Patent Publication Number: US-2022217831-A1

Title: Maintaining a given focal spot size during a kvp switched spectral (multi-energy) imaging scan

Description:
FIELD OF THE INVENTION 
     The following generally relates to imaging and more particularly to maintaining a given focal spot size during a kVp switched spectral (multi-energy) imaging scan and is described with particular application to computed tomography (CT). 
     BACKGROUND OF THE INVENTION 
     A computed tomography (CT) scanner generally includes an X-ray tube mounted on a rotatable gantry opposite one or more rows of detectors. The X-ray tube rotates around an examination region located between the X-ray tube and the one or more rows of detectors and emits broadband radiation that traverses the examination region. The one or more rows of detectors detect radiation that traverses the examination region and generate projection data indicative thereof. A reconstructor reconstructs the projection data to generate volumetric image data, which can be displayed, filmed, archived, conveyed to another device, etc. 
       FIG. 1  diagrammatically depicts certain elements of an X-ray tube  100 , including a cathode  102  with a focusing cup  104  and a filament  106  and an anode  108 . A tube current (mA) is applied to the filament  106 , which heats the filament  106 , causing the filament  106  to expel electrons (thermionic emission), creating a space charge (or cloud a negative charge) a short distance away from the filament  106 . A peak tube voltage (kVp) is applied across the cathode  102  and the anode  108  and causes a beam of the electrons  110  to accelerate from the cathode  102  and impinge the anode  108 . A grid voltage is applied to electrodes of the focusing cup  104  to control a size of and steer the beam of electrons  110 . An interaction of the electrons  110  with the material of the anode  108  produces heat and radiation, including X-rays  112 , which pass through a tube window  114 , into an examination region  116 , to a detector  222 . 
     A surface area  120  of the anode  108  that receives the beam of electrons  110  is referred to as a focal spot. The size of the focal spot is one factor that affects the image quality of the volumetric image data. For example, the focal spot size affects the spatial resolution, where a smaller focal spot size results in a greater spatial resolution than a larger focal spot size, e.g., due to less focal spot blur from geometric magnification. The size of the focal spot depends on the X-ray tube voltage and the grid voltage. For a given focal spot size and a given X-ray tube voltage for a scan, the same grid voltage is applied to the focusing cup  104  the entire scan to maintain the focal spot size at a predetermined size for that X-ray tube voltage for the entire scan, e.g., to achieve a desired image quality of the volumetric image data. 
     The voxels of the volumetric image data are displayed using gray scale values corresponding to relative radiodensity. The gray scale values reflect the attenuation characteristics of the scanned subject and represent anatomical structures. The detected radiation also includes spectral information as the absorption of a photon by a material of a subject and/or an object is dependent on the energy of the photon traversing the material. Such spectral information provides additional information such as information indicative of the atomic, elemental or material composition of the material. However, the projection data does not reflect the spectral characteristics as the projection data are proportional to the energy fluence integrated over the energy spectrum (e.g., 40 keV to 120 keV), and the volumetric image data will not reflect the energy dependent information. 
     A CT scanner configured for spectral (multi-energy) imaging (a spectral CT scanner) leverages the spectral characteristics in the detected radiation to provide further information such as atomic or elemental composition information. In general, a spectral CT scanner is configured to detect different bands of X-ray radiation (instead of just the entire spectrum) and generate projection data for each of the different energy bands (instead of just the entire spectrum). In one instance, this is achieved through kVp switching. For example, with a dual-energy configuration, the X-ray tube voltage (i.e. the kVp) is switched back and forth between two kVp&#39;s, such as between a 80 kVp for odd number data acquisition periods and a second 140 kVp for even number data acquisition periods, or vice versa. 
     However, switching the kVp as such, for a given grid voltage, will cause the focal spot size to change, i.e. increase and/or decrease, depending on whether the kVp is switched to a higher or lower kVp, since, as described above, the focal spot size depends on both the kVp and the grid voltage. Unfortunately, this will lead to a varying spatial resolution because, as discussed above, an increased focal stop size decreases spatial resolution. Furthermore, this may introduce artifact into the reconstructed volumetric image data because the calibration table, which is created for a particular focal spot size, will not be correct for all focal spot sizes. Both of these may reduce an image (and thus diagnostic) quality of the reconstructed volumetric image data. Furthermore, a decreased focal spot may result in damage to the anode due to the concentration of electrons at small surface area on the anode. 
     SUMMARY OF THE INVENTION 
     Aspects described herein address the above-referenced problems and/or others. For instance, the following describes an approach for controlling the X-ray tube grid voltage in coordination with the kVp during a kVp switched based spectral (multi-energy) scan to maintain a given focal spot size (within tolerance) during the spectral (multi-energy) entire scan. 
     In one aspect, an imaging system includes an X-ray radiation source configured to emit radiation that traverses an examination region. The imaging system further includes a controller. The controller is configured to control an X-ray tube peak voltage of the X-ray radiation source to switch between at least two different X-ray tube peak voltages during a kVp switched spectral scan. The controller is further configured to control a grid voltage of the X-ray radiation source to follow the X-ray tube peak voltage during the spectral scan. The controller adjusts the grid voltage based on a predetermined mapping between a currently applied X-ray tube peak voltage and a corresponding grid voltage for a given focal spot size, thereby maintaining the given focal spot size throughout the spectral scan. 
     In another aspect, a method includes controlling an X-ray tube peak voltage of an X-ray radiation source to switch between at least two different X-ray tube peak voltages during a kVp switched spectral scan, and controlling a grid voltage of the X-ray radiation source to follow the X-ray tube peak voltage during the spectral scan based on a predetermined mapping between X-ray tube peak voltages and grid voltages for a given focal spot size to maintain the given focal spot size throughout the spectral scan. 
     In another aspect, a computer-readable storage medium stores instructions that when executed by a processor of a computer cause the processor to: control an X-ray tube peak voltage of an X-ray radiation source to switch between at least two different X-ray tube peak voltages during a kVp switched spectral scan, and control a grid voltage of the X-ray radiation source to follow the X-ray tube peak voltage during the spectral scan based on a predetermined mapping between X-ray tube peak voltages and grid voltages for a given focal spot size to maintain the given focal spot size throughout the spectral scan. 
     Those skilled in the art will recognize still other aspects of the present application upon reading and understanding the attached description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the embodiments and are not to be construed as limiting the invention. 
         FIG. 1  diagrammatically depicts certain elements of an example X-ray tube. 
         FIG. 2  diagrammatically illustrates an example imaging system configured to maintain a given focal spot size of interest during a kVp switched spectral (multi-energy) scan, in accordance with an embodiment(s) described herein. 
         FIG. 3  shows an example plot of kVp as a function of time for a dual energy scan using a grid voltage that changes with kVp along with a graphic representing focal spot size, in accordance with an embodiment(s) described herein. 
         FIG. 4  shows a mapping of grid voltage to kVp for a given focal spot size, in accordance with an embodiment(s) described herein. 
         FIG. 5  shows a simulated plot of kVp as a function of time for dual energy scans using only a single constant grid voltages (only for a lower kVp) along with a graphic representing focal spot size. 
         FIG. 6  shows a simulated plot of kVp as a function of time for dual energy scans using only a single constant grid voltages (only for a higher kVp) along with a graphic representing focal spot size. 
         FIG. 7  shows a simulated plot of kVp as a function of time for dual energy scans using two discrete grid voltages (one for a lower kVp and one for a higher kVp) along with a graphic representing focal spot size. 
         FIG. 8  illustrates an example method for kVp switching. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The following describes an example imaging system configured to control the X-ray tube grid voltage in coordination with the kVp during a kVp switched based spectral (multi-energy) scan to maintain a given focal spot size (within tolerance) during the entire spectral (multi-energy) scan. 
       FIG. 2  diagrammatically illustrates an imaging system  202 , such as a computed tomography (CT) scanner. The imaging system  202  includes a generally stationary gantry  204  and a rotating gantry  206 , which is rotatably supported by the stationary gantry  204  and rotates around an examination region  208  about a z-axis “Z”. 
     A radiation source  210 , such as an X-ray tube, is rotatably supported by the rotating gantry  206 , rotates with the rotating gantry  206  around the examination region  208 , and emits X-ray radiation that traverses the examination region  208 . The radiation source  210  includes a cathode  212  with a focusing cup  214  and at least one a filament  216  and an anode  218  and is configured for kVp switching during a spectral scan, e.g., between data acquisition periods, within a data acquisition period, etc. 
     A controller (“CTRL”)  220  controls the radiation source  210 , including at least a kVp applied across the cathode  212  and the anode  218  and a grid voltage applied across electrodes of the focusing cup  214 . In the illustrated embodiment, the controller  220  includes a mapping that maps kVps to grid voltages for each available focal spot size. As described in greater detail below, for kVp switching, the controller  220 , for each kVp for a scan, including transitions between kVps, applies the corresponding grid voltage from the mapping so that the grid voltage follow the X-ray tube peak voltage during the spectral scan based on the mapping. In one instance, switching the grid voltage in coordination with switching the kVp ensures that the given focal spot size for the scan is maintained (within tolerance) for the entire kVp switched spectral (multi-energy) scan. As such, the approach described herein mitigates degradation of image quality (artifact into and/or a decreased spatial resolution) and/or anode damage, relative to a configuration in which the grid voltage is not controlled as such. 
     A detector  222  includes a one- or two-dimensional array of rows of elements, each row extending in an x-y plane, and each row arranged with respect to each other along the z-axis, and each row including one or more detection layers. The detector  222  is rotatably supported by the rotating gantry  206  along an angular arc opposite the radiation source  210  across the examination region  208 . The detector  222  rotates in coordination with the radiation source  210 , detects radiation that traverses the examination region  208 , and generates a different set of projection data (line integrals) for each kVp. 
     A reconstructor  224  includes a projection domain (PD) decomposer  226  configured to decompose the projection data into different contributions such as photo-electric effect and Compton scattering and/or other bases. The reconstructor  224  reconstructs the decomposed projection data to generate spectral and non-spectral volumetric image data. Examples of spectral volumetric image data include low and high energy, mono-energetic, virtual non-contrast, effective Z (atomic number), iodine only, etc. In one instance, the reconstructor  224  is implemented with a processor (e.g., a central processing unit (CPU), a microprocessor (μCPU), etc.) configured to execute computer executable instructions stored, embedded, encoded, etc. on computer readable storage medium (which excludes transitory medium), such as physical memory and/or other non-transitory memory. The reconstructor  224  is part of the system  202  as shown and/or remote therefrom. 
     An operator console  228  includes an output device  230  such as a display monitor, a filmer, etc. and an input device  232  such as a keyboard, mouse, etc. The console  228  further includes a processor  234  (e.g., a CPU, μCPU, GPU, etc.) and computer readable storage medium  236  (which excludes transitory medium) such as physical memory. In this example, the computer readable storage medium  236  also includes an image domain (IM) decomposition module  238  for performing the decomposition in the image domain instead of the projection domain (PD). In a variation, the system  202  includes only one of the PD decomposer  226  or the IM decomposition module  238 . The operator console  228  allows an operator to control an operation of the system  202  such as selecting a kVp switching imaging protocol, etc. 
     A subject support  240 , such as a couch, supports an object or subject in the examination region  208 . The subject support  240  is movable in coordination with performing an imaging procedure so as to guide the subject or object with respect to the examination region  208  for loading, scanning, and/or unloading the subject or object. 
     As briefly described above, the controller  220  controls at least the kVp and the grid voltage during a kVp switched spectral scan to maintain a given focal spot size for the entire scan. The following describes an example for a dual-energy kVp switched spectral scan. 
       FIG. 3  shows a plot  302  of kVp as a function of time. A first axis  304  represents kVp. A second axis  306  represents time. In this example, a first kVp  308  is applied during a first time duration  310 . At a first time  312 , the controller  220  switches the kVp from the first kVp  308  to a second kVp  314 . During a second time duration  316 , the kVp transitions from the first kVp  308  to the second kVp  314  takes place. During a third time duration  318 , the second kVp  314  continues to be applied. At a second time  320 , the controller  220  switches the kVp from the second kVp  314  to the first kVp  308 . During a fourth time duration  322 , the kVp transitions to the first kVp  308  takes place. This pattern repeats for the dual energy scan. 
     In this example, the controller  220  identifies and applies the grid voltage corresponding to the first kVp  308  during the first time duration  310 , each grid voltage corresponding to each kVp transition during the second time duration  316 , the grid voltage corresponding to the second kVp  314  during the third time duration  318 , and each grid voltage corresponding to each kVp transition during the fourth time duration  322 . The controller  220  identifies the grid voltages from the mapping, which can be stored in the controller  220  (and/or the console  228 ). Also shown in  FIG. 3 , is a graphic  324 , for each time duration, with a size corresponding to the focal spot size. As shown, a size of the graphic  324  is the same (within a tolerance) across the time durations. 
     In one instance, where the first kVp  308  is 80 kVp and the second kVp is 140 kVp, the transitions between the first kVp  308  and the second kVp  314  (i.e. the second time duration  316 ) is on an order of fifty microseconds (50 μs) to one hundred microseconds (100 μs). Depending on the actual emission current, the transitions between the second kVp  314  and the first kVp  308  (i.e. the fourth time duration  322 ) is on an order of one hundred and fifty microseconds (150 μs) to three hundred microseconds (300 μs). 
     An example integration period (IP) may be on the order of one hundred and fifty microseconds (150 μs) to two hundred and fifty microseconds (250 μs). In general, an IP refers to a period of time when the detector  222  detects radiation while rotating through a predetermined angular increment for a measurement. For each IP, each detector element produces a line integral, a set of line integrals for an IP/angular increment is a view, and the projection data includes a set of views acquired over at least 180° plus a fan angle for each of the different energy spectrums. 
     In one instance, the mapping between kVps and grid voltages for each focal spot size is determined during manufacture. For this, in one instance, a scan is performed at a given kVp and the focal spot size is measured for different grid voltages. This is repeated for a set of kVps. A mapping between the kVp and the grid voltages for a particular focal spot size can be determined from this data. This can be repeated for other focal spot sizes.  FIG. 4  shows an example plot of a set of measured points  402  that maps grid voltages (first axis  404 ) to kVps (second axis  406 ) for a given focal spot size. 
     Interpolation and/or other technique can be used to estimate grids voltage for kVps between the measured points  402  to generate a curve  408 . The curve  408  can be stored as the LUT, polynomial, and/or otherwise in the controller  220  and/or elsewhere, e.g., in the memory  236  of the console  228 . Continuing with the above example, where the first kVp  308  is 80 kVp and the second kVp is 140 kVp, the grid voltages, in one instance, may range from a few hundred volts (400-800 V) to a few thousand voltages (1000 V-2000 V). 
     The adjustment of the grid voltage can be achieved with standard and/or specialized electronics without introducing a noticeable delay between kVp values and grid values. In one instance, the bandwidth of the grid voltage control is below ten megahertz (10 MHz). The focal spot size will be constant because the correct grid voltage is applied for each kVp value. A rapid change of grid voltage does not occur at any time point. Risk of uncertainties in the spectrum due to unknown emission currents arising due to random delays in the switching of the grid voltages is suppressed. In the illustrated embodiment, the kVp is switched electrostatically. In a variation, the kVp is switched electromagnetically. 
     For comparative purposes,  FIG. 5  shows a simulated plot  502  of kVp  504  as a function of time  506  for a dual energy scan using only a single constant grid voltages (only for the lower kVp) along with a graphic  508  representing focal spot size.  FIG. 5  shows the focal spot size increases (relative to the desired focal spot size) in a region  510  outside of regions  512  corresponding to the lower kVp.  FIG. 6  shows a simulated plot  602  of kVp  604  as a function of time  606  for a dual energy scan using only a single constant grid voltages (only for the higher kVp) along with a graphic  608  representing focal spot size.  FIG. 6  shows the focal spot size decreases (relative to the desired focal spot size) in regions  610  outside of region  612  corresponding to the higher kVp. As discussed herein, these situations may lead to image artifact, reduce spatial resolution and/or anode damage. 
       FIG. 7  shows another simulated plot  702  of kVp  704  as a function of time  706  for a dual energy scan using only two discrete grid voltages, one for the lower kVp and one for the higher kVp, along with a graphic  708  representing focal spot size.  FIG. 7  shows the focal spot size changes (relative to the desired focal spot size) in regions  710 ,  712 ,  714  and  716  outside of regions  718  corresponding to the lower and higher kVps. In this example, the grid voltage is switched at  720  and  722  during the transitions between kVps. 
     As a consequence, the focal spot size is too large in the region  710  as the kVp increases from the lower kVp voltage towards the higher kVp and the grid voltage for the lower kVp is maintained. In addition, the focal spot size is too small in the region  712  as the kVp continues to increase towards the higher kVp and the grid voltage is now maintained for the higher kVp. In addition, the focal spot size is too small in the region  714  as the kVp now decreases from the higher kVp voltage towards the lower kVp and the grid voltage for the higher kVp is maintained. In addition, the focal spot size is too large in the region  716  as the kVp continues to decrease towards the lower kVp and the grid voltage is now maintained for the lower kVp. 
     The emission current decreases with an increasing grid voltages and increases with an increasing kVp. The transition from higher to lower kVp is mainly driven by the discharge of the electrodes by the emission current. Changes of the emission current during the transition will changes the slope of the kVp as this transition as shown by the change in slope at  724 . Here, the switching of the grid voltage from high to low increases the emission current by this the velocity of the discharge of the electrodes. 
       FIG. 8  illustrates an example method in accordance with an embodiment(s) described herein and/or otherwise. 
     At  802 , the controller  220  applies a first kVp across the cathode  212  and the anode  218  and a corresponding first grid voltage across electrodes of the focusing cup as described herein and/or otherwise. 
     At  804 , the controller  220  transitions the kVp to a second kVp and concurrently transitions the first grid voltage to a corresponding second grid voltage based on a mapping between kVps and grid voltages, as described herein. 
     At  806 , the controller  220  applies the second kVp across the cathode  212  and the anode  218  and the corresponding second grid voltage across electrodes of the focusing cup, as described herein and/or otherwise. 
     At  808 , the controller  220  transitions the kVp back to the first kVp and concurrently transitions second grid voltage back to the first grid voltage based on a mapping between kVps and grid voltages, as described herein and/or otherwise. 
     One or more of the above acts is performed until the scan is complete. Once complete, the kVp and grid voltage are removed. 
     The above may be implemented by way of computer readable instructions, encoded or embedded on computer readable storage medium (which excludes transitory medium), which, when executed by a computer processor(s) (e.g., central processing unit (CPU), microprocessor, etc.), cause the processor(s) to carry out acts described herein. Additionally, or alternatively, at least one of the computer readable instructions is carried by a signal, carrier wave or other transitory medium, which is not computer readable storage medium. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
     In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
     A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.