X-ray CT apparatus and method

Abstract

To determine a timing at which to shift to main scanning easily in an examination using a contrast medium.SOLUTION: An X-ray CT apparatus includes: a photon counting type detector for outputting a signal capable of measuring an energy value of emitted X-ray photons; and a control unit for determining a timing at which to shift from pre-scanning to main scanning by estimating an amount of presence of a contrast medium on an X-ray route including a monitoring region from projection data generated when the photon counting type detector detects an X-ray transmitted through the monitoring region set to a subject in the pre-scanning.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD

[0001] Embodiments disclosed in the present specification and the drawings relate to an X-ray CT apparatus and a method.

BACKGROUND ART

[0002] In examinations using an X-ray CT (Computed Tomography) apparatus, a contrast agent may sometimes be used. Since a large amount of contrast agent may cause adverse reactions in a subject, a smaller usage amount is more preferable. On the other hand, the smaller the amount of the contrast agent, the shorter the time during which blood vessels and the like are contrasted, making it more difficult to adjust the timing for performing a CT scan.

[0003] As a technique for performing a CT scan at an appropriate timing, a method of performing a pre-scan prior to the main CT scan is known. Specifically, a method is known in which X-ray CT images (image data) are sequentially generated in the pre-scan, changes in pixel values in a region of interest to be examined or a region in the vicinity thereof are monitored, and the main scan is started when the inflow of the contrast agent is detected based on the changes in the pixel values.

PRIOR ART DOCUMENTS

PATENT DOCUMENTS

[0004]

PATENT DOCUMENT 1

PATENT DOCUMENT 2

PATENT DOCUMENT 3

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] One of the problems that the embodiments disclosed herein and in the drawings aim to solve is to easily determine the timing for transitioning to the main scan in an examination using a contrast agent. However, the problems that the embodiments disclosed herein and in the drawings aim to solve are not limited to the above problem. Problems corresponding to the effects of each configuration shown in the embodiments described later can also be positioned as other problems. [Means for solving the problem]

[0006] The X-ray CT apparatus according to this embodiment includes a photon counting detector that outputs a signal capable of measuring the energy value of emitted X-ray photons, and a control unit that determines the timing of transitioning from the pre-scan to the main scan by estimating the amount of contrast agent present in the X-ray path including the monitoring area from projection data generated by the photon counting detector detecting X-rays that have passed through a monitoring area set for the subject during the pre-scan. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a block diagram showing an example of the configuration of an X-ray CT apparatus according to this embodiment. [Figure 2] Figure 2 is a diagram illustrating the region of interest and the monitoring region according to the embodiment. [Figure 3] Figure 3 is a diagram illustrating the projection data according to the embodiment. [Figure 4] Figure 4 shows an example of a substance discrimination process according to an embodiment. [Figure 5A] Figure 5A shows an example of the energy bin configuration according to this embodiment. [Figure 5B] Figure 5B shows an example of the energy bin configuration according to the embodiment. [Modes for carrying out the invention]

[0008] The embodiments of the X-ray CT apparatus and method will be described in detail below with reference to the attached drawings.

[0009] (Embodiment) The X-ray CT apparatus 10 will be described using Figure 1. Figure 1 is a block diagram showing an example of the configuration of the X-ray CT apparatus 10 according to this embodiment. As shown in Figure 1, the X-ray CT apparatus 10 includes a gantry unit 110, a patient bed unit 130, and a console unit 140.

[0010] In Figure 1, the rotation axis of the rotating frame 113 in the non-tilted state or the longitudinal direction of the top plate 133 of the patient device 130 is defined as the Z-axis direction. The axis direction perpendicular to the Z-axis direction and horizontal to the floor surface is defined as the X-axis direction. The axis direction perpendicular to the Z-axis direction and perpendicular to the floor surface is defined as the Y-axis direction. Note that Figure 1 shows the gantry device 110 from multiple directions for illustrative purposes, and illustrates the case where the X-ray CT apparatus 10 has one gantry device 110.

[0011] The rigging device 110 includes an X-ray tube 111, an X-ray detector 112, a rotating frame 113, an X-ray high-voltage device 114, a control device 115, a wedge 116, a collimator 117, and a DAS (Data Acquisition System) 118.

[0012] The X-ray tube 111 is a vacuum tube having a cathode (filament) that generates thermionic electrons and an anode (target) that generates X-rays upon collision with thermionic electrons. The X-ray tube 111 generates X-rays to irradiate the subject P by irradiating thermionic electrons from the cathode to the anode when a high voltage is applied from the X-ray high-voltage device 114.

[0013] The X-ray detector 112 is a photon-counting type detector that outputs a signal capable of measuring the energy value of an X-ray photon each time one is incident on it. The X-ray photon is, for example, an X-ray photon that is irradiated from the X-ray tube 111 and passes through the subject P. The X-ray detector 112 has multiple detection elements that output one pulse of electrical signal (analog signal) each time one is incident on an X-ray photon. By counting the number of electrical signals (pulses), it is possible to count the number of X-ray photons incident on each detection element. Furthermore, by performing calculation processing on this signal, it is possible to measure the energy value of the X-ray photon that caused the output of the signal. For example, the X-ray detector 112 is a surface detector in which multiple detection elements are arranged in the channel direction and the slice direction.

[0014] The detection element described above is composed of, for example, a scintillator and a photosensor such as a photomultiplier tube. In this case, the X-ray detector 112 is an indirect conversion type detector that converts incident X-ray photons into scintillator light using a scintillator, and then converts the scintillator light into an electrical signal using a photosensor such as a photomultiplier tube. Alternatively, the detection element described above may be a semiconductor detection element such as CdTe (cadmium telluride) or CdZnTe (cadmium zinc telluride) with electrodes arranged on it. In this case, the X-ray detector 112 is a direct conversion type detector that directly converts incident X-ray photons into an electrical signal.

[0015] The X-ray detector 112 has multiple detection elements and Application Specific Integrated Circuits (ASICs) connected to the detection elements that count the X-ray photons detected by the detection elements. The ASIC counts the number of X-ray photons incident on the detection elements by discriminating the individual charges output by the detection elements. The ASIC also measures the energy of the counted X-ray photons by performing calculations based on the magnitude of each charge. Furthermore, the ASIC outputs the X-ray photon counting result as digital data to the DAS118.

[0016] The rotating frame 113 is an annular frame that supports the X-ray tube 111 and the X-ray detector 112 to face each other, and causes the X-ray tube 111 and the X-ray detector 112 to rotate under the control of the control device 115. For example, the rotating frame 113 is a casting made of aluminum. In addition to the X-ray tube 111 and the X-ray detector 112, the rotating frame 113 can further support the X-ray high-voltage device 114, the wedge 116, the collimator 117, the DAS 118, and the like. Furthermore, the rotating frame 113 can also further support various configurations not shown in FIG. 1. Hereinafter, in the gantry device 110, the rotating frame 113 and the portions that rotate and move together with the rotating frame 113 are also referred to as a rotating unit.

[0017] The X-ray high-voltage device 114 has electric circuits such as a transformer and a rectifier, and includes a high-voltage generator that generates a high voltage to be applied to the X-ray tube 111, and an X-ray control device that controls the output voltage corresponding to the X-rays generated by the X-ray tube 111. The high-voltage generator may be of a transformer type or an inverter type. The X-ray high-voltage device 114 may be provided on the rotating frame 113, or may be provided on a fixed frame not shown in the drawings.

[0018] The control device 115 includes a processing circuit having a CPU (Central Processing Unit) and the like, and a drive mechanism such as a motor and an actuator. The control device 115 receives an input signal from the input interface 143, and controls the operations of the gantry device 110 and the couch device 130. For example, the control device 115 controls the rotation of the rotating frame 113, the tilting of the gantry device 110, the operations of the couch device 130 and the top plate 133, and the like. The control device 115 may be provided in the gantry device 110, or may be provided in the console device 140.

[0019] The wedge 116 is a filter for adjusting the X-ray dose emitted from the X-ray tube 111. Specifically, the wedge 116 is a filter that transmits and attenuates the X-rays emitted from the X-ray tube 111 so that the X-rays emitted from the X-ray tube 111 to the subject P have a predetermined distribution. For example, the wedge 116 is a wedge filter or a bow-tie filter, and is a filter formed by processing aluminum or the like to have a predetermined target angle and a predetermined thickness.

[0020] The collimator 117 is a lead plate or the like for narrowing down the irradiation range of the X-rays that have passed through the wedge 116, and a slit is formed by a combination of a plurality of lead plates or the like. Note that the collimator 117 may also be referred to as an X-ray diaphragm. Further, although FIG. 1 shows a case where the wedge 116 is disposed between the X-ray tube 111 and the collimator 117, the collimator 117 may be disposed between the X-ray tube 111 and the wedge 116. In this case, the wedge 116 transmits and attenuates the X-rays that are emitted from the X-ray tube 111 and whose irradiation range is limited by the collimator 117.

[0021] The DAS 118 collects X-ray signals detected by each detection element included in the X-ray detector 112. For example, the DAS 118 includes an amplifier that performs amplification processing on the electrical signals output from each detection element, and an A / D converter that converts the electrical signals into digital signals, to generate detection data. The DAS 118 is implemented by, for example, a processor.

[0022] The data generated by DAS118 is transmitted via optical communication from a transmitter having a light-emitting diode (LED) on the rotating frame 113 to a receiver having a photodiode located on the non-rotating part of the mounting device 110 (e.g., a fixed frame, which is not shown in Figure 1), and then transferred to the console device 140. Here, the non-rotating part is, for example, a fixed frame that rotatably supports the rotating frame 113. Note that the method of transmitting data from the rotating frame 113 to the non-rotating part of the mounting device 110 is not limited to optical communication; any non-contact data transmission method or a contact-type data transmission method may be used.

[0023] The examination bed device 130 is a device for placing and moving the subject P to be scanned, and comprises a base 131, an examination bed drive device 132, a tabletop 133, and a support frame 134. The base 131 is a housing that supports the support frame 134 so that it can move vertically. The examination bed drive device 132 is a drive mechanism that moves the tabletop 133 on which the subject P is placed in the direction of the long axis of the tabletop 133, and includes a motor and actuator, etc. The tabletop 133, which is provided on the upper surface of the support frame 134, is a plate on which the subject P is placed. In addition to moving the tabletop 133, the examination bed drive device 132 may also move the support frame 134 in the direction of the long axis of the tabletop 133.

[0024] The console device 140 includes a memory circuit 141, a display 142, an input interface 143, and a processing circuit 144. Although the console device 140 is described separately from the stand device 110, the stand device 110 may include the console device 140 or some of its components.

[0025] The memory circuit 141 can be implemented using, for example, semiconductor memory elements such as RAM or flash memory, a hard disk, or an optical disc. The memory circuit 141 stores, for example, projection data and image data reconstructed based on the projection data. Alternatively, the memory circuit 141 can store programs for circuits included in the X-ray CT scanner 10 to perform their functions. The memory circuit 141 may also be implemented using cloud computing.

[0026] The display 142 displays various types of information. For example, the display 142 may display various images generated by the processing circuit 144, or it may display a GUI for receiving various operations from the operator. For example, the display 142 may be an LCD display or a CRT display. The display 142 may be a desktop type, or it may be composed of the console device 140 main unit and a tablet terminal or the like that can communicate wirelessly.

[0027] The input interface 143 receives various input operations from the operator, converts the received input operations into electrical signals, and outputs them to the processing circuit 144. For example, the input interface 143 can be implemented using a mouse, keyboard, trackball, switch, button, joystick, touchpad for input operations by touching the operating surface, touchscreen with an integrated display screen and touchpad, non-contact input circuit using an optical sensor, audio input circuit, etc. The input interface 143 may also be provided on the mounting device 110. Furthermore, the input interface 143 may consist of the console device 140 main unit and a tablet terminal or the like that can communicate wirelessly. Moreover, the input interface 143 is not limited to those equipped with physical operating components such as a mouse or keyboard. For example, an electrical signal processing circuit that receives electrical signals corresponding to input operations from an external input device provided separately from the console device 140 and outputs these electrical signals to the processing circuit 144 is also included as an example of the input interface 143.

[0028] The processing circuit 144 controls the operation of the entire X-ray CT apparatus 10 by executing system control function 144a, preprocessing function 144b, generation function 144c, and control function 144d.

[0029] For example, the processing circuit 144 reads a program corresponding to the system control function 144a from the memory circuit 141 and executes it to perform a scan on the subject P. For example, the system control function 144a supplies high voltage to the X-ray tube 111 by controlling the X-ray high-voltage device 114. As a result, the X-ray tube 111 generates X-rays to irradiate the subject P. The system control function 144a also moves the subject P into the imaging port of the pallet unit 110 by controlling the bed drive device 132. The system control function 144a also adjusts the aperture and position of the collimator 117. The system control function 144a also rotates the rotating part by controlling the control device 115. While the scan is being performed by the system control function 144a, the DAS 118 collects X-ray signals from each detection element in the X-ray detector 112 and generates detection data.

[0030] Furthermore, the processing circuit 144 reads a program corresponding to the preprocessing function 144b from the memory circuit 141 and executes it, thereby performing preprocessing on the detection data output from the DAS 118. For example, the preprocessing function 144b performs preprocessing such as logarithmic transformation, offset correction, inter-channel sensitivity correction, and beam hardening correction on the detection data output from the DAS 118. The data after preprocessing is also referred to as raw data. The detection data before preprocessing and the raw data after preprocessing are collectively referred to as projection data.

[0031] Furthermore, the processing circuit 144 generates image data (volume data) based on the projection data by reading and executing a program corresponding to the generation function 144c from the storage circuit 141. For example, the generation function 144c reconstructs the image data by performing reconstruction processing on the projection data using methods such as filtered back projection and iterative reconstruction.

[0032] Furthermore, for example, the processing circuit 144 determines the timing for transitioning from prescan to main scan by reading and executing a program corresponding to the control function 144d from the memory circuit 141. The system control function 144a first starts a prescan for the subject P, and then transitions from prescan to main scan at the timing determined by the control function 144d. Details of the processing by the control function 144d will be described later.

[0033] In this embodiment, the main scan is a scan that collects projection data to be used for image interpretation. Image data is reconstructed from the projection data collected by the main scan, and the reconstructed image data is referenced by a physician for purposes such as diagnosis and treatment planning. On the other hand, a pre-scan is a scan performed prior to the main scan to determine when to proceed to the main scan. The projection data collected by the pre-scan may or may not be used for image interpretation. Pre-scans are usually performed at a lower dose than the main scan.

[0034] In the X-ray CT scanner 10 shown in Figure 1, each processing function is stored in the memory circuit 141 in the form of a program that can be executed by a computer. The processing circuit 144 is a processor that realizes the function corresponding to each program by reading and executing the program from the memory circuit 141. In other words, the processing circuit 144, when a program has been read, has the function corresponding to the read program.

[0035] In Figure 1, the system control function 144a, preprocessing function 144b, generation function 144c, and control function 144d are described as being realized by a single processing circuit 144. However, the processing circuit 144 may be configured by combining multiple independent processors, with each processor executing a program to realize the functions. Furthermore, each processing function of the processing circuit 144 may be appropriately distributed or integrated across one or more processing circuits to realize the functions.

[0036] In the above description, the term "processor" refers to circuits such as CPUs, GPUs (Graphics Processing Units), Application Specific Integrated Circuits (ASICs), and Programmable Logic Devices (e.g., Simple Programmable Logic Devices (SPLDs), Complex Programmable Logic Devices (CPLDs), and Field Programmable Gate Arrays (FPGAs)). The processor performs its functions by reading and executing programs stored in the memory circuit 141.

[0037] In Figure 1, a single memory circuit 141 was described as storing programs corresponding to each processing function. However, the embodiments are not limited to this. For example, multiple memory circuits 141 may be distributed, and the processing circuit 144 may read the corresponding programs from the individual memory circuits 141. Alternatively, instead of storing programs in the memory circuits 141, the program may be directly incorporated into the processor's circuitry. In this case, the processor realizes its functions by reading and executing the program incorporated into the circuitry.

[0038] Furthermore, the processing circuit 144 may also implement its functions by utilizing the processor of an external device connected via a network NW. For example, the processing circuit 144 reads and executes programs corresponding to each function from the memory circuit 141, and also utilizes a group of servers (cloud) connected to the X-ray CT device 10 via a network NW as computing resources to implement the functions shown in Figure 1.

[0039] The overall configuration of the X-ray CT scanner 10 has been described above. With this configuration, the X-ray CT scanner 10 makes it possible to easily determine the timing for transitioning to the main scan during examinations using contrast agents.

[0040] This section describes one example of how to determine the timing for transitioning to the main scan. In an examination using the X-ray CT scanner 10, a region of interest is first set. The region of interest is an area of ​​the subject P that the user, such as a physician, is interested in, for example, an area with disease or an area suspected of having disease. If the region of interest is the heart or blood vessels, these will not be visualized in the image data if a CT scan is performed alone, so a contrast agent is used.

[0041] When performing a CT scan using a contrast agent, it is preferable to identify the timing when the contrast agent has filled the region of interest and then begin the scan. While using a large amount of contrast agent makes it easier to obtain image data of the region of interest filled with contrast agent, there are concerns about side effects on the subject P due to the use of large amounts of contrast agent. Similarly, while performing a CT scan for a long period makes it easier to obtain image data of the region of interest filled with contrast agent, there are concerns about increased radiation exposure to the subject P. Therefore, to suppress the amount of contrast agent used and the radiation exposure to the subject P, it is preferable to identify the appropriate timing when the contrast agent has filled the region of interest, begin the scan, and complete the scan in a short time.

[0042] To ensure that the CT scan is performed at the appropriate time, it is advisable to perform a pre-scan prior to the main CT scan. For example, the system control function 144a first initiates a pre-scan for the subject P. Specifically, the system control function 144a rotates the X-ray tube 111 around the subject P and irradiates the subject P with X-rays. The X-rays that pass through the subject P are detected by the X-ray detector 112, and projection data at various irradiation angles (views) is generated. The generation function 144c then reconstructs the image data based on the projection data.

[0043] In the pre-scan phase, projection data is collected and image data is reconstructed sequentially. Specifically, the generation function 144c generates multiple time-series image data based on the multiple time-series projection data collected by the pre-scan. The control function 144d determines the timing for transitioning from the pre-scan to the main scan based on these multiple time-series image data. For example, as shown in Figure 2, the control function 144d starts imaging when the pixel value of the monitoring area clears a threshold.

[0044] The monitoring area may be the region of interest (ROI) as shown in Figure 2, or it may be an area near the region of interest. For example, when the right atrium is the region of interest, the control function 144d may monitor the right atrium, or it may monitor the vena cava, which supplies blood to the right atrium. The pixel value used for comparison with the threshold may be the CT value, or it may be a grayscale value obtained by converting the CT value for the display image.

[0045] Furthermore, the threshold can be a preset value or a value that the user adjusts as needed. For example, the threshold can be empirically set for each combination of region of interest and monitoring region. Specifically, the threshold can be optimized by setting a threshold at which the transition from prescan to main scan occurs, increasing the threshold if the transition is too early, and decreasing the threshold if it is too late.

[0046] If the monitoring area is a region near the region of interest, the control function 144d can calculate the time from when the contrast agent enters the monitoring area until it enters the region of interest, based on the distance between the monitoring area and the region of interest, blood flow velocity, etc., and determine the timing to proceed to the main scan. Alternatively, if the monitoring area is the region of interest, the control function 144d can calculate the time from when the initial contrast agent enters the region of interest until it diffuses throughout the entire region of interest, based on the size of the region of interest, blood flow velocity, etc., and determine the timing to proceed to the main scan.

[0047] If the control function 144d determines the timing for transitioning to the main scan before the determined timing, the system control function 144a waits until the determined timing and then starts the main scan. However, due to the computation time required for reconstruction processing, it is also possible that the control function 144d determines the timing for transitioning to the main scan after the determined timing. In this case, the system control function 144a starts the main scan immediately after the control function 144d has determined the timing for transitioning to the main scan.

[0048] As mentioned above, by monitoring changes in pixel values ​​within the monitoring area on the image data collected during the pre-scan and detecting the inflow of contrast agent, the timing for transitioning to the main scan can be determined. However, generating image data takes a certain amount of time. For example, in the case of full reconstruction, projection data for "360°" must be collected before reconstruction can be performed, and in the case of half reconstruction, projection data for "180° + fan angle" must be collected before reconstruction can begin. In addition, computation time is required for the reconstruction process. As a result, there was a time lag between the inflow of contrast agent into the monitoring area and the determination of the timing for transitioning to the main scan, and there were cases where the main scan could not be started at the appropriate time.

[0049] Furthermore, image data may contain artifacts (e.g., metal artifacts). Pre-scans are typically performed at lower doses than main scans, making them more susceptible to artifacts. While various reconstruction methods have been proposed to reduce artifacts, it is difficult to suppress artifacts through reconstruction methods alone, as the reconstruction process must be completed quickly to minimize the aforementioned time lag. Moreover, if image data contains artifacts, these may be mistakenly identified as contrast agent inflow, preventing the main scan from starting at the appropriate time.

[0050] Therefore, the X-ray CT scanner 10 can easily determine the timing to proceed to the main scan based on the projection data, without performing any reconstruction processing.

[0051] Specifically, in the pre-scan, as shown in Figure 3, for example, X-rays are irradiated so as to pass through the monitoring area of ​​the subject P. Then, the X-ray detector 112 detects the X-rays that have passed through the monitoring area, and projection data is generated.

[0052] For example, the system control function 144a rotates the X-ray tube 111 around the subject P and irradiates the subject P with X-rays. Here, since it is not necessary to collect reconstructible projection data (such as projection data for "360°" or projection data for "180° + fan angle"), the system control function 144a may irradiate with X-rays intermittently to reduce the radiation dose.

[0053] For example, the system control function 144a irradiates X-rays when the X-ray tube 111 is positioned at a predetermined irradiation angle. That is, the system control function 144a irradiates X-rays once each time the X-ray tube 111 makes one rotation around the subject P. In this case, all the projection data collected will be from the same irradiation angle. As will be described in detail later, the control function 144d estimates the amount of contrast agent present each time projection data is collected and compares it with a threshold. Although there is a possibility of influence from the subject P's body movement, by collecting multiple projection data at the same irradiation angle, the calculation conditions for estimating the amount of contrast agent present can be standardized.

[0054] Here, the system control function 144a may collect projection data in such a way that bone is not included in the X-ray path. For example, when collecting projection data for the abdomen, by irradiating the subject P from the left and right directions (i.e., by irradiating with X-rays at a 90° angle), it is possible to collect projection data that does not include bones such as the spine in the X-ray path. This eliminates the need to set "bone (Calcium)" in the material discrimination process described later, making calculations easier.

[0055] In the pre-scan stage, it is also possible to collect projection data by irradiating the X-ray tube 111 with X-rays while the rotating part, including the X-ray tube 111, is fixed in place. However, in this case, time is required to accelerate the rotating part to a predetermined rotation speed before transitioning to the main scan. If it is not possible to complete the acceleration of the rotating part between the time the timing for transitioning to the main scan is determined and the time the rotating part is determined, it is preferable to start rotating the rotating part from the pre-scan stage.

[0056] To give another example, the system control function 144a irradiates with X-rays at regular intervals or at regular angles. In this case, the collected projection data will be data from various irradiation angles. By increasing the interval between X-ray irradiations, the radiation dose to subject P can be reduced. On the other hand, by shortening the interval between X-ray irradiations, it becomes possible to estimate the amount of contrast agent present and compare it with the threshold, as described later, at shorter intervals, and to determine the timing of the transition from pre-scan to main scan more accurately.

[0057] Figure 3 shows an example where fan-beam-shaped X-ray irradiation is performed on an area including the monitoring region. However, the estimation of the amount of contrast agent present, as described later, can be performed with projection data for one path that has passed through the monitoring region (data for one detection element). That is, the system control function 144a may collect projection data using laser-like X-rays with a fan angle of approximately 0. For example, the system control function 144a may set the fan angle so that the monitoring region is included in the X-ray irradiation range even with the influence of the subject P's body movement, and irradiate a narrow area including the monitoring region with X-rays.

[0058] The monitoring area can be defined, for example, based on pre-collected image information. Examples of such image information include pre-collected X-ray CT images, low-dose helical scans for positioning such as 3D landmark scans, and 3D scans.

[0059] Furthermore, the monitoring area can be defined without using image information. In other words, the monitoring area can be defined based on anatomical knowledge about the positional relationships of various organs and blood vessels.

[0060] Furthermore, if there are substances with a high absorption coefficient in the subject P's body (for example, medical devices such as pacemakers or metal immobilization devices), it is preferable that these substances do not overlap with the monitoring area. This is because the X-rays would be greatly attenuated, resulting in a weaker detected signal, which could reduce the accuracy of the calculations used to estimate the amount of contrast agent, as described later. If pre-collected image information is available, the position of the monitoring area and the X-ray irradiation angle can be adjusted based on this image information so that the monitoring area does not overlap with substances with a high absorption coefficient.

[0061] Furthermore, if it is known that a substance with a high absorption coefficient exists in the body of subject P, and pre-collected image information cannot be used, the image data may be reconstructed based on projection data collected by pre-scan to identify the location of the substance with the high absorption coefficient. Note that the pre-scan in this case is a non-CE (Contrast Enhanced) scan, and blood vessels will not be depicted in the reconstructed image data.

[0062] If projection data is collected at various irradiation angles, and there is an irradiation angle where a substance with a high absorption coefficient overlaps with the monitoring area, the projection data at that irradiation angle may be excluded from the estimation of the amount of contrast agent, as described later. For example, if the collected projection data contains portions where the CT value exceeds a threshold (such as the CT value equivalent to metal), the projection data may be excluded from the estimation of the amount of contrast agent, as described later.

[0063] Next, the control function 144d obtains the count value for each energy bin from the projection data, performs substance discrimination processing, and estimates the amount of contrast agent present. For example, the control function 144d estimates the amount of each of the two substances present by solving equations (1) and (2) below.

[0064]

number

[0065]

number

[0066] Equations (1) and (2) define two energy bins. In equations (1) and (2), "N(bin1)" is the count value of energy bin "bin1". That is, "N(bin1)" is the number of X-ray photons that have the X-ray energy contained in energy bin "bin1" among the X-ray photons incident on the X-ray detector 112. Similarly, "N(bin2)" is the count value of energy bin "bin2". "u1(bin1)" represents the linear attenuation coefficient of substance 1 in energy bin "bin1", "u2(bin1)" represents the linear attenuation coefficient of substance 2 in energy bin "bin1", "u1(bin2)" represents the linear attenuation coefficient of substance 1 in energy bin "bin2", and "u2(bin2)" represents the linear attenuation coefficient of substance 1 in energy bin "bin2". The values ​​of these linear attenuation coefficients are known.

[0067] "L1" is the path length through which substance 1 exists, and "L2" is the path length through which substance 2 exists. "L1" and "L2" can be determined by solving equations (1) and (2). For example, when an iodine-based contrast agent is used, equations (1) and (2) can be solved by setting substance 1 to "water" and substance 2 to "iodine".

[0068] The control function 144d solves equations (1) and (2) each time projection data is collected to determine the path length of the contrast agent (the path length of "iodine," "L2"). For example, before the contrast agent flows into the monitoring area, the path length of the contrast agent is approximately 0. Subsequently, as the contrast agent flows into the monitoring area, the path length of the contrast agent increases. The control function 144d can then determine the timing of the transition from pre-scan to main scan depending on whether the path length of the contrast agent exceeds a threshold.

[0069] It should be noted that the contrast agent pathway length is not limited to the actual presence of the contrast agent within that length. For example, suppose a contrast agent flows into a blood vessel with a diameter of "2 mm" and mixes with the blood to a concentration of "10%". In this case, the contrast agent pathway length is calculated as "0.2 mm", which is "10%" of "2 mm", but the actual contrast agent is dispersed within a "2 mm" area.

[0070] While an example of comparing the contrast agent pathway length with a threshold has been described, the embodiments are not limited to this. For example, as mentioned above, if the vessel diameter is known to be "2 mm", then the contrast agent pathway length of "0.2 mm" can be estimated to be "10%". The timing of transitioning from prescan to main scan may be determined by comparing such a ratio of pathway length to vessel diameter with a threshold.

[0071] The length of the contrast agent pathway and the ratio of the pathway length to the vessel diameter are examples of the amount of contrast agent present. In other words, the amount of contrast agent present is not limited to the mass or volume of the contrast agent, but includes various parameters that fluctuate in response to increases or decreases in the contrast agent. For example, it is clear that the amount of contrast agent increases as the length of the contrast agent pathway increases. Any parameter that correlates with the increase or decrease in the contrast agent is included as an example of the amount of contrast agent present.

[0072] We have explained an example of setting two substances and estimating their respective abundances, but the number of substances to set can be changed arbitrarily. For example, in the chest and head, as shown in Figure 4, "bone" is often included within the X-ray irradiation range. Therefore, in addition to the "water" and "iodine" mentioned above, "bone (calcium)" can be added as substance 3 to the calculation, and the abundance of each can be calculated. In principle, there is no limit to the number of substances that can be set, and by setting the number of energy bins corresponding to the number of substances, it becomes possible to solve simultaneous equations.

[0073] An example of determining the path length of each substance using simultaneous equations such as equations (1) and (2) has been described, but the embodiments are not limited to this. For example, an energy bin corresponding to the contrast agent may be set up, and the amount of contrast agent present may be estimated based on the count value in the energy bin. For example, by determining the correlation between the count value in a certain energy bin and the amount of contrast agent present in advance, the count value in the energy bin can be converted to the amount of contrast agent present and used for comparison with a threshold.

[0074] Alternatively, multiple energy bins can be set for a single contrast agent, and the calculated values ​​from each energy bin can be used for comparison with a threshold. For example, some contrast agents may exhibit a k-edge, as shown in Figures 5A and 5B. Here, by setting two energy bins before and after the k-edge and subtracting the count values ​​from each energy bin, a value emphasizing the k-edge can be obtained.

[0075] Specifically, in Figures 5A and 5B, when the k-absorption edge is at energy E2, energy bin "bin1" with energies E1-E2 is set as the first energy bin, and energy bin "bin2" with energies E2-E3 is set as the first energy bin. Here, if contrast agent is present in the X-ray path, the difference between the count value of energy bin "bin1" and the count value of energy bin "bin2" will be large. On the other hand, if no contrast agent is present, the k-absorption edge will not appear, and the difference between the count value of energy bin "bin1" and the count value of energy bin "bin2" will be small. As described above, the difference between the count value of energy bin "bin1" and the count value of energy bin "bin2" is an example of the amount of contrast agent present, and by comparing it with a threshold, the timing of transitioning from prescan to main scan can be determined.

[0076] Furthermore, when collecting projection data at various irradiation angles, the substances contained in the X-ray path and the amount of noise will vary for each projection data. This may cause fluctuations in the accuracy of the estimation of the amount of contrast agent present, as described above. Therefore, the control function 144d may combine and process multiple projection data with different irradiation angles to mitigate the effects of fluctuations in the substances contained in the X-ray path and the amount of noise. For example, the control function 144d may average the count values ​​for each energy bin of several adjacent views to reduce noise and then perform the estimation of the amount of contrast agent present, as described above.

[0077] As described above, the X-ray CT apparatus 10 of the embodiment includes a photon counting type X-ray detector 112 that outputs a signal capable of measuring the energy value of incident X-ray photons, and a control function 144d that determines the timing of transitioning from prescan to main scan. The control function 144d determines the timing of transitioning from prescan to main scan by estimating the amount of contrast agent present on the X-ray path including the monitoring area from projection data generated when the X-ray detector 112 detects X-rays that have passed through the monitoring area set for the subject P during the prescan. Therefore, the X-ray CT apparatus 10 of the embodiment can easily determine the timing of transitioning to the main scan without performing reconstruction processing.

[0078] Furthermore, by omitting the reconstruction process, the X-ray CT scanner 10 can quickly determine whether or not contrast agent has flowed into the monitoring area. As mentioned above, if the reconstruction process takes time, it can cause a time lag before the timing to transition to the main scan can be determined, and the main scan may not be able to start at the appropriate time. In other words, it is conceivable that by the time the timing to transition to the main scan is determined, the determined timing may have already passed. In contrast, the X-ray CT scanner 10 of this embodiment shortens the time lag by omitting the reconstruction process and speeding up the processing, making it possible to transition to the main scan at the appropriate time.

[0079] Furthermore, artifacts may occur or be amplified during the reconstruction process, which is particularly noticeable when the reconstruction process is performed quickly. In contrast, the X-ray CT apparatus 10 of this embodiment omits the reconstruction process, thereby suppressing the effects of artifacts and enabling accurate determination of the timing for transitioning to the main scan.

[0080] Furthermore, the X-ray CT apparatus 10 of this embodiment can intermittently irradiate with X-rays to collect projection data and determine the timing of transitioning from pre-scan to main scan based on this projection data. In other words, since it is not used for reconstruction processing, it is possible to reduce the amount of projection data to be collected, thereby reducing the radiation exposure of the subject P.

[0081] The embodiments described above describe a case in which one type of contrast agent is injected, but the embodiments are not limited to this. That is, when multiple types of contrast agents are injected, the timing of performing the main scan may be determined for each contrast agent.

[0082] For example, a subject P is injected with a first iodine-based contrast agent and a second gadolinium-based contrast agent. By comparing the amount of the first contrast agent with a threshold, the timing for transitioning to the first main scan can be determined, and image data with visualized blood vessels in the region of interest can be obtained. Similarly, by comparing the amount of the second contrast agent with a threshold, the timing for transitioning to the second main scan can be determined, and image data with the second contrast agent accumulated in a specific area (e.g., a lesion) in the region of interest can be obtained.

[0083] As described above, the control function 144d can determine the timing of the transition from prescan to main scan based on projection data without performing reconstruction processing. Alternatively, the control function 144d can perform reconstruction processing and determine the timing of the transition from prescan to main scan based on image data. The control function 144d may switch between these methods as appropriate.

[0084] The method of determining the timing of transitioning to the main scan based on projection data has many advantages, including its simplicity, high processing speed, accurate timing determination due to high processing speed, and reduced radiation exposure because intermittent X-ray irradiation is sufficient. However, since reconstruction processing is not performed and 3D information cannot be obtained, it is conceivable that the monitoring area may not be able to be identified, for example, when the vascular shape is complex. For example, when a certain vascular area is designated as the monitoring area, it may be impossible to distinguish whether the contrast agent has flowed into that vascular or only into a nearby vascular. In such cases, the control function 144d can perform reconstruction processing, distinguish vasculars based on 3D image data, and then determine the timing of transitioning from pre-scan to main scan. The control function 144d may switch between these methods automatically based on the complexity of the vascular shape, etc., or a user such as a physician may switch manually.

[0085] In the embodiments described above, the type of contrast agent is not particularly limited and may be a positive contrast agent mainly composed of iodine or barium sulfate, or a gaseous contrast agent such as carbon dioxide. Furthermore, the injection of the contrast agent may be performed manually by a user such as a physician, or it may be performed automatically by an injector provided in the X-ray CT apparatus 10.

[0086] Furthermore, the embodiments described above describe a case in which the amount of contrast agent present on the X-ray path including the monitoring area is estimated, and the timing of transitioning from prescan to main scan is determined by comparing the estimated amount with a threshold. However, the embodiments are not limited to this. For example, the timing of transitioning from prescan to main scan may be determined based on the estimated rate of increase (time gradient) of the amount present.

[0087] Furthermore, the above-described embodiment described an example in which material discrimination of projection data is performed using a photon-counting type X-ray detector 112. However, the embodiment is not limited to a photon-counting type X-ray detector 112 as long as material discrimination of projection data is possible. For example, the X-ray CT apparatus 10 may realize the above-described embodiment by performing dual-energy imaging, collecting high-energy / low-energy projection data, and performing material discrimination processing.

[0088] For example, the X-ray CT apparatus 10 may be equipped with a stacked detector as the X-ray detector 112. Such a stacked detector consists of a first layer and a second layer, and detects X-rays irradiated from the X-ray tube 111 by spectral analysis. For example, the first layer detects low-energy X-rays, and the second layer 112b detects high-energy X-rays that have passed through the first layer. Then, high-energy projection data can be obtained based on the output from the first layer, and low-energy projection data can be obtained based on the output from the second layer.

[0089] Each component of the apparatus according to the above embodiment is a functional concept and does not necessarily have to be physically configured as shown in the illustration. That is, the specific form of distribution and integration of each apparatus is not limited to that shown in the illustration, and all or part of it can be functionally or physically distributed and integrated in any unit according to various loads and usage conditions. Furthermore, each processing function performed by each apparatus can be implemented in whole or in any part by a CPU and a program that is analyzed and executed by the CPU, or by hardware using wired logic.

[0090] Furthermore, the method described in the above-mentioned embodiments can be implemented by executing a pre-prepared program on a computer such as a personal computer or workstation. This program can be distributed via a network such as the Internet. Alternatively, this program can be recorded on a computer-readable non-transient recording medium such as a hard disk, flexible disk (FD), CD-ROM, MO, or DVD, and executed by reading it from the recording medium by a computer.

[0091] According to at least one embodiment described above, the timing for transitioning to the main scan in an examination using a contrast agent can be easily determined.

[0092] While several embodiments have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be implemented in a variety of other forms, and various omissions, substitutions, modifications, and combinations of embodiments can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of symbols]

[0093] 10:X-ray CT device 111:X-ray tube 112: X-ray detector 144: Processing Circuit 144a: System control function 144b: Preprocessing function 144c:Generation function 144d: Control function

Claims

1. An X-ray detector that outputs a signal corresponding to the energy of the incident X-rays, In pre-scanning, projection data acquisition means acquires projection data generated by the detection of X-rays that have passed through a set monitoring area for the subject by the X-ray detector, An index acquisition means for acquiring a contrast agent quantity index that fluctuates in correlation with the increase or decrease of the contrast agent component in the X-ray path represented by the projection data, based on detection information corresponding to the energy of the X-rays contained in the projection data, A timing determination means that determines the timing of transitioning from the pre-scan to the main scan based on the contrast agent volume index, An X-ray CT scanner equipped with [the following features].

2. The X-ray CT apparatus according to claim 1, wherein the index acquisition means performs a substance discrimination process on the projection data and acquires the contrast agent amount index by obtaining the component corresponding to the contrast agent through the substance discrimination process.

3. The X-ray CT apparatus according to claim 2, wherein the contrast agent volume index is the path length of the contrast agent estimated by the substance discrimination process.

4. The X-ray CT apparatus according to claim 1, wherein the index acquisition means acquires the contrast agent amount index from the acquired detection information based on a predetermined correlation between the detection information in a predetermined energy bin obtained from the projection data and the contrast agent amount index.

5. The X-ray CT apparatus according to claim 4, wherein the index acquisition means converts the detection information in the predetermined energy bin into the contrast agent amount index based on the correlation.

6. The X-ray CT apparatus according to claim 1, wherein the index acquisition means acquires the contrast agent amount index by the difference between first detection information on the low-energy side of the k-absorption edge of the contrast agent and second detection information on the high-energy side of the k-absorption edge, obtained from the projection data.

7. The X-ray CT apparatus according to claim 1, wherein the timing determination means determines the timing of transitioning from the pre-scan to the main scan by comparing the contrast agent volume index with a threshold.

8. The projection data acquisition means acquires a plurality of time-series projection data in the prescan, The index acquisition means acquires the contrast agent amount index based on each of the plurality of projection data, The X-ray CT apparatus according to claim 1, wherein the timing determination means determines the timing of transitioning from the pre-scan to the main scan based on the change in the contrast agent volume index acquired in a time series.

9. The X-ray CT apparatus according to claim 8, wherein the timing determination means determines the timing of transitioning from the prescan to the main scan based on the rate of increase of the contrast agent volume index acquired in a time series.

10. The X-ray CT apparatus according to claim 1, wherein the projection data is a plurality of time-series projection data collected at the same irradiation angle.

11. The X-ray CT apparatus according to claim 1, wherein the projection data is collected by intermittently irradiating the subject with X-rays while rotating the X-ray tube around the subject.

12. The X-ray CT apparatus according to claim 1, wherein the X-ray detector is a photon counting type detector that outputs a signal capable of measuring the energy value of incident X-ray photons.

13. The X-ray CT apparatus according to claim 1, wherein the main scan is a scan that collects projection data to be used for image interpretation.

14. In the pre-scan, projection data is acquired by detecting X-rays that have passed through a set monitoring area for the subject using an X-ray detector that outputs a signal corresponding to the energy of the incident X-rays. Based on detection information corresponding to the X-ray energy contained in the projection data, an index of contrast agent volume is obtained that fluctuates in correlation with the increase or decrease of the contrast agent component in the X-ray path represented by the projection data. Based on the aforementioned contrast agent dosage index, the timing of the transition from the pre-scan to the main scan is determined. Methods that include...

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