System and method for calibration of an X-ray tube

A system and method for calibrating an X-ray tube is provided in which the X-ray tube includes an electronic storage medium associated with the X-ray tube on which calibration information for the X-ray tube is stored. The calibration information includes operating parameters for the focusing elements of the X-ray tube for desired focal spots, tolerance limits for variations in the focal spots and a number of gradient coefficient values corresponding to certain modulation transfer functions (MTF) for the X-ray tube that the imaging system can employ in an iterative manner to correct the operating parameters of the focusing elements to achieve the desired focal spot. This automatic iterative process significantly reduces the time required for the calibration of the X-ray tube. The system and method also employs scan sequencing to minimize the heat generated enabling the scans to be completed in a shorter amount of time than prior calibration processes.

BACKGROUND OF THE DISCLOSURE

The disclosure relates generally to diagnostic imaging systems and methods, and more particularly, to a system and method for calibrating an X-ray tube.

X-ray systems typically include an X-ray tube, a detector, and a support structure for the X-ray tube and the detector. In operation, an imaging support, on which an object is positioned, is located between the X-ray tube and the detector. The X-ray tube typically emits radiation, such as X-rays, toward the object. The radiation typically passes through the object on the support and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure or an inanimate object as in, for instance, a package in an X-ray scanner or computed tomography (CT) package scanner.

X-ray tubes include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor that supports a disc-shaped anode target and a stator structure that surrounds an elongated neck of the X-ray tube. The rotor of the rotating anode assembly is driven by the stator.

An X-ray tube cathode provides an electron beam that is accelerated using a high voltage applied across a cathode-to-anode vacuum gap to produce X-rays upon impact with a target track of the anode. The area where the electron beam impacts the target track is often referred to as the focal spot. Typically, the cathode includes one or more resistive filaments positioned within a cup for providing electron beams to create a high-power, large focal spot or a high-resolution, small focal spot, as examples. Typically, an electrical current is passed through the resistive elements, thus causing the resistive elements to increase in temperature and emit electrons when in a vacuum.

Imaging applications may be designed that include real-time control of focal spot size (length and width) and position on the target track. The position of the focal spot may be kept at the same track location (ignoring track rotation) or dynamically deflected view-by-view between two or three or more positions. In some X-ray tubes, focal spot control is enabled via electrodes surrounding the filament within the cathode structure or via electromagnets in the electron beam drift region. Changes in current (mA) and voltage (kVp) to the cathode filaments affect the position and size of the focal spot.

According to one example, to compensate for current and voltage adjustments, electrode voltages within the cathode are adjusted to achieve a desired or targeted focal spot size and position. According to another example, focal spot size and position may be controlled using magnetic lenses (dipole, quadrupole, multipole) instead of or additional to electrostatic control as described with respect to the electrode voltages. Such adjustments may occur at the start of the scan (dependent upon user selection of mA and kVp) or during an exam (e.g., mA adjustment during the exam). For a modern X-ray tube capable of microsecond X-ray intensity switching, quadrupole magnets are used to control focal spot size. To achieve this the quadrupole currents vary over a range of several amps over the full application range (typically: 70-140 kVp, 10-1300 mA, small to large focal spot sizes). The currents supplied to the magnets are required to be controlled within a few milliamps to achieve proper focal spot size on the target.

Due to manufacturing variability for the components of the X-ray tubes, the values for the focal spot control are typically determined for each X-ray tube and imaging system combination to achieve the targeted focal spot sizes and positions (within a predetermined tolerance) for a plurality of currents and voltages. The values determined for a particular X-ray tube within one imaging system, however, may cause the X-ray tube to exceed focal spot tolerances when the particular X-ray tube is coupled to another generator. For example, values determined using a testing imaging system during a manufacturing process of the X-ray tube may be different from those required for the same X-ray tube within an imaging system into which the X-ray tube is to be installed.

When a new X-ray tube is installed within an imaging system, when another component of the imaging system relating to the X-ray tube is replaced, such as a voltage tank or magnet control board, among others, or in order to evaluate any degradation of performance of an X-ray tube already installed within an imaging system due to aging, calibration of the X-ray tube is required to ensure the required image quality for the imaging system. However, with the fine adjustments required for the proper operation of the X-ray tube to achieve the desired focal spot size, once the X-ray tube is installed within the imaging system a large number of calibration points across the operational ranges for the X-ray tube must be obtained to determine the proper operation of the X-ray tube within the system. These calibration points are then stored within the imaging system for the X-ray tube for later use in determining whether the X-ray tube is within the proper operating parameters during the useful life of the X-ray tube. One example of such a calibration system is disclosed in U.S. Pat. No. 7,409,043, entitled Method and Apparatus to Control Radiation Tube Focal Spot Size, the entirety of which is expressly incorporated by reference herein

However, the process for the calibration of the X-ray tube is highly time and effort intensive. In particular, in each of these situations where calibration is necessary, the number of calibration points that need to be determined for proper use of the X-ray tube requires significant time to operate the X-ray tube at each point to obtain the information necessary for the storage within the system for calibration purposes. Further, the calibration information is only utilized to correct for ongoing variations of focal spot size during operation of the X-ray tube, rather than enabling a calibration of the overall functioning of the X-ray tube.

Therefore, it would be desirable to design a system and method capable of efficiently calibrating the overall functionality of an X-ray tube particular to the imaging system into which the X-ray tube is to be or has been installed.

BRIEF DESCRIPTION OF THE DISCLOSURE

In one exemplary embodiment of the present disclosure, a system and method for calibrating an X-ray tube is provided in which the X-ray tube includes an electronic storage medium associated with the X-ray tube on which calibration information for the X-ray tube is stored. The calibration information includes values representing the operating parameters for the focusing elements of the X-ray tube for desired focal spots that are determined during the initial testing of the X-ray tube after manufacture of the X-ray tube, as well as tolerance limits for variations in the focal spots. The stored calibration information also includes a number of gradient or sensitivity coefficient values corresponding to certain focal spot size functions, such as modulation transfer functions (MTF), for the X-ray tube that are employed to correct the operating parameters to achieve the desired focal spot size. This data is stored in a computer-readable format in the electronic storage medium and is accessed by the imaging system during a calibration procedure performed for the X-ray tube. In the procedure, the imaging system can operate the X-ray tube and focusing elements at the operating parameters specified for each of the focal spot sizes. Should a focal spot be determined to be out of the tolerance range(s), the imaging system can employ the gradient or sensitivity coefficients with the MTF in an iterative manner to correct the operating parameters of the focusing elements to achieve the desired focal spot. This automatic iterative process provides the technical effect of significantly reducing the time required for the calibration of the X-ray tube within the imaging system in which it has been installed.

In another exemplary embodiment of the present disclosure, the system and method provides a sequencing algorithm for the scans performed during the calibration process. This sequencing of the scans accommodates the heat generated by each scan in order to minimize the potential of damage being done to the X-ray tube as a result of overheating. The scan sequencing has the technical effect of ordering the scans performed in the calibration process to minimize the heat generated in order to enable the scans to be completed in a shorter amount of time than prior calibration processes and/or by preventing any source protection algorithm from stopping the calibration process prior to its completion.

In one exemplary embodiment of the disclosure, a system for the calibration of an X-ray tube includes an imaging system having a control mechanism, a computer and an electronic storage device operably connected to one another and an X-ray tube connected to the imaging system, wherein the computer is configured to access initial parameters of operation for the X-ray tube at a number of focal spots, the initial parameters including values for X-ray tube voltage, X-ray tube emission, operating currents and linearized focal spot size functions and gradients therefor, determining a calibration state of the X-ray tube, optionally operating the X-ray tube at a first portion of the number of focal spots to determine any offsets for the initial parameters, optionally updating the initial parameters with the offsets, operating the X-ray tube at each of a second portion of the number of focal spots and determining if any of the second portion of focal spots are outside of calibration tolerance limits.

In another exemplary embodiment of the disclosure, a method for the calibration of an X-ray tube includes the steps of providing original parameters of operation for a first number of focal spots, the original parameters including values for X-ray tube voltage, X-ray tube emission, operating currents and linearized focal spot size functions and gradients therefor, operating the X-ray tube at the original parameters, determining values for any offsets in the original parameters and updating the original parameters to provide updated parameters for the first number of focal spots.

In an exemplary embodiment of the method of calibrating an X-ray tube including magnetic focusing elements includes the steps of providing initial parameters of operation for the X-ray tube at a number of focal spots, the initial parameters including values for X-ray tube voltage, X-ray tube emission, operating currents and linearized focal spot size functions and gradients therefor, determining a calibration state of the X-ray tube, optionally operating the X-ray tube at a first portion of the number of focal spots to determine any offsets for the initial parameters, optionally updating the initial parameters with the offsets, operating the X-ray tube at each of a second portion of the number of focal spots and determining if any of the second portion of focal spots are outside of calibration tolerance limits.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present disclosure relate to calibration systems and processes for an X-ray tube operated with microsecond X-ray intensity switching. An exemplary X-ray tube and a computed tomography system employing the exemplary calibration system and method are presented.

Referring now toFIGS. 1 and 2, as disclosed in U.S. Pat. No. 8,401,151 entitled X-ray Tube for Microsecond X-ray Intensity Switching, the entirety of which is expressly incorporated herein by reference, a computed tomography (CT) imaging system10is illustrated. The CT imaging system10includes a gantry12. The gantry12has an X-ray source14, which typically is an X-ray tube that projects a beam of X-rays16towards a detector array18positioned opposite the X-ray tube on the gantry12. In one embodiment, the gantry12may have multiple X-ray sources (along the patient theta or patient Z axis) that project beams of X-rays. The detector array18is formed by a plurality of detectors20which together sense the projected X-rays that pass through an object to be imaged, such as a patient22. During a scan to acquire X-ray projection data, the gantry12and the components mounted thereon rotate about a center of rotation24. While the CT imaging system10described with reference to the medical patient22, it should be appreciated that the CT imaging system10may have applications outside the medical realm. For example, the CT imaging system10may be utilized for ascertaining the contents of closed articles, such as luggage, packages, etc., and in search of contraband such as explosives and/or biohazardous materials.

Rotation of the gantry12and the operation of the X-ray source14are governed by a control mechanism26of the CT system10. The control mechanism26includes an X-ray controller28that provides power and timing signals to the X-ray source14and a gantry motor controller30that controls the rotational speed and position of the gantry12. A data acquisition system (DAS)32in the control mechanism26samples analog data from the detectors20and converts the data to digital signals for subsequent processing. An image reconstructor34receives sampled and digitized X-ray data from the DAS32and performs high-speed reconstruction. The reconstructed image is applied as an input to a computer36, which stores the image in an electronic mass storage device, database or memory38or subdivision thereof.

Moreover, the computer36also receives commands and scanning parameters from an operator via operator console40that may have an input device such as a keyboard (not shown inFIGS. 1-2). An associated display42allows the operator to observe the reconstructed image and other data from the computer36. Commands and parameters supplied by the operator are used by the computer36to provide control and signal information to the DAS32, the X-ray controller28and the gantry motor controller30. In addition, the computer36operates a table motor controller44, which controls a motorized table46to position the patient22and the gantry12. Particularly, the table46moves portions of patient22through a gantry opening48. It may be noted that in certain embodiments, the computer36may operate a conveyor system controller44, which controls a conveyor system46to position an object, such as, baggage or luggage and the gantry12. More particularly, the conveyor system46moves the object through the gantry opening48.

The X-ray source14is typically an X-ray tube that includes at least a cathode and an anode. The cathode may be a directly heated cathode or an indirectly heated cathode. Currently, X-ray tubes include an electron source to generate an electron beam and impinge the electron beam on the anode to produce X-rays. These electron sources control a beam current magnitude by changing the current on the filament, and therefore emission temperature of the filament. Unfortunately, these X-ray tubes fail to control electron beam intensity to a view-to-view basis based on scanning requirements, thereby limiting the system imaging options. Accordingly, an exemplary X-ray tube is presented, where the X-ray tube provides microsecond current control during nominal operation, on/off gridding for gating or usage of multiple X-ray sources, 0 percent to 100 percent modulation for improved X-ray images, and dose control or fast voltage switching for generating X-rays of desired intensity resulting in enhanced image quality.

FIG. 3is a diagrammatical illustration of an exemplary X-ray tube50, in accordance with aspects of the present technique. In one embodiment, the X-ray tube50may be the X-ray source14(seeFIGS. 1-2). In the illustrated embodiment, the X-ray tube50includes an exemplary injector52disposed within a vacuum wall54. Further, the injector52includes an injector wall53that encloses various components of the injector52. In addition, the X-ray tube50also includes an anode56. The anode56is typically an X-ray target. The injector52and the anode56are disposed within an X-ray tube casing72. In accordance with aspects of the present technique, the injector52may include at least one cathode in the form of an emitter58. In the present example, the cathode, and in particular the emitter58, may be directly heated. Further, the emitter may be coupled to an emitter support60, and the emitter support60in turn may be coupled to the injector wall53. The emitter58may be heated by passing a large current through the emitter58. A voltage source66may supply this current to the emitter58. In one embodiment, a current of about 10 amps (A) may be passed through the emitter58. The emitter58may emit an electron beam64as a result of being heated by the current supplied by the voltage source66. As used herein, the term “electron beam” may be used to refer to a stream of electrons that have substantially similar velocities.

The electron beam64may be directed towards the target56to produce X-rays84. More particularly, the electron beam64may be accelerated from the emitter58towards the target56by applying a potential difference between the emitter58and the target56. In one embodiment, a high voltage in a range from about 40 kVp to about 150 kVp may be applied via use of a high voltage feedthrough68to set up a potential difference between the emitter58and the target56, thereby generating a high voltage main electric field78. In one embodiment, a high voltage differential of about 140 kVp may be applied between the emitter58and the target56to accelerate the electrons in the electron beam64towards the target56. It may be noted that in the presently contemplated configuration, the target56may be at ground potential. By way of example, the emitter58may be at a potential of about −140 kVp and the target56may be at ground potential or about zero volts.

In an alternative embodiment, emitter58may be maintained at ground potential and the target56may be maintained at a positive potential with respect to the emitter58. By way of example, the target may be at a potential of about 140 kVp and the emitter58may be at ground potential or about zero volts.

Moreover, when the electron beam64impinges upon the target56, a large amount of heat is generated in the target56. Unfortunately, the heat generated in the target56may be significant enough to melt the target56. In accordance with aspects of the present technique, a rotating target may be used to circumvent the problem of heat generation in the target56. More particularly, in one embodiment, the target56may be configured to rotate such that the electron beam64striking the target56does not cause the target56to melt since the electron beam64does not strike the target56at the same location. In another embodiment, the target56may include a stationary target. Furthermore, the target56may be made of a material that is capable of withstanding the heat generated by the impact of the electron beam64. For example, the target56may include materials such as, but not limited to, tungsten, molybdenum, or copper.

In the presently contemplated configuration, the emitter58is a flat emitter. In an alternative configuration the emitter58may be a curved emitter. The curved emitter, which is typically concave in curvature, provides pre-focusing of the electron beam. As used herein, the term “curved emitter” may be used to refer to the emitter that has a curved emission surface. Furthermore, the term “flat emitter” may be used to refer to an emitter that has a flat emission surface. In accordance with aspects of the present technique shaped emitters may also be employed. For example, in one embodiment, various polygonal shaped emitters such as, a square emitter, or a rectangular emitter may be employed. However, other such shaped emitters such as, but not limited to elliptical or circular emitters may also be employed. It may be noted that emitters of different shapes or sizes may be employed based on the application requirements.

In accordance with aspects of the present technique, the emitter58may be formed from a low work-function material. More particularly, the emitter58may be formed from a material that has a high melting point and is capable of stable electron emission at high temperatures. The low work-function material may include materials such as, but not limited to, tungsten, thoriated tungsten, lanthanum hexaboride, and the like.

With continuing reference toFIG. 3, the injector52may include at least one focusing electrode70. In one embodiment, the at least one focusing electrode70may be disposed adjacent to the emitter58such that the focusing electrode70focuses the electron beam64towards the target56. As used herein, the term “adjacent” means near to in space or position. Further, in one embodiment, the focusing electrode70may be maintained at a voltage potential that is less than a voltage potential of the emitter58. The potential difference between the emitter58and focusing electrode70prevents electrons generated from the emitter58from moving towards the focusing electrode70. In one embodiment, the focusing electrode70may be maintained at a negative potential with respect to that of the emitter58. The negative potential of the focusing electrode70with respect to the emitter58focuses the electron beam64away from the focusing electrode70and thereby facilitates focusing of the electron beam64towards the target56.

In another embodiment, the focusing electrode70may be maintained at a voltage potential that is equal to or substantially similar to the voltage potential of the emitter58. The similar voltage potential of the focusing electrode70with respect to the voltage potential of the emitter58creates a parallel electron beam by shaping electrostatic fields due to the shape of the focusing electrode70. The focusing electrode70may be maintained at a voltage potential that is equal to or substantially similar to the voltage potential of the emitter58via use of a lead (not shown inFIG. 3) that couples the emitter58and the focusing electrode70.

Moreover, in accordance with aspects of the present technique, the injector52includes at least one extraction electrode74for additionally controlling and focusing the electron beam64towards the target56. In one embodiment, the at least one extraction electrode74is located between the target56and the emitter58. Furthermore, in certain embodiments, the extraction electrode74may be positively biased via use of a voltage tab (not shown inFIG. 3) for supplying a desired voltage to the extraction electrode74. In accordance with aspects of the present technique, a bias voltage power supply90may supply a voltage to the extraction electrode74such that the extraction electrode74is maintained at a positive bias voltage with respect to the emitter58. In one embodiment, the extraction electrode74may be divided into a plurality of regions having different voltage potentials to perform focusing or a biased emission from different regions of the emitter58.

It may be noted that, in an X-ray tube, the energy of an X-ray beam may be controlled via one or more of multiple ways. For instance, the energy of an X-ray beam may be controlled by altering the potential difference (that is acceleration voltage) between the cathode and the anode, or by changing the material of the X-ray target, or by filtering the electron beam. This is generally referred to as “kVp control.” As used herein, the term “electron beam current” refers to the flow of electrons per second between the cathode and the anode. Furthermore, an intensity of the X-ray beam is controllable via control of the electron beam current. Such a technique of controlling the intensity is generally referred to as “mA control.” As discussed herein, aspects of the present technique provide for control of the electron beam current via use of the extraction electrode74. It may be noted that, the use of such extraction electrode74enables a decoupling of the control of electron emission from the acceleration voltage.

Furthermore, the extraction electrode74is configured for microsecond current control. Specifically, the electron beam current may be controlled in the order of microseconds by altering the voltage applied to the extraction electrode74in the order of microseconds. It may be noted that the emitter58may be treated as an infinite source of electrons. In accordance with aspects of the present technique, electron beam current, which is typically a flow of electrons from the emitter58towards the target56, may be controlled by altering the voltage potential of the extraction electrode74. Control of the electron beam current will be described in greater detail hereinafter.

With continuing reference toFIG. 3, the extraction electrode74may also be biased at a positive voltage with respect to the focusing electrode70. As an example, if the voltage potential of emitter58is about −140 kVp, the voltage potential of the focusing electrode70may be maintained at about −140 kVp or less, and the voltage potential of the extraction electrode74may be maintained at about −135 kVp for positively biasing the extraction electrode74with respect to the emitter58. In accordance with aspects of the present technique, an electric field76is generated between the extraction electrode74and the focusing electrode70due to a potential difference between the focusing electrode70and the extraction electrode74. The strength of the electric field76thus generated may be employed to control the intensity of electron beam64generated by the emitter58towards the target56. The intensity of the electron beam64striking the target56may thus be controlled by the electric field76. More particularly, the electric field76causes the electrons emitted from the emitter58to be accelerated towards the target56. The stronger the electric field76, the stronger is the acceleration of the electrons from the emitter58towards the target56. Alternatively, the weaker the electric field76, the lesser is the acceleration of electrons from the emitter58towards the target56.

In addition, altering the bias voltage on the extraction electrode74may modify the intensity of the electron beam64. As previously noted, the bias voltage on the extraction electrode may be altered via use of the voltage tab present on the bias voltage power supply90. Biasing the extraction electrode74more positively with respect to the emitter58results in increasing the intensity of the electron beam64. Alternatively, biasing the extraction electrode74less positively with respect to the emitter58causes a decrease in the intensity of the electron beam64. In one embodiment, the electron beam64may be shut-off entirely by biasing the extraction electrode74negatively with respect to the emitter58. As previously noted, the bias voltage on the extraction electrode74may be supplied via use of the bias voltage power supply90. Hence, the intensity of the electron beam64may be controlled from 0 percent to 100 percent of possible intensity by changing the bias voltage on the extraction electrode74via use of the voltage tab present in the bias voltage power supply90.

Furthermore, voltage shifts of 8 kVp or less may be applied to the extraction electrode74to control the intensity of the electron beam64. In certain embodiments, these voltage shifts may be applied to the extraction electrode74via use of a control electronics module92. The control electronics module92changes the voltage applied to the extraction electrode74in intervals of 1-15 microseconds to intervals of about at least 150 milliseconds. In one embodiment, the control electronics module92may include Si switching technology circuitry to change the voltage applied to the extraction electrode74. In certain embodiments, where the voltage shifts range beyond 8 kVp, a silicon carbide (SiC) switching technology may be applied. Accordingly, changes in voltage applied to the extraction electrode74facilitates changes in intensity of the electron beam64in intervals of 1-15 microseconds, for example. This technique of controlling the intensity of the electron beam in the order of microseconds may be referred to as microsecond intensity switching.

Additionally, the exemplary X-ray tube50may also include a magnetic assembly80for focusing and/or positioning and deflecting the electron beam64on the target56. In one embodiment, the magnetic assembly80may be disposed between the injector52and the target56. In one embodiment, the magnetic assembly80may include one or more multipole magnets for influencing focusing of the electron beam64by creating a magnetic field that shapes the electron beam64on the X-ray target56. The one or more multipole magnets may include one or more quadrupole magnets, one or more dipole magnets, or combinations thereof. As the properties of the electron beam current and voltage change rapidly, the effect of space charge and electrostatic focusing in the injector will change accordingly. In order to maintain a stable focal spot size, or quickly modify focal spot size according to system requirements, the magnetic assembly80provides a magnetic field having a performance controllable from steady-state to a sub-30 microsecond time scale for a wide range of focal spot sizes. This provides protection of the X-ray source system, as well as achieving CT system performance requirements. Additionally, the magnetic assembly80may include one or more dipole magnets for deflection and positioning of the electron beam64at a desired location on the X-ray target56. The electron beam64that has been focused and positioned impinges upon the target56to generate the X-rays84. The X-rays84generated by collision of the electron beam64with the target56may be directed from the X-ray tube50through an opening in the X-ray tube casing72, which may be generally referred to as an X-ray window86, towards an object (not shown inFIG. 3).

With continuing reference toFIG. 3, the electrons in the electron beam64may get backscattered after striking the target56. Therefore, the exemplary X-ray tube50may include an electron collector82for collecting electrons that are backscattered from the target56. In accordance with aspects of the present technique, the electron collector82may be maintained at a ground potential. In an alternative embodiment, the electron collector82may be maintained at a potential that is substantially similar to the potential of the target56. Further, in one embodiment, the electron collector82may be located adjacent to the target56to collect the electrons backscattered from the target56. In another embodiment, the electron collector82may be located between the extraction electrode74and the target56, close to the target56. In addition, the electron collector82may be formed from a refractory material, such as, but not limited to, molybdenum. Furthermore, in one embodiment, the electron collector82may be formed from copper. In another embodiment, the electron collector82may be formed from a combination of a refractory metal and copper.

Furthermore, it may be noted that the exemplary X-ray tube50may also include one or more ion management electrodes (not shown inFIG. 3) either to repel or to attract positive ions that may be produced due to collision of electrons in the electron beam64with the target56and with the residual gas. A positive ion barrier for example is generally placed along the electron beam path and prevents the positive ions from striking various components in the X-ray tube50, thereby preventing damage to the components in the X-ray tube50, particularly components that are part of the injector52.

Table 1, below, is an overview of the aspects and associated processes for calibrating the X-ray source or X-ray tube14within the imaging system10. The first aspect/step (Sweeper) is employed after initial manufacture of the X-ray tube14and generates the necessary magnet current seed values and sensitivity/gradient coefficients that are required for any later calibration of the X-ray tube14needed in the field. The second aspect/step (Calibrator) can be employed or performed at any later time in the field with the X-ray tube14installed in a system10to correct for any drift in X-ray tube performance over time. The second aspect/step uses the seed values produced in the Sweeper or the magnet currents from the latest most recent calibration as a starting point and then calibrates the X-ray tube14using the sensitivities from provided by the Sweeper in aspect/step1. The last aspect/step (Tabulator) provides an interpolation algorithm to compute magnet calibration values for any intermediate X-ray tube emission values that were not calibrated in the prior aspects/steps.

TABLE 1Calibration System AspectsFocal Spot CalibrationSoftware ModulesFunctionSweeperFind QC, QT seed values and gradients forall focal spots across kVp/mA space.Sweeper seed and gradient data passed toCalibrator fir subsequent calibrationprocessesCalibratorIterative, gradient-based process that refinesQC, QT to within the calibration tolerancefor MTF at 50% intensity for all focal spots.Can be performed during manufacturing,upon initial installation in CT system andwhen necessary after installation. Data fromcalibrator stored in X-ray tube interface,magnet control board and/or CT system forimport to tabulator and back-upTabulatorInterpolates data from Calibrator and appliesany required offsets to create patient look uptable for patient scan

FIGS. 4A, 4B and 5show an exemplary block diagram of a process500for calibrating the X-ray source or X-ray tube14within the imaging system. Initially, from start block1000where the calibration process is initiated manually or automatically, such as in response to the expiration of a predetermined time period for operation of the source14, the system10moves to block1002to access the configuration parameters for the calibration procedure upon connection of the source14to the system10. The Configuration parameters are stored in the form of an X-ray tube look up table (LUT)1001that is found in a suitable electronic storage medium disposed on an X-ray tube interface (TIF) board (not shown) formed as part of the source14and operably interconnected to the system10upon engagement of the source14to the system10, thereby enabling access to the X-ray tube LUT1001by the system10. These parameters can include, but are not limited to a determination of the upper and lower emission values for the source14to be utilized in the scan techniques for the various calibration steps on the given focal spot sizes to be calibrated, as well as the nominal values, target values, safety focal spot tolerances, calibration tolerances and system validation tolerances for the focal spots being calibrated within specified source emission and voltage ranges and the magnet settings i.e., QC/QT, that are determined during initial testing of the source14that can occur during or immediately after initial production or manufacturing of the X-ray source14for selected focal spots. In block1002, the system10accesses these parameters and determines if the scan(s) would cause the emission from the source14to fall outside of the minimum and maximum emission ranges set for the scan(s) at which point the system10stop the calibration procedure set up. In block1002, if the emissions for the scan techniques fall within the minimum and maximum values, the system10can additionally locate an index for emission bracketing techniques and compute the linear interpolation for the magnetic currents to be used for the scans.

Subsequently, the system10moves to block1004in order to perform the various pre-requisite set up procedures for the calibration of the source14. These pre-requisites include, but are not limited to confirming the emitter and/or extractor calibration and source alignment are complete, performing a number of air scans at various voltages, current, and/or focal spot sizes defined within the X-ray tube LUT1001on the X-ray tube interface (TIF) board, and employing preset adaptive source warmup or cooldown procedures to ensure the source14is within set cathode and target track temperature requirements located within the X-ray tube LUT1001on the TIF board. These temperature requirements for the cathode and target track in the source14are controlled by a source protection algorithm (SPA) stored on the X-ray tube LUT1001and executed by the system10during the entire calibration procedure to protect the source14and ensure stable MTF performance during the calibration process.

After the prerequisites for the calibration procedure have been met in block1004, the system10proceeds to block1006to determine if the source14has recently been calibrated and/or block1008to determine if a system component (not shown) related to the proper operation of the source14, such as the high voltage tank or magnet control board, among others, has recently been replaced. If the source14was recently calibrated and no components have been replaced, the system10proceeds with a more detailed, second stage calibration process1024, to be described.

However, if either the source14has not been recently calibrated or a related component of the system10has recently been replaced, as shown in the exemplary embodiment ofFIG. 4Athe system10moves to block1010to perform a preliminary, first stage calibration process. In the first stage1010, a limited number of scans are performed with the source14to detect any shift or current offset in the focusing elements80, such as due to the source14being operated with a different MCB than used during initial testing, that may generate high impact temperature focal spots exceeding X-ray tube limitations. The list of scan techniques1009for the first stage1010are stored in the X-ray tube LUT1001and include data on the X-ray tube voltage (kVp), X-ray tube emission (mA) and size of the focal spot (fs) for each scan, as well as the current values for the focusing elements80to be utilized. In an exemplary embodiment, the focusing elements80are quadrupole magnets, paired as QC1011and QT magnets1013(FIG. 3), respectively, and the current values for the focusing elements80are provided as values QC, QT for the QC magnets1011and the QT magnets1013.

Along with the scan technique parameters, the X-ray tube LUT1001includes various gradient coefficients W0 (lp/cm), W1 (lp/cm*mA), W2 (lp/cm*mA), L0 (lp/cm), L1 (lp/cm*mA) and L2 (lp/cm*mA) that are utilized in linearized focal spot size functions, such as MTF equations standardized at 50% intensity for the width and length values, i.e., MTFW (lp/cm) and MTFL (lp/cm), of the focal spots for each scan technique. The data associated with each scan technique stored in the X-ray tube LUT is determined during the initial testing/calibration of the source14and stored in the X-ray tube LUT to be accessed by the system10during the calibration process, e.g., data generated during manufacturing of the source at nominal alignment, or zero deflection currents in the dipoles.

Once the scan techniques for the first stage1010have been determined from the X-ray tube LUT1001, which can be as few as six (6) in an exemplary embodiment, the system10proceeds in block1012to operate the source14to perform those scans and obtain actual or measured MTFW and MTFL values from each scan. In block1014this data along with the gradient coefficients for each is utilized in the following equations to compute a quadrupole offset QPoffsetthat minimizes the following equations:
MIN((—MTFW−MTFW_spec)2+(MTFL−MTFL_spec)2(QPoffset)
where
MTFW=MTFW_measured+QPoffset(which is a function ofW1,W2, . . . )
and
MTFL=MTFL_measured+QPoffset(which is a function ofL1,L2, . . . )

The minimum sum is evaluated within a range of −10 mA to +10 mA, for example. The current offset that minimizes the sum (QPmin) is then added to all QC and QT currents in the X-ray tube LUT1001according to the following equations:
QCnew=QCold+QPoffset
QTnew=QTold+QPoffset
And the resulting values for QC and QT associated with each scan technique are recorded, such as temporarily in computer memory38, by the system1000in block1015.

Once the QC and QT values are updated, the first stage scan techniques are re-run in block1016, with the results recorded, such as temporarily in computer memory38, by the system1000in block1018. The results for the focal spots obtained in these re-run scans are compared with the tolerances for the scan techniques, also stored within the X-ray tube LUT1001in block1020. If the focal spots are found to be outside of the tolerance ranges, in block1022the system1000provides an alert concerning the error and stops the calibration process until a suitable intervention has been performed on the system10.

Alternatively, if the measured focal spots fall within the stored tolerances, the system1000updates all of the stored QC and QT values for all scan techniques in the X-ray tube LUT1001with the offset in block1023, and then proceeds to block1024to perform a more detailed, second stage calibration procedure. This second stage procedure is performed similarly to the first stage procedure in block1010but with the addition of multiple scan techniques associated with techniques associated with approximately three hundred (300) focal spots that establish a complete characterization of all focal spot sizes over the full performance range of the source14.

In the second stage calibration1024(FIG. 4B), the system1000in block1026initially accesses the X-ray tube LUT1001to locate the values of focal spot size (fs), X-ray tube voltage (kVp), X-ray tube emission (mA), QC, QT, MFTWgoal, MFTLgoal, W0, W1, W2, L0, L1 and L2 for each scan technique, which is also the step to which the overall process proceeds in the event that a determination is made in blocks1006and1008that the source14has been recently calibrated and that no components of the system10have recently been replaced. In addition, all of the information regarding the latest calibration is copied into the storage38, such as in a CT look up table (CT-LUT) on the system10to provide an additional record of the last calibration of the source14.

Initially, in block1028the system1000performs each scan stored in the X-ray tube LUT1001for the second stage calibration process using the stored values. For each scan, the data on the focal spot measurement provides values for MTFW and MTFL. In block1030, these values can be used to provide quadrupole offsets in the following equations to solve for QC and QT:
MTFW=W0+QC*W1+QT*W2
MTFL=L0+QC*L1+QT*L2
These equations (MTF linearization) can be solved to provide the quadrupole currents QC, QT that yield MTFW and MTFL. Likewise, the quadrupole currents for the goal MTF (MFTWgoal, MFTLgoal) can be obtained as QCgoal, QTgoal. The predicted change in the quadrupole current to focus the actual focal spot within the tolerances provided is therefore:
dQC=QC−QCgoal
dQT=QT−QTgoal

After transforming the linear equations for MTFW and MTFL, the quadrupole current changes can be calculated as follows:
dw=MTFWgoal−MTFW
dl=MTFLgoal−MTFL
dQT=(L1*dw−W1*dl)/(L1*W2−W1*L2)
dQC=dw/W1−W2/W*dQT
Thus, for any given MTF (MTFW, MTFL) measurement, the measurement returns values for the magnet current change (dQC, dQT) to bring the focal spot within the required tolerances. Any initially out of tolerance focal spots determined by the scan (block1032) are stored in an internal list within the X-ray tube LUT1001(block1034). If the number of scans performed for calibration of an out of tolerance focal spot on the list have not exceeded the maximum number of scans/iterations allowed (block1036), the scan for the focal spot is retaken (block1038) until the focal spot with within tolerances or the number of iterations for the calibration scan has been exceeded. This process employed in blocks1030,1032,1034,1036and1038is iterative due to the existing non-linearities and measurement uncertainties, but normally converges within a small number of iteration steps, such as less than five (5) steps. For each iteration, the X-ray tube LUT1001employs a convergence tolerance of 2% of the goal value, as convergence is only possible within the MTF measurement error and the MTF variability that is influenced by various factors, e.g., the temperature of the source14, such that any scan results reaching this threshold will result in stoppage of the scans for that focal spot. Further, the X-ray tube LUT1001can contain a maximum for the number of iterations that will be performed for any focal spot measurement.

To avoid any oscillation around the result, a common factor scaled can be employed to dampen a calculated current step (dQC, dQT) if the preceding measurement overshot the MTF goal in width and/or in length. In employing the dampening, the following equation can be utilized:
dQC=scaleDQ*dQC
dQT=scaleDQ*dQT

Further, in block1030, to avoid inadvertent large changes to the focal spot during calibration a limit can be applied to the magnet steps dQC,dQT after dampening. In doing so, the magnet step is limited to the maximum step allowable either for QC or QT while the other is adjusted according to its sensitivity. This ensures that a new, safe calibration point is generated that moves the calibration in the proper direction towards the solution.

After the initial scan and necessary subsequent iterations thereof have been performed, the updated calibration information is copied over the CT-LUT present within the storage38on the system10in block1040. Further, any out of tolerance focal spots that have exceeded the maximum number of iterations can be compared with system validation tolerances, as even if individual focal spot scan techniques did not converge during the iterations performed, the focal spots used for system validation may still fall within system tolerance specifications.

In the situation where the second stage calibration fail to converge or diverges, the sum of squares, i.e., Σ(i)=dw(i)2+−dl(i)2is calculated for each scan/iteration performed for a particular focal spot and the result with the minimum sum is selected for storage in the X-ray tube LUT1001.

During the process provided by blocks1030-1038, the system10employs a scan sequencing technique to avoid violating thermal limitation for the source14. The sequencing ensures the calibration process is not inadvertently interrupted by the source protection algorithm to enable quality calibration results to be obtained.

To perform the scan sequencing using the sequencing rules stored in the X-ray tube LUT1001, as illustrated in the exemplary embodiment ofFIG. 5, initially the system10initially orders all of the scans to be performed in the second stage calibration in order of decreasing expected focal spot temperature. With this list, the highest power scan to be performed is ordered first, with the next three scans performed being the three lowest power scans. This process is repeated by grouping the next highest power scan with the next three lowest power scans, until all initial scans to be performed in the second stage calibration are ordered into group of four scans, with the first scan being a high-power scan followed by three successive low-power scans. In one exemplary embodiment, each scan performed has a duration of between 10 ms to 500 ms, and can be approximately 150 ms, with a delay of between 500 ms to 5000 ms, and can be 800 ms between successive scans. In this manner the heat generated by the initial high-power scan is allowed to dissipate during performance of the low-power scan prior to any subsequent high-power scan being performed, thereby maintaining the temperature within the source14at levels within the tolerances for the source protection algorithm.

Additionally, because certain focal spots will require subsequent iterations, the scan sequencing algorithm also accommodates for multiple sets or iterations of scan to be performed with the source14by the system10. To provide the necessary temperature control, the scan sequencing algorithm employs an inter-iteration delay before beginning a subsequent sequence of scans. This inter-iteration delay allows the source14to cool effectively, and in one exemplary embodiment is a time period of approximately fifteen (15) minutes between scan iterations.

Further, as during the second stage calibration process certain scan techniques for particular focal points will reach convergence prior to others, in order to accommodate for those scans that do not require any further iterations and will not be re-executed, an empty scan, the scan sequencing algorithm will initially determine if the empty scan is a high or low temperature impact scan, i.e., if the scan would be the first high-power scan of the four scan grouping, or one of the three low-power scan of the grouping. If the empty scan is the high-power scan, the scan sequencing algorithm replaces the empty exposure/scan with a delay of 0 ms, as no delay is required due to the absence of the high-power scan. Alternatively, if the empty scan is a low-power scan, the scan sequencing algorithm replaces the empty scan with an exposure time of 0 ms, but maintains the interscan delay of 800 ms.

After the second stage calibration process1024has been completed, i.e., all focal spots are determined to be within tolerances or have exceeded the maximum number of iterations for the second stage calibration process, the system1000proceeds to block1040to record the second stage calibration data within the CT-LUT on the system. This data can subsequently be utilized by the system10during a patient scan.

Further, with all the focal spots within the second stage calibration having been scanned, the system10can move to a focal spot validation process1042. The focal spot validation process1042is similar to the second stage calibration process1024, but uses only a subset of the focal spots scanned in the second stage, i.e., a validation focal spot list, stored in the X-ray tube LUT1001. This focal spot list provides the system10with the focal spots, which may or may not correspond to techniques utilized in the second stage calibration process1024, for direct comparison with the system validation tolerances also stored within the X-ray tube LUT1001. To do so, in block1044, the system10retrieves validation focal spot table/list. In one exemplary embodiment, the validation focal spots/techniques corresponding to second stage calibration results stored within the CT-LUT on the system10for the focal spots listed in the validation table results and the scan results for any other techniques performed according to the validation focal spots/techniques are compared with the validation focal spot tolerances in block1046to determine if the results for each of the focal spots fall within the validation tolerance limits. From block1048, if the results do all fall within tolerance, the system10overwrites the CT-LUT with the results from the second stage calibration in block1050and the overall calibration process ends in block1052. Alternatively, if one or more of the focal spots do not fall within the specified validation tolerances, in block1054the system10removes the second stage calibration results from the CT-LUT and restores the original results, and provides an alert regarding the calibration failure and optionally required maintenance for the system10and our source14in block1056.

Further, separate from the overall calibration process500inFIGS. 4A-4B, inFIG. 6an exemplary embodiment of a standalone focal spot validation process2000is illustrated. This process2000is utilized as a recurring check on the focal spot stability of the source14after the source14has undergone the complete calibration process500and when no other components of the system10, e.g., the MCB, have been replaced. The validation process2000from start2001initially retrieves the validation focal spot table or list from the X-ray tube LUT1001in block2002and the corresponding second stage calibration results for those focal spots from the CT-LUT in block2004. Similar to the validation process1042, the system10in block2006then compares the results to the validation tolerances stored in the X-ray tube LUT1001to determine if the results for each of the focal spots fall within the validation tolerance limits. From block2008, if the results do all fall within tolerance, the validation process ends in block2010. Alternatively, if one or more of the focal spots do not fall within the specified validation tolerances, in block2012the system10proceed to initiate the complete calibration process500, or alternatively provides an indication that the process500needs to be initiated.