurality of electrons from a
corresponding one of the plurality of cathodes. Further, the
windows 422 can provide a collimating feature to the apparatus. The
collimating feature arises from the positioning of the window 422
with respect to the propagating path of the generated x-ray.
[0061] Field emissive electron sources field emit electrons by
applying a bias to a field emissive material and can be
individually addressed to emit electrons varying temporally and
spatially. The field emissive electron sources or portions thereof
can be addressed by a control voltage individually, or in
predetermined sequences or groupings, to provide control over the
emitted electron beam and so as to direct the beam to impact at any
one position on the anode target. By this method, multiple points
of incidence on a target may be impacted in the same device. If the
target is made from several different materials, a broader spectrum
of emitted x-rays can be generated without having to add and remove
targets of different materials from the chamber. Additionally, the
use of an array of field emissive sources can be used to reduce the
time any one area of the target is bombarded, thus contributing to
reduced heating of the anode.
[0062] For example, in an embodiment in which the cathode structure
is circular, the field emissive material can be pulsed in a stepped
fashion in which, in every revolution around the cathode structure,
the angular distance between field emissive material addressed by
successive pulses varies either in a predetermined manner or
randomly.
[0063] In an additional example, where the field emissive material
is positioned on the cathode structure at a desired interval, the
field emissive material can be pulsed in successive or
non-successive fashion to produce field emission electrons that
produce x-rays from the target corresponding in angular position
and in pulse frequency to the electrons emitted from the field
emissive material. Additionally, the field emissive material can be
pulsed individually or in groups, sequentially or randomly, in a
clockwise or counter clockwise rotation, or can be pulsed at a
predetermined angular separation. Desired intervals can include
angular separations from approximately 10.degree. to 120.degree.
and can include 10.degree., 15.degree., 30.degree., 45.degree.,
60.degree., 90.degree., or 120.degree. or variations thereof.
[0064] Further, the cathode can have a rasterization capability.
Through the use of individual, group, or matrix addressing of
selected regions of the cathode, a rasterized emitted electron beam
can be generated and directed to impinge distinctly at a point of
incidence on the target. By this arrangement, the stationary
cathode can be electrically switched at a predetermined frequency
to generate emitted electrons and accelerate them toward a
corresponding incident point on a stationary target.
[0065] The emitted x-rays from the stationary target accordingly
correspond in position and frequency to the electrically switched
cathodes and generate emitted x-rays that can be sufficiently
discriminated about the object to be imaged so as to produce a
three dimensional image. The emitted x-rays are discriminated both
in angular position about the object to be imaged and/or by
elevation angle with respect to the object to be imaged. Further,
the predetermined frequency is sufficiently rapid to dynamically
image physiological functions of the object to be imaged. In one
example, the power can be applied to all of the plurality of
cathodes simultaneously. In another example, power can be applied
to a subset (e.g., less than all) of the cathodes sequentially at a
frequency and thereby to electrically switch on and off the
generation of x-rays. The frequency can be either pre-set or
variably-set (e.g., changed in response to a feedback mechanism,
either automatically or manually). In one aspect, the frequency can
be sufficiently rapid to image the beating of a human heart in a CT
application. In an exemplary embodiment, the predetermined
frequency in the range of 0.1 Hz to 100 kHz.
[0066] FIG. 7 shows a top view of an apparatus for generating
x-rays 700 depicting a target structure 702 in a substantially
circular form. A detector 704 can be positioned within the hollow
706 of the apparatus 700 and can be adapted to allow positioning of
an object 708 within the imaging zone. The detector 704 can be
operatively positioned to receive emitted x-rays 710 from the
target structure 702 after they have passed through the imaging
zone. In the FIG. 7 embodiment, two discrete positions are shown
generating x-rays. Although two positions are shown, it is to be
understood by one skilled in the art that any number of spatially
arranged x-ray generating positions can be utilized. Further, when
the cathode is electrically switched, the detector system can be
moved or appropriately activated in a corresponding frequency to
the frequency of switching the power so as to position or activate
the detector to capture the x-ray image.
[0067] Additionally, any form of x-ray beam can be generated by the
apparatus. For example, an area array of targets can each generate
a pencil-like x-ray beam which may correspond to one or more pixels
in the detector, preferably no more than ten pixels. Alternately, a
line array of target material can each generate a fanlike x-ray
beam which corresponds to one or more lines of pixels in the
detector, preferably no more than ten lines of pixels. Finally, an
area array of targets can each generate a fan-like x-ray beam which
corresponds to an area of no more than 128.times.128 square pixels,
preferably no more than 64.times.64 square pixels. FIG. 8 depicts
an embodiment of an apparatus 800 in which the target material 802
generates a fan-like x-ray beam 804 when bombarded by electrons
806. In FIG. 8, the fan-like x-ray beams 804 image the object 808
in transmission mode and impact a large area array detector 810.
Accordingly, the detector 810 is individually addressable (e.g.,
pixilated).
[0068] The object positioner adjusts the position of an object to
be imaged within the imaging zone. For example, the object
positioner can place an object between the x-ray source and the
array or matrix of the detector. In one example, the object to be
imaged is stationarily mounted onto the object positioner such that
a centroid of the object to be imaged is positioned centrally
within the imaging zone. In a CT application, the object can be a
patient suitably oriented within the imaging zone. Further, the
centroid can correspond to the physiological function to be
monitored, such as the heart.
[0069] A detector is operatively positioned to detect the x-ray
after it has passed through the imaging zone. In one embodiment,
the detector can be at a elevation angle to the plane defined by
the target structure. The angle .beta. and corresponding angle
.gamma. can be chosen to cause the emitted x-rays to propagate at
an angle out of plane to that of the target structure, such that
the propagating x-rays can be detected by a detector positioned so
as not to obstruct the field of view of the target, e.g., located
above the target structure and across the imaging zone from the
origin of the x-rays to be detected. By such an arrangement, the
stationarily positioned detector array or matrix does not block the
x-ray from the target structure (e.g., obstruct the target with the
backside of the detector) and allows simultaneous imaging of the
object from all directions (e.g., 360.degree. imaging).
[0070] An exemplary method to obtain an x-ray image performed
consistent with the present invention stationarily positions an
object to be imaged on an object positioner and positions the
object to be imaged within an imaging zone, electrically switches a
plurality of stationary and individually addressable field emissive
electron sources at a predetermined frequency, each field emissive
electron source electrically switched in a programmable sequence to
field emit electrons toward an incidence point on a stationary
target. The beam of field emitted electrons which impact the target
generate x-rays by means well known in physics. The generated
x-rays then exit the chamber through the collimating windows and
image the object to be imaged within the imaging zone. The emitted
x-rays are then detected by a suitable detector. The emitted x-rays
correspond in position and frequency to the electrical addressing
of the field emissive electron sources. Additionally, the
circumferential position and/or the elevation angle of the emitted
x-rays is sufficiently discriminated with respect to the object to
be imaged to produce a three dimensional image.
[0071] In an additional method to obtain an x-ray image performed
consistent with the present invention, an object is placed in a
device to record x-ray images, such as the device to record x-ray
images as substantially described herein, power is applied to at
least one of the plurality of cathodes of the x-ray source to
generate x-ray radiation for a pre-set exposure time, the object is
exposed to the x-ray radiation, and an x-ray image corresponding to
the object is captured by either the x-ray detectors or the x-ray
sensitive films of the device to record x-ray images.
[0072] In one aspect of the method, the power is applied to all of
the plurality of cathodes simultaneously or to a a subset of the
plurality of cathodes sequentially at a frequency. When applying
power sequentially at a frequency, the x-ray detectors or the x-ray
sensitive films can be moved and/or activated at a corresponding
frequency to the power application frequency so as to capture the
x-rays produced. The frequency can be in the range or 0.1 Hz to 100
kHz. Further, the frequency can be sufficiently rapid to
dynamically image a physiological function, such as a beating
heart.
[0073] In a still further aspect, the method provides an imaging
environment in which the emitted x-rays are sufficiently
discriminated with respect to the object to be imaged to produce a
three dimensional image. This can be a result of the
circumferential position and/or the elevation angle of incidence
point with respect to the object to be imaged.
[0074] Variations of the above described method, as well as
additional methods, are evident in light of the above described
devices of the present invention.
[0075] While the present invention has been described by reference
to the above-mentioned embodiments, certain modifications and
variations will be evident to those of ordinary skill in the art.
Therefore, the present invention is to limited only by the scope
and spirit of the appended claims.
* * * * *