CT X-RAY TUBE WITH AN ANODE PLATE WHERE ANGULAR VELOCITY VARIES WITH TIME

Abstract

A computer tomography x-ray tube for generating pulsed x-rays is presented. The x-ray tube comprises an anode and an electron emission unit for generating a pulsed electron beam onto the anode. Furthermore, a rotation mechanism for rotating the anode characterized in that the rotation mechanism is configured for rotating the anode with an angular velocity that varies in time is comprised. The rotation mechanism may also be configured for rotating the anode such that the variation of the angular velocity in time is a continuous oscillation around a mean angular velocity ω.sub.0 in time. In a preferred embodiment the angular velocity ω (t) varies in time according to the following formula:

ω(t)=ω.sub.0+Δω sin Ωt,

wherein ω.sub.0 is a mean angular velocity. In a particular embodiment, the grid switch for generating the pulsed electron beam is comprised and the x-ray tube may be embodied as a stereo tube, in which two focal spots of electron beams are generated in an alternating manner.

Claims

1. A computed tomography x-ray tube for generating pulsed x-rays, comprising: an anode; an electron emission configured to generate a pulsed electron beam onto the anode, wherein the electron emission comprises a grid switch for generating the pulsed electron beam; and a rotation mechanism for rotating the anode, wherein the rotation mechanism is configured to rotate the anode with an angular velocity which varies in time, such that the variation of the angular velocity in time is a continuous periodic oscillation around a mean angular velocity in time, and wherein a rotational frequency of the anode and a switching frequency of the grid switch do not coincide.

2. The computed tomography x-ray tube according to claim 1, wherein the rotation mechanism comprises a stator-rotor combination configured to rotate the anode, and wherein the rotation mechanism is configured to vary in the stator at least one of a frequency of an electrical current and an electrical power.

3. The computed tomography x-ray tube according to claim 1, wherein the rotation mechanism is configured to vary the angular velocity in time such that the angular velocity of the anode follows a predefined time development and does not require measuring and controlling a rotation frequency of the anode.

4. The computed tomography x-ray tube according to claim 1, wherein the x-ray tube is a stereo tube having two focal spots of electron beams generated in an alternating manner.

5. The computed tomography x-ray tube according to claim 1, wherein the angular velocity varies in time according to a following formula:
ω(t)=ω.sub.0+Δω sin Ωt, wherein ω.sub.0 is the mean angular velocity.

6. The computed tomography x-ray tube according to claim 5, wherein Δω fulfils one of 1% ω.sub.0≤Δω≤6% ω.sub.0, 2% w.sub.0≤Δω≤5% w.sub.0, and 3% ω.sub.0≤Δω≤4% ω.sub.0.

7. The computed tomography x-ray tube according to claim 5, wherein Ω is 2π2 Hz.

8. The computed tomography x-ray tube according to claim 1, wherein the electron emission is configured to generate the pulsed electron beam with a pulse duration between 10 microseconds and 500 milliseconds, between 10 microseconds and 250 milliseconds, or between 10 microseconds and 100 milliseconds.

9. The computed tomography device for generating images of a patient, the computer tomography device comprising: an x-ray tube according to claim 1; and a gantry, wherein the computed tomography device is configured to cause the gantry to undergo a rotational movement during imaging, and wherein the angular velocity of the anode and the rotational movement of the gantry during imaging are de-synchronized due to the variation in time of the angular velocity of the anode.

10. A method of generating pulsed x-ray radiation with a rotating anode and a pulsed electron beam, comprising: emitting the pulsed electron beam onto the anode; and rotating the anode with an angular velocity which varies in time, wherein the anode is rotated such that the variation of the angular velocity in time is a continuous oscillation around a mean angular velocity in time, and wherein the electron beam is pulsed by a grid switch, wherein a rotational frequency of the anode and a switching frequency of the grid switch do not coincide.

11. The method according to claim 11, further comprising: driving the anode rotation by a stator-rotor combination, and varying in the stator at least one of a frequency of an electrical current and an electrical power to cause a continuous oscillation in time of the angular velocity of the anode around the mean angular velocity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1 schematically shows a computer tomography x-ray tube according to an exemplary embodiment of the present invention.

[0044] FIG. 2 schematically shows a computer tomography device for generating images of a patient according to an exemplary embodiment of the present invention.

[0045] FIG. 3 schematically shows a flow diagram of a method of generating pulsed x-ray radiation with a rotating anode and a pulsed electron beam according to an exemplary embodiment of the present invention.

[0046] Exemplary embodiments of the invention will be described in the following drawings.

DETAILED DESCRIPTION OF EMBODIMENTS

[0047] FIG. 1 schematically shows a computer tomography x-ray tube 100 for generating pulsed x-ray radiation. The x-ray tube 100 comprises an anode 104, an electron emission unit 102 for generating a pulsed electron beam 103 onto the anode 104. A rotation mechanism 107 for rotating the anode 104 is comprised as well. The rotation mechanism 107 is configured for rotating the anode 104 with an angular velocity, which varies in time. The rotation of the anode 104 is shown with arrow 106. The pulsed electron beam 103 is focused onto focal spot 105 of the anode 104 where the x-ray radiation 101 is generated. The x-ray radiation may exit the x-ray tube 100 via the radiation window 110. In the embodiment shown in FIG. 1, the rotation mechanism 107 comprises a stator 109 as well as a rotor 108, which is configured for rotating the anode 104. In this embodiment, the stator-rotor combination is configured for varying the frequency of the electrical current running through the stator 109 such that the angular velocity is varying in time as desired. Furthermore, the rotation mechanism is configured for varying the electrical power in the stator 109 such that the angular velocity of the anode is varying in time. The electric motor used in the embodiment of FIG. 1 for creating the rotation of the anode may comprise a controller (not shown), which ensures that the angular velocity in time is a continuous oscillation around a mean angular velocity ω.sub.0 in time.

[0048] The electron emission unit 102 may comprise several different components. In particular, the cathode, which emits the electron of the pulsed electron beam is comprised by the electron emission unit 102. Preferably, also a grid switch is comprised by the electron emission unit 102, which allows for a switching of the electron beam in an on and off state in very short time intervals. Care must be taken that the electron beam does not hit the same positions of the anode plate after each rotation since this would lead to non-uniform heating. A special case is given by a stereo tube, in which two focal spots are used in an alternating manner. Here, the targeted power for each focal spot can get quite high during periods of illumination. Therefore, heating up identical areas of the anode after each rotation can mean a major drawback. Therefore, the embodiment of FIG. 1 provides for the rotation mechanism, which is configured for rotating the anode with an angular velocity which varies in time. Thus, the likelihood of an unfortunate heating of the anode will significantly be reduced. This is true for nearly all electron beam switching patterns. The proposed solution of the CT x-ray tube 100 with an anode plate where the angular velocity varies with time is cost-efficient and no complex controlling mechanism is required.

[0049] In other words, the embodiment shown in FIG. 1 as an exemplary example can have a grid switch for generating the pulsed electron beam 103. Furthermore, the x-ray tube can be embodied as a stereo tube, in which two focal spots of electron beams are generated in an alternating manner. The computer tomography x-ray tube 100 is particularly used in computer tomography devices for generating images of a patient, as will be described in more detail hereinafter in the context of FIG. 2.

[0050] According to an exemplary embodiment of an aspect of the present invention, FIG. 2 shows a computer tomography device 200 for generating images of a patient. The computer tomography device 200 comprises an x-ray tube 201, which is located in an upper part of gantry 206. Gantry 206 is rotatable around an axis, which extends along the patient positioning table 203. The rotational movement of gantry 206 is indicated by arrow 207. The x-ray radiation 208 emitted by the computer tomography x-ray tube 201 can be detected after being transmitted through the patient by x-ray detector 202. A movement mechanism 205, which is capable of positioning the table 203 with respect to x-ray tube 201 allows an accurate positioning of the patient. Furthermore, the created CT images can be shown on display 204 to the medical practitioner after image acquisition. The computer tomography device 200 is configured to cause the gantry 206 to undergo a rotational movement 207 during imaging. Furthermore, the angular velocity of the anode 104 and the rotational movement of the gantry 206 during imaging are desynchronized due to the variation in time of the angular velocity of the anode taking place in the x-ray tube 201.

[0051] In particular, the variation of the angular velocity in time of the anode ensures that during a rotational movement of a gantry in a CT device during imaging, the angular velocity of the anode and said rotational movement are desynchronized. This cost-efficient solution does not require a complex controlling mechanism of the angular velocity of the anode but at the same time reduces the unfortunate heating of the anode plate significantly. This is true for nearly all electron beam switching patterns and is of particular advantage if grid switches and/or stereo tubes with two focal spots of electron beams are used.

[0052] In an exemplary embodiment, the CT may comprise a grid switch with a rotating anode plate within the tube drivable by a stator-rotor combination with a mechanism for varying the angular velocity of the anode plate. In a particular embodiment, the angular velocity of the anode plate varies like in the following equation

ω(t)=ω.sub.0+Δω sin Ωt,

wherein ω.sub.0 is a mean angular velocity. According to another exemplary embodiment of the present invention, Δω fulfils one of the following criteria 1% ω.sub.0≤Δω≤6% ω.sub.0, 2% ω.sub.0≤Δω≤5% ω.sub.0, and 3% ω.sub.0≤Δω≤4% ω.sub.0. The exemplified values for Δω are chosen such that sufficient variation is realized for obtaining the targeted benefits with respect to heating, while values for Δω are kept as small as possible for staying as close as possible to the target frequency ω.sub.0. In a further preferred embodiment Ω=2π2 Hz. This preferred value for Ω is chosen such that the targeted variation can be obtained with adequate electrical power.

[0053] The proposed solution can preferably be realized by varying the frequency of the electrical current in the stator of the rotation mechanism or by varying the electrical power in the stator of the rotation mechanism or by varying both. In any case, the likelihood of local overheating of the anode is reduced significantly by varying the angular velocity in time.

[0054] According to another exemplary embodiment of the present invention, FIG. 3 shows a flow diagram of a method of generating pulsed x-ray radiation with a rotating anode and a pulsed electron beam. The method comprises the steps of emitting the pulsed electron beam onto the anode S1 and rotating the anode with an angular velocity which varies in time S3. In the embodiment of FIG. 3, the anode rotation is caused by driving the anode by a stator-rotor combination. Moreover, by varying a frequency of the electrical current and the stator, a continuous oscillation in time of the angular velocity of the anode around a mean angular velocity is caused. Alternatively or in addition, varying the electrical power in the stator is comprised by the method thereby causing a continuously oscillation in time of the angular velocity of the anode around a mean angular velocity ω.sub.0. In step S2 the anode rotation is driven by a stator-rotor combination. The step of varying the frequency of the electrical current in the stator thereby causing a continuous oscillation in time of the angular velocity of the anode around a mean angular velocity ω.sub.0, and/or varying the electrical power in the stator thereby causing a continuous oscillation in time of the angular velocity of the anode around a mean angular velocity ω.sub.0 is shown in FIG. 3 with S3a.

[0055] In a particular embodiment of the method of FIG. 3, the electron beam is pulsed by using grid switch. Such grid switch, which allows quickly turning x-ray radiation on and off. In particular, the grid switch consists of a grid aperture, which is mounted in the space between cathode and anode. The electronics of the grid switch allows changing the voltage at this aperture quickly. Typical values of these voltages are +12 kV and −12 kV. Electrical fields arising from the aperture either allow electrons originating from the cathode to pass through to the anode, or these fields prevent the electrons from passing the aperture such that no x-ray radiation is generated.