C-arm imaging system and method

Abstract

The invention is entitled "C-arm imaging system and method". The invention discloses a system for imaging an object, comprising: an X-ray source operative to transmit X-rays through the object; and a detector to receive X-ray energy of the X-rays after passing through the object and generate corresponding object X-ray intensity. The system further comprises: a controller to measure a detector entrance dose without the object placed in the X-ray beam path and determine a relationship between X-ray tube electrical parameters and the detector entrance dose. The controller further determines a relationship between the X-ray tube electrical parameters, the detector entrance dose and the detector average pixel intensity and obtains a normalized air map as a function of the X-ray tube electrical parameters based on calibration image data. The controller also generates an air map based on the normalized air map, the detector entrance dose and the detector average pixel intensity and reconstructs an image of the object based on the air map and the measured object X-ray intensity.

Description

BACKGROUND

[0001] The subject matter disclosed herein relates to X-ray imaging systems with C-arms, and more particularly to air calibration of X-ray imaging systems in 3D image reconstruction.

[0002] Medical diagnostic imaging systems generate images of objects, such as patients, for example, by exposure to an energy source, such as X-rays passing through, for example, a patient. The generated images can be used for many purposes. Typically, when a physician acquires an X-ray of a patient, it is desirable to acquire several X-rays of one or more portions of the patient's body from multiple different positions and angles, and preferably without the need to frequently reposition the patient. To meet this need, C-arm X-ray systems have been developed. The term C-arm generally refers to an X-ray imaging device having a rigid and / or articulated structural member with an X-ray source and an image detector assembly each located at opposite ends of the structural member, such that the X-ray source and image detector face each other. The structural member is generally "C" shaped, hence the term C-arm. In this way, X-rays emitted from the X-ray source can be projected on the image detector and provide an X-ray image of one or more objects placed between the X-ray source and the image detector.

[0003] C-arm X-ray systems include various electronic and optical components that have gain values and offset values that are only applicable to a certain range of operating conditions. Gain drift of the electronic and optical components introduces undesirable artifacts in the final images produced by the C-arm X-ray system. Therefore, to remove these artifacts, it is necessary to periodically calibrate the C-arm X-ray system. Furthermore, even without drift, calibration is needed to establish the relationship between the X-ray technique used and the image detector output. Moreover, calibration helps to compensate for the non-ideal response of the X-ray tube and X-ray detector. SUMMARY

[0004] According to an embodiment of the present technology, a system for imaging an object is provided. The system includes an x-ray source operative to transmit x-rays through the object and a detector operative to receive x-ray energy of the x-rays after the x-rays pass through the object and generate corresponding object x-ray intensities. The system further includes a controller operative to measure a detector entrance dose without the object placed in the x-ray beam path and determine a relationship between x-ray tube electrical parameters and the detector entrance dose. The controller is further operative to determine a relationship between the x-ray tube electrical parameters, the detector entrance dose, and detector average pixel intensities and obtain a normalized air map as a function of the x-ray tube electrical parameters based on calibration image data. The controller is also operative to generate an air map based on the normalized air map, the detector entrance dose, and the detector average pixel intensities and reconstruct an image of the object based on the air map and the measured object x-ray intensities.

[0005] According to another embodiment of the present technology, a method for imaging an object is provided. The method includes transmitting x-rays from an x-ray source to the object and acquiring measurement data related to the object. The method also includes measuring a detector entrance dose without the object placed in the x-ray beam path and determining a relationship between x-ray tube electrical parameters and the detector entrance dose. The method further includes determining a relationship between the x-ray tube electrical parameters, the detector entrance dose, and detector average pixel intensities and obtaining a normalized air map as a function of the x-ray tube electrical parameters based on calibration image data. Finally, the method includes generating an air map based on the normalized air map, the detector entrance dose, and the detector average pixel intensities and reconstructing an image of the object based on the air map and the measurement data related to the object. BRIEF DESCRIPTION OF DRAWINGS

[0006] These and other features, aspects, and advantages of the present application will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0007] Figure 1 is a schematic diagram of a radiological imaging system according to aspects of the present method;

[0008] Figure 2 is a flowchart depicting a method for imaging an object according to aspects of the present method;

[0009] Figure 3 is a graphical plot depicting a relationship between x-ray tube electrical parameters and detector entrance dose according to aspects of the present method;

[0010] Figure 4 is a graphical plot depicting a relationship between x-ray tube electrical parameters and detector entrance dose normalized average pixel values according to an embodiment of the present technology;

[0011] Figure 5 is a graphical plot depicting a normalized air map according to an embodiment of the present technology; and

[0012] Figure 6 is a schematic of a comparison of a reconstructed image to a true image according to an embodiment of the present technology. DETAILED DESCRIPTION

[0013] One or more specific embodiments will be described below. To provide a context for the various embodiments, Figure 1 shows, in simplified form, a diagram of a system 100 that can be used in accordance with an embodiment of the present technology. In general, system 100 is representative of a system that can be used in accordance with an embodiment of the present technology. As shown in Figure 1, system 100 includes a processor 102, a memory 104, and a communication interface 106. Processor 102 can be any suitable processor, such as a central processing unit (CPU), a graphics processing unit (GPU), a digital signal processor (DSP), a field-programmable gate array (FPGA), a complex programmable logic device (CPLD), a microprocessor, a microcontroller, a microcontroller unit (MCU), a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a programmable logic device (PLD), a programmable logic array (PLA), a programmable array logic (PAL), a generic array logic (GAL), a high-speed

[0014] When introducing elements of various embodiments of the present embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there can be additional elements other than the listed elements. Additionally, any numerical examples in the following discussion are intended to be non-limiting, and thus additional numerical values, ranges, and percentages are within the scope of the disclosed embodiments. Furthermore, the terms "circuit" and "circuitry" and "controller" can include a single component or multiple components that are either active and / or passive and are connected or otherwise coupled together to provide the described functionality.

[0015] Figure 1An exemplary radiological imaging system 200 used, for example, in an interventional medical procedure is shown. In one embodiment, the system 200 can include a C-arm radiographic system 102 configured to acquire projection data from one or more viewing angles around a subject, such as a patient 104 positioned on a table 105, for further analysis and / or display. To this end, the C-arm radiographic system 102 can include a gantry 106 having a movable support, such as a movable C-arm 107, including at least one radiation source 110, such as an X-ray tube, and a detector 108 at opposite ends of the C-arm 107. In exemplary embodiments, the radiographic system 102 can be an X-ray system, a positron emission tomography (PET) system, a computed tomography (CT) system, an angiographic or fluoroscopic system, or the like, or combinations thereof, which can be operable to generate static images acquired prior to a medical procedure by a static imaging detector (e.g., a CT system, an MRI system, or the like), or real-time images acquired during a medical procedure using a real-time imaging detector (e.g., an angioplasty system, a laparoscopic system, an endoscopic system, or the like), or combinations thereof. Thus, the type of acquired images can be diagnostic or interventional.

[0016] In certain embodiments, the radiation source 110 can include a plurality of emission devices, such as one or more independently addressable solid state emitters arranged in a one- or multi-dimensional field emitter array configured to emit an X-ray beam 112 toward the detector 108. Further, the detector 108 can include a plurality of detector elements, which can be similar or different in size and / or energy sensitivity, for imaging a region of interest (ROI) of the patient 104 at a desired resolution. In one embodiment, a dosimeter 114 is provided in proximity to the detector 108 to measure the X-ray dose per frame at the entrance of the detector 108.

[0017] In certain embodiments, the C-arm 107 can be configured to move along a desired scan path for orienting the X-ray source 110 and the detector 108 at different positions and angles around the patient 104 to acquire information for 3D imaging of dynamic processes. Thus, in one embodiment, the C-arm 107 can be configured to rotate around a first rotational axis. Additionally, the C-arm 107 can also be configured to rotate around a second axis with angular motion in a range of about plus or minus 60 degrees relative to a reference position. In certain embodiments, the C-arm 107 can also be configured to move forward and / or backward along the first axis and / or the second axis.

[0018] Accordingly, in one embodiment, the C-arm system 102 can include control circuitry 204 configured to control movement of the C-arm 107 along different axes based on inputted and / or protocol based instructions. To this end, in certain embodiments, the C-arm system 102 can include circuitry configurable to use various input mechanisms to provide signals to the control circuitry 204 for adaptive and / or interactive control of imaging and / or processing parameters, such as the tableside controls 116. For example, the imaging and / or processing parameters can include display characteristics, X-ray technique and frame rate, scan trajectory, and gantry motion and / or position.

[0019] In certain embodiments, the detector 108 can include a plurality of detector elements 202, e.g., arranged as a 2D detector array for sensing the projected X-ray beam 112 that passes through the patient 104. In one embodiment, the detector elements 202 generate electrical signals representative of the intensity of the projected X-ray beam 112, which can in turn be used to estimate the attenuation of the X-ray beam 112 as it passes through the patient 104. In another embodiment, the detector elements 202 determine counts and / or determine corresponding energies of incident photons in the X-ray beam 112.

[0020] In particular, in one embodiment, the detector elements 202 can acquire electrical signals corresponding to the generated X-ray beam 112 at various angular positions around the patient 104 to collect a plurality of radiographic projection views for constructing an X-ray image, such as to form a fluoroscopic image. To this end, the control circuitry 204 of the system 200 can include control mechanisms configured to control the position, orientation, and / or rotation of the gantry 106, the C-arm 107, and / or components mounted thereon in certain specific acquisition trajectories.

[0021] In certain embodiments, the X-ray source 110 and detector 108 for interventional imaging can be controlled using an X-ray controller 207 in the control mechanisms 204, where the X-ray controller 207 is configured to provide power signals and timing signals to the radiation source 110 for controlling X-ray exposure during imaging. In addition, the control mechanisms 204 can also include a gantry motor controller 208, which can be configured to control the rotational speed, tilt, view angle, and / or position of the gantry 106. In certain embodiments, the control mechanisms 204 also include a C-arm controller 210, which can be configured to move the C-arm 107 for real-time imaging of dynamic procedures in cooperation with the gantry motor controller 208.

[0022] In one embodiment, the control mechanism 204 can include a data acquisition system (DAS) 212 for acquiring projection data from the detector elements 206 and processing the data for image reconstruction by a 2D image processor 220 for real-time reconstruction of high-fidelity 2D images for use during an interventional procedure, and / or by a 3D image processor / reconstructor 222 for generating 3D cross-sectional images (or a 3D volume), and then showing the images on a display 218. In addition, in certain embodiments, the data acquired by the DAS 212 can be input to a computing device 214. Alternatively, in certain embodiments, the computing device 214 can store the projection data in a storage device 216, such as a hard drive, a floppy drive, a compact disk - read / write (CD-R / W) drive, a digital versatile disk (DVD) drive, a flash drive, or a solid state storage device for further evaluation.

[0023] In one embodiment, the system 200 can include an operator console 224 that can be configured to allow selection and display of scan modes, FOVs, prior examination data, and / or interventional paths. The operator console 224 can also allow immediate access to 2D and 3D scan parameters and selection of ROIs for subsequent imaging, for example, based on operator commands and / or system commands.

[0024] In addition, in certain embodiments, the system 200 can be coupled to a plurality of displays, printers, workstations, picture archiving and communication systems (PACS) 226, and / or similar devices, for example, located locally or remotely within an institution or hospital, or in a completely different location, via communication links in one or more configurable wired and / or wireless networks, such as a hospital network and a virtual private network.

[0025] In operation, during a 3D scan of a subject (e.g., a patient), an X-ray detector measures image data after an X-ray passes through the subject to generate an actual subject X-ray intensity I t . The 3D image processor / reconstructor 222 utilizes this actual subject X-ray intensity I t to generate a 3D image of the subject. This actual subject intensity I t is related by the Beer-Lambert law to the unattenuated X-ray intensity I0 along the path from the X-ray source to the X-ray detector pixel:

[0026] I t = I0.e -μt Equation 1

[0027] where t is the thickness of the subject and μ is the attenuation coefficient of the subject.

[0028] According to embodiments of the present technology, the C-arm radiography system 102 is performed an air calibration by measuring the X-ray intensity, i.e., the unattenuated X-ray intensity I0, with no subject positioned in the path of the X-ray beam 112. The air calibration compensates for X-ray field inhomogeneities, X-ray detector pixel gain (including analog-to-digital (A / D) converter gain inhomogeneities), and tube-detector alignment variations from view-to-view. The result of the calibration is a series of two-dimensional maps, referred to as air maps, that represent the unattenuated X-ray intensity I0. In general, the air calibration is designed to obtain the unattenuated intensity I0 to normalize the scan data to the unobstructed beam intensity, which is used to determine the amount of attenuation caused by the subject in the beam path. Based on the determined unattenuated intensity I0 and the measured actual subject X-ray intensity I t The X-ray attenuation caused by the subject in the beam can be calculated. The X-ray attenuation of the subject is further used to generate or reconstruct a 3D image of the subject.

[0029] Figure 2 A flowchart 400 according to embodiments of the present technology is shown that depicts an exemplary method for imaging a subject. In one embodiment, the method of the flowchart 400 can be implemented in the computing device 214 of the C-arm system 102. Embodiments of the exemplary method can include computer-executable instructions on a computing system or processor. Generally, computer-executable instructions can include routines, programs, objects, components, data structures, procedures, modules, functions, and the like that perform particular functions or implement particular abstract data types. Embodiments of the exemplary method including computer-executable instructions can also be practiced in distributed computing environments where functions are performed by remote processing devices that are linked through a wired and / or wireless communication network. In a distributed computing environment, computer-executable instructions can be located in both local and remote computer storage media, including memory storage devices. Figure 1

[0030] Embodiments of the present method describe techniques for enhanced imaging of high quality 3D cross-sectional images using the C-arm system 102. To this end, at step 402, a detector entrance dose is measured with no subject placed in the path of the X-ray beam 112. In one embodiment, the detector entrance dose is measured by the dosimeter 114 placed at the entrance of the detector 108. In one embodiment, the dosimeter measures the detector entrance dose in micro-Gray per frame (pGy / frame).

[0031] ​At step 404, a relationship between the X-ray tube electrical parameters and the detector entrance dose is determined based on the measured dose data. The X-ray tube electrical parameters include electrical parameters applied to the X-ray tube, such as tube voltage, tube current, or a combination thereof. As will be appreciated by those skilled in the art, the unit for tube voltage is kilovoltage peak (kVp), and the unit for tube current is milliampere (mA). In general, the tube voltage controls the energy and the quality of the X-ray beam generated by the X-ray tube, while the tube current controls the quantity of the X-ray beam.

[0032] Figure 3 A graphical plot 500 showing an exemplary relationship between the X-ray tube electrical parameters and the detector entrance dose is shown. The horizontal axis 502 of the plot 500 represents the tube voltage in kVp and a given tube current in mA, while the vertical axis 504 represents the detector entrance dose in μGy / frame. In general, as the tube voltage of the X-ray tube increases, the detector entrance dose at the entrance of the detector entrance is measured by a dosimeter at the surface of the detector.

[0033] It is noted that in one embodiment, the detector entrance dose can be measured while both the tube voltage and the tube current are changed. In this case, the detector entrance dose becomes a function of both the tube voltage and the tube current. The tube voltage and the tube current can be changed according to a 3D automatic brightness system (ABS) table as a predefined pair. The 3D ABS is used to keep the brightness of the displayed image at a constant level during the X-ray examination. In one embodiment, the 3D ABS also adjusts the digital gain to adjust the brightness of the image. The kV and mA can be adjusted as a pair depending on the patient being examined and the part of the patient's anatomy. It is noted that the tube current mA needs to be adjusted so that the pixel value is near the middle of the dynamic range of the detector to avoid the non-linearity near the unsaturation. In this case, the horizontal axis 502 can represent the tube voltage and tube current (kVp / mA) pair, instead of the tube voltage (kVp) fixed for a given tube current (mA).

[0034] In one embodiment, a linear interpolation of the measured points of the plot 500 in Figure 3 is performed to find the relationship between the X-ray tube electrical parameters and the detector entrance dose. For example, let kVp be the actual kVp in the projection, and let kVp(n) and kVp(n+1) be two consecutive kVp values obtained during the air calibration, i.e., n is the index. Also let D(n) and D(n+1) be two detector entrance dose values in μGy / frame corresponding to kVp(n) and kVp(n+1), respectively, obtained from the air calibration. Then, the detector entrance dose D in μGy / frame corresponding to kVp in the projection is given by the following linear interpolation:

[0035]

[0036] Returning to Figure 2 At step 406, the relationship between the X-ray tube electrical parameter, the detector entrance dose, and the detector average pixel intensity as measured by the detector 108 when no object is placed on the X-ray beam path is determined. The average pixel intensity is computed by averaging the pixel values of all detector elements. In one embodiment, the average pixel value is determined from a central region of interest (e.g., 512 x 512 pixels, specifically, in a 1536 x 1536 pixel image) rather than using all pixels from the detector because the X-ray beam is collimated in the corners (producing a "square-round"). For a given X-ray spectrum, the average pixel intensity of an X-ray detector is known to be linearly proportional to the detector entrance dose as measured by the dosimeter 114. In other words, for any given X-ray tube electrical parameter, such as tube voltage kVp, the detector pixel intensity is linearly proportional to the detector entrance dose. Therefore, for a given tube voltage kVp, the relationship between tube voltage kVp and average detector pixel intensity can be established by dividing the average detector pixel intensity by the detector entrance dose and the ABS digital gain when the average detector pixel intensity is measured.

[0037] Figure 4 A graphical plot 600 showing an exemplary relationship between the X-ray tube electrical parameter and the average pixel value normalized by the detector entrance dose is shown. The horizontal axis 602 of the plot 600 represents the tube voltage in kVp, while the vertical axis 604 represents the average pixel value with units of dose in pixel counts. The average pixel value with units of dose is determined by dividing the average pixel value by the detector entrance dose and the ABS digital gain when the average detector pixel intensity is measured.

[0038] In one embodiment, the plot 600 of the X-ray tube electrical parameter versus the average pixel value with units of dose and units of gain is represented by a mathematical equation. For example, as previously described, let kVp be the actual kVp in the projection, and let kVp(n) and kVp(n+1) be two consecutive kVp values obtained during the air calibration. Also let P(n) and P(n+1) be the two detector average pixel values corresponding to kVp(n) and kVp(n+1) obtained from the air calibration, respectively. Then the detector average pixel value P for the projection corresponding to kVp in the projection is given by the following linear interpolation:

[0039]

[0040] Returning to Figure 2At step 408, a normalized air map based on the calibration image data is obtained as a function of the X-ray tube electrical parameters. The air map is essentially an X-ray intensity map proportional to the unattenuated X-ray intensity I0. In general, based on the detector entrance dose D (from equation 2) and the average detector pixel value P (from equation 3), any triad {kVp, mA, K} defined by the 3D ABS table can be obtained, where kVp is the X-ray tube voltage, mA is the X-ray tube current, and K is the ABS digital gain. However, to include pixel-to-pixel variations due to detector pixel gain (including A / D gain), detector scintillator inhomogeneity, and X-ray field inhomogeneity, a two-dimensional air map or X-ray intensity map for the X-ray detector also needs to be obtained.

[0041] Figure 5 A graphical plot 700 of the normalized air map versus average pixel value is shown at an X-ray tube voltage equal to 80 kVp. The horizontal axis 702 and the vertical axis 704 of the plot 700 represent the coordinates of the image pixels in terms of pixel position, respectively. To obtain the normalized air map, for a given tube voltage kVp, the tube current mA is adjusted so that the detector output pixel value is near the middle of the detector measurement range when no object is placed in the X-ray beam path. A series of calibration images (i.e., calibration image data) are acquired at the same operating conditions. The resulting images are first averaged to generate one air image with reduced image noise. As will be appreciated by those skilled in the art, image averaging works on the assumption that the noise in the images is truly random. In this way, as more and more images are averaged, the random fluctuations in the image data are gradually smoothed out. Then, the average pixel value of the obtained air image is calculated. Finally, the normalized air map is obtained by dividing the average pixel value by the air image.

[0042] In one embodiment, the normalized air map can be expressed in terms of a mathematical equation. For example, as previously mentioned, let kVp be the actual kVp in the projection, and let kVp(n) and kVp(n+1) be two consecutive kVp values obtained during the air calibration. Also assume and are the two normalized air maps corresponding to kVp(n) and kVp(n+1) obtained from the air calibration, respectively. Then, the normalized air map corresponding to the kVp of the projection is given by the following linear interpolation

[0043]

[0044] Returning to Figure 2 At step 410, based on the detector entrance dose D (from equation 2), the average detector pixel value P (from equation 3), and the normalized air map, the X-ray tube voltage kVp is calculated as (from Equation 4), the air map is generated using the following equation:

[0045]

[0046] Where I0(i, j) is the air map and (i, j) is the coordinate of the image pixel. Finally, at step 412, based on the air map and the measured object X-ray intensity I t (i, j) reconstructs the 3D image of the subject. Compared with the actual image data I t (i, j), the air map I0(i, j) is used together with the air map I0(i, j) to determine the x-ray attenuation μt(i, j):

[0047]

[0048] The x-ray attenuation μt(i,j) is then further used by the 3D image processor / reconstructor 222 to generate a reconstructed 3D image, as will be understood by those skilled in the art.

[0049] Figure 6 Schematic diagram 800 showing a comparison of a reconstructed image and a real image according to an embodiment of the present technology. Generally speaking, image 802 represents a reconstructed image and image 804 represents a real image, both having an offset of +1000. In other words, both the reconstructed image and the real image are expressed in "offset" Hounsfield units (sHU). As can be seen, the reconstructed image 802 is nearly identical to the real image 804. For example, the real image shows a CT number of 1550 for bone, 0 for air, and 1000 for water, while the reconstructed image shows CT numbers of 1471, 19, and 1022 for these same elements, respectively.

[0050] It is noted that the foregoing examples, demonstrations, and process steps that may be performed by certain components of the present system, for example, by the control mechanism 207, the DAS 212, the computing device 214, the processor 220, and / or the image reconstructor 222, may be implemented by suitable code on a processor-based system (such as a general-purpose or special-purpose computer). It is also noted that different embodiments of the present technology may perform some or all of the steps described herein in a different order or substantially simultaneously (i.e., in parallel).

[0051] Additionally, these functions can be implemented in various programming languages, including, but not limited to, Ruby, Hypertext Preprocessor (PHP), Perl, Delphi, Python, C, C++, or Java. Such code can be stored or adapted to be stored on one or more tangible machine-readable media, such as data storage chips, local or remote hard disks, optical disk (i.e., CD or DVD), solid state drives, or other media that can be accessed by a processor-based system to execute the stored code.

[0052] This written description uses examples to disclose the application, including the best mode, and also to enable any person skilled in the art to practice the application, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the application is defined by the claims, and can include other examples that occur to those skilled in the art. Such other examples are intended to fall within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

1. A system for imaging an object, comprising: an X-ray source operative to transmit X-rays through the object; a detector operative to receive X-ray energy of the X-ray after the X-ray passes through the object and generate a corresponding object X-ray intensity; and A controller operative to: Detector entrance dose is measured without an object placed in the X-ray beam path; determining a relationship between an X-ray tube electrical parameter and an entrance dose to the detector; determining a relationship between the X-ray tube electrical parameters, the detector entrance dose, and the detector average pixel intensity without an object placed in the X-ray beam path; obtaining a normalized air map as a function of electrical parameters of the X-ray tube based on calibration image data, wherein the calibration image data is a series of calibration images acquired under identical operating conditions without an object placed in a path of the X-ray beam; generating an air map based on the normalized air map, the detector entrance dose, and the detector average pixel intensity; and An image of the object is reconstructed based on the air map and the measured object X-ray intensities. 2 . The system of claim 1 , wherein the X-ray tube electrical parameters include tube voltage, tube current, or a combination thereof. 3 . The system of claim 1 , wherein the detector entrance dose comprises a plurality of detector entrance dose values ​​corresponding to a plurality of X-ray tube electrical parameter values.

4. The system of claim 3, wherein the controller is operative to determine the relationship between the X-ray tube electrical parameter and the detector entrance dose based on linear interpolation of the plurality of detector entrance dose values.

5. The system of claim 1 , wherein the detector average pixel intensity is divided by the detector entrance dose and a digital gain to determine the relationship between the X-ray tube electrical parameters, the detector entrance dose, and the detector average pixel intensity.

6. The system of claim 5, wherein the controller is operative to determine the relationship between the X-ray tube electrical parameter and the detector average pixel intensity based on a linear interpolation of a plurality of average pixel intensity values ​​divided by the detector entrance dose and the digital gain.

7. A method for imaging an object, comprising: transmitting X-rays from an X-ray source to the object; acquiring measurement data associated with the object; Detector entrance dose is measured without an object placed in the X-ray beam path; determining a relationship between an X-ray tube electrical parameter and an entrance dose to the detector; determining the relationship between the X-ray tube electrical parameters, the detector entrance dose, and the detector average pixel intensity without an object placed in the X-ray beam path; obtaining a normalized air map as a function of electrical parameters of the X-ray tube based on calibration image data, wherein the calibration image data is a series of calibration images acquired under identical operating conditions without an object placed in a path of the X-ray beam; generating an air map based on the normalized air map, the detector entrance dose, and the detector average pixel intensity; and An image of the object is reconstructed based on the air map and the measurement data related to the object. 8 . The method according to claim 7 , wherein the X-ray tube electrical parameters include tube voltage, tube current, or a combination thereof.

9. The method of claim 7, wherein the detector entrance dose comprises a plurality of detector entrance dose values ​​corresponding to a plurality of X-ray tube electrical parameter values.

10. The method of claim 7, wherein the detector average pixel intensity is divided by the detector entrance dose and a digital gain to determine the relationship between the X-ray tube electrical parameters, the detector entrance dose, and the detector average pixel intensity.

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