JP2001190543A - Method for detecting x-ray beam quantity and x-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily obtain image data in the optimum range at all times in a method for detecting X-ray beam quantity and in an X-ray CT apparatus. SOLUTION: In this method for detecting X-ray beam quantity, an X-ray beam is irradiated for a subject 100 from an X-ray tube 40, the X-ray beam passing through the subject is detected, the detected output is integrated by integrators 811 to 81n to generate an X-ray beam quantity detection voltage, and the output is A/D converted by an A/D converter 83 to obtain X-ray beam quantity detection data. Time constants of the integrators 811 to 81n are changed in accordance with one or two or more imaging parameters of a tube voltage kV of the X-ray tube, a tube current mA, a detection width Thic in axis direction of the subject, and X-ray irradiation time Sec for the subject which are imaging parameters to generate an X-ray beam quantity detection voltage in the optimum dynamic range which does not exceed the range for the A/D converter 83 per channel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray beam amount detection method and an X-ray CT (Computed Tomography) apparatus, and more particularly to an X-ray beam emitted from an X-ray tube and transmitted through a subject.
The present invention relates to an X-ray beam amount detection method and an X-ray CT apparatus that detect a line beam by an opposing detector, and integrate and A / D convert the output to obtain X-ray beam amount detection data (projection data).

In general, the imaging conditions (X-ray tube voltage kV, tube current mA, detection width Thic in the body axis direction of the subject, X-ray irradiation time Sec on the subject, etc.) of the X-ray CT apparatus are as follows:
It differs greatly depending on the physique of the subject and the purpose of imaging based on medical treatment, and it is desired that an appropriate X-ray CT image can be obtained under any imaging conditions.

[0003]

2. Description of the Related Art FIG. 10 is a diagram showing the configuration of a main part of a conventional X-ray CT apparatus, and mainly shows the configuration of a data acquisition system (DAS). In the figure, 40 is an X-ray tube, 50 is an X-ray tube.
A collimator for limiting a line irradiation range (fan direction and slice thickness direction), 100 is a subject, 20 is a subject 100
An X-ray detector array in which a large number (for example, n = 1000) of X-ray detectors are arranged in, for example, a single row in an arc shape, XD1 to XD
Dn is an X-ray detector including a scintillator and a photodiode, 81 _{1 to} 81 _n are integrators for detecting the amount of X-ray beams, A 1 to An are amplifiers, SH 1 to SHn are sample-hold circuits, and 82 is a signal multiplexer (MPX). ), 8
3 is an A / D converter (A / D), 15 is a data acquisition buffer, 11 is a central processing unit that performs main control and processing (scan control, CT image reconstruction processing, etc.) of the X-ray CT apparatus, 11
a is a CPU, 11b is a main memory (MEM) storing a control program executed by the CPU, and 14 is a CP.
A control interface 84 of U11a is a timing generator (TG) for generating various timing signals related to X-ray data detection / collection control.

[0004] In summary, the fan beam emitted from the X-ray tube 40 and restricted by the collimator 50 is simultaneously incident on the X-ray detector array 70 through the subject 100. Focusing on the signal processing of the X-ray beam XB1, the X-ray detector XD1 outputs a current signal IB1 corresponding to the intensity of the X-ray beam XB1, and the integrator 81 ₁ converts the input current signal IB1 into a constant time constant. (Capacity C).

[0005] Here, this integration operation is described by a general equation. Equation (1) is defined between the output voltage Vo and the input current Ii of the integrator.

[0006]

(Equation 1)

[0007] There is a relationship. This is a case where the input (signal source) of the integrator is a current source. According to the equation (1), the output voltage Vo of the integrator is in a relationship of integrating the input current Ii at a ratio of (1 / C). . Therefore, in this specification, this capacitance C is also called a time constant of integration. In this case, the resistance R
Is a protection resistance of a so-called circuit having a relatively small resistance value, and does not contribute to the integration operation (time constant).

Furthermore, the amplifier A1 amplifies the output of the integrator 81 _1, the sample-hold circuit SH1 samples and holds the output of the amplifier A1 at a predetermined timing. Other X
The same applies to each signal processing of the line beams XB2 to XBn. Then, the signal multiplexer 82 scans each sample output of the sample hold circuits SH1 to SHn at high speed, and the A / D converter 83 performs A / D conversion on each output of the signal multiplexer 82 at high speed. A series of main signal data (X-ray beam amount detection data) thus obtained is stored in the data collection buffer 15 and processed by the CPU 11a.

In the integrators 81 _{1 to} 81 _n , a block “RESET” is a reset circuit of a capacitance (capacitor) C, and a reset signal R of a timing generator 84.
When S is energized, the analog switch S5 is closed,
The charge Q of the capacitor C is discharged via the reset circuit. The block "AUTO ZERO" is an input offset voltage calibration circuit of the operational amplifier OA, and when the calibration signal CAL of the timing generation unit 84 is activated, the analog switches S6 and S7 are switched to the opposite sides shown in FIG. The bias state of the calibration circuit is updated so that the output voltage of the operational amplifier OA becomes 0V.

In the above, the case where the integrator 81 ₁ directly integrates the detected current output IB1 of the X-ray detector XD1 to generate the corresponding integrated voltage signal V1 has been described, but the present invention is not limited to this. Depending on the X-ray CT device, the detection current output IB1 of the X-ray detector XD1 is once converted into a voltage signal VB1 (including logarithmic conversion) by the preamplifier PA1 as shown by an arrow in the figure, and then converted to the integrator 81 ₁ . There are input methods.

In this case, the integrator 81 ₁ (the operational amplifier OA)
In the input circuit of 1), the relationship of IB1 = VB1 / R always holds. That is, generally speaking, the input current Ii and the input voltage Vi of the integrator 81 ₁ always have Ii. = Vi / R holds, and when this is substituted into the above-described equation (1) of current integration, the equation (2) is obtained between the output voltage Vo of the integrator and the input current Ii,

[0012]

(Equation 2)

There is a relationship as follows. This is a case where the input (signal source) of the integrator is a voltage source. According to the equation (2), the output voltage Vo of the integrator is a relation that integrates the input voltage Vi at a ratio of (1 / CR). It is in. Therefore, in this specification, the product (CR) of the capacitance C and the resistance R is also referred to as a time constant of integration.
It is clear that the resistance R in this case contributes to the integration operation (time constant). In the following description, the configuration corresponding to the current integration of the above equation (1) will be mainly described.
If necessary, a configuration corresponding to the voltage integration of the above equation (2) will be described.

Conventionally, the X of the subject 100 is
X-ray CT imaging is performed. As described above, imaging conditions of an X-ray CT apparatus generally vary greatly depending on the physique of a subject, imaging purposes based on medical treatment, and the like. Tube voltage kV, tube current mA, detection width Thic of the subject in the body axis direction, and X to the subject
The line irradiation time Sec is changed over a wide range.

[0015]

However, under such circumstances, the conventional DAS gain (that is, the time constant C of the integrator 81) is fixed to a constant value regardless of the imaging conditions. Depending on the magnitude of the imaging conditions (tube voltage kV, tube current mA, etc.), the output voltage of the amplifier A (that is, the sample-and-hold circuit SH) is changed to the conversion upper limit value LI of the A / D converter 83.
When the CT image is reconstructed with the acquired data, the streak artifact or an arch partly blackened was obtained.

Hereinafter, the operation characteristics of the conventional integrator 81 will be specifically described. In the following description, the relationship between the integration output V of the integrator 81 and the conversion output upper limit LIM of the A / D converter 83 becomes a problem, and therefore, the gain of the amplifier A is 1 for simplicity. The description is made as follows. That is, in the following description, the integral output V of the integrator 81 is the input of the A / D converter 83.

FIG. 11 is a diagram for explaining the operation characteristics of the conventional integrator. FIG. 11A shows an integrated output when currents having different magnitudes (i1 <i2 <i3) are input per unit integration time t2. The relationship of voltage (v1 <v2 <v3) is shown.
Here, LIM is the upper limit of the A / D conversion output. The conventional DAS gain (corresponding to the time constant C of the integrator 81) is the integral output v when the magnitude of the input current IB is, for example, i2.
2 was fixed to be equal to the upper limit value LIM. Therefore, if, for example, i3 larger than i2 is input, the integrated output voltage v3 exceeds the upper limit LIM as shown in the figure, and the A / D conversion output becomes the upper limit LIM.
Was clamped to.

FIG. 11B shows main signal data (profile) for one view, and the horizontal axis represents the X-ray detector XD.
1 to XDn (that is, detection channels CH1 to CHn). In the figure, an appropriate A / D conversion output is obtained in each of the detection channels whose input current does not exceed i2, but the A / D conversion output is obtained in each of the detection channels whose input current exceeds i2.
Since the D-converted output is clamped at the upper limit LIM, such main signal data causes an artifact.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to provide an X-ray beam amount detection method and an X-ray beam amount detection method capable of easily obtaining imaging data in an optimum range.
An object of the present invention is to provide a line CT apparatus.

[0020]

The above-mentioned problem is solved, for example, by referring to FIG.
Is solved. That is, the X-ray beam amount detection method of the present invention (1) irradiates the subject 100 with an X-ray beam from the X-ray tube 40 under predetermined imaging parameters, and transmits the X-ray beam transmitted through the subject 100. Is detected, the detection output is integrated by integrators 81 _{1 to} 81 _n to generate an X-ray beam amount detection voltage, and the output is A / D converted by an A / D converter 83 to detect the X-ray beam amount. In the X-ray beam amount detection method for obtaining data, tube voltages kV and tube current mA of the X-ray tube 40, the detection width Thic in the body axis direction of the subject 100, and the X-ray irradiation time Sec on the subject 100 are imaging parameters. The time constants of the integrators 81 _{1 to} 81 _n are changed according to any one or more of the imaging parameters.

According to the present invention (1), X-ray CT imaging
Integrator 81 according to imaging parameters ₁~ 81_nTime constant
Can be changed based on the physique and medical treatment of the subject.
Imaging parameters during X-ray CT imaging depending on the imaging purpose
Is changed over a wide range, the A / D converter 83
X-ray beam always in the optimal range to avoid overrange
It is possible to generate a beam amount detection voltage.

The integrators 81 _{1 to} 81 _n integrate the detection output of the X-ray beam transmitted through the subject 100 to generate an X-ray beam amount detection voltage. , X-ray beam detection current and X-ray beam detection voltage.

Preferably, in the present invention (2), when the conversion upper limit value of the A / D converter 83 is V _{LIM in the} present invention (1), the first imaging parameter is set to X in advance.
The tube voltage kV _s , tube current mA _{s of} the tube 40,
The first subject by the imaging to the X-ray irradiation time Sec _s of the detection width THIC _s and the subject 100 in the body axis direction 1
X-ray beam amount detection data for the desired imaging region of No. 00 is collected, and the second imaging parameter is later set to the X-ray tube 40.
Tube voltage kV _a, tube current mA _a, when performing a second imaging of the X-ray irradiation time Sec _a to the detection width THIC _a and the subject 100 in the body axis direction of the subject 100, the first, The ratio Ratio of the second imaging parameter is obtained from the ratio of one or more corresponding item values of the two imaging parameters or the product of the respective ratios, and X to be detected in the second imaging.
The time constant of the integrators 81 _{1 to} 81 _n for preventing the line beam amount detection voltage from exceeding the upper limit value V _LIM is determined by the above-described ratio Ratio.
Is determined based on

According to the present invention (2), at least one or more of the first imaging parameter previously adopted in the first imaging and the second imaging parameter set in the second imaging later. the ratio of the corresponding item value (e.g. mA _a / mA _s) or the product of the ratio {e.g. _{(mA a / mA s) ×} (Thic a / Thi
c _s ) ×... are determined, and the time constants of the integrators 81 _{1 to} 81 _n during the second imaging are determined based on the obtained ratio Ratio. No matter how large the parameter is changed, not only does the X-ray beam amount detection voltage to be detected in the second imaging not overrange the A / D converter 83, but also the integrator 8
It is possible to operate the X-ray beam amount detection voltage of 1 _{1 to} 81 _{n in} a linear full range that is lower than the upper limit value V _LIM of the A / D conversion output, so that the imaging data (X-ray Beam amount detection data).

Preferably, in the present invention (3), in the above-mentioned present invention (2), the time constants are the first and second time constants.
And the maximum value V _MAX and / or the minimum value V _{MIN of} the X-ray beam amount detection data collected in the first imaging.

According to the present invention (3), in determining the time constant, the ratio between the first and second imaging parameters is used as a reference, and the amount of X-ray beams collected in the first imaging is detected. By further taking into account the maximum value V _MAX and / or the minimum value V _MIN of the data, the X
The line beam amount detection data can be used effectively,
, The time constant can be set more finely at the time of imaging.

More specifically, for example, the difference between the two (V _MAX
When −V _MIN ) is small, it is highly possible that the physique of the subject 100 is lower than normal. Therefore, even if the ratio Ratio between the two imaging parameters is slightly large when this ratio is evaluated by itself, the second
Is unlikely to be over-ranged in the data collection at the time of imaging, so that the capacity (time constant) C in this case can be kept small. On the other hand, the difference (V _MAX −V
_{When MIN} ) is large, there is a high possibility that the physique of the subject 100 is large, and therefore, even if the ratio Ratio of the two imaging parameters is not so large when evaluated independently, the overrange in the data collection at the second imaging is large. Therefore, in this case, it is possible to increase the capacity (time constant) C.

The above-mentioned problem is solved by, for example, the configuration of FIG.
Is done. That is, the X-ray beam amount detection method of the present invention (4)
Is an object from the X-ray tube 40 under predetermined imaging parameters.
100 is irradiated with an X-ray beam,
The passed X-ray beam is detected, and the detected output is
₁~ 81_nTo generate an X-ray beam amount detection voltage,
The output is A / D converted by an A / D converter 83, and the
In the X-ray beam amount detection method for obtaining the system amount detection data,
The conversion upper limit value of the A / D converter 83 is V_LIMAnd if
The tube current of the X-ray tube 40 is set to a predetermined value mA in advance._sFirst shooting
An X-ray image of a desired imaging area of the
Collect the volume detection data, and
Maximum value V_MAXIs detected, and a second photographing performed later is performed.
X-ray beam amount detection voltage to be detected in an image is the upper limit value
V_LIMTube current mA not to exceed _aIs the upper limit value V
_LIMAnd the maximum value V_MAXIs calculated based on the ratio
You.

According to the present invention (4), the tube for the second imaging is provided.
Current mA_aIs the upper limit value V of the A / D conversion output._LIMAnd in advance
Maximum value of both X-ray beam detection data collected in the first imaging
V_{MA X}(For example, V_LIM/ V_MAX) Based on
(Predicting) the proper tube for the second imaging.
Current mA_aCan be easily set. Therefore, the second shooting
The X-ray beam amount detection voltage at the time of image is supplied to the A / D converter 83
Not only overrange, but also
0 is the upper limit value V of the A / D conversion output_LIMSlightly below
Images can be taken with the optimal dynamic range, and therefore the optimal lens
Image data (X-ray beam amount detection data)
Can be easily obtained.

The above-mentioned problem can be solved, for example, by the structure shown in FIG. That is, the X-ray CT apparatus of the present invention (5) includes the X-ray tube 40 and the detector 7 which face each other across the subject 100.
0, the integrators 81 _{1 to} 81 _n for integrating the detection output of the X-ray beam intensity by the detector 70 to generate a corresponding X-ray beam amount detection voltage, and the output voltages of the integrators 81 _{1 to} 81 _n as A An A / D converter 83 for performing A / D conversion, and an X-ray C that reconstructs a tomographic image of the subject 100 based on X-ray beam amount detection data (main signal data) output from the A / D converter 83
In the T device, tube voltages kV and tube current mA of the X-ray tube 40, which are imaging parameters, a detection width Thic in the body axis direction of the subject 100, and an X-ray irradiation time Sec on the subject 100
The time constants of the integrators 81 _{1 to} 81 _n (for example, the resistance values R _{1 to} R 4) in accordance with one or more of the imaging parameters
And / or the capacitance values C1 to C4) are variably configured.

The integrators 81 _{1 to} 81 _n integrate the detection output of the X-ray beam intensity by the detector 70 to generate a corresponding X-ray beam amount detection voltage. Includes the detection current of the X-ray beam detected by the detector 70 and the detection voltage obtained by current-voltage conversion (including logarithmic conversion or the like) of the detection current of the X-ray beam detected by the detector 70 by, for example, the preamplifiers PA1 to PAn.

Preferably, in the present invention (6), in the above-mentioned present invention (5), an A / D converter 83 having an upper limit value of A / D conversion of V _LIM and an X-ray tube voltage kV _s of the tube, tube current mA _s, X for a desired imaging area of the first object by the imaging to the X-ray irradiation time Sec _s to detection width THIC _s and the subject in the body axis direction of the subject An imaging control unit (central processing unit) 11 for collecting line beam amount detection data, and the imaging control unit later sets _a second imaging parameter to a tube voltage kV _a of the X-ray tube, _a tube current m
A _a, when performing a second imaging of the X-ray irradiation time Sec _a to the detection width THIC _a and subject in the body axis direction of the subject, the first, the ratio Ratio of the second imaging parameters the both A ratio calculating means 3 for calculating a ratio of one or more corresponding item values of the imaging parameters or a product of the respective ratios, and an X-ray beam amount detection voltage to be detected in the second imaging is the upper limit value Integrators 81 _{1 to} 81 for not exceeding V _LIM
selection control means 4 for selecting a time constant of _n based on the determined ratio Ratio.

Preferably, in the present invention (7), in the above present invention (6), the selection control means 4
The time constant of the integrators 81 _{1 to} 81 _n is selected based on the ratio Ratio of the imaging parameters and the maximum value V _MAX and / or the minimum value V _{MIN of} the X-ray beam amount detection data collected in the first imaging. I do.

The above-mentioned problem can be solved, for example, by the structure shown in FIG. That is, the X-ray CT apparatus of the present invention (8) includes the X-ray tube 40 and the detector 7 that face each other across the subject 100.
0, the integrators 81 _{1 to} 81 _n for integrating the detection output of the X-ray beam intensity by the detector 70 to generate a corresponding X-ray beam amount detection voltage, and the output voltages of the integrators 81 _{1 to} 81 _n as A An A / D converter 83 for reconstructing a tomographic image of a subject based on the X-ray beam amount detection data output from the A / D converter 83 is provided. A / D converter 83 whose upper limit value is V _LIM ,
An imaging control unit 11 for collecting X-ray beam quantity detection data of the desired imaging region of the object 100 by the first imaging of the tube current of the line pipe 40 to a predetermined value mA _s, the collected X-ray beam quantity detection A maximum value detecting means 1 for detecting a maximum value V _MAX in the body axis direction of the data; and an X-ray beam amount detection voltage to be detected in a second imaging performed later by the imaging control unit 11 is the upper limit value. the tube current mA _a for not exceeding V _LIM upper limit V _LIM and the maximum value V
And a tube current calculating means 2 for obtaining a value based on the ratio to _MAX .

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Note that the same reference numerals indicate the same or corresponding parts throughout the drawings. FIG. 2 is a block diagram of the X-ray CT apparatus according to the embodiment. In the figure, reference numeral 10 denotes an operation console operated by a user;
Reference numeral 0 denotes an imaging table on which the subject is placed and moved in the body axis direction. Reference numeral 30 denotes an Axial of the subject by an X-ray fan beam or the like.
/ Hericl A scanning gantry for scanning and reading.

In the scanning gantry 30, 40 is an X-ray tube, 41 is X-ray irradiation timing (corresponding to the X-ray irradiation time Sec to the subject) and X-ray intensity (tube voltage kV, tube current m
A) an X-ray control unit for controlling A), 50 a collimator for limiting the X-ray exposure range, and 51 adjusting the X-ray transmission slit width (corresponding to the detection width Thic in the body axis direction of the subject) and position. A collimator control unit 60 is a rotation control unit for rotating the X-ray tube 40, the X-ray detector array 70, and the like around the body axis of the subject, and a large number (for example, n = 1000) of X-ray detectors are arc-shaped For example, an X-ray detector array 80 is arranged in a line, and a data acquisition unit (DAS) 80 collects detection data of the X-ray detector array.

In the operation console 10, reference numeral 11 denotes a central processing unit that performs main control and processing (scan control, CT image reconstruction processing, etc.) of the X-ray CT apparatus, 12 denotes an input device that receives instructions and information of an operator, 13 is a scanning protocol (imaging parameters kV, mA, Sec, Thic, etc.) and C
A display device (CRT) for displaying a T-reconstructed image;
Reference numeral 4 denotes a control interface for outputting various control signals and the like to the scanning gantry 30 and the imaging table 20, reference numeral 15 denotes a data collection buffer for storing main signal data from the data collection unit 80, and reference numeral 16 denotes various types of data necessary for operation of the X-ray CT apparatus. A secondary storage device (such as a disk) that stores data, application programs, and the like.

FIG. 3 shows a data collection unit (D) according to the embodiment.
AS) is a block diagram. The integrators 81 _{1 to} 81 _n according to the present embodiment have capacitors C 1 to C 4 (hereinafter, referred to as capacitors C 1 to C 4) including a plurality of capacitance values C 1 to C 4 so that the integration time constant C can be set and changed as desired. ) Are provided in parallel, and the capacitors C1 to C4 are connected to the analog switch AS.
It is configured to be switchable and connectable to a resistor R by W. That is, the selection command SL from the CPU 11a is transmitted via the control interface 14 to each of the decoders D of the integrators 81 _{1 to} 81 _n.
In addition to EC, one or more switches S1 to S4 corresponding to the selection command SL are closed. Thereby, the time constant C of the integrators 81 _{1 to} 81 _n can be changed in a minute to wide range. Here, the values of the capacitors C1 to C4 are C1 <C2 <C3.
<C4 is assumed.

As shown by the insertion arrows in the figure, the preamplifiers PA1 to PAn are connected to the respective outputs of the X-ray detectors XD1 to XDn.
And input current I to integrators 81 _{1 to} 81 _{n in} advance.
In an X-ray CT apparatus that converts B1 to IBn into corresponding voltage signals VB1 to VBn (including logarithmic conversion),
Instead of the configuration of selecting one or two or more capacitors C1 to C4, for example, a single capacitor C and a plurality of resistors R1 to R4 or the like are provided in series or in parallel in the circuit instead of the resistor R in the drawing, The resistors R1 to R4 may be configured to be selected by the analog switch ASW. In this case, the integrators 81 _1-
The time constant (CR) of 81 _n can be changed. Alternatively, a plurality of capacitors C1 to C4 and resistors R1 to R4 may be provided to change the combination of these connections.

Hereinafter, the operation of imaging using the X-ray CT apparatus according to the present embodiment will be specifically described. 4 and 5 are operation explanatory diagrams (1) and (2) of a scout scan according to the embodiment, and FIG.
The case where the physique of 0 is normal is shown. Here, the scout scan refers to fluoroscopic imaging performed before the main scan in order to obtain optimal imaging parameters for appropriately performing the subsequent main scan. In addition, this scan (Axial / Helical
“Scan” refers to imaging performed for the purpose of obtaining desired imaging data from the subject 100 in accordance with a medical purpose.

In FIG. 4C, a subject 100 having a normal physique is placed on the imaging table 20. When the reference point in the body axis (Z-axis) direction of the subject 100 is Z0,
Now, assuming that the main scan is performed in the range of Z0−10 cm to Z0 + 10 cm, a scout scan with coarse accuracy (intermittent slice positions) is performed in advance in this range. At this time, the imaging table 20 moves in the direction of arrow a. Also, this scout scan is performed at an angle (for example, 0 °) at which the subject 100 is looked down from directly above as shown in FIG. Further, if necessary, for example, as shown in FIG. 4B, the angle at which the subject 100 is viewed from the side (for example, 90 °).
May be followed.

FIG. 4D shows a display example of the imaging parameters on the display device 13. Preferably, this scout scan is performed in a mode in which the amount of exposure of the subject 100 is small, and the protocol (imaging parameter) setting is as follows, for example.

Tube voltage of the X-ray tube (kV _s ) = 120 kV Filament current of the X-ray tube (mA _s ) = 20 mA Slice thickness of the subject (Thic _s ) = 1 mm The X-ray irradiation time of the scout scan is set for one rotation of the gantry. Time (corresponding to scan time) (Sec _s ) = 1
The scan time indicates the X-ray irradiation time for one rotation of the gantry in the Axial scan, and the total X-ray irradiation time in the Scout scan. Therefore,
The scan time in the scout scan is set to a time obtained by converting the total X-ray irradiation time of the scout scan into one rotation of the gantry, and can be compared with a scan time in a main (Axial) scan described later. .

The above protocol setting is performed by the integrator 81.
Is within a range where the DAS gain does not over-range even if the time constant of the integrator 81 is relatively small, and for example, the optimum capacity C2 of the integrator 81 is selected. The user may set an arbitrary value for this protocol setting every time a scout scan is performed, or hold the above protocol setting value as default information and automatically set the protocol setting every time a scout scan is performed. May be.

FIGS. 4E to 4G show examples of profiles of X-ray beam amount detection data (main signal data after A / D conversion) obtained by the above scout scan. FIG.
(E) is a certain angle (for example, 0 °) at the imaging start position
, And a data profile P1s as shown is obtained for the channels CH1 to CHn. Here, LIM is a conversion upper limit value of the A / D converter 83,
MAXV1 is the maximum value of the X-ray beam amount detection data in the region of interest in imaging (channel direction). Similarly, FIGS. 4F and 4G respectively correspond to VEWi and VEWk at the above-mentioned certain angle (0 °) in the middle and at the end of the imaging, and each data profile as shown for the channels CH1 to CHn. Pis, Pks and their respective maximum values MAXVi, MAXVk were obtained. Actually, data profiles of more angles and slice positions can be collected.

The maximum values MAXV1-MA of this example
XVk corresponds to X-ray beam amount detection data that has passed through the air (that is, does not pass through the subject 100), and these uniformly reflect the substantially irradiation energy of the X-ray tube 40. I have. That is, each maximum value MAXV1 in this case
MAXVk are substantially the same.

In some cases, a required filter (water or the like) is provided on the emission surface of the X-ray tube 40. In this case, both ends of each data profile P1s to Pks are attenuated as shown by the dotted lines in the figure. It has become something. Accordingly, the data profiles P1s to Pks in this case are obtained by superimposing the respective attenuation characteristics of the filter and the subject 100,
Each of the maximum values MAXV1 to MAXVk in the attention area is not always substantially the same.

Further, in this example, since the physique of the subject 100 is normal, the amount of attenuation of the X-rays by the subject 100 is not so large, so that the data profiles P1s to P1s
For ks, the minimum values MINV1 to MINVk whose X-ray beam amount detection data has a significant value other than “0” are detected. Therefore, in this case, the amplitude of the attenuation (MAX
V-MINV) can be detected, and conversely, the present apparatus can recognize that the physique of the subject 100 is normal (or somewhat small) from this.

Further, referring to FIGS. 4 (e) to 4 (g),
Each data profile P1s~Pks is, X-rays of the exposure output energy (tube current mA _s etc.) is low results, X-rays beam weight detection data has remained at a relatively low dynamic range, the A / D converter 83 It does not fully utilize the conversion range. Therefore, this is not necessarily the optimal imaging protocol for the subsequent main scan.

FIG. 5 shows a case where the size of the subject (patient) 100 is large. Here, the description of FIGS. 5 (a) to 5 (c) is the same as that of FIGS. 4 (a) to 4 (c). FIG.
In (d), in this scout scan, the subject 10
A slightly larger tube current m in consideration of the large physique of 0
A (= 30 mA) is adopted.

FIGS. 5E to 5G show examples of profiles of X-ray beam amount detection data obtained by the scout scan. In this case, each data profile P1s ~
Pks is the irradiation energy of the X-ray tube 40 (tube current m
Result of enhanced A _s), each of the maximum value to reflect this MAXV
The levels of 1 to MAXVk are uniformly higher.
However, since the physique of the subject 100 is extremely large, the amount of X-ray attenuation by the subject 100 is extremely large,
For example, as shown in FIGS. 5F and 5G, at these slice positions, some of the profile data Pis and Pks are at level 0, and a significant minimum value MINVi, MINVk cannot be detected. That is, on the contrary, the present apparatus can recognize that the physique of the subject 100 is large from this. Also in this case, the protocol adopted in this scout scan is not necessarily the optimal imaging protocol for the subsequent main scan.

Therefore, in the present embodiment, based on the imaging data obtained by such a scout scan, the range in which the X-ray beam amount detection data (accurately, the input of the A / D converter 83) does not over-range. Perform the most suitable main scan within. Hereinafter, this control and processing will be specifically described.

FIG. 6 is a flowchart of the overrange avoiding process according to the embodiment. Here, a series of processes for performing the subsequent main scan in the optimum range based on the main signal data and the like obtained by the scout scan are described. This processing is executed by the CPU 11a of FIG. Step S
In step 1, the above-described protocol (imaging parameters and the like) of the scout scan is set. At this time, the tube current mA
Each imaging parameter other than _s can be set to be the same as the protocol for the subsequent main scan. In this way, when the scanning it is sufficient only to change (increase) the tube current mA _a, this by adopting a method for setting the process described below in the main scan optimum tube current mA _a (simultaneously, D
AS gain) can be accurately predicted. Of course, it goes without saying that the protocol setting at the time of the scout scan can be set to an arbitrary value irrelevant to the time of the main scan.

In step S2, scanning of one view (a certain slice position at a certain angle) is performed. Step S
In step 3, the X-ray beam amount detection data (A / D conversion output) of each of the channels CH1 to CHn is collected in the data collection buffer 15. In step S4, if necessary, predetermined data correction processing (so-called reference correction, inter-channel sensitivity correction, etc.) is performed on the collected data. In step S5, it is determined whether or not all scans in the desired (scout scan plan) area have been completed. If not, the process returns to step S2.

When all the scans are completed, the flow advances to step S6 to extract the maximum X-ray beam amount detection data Vmax for all the profile data in the target area.
In the examples of FIGS. 4E to 4G, for example, MAXVi>
MAXVk> MAXV1, so that the maximum value Vmax = MAXVi of the attention area. Further, in this step S6, the minimum X-ray beam amount detection data Vmin is extracted from all the profile data. The CPU 11a
When the minimum value Vmin> 0, the physique of the subject 100 can be determined to be less than normal, and when the minimum value Vmin = 0, the physique of the subject 100 can be determined to be large. Then, in step S7 the tube current mA _a of the main scan (3),

[0056]

(Equation 3)

Is obtained by Here, if the a coefficient alpha = 1, the tube current mA _a maximum in the tube current mA _s V
_max is the upper limit value V _LIM of the A / D converter 83 during the main scan.
The tube current is as high as In practice, for example, by setting α = 0.9, a margin is given to the dynamic range at the time of the main scan. In step S8 is displayed on the display device 13 the resulting tube current mA _a. At the same time, the extracted maximum value V _max and minimum value Vmin (=
0 (including 0).

In step S9, the user sets the main scan
Set the protocol for In that case, preferably
Is the tube current m for obtaining the above optimum dynamic range.
A_aThe displayed value of is referred to. And if the tube current mA
Each imaging parameter other than
In the case where the same may be used at the time of the scan, the user may select the tube current mA. _a
All you need to do is set. Or the tube current mA indicated above
_aCan be automatically adopted. Of course, this full scan
The setting of the image parameter is
It goes without saying that the relationship can be set to an arbitrary value.

In any case, the following step S10
Then, the ratio Ratio of the imaging parameters (imaging energy) between the main scan and the scout scan is expressed by Equation (4),

[0060]

(Equation 4)

Is determined by Here, for example, β = 3. In practice, there may be a parameter element such that the ratio always becomes 1 or another constant value between the two scans. Therefore, the above expression of the ratio Ratio is at least 1 or 2 where a change may occur between the two scans. What is necessary is just the ratio of the above parameter elements or the product of each ratio. By the way, in an application where the ratio of all parameters other than the tube current mA is a constant value γ (invariant), Ratio = γ × (mA _a / mA _s ) = γ × α × (V _LIM /
Becomes a relationship of V _max), the user even without setting the tube current mA _a during the scan input in step S9, the optimum tube current mA _a for the main scan at step S7
However, in step S10, the ratio of the imaging energy between both scans is automatically obtained.

In step S11, based on the ratio of the imaging energies thus obtained and, if necessary, taking into account the ratio of each parameter element, an optimum value for preventing the outputs of the integrators 81 _{1 to} 81 _n from being overranged. Select the DAS gain (ie, capacitance C). The mode of this selection will be described later. In step S12, the main scan of the subject 100 is performed according to the imaging protocol used in the main scan.

FIG. 7 is a diagram for explaining the operating characteristics of the integrator according to the embodiment. FIG. 7A shows a difference in magnitude per unit integration time t2 as in the case of FIG. 11A. The relationship of the integrated output voltage (v1 ′ <v2 ′ <v3 ′) when the current (i1 <i2 <i3) is input is shown. In FIG. 7 (A), the integral characteristic in the same state as that of FIG. 11 (A) is drawn with a thick dotted line superimposed.
Assuming that the time constant of the integrator 81 in the present embodiment remains C2 (corresponding to the conventional C) at the time of the above-described scout scan, the current i3 larger than the current i2 is input to the integrator 81, for example. The integrated output voltage v3 exceeds the upper limit value LIM as shown in FIG.
The converted output is clamped to the upper limit LIM.

However, according to the present embodiment, FIG.
Ratio of imaging energy obtained in step S10
(Preferably tube voltage kV, tube current mA, slice thickness Thi
Based on the fact that each ratio of c is greater than a predetermined value, the selected capacitance (time constant) in the main scan is set to, for example, C3 (> C2) in step S11. As a result, the input current (i1 <i2) in this case is obtained. <I3) is an integrated output voltage (v1 ′ <v2 ′ <v) in which the respective amplitudes are compressed at a fixed ratio as shown.
3 ′), and thus the overrange can be effectively avoided.

FIG. 7B shows main signal data (profile) for one view, and the horizontal axis represents each X-ray detector X.
It corresponds to the detection channels CH1 to CHn of D1 to XDn. In the figure, each integrated output voltage (v1 '
<V2 ′ <v3 ′) have a fixed ratio (for example, 2 /
As a result of the compression in 3), all the detection channels CH1 to CHn
, Proper main signal data is obtained. Thus,
Optimal range that does not always overrange (optimal DA
(S gain) integrated voltage V is obtained.

The capacity of the integrators 81 _{1 to} 81 _n is selected automatically by the CPU 11a. For example, as shown in FIG. 3, the number of each capacity displayed on the display device 13 is selected. A user who is skilled in imaging may arbitrarily select any of 1 to 4 forcibly.

FIGS. 8 and 9 show a book (Axial) according to the embodiment.
/ Herical) Scan operation explanatory diagrams (1), (2),
FIG. 8 shows a main scan corresponding to the scout scan (subject having a normal physique) of FIG. 8A, the subject 100 having the same normal physique as that of FIG. 4C is mounted on the imaging table 20 in the same state. When the reference point in the body axis direction of the subject 100 is Z0, a precise book (Axial / Herical) scan is performed in the range of Z0-10 cm to Z0 + 10 cm. At this time,
The photographing table 20 moves in the direction of arrow a, for example.
In the case of xial scan, the imaging table 20 is intermittently moved each time one rotation of the scanning gantry 30 is completed. In the case of Herical scanning, the imaging table 20 is continuously moved simultaneously with the rotation of the scanning gantry 30.

The protocol setting at the time of the main scan is shown in FIG.
In step S9 of FIG. 4, the process is performed in the manner shown in FIG. This main scan is performed in a desired mode according to the physique and medical purpose of the subject 100. In this example, since the physique of the subject 100 is less than normal, the protocol setting is as follows, for example.

X-ray tube voltage (kV _a ) = 120 kV X-ray tube filament current (mA _a ) = 40 mA Slice thickness of subject (Thic _a ) = 1 mm Scan time per gantry rotation (Sec _a ) = 1
Here, only the tube current mA _a = 40 mA is twice as large as that in the scout scan shown in FIG. 4, and the tube current mA _a = 40 mA is used as the CP in step S7 in FIG.
The one determined by U11a is automatically adopted. Therefore, the energy ratio between both scans Ratio = (2
/ 1) × 1 × 1 × 1 = 2.

Further, in step S11 in FIG. 6, the integrator 81 based on the energy ratio Ratio = 2 between both scans.
A time constant (capacity) of _{1 to} 81 _n is selected. In this example, furthermore, the ratio of the tube current mA _a twice as small, and the ratio of the scan time Sec _a Considering that 1,
For example, the same capacitance C2 as in the scout scan is selected. Then, in the subsequent step S12, a main scan is performed.

FIGS. 8B to 8D show partial profiles P1a to Pka of the profiles of the X-ray beam amount detection data (accurately, the input of the A / D converter 83) obtained by the main scan.
Are indicated by solid lines. In addition, data profiles P1s to Pks obtained by the scout scan in FIG. 4 are indicated by dotted lines. In FIG. 8C, focusing on the maximum value Vmax (= MAXVi) at the time of the scout scan, the X-ray beam amount detection data at the time of the main scan on the same channel is the optimum value obtained at the step S7 of FIG. As a result of adopting the tube current mA _a = 40 mA as it is, without exceeding (over-ranging) the upper limit LIM of the A / D conversion output, it is about 90% of the A / D conversion upper limit LIM according to the above energy ratio Ratio = 2. Level has been raised. Also, the minimum value MI of the data profile Pis
NVi is also raised at the same time by this energy ratio Ratio, so that the data profile P at the time of the main scan is
The entirety of ia evolves to an optimal (desired) level within the full effective dynamic range. The same applies to other data profiles P1a and Pka.

FIG. 9 shows a main scan corresponding to the scout scan (subject having a large size) shown in FIG. In FIG. 9A, since the physique of the subject 100 is large in this example, the protocol setting is as follows, for example.

X-ray tube voltage (kV _a ) = 120 kV X-ray tube filament current (mA _a ) = 90 mA Slice thickness of subject (Thic _a ) = 1 mm Scan time per gantry rotation (Sec _a ) = 2
Sec Here, the tube current mA _a = 90 mA is three times that of the above scout scan, the scan time Sec _a is twice that of the scout scan, and other parameters are the same as those of the scout scan. Energy ratio Ratio =
(3/1) × 1 × (2/1) × 1 = 6.

Further, in step S11 in FIG. 6, the integrator 81 based on the energy ratio Ratio = 6 between both scans.
A time constant (capacity) of _{1 to} 81 _n is selected. In this example, further, that the tube current mA _a is 3 times (= 90 mA), and in consideration of the fact scan time Sec _a is twice, for example, capacitance C3 (> C2) is selected. Then, in the subsequent step S12, a main scan is performed.

FIGS. 9B to 9D show a part P of the profile of the X-ray beam amount detection data obtained by the main scan.
1a to Pka are indicated by solid lines. In addition, the data profiles P1s to P1s obtained by the scout scan in FIG.
ks are indicated by dotted lines. In FIG. 9C, if attention is paid to the maximum value Vmax (= MAXVi) at the time of the scout scan, the level of the X-ray beam amount detection data at the time of the main scan in the same channel is unchanged according to the above energy ratio Ratio = 6. If it is raised, the upper limit LIM of the A / D conversion output will be greatly overranged as shown by the broken line in the figure.

However, according to the present embodiment, each of the time constants of the integrators 81 _{1 to} 81 _n is _equal to C in the scout scan.
As a result of the setting change to the value C3 larger than 2, the X-ray beam amount detection data at the time of the main scan corresponding to the MAXVi is obtained in accordance with the automatic changing operation of the DAS gain described in FIG. As shown by the solid line, the A /
Do not exceed D conversion upper limit LIM (do not overrange)
It has become something. In addition, the above data profile Pis
The X-ray beam amount detection data at the time of the main scan corresponding to the minimum value MINVi that falls below the level 0 is raised accordingly by the above-mentioned DAS gain changing operation. Therefore, the entire data profile Pia at the time of the main scan is It develops to the optimal (desired) level and full resolution within the full effective dynamic range. The same applies to other data profiles P1a and Pka.

When the capacitance C is selected in step S11 in FIG. 6, the ratio of the two imaging parameters is used as a reference, and in addition to the total X-ray beam amount detection data acquired in the scout scan. The maximum value V _MAX (including LIM) and the minimum value V _MIN (= 0
) Can be further taken into account. Specifically, for example, when the difference (V _MAX −V _MIN ) between the two is small, it is highly possible that the physique of the subject 100 is less than normal, and therefore, the ratio Ratio of the two imaging parameters is evaluated independently. Then, even if it is somewhat large, the possibility of overrange in the DAS at the time of the main scan is low, and therefore, it is possible not to increase the capacity (time constant) C in this case.
On the other hand, when the difference (V _MAX −V _MIN ) between the two is large, there is a high possibility that the physique of the subject 100 is large. There is a high possibility that the DAS at the time of scanning will overrange, and in this case, the capacity (time constant) C can be increased. Also, for example, in the case where the maximum value V _MAX = LIM, there is a high possibility that the DAS at the time of the main scan will be over-ranged. Therefore, also in this case, the capacity (time constant) C can be increased. Thus, according to the present embodiment, the X-ray beam amount detection data collected by the scout scan can be effectively used, and the time constant during the main scan can be set more finely.

In the above embodiment, an example of the integrator 8
1 is shown, the integrator 8
As for one basic circuit configuration, various other known circuit configurations can be adopted.

In the above embodiment, the case where the integrator 81 integrates the input current IB has been described. However, it is apparent that the present invention can be applied to the case where the integrator 81 integrates the input voltage VB.

In general, a high accuracy is required for the time constant of this type of integrator. Therefore, in the above-described embodiment, a plurality of discrete capacitors C (or resistors R) are generally provided so that a high-precision one can be easily obtained. Thus, the case where these connections are switched has been described. However, in the present invention, "changing the time constant of the integrator" is performed.
This does not exclude, for example, a configuration in which the time constant of the integrator can be changed using a single variable capacitor VC (or variable resistor VR).

In the above embodiment, an example of application to an X-ray CT apparatus using a Rotate / Rotate method using a fan beam has been described. However, the present invention provides another configuration having a configuration for integrating a detection signal of an X-ray beam. Any method (Stationary / Rotat
It is apparent that the present invention can be applied to an X-ray CT apparatus of an e-type or a parallel type.

Although the preferred embodiments of the present invention have been described, it goes without saying that various changes in the configuration, control, processing, and combinations thereof can be made without departing from the spirit of the present invention. There is no.

[0083]

As described above, according to the present invention, the X-ray C
By changing the time constant of the integrator for detecting the amount of X-ray beam in accordance with the imaging parameters of T, imaging data in the optimum range can be easily obtained at all times, thus contributing to the improvement of the image quality of the X-ray CT image. However, it is extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a block diagram of the X-ray CT apparatus according to the embodiment.

FIG. 3 is a block diagram of a data collection unit (DAS) according to the embodiment.

FIG. 4 is an explanatory diagram (1) of a scout scan operation according to the embodiment;

FIG. 5 is an explanatory diagram (2) of a scout scan operation according to the embodiment;

FIG. 6 is a flowchart of overrange avoidance processing according to the embodiment.

FIG. 7 is a diagram illustrating the operation characteristics of the integrator according to the embodiment.

FIG. 8 is an explanatory diagram (1) of a main scan operation according to the embodiment.

FIG. 9 is an explanatory diagram (2) of the operation of the main scan according to the embodiment.

FIG. 10 is a diagram showing a main part configuration of a conventional X-ray CT apparatus.

FIG. 11 is a diagram illustrating the operation characteristics of a conventional integrator.

[Explanation of symbols]

Reference Signs List 10 operation console 11 central processing unit 12 input device 13 display device (CRT) 14 control interface 15 data collection buffer 16 secondary storage device 20 imaging table 40 X-ray tube 30 scanning gantry 50 collimator 60 rotation control unit 70 X-ray detector array Reference Signs List 80 Data acquisition unit (DAS) 81 _{1 to} 81 _n Integrator 82 Signal multiplexer (MPX) 83 A / D converter (A / D) 84 Timing generation unit (TG) 100 Subject A1 to An amplifier ASW Analog switch DEC decoder SH1 to SHn sample hold circuit XD1 to XDn X-ray detector

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hirofumi Yanagida 127 Gee Yokogawa Medical System Co., Ltd., 4-7-7 Asahigaoka, Hino-shi, Tokyo (72) Inventor Masaya Kumazaki 4-7-7 Asahigaoka, Hino-shi, Tokyo 127 GE Yokogawa Medical System Corporation F-term (reference) 4C093 AA22 CA13 FA18 FA32 FA44 FA52 FA59 FC04 FD01 FD09

Claims (8)

[Claims] An X-ray tube is irradiated with an X-ray beam from an X-ray tube under predetermined imaging parameters, an X-ray beam transmitted through the object is detected, and the detection output is integrated by an integrator. An X-ray beam amount detection method for generating an X-ray beam amount detection voltage and A / D converting an output of the voltage with an A / D converter to obtain X-ray beam amount detection data, comprising: Voltage kV, tube current m
A: The time constant of the integrator is changed in accordance with one or more imaging parameters of the detection width Thic in the body axis direction of the subject and the X-ray irradiation time Sec on the subject. X-ray beam amount detection method. 2. When the upper limit of the conversion of the A / D converter is set to V _LIM , the first imaging parameters are set in advance to the tube voltage kV _s , the tube current mA _s of the X-ray tube, and the body axis direction of the subject. X-ray beam amount detection data for a desired imaging region of the subject is collected by the first imaging with the detection width Thic _s and the X-ray irradiation time Sec _s to the subject, and the second imaging parameter is changed to X-ray later. When performing the second imaging with the tube voltage kV _{a of} the tube, the tube current mA _a , the detection width Thic _a in the body axis direction of the subject, and the X-ray irradiation time Sec _a to the subject, the first and second imagings are performed. The ratio Ratio of the two imaging parameters is obtained from the ratio of any one or two or more corresponding item values of the two imaging parameters or the product of each ratio, and X to be detected in the second imaging.
The X-ray beam amount detection method according to claim 1, wherein a time constant of an integrator for preventing the line beam amount detection voltage from exceeding the upper limit value V _LIM is determined based on the determined ratio. 3. The time constant is defined as first and second imaging parameters.
Ratio and the amount of X-ray beam collected in the first imaging
Maximum value V of data_MAXAnd / or minimum value V _MINAnd base
3. The X-ray according to claim 2, wherein
Beam amount detection method. 4. An X-ray tube is irradiated with an X-ray beam from an X-ray tube under predetermined imaging parameters, an X-ray beam transmitted through the subject is detected, and the detection output is integrated by an integrator. In the X-ray beam amount detection method of generating an X-ray beam amount detection voltage and A / D converting an output of the voltage by an A / D converter to obtain X-ray beam amount detection data, a conversion upper limit value of the A / D converter to the case of a V _LIM, collects X-ray beam quantity detection data of the desired imaging region of the object by the first imaging of the tube current of the advance X-ray tube with a predetermined value mA _s, body of these Maximum value V in the axial direction
It detects the _MAX, the upper limit value the tube current mA _a for the X-ray beam quantity detection voltage to be detected does not exceed the upper limit value V _LIM at second imaging performed after V _LIM and the maximum value V
An X-ray beam amount detection method characterized in that the X-ray beam amount is obtained based on a ratio with _MAX . 5. An X-ray tube and a detector facing each other across an object, an integrator for integrating a detection output of an X-ray beam intensity by the detector to generate a corresponding X-ray beam amount detection voltage, An A / D converter for A / D converting the output voltage of the integrator, wherein the X-ray CT apparatus reconstructs a tomographic image of the subject based on the X-ray beam amount detection data output from the A / D converter X-ray tube voltage kV and tube current m as imaging parameters
A, the time constant of the integrator is configured to be variable according to any one or two or more imaging parameters of the detection width Thic in the body axis direction of the subject and the X-ray irradiation time Sec to the subject. An X-ray CT apparatus characterized by the above-mentioned. 6. An A / D converter in which the upper limit value of A / D conversion is V _LIM.
The D converter and the first imaging parameters are set in advance as the tube voltage kV _s and tube current mA _s of the X-ray tube, and the detection width Thic _s in the body axis direction of the subject.
And an imaging control unit for collecting X-ray beam quantity detection data of the desired imaging region of the object by the first imaging of the X-ray irradiation time Sec _s to a subject, a second imaging after the imaging control unit In performing the second imaging, parameters are a tube voltage kV _a of the X-ray tube, _a tube current mA _a , _a detection width Thic _a in the body axis direction of the subject, and an X-ray irradiation time Sec _a to the subject. A ratio calculating means for calculating a ratio Ratio of the first and second imaging parameters by a ratio of one or more corresponding item values of the two imaging parameters or a product of the respective ratios; 6. The apparatus according to claim 5, further comprising: selection control means for selecting a time constant of an integrator based on the obtained ratio to prevent the X-ray beam amount detection voltage from exceeding the upper limit value V _LIM. An X-ray CT apparatus according to claim 1. 7. The selection control means includes a ratio Ratio of the first and second imaging parameters and a maximum value V _MAX and / or a minimum value V _{MIN of} the X-ray beam amount detection data collected in the first imaging.
7. The X-ray CT apparatus according to claim 6, wherein the time constant of the integrator is selected based on the following. 8. An X-ray tube and a detector facing each other across a subject, an integrator for integrating a detection output of X-ray beam intensity by the detector to generate a corresponding X-ray beam amount detection voltage, An A / D converter for A / D converting the output voltage of the integrator, wherein the X-ray CT apparatus reconstructs a tomographic image of the subject based on the X-ray beam amount detection data output from the A / D converter , an a / D converter for the upper limit value of the a / D converter and V _LIM, the X-ray beam for a desired imaging region of the object by the first imaging of the tube current of the advance X-ray tube with a predetermined value mA _s An imaging control unit that collects the amount detection data; a maximum value detection unit that detects a maximum value V _MAX in the body axis direction of the collected X-ray beam amount detection data; and a second value detection unit that is performed later by the imaging control unit. The X-ray beam amount detection voltage to be detected in Limited value V
The tube current mA _a for not exceeding _LIM is set to the upper limit value V _LIM
An X-ray CT apparatus comprising: a tube current calculating means for obtaining a tube current based on a ratio between a maximum value and a maximum value V _MAX .