JP2622064B2 - CT device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CT apparatus (computer tomography apparatus) such as a helical scan type X-ray CT apparatus, and more particularly to observing a temporal change of a subject such as a contrast effect by a contrast agent. The present invention relates to a dynamic helical scan, which is a projection method that repeatedly performs a helical scan.

[0002]

2. Description of the Related Art Helical scan (Helical Scan) is, in a narrow sense, a spiral scan, that is, a scan method in which the volume of an object is scanned by moving the object while continuously rotating the projection direction in one direction. In particular, when there is a radiation source outside the subject, such as XCT, this name is used because the movement trajectory of the radiation source draws a screw surrounding the subject with respect to the subject.

[0003] This concept is easily extended to refer to a general scanning method for scanning the volume of a subject by moving the subject while performing scanning. For example, as actually performed in the third, fourth, and fifth generation XCT, the X-ray source may alternately repeat clockwise and counterclockwise, with a “rotation” of less than 360 °. There may be. In these cases, the trajectory of the X-ray source generally draws a three-dimensional curve on a cylinder with respect to the subject. Furthermore, this curve is not always continuous.

[0004] This is implemented in the fifth generation XCT, in which the focal point moves repeatedly in the same direction on an arc of less than 360 ° (for example, 210 °), and the bed is continuously moved. It occurs when helical scan is performed by combining things.

That is, it is rather difficult to define the category of Helical Scan based on what kind of curve the trajectory describes. In particular, it is not appropriate to use the term "spiral" when defining Helical Scan.

Further considering SPECT and PET,
It turns out that the concept of “trajectory of the radiation source” itself should not be used to characterize Helical Scan.

Helical Scan in a broad sense includes (1) a CT apparatus (where at least one tomographic image of a subject can be obtained without moving the subject).
Device that can capture images for one slice position)
(2) By moving the subject continuously during the scanning operation, (3) Data collection is performed over the range of slice positions that can be photographed without moving the subject, and (4) Based on this data It is reasonable to assume that an image reconstructing device capable of producing a tomographic image at any slice position within the range (at least potentially) is provided, and (5) the above can be defined as performing three-dimensional imaging of a subject. is there.

Accordingly, Helical Scan (in a broad sense) is possible (although it is necessary to add the image reconstructing device described in (4)) in all the CT devices described above.

Note that the third, fourth, and fifth generation XCT, SP
ECT and PET are as described below.

[1] Third generation (Rotate-Rotate system) XC
When using an X-ray tube as the T X-ray source, there is a focus in the X-ray tube, which is the X-ray source (usually the focus is stationary with respect to the X-ray tube, but the focus is There is also a method that moves a little repeatedly.) To rotate the X-ray source around the subject, the X-ray tube is rotated around the subject.

The detector array includes a plurality of detectors (channels).
Are arranged on a plane curve (usually an arc or line segment), and X
Position unchanged with respect to the tube. Thus, as the x-ray tube rotates around the subject, the detector array also rotates around the subject.

The rotation here is not limited to continuous rotation in one direction, but may be, for example, a clockwise rotation and a counterclockwise rotation alternately. Further, the rotation of 360 ° is not always required, and may be 180 ° + (angle of spread of the X-ray fan) or more.

[2] 4th generation (Stationary-Rotate method)
When using an X-ray tube as an XCT X-ray source, there is a focus in the X-ray tube, which is the X-ray source (usually the focus is stationary with respect to the X-ray tube, but the focus is There is also a method that moves a little repeatedly.) To rotate the X-ray source around the subject, the X-ray tube is rotated around the subject.

The detector array includes a plurality of detectors (channels).
Are arranged on a plane curve (usually the circumference), and the detector array is spatially immobile (ie, relative to the earth).

The meaning of the term "rotation" is the third generation XC
As described in the section of T.

When using an X-ray tube as a fourth generation (Nutate-Rotate type) XCT X-ray source, the X-ray tube has a focal point, which is an X-ray source (usually, the focal point is relative to the X-ray tube). Although there is no movement, there is also a method in which the focal point is slightly moved repeatedly with respect to the X-ray tube). To rotate the X-ray source around the subject, the X-ray tube is rotated around the subject.

The detector array comprises a plurality of detectors (channels)
Are arranged on a plane curve (usually a circle), and the detector array does not rotate or move spatially, but does so-called nutation. That is, only the direction of the normal vector of the plane determined by the plane curve formed by the array of detectors changes. More specifically, if one end of a normal vector (length 1) 101 is placed at the origin 102 as shown in FIG. 9, the other end draws a circle 103 on a spherical shell 105 having a radius of 1 centered on the origin. Exercise. In addition, this movement is linked to the movement of the X-ray tube 104 rotating around the object, and the other end of the normal vector also moves one circle on the circumference of the spherical shell 105 while the X-ray tube 104 makes one rotation. Rotate.

The meaning of the term "rotation" is the third generation XC
As described in the section of T. Therefore, in the above description, "one rotation" is a special case of "one cycle of operation" for convenience.

[3] Fifth generation (electron beam scanning) type XCT X-ray tube is spatially immobile. The focus is inside the X-ray tube, and this is the X-ray source. The focal point rotates around the subject by moving with respect to the X-ray tube.

The detector array comprises a plurality of detectors (channels)
Are arranged on a plane curve (usually a circle or arc), and the detector array is spatially immobile.

The meaning of the term "rotation" is the third generation XC
As described in the section of T. There is an actual commercial machine that reciprocates or repeats on an arc with a focal point of about 215 °.

[4] Modifications There are various modifications to these basic forms. For example, there are those having a so-called two-dimensional detector array in which detectors are arranged on a curved surface, those having a plurality of X-ray sources, and the like.

As the X-ray source, in addition to the X-ray tube, a pellet containing a radioisotope emitting X-rays, a device for generating X-rays by accelerating an electron beam with an accelerator, and an X-ray laser device are considered.

Gamma ray CT having an external radiation source The above third generation and fourth generation XCTs (three in total) except that a gamma ray is used in place of the X-ray and a pellet containing a normal radioisotope is used as the gamma ray source. Is the same as

A gamma ray CT having an external radiation source is often constructed by diverting a gamma camera, SPECT, or PET detection unit and combining this with an external radiation source.

Of course, there may be more than one source.

[5] SPECT (Single Photon Emission)
CT) A gamma ray emitted from a gamma ray source distributed in the subject is detected, and a tomographic image of the distribution of the gamma ray source in the subject is reconstructed by calculation.

[6] Rotating camera type SPECT device A gamma camera (scintillation camera) is rotated around a subject to scan.

Normally, a gamma camera is provided with a plate-shaped, ie, two-dimensional, single detector, and it is possible to know at which position on the detector a gamma ray photon hits. Further, gamma cameras are equipped with a collimator in front of the detector so that only gamma rays coming from a particular direction (usually perpendicular to the surface of the detector) can be incident on the detector. A typical collimator has a shape (not a manufacturing method) in which a thick plate is densely provided with a large number of holes whose central axes are parallel to each other. The meaning of the term "rotation" is as described in the section of the third generation XCT, and usually, at least 18
What is necessary is just to turn 0 degree.

Rotating Collimator SPECT Apparatus The detector usually comprises a plurality of elements, which are arranged to form a circumference. Inside the circumference, there is a circumferential collimator concentric with the circumference, and this collimator has a structure in which only a gamma ray radiated in a specific direction from a plane formed by the collimator and the detection can enter the detector. It has become. A typical collimator, specifically,
It is in the shape of a ring as a whole, but there is a difference between its inner diameter and outer diameter, that is, it is a thick and extremely short pipe, and many parallel to the plane (end face of the pipe) formed by the collimator and parallel to each other. The holes are dense.

The detector is usually immobile, and by rotating a collimator inside the circumference formed by the detector, scanning is performed by continuously changing the direction of gamma rays which can be incident on the detector.

The meaning of the term "rotation" is the third generation XC
As described in the section of T, usually, it is sufficient to rotate at least 180 °.

Conventionally, before the introduction of the helical scan as described above, a contrast agent is injected, inhaled, or accumulated into a site to be inspected of a subject such as a human body, and a contrast effect (contrast of the target site) by the CT agent is obtained by a CT apparatus. In order to photograph or observe the shadow over time, dynamic scanning has been performed. Dynamic scan is a part (position) of the subject
It is a method that scans repeatedly, and by observing the change over time of the subject at one or more scan positions, observes the dynamics of inflow, outflow, or retention of the contrast agent into the tissue at the target site. measure. In particular,
From each of a series of images taken repeatedly after administration of the contrast agent, a difference image obtained by subtracting “the image at the slice position before the administration of the contrast agent” is created, and the difference image is obtained. Observe the change (which changes with time t).

For example, in FIG. 7, FIG. 7A is a diagram showing an example of a dynamic scanning imaging method, in which the vertical axis represents a slice position, the horizontal axis represents time, and A0, A1,.
A2, A3..., B0, B1, B2, B3... Are times tA0, tA1, tA2, tA2... TB0, tB1, tB2,
The observation image at tB3. Assuming that the contrast agent is administered between time tB0 and tA1, the images A0 and B0 are "images before administration of the contrast medium", and therefore, as shown in FIG. 7B, A1 -A0, B1 -B0, A2 -A0, B2 -B0
, A3 -A0, B3 -B0. FIG.
A is A1-A0, A2-A0, A3-A shown in FIG. 7B.
This is a difference image of 0. From these difference images, a characteristic value of the change of the image is extracted as a pixel, and an image (functional image) in which the value is expressed as a gray value is also performed. For example, FIG. 8B shows that the density of the contrast agent is maximum for each pixel of the image (the CT value is maximum).
It is a time (peak time image) that represents the time with a corresponding gray value. According to this, it is possible to easily know the place where the contrast agent has reached the earliest.

A problem when performing dynamic scan in a conventional helical scan type X-ray CT apparatus is an artifact appearing in a difference image. When the same subject is scanned twice or more without a contrast agent, and the obtained images are compared. In this case, since no contrast agent is added, the difference between the two is ideally zero except for random noise. Should be.

[0036]

However, in the helical scan system, the difference between them does not always become zero. When such a difference occurs in the image, the following problem occurs in practical use. For example, scan the subject once by helical scan, then administer the contrast agent and
Suppose that a helical scan is performed, the flow of the contrast agent between the image I ₁ obtained from the first helical scan and the image I ₂ obtained from the second helical scan for the same slice position It is expected that a difference will occur, but other differences that would have occurred even if no contrast agent was administered will cause the operator to mistakenly interpret the difference as "due to the flow of (original) contrast agent." There is a risk of doing it.

Therefore, if there is no change in the subject, it has been required to eliminate such a difference between the image obtained by the first helical scan and the second and subsequent helical scans.

The above problem is not limited to the helical scan type X-ray CT apparatus, but is a common problem in performing dynamic helical scan using SPECT, gamma ray CT, optical CT apparatus, etc., which generally perform helical scan. is there.

The present invention has been made in order to solve the above problems.
In a dynamic helical scan for observing a subject over time using a computer tomography apparatus that repeatedly performs a helical scan, an artifact does not appear in a difference between images of the first helical scan and the second and subsequent helical scans. An object of the present invention is to provide a helical scan type computed tomography apparatus.

(Causes of the above-mentioned problems) The helical scan X-ray CT shown in FIG.
A description will be given of the cause of the above-described problem by taking a case where a dynamic helical scan is performed by the apparatus as an example. FIG.
The X-ray source 41 is rotating at a constant speed on an orbit 42, and its position is represented by an angle θ. In addition, bed (couc)
h) 43 moves in the Z direction. X-ray source position θ and time t
Is shown in FIG. Also, the position of the bed and the time t
Is shown in FIG. A ≦ Z ≦ B is a range to be photographed, and X-rays are emitted at least while the couch is in this range (FIG. 5 (c)). During this time, the speed of the couch 43 is constant (FIG. 5 (b) )). Further, in FIG. 5 (b), the movement of the bed because it is dynamic scanning is repeated, Z = z become time t _z1 in first helical scan, the next iteration is t _z2. The position of the X-ray source at time t _z1 during the first helical scan is θ _tz1 , and the position of the X-ray source at the next repetition time t _z2 is θ _tz2
In general, θ _tz1 ≠ θ _tz2 .

As described above, if the positions θ _tz1 and θ _{tz 2} of the X-ray source 41 are not the same at the same slice position Z = z, 1
There is a difference in the image between the first scan and the second scan that is not dependent on the contrast agent. Considering the CT image at the slice position z when considering the cause of the difference, when the X-ray source 41 is instantaneously photographed at the slice position z during the first scan at the angle θ _tz1 , The projection data for one rotation in z necessary to form an image has not been obtained. Therefore, the data for one rotation at z, that is, the data that would be obtained if the X-ray source was rotated once without moving the bed at the position z, is represented by the angle θ _tz1.
Is calculated using data for about one rotation before and after the above, and an image at z is created using the result of the calculation. Since the estimated data has some difference compared to the original data for one rotation in z, this difference appears as an artifact on the created image, but the artifact is too small during the estimation. We are trying to make it less noticeable (this artifact is not the problem in this case). The same processing is performed in the second scan, but it is approximately one rotation before and after the angle θ _tz2 at time t _z2 that is used to estimate projection data for one rotation at z.

The data used for estimating data for one rotation at the slice position has different angles of the X-ray tube in two helical scans.

For example, to estimate projection data from the direction of θ = 0, the first scan in FIG.
The data of Otsu1 is used. The data of Exhibit A1 is projection data from the direction of θ = 0 at the slice position z−ΔYA1, and the data of Exhibit 1 is projection data from the direction of θ = 0 at the slice position z + Δz. In the second scan, the data of Exhibits 2 and 2 are used to estimate the projection data from the direction of θ = 0, and the slice positions are z-Δz A2 and z-Δz 2. Here, if Δz A1 = Δz A2 and Δz O1 = Δz O2, then Data A1 is equal to Data A2, and Data O1 is equal to Data A2, and the estimated “Z = z Projection data from the direction of θ = 0 ”should match between the first helical scan and the second helical scan. However, in FIG. 5, Δz A1 ≠ Δz A2, Δz
Otsu 1 ≠ Δz Otsu 2, which is not equal to Data A 1 and Data A 2, and Data A 1 and Data A 2, resulting in a difference in the estimation result.

FIG. 6 assumes that time t is a parameter and θ and Z
(Hereinafter referred to as a Z-θ graph). For example, focusing on the slice position z, the angle of the X-ray source differs between the first helical scan and the second helical scan (θ _tz1 , θ _tz2 ).

The data (C) in the projection direction θ = 0 at the slice position z are shown in FIG.
Estimated from the data at Otsu 1, the second time
Estimated from data of 2. The data of Exhibits A1 and A2 are the same in the position of the X-ray source, but are not the same because the position Z of the bed is slightly different, and are the same for Otsu1 and Otsu2. For this reason, in the first and second helical scans, the difference in the slice position z is generated in exactly the same manner for all angles θ, and the images calculated (reconstructed) from these data are slightly different. In other words, even if there is no change in the subject, the difference is different between the first and second helical scans at the same slice position, and thus these difference images do not become zero.

When such a difference occurs in the image, practically,
The following problems occur. For example, if the subject is scanned once by a helical scan, then a contrast agent is administered, and then another helical scan is performed, an image I ₁ obtained from the first helical scan at the same slice position is obtained. It is expected that there will be a difference in the inflow of contrast agent between and the image I ₂ obtained from the second helical scan, but other differences that would have occurred without, for example, administering contrast agent. Overlap, the latter may be erroneously interpreted as "difference due to the inflow of the original contrast agent". Therefore, it has been required to eliminate such a difference between the image obtained by the first helical scan and the second and subsequent times if there is no change in the subject.

[0047]

According to a first aspect of the present invention, an X-ray source is rotated around an object while irradiating an X-ray, and data is transmitted during the operation. The scanning unit performs scanning by continuously moving the X-ray source and the subject in the body axis direction of the subject while collecting the projection data, and based on the projection data obtained from the data collecting unit by the scanning. In a CT apparatus configured to reconstruct a cross-sectional image of a subject within a scanned range, when the same subject is repeatedly scanned in the range multiple times, the first scan is performed. To recognize the relative position between the subject and the X-ray source in the body axis direction and the rotational angle position of the X-ray source, and adjust the body axis so as to match the relative position and the rotational angle position in the second scan. Relative position of subject and X-ray source in different directions And a control unit for controlling the rotational angular position of the X-ray source, even in many times the scan is repeated to subject matter to be so in the same positional relationship.

According to a second aspect of the present invention, the data collection unit performs an operation of collecting projection data while changing the direction of projection, and continuously connects the data collection unit and the subject in the body axis direction of the subject. In the CT apparatus configured to reconstruct a cross-sectional image of the subject within the scanned range based on the projection data obtained from the data collection unit by this scanning, In the case where the scanning of the range is repeatedly performed a plurality of times on the same subject, the relative position between the subject and the data collection unit in the body axis direction and the rotation angle position of the data collection unit in the first scan And controls the relative position between the subject and the data acquisition unit in the body axis direction and the rotation angle position of the data acquisition unit in the second scan so as to match the relative position and the rotation angle position in the second scan. Have that control unit, and the gist to be in so that even in many times the scan is repeated in the same positional relationship.

According to a third aspect of the present invention, in the CT apparatus according to the first or second aspect, based on the projection data collected in the first scan before the contrast medium is injected into the subject. Reconstructing a first cross-sectional image, reconstructing a second cross-sectional image based on the projection data collected in the second scan after injecting a contrast agent into the subject,
The gist is that the image processing apparatus further includes a calculation unit that obtains a difference image that is a difference between the cross-sectional image and the second cross-sectional image, and obtains a distribution of the contrast agent in the subject based on the difference image.

[0050]

In the CT apparatus having the above configuration, when two scans are performed on the same subject without administering a contrast agent, the first scan and the second scan take pictures as shown in FIG. Since the position of the X-ray source is the same (θ _z ) at any of the couch positions z within the desired ranges A and B, a series of projection data obtained from the first scan and the second scan is the same. That is, C
In the T apparatus, when two helical scans are performed on the same subject, if there is no change in the subject, the first scan and the second scan may use any one of the ranges A to B to be imaged. Since the direction of projection is the same at the couch position z, the series of data obtained is exactly the same (excluding noise). Therefore, for example, when a contrast agent is administered, an accurate “difference” occurs between the first scan and the second scan due to X-ray absorption by the contrast agent. In other words, when a contrast image is administered and dynamic helical scanning is performed using the apparatus of the present invention to create a difference image, only the difference caused by the inflow / outflow of the contrast agent appears as the difference.

Further, by using the CT apparatus according to the present invention, a cross-sectional image reconstructed based on the projection data before the injection of the contrast agent and a projection image reconstructed based on the projection data after the injection of the contrast agent. By obtaining a difference image that is a difference from the cross-sectional image, it is possible to extract only an image using a pure contrast agent without artifacts or other disturbances. Then, the distribution of the contrast agent in the subject can be obtained based on the difference image.

[0052]

FIG. 1 is a block diagram showing a schematic configuration of a first embodiment of an X-ray CT apparatus according to the present invention, and FIG. 2 shows the relationship between the position of a couch and the angle of an X-ray tube.
FIG. 1A shows X while continuously rotating around the subject 10.
A gantry 2 including an X-ray tube 3 and an X-ray source angle detector 8 as an X-ray source for irradiating a ray, and a bed capable of continuously moving a subject 10 while the X-ray tube 3 rotates. 4, a bed driving unit 5, a control unit 6 for controlling the rotation of the X-ray tube 3 and the bed driving unit 5, a timing clock 7, a DAS circuit 1,
2, an X-ray CT apparatus 1 including a computer 12, a drive circuit 16, and a servomotor 17 for rotating an X-ray tube is shown. Although not shown in FIG. 1, an X-ray detector composed of a plurality of detection elements is provided so as to face the rotating X-ray tube 3. X-ray CT apparatus 1
Is provided with an image reconstructing device (not shown) that scans the subject 10 and reconstructs an image based on the projection data obtained by the scanning and the position data of the bed 4.

In FIG. 1, the X-ray tube 3 has an orbit (broken line portion) 11.
Are rotated at a constant angular velocity, and the position of the X-ray tube 3 at a certain time is represented by an angle θ. Figure 2 shows the relationship between θ and time t.
It is shown in (a). The bed 4 moves in the Z direction. The relationship between the position of the bed and the time t is shown in FIG. In this case, A ≦ z ≦ B is a range to be photographed, and at least during this period (T = | tA2−tA1 |), X-rays are emitted (FIG. 2).
(C)), the speed of the bed 4 is constant (FIG. 2 (b)).

According to the present invention, the control unit 6 controls the X-ray tube angle θ (when the position of the bed is z) throughout the period in which the X-ray CT apparatus 1 emits X-rays and collects projection data. Less than,
θ (z)) is controlled so that z and θ are the same in any number of helical scans. In the present embodiment, the control unit 6 also controls the amount of X-ray irradiation. In order to control the above-mentioned z and θ, the control unit 6 controls T / C in FIG.
The angle θ of the X-ray tube 3 or the detector and the position z of the bed are controlled so that is an integer (T is FIG. 2 (b); C is FIG. 2).
(B)). Specifically, the control unit 6 performs the following (1),
The angle θ and the bed position z are controlled so as to satisfy (2) and (3).

(1) The speed of the bed is constant while collecting X-ray exposed data. In other words, in FIG. 2B, the speed between tA1 and tB1 and the speed between tA2 and tB2 are constant, and the speed between tA1 and tB1 and the speed between tA2 and tB2 are constant.
The speeds at tB2 are equal. The same applies to the second and subsequent repetitions.

(2) The angular velocity of rotation of the X-ray source is constant (FIG. 2A), and its cycle is C.

(3) The start time tAn of the n-th scan and the start time tAn + 1 of the (n + 1) -th scan are expressed as “T = t
T is an integral multiple of C when An-1−tAn is satisfied ”(FIG. 2B).

In this way, as shown in FIG. 3, the data collection portion (straight lines 31 to 34) can be obtained in any scan.
The orbit at is the same.

In FIG. 1, when the X-ray tube 3 rotates, the position z of the bed 4 is associated with the angle θ of the X-ray source, and the angle θ
Is detected by the X-ray source angle detector 8 and feedback control is performed by the control unit 6. In addition, a control signal is sent to the servo motor drive circuit 16 in accordance with the angle θ, and the bed 4 is moved in the body axis direction by the servo motor 5 as a bed driving unit. When the bed returns to the original position (tB1 to tA2 in FIG. 2B), the control unit 6 drives the servomotor 17 to return to the home position. Then, waiting for the angle θ to reach a predetermined value, z changes according to the angle θ of the X-ray source (FIG. 2).
(B) tA2 to tB2).

In the above embodiment, the spiral scanning X-ray C
Although the T apparatus has been described as an example, the detector is not limited to the X-ray CT apparatus. For example, the detector of the subject can be used without the third, fourth, and fifth generation XCT apparatuses and the radiation source described above in “Prior art”. The present invention is also applicable to a computer tomography apparatus such as a SPECT apparatus that rotates around.

Further, the present invention is not limited to the spiral scanning, but the X-ray source may scan so as to alternately rotate clockwise and counterclockwise, and the rotation may be less than 360 °. You may scan. Also, like the 5th generation method,
By moving the focal point in the X-ray tube relative to the spatially immobile X-ray tube so that the X-ray source rotates around the subject and scanned, it is included in the present invention. Of course. In addition, C having a so-called two-dimensional detector array
The present invention can be applied to a T device and a CT device having a plurality of X-ray sources. As the X-ray source, a pellet containing a radioisotope that emits X-rays, a device that generates X-rays by accelerating an electron beam with an accelerator, an X-ray laser device, or the like may be used. Further, the present invention may be applied to a gamma ray CT having an external radiation source, or may have a plurality of radiation sources. That is, in the present invention, any CT apparatus capable of performing a helical scan in a broad sense as described in the section of "Prior Art" can be applied to all except PET in which a detector or a collimator does not move at all. Needless to say, various modifications can be made.

In this embodiment, the subject is fixed, and X
Although the example in which the helical scan is performed while rotating the X-ray detector and the X-ray detector has been described, the present invention is not limited to this, and can be applied to the case where the helical scan is performed by moving the subject.

Next, a second embodiment of the present invention will be described. In this embodiment, the blood volume of a three-dimensional tissue is measured using the CT apparatus shown in the first embodiment.

Generally, the blood volume distribution in the brain is determined by CBV (Cevebr
al Blood Volume), which is useful information for diagnosis and treatment planning of various diseases. As such a CBV measurement method, a method using SPECT or PET is conventionally known. In such SPECT and PET, drugs that do not leak out of the blood vessels of the brain to R. I. (Radioactive substance) is injected into blood, and a three-dimensional image is taken to obtain R.R. I. Know the distribution. Then, the blood is collected and the R.V. I. If you measure the amount of CBV
You can know. As a drug that does not leak out of the blood vessels of the brain to the outside, for example, a drug obtained by taking out only a membrane of red blood cells and injecting an appropriate radiopharmaceutical into the membrane using this membrane as a capsule may be used.

However, in such CBV measurement, the cost of the drug is high, the measurement takes a long time, large facilities for handling radioactive substances are required, and the spatial resolution is low (5 mm to 20 mm). There are drawbacks.

As a second method, there is also known a method of measuring CBV on some tomographic slices using an X-ray CT apparatus. In this method, a contrast agent such as a nonionic iodine contrast agent that does not leak out from the cerebral blood vessels is injected into blood, and a distribution of the contrast agent is known by taking a tomographic image. Then, the CBV is measured by measuring how much the CT value has changed before and after the injection of the contrast agent.

However, according to this method, CBV can be measured only in a small number of cross sections. In addition, a long time is required for the measurement.

Therefore, in the second embodiment of the present invention, the CBV is measured using the CT apparatus of the first embodiment. That is, the helical CT scan according to the first embodiment is performed before and after the injection of the contrast agent, and the difference between them is calculated as 3
CBV can be measured dimensionally.

FIG. 10 is a characteristic diagram showing the change in density when the contrast agent is injected at time t _1. Curve S ₁ indicates the case where no contrast agent is injected, and curve S ₂ indicates that the injection is not performed. FIG.

As a measurement procedure, first, a helical CT of the brain is taken before injection of a contrast agent (this is referred to as pre-data). When injecting the contrast medium at time t _1, the contrast medium flows in the blood vessel become lumps.

After about one minute, the contrast agent is almost uniformly distributed in the blood of the whole body, and is gradually excreted from the kidney at the same time. At this point, a helical CT is taken using the method of the first embodiment so as to draw the same trajectory as the helical CT performed before the injection of the contrast agent (this is referred to as post data). Incidentally, as shown by the curve S _2, the contrast medium was instilled, may be kept the concentration of the contrast medium in the blood constant.

When the pre CT image is subtracted from the post CT image, a three-dimensional distribution of the contrast agent is obtained as a difference map as shown in FIG. This difference map is referred to as dif.

Since dif is a change amount of the CT value, it is converted into a blood amount CBV. That is, if the CT value of blood at the time of helical CT imaging before contrast agent injection is Bpost, and the CT value of blood at the time of helical CT imaging after injection of contrast agent is Bpre,
The following equation holds.

CBV = dif / △ B △ B = (Bpost−Bpre) Here, there are, for example, the following three methods for finding △ B.

(A) Blood is collected at times t ₂ and t ₃ , their CT values are measured with a CT scanner, and the difference is expressed as ΔB
And

(B) At time t ₃ , blood is collected and subjected to chemical analysis to obtain a contrast agent concentration, which is converted into a change in CT value.

(C) Blood is not collected, and the difference map di is used instead.
The ΔCT value of the large blood vessel included in f is used as ΔB.

Thus, the CBV can be obtained by using the method shown in the first embodiment. As described above, according to the second embodiment, the CBV can be obtained without increasing the spatial resolution as compared with the related art, requiring a long time, and requiring no facility for handling radioactive materials. In addition, since the drug is relatively inexpensive and the measurement time is short, accurate results can be obtained with less body movement. Further, there is an advantage that a three-dimensional distribution can be understood.

[0079]

As described above, when a difference image is created by performing the dynamic helical scan according to the present invention, only a difference caused by a temporal change of the subject, for example, an inflow or a loss of the contrast agent appears as a difference. Inject, inhale or accumulate the contrast agent into the examination target site
When imaging or observing a contrast effect (shadow of a target portion) by a contrast agent over time by a computer tomography apparatus such as a line CT apparatus, only a change in a subject, that is, in this case, only an imaging effect can be imaged or observed. Therefore, a more precise examination can be performed than before, and more accurate diagnosis can be expected.

In addition, the blood volume distribution can be measured easily and in a short time, and the effect that the blood volume distribution can be measured with high accuracy can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of a spiral scanning X-ray CT apparatus according to the present invention.

FIG. 2 is an explanatory diagram of a position of a bed, an angle of an X-ray tube, and an X-ray irradiation state.

FIG. 3 is a Z-θ graph in which the relationship between θ and z is plotted by regarding time t as a parameter.

FIG. 4 is a perspective view showing an example of a helical scan type X-ray CT apparatus.

FIG. 5 is an explanatory diagram of a position of a bed, an angle of an X-ray tube, and an X-ray irradiation state in a conventional dynamic helical scan.

FIG. 6 is a Z-θ graph in a dynamic helical scan according to the related art.

7A is a plot diagram showing a result of the dynamic scan at slice positions A and B, and FIG. 7B is a plot diagram of a difference image of the result shown in FIG. 7A.

8A shows the difference image shown in FIG. 7B, FIG.
Is a functional image.

FIG. 9 is an explanatory diagram of a detector array in the fourth generation (Nutate-Rotate method) XCT.

FIG. 10 is a characteristic diagram showing a temporal change in a cerebral blood contrast medium concentration according to the second embodiment of the present invention.

FIG. 11 is an explanatory diagram showing a procedure for creating a CBV map.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 X-ray CT apparatus (spiral scanning computer tomography apparatus) 2 gantry 3 X-ray tube (X-ray source) 4 couch 5 couch drive unit (servo motor) 6 control unit 10 subject

Claims (3)

(57) [Claims] An X-ray source is rotated around an object while irradiating the X-ray, and during this operation, the data collection unit collects projection data while the X-ray source and the object are exposed to the object. Scanning is performed by continuously moving the body in the body axis direction, and a cross-sectional image of the subject within the scanned range is reconstructed based on projection data obtained from the data collection unit by this scanning. In the CT apparatus, when the same subject is repeatedly scanned in the range multiple times, the relative position between the subject and the X-ray source in the body axis direction and the X-ray The relative position between the subject and the X-ray source in the body axis direction and the rotation angle of the X-ray source so as to recognize the rotational angle position of the source and to match the relative position and the rotational angle position in the second scan. It has a control unit to control the position, A CT apparatus having the same positional relationship regardless of the number of times of scanning. 2. Scanning is performed by continuously moving the data collection unit and the subject in the body axis direction of the subject while the data collection unit operates to collect projection data while changing the direction of projection. And a CT apparatus configured to reconstruct a cross-sectional image of the subject within the scanned range based on the projection data obtained from the data collection unit by this scanning. In the case where the scanning of the range is repeatedly performed a plurality of times, the relative position between the subject and the data acquisition unit in the body axis direction and the rotation angle position of the data acquisition unit are recognized in the first scan, and in the second scan, A control unit that controls a relative position between the subject and the data collection unit in the body axis direction and a rotation angle position of the data collection unit so as to match the relative position and the rotation angle position; A CT apparatus having the same positional relationship regardless of the number of times of scanning. 3. The method according to claim 1, wherein the first step is performed before injecting a contrast agent into the subject.
A first cross-sectional image is reconstructed based on the projection data collected in the scan, and a second image is reconstructed based on the projection data collected in the second scan after the contrast agent is injected into the subject.
Calculating means for reconstructing the cross-sectional image of the above, obtaining a difference image which is a difference between the first cross-sectional image and the second cross-sectional image, and calculating a distribution of the contrast agent in the subject based on the difference image. C according to claim 1 or claim 2, wherein
T device.