JPH1128202A - X-ray ct - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray CT capable of reducing the exposure to X-rays of a patient, etc., without missing the optimum timing of visualization after injection of a contrast medium and degrading picture quality. SOLUTION: In an X-ray CT which displays tomograms of a subject B on a display whenever necessary by rotating an X-ray source 200 continuously, measuring the data about the projection of the subject B plural times in succession by use of detectors 250, and reconfiguring the tomograms based on the data about the projection, photographing can be performed in such a way that the radiation field of X-rays is limited to an area A of interest within the subject B by a channel collimator 210. Further, an area-of-interest computing means which monitors CT values within the area of interest and which then computes the average value of the CT values inside the area is provided to offer an X-ray CT value monitoring function, so that reducing the exposure to X-rays of a patient or the like is made possible while monitoring a CT scanner after injection of a contrast medium so that the optimum timing of visualization is not missed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus for obtaining a tomographic image of the inside of a body such as a patient using X-rays.
When continuously measuring substantially the same cross section, the irradiation of X-rays in the set (interest) region is suppressed as much as possible,
Alternatively, the present invention relates to an X-ray CT apparatus capable of reducing the amount of exposure to a specific tissue.

[0002]

2. Description of the Related Art X-ray CT apparatuses have already been widely used in medical treatment and the like, and various usages have been made by users. For example, recently, a technique for displaying an image in real time using a high-speed image reconstruction technique has been proposed.

That is, by applying the image displayed in real time, the CT of a certain region of interest is monitored after the injection of the contrast agent, and if the region of interest reaches a preset CT value, the line is changed. A scanning method for starting a scan or a normal scan has been proposed. This method is an example devised so as not to miss the optimal contrast timing.

[0004]

The above-mentioned method of using the X-ray CT apparatus is effective as a method for keeping the timing of the imaging, however, the patient suffers from an increase in the amount of exposure. ing. In order to reduce the exposure dose, the X-ray tube current may be reduced, but this results in a decrease in the irradiation dose (mAs = mA × sec), which means an increase in X-ray fluctuation noise, and the image quality is significantly deteriorated. There was a problem of doing.

In view of the above-mentioned problems in the prior art, the present invention provides a method for reconstructing a CT image in real time by a high-speed image reconstruction technique using the above-mentioned X-ray CT apparatus. It is an object of the present invention to provide an X-ray CT apparatus capable of reducing the amount of X-ray exposure.

[0006]

According to the present invention, in order to achieve the above object, the X-ray source is continuously rotated to measure the projection data of the subject a plurality of times continuously. In an X-ray CT apparatus that reconstructs a tomographic image of a subject based on the projection data and sequentially displays the tomographic images on a display device, an X-ray CT apparatus for setting an irradiation range of the X-ray from the X-ray source in the subject. A region-of-interest setting means, a channel collimator for blocking irradiation of X-rays outside the region set by the region-of-interest setting means when measuring the projection data by rotating the X-ray source; There is proposed an X-ray CT apparatus including a region-of-interest calculation means for monitoring a CT value and calculating a change amount of the CT value in a specific region of the region of interest.

Further, according to the present invention, the above-described X
In the X-ray CT apparatus, the X-ray CT apparatus starts a continuous scan according to a preset condition when a CT value in a specific region of the region of interest by the region of interest calculation means reaches a set CT value.

That is, according to the present invention, it is possible to take an image with the X-ray irradiation field limited to a range (region of interest) including the effective field of view, thereby performing no measurement operation outside the irradiation range, and / or The X-ray CT apparatus scans while monitoring the CT value in the region of interest by the region-of-interest calculation means for calculating the amount of change such as the average value of the CT value in the specific region of the region of interest. An object of the present invention is to provide an X-ray CT apparatus capable of reducing the amount of X-ray exposure to a patient or the like even when a CT image is reconstructed in real time by a high-speed image reconstruction technique.

Further, as will be described in detail in the following embodiments, the data outside the X-ray irradiation range is reconstructed by applying the previously measured data, that is, substantially the same part measured earlier in time is used. By applying and compensating for the projection data, the occurrence of an artifact can be suppressed without performing any special correction even when the subject is out of the irradiation area.

[0010]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. First, FIG. 2 shows an overall configuration of an X-ray CT apparatus according to an embodiment of the present invention. As is clear from the figure, this X-ray C
The T device is equipped with a display device 100, a host computer 101 controlling the whole device, an X-ray generation system, an X-ray detection system, etc., a scanner system 102 capable of continuous scanning by a slip ring, and a pre-processing of images. Processing apparatus 103 including a preamplifier (indicated by reference numeral 106 in FIG. 3) which is responsible for image reconstruction processing or various analysis processing
And a high voltage generator 104 for supplying a high voltage to the X-ray generation system, and a patient table 105 on which a subject is placed
Etc. Although not shown, the host computer 101 includes a keyboard,
A mouse, tracking ball, etc. are provided.

FIG. 3 is a detailed explanatory view of the scanner system 102. According to the X-ray CT apparatus of the present invention, an X-ray tube for generating X-rays by a high voltage supplied from a high voltage generator 104 A channel collimator 210 is provided close to the sample 200 and between the sample B and the subject. The movement of the channel collimator 210 in the X-axis direction in the figure is controlled by the controller 211. That is, the channel collimator 210 can limit the X-ray irradiation range in the channel direction to the region of interest A that can be set in advance. Reference numeral 250 in the drawing indicates a detector of the scanner system 102.

Next, the structure of the channel collimator 210 which characterizes the present invention is schematically shown in FIGS. As shown in FIG. 4A, the channel collimator 210 has a trapezoidal outer collimator case 212.
A compensating filter 213 is provided, and a lead shielding plate 214 is provided at a bottom portion thereof. FIG. 4B shows the structure of the lower surface of the channel collimator 210. A pair of guides 2 are provided on both sides of the lead shielding plate 214.
15 and 215 are provided, and the expansion and contraction of the lead shielding plate 214 is controlled by the operation of the motor 216 having an encoder and the like and the timing belt 217. FIGS. 5A and 5B show a state where the lead shielding plate 214 is contracted and a state where the lead shielding plate 214 is extended, respectively.

As shown in the above-mentioned figure, a collimator case 212 further includes a system controller 2 for overall control of the channel collimator 210.
11 and a motor driver 2 for driving the motor 216
18 are provided. Note that the system controller 211 and the motor driver 218 may be attached to other than the collimator case 212.

The system controller 211
Performs overall control of the entire channel collimator 210 as described above. For example, when setting the region of interest A of the present invention, several types of tables are stored in the controller as the first stage. Further, the motor driver 218 drives the motor 216 to rotate, and the rotation of the motor is transmitted to the timing belt 217, whereby the timing belt 217 is connected to the curtain-shaped lead shielding plate 2.
14 is expanded or contracted, thereby enabling a region of interest scan during scanning.

Next, the procedure of an application example in the X-ray CT apparatus of the present invention whose configuration has been described above will be described with reference to FIG. According to the X-ray CT apparatus of the present invention, CT value monitoring imaging (scan) and precision imaging can be performed in a large procedure. In this precise imaging, an accurate tomographic image is obtained by taking an image under sufficient X-ray conditions.
In the T value monitoring scan, the tube current is reduced to suppress the exposure dose, thereby enabling continuous imaging. Hereinafter, description will be made in order.

As for the flow of shooting, when shooting is started,
First, a scanogram is displayed (step S11), and then a precise shooting range is determined (step S12). In this precise imaging range determination, after the patient has been set on the table 105, in order to determine the imaging position of the tomographic image, first, the scanogram (the X-ray tube is stationary and the fluoroscopic imaging performed while moving the table) is performed. Image). The setting of the number of shots is performed on the displayed scanogram. The shooting range includes items related to the start of shooting, the shooting interval, the number of shots, and the like. For example, in the case of a spiral scan, the shooting start position, the table moving speed, the number of scans, and the like are also set. In the case of continuously measuring the same cross section, setting excluding the table moving speed is performed.

Next, the patient table 105 moves its top plate to the selected slice position according to an instruction from the host computer 101 (step S13). Here, imaging at a normal position of the channel collimator 210, that is, a so-called reference scan is performed under a low dose condition (step S14).

Subsequently, an irradiation range and a region of interest are determined. That is, when the pre-scan is completed, the doctor observes the photographed image, determines a range (irradiation range) for irradiating X-rays (step S15), and then sets C within the irradiation range.
A specific region is set in the region of interest for T value monitoring imaging (step S16). For setting the irradiation range and the region of interest, as shown in FIG. 6, for example, a circular shape is displayed on a display screen of the display device 100 by using a pointing device PD such as a mouse or a trackball as an input device. By drawing an area or an elliptical area, a setting is made so that X-rays are not directly irradiated. At this time, for the specific region in the above-mentioned region of interest, a setting is made to calculate the amount of change such as the average value of CT values.

At this time, imaging (reference scan) is again performed at the low X-ray dose condition and at the normal position of the channel collimator (step S17), and the irradiation position is further checked (step S1) together with the confirmation of the slice position.
8) And the setting of the specific region of the region of interest is confirmed (step S19). Then, when a desired image is obtained by this reference scan, CT value monitoring imaging is performed there.

In this CT value monitoring imaging, the doctor injects a contrast agent (step S20), and then shifts to continuous scanning of CT value monitoring imaging to sequentially reconstruct the image data (step S21). In addition, while displaying temporally continuous tomographic images, at the same time, an average value of CT values in a region of interest or a specific region in the region of interest is also calculated.

Thereafter, the CT value in the region of interest or in a specific region within the region of interest gradually changes due to the influence of the injected contrast agent, and the CT value in the related region of the set CT value becomes equal to the set value. If it has come (the average value of the CT value in the related area = set value), the precise photographing is started under the photographing conditions set in advance in the precise photographing range determination (step S12) (step S22). Note that the precision imaging is an imaging at the normal position of the channel collimator 210, and may be a spiral scan or a normal scan that captures projection data while the patient bet is stopped. Either way, a reconstructed image is a precise image that can be sufficiently diagnosed.

Here, the control of the channel collimator 210 whose detailed configuration has been described with reference to FIG. 4 will be described below with reference to FIG. First, as shown in FIG. 7, a line connecting the focal position (Xs, Ys) of the X-ray tube, which is a source of X-rays, and the center of rotation is a y-axis, and a line intersecting the y-axis at right angles to the center of rotation is x. Axis. Here, the collimator 210 (corresponding to the lead shielding plate 214 of the channel collimator 210) is movable in the x-direction as described above, and is a mechanism capable of arbitrarily limiting the X-ray irradiation range. .

Now, assuming that the region of interest is defined by a circle having a radius r centered on the center coordinates (Xc, Yc) in FIG. 7, the set X-ray irradiation range is as follows. , ΘL to θR. Where X
The calculation of the focal point of the line is assumed to be a point focal point as described above, and the coordinates thereof are defined as (Xs, Ys). On the other hand, the spread positions of the X-ray on the collimator are defined as XL0 and XR0. In other words, the position of the collimator 210 on the x-axis is defined by a right end (XR0- △ XR) and a left end (XL0- △ X).
L).

The amount of movement of the collimator 210 △ X
As means for obtaining L, △ XR, which can be obtained by the following calculation formula, these are obtained in advance. Further, it is also possible to approximate with a periodic function such as a trigonometric function without using the following calculation formula.

That is, △ XL and △ XR are obtained as follows.

(Equation 1) Here, d is a distance between two points of the focal point and the center coordinate of the region of interest. And, if θL and θR are expressed using this d,
It looks like this:

(Equation 2)

(Equation 3) Here, d ₀ is the distance from the X-ray focal point to the rotation center of the scanner. Then, if △ XL and △ XR are expressed using these θL and θR, the following is obtained.

(Equation 4)

(Equation 5)

That is, by controlling the position of the collimator 210 of the channel collimator at each projection angle in accordance with 求 め XL and △ XR obtained as described above, as shown in FIG. 3, X-rays are emitted only to the region of interest of the subject. Will be irradiated. In the above example, the region of interest has been described as a circle. However, the shape of the region of interest is not limited to a circle, and may be set to, for example, an ellipse. In this case, a parameter is added to the following procedure for obtaining $ XL and $ XR.

Also, in the above embodiment, the offset correction by the preamplifier dark current is added to the measured projection data.
Packing (embedding) processing was used in addition to preprocessing such as X-ray fluctuation correction or radiation quality correction and log conversion.
As described above, this packing process is a process of obtaining an invalid data range at the time of reconstructing the screen because the irradiation range of the X-ray is limited, and replacing the data with data measured in advance.

Here, the packing (embedding) processing will be described. As described above, the projection data obtained by irradiating only the region of interest with the X-rays limited is, as shown by the thick solid line in FIG. 8, the data in the region shielded by the above-described collimator, after the offset correction. Becomes almost zero. Therefore, very high frequency components are generated in the channels ia and ib in FIG. 8. Therefore, if the obtained projection data is used as it is for image reconstruction processing, artifacts will be generated on the image. . In order to solve this, as already proposed by the present applicant, a process of embedding the projection data measured in advance is performed for the data of the area shielded by the collimator.

This is realized, for example, by the image processing apparatus 103 shown in FIG. That is, in the figure, the image processing apparatus 103 includes a reconstruction operation unit 11 and a weighted image adder 12, and outputs the result to the display device 100. The reconstruction calculator 11 includes a projection data memory 20, a pre-processing calculator 21, a fan beam-parallel beam converter 2,
2, a filter correction calculator 23 and a back projection calculator 24. On the other hand, the weighted image adder 12 includes seven image memories 10 (# 1 to # 7) and seven weight coefficient multipliers 13 (W
1 to W7 are weighting factors) and an adder 25.

In this example, the image processing device 103
Can be reconstructed in less than one second per image, for example. This is to sequentially and sequentially obtain the divided reproduction constituent images at a 30 ° width, and further, in one reconstruction screen, add the latest newly obtained 30 ° width divided reconstructed image to the previous one. It is because of what can be achieved. For example, when the angle is updated at a width of 30 °, a reconstructed image of 12 sheets / second can be obtained.

The reconstruction arithmetic unit 11 includes a pre-processing unit 21, a parallel beam conversion by a fan beam-parallel beam conversion unit 22, a filter correction process by an arithmetic unit 23, and a back projection arithmetic unit 24. To obtain a reconstructed image. This reconstruction operation is 3
This is not a collective reconstruction for 60 °, but an addition operation for the divided reconstructed images obtained from the parallel beam data with a partial angular width (for example, 30 ° width).

Then, the weighted image adder 12 sequentially assigns the divided reconstructed images obtained by the reconstruction operation unit 11 to the memories # 1 to # 7 of the image memory 10 in order. For example, the divided reconstructed image g1 is assigned to # 1, the divided reconstructed image g2 is assigned to # 2,..., And the divided reconstructed image g7 is assigned to # 7 and stored. .., G8 is assigned to # 1 in place of g1, g9 is assigned to # 2 in place of g2, and so on.

The multiplier 13 weights each of the divided images by multiplying the images of the memories # 1 to # 7 in association with the weighting factors W1 to W7. The adder 25 performs total addition to obtain one reconstructed image.

The concept of the reconstructed image processing by the image processing apparatus 103 described above will be described again with reference to FIG. That is, the effective data range obtained by the pre-scan, the reference scan, or the normal scan at the time of precision imaging is shown by a thick solid line in FIG. The projection image data obtained by being limited and irradiated is a continuous scan, or, if the scan is repeated continuously such as fluoroscopy,
As shown in the figure, the data becomes temporally discontinuous outside the region of interest A, and high image quality cannot be obtained.

Therefore, the above continuous scan or
During continuous scans such as fluoroscopic imaging, the channel collimator 210 embeds projection data of a previous time obtained by scanning at a normal position or scanning over a range larger than the irradiation range, thereby enabling low exposure, and In addition, reconstructed image processing capable of improving the image quality even in the region outside the region of interest A can be realized. In other words, this makes it possible to obtain images with low exposure, and to obtain more easily-viewable images even in regions inside and outside the region of interest A.

[0036]

As is apparent from the above detailed description,
According to the X-ray CT apparatus according to the present invention, the CT of a certain region of interest is monitored after the injection of the contrast agent by applying a technique for displaying an image in real time using a high-speed image reconstruction technique. When the region reaches a preset CT value, the scan method for starting a spiral scan or a normal scan does not miss the optimal contrast timing, does not degrade the image quality, and performs X-ray irradiation on the patient or the like. It is possible to provide an X-ray CT apparatus capable of reducing the amount of exposure.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a flow of imaging, which is an example of a usage example of an X-ray CT apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing an entire configuration including a circuit configuration of the X-ray CT apparatus of the present invention.

FIG. 3 is a detailed explanatory view of a scanner system of the X-ray CT apparatus of the present invention.

FIG. 4 is a detailed explanatory view of a channel collimator of the X-ray CT apparatus of the present invention.

FIG. 5 is an explanatory diagram illustrating an operation of a lead shielding plate of the channel collimator.

FIG. 6 is a flowchart illustrating a flow of imaging in the X-ray CT apparatus of the present invention.

FIG. 7 is a geometric explanatory view of the operation of the channel collimator in the X-ray CT apparatus of the present invention.

FIG. 8 is a diagram illustrating a method of correcting projection data and data in a region of interest in a scan obtained by the channel collimator.

FIG. 9 is a circuit block diagram illustrating an example of an image processing apparatus in the X-ray CT apparatus according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 Display device 101 Host computer 102 Scanner 103 Image processing device 104 High voltage generator 105 Patient table 200 X-ray source 210 Channel collimator 211 Controller 214 Lead shielding plate 250 Detector A Region of interest B Subject

Claims (4)

[Claims] An X-ray source is continuously rotated to continuously measure projection data of a subject a plurality of times, and a tomographic image of the subject is reconstructed on the basis of the projection data and displayed on a display device. In an X-ray CT apparatus for sequentially displaying, when a region of interest setting means for setting an irradiation range of the X-ray from the X-ray source in the subject, and measuring the projection data by rotating the X-ray source, A channel collimator for blocking the irradiation of X-rays to the outside of the region set by the region of interest setting unit, and monitoring a CT value in a specific region of the region of interest; An X-ray CT apparatus comprising: a calculation unit in a region of interest for calculating a change amount. 2. The X-ray CT apparatus according to claim 1, wherein the X-ray CT apparatus sets a CT value in a specific region of the region of interest by the region-of-interest calculation means to a set CT value. An X-ray CT apparatus for starting a continuous scan under a preset condition. 3. An X-ray CT apparatus according to claim 2, wherein the continuous scan is a helical scan. 4. The X-ray CT apparatus according to claim 2, wherein the continuous scan measures the same cross section continuously.