Fluid injector for x-ray tubes and method to provide a liquid anode by liquid metal injection

The present invention relates to a fluid injector for x-ray tubes and a method to provide a liquid anode by liquid metal injection, with a device to inject fluid from an opening in a chamber of the device in form of a fluid jet generated by an arrangement to change the volume within the chamber, and comprising a reservoir to store the anode material, which is fluidically connected by a pipe with the chamber of the device . X-rays are used for example in clinical diagnostics and visualization. X-rays are generated usually by applying a high voltage to an x-ray tube. The x-ray tube is a capsulated device comprising vacuum and with an electron source, i.e. cathode as well with an electron target, i.e. anode.

Electrons emitted from the cathode are accelerated by an applied high voltage between anode and cathode, and hit the anode with high velocity, i.e. energy. A high thermal load is generated at the anode material of the x-ray tube as an impact of the electron beam.

The interaction of electrons with anode material is

accompanied by radiation, the so called "Bremsstrahlung" with a continuous spectrum and the so called "Characteristic" radiation with a discrete monochromatic spectrum. The

"Bremsstrahlung" radiation spectrum is inefficient for various diagnostic applications in healthcare. Only some portion of the "Bremsstrahlung" radiation spectrum is used for quality imaging, while low energy photons overexpose the patient without contribution to the image quality. More than 99% of electron energy is converted into heat in the anode material, leading to a high amount of thermal load at the target material. This is especially the case when the x-ray focal spot is in the range of micrometer in diameter, to obtain high resolution x-ray images.

To reduce the thermal load at the anode, the target material can be changed rapidly in order not to accumulate the thermal load in a specific volume part of the target material. The most effective ways to provide rapidly changing anode

material facing the electron beam are the use of rotating or moving solid state anodes, and another way is the use of a target formed by a flowing liquid material, for example a high Z material or a combination of a low and high Z

material .

X-ray tubes with rotating anodes are known from the state of the art, for example US3836805A, DE3429799A1 and US6735283B2. The limiting factor in the described arrangements is the maximum of rotational frequency, which is sensible to

external acceleration for example of the whole tube, and it is difficult to fabricate tubes with a reliable transmission microfocus source.

X-ray tubes with liquid metal in form of a jet used as anode are also known from the state of the art, for example

US8170179B2, US7929667B1 and US7412032B2. The advantages of a liquid jet as target material are the excellent heat transfer properties of liquid metals and the possibility to generate thin and fast liquid jets flowing free in vacuum or inside an electron- and x-ray transparent casing, for example with a jet with less than 0.1 mm diameter and more than 50 m/s velocity of the liquid flow. Disadvantage of the described arrangements is the use of complex recirculation systems, comprising pumps for high temperature liquid metal. The flow rate of liquid metal is limited by the pumps, the reliability of the arrangement is limited and pumps increase costs and complexity.

The object of the present invention is to present a fluid injector for x-ray tubes and a method to provide a liquid anode by liquid metal injection solving the above described problems. Particularly an object is to resent an injector and a method to use the injector to produce x-rays, preventing high thermal loads, without moving parts like valves, with a simple and easy to use design, with low complexity and long lasting without substantial wear, and cost-effective in production.

The above objects are achieved by a fluid injector for x-ray tubes according to claim 1 and a method to provide a liquid anode according to claim 10.

Advantageous embodiments of the present invention are given in dependent claims . Features of the main claims can be combined with each other and with features of dependent claims, and features of dependent claims can be combined together.

The fluid injector for x-ray tubes to provide a liquid anode by liquid metal injection according to the present invention comprises a device to inject fluid from an opening in a chamber of the device in form of a fluid jet, generated by an arrangement to change the volume within the chamber. The fluid injector further comprises a reservoir to store the anode material, which is fluidically connected by a pipe with the chamber. The pipe comprises a part formed in fluid flow direction with a shape to block fluid flow from the chamber to the reservoir during injection. The described fluid injector according to the present

invention solves the above described problems. Particularly the fluid injector provides a liquid anode by liquid metal injection to an x-ray tube for the generation of x-rays, preventing high thermal loads. The injector has a simple and easy to use design, with low complexity. The described injector is long lasting without substantial wear, and cost- effective in the production due to no moving parts with high wear like pumps or valves. The design of the injector without valves and pumps allows the use with a high frequency of injection, is reliable and due to the fast change of anode material in contact with an electron beam, the summing up of thermal load is reduced in the material. X-rays can be produced with high intensity and focused, without high effort .

The part of the pipe with a shape to block fluid flow from the chamber to the reservoir during injection can be formed in fluid flow direction with a curved and/or angled shape, particularly in form of repeated loops of the pipe,

particularly in spiral form.

This form is easy and cost-effective to produce, especially with commercially available pipe material, and is effective in blocking fluid flow from the chamber to the reservoir during injection, particularly short injection phases with high frequency alternating to refilling phases. The fluidic connection between the chamber and the reservoir can be an uninterrupted direct connection through the pipe and/or permanent. This means, there are no valves or other fluidically disrupting components arranged in the fluidic connection between the chamber and the reservoir. A pipe with curved and/or angled shape without moving parts like valves is easy to use, cheap to produce, and reliable, without high complexity, and is long lasting due to no moving parts, reducing wear. The pipe can comprise a part with spiral shape with a number of full loops in the range of 5 to 15 and/or a predefined radius of curvature and cross-section to result in a limited and/or turbulent fluid flow in direction to the reservoir in a phase of injection and to a laminar fluid flow in direction to the chamber during a phase of refilling. The repeated loops are well able to block fluid flow back to the reservoir during injection, even when fluid would flow with high velocity in a pipe without loops . Turbulent flow is blocking fluid from flowing back to the reservoir from the injector chamber during injection. A laminar flow allows a good refilling of the chamber with fluid from the reservoir due to a good flow of fluid from the reservoir to the device with chamber. A number of loops between 5 to 15 is high enough to block fluid flow during injection and low enough to allow a good fluid flow within the pipe during refilling without high resistance to the fluid to flow. Depending among others on the kind of fluid used, the dimensions of the device to inject fluid and of the reservoir, the material of the pipe, the cross-section and radius of curvature as well as number of loops needed to block fluid flow during injection can be calculated and pre-determined. The fluid can be and/or can comprise a liquid metal,

particularly Gallium and/or a Gallium alloy and/or Lithium and/or a Lithium alloy. These materials are well suitable as anode materials for x-ray generation. The device can be designed for high pressure pulsed fluid injection, particularly with an injection frequency in the range of 10 to 1000 Hz. At this frequency the use of valves is difficult to handle and involves a high amount of wear. The curved and/or angled shape part in the pipe is well able to block fluid flow during injection due to a hydraulic hammer effect.

The arrangement to change the volume within the chamber can comprise a metal sheet, a membrane and/or a piezo element for a volume change within the chamber, particularly with a frequency in the range of 10 to 1000 Hz and/or to produce a high pressure within the chamber for pulsed fluid injection through the opening. The device to inject fluid can comprise a nozzle cup,

particularly with a sharp edge orifice, and/or a clamped circular membrane, and/or a piston particularly driven by a piezo-actuator. Such design of the device enables the injection of a jet of anode material with small cross section at high frequency.

The injector and/or components of the injector, particularly the opening, can be arrangeable in or fluidically connectable to the inner part of a vacuum tube, to inject fluid as anode material into and/or to an electron beam, particularly generated by an electron source . The method to provide a liquid anode by liquid metal

injection in an x-ray tube according to the present

invention, particularly using a fluid injector as described before, comprises a step of injecting liquid metal in form of a fluid jet from an opening of a chamber comprised by a device to inject fluid in the direction towards an electron beam. The injection is generated by changing the volume of the chamber with an arrangement producing a high pressure in the fluid in the chamber. The method further comprises a step of refilling the chamber with liquid metal from a reservoir, the liquid metal flowing from the reservoir to the chamber through a pipe .

The liquid metal flow in the pipe can be laminar during refilling and at least partly turbulent during injection, particularly with liquid metal flow in the pipe limited by a part of the pipe with curved and/or angled shape in flow direction during injection. The curved and/or angled shape can be in form of repeated loops of the pipe, particularly in spiral form, i.e. formed as a spiral.

A pulsed liquid metal injection, particularly with a

frequency of injection pulses in the range of 10 to 1000 Hz, can be followed in time by refilling of the chamber with liquid metal from the reservoir, particularly the metal liquidized by heating up solid metal in the reservoir. The metal can be liquidized according to the amount needed for liquid injection. The electron beam can converge at the injected liquid metal jet, particularly pulsed liquid metal jet, in an angle of substantially 90 degree. Electrons hitting the liquid metal, i.e. anode target material yield energy to the material resulting in thermal load and x-ray generation. Due to the movement of material the target material hit by the electron beam is changing, preventing respectively reducing the sum up of thermal load in a specific volume element of material. An angle of substantially 90 degree of electron beam impact to the liquid metal jet can results in a high x-ray yield, particularly in transmission mode. Depending on energy distribution, geometrical restrictions and other

circumstances other angels of coincidence are possible too. The electron beam can converge at the injected liquid metal jet, the liquid metal acting as anode material and/or target, and x-rays are generated, particularly the electron beam hitting the target with high intensity in a small volume of metal and/or with low thermal load at the injected metal.

During injection a piston driven by a piezo-actuator can produce high pressure in the chamber by compressing a

hydraulic fluid volume, deforming a membrane, particularly a clamped circular membrane, to reduce the volume of the chamber with liquid metal being ejected from the chamber through an opening, particularly in form of a nozzle cup with sharp edge orifice, and liquid metal being blocked from flowing to the reservoir by a part of the pipe with curved and/or angled shape .

The advantages in connection with the described method to provide a liquid anode by liquid metal injection in an x-ray tube according to the present invention are similar to the previously, in connection with the fluid injector described advantages and vice versa. The present invention is further described hereinafter reference to illustrated embodiments shown in the

accompanying drawings, in which: FIG 1 illustrates a fluid injector 1 according to the present invention, with a pipe 7 comprising a spiral part 9 fluidically connecting a device to inject a fluid 2 and a reservoir 6, and FIG 2 shows an embodiment of the device 2 to inject a

fluid of FIG 1 in more detail with an opening 10 in form of a nozzle cup with sharp edge orifice, and

FIG 3 shows the fluid injector 1 arranged in an x-ray

tube combined with an electron source 1 , with generated electrons 15 interacting with injected anode material 8 to generate x-rays 16.

In FIG 1 a fluid injector 1 according to the present

invention is shown, with a pipe 7 comprising a spiral part 9 fluidically connecting a device 2 to inject a fluid and a reservoir 6. The spiral part 9 allows a laminar fluid flow during refilling of the device 2 with liquid from the reservoir 6, and blocks fluid flow from the device 2 to the reservoir 6 during injection of fluid from the injector 2.

The device 2 comprises a chamber 3 filled with fluid in liquid form to be injected. The volume of the chamber 3 is for example in the range of 1 cm ³ . The fluid is a liquid metal which is used as anode, for example a Gallium based alloy or a Lithium based alloy. Injected means ejected from the chamber 3 via an opening 4 in the chamber 3 to the outside of the chamber 3, i.e. the outside of the device 2. The device 2 comprises an arrangement 5 to change the volume within the chamber 3. The arrangement 5 can be or comprise for example a piezo-electric device, formed to reduce the volume of the chamber 3 particularly after applying a first electrical voltage. The volume reduction increases the pressure in the chamber 3 and a stream of liquid with a flow direction 8 as shown in Fig 1 is ejected out of the chamber 3 through the opening 4. The liquid stream is injected when the surface tension of the liquid at the opening 4 is overcome. The injection leads to a reduction of pressure in the chamber

3 up to a point the pressure is below a value to overcome the surface tension, when the injection stops.

In a next step the arrangement 5, for example the piezo- electric device can increase the volume of the chamber 3 for example after applying a second electrical voltage,

particularly with opposite sign of the electrical voltage. The volume increase leads to a pressure reduction in the chamber 3. Liquid is sucked from the reservoir 6, which is filled with liquid, through the pipe 7 to the chamber 3 of the device 2. The chamber 3 is refilled with liquid metal and the process can start again from the beginning, injecting liquid. As a result a pulsed injection of liquid metal can be generated continuously or interrupted. Various periods of injection and refilling can be chosen according to the needs during application of the fluid injector 1. The injection and refilling can be periodical with the same time intervals or with changing time intervals. The pressure in reservoir 6 is chosen to be low enough, not to overcome surface tension at the opening 4 without moving of parts of the arrangement 5. As long pressure in the reservoir 6 is below a limit, set by the diameter of opening

4 and liquid surface tension, no liquid is leaving the fluid injector 1. The limit depends among others from the

environment of the liquid injector 1, particularly the pressure for example in vacuum.

For a laminar liquid flow during refilling, which results in a liquid flow with low friction and a high refilling rate, the cross section of the pipe 7 is larger than the cross section of the opening 4 in the chamber 3. With circular diameter of the opening 4 Dl and pipe 7 D2, an example for an inner diameter of the pipe 7 D2 is 200 Micrometer and a diameter of the opening 4 Dl is 50 Micrometer.

For a liquid flow to be blocked during injection, the time interval to refill is for example in the order of ten times longer than the time interval to inject. A short time

interval to inject liquid, i.e. eject liquid from the chamber 3, results from a fast volume change by the arrangement 5, inducing a liquid flow pulse breaking through the opening 4 after overcoming the surface tension of the liquid and pushing liquid in the pipe 7 in direction from the chamber 3 to the reservoir 6. The liquid pushed fast into the pipe 7 from the camber 3 , particularly into the pipe with higher cross-section than the cross-section of the opening 4, results in a kind hydraulic hammer and/or turbulent flow, which is blocked by the spiral part 9 of the pipe 7. In contrast, the more slow laminar flow during refilling is no or at least only very little reduced by the spiral part 9 of the pipe 7.

The number of windings of the spiral 9, the cross-section for fluid flow in the pipe 7 in relation to the cross-section of opening 4 and/or the volume of the chamber 3, particularly depending on the speed of volume change by the arrangement 5 and/or the time interval for refilling and injection, are calculated and/or predefined to get an injection of liquid from the device 2, particularly overcoming the surface tension of the liquid at opening 4, and to result in a blocking of liquid flow in the pipe 7 at the spiral part 9 during injection. The values, particularly the cross-section, i.e. inner cross-section of the pipe 7, the refilling time period and the number of windings of the spiral 9 are chosen to result during refilling in a laminar liquid flow to refill the chamber 3 from the reservoir 6 without or with little flow resistance and/or friction losses. A good refilling results from the described fluid injector 1 during refilling, with a high amount of injected liquid without and/or with little liquid flowing from the chamber 3 to the reservoir 6 during injection.

In FIG 2 an embodiment of the device 2 is shown, with an opening 10 in form of a nozzle cup with sharp edge orifice. The arrangement 5 to change the volume within the chamber 3 comprises a hydraulic liquid volume 13, for example filled with air, oil or water, enclosed by a clamped circular membrane 11, particularly steel membrane, and a piston driven by a piezo-actuator 12. Other parts like the pipe 7 or reservoir 6 are not shown for reasons of simplicity. With a first voltage applied, for example with positive sign, the piezo-actuator drives the piston 12 down, in direction to the membrane 11. The hydraulic liquid volume is pushed in

direction to the membrane 11, deforming the membrane 11 in the direction away from the piston 12. The chamber 3 with liquid to be injected by the injector is arranged opposite to the hydraulic liquid volume 13, separated by the membrane 11. The membrane 11 compresses the liquid to be injected in the chamber 3, increasing the pressure to a value above the fluid surface tension at the opening 10. Fluid breaks trough and is ejected from the chamber 3, i.e. injected by the device 2.

With a second voltage applied to the piezo-actuator, for example with negative sign, the piston 12 moves up, in the direction away from the membrane 11. The hydraulic liquid volume is expanded, deforming the membrane 11 towards the piston 12. The membrane 11 expands the liquid to be injected in the chamber 3, decreasing the pressure slowly to suck liquid from the reservoir 6 via the pipe 7 to the chamber 3 without overcoming the surface tension of the liquid at the opening 10. No vacuum or air with low pressure is sucked into the chamber 3 via the opening 10. A slow movement of the membrane 11, i.e. slow expansion of the volume in chamber 3 and liquid sucking results in a laminar liquid flow in the pipe 7 without blocking of the liquid by the spiral part 9. The chamber 3 is refilled with liquid from the reservoir 6, to be ready for the next injection. The process can be repeated as long liquid is in the reservoir 6, which can be refilled.

The refilling of chamber 3 can be actively, directly induced by the arrangement 5 with liquid flow synchronous with the movements of arrangement 5. In high frequency operation, the refilling can be slow over time after a fast movement of arrangement 5 is inducing a pressure difference between chamber 3 and reservoir 6.

With piezo-electric stacks high frequency movements are possible, depending on electric voltage change and its frequency. Typical expansion distances of piezo-electric stacks in the arrangement 5 are for example in the range of 0.1 mm, with a force in the range of 50 kN, creating pressure up to 500 to 1000 Atm. This allows high pressure injection in pulsed manner at high frequencies, for example 10 to 1000 Hz. A linear piezo-actuator is able at high voltage changes to expand and/or contract with high frequency, pushing and/or pulling a piston 12 with high constant force. This force is converted to high pressure changes of for example a hydraulic liquid in a hydraulic liquid volume 13. A pressure difference between hydraulic liquid volume 13 and chamber 3 deforms for example a clamped disc membrane, particularly made of a thin steel sheet. The deformation induces high or low pressure in the chamber 3, inducing the liquid metal injection pulse respectively refilling.

The dimension of opening 10 in a chamber 3 is for example in the order of 0.01 to 0.1 mm and can be produced for example by laser drilling. The opening 10 can have conical shape with a cone base at the inner side of chamber 3, to provide a vena contracta flow. High injection pressure and small diameter of the opening 10 enable a high speed microjet.

The fluid injector 1 as shown in FIG 1 allows a liquid metal flow, for example liquid injection via opening 4 and

refilling from reservoir 6, at high frequency without the use of vales or moving parts to prevent air to be sucked into the chamber 3 during refilling and/or liquid to be pushed back into the reservoir 6 form chamber 3 during injection. The fluid injector 1 is less complex than with moving parts like valves to block liquid flow, easier to produce, less

expensive in production, and long lasting without wear parts like valves.

The change of direction of liquid flow in the pipe 7 part with curved and/or angled shape 9, with the pipe 7 being curved and/or angled shaped along the fluid flow direction, will cause significant hydraulic losses in addition to friction losses occurring in the pipe 7 along the length of the pipe 7. These losses during injection phase are orders of magnitude higher than during the refilling phase. This is caused by the fact that during injection the liquid outflow will be turbulent opposite to a laminar low speed refilling flow, i.e. charging of the chamber 3. A part of the pipe with spiral shape 9 can be made, i.e.

produced by spiraling a capillary tube with for example an inner diameter of 0.1 to 1 mm around a cylindrical rod with for example a diameter of 16 mm. A spiraled tube with 1 mm inner diameter and 2 mm outer diameter with 15 x 360 grad full turns will results in a pipe 7 with a length of

approximately 0.85 m, and in 60 x 90 curved and/or angled shaped parts . Hydraulic losses in the curved and/or angled shape parts, which can also be called elbows and sum up to the spiral part 9, will give rise to additional 50 % losses compared to pure friction losses in the pipe 7, assuming high speed turbulent flow when the pressure difference between the device 2 and reservoir 6 is high, e.g. 100 Atm. During refilling, i.e. charge phase the curved and/or angled shape parts will not generate any additional losses due to laminar regime of liquid flow.

With pressure inside chamber 3 being increased very fast, the effect of hydraulic hammer in the pipe 7 will appear, particularly in the spiral part 9 of the pipe 7. This effect originates from the fact that any disturbances in the liquid are propagating through the liquid with finite speed, which corresponds to the specific velocity of sound in the liquid, depending on physical and thermodynamics properties of the liquid and mechanical properties of the pipe 7. Due to the hydraulic hammer effect, a compression wave moves in

direction from the device 2 to the reservoir 6, and an expansion wave moves in reverse direction. Liquid is

accelerated behind the front of the waves, taking a certain period of time to establish the outflow of the liquid to the reservoir 6. The propagation of shock waves in spiraled tubes is more complex than in straight tubes. This fact requires additional time to establish the outflow, minimizing total liquid losses during the injection and increasing a possible operating frequency.

This allows an operation of the device 2 in pulse mode, with a high frequency depending on the ratio of liquid loss during injection and compensation during refilling. Spiraled pipes 9 are easy to manufacture and limit the liquid outflow to the reservoir 7, i.e. storage tank, during injection phase when the flow regime is turbulent, but do not bring any essential liquid flow losses during refilling when the flow regime is laminar.

In FIG 3 the fluid injector 1 is shown, arranged in an x-ray tube combined with an electron source 14. The electron source 14 generates electrons 15, which interact with the injected anode material 8 to generate x-rays 16. The casing of the x- ray tube is not shown in FIG 3 for simplicity reasons. The injection of liquid metal as anode material, in form of for example a liquid high-speed jet 8, with injection into the electron beam 15 in the x-ray tube, provides an anode

material well defined in shape and velocity. The anode material interacting with the electron beam 15 is fast changing, reducing the thermal load in the anode material volume in interaction with the electrons . The thermal load is distributed over the anode material. The electron beam 15 is generated by the electron source 14 and can be focused, striking the liquid anode material jet with flow direction 8 as shown in FIG 3 in an angle of for example 90 degree. X- rays are generated by the interaction of electrons with the anode material .

Due to reduced thermal load less "Bremsstrahlung" and a higher specific x-ray radiation with well defined wave length can be produced. A well defined, specific wavelength of x- rays increases the resolution of for example x-ray computer tomographs or other x-ray examination devices.

To produce an x-ray image just one anode material injection can be used for the imaging. The amount of anode material stored in reservoir 6 can last for a lifetime of an x-ray examination device. Alternative it can also be refilled. The fluid injector 1 according to the present invention for x-ray tubes can be attached to the x-ray tube or arranged within the x-ray tube. The whole system can be part of an x-ray examination device, for example build into the device. The compact set-up of the fluid injector 1, without moving parts like valves enables a set-up of an x-ray tube comprising the injector 1, with long lifetime.

The above described features of embodiments according to the present invention can be combined with each other and/or can be combined with embodiments known from the state of the art . For example, the dimension of components of the fluid

injector 1 and frequencies for injection can be chosen according to the kind of liquid metal used as anode material and according to the application of the x-ray tube. The membrane material can for example be instead of steel made of other metals and/or non metallic materials. The fluid

injector 1 can be used in inert atmosphere instead of vacuum, the atmosphere influencing the surface tension of the fluid and necessary dimensions of the opening 4, 10. The x-ray produced can be a microfocus x-ray. The liquid injector 1 can replace complex and bulky pump based

recirculation systems, which are used for liquid metal injection. The operation mode of the liquid metal injector 1 can be pulse mode, with a frequency depending on the ratio of liquid loss in chamber 3 during injection and compensation during refilling, i.e. charging. A valveless injector 1 is able to generate thin, high speed liquid jets, which can be utilized as anode material for microfocus x-ray generation. The use of an injector 1 without moving parts like valves increases the system reliability in comparison to tubes with rotating anodes or to liquid anode tubes with high pressure pumps. The injector 1 according to the present invention is not sensitive to external acceleration, thus improving operational limits for different applications for example in computer tomography with fast rotating gantries.

The use of liquid metallic jets as anode material allows applying significantly higher loads to x-ray tubes compared with conventional microfocus solutions. The use of optimized combinations of different components of metallic alloys, for example of Lithium and Lanthanum, results in an optimized x- ray spectrum, which is essential for a high image quality and a low patient dose load during medical diagnostics.