Patent Application
20100098218
Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to a modular multispot x-ray source for use in an imaging system.
Traditional x-ray imaging systems include an x-ray source and a detector array. X-rays are generated by the x-ray source, passed through and attenuated by an object, and are detected by the detector array. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the object. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis, which ultimately produces an image.
Generally, as in a CT application, the x-ray source and the detector array are mounted on a gantry and rotated about an imaging plane and around the object. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator for converting x-rays to light energy, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. The X-ray detectors may also include a direct conversion device for discriminating the energy content of the x-ray beam. The outputs of the detector array are then transmitted to the data processing system for image reconstruction. Electrical signals generated by the detector array are conditioned to reconstruct an x-ray image of the object.
In CT imaging systems, the gantry rotates at various speeds in order to create a 360° image of the object. The gantry contains an x-ray source having an electron source or cathode assembly that generates electrons that are accelerated across a vacuum gap to a target or anode assembly via a high voltage potential. In releasing the electrons, a filament contained within the electron source is heated to incandescence by passing an electric current therethrough. The electrons are accelerated by the high voltage potential and impinge upon a target surface of the target at a focal spot. Upon impingement, the electrons are rapidly decelerated, and in the process, x-rays are generated therefrom.
The process of deceleration typically results in heating of the focal spot to very high temperatures. Thus, x-ray tubes include a rotating target or anode structure for the purpose of distributing heat generated at the focal spot. The target is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating target is driven by the stator. Because of the high temperatures generated when the electron beam strikes the target, the target is typically rotated at high rotational speed.
Newer generation x-ray tubes have increasing demands for providing higher peak power, thus generally higher average power as well. Higher peak power, though, would result in higher peak temperatures occurring in the target, particularly at the “track” or the point of impact on the target, unless the target design is altered. Because x-ray tubes are typically designed having peak temperatures at limits imposed by material capabilities and high voltage considerations, higher peak power typically calls for a re-design of the target. For a rotating target, the re-design may include higher rotation speed, larger track radius, or novel x-ray production means. These designs may reduce life and reliability of the rotating target. For stationary target sources, the re-design options are generally limited to material improvements or novel approaches to backscattered electron energy management.
Furthermore, newer generation CT systems have increased gantry speed requirements to better enable, for instance, cardiac imaging. Thus, systems have been designed having applications wherein the gantry is spun at or below 0.5 seconds rotational speed. Such applications may include yet faster gantry rotation, thereby increasing the g-load demands to, for instance, 0.2 second rotation, which represents a g-load well in excess of what can be withstood in many current CT systems.
Accordingly, to counter the need for high g-load capability x-ray sources, multispot systems have been designed having stationary imaging components therein. For instance, scanning electron beam (e-beam) x-ray sources include an electron gun positioned at a gantry center that emits an e-beam that is magnetically deflected toward a target. In such a system, the target typically forms a continuous ring surrounding a patient, and the e-beam is rapidly deflected to circumferential locations on the target and around the patient. The e-beam may be deflected in the z-direction as well. As such, multispot imaging may be performed very rapidly using stationary components. However, not only are such systems expensive, they may be prone to performance degradation as well. For instance, the continuous target may have thermal distortion that can degrade image quality through excessive focal spot motion.
Furthermore, other known systems having stationary components include a thin transmission-style target for x-ray generation. However, such a continuous target is likewise prone to thermal loading and distortion effects resulting, as well, in degraded image quality through excessive focal spot motion.
As such, modular multispot devices have been developed to reduce the thermal distortion effects resulting from large, continuous targets or anodes. In such a system, individual, modularized x-ray sources may be positioned within a gantry, each module having a plurality of individual or discrete focal spots that have reduced relative motion. As such, the overall system thermal distortion may be minimized and image quality may be improved. A modular design has the benefit of simplifying manufacturing and assembly procedures because the individual modules may be assembled and tested as sub-units before being installed into the overall system. Such a design further simplifies troubleshooting and repair of the system in the field, as a field engineer may be able to test and replace individual modules within the system. Thus, the need to return all of the sources or even the entire system back to a manufacturing site may be precluded, resulting less in system downtime, cost of repair, and frustration.
However, a multispot source typically results in the need to provide x-ray shielding of many spatially distributed focal spots. Adopting a traditional shielding approach would require covering the vacuum chamber containing the modules with lead or other high-density shielding material to eliminate the openings from which undesired x-rays could emanate. This presents at least two issues: first, the basic amount of shielding material would be large; and second, the amount of scattered radiation produced by objects inside the vacuum chamber makes the determination of the minimum thickness of shielding material required at all locations difficult.
Thus, not only is the basic amount of shielding material prohibitive, but because of the variation from system to system and the resulting uncertainty of sources of scattered radiation and to be conservative, designs typically include excess amounts of shielding. This results in increased system cost and an unnecessary amount of shielding mass being included in the system. As such, the desire for increased g-load capability may be limited due to the excess shielding required in a modular source design.
Furthermore, modular source designs typically include a pre-patient collimator to collimate scatter and off-focal radiation that may emit from the anodes. However, to collimate each spot within a multispot source, a separate collimator is provided for each spot, resulting in a series of individually constructed collimators. Further, in order to collimate in both the X and Z dimensions, respective collimating plates or elements must be provided in each orientation. Such a construction is complex and expensive to build, and cumbersome and difficult to operate.
Therefore, it would be desirable to design a cost-effective and low-mass shield for a modular multispot x-ray source.
Embodiments of the invention provide a apparatus and method that overcome the aforementioned drawbacks. Embodiments of the invention are directed to an apparatus and method of manufacturing a cost-effective modular multispot x-ray source having robust g-load capability and improved.
According to one aspect of the invention, a modular x-ray source for an imaging system includes a structure forming a cavity and having a first wall and a second wall, at least one target positioned on the first wall within the cavity and configured to receive a first electron beam at a first spot position and a second electron beam at a second spot position, and a shielding material positioned on the second wall.
In accordance with another aspect of the invention, a method of manufacturing a modular x-ray source includes forming a target mounting material having at least one cavity therein, positioning a plurality of targets within the at least one cavity, each spaced one from the other in substantially the same pattern as an array of electron sources, and attaching a shielding material to a wall within the at least one cavity.
Yet another aspect of the invention includes an x-ray imaging system that includes a rotatable gantry, a detector mounted to the rotatable gantry, and a modular x-ray source mounted to the rotatable gantry. The modular x-ray source includes a structure forming a cavity, a target positioned on the structure and within the cavity, configured to receive two or more electron beams from respective electron sources and forming two or more focal spots, and a shielding material positioned on a wall within the cavity.
Various other features and advantages of the invention will be made apparent from the following detailed description and the drawings.
The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.
In the drawings:
The operating environment of the invention is described with respect to a sixty-four-slice computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the invention is equally applicable for use with other multi-slice configurations. The invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems.
Referring to
Rotation of gantry 12 and the operation of x-ray source 14 are governed by a control mechanism 26 of CT system 10. Control mechanism 26 includes an x-ray controller 28 that provides power and timing signals to an x-ray source 14 and a gantry motor controller 30 that controls the rotational speed and position of gantry 12. An image reconstructor 34 receives sampled and digitized x-ray data from DAS 32 and performs high-speed reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a mass storage device 38.
Computer 36 also receives commands and scanning parameters from an operator via console 40 that has some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus. An associated display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 32, x-ray controller 28 and gantry motor controller 30. In addition, computer 36 operates a controller 44 to position a motorized table 46 and hence patient 22 and gantry 12. Particularly, table 46 moves patients 22 through a gantry opening 48 of
The x-ray source 14 may include a modular design according to an embodiment of the invention. In this embodiment, referring to
The electron sources 110 are configured as sub-modules, three of which are illustrated 114, 115, 117, and each of which includes, in the illustrated embodiment, four electron sources 110. Each electron source 110 is positioned opposite a respective target 112. As described, targets 112 may include separate structures corresponding to respective electron sources 110, or a single target 112 may span along multiple electron sources 110 within each sub-module 114, 115, 117 such that multiple focal spots emanate from a single target 112. The electron source sub-modules 114, 115, 117 are mounted on the electron source mounting plate 102 via electron source support blocks 116. The electron source sub-modules 114, 115, 117 and their respective electron source support blocks 116 may be mounted on additional spacers 118, 119 such as illustrated for electron source sub-modules 114, 115, such that target-electron source spacing may be controlled independently for each electron source sub-module 114, 115. As illustrated, the spacers 118, 119 are designed to position each electron source 110 within each electron source sub-module 114, 115, 117 at a proper spacing with respect to its respective target 112. The electron source sub-modules 114, 115, 117 are positioned opposite respective target sub-modules 130, 132, 134. Thus, a 4×3 array of 12 target-electron source pairs are illustrated in the module 100.
One skilled in the art will recognize that the module 100 need not be limited to three source sub-modules 114, 115, 117, and respective target sub-modules 130, 132, 134. Nor does the number of electron sources 110 need to be limited to four within each sub-module 114. As such, a module 100 may include more or less than the 12 pairs illustrated in
The electron sources 110 are positioned such that electrons are emitted substantially orthogonal therefrom and received from each respective electron source 110 on a focal spot surface of targets 112 at an angle of between 0° and 90°. In a preferred embodiment the angle is between 10° to 40°. Each target 112 includes tungsten, molybdenum, and/or alloys thereof including other materials, for generation of x-rays, as is commonly understood within the art. Alternatively, each electron source 110 may include field emitters. The target block 104, with its plurality of targets 112, further includes a target cover 120, positioned on the target block 104 and having a plurality of holes or passageways 122 therein. The passageways 122 are positioned to allow passage of electrons from each electron source 110 to its respective target 112, while limiting the flow of backscattered electrons and ions away from the target to the tube frame and electron source, respectively.
A high voltage, such as a monopolar operation having up to 140 kV or more, is applied between the electron sources 110 and the targets 112 via the electron source plate 102 and the target block 104. In this embodiment, the 140 kV voltage difference is applied by grounding the electron source plate 102 and applying +140 kV to the target block 104. However, one skilled in the art will recognize that the voltage differential may be applied in other fashions, such as by splitting the applied kV between the target block 104 and the electron source plate 102 (i.e. a bipolar operation having +70 kV to the target block 104 and −70 kV to the electron source plate 102) or by grounding the target block 104 while applying a −140 kV bias to the electron source plate 102. The split-potential embodiment may include an additional set of insulators between the target or electron source block and the vacuum chamber and attendant changes in the electrical feedthroughs from the high voltage power supply. In one embodiment, the total applied voltage differential is 450 kV or more for, for instance, a baggage scanner in a security application, and in such embodiment the differential may be applied by grounding the anode, grounding the cathode, or splitting the applied voltage between them as discussed above.
In one embodiment, coolant (such as water, dielectric oil, or glycol, as examples) is flowed through a plurality of coolant lines 124 to remove heat generated at the targets 112. Such coolant lines may be connected via a manifold that may feed several modules, and the coolant lines may be connected to the manifold via, for instance, a vacuum-compatible connector. Accordingly, the coolant lines 124 may further serve as a means to apply a bias voltage to the module 100. Thus, as an example, in such an embodiment the electron source plate 102 may be grounded and the target block 104 may be biased to +140 kV via the cooling lines 124.
Filaments (not shown) within each electron source 110 are caused to emit beams of electrons 128 toward respective targets 112. The beams of electrons 128 emit from the electron sources 110 and are accelerated toward and impinge upon the targets 112 while passing through passageways 122. As such, x-rays 126 are generated and are emitted toward an imaging object, such as the object 22 of
The module 100 may include a shielding material according to an embodiment of the invention.
Shielding material 146 is selected based on its ability to absorb high energy electrons and high energy x-rays. Material 146 is also selected based on its melt temperature, cost, and ease of manufacture. Thus, materials of choice include molybdenum and tungsten. In the case of tungsten, the thickness is selected to be between 1.0 mm and 4.0 mm, preferably between 2.0 mm and 3.2 mm. Molybdenum, having a lower density than tungsten, is preferably proportionately thicker than tungsten. Lead at 4.26 mm may provide adequate shielding, but may not be a preferred material because of its low melt temperature, which may cause sublimation at operating temperatures.
Target sub-module 130 is configured with shielding material 146 to absorb backscatter electrons 147 and radiation emitting therein and configured with passageways 122 to allow electron beam 128 to pass to the target 112. Target sub-module 130 is also configured to allow passage of x-rays 126, as described with respect to
The module 100 is thus a single or stand-alone unit that may be fabricated with a vacuum chamber and inserted into, for example, a CT system such as the CT system 10 of
One skilled in the art will recognize that each module 100 may house its own vacuum region. In such an embodiment, a plurality of modules 100 may be positioned within a gantry, having the advantage of enabling replacement of individual modules without having to access the vacuum region 202 as discussed above.
As discussed with respect to
Furthermore, because of the compact and stand-alone nature of the module 100, the module 100 may be structurally designed to have g-load capability in a system having 0.35 second rotation and faster. Accordingly, the multi-spot source 200 illustrated in
Referring still to
As such, each focal spot 151 of
Referring again to
Collimating elements 156, 162, 166 of
Referring now to
Each plate 302, 304 has a respective array 306, 308 of passageways, or apertures 310, 311 passing therethrough. The arrays 306, 308 of apertures 310, 311 are configured in a pattern that corresponds to the plurality 300 of focal spots 151 within each module 100. Consistent with the X-Y-Z coordinates illustrated in
Referring now to
Referring back to
A collimator 350 may be fabricated having plates 302 and 304 in contact with one another. In embodiments where the plates 302, 304 are in contact, thus forming a single unit, the collimator 350 may be positioned on either a vacuum side or an air side of a multi-spot system. Referring now to
Additionally, although the plates 302, 304 are illustrated as being joined together in
However, first plate 302 includes a neighboring aperture 318, and second plate 304 likewise includes a neighboring aperture 320. The neighboring apertures 318, 320 are positioned to form another composite opening 354 that is positioned to allow passage of x-rays 126 that emit from another focal spot 151, labeled as position 322. However, in this embodiment, the plates are positioned such that, while x-rays 126 that emit from focal spot 151 at position 301 may pass through aperture 318 of the first plate 302, they are obstructed from passing all the way to detector 160 of
Further, although the composite opening 352 of collimator 350 is illustrated with respect to the Z direction of the sources 200, 400 of
The collimators described herein need not be static, but may be designed in such a fashion that one or both plates of the collimator may be dynamically positionable. As such, one or both plates may be re-positioned during a scan, or between scans, depending on the application.
Referring now to
According to one embodiment of the invention a modular x-ray source for an imaging system includes a structure forming a cavity and having a first wall and a second wall, at least one target positioned on the first wall within the cavity and configured to receive a first electron beam at a first spot position and a second electron beam at a second spot position, and a shielding material positioned on the second wall.
In accordance with another embodiment of the invention a method of manufacturing a modular x-ray source includes forming a target mounting material having at least one cavity therein, positioning a plurality of targets within the at least one cavity, each spaced one from the other in substantially the same pattern as an array of electron sources, and attaching a shielding material to a wall within the at least one cavity.
Yet another embodiment of the invention includes an x-ray imaging system that includes a rotatable gantry, a detector mounted to the rotatable gantry, and a modular x-ray source mounted to the rotatable gantry. The modular x-ray source includes a structure forming a cavity, a target positioned on the structure and within the cavity, configured to receive two or more electron beams from respective electron sources and forming two or more focal spots, and a shielding material positioned on a wall within the cavity.
The invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.
