JP2007185358A - X-ray ct system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT system that attains a desired image quality index value expected at the time of scan planning even in the event when the subject lies out of position with regard to the scanner rotational center. <P>SOLUTION: The X-ray CT scanner comprises an operation means 4a to input X-ray photographing conditions and desired image quality index values, a memory means 25 for storing CT images reconstructed by an image reconstruction means 24, various data and scanogram data acquired from a subject 2 in the process of operating a preparation section, and a scan programming means 28 that analyzes the scanograms stored in the memory means 25, generates appropriate three-dimensional models of the subject 2 even in the event when the subject 2 lies out of position with regard to the scanner rotational center, forecasts image noises from the obtained three-dimensional models of the subject 2 corresponding to his/her photographed bodily part, and a scan programming means 28 for setting an appropriate modulation pattern of irradiating X-ray volume based on the comparison with desired image index values input from the operation means 4a. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention irradiates a subject with X-rays of a fan beam (fan shape) or cone beam (conical shape or pyramid shape), measures the X-rays transmitted through the subject with an X-ray detector, and multi-directionally. The present invention relates to a single-slice type or multi-slice type X-ray CT apparatus that obtains a tomographic image of a subject by back-projecting measurement data from the subject.

As shown in FIG. 4, the conventional multi-slice X-ray CT apparatus irradiates the subject 2 with a cone beam, for example, a pyramid-shaped X-ray beam, from the X-ray tube 6 and moves the detection element 7a in a two-dimensional direction (channel direction and channel direction). The X-ray detector 7 arranged in a direction perpendicular to the X-ray detector 7 measures X-rays transmitted through the subject 2 to obtain projection data of the subject 2.
The single-slice X-ray CT apparatus uses the X-ray detector 7 in which the detection elements 7a are arranged in one row, that is, in a one-dimensional direction (channel direction), and a fan-shaped X-ray beam is applied to the subject 2 in the X-ray tube 6. The X-ray detector 7 irradiates the subject 2 with the X-ray and transmits the subject 2 to obtain projection data of the subject 2.

In any X-ray CT apparatus, the X-ray tube 6 and the X-ray detector 7 facing each other are rotated around the subject 2 to acquire projection data from multiple directions, and a reconstruction filter is used to correct blur. After processing, back projection is performed to reconstruct a tomographic image of the subject 2.
The projection data is acquired at discrete X-ray tube positions (hereinafter referred to as views), and the number of views per rotation of the obtained projection data (hereinafter referred to as projection data in views) is usually several hundred to several. Thousands. The operation of acquiring projection data for the number of views necessary to reconstruct one CT image is called “scanning”.

The projection data for one view consists of data for the number of channels of the X-ray detector 7 × the number of columns (a single slice X-ray CT apparatus can be considered as the number of columns = 1 as described above).
Here, in order to perform a scan that satisfies an image SD (Standard Deviation) value required for the reconstructed image, an elliptical cross-section model of the subject 2 is calculated from the scanogram projection data obtained by one-way scanogram imaging, An X-ray CT apparatus capable of calculating an appropriate tube current value from a projected area of an elliptical section, an aspect ratio of the elliptical section, and an image SD desired value input by an operator of the X-ray CT apparatus and executing a scan. For example, it is proposed in Patent Document 1.
In this way, the function of inputting the desired image quality index value within the scan range at the scanning planning stage and performing the optimum irradiation X-ray dose modulation in order to achieve the desired image quality index value will be described below. Called a function.
JP 2001-43993 A

However, in the conventional X-ray CT apparatus, when the position of the subject 2 moves away from the scanner rotation axis, optimal irradiation X-ray dose modulation cannot be performed, and thus there is a problem that the desired image quality index value assumed at the time of scan planning cannot be achieved.
In particular, the top plate on which the subject 2 is placed does not move in the horizontal direction orthogonal to the scanner rotation axis, but the vertical position can be changed by the subject table vertical movement means. Scanning may be performed in a state where the cross-sectional position of 2 is shifted in the vertical direction from the scanner rotation axis, and as a result, there is a problem that a tomographic image that achieves the desired value of the image quality index assumed at the time of scanning planning cannot be obtained. .
The present invention has been made to remedy such a problem. Even when the position of the subject is deviated from the scanner rotation center, optimal irradiation X-ray dose modulation is performed to achieve the desired value of the image quality index assumed at the time of scanning planning. An object of the present invention is to realize an X-ray CT apparatus capable of performing the above.

  The X-ray CT apparatus according to the present invention detects a rotating body that rotates around the subject, an X-ray tube and an X-ray detector that are arranged on the rotating body so as to face each other with the subject interposed therebetween, and an X-ray detector An X-ray CT apparatus comprising image reconstruction means for reconstructing a tomographic image of a subject using the projection data and display means for displaying the tomographic image reconstructed by the image reconstruction means, Operation means for inputting imaging conditions and desired image quality index values, storage means for storing CT images reconstructed by the image reconstruction means, various data, and scanogram data acquired from the subject at the preparation unit operation stage; By analyzing the scanogram data stored in the storage means, even when the subject is deviated from the scanner rotation center, a means for generating an appropriate subject three-dimensional model and the generated subject three-dimensional model Images according to the part to be imaged of the subject A scan planning unit that calculates noise and sets an appropriate irradiation X-ray dose modulation pattern to achieve the desired image quality index value based on comparison with the desired image quality index value input from the operation unit. is there.

  With the above configuration, even if the subject is deviated from the scanner rotation center, an appropriate subject three-dimensional model is generated, and imaging conditions are calculated from the obtained subject three-dimensional model so as to achieve a desired image quality index value. As a result, a tomographic image that achieves the desired value of the image quality index assumed at the time of scan planning can be easily obtained with the minimum necessary exposure dose.

  In the X-ray CT apparatus of the present invention, the amount of displacement of the subject from the rotation center of the rotating body is calculated using the position information of the top plate on which the subject is placed.

  With the above configuration, the amount of displacement of the subject from the scanner rotation center can be calculated easily and accurately without using complicated means.

  According to the X-ray CT apparatus of the present invention, a tomographic image that achieves the desired value of the image quality index assumed at the time of scan planning can be easily obtained with the minimum necessary exposure dose even if the subject is deviated from the scanner rotation center. become.

Embodiments of the present invention will be described in detail with reference to the drawings.
1 is an overall perspective view of an X-ray CT apparatus, FIG. 2 is the same configuration diagram, FIG. 3 is a side view thereof, and FIG. 4 is a schematic diagram illustrating the configuration of detection means and the relationship with X-ray irradiation and a subject. FIG. 5 is a flowchart showing the action at the time of the preparation operation, and FIGS. 6 to 9 are action explanatory diagrams at the time of generating the subject three-dimensional model.
The X-ray CT apparatus shown in FIGS. 1 and 2 includes a scanner main body 1 that scans a subject 2, a subject table 3 that places the subject 2, a console 4 that operates the scanner main body 1, and a console. A circular opening 1b is opened at the center of a cover 1a that covers the scanner body 1 and includes an operation means 4a including a keyboard installed on the display 4 and a display means 27.
In the cover 1 a of the scanner body 1, a disk-like rotating body 5 that rotates about a scanner rotation center (not shown) is provided, and X-rays are provided so as to face the rotating body 5 with the subject 2 interposed therebetween. A tube 6 and an X-ray detector 7 are arranged.

As shown in FIG. 6, the X-ray tube 4 is controlled by an X-ray tube control means 8 mounted on the rotator 5, and passes through a circular hole 5 a opened at the center of the rotator 5 along the scanner rotation axis. X-rays are irradiated toward the moving subject 2.
In the vicinity of the X-ray tube 6, a collimator 10 controlled by a collimator control means 9 mounted on the rotating body 5 is installed, and the collimator 10 irradiates the subject 2 from the X-ray tube 6. The X-ray beam 11 is collimated into, for example, a pyramid-shaped cone beam, and the X-ray beam 11 transmitted through the subject 2 is detected by the X-ray detector 7.
As shown in FIG. 4, the X-ray detector 7 is formed in an arc shape centered on the focal point 6 a of the X-ray tube 6, and is orthogonal to the body axis of the subject 2 on the light receiving surface of the X-ray beam 11. A number of X detection elements 7a are arranged in the channel direction, and the X-ray detection elements 7a are two-dimensionally arranged in a plurality of rows in the body axis direction (column direction).

Further, the X-ray detection element device 7 has an X-ray incident surface curved in a cylindrical surface shape or a polygonal line shape with respect to the channel direction as a whole, the channel number is, for example, about 1 to 1000, and the column number is, for example, 1 to 1. For example, it is composed of a combination of a scintillator and a photodiode, and the spread angle in the channel direction of the X-ray beam 11 that coincides with the channel arrangement direction of the X-ray detector 7, that is, the fan angle is α, The row direction spread angle of the X-ray beam 11 that coincides with the arrangement direction of the rows of the line detector 7, that is, the cone angle is γ.
Data collection means 12 is installed on the back side of the X-ray detector 7 so that the projection data detected by the X-ray detector 7 is collected and sent to the console 4. The rotating body drive means 15 whose rotation is controlled by the rotating body control means 14 is rotated through a power transmission means 16 such as an endless belt.

  On the other hand, as shown in FIG. 2, the subject table 3 is moved in the horizontal direction parallel to the scanner rotation axis by the top plate driving means 18 and the vertical direction (vertical direction) perpendicular to the scanner rotation axis by the top plate vertical movement driving means 19. The object table 2 is moved back and forth by the object table control means 17 installed in the object table 3 by the object table control means 17 and the object table 2 is moved to the scanner body. The top plate 3a can be adjusted to an appropriate height by carrying it in and out of the circular hole 5a of the rotator 5 from the opening 1b of the one or by controlling the top plate vertical movement means 19. Are moved one by one by the top position sensor 20, and the detected position information is sent to the system control means 22 for controlling the entire system provided in the console 4 shown in FIG. Yes.

The system control unit 22 includes an X-ray tube control unit 8 installed in the scanner body 1, a collimator control unit 9, a data collection unit 12, a rotating body control unit 14, and a subject installed in the subject table 3. The table control means 17 and the like are connected by a cable 23, and projection data collected by the data collection means 12 in the scanner main body 1 is sent to the console 4 via the cable 23 and stored in the console 3. The image is output from the system control means 22 to the image reconstruction means 24.
The image reconstruction unit 24 creates a scanogram image using the scanogram projection data collected by the data collection unit 12 at the time of scanogram imaging, and uses a plurality of views of projection data collected by the data collection unit 12 at the time of scanning to reconstruct a CT image. The system control means 22 includes a scanogram image created by the image reconstruction means 24, a reconstructed CT image, various data, and a program for realizing the functions of the X-ray CT apparatus. Is stored in the storage means 25 connected to the.

The system control means 22 includes an operation means 4a that is operated by an operator and inputs various instructions and information to the system control means 22, a reconstructed image output from the image reconstruction means 24, and a system control means. The display means 27 for displaying various information handled by the terminal 22 is connected, and the operator can operate the X-ray CT apparatus interactively by operating the operation means 4a while looking at the display means 27. ing.
Further, the system control means 22 is connected with a scan planning means 28, executes an instruction input by the operator operating the operation means 4 a, and uses a scanogram image read from the storage means 25. A scan condition can be prepared in advance.
That is, the scanogram image read from the storage unit 25 is displayed on the display unit 27, and the operator uses the operation unit 4a to set the coordinates of the CT image reconstruction position (hereinafter referred to as the slice position) on the displayed subject scanogram image. Since the slice position information planned here is stored in the storage means 25, the scan planning means 28 can set the X-ray dose control conditions and the like. It can also be used when planning.

Next, the operation of the X-ray CT apparatus constructed as described above will be described.
Before the scanning operation for acquiring the CT image of the subject 2 with the X-ray CT apparatus, various preparation operations are performed in order to set the imaging conditions. As this preparation operation, the imaging position of the subject 2 is set. Of the scanogram image, analysis of the scanogram data, determination of a change pattern of the optimum irradiation X-ray dose as an imaging condition based on the scanogram data, and the like, which are performed under the control of the system control means 22, In particular, the scan plan data 28 connected to the system control means 22 performs analysis of scanogram data and determination of an optimal tube current change pattern as an imaging condition based thereon.
In this preparation operation, first, X-ray imaging conditions such as an X-ray tube voltage and an X-ray tube current set value are input to the system control unit 22 from the operation unit 4a.

Next, the subject table 4 and the rotator 5 are relatively moved along the body axis of the subject 2 without rotating the rotator 5, and a scanogram image is taken by the X-ray tube 6 and the X-ray detector 7. The scanogram projection data and the scanogram image data are stored in the storage means 25.
The scan planning unit 28 analyzes the scanogram projection data in consideration of the height information of the top 4 a calculated by the subject table control unit 17 based on the position information from the top position sensor 20, and An estimated cross section at an arbitrary position along the body axis is modeled as an elliptical cross section having an X-ray absorption coefficient equivalent to water, for example. This model is a position along the body axis of the subject 2 (hereinafter, z position). The long axis length and short axis length of the elliptical cross section changes (hereinafter referred to as a “subject 3D model”), and the data of the subject 3D model is stored in the storage means 25. The

The scan planning unit 28 includes an image index desired value, a tube voltage, a tube current setting value, an X-ray collimation condition, a time per one rotation of the scanner (hereinafter referred to as a scan time), and the scan planning unit 28 input from the operation unit 4a. A series of tube current values that change over time according to changes in the transmitted X-ray dose of the imaging region of the subject 2 during the scan based on the data of the three-dimensional model of the subject created in advance, that is, a change pattern of the tube current The scan planning means 28 having this function is an important component.
FIG. 5 is a flowchart showing a series of preparatory operations prior to scanning by the X-ray CT apparatus. The preparatory operations described above will be described with reference to this flowchart. First, in the scanogram imaging step of step S1, a scanogram image of the subject 2 is obtained. However, the procedure for capturing a scanogram image of the subject 2 is basically the same as the procedure for capturing a CT image.

Further, the scanogram projection data photographed in step S1 is obtained by irradiating the subject 2 with X-rays from a certain direction, for example, the back direction, without rotating the rotator 5, and capturing the detection data with the X-ray detector 7. This scanogram projection data is sent from the X-ray detector 7 to the image reconstruction means 24 via the system control means 22, and a scanogram image is created by the image reconstruction means 24.
The scanogram image obtained at this time is an X-ray image transmitted from the back to the front in a certain direction, for example, from the front. The scanogram image is obtained by slicing the slice position of the subject 2 at the time of scanning (CT image reconstruction). In addition to being used for creating a scanogram image, the scanogram projection data is used not only for creating a scanogram image, but also for determining an irradiation X-dose change pattern for irradiation X-dose control particularly in a scan.

In the process from step S2 to step S4, when the operator refers to the scanogram image and inputs the top plate movement pitch, the scan start position, and the scan end position as the photographing conditions from the operation means 4a, these input data are input. Using the scan planning means 28, the CT image capturing range of the subject 2, the slice position z, and the phase angle of the X-ray tube 6 (phase angle of the rotating body 5) β are determined.
The scan start position and the scan end position mean the z position of the first CT image and the z position of the last CT image obtained by a series of scans, respectively.
In step S5, the operator inputs the tube voltage setting value, scan time, X-ray collimation condition, reconstruction filter function type, field size, and the like as imaging conditions from the operation means 4a. In step S6, the operator An image quality index desired value as an image quality target is input from the operation means 4a.

Next, in the scanogram projection data analysis step of step S7 and the subject three-dimensional model generation step of step S8, the scanogram projection data is analyzed by the scan planning means 28, and a subject three-dimensional model of the subject 2 is generated. However, this subject three-dimensional model approximates each cross section of the subject 2 corresponding to the z position as an elliptical cross section having an X-ray absorption coefficient equivalent to water, for example.
In the present embodiment, in calculating the elliptical cross section, the height of the top 3a, that is, the distance from the installation floor 30 of the subject table 3 to the top of the top 3a and the position of the scanner rotation axis (horizontal direction, vertical direction). ) Is also calculated, and an appropriate elliptical cross-sectional size is calculated even when the subject 2 is not located at the center of rotation.

Here, the calculation method of the elliptical cross section will be described with reference to FIGS. 6 and 7. From the scanogram projection data corresponding to a certain cross sectional position A, the center transmission length and X of the approximate elliptic cross section of the subject 2 at the cross sectional position A are described. A method of determining the opening angle obtained by viewing the approximate elliptical cross section from the line focus 6a is as shown in FIG. 6, for example.
That is, the projection data at the cross-sectional position A of the subject 2 shown in FIG. 6A is as shown in FIG. 6B, and the projection data value near the maximum value is Rg and the scanogram data of channel number k. R _A (k), n is an appropriate number of channels, and a projection data value Rg near the maximum value is calculated by the following equation (Equation 1).

そして得られた投影データ値Ｒｇから一様な楕円断面モデルを推定すると、図６の（ｃ）に示すようになる。
なお中心透過長Ｄ＝Ｒｇ／（Ｇ＊μ_ｗ）、Ｘ線焦点から見た楕円の開き角度α_E＝α_P＊S/Rg
ただしRgは、最大値付近の投影データ値、Gｇはｌｏｇ変換時の係数、μ_ｗは水のＸ線吸収係数、α_PはＸ線検出器チャネルの開き角、Ｓは投影データの和、である。

Then, when a uniform elliptical cross-section model is estimated from the obtained projection data value Rg, it is as shown in FIG.
The center transmission length D = Rg / (G * μ _w ), the opening angle of the ellipse viewed from the X-ray focal point α _E = α _P * S / Rg
However Rg is projection data values near the maximum value, Gg is the coefficient at the time of log-transformed, mu _w X-ray absorption coefficient of water, alpha _P is the opening angle of the X-ray detector channels, S is the sum of projection data in, is there.

However, as shown in FIG. 7, even if the central transmission length of the elliptical cross section and the opening angle (as viewed from the X-ray focal point 6a) are calculated from the projection data at the time of scanogram imaging, there are an infinite number of ellipses that satisfy them (note that In FIG. 7, only three ellipses corresponding to the same projection data are drawn, but in reality, an ellipse having the same center opening length and the same opening angle value as viewed from the X-ray focal point is from the center of rotation. Infinitely depending on the distance Conventionally, by assuming that the subject 2 is positioned at the scanner rotation center (that is, the scanner rotation center coincides with the center of the ellipse), the vertical and horizontal axis lengths of the ellipse are obtained. However, when the subject 2 is not actually located at the scanner rotation center, it is clear that the estimation of the ellipse is not appropriate in the conventional method.

Therefore, in this embodiment, as shown in FIG. 8, the vertical distance between the floor surface 30 and the scanner rotation center, the vertical distance between the X-ray focal point and the scanner rotation center, and the vertical distance between the floor surface 30 and the top plate 3a are taken into consideration. By doing so, the position and size of the ellipse can be appropriately calculated even if the subject 2 is not located at the scanner rotation center.
That is, the distance from the floor surface 30 to the scanner rotation center is known, and the distance from the X-ray focal point 6a to the scanner rotation center is also known.
Since the position of the top 3a can be detected by the top position sensor 20, the distance from the X-ray focal point 6a to the center of the ellipse can be calculated as follows.

Distance from the floor surface to the center of the ellipse = Vertical distance between the floor surface and the top surface of the top plate + 0.5 * Transmission length from the center of the ellipse Distance from the X-ray focus to the center of the ellipse = Distance from the X-ray focus to the center of rotation of the scanner + (Floor (Distance from the surface to the ellipse center-distance from the floor surface to the scanner rotation center)
Conventionally, even when the position of the subject 2 is deviated from the scanner rotation center, the subject model is estimated on the assumption that the subject 2 is at the scanner rotation center. In the present embodiment, the object model is estimated based on the distance from the X-ray focal point 6a to the ellipse center by the above calculation, and therefore the object model estimation result is shown in FIG. The object model estimation result with the correct position and size can be obtained.

Returning to FIG. 5, in the process of step S9, the X-ray attenuation index is calculated for each position z in the body axis and the phase angle β of the X-ray tube 6. Here, the X-ray attenuation index is calculated based on the three-dimensional model of the subject. This is the integral value of the X-ray absorption coefficient distribution along the X-ray transmission path passing through the center of the elliptical cross section at (z, β), and this data can be obtained from the previously generated three-dimensional model of the subject. The scan planning unit 28 calls and calculates the subject three-dimensional model from the storage unit 25. The X-ray attenuation index calculation result is represented by T = T (z, β).
Next, in step S10, the function of the X-ray attenuation index T is calculated from T = T (z, β) to time t based on the scan start position, scan end position, top plate movement pitch, and scan time. Convert to function T = T (t).

Next, an example of the tube current change pattern setting method in step S11 when the image SD desired value is used as the image quality index desired value will be described.
Here, the number of views used to reconstruct the CT image Img (z) at the slice position z is M, and the convenient view number m is m = 0 to M−1. When the number of views per rotation is N, the number of used views M is not necessarily equal to the number of views N per rotation.
The aforementioned X-ray attenuation index T can also be expressed as a function T (m) of the view number to be used, and the maximum value of the X-ray attenuation index T in the view numbers m = 0 to M−1 is represented by TmaX (0: Assuming that the reference tube current value i_ref is made to correspond at this time, the tube current value i _v (m) for the view number m is as follows.

  On the other hand, the time trot for one rotation of the rotating body 5 of the scanner body 1 is equal to the reference time trot_ref, and during that time, the X-ray attenuation index T is a constant value. Assuming that the image noise variance value V when the image thickness thk is reconstructed as the reference image thickness thk_ref using the reconstruction filter function g with equal weighting on the number of views N_ref in one rotation is X-ray It is expressed as the following equation as a function of the attenuation index T.

Further, the predicted image noise variance value V ^* when the tube current value i _v (m) represented by the above (Equation 2) is used is represented by the following equation.

ここで（数４）のｗ（ｍ）は各ビューに対して適用されるビュー方向重みであり、ビュー方向重みは再構成に使用するビュー数Ｍが１回転あたりのビュー数Ｎと異なる場合（非特許文献）や、被検体の動きによるアーチファクトを補正する場合（特許文献２）に用いられる。

Here, w (m) in (Expression 4) is a view direction weight applied to each view, and the view direction weight is determined when the number of views M used for reconstruction is different from the number of views N per rotation ( Non-patent literature) and when correcting artifacts caused by movement of the subject (Patent literature 2).

Non-patent document 1
G.Wang et al. “Half-Scan Cone-Beam X-ray Microtomography Formula”
Journal of Scanning Microscopies Vol.16, 216-220 (1994)
Japanese Patent Application Laid-Open No. 08-280664

とすることにより、いわゆるフルスキャン再構成を行うこともできる。
ここで、操作者が入力した画像ＳＤ所望値ＳＤｔｇｔから定まる画像ノイズ分散所望値Ｖｔｇｔ（ＳＤｔｇｔの二乗値）と（数３）の画像ノイズ分散予測値Ｖ^＊から、実際に適用すべき管電流値ｉ_ａ（ｍ）は次式のように定められる。 If the number of used views M is equal to the number of views N per rotation,

By so doing, so-called full scan reconstruction can also be performed.
Here, the tube current value to be actually applied is determined from the desired image noise variance value Vtgt (square value of SDtgt) determined from the desired image SD value SDtgt input by the operator and the predicted image noise variance value V ^{* of} (Equation 3). i _a (m) is determined as follows.

As described above, the tube current modulation pattern for realizing the image SD desired value input by the operator in the CT image at each slice position is determined. If this tube current modulation pattern is I, I is scanned. The tube current modulation pattern I = I (t) determined in this way can be expressed as a function I (t) of the elapsed time t after the start, and is stored in the storage means 25, and the imaging region of the subject 2 at the time of scanning In response to this, the system control means 22 sequentially calls and controls the tube current during scanning via the X-ray tube control means 8.
As described above, by inputting an image quality index desired value before scanning the subject 2 with X-rays and determining an irradiation X-ray dose modulation pattern suitable for it, an optimal scan for realizing an appropriate image quality is executed. It becomes possible.

The present invention is not limited to the embodiment described above. For example, as shown in FIG. 9, the subject 2 is displaced not only in the vertical direction but also in the horizontal direction with respect to the scanner rotation axis. Even so, an appropriate subject model is uniquely determined.
That is, when the subject 2 is displaced from the scanner rotation center, for example, diagonally upward to the left, it is conventionally assumed that the subject is at the scanner rotation center even if the position of the subject 2 is displaced from the scanner rotation center. Since the object model was estimated, the object model estimation result was at the position indicated by the broken line in FIG. 9, but in the present invention, as in the case of FIG. 8, the elliptical center transmission length and the X-ray focal point 6a. Is calculated from the projection data, and an ellipse that passes through the X-ray focal point 6a, is in contact with the two straight lines in contact with the ellipse, and the top surface of the top 3a, and has a central transmission length of a specific value. Since it is uniquely determined, the object model estimation result is the position indicated by the solid line in FIG. 9, and the object model estimation result having the correct position and size can be obtained.
Further, at the time of scanogram imaging, in the above-described embodiment, the case where the X-ray tube 6 is located below the top plate 3a has been described. However, the X-ray tube 6 may be executed so as to be located above the top plate 3a. .

As described above, in the X-ray CT apparatus of the present invention, the scan planning means 28 has a scanogram analysis / subject three-dimensional model generation function for generating a three-dimensional model of the subject 2 from the scanogram projection data of the subject 2. The image noise corresponding to the imaging region of the subject 2 is predicted from the three-dimensional model of the subject 2, and the appropriate modulation pattern of the irradiation X-ray dose is compared with the comparison between the predicted image noise value and the image quality index desired value input by the operator. A scan planning means 28 having a function of generating an appropriate subject three-dimensional model even when the subject 2 is not located at the center of scanner rotation. Therefore, the tomographic image with the optimum image quality can be easily obtained with the minimum necessary exposure dose.
In addition, by appropriately considering the influence of filtering processing when creating a scanogram image, the scanogram image is analyzed instead of the scanogram projection data to obtain a three-dimensional model of the subject, and X-ray CT that meets the spirit of the present invention It is also possible to implement the device.

1 is an overall perspective view of an X-ray CT apparatus according to an embodiment of the present invention. 1 is an overall configuration diagram of an X-ray CT apparatus according to an embodiment of the present invention. 1 is a partially omitted side view of an X-ray CT apparatus according to an embodiment of the present invention. It is a schematic diagram explaining the structure of the detector of X-ray CT apparatus which becomes embodiment of this invention, and the conventional X-ray CT apparatus, and a relationship with X-ray irradiation and a test object. It is a flowchart which shows the effect | action at the time of preparatory operation of the X-ray CT apparatus which becomes embodiment of this invention. It is explanatory drawing at the time of producing | generating a to-be-examined object three-dimensional model by preparation operation of the X-ray CT apparatus which becomes embodiment of this invention. It is explanatory drawing at the time of producing | generating a to-be-examined object three-dimensional model by preparation operation of the X-ray CT apparatus which becomes embodiment of this invention. It is explanatory drawing at the time of producing | generating a to-be-examined object three-dimensional model by preparation operation of the X-ray CT apparatus which becomes embodiment of this invention. It is explanatory drawing at the time of producing | generating a to-be-examined object three-dimensional model by preparation operation of the X-ray CT apparatus which becomes embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Scanner main body 2 Subject 5 Rotating body 6 X-ray tube 7 X-ray detector 20 Top plate position sensor 24 Image reconstruction means 25 Memory | storage means 28 Scan plan means

Claims (2)

  Using a rotating body that rotates around the subject, an X-ray tube and an X-ray detector arranged on the rotating body so as to face each other with the subject interposed therebetween, and projection data detected by the X-ray detector An X-ray CT apparatus comprising image reconstruction means for reconstructing a tomographic image of the subject and display means for displaying a tomographic image reconstructed by the image reconstruction means, wherein the X-ray imaging conditions and image quality An operation means for inputting a desired index value, a storage means for storing CT images reconstructed by the image reconstruction means, various data, and scanogram data acquired from the subject in a preparation unit operation stage, and the storage Means for analyzing the scanogram data stored in the means and generating an appropriate subject three-dimensional model even when the subject is deviated from the scanner rotation center; and the generated subject cubic Previous model Image noise corresponding to the imaging region of the subject is calculated, and an appropriate irradiation X-ray dose modulation pattern is set to achieve the image quality index desired value based on comparison with the image quality index desired value input from the operation means An X-ray CT apparatus comprising: The X-ray CT apparatus according to claim 1, wherein an amount of positional displacement of the subject from the rotation center of the rotating body is calculated using position information of a top plate on which the subject is placed. .

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