Patent Publication Number: US-2019192091-A1

Title: Method and apparatus for performing computed tomography (ct) imaging by injecting contrast medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2017-0181522, filed on Dec. 27, 2017, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     The disclosure relates to methods and apparatuses for performing CT imaging by injecting a contrast medium into an object, and more particularly, to methods and apparatuses for initiating a CT scan by tracking an optimal time point of contrast enhancement based on acquired projection images of an object. 
     2. Description of Related Art 
     Medical imaging apparatuses are used to acquire images showing an internal structure of an object. The medical imaging apparatuses are non-invasive examination apparatuses that capture and process images of details of structures, tissue, fluid flow, etc., inside a body and provide the images to a user. A user, e.g., a medical practitioner, may use medical images output from the medical imaging apparatuses to diagnose the patient&#39;s condition and diseases. 
     A CT apparatus is an example of an apparatus for imaging an object by emitting X-rays toward a patient. The CT apparatus is capable of providing a cross-sectional image of an object and may represent an internal structure (e.g., organs such as a kidney, a lung, etc.) of the object without superimposition of adjacent structures, as compared to a typical X-ray apparatus. Due to these advantages, the CT apparatus is widely used for diagnostic purposes. 
     CT angiography is used to obtain a CT image of a blood vessel such as a carotid artery, a pulmonary artery, etc., via a CT apparatus by injecting a contrast medium into the blood vessel. In CT imaging using injection of a contrast medium, a bolus tracking technique is used to more clearly visualize a region of interest (ROI), i.e., a blood vessel, by tracking a time point at which a contrast effect due to the contrast medium is maximized. A related art bolus tracking method involves acquiring a pre-scan image by reconstructing an X-ray projection image generated by emitting X-rays toward an object into which a contrast medium is injected and determining a time point for initiation of a diagnostic CT scan by measuring a Hounsfield Unit (HU) value in an ROI that is set in the acquired pre-scan image. In detail, the bolus tracking method is achieved by monitoring whether an HU value in the ROI is greater than a preset threshold and determining the start of a diagnostic CT scan at a time point when the HU value is greater than the preset threshold. 
     However, according to the bolus tracking method, a user suffers the inconvenience of having an ROI individually set in a pre-scan image, and when the ROI is set in a wrong region other than a region to be observed in the pre-scan image, a CT scan has to be performed again. Furthermore, it takes a long time to acquire a pre-scan image by reconstructing an X-ray projection image, and the user may miss a time point at which an attenuation coefficient for a contrast medium reaches its maximum value during reconstruction of the pre-scan image. In addition, since an X-ray source emits X-rays while rotating on a rotating frame through 360 degrees to acquire a pre-scan image, the amount of radiation to which a patient is exposed may be excessive. 
     SUMMARY 
     Provided are CT methods and apparatuses for determining an optimal time point for initiating a diagnostic CT scan by using pixel values in at least one X-ray projection image respectively obtained before and after injection of a contrast medium into an object. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
     In accordance with an aspect of the disclosure, a method of performing CT imaging by injecting a contrast medium into an object includes: setting an ROI in a scout image of the object; acquiring at least one X-ray projection image by detecting X-rays that have passed through the ROI; measuring a difference between pixel values of at least one X-ray projection image acquired before injection of a contrast medium into the ROI and at least one X-ray projection image acquired after the injection of the contrast medium into the ROI; and determining whether to initiate a diagnostic CT scan by comparing the measured difference with a preset threshold. 
     The method may further include acquiring the scout image by emitting X-rays toward a predetermined region of the object including the ROI. 
     The acquiring of the at least one X-ray projection image may include: emitting, via an X-ray source, the X-rays toward the ROI at at least one angular position; and acquiring the at least one X-ray projection image by detecting, by using an X-ray detector, the X-rays that are emitted at the at least one angular position and pass through the ROI. 
     The measuring of the difference between the pixel values may include: setting, as a reference value, a sum of pixel values of the at least one X-ray projection image acquired before the injection of the contrast medium into the ROI; calculating a difference between the reference value and a sum of pixel values of the at least one X-ray projection image acquired at each of a plurality of time points following the injection of the contrast medium into the ROI; and determining, among the plurality of time points, a time point when the calculated difference exceeds the preset threshold. The determining of whether to initiate the diagnostic CT scan may include determining initiation of the diagnostic CT scan for examining the object at the determined time point. 
     The setting of the reference value may include setting, as the reference value, a sum of pixel values of an X-ray projection image acquired at a first angular position from among the at least one X-ray projection image acquired before the injection of the contrast medium into the ROI. The calculating of the difference may include adding up, for each of the plurality of time points, pixel values of the X-ray projection image acquired at the first angular position from among the at least one X-ray projection image acquired at the plurality of time points following the injection of the contrast medium into the ROI and calculating a difference between a result of the adding up and the reference value. 
     The acquiring of the at least one X-ray projection image may include detecting the X-rays that have passed through the ROI by using a photon counting detector configured to detect X-rays having a plurality of energy bands. 
     The acquiring of the at least one X-ray projection image may include detecting, by using the photon counting detector, X-rays having an energy corresponding to a first energy band among the plurality of energy bands and acquiring the at least one X-ray projection image by using the detected X-rays. 
     The acquiring of the at least one X-ray projection image may include detecting X-rays having an energy band corresponding to a k-edge that is in an energy band in which an attenuation coefficient of the contrast medium increases rapidly among the plurality of energy bands and acquiring the at least one X-ray projection image by using the detected X-rays. 
     The acquiring of the at least one X-ray projection image may include: emitting X-rays from a plurality of X-ray sources having different tube voltages; and acquiring a plurality of X-ray projection images respectively corresponding to the plurality of energy bands by detecting, via a plurality of X-ray detectors, the X-rays that have passed through the ROI. The measuring of the difference between the pixel values may include measuring a difference between a reference value, which is a sum of pixel values of the at least one X-ray projection image acquired before the injection of the contrast medium into the ROI, and a sum of pixel values of an X-ray projection image acquired, among the plurality of X-ray projection images, by detecting X-rays having an energy corresponding to an energy band in which an attenuation coefficient of the contrast medium undergoes a largest variation. 
     The method may further include separating, from the acquired at least one X-ray projection image, a contrast projection image generated by detecting X-rays that have passed through the contrast medium. The measuring of the difference between the pixel values may include: setting, as a reference value, a sum of pixel values of the at least one X-ray projection image acquired before the injection of the contrast medium into the ROI; and calculating a difference between a sum of pixel values of the contrast projection image and the reference value. 
     The method may further include calculating an average of values of a preset number of adjacent pixels among pixels of the acquired at least one X-ray projection image and replacing the values of the preset number of adjacent pixels with the calculated average. 
     In accordance with an aspect of the disclosure, a CT apparatus includes: an X-ray source configured to emit X-rays toward an ROI of an object at at least one angular position arranged around the object; an X-ray detector configured to detect the X-rays that have passed through the ROI at the at least one angular position; a data acquisition unit configured to acquire at least one X-ray projection image by using the X-rays detected by the X-ray detector; and a processor configured to set the ROI in a scout image of the object, measure a difference between pixel values of at least one X-ray projection image acquired before injection of a contrast medium into the ROI and at least one X-ray projection image acquired after the injection of the contrast medium into the ROI, and determine whether to initiate a diagnostic CT scan by comparing the measured difference with a preset threshold. 
     The X-ray source may be further configured to emit X-rays toward a predetermined region of the object including the ROI, the X-ray detector may be further configured to detect the X-rays that have passed through the predetermined region of the object, and the processor may be further configured to generate the scout image by using the detected X-rays. 
     The processor may be further configured to set, as a reference value, a sum of pixel values of the at least one X-ray projection image acquired before the injection of the contrast medium into the ROI, calculate a difference between the reference value and a sum of pixel values of the at least one X-ray projection image acquired at each of a plurality of time points following the injection of the contrast medium into the ROI, and determine initiation of the diagnostic CT scan for examining the object at a time point when the calculated difference exceeds the preset threshold among the plurality of time points. 
     The processor may be further configured to set, as the reference value, a sum of pixel values of an X-ray projection image acquired at a first angular position from among the at least one X-ray projection image acquired before the injection of the contrast medium into the ROI, add up, for each of the plurality of time points, pixel values of the X-ray projection image acquired at the first angular position from among the at least one X-ray projection image acquired at the plurality of time points following the injection of the contrast medium into the ROI, and calculate a difference between a result of the adding up and the reference value. 
     The X-ray detector may be a photon counting detector (PCD) configured to detect X-rays having a plurality of energy bands. 
     The X-ray detector may be further configured to detect X-rays having an energy corresponding to a first energy band among the plurality of energy bands, and the data acquisition unit may be further configured to acquire the at least one X-ray projection image by using the X-rays detected by the X-ray detector. 
     The X-ray detector may be further configured to detect X-rays having an energy band corresponding to a k-edge that is in an energy band in which an attenuation coefficient of the contrast medium increases rapidly among the plurality of energy bands, and the data acquisition unit may be further configured to acquire the at least one X-ray projection image by using the X-rays detected by the X-ray detector. 
     The X-ray source may include a plurality of X-ray sources configured to emit X-rays at different tube voltages, and the X-ray detector may include a plurality of X-ray detectors configured to detect the X-rays that are respectively emitted by the plurality of X-ray sources and pass through the ROI. The data acquisition unit may be further configured to acquire a plurality of X-ray projection images respectively corresponding to the plurality of energy bands by using the X-rays detected by the plurality of X-ray detectors, and the processor may be further configured to measure a difference between a pixel value for an X-ray projection image, which is acquired, among the plurality of X-ray projection images, by detecting X-rays having an energy corresponding to an energy band in which an attenuation coefficient of the contrast medium undergoes a largest variation, and a pixel value for the at least one X-ray projection image acquired before the injection of the contrast medium into the ROI. 
     The processor may be further configured to separate, from the acquired at least one X-ray projection image, a contrast projection image including only X-rays detected when passing through only the contrast medium, set, as a reference value, a sum of pixel values of the at least one X-ray projection image acquired before the injection of the contrast medium into the ROI, and calculate a difference between a sum of pixel values of the contrast projection image and the reference value. 
     In accordance with an aspect of the disclosure, a computer program product includes a computer-readable recording medium including instructions for performing: setting an ROI in a scout image of the object; acquiring at least one X-ray projection image by detecting X-rays that have passed through the ROI; measuring a difference between pixel values of at least one X-ray projection image acquired before injection of a contrast medium into the ROI and at least one X-ray projection image acquired after the injection of the contrast medium into the ROI; and determining whether to initiate a diagnostic CT scan by comparing the measured difference with a preset threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a conceptual diagram of a method, performed by a CT apparatus, of determining a time point to start a diagnostic CT scan, according to an embodiment; 
         FIG. 2  is a block diagram illustrating components of a CT apparatus according to an embodiment; 
         FIG. 3  is a flowchart of a method, performed by a CT apparatus, of determining a time point to start a diagnostic CT scan is performed, according to an embodiment; 
         FIG. 4  is a flowchart of a method, performed by a CT apparatus, of determining a time point to start a diagnostic CT scan, according to an embodiment; 
         FIG. 5  is a diagram of a method, performed by a CT apparatus, of determining a time point to start a diagnostic CT scan, according to an embodiment; 
         FIG. 6  is a graph of attenuation coefficients of a bone of an object, water, and a contrast medium, with respect to energy; 
         FIG. 7A  is a block diagram showing components of a CT apparatus according to an embodiment; 
         FIG. 7B  is a diagram of a method, performed by the CT apparatus, of performing a CT scan according to an embodiment; 
         FIG. 7C  is a graph of the number of photons contained in X-rays, which are emitted by the CT apparatus, pass through an object, and are detected, with respect to an energy band; 
         FIGS. 8A, 8B, and 8C  are conceptual diagrams of methods, performed by CT apparatus, of obtaining a plurality of X-ray projection images, according to embodiments; 
         FIG. 9  is a flowchart of a method, performed by a CT apparatus, of separating a contrast projection image from an X-ray projection image of an object, according to an embodiment; 
         FIG. 10  illustrates a method, performed by a CT apparatus, of binning pixels in an X-ray projection image such that noise in the X-ray projection image is reduced, according to an embodiment; 
         FIG. 11  is a diagram of a method of setting an ROI in a scout image of an object by using a CT apparatus, according to an embodiment; 
         FIG. 12  is a flowchart of a method, performed by a CT apparatus, of performing a diagnostic CT scan on an object, according to an embodiment; and 
         FIG. 13  illustrates a structure of a CT system according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout the specification, like reference numerals or characters refer to like elements. In the present specification, all elements of embodiments, general matters in the technical field of the disclosure and redundant matters between embodiments are not described. Terms ‘part’ and ‘portion’ used herein may be implemented using software or hardware, and, according to embodiments, a plurality of ‘parts’ or ‘portions’ may be implemented using a single unit or element, or a single ‘part’ or ‘portion’ may be implemented using a plurality of units or elements. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. For example, the expressions “at least one of a, b, and c” and “at least one of a, b, or c” should be understood as including only a, only b, only c, both a and b, both a and c, both b and c, or all of a, b, and c. 
     In the present specification, an image may include a medical image obtained by a medical imaging apparatus, such as a CT apparatus, a magnetic resonance imaging (MRI) apparatus, an ultrasound imaging apparatus, or an X-ray apparatus. 
     Throughout the specification, the term ‘object’ is a thing to be imaged, and may include a human, an animal, or a part of a human or animal. For example, the object may include a part of a body (i.e., an organ), a phantom, or the like. 
     In the present specification, a ‘CT system’ or ‘CT apparatus’ refers to a system or apparatus configured to emit X-rays while rotating around at least one axis relative to an object and image the object by detecting the X-rays. 
     In the specification, a ‘CT image’ refers to an image constructed from raw data obtained by imaging an object by detecting X-rays that are emitted as the CT system or apparatus rotates about at least one axis with respect to the object. 
       FIG. 1  is a conceptual diagram for explaining a method, performed by a CT apparatus  200 , of determining a time point when a diagnostic CT scan is performed by injecting a contrast medium into an object  1 , according to an embodiment.  FIG. 2  is a block diagram illustrating components of a CT apparatus  200  according to an embodiment. 
     Referring to  FIGS. 1 and 2 , the CT apparatus  200  sets an ROI  10 R in a scout image  10 , acquires at least one X-ray projection image of the set ROI  10 R, and measures a difference between pixel values of at least one X-ray projection image respectively obtained before and after injection of a contrast medium into the object  1  to determine whether to initiate a diagnostic CT scan based on the measured difference. 
     In detail, first, the CT apparatus  200  may capture the scout image  10  of the object  1  and set the ROI  10 R in the captured scout image  10 . The scout image  10  may be a preliminary image obtained by emitting X-rays toward a region to be scanned, i.e., a predetermined region of the object  1  including the ROI  10 R, and detecting X-rays that have passed through the object  1 . The scout image  10  may be an image captured for positioning an X-ray source  210  and an X-ray detector  220  with respect to the region to be scanned including the ROI  10 R. The X-ray source  210  may be included in an X-ray generator  1312  (refer to  FIG. 13 ). 
     The CT apparatus  200  may receive a user input of setting the ROI  10 R in the scout image  10  and set the ROI  10 R based on the received user input. In an embodiment, the CT apparatus  200  may set only a region to be observed by injecting a contrast medium into the object  1 , e. g., a carotid artery or pulmonary artery, as the ROI  10 R, or may set the ROI  10 R to be a relatively wide portion including both a patient&#39;s shoulders and maxillary bones. 
     After setting the ROI  10 R, the CT apparatus  200  may acquire at least one among X-ray projection images  20  respectively at at least one among angular positions  212  arranged around the object  1 . According to an embodiment, the CT apparatus  200  may emit, via the X-ray source  210 , X-rays toward the object  1  at a first angular position θ 1  and detect, by using the X-ray detector  220 , the X-rays that have passed through the object  1  to acquire a first X-ray projection image  21 . Then, after moving the X-ray source  210  by a preset angle θ on a rotating frame  130 , the CT apparatus  200  may emit X-rays toward the object  1  at a second angular position θ 2  and detect the X-rays that have passed through the object  1  to acquire a second X-ray projection image  22 . In the same manner, the CT apparatus  200  may acquire a third X-ray projection image  23  at a third angular position θ 3 . Although  FIG. 1  shows that the CT apparatus  200  acquires a total of 3 X-ray projection images including the first through third X-ray projection images  21  through  23  respectively at the first through third angular positions θ 1  through θ 3 , this is merely an example, and the number of acquired X-ray projection images are not limited to 3, but may be any other appropriate natural number. 
     The CT apparatus  200  may inject a contrast medium into the ROI  10 R of the object  1 , such as a blood vessel, and measure a difference between pixel values of at least one X-ray projection image respectively obtained before and after injection of the contrast medium. According to an embodiment, the CT apparatus  200  may set, as a reference value, the sum of pixel values of at least one X-ray projection image, e.g., of a pre-contrast image, obtained before injection of a contrast medium into the ROI  10 R. The CT apparatus  200  may sum up pixel values of at least one X-ray projection image respectively obtained at a plurality of time points following injection of the contrast medium into the ROI  10 R and calculate a difference between a reference value and a result of summing up the pixel values obtained for each of the plurality of time points, as described below in detail with reference to  FIG. 3 . 
     According to an embodiment, the CT apparatus  200  may set, as a reference value, a pixel value for the first X-ray projection image  21  obtained by emitting X-rays at a specific angular position, e.g., the first angular position θ 1 , add up pixel values of an X-ray projection image acquired at the first angular position θ 1  at each of a plurality of time points following injection of a contrast medium, and calculate a difference between a result of adding up the pixel values and the reference value, as described in detail below with reference to  FIG. 5 . 
     The CT apparatus  200  may determine whether to initiate a diagnostic CT scan by comparing the calculated difference with a preset threshold. In this case, a diagnostic CT image may be reconstructed from a sinogram acquired by emitting X-rays via the X-ray source  210  that rotates 360 degrees around the object  1  and detecting the X-rays that have passed through the object  1  by using the X-ray detector  220 . 
     A difference between a reference value and the sum of pixel values of an X-ray projection image obtained at a time point after injection of a contrast medium into the ROI  10 R of the object  1  may exceed a threshold Δ th  at a particular time point t k . The CT apparatus  200  may calculate a difference between the reference value and the sum of pixel values of an X-ray projection image obtained at each of a plurality of time points after injection of the contrast medium and determine the start of a diagnostic CT scan at the particular time point t k  by tracking the calculated difference. The particular time point t k  may be a time point when an attenuation coefficient of the contrast medium undergoes a rapid change, i.e., a time point when a contrast effect due to the contrast medium is maximized, as described in detail below with reference to  FIG. 6 . 
     According to an embodiment, the CT apparatus  200  may start a diagnostic CT scan after a lapse of a preset delay from the particular time point t k . 
     According to a bolus tracking technique, to obtain a high quality CT image exhibiting a maximized contrast effect, the X-rays are emitted at a time point when an attenuation coefficient of a contrast medium injected into an object reaches its maximum value. A related art bolus tracking method involves injecting a contrast medium into an ROI such as a carotid artery or a pulmonary artery, acquiring a pre-scan image by reconstructing an X-ray projection image generated by emitting X-rays in order to specify a time point when a contrast effect due to the contrast medium is maximized, and determining a time point for initiation of a diagnostic CT scan by setting an ROI in the acquired pre-scan image and measuring an (HU value in the ROI. In detail, the related art bolus tracking method is achieved by monitoring whether an HU value in the ROI is greater than a preset threshold and determining start of a diagnostic CT scan at a time point when the HU value is greater than the preset threshold. However, according to the related art bolus tracking method, a user suffers the inconvenience of having an ROI manually set in a pre-scan image, and when the ROI is set in a wrong region other than a region to be observed in the pre-scan image, a CT scan has to be performed again. Furthermore, it takes a long time to acquire a pre-scan image by reconstructing an X-ray projection image, and the user may miss a time point at which an attenuation coefficient for a contrast medium reaches its maximum value during reconstruction of the pre-scan image. In addition, since an X-ray source emits X-rays while rotating on a rotating frame through 360 degrees to acquire a pre-scan image, the amount of radiation to which a patient is exposed may increase excessively. 
     The CT apparatus  200  according to an embodiment is configured to determine a time point when a contrast effect due to a contrast medium is maximized by using a difference between pixel values of X-ray projection images respectively obtained before and after injection of the contrast medium into an ROI, reducing the amount of time required for reconstruction of a CT image. Since the reconstruction is accomplished in a short time, it is possible to solve the problem of missing the time point when a contrast effect due to the contrast medium is maximized. 
     Unlike a related art technique whereby a reconstruction process is performed after emitting X-rays during 360 degree rotation around the object  1  and detecting the X-rays that have passed through the object  1 , the CT apparatus  200  according to an embodiment is configured to emit X-rays only at at least one of angular positions  212  around the object  1  and determine a time point at which a diagnostic CT scan is initiated by using at least one of X-ray projection images  20  acquired based on the emitted X-rays, thereby reducing the amount of X-ray radiation to which a patient is exposed. 
     Referring to  FIG. 2 , the CT apparatus  200  according to an embodiment may include an X-ray source  210 , an X-ray detector  220 , a data acquisition unit  230 , and a processor  240 . However, the CT apparatus  200  may further include a gantry  1310 , a rotating frame  130 , a table  1305 , a display  1370 , and a communication interface  1380 , as shown in  FIG. 13 . In an embodiment, the CT apparatus  200  may further include a user input device or an input interface  1360  configured to receive a user input of setting an ROI in the scout image ( 10  of  FIG. 1 ). 
     The X-ray source  210  may generate X-rays and emit the generated X-rays toward an object (e.g., a patient) while rotating on the rotating frame ( 130  of  FIG. 1 ) positioned around the object. According to an embodiment, the X-ray source  210  may emit X-rays toward the object at the plurality of angular positions θ 1  through θ 3  while rotating about a rotation axis on the rotating frame  130  at preset angular intervals. For example, the X-ray source  210  may emit X-rays toward the object while rotating about a rotation axis at one-degree intervals. The X-ray source  210  may emit X-rays toward the object at angular positions corresponding to 1°, 2°, 3°, etc. 
     The X-ray detector  220  may detect the X-rays emitted by the X-ray source  210  toward the object. In an embodiment, the X-ray detector  220  may be formed as a photon counting detector (PCD) for detecting X-rays having a plurality of different energy bands. The X-ray detector  220  may detect X-rays emitted at a plurality of angles by which the X-ray source  210  rotates. For example, when the X-ray source  210  emits X-rays toward the object at angular positions of 1°, 2°, 3°, etc., the X-ray detector  220  may detect the X-rays emitted at the angular positions of 1°, 2°, 3°, etc. 
     The data acquisition unit  230  may acquire X-ray projection data output from the X-ray detector  220 . The data acquisition unit  230  may include at least one amplifying circuit that may be used to amplify the X-ray projection data. The data acquisition unit  230  shown in  FIG. 2  may be the same component as a data acquisition system (DAS)  1315 - 1  shown in  FIG. 13 . According to an embodiment, the data acquisition unit  230  may acquire at least one X-ray projection image from the X-ray detector  220  that detects X-rays emitted by the X-ray source  210  at at least one angular position as the X-ray source  210  rotates about an axis disposed at a center of the object. In an embodiment, the data acquisition unit  230  may form a sinogram by sequentially stacking at least one X-ray projection image respectively corresponding to at least one angular position. 
     The processor  240  may set an ROI in a scout image of the object, measure a difference between pixel values of at least one X-ray projection image respectively obtained before and after injection of a contrast medium into the ROI, and compare the measured difference with a preset threshold to determine whether to initiate a diagnostic CT scan. The processor  240  may be formed as a hardware unit having computational capabilities of converting pixel values of the at least one X-ray projection image acquired by the data acquisition unit  230  into numbers, adding up pixel values, and measuring a difference between pixel values before and after injection of a contrast medium. For example, the processor  240  may be constituted by at least one of a central processing unit (CPU), a microprocessor, and a graphic processing unit. 
     According to an embodiment, the X-ray source  210  may emit X-rays towards a predetermined region of the object including an ROI, and the X-ray detector  220  may detect the X-rays that have passed through the predetermined region of the object. The processor  240  may reconstruct a scout image from an X-ray projection image acquired based on the detected X-rays. In an embodiment, the user input device may receive a user input of setting an ROI in the scout image, such as a predetermined region including a blood vessel to be observed, and the processor  240  may set the ROI in the scout image based on the received user input. 
     According to an embodiment, the processor  240  may set, as a reference value, the sum of pixel values of at least one X-ray projection image obtained before injection of a contrast medium into an ROI, and calculate a difference between the reference value and the sum of pixel values of an X-ray projection image obtained at each of a plurality of time points after injection of the contrast medium into the ROI. The processor  240  may determine initiation of a diagnostic CT scan for examining the object at a time point when the calculated difference exceeds a preset threshold among the plurality of time points after injection of the contrast medium. 
     According to an embodiment, the processor  240  may set, as a reference value, the sum of pixel values of an X-ray projection image acquired at a particular angular position among X-ray projection images obtained before injection of a contrast medium into an ROI, and add up, for each of a plurality of time points, pixel values of an X-ray projection image acquired at the particular angular position among X-ray projection images acquired at the plurality of time points after injection of the contrast medium into the ROI. The processor  240  may then calculate a difference between the reference value and a result of adding up the pixel values obtained for each of the plurality of time points and determine initiation of a diagnostic CT scan at a time point when the calculated difference exceeds a preset threshold. 
     According to an embodiment, the X-ray detector  220  may be formed as a PCD, and may detect X-rays having an energy band in which an attenuation coefficient of a contrast medium reaches its maximum value among a plurality of energy bands. The data acquisition unit  230  may acquire at least one X-ray projection image based on the X-rays detected by the X-ray detector  220 . The processor  240  may add up pixel values of the at least one X-ray projection image acquired by the data acquisition unit  230  and determine whether to initiate a diagnostic CT scan by calculating a difference between the sum of pixel values and a reference value. 
     According to an embodiment, the processor  240  may separate a contrast projection image to include only image data of those X-rays which have been detected when passing through only a contrast medium, from the acquired X-ray projection image. The processor  240  may respectively add up pixel values of the at least one X-ray projection image acquired at a plurality of time points following injection of the contrast medium and calculate a difference between a reference value and the sum of pixel values obtained for each of the plurality of time points. The processor  240  may determine the start of a diagnostic CT scan at a time point when the calculated difference exceeds a preset threshold. 
       FIG. 3  is a flowchart of a method, performed by the CT apparatus  200 , of determining a time point when a diagnostic CT scan is performed by injecting a contrast medium into an object, according to an embodiment. 
     The CT apparatus  200  sets an ROI in a scout image of an object (operation S 310 ). In an embodiment, the CT apparatus  200  may emit X-rays toward a predetermined region of the object including the ROI and detect the X-rays that have passed through the object to capture a scout image. The scout image may be an image captured for positioning the X-ray source  210  and the X-ray detector  220  with respect to a region to be scanned including the ROI. 
     The CT apparatus  200  may receive a user input of setting an ROI in a scout image and set the ROI in the scout image based on the received user input. According to an embodiment, the CT apparatus  200  may set as the ROI only a region to be observed by injecting a contrast medium into the object, e. g., a carotid artery or pulmonary artery, or may set the ROI to be a relatively wide portion including both a patient&#39;s shoulders and maxillary bones. 
     The CT apparatus  200  emits X-rays toward the ROI and detects the X-rays that have passed through the ROI to acquire at least one X-ray projection image (operation S 320 ). According to an embodiment, the CT apparatus  200  may emit, via the X-ray source  210 , X-rays at at least one angular position arranged around the object and detect, via the X-ray detector  220 , the X-rays that have passed through the object. The CT apparatus  200  may generate at least one X-ray projection image by using the X-rays respectively detected at the at least one angular position. For example, the CT apparatus  200  may emit, via the X-ray source  210 , X-rays toward the object at a first angular position and detect, by using the X-ray detector  220 , the X-rays that have passed through the object to acquire a first X-ray projection image. Similarly, the CT apparatus  200  may emit, via the X-ray source  210 , X-rays toward the object at a second angular position and detect, via the X-ray detector  220 , the X-rays that have passed through the object to acquire a second X-ray projection image. 
     The CT apparatus  200  measures a difference between pixel values of at least one X-ray projection image respectively acquired before and after injection of a contrast medium into the ROI (operation S 330 ). According to an embodiment, the CT apparatus  200  may set, as a reference value, the sum of pixel values of at least one X-ray projection image acquired before injection of the contrast medium into the ROI. The CT apparatus  200  sums up pixel values of at least one X-ray projection image respectively obtained at a plurality of time points following injection of the contrast medium into the ROI and calculate a difference between a reference value and the sum of pixel values obtained for each of the plurality of time points. For example, the CT apparatus  200  may emit, via the X-ray source  210 , X-rays at at least one angular position at a first time point t 1 , add up all pixel values of at least one X-ray projection image acquired by detecting the X-rays that have passed through the object, and calculate a difference between a first pixel value that is the resulting sum and a reference value. The CT apparatus  200  may add up all pixel values of at least one X-ray projection image acquired at a second time point t 2  and calculate a difference between a second pixel value that is the resulting sum and the reference value. In the same way, the CT apparatus  200  may add up all pixel values of at least one X-ray projection image acquired at a k-th time point t k  and calculate a difference between a k-th pixel value that is the resulting sum and the reference value. 
     The CT apparatus  200  determines whether to initiate a diagnostic CT scan by comparing the measured difference with a preset threshold (operation S 340 ). According to an embodiment, the CT apparatus  200  may compare a difference, which is calculated for each of a plurality of time points after injection of a contrast medium into the ROI, with a preset threshold. The CT apparatus  200  may determine to perform a diagnostic CT scan at a time point t k  that is a time point when a calculated difference exceeds the preset threshold among the plurality of time points. 
       FIG. 4  is a flowchart of a method, performed by the CT apparatus  200 , of determining a time point when a diagnostic CT scan is initiated by using X-ray projection images respectively obtained before and after injection of a contrast medium into an object, according to an embodiment. 
     The CT apparatus  200  adds up pixel values of at least one X-ray projection image acquired before injection of a contrast medium into an ROI and sets a result of adding up the pixel values as a reference value (operation S 410 ). According to an embodiment, before injection of the contrast medium into the object, the CT apparatus  200  may emit, via the X-ray source  210 , X-rays at at least one angular position arranged around the object and detect, via the X-ray detector  220 , the X-rays that have passed through the object. The CT apparatus  200  may generate at least one X-ray projection image by using X-rays respectively detected at the at least one angular position. The CT apparatus  200  may sum up all pixel values of the at least one X-ray projection image acquired before injection of the contrast medium and set the resulting sum as a reference value. According to an embodiment, the CT apparatus  200  may sum up only pixel values of an X-ray projection image acquired, among X-ray projection images obtained before injection of the contrast medium, by emitting, via the X-ray source  210 , X-rays toward the object at a particular angular position and detecting the X-rays that have passed through the object at the particular angular position, and set the resulting sum as a reference value. For example, the CT apparatus  200  may add up pixel values of a first X-ray projection image acquired by emitting X-rays at a first angular position θ 1  and set the resulting sum as a reference value. 
     The CT apparatus  200  adds up pixel values of at least one X-ray projection image acquired after injection of the contrast medium into the ROI for each of a plurality of time points (operation S 420 ). According to an embodiment, the CT apparatus  200  may emit X-rays toward the object at a plurality of time points following injection of the contrast medium into the ROI and detect the X-rays that have passed through the object to acquire X-ray projection images. For example, the CT apparatus  200  may emit, via the X-ray source  210 , X-rays at at least one angular position at a first time point t 1  after injection of the contrast medium into the ROI and detect the X-rays that have passed through the object to acquire at least one X-ray projection image. In the same manner, the CT apparatus  200  may emit, via the X-ray source  210 , X-rays toward the object at a second time point t 2  after the first time point t 1  at the same angular position as for the first time point t 1  and detect the X-rays that have passed through the object to acquire at least one X-ray projection image. 
     The CT apparatus  200  calculates a difference between the sum of pixel values for each of the plurality of time points and the reference value (operation S 430 ). According to an embodiment, the CT apparatus  200  may calculate a first pixel value by adding up all pixel values of at least one X-ray projection image acquired at a first time point t 1  and determine a difference between the first pixel value and the reference value. 
     The CT apparatus  200  compares the calculated difference with a preset threshold (operation S 440 ). According to an embodiment, the CT apparatus  200  may track whether a difference calculated for each of a plurality of time points exceeds a preset threshold. For example, the CT apparatus  200  may determine whether a difference between the reference value and a first pixel value, which is calculated at the first time point t 1 , exceeds a threshold. 
     When the calculated difference does not exceed the preset threshold (no in operation S 440 ), the CT apparatus  200  returns to operation S 420  of acquiring at least one X-ray projection image at a second time point t 2 , calculating a second pixel value by adding up pixel values of the acquired at least one X-ray projection image, and determining a difference between the second pixel value and the reference value. The CT apparatus  200  may determine whether a difference between the reference value and a second pixel value calculated at the at the second time point t 2  exceeds the threshold in operation S 440 . Until reaching a time point when the calculated difference exceeds the preset threshold, the CT apparatus  200  may continue to acquire an X-ray projection image at a plurality of time points, calculate a difference between the sum of pixel values of the acquired X-ray projection image and a reference value, and compare the calculated difference with the preset threshold. 
     When the calculated difference exceeds the preset threshold (yes in operation S 440 ), the CT apparatus  200  determines initiation of a diagnostic CT scan (operation S 450 ). In operation S 450 , a diagnostic CT image may be acquired as an image reconstructed from a sinogram acquired by emitting X-rays as the X-ray source  210  rotates 360 degrees around the object and detecting, by using the X-ray detector  220 , the X-rays that have passed through the object. 
     The CT apparatus  200  may initiate a diagnostic CT scan immediately after a time point t k  when the calculated difference exceeds the preset threshold in operation S 440 , but embodiments are not limited thereto. In an embodiment, the CT apparatus  200  may determine initiation of a diagnostic CT scan after a lapse of a preset delay from the time point t k  when the calculated difference exceeds the preset threshold. 
       FIG. 5  is a diagram for explaining a method, performed by the CT apparatus  200 , of determining a time point when a diagnostic CT scan is initiated by using X-ray projection images  20  including an X-ray projection image  500  acquired at time point t 0  and an X-ray projection image  500   k  acquired at time point t k , by emitting X-rays toward an object  1  at a particular angular position, according to an embodiment. Although  FIG. 5  shows only two X-ray projection images acquired at two time points t 0  and t k , a different number of X-ray projection images may be acquired at the time points between t 0  and t k  or additional X-ray projection images may be acquired after the time point t k . 
     Referring to  FIG. 5 , the CT apparatus  200  may emit, via the X-ray source  210 , X-rays toward the object  1  before a time point t 0  that is before injection of a contrast medium into an ROI of the object  1 , and detect, via the X-ray detector  220 , the X-rays that have passed through the object  1  to acquire an X-ray projection image  500 . The X-ray source  210  may emit X-rays toward the object  1  at a first angular position θ 1  on a rotating frame  130  positioned around the object, and the X-ray detector  220  positioned opposite the first angular position θ 1  may detect the X-rays that have passed through the object  1  to acquire the X-ray projection image  500 . The first angular position θ 1  may be a position obtained as the X-ray source  210  rotates from a zero-degree position on the rotating frame  130  by a preset angle θ. 
     The CT apparatus  200  may add up pixel values of the X-ray projection image  500  acquired at the time point t 0  that is before injection of the contrast medium into the ROI and set the resulting sum as a reference value. 
     The CT apparatus  200  may inject the contrast medium into the ROI of the object  1  and acquire X-ray projection images at a plurality of time points following injection of the contrast medium. According to an embodiment, the CT apparatus  200  may move the X-ray source  210  to the first angular position θ 1  at a particular time point t k  following the injection of the contrast medium into the ROI and then detect, by using the X-ray detector  220  positioned opposite the first angular position θ 1 , X-rays that have passed through the ROI of the object  1  to acquire an X-ray projection image  500   k . 
     The CT apparatus  200  may add up pixel values of the X-ray projection image  500   k  acquired at the particular time point t k  following injection of the contrast medium into the ROI and calculate a difference between a result of adding up the pixel values and the reference value. The CT apparatus  200  may determine to perform a diagnostic CT scan at a time point when the calculated difference exceeds a preset threshold. 
     In the embodiment shown in  FIG. 5 , the CT apparatus  200  may emit X-rays at the same angular position, i.e., the first angular position θ 1  at both the time points t 0  and t k  that are respectively before and after injection of the contrast medium into the ROI of the object  1 , and determine a time point when the a diagnostic CT scan is initiated based on a difference between pixel values of the X-ray projection images  500  and  500   k  acquired by detecting the X-rays that have passed through the object  1 . 
       FIG. 6  is a graph  600  of attenuation coefficients of a bone of an object, water, and contrast medium with respect to energy. 
     Referring to the graph  600  of  FIG. 6 , attenuation coefficients  610  and  630  of a bone and water tend to decrease gradually as energy increases. However, an attenuation coefficient  620  of a contrast medium undergoes a rapid change in a particular energy band among a plurality of energy bands E 1  through E 3 . In other words, the attenuation coefficient  620  of the contrast medium may increase or decrease rapidly in a particular energy band when energy increases. For example, the attenuation coefficient  620  of the contrast medium may increase rapidly in a second energy band E 2  and decrease again as an energy band becomes higher. An energy in an energy band in which the attenuation coefficient  620  of the contrast medium undergoes a rapid change may be defined as a k-edge energy E k , and the attenuation coefficient  620  of the contrast medium may increase rapidly at k-edge energy E k . 
     While the contrast medium in  FIG. 6  may be iodine, the type of contrast medium used is not limited thereto. In an embodiment, the contrast medium injected into the ROI may be gadolinium. When the contrast medium is iodine, iodine has a k-edge at which an attenuation coefficient undergoes a sudden change at about 33 kilo-electronvolts (keV) contained in the second energy band E 2 . In other words, iodine has a k-edge energy E k  of about 33 keV. When the contrast medium is gadolinium, gadolinium has a k-edge energy E k  at 30.2 keV at which an attenuation coefficient undergoes a rapid change. In an embodiment, the second energy band E 2  may be in a range of between 30 keV and 50 keV. However, the range of the second energy band E 2  is not limited thereto. 
     The X-ray detector  220  included in the CT apparatus  200  may be PCD for detecting X-rays having a plurality of different energy bands. The CT apparatus  200  may detect, via a PCD, X-rays having different energy bands and which have passed through an ROI of an object and acquire a plurality of X-ray projection images by using the detected X-rays. 
     The CT apparatus  200  may determine a time point when a diagnostic CT scan is initiated by using an X-ray projection image generated, among a plurality of X-ray projection images, by using X-rays having an energy corresponding to a k-edge that is in an energy band in which an attenuation coefficient of a contrast medium injected into an ROI increases rapidly. According to an embodiment, when an iodine-based contrast medium is injected into the ROI, the CT apparatus  200  may acquire a second X-ray projection image by using X-rays having the second energy band E 2  including 33 keV that is a k-edge energy of iodine. The CT apparatus  200  may add up pixel values of an X-ray projection image acquired before injection of the contrast medium into the object to set the resulting sum as a reference value, and add up pixel values of the second X-ray projection image acquired after injection of the contrast medium to calculate a difference between a result of adding up the pixel values and the reference value for each of a plurality of time points. The CT apparatus  200  may then determine initiation of a diagnostic CT scan at a time point when the difference between the result of adding up the pixel values and the reference value exceeds a preset threshold. 
     Since the degree of attenuation due to a contrast medium increases with an increase in an attenuation coefficient of the contrast medium, pixel values of an ROI, i.e., a blood vessel, in a CT image may increase. In other words, since the blood vessel appears white on the CT image, it is possible to achieve a maximum contrast effect between the blood vessel and a bone or water that appears black or gray on the CT image. To maximize a contrast effect due to the contrast medium in a CT image, a CT scan needs to be performed in an energy band (e. g., the second energy band E 2 ) in which an attenuation coefficient of the contrast medium increases to the greatest extent. According to an embodiment, the CT apparatus  200  may detect, via a PCD, X-rays having a plurality of different energy bands and determine a time point for initiation of a diagnostic CT scan by using pixel values of an X-ray projection image acquired in an energy band (e.g., the second energy band E 2 ) in which the attenuation coefficient of the contrast medium increases to the greatest extent, thereby allowing acquisition of a high quality CT image exhibiting a maximum contrast effect due to the contrast medium. 
       FIG. 7A  is a block diagram showing components of a CT apparatus  700  according to an embodiment.  FIG. 7B  is a diagram for explaining a method, performed by the CT apparatus  700  of  FIG. 7A , of performing a CT scan by emitting X-rays toward an object  1  at different tube voltages, and  FIG. 7C  is a graph of the number of photons contained in X-rays, which are emitted by the CT apparatus  700  of  FIG. 7A , pass through an object, and are detected thereby, with respect to an energy band. The CT apparatus  700  may correspond to the CT apparatus  200 . 
     Referring to  FIG. 7A , the CT apparatus  700  may include the X-ray generator  1312  including first and second X-ray sources  711  and  712 , first and second X-ray detectors  721  and  722 , a data acquisition unit  730 , and a processor  740 . The data acquisition unit  730  and the processor  740  may be the same components as the data acquisition unit  230  and the processor  240  described with reference to  FIG. 2 , respectively. The descriptions that are provided above with respect to  FIG. 2  will be omitted here. 
     The first and second X-ray sources  711  and  712  may respectively emit X-rays toward an object at different tube voltages (kilovoltage peak; kVp). For example, the first and second X-ray sources  711  and  712  may emit X-rays toward the object at 80 kVp and 140 kVp, respectively. Referring to  7 B, the first X-ray source  711  may emit X-rays toward the object  1  at a first position at a tube voltage of 80 kVp, and the second X-ray source  712  may emit X-rays toward the object  1  at a second position at a tube voltage of 140 kVp. 
     The first and second X-ray detectors  721  and  722  may respectively detect the X-rays that are respectively emitted by the first and second X-ray sources  711  and  712  and pass through the object. Referring to  FIG. 7B , the first X-ray detector  721  may detect the X-rays that are emitted by the first X-ray source  711  and pass through the object  1 . The second X-ray detector  722  may detect X-rays that are emitted by the second X-ray source  712  and pass through the object. The number of photons in X-rays respectively detected by the first and second X-ray detectors  721  and  722  may vary according to an energy band, as described in detail below with reference to  FIG. 7C . 
     The data acquisition unit  730  may generate a plurality of X-ray projection images based on X-rays respectively detected by the first and second X-ray detectors  721  and  722 . Referring to  FIG. 7C , X-rays emitted by the first X-ray source  711  at a tube voltage of 80 kVp may have a lower energy band than X-rays emitted by the second X-ray source  712  at a tube voltage of 140 kVp. The data acquisition unit  730  may generate a first X-ray projection image by using X-rays having a first energy band that is a low energy band, and produce a second X-ray projection image by using X-rays having a second energy band that is a relatively high energy band. 
     The processor  740  may measure a difference between pixel values of an X-ray projection image acquired before injection of a contrast medium, among the first and second X-ray projection images, and an X-ray projection image acquired by detecting X-rays having an energy corresponding to a k-edge energy E k  that is in an energy band in which the degree of attenuation of the contrast medium injected into the object  1  is at a maximum. For example, when the contrast medium injected into the object  1  is iodine, since iodine has a k-edge energy E k  at about 33 keV, the processor  740  may add up pixel values of the first X-ray projection image acquired by detecting the X-rays emitted at a tube voltage of 80 kVp. The processor  740  may add up pixel values of an X-ray projection image generated by the data acquisition unit  730  before injection of the contrast medium into the object  1  and set the resulting sum as a reference value. The processor  740  may add up pixel values of the first X-ray projection image respectively acquired at a plurality of time points following injection of the contrast medium into the object  1  and calculate a difference between a result of adding up the pixel values and the reference value for each of the plurality of time points. 
     According to an embodiment, the processor  740  may measure a time point when the calculated difference exceeds a preset threshold and determine initiation of a diagnostic CT scan at the measured time point. 
     The CT apparatus  700  according to the embodiment shown in  FIGS. 7A through 7C  may emit X-rays at different tube voltages via the plurality of X-ray sources (the first and second X-ray sources  711  and  712 ), detect the X-rays having different energy bands via the plurality of X-ray detectors (the first and second X-ray detectors  721  and  722 ), and acquire a plurality of X-ray projection images by using the detected X-rays. The CT apparatus  700  may determine a time point when the diagnostic CT scan is initiated by using pixel values of an X-ray projection image acquired, among the plurality of X-ray projection mages, by using X-rays having an energy band corresponding to a k-edge energy E k  of the contrast medium. 
       FIGS. 8A through 8C  are conceptual diagrams for explaining methods, performed by CT apparatus  200 , of obtaining a plurality of X-ray projection images by emitting X-rays having a plurality of different energy bands toward an object, according to embodiments. 
     Referring to  FIG. 8A , the CT apparatus  200  may emit X-rays toward an object  1  by applying different tube voltages to an X-ray source  210 . In an embodiment, the CT apparatus  200  may apply tube voltages of 80 kVp and 140 kVp to the X-ray source  210  and emit X-rays toward the object at 80 kVp and 140 kVp. An X-ray detector  220  may detect X-rays that are emitted at the different tube voltages and penetrate the object  1 . According to an embodiment, the X-ray detector  220  may detect X-rays having different energy bands. 
     The CT apparatus  200  may acquire at least one X-ray projection image by using, among X-rays having different energy bands detected by the X-ray detector  220 , X-rays having an energy corresponding to a k-edge that is in an energy band in which an attenuation coefficient of a contrast medium injected into the object  1  undergoes a largest variation. According to an embodiment, the CT apparatus  200  may add up pixel values of the acquired at least one X-ray projection image and determine a time point when a diagnostic CT scan is initiated based on a difference between a result of adding up the pixel values and a reference value which is a sum of pixel values for an X-ray projection image acquired before injection of the contrast medium. 
     Referring to  FIG. 8B , the CT apparatus  200  may include an X-ray source  210  and an X-ray detector  220 . The X-ray detector  220  may include first and second X-ray detection layers  841  and  842 . The first and second X-ray detection layers  841  and  842  may respectively detect X-rays having different energy bands. In an embodiment, the first X-ray detection layer  841  may detect X-rays having a lower energy band than X-rays detected by the second X-ray detection layer  842 . 
     The CT apparatus  200  may acquire at least one X-ray projection image by using the X-rays respectively detected by the first and second X-ray detection layers  841  and  842 . The CT apparatus  200  may acquire at least one X-ray projection image via an X-ray detector that has detected, among the X-rays respectively detected by the first and second X-ray detection layers  841  and  842 , X-rays having an energy band corresponding to a k-edge of a contrast medium injected into an object  1 . According to an embodiment, the CT apparatus  200  may add up pixel values of the acquired at least one X-ray projection image and determine a time point when a diagnostic CT scan is initiated based on a difference between a result of adding up the pixel values and a pixel value for an X-ray projection image acquired before injection of the contrast medium. 
     Referring to  FIG. 8C , the CT apparatus  200  may include the X-ray generator  1312  including a plurality of X-ray sources, i.e., first and second X-ray sources  711  and  712 . The first and second X-ray sources  711  and  712  may respectively emit X-rays toward an object  1  at different tube voltages. The first and second X-ray sources  711  and  712  may rotate on a rotating frame within a gantry. The first and second X-ray sources  711  and  712  may alternately emit X-rays toward the object  1  while rotating on the rotating frame  130 . 
     The CT apparatus  200  may detect X-rays having different energy bands that are respectively emitted by the first and second X-ray sources  711  and  712  and pass through the object  1 , by a single X-ray detector or two X-ray detectors. The CT apparatus  200  may then acquire at least one X-ray projection image by using the X-rays having different energy bands. 
     According to an embodiment, the CT apparatus  200  may acquire at least one first X-ray projection image by detecting X-rays emitted by the first X-ray source  711  and at least one second X-ray projection image by detecting X-rays emitted by the second X-ray source  712 . The CT apparatus  200  may also determine a time point when a diagnostic CT scan is initiated based on pixel values of an X-ray projection image generated, among the at least one first X-ray projection image and the at least one second X-ray projection image, by using X-rays having an energy band corresponding to a k-edge of a contrast medium injected into the object  1 . 
     According to an embodiment, the CT apparatus  200  may add up pixel values of the X-ray projection image generated using the X-rays having an energy band corresponding to the k-edge of the contrast medium at each of a plurality of time points, measure a difference between a result of adding up the pixel values and a pixel value for an X-ray projection image acquired before injection of the contrast medium, and determine initiation of a diagnostic CT scan at a time point when the calculated difference exceeds a preset threshold. 
     In general, iodine is used as a contrast medium during a CT scan, and has a k-edge energy E k  of 33 keV. Thus, by detecting, among X-rays having a plurality of different energy bands, X-rays having an energy band containing 33 keV and generating an X-ray projection image based on the detected X-rays, it is possible to maximize a contrast effect between a blood vessel that is an ROI of the object  1  and the remaining regions other than the ROI, such as a bone, skin, water, etc. 
     The CT apparatus  200  according to embodiments described with reference to  FIGS. 8A through 8C  may detect X-rays having different energy bands and determine a time point for initiation of a diagnostic CT scan by using pixel values of an X-ray projection image acquired using, among the detected X-rays, X-rays having an energy band in which an attenuation coefficient of a contrast medium injected into the object  1  increases rapidly, e.g., X-rays having an energy corresponding to 33 keV when the contrast medium is iodine or X-rays having an energy corresponding to a different keV value when the contrast medium is other than iodine. 
       FIG. 9  is a flowchart of a method, performed by a CT apparatus, of separating a contrast projection image from an X-ray projection image of an object and determining whether to initiate a diagnostic CT scan by using the separated contrast projection image, according to an embodiment. 
     The CT apparatus sets an ROI in a scout image of an object (operation S 910 ). According to an embodiment, the CT apparatus may emit X-rays toward a predetermined region of the object including the ROI and detect the X-rays that have passed through the object to capture a scout image. The scout image may be an image captured for positioning an X-ray source and an X-ray detector with respect to a region to be scanned including the ROI. 
     The CT apparatus emits X-rays toward the ROI and detects the X-rays that have passed through the ROI to acquire at least one X-ray projection image (operation S 920 ). In an embodiment, the CT apparatus may emit, via the X-ray source, X-rays at at least one angular position arranged around the object, detect, by using the X-ray detector, the X-rays that have passed through the object, and generate at least one X-ray projection image by using the detected X-rays. 
     The CT apparatus adds up pixel values of the at least one X-ray projection image and sets the resulting sum as a reference value (operation S 930 ). According to an embodiment, before injection of a contrast medium into the object, the CT apparatus may emit, via the X-ray source, X-rays at at least one angular position arranged around the object and detect, by using the X-ray detector, the X-rays that have passed through the object. 
     The CT apparatus may sum up all pixel values of the at least one X-ray projection image acquired before injection of the contrast medium and set the resulting sum as a reference value. According to an embodiment, the CT apparatus may sum up only pixel values of an X-ray projection image acquired, among the at least one X-ray projection image obtained before injection of the contrast medium, by emitting, via the X-ray source, X-rays toward the object at a particular angular position and detecting the X-rays that have passed through the object at the particular angular position, and set the resulting sum as a reference value. 
     The CT apparatus separates a contrast projection image from X-ray projection images acquired after injection of the contrast medium into the ROI (operation S 940 ). The contrast projection image may mean a projection image generated using X-rays detected when passing through only the ROI of the object into which the contrast medium is injected. 
     According to an embodiment, the CT apparatus may separate only a contrast projection image from at least one X-ray projection image acquired at a plurality of time points following injection of the contrast medium into the ROI. For example, when a CT scan of a patient&#39;s neck is performed by injecting a contrast medium into a blood vessel including a carotid artery in order to observe the carotid artery, the CT apparatus may separate a contrast projection image that is an X-ray projection image of the carotid artery from an X-ray projection image acquired by emitting X-rays toward the patient&#39;s neck. 
     The CT apparatus calculates a difference between the sum of pixel values of the contrast projection image and the reference value (operation S 950 ). According to an embodiment, the CT apparatus may add up pixel values of a contrast projection image separated at each of a plurality of time points following injection of the contrast into the ROI. The CT apparatus may calculate a difference between a result of adding up the pixel values and the reference value for each of the plurality of time points. 
     For example, the CT apparatus may add up pixel values of a contrast projection image at a first time point t 1 , and calculate a difference between a first pixel value that is the resulting sum and the reference value. The CT apparatus may add up pixel values of a contrast projection image at a second time point t 2  and calculate a difference between a second pixel value that is the resulting sum and the reference value. Similarly, the CT apparatus may add up all pixel values of a contrast projection image at a k-th time point t k  and calculate a difference between a k-th pixel value that is the resulting sum and the reference value. 
     The CT apparatus compares the calculated difference with a preset threshold (operation S 960 ). According to an embodiment, the CT apparatus may track whether a difference calculated for each of a plurality of time points exceeds a preset threshold. For example, the CT apparatus may determine at a first time point t 1  whether a difference between the reference value and a first pixel value that is the sum of pixel values of a contrast projection image calculated at the first time point t 1  exceeds the preset threshold. Similarly, the CT apparatus may determine at a second time point t 2  whether a difference between the reference value and a second pixel value that is the sum of pixel values of a contrast projection image calculated at the second time point t 2  exceeds the preset threshold. 
     When the calculated difference does not exceed the preset threshold (no in operation S 960 ), the CT apparatus returns to operation S 940  of separating a contrast projection image at a third time point t 3  and calculating a third pixel value by adding up pixel values of the contrast projection image at the third time point t 3 . Until reaching a time point when the calculated difference exceeds the preset threshold, the CT apparatus may continue to separate a contrast projection image at a plurality of time points, calculate a difference between the sum of pixel values of the contrast projection image and the reference value, and compare the calculated difference with the preset threshold. 
     When the calculated difference exceeds the preset threshold (yes in operation S 960 ), the CT apparatus determines initiation of a diagnostic CT scan (operation S 970 ). In operation S 970 , a diagnostic CT image may be an image reconstructed from a sinogram acquired by emitting X-rays as an X-ray source rotates around the object through 360 degrees and detecting, via an X-ray detector, the X-rays that have passed through the object. 
       FIG. 10  illustrates a method, performed by a CT apparatus, of binning pixels in an X-ray projection image such that noise in the X-ray projection image is reduced, according to an embodiment. 
     Referring to  FIG. 10 , the CT apparatus may change values of a preset number of adjacent pixels  1001 ,  1002 ,  1003 , and  1004  among pixels of an X-ray projection image of an ROI of an object. For example, the CT apparatus may calculate an average of values of four pixels  1001  through  1004  included in a pixel group P of the X-ray projection image and replace all the values of the four pixels  1001  through  1004  with the calculated average. 
     When at least one X-ray projection image is generated by emitting X-rays at only one or some of angular positions θ 1 , θ 2 , θ 3 , . . . on a rotating frame instead of entire 360 degrees, noise occurs in the X-ray projection image as compared to that acquired in a complete 360-degree sinogram. The CT apparatus of  FIG. 10  according to an embodiment may calculate an average of values of the preset number of adjacent pixels  1001  through  1004  and replace the values of the adjacent pixels  1001  through  1004  with the calculated average, thereby reducing noise in the X-ray projection image. 
       FIG. 11  is a diagram for explaining a method of setting an ROI in a scout image of an object by using a CT apparatus, according to an embodiment, and  FIG. 12  is a flowchart of a method, performed by a CT apparatus, of performing a diagnostic CT scan on an object, according to an embodiment. 
     Referring to  FIG. 12 , the CT apparatus sets a first ROI in a scout image of an object (operation S 1210 ). According to an embodiment, the CT apparatus may emit X-rays toward a region of an object to be scanned, e.g., a predetermined region including a patient&#39;s neck, chest, brain, etc., and detect the X-rays that have passed through the object to capture a scout image. The scout image may be an image captured for positioning an X-ray source and an X-ray detector with respect to a region of the patient to be scanned. 
     The CT apparatus acquires a first sinogram with respect to the first ROI and reconstructs a CT image from the first sinogram (operation S 1220 ). According to an embodiment, the CT apparatus may rotate an X-ray source 360 degrees on a rotating frame positioned around the object and emit X-rays toward the first ROI via the X-ray source. The CT apparatus may detect, by using an X-ray detector, the X-rays that have passed through the first ROI to acquire a first sinogram that is a set of a plurality of X-ray projection images corresponding to angular positions from 0 to 360 degrees. The CT apparatus may then reconstruct a CT image from the first sinogram by using a filtered back-projection (FBP) algorithm, etc. 
     The CT apparatus sets a second ROI in a predetermined region on a CT image, including a region into which a contrast medium is to be injected (operation S 1230 ). Referring to  FIG. 11 , the CT apparatus may set a second ROI  1100 R in a CT image  1100  displayed on a screen of a display  1370  (refer to  FIG. 13 ). For example, during a CT scan of a patient&#39;s neck, the CT apparatus may set the second ROI  1100 R in a relatively wide region including blood vessels  1101  and  1102  into which a contrast medium is to be injected without receiving a user input of specifying the blood vessels  1101  and  1102 . When the CT scan is performed by injecting the contrast medium into the blood vessels  1101  and  1102  of the patient, a user of the CT apparatus described with reference to  FIGS. 11 and 12  does not need to individually specify the blood vessels  1101  and  1102 . Thus, user&#39;s convenience may be improved. 
     The CT apparatus emits X-rays toward the second ROI ( 1100 R of  FIG. 11 ) and detects the X-rays that have passed through the second ROI  1100 R to acquire a second sinogram (operation S 1240 ). According to an embodiment, the CT apparatus may emit, via the X-ray source, X-rays toward the second ROI  1100 R of the object at angular positions from 0 to 360 degrees and detect the X-rays that have passed through the second ROI  1100 R to acquire a second sinogram that is a set of plurality of X-ray projection images. 
     The CT apparatus reconstructs a contrast CT image from the second sinogram (operation S 1250 ). 
     The CT apparatus measures an HU value in the contrast CT image (operation S 1260 ). According to an embodiment, the CT apparatus may acquire a sinogram at a time point following injection of the contrast medium into the second ROI, e.g., blood vessels, and reconstruct a contrast image from the acquired sinogram. The CT apparatus may also measure an HU value of the contrast CT image. 
     The CT apparatus compares the measured HU value with a preset threshold (operation S 1270 ). According to an embodiment, the CT apparatus may respectively compare HU values of first through k-th CT images with a preset threshold for a plurality of time points t 1  through t k . When the measured HU value does not exceed the preset threshold (no in operation S 1270 ), the CT apparatus repeats operation S 1240  of emitting X-rays toward the second ROI and detecting the X-rays that have passed through the second ROI to acquire a second sinogram, operation S 1250  of generating a contrast CT image, and operation S 1250  of measuring an HU value of the contrast CT image. 
     According to an embodiment, the CT apparatus may repeat operations S 1240  through S 1270  at a plurality of time points following injection of the contrast medium into the second ROI. For example, the CT apparatus may acquire a sinogram with respect to the second ROI at a first time point t 1  after injection of the contrast medium into the second ROI and reconstruct a first contrast CT image from the acquired sinogram. The CT apparatus may then compare an HU value of the first contrast CT image with the preset threshold (operation S 1270 ) and, when the HU value of the first contrast CT image does not exceed the preset threshold (no in operation S 1270 ), return to operation S 1240  to generate a second contrast CT image. When an HU value of a k-th CT image at a k-th time point t k  exceeds the preset threshold (yes in operation S 1270 ), the CT apparatus determines initiation of a diagnostic CT scan (operation S 1280 ). 
     According to a related art bolus tracking method, when an ROI is set in a pre-scan image, the user suffers the inconvenience of having to directly select a blood vessel such as carotid artery into which a contrast medium is to be injected for observation. When the blood vessel such as the carotid artery is not set correctly as the ROI, a CT scan has to be performed again. 
     According to the embodiment described with reference to  FIGS. 11 and 12 , the CT apparatus is configured to detect X-rays having a plurality of different energy bands via a PCD, generate an X-ray projection image by using, among the detected X-rays, X-rays having an energy region in which the degree of attenuation due to the contrast medium is maximized, and reconstruct a CT image from the X-ray projection image, thereby eliminating the need for directly setting the blood vessels ( 1101  and  1102  of  FIG. 11 ) as an ROI. In other words, the CT apparatus according to the embodiment may set the second ROI  1100 R to be a relatively wide region including not only the blood vessels  1101  and  1102  but also the surrounding region, thereby improving user convenience. Furthermore, it is possible to prevent repetition of a CT scan due to erroneous setting of an ROI, thereby reducing the amount of patient&#39;s radiation exposure. 
       FIG. 13  illustrates a structure of a CT system  1300  according to an embodiment. 
     The CT system  1300  may include a gantry  1310 , a table  1305 , a controller  1330 , a storage  1340 , an image processor  1350 , an input interface  1360 , a display  1370 , and a communication interface  1380 . 
     The gantry  1310  may include a rotating frame  130 , an X-ray generator  1312 , an X-ray detector  220 , a rotation driver  1314 , and a readout device  1315 . 
     The rotating frame  130  may receive a driving signal from the rotation driver  1314  and rotate around a rotation axis (RA). 
     An anti-scatter grid  1316  may be disposed between an object and the X-ray detector  220  and may transmit most of primary radiation and attenuate scattered radiation. The object may be positioned on the table  1305  which may move, tilt, or rotate during a CT scan. 
     The X-ray generator  1312  receives a voltage and a current from a high voltage generator (HVG) to generate and emit X-rays. 
     The CT system  1300  may be implemented as a single-source CT system including one X-ray source  210  and one X-ray detector  220 , a dual-source CT system including two X-ray sources, e.g., the first and second X-ray sources  711  and  712 , and two X-ray detectors, e.g., the first and second X-ray detectors  721  and  722 , or a dual-source CT system including two X-ray sources, e.g., the first and second X-ray sources  711  and  712 , and a single X-ray detector  220 . 
     The X-ray detector  220  detects radiation that has passed through the object. For example, the X-ray detector  220  may detect radiation by using a scintillator, a photon counting detector, etc. 
     Methods of driving the X-ray generator  1312  and the X-ray detector  220  may vary depending on scan modes used for scanning of the object. The scan modes are classified into an axial scan mode and a helical scan mode, according to a path along which the X-ray detector  220  moves. The scan modes are classified into a prospective mode and a retrospective mode, according to a time interval during which X-rays are emitted. 
     The controller  1330  may control an operation of each of the components of the CT system  1300 . The controller  1330  may include a memory configured to store program codes for performing a function or data and a processor (e.g., a processor  240  or  740 ) configured to process the program codes or the data. The controller  1330  may be implemented in various combinations of at least one memory and at least one processor. The processor may generate or delete a program module according to an operating status of the CT system  1300  and process operations of the program module. 
     The readout device  1315  receives a detection signal generated by the X-ray detector  220  and outputs the detection signal to the image processor  1350 . The readout device  1315  may include a DAS  1315 - 1  and a data transmitter  1315 - 2 . The DAS  1315 - 1  uses at least one amplifying circuit to amplify a signal output from the X-ray detector  220 , and outputs the amplified signal. The data transmitter  1315 - 2  uses a circuit such as a multiplexer (MUX) to output the signal amplified in the DAS  1315 - 1  to the image processor  1350 . According to a slice thickness or a number of slices, only some of a plurality of pieces of data collected by the X-ray detector  220  may be provided to the image processor  1350 , or the image processor  1350  may select only some of the plurality of pieces of data. 
     The image processor  1350  obtains tomography data from a signal obtained by the readout device  1315  (e.g., pure data that is data before being processed). The image processor  1350  may pre-process the obtained signal, convert the obtained signal into tomography data, and post-process the tomography data. The image processor  1350  may perform some or all of the processes described herein, and the type or order of processes performed by the image processor  1350  may vary according to embodiments. 
     The image processor  1350  may perform pre-processing, such as a process of correcting sensitivity irregularity between channels, a process of correcting a rapid decrease of signal strength, or a process of correcting signal loss due to an X-ray absorbing material, on the signal obtained by the readout device  1315 . 
     According to embodiments, the image processor  1350  may perform some or all of the processes for reconstructing a tomography image, to generate the tomography data. According to an embodiment, the tomography data may be data that has undergone back-projection, or a tomography image. According to embodiments, additional processing may be performed on the tomography data by an external device such as a server, a medical apparatus, or a portable device. 
     Raw data is a set of data values corresponding to intensities of X-rays that have passed through the object, and may include projection data or a sinogram. The data that has undergone back-projection is obtained by performing back-projection on the raw data by using information about an angle at which X-rays are emitted. The tomography image is obtained by using image reconstruction techniques including back-projection of the raw data. 
     The storage  1340  is a storage medium for storing control-related data, image data, etc., and may include a volatile or non-volatile storage medium and/or a memory. 
     The input interface  1360  receives control signals, data, etc., from a user. The display  1370  may display information indicating an operational status of the CT system  1300 , medical information, medical image data, etc., and may serve as a user input device if the display  1370  includes touchscreen. 
     The CT system  1300  includes the communication interface  1380  and may be connected to external devices, such as a server, a medical apparatus, and a portable device (smartphone, tablet personal computer (PC), wearable device, etc.), via the communication interface  1380 . 
     The communication interface  1380  may include one or more components that enable communication with an external device. For example, the communication interface  1380  may include a short distance communication module, a wired communication module, and a wireless communication module. 
     The communication interface  1380  may receive control signals and data from an external device and transmit the received control signals to the controller  1330  so that the controller  1330  may control the CT system  1300  according to the received control signals. 
     Alternatively, by transmitting a control signal to an external device via the communication interface  1380 , the controller  1330  may control the external device according to the control signal. 
     For example, the external device may process data according to a control signal received from the controller  1330  via the communication interface  1380 . 
     A program for controlling the CT system  1300  may be installed on the external device and may include instructions for performing some or all of the operations of the controller  1330 . 
     The program may be preinstalled on the external device, or a user of the external device may download the program from a server that provides an application for installation. The server that provides an application may include a recording medium having the program recorded thereon. 
     The above-described embodiments of the disclosure may be embodied as a computer-readable recording medium for storing computer executable command languages and data. The command languages may be stored as program codes and, when executed by a processor, may perform a certain operation by generating a certain program module. Also, when executed by a processor, the command languages may perform certain operations of embodiments. 
     While embodiments of the disclosure have been particularly shown and described with reference to the accompanying drawings, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims. The embodiments should be considered in descriptive sense only and not for purposes of limitation.