JP2770935B2 - 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 for reducing image artifacts in spiral scanning.

[0002]

2. Description of the Related Art Conventional examples of a helical scanning CT apparatus include JP-A-59-111538 and JP-A-62-87137. JP-A-59-11738 discloses an example in which a CT image is obtained by helical scanning. JP-A-62-87137
Discloses an example of interpolation according to distance distribution for obtaining a CT image by helical scanning.

[0003]

Problems to be Solved by the Invention
No. 38 discloses various techniques for obtaining a CT image by helical scanning, but the specific method is uncertain. Japanese Patent Application Laid-Open No. 62-87137 provides a practical method in that an interpolation example to a vertical tomographic plane is described, but various methods for examples other than the vertical tomographic plane are not clear.

[0004] An object of the present invention is to provide a CT apparatus capable of obtaining a CT image by spiral scanning that is closely connected.

[0005]

According to the present invention, there is provided an X-ray source for generating X-rays while rotating around an object, and an X-ray source provided opposite to the X-ray source and transmitted through the object. An X-ray detector for detecting X-rays, and a bed on which the subject is moved, wherein the bed is continuously moved during the rotation of the X-ray source in the body axis direction of the subject; In a CT apparatus that irradiates the subject with X-rays from the X-ray source in the process of moving the bed and performs a spiral scan on the subject,
Means for measuring and holding helical data by helical scanning when the rotation exceeds at least one rotation of the X-ray source; The first spiral data range is set, and a second spiral data range having the same range width as that of the first spiral data range is set to the first spiral data range with respect to the spiral data. Means for setting the sections so as to overlap each other, and means for reading out the spiral data of each of the first spiral data range and the second spiral data range and separately reconstructing the image. A CT device is disclosed.

Further, according to the present invention, the spiral data ranges to be set are set to be n (n ≧ 3) in such a manner that adjacent spiral data ranges are sequentially shifted in the direction of the subject body with the same overlapping range. And

Further, according to the present invention, the helical data range used for reconstructing one tomographic image includes helical data of one rotation of the X-ray source.

[0008]

FIG. 1 is a view showing an embodiment of an X-ray CT apparatus according to the present invention. The scanner body 100 includes a rotating X-ray source 1,
The gantry includes a multi-channel X-ray detector 2 rotating in opposition to the X-ray source, a gantry opening 4 having a measurement space, and a bed 3 for moving a subject 32 into and out of the gantry opening 4. In addition, a rotary mechanism and various control mechanisms are provided, but omitted.

In the present embodiment, the X-ray source 1 and the X-ray detector 2 rotate continuously, while the bed 3 continuously moves forward or backward. Thereby, a spiral scan is realized. Further, it is assumed that X-rays are continuously or discontinuously (pulsed) generated from the X-ray source 1. The X-ray source 1 generates a fan-shaped X-ray beam, and its slice width SW (the width of the X-ray beam in a direction perpendicular to the spreading direction of the fan-shaped X-ray beam) is 0 <SW ≦ 20.
It has a mechanism that can be varied to about mm.

In FIG. 1, a data processing unit 200 includes a data collection circuit 2A, a two-dimensional buffer 12, a projection data formation circuit 1
3, filter correction circuit 14, back projection operation circuit 15, CT
An image display unit 16 is provided.

The two-dimensional buffer 12 stores an X-ray detection signal (converted into data) obtained by the data collection circuit 2A at an address (i) determined by a measurement cycle number j and a projection number (projection angle number) i. , J). Here, the X-ray detection signal (data) means data obtained by spiral scanning,
These spiral data SR are displayed. The helical data SR includes a measurement cycle number j indicating how many times the X-ray source 1 has rotated (if one rotation is defined as one measurement cycle, the number of measurement cycle numbers), and the position during one rotation (I.e., this is the position of the X-ray source, so-called projection angle number) i.

Therefore, the rotation speed of the X-ray source 1 and the moving speed of the bed 3 are controlled, and the corresponding address (i, j) of the spiral data SR is uniquely determined according to the speed.
The spiral data SR is stored. The easiest to manage is X
This is a case where both the radiation source 1 and the bed 3 are controlled to move at a constant speed, and has an advantage that the control method is easy.
It goes without saying that the spiral data SR exists in one address (i, j) for the number of channels.

When stored in the two-dimensional buffer 12, the projection data forming circuit 13 creates projection data for CT calculation from the rotation data SR. To reconstruct one CT image,
Basically, 360 ° data is required for one slice plane. Since the spiral data SR cannot be essentially data of one slice plane, it is necessary to process the spiral data SR into data on one slice plane by performing some kind of processing. is there. The projection data forming circuit 13 is provided to achieve this purpose. It is a feature of the present invention to provide a plurality of such certain treatment methods.

The filter correction circuit 14 is a spatial filtering for removing a so-called image blur.
Previously known. The back projection operation circuit 15 performs a reconstruction operation on the projection data subjected to the blur removal processing to obtain CT image data. This is displayed on the display unit 16 as a CT image or stored in various memories.

FIG. 2 is a diagram showing the trajectory of the spiral scan.
The subject 32 is rotated by the rotation of the X-ray source 1 and the movement of the bed 3.
It can be seen that X-ray scanning is being performed according to the spiral scanning trajectory. By this helical scanning, the rotation data SR is obtained along the body axis direction of the subject 32.

FIG. 3 is an example showing a scanning range at the head of the subject 32. The subject 32 is advanced in the direction of the arrow,
An example in which a section of BB ′ is photographed is shown. in this case,
An AB section indicates a shooting start-up section, and B′-A ′ indicates a falling section at the end of shooting, and is a section included in the scanning range (A′-A) but excluded from the shooting area. The reason for exclusion is because of the unstable control region for speed rising and speed falling.

FIG. 4 shows a sequence in the example of FIG. In the section of BB ', the speed is constant (both the scanner and the bed are constant). X-ray irradiation is performed in this section. In the sections AB and B'-A, the speed is not constant. It is shown not to do. FIGS. 3 and 4 show examples of the head, but it goes without saying that the position of the imaging region and the size of its width can be freely set for the chest, abdomen, and the like. Further, it is possible to freely set where and how many slice planes are obtained from the set imaging region. still,
a and b are section distances of A'-B 'and BA and B'-B, respectively.

FIG. 5 is a diagram showing a control mechanism of the CT apparatus of FIG. The high-voltage generating unit 110 generates a prescribed high-voltage under the control of the X-ray control unit 101, and supplies a voltage for X-ray irradiation by the X-ray source 1. The X-ray control unit 10 controls the timing of X-ray generation (including both continuous and discontinuous) and the magnitude of the generated voltage. Rotation mechanism 11
Reference numeral 1 denotes a mechanism for rotating the X-ray source 1 and the X-ray detector 2 of the scanner, and is driven under the control of the rotation mechanism control unit 102. If the rotation is at a constant speed, the constant speed is controlled. The bed moving mechanism 112 performs forward (or backward) movement of the bed 3 and is controlled by the bed moving mechanism control unit 103. If the speed is constant, the moving direction is determined and controlled so as to be constant. Bed position detection unit 113,
The rotation position detector 114 calculates the projection number i and the measurement cycle number j
The constant speed control is effective for monitoring the unstable period of start-up and fall. But,
In the variable speed control described later, it is used for creating i and j (or for correcting i and j).

The system control unit 100 manages and controls the entire system. The control has a function of synchronizing the entire system. However, synchronization refers to a function used to recognize the timing of the entire system and to integrate and manage them (see FIG. 4).

An embodiment on the creation side by the perspective data forming circuit 13 will be described below. (1), Preparation Example 1 Direct placement method. The direct arrangement method is a method in which, when acquired in one measurement cycle (360 °), the spiral data obtained by the rotational scan is directly arranged as it is obtained on one slice plane, and this is used as 360 ° perspective data. It is. This will be described with reference to FIGS. FIG. 6A shows that the projection number for 360 ° is PP.
A spiral scan example with a total of n projections is shown. FIG. 6B shows the bed position and X in the case according to FIG.
FIG. 3 is a diagram illustrating a relationship with a position with respect to a radiation source 1. (B) of FIG.
According to the above, the relationship between the bed position and the X-ray source position can be indicated by a locus of a sine curve. Here, it is assumed that the bed position advances by a distance D when the X-ray source rotates by 360 °. That is, the subject can be spirally scanned by a distance of D in one measurement cycle.

In the direct placement method of this embodiment, the bed position moves by D in one measurement period (1 to pp), but this movement is ignored, and the surrounding 360 at a certain slice position is ignored.投影, and is forcibly treated as projection data obtained from 1 to pp. Here, a certain slice position is a middle position X1 in a section from ₁ to pp.
The middle position is the 180 ° position, and the projection angles are 0 ° to 3 °.
This is an intermediate position with respect to 60 °. Hereinafter, similarly, the following 3
For 60 ° interval, the intermediate position X ₂ to the slice position, forcibly handle spiral data before and after the 180 ° section of the intermediate position as 0-360 ° projection data.

Thus, projection data at virtual slice positions such as positions X ₁ , X ₂ ,... Are obtained, and if reconstruction (including filtering) is performed for each position, slice position X ₁ , X ₂ X ₂ ... CT images can be obtained. The CT images at this time are shown as images 1, 2,... In FIG.

(2), Preparation example 2 ... Overlap arrangement method. The overlapping arrangement method is a method shown in FIG. FIG. 6C shows an example in which the bed position is divided every 360 °, and one CT image having the center position as the slice position is obtained every 360 °. In the overlapping arrangement method, as shown in FIG. 6D, for example, the first CT image uses the spiral data SR of 360 ° from the projection angle numbers 1 to pp, but the next second image The CT image has a projection angle of 2 to (pp + 1)
, And the next third CT image has projection angle numbers 3 to (pp +
360 from a position shifted by an arbitrary projection number as in 2)
The method of using minute measurement data is adopted. That is, the spiral data SR partially overlaps the CT image 1 and the CT image 2. As a result, the slice position of the first CT image is 180 °, and the second CT image is
The slice position of the image is (180 ° position) + (distance to go to one projection). Thus, there is an advantage that a CT image can be calculated at the projection angle pitch, and the tomographic plane position at this time has the following relationship. Xn = D / 2 + (D / pp) (n-1) (1) where n is the number of movements of the projection number, and pp is the number of projections in one scan. Thus, if the data range (360 °) used for image reconstruction is arbitrarily set, image reconstruction at an arbitrary position becomes possible.

In the methods of the first and second production examples, since the start position and the end position of 360 ° are different, artifacts are easily generated, and the larger the movement amount in one measurement cycle, the stronger the artifacts.

(3) Creation Example 3—Start Point / End Point Matching Method This method virtually matches the start point and the end point with respect to the data of 360 ° in each area of the creation examples 1 and 2. This is a technique for reducing the occurrence of image artifacts. Here, the start point indicates a position at a start projection angle of 0 ° when one CT image is to be obtained from 360 ° projection data, and the end point indicates a position at an end projection angle of 360 °. . This is based on the theory that if the error in the data between the start point and the end point is small, the image artifact will be eliminated. The problem with spiral scanning is that 36
Since the subject moves while the X-ray source performs the rotation of 0 ° to 360 ° in the image reconstruction data of 0 °, the 0 °
In the section of {-360}, the subject is not the same part between 0-360 °. That is, the coincidence between the start point and the end point is essentially impossible, which is a feature of spiral scanning. Therefore, it is the start point end point matching method according to the present embodiment that attempts to eliminate this mismatch in the handling of data.

Hereinafter, the concept of this method will be described with reference to FIG. FIG. 7 is a diagram showing an example of a non-helical scan shown in (a), an example of a spiral scan shown in (b), and an example of a spiral scan process using the present technique shown in (c). , Slice direction measurement range, and image reconstruction data contribution ratio.

FIG. 7A shows an example of fixed scanning of a subject which is not helical scanning. Although the two slice planes A and B are different slice planes, each slice plane, for example, the slice plane A, has a start point S and an end point as indicated by its measurement trajectory (the position indicated by a dotted line is a slice position). It is expected that the data always coincides with the point E and there is almost no data error. Similarly, the slice plane B also differs only in the slice position, and the start point and the end point on this plane B always coincide. On the other hand, the slice direction measurement ranges are slice planes A and B, each of which has a slice width SW, and each slice plane is an example having no overlap. FIG. 7 shows that the image reconstruction contribution rate is 100% for both slice planes A and B.
(A) of FIG. (Contribution ratio 1.0 is 100%
And).

Next, FIG. 7B shows an example of a spiral scan.
An example is shown in which the start point S and the end point E do not match as in the first and second creation examples. This example is an example of SE = D (bed movement amount in one measurement cycle). According to (b), the measurement trajectory naturally has a mismatch between the start point S and the end point E as shown in the figure. is there. The slice direction measurement range is shifted according to the helical scan, and becomes like the first slice plane A1 and the next slice plane B1. Looking at the center position of the slice plane (shown by the dotted line), it can be seen that the start point S and the end point E are significantly different. In addition, the contribution ratio is 100% because all data for 0 to 360 degrees can be used for image reconstruction. This example shows the methods of creation 1 and 2. In order to reduce artifacts, it is necessary to reduce the bed movement amount D.

Next, FIG. 7C shows an example according to this method. In this method, the slice position relationship between the start point S and the end point E of the SR data (360 minutes) used for image reconstruction is forcibly matched, and the relative position relationship is determined by the first slice plane. A2 and the second slice plane B2. At this time, regarding the contribution ratio of the SR data, there are various ways of contribution, but in FIG.
In {-360}, the continuity of data is maintained by setting the contribution rate to 1.0 at the center and decreasing the contribution rate toward the end (defined as a specific range). There is a need to.

Further, with respect to the slice direction measurement range, A1 and B1 whose relative positional relationship is (b) draw a triangular locus as shown by an arrow in the example of B1, whereas A2 and (b) In B2, the relationship of drawing a lozenge locus as shown by an arrow in the example of B2. The locus of the rhombus means that the calculation start point and the end point position are the same, a position of 180 ° from the start point to the end point, and 180 ° from the end point to the start point in the reverse direction.゜ position has the same slice position relationship, and other points (that is, positions 180 ° apart, with the start point of the 360 ° data cut out as image reconstruction data being 0 ° and the end point being 360 °) In other words, if the other points are represented by x, 0 点 <x <180 ゜ and 180 ゜ <x <360 ゜), the calculation start point and end point This is satisfied under the three conditions of creating data that moves away from the image reconstruction sectional position (slice position) as the distance increases. Thus, one closed loop is formed over the entire interval from 0 ° to 360 °, and the occurrence of image artifacts is eliminated because the continuity between projection data is maintained.

The trajectory of the measurement range and the contribution ratio characteristics shown in FIG. 7C are used to calculate how realistically the projection data is obtained from the SR data when the start point S and the end point E match. Determined by law.

Further, the specific range of the end portion in FIG. 7C is used to provide continuity as projection data in the vicinity of the coincidence point when the measurement start point and the end point coincide with each other. And an area where the continuity of projection data is maintained by processing SR data such as interpolation processing and weighted averaging processing to be described later.

In FIG. 7, the constant slice width SW
It is needless to say that the example of the locus of lozenges can be applied as it is even when the slice width is so small that the value of SW can be ignored.

Next, a specific example of the processing in the specific range of the above-mentioned preparation example 3 will be described. (3-1), specific example 1; This specific example satisfies the relationship of FIG. 7 (c) of the above-described creation example 3 by interpolating between measurement data having the same projection angle between different measurement cycle data, and by slicing the data in a specific range. It is a way to make it. This interpolation method will be described with reference to FIG. As shown in FIG. 6B, the relationship between the position of the X-ray tube and the measurement slice position (bed position) is represented by a sine curve. X ₀ is the measurement start position (corresponding to the position B in FIG. 3), and β = 0 °. As an example, the fault plane X _n
An example of reconstructing the image at the position will be described. In this case, _{assuming that} the position of this image is the position at the projection angle β _n = 450 ° of the second scan, the image reconstruction area is 270 ° ≦ β _n ≦ 630.
It indicates that the subject has moved by the distance D during the measurement of one image for 360 ° in the range of ゜.

Here, the central part of the 360 ° SR data is used as it is, and the measurement start point (β _s = 270)
+) Within a certain range from ゜), the measurement end point (β _e = 630)
The range from-) to -β is determined by interpolation. FIG.
Shows an example in which β = 90 °. The interpolation curve in this case is set to be the same as the position of the subject between β _s + β and β _e −β. In this example, it becomes a sine wave indicated by a broken line. As the interpolation area, an area of β _s −β to β _e + β is required, and the data of β _s to β in the section B is between the section B and the section D advanced by 360 ° from B, and The data of C is obtained by interpolating between the same projection data in the section C and the section A delayed by -360 ° from C. In the figure, _Kn1 and _Kn2
2 shows an example of interpolation at two points, and the point K _n1 is obtained as follows.

SR ₁₁ (β _s + β ₁ , X ₁₁ ) = {SR ₁₁ (β _s + β ₁ , X ₁₁ ) · a ₁ + SR _m1 (β _s + β ₁ +360 ゜, X _{m 1} ) · b ₁ } / ( a ₁ + b ₁ ) (2) where a ₁ = X _m1 −X _k1 b ₁ = X _k1 −X _l1 a ₁ + b ₁ = D

Similarly, the point R _n2 is SR _m2 (β _s + β ₂ , X _m2 ) = {SR _m2 (β _s + β ₂ , X _m2 ) · a ₂ + SR _l2 (β _s + β ₂ −360), X _l2 ) · B ₂ ｝ / (a ₂ + b ₂ ) (3) where a ₂ = X _m2 −X _k2 b ₂ = X _k2 −X _l2 a ₂ + b ₂ = D

In this manner, over the range of the section E (projection angle of 180 °), the positional relationship with respect to the subject becomes equivalent to the closed relationship with respect to the tomographic plane _Xn over 360 °. It is possible to reconstruct an image using the above data, and the measurement start point and the end point, and the subject position of the data shifted by 180 ° from the measurement start point can be equivalently made the same. Further, β _s can be set arbitrarily, and the interpolation method can reconstruct an image at an arbitrary position in exactly the same manner. However, this method requires interpolation when creating data (360 minutes) for image reconstruction. The area is the image reconstruction data 360
Since β is required before and after ゜, the start of the shooting area
At the end, there is a region that is used only for interpolation. Further, the interpolation curve may be a straight line or another curve, that is, it is only necessary to maintain the continuity of the position. FIG. 9 shows the relationship between the angle sample points and the interpolation points in an easy-to-understand manner.
This is for understanding the description of FIG.

(3-2), Specific Example 2 This embodiment is characterized in that the weighted averaging method using a weighting function is used and that the data to be subjected to the weighted averaging is data having a 180 ° facing positional relationship.

First, data on the 180 ° facing positional relationship will be described. FIG. 10 is an explanatory view of this case. FIG. 2A shows that the X-ray source has projection angles β = 0 °, β = 180 °, and β = 180.
FIG. 3 is a diagram illustrating a positional relationship between an X-ray source 1 and an X-ray detector 2 in the case of + 2θ. The X-ray of the X-ray source 1 is a fan beam having an opening angle of 2θ, and the X-ray detector 2 is a multi-channel type for detecting a fan beam X-ray. FIG. 2B is an explanatory diagram of the width of the fan beam (the so-called slice width) as viewed from a direction perpendicular to the fan beam spreading direction.
With respect to the opening angle, the angle of the beam passing through the center point C is set to 0.
°, the left half of which is treated as a + direction and the right half thereof is treated as one direction for convenience. Therefore, the first channel has an opening angle of + θ, and the final channel has an opening angle of -θ.

Now, the measurement path of the center beam at β = 0 ° and β = 180 ° is the same path P1. This is called a 180 ° facing positional relationship. This exists in addition to the center beam. For example, the measurement path of the first channel (+ θ) at β = 0 ° is P2. On the other hand, the measurement path of the n-th channel (final channel) (−θ) at the projection angle of β = 180 ° + 2θ is P2, and the two have a positional relationship of 180 °. In general, any i-th channel θ (i) at the projection angle β
Is 180 ° opposite to the projection angle β + 180 ° + 2
θ, the corresponding channel is a channel of −θ (i) at this projection angle.

The X-ray fan beam has a slice width SW as shown in FIG. As a result, β = 0 °
And β = 180 °, the beam 2B at 0 ° and the beam 2A at 180 ° have the same slice width SW, but the range measured by the slice width is completely the same. do not do. Fortunately, since there are few sharp changes in the body of the subject, they can often be regarded as almost the same value in an arrangement using a normal CT apparatus.

However, in the spiral scan, the bed moves during the measurement, so that at the position of β = 180 °, the X-ray source and the X-ray are moved by the amount of movement as shown by the dotted line in FIG.
The line detector is shifted in the slice direction. If the bed feed amount is not larger than the slice width, the overlap with the measurement path at the opposing position increases, and it can be seen that the value approaches the same value. For this reason, the slice position must originally be on a complete vertical tomographic plane, but if it has a slice width SW, a CT image can be calculated even if it is not a complete vertical tomographic plane. Therefore, in the present embodiment, 18 ° from the start point in the data of 360 °
Since the data at the time of shifting by 0 ° is naturally the center of the movement of the slice position during the rotation scan, a weighted average is performed with the data having the 180 ° facing relationship, and the result is replaced as projection data. I did it.

FIG. 11 shows an example of a weighting function for weighted averaging. FIG. 11A shows an example of a weighting function on the measured data side, and FIG. 11B shows an example of a weighting function on the measured data on the side of 180 ° facing the measured data. Here, the actual measurement data is the actual measurement data SR, 18 at β = 0 ° when β = 0 ° is the actual measurement as shown in the example of FIG.
The measurement data on the side of the 0 ° facing positional relationship indicates the actually measured data SR at β = 180 ° (however, it is assumed that the channel is θ (i) = 0). Therefore, for channel data other than θ = 0, it is necessary to consider a 180 ° facing positional relationship.

In FIG. 11A, the section of 0.ltoreq.180.degree. Is a straight line L1, 180.degree. <Pp (360.degree.
The section of (Position) is an example in which a weight function of a straight line L2 having a negative inclination angle is used.

On the other hand, FIG. 11B shows a weight function of a straight line L3 having a negative inclination angle in a section of 0 ≦ 180 ° and a straight line L4 of a positive inclination angle in a section of 180 ° <pp.

The weighting function (weighting function at an arbitrary position) is shown in FIG. 11 (A) as W1, and FIG.
Assuming that 2, the weighted average value D is as follows: D = (W1 × SR1 + W2 × SR2) (4) W1 + W2 = 1 (5) Here, SR1 is an actually measured data value, and SR2 is 18
This is a measured data value having a 0 ° facing relationship.

There may be examples other than the weight function and the straight line.
There are various examples of continuous changes (quadratic function,
Cubic function, etc.).

According to the present embodiment, the projection data at the bed position closer to the desired image reconstruction plane position (slice position) is calculated for the data at the reconstruction start and end positions, and the image reconstruction is performed. It replaces the configuration data and has a closed loop relationship of the image reconstruction data.

(3-3) Specific Example 3, This method has a feature in processing relating to a specific range. Specifically, the following method is employed. First, as processing in a specific range, average values SRa (β (1), X) and SRa (β (p (p)) between the start and end data of image reconstruction.
Replace p) and X) with both measurement data. This is the same as using the same interpolation coefficient in the interpolation of the first specific example, and the bed position is halfway between the image reconstruction start position and the end position. In this state, discontinuity with the image reconstruction start and end data occurs, so that the measurement data of adjacent projection numbers is calculated as follows by weighted averaging with this average value.

SRa (β (n + 1), X) = K · SRa (β (n), X) + (1−K) · SR (β (n + 1), X) (6) SRa (β (pp− n), X) = K · SRa (β (pp−n), X) + (1−K) · SR (β (pp−n), X) (7) where K is 0 ≦ K ≦ 1, P is a target range, and n is defined as 1 ≦ n ≦ p. This weighted averaging process is performed in the specific range portion described above, and has a shape as shown in FIG. The weight coefficient may be constant within a specific range, or the weight may be changed. As described above, it is important to perform the correction such that the bed position error gradually increases from the bed position data of the image reconstruction start and end data.
According to the present embodiment, since continuity is maintained, there is no occurrence of image artifacts.

(3-4) Regarding the width of a specific range. Finally, if the specific range is expanded, data close to the start and end of image reconstruction will be ignored, and the data will not be used effectively. As described with reference to FIG. 13, the smaller the bed movement amount or the smaller the slice thickness, the smaller the influence of the displacement becomes. Therefore, according to the bed movement amount and the slice width, in the above three examples, It is possible to change the specific range. Further, in head measurement, it is expected that the influence of bones is large and the change in data due to displacement is large. For this reason, when the head measurement or the bed movement amount per scan is 10 mm, this specific range is extended to a projection angle of 90 ° or more from the image reconstruction start and end points, respectively, and the abdominal measurement ゃ and the bed movement amount are small. Is 90
It is possible to reduce the correction processing time by lowering the value to a value smaller than °. Further, the specific range needs to be increased as the bed movement amount increases.

In the first specific example, the specific interpolation range is 180 ° in total by 90 ° from the image reconstruction start and end positions.
However, the specific interpolation range can be changed in a range where the influence of the image is small. In this case, it is natural that the interpolable range is also changed. Also, specific example 2
When the specific interpolation range is narrowed in the case of (1), the shape becomes the shape of the slice direction measurement range as shown in (d) of FIG. 7 and the shape does not become completely closed but the artifact can be reduced. It is.

(4) Modifications and application examples. The interpolation example is linear interpolation, but higher-order interpolation such as secondary interpolation and tertiary interpolation can also be adopted. At the time of interpolation,
Although the 2D example has been described, interpolation may be performed from spiral data in a plurality of scan measurement ranges such as 2D and 3D. The X-ray CT apparatus of the present invention is applicable to each generation such as the first, second, and third generations, and is also applicable to a cone beam format. In the present embodiment, an example of the X-ray CT apparatus has been described. However, it is needless to say that the content of the image reconstruction correction processing can be applied to another apparatus that performs a vertical cross-section measurement and displays a cross-sectional image. The present invention can be applied to an apparatus, a positron CT apparatus, an electronic scanning X-ray CT apparatus, and the like.

According to the present embodiment, it is not necessary to interpolate from the measurement data of two or more cycles to the entire projection angle in order to make the image reconstruction section vertical, so that the image reconstruction time is large. Since the number of projections can be reduced and data can be calculated from the data of the number of projections that are equal to or close to the projection angle originally required for image reconstruction, a CT apparatus with high calculation efficiency can be constructed. In addition, since the specific range can be changed depending on the imaging conditions, more optimal correction processing can be performed in the shortest calculation time according to the measurement site, so that a good image with accurate positional relationship and no artifact due to positional deviation can be obtained. can get.

[0056]

According to the present invention, a CT image connected along the body axis direction of a subject can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a CT apparatus according to the present invention.

FIG. 2 is a diagram showing a trajectory of spiral scanning.

FIG. 3 is a diagram illustrating a relationship between a shooting region and a scanning region in spiral scanning.

FIG. 4 is a diagram showing a time chart of FIG. 3;

FIG. 5 is a control system diagram of the CT apparatus of FIG. 1;

FIG. 6 is a diagram showing an example of projection data creation according to the present invention.

FIG. 7 is a diagram showing a comparison between an example of coincidence of a start point and an end point of the present invention and a conventional example.

FIG. 8 is a diagram showing an interpolation example of the present invention.

FIG. 9 is an explanatory diagram of an interpolation process according to the present invention.

FIG. 10 is a diagram showing a 180 ° facing relationship used in an embodiment of the present invention.

FIG. 11 is a diagram illustrating an example of a weight relationship according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 X-ray source 2 X-ray detector 3 Bed (bed) 4 Opening 12 12-dimensional buffer 13 Transparent data creation circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-59-111738 (JP, A) JP-A-62-187137 (JP, A) JP-A-60-111640 (JP, A) JP-A-62-187 227324 (JP, A) JP-A-62-272326 (JP, A) JP-A-62-272327 (JP, A) JP-A-4-49952 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) A61B 6/03

Claims (3)

(57) [Claims] 1. An X-ray source that generates X-rays while rotating around an object, and an X-ray detector that is provided to face the X-ray source and detects X-rays transmitted through the object. And a bed on which the subject is moved. The bed is continuously moved in the body axis direction of the subject during rotation of the X-ray source. In a CT apparatus that irradiates the subject with X-rays from a radiation source to perform a spiral scan on the subject, at least one of the X-ray sources
Means for measuring and holding helical data by helical scanning if the rotation is exceeded, and first helical data for reconstructing a tomographic image of a specific position of the subject with respect to the helical data when held by the measurement and holding means A range is set, and a second helical data range having the same range width as the first helical data range is set so as to partially overlap the first helical data range with respect to the helical data. And a means for reading the helical data of each of the first helical data range and the second helical data range and separately reconstructing the image. 2. The method according to claim 1, wherein the helical data ranges to be set are set to be n (n ≧ 3) in such a manner that adjacent helical data ranges are sequentially shifted in the subject body axis direction with the same overlapping range. The CT apparatus according to claim 1, wherein: 3. The CT apparatus according to claim 1, wherein the helical data range used for reconstructing one tomographic image includes helical data of one rotation of the X-ray source.