JP2009095572A - Radiation imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radiation imaging apparatus capable of acquiring a clear tomographic image having a long visual field without generating a wave-shaped artifact. <P>SOLUTION: Whenever an X-ray tube 2 and a FPD 3 are relatively moved to a subject M at prescribed pitch, radiation is applied from the X-ray tube and projection images are acquired by the FPD. A projection image analysis part 9b decomposes the respective projection images every projection angle of the radiation corresponding to the prescribed pitch and generates partial projection images. An image composing part 9c connects the partial projection images of the same projection angle in the order of acquisition of the projection images one after another to form a composite projection image. An image pretreatment part 9e detects areas where the subject is not present in the composite projection image formed by the image composing part, and pretreats the composite projection value by replacing the pixel value of the region of the image with a predetermined value. A reconstruction processing part 9d executes the reconstruction processing based on the pretreated composite projection image and generates the tomographic image of the subject. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  In the present invention, the radiation irradiating means is disposed on one side of the subject, the radiation detecting means is disposed on the opposite side, and the radiation irradiating means and the radiation detecting means are disposed in the same direction along the longitudinal direction of the subject. Each time the radiation irradiating means and the radiation detecting means move relative to the subject by a predetermined pitch, radiation is emitted from the radiation irradiating means, and a projection image is acquired by the radiation detecting means. The present invention relates to a radiation imaging apparatus that performs reconstruction processing on the basis of it and generates a tomographic image of a subject.

  An example of the radiation imaging apparatus is an X-ray CT (Computed Tomography) apparatus. In the X-ray CT apparatus, an X-ray tube is disposed on one side of the subject, and an X-ray detector is disposed on the opposite side of the X-ray tube with respect to the subject. The detector is integrally rotated around the subject. During this rotational movement, the subject is irradiated with radiation from the X-ray tube, and transmitted X-rays are detected by the X-ray detector. A projection image of the subject is acquired at every fixed rotation angle. Then, a tomographic image of the subject is obtained from the projection image by reconstruction processing.

  In the X-ray CT apparatus, a rotation drive mechanism for rotating the X-ray tube and the X-ray detector is required. However, such a rotation drive mechanism is generally expensive, and particularly for obtaining an accurate tomographic image. Requires a more expensive rotary drive mechanism that operates with high accuracy.

  In contrast, the X-ray tube and the X-ray detector are translated relative to each other along the body axis direction of the subject, and a tomographic image at an arbitrary height position is reconstructed from a plurality of acquired projection images. An X-ray tomography apparatus that generates the image is known (for example, see Patent Document 1). Although this X-ray tomography apparatus is inferior in resolution of the generated tomographic image as compared with the X-ray CT apparatus, it is only necessary to translate the X-ray tube and the X-ray detector. It has the advantage that a rotational drive mechanism for the detector is unnecessary, and is effectively used for diagnosis of many human body parts such as the chest, joints, digestive organs and the like.

  However, in this X-ray tomography apparatus, a platform type X-ray detector (FPD) has recently been used as an X-ray detector, and the field of view is larger than that of the previous image intensifier (II). However, the field of view is limited because the resolution decreases with increasing distance from the region of interest, and it has not been possible to cope with long-field tomography along the body axis direction of the human body.

  In addition, this X-ray tomography apparatus has a problem in that when a tomographic image having an arbitrary height is formed from a plurality of projection images, a seam is visible at a connected portion of the images. In particular, this becomes prominent in a portion of a projected image by X-rays that has passed through a region where no subject exists, and when a tomographic image is reconstructed, a wavy artifact appears in a region where the subject does not exist.

JP 2004-236929 A

  Therefore, an object of the present invention is to provide a radiation imaging apparatus capable of obtaining a clear tomographic image having a long field of view without generating wavy artifacts.

  In order to solve the above problems, the present invention provides a radiation irradiating means on one side of a subject, and detects radiation transmitted through the subject on the opposite side of the radiation irradiating means with respect to the subject. A radiation detecting means for acquiring a projection image, and relatively moving the radiation irradiating means and the radiation detecting means in the same direction along the longitudinal direction of the subject. Each time the radiation detection means moves relative to the subject by a predetermined pitch, radiation is emitted from the radiation irradiation means, a projection image is acquired by the radiation detection means, and reconstruction processing is performed based on the projection image. In the radiation imaging apparatus that performs the tomographic image of the subject, each of the projection images is decomposed at the predetermined pitch along the longitudinal direction of the subject. Thus, the projection image decomposing means for generating the partial projection images decomposed for each radiation projection angle corresponding to the predetermined pitch, and the partial projection images of the projection image for each same projection angle An image composition unit that sequentially combines in accordance with the order in which the projection images are acquired to form a composite projection image, and a region where the subject does not exist is detected in the composite projection image formed by the image composition unit, and the image region Image pre-processing means for pre-processing the composite projection image by replacing the pixel value of a predetermined value, and a reconstruction process based on the composite projection image pre-processed by the image pre-processing means, And a reconstruction processing unit configured to generate a tomographic image of the subject.

In the above configuration, preferably, the image preprocessing unit sequentially detects a position at which a difference in pixel values of adjacent pixels is equal to or greater than a predetermined value in the composite projection image, and the subject and the subject. The region where the subject does not exist is detected by obtaining a boundary line with the region where the subject does not exist, or the image preprocessing means detects the pixel value of the region where the subject exists in the composite projection image. A region having a pixel value larger than a reference value determined in advance as a maximum value is detected as a region where the subject does not exist.
Further, it is preferable that the radiation emitting means and the radiation detecting means are relatively translated with respect to the subject at the same speed.

  According to the present invention, the radiation irradiating means and the radiation detecting means are relatively translated in the same direction along the longitudinal direction of the subject, and the radiation irradiating means and the radiation detecting means have a predetermined pitch with respect to the subject. Each time relative movement is performed, radiation is emitted from the radiation irradiating means, a projection image is acquired by the radiation detecting means, and each projection image is decomposed at each radiation projection angle corresponding to a predetermined pitch to generate a partial projection image. The partial projection images are sequentially combined at the same projection angle according to the order in which the projection images are acquired to form a composite projection image, and an area where no subject exists in the composite projection image is detected. Preprocessing the composite projection image by replacing the pixel value with a predetermined value, and performing a reconstruction process based on the preprocessed composite projection image to generate a tomographic image of the subject Because, it can very clearly not appear wavy artifacts, it obtained a long field of view of the tomographic image, to achieve a diagnosis by faster and more accurate tomographic image.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a block diagram showing a configuration of a radiation imaging apparatus according to one embodiment of the present invention. In this embodiment, the radiation imaging apparatus is configured as an X-ray tomography apparatus. However, in addition to the X-ray tomography apparatus, the present invention is a PET (Positron Emission Tomography) apparatus or a SPECT (Single Photon Emission CT). The present invention can also be applied to a radiation imaging apparatus that generates a tomographic image using radiation other than X-rays in an ECT (Emission Computed Tomography) apparatus typified by an apparatus.
With reference to FIG. 1, the X-ray tomography apparatus of the present invention has a support 1 on which a subject M is placed, and an X placed on the support 1 to irradiate the subject M with X-rays. An X-ray detection unit 3 that detects X-rays that are disposed below the support table 1 and that passes through the subject M is provided.

  The support table 1 can be moved up and down, rotated and horizontally by a support table moving mechanism 14. The operation of the support table moving mechanism 14 is controlled by the support table control unit 4.

  The X-ray irradiation means 2 is composed of an X-ray tube, and can be moved in both directions along the longitudinal direction (arrow z) of the subject by an X-ray tube moving mechanism 15. As shown in FIG. 2, the X-ray tube moving mechanism 15 is a horizontal screw that is arranged beside the support base 1 and extends in the longitudinal direction (arrow z) of the subject, and is driven to rotate around the axis by the motor 20. The rod 21 includes a column 22 having a lower end engaged with the screw rod, and the X-ray tube 2 is supported and fixed to the upper end of the column 22. Then, when the screw rod 21 is rotationally driven in the forward or reverse direction, the X-ray tube 2 moves along the longitudinal direction of the subject as indicated by the one-dot chain line in the drawing. In addition, an encoder 23 for detecting the rotation direction and the rotation amount of the motor 20 is provided. The X-ray tube control unit 7 obtains the current position of the X-ray tube 2 based on the detected output value. The X-ray tube controller 7 has a high voltage generator 6 that applies a tube voltage and a tube current to the X-ray tube 2.

The X-ray detector 3 is composed of an FPD (flat panel X-ray detector). As the X-ray detector 3, any known X-ray detection means such as an image intensifier (II) can be used instead of the FPD.
The FPD 3 can be moved along the longitudinal direction (arrow z) of the subject by the FPD moving mechanism 16. As shown in FIG. 3, the FPD moving mechanism 16 includes a rack and pinion mechanism, the rack 30 is attached to the support base 1 and extends in the longitudinal direction (arrow z) of the subject, and the pinion 31 is attached to the FPD 3. It is attached. Then, when the pinion 31 is rotationally driven in the forward or reverse direction by the motor 32, the FPD 3 moves along the longitudinal direction (arrow z) of the subject, as shown by the one-dot chain line in the figure. In addition, an encoder 33 that detects the rotation direction and amount of rotation of the motor 32 is provided, and an output detection value of the encoder 33 is sent to the FPD control unit 5. The FPD controller 5 obtains the current position of the FPD 3 based on this output detection value.

  The X-ray tomography apparatus further digitizes an X-ray detection signal, which is a charge signal from the FPD 3, and extracts it based on the X-ray detection signal output from the A / D converter 8. A tomographic image generation unit 9 that generates a tomographic image, a controller 10 that performs overall control of the above-described components, a memory unit 11 that stores processed image data and the like, and an input unit 12 for an operator to make various inputs. And a monitor 13.

  The tomographic image generation unit 9 performs lag correction, gain correction, and the like on the X-ray detection signal, and outputs a projection image projected on the detection surface of the FPD 3, and the corrected projection image for each pitch. A pre-process for each of the combined projection images An image preprocessing unit 9e and a reconstruction processing unit 9d that performs a reconstruction process based on the preprocessed composite projection image and generates a tomographic image of the subject are included.

  Next, operations of the projection image decomposition unit 9b, the image composition unit 9c, and the reconstruction processing unit 9d will be described. FIG. 4 is a diagram for explaining the principle of imaging by an X-ray tube and a flat panel X-ray detector (FPD), and FIGS. FIG. Note that the projection image obtained by FPD will be described on the assumption that processing such as lag correction and gain correction has already been performed by the correction unit.

Each time the X-ray tube 2 and the FPD 3 move by a predetermined pitch (see FIGS. 4A to 4D), X-rays are irradiated from the X-ray tube 2 to the subject and pass through the subject. X-rays are detected by the FPD 3 to obtain projection images O ₁ , O ₂ ,..., O _I ,..., O _M (1 ≦ I ≦ M) (see FIGS. 4E to 4H). .
Specifically, when the X-ray tube 2 and the FPD 3 are at the position shown in FIG. 4A, X-rays are emitted from the X-ray tube 2, and the X-rays are detected by the FPD 3 and projected image O ₁ (FIG. 4). (E)) is acquired, and when the X-ray tube 2 and the FPD 3 move by the pitch d from the position of FIG. 4A to reach the position of FIG. Then, the X-ray is detected by the FPD 3 to obtain a projection image O ₂ (FIG. 4F). Similarly, when the X-ray tube 2 and the FPD 3 move by the pitch d and reach the I-th position (see FIG. 4C), the projection image O _I (FIG. 4G) is obtained by the FPD 3. And the projection image O _M (FIG. 4H) is acquired by the FPD 3 when the Mth position is reached (see FIG. 4D). In this embodiment, the position of FIG. 4A is on the head of the subject M, the position of FIG. 4D is on the foot of the subject M, and the X-ray tube 2 and the FPD 3 are sequentially shown in FIG. As it moves to each position of (A) to (D), it moves from the head of the subject M to the foot.

The image decomposition unit 9b decomposes the acquired projection images O ₁ , O ₂ ,..., O _I ,..., O _M for each length corresponding to one pitch d, thereby projecting X-rays. A partial projection image decomposed for each angle is generated.
This will be described with reference to FIG. 4I, which is a partially enlarged view of FIG. Referring to FIG. 4I, the detection surface (corresponding to the projection image) of FPD 3 is decomposed every pitch d along the longitudinal (body axis) direction of the subject. At this time, the X-ray projection angle, which is the angle formed between the irradiation axis from the X-ray tube 2 to the FPD 3 and the body axis z of the subject, corresponds to θ ₁ , θ ₂ ,. _J, ··, the θ _N. That is, decomposing the projection image at every pitch d along the longitudinal direction of the subject corresponds to decomposing the projection image at every X-ray projection angle corresponding to the pitch d.

For example, as shown in FIG. 4 (E), the projection image _{O 1,} for each pitch d, the partial projection image _{O 11, O 12, ··,} O 1J, is decomposed _..., the _{O 1N,} partial projection image O ₁₁ corresponds to an image obtained by X-rays irradiated at a projection angle θ ₁ , partial projection image O ₁₂ corresponds to an image obtained by X-rays irradiated at a projection angle θ ₂ , and so on. The partial projection image O _1J corresponds to an image obtained by X-rays irradiated at the projection angle θ _J , and the partial projection image O _1N corresponds to an image obtained by X-rays irradiated at the projection angle θ _N. .

Further, for example, as shown in FIG. 4 (H), the projected image _{O M,} for each pitch d, the partial projection image _{O M1,} O _M2, is decomposed _{· ·,} O MJ, · _·, to the _{O MN,} part The projection image O _M1 corresponds to an image obtained by X-rays irradiated at the projection angle θ ₁ , the partial projection image O _M2 corresponds to an image obtained by X-rays irradiated at the projection angle θ ₂ , and Similarly, the partial projection image O _MJ corresponds to an image obtained by X-rays irradiated at the projection angle θ _J , and the partial projection image O _MN corresponds to an image obtained by X-rays irradiated at the projection angle θ _N. Correspond.

Then, the image composition unit sequentially combines the partial projection images at the same projection angle according to the order in which the projection images are acquired to form a composite projection image.
For example, with reference to FIG. 5, the projected image _{O 1, O 2, ··,} O, ··, among the respective decomposition projected image _{O M,} the partial projection image _{O 11} corresponding to the projection angle theta _1, O ₂₁ ,..., O _I1 ,..., O _M1 are sequentially combined in the order in which the projection images are acquired to form _one composite projection image P ₁ (FIGS. 5A to 5E). reference). Further, the partial projection images O ₁₂ , O ₂₂ ,..., O _I2 ,..., O _M2 corresponding to the projection angle θ ₂ are sequentially combined according to the order in which the projection images are acquired, and one combined projection image P ₂ is formed (see FIGS. 5F to 5J).

Further, referring to FIG. 6, the projection image _{O 1, O 2, ··,} O I, ··, among the respective decomposition projected image _{O M,} the partial projection image _{O 1 J} corresponding to the projection angle theta _{J, O 2J, ··, O IJ,} ··, O MJ is sequentially coupled in the order in which the projected image is acquired, one of the synthetic projection images _{P J} is formed (FIG. 6 (a) ~ (E) reference). Further, the partial projection images O _1N , O _2N ,..., O _IN ,..., O _MN corresponding to the projection angle θ _N are sequentially combined according to the order in which the projection images are acquired, and one combined projection image P _N is formed (see FIGS. 6F to 6J).

Thus, the image combining unit 9c is a respective partial projection image, the same projection angles _{θ 1, θ 2, ··,} θ J, ··, each theta _N, sequentially combined in the order in which the projected image is acquired synthetic projection images _{P 1,} P 2 _Te, ··, P J, ··, to form a _{P N.}

Next, the image preprocessing section 9e is synthesized projected image _{P 1, P 2, ··,} P J, ··, for each _{P N,} and the difference between the pixel values of adjacent pixels predetermined value or more The region where the subject M does not exist is detected by sequentially detecting the position to obtain the boundary line between the subject M and the region where the subject M does not exist. Alternatively, the image preprocessing unit 9e detects, in each composite projection image, a region having a pixel value larger than a reference value determined in advance as the maximum pixel value of the region where the subject exists as a region where the subject does not exist. .
In this case, the pixel value difference value and the reference value of adjacent pixels, each pixel value of the composite projection image varies considerably depending on the intensity of X-rays irradiated from the X-ray tube 2 and the thickness of the subject. Must be determined in advance.

  The image preprocessing unit 9e further replaces the pixel value of the area where the subject M does not exist with the same predetermined value. The predetermined identical value is an appropriate value at a level at which no wavy artifact appears in the tomographic image obtained by the reconstruction process.

  Then, the reconstruction processing unit 9d performs a known image reconstruction process based on the composite projection image preprocessed by the image preprocessing unit 9e, and generates a tomographic image of the subject M. The reconstruction process will be described later.

According to the X-ray tomography apparatus of the present invention, the X-ray tube 2 and the FPD 3 are translated in the same direction along the longitudinal direction (body axis z direction) of the subject M, thereby along the body axis z. Projection image data with a long field of view can be acquired from the FPD 3.
This will be described numerically. The number of pre-processed composite projection images P ₁ , P ₂ ,..., P _J ,..., P _N is N [Frame], and the X-ray tube 2 and FPD 3 If the moving speed of the imaging system is v [mm / sec], the visual field size of the FPD 3 is V [mm], and the imaging cycle (also called “pulse time width”) is T [sec / Frame], the moving speed v [mm / sec]
v [mm / sec] = V [mm] / N [Frame] × (1 / T [sec / Frame])
It is expressed. The reciprocal of the imaging cycle is the imaging speed. If the imaging speed is F [Frame / sec], the moving speed v [mm / sec] is
v [mm / sec] = V [mm] / N [Frame] × F [Frame / sec]
And the pitch d [mm / Frame] is
d [mm / Frame] = V [mm] / N [Frame]
It is expressed.

For example, if the field size V is 17 inches (= 430 [mm]), the number N of composite projected images is 50 [Frame], and the imaging speed F is 15 [Frame / sec], the moving speed v is v [mm]. / sec] = 430 [mm] / 50 [Frame] × 15 [Frame / sec] = 129 [mm / sec], and the pitch d is d [mm] = 430 [mm] / 50 [Frame] = 8.6. [mm / Frame].
Accordingly, the X-ray tube 2 and the FPD 3 are translated at the same speed of 129 [mm / sec], and every time the X-ray tube 2 and the FPD 3 move at a pitch of 8.6 [mm / Frame], 15 [Frame / 50 composite projection images are obtained by intermittently irradiating X-rays at the timing of [sec].

Next, the operation of the reconstruction processing by the reconstruction processing unit 9d will be described. In this embodiment, the reconstruction processing unit 9d performs reconstruction processing by the Feldkamp method.
The Feldkamp method is generally applied when tomography is performed by moving the X-ray tube 2 and the FPD 3 along an arc orbit as shown in FIG. As shown in FIG. 7B, the present invention is applied when tomography is performed by moving the X-ray tube 2 and the FPD 3 in parallel along a linear trajectory.

  First, when Feldkamp (Feldkamp method) is applied to tomography along an arc orbit, the reconstruction algorithm is expressed by the following equations (1) to (3).

  Here, the xyz coordinate system is set fixed in space as shown in FIG. Then, the imaging system including the X-ray tube 2 and the FPD 3 rotates around the center of the z-axis, and the X-ray tube 2 performs a circular motion with a radius D around the origin in the xy plane. In the following description of the reconstruction process, the projection angle is defined as an angle formed by the central beam of the X-rays incident on the detection surface of the FPD 3 with respect to the normal line of the detection surface, and is represented by φ. The body axis is taken in the y-axis direction.

In the expression (1), f (r ↑) (when the symbol “↑” is added after the character, it represents a vector display. The same applies hereinafter) indicates the position r of the tomographic image to be reconstructed. Represents the pixel value at ↑. Y (r ↑) and Z (r ↑) are the coordinates of the point at which the pixel at the position r ↑ is projected on the detection surface of the FPD 3. P _φ is a projection image on the detection surface of the FPD 3 at the projection angle φ. g _y is a filter function of the FBP (Filtered Back Projection). FIG. 9 shows an example of this filter function. In FIG. 9, φ = β. W ₁ and W ₂ are coefficients for correcting the influence of the spread of the X-ray beam.

  When the Feldkamp method is applied to tomography along a linear trajectory, the tomography along the arc trajectory is lowered from the X-ray tube 2 to the center of rotation as shown in FIG. Whereas the perpendicular is perpendicular to the detection surface of the FPD 3, in the tomography along the straight trajectory, as shown in FIGS. 7 (B) and 11 (B), the X-rays irradiated from the X-ray tube 2 are irradiated. The center beam does not always pass through the center point C of the specific tomographic plane of the subject M and always enter the center point of the detection surface of the FPD 3 perpendicularly. That is, the incident angle with respect to the center point of the detection surface of the FPD 3 is different for each photographing.

Therefore, as shown in FIG. 11 (B), an axis orthogonal to the X-ray beam perpendicularly incident on the center point of the detection surface of the FPD 3 and orthogonal to the moving direction of the X-ray tube 2 and the FPD 3 every imaging. The axis passing through the tomographic plane at the center of the region of interest of the subject M is set as the virtual rotation center axis VC. In addition, the distance from the X-ray tube 2 to the center C of the specific tomographic plane along the length of the first perpendicular drawn from the X-ray tube 2 to the center point of the detection surface of the FPD 3 for each radiograph is taken as SID. Is the SOD (= D), and the length of the second perpendicular drawn from the center point of the FPD 3 for each photographing to the first perpendicular is YA. At this time, the length YA of the second perpendicular is
YA = SID × tanφ (4)
It becomes.

  Therefore, when performing the back projection process for each photographing, the Y (r ↑) is corrected by using YA converted by the equation (4) on the basis of the virtual rotation center axis VC, and the Feldkamp (Feldkamp) ) Law can be applied.

Next, correction of Y (r ↑) will be described. With reference to the virtual rotation center axis VC, in FIG. 11B, the point represented by r ↑ in B ′ and the point represented by rc ↑ in C ′ are regarded as the same point. At this time, the following relational expression is established for the y coordinates of these two points.
rc _y = r _y + SOD × tan φ (5)
However, rc _y is the distance of the y-axis direction from the VC to rc ↑, _{r y} is the distance of the y-axis direction from the VC to r ↑. The center C of the specific tomographic plane of the subject M is on the VC.

The y coordinate of the point at which X-rays passing through rc ↑ from the X-ray tube 2 are projected on the detection surface of the FDP 3 is Yc (r ↑). At this time, since the actual detection surface of the FPD 3 is shifted by YA = SID × tanφ, the coordinates on the detection surface of the actual FPD 3 are
Y (r ↑) = Yc (r ↑) −SID × tanφ (6)
Here, Yc (r ↑) = SID × rc _y / SOD.

  In this way, by correcting Y (r ↑) in equation (1) according to equation (6) for each shooting, the Feldkamp method is applied as it is, and back projection processing is performed on each pixel of the projected image. It can be carried out.

12 and 13, the X-ray tomography apparatus of the present invention, the X-ray tube 2 and FPD3 moves every pitch, sequentially projected images _{O 1,} O 2, · · ·, _{O M} is acquired Then, the relationship between the center cut section (horizontal plane parallel to the body axis y of the subject) and the projection image when the partial projection image in which the acquired projection image is decomposed for each X-ray projection angle is generated is shown. FIG. FIG. 14 is a diagram showing a relationship between one fragment of the central cut section and the projection image, and FIG. 15 is a table showing a relationship between one fragment of the central cut section and the projection image. FIG. 16 is a diagram showing the relationship between the Kth and (K + 1) th fragments in the central section and the projection image.

  Assuming that the movement distance of one pitch of the X-ray tube 2 and the FPD 3 is L, as shown in FIG. 16, a strip-shaped image having a width L is Ls = L × enlarged on the detection plane of the FPD 3 as shown in FIG. Projected with rate width.

In FIG. 14, the eighth (N = 8) fragment T _{8 of} the central section is projected to form partial projection images O ₈₁ , O ₇₂ , O ₆₃ ,..., O ₁₈ . Here, N represents an integer part of a value obtained by dividing the length of the FPD 3 by the image O _IJ (the length of the FPD 3 / the image O _IJ ), and how many O _IJ can be acquired on one detection surface. Represents. FIG. 15 shows the relationship between the cut surface and the projected image O _IJ . In Figure 15, the projected image of the T _I is an image within the range indicated by the arrow. This is illustrated in FIG. FIG. 16 shows the K-th and (K + 1) -th fragments T _K and T _{K + 1} of the central section.

When the X-ray tube 2 and the FPD 3 are moved by one pitch (distance L), as described above, on the subject surface (that is, the central section on the subject M) T, as shown in FIG. The strip-shaped portion forms a projection image having a width Ls. As described above, O _IJ is a J-th strip image of the I-th projection image, that is, a partial projection image at a projection angle θ _N (corresponding to φ _N ) obtained by shifting the pitch to the I-th, The width L is the above-described pitch d (see FIG. 6).

Information around the subject face _{T K, as} shown in FIG. _{16 (A), O KI,} O (K-1) 2, O (K-2) 3, ···, O (K- (N _-1) Reconstructed by _N The above-described Feldkamp method is applied to this reconstruction. Then, as shown in FIG. 16B, the peripheral information on the next subject surface T _{K + 1} is represented by O _{(K + 1) I} , O _K2 , O _{(K−1) 3} ,..., O _{(K + 1). -(N-1)) N} is reconstructed using the Feldkamp method.

From strip portion T ₁ of the endmost on the subject surface (the starting point), until the strip portion T _I are separated by a distance L × (I-1), thus, each image L × in the y direction (I- 1) Deviation by xSID / SOD. To obtain the entire reconstructed image of the subject M (tomographic image), the reconstructed image of the periphery of each of the strips portion T _I determined, may be bonded to each (synthesis). When the reconstructed image T _I near the fi (r ↑), (1 ) from the equation, the following equation is established.

  The entire reconstructed image f (r ↑) obtained by combining (combining) fi (r ↑) is obtained by the following equation.

  In this equation, the order of back projection according to φ and the combination of images can be changed. When the order of operations is changed, the following equation is established.

ΣO _{(I− (φ−1)) φ} in the equation (9) is a combined projection image P _φ obtained by combining the partial projection images corresponding to the projection angle φ. Therefore, equation (9) is

Can be rewritten.

  In the X-ray tomography apparatus according to the present invention, the partial projection images of the subject M can be reconstructed by the Feldkamp method, and the reconstruction of the equations (9) and (10) The tomographic image obtained by combining (combining) the results means that it is equivalent to the tomographic image obtained by reconstructing the composite projection image of the subject M by the Feldkamp method.

  Referring to FIG. 10, reconstruction processing unit 9d performs first weighting processing on the composite projection image collected for each projection angle in first weighting processing unit 9A according to the above-described method, and performs convolution processing. In the unit 9B, convolution processing (convolution integration) is performed on each composite projection image after the first weighting process, and in the second weighting processing unit 9C, the second combined projection image is subjected to the first convolution projection image. 2 is performed, and the back projection processing unit 9D performs back projection processing on each composite projection image after the second weighting processing to generate a reconstructed image (tomographic image).

  FIG. 17 is a tomographic image of the lower limb of the subject obtained by the X-ray tomography apparatus of the present invention, and FIG. 18 is a tomographic image of the same part obtained by the conventional X-ray tomography apparatus. From comparison between FIG. 17 and FIG. 18, the wavy artifact appears in the tomographic image obtained by the conventional X-ray tomography apparatus, whereas according to the present invention, there is no wavy artifact in the tomographic image and the image is clear. It can be seen that a tomographic image can be obtained.

It is a block diagram which shows the structure of the radiation imaging device by one Example of this invention. It is a figure which shows schematic structure of the X-ray tube moving mechanism of the apparatus of FIG. It is a figure which shows schematic structure of the FPD moving mechanism of the apparatus of FIG. It is a figure explaining the imaging principle by X-ray tube and FPD. It is a figure explaining the decomposition | disassembly of a projection image, and the production method of a new synthetic | combination projection image. It is a figure explaining the decomposition method of a projection image, and the production method of a new synthetic projection image. (A) is a figure which shows the principle of circular orbit tomography, (B) is a figure which shows the principle of linear orbit tomography. It is a figure explaining the algorithm of the Feldkamp method (Feldkamp) method. It is a graph which shows one example of a filter function. It is a block diagram which shows the structure of a reconstruction process part. It is a figure explaining the application method to the linear orbit tomography of the Feldkamp method (Feldkamp) method. In the X-ray tomography apparatus of the present invention, the X-ray tube and the FPD move for each pitch, and projection images are sequentially acquired, and the acquired projection images are each decomposed for each X-ray projection angle. It is a figure which shows the relationship between the center section and projection image when a projection image is produced | generated. In the X-ray tomography apparatus of the present invention, the X-ray tube and the FPD move for each pitch, and projection images are sequentially acquired, and the acquired projection images are each decomposed for each X-ray projection angle. It is a figure which shows the relationship between the center section and projection image when a projection image is produced | generated. It is a figure which shows the relationship between 1 fragment of a center cut section, and a projection image. It is a table | surface which shows the relationship between 1 fragment | piece of a center section, and a projection image. It is a figure which shows the relationship between the Kth and (K + 1) th fragment | piece in a center section, and a projection image. It is a tomographic image of a lower limb of a subject obtained by the X-ray tomography apparatus of the present invention. FIG. 6 is a tomographic image of the same part as in FIG. 5 obtained by a conventional X-ray tomography apparatus.

Explanation of symbols

1 Support stand 2 X-ray tube 3 FPD
4 support table control unit 5 FPD control unit 6 high voltage generation unit 7 X-ray tube control unit 8 A / D converter 9 tomographic image generation unit 9a correction unit 9b projection image decomposition unit 9c image composition unit 9d reconstruction processing unit 9e image Pre-processing unit 10 Controller 11 Memory unit 12 Input unit 13 Monitor

Claims (4)

A radiation detecting unit is disposed on one side of the subject, and a radiation detecting unit is provided on the opposite side of the subject to the radiation irradiating unit to detect radiation transmitted through the subject and obtain a projection image. The radiation irradiating means and the radiation detecting means are relatively translated in the same direction along the longitudinal direction of the subject, and the radiation irradiating means and the radiation detecting means are predetermined with respect to the subject. Each time the relative movement of the pitch is performed, radiation is emitted from the radiation irradiating means, a projection image is acquired by the radiation detecting means, a reconstruction process is performed based on the projection image, and a tomographic image of the subject is generated In the radiation imaging apparatus,
Projection that generates a partial projection image decomposed for each projection angle of radiation corresponding to the predetermined pitch by resolving each of the projection images at the predetermined pitch along the longitudinal direction of the subject. Image decomposition means;
Image combining means for sequentially combining the partial projection images of the projection image at the same projection angle in accordance with the order in which the projection images were acquired to form a composite projection image;
In the combined projection image formed by the image combining means, an area before the subject is detected, and a pre-process of the combined projection image is performed by replacing a pixel value of the image area with a predetermined value. Processing means;
A radiation imaging apparatus comprising: reconstruction processing means for performing reconstruction processing based on the composite projection image preprocessed by the image preprocessing means and generating a tomographic image of the subject.   The image preprocessing means sequentially detects a position where a difference between pixel values of adjacent pixels is equal to or greater than a predetermined value in the composite projection image, and a boundary between the subject and the region where the subject does not exist The radiation imaging apparatus according to claim 1, wherein a region where the subject does not exist is detected by obtaining a line.   The image preprocessing means detects an area having a pixel value larger than a reference value determined in advance as a maximum value of a pixel value of an area where the subject exists in the composite projection image as an area where the subject does not exist. The radiation imaging apparatus according to claim 1.   The radiation imaging apparatus according to claim 1, wherein the radiation emitting unit and the radiation detecting unit are relatively translated with respect to the subject at the same speed.