JP2825352B2 - CT device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention radiates a fan-shaped radiation from a plurality of directions to a subject, measures radiation intensity data of the radiation transmitted through the subject, and obtains projection data therefrom.
The present invention relates to a CT apparatus that images a distribution of radiation absorption coefficients on a tomographic plane of a subject from reconstruction data from projection data from these multiple directions.

2. Description of the Related Art Conventional CT apparatuses have several scanning methods depending on the collection of projection data. For example, in an X-ray CT apparatus using a rotating / rotating method or a third-generation method as one scanning method, an X-ray source that irradiates a subject with fan-shaped X-rays, and an X-ray transmitted through the subject the X-ray detector elements to measure X-ray intensity and the arc-shaped in the X-ray detector element array formed by arranging one-dimensionally, placed on a plane, they without changing the relative position to each other, The subject is rotated on this plane. This plane is called a scanning plane. By radiating X-rays from the X-ray source in a fan shape and collecting X-ray intensity data for each element of the X-ray detector element array, a group of tomographic images for one slice plane of the subject is obtained. Projection data is obtained.

On the other hand, in an X-ray CT apparatus of the stationary / rotating type or the fourth generation type, an X-ray
The radiation source alone rotates around the subject, and the array of X-ray detector elements arranged to surround the subject does not rotate.

A conventional third-generation or fourth-generation X-ray CT apparatus uses X-ray detection means in which X-ray detector elements are arranged one-dimensionally. The X-ray source must be rotated at least once around the subject on a plane. Therefore, in order to obtain a group of projection data for obtaining a tomographic image for one slice plane of the subject, in order to obtain tomographic images for a plurality of different slice planes of the subject, scan the scanning plane After moving the X-ray source one time on this plane.

[0005] Therefore, by using such an X-ray CT apparatus, it is possible to collect in a short time X-ray intensity data necessary for creating tomographic images on a plurality of different slice planes of a subject. As a scanning method, there is a spiral scan (referred to as a helical scan, a spiral scan, or the like by an X-ray CT apparatus manufacturer).

The helical scan is, for example, for moving the subject while continuously rotating the X-ray source in the case of the third or fourth generation X-ray CT apparatus described above. In the spiral scan, the position of the subject continuously changes according to the rotation angle of the X-ray source during the operation of irradiating X-rays. That is, the position of the scanning plane with respect to the subject continuously changes. The X-ray intensity data collected by the spiral scan is processed by a predetermined method, and tomographic images on a plurality of slice planes are reconstructed. The spiral scan can collect projection data necessary for reconstructing tomographic images of a plurality of slice planes in a shorter time than the scan method described above.

[0007]

However, the above-mentioned third method
When a spiral scan is performed by a generation or fourth generation X-ray CT apparatus, the X-ray detection means uses a one-dimensional array, and during the rotation of the X-ray source, it substantially corresponds to the thickness of the slice plane. Since the subject can be moved only to the extent, the X-ray source has to be rotated as many times as the number of tomographic images in order to obtain a plurality of tomographic images.

For this reason, a large amount of time is required for the spiral scan. And it takes time to scan,
At that time, the posture of the patient (subject) or the organ of the patient may change, and a correct tomographic image may not be obtained.

Further, since the heat generated by the X-ray source becomes enormous, an X-ray source having a large heat capacity must be used.

An object of the present invention is to provide a CT apparatus capable of shortening the time required for a spiral scan.

[0011]

According to the first aspect of the present invention, there is provided:
X-ray source that emits X-rays to the subject
The X-ray source draws a spiral trajectory around the subject
Driving means for driving the X-ray source or the subject;
X-ray detection element arrays are arranged in a plurality of rows in the body axis direction of the subject.
Detects X-rays from multiple directions transmitted through the subject
X-ray detecting means for detecting
At the desired position and at the slice plane with the specified width.
In generating projection data at various angles,
The projection angle of the projection data to be generated on the slice plane
Means for recognizing, and the recognized projection angle
Means for specifying a plurality of detection element rows included in a predetermined width
And the projections obtained from the identified plurality of detection element arrays
Data bundling means for bundling data and this data bundling
Bundled at said various projection angles obtained by means
Image reconstruction method for reconstructing tomographic images based on projection data
And a step. Here, the predetermined
The detection element rows included in the width are two or three or more rows.
Above. The data bundling means calculates the data for each projection angle.
Between the data for each detector element row based on the
The inter-operation is performed. The weight is a linear contribution
You.

[0012]

[0013]

[0014]

Here, the above-described three-dimensional spiral will be described with reference to FIGS. That is, as shown in FIG. 1, when a plane P translates along a straight line Z that is not parallel to the plane P, and there is a point q that makes a circular motion on the plane P, the locus drawn by the point q in space is It is a three-dimensional spiral. In the case of the driving means of the present invention, the circular movement of the point q is realized by, for example, a rotating mechanism provided in the gantry of the X-ray CT apparatus, and the translation movement of the plane P is caused by the movement of the gantry itself or the movement of the subject. This is realized by at least one of the movement of the couch top serving as the holding means.

[0016]

In the CT apparatus according to the present invention, the X-ray detecting element
The X-ray detection means, which is a two-dimensional array of
Projection data transmitted through multiple slice planes of the body in a short time
Obtainable.

At this time, while the radiation source rotates around the subject, the subject is moved not by the distance of the thickness of the slice plane but by the distance of the thickness which is simultaneously scanned by the two-dimensional detector array. .

Here, the thickness simultaneously scanned by the two-dimensional detector array has the following concept. That is, at one moment, in a cone spanned by a two-dimensional detector array receiving radiation about a radiation source,
Contains part of the subject. This is the length of the portion inside the cone of the subject along the direction in which the subject moves.

From the group of radiation intensity data obtained in this manner, a tomographic image cannot be created by a conventional image reconstruction method. However, the image reconstruction means provided in the present invention can form a tomographic image from the group of intensity data by performing appropriate approximation. Accordingly, the amount of movement of the subject per one rotation of the radiation source can be increased, so that the time for collecting projection data in a desired range of the subject is greatly reduced.

When an X-ray tube is used as an element of a radiation source, conventionally, all X-rays other than those incident on a one-dimensional radiation detector element array have not been shielded and used.
In the present invention, since the X-rays are received by the two-dimensional radiation detector element array, the X-ray cone used is much larger, and the X-rays generated by the X-ray tube can be used effectively. Therefore, the amount of X-ray to be generated by the X-ray tube is greatly reduced, and the amount of heat generated in the X-ray tube is reduced accordingly. Therefore, the exposure can be continued for a longer time. Therefore, it is convenient when performing repeated shooting. Alternatively, an X-ray tube with a small heat capacity may be used.
Since the X-ray tube is small and light, the strength of the entire gantry may be small, and the whole apparatus is light.

[0021]

Embodiments will be described below with reference to the drawings.

FIG. 3 shows an embodiment in which the present invention is applied to a fourth generation X-ray CT apparatus. <Explanation of radiation detecting means> FIG.
As shown in the figure, a detector array in which X-ray detector elements (hereinafter, referred to as “detector elements”) for measuring the intensity of X-rays are two-dimensionally arranged has an axis h in cylindrical coordinates as shown in FIG.
And a part of a cylindrical surface at a fixed distance r _D from.

The position of the detector element is determined by the following equation. r = r _D , h = nΔh + h ₀ (n = 0, 1,..., N−1), φ = m
Δφ (m = 0, 1,..., M−1), where h ₀ is a predetermined constant and MΔφ is 2π.

That is, the detector elements are, for example, scintillator / photodiode detectors arranged on the N × M lattice points toward the inside of the cylinder. Thus, individual detector elements are labeled as (n, m).

Now, when all the detector elements in which the sign (n, m) n is a certain value n ₀ are collected, they form a ring with the h axis as the central axis. This will be referred to as a detector array n _0. That is, it can be considered that the detector array as the radiation detecting means is composed of N detector rows. <Description of radiation irradiation means>
An X-ray source as an irradiation means is an X-ray tube, a collimator,
It is composed of an aperture device and the like. An X-ray source is located inside the detector array and rotates about the h-axis. That is, the position S of the X-ray source is expressed as follows using the cylindrical coordinate system in FIG. 3 (the origin O of which represents the center point of the gantry). r = r _S (0 <r _S <r _D ) h = 0 θ = ωt where t is time and ω is angular velocity. <Description of Data Collection Means> The data collection means is capable of obtaining, for example, digital signals from all the elements of a detector array including detector elements such as the aforementioned scintillator / photodiode detector.

In collecting data, the intensity of the X-rays emitted from the X-ray source is measured by a group of detector elements on the opposite side of the central axis. That is, (ωt + π +
Outputs are obtained from all detector elements having a marker (n, m) for all integers j satisfying α) / Δφ ≧ j ≧ (ωt + π−α) / Δφ (see FIG. 4).

Here, m = j (mod N) 0 ≦ n ≦ N−1 FIG. 5 schematically shows a fourth-generation X-ray CT apparatus according to an embodiment of the present invention having the above-described units. It is a schematic perspective view. This apparatus has a detector array 1 in which four detector rows in which detector elements are arranged in an annular shape, for example, are arranged as shown in the figure. In order to irradiate the detector elements of the detector array 1 with X-rays, an X is provided inside the detector array 1 so as to be capable of circular movement.
A radiation source 2 is provided. The X-ray source 2 is supplied with a high voltage from a high voltage generator 6. A bed apparatus 4 equipped with a top plate 4A is provided as holding means for holding the subject 3 in an area sandwiched between the detector array 1 and the X-ray source 2. When the movement of the X-ray source 2 is viewed with respect to the subject 3, at least one of the X-ray source 3 and the top 4 </ b> A draws a locus along a three-dimensional spiral surrounding the subject 3 with respect to the subject 3. An X-ray source rotation controller 5 and a bed controller 7 are provided as driving means for driving one of them to move. The X-ray source rotation controller 5, the high voltage generator 6, and the bed controller 7 are totally controlled by the controller 8.

A DAS (D) for collecting, for each element, intensity data of radiation transmitted through the subject 3 detected by the detector array 1 during the above-described three-dimensional helical movement related to the scanning operation.
data Acquisition System) 9.

Further, a reconstructing apparatus 10 for calculating a tomographic image comprising a distribution of radiation absorption coefficients on a predetermined slice plane of the subject 3 from a group of radiation intensity data obtained by the DAS 9, and a display for displaying the tomographic image 11 is provided. <Description of Scan Operation> In the above-described apparatus, for example, the following scan operation is performed. In response to a command from the controller 8, the X-ray tube makes a rotary motion according to the following equation (see FIG. 6). θ = ωt In addition, the top plate 4A as the holding means also performs a translational motion according to the following equation. l = vtv is the speed of the translational movement.

When the rotation of the X-ray source 2 and the translation of the top 4A proceed simultaneously, the origin fixed to the subject 3 is set to o.
In the coordinate system described above, the position of the X-ray source 2 is described by a three-dimensional helical motion represented by the following equation. x = r _S sin ωty = −r _S cos ωt z = −vt Here, as shown in FIG. 7, considering a coordinate system (x, y, z) fixed to the subject 3, Can be regarded as nothing but the three-dimensional distribution μ (x, y, z) of the X-ray absorption coefficient.

The coordinate system (X, Y, Z) is a rectangular coordinate system, and each axis (X axis, Y axis, Z axis) is an x axis, a y axis,
It is parallel to the z-axis, and the origin O is completely coincident with the center of the gantry (O in FIGS. 2, 3 and 4). o ((x, y,
The origin of z) moves on the z-axis.

Here, the z-axis is placed so as to coincide with the Z-axis, and the position 1 of θ with respect to O is changed by the top plate 4A as the holding means (see FIG. 8). <Explanation of the subject and its holding means> As shown in FIG. 9, the subject 3 is supported by the top plate 4A and inserted into the gantry opening 1A. The top plate 4A can translate the subject 3 along the axis Z.
The axis Z passes through O and intersects the axis h at an angle of the tilt angle τ. The tilt angle τ may take any value as long as it does not hinder the subject 3 from being inserted into the hole of the detector array 1. However, below, for simplicity, τ = 0
Is described.

In such a configuration, the X-ray source 2 emits X-rays while continuously rotating. Further, the top plate 4A continuously translates along the Z axis at a constant speed.

Therefore, as shown in FIG.
Moves relatively from the position S ₀ to the position S ₁ in the first rotation, and continuously moves from the position S ₁ to the position S ₃ relatively in the second rotation. Thus, a spiral scan is performed on the subject 3.

FIG. 11 is a sectional view of the subject 3 and the detector array 1 when the table 4A is translated, and is a section taken along a plane including the h-axis and the position S of the X-ray source. FIG.
In this example , only the z-axis is shown because the h-axis, the Z-axis, and the z-axis all overlap each other. That is, in the following, for simplicity of description, of the X-rays emitted from the X-ray source S, only the beam that intersects the z-axis will be described. An X-ray beam from an X-ray source 2 penetrates a subject 3, and a detector row 1 in which transmitted X-rays are arranged along the Z-axis direction
(Detector rows k _{1 to} k _n ). Since the detector array 1 is a two-dimensional array of detector elements, the detector array in the beam spread direction of the X-ray beam is k
It is the column.

[0036] wherein each of the rear end B ₁ .about.B _n detector element k ₁ to k _n of the focus S and each of the X-ray source 2, the front end F ₁ ~
Triangle formed by a line connecting the F _n (e.g., triangle
SB _n F _n ), a portion included in the subject 3, for example, a hatched portion corresponding to the triangle SB _n F _n is defined as a slice plane SL.
_Let it be _n . Z _i is the rear end B of the slice plane SL _{i in} the i-th row
The front end F _i-1 of the slice plane SL _{i-1 in} the _{i- th} and B _i−1 columns intersects the Z-axis. The slice thickness △ Z of one slice plane is obtained by calculating the distance between a point Z _i and its adjacent point Z _i-1 ,
That is, ΔZ = | Z _i −Z _i−1 |, which is a constant value in the present embodiment.

Then, the top plate 4A is always set to v (2π / ω) =
In order to satisfy | Zn-Z ₀ | (= nΔZ), in other words, the lens is moved at a speed of v = (ω / 2πω) (Zn-Z ₀ ) and a spiral scan is performed. In the following description, the rotation angle per unit time is ω = 2π for easy understanding. In this case, just one rotation per unit time, the top plate 4A moves by 1 = v = nΔZ during this time.

FIG. 12 shows the points Z ₀ ,.
Z _n , that is, the point (front end point) at which the plane formed by the X-ray source S and the i-th detector row intersects the Z-axis moves along with the spiral scan on the coordinate z on the subject. FIG.

The vertical axis is obtained by measuring the rotation angle of the X-ray source so as to fall within the range of -π to π, and is referred to as an irradiation angle. That is, if the rotation angle is θ and the emission angle is β, β = (θ + π) mod 2π−π = (ωt + π) mod 2π−π where the operator xmod y represents the remainder of x by y.

The horizontal axis is a coordinate system (x,
y, z). Curve Z _i shown in the figure
(M, theta) In the first rotating X-ray source is the m, yet when in the position forming the exposure angle theta, on the z axis of the front end point Z _i corresponding to the detector row of the i-th column Position.

FIG. 13 shows four rows of X-ray detectors k _{1 to} k _4.
FIG. 6 is a diagram showing the relationship between the irradiation angle θ of the X-ray source 2 and the position on the Z axis when a spiral scan is performed using the scan. As shown in the figure, during the first rotation of the X-ray source 2, the top 4A
Move from ₀ (1, θ) to position Z ₃ (1, θ). Therefore, the position Z ₀ (1, θ) to the position Z ₃ (1,
θ) for the four slice planes corresponding to θ)
All projection data from π to + π are collected.

Similarly, during the second rotation of the X-ray source 2,
The top plate 4A is at a position Z ₀ (2, θ) to a position Z ₃ (2, θ).
, All projection data from the irradiation angles −π to + π are collected for the four slice planes corresponding to the positions Z ₀ (2, θ) to Z ₃ (2, θ) of the top 4A. Is done.

Therefore, four projection data are obtained by one rotation of the X-ray source 2, so that the time required for the spiral scan can be reduced.

The DAS 9 collects projection data P _i (m, φ, θ) on four slice planes every time the X-ray source 2 rotates. Here, φ is a line connecting the X-ray source 2 and the center O ₁ ,
It is an angle formed between the line connecting the arbitrary example X-ray detector k ₁ in the range of X-ray source 2 and the predetermined angle alpha. For example, in the case of the first rotation of the X-ray source 2, four projection data P
₁ (1, φ, θ), P ₂ (1, φ, θ), P ₃ (1,
φ, θ) and P ₄ (1, φ, θ) are collected by the DAS 9.

The reconstruction device 10 fetches projection data from the DAS 9 and reconstructs a desired slice image based on the projection data.

[0046] Here, in one slice to a thickness of up to a position _{z b} from the position _{z a,} position _(z a ₊ z b)
The reconstruction of one slice image at / 2 will be described.

[0047] FIG. 14 is a diagram for explaining one slice to a thickness of up to a position z _b from the position z _a. First, in order to synthesize projection data corresponding to a slice plane to be reconstructed, a slice position z _a shown in FIG.
Corresponding to a range of up to slice position z _b from the projection data, i.e. the first column the second rotation of the projection data P _{1 (2,}
φ, θ), the second-row second rotation data P ₂ (2, φ, θ),
Third row second rotation data P ₃ (2, φ, θ), fourth row second rotation data P ₄ (2, φ, θ), first row third rotation data P ₁ (3, φ, θ) Is used.

FIG. 15 shows projection data of slices (a + b + 2Δz) having a thickness of (a + b + 2Δz) centered on the position of z _c on the z-axis of the subject and having an exposure angle of θ ₁ , p (φ, θ).
₁ ) A diagram showing a method of specifying a set of data used for synthesizing (φ = mΔφ, m = 0,..., M−1) and calculating their contribution. When the contribution L from the position z _a to the position z _b corresponds to the line L corresponding to the position of each rotation angle θ of the X-ray source 2 and the position on the z-axis for each rotation of the X-ray source 2, A, ΔZ, ΔZ, b from the position z _a side, which contribute to data synthesis for obtaining new projection data. The contribution is determined for each rotation, each angle, and each column of the X-ray source 2.

Then, new projection data is synthesized using the contribution. In this case, the projection data P (φ, θ ₁ ) at the irradiation angle θ ₁ is P (φ, θ ₁ ) = ｛aP ₃ (1, φ, θ ₁ ) + ΔZP ₄ (1, φ, θ _{1) ) + ΔZP 1 (2, φ} , θ 1) + bP 2 (2, φ, θ 1) × 1} becomes / (z _b -z _a). Similarly, for other angles θ, θ = −π to
Construct projection data P (φ, θ) over + π.

The synthesized projection data P (φ, θ) closely approximates projection data obtained by scanning one slice having a thickness from position z _a to position z _b without moving the tabletop. Synthetic projection data P (φ, θ) predetermined positions by reconstructing an _{(z a + z b) /} 2 slice image is obtained.

The slice thickness is determined by a normal image reconstruction method (for example, convolution-back projection method) as if it were projection data of a single slice taken without a spiral scan by a conventional CT apparatus. A tomographic image of T can be obtained.

Also, once such data is collected, synthesized projection data corresponding to a slice of an arbitrary position and an arbitrary thickness can be created.

Next, referring to FIG. 16 and FIG.
A method of reconstructing a tomographic image having a smaller slice thickness than the tomographic image shown in FIG. 15 will be described. FIG.
FIG. 17 corresponds to FIG. The data to be used are the fourth column in the first rotation, the first column in the second rotation, the second column in the second rotation, and the third column in the second rotation.

[0054] Then, the weight is subjected to data of each column, a second column of the second rotation In the angle theta _1, the weight because 2 3 column of revolution at all no contribution is zero. The fourth row in the first rotation is a / (a + b), and the first row in the second rotation is b / (a + b)
Gives the weight.

As described in detail above, the four X-ray detectors k
The ₁ to k ₄ provided, because the helical scan can 3-dimensionally scanned along the object 3 in the z-axis direction in a short time. The scan time of the present embodiment can be reduced to about 1/4 times as compared with the scan time in the case of using a single-row X-ray detector as in the related art. Further, since the X-rays emitted from the X-ray source 2 are detected by the X-ray detectors in a plurality of rows, the X-rays can be used effectively.

In the above-described embodiment, the fourth-generation X-ray CT apparatus has been described. However, as shown in FIGS. 18 and 19, for example, an arc-shaped detector array 1b is not shown for the X-ray source 2b. Confront each other across the subject, and X
The present invention is also applicable to a third-generation X-ray CT apparatus which continuously rotates and scans the source 2b and the detector array 1b as a center of rotation defined in the spread area of the X-ray beam while maintaining the mutual positional relationship. Is applicable. The third generation X
The X-ray CT apparatus includes, instead of the X-ray source rotation controller 5 in FIG. 5, a rotating body rotation controller 12 to which, for example, a rotating body is attached, to which an X-ray source 2b and a detector array 1b are attached.

Further, according to the present embodiment, since four X-ray detectors corresponding to four slice thicknesses are used, X-rays corresponding to one thick slice thickness corresponding to four slice thicknesses are used.
More partial volume effect than using a line detector
Can be improved . Hereinafter, the reason why the partial volume effect is improved will be described. When an X-ray detector corresponding to one thick slice thickness corresponding to the four slice thicknesses is used, as shown in FIG.
Transmitted through, the X-ray intensity I ₁ ~I ₄ in the X-ray detector 1 _a is incident, detected data is _{I 1 + I 2 + I 3} + I 4, the projection data P ₁ is P ₁ = - ( ln (I ₁ + I ₂ + I ₃ + I ₄ )) / 4I.

The obtained projection data P ₁ is expressed as P ₁ ≦ − (l
nI ₁ / 4I + lnI ₂ / 4I + lnI ₃ / 4I + ln
I ₄ / 4I). That projection data P ₁ obtained by one thick slice X-ray detector corresponding to the thickness 1 _a is projected when detecting X-rays by four X-ray detectors corresponding example four slice thickness It is the same as or smaller than the projection data. Due to such an error, although only the objects Q ₁ and Q ₂ exist originally in the slice plane as shown in FIG. 22, the image obtained by the image reconstruction is shown in FIG. image _S 1 of the object _Q 1, _{Q 2} as shown, _{S 2}
However, according to this embodiment, the projection data of four slice thicknesses is measured for each of the four slice planes.
₂ is P ₂ = − (lnI ₁ / 4I + lnI ₂ / 4I + 1
_{nI 3 / 4I + lnI 4 /} 4I) to become.

Therefore, in the present embodiment, the projection data is measured close to the true value, thereby reducing the artifacts due to the partial volume effect, so that the image quality can be improved.

Next, FIG. 13 to FIG. 15 or FIG. 16 and FIG.
FIG. 23 shows a method of reconstructing a tomographic image from a group of data obtained by a spiral scan, which is different from the method described in FIG.
This will be described with reference to FIGS. For example, a configuration shown in FIG. 18 will be described. That is, the X-ray source 2b and the detector array 1b rotate around the axis Z integrally without changing their relative positional relationship. Further, a translational motion is performed in parallel with the Z axis with this rotation. Thus, the X-ray source 2b draws a spiral (see FIG. 23).

Consider a case where the X-ray source 2b and the detector array 1b move L at a constant speed in parallel with the Z-axis while the X-ray source 2b makes one rotation. In particular, it is assumed that Z = Lβ / (2π) holds.

Here, Z is the Z-axis component of the position of the X-ray source 2b, and β is the rotation angle of the X-ray source 2b.

Referring to FIG. 24, F (t) is the position of X-ray source 2b at time t, x is a point on plane P, and F 'is F
The perpendicular leg lowered from (t) to the plane P, E is F (t) x
, A detector element on the extension of E, a perpendicular foot from E to the plane P, and O an intersection of the plane P and the Z axis.

Next, back projection is performed.

That is, as shown in FIG. 25, P (α, Z) used to reconstruct the plane P is expressed by the planes P and X
The point of intersection with the source 2b is C, the line segment CO is extended, and a point C 'that intersects with the cylindrical surface S on the opposite side to C is determined. when the point of first intersection and the trajectory of the X-ray source 2b in the vertical and _Q 1, _{Q 2,} X-ray source 2b
Are data in the range of Q _{1 to} C to Q ₂ k. That is,
When the Z coordinate of the plane P is Z, and β ₀ = 2πZ / L,
Data corresponding to β in the range β ₀ −π ≦ β ≦ β ₀ + π is used.

The operation of the back projection is represented as follows.

That is, the distribution f of the CT values on the plane P
(X, y, z) is obtained by converting (x, y) to (rsin φ, rco
s φ) is as follows.

[0068]

(Equation 1) The projection data of the beam F (t) E is represented by P _β (α,
Write Z). Here, α and β are illustrated. Z is F
(T) F '+ E'E, which indicates how far E is in the detector row relative to the X-ray source 2'.

The image reconstruction is performed in the following procedure.

First, the fact that the length of the beam passing through the subject due to the beam being oblique to the plane P is corrected.

[0071]

(Equation 2) Then do the convolution.

[0072]

(Equation 3) Where h (α) is a convolution function,

[0073]

(Equation 4) Alternatively, this is smoothed.

Both the above method and this method give a completely correct reconstructed image if the subject is a kind of candy which does not change along the z-axis. If the subject C
If the distribution of the T value changes along the z-axis, either method gives a slight error to the extent that there is no practical problem. However, although this method has a disadvantage that the calculation time is longer than that of the above-described method, it has an advantage that even when the width of the detector array in the Z direction is considerably large, an error generated is small.

The present invention is not limited to the embodiment described above. In the above embodiment, the top plate 4A is moved in the z-axis direction, but the top plate 4A may be moved along a direction making a predetermined angle to the region. In addition, it goes without saying that various modifications can be made without departing from the spirit of the present invention.

As described above, according to the present invention, a plurality of projection data corresponding to a plurality of slice planes can be obtained by one rotation movement, and the time for spiral scanning can be reduced. Further, since radiation is detected by a plurality of detector rows, which are a two-dimensional array, the radiation can be used effectively. Therefore, the radiation dose to be generated by the radiation irradiating means can be reduced, and the heat generated by the radiation irradiating means can be suppressed. And so on .

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a three-dimensional spiral defined in the present invention.

FIG. 2 is a diagram illustrating a three-dimensional spiral defined in the present invention.

FIG. 3 is a schematic diagram showing the application of the present invention to a fourth-generation X-ray CT apparatus.

FIG. 4 is a diagram showing the relationship between the movement of an X-ray source and a group of elements that perform a detection operation during the movement.

FIG. 5 is a schematic perspective view of a fourth generation X-ray CT apparatus to which the present invention is applied.

FIG. 6 is a diagram showing a trajectory of an X-ray source.

FIG. 7 is a diagram showing a relationship between a subject and a cylindrical coordinate system.

FIG. 8 is a view showing a translational movement of a subject holding means.

FIG. 9 is a diagram illustrating a tilt angle.

FIG. 10 is a diagram showing a positional relationship of a motion of an X-ray source on a subject.

FIG. 11 is a diagram showing a relationship between an X-ray source, a subject, and a detector array in X-ray irradiation.

FIG. 12 is a diagram showing the relationship between the rotational movement of the X-ray source and the amount of top movement.

FIG. 13 is a diagram showing a relationship between projection data of each row of a detector array corresponding to the rotational movement of the X-ray source and the amount of top movement.

FIG. 14 is a view for explaining a method for reconstructing a tomographic image of a certain slice plane from the projection data shown in FIG. 13;

FIG. 15 is a view for explaining a method for reconstructing a tomographic image of a certain slice plane from the projection data shown in FIG. 13;

FIG. 16 is a diagram corresponding to FIG. 14 and illustrating a method of reconstructing a tomographic image having a smaller slice thickness than the tomographic image shown in FIG. 14;

FIG. 17 is a diagram corresponding to FIG. 15 and illustrating a method of reconstructing a tomographic image having a smaller slice thickness than the tomographic image shown in FIG. 15;

FIG. 18 is a view for explaining a case where the present invention is applied to a third-generation X-ray CT apparatus, in which rows of detector elements in a third-generation X-ray CT apparatus to which the present invention is applied are arranged in an arc shape. FIG. 4 is a diagram showing a relationship between radiation detecting means and an X-ray source in which, for example, four are arranged in parallel.

FIG. 19 is a perspective view illustrating a case where the present invention is applied to a third generation X-ray CT apparatus, and is a schematic perspective view of the third generation X-ray CT apparatus to which the present invention is applied.

FIG. 20 is a view for explaining occurrence of an artifact due to a partial volume effect.

FIG. 21 is a view for explaining occurrence of an artifact due to a partial volume effect.

FIG. 22 is a view for explaining occurrence of an artifact due to a partial volume effect.

FIG. 23 shows a method of reconstructing a tomographic image from a group of data by helical scanning, which is different from the method described with reference to FIGS. 13 to 15 or FIGS. 16 and 15, and is shown in FIG. The figure which looked at the X-ray source in the 3rd generation X-ray CT apparatus, and the arc-shaped detector array from the Z-axis direction.

FIG. 24 is a bird's-eye view showing a method of reconstructing a tomographic image from a group of data obtained by spiral scanning, which is different from the method described with reference to FIGS. 13 to 15 or FIGS.

FIG. 25 shows a method of reconstructing a tomographic image from a group of data by helical scanning, which is different from the method described with reference to FIG. 13 to FIG. 15 or FIG. 16 and FIG. The figure which shows the calculation method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Detector arrangement, 2 ... X-ray source, 3 ... Subject, 4 ... Bed apparatus, 4A ... Top plate, 5 ... X-ray source rotation control apparatus, 6 ... High voltage generator, 7 ... Bed controller, 8 ... Controller, 9 ... DA
S, 10: Reconstruction device, 11: Display, 12: Rotating body rotation controller.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References IEICE Transactions D-II, 72 [6] (1989) “Reconstruction of 3D CT Images from Conical Beam Projection by Helical Scan” p. 954-962 Image Engineering Conference, 21 (1990) “3D Helical Scan CT Using Conical Beam Projection”, p. 165-168 (58) Field surveyed (Int. Cl. ⁶ , DB name) A61B 6/03 JICST file (JOIS)

Claims (5)

(57) [Claims] (1)X-ray source that emits X-rays to the subject
When, Before the X-ray source draws a spiral trajectory around the subject
Driving means for driving the X-ray source or the subject; X-ray detection element arrays are arranged in a plurality of rows in the body axis direction of the subject.
Detects X-rays from multiple directions transmitted through the subject
X-ray detecting means for detecting In the body axis direction
At the desired position and at the slice plane with the specified width.
In generating projection data at various angles,
The projection angle of the projection data to be generated on the slice plane
Means to recognize, The recognized projection angle, which is included in the predetermined width.
Means for identifying a plurality of detector element rows, Projection data obtained from the specified detection element rows
Data bundling means for bundling data At the various projection angles obtained by the data bundling means,
Reconstruction of tomographic image based on bundled projection data
Image reconstruction means, A CT apparatus comprising: 2. A plurality of detecting elements included in the predetermined width.
2. The method according to claim 1, wherein the number of rows is two.
T device. 3. A plurality of detecting elements included in the predetermined width.
2. The method according to claim 1, wherein the number of rows is three or more.
CT device. 4. The data bundling means is provided for each projection angle.
Between the data for each detector element row based on the obtained weight
Wherein an interpolation operation is performed.
The CT apparatus according to any one of 1, 2, and 3. 5. The method according to claim 1, wherein the weight is a linear contribution.
5. The CT apparatus according to claim 4, wherein