JP2013215480A - X-ray ct apparatus - Google Patents

Abstract

An X-ray CT apparatus capable of collecting data in synchronization with an effective tube voltage switching timing even when the tube voltage is switched at high speed.
An X-ray CT apparatus comprising an X-ray tube, a tube voltage generation unit, an X-ray detector, a tube voltage monitoring unit, and a data collection unit. The X-ray tube emits X-rays toward the subject. The tube voltage generator applies different tube voltages to the X-ray tube in a time-sharing manner. The X-ray detector is arranged corresponding to the X-ray tube with the subject interposed therebetween, and detects X-rays transmitted through the subject. The tube voltage monitoring unit monitors a change in tube voltage along a time series, and identifies a timing at which the tube voltage is effectively switched in response to switching. The data collection unit collects projection data based on different tube voltages in synchronization with the specified timing.
[Selection] Figure 1

Description

  The present invention relates to a technique of an X-ray CT apparatus.

  An X-ray computed tomography apparatus (hereinafter referred to as “X-ray CT apparatus”) irradiates a subject with X-rays and detects X-rays transmitted through the subject, thereby detecting X-rays within the subject. Projection data consisting of absorption coefficients is obtained.

  There is a so-called dual energy system in the X-ray CT apparatus. In the dual energy method, two different X-ray images are generated in one scan using two X-rays having different energies (different from each other in quality). Thereby, for example, by obtaining a difference between these image data, it is possible to extract a portion composed of a predetermined substance (for example, calcium carbonate).

  When photographing with the dual energy method, there is a method in which X-rays having different radiation qualities are exposed in a time-sharing manner by switching a tube voltage applied to the X-ray tube between a high voltage and a low voltage in time series. . In such a case, the data collection unit recognizes whether the collected data is a high voltage or a low voltage according to a predetermined timing based on the switching timing of the tube voltage. Acquire as different data. By operating in this way, different data is collected in a time division manner in synchronization with the switching of the tube voltage.

JP 2010-274108 A

  On the other hand, in the case of such a method, it is ideal that the low voltage and the high voltage can be switched instantaneously, but actually, it takes time to switch the tube voltage. When switching between a high voltage and a low voltage at high speed as in a method called Fast kV Switching, the time required for this switching cannot be ignored, and data may be collected for this time as well. In such a case, a desired voltage value between the high voltage and the low voltage is set as a threshold value, and each threshold value is recognized as a high voltage or a low voltage.

  However, the time required for switching the tube voltage is not always constant, and varies depending on the period of the high voltage and the low voltage, for example, due to a change in the effective tube voltage due to a temperature change or a disturbance derived from a circuit. Occurs. For this reason, the actual tube voltage switching timing may not match the data collection switching timing. If the period of data collection at each voltage is sufficiently long with respect to this timing shift, the effect of this shift can be ignored. However, when the tube voltage is switched at high speed, this timing shift causes the projection data Appears as noise on top.

  An object of the embodiment of the present invention is to provide an X-ray CT apparatus capable of collecting data in synchronization with an effective tube voltage switching timing even when the tube voltage is switched at high speed.

  An embodiment of the present invention is an X-ray CT apparatus including an X-ray tube, a tube voltage generation unit, an X-ray detector, a tube voltage monitoring unit, and a data collection unit. The X-ray tube emits X-rays toward the subject. The tube voltage generator applies different tube voltages to the X-ray tube in a time-sharing manner. The X-ray detector is arranged corresponding to the X-ray tube with the subject interposed therebetween, and detects X-rays transmitted through the subject. The tube voltage monitoring unit monitors a change in tube voltage along a time series, and identifies a timing at which the tube voltage is effectively switched in response to switching. The data collection unit collects projection data based on different tube voltages in synchronization with the specified timing.

It is a block diagram of the X-ray CT apparatus concerning this embodiment. It is an example of the result of having measured the permeation | transmission amount at the time of changing tube current and tube voltage about two filters from which a characteristic differs. It is an example of the relationship between dose ratio REFa / RFFb, tube voltage, and tube current. It is the graph which showed the relationship between dose ratio and tube voltage. It is the figure shown about the change along the time series of a tube voltage and a dose ratio, and the relationship of collection timing.

  The configuration of the X-ray CT apparatus according to the present embodiment will be described with reference to FIG. As shown in the block diagram of FIG. 1, the X-ray CT apparatus according to the present embodiment includes an imaging unit 500, a scan control unit 501, a preprocessing unit 31, an X-ray projection data storage unit 32, and a reconstruction process. The unit 33, the image storage unit 34, the image processing unit 35, and the display unit 36 are configured.

  The imaging unit 500 includes a gantry 1, a tube voltage generation unit 7, an X-ray control unit 8, a gantry / bed controller 9, a data collection unit 10, and a tube voltage monitoring unit 20. The gantry 1 includes a rotating ring 2, an X-ray source (X-ray generation unit) 3, an X-ray filter 4, an X-ray detector 5, and a scan control unit 501. The X-ray detector 5 is an array type X-ray detector. That is, in the X-ray detector 5, detection elements are arranged in a matrix of m rows in the channel direction and n columns in the slice direction.

  The X-ray source 3 and the X-ray detector 5 are installed on the rotating ring 2 and are arranged opposite to each other with the subject lying on the slide bed 6 interposed therebetween. Each channel is associated with each detection element constituting the X-ray detector 5. The X-ray source 3 is opposed to the subject via the X-ray filter 4. The tube voltage generator 7 drives the X-ray source 3 based on the control from the X-ray controller 8. The tube voltage generator 7 applies a high voltage to the X-ray source 3 at a predetermined timing based on control from the X-ray controller 8. As a result, X-rays are generated by the X-ray source 3, and the gantry / bed controller 9 controls the rotation of the rotating ring 2 of the gantry 1 and the sliding of the sliding bed 6 synchronously.

  The imaging unit 500 according to the present embodiment is configured to be operable in a dual energy system. Specifically, the voltage applied by the tube voltage generator 7 to the X-ray source 3 based on the timing determined in advance based on the control by the X-ray controller 8 is a high voltage and a low voltage in time series. Vary between. Thereby, the X-ray in the case of a high voltage and the X-ray in the case of a low voltage are exposed from the X-ray source 3 in a time division manner.

  The scan control unit 501 constitutes the control center of the entire system, and is based on the pre-designated projection data acquisition conditions (hereinafter sometimes referred to as “scan conditions”), the X-ray control unit 8, the gantry / bed The controller 9 controls the sliding bed 6. That is, the scan control unit 501 rotates the rotating ring 2 along a predetermined path around the subject while irradiating the X-ray from the X-ray source 3. Note that the resolution and resolution of the projection data are determined based on predetermined scanning conditions. In other words, scanning conditions are determined in advance according to the required resolution and resolution, and the scan control unit 501 controls the operation of each unit based on the scanning conditions.

  In the case of operating in the dual energy system, the scan condition includes information indicating the exposure timing of the X-rays when the tube voltage is high and the X-rays when the tube voltage is low. Based on this information, the X-ray control unit 8 specifies the timing for switching the tube voltage applied to the X-ray source 3 between the high voltage and the low voltage, and controls the tube voltage in accordance with the specified timing.

  In addition, when an instruction to stop scanning is given, the scan control unit 501 controls the X-ray control unit 8, the gantry / bed controller 9, and the sliding bed 6 to stop imaging. Accordingly, the scan control unit 501 automatically stops scanning with this instruction as a trigger.

  The detection elements constituting the X-ray detector 5 have the intensity of X-rays generated by the X-ray source 3 both when the subject is interposed between the X-ray source 3 and the detection element and when the subject is not interposed. Can be measured. Therefore, each detection element measures at least one X-ray intensity and outputs an analog output signal corresponding to this intensity. The output signal from each detection element is read by the data collection unit 10 described later.

  The tube voltage monitoring unit 20 includes transmission amount measuring units 21 a and 21 b, a timing specifying unit 22, and a tube voltage information storage unit 23. The tube voltage monitoring unit 20 monitors a change in dose accompanying the switching of the tube voltage, and specifies the timing at which the tube voltage is effectively switched based on this change. Hereinafter, the details of each part constituting the tube voltage monitoring unit 20 will be described.

  The transmission amount measuring units 21a and 21b do not block the X-rays that are exposed to the subject from the X-ray source 3, and the dose of X-rays that are exposed from the X-ray source 3 and are not transmitted through the subject Is provided at a position where it can be measured. For example, in FIG. 1, it is provided in the vicinity of the X-ray source 3. Note that the installation location is not limited as long as the dose of X-rays that do not pass through the subject can be measured. For example, the X-ray detector 5 may be provided at the end of the X-ray detector 5.

The transmission amount measuring units 21a and 21b are provided with filters having different amounts of transmitted dose (hereinafter referred to as “transmission amount”). The amount of transmission of the filter varies depending on the material constituting the filter and the size (strictly, density × thickness) of the filter. In other words, the filters provided in the transmission amount measuring units 21a and 21b have different transmission amounts by, for example, “changing the size with the same material”, “changing the material with the same size”, or “changing the material and the size”. In general, there is no problem if the material constituting the filter has different atomic numbers. More precisely, when the attenuation coefficient of one material is μa (E) [cm ⁻¹ ] and the attenuation coefficient of the other material is μa (E) [cm ⁻¹ ], μa (E) = Any combination of materials that does not become k · μb (E) (k is an integer) may be used.

  The transmission amount measuring units 21a and 21b receive the X-rays that are irradiated from the X-ray source 3 and do not pass through the subject, and respectively measure the dose (that is, the transmission amount) after passing through the filter. At this time, different transmission amounts are measured depending on the difference of the filters provided in each. Hereinafter, the transmission amount measured by the transmission amount measurement unit 21a is referred to as REFa, and the transmission amount measured by the transmission amount measurement unit 21b is referred to as REFb.

  The information indicating the transmission amounts REFa and REFb measured by the transmission amount measuring units 21a and 21b is at least a predetermined time resolution capable of detecting variations in periods of high voltage and low voltage due to an effective change in tube voltage. Is read by the timing specifying unit 22 at each timing.

  The timing specifying unit 22 reads information indicating the X-ray transmission amounts REFa and REFb from the transmission amount measurement units 21a and 21b at every predetermined timing. Based on the information indicating the read transmission amounts REFa and REFb, the timing specifying unit 22 calculates a transmission amount ratio (hereinafter referred to as “dose ratio”) REFa / RFFb. The timing specifying unit 22 monitors the change in the calculated dose ratio REFa / RFFb and detects the timing of switching from the high voltage to the low voltage and from the low voltage to the high voltage with the desired threshold as a boundary. In other words, the timing specifying unit 22 indirectly monitors the change in tube voltage via the dose ratio REFa / RFFb and detects the tube voltage switching timing. The details are summarized below.

  The dose of X-rays exposed from the X-ray source 3 varies depending on the tube voltage and tube current. When the tube voltage is constant and only the tube current changes, the transmission amounts REFa and REFb change at the same ratio. Therefore, in this case, the dose ratio REFa / RFFb does not change. On the other hand, when the tube current is constant and the tube voltage changes, the ratios of changes in the transmission amounts REFa and REFb are different from each other. Therefore, the influence of the change in the tube current is separated, and the dose ratio REFa / RFFb changes in conjunction with the change in the tube voltage.

Here, a specific example of the relationship between the above-described transmission amounts REFa and REFb, the tube voltage and the tube current (that is, the filter characteristics of the transmission amount measurement units 21a and 21b) will be described with reference to FIG. 2A. I will explain. FIG. 2A shows an example of a result of measuring the transmission amount when the tube current and the tube voltage are changed for two filters having different characteristics. In the example of FIG. 2A, a material of the material Al (aluminum), a thickness of 5 mm, and a density of 2.699 [g / cm ³ ] is used for the filter of the transmission amount measuring unit 21a. The filter of the transmission amount measuring unit 21b is made of a material Al (aluminum), a thickness of 20 mm, and a density of 2.699 [g / cm ³ ]. That is, the material and density are constant and only the thickness is changed. In addition, when the tube voltage is 80 [kVp], 100 [kVp], 120 [kVp], and 135 [kVp], the tube current is 50 [mA], 100 [mA], and 150 [mA]. The amount of X-ray transmission [lsb] was measured. For example, when the tube voltage is 80 [kVp] and the tube current is 50 [mA], the transmission amount REFa = 65, 136 [lsb] and the transmission amount REFb = 13, 361 [lsb] as shown in FIG. 2A. It becomes. Also, when the tube voltage is 100 [kVp] and the tube current is 100 [mA], the transmission amount REFa = 248, 199 [lsb] and the transmission amount REFb = 67130 [lsb].

  Reference is now made to FIG. FIG. 2B shows the results of calculating the dose ratio REFa / RFFb of the transmission amounts REFa and REFb shown in FIG. 2A for each combination of tube voltage and tube current shown in FIG. 2A. That is, FIG. 2B shows an example of the relationship between the dose ratio REFa / RFFb, the tube voltage, and the tube current. For example, when the tube voltage is fixed at 80 [kVp] and only the tube current is changed, the dose ratio REFa / RFFb is about 4.87 in any case as shown in FIG. 2B. That is, even if the tube voltage is constant and only the tube current is changed, the dose ratio REFa / RFFb does not change. This characteristic is the same for other tube voltage values. On the other hand, when the tube current is fixed to 50 [mA] and only the tube voltage is changed, the dose ratio REFa / RFFb varies from about 4.87 to about 2.94 according to the tube voltage. That is, the dose ratio REFa / RFFb does not depend on a change in tube current but depends on a change in tube voltage. Therefore, in the X-ray CT apparatus according to the present embodiment, the relationship between the dose ratio REFa / RFFb and the tube voltage is examined in advance, and the change in the tube voltage is monitored indirectly via the dose ratio REFa / RFFb. Detect voltage switching timing. Information indicating the relationship between the tube voltage and the dose ratio REFa / RFFb (hereinafter referred to as “tube voltage information”) is generated in advance and stored in the tube voltage information storage unit 23. FIG. 3 shows an example of the tube voltage information.

  The timing specifying unit 22 stores information indicating a threshold value of the tube voltage indicating a threshold for switching between a predetermined high voltage and a low voltage. This information may be designated by the operator via the U / I before the inspection, or predetermined information (fixed value) may be stored. The timing specifying unit 22 compares the threshold of the tube voltage with the tube voltage information stored in the tube voltage information storage unit 23, so that the threshold of the dose ratio REFa / RFFb corresponding to the voltage value is at least before the examination. Specify in advance.

  Reference is now made to FIG. For example, when the predetermined tube voltage threshold is “110 [kVp]”, the timing specifying unit 22 specifies “3.5” as the dose ratio threshold based on the tube voltage information.

  The timing specifying unit 22 sequentially compares the dose ratio REFa / RFFb calculated based on the information indicating the transmission amounts REFa and REFb read at each predetermined timing with a dose ratio threshold value specified in advance. By this comparison, the timing specifying unit 22 detects the timing at which the tube voltage is switched from the high voltage to the low voltage and from the low voltage to the high voltage. Specifically, in the example illustrated in FIG. 3, the timing specifying unit 22 indicates the timing at which the dose ratio REFa / RFFb has changed from “less than 3.5” to “3.5 or more”, and the tube voltage is “high voltage ( 110 [kVp]) ”to“ low voltage (110 [kVp] or less) ”. In addition, the timing specifying unit 22 changes the timing at which the dose ratio REFa / RFFb changes from “3.5 or more” to “less than 3.5” from “low voltage (110 [kVp] or less)” to “high”. This is detected as the timing of switching to “voltage (exceeding 110 [kVp])”.

  When each timing related to the switching of the tube voltage is detected, the timing specifying unit 22 synchronizes with this timing, and the data collection unit 10 generates a signal based on the X-ray when the voltage is high (hereinafter, “high voltage ) And a signal based on an X-ray in the case of a low voltage (hereinafter, referred to as “signal in the case of a low voltage”) are generated to generate a trigger signal.

  Here, with reference to FIG. 4, the relationship between the change in the tube voltage and the dose ratio along the time series and the trigger signal (that is, the collection timing) will be described. FIG. 4 is a diagram showing the relationship between changes in tube voltage and dose ratio along the time series and collection timing (in other words, integration period). When the tube voltage is switched at a high speed, as shown in FIG. 4, the change of the voltage value along the time series becomes close to a triangular wave. The ratio of the transmission amounts REFa and REFb, that is, the dose ratio REFa / RFFb changes together with the change in the tube voltage. Specifically, when the tube voltage increases, the dose ratio REFa / RFFb decreases in conjunction with this, and when the tube voltage decreases, the dose ratio REFa / RFFb increases. That is, the change in the tube voltage and the change in the dose ratio REFa / RFFb are synchronized.

  R1 in FIG. 4 indicates the threshold value of the dose ratio REFa / RFFb. For example, when the tube voltage threshold is “110 [kVp]”, the threshold R1 of the dose ratio REFa / RFFb is “3.5” as described above (see FIG. 3).

  For example, in FIG. 4, at time t1, the dose ratio REFa / RFFb is switched from the threshold value R1 to the threshold value R1. That is, the timing specifying unit 22 detects that the tube voltage has been switched from the low voltage to the high voltage at this timing. At time t2, the dose ratio REFa / RFFb is switched from less than the threshold value R1 to the threshold value R1 or more. That is, the timing specifying unit 22 detects that the tube voltage has been switched from a high voltage to a low voltage at this timing. Thereby, the timing specific | specification part 22 produces | generates a trigger signal so that the data collection part 10 may collect the signal from the X-ray detector 5 as a signal in the case of a high voltage between time t1 and t2. That is, the time t1 to t2 is a signal collection period (signal integration period) in the case of a high voltage.

  Further, at time t3, the dose ratio REFa / RFFb is switched from the threshold value R1 or more to less than the threshold value R1. That is, the timing specifying unit 22 detects that the tube voltage has been switched from the low voltage to the high voltage at this timing. Thereby, the timing specific | specification part 22 produces | generates a trigger signal so that the data acquisition part 10 may collect the signal from the X-ray detector 5 as a signal in the case of a low voltage between time t2 and t3. That is, the period from t2 to t3 is a signal collection period (signal integration period) when the voltage is low.

  It should be noted that the cycle of the process related to the monitoring of the tube voltage, that is, the cycle for specifying the timing by calculating the dose ratio REFa / RFFb and comparing it with the threshold value R1, is set to be shorter than the cycle for switching the tube voltage. Needless to say.

  Moreover, the change in tube voltage may be accompanied by, for example, a change in temperature or noise derived from a circuit. That is, there is a case where the waveform (a triangular wave in the example of FIG. 4) shown in FIG. 4 has a higher frequency noise than this waveform, and this noise is also applied to the dose ratio REFa / RFFb. Appear in Therefore, due to this noise, the dose ratio REFa / RFFb may change at a high speed around the threshold value R1 in the vicinity of the timing at which the voltage value is switched. When such an operation is assumed, the timing specifying unit 22 may specify the timing immediately after the predetermined number of times of switching as the timing at which the tube voltage is switched to improve the robustness.

  Moreover, if the timing which a tube voltage switches can be specified, it will not be limited to the specific method based on above-mentioned dose ratio REFa / RFFb. For example, the voltage applied to the X-ray source 3 by the tube voltage generator 7 may be directly measured, and the timing at which the tube voltage is switched may be specified based on the measured value.

  The timing specifying unit 22 outputs a trigger signal generated based on the detected timing to the data collection unit 10.

  The data collecting unit 10 receives a trigger signal for collecting data from the timing specifying unit 22. Based on this trigger signal, the data collection unit 10 recognizes the timing at which the X-rays are switched with the change in the tube voltage. That is, in synchronization with this trigger signal, the data collection unit 10 distinguishes the signal in the case of a high voltage and the signal in the case of a low voltage from each detection element constituting the X-ray detector 5 and reads them in a time-sharing manner. .

  The data collection unit 10 amplifies each signal read in synchronization with the notified timing and integrates each signal to generate digital data. Hereinafter, the digital data converted from the signal based on the X-ray in the case of high voltage is referred to as “digital data in the case of high voltage”, and the digital data converted from the signal based on the X-ray in the case of low voltage is referred to as “low voltage”. In this case, it is called “digital data”. The data collection unit 10 distinguishes the digital data in the case of a high voltage and the digital data in the case of a low voltage, and outputs them to the preprocessing unit 31.

  The preprocessing unit 31 receives the digital data in the case of a high voltage and the digital data in the case of a low voltage from the data collection unit 10 separately from each other. The preprocessing unit 31 performs processing such as sensitivity correction on each of these digital data to obtain projection data. The preprocessing unit 13 stores these projection data in the X-ray projection data storage unit 32 so that they can be distinguished and read out. The X-ray projection data storage unit 32 is a storage unit for storing the acquired projection data.

  The reconstruction processing unit 33 reads the projection data stored in the X-ray projection data storage unit 32. The reconstruction processing unit 33 back-projects the read projection data using, for example, a reconstruction algorithm called the Feldkamp method to generate image data. The reconstruction processing unit 33 causes the image storage unit 34 to store the reconstructed image data. The image storage unit 34 is a storage unit for storing image data.

  The image processing unit 35 reads image data from the image storage unit 34. The image processing unit 35 generates an image such as a tomographic image or a three-dimensional image such as a still image or a moving image based on the image data. The image processing unit 35 causes the display unit 36 to display the generated image.

  In the X-ray CT apparatus according to the present embodiment, as described above, when imaging by the dual energy method, the tube voltage applied to the X-ray tube is switched between the high voltage and the low voltage in time series. Then, X-rays with different radiation qualities are exposed in a time-sharing manner. In such a system, data acquisition is performed in synchronization with the switching timing of the X-rays, so that projection data based on X-rays in the case of high voltage and projection data based on X-rays in the case of low voltage are obtained. Collect in time division.

  The timing at which the tube voltage applied to the X-ray source 3 is switched is controlled by the X-ray controller 8 as described above. However, the time required for switching the tube voltage is not necessarily constant. For example, there are variations in the periods of the high voltage and the low voltage due to an effective change in the tube voltage due to a temperature change or a disturbance derived from a circuit. Arise. For this reason, if data collection is switched based on a predetermined timing, the data collection timing may be different from the actual tube voltage switching timing. If the timing is shifted in this way, for example, X-rays generated at a high voltage may be recognized as X-rays generated at a low voltage and data may be collected. If the period of data collection at each voltage is sufficiently long with respect to this deviation, the influence of this deviation can be ignored.However, if the tube voltage is switched at a high speed, this timing deviation will appear on the projection data. It becomes manifest as noise.

  Therefore, in the X-ray CT apparatus according to the present embodiment, the X-ray dose exposed from the X-ray source 3 is monitored as described above, and the tube voltage is switched based on the change in dose accompanying the switching of the tube voltage. Identify timing. The X-ray CT apparatus according to the present embodiment collects data in synchronization with the specified timing. By operating in this way, the timing of data collection is synchronized with the effective switching timing of the tube voltage even when there are variations in the high voltage and low voltage periods due to changes in the effective tube voltage. Thus, it becomes possible to collect each data by distinguishing X-rays generated at a high voltage and X-rays generated at a low voltage.

  As described above, according to the ultrasonic diagnostic apparatus according to the present embodiment, the change in the effective tube voltage along the time series is monitored, and the timing for switching the data collected by the data collection unit 10 is specified based on the monitoring result. To do. Thus, as in the case of switching the tube voltage at a high speed, even when variations occur in the high voltage and low voltage periods due to changes in the effective tube voltage due to temperature changes, disturbances derived from the circuit, etc., Projection data can be collected in synchronization with this timing.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention and are included in the equivalent scope described in the claims.

DESCRIPTION OF SYMBOLS 1 Gantry 2 Rotating ring 3 X-ray source 4 X-ray filter 5 X-ray detector 6 Sliding bed 7 Tube voltage generation part 8 X-ray control part 9 Gantry / bed controller 10 Data collection part 20 Tube voltage monitoring part 21a, 21b Transmission Quantity measuring unit 22 Timing specifying unit 23 Tube voltage information storage unit 31 Preprocessing unit 32 X-ray projection data storage unit 33 Reconstruction processing unit 34 Image storage unit 35 Image processing unit 36 Display unit 500 Imaging unit 501 Scan control unit

Claims (4)

An X-ray tube that emits X-rays toward the subject;
A tube voltage generator for switching and applying different tube voltages to the X-ray tube in a time-sharing manner;
An X-ray detector arranged corresponding to the X-ray tube across the subject and detecting X-rays transmitted through the subject;
A change in the tube voltage along a time series, and a tube voltage monitoring unit that identifies the timing at which the tube voltage is effectively switched in response to the switching;
A data collection unit for collecting projection data based on each of the different tube voltages in synchronization with the identified timing;
An X-ray CT apparatus comprising: The tube voltage monitoring unit is
A first filter that includes a predetermined filter whose transmission amount changes according to the exposed X-ray, receives the X-ray that has not passed through the subject, and measures a first transmission amount that has passed through the filter. A measuring section of
A second measurement unit that includes a filter having a transmission amount different from that of the first measurement unit, receives the X-ray, and measures a second transmission amount that has passed through the filter;
A timing specifying unit that specifies the timing based on a dose ratio that is a ratio between the first transmission amount and the second transmission amount;
The X-ray CT apparatus according to claim 1, comprising:   The timing specifying unit stores in advance the dose ratio in the tube voltage at the time of the effective switching as a threshold value, and sets the dose ratio based on the first transmission amount and the second transmission amount as the threshold value. The X-ray CT apparatus according to claim 2, wherein the timing is specified by comparison. The tube voltage monitoring unit is
A storage unit for storing tube voltage information indicating a relationship between the dose ratio and the tube voltage;
The X-ray CT apparatus according to claim 3, wherein the timing specifying unit obtains the threshold value based on the tube voltage information.

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