FIELD OF THE INVENTION
[0001] The invention relates to an X-ray tube with a hybrid anode structure.
BACKGROUND OF THE INVENTION
[0002] Spectral imaging seems to become mainstream in X-ray computed tomography. Fast-kilovolt-peak,
abbreviated kVp, switching is the easiest route for implementing spectral capabilities
to the mid-range segment based on a voltage switching between energy levels and acquiring
imaging data at more than one energy range. The drawback of the simplest implementations
is that not all needed features are readily implemented in such an X-ray system.
[0003] The main problem with a fast-kilovolt-peak implementation is that is very difficult
to implement a fast dose modulation which is necessary for low dose scans. It is also
difficult to use very low current protocols as this increases the time to discharge
the high voltage capacitor and allows only for slow change in voltage.
[0004] US 2012/0326031 A1 describes a medical imaging method comprising generating a radiation at a first energy
level by a radiation source, generating a radiation at a second energy level different
from the first energy level by the radiation source, emitting the generated radiations
at an output of the radiation source towards a detector, and blocking or diverting
the emitted radiations during at least one intermediate phase during which the radiation
source switches in a transient way from one of the first energy level and the second
energy level to the other of the first energy level and the second energy level.
US 2005/0163281 A1 describes an X-ray tube which includes a device for substantially protecting an object
to be examined against the incidence of undesirable X-rays which can be produced notably
by the decay of a residual or surplus charge present in a high-voltage circuit after
an X-ray exposure. To this end there is provided at least one for deflecting and/or
defocusing the electron beam produced by the residual and/or surplus charge in such
a manner that at least it is not incident to a significant extent on a region of an
anode wherefrom X-rays excited thereby are directed towards an object to be examined.
US 2010/0172475 A1 describes a fast dose modulation method using a z-deflection in a rotating anode
or a rotating frame tube, where the electron beam is deflected from a first focal
spot region to a second focal spot region being formed on the anode, wherein only
the electromagnetic beam generated in the first focal spot region contributes to the
useful electromagnetic exposure beam, wherein the second focal spot region is designed
to avoid emission of electromagnetic beams into the direction of a useful electromagnetic
beam direction.
[0005] Sending X-rays of different spectra allows imaging the patient with spectral material
decomposition. This technique has proven to yield better diagnostics, and save toxic
contrast die.
[0006] Switching the tube high voltage as provided by fast-kilovolt-peak implementation
is challenging, however. Smoothening capacitance in the generator and cables have
to be charged and discharged at a fast pace.
[0007] Charging up, for example from 80 kV to 140 kV is merely a matter of generator power.
In contrast, discharging from 140 kV down to 80 kV is a challenge unless extra expensive
means like additional switchable resistors are used. The discharge current is then
the tube current through the X-ray tube. This provides a lower limit to the time for
ramp-down.
[0008] Higher tube current usually produces higher X-ray flux to the patient. This is mostly
unwanted. Instead, the X-ray flux in the used beam should be rather independent of
the higher tube current.
SUMMARY OF THE INVENTION
[0009] There may be a need to overcome the drawbacks from existing switching or fast-kilovolt-peak
implementations for X-ray tubes.
[0010] This need is met by the subject-matter of the independent claims. Further exemplary
embodiments are evident from the dependent claims and the following description.
[0011] A first aspect of the invention relates a hybrid anode structure for fast kVp switching
for dual energy CT, the hybrid anode structure comprising:
- i) an auxiliary anode, which comprises a first target area, which is configured to
receive a first portion of an electron beam;
- ii) a main anode, which comprises a second target area, which is configured to receive
a second portion of the electron beam and to convert the second portion of the electron
beam into X-rays; and
- iii) a deflector, which is configured to deflect an incident electron beam and to
spread the incident electron beam between the first target area of the auxiliary anode
and the second target area of the main anode.
[0012] The present invention advantageously uses an X-ray tube, wherein the current of the
X-ray tube is kept at maximum and the X-ray flux is controlled by at least partially
deflecting the electron beam across the anode such that the electron beam only partially
hits the anode and sending the rest of the electron beam into a beam dump under the
edge of the anode, which is biased such that its absolute voltage is slightly less,
for instance 1 kV, than the absolute voltage of the cathode.
[0013] Based on the present invention, an X-ray tube may be provided with an auxiliary anode
in terms of a fixed anode and a main, e.g. rotating anode, and a magnetic or electric
field, for example by z-deflection, for directing an electron beam to either anode,
or distributing the current between the first and the second anode ,whereby the electron
beam passes, at least partially, at the rim of the second anode, which serves as X-ray
generator, and with variable and adjustable ratio also hits the first anode, which
serves as an electron dump. The present invention advantageously provides a minimal
waste of energy as well as a minimal leakage radiation due to low impact electron
beam energy. A practical point to consider is also the effort in calibration needed.
As any current setting needs its own calibration, it is desired to have only a few
of them while all clinical protocols shall be feasible. A grooved anode is a solution
to this problem, but it imposes a high load to the rotating anode and cooling becomes
an issue.
[0014] According to an embodiment of the invention, the auxiliary anode is for instance
V-shaped and is located below and at the side of the second anode. The thermal load
of the second anode is substantially reduced.
[0015] According to an embodiment of the invention the auxiliary anode and the main anode
are configured to have an at least partially gapless overlap between the first target
area of the auxiliary anode and the second target area of the main anode within the
beam path of the electron beam
[0016] According to an embodiment of the invention, the auxiliary anode is configured to
be biased with an offset voltage to an auxiliary anode voltage, which is less than
a main anode voltage of the main anode.
According to an embodiment of the invention, the auxiliary anode is configured to
be biased with the offset voltage to the auxiliary anode voltage, which is varied
between a first voltage value and a second voltage value.
[0017] According to an embodiment of the invention, the first target area of the auxiliary
anode is configured to receive the first portion of the electron beam in a grazing
incidence and wherein the auxiliary anode is configured to dissipate energy of the
first portion of the electron beam. This advantageously provides improved heat dissipation
inside the anode structure.
[0018] According to an embodiment of the invention, the first target area of the auxiliary
anode is configured to receive the first portion of the electron beam in the grazing
incidence by means of an inclined plane, tilted to a beam axis of the first portion
of the electron beam. This advantageously provides improved heat dissipation inside
the anode structure.
[0019] According to an embodiment of the invention, the first target area of the auxiliary
anode is configured to receive the first portion of the electron beam in the grazing
incidence by means of a groove, at least one side wall of which is tilted to an beam
axis of the first portion of the electron beam. This advantageously provides improved
heat dissipation inside the anode structure.
[0020] According to an embodiment of the invention, the auxiliary anode comprises a shielding
portion, which is configured to prevent generated X-rays to leave the hybrid anode
structure. This advantageously provides improved X-ray shielding. According to an
embodiment of the invention, the shielding portion comprises carbon and/or wherein
the shielding portion is configured to prevent back scattering of electrons. This
advantageously provides improved X-ray shielding.
[0021] According to an embodiment of the invention, the auxiliary anode is a circular ring
segment, which is optionally V-shaped, and the main anode is a rotating anode tube.
[0022] According to an embodiment of the invention, the auxiliary anode is configured to
be cooled. This advantageously provides higher X-ray fluxes.
[0023] According to an embodiment of the invention, the auxiliary anode comprises molybdenum
or tungsten. This advantageously provides improved X-ray tube reliability.
[0024] According to an embodiment of the invention, the deflector is configured to spread
first portion of an electron beam over the first target area of the auxiliary anode
and configured to increase an irradiated surface of the first target area of the auxiliary
anode.
[0025] According to an embodiment of the invention, the deflector comprises a single-stage
or a multistage collector and/or the deflector is configured to spread the beam by
an angled structure and/or the deflector is configured to use:
a static electric field; and/or
a dynamic electric field; and/or
a magnetic field
to spread the incident electron beam between the first target area of the auxiliary
anode and the second target area of the main anode.
[0026] Besides spreading the beam by the angled structure, it is also possible to use a
static and or dynamic electric or magnetic field to spread the electrons over the
surface to reduce power density. This is best combined with a shallow angle of the
electron impact.
[0027] According to an embodiment of the invention, a first target is constructed in a shallow
angle where most of the electrons get scattered off and a second electron target surface
is provided at a distance with even higher surface area, essentially a "hohlraum"
absorber, a black body or blackbody as an idealized physical body that absorbs all
incident electromagnetic radiation.
[0028] A "hohlraum" - a non-specific German word for a "hollow space" or "cavity" - is a
cavity whose walls are in radiative equilibrium with the radiant energy within the
cavity.
[0029] According to an embodiment of the invention electron scattering is achieved by using
a set of fine, for instance Wollaston wires, which comprise a diameter of 50 nm to
100 nm in an example, to scatter the electrons to the target surface. Such small wires
can radiate the energy away without being harmed. A Wollaston wire is a very fine,
for instance less than 0.01 mm thick, platinum wire clad in silver.
[0030] The density of the wires in the hohlraum is such that the almost all of the primary
beam touches a small wire while the scattered electrons are likely to strike the surface
of the hohlraum.
[0031] A second aspect of the invention relates an X-ray tube comprising the hybrid anode
structure according to the first aspect or according to any implementation of the
first aspect.
[0032] These and other aspects of the invention will be apparent from and elucidated with
reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Below, embodiments of the present invention are described in more detail with reference
to the attached drawings.
FIG. 1 shows a schematic diagram of a diagram of the ideal tube voltage versus time
according to an embodiment of the invention.
FIG. 2 shows a schematic diagram of a hybrid anode structure for fast fast-kilovolt-peak
switching for dual energy CT according to an embodiment of the invention.
FIG. 3 shows a schematic diagram of a hybrid anode structure for fast fast-kilovolt-peak
switching for dual energy CT according to an embodiment of the invention.
FIG. 4 shows a schematic diagram of a hybrid anode structure for fast fast-kilovolt-peak
switching for dual energy CT according to an embodiment of the invention.
In principle, identical parts are provided with the same reference symbols in the
figures.
DETAILED DESCRIPTION OF EMBODIMENTS
[0034] FIG. 1 shows a schematic diagram of a diagram of the ideal tube voltage versus time
according to an embodiment of the invention.
[0035] Sending X-rays of different spectra allows imaging the patient with spectral material
decomposition. This technique has proven to yield better diagnostics, and save toxic
contrast die.
[0036] Switching the tube high voltage is challenging, however. Smoothening capacitance
in the generator and cables have to be charged and discharged at a fast pace. Charging
up, for example from 80 kV to 140 kV in times of several 10 µs or 100 µs is merely
a matter of generator power. However, discharging from 140 kV down to 80 kV is a challenge
unless extra expensive means like additional switchable resistors are used. The discharge
current is then the tube current through the X-ray tube. This provides a lower limit
to the time for ramp-down.
[0037] High tube current usually produces high X-ray flux to the patient. This is mostly
unwanted. Instead, the X-ray flux in the used beam should be rather independent of
the high tube current.
[0038] To have a well performing kVp switching system, it is a criterion that the current
consumed by the X-ray tube, or in other words, the current over voltage, is the same
at all times of the scan. This ensures that the high kVp and low kVp times as well
as the transition times remain constant and therefore the average high kVp and low
kVp spectra.
[0039] This is advantageously achieved, if the condition at the cathode - for instance to
be defined by parameters like filament current, acceleration voltages, temperature,
pressure, electric resistance, heat capacitance - are kept constant. To modulate the
X-ray flux, a part of the electron beam is dumped where it cannot produce image relevant
X-rays. According to an embodiment of the invention, this can be realized with a groove
at the anode and a large-scale z-deflection unit, best a magnetic one.
[0040] However, as the current is always set to the highest value needed for the scan, and
all energy is dumped to the rotating anode, issues with cooling arise which may impair
work flow. As a further drawback, the groove impairs heat flow towards the axle of
the anode and further increases heat issues. To overcome this, according to an embodiment
of the invention, the fixed anode is provided below and to the side of the rotating
anode, as shown in Fig. 1.
[0041] According to an embodiment of the invention, the - optionally dynamic - electric
or magnetic field deflects the electron beam at least partially off the rotating anode.
However, the fixed anode needs to have a special shape to cope with the high power
density of the electron beam.
[0042] According to an embodiment of the invention, it is a V-shaped groove directed to
the axis. According to an embodiment of the invention, the width of the V-shaped groove
is more than the width of the electron beam, for example the width of the V-shaped
groove more than 1 mm or more than 0.5 mm or more than 100 µm.
[0043] According to an embodiment of the invention, the length of the groove is at least
the length of the electron beam i.e. about at least 1 cm or at least 5 mm or at least
1 mm. The energy absorbing surface needs to be at least 30 times of the equivalent
area as provided by rotation of the rotating anode.
[0044] This is for example achieved by making the groove at least 15 mm deep or at least
20 mm deep or at least 30 mm deep. The depth of the groove of at least 15 mm is enough
as due to the scattered electrons, both surfaces of the V-structure are about equally
subjected to the beam power, i.e. the depth of the groove of at least 15 mm is larger
than any characteristic electron scattering length of the decay of hot electron in
the anode.
[0045] According to an embodiment of the invention, the V-structure is sealed at both ends
with an X-ray absorbing material to prevent the generated X-rays to leave the X-ray
tube. The V-structure is made of a highly heat resistant material like molybdenum
or preferably tungsten.
The fixed anode has to be actively cooled and therefore should be connected to the
oil system of the tube to prevent overheating. The needed length of the V-structure,
for example 15 mm plus structural thickness of 20 to 25 mm, may be difficult to implement
in some X-ray tube designs. In order to reduce the power density at the fixed anode,
it is possible to use only one side of the V-structure to be exposed to the electron
beam.
[0046] According to an embodiment of the invention, as the majority of the electrons are
scattered away from the surface, the power density on this surface is rater low. The
other V-structure face is retracted from this side and possibly some more grooves
are added to this. This helps the electrons to spread out over a larger surface.
[0047] FIG. 2 shows a schematic diagram of a hybrid anode structure for fast fast-kilovolt-peak
switching for dual energy CT according to an embodiment of the invention. According
to an embodiment of the invention, maintaining the highest tube current may be applied,
a current which the tube can deliver and sustain at highest tube voltage, e.g. 140
kV). This is typically done by setting the temperature of the thermionic electron
emitter accordingly.
[0048] According to an embodiment of the invention, bias voltages in the cathode are adjusted
as well. According to an embodiment of the invention, the maximal tube current discharges
the generator capacitance in the fastest way possible.
[0049] According to an embodiment of the invention, in order to reduce the used X-ray flux,
steering the electron beam across the anode such that the electron beam only partially
hits the anode is performed. This portion creates the finally used X-rays, while steering
the rest of the electron beam into a beam dump is conducted. According to an embodiment
of the invention, the beam dump may be biased or charged in such a way the electrons
hits with minimal power.
[0050] According to an embodiment of the invention, the absolute of its negative voltage
should be only a little less than the absolute of the momentary cathode voltage. Electrons
will be absorbed, but their impact energy will be minimal, as will be the heat generated
[0051] According to an embodiment of the invention, the beam dump is configured such that
electron backscattering is minimal, for instance by using carbon-based materials,
or by using a grazing impact of the electron beam towards the auxiliary anode. According
to an embodiment of the invention, the beam dump in terms of the auxiliary anode is
configured to be fluid-cooled.
FIG. 3 a schematic diagram of a hybrid anode structure for fast kilo-volt-peak switching
for dual energy CT according to an embodiment of the invention.
[0052] According to an embodiment of the invention, the beam dump is a current source. Actively
steer the bias of the beam dump by the generator. This must be done according to the
predicted discharge pattern and the dumped portion of the electron beam. Re-use the
energy supplied in the generator.
[0053] According to an embodiment of the invention, the beam dump may be configured to be
basically float, and discharged by passive load.
[0054] According to an embodiment of the invention, the beam dump may be coupled to a resistor
and/or to a capacitor and/or to an inductor to be discharged by passive load.
[0055] According to an embodiment of the invention, an electric current through the beam
dump maybe used for monitoring purposes or the active tube current. For instance,
an X-ray quantity of the generate X-rays may be measured by an X-ray sensor quantifying
the X-ray flux or the tube voltage may be used and measured.
[0056] FIG. 4 a schematic diagram of a hybrid anode structure for fast kVp switching for
dual energy CT according to an embodiment of the invention.
[0057] According to an embodiment of the invention, the X-ray tube is rotating about the
patient, generating a fan beam of X-rays. Opposite to the X-ray tube and with it on
a gantry rotor rotates a detector system, which converts the attenuated X-ray into
electrical signals. A computer system reconstructs an image of the patients anatomy.
[0058] According to an embodiment of the invention, the X-ray tube as shown in Fig. 4 comprises
a hybrid anode structure, a house, an anode support shaft and a rotor body. The rotor
body may for instance comprise cooper. The X-ray tube as shown in Fig. 4 comprises
an anode disc which is rotating and a fixed anode.
[0059] It has to be noted that embodiments of the invention are described with reference
to different subject matters. In particular, some embodiments are described with reference
to method type claims whereas other embodiments are described with reference to the
device type claims.
[0060] However, a person skilled in the art will gather from the above and the following
description that, unless otherwise notified, in addition to any combination of features
belonging to one type of subject matter also any combination between features relating
to different subject matters is considered to be disclosed with this application.
However, all features can be combined providing synergetic effects that are more than
the simple summation of the features.
[0061] While the invention has been illustrated and described in detail in the drawings
and foregoing description, such illustration and description are to be considered
illustrative or exemplary and not restrictive; the invention is not limited to the
disclosed embodiments. Other variations to the disclosed embodiments can be understood
and effected by those skilled in the art and practicing the claimed invention, from
a study of the drawings, the disclosure, and the appended claims.
[0062] In the claims, the word "comprising" does not exclude other elements or steps, and
the indefinite article "a" or "an" does not exclude a plurality. A single processor
or controller or other unit may fulfil the functions of several items recited in the
claims. The mere fact that certain measures are recited in mutually different dependent
claims does not indicate that a combination of these measures cannot be used to advantage.
Any reference signs in the claims should not be construed as limiting the scope.
1. A hybrid anode structure (100) for fast-kilovolt-peak switching for dual energy CT,
the hybrid anode structure comprising:
- an auxiliary anode (10), which comprises a first target area (12), which is configured
to receive a first portion (42) of an electron beam;
- a main anode (20), which comprises a second target area (22), which is configured
to receive a second portion (44) of the electron beam and to convert the second portion
of the electron beam into X-rays (50); and
- a deflector (30), which is configured to deflect an incident electron beam (40)
and to spread the incident electron beam (40) between the first target area (12) of
the auxiliary anode (10) and the second target (22) area of the main anode (20).
2. The hybrid anode structure according to claim 1,
wherein the auxiliary anode (10) and the main anode (20) are configured to have an
at least partially gapless overlap between the first target area (12) of the auxiliary
anode (10) and the second target (22) area of the main anode (20) within the beam
path of the electron beam.
3. The hybrid anode structure according to claim 1 or 2,
wherein the auxiliary anode is configured to be biased with an offset voltage to an
auxiliary anode voltage, which is less than a main anode voltage of the main anode
(20).
4. The hybrid anode structure according to any one of the claims 1 to 3,
wherein the auxiliary anode is configured to be biased with the offset voltage to
the auxiliary anode voltage, which is varied between a first voltage value and a second
voltage value.
5. The hybrid anode structure according to any one of the claims 1 to 4,
wherein the first target area of the auxiliary anode is configured to receive the
first portion of the electron beam in a grazing incidence and wherein the auxiliary
anode is configured to dissipate energy of the first portion of the electron beam.
6. The hybrid anode structure according to claim 5,
wherein the first target area of the auxiliary anode is configured to receive the
first portion of the electron beam in the grazing incidence by means of an inclined
plane, tilted to a beam axis of the first portion of the electron beam.
7. The hybrid anode structure according to claim 6,
wherein the first target area of the auxiliary anode is configured to receive the
first portion of the electron beam in the grazing incidence by means of a groove,
at least one side wall of which is tilted to a beam axis of the first portion of the
electron beam.
8. The hybrid anode structure according to any one of the claims 1 to 7,
wherein the auxiliary anode comprises a shielding portion, which is configured to
prevent generated X-rays to leave the hybrid anode structure.
9. The hybrid anode structure according to claim 8,
wherein the shielding portion comprises carbon and/or wherein the shielding portion
is configured to prevent back scattering of electrons.
10. The hybrid anode structure according to any one of the claims 1 to 9,
wherein the auxiliary anode (10) is a circular ring segment, which is optionally V-shaped,
and the main anode (20) is a rotating anode tube.
11. The hybrid anode structure according to any one of the claims 1 to 10,
wherein the auxiliary anode (10) is configured to be cooled.
12. The hybrid anode structure according to any one of the claims 1 to 11,
wherein the auxiliary anode (10) comprises molybdenum or tungsten.
13. The hybrid anode structure according to any one of the claims 1 to 12,
wherein the deflector (30) is configured to spread first portion of an electron beam
over the first target area (12) of the auxiliary anode and configured to increase
an irradiated surface of the first target area (12) of the auxiliary anode.
14. The hybrid anode structure according to any one of the claims 1 to 14,
wherein the deflector (30) comprises a single-stage or a multistage collector and/or
the deflector (30) is configured to spread the beam by an angled structure and/or
the deflector (30) is configured to use:
a static electric field; and/or
a dynamic electric field; and/or
a magnetic field
to spread the incident electron beam (40) between the first target area (12) of the
auxiliary anode (10) and the second target (22) area of the main anode (20).
15. An X-ray tube comprising the hybrid anode structure according to any one of the claims
1 to 14.