[0001] The present invention relates generally to X-ray tubes, and more particularly to
an X-ray tube having a higher ratio of X-ray energy flux to power deposited in the
target.
[0002] X-ray devices used in the medical field contain an X-ray tube which typically includes
a cathode which is heated to emit a beam of electrons, a (typically rotating) anode
having a target with a surface facing the cathode, and a surrounding glass and/or
metal frame containing an X-ray-transparent window secured by a window mount. Typically,
the cathode is oriented such that the electrons strike a focal spot on the target
surface at an angle which is generally ninety degrees with respect to the target surface.
Some emitted electrons strike the target surface and produce X-rays, and some of the
X-rays exit the frame as an X-ray beam through the X-ray-transparent window. Typically,
the X-ray window is positioned such that it receives X-rays which leave the target
surface at an angle of generally seven degrees with respect to the target surface.
Some emitted electrons do not produce X-rays and may be back-scattered when they strike
the target surface. Many of the back-scattered electrons go on to strike and heat
the frame including the X-ray-transparent window and the window mount. The frame is
also heated from within by other sources such as thermal radiation. The heated frame
is typically cooled by a liquid coolant, such as oil or water, located between the
frame and a surrounding casing having its own X-ray-transparent window.
[0003] Generally less than one percent of the power of the electrons striking the target
surface is converted into X-ray power. Increasing the power of the electron beam will
increase the X-ray power output of the tube. However, increasing the power of the
electron beam leads to unacceptably high thermal loading of the target which ultimately
limits the X-ray power output. What is needed is an X-ray tube assembly, and a method
for producing X-rays, which increases the ratio of X-ray tube power per target thermal
load.
[0004] According to a first aspect of the invention, there is provided an X-ray tube assembly
comprising: a) an X-ray tube anode including an X-ray-producing target having a surface;
b) an X-ray tube cathode having an electron-beam axis, wherein the electron-beam axis
intersects the surface of the target at a focal point, wherein the electron-beam axis
is oriented at a first angle with respect to the surface of the target, and wherein
the first angle is between and including fifteen degrees and sixty degrees; and c)
an X-ray tube window including a surface having a center point, wherein a line between
the focal and center points makes a second angle with respect to the surface of the
target.
[0005] The first angle may be between and including fifteen degrees and thirty degrees,
and the second angle may be between and including five degrees and fifteen degrees.
[0006] The first angle may be generally twenty degrees, and the second angle may be generally
seven degrees.
[0007] The electron-beam axis and the center point may define a plane which is oriented
generally perpendicular to the surface of the target.
[0008] The X-ray tube cathode may generate electrons which strike the target producing X-rays
having energies less than generally two hundred kilovolts.
[0009] According to a second aspect of the invention, there is provided an X-ray tube assembly
comprising: a) an X-ray tube anode including an X-ray-producing target having a surface;
b) an X-ray tube cathode having an electron-beam axis, wherein the electron-beam axis
intersects the surface of the target at a focal point, wherein the electron-beam axis
is oriented at a first angle with respect to the surface of the target, wherein the
first angle is between and including fifteen degrees and sixty degrees, and wherein
the X-ray tube cathode generates electrons which strike the target producing X-rays
having energies less than generally two hundred kilovolts; and c) an X-ray tube window
including a surface having a center point, wherein a line between the focal and center
points makes a second angle with respect to the surface of the target, wherein the
second angle is less than the first angle, and wherein the electron-beam axis and
the center point define a plane which is oriented generally perpendicular to the surface
of the target.
[0010] The first angle may be between and including fifteen degrees and thirty degrees,
and the second angle may be between and including five degrees and fifteen degrees.
[0011] The first angle may be generally twenty degrees, and the second angle may be generally
seven degrees.
[0012] According to a third aspect of the invention, there is provided a method for producing
X-rays comprising the following steps: a) generating a beam of electrons, wherein
the beam has an electron-beam axis; b) orienting the beam of electrons to strike a
focal spot on a surface of an X-ray-producing target to produce X-rays such that the
electron-beam axis makes a first angle with respect to the surface of the target and
such that the first angle is between and including fifteen degrees and sixty degrees;
and c) utilizing those X-rays which make a second angle with respect to the surface
of the target.
[0013] The first angle may be between and including fifteen degrees and thirty degrees,
and the second angle may be between and including five degrees and fifteen degrees.
[0014] The first angle may be generally twenty degrees, and the second angle may be generally
seven degrees.
[0015] The step c) may include utilizing those X-rays which, together with the electron-beam
axis, define a plane oriented generally perpendicular to the surface of the target.
[0016] The step b) may produce X-rays having energies less than generally two hundred kilovolts.
[0017] According to a fourth aspect of the invention, there is provided a method for producing
X-rays comprising the following steps: a) generating a beam of electrons, wherein
the beam has an electron-beam axis; b) orienting the beam of electrons to strike a
focal spot on a surface of an X-ray-producing target to produce X-rays having energies
less than generally two hundred kilovolts such that the electron-beam axis makes a
first angle with respect to the surface of the target and such that the first angle
is between and including fifteen degrees and sixty degrees; and c) utilizing those
X-rays which make a second angle with respect to the surface of the target, wherein
the second angle is less than the first angle, and which, together with the electron-beam
axis, define a plane oriented generally perpendicular to the surface of the target.
[0018] The first angle may be between and including fifteen degrees and thirty degrees,
and the second angle may be between and including five degrees and fifteen degrees.
[0019] The first angle may be generally twenty degrees, and the second angle may be generally
seven degrees.
[0020] In a first expression of an embodiment of the invention, an X-ray tube assembly includes
an X-ray tube anode, an X-ray tube cathode, and an X-ray tube window. The anode includes
an X-ray-producing target having a surface. The cathode has an electron-beam axis.
The electron-beam axis intersects the target surface at a focal point, and the electron-beam
axis is oriented at a first angle with respect to the surface of the target. The first
angle is between and including fifteen degrees and sixty degrees. The window includes
a surface having a center point, and a line between the focal and center points makes
a second angle with respect to the target surface.
[0021] In a second expression of an embodiment of the invention, an X-ray tube assembly
includes an X-ray tube anode, an X-ray tube cathode, and an X-ray tube window. The
anode includes an X-ray-producing target having a surface. The cathode has an electron-beam
axis. The electron-beam axis intersects the target surface at a focal point, and the
electron-beam axis is oriented at a first angle with respect to the surface of the
target. The first angle is between and including fifteen degrees and sixty degrees.
The X-ray tube cathode produces electrons which strike the target producing X-rays
having energies less than generally two hundred kilovolts. The window includes a surface
having a center point, and a line between the focal and center points makes a second
angle with respect to the target surface. The second angle is less than the first
angle. The electron-beam axis and the center point define a plane which is oriented
generally perpendicular to the target surface.
[0022] A first method of the invention is for producing X-rays and includes steps a) through
c). Step a) includes generating a beam of electrons, wherein the beam has an electron-beam
axis. Step b) includes orienting the beam of electrons to strike a focal spot on a
surface of an X-ray-producing target to generate X-rays such that the electron-beam
axis makes a first angle with respect to the surface of the X-ray target and such
that the first angle is between and including fifteen degrees and sixty degrees. Step
c) includes utilizing those X-rays which make a second angle with respect to the surface
of the target.
[0023] A second method of the invention is for producing X-rays and includes steps a) through
c). Step a) includes generating a beam of electrons, wherein the beam has an electron-beam
axis. Step b) includes orienting the beam of electrons to strike a focal spot on a
surface of an X-ray-producing target to generate X-rays having energies less than
generally two hundred kilovolts such that the electron-beam axis makes a first angle
with respect to the surface of the X-ray target and such that the first angle is between
and including fifteen degrees and sixty degrees. Step c) includes utilizing those
X-rays which make a second angle with respect to the surface of the target, wherein
the second angle is less than the first angle, and which, together with the electron-beam
axis, define a plane oriented generally perpendicular to the surface of the target.
[0024] Several benefits and advantages are derived from choosing the first angle (which
typically is called the electron-beam incident angle and referred to as "alpha") and
the second angle (which typically is called the X-ray emission angle and referred
to as "beta") in accordance with the invention. For example, computer simulations
benchmarked by experimental data show an X-ray energy flux enhancement of generally
1.5 when beta equals seven degrees and when alpha equals fifteen to twenty degrees.
The enhancement is computed in comparison to the X-ray energy flux of the prior art
design wherein beta is seven degrees and alpha is ninety degrees, wherein the deposited
power (i.e., the thermal load measured by temperature) and focal-spot temperature
in the target is the same in the inventive and prior-art designs, and wherein the
X-ray spectra of the inventive design is filtered to obtain the same mean photon (i.e.,
X-ray) energy as that of the prior-art design, for proper comparison, as can be appreciated
by those skilled in the art. An enhancement of 1.5 means a fifty percent increase
in X-ray power output for the same thermal load and focal-spot temperature in the
target for the inventive design compared to the prior-art design. It also means the
X-ray tube of the inventive design can be operated at the same X-ray power output,
but at a lower temperature (to increase tube life) compared to the X-ray tube of the
prior-art design.
[0025] The invention will now be described in greater detail, by way of example, with reference
to the drawings, in which:-
Figure 1 is a schematic view of an embodiment of an X-ray tube assembly of the present
invention;
Figure 2 is a contour plot of X-ray energy flux enhancement for various values of
alpha and beta (given in degrees), wherein the azimuth angle was zero degrees and
wherein certain contour lines have been omitted for clarity; and
Figure 3 is s plot of X-ray energy flux enhancement for various values of alpha (given
in degrees) wherein beta is seven degrees.
[0026] Referring now to the drawings, Figure 1 schematically shows an embodiment of the
X-ray tube assembly 10 of the present invention. In a first expression of the embodiment
shown in Figure 1, the X-ray tube assembly 10 includes an X-ray tube anode 12, an
X-ray tube cathode 14, and an X-ray tube window 16. The anode 12 includes an X-ray-producing
target 18 having a surface 20. The cathode 14 has an electron-beam axis 22. The electron-beam
axis 22 intersects the surface 20 of the target 18 at a focal point 24, and the electron-beam
axis 22 is oriented at a first angle 26 with respect to the surface 20 of the target
18. The first angle 26 is between and including fifteen degrees and sixty degrees.
The window 16 is an X-ray transparent window, as is known to those skilled in the
art, and includes a surface 28 having a center point 30. The center point 30 is a
geometric center point. For example, when the surface of the window has a shape of
a rectangle, the center point is the intersection of the diagonals of the rectangle.
A line 32 between the focal and center points 24 and 30 makes a second angle 34 with
respect to the surface 20 of the target 18.
[0027] In one design, the first angle 26 is between and including fifteen and thirty degrees,
and the second angle 34 is between and including five degrees and fifteen degrees.
In another design, the first angle 26 is generally twenty degrees, and the second
angle is generally seven degrees. For purposes of describing the invention, the terminology
"generally x degrees" means x degrees plus or minus two degrees. In one construction,
the electron-beam axis 22 and the center point 30 define a plane (i.e., the plane
of the paper of Figure 1) which is oriented generally perpendicular to the surface
20 of the target 18. For purposes of describing the invention, the terminology "generally
perpendicular" means perpendicular plus or minus two degrees. In one example, the
cathode 14 produces electrons (aligned with and centered about the electron-beam axis
22) which strike the target 18 producing X-rays having energies less than generally
two hundred kilovolts, wherein some of the X-rays which are aligned with and centered
about line 32 pass through the window 16 and are used for various purposes such as
medical diagnosis. Typically, X-rays used for medical diagnosis have energies less
than generally two hundred kilovolts. For purposes of describing the invention, the
terminology "less than generally two hundred kilovolts" means less than two hundred
five kilovolts. It is noted that the electron-beam axis 22 is a directional line indicating
the direction of travel of those electrons whose trajectories coincide with the electron-beam
axis 22. It also is noted that line 32 is a directional line indicating the direction
of travel of those X-rays whose trajectories coincide with line 32. In one embodiment,
the second angle 34 is less than the first angle 26.
[0028] In a second expression of the embodiment shown in Figure 1, the X-ray tube assembly
10 includes an X-ray tube anode 12, an X-ray tube cathode 14, and an X-ray tube window
16. The anode 12 includes an X-ray-producing target 18 having a surface 20. The cathode
14 has an electron-beam axis 22. The electron-beam axis 22 intersects the surface
20 of the target 18 at a focal point 24, and the electron-beam axis 22 is oriented
at a first angle 26 with respect to the surface 20 of the target 18. The first angle
26 is between and including fifteen degrees and sixty degrees. The cathode 14 produces
electrons which strike the target 18 producing X-rays having energies less than generally
two hundred kilovolts. The window 16 is an X-ray transparent window, as is known to
those skilled in the art, and includes a surface 28 having a center point 30. The
center point 30 is a geometric center point. A line 32 between the focal and center
points 24 and 30 makes a second angle 34 with respect to the surface 20 of the target
18. The second angle 34 is less than the first angle 26. The electron-beam axis 22
and the center point 30 define a plane (i.e., the plane of the paper of Figure 1)
which is oriented generally perpendicular to the surface 20 of the target 18. In one
design, the first angle 26 is between and including fifteen and thirty degrees, and
the second angle 34 is between and including five degrees and fifteen degrees.
[0029] In another design, the first angle 26 is generally twenty degrees, and the second
angle is generally seven degrees.
[0030] A first method of the invention is for producing X-rays and includes steps a) through
c). Step a) includes generating a beam of electrons, wherein the beam has an electron-beam
axis 22. Step b) includes orienting the beam of electrons to strike a focal spot (having
a geometric center called a focal point 24) on a surface 20 of an X-ray-producing
target 18 to produce X-rays such that the electron-beam axis 22 makes a first angle
26 with respect to the surface 20 of the target 18 and such that the first angle 26
is between and including fifteen degrees and sixty degrees. Step c) includes utilizing
those X-rays which make a second angle 34 with respect to the surface 20 of the target
18.
[0031] In one application of the first method of the invention, the first angle 26 is between
and including fifteen degrees and thirty degrees, and the second angle 34 is between
and including five degrees and fifteen degrees. In another application of the first
method, the first angle 26 is generally twenty degrees, and the second angle is generally
seven degrees. In one example of the first method of the invention, step c) includes
utilizing those X-rays which, together with the electron-beam axis 22, define a plane
oriented generally perpendicular to the surface 20 of the target 18. In another example
of the first method, step b) producing X-rays having an energy less than generally
two hundred kilovolts. In one employment, the second angle 34 is less than the first
angle 26
[0032] A second method of the invention is for producing X-rays and includes steps a) through
c). Step a) includes generating a beam of electrons, wherein the beam has an electron-beam
axis 22. Step b) includes orienting the beam of electrons to strike a focal spot (having
a geometric center called a focal point 24) on a surface 20 of an X-ray-producing
target 18 to produce X-rays having energies less than generally two hundred kilovolts
such that the electron-beam axis 22 makes a first angle 26 with respect to the surface
20 of the target 18 and such that the first angle 26 is between and including fifteen
degrees and sixty degrees. Step c) includes utilizing those X-rays which make a second
angle 34 with respect to the surface 20 of the target 18, wherein the second angle
34 is less than the first angle 26, and which, together with the electron-beam axis
22, define a plane oriented generally perpendicular to the surface 20 of the target
18. In one application of the second method of the invention, the first angle 26 is
between and including fifteen degrees and thirty degrees, and the second angle 34
is between and including five degrees and fifteen degrees. In another application
of the second method, the first angle 26 is generally twenty degrees, and the second
angle is generally seven degrees.
[0033] Applicants conducted experiments to obtain data on X-ray energy flux enhancement
for different values of the first angle 26 (which typically is called the electron-beam
incident angle and referred to as "alpha") and the second angle 34 (which typically
is called the X-ray emission angle and referred to as "beta"). By "enhancement" is
meant the X-ray energy flux for different values of the first and second angles were
divided by the X-ray energy flux obtained using a prior-art design of a ninety-degree
first angle 26 (i.e., alpha equals ninety degrees) and a seven-degree second angle
34 (i.e., beta equals seven degrees), wherein the deposited power (i.e., the thermal
load measured by temperature) in the target 18 is the same in the inventive and prior-art
designs, and wherein the X-ray spectra of the inventive design is filtered to obtain
the same mean photon (i.e., X-ray) energy as that of the prior-art design, for proper
comparison, as can be appreciated by those skilled in the art. An enhancement of 1.5
means a fifty percent increase in X-ray power output for the same thermal load on
the target 18 for the inventive design compared to the prior-art design. It also means
the X-ray tube assembly 10 of the inventive design can be operated at the same X-ray
power output, but at a lower temperature (to increase tube life) compared to the X-ray
tube assembly of the prior-art design.
[0034] Applicants also performed Monte-Carlo computer-program simulations based on an electron-microscopy
computer code optimized for 100 - 150 kilovolts using the experimental data to benchmark
the computer program. The results of the benchmarked Monte-Carlo simulations are presented
in Figure 2 as an x-y contour-map plot of X-ray energy flux enhancement with the y-axis
representing alpha (i.e., the first angle 26) and the x-axis representing beta (i.e.,
the second angle 34). Surprisingly, Applicants found a "sweet spot" where the enhancement
is at least 1.5 (i.e. the area on and within the enclosed 1.50 contour line in Figure
2. It is noted that constructing X-ray tubes with an alpha (i.e., first angle 26)
of less than fifteen degrees and/or with a beta (i.e., second angle 34) of less than
five degrees presents mechanical-design difficulties, as can be appreciated by those
skilled in the art. It also is noted that modifying a prior-art X-ray tube design
having an alpha of ninety degrees and a beta of seven degrees to have an alpha of
generally twenty degrees while keeping beta at generally seven degrees will result
in an X-ray energy flux enhancement of close to 1.50 as seen from Figures 2 and 3
while minimizing mechanical-design difficulties. A broader design envelope offering
improved X-ray energy-flux enhancement, as seen from Figure 2, with modest mechanical
redesign, requires an alpha (first angle 26) between and including fifteen and thirty
degrees and a beta (second angle 34) which is less than alpha and which is between
and including five and fifteen degrees. Although off the scale of Figure 2, Applicants
found improvement in X-ray energy flux when alpha was between and including fifteen
and sixty degrees and when beta was less than alpha.
1. An X-ray tube assembly comprising:
a) an X-ray tube anode including an X-ray-producing target having a surface;
b) an X-ray tube cathode having an electron-beam axis, wherein the electron-beam axis
intersects the surface of the target at a focal point, wherein the electron-beam axis
is oriented at a first angle with respect to the surface of the target, and wherein
the first angle is between and including fifteen degrees and sixty degrees; and
c) an X-ray tube window including a surface having a center point, wherein a line
between the focal and center points makes a second angle with respect to the surface
of the target.
2. An X-ray tube assembly comprising:
a) an X-ray tube anode including an X-ray-producing target having a surface;
b) an X-ray tube cathode having an electron-beam axis, wherein the electron-beam axis
intersects the surface of the target at a focal point, wherein the electron-beam axis
is oriented at a first angle with respect to the surface of the target, wherein the
first angle is between and including fifteen degrees and sixty degrees, and wherein
the X-ray tube cathode generates electrons which strike the target producing X-rays
having energies less than generally two hundred kilovolts; and
c) an X-ray tube window including a surface having a center point, wherein a line
between the focal and center points makes a second angle with respect to the surface
of the target, wherein the second angle is less than the first angle, and wherein
the electron-beam axis and the center point define a plane which is oriented generally
perpendicular to the surface of the target.
3. The X-ray tube assembly of claim 1 or 2, wherein the first angle is between and including
fifteen degrees and thirty degrees, and wherein the second angle is between and including
five degrees and fifteen degrees.
4. The X-ray tube assembly of claim 3, wherein the first angle is generally twenty degrees,
and wherein the second angle is generally seven degrees.
5. The X-ray tube assembly of any preceding claim, wherein the electron-beam axis and
the center point define a plane which is oriented generally perpendicular to the surface
of the target.
6. A method for producing X-rays comprising the following steps:
a) generating a beam of electrons, wherein the beam has an electron-beam axis;
b) orienting the beam of electrons to strike a focal spot on a surface of an X-ray-producing
target to produce X-rays such that the electron-beam axis makes a first angle with
respect to the surface of the target and such that the first angle is between and
including fifteen degrees and sixty degrees; and
c) utilizing those X-rays which make a second angle with respect to the surface of
the target.
7. A method for producing X-rays comprising the following steps:
a) generating a beam of electrons, wherein the beam has an electron-beam axis;
b) orienting the beam of electrons to strike a focal spot on a surface of an X-ray-producing
target to produce X-rays having energies less than generally two hundred kilovolts
such that the electron-beam axis makes a first angle with respect to the surface of
the target and such that the first angle is between and including fifteen degrees
and sixty degrees; and
c) utilizing those X-rays which make a second angle with respect to the surface of
the target, wherein the second angle is less than the first angle, and which, together
with the electron-beam axis, define a plane oriented generally perpendicular to the
surface of the target.
8. The method of claim 6 or 7, wherein the first angle is between and including fifteen
degrees and thirty degrees, and wherein the second angle is between and including
five degrees and fifteen degrees.
9. The method of claim 8, wherein the first angle is generally twenty degrees, and wherein
the second angle is generally seven degrees.
10. The method of any one of claims 6 to 9, wherein step b) produces X-rays having energies
less than generally two hundred kilovolts.