Patent Publication Number: US-6983035-B2

Title: Extended multi-spot computed tomography x-ray source

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to computed tomography (CT) imaging and volume computed tomography (VCT) imaging. More specifically, the present invention relates to multi-spot x-ray sources for CT imaging. Even more specifically, the present invention relates to a stand-alone, self-contained electron gun, having electron beams focusable at different distances, which impinge on multiple targets to generate near-linear multi-spot x-rays for CT and VCT imaging. 
   BACKGROUND OF THE INVENTION 
   Computed tomography (CT), sometimes called computed axial tomography (CAT) or CAT scan, and volume computed tomography (VCT), use special x-ray equipment to obtain image data from different angles around a person&#39;s body, and then use computer processing of the data to create a two-dimensional cross-sectional image (i.e., slice) or three-dimensional image of the body tissues and organs that were scanned. CT and VCT imaging are particularly useful because they can show a combination of several different types of tissue (i.e., heart, lungs, stomach, colon, kidneys, liver, bone, blood vessels, muscles, etc.) with high spatial resolution and a great deal of clarity and contrast. Radiologists can interpret CT and VCT images to diagnose various injuries and illnesses, such as cardiovascular disease, trauma, cancer, and musculoskeletal disorders. CT and VCT images can also be used to aid in minimally invasive surgeries, and to allow for accurate planning and pinpointing of tumors for radiation treatment, among other things. 
   CT and VCT imaging allow structures within a body to be identified and delineated without superimposing other structures on the images created thereby. In a typical conventional CT or VCT imaging system, an x-ray source emits a fan-shaped x-ray beam that is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the “imaging plane.” The x-ray beam passes through a section of the object being imaged, typically a patient. After passing through the object and being attenuated thereby, the x-ray beam impinges upon an array of radiation detector elements. The intensity of the attenuated x-ray beam radiation that is received by each detector element varies since different parts of the body absorb and attenuate the x-rays differently. Each detector element in the array produces a separate electrical signal that is a measurement of the x-ray beam&#39;s attenuation at each detector element. The attenuation measurements from all the detector elements are acquired separately, and are then combined to produce an image transmission profile. 
   Currently, x-ray sources for CT and VCT are limited to fairly narrow “slices” for each revolution of the gantry because of the well-understood “cone-beam artifact” problem, in which the “edges” of the cone-like x-ray beam that emerges from a point source cannot produce enough attenuation data, thereby resulting in portions of the imaged object not being imaged at all. It would be desirable, particularly for VCT, to have an extended or “linear” x-ray source to eliminate or minimize the cone-beam artifact problem. That would make it possible to obtain CT or VCT scans that cover an entire organ in a single scan or revolution of the gantry. For example, while existing CT and VCT imaging systems and methods allow multi-slice images, having a total thickness of about 10–40 mm, to be obtained in a single gantry rotation, it would be desirable to have CT and VCT imaging systems and methods that allowed multi-slice images having a total thickness as thick as 80–160 mm or thicker to be obtained in a single gantry rotation. However, improved CT and VCT imaging systems and methods are needed in order for thicker multi-slice images to be realized. 
   Since existing CT and VCT imaging systems and methods have many drawbacks, it would be desirable to have improved CT and VCT imaging systems and methods that lack such restrictions. This invention provides a single, near-linear, multi-spot x-ray source that utilizes multiple x-ray targets having varying focal spots thereon so as to improve the imaging data around the edges of the object being imaged, thereby allowing thicker multi-slice images to be obtained than currently possible. 
   SUMMARY OF THE INVENTION 
   Accordingly, the above-identified shortcomings of existing CT and VCT imaging systems and methods are overcome by embodiments of the present invention, which relates to a single, near-linear, multi-spot x-ray source comprising multiple targets that have varying focal spots thereon. Embodiments of this invention allow thicker multi-slice images (up to about 80–160 mm thick or thicker) to be obtained with each gantry rotation than currently possible with existing CT and VCT imaging systems. 
   Embodiments of this invention comprise systems and methods for obtaining thick total volume slices (i.e., up to about 160 mm or thicker) in a single gantry rotation in computed tomography or volume computed tomography. Embodiments of this invention comprise an extended, multi-spot x-ray source for computed tomography and/or volume computed tomography imaging. This x-ray source comprises: an electron gun capable of producing a plurality of electron beams, each electron beam focused at a predetermined distance and aimed in a predetermined direction; and a plurality of targets positioned to receive the electron beams and generate x-rays in response thereto, each target comprising a predetermined focal spot thereon, wherein each electron beam is synchronized to strike, at an appropriate time, a predetermined target comprising a predetermined focal spot thereon. 
   The plurality of targets rotate about an axis of rotation. Each target comprises a different focal spot thereon, each electron beam is focused at a different distance, and each electron beam is aimed in a different direction. Each electron beam also strikes a different target having the appropriate focal spot thereon. A single electron beam, focused at a predetermined distance, strikes only one target, comprising a matching predetermined focal spot thereon, at a time. 
   At least one target is designed to let electron beams pass therethrough and strike another target at predetermined intervals. At least one target may comprise a cut-out section that allows electron beams to pass therethrough and strike another target at predetermined intervals. 
   The x-ray source may comprise a sensing device for identifying a rotational position of the targets. The sensing device may comprise: a magnetic material disposed on a rotor; and a magnetic pick-up device disposed in close proximity to the magnetic material, wherein when the rotor spins around its axis of rotation, the magnetic pick-up device obtains a voltage or current signal as the magnetic material passes thereby, and then the magnetic pick-up device transmits an appropriately treated and amplified signal to the electron gun to change electron beam focusing parameters and/or to make deflection corrections. 
   Adjusting a focal bias voltage or an accelerating voltage placed on the electron gun focuses at least one electron beam, and/or changes the electron beam properties. A total volume slice (i.e., a total thickness of the multi-slice images) of up to about 80 mm to about 160 mm thick or thicker can be obtained in a single gantry rotation. 
   Embodiments of this invention also comprise a computed tomography or volume computed tomography imaging system. These systems comprise an extended, multi-spot x-ray source. This x-ray source comprises: an electron gun capable of producing a plurality of electron beams, each electron beam focused at a predetermined distance and aimed in a predetermined direction; and a plurality of targets positioned to receive the electron beams and generate x-rays in response thereto, each target comprising a predetermined focal spot thereon, wherein each electron beam is synchronized to strike, at an appropriate time, a predetermined target comprising a predetermined focal spot thereon; and an x-ray detector, wherein the x-ray source projects a multi-spot beam of x-rays towards the x-ray detector, the x-ray detector detects the x-rays, and an image is created therefrom. 
   Further features, aspects and advantages of the present invention will be more readily apparent to those skilled in the art during the course of the following description, wherein references are made to the accompanying figures which illustrate some preferred forms of the present invention, and wherein like characters of reference designate like parts throughout the drawings. 

   
     DESCRIPTION OF THE DRAWINGS 
     The systems and methods of the present invention are described herein below with reference to various figures, in which: 
       FIG. 1  is a schematic drawing showing one embodiment of a CT imaging system that may be utilized in embodiments of this invention; 
       FIG. 2  is a schematic drawing showing the architecture of the CT imaging system shown in  FIG. 1 ; 
       FIG. 3  is a schematic diagram showing an embodiment of a self-contained electron gun that produces an electron beam that can be sequentially focused at different distances, wherein the electron beam sequentially strikes a different target having a different focal spot thereon, which yields a near-linear multi-spot x-ray source useful for CT and VCT imaging; 
       FIG. 4  is a schematic diagram showing multiple targets, some notched, each having a different focal spot thereon, as utilized in embodiments of this invention; and 
       FIG. 5  is a schematic diagram showing a sensing coil that produces a rotation-angle-dependent signal, which is used to trigger changes in electron beam focusing in the electron gun from one target to the next, as utilized in embodiments of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   For the purposes of promoting an understanding of the invention, reference will now be made to some preferred embodiments of the present invention as illustrated in  FIGS. 1–5  and specific language used to describe the same. The terminology used herein is for the purpose of description, not limitation. Specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims as a representative basis for teaching one skilled in the art to variously employ the present invention. Any modifications or variations in the depicted support structures and methods, and such further applications of the principles of the invention as illustrated herein, as would normally occur to one skilled in the art, are considered to be within the spirit and scope of this invention. 
   This invention relates to systems and methods for minimizing or eliminating the cone-beam artifact problem in CT images, particularly VCT images, to allow thicker multi-slice images to be obtained with each gantry rotation. Referring now to  FIG. 1 , there is shown a schematic diagram showing an exemplary CT imaging system  10  that may be utilized in embodiments of this invention. Such systems generally comprise a gantry  12 , a gantry opening  48 , and a table  46  upon which a patient  22  may lie. Gantry  12  comprises an x-ray source  14  that projects a beam of x-rays  16  toward an array of detector elements  18 . During operation, gantry  12  rotates about a center of rotation  24  to obtain an image of one or more “slices” of an area of interest in a patient  22 . Generally, the array of detector elements  18  comprises a plurality of individual detector elements  20  that are arranged in a side-by-side manner in the form of an arc that is essentially centered on x-ray source  14 . In multi-slice imaging systems, parallel rows of arrays of detector elements  18  can be arranged so that each row of detectors can be used to simultaneously generate multiple thin slice images through patient  22  in the X-Y plane. Each detector element in the array of detector elements  18  senses and detects the x-rays  16  that pass through an object, such as patient  22 , and then an image is created therefrom. While this figure shows the x-ray source  14  and the array of detector elements  18  aligned in the X-Y plane, some CT imaging systems may align the x-ray source  14  and the array of detector elements  22  differently, without deviating from the spirit and scope of this invention. 
   Referring now to  FIG. 2 , there is shown a schematic diagram showing the architecture of the CT imaging system shown in  FIG. 1 . The rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT imaging system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to x-ray source  14 , and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from the individual detector elements  20 , and converts that analog data to digital signals for subsequent processing in accordance with the methods and systems of this invention. An image reconstructor  34  receives the sampled and digitized x-ray data from DAS  32  and performs high speed image reconstruction thereon. The reconstructed image is then applied as input to a computer  36 , which can store the image in a mass storage device  38 . Computer  36  may also retrieve stored images from the mass storage device  38  for later viewing. 
   Computer  36  may also receive commands and scanning parameters from an operator via an operator console  40 , which may comprise a keyboard, touchpad, or other suitable input device. An associated cathode ray tube display  42  (or other suitable display) may allow the operator to view the reconstructed image and other data from computer  36 . The operator supplied commands and parameters may be used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . Additionally, computer  36  may operate a table motor controller  44  which can control a motorized table  46 , thereby allowing the patient  22  to be properly positioned within gantry  12  or moved therethrough. For example, table  46  may move portions of patient  22  through gantry opening  48  in the Z-direction. 
   Embodiments of the present invention may make use of software or firmware running on computer  36 . A mouse or pointing device may be employed to facilitate the entry of data and/or image locations. Other embodiments of this invention may utilize a general purpose computer or workstation having a memory and/or printing capability for storing or printing images. Suitable memory devices are well known and include, but are not limited to, RAM, diskettes, hard drives and optical media. Embodiments using such stand-alone computers or workstations may receive data from CT imaging system  10  via conventional electronic storage media or via a conventional communications link, and images may then be reconstructed therefrom. 
   Generally, x-ray sources for CT and VCT comprise single focal spot x-ray tubes  14  mounted on gantry  12 . Such x-ray sources produce a single fan-like x-ray beam that is aimed at the array of detector elements  18 . However, there are drawbacks for such single focal spot x-ray sources: (1) such x-ray sources limit the image that can be obtained to fairly narrow “slices” per each gantry revolution (i.e., slices having a total combined thickness of about 10–40 mm); and (2) such sources also lead to the cone-beam artifact problem, in which there is not enough data to be detected on the “edges” of the cone-like beam emerging from such point sources. Therefore, in order to increase the z-axis coverage, an extended x-ray source is needed to produce a linear or near-linear x-ray source effect so that sufficient information for large organ scans can be gathered with a single gantry revolution. While using multiple x-ray sources arranged in a linear fashion is one possible solution, it is a very expensive and cumbersome solution to the problem, and is therefore not very practical. This invention, on the other hand, provides a much less expensive and less cumbersome solution to the problem, making it ideal for extending the x-ray source in the z-direction. Additionally, this invention allows multi-slice images as large as 80–160 mm thick, or sometimes even thicker, to be obtained in a single gantry rotation. 
   Referring now to  FIG. 3 , there is shown a schematic diagram showing an embodiment of this invention comprising a single self-contained electron gun  50  that produces an electron beam  52  that can be sequentially focused at different distances, wherein the electron beam  52  sequentially strikes a different target  62 A,  62 B,  62 C having a different focal spot thereon. While there are three targets  62 A,  62 B,  62 C shown in this embodiment, this is in no way meant to be limiting on this invention. In fact, other embodiments of this invention may comprise other numbers of targets, such as anywhere from 2–6 different targets  62 , with each target  62  having a different focal spot thereon. Also, depending on the application, even more than  6  targets could be used, if desired. 
   This invention comprises a single, self-contained electron gun  50  that produces focused electron beams  52  independent of most tube geometry features. This electron gun  50  may comprise a General Electric Imatron electron gun. As shown in  FIG. 3 , the electron gun  50  comprises an electron source  54 , apertures  56 , accelerating and/or focusing electrodes  58 , and steering electrodes  60 . This electron gun  50  produces focused electron beams  52 , each having a different focal length and direction. The electron beams  52  produced hereby can be focused, and the electron beam properties can be changed rapidly, by adjusting the focal bias voltages placed on parts inside the electron gun  50 . By focusing in this manner, the electron gun  50  is free from the focusing effect of the tube geometry, and can therefore be controlled by simply changing the accelerating and bias voltages within the electron gun  50  structure. In embodiments, the electron gun  50  may be aimed at a stack of slotted targets  62  that are mounted on a straddle support  64  for ideal gantry movement load distribution. The targets  62  may comprise molybdenum (Mo), and the targets  62  may be mounted on a shaft  66  comprising tungsten (W) alloyed with about 5–10% rhenium (Re). The targets  62  and shaft  66  may also comprise any other suitable materials. The electron gun  50  may be isolated from nearby objects at ground potential with high-density alumina or other insulation material suitable for high voltage electrostatic isolation. 
   In embodiments, the electron gun  50  may be aimed roughly parallel to the axis of rotation  65  of a stacked ensemble of multiple targets  62  that form an anode having several different focal spots. X-rays  16  may emerge from the targets  62  at a proper range of angles between the cut-off due to the heel effect, and that angle plus the usable angle imposed by cone-beam artifacts and reconstruction limits. Several targets  62 A,  62 B,  62 C may be mounted on shaft  66 , which is mounted on a straddle support  64 . The straddle support  64  may comprise one or more sets of ball bearing assemblies, and ideally, distributes the mechanical load over the ball bearing assemblies to improve the bearing operation and yield longer bearing life. The shaft  66 , on which the targets are stacked and mounted, may comprise a hollow channel  67  therein so that liquid coolant, water or other suitable substance  68  can circulate freely therein to cool the targets  62 A,  62 B,  62 C. Since the targets  62 A,  62 B,  62 C and anode structure are at ground potential, cooling fluid  68  may be supplied thereto via pumps and hoses/lines. This grounded target design is a simplified high efficiency motor (HEM) design, since a close distance between the rotor (enclosed in a vacuum vessel) and the stator (in atmosphere or in oil or other cooling fluid) provides close magnetic coupling between the two motor elements. 
   Referring now to  FIG. 4 , there is shown a schematic diagram showing multiple targets  62 A,  62 B,  62 C, some notched, each having a different focal spot thereon, as utilized in embodiments of this invention. As shown herein, the first target  62 A comprises large notches  80 , while the second target  62 B comprises small notches  82 , and the third target  62 C is not notched at all. The large notches  80  in the first target  62 A allow the electron beam  52  to pass through the first target  62 A and either strike or pass through the second target  62 B, as appropriate, while the small notches  82  in the second target  62 B allow the electron beam  52  to pass through the second target  62 B and strike the third target  62 C. In embodiments, the third target  62 C comprises a focal spot  83 C thereon from about 0–40°, the second target  62 B comprises a focal spot  83 B thereon from about 40–80°, and the first target  62 A comprises a focal spot  83 A thereon from about 80–120°. The large notches  80  in the first target  62 A are shown here in this embodiment as comprising cut-out sections from about 0–80°, 120–200°, and 240–320°, while the small notches  82  in the second target  62 B are shown here as comprising cut-out sections from about 0–40°, 120–160°, and 240–280°. While the notches  80 ,  82  herein are shown as pie-shaped cut-outs, various other cut-outs are possible without deviating from the spirit and scope of this invention. For example, the notches could comprise windows cut-out from around the periphery of the targets  62 , or could comprise any other suitable shape or design that allows the electron beam  52  to pass through the target  62  and strike or pass through the next target  62 . Additionally, while each target  62 A,  62 B herein is shown having three notches therein  80 ,  82  respectively, numerous other cut-out/notching arrangements are possible within the scope of this invention. 
   The electron gun  50  is designed to allow the focal spot of the electron beam  52  that is being emitted at a specific time to be synchronized with the target  62  comprising that particular focal spot thereon. For example, while the targets  62 A,  62 B,  62 C are rotating with shaft  66 , there are predetermined times when the third target  62 C is to be struck by the electron beam  52  (and accordingly, the electron beam  52  passes through the first target  62 A and the second target  62 B at that time), then when the second target  62 B is to be struck by the electron beam  52  (and accordingly, the electron beam  52  passes through the first target  62 A at that time), and then when the first target  62 A is to be struck by the electron beam  52 . Since all three targets  62 A,  62 B,  62 C have different focal spots  83  thereon, the electron beam focus is controlled so that the electron gun  50  emits an electron beam  52  having the appropriate focal length for the given target it is to strike at that time. 
   In embodiments of this invention, the electron gun  50  is controlled by obtaining a signal from a magnetic pick-up device such as the one shown in  FIG. 5 , which functions as an odometer or tachometer and produces a rotational phase-determined signal. As shown herein in this non-limiting embodiment, a sensing device for identifying the rotational position of the targets comprises a slug or pin of magnetic material  90  embedded in the rotor  92 , and a magnetic pick-up device  94  disposed in close proximity thereto. As the rotor  92  spins around its axis of rotation  95 , the magnetic pick-up device  94  (shown here as being a B-flux sensing coil), obtains a voltage or current signal each time the magnetic slug  90  passes the sensing coil  94 . An appropriately treated and amplified signal can then be transmitted to the electron gun  50  to change the electron beam focusing parameters and, if necessary, to make any deflection corrections that may be needed to optimize the performance of this multi-spot x-ray source. In this manner, an entire revolution of the rotor  92  can be accounted for, and the focal length of the electron beam  52  can be adjusted and controlled so that the electron gun  50  emits an electron beam  52  having the appropriate focal length, depending upon which target  62 A,  62 B,  62 C the electron beam  52  is supposed to strike at a particular time. 
   For example, initially, and while the rotating target assembly has an angular orientation of about 0–40°, the electron gun  50  may emit an electron beam  52  that strikes the third target  62 C. Then, after a predetermined period of time, and while the rotating target assembly has an angular orientation of about 40–80°, the electron gun focusing parameters could change and cause the electron gun  50  to emit an electron beam  52  that strikes the second target  62 B. Then, after another predetermined period of time, and while the rotating target assembly has an angular orientation of about 80–120°, the electron gun focusing parameters could change again and cause the electron gun  50  to emit an electron beam  52  that strikes the first target  62 A. This can continue in 40° increments until the rotor  92  has made one complete revolution, after which the cycle may start over again from the beginning, with the electron gun  50  emitting an electron beam  52  that strikes the third target  62 C, then the second target  62 B, then the first target  62 A, etc. While 40° increments have been described herein, this is in no way meant to be limiting on this invention as other angular increments could clearly be used too. 
   The bias voltages of the electron gun  50  that determine the focal length of the electron beam may be established in 10&#39;s of μseconds. This is fast enough to accomplish the necessary switching of the focusing parameters since the targets  62 A,  62 B,  62 C rotate at about 120 Hz or 8.0 msec/revolution, which is approximately 20 μsec/degree. The electron guns  50  of this invention may allow the electron beam source to be handled as a complete sub-assembly, thereby making it easier to replace, align, design and improve the electron beam source independent of the remaining x-ray tube insert geometry. 
   As described above, this invention provides an extended, near-linear multi-spot x-ray source that allows thicker multi-slice images to be obtained than currently possible with existing CT and/or VCT imaging systems. Advantageously, this invention utilizes a combination of known target and x-ray source technology to yield a near-linear x-ray source, which can ideally be utilized in CT and/or VCT imaging systems. This invention comprises a single self-contained electron gun that produces focused electron beams that are independent of most tube geometry features. These electron beams have different focal lengths, with each beam being designed to strike a different target, which creates a near-linear, multi-spot x-ray source. The targets are designed to allow the electron beams to pass therethrough when required, so that more distant targets can be struck by the electron beam. The multiple targets in this invention allow multi-spot x-rays to be generated from a single source, and the multi-spot x-ray source of this invention allows a number of previously inaccessible diagnostic techniques to be realized, of which whole organ scanning in a single CT or VCT scan is only one. Many other advantages will also be apparent to those skilled in the relevant art. 
   Various embodiments of this invention have been described in fulfillment of the various needs that the invention meets. It should be recognized that these embodiments are merely illustrative of the principles of various embodiments of the present invention. Numerous modifications and adaptations thereof will be apparent to those skilled in the art without departing from the spirit and scope of the present invention. For example, while the embodiments shown and described herein utilized three targets, it will be appreciated by those skilled in the art that this invention may comprise other numbers of targets without deviating from the spirit and scope of this invention, and all such variations are intended to be covered herein. Additionally, while pie-shaped cut-out notches were described herein as a means of letting the electron beams pass through a particular target, numerous other designs are possible, and are also intended to be covered herein. Thus, it is intended that the present invention cover all suitable modifications and variations as come within the scope of the appended claims and their equivalents.