Patent Grant
7197116
The present invention relates generally to x-ray imaging systems, and more particularly, to a system and method of performing a wide scan of an object within an x-ray 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 an object, and are detected by the detector array. Electrical signals generated by the detector array are conditioned to reconstruct an x-ray image of the object.
CT imaging systems include a gantry that rotates at various speeds in order to create a 360° image. The gantry contains an x-ray source having a single focal spot CT tube assembly that generates x-rays across a vacuum gap between a cathode and an anode. In order to generate the x-rays, a large voltage potential is created across the vacuum gap allowing electrons, in the form of an electron beam, to be emitted from the cathode to a single target surface on the anode. In releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therethrough. The electrons are accelerated by the high voltage potential and Impinge on the target surface at a single focal spot, whereby they are abruptly slowed down, directed at an impingement angle α of approximately 90°, to emit x-rays through a CT tube window.
Traditionally, scanning widths of an object have been limited due to the feasibly usable maximum angle of the x-ray beam and capabilities of the detector array, which in combination affect quality of a reconstructed image. Typical scanning widths of an imaging tube are approximately 10 mm. The width of the x-ray beam at the detector array is 10 mm and thus the width of the detector array is also 10 mm. With recent developments in CT detector arrays that indicate that the total detector array width or number of slice capability is increasing, limitation of scanning width has become increasingly more dependent upon maximum angle of the x-ray beam. Current CT imaging systems have 16-slice capability, and larger slice capability is foreseeable in the future.
It has been suggested to utilize the current x-ray source with updated larger width detector arrays. A fundamental limit exists when using a single focal spot tube with larger width detector arrays. The larger the width of the detector arrays the more cone-beam artifacts that are produced, causing a reduction in image quality. Another limit associated with single focal spot tubes is that the resolving power of the electron beam decreases from a center ray, extending through the center of the focal spot, towards outer edges of the focal spot. Therefore, detector elements farther away from a center of the focal spot receive a lower resolving power causing poorer image quality for the elements with lower resolving power.
It is also desirable in CT imaging to increase speed of an imaging system without degradation of image quality. CT imaging systems are limited in scanning speed of an image due to the maximum angle of the x-ray beam. With the current scanning angle, for example, only a portion of an organ can be scanned for a single revolution of the gantry, thus requiring multiple rotations and significant amounts of scanning time.
Additionally, in design of an imaging system several other concerns are to be taken into account. One is the desire to mitigate problems associated with conditioning surfaces of a target in preparation for high voltage application.
Another desire is to minimize high voltage instability within the imaging tube. One mechanism for high voltage instability is high vapor pressure, due to gas species such as background gas, surface-absorbed gas, target surface bulk absorbed gas, or track material atoms. These gas species provide ionization targets for incident electron flux producing charged ions. The charged ions and excess electrons produce a low impedance path between high anode and cathode direct current (DC) potentials, which generates “spit” activity. Spit activity can reduce image quality and potentially prevent image reconstruction.
Thus, there exists a need for an improved imaging system that is capable of performing a wide scan of a patient organ or of an object with increased scanning speed while at least maintaining current image quality.
The present invention provides a system and method of performing a wide scan of an object within an x-ray imaging system. One embodiment of the present invention provides an imaging tube that includes a cathode and an anode. The cathode emits an electron beam. The anode includes multiple target surfaces. Each of the target surfaces has a focal spot that receives the electron beam. The target surfaces generate multiple x-ray beams in response to the electron beam. Each x-ray beam is associated with one of the target surfaces.
Another embodiment of the present invention provides an x-ray imaging system that includes the cathode and the anode. A controller is electrically coupled to the cathode and adjusts emission of the electron beam on the anode.
The embodiments of the present invention provide several advantages. One such advantage is the provision of an imaging tube having an adjustable cathode and an anode having multiple target surfaces, together providing a relatively wide scan as compared to traditional imaging tubes. The electron beam of the cathode may be steered and the focal spot of that electron beam may be altered in response to electrical potentials within the electron beam “gun” or cathode. In providing a wide scan, the present invention is capable of scanning a full organ in a single rotation of a gantry, thereby, increasing scanning speed and minimizing x-ray exposure to a patient.
Another advantage provided by an embodiment of the present invention is the provision of a cathode that is a member of a replaceable subassembly, which allows the cathode to be easily maintained and replaced.
Furthermore, another advantage provided by an embodiment of the present invention is the efficient x-ray production by incorporating forward angle x-ray generation for incident angles less than 90°, which allows for greater x-ray radiation output per unit of heat or power input into a target surface. This increases efficiency of an imaging tube.
Moreover, another advantage provided by an embodiment of the present invention is the provision of an x-ray window that has a length that corresponds with a width associated with multiple adjacently emitted x-ray beams and as such minimizes x-ray absorption or thermal absorption by the x-ray window, thus minimizing the temperature of the window so that the window does not experience thermal or heat-related mechanical stresses, cracks, fractures, or other undesirable characteristics.
The present invention itself, together with attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying figures.
For a more complete understanding of this invention reference should now be had to the embodiments illustrated in greater detail in the accompanying figures and described below by way of examples of the invention wherein;
In each of the following figures, the same reference numerals are used to refer to the same components. While the present invention is described with respect to a method and system for performing a wide scan of an object within an computed tomography (CT) imaging system, the following apparatus and method is capable of being adapted for various purposes and is not limited to the following applications: CT systems, radiotherapy systems, x-ray imaging systems, ultrasound systems, nuclear imaging systems, magnetic resonance spectroscopy systems, and other applications known in the art.
Also, the present invention although described as being used in conjunction with a CT tube may be used in conjunction with other imaging tubes including x-ray tubes and vascular tubes.
Additionally, the terms “wide scan” refer to an x-ray source scanning width that is approximately greater than 10 mm. For example, in one embodiment of the present invention an x-ray source of the present invention has a scanning width of approximately 90 mm, which is significantly larger than scanning widths of traditional x-ray sources.
In the following description, various operating parameters and components are described for one constructed embodiment. These specific parameters and components are included as examples and are not meant to be limiting.
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The cathode 30 is in the form of a self-contained electron gun and is a member of a replaceable subassembly 46 that is easily maintained and replaced. Although a single electron gun 30 is shown, any number of electron guns may be utilized. The electron gun 30 may have an insulating layer (not shown), which may be formed of ceramic and be of various shape. An advantage to using a self-contained electron gun is mitigation of associated problems in conditioning surfaces in a target in preparation for high voltage application. By using an electron gun there is no static electric field gradient present at the target surfaces 36 when the electron beam 32 is incident, thereby reducing the necessity for target and cathode surface conditioning, Also, since there is no DC electric field gradient present, incidence of “spit” activity is reduced.
The cathode 30 is not limited to use of a thermionic tungsten wire coil that is traditionally used in imaging tubes, many other electron sources may be used. One electron source that may be used includes field emitter arrays formed from Spindt cones, barrel or hollow cylinders, carbon nanotubes, or physical vapor deposition or chemical vapor deposition layers that give rise to field emitting carbon structures or photoemitters. In a sample embodiment, an electron source having a focusing electrode with a variable potential applied therein is used. With a variable potential applied to the focusing electrode usefulness of the imaging tube is increased to include cardiac and angio functions, where focal spot variability is necessary. In addition, the focusing electrode may be utilized in standard fluoroscopic and radiographic modalities.
Current of the cathode 30 may be varied similar to a traditional cathode having a tungsten coil wire filament. Emission current or ‘beam current’ is controlled by the direct heating of the coiled tungsten wire (not shown) within the cathode 30. Emission current or beam current is controlled in different fashions using different emission techniques. For example, the local electric field is changed to produce more or less current from field emission devices. For Spindt-type cone-shaped field emitters, this implies controlling the gate voltage near the tip of the emitter cone. The metal in a dispenser cathode is raised to higher or lower temperatures by means of a direct or indirect heater. The low work function of the metal in the dispenser cathode promotes electron emission at lower temperatures than from a bare tungsten emitter.
The cathode 30 is capable of accelerating the electron beam 32 at various kinetic energies, depending upon the patient or object being scanned, as opposed to using a cathode with a fixed high-voltage for all objects. Independent of the final electron energy the use of an electron gun that is substantially isolated from the remainder of the chamber or tube volume lowers the voltage drop from the gun 30 to the target surfaces 32, in turn suppressing discharge or arc-forming mechanisms. This in turn reduces the number of vacuum arc events or ‘spits’. By minimizing arc-forming mechanisms a diagnostician is better able to observe organ motion during a “cine” mode exam, for example, during a cardiac scan due to fewer interruptions in image brightness. Reduction or elimination of the high electric fields at the target surfaces 36 and near the high-temperature regions of the cathode 30 reduces the amount of accelerating electric field non-uniformities and increases ease of tailoring electric fields near the target surfaces 36 for increased accuracy in focal spot size, location, and shape generation.
Ability of the cathode 30 to alter direction of the electron beam 32 at various angles allows for generation of the electron beam 32 at the shallow incident angles 40. Direction alteration of the electron beam 32 may be performed within the cathode 30, by rastering the electron beam 32 across the target surfaces 36, or through movement of the cathode 30 via a cathode steering mechanism 50. Direction of the electron beam 32 may also be altered within an imaging tube, allowing for an electron beam angle other then approximately 90° without altering shape of the resulting electron beam.
The cathode 30 is positioned a predetermined distance from the target surfaces 36 to minimize a maximum steering angle and a maximum rastering angle of the cathode 30 and electron beam 32, respectively. In positioning the cathode 30 at the predetermined distance the cathode 30 is positioned such that the electron beam 32 may be directed at each target surface 36 without interference from other target surfaces.
Focusing level and shape of the electron beam 32 is adjustable within the cathode 30, which further allows the electron beam 32 to be rastered over the target surfaces 36. Adjustment of electron acceleration, electron beam focusing, and cathode steering is minimized between the cathode 30 and the anode 38 by generating uniform focal spots for each target surface 36 such that each target surface 36 has a similar focal spot.
The anode 38 rotates to increase average power capacity of the target surfaces 36, as well as to increase the amount of temperature abatement. The anode 38 may be internally cooled via a support shaft (not shown) coupled to a liquid metal by rotating seals (not shown). Thermal energy generated at the target surfaces 36 is transferred through the shaft to the liquid metal and finally to a cooling oil through a transfer component (not shown) where the energy is removed from the corresponding imaging tube. For further explanation of the described cooling apparatus see U.S. Pat. No. 6,160,868, entitled “X-ray Tube Apparatus Employing Liquid Metal for Heat Removal”. Of course, other cooling methods known in the art may be used, such as x-ray tube apparatuses that use water, mixtures of water and ethylene glycol, or mixtures of water and other cooling fluids and/or materials to increase heat capacity and cooling power. When rotating seals are used the anode is coupled to ground or is at ground potential.
Although the anode 38 is shown in the form of a cylinder or drum having multiple target surfaces, the anode 38 may be in some other form, also having multiple target surfaces. The anode 38 may be in the form of a single support element or may be in the form of multiple rings, as shown in
The target surfaces 36 are on multiple tracks 52 that may be wedge shaped, as shown. The tracks 52 may correspond to multiple rings forming the anode 38, as shown in
The window 44 is transmissive for electrons at a beam potential between approximately 80 kV and 120 kV, without electron beam loss or heating of the window 44. The window 44 may be formed of beryllium or similar material known in the art. When beryllium is not used or the window 44 is not as transmissive for the above potentials window-cooling methods known in the art may be utilized,
In operation, as the anode 38 rotates about the patient 24, a protocol within the controller 28 determines appropriate steering angle, how the electron beam 32 is rastered across the target surfaces 36, focal spot positioning, focusing level of the gun 30, shape of the electron beam 32, and other parameters affecting x-ray generation.
The controller 28 may be microprocessor based such as a computer having a central processing unit, memory (RAM and/or ROM), and associated input and output buses. The controller 28 may be an application-specific integrated circuit or be formed of other logic devices known in the art. The controller 28 may be a portion of a central main control unit, an interactive dynamics module, or may be a stand-alone controller as shown.
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In step 150, the target surfaces of an anode, such as the anodes 38, 38′, and 38″ are rotated about a center axis, such as the z-axis, to provide cooling of the target surfaces.
In step 152, the controller 28 sequentially steers the cathode or rasters the electron beam, such as beams 32 and 32′, at each target surface that is being utilized for a particular scan. The cathode may be similar to the electron guns 30 and 30′. Sequentially adjusting direction of the electron beam on the target surfaces can occur in approximately a few milliseconds, which allows a full organ to be scanned in a fraction of the time required of prior CT scanning systems.
In step 154, an electron beam is emitted from the cathode and is incident upon the target surfaces. The target surfaces that are used for a particular scan have an associated scan width that corresponds with an active detector width of the detector array. The controller 28 determines which target surfaces to utilize in response to a determination of the active detector width. The electron beam is formed to uniformly generate the focal spots on each of the target surfaces.
In step 156, multiple tasks may also be performed including: adjusting emission of the electron beam; focusing the cathode; adjusting voltage potential of various imaging tube components; adjusting current of the cathode; steering the electron beam, or other various tasks. For example, the voltage potential of the cathode may be gated. The electron beam is gated by gridding or pulsing high-voltage potential of the cathode to correspond in time with the rotation of used x-ray exposed portions of an anode.
In step 158, the x-ray beams are generated in response to the impact of the electron beam on the target surfaces. The x-ray beams are emitted from the target surfaces to exit an imaging tube window, such as window 44.
The above-described steps are meant to be an illustrative example; the steps may be performed synchronously or in a different order depending upon the application.
The present invention therefore provides an imaging system including an imaging tube with increased coverage that has increased scanning speed. The present invention is capable of scanning a whole organ in relatively a small number of scans rendering procedures such as organ perfusion feasible on short time scales allowing capture of organ function to occur quickly. By using a cylindrically symmetric target a single electron source may be used, simplifying an imaging tube over use of multiple electron sources. The use of a single electron source also minimizes power dissipation necessary in scanning of an object.
The present invention provides high voltage stability and events such as arcs, discharges, and spits are reduced. The present invention prevents high vapor pressures that often develop within an imaging tube insert from creating a path to ground or a high voltage opposite polarity, which in turn prevents the perturbing of high voltage stability. This is especially beneficial in applications, such as cardiac scanning, where the time of the patient subjected to contrast media is limited by body tolerance to the injected or ingested contrast medium and the scan time. High voltage stability allows for increased imaging tube life. High voltage stability also strengthens sub-component design and provides increased robustness and longer tube operating life from a reduction in arcing or discharges that tend to shorten lifetime of high-voltage generator equipment and damage surfaces internal to a tube vacuum enclosure.
By simplifying the imaging tube manufacturing time and costs of the imaging tube are also decreased. Exhaust times and temperatures are reduced for high vacuum preparation. Lengthy high-voltage seasoning is also minimized.
The x-ray source of the present invention simplifies the implementation of glancing angle x-ray production (HEXLAB effect), since the electric fields are confined to the interior of the x-ray source. One can choose a beam angle different than perpendicular and not alter the shape of the resulting electron beam.
Since the electron emission source of the present invention is protected from the harsh high electric field environment of the tube, emitters other than coiled tungsten wire filaments may be utilized. Since the emitter is partially isolated from the parts of the tube that are traditionally at high temperature and subject to high E-field stress other emitter types, such as field emitter, field emitter arrays, carbon nanotube emitters and arrays, and dispenser cathodes, can be used.
While the invention has been described in connection with one or more embodiments, it is to be understood that the specific mechanisms and techniques which have been described are merely Illustrative of the principles of the invention, numerous modifications may be made to the methods and apparatus described without departing from the spirit and scope of the invention as defined by the appended claims.
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