JPH09154838A - Projection data preparing method of helical scan and x-ray ct device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To sharply restrain an artifact generated by a data losing area, and contribute to an improvement in an image quality by preparing interpolative projection data with respective rotational phases by simply and additively averaging first and second interpolative data prepared by interpolative processing. SOLUTION: A projection data correction-interpolative processing part 12 linearly interpolates projection data based on a former beam fU selected with respective rotational phases and projection data base on a former beam fL in the reciprocal ratio of a distance between the respective beams and an object slice position, and prepares first interpolative data on the rotational phases and the slice position. Projection data base on an opposed beam gU selected with respective rotational phases and projection data based on an opposed beam gL are linearly interpolated in the reciprocal ratio of a distance between the respective beams and the object slice position, and second interpolative data of the rotational phases and the slice position is prepared. The first and the second interpolative data are additively averaged with respective rotational phases, and are formed as projection data on the respective rotational phases of the object slice position.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a helical in an X-ray CT apparatus for arranging a plurality of X-ray detectors having a plurality of detection channels in a row along an XY plane orthogonal to the Z-axis to perform a helical scan. The present invention relates to a scan projection data creating method and an X-ray CT apparatus.

[0002]

2. Description of the Related Art In a normal scan in an X-ray CT apparatus, a bed is held fixed and a slice position 360
After collecting the projection data of a plurality of slices, the bed is moved to the next slice position, the projection data of a 360 ° is collected at that position, and the projection data of a large number of slices is collected by repeating this operation. Is.

On the other hand, the helical scan first made possible by continuous-rotation CT, as its name implies, scans the subject in a helical form and collects projection data. Here, the slice position in the helical scan (center of the X-ray beam at the rotation center) is shown where the vertical axis represents the rotation phase (rotation angle of the X-ray tube) and the horizontal axis represents the Z axis (body axis direction) ( (Solid line in the figure). As shown in FIG. 17, in the helical scan, projection data of 360 degrees or more is collected while continuously moving the bed in the Z-axis direction. Note that FIG. 17 is referred to as a scan diagram in the present specification, and clinically, the normal helical pitch (unit: mm, bed feed amount per rotation) is the same as the slice thickness (mm), and is basically the same. It is called pitch.

In the helical scan, in order to reconstruct the image at the slice position s, projection data for one vertical column of the slice position s is necessary, but this projection data does not exist. Therefore, the required projection data is created by interpolating from a plurality of projection data close to s, and the interpolation data is used to reconstruct.

An X-ray detector having a plurality of detection channels is arranged in one row along an XY plane orthogonal to the Z axis.
The helical scan interpolation method in the T-apparatus, so-called single slice CT, is (1) 360 ° interpolation method, (2)
There is a counter beam interpolation method.

FIGS. 18 (a) and 18 (b) show a pair of fan beams (f1, f2) having a rotation phase θ when the 360 ° interpolation method is used. The projection data of the rotation phase θ of the slice position s is generated by using the projection data d1 and d2 of the fan beams f1 and f2 of the same phase sandwiching the slice position s. The distance between the fan beams f1 and f2 is the helical pitch t as shown in FIGS. 18 (a) and 18 (b).

That is, the projection data d1 and d2 by the fan beams f1 and f2 are linearly interpolated at the inverse ratio of the distance to obtain the data of the rotation phase θ at the same slice position s. In addition,
The same interpolation processing is performed for other rotation phases,
The projection data of the slice position s is created.

Further, FIG. 19 (a) shows a fan beam f3 having a rotation phase θ when the counter beam interpolation method is used, and counter beams (generally fo) of respective scanning beams constituting the fan beam f3. 19 (b) shows the fan beam f _b and its opposite beam f _b ′ detected in the central channel of the detector. Further, in FIG. 20, the spread angle (maximum fan angle) each scanning beam f _a of α fan beam f3 (rotation phase theta), f _b, f _c and its opposite beam _{f a ', f b',} f c ' Indicates.

According to FIG. 20, the fan beam f3 (position P (θ)) of the rotation phase θ is P (θ1; rotation phase θ1 = 1
80 ° + θ−α), the beam f _a ′ is opposite to the scanning beam f _a in the fan beam f3 having the rotation phase θ.
The projection data by can be acquired. Below, fan beam f
3 goes from P (θ1) to P (θ2; rotational phase θ2 = 180 degrees + θ) and then P (θ3; rotational phase θ3 = 180 degrees + θ +
During the process proceeds to alpha), the scanning beam f _a ~f opposed to _c beam f _a '~f _c' can be obtained. The scanning beam f
_b and f _b 'the distance between, as shown in FIG. 19 (a), (b) , is 1/2 of the helical pitch t.

[0010] That is, opposite beam interpolation method, projection data of each scanning beam f _a ~f _b ~f _c constituting the rotational phase θ fan beam f3 (divergence angle alpha), and opposite beam opposite to the beams _{f a '~f b' ~f c} '
The projection data of the rotation phase θ at the slice position s is created from the projection data of (the scanning beams forming the fan beam of the rotation phase θ1 to the rotation phase θ3).

On the other hand, the X-ray detector uses the Z-ray detector in order to capture a high-definition and wide-range image per unit time.
A CT device (so-called multi-slice CT) arranged in a plurality of rows (two rows, three rows, four rows, ...) Along the axial direction has been devised. 21A shows the case where the X-ray detector 50 has one row (single slice CT), and FIG. 21B shows the case where the X-ray detector 50a has two rows (multi-slice CT). As shown in FIG. 21B, the two rows of the X-ray detectors 50a include a first X-ray detector (seg) along the Z-axis direction.
1) and the second X-ray detector (seg2) are arranged in two rows.

In multi-slice CT, the X-ray beam sent from the X-ray tube is detected as fan beams having different phases for each detector row. Multi-slice CT
In, since the concept of the basic pitch cannot be applied as it is, the concept of the basic pitch is extended. That is, in the present specification, the basic pitch is the basic pitch = “the number of detector rows” × “slice thickness” (1).

Helical interpolation in multi-slice CT includes an adjacent interpolation method, which is a development of the 360 ° interpolation method in single-slice CT. The adjacent interpolation method is an interpolation method that linearly interpolates two fan beams having the same phase and sandwiching the slice position s with the inverse ratio of the distance to obtain the data of the phase.

22 (a), (b), and (c), in the multi-slice CT with two detectors, the fan beam f4 (seg1) of the rotation phase θ1 when the adjacent interpolation method is used.
Fan beam and f5 (fan beam detected in seg1), and fan beams f6 (fan beam detected in seg2) and f7 having a rotation phase θ2.
(Fan beam detected by seg1) is shown.

That is, the projection data d4 and d5 by the fan beams f4 and f5 are linearly interpolated by the inverse ratio of the distance to obtain the data of the rotation phase θ1 at the same slice position s, and the projection data d6 and d7 by the fan beams f6 and f7 are obtained. Rotation phase θ at the same slice position s after linear interpolation with the inverse ratio of distance
The data is 2.

As can be seen from FIGS. 22 (b) and 22 (c), in a helical scan in a multi-slice CT having two detector rows, the slices are adjacent to the slice position s when viewed from the direction orthogonal to the Z-axis direction. Two fan beams having the same phase (interposing the slice position s) differ depending on the rotation phase. That is, from the rotational phase 0 ° to a certain rotational phase θt, two fan beams having the same phase sandwiching the slice position s are detected by seg2 located on the upper side (hereinafter simply referred to as the upper side) along the Z axis of the slice position s. Fan beam (upper beam; for example, fan beam f4 with rotation phase θ1 (<θt)) and fan detected by seg1 on the lower side (hereinafter simply referred to as the lower side) along the Z axis of the slice position s. Beam (lower beam; for example, fan beam f5 with rotation phase θ1), the rotation phase θt to rotation phase 3
Up to 60 °, two fan beams having the same phase sandwiching the slice position s are detected by seg1 above the slice position s (for example, a rotation phase θ 2 (> θ).
fan beam f7) of t) and the fan beam detected by seg2 below the slice position s (for example, fan beam f7 of rotation phase θ2).

The basic pitch is “the number of detector rows is 2” ×
Since it is the “slice thickness t”, it is 2t.

On the other hand, in the X-ray CT apparatus, an area (imaging area) to be imaged can be designated according to the physique of the subject.
This shooting area is usually circular and has a FOV (Field of V
iew) is called.

FIG. 23 is a view showing the X-ray focal point, the X-ray detector, the X-ray fan beam, and the FOV as seen from the Z-axis direction in the X-ray CT apparatus. In FIG. 23, a normal size FOV (M) is shown, but the FOV has, for example, FIG.
As shown in, there are five sizes from the minimum SS to the maximum LL.

[0020]

Problem to be Solved by the Invention Single-slice CT
FIG. 25 shows a schematic diagram when the is viewed from the direction perpendicular to the Z axis. For example, the target slice of an LL size FOV (LL) (which is a rectangle when viewed from the direction orthogonal to the z axis) is added. As shown in FIG. 25, since the X-ray beam spreads in the Z-axis direction, it can be seen that it does not pass through the entire target slice, and the X-ray (beam)
It can be seen that the range passing through the slice is a trapezoid that is symmetrical with respect to the XY directions orthogonal to the Z axis. That is, the target shoulder region is a region where the X-ray beam does not pass, that is, a data loss region. Since CT requires X-ray data that has passed through the entire target slice, the presence of this data loss region causes deterioration of image quality. It should be noted that this data loss area has a size of F regardless of the size of the FOV.
It is also present when using OV.

Further, when the adjacent interpolation method is used when the helical scan is executed, two projections by the fan beams of two slices of the same phase (an upper fan beam and a lower fan beam sandwiching the target slice position) are projected. The data is interpolated, and the fan beam for the two slices and the relationship between the fan beam and the slice are as shown in FIG. That is, a data loss region is generated biased toward the X-ray focal point side of the range including the target slice position (center of the slice), and biased toward the detector side of the range including the target slice position (center of the slice). As a result, a region where the upper fan beam and the lower fan beam overlap (data overlapping region) occurs.

The data lost area and the data overlap area are one of the causes of image quality deterioration during helical scanning. Note that, although FIG. 27 shows fan beams for 4 slices continuously, as in the case of 2 slices, a data loss region is biased toward the X-ray focal point side, and a data overlapping region is generated on the detector side. Occur.

The data loss area and the data overlap area that occur in the single slice described above also occur in the multi-slice CT.

28A and 28B are schematic views of the multi-slice CT seen from the direction perpendicular to the Z axis. FIG.
(A) and (b) show a multi-slice CT with two detector rows (first segment, second segment). For example, a target slice of an LL size FOV (LL) is added. There is. 28 (a) and (b),
The range in which the X-ray beam passes through the slice is a trapezoid that is asymmetrical with respect to the XY direction orthogonal to the Z axis, and the target shoulder region is the data loss region.

Further, two fan beams of the same phase (an upper fan beam and a lower fan beam sandwiching a target slice position) are used when the adjacent interpolation method is used when performing the helical scan in the multi-slice CT. The two projection data are interpolated, and the relationship between the two fan beams and the slice by the fan beams is as shown in FIGS. 29 (a) and 29 (b).

FIG. 29A shows the first segment (seg).
FIG. 29 shows a region in which each fan beam passes through the target slice when the fan beam detected in 1) is the upper side and the fan beam detected in the second segment (seg2) is the lower side. In b), the fan beam detected in the second segment (seg2) is on the upper side,
It shows a region where each fan beam when the fan beam detected in the first segment (seg1) is on the lower side passes through the target slice. As shown in FIGS. 29A and 29B, even in the case of multi-slice CT, X
Since the line beam spreads in the Z-axis direction, it can be seen that it does not pass through the entire target slice. In particular, FIG.
In (a), the data loss region occurs biased toward the X-ray focal point side of the range (center of the slice) including the target slice position, and the data overlap region detects the range (center of the slice) including the target slice position. It occurs on the vessel side. FIG.
In FIG. 9 (b), the range in which the X-ray beam passes through the slice is a trapezoid that is symmetrical with respect to the XY direction orthogonal to the Z axis, and the target shoulder region is the data loss region. .

When FIG. 26 and FIG. 29 (a) are compared, FIG.
In FIG. 9 (a), the passage range of each X-ray beam is a trapezoid that is more biased than that in FIG. 26, and therefore the data loss region is also biased. That is, in FIG. 29A, the data loss region is largely generated in the central portion of the slice. Therefore, even if the adjacent interpolation is performed in the positional relationship shown in FIG. 29A (when the target slice position is the position shown in the drawing), the data loss region occupying the slice data at the target slice position The ratio becomes relatively large. Since the slice data of the target slice position includes a large data loss region, an artifact occurs in the obtained image of the slice position, and the accuracy of interpolation deteriorates.

Further, FIG. 30 shows fan beams for four slices in multi-slice CT. However, as in the case of two slices, a data loss region is biased toward the X-ray focal point side, and a data loss region is generated, so that the detector side is affected. Data overlap area occurs. Comparing FIG. 30 and FIG. 27, single slice CT
In FIG. 27, the data loss areas are evenly distributed along the Z-axis direction, so that the artifacts caused by the data loss areas do not appear strongly in a specific range and direction. However, since multi-slice CT (in FIG. 30, the data loss area is largely generated in one place (the central portion of the slice), the artifacts caused by the data loss area are strong in a specific range and direction. It has appeared.

The present invention has been made in view of the above-mentioned circumstances, and by recovering the data loss area generated when performing interpolation in the helical scan in the multi-slice CT, the data loss area is caused by the data loss area. An object of the present invention is to provide a helical scan projection data creation method and an X-ray CT apparatus, which prevent generated artifacts from appearing strongly in a specific range and direction and contribute to improvement of image quality.

[0030]

The present invention has been made to achieve the above object, and is characterized in that a scanning beam and an opposite beam opposite to the scanning beam are averaged to perform mutual averaging. The purpose is to recover the lost data area.

That is, in the helical scan projection data creating method according to the first aspect, the supporting means for supporting the subject, the X-ray tube for irradiating the subject with an X-ray beam, and the X-rays A two-dimensional array comprising a tube and an X-ray detector arranged at a position opposed to each other with the subject in-between, wherein the X-ray detector has a plurality of detection element rows each having a plurality of detection channels arranged along the body axis direction. Helical scan projection in an X-ray CT apparatus, which is a detector and includes a gantry that rotatably supports the X-ray tube around the subject while holding the X-ray tube facing the X-ray detector. In the data creating method, the X-ray tube is rotated and moved through the gantry, and one of the support means and the gantry is connected to the subject while the X-ray tube is rotated and moved. By linearly moving along the axial direction, helically scanning the subject with the X-ray beam emitted from the X-ray tube; and the helical scan via the X-ray detector. The X
Pre-processing the helical scan data obtained for each rotational phase of the ray tube, collecting as projection data, and a step of creating interpolation projection data for each rotational phase of the target slice position from the projection data The step of creating the interpolated projection data for each of the rotation phases is performed by interpolation processing using the projection data of a pair acquired based on the X-ray beams of a pair adjacent to each other with the target slice position interposed therebetween. By the step of creating the first interpolation data for each rotational phase of the position and the interpolation processing using the projection data of the pair collected based on the counter beam of the pair opposed to the X-ray beam of the pair, Creating a second interpolation data for each rotation phase of the slice position, and the first interpolation data and the second interpolation data for each rotation phase. Simple averaging process to and a step of generating an interpolation projection data of the respective rotational phases.

Further, in order to achieve the above object, a helical scan projection data creating method according to a second aspect of the present invention is a supporting means for supporting a subject and an X-ray for irradiating the subject with an X-ray beam. A tube and an X-ray detector arranged at a position opposed to the X-ray tube with the subject in-between, the X-ray detector having a detection element array composed of a plurality of detection channels along the body axis direction. X-ray CT including a plurality of two-dimensional detectors arranged and a gantry that rotatably supports the X-ray tube around the subject while holding the X-ray tube facing the X-ray detector. In a method for creating projection data of helical scan in an apparatus, the X-ray tube is rotatively moved via the gantry, and any one of the supporting means and the gantry in a state where the X-ray tube is rotatively moved. By linearly moving the subject along the body axis direction of the subject so as to helically scan the subject with an X-ray beam emitted from the X-ray tube; Pre-processing the helical scan data obtained for each rotational phase of the X-ray tube via the X-ray detector and collecting it as projection data; and interpolation from the projection data for each rotational phase of the target slice position. A step of creating projection data, wherein the step of creating the interpolated projection data for each of the rotational phases is performed by converting the projection data of a pair acquired based on the X-ray beams of the pair adjacent to each other with the target slice position interposed therebetween. A step of creating first interpolation data of a rotation phase having a target slice position by the interpolation processing used, and a step of generating a pair of X-ray beams facing the X-ray beam of the pair. A step of creating second interpolation data of a rotation phase having the target slice position by interpolation processing using a pair of projection data collected based on the directed beams; the first interpolation data and the second interpolation data; The step of creating the interpolated projection data of the certain rotation phase by simple addition and averaging with the interpolated data, the step of creating the first interpolated data, the step of creating the second interpolated data, and the step of creating the interpolated projection data Is repeatedly performed while changing the above, and a step of creating the interpolated projection data for each rotation phase is provided.

Further, in order to achieve the above object, the helical scan projection data generating method according to the third aspect of the present invention is such that the supporting means for supporting the subject and the X-ray for irradiating the subject with an X-ray beam. The X-ray detector includes a X-ray tube and an X-ray detector that is arranged at a position facing the X-ray tube with the subject in between, and the X-ray detector includes a detection element array including a plurality of detection channels in the body axis direction. An X-ray including a plurality of two-dimensional detectors arranged along the X-ray tube, and a gantry that rotatably supports the X-ray tube around the subject while holding the X-ray tube facing the X-ray detector. In a method for creating projection data of a helical scan in a CT apparatus, the X-ray tube is rotatively moved via the gantry, and the support means and the gantry are installed while the X-ray tube is rotatively moved. Linearly moving one of them along the body axis direction of the subject to scan the subject helically with the X-ray beam emitted from the X-ray tube; and the helical scan. Pre-processing the helical scan data obtained for each rotational phase of the X-ray tube via the X-ray detector, and collecting it as projection data; and for each rotational phase of the target slice position from the projection data. And a step of reconstructing an image of a target slice position based on the interpolation projection data for each rotation phase, and a step of creating the interpolation projection data for each rotation phase Is a target slice by an interpolation process using projection data of a pair acquired based on X-ray beams of a pair adjacent to each other across the target slice position. By the step of creating the first interpolation data for each rotational phase of the position and the interpolation processing using the projection data of the pair collected based on the counter beam of the pair opposed to the X-ray beam of the pair, Creating a second interpolation data for each rotation phase of the slice position, and weighting the first interpolation data and the second interpolation data for each rotation phase according to the target slice position. And adding and averaging to create interpolated projection data for each rotation phase.

In particular, in the helical scan projection data creation method according to the fourth aspect, the interpolation projection data creation step includes the step of creating the first interpolation data with respect to the first interpolation data and the second interpolation data. X of the original pair of
Position of either one of the X-ray tube of each beam in the line beam (original beam) and the X-ray tube of each beam in the pair of opposing beams that is the source of the second interpolation data and the target slice position Weighting is applied according to the relationship, and the weighted first interpolation data and the second interpolation data are added and averaged for each rotational phase to create the interpolated projection data for each rotational phase. It is a step to do.

Further, in particular, in the helical scan projection data creating method according to the fifth aspect, the interpolation projection data creating step creates the interpolation projection data by linearly changing the weight based on the following equation. The helical scan projection data creating method according to claim 4, which is a step.

Interpolation projection data = (first interpolation projection data) × (1-γ) + (second interpolation projection data) × γ (1) where γ = min (x, y) ...... ( 2) x = | position of the X-ray tube of one of the original beams) − (target slice position) | / {(number of detector rows) × slice thickness × 1/2)} (3) y = | (target slice position)-(position of the X-ray tube of the other beam in the original beam) | / {(number of detector rows) × slice thickness × 1/2)} (4) Further, in the helical scan projection data creation method according to claim 6, the interpolation projection data creation step includes a positional relationship between the position of the X-ray tube of each beam in the pair of X-ray beams and the target slice position. And the weighting according to the rotation phase when the original beam adjacent to the target slice position is switched Even without a step of creating the interpolated projection data by varying a part nonlinearly.

On the other hand, according to the X-ray CT apparatus of the present invention for achieving the above object, the X-ray tube for irradiating the subject with the supporting means for supporting the subject and the X-ray beam. And an X-ray detector arranged at a position opposed to the X-ray tube with the subject interposed therebetween, the X-ray detector having a detection element array including a plurality of detection channels along the body axis direction. An X-ray CT apparatus including a plurality of two-dimensional detectors and a gantry that rotatably supports the X-ray tubes around the subject while holding the X-ray tubes in a state of facing the X-ray detectors. In which the X-ray tube is rotationally moved through the gantry, and one of the supporting means and the gantry is linearly moved along the body axis direction of the subject while the X-ray tube is rotationally moved. By moving to Helical scanning means for helically scanning the subject with an X-ray beam emitted from the X-ray tube, and the helical scan is obtained for each rotational phase of the X-ray tube via the X-ray detector. Data collection means for collecting helical scan data, interpolation projection data creation means for creating interpolation projection data for each rotational phase of the target slice position from the projection data, and for each rotational phase of the target slice position Image reconstruction means for reconstructing an image at the target slice position based on the interpolated projection data, wherein the interpolated projection data creation means is based on a pair of X-ray beams adjacent to each other across the target slice position. For each rotational phase of the target slice position by interpolation processing using the collected projection data of the pair based on the collected projection data of the pair A first interpolation data generating means for generating first interpolated data, X of the pair
Second interpolation data creating means for creating second interpolation data for each rotation phase of the target slice position by interpolation processing using a pair of opposed beams opposed to the line beam; the first interpolation data and the above The second interpolation data and the data creating means for creating the interpolated projection data for each rotational phase by simple arithmetic mean processing for each rotational phase.

According to the invention described in claim 1, 2, 3 or 7, the first data obtained from the projection data based on the X-ray beam (original beam) of the pair adjacent to each other with the target slice position sandwiched therebetween. Since the interpolation data and the second interpolation data obtained from the projection data based on the opposite beam opposite to the original beam of the pair are added and averaged to obtain the interpolated projection data, the data loss region of the original beam is The interpolation data of the opposite beam is used for recovery, and the data loss area of the opposite beam is recovered by the interpolation data of the original beam.

According to the invention described in claims 4 to 6, according to the positional relationship (for example, distance) between the target slice position and the X-ray tube of the original beam sandwiching the slice position, for example, (2) ) To (4), the value of γ is changed to linearly change the weight applied to the first interpolation data and the second interpolation data shown in (1), or the target slice At least a part of the weight is changed non-linearly depending on the positional relationship between the position and the slice position, for example, the positional relationship between the original beam and the X-ray tube and the rotation phase when the original beam adjacent to the target slice position is switched. As a result, the interpolation data (either the first interpolation data or the second interpolation data) based on the beam (the original beam or the counter beam) having a small data loss area near the target slice position is obtained. High projection data weights can be created. Therefore, the data lost area can be recovered more effectively, and the artifacts generated in the reconstructed image are significantly suppressed. As a result, the image quality is significantly improved.

[0040]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

(First Embodiment) An X-ray CT apparatus 1 shown in FIGS. 1 and 2 includes a gantry 2 and a bed 3.
It is a device driven by the -R method. A top plate 3a is disposed on the upper surface of the bed 3 so as to be slidable in the longitudinal direction (corresponding to the slice (body axis) direction (Z-axis direction)) of the top plate 3a. A patient who is a subject is placed on the upper surface. The top plate 3a is driven by a bed driving device (not shown) represented by an electric motor based on the control of the gantry / bed driving section 4 to open the diagnostic opening 2 of the gantry 2.
It is inserted into a so that it can move forward and backward. The bed 3 has a top plate 3a.
The bed is provided with a position detector (not shown) such as a potentiometer for detecting the position in the longitudinal direction of the bed with an electric signal.

The gantry 2 is provided with an X-ray tube 5 and an X-ray detector 6 which face each other with the patient inserted in the opening 2a interposed therebetween. In addition, between the X-ray tube 5 in the gantry 2 and the patient, the width of the X-ray beam emitted from the X-ray focal point of the X-ray tube 5 in the slice direction is narrowed down to obtain a fan beam of a desired width (divergence angle). Is defined as α).

The X-ray CT apparatus 1 also includes a high voltage generator 10 for supplying a high voltage to the X-ray tube 5. The high voltage supply to the X-ray tube 5 by the high voltage generator 10 is performed by a non-contact type slip ring mechanism. High voltage generator 10
An X-ray controller 11 for controlling the high voltage supply timing of the high voltage generator 10 is connected to the.

As shown in FIG. 3, the X-ray detector 6 is a two-dimensional detector (100 in FIG. 3) in which a plurality of detection elements having a plurality of detection channels are arranged in the Z-axis direction (column direction).
It shows an example of 0 channels and 10 columns).

The X-ray tube 5 and the X-ray detector 6 are arranged around a central axis parallel to the body axis direction of the patient inserted into the gantry 2 by a gantry drive device (not shown) based on the control of the gantry / bed drive unit 4. It is possible to rotate as a unit.

The weak current signal corresponding to the transmitted X-ray detected by the X-ray detector 6 is subjected to pre-processing such as A / D conversion processing and signal amplification processing in the data collecting section (DAS) 8. It is designed to be converted into digital projection data. A projection data correction / interpolation processing unit 12, a reconstruction processing unit 13, and a display device 14 are provided on the output side of the data collection unit (DAS) 8.

Further, the X-ray CT apparatus 1 is equipped with a system controller 15. This system control unit 15 is a CPU
Equipped with a computer circuit including a gantry / bed control unit 4, an X-ray controller 11, a projection data correction / interpolation processing unit 1
2 and the reconstruction processing unit 13. further,
An input unit 1 capable of inputting a scan mode (for example, a single scan mode or a helical scan mode), a size of an imaging region (FOV), scan setting conditions such as X-ray tube voltage and current, and the like to the system control unit 15.
6 are connected.

The system controller 15 controls the gantry / bed controller 4 and the X-ray controller 11 based on the scan setting conditions.
By controlling the feed amount and feed rate of the top plate 3a, the rotation speed of the gantry 2, the rotation pitch, the X-ray exposure timing, and the like via the X-ray, normal scanning and gantry rotation, X-ray exposure, and heaven exposure. By performing plate movement at the same time, it is possible to perform helical scan or the like in which data is acquired in a spiral shape.

The projection data correction / interpolation processing unit 12 has, for example, an arithmetic unit having a CPU and a memory for storing data, and is sent from the data collection unit 8 based on data such as scan conditions sent from the system control unit 15. Correction processing such as detector sensitivity correction and X-ray radiation hardening correction is performed on the projection data thus obtained. The corrected projection data is a parameter based on various scan conditions sent from the system control unit 15 such as the rotation phase (θ), the spread angle of the fan beam forming the fan beam, the position of the X-ray tube 5 of each fan beam, and the like. Is stored in each storage area of the memory. Then, the projection data correction / interpolation processing unit 12 reads the projection data from the memory by addressing the projection data in accordance with the parameter when executing the helical scan. Then, by the interpolation processing shown in FIG. 5, which will be described later, from the corrected projection data (data collected in a spiral for helical scanning) to projection data at a target slice position (one row of data orthogonal to the Z axis). ) Is created.

Here, the projection data correction / interpolation processing unit 12
The principle of the interpolation processing performed in 1. will be described. FIG.
Is a scanning beam (original beam) f of a certain rotation phase (θk)
The positional relationship between A and the counter beam fB facing the original beam fA is shown. According to FIG. 4, the original beam fA has a data loss area A, and the opposite beam fB also has a data loss area B. However, the original beam f
The counter beam fB passes through the data loss area A of A, and the original beam f passes through the data loss area B of the counter beam fB.
A is passing. That is, the two (the original beam and the opposite beam) have a mutual interpolation relationship, and the average of the two is restored to recover the data loss region, which is a feature of the interpolation processing performed by the projection data correction / interpolation processing unit 12. Is.

The reconstruction processing unit 13 has a high-speed image processing device and the like, and in response to a control command from the system control unit 15, the projection data that has been corrected and interpolated as necessary is converted into a FOV of a specified size. The image is reconstructed accordingly to generate an image at the target slice position. The reconstructed image is sent to the display device 14 and displayed.

Next, the overall operation of this embodiment will be described with reference to FIG.
This will be described with reference to FIG. In this embodiment, the X-ray detector has two detector rows (seg1 to seg2).
The case of using the detector will be described.

A helical scan mode, for example, an M size FOV (M), is input by the operator via the input unit 16.
When scan setting conditions such as a predetermined X-ray tube voltage value and current value, bed (top) feed rate, and (target) slice position s for image reconstruction are input, the system control unit 15
Controls the bed / cradle control unit 4 and the X-ray controller 11 to execute a helical scan. That is, the X-ray tube 5
The X-ray detector 6 and the X-ray detector 6 rotate integrally around the central axis, and the top plate 3
a continuously moves in the body axis direction at a predetermined speed. Then, the X-ray tube 5 and the X-ray detector 6 are integrally rotated, and the top plate 3 is
Simultaneously with the movement of a in the body axis direction, X-rays are radiated from the X-ray tube 5 at constant rotation angles (sampling angles), and projection data is collected by the X-ray detector 6 and the data collection unit 8. .

The collected projection data is sent to the projection data correction / interpolation processing unit 12.

The projection data correction / interpolation processing unit 12 performs correction processing such as detector sensitivity correction and X-ray radiation hardening correction on the sent projection data. Then, the correction processing is performed and the projection data stored in the memory is read out, and the processing shown in FIG. 5 is performed.

That is, the projection data correction / interpolation processing unit 1
Reference numeral 2 refers to scan conditions such as position data of the target slice position s sent from the system control unit 15 to select each rotation phase θ (n) from the corrected projection data.
For each (n is 1, 2, ..., Sampling number), the original beam f _U (θ (n) that is adjacent to the target slice position s is sandwiched.
), F _L (θ (n)) based projection data d _U (θ (n))
), D _L (θ (n)) and the original beams f _U (θ (n)), f
Opposing beam g _U (180 ° + θ) facing _L (θ (n))
(n) −α) to g _U (180 ° + θ (n) + α) {collectively referred to as g _U }, g _L (180 ° + θ (n) −α) to g
Projection data d _{U1 to} d _U2 {projection data facing d _U (θ (n))}, d _{L1 to} d _L2 {based on _L (180 ° + θ (n) + α) {collectively referred to as g _L } Projection data facing d _L (θ (n))} is selected and read from the memory.

For example, FIGS. 6A and 6B show scan diagrams in the present embodiment. In FIG. 6, the slice position of the counter beam is the position of the counter beam of the detector central channel.

6A and 6B, the rotation phase θ10
The original beam f _U (θ10), which is the lower beam adjacent to the target slice position s in
The upper beam adjacent to the target slice position s is s
original beam f _L detected by eg1 ([theta] 10), and based on the beam f _U ([theta] 10) and opposite beam g _U detected by the oppositely seg1 (180 ° + θ10-α ) ~g U (180 ° + θ10
+ Α) {collectively referred to as counter beam g _U }, original beam f
G _L (18) detected by seg2 in opposition to _L (θ10)
0 ° + θ10−α) to g _L (180 ° + θ10 + α) {collectively referred to as opposed beam g _L } are shown.

At the rotation phase θ10, the original beam f _U shown in the figure is obtained.
Projection data d _U detected by seg2 based on (θ10)
(Θ10), the projection data d _L (θ10) detected by seg1 is selected based on the original beam f _L (θ10), and
The projection data d _{U1 to} d _U2 detected by seg1 based on the counter beam g _U and the projection data d _{L1 to} d _L2 detected by seg2 based on the counter beam g _L are selected (step S1).

Subsequently, the projection data correction / interpolation processing unit 12
Is the original beam f _U (θ (n) selected for each rotation phase
) Based projection data d _U (θ (n)) and original beam f _L
The projection data d _L (θ (n)) based on (θ (n)) is linearly interpolated by the inverse ratio of the distance between each beam and the target slice position s, and the rotation phase θ (n) and slice First interpolation data d1t (θ (n)) at the position s is created (step S2).

Further, the projection data correction / interpolation processing unit 12
Is the projection data d _{U1 to} d _U2 based on the counter beam g _U selected for each rotation phase and the projection data d _{L1 to} d _L2 based on the counter beam g _L for each beam and the target slice position s. Rotation phase θ
(n) and the second interpolation data d2t (θ
(n)) is created (step S3).

The first interpolation data d1t (θ (n)) is the interpolation data created from the original beam, and the second interpolation data d2t (θ (n)) is the counter beam opposite to the original beam. Since the interpolation data is created from the above, the data loss area of the mutual beams can be recovered by averaging these interpolation data from the relationship of FIG.

That is, the projection data correction / interpolation processing unit 1
2 is the first interpolation data d1t (θ (n)) created by the process of step S2 and the second interpolation data d2t (θ (n)) created by the process of step S3.
For each rotation phase, the arithmetic mean is calculated according to the following equation (5) to obtain projection data of each rotation phase at the target slice position s (step S4).

Projection data d (θ (n)) = {first interpolation data d1t (θ (n)) + second interpolation data d2t (θ (n))} / 2 (5) The projection data correction / interpolation processing unit 12 sends the projection data d (θ (n)) created for each rotation phase to the reconstruction processing unit 13 by the interpolation processing of steps S1 to S4 described above (step S5). , The process ends.

The reconstruction processing unit 13 performs image reconstruction processing such as convolution back projection processing on the sent projection data d (θ (n)) to generate a reconstructed image, and this reconstruction image is generated. The image is sent to the display unit 14.
As a result, the reconstructed image at the target slice position s is displayed on the display unit 4.

As described above, according to this embodiment,
In the interpolation processing of the helical scan in the multi-slice CT, it is obtained from the first interpolation data obtained from the projection data based on the original beam sandwiching the target slice position and the projection data based on the counter beam opposite to the original beam. Since the projection data is interpolated by averaging the second interpolation data, the data loss area of the original beam is recovered by the interpolation data of the counter beam, and the data loss area of the counter beam is recovered by the interpolation data of the original beam. Recover. Therefore, in the obtained reconstructed image, the artifacts caused by the data loss regions of the mutual beams are significantly suppressed, and the image quality is improved.

In this embodiment, a detector having two detector rows is used as the X-ray detector, but the present invention is not limited to this, and a plurality of detector rows of two or more rows are used. X
A line detector may be used.

FIGS. 7A and 7B show, for example, four columns (s).
It is a figure which shows the scan figure of multi-slice CT of eg1-seg4). In FIG. 7, the slice position of the counter beam is the position of the counter beam of the detector central channel.

In FIGS. 7A and 7B, the rotation phase θ15
The original beam f _U3 (θ15), which is the lower beam adjacent to the target slice position s1 in
The upper beam adjacent to the target slice position s is s
original beam f _4L detected by eg4 (θ15) and original beam f _U3 (θ15) and facing opposite beam g _U1 detected by seg1 to (180 ° + θ15-α) ~g U1 (180 ° + θ15
+ Α) {collectively referred to as counter beam g _U1 }, original beam f
G _L2 (18 detected by seg2 facing the _{4L (θ15)}
0 ° + θ15-α) ~g L2 (180 ° + θ15 + α) { collectively and opposite beam g _L2} are shown, respectively.

FIG. 8 shows a region where the original beams f _{U1 to} f _U4 and the counter beams g _{L1 to} g _L4 pass through the slice.
According to FIG. 8, a data loss area C between the original beam f _U1, f _U4 is understood to have been restored by the opposing beam g _L3, g _L2 facing the original beam f _U1, f _U4, opposite beam It can be seen that the data loss area D due to g _L4 and g _L1 has been recovered by the original beams f _U2 and f _{U3 in the} opposing relationship.

Therefore, by the same interpolation method as that shown in FIGS. 5 to 6, the data loss area is recovered, and the reconstructed images obtained are generated due to the data loss areas of the mutual beams. The artifacts that were used are greatly suppressed.

Further, in the present embodiment, the equation of the addition average is the equation shown in the above (5), but the present invention is not limited to this. For example, the following equation (6) or (7)
Weighted addition may be performed as in the formula.

Projection data d (θ (n)) = first interpolation data d1t (θ (n)) × (1-β) + second interpolation data d2t (θ (n)) × β (6 ) Projection data d (θ (n)) = first interpolation data d1t (θ (n)) × β + second interpolation data d2t (θ (n)) × (1-β) (7) where β Is a constant value that satisfies 0 ≦ β ≦ 1.

The position of the opposite beam is determined by the X-ray detector 6
Not the slice position of the opposite beam of the center channel of
The slice position of each of the opposing beams may be used. FIG. 9 shows a scan diagram of four rows of multi-slice CT in this case. FIG. 9 shows original beams f _U3 and f _U4 sandwiching the target slice position s1 in the rotation phase θk.
Then, the counter beams g _L1 and g _L2 that oppose the original beam f _U3
And opposite beams g _L3 and g _L4 opposite to the original beam f _U4 are shown.

(Second Embodiment) The X-ray CT apparatus 1 of this embodiment is different from that of the first embodiment only in the processing of the projection data correction / interpolation processing unit 12, and other configurations and respective constituent elements. Since the operation of is similar to that of the 464 first embodiment, the description thereof will be omitted.

Here, the principle of the interpolation processing of the projection data correction / interpolation processing unit 12 in this embodiment will be described.

FIG. 10A shows an upper original beam f _U1 (detected by seg1) adjacent to the target slice position and a lower original beam f _L1 (detected by seg2) adjacent to the target slice position. the detected) and the opposite beam g _U1 and opposite beam g _L1 of the lower original beam f _L1 of the upper original beam f _U1 respectively show. In addition, FIG.
The upper original beam f _U2 (detected by seg2) and the lower original beam f _L2 (detected by seg1), the counter beam g _U2 of the upper original beam f _U2 , and the counter beam g _L2 of the lower original beam f _L2 Shown respectively.

In the beam positional relationship shown in FIG. 10 (a),
The data loss area E of the original beam spreads in the vicinity of the target slice position s2, and the data loss area F of the opposite beam is separated above and below the target slice position. Therefore, in this positional relationship, the interpolation data d2t 'created by interpolating the opposite beams g _U2 and g _L2 is superior.

On the other hand, in the beam positional relationship shown in FIG. 10B, conversely, the data loss region G of the original beam is separated above and below the target slice position, and the data loss region H of the opposite beam is It spreads near the target slice position. Therefore, in this positional relationship, the interpolated data d1t 'created by interpolating the original beams f _U2 and f _L2 is superior.

For the above reason, the projection data correction / interpolation processing unit 12 in this embodiment performs the process shown in FIG. 11 instead of the process of step S4 of FIG. The processing up to step S3 is the same as that in the first embodiment, and the description thereof is omitted.

That is, the projection data correction / interpolation processing unit 1
2 represents the first interpolation data d1t (θ (n)) created by the process of step S2 and the second interpolation data d2t (θ (n)) created by the process of step S3 by the following equation ( 3), weighted addition averaging is performed, and projection data of each rotation phase of the target slice position s is obtained (step S).
4a).

Projection data d (θ (n)) = first interpolation data d1t (θ (n)) × (1-γ) + second interpolation data d2t (θ (n)) × γ (8 ) However, γ = min (x, y) (9) x (= weight 1) = {| (position of X-ray tube 5 of upper original beam)-(target slice position) |} / { (Number of detector rows) × (slice thickness) × 1/2)} (10) y (= weight 2) = {| (target slice position)-(lower original beam X-ray tube 5 Position) |} / {(number of detector rows) × (slice thickness) × 1/2)} (11) Note that “γ = min (x, y)” in the equation (8) is γ = x.
(X ≦ y), γ = y (x> y), that is, a function that compares variables x and y and selects a smaller value.

That is, the equation (8) shows that the first interpolation data d1t (θ (n)) and the second interpolation data d2t (θ (n)) are weighted according to the value of γ. Equations (9) to (11) represent the target slice position and the position of the X-ray tube of the upper original beam, and the target slice position and the X-ray of the lower original beam. It shows that it changes depending on the position of the tube.

It linearly changes according to the positions of the upper and lower original beams of the X-ray tube in the equations (8) to (11) and the positional relationship between the X-ray tube position and the target slice position. The γ curve (straight line) is shown in FIG. 12, and the weight "(1-γ)" of the scan diagram in this embodiment and the first interpolation data d1t (θ (n)) corresponding to the scan diagram.
The curve (straight line) is shown in FIG. In FIG. 13, the original beam f detected by seg1 is the lower beam adjacent to the target slice position s2 in the rotation phase θx.
_x1 (θx), the original beam f _x2 (θx which is the upper beam adjacent to the target slice position s2 and is detected by seg2
) And the original beam f _x1 ([theta] x) and facing opposite beam g _x1 detected by seg2 to (180 ° + θx -α) ~g x1
(180 ° + θx + α) { a collectively opposite beam _{g x1}, g x2 (180} ° + θx -α) which is detected by seg1 to face with the original beam _{f x2 (θx) ~g x2 (} 180 ° +
θx + α) {collectively referred to as the counter beam g _x2 } is shown.

According to FIGS. 12 and 13, the value of the weight “(1-γ)” of the first interpolation data d1t (θ (n)) generated from the original beam is the right side (1 .0) (the value of γ approaches the left side (0.0) in FIG. 12 in FIG. 12), the weight of the first interpolation data d1t (θ (n)) becomes large, and conversely When approaching the left side (0.0) (value of γ in FIG. 12 → right side (0.0)), the weight of the second interpolation data d2t (θ (n)) generated from the opposite beam becomes large.

For example, in the positional relationship between the slice position shown in FIG. 10A and the X-ray tube (X-ray focus) of the original beam (slice position sa in FIG. 12), γ = 1.0, and (8)
From the formula: projection data d (θ (n)) = first interpolation data d1t (θ (n)) × (1-1) + second interpolation data d2t (θ (n)) × 1 = second interpolation The data becomes d2t (θ (n)) (12). In other words, the weight “(1-γ)” of the first interpolation data d1t (θ (n)) becomes 0, and the data loss region is created by interpolating the counter beam that is rarely present in the vicinity of the target slice position. The second interpolation data d2t (θ (n)) becomes the projection data d (θ (n)).

Further, for example, in the positional relationship between the slice position shown in FIG. 10B and the X-ray tube (X-ray focus) of the original beam (slice position sb in FIG. 12), γ = 0.0,
From equation (8), projection data d (θ (n)) = first interpolation data d1t (θ (n)) × (1-0) + second interpolation data d2t (θ (n)) × 1 = first The interpolation data of 1 is d1t (θ (n)) (13). That is, the weight “γ” of the second interpolation data d2t (θ (n)) becomes 0, and the first interpolation created by interpolating the original beam in which the data loss region is almost not present near the target slice position. The data d1t (θ (n)) is the projection data d
(Θ (n))

As described above, the weighting is linearly changed according to the distance between the target slice position and the X-ray tube of the original beam sandwiching the target slice position, as shown in FIG. 12 and FIG. A beam with a small data loss area near the slice position (original beam or counter beam)
It is possible to create projection data in which the weight of the interpolation data (first interpolation data or second interpolation data) based on is increased. Therefore, as compared with the first embodiment, the data lost area at the target slice position can be restored more effectively, and the artifacts generated in the reconstructed image can be significantly suppressed.

In the equation (8), the weight of the first interpolation data d1t (θ (n)) is set to “γ”, and the weight of the second interpolation data d2t (θ (n)) is set to “(1 -Γ) ". That is, projection data d (θ (n)) = first interpolation data d1t (θ (n)) × γ + second interpolation data d2t (θ (n)) × (1-γ) (14) Even if the projection data d (θ (n)) is obtained, the same effect as the above embodiment can be obtained (however, when γ → 0, the projection data d (θ (n)) → the second interpolation projection data d2t (Θ
(n)), and when γ → 1, projection data d (θ (n))
→ The first interpolation projection data d1t (θ (n))).

Further, in the present embodiment, the weights are changed according to the positional relationship between the target slice position and the X-ray tube of the original beam, but the present invention is not limited to this. The weighting may be changed in accordance with the positional relationship between the slice position and the opposite beam X-ray tube.

Further, in the present embodiment, the weights are linearly changed as shown in the equations (8) to (11) according to the position of the X-ray tube of the original beam, but the present invention is not limited to this. The point is that if the weight is linearly changed according to the position of the X-ray tube of the original beam and the weight of the interpolation data with less data loss area near the target slice position can be set higher. The setting of the weighting may be performed by using any formula.

Furthermore, in the present embodiment, the weighting is linearly changed according to the position of the X-ray tube of the original beam, but the present invention is not limited to this, and, for example,
Linear according to the position of the X-ray tube of the original beam (or the position of the X-ray tube of the opposite beam), the rotation phase of the switching position of the projection data based on the original beam, and the rotation phase where the weight of the opposite beam becomes "0". It may be changed dynamically and non-linearly.
The rotation phase of the projection data switching position based on the original beam is, for example, θxa shown in FIG.
The lower original beam adjacent to the target slice position s2 with the target slice position s2 as the boundary is the original beam (fx
1) is switched to the original beam (fx2 ′) detected by seg2 (thus, the projection data is also changed from the projection data corresponding to the original beam (fx1) detected by seg1 to se
Switching to projection data corresponding to the original beam (fx2 ') detected by g2). Similarly, the upper original beam adjacent to the target slice position s2 at the boundary of θxa is se.
The original beam (fx2) detected by g2 is switched to the original beam (fx1) detected by seg1 (projection data is also s
The projection data corresponding to the original beam (fx2) detected by eg2 is switched to the projection data corresponding to the original beam (fx1) detected by seg2).

Θxb in FIG. 13 also indicates the rotational phase of a similar projection data switching position, and such a projection data switching gap used when generating interpolation data may become an artifact (see FIG. 13 out of A1, A
2).

Further, as shown in FIG. 10A, the upper original beam f _U1 detected at seg1 is adjacent to the target slice position, and the upper original beam f _U1 detected at seg1 is detected at seg2. the positional relationship between the lower source beam f _L1 is, the X-ray focal point and the X-ray focal point of the lower original beam f _L1 of the upper original beam f _U1 is spaced apart from the slice location of interest. Therefore, in the rotation phase having such a beam positional relationship, if projection data based on the original beam is used as interpolation data, the data loss region shown in FIG. 10A causes an artifact. The data loss region in the rotation phase in which the X-ray focus of the original beam has a positional relationship away from the target slice position is indicated by "B1" in FIG.

Similarly, the data loss area in the rotational phase in which the X-ray focal point of the opposite beam shown in FIG. 10 (b) has a positional relationship apart from the target slice position is shown by "B2".

In the switching gap and the data loss area and their vicinity as described above, if interpolation data is generated without using the projection data of the beam which is the source of the switching gap and the data loss area, reconstruction with better image quality can be achieved. Images can be generated. Therefore, an example in which the weighting is changed according to the position of the switching gap or the data loss area described above will be shown below.

(1) A method of weighting so as to avoid only the rotation phase where the data loss area is generated and its vicinity.

FIG. 14 shows a scan diagram for explaining the weighting method and a weight “(1-γ)” curve (straight line) of the first interpolation data d1t (θ (n)) corresponding to the scan diagram. . In FIG. 14, the lower beam adjacent to the target slice position s3 in the rotation phase θy is represented by s
eg1 original beam f _y1 detected in ([theta] y), the original beam f _y2 ([theta] y) and the original beam f _y1 (theta detected by seg2 be beams of the upper adjacent to the slice position s3 object
opposite beams g _y1a and g _Y1b detected by seg1 and seg2 facing a y), g _y2a and g detected by seg1 and seg2 opposite to the original beam f _y2 ([theta] y)
_y2b is shown respectively.

According to FIG. 14, at the projection angle θx0 of the region (B2) where the data loss region is caused by the projection data of the opposite beam and its vicinity, the weighting of the projection data of the opposite beam becomes 0.0. The curve is changed linearly, and further, in the projection angle θxc of the region (B1) where the data loss region is caused by the projection data of the original beam and its vicinity, the weighting of the projection data of the original beam is approximately 0.0.
The weighting curve is changed non-linearly so that In the case of this example, the influence of the switching gap in the original beam is small and is ignored.

(2) A method of weighting so as to avoid only the rotational phase in which the switching gap occurs and its vicinity.

FIG. 15 shows a scan diagram for explaining the weighting method, and a weight “(1-γ)” curve (solid line) of the first interpolation data d1t (θ (n)) corresponding to the scan diagram. . In FIG. 15, the lower beam adjacent to the target slice position s3 at the rotation phase θy is represented by s
eg1 original beam f _z1 detected in ([theta] z), the original beam f _z2 ([theta] z) and original beam f _z1 (theta detected by seg2 be beams of the upper adjacent to the slice position s4 object
opposite beams g _Z1a and g _Z1b detected by seg1 and seg2 facing a z), g _Z2a and g detected by seg1 and seg2 opposite to the original beam f _z2 ([theta] z)
Each _z2b is shown.

According to FIG. 15, the projection data of the original beam is weighted to approximately 0.0 at the projection angle θza where the projection data switching gaps A1 and A2 of the original beam are generated and the vicinity thereof and θzb and the vicinity thereof. The weighting curve is changed nonlinearly. In the case of this example, the influence of the data loss region is small due to the projection data of the original beam and the opposite beam, and is ignored.

(3) A method of weighting so as to avoid the rotation phase and its vicinity where the data loss area and the switching gap occur.

FIG. 15 is a scan diagram for explaining the weighting method, and the weight “(1-γ)” curve (straight line) of the first interpolation data d1t (θ (n)) corresponding to the scan diagram.
(Weighting curve G1, broken line in the figure) is shown. As shown by the broken line in FIG. 15, at the projection angle θz0 of the region (B2) where the data loss region occurs due to the projection data of the opposite beam and in the vicinity thereof, the weighting of the projection data of the opposite beam is approximately 0.
The weighting curve is changed non-linearly so that the weighting curve of the original beam is approximately 0.0 at and near the projection angle θza at which the projection data switching gap A1 of the original beam is generated. Is changed nonlinearly and linearly. Then, in the projection angle θzc of the area (B1) where the data loss area is generated by the projection data of the original beam and its vicinity, the weighting curve is linearly changed so that the weighting of the projection data of the original beam becomes 0.0. , Original beam projection data switching gap A2
At the projection angle θzb where is generated and the vicinity thereof, the weighting curve is changed nonlinearly and linearly so that the weighting of the projection data of the original beam becomes approximately 0.0.

In addition, FIG. 16 shows a scan diagram for explaining the weighting methods of (2) and (3) above, and the weight “(1 of the first interpolation data d1t (θ (n)) corresponding to the scan diagram. −γ) ”curve (straight line). In FIG. 16, the lower beam adjacent to the target slice position s5 at the rotation phase θp, the original beam f _p1 (θp) detected by seg1, and the upper beam adjacent to the target slice position s5 are detected. Original beam f _p2 which is a beam and is detected by seg2
(Θp) and the original beam f _p1 (θp) in opposition to seg1
And seg2 are opposite beams g _p1a and g _p1b , and g _p2a and g _p2b are detected respectively at seg1 and seg2 in opposition to the original beam f _p2 ( _{θ p} ). In FIG. 16, the weighting curve by the above-mentioned weighting method (2) is shown by a solid line, and the weighting curve by the above-mentioned weighting method (3) is shown by a broken line (arrow G2).

Although the above method can be applied to the gap of the counter beam, the frequency of the gap of the counter beam is smaller than the frequency of the gap of the original beam, and therefore the adverse effect of the gap is small. Conceivable.

By the way, in the above-described first and second embodiments, the first interpolation data creating process, the second interpolation data creating process, and the interpolated projection data creating process are performed on the original beam for each rotation phase and this. Although it was performed simultaneously for each projection data group based on the opposite beam opposite to the original beam, the present invention is not limited to this. For example, the rotation phase θ
First, interpolated projection data for the rotation phase θ (1) is created by the projection data group based on the original beam of (1) and the opposite beam that opposes the original beam.
Projection data may be created by sequentially interpolating θ (3), ..., θ (n) for each rotation phase.

Further, in the first and second embodiments, the bed is moved in the slice direction to perform the helical scan, but the gantry may be moved in the slice direction to perform the helical scan.

[0109]

As described above, according to the helical scan projection data creating method and the X-ray CT apparatus of the present invention,
The data loss area of the X-ray beam (original beam) of the pair adjacent to each other across the target slice position is recovered by the interpolation data of the opposite beam of the pair opposite to the original beam of the pair, and the data loss of the opposite beam of the pair is recovered. Since the region is recovered by the interpolation data of the original beam of the pair, that is, the so-called mutual interpolation relationship, the first interpolation data obtained from the projection data of the original beam of the pair and the projection data of the opposite beam of the pair are used. By adding and averaging the obtained second interpolation data, it is possible to create the interpolation projection data in which the data loss areas of the beams are restored. Therefore, in the reconstructed image obtained by reconstructing the interpolated projection data, the artifacts caused by the data loss regions of the mutual beams are significantly suppressed, and as a result, the reconstructed image is reconstructed. Image quality is improved.

In particular, the weights applied to the first interpolation data and the second interpolation data are changed, for example, linearly according to the positional relationship between the target slice position and the X-ray tube of the original beam that sandwiches the slice position. As a result, the data loss area at the target slice position can be recovered more effectively, and artifacts that occur in the reconstructed image at the target slice position can be greatly suppressed. As a result, the quality of the reconstructed image is improved.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an external schematic configuration of an X-ray CT apparatus according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a functional schematic configuration of the X-ray CT apparatus according to the first embodiment of the present invention.

FIG. 3 is a perspective view showing an example of an X-ray detector according to the first embodiment.

FIG. 4 is a diagram schematically showing a positional relationship between an original beam and an opposite beam and a data loss region of these beams.

FIG. 5 is a schematic flowchart showing an example of processing of a projection data correction / interpolation processing unit in the first embodiment.

6A and 6B are scan diagrams in the first embodiment, and in particular, FIG. 6A is a scan diagram showing a relationship between an original beam and a target slice position;
FIG. 6B is a scan diagram showing the relationship between the counter beam and the target slice position.

7 (a) and 7 (b) are scan diagrams in a modified example (four-row multi-slice CT) of the first embodiment, and in particular, (a) shows an original beam and a target slice. FIG. 3B is a scan diagram showing the relationship with the position, and FIG. 6B is a scan diagram showing the relationship between the counter beam and the target slice position.

FIG. 8 is a diagram schematically showing regions where original beams and counter beams pass through a slice and data loss regions of those beams in a modified example.

FIG. 9 is a scan diagram of a four-row multi-slice CT in another modification.

10A and 10B are diagrams schematically showing a positional relationship between an original beam and a counter beam in the second embodiment, and a data loss region and a data duplication region of these beams.

FIG. 11 is a schematic flowchart showing a part of the processing of a projection data correction / interpolation processing unit in the second embodiment.

FIG. 12 is a diagram showing the positions of the upper and lower original beams of the X-ray tube, and the value of γ that linearly changes according to the positional relationship between the position of the X-ray tube and the target slice position.

FIG. 13 is a diagram showing a scan diagram according to the second embodiment and a value of γ that linearly changes according to a rotation phase in the scan diagram.

FIG. 14 is a scan diagram in a modified example of the second embodiment,
And a diagram showing the value of γ that changes non-linearly and linearly according to the rotation phase in the scan diagram.

FIG. 15 is a scan diagram in a modified example of the second embodiment,
And a diagram showing the value of γ that changes nonlinearly (solid line) and the value of γ that changes nonlinearly and linearly (broken line) according to the rotation phase in the scan diagram.

FIG. 16 is a scan diagram in a modified example of the second embodiment,
And a diagram showing the value of γ that changes nonlinearly (solid line) and the value of γ that changes nonlinearly and linearly (broken line) according to the rotation phase in the scan diagram.

FIG. 17 is a scan diagram in a helical scan.

FIG. 18 (a) is 360 ° in single CT.
A scan diagram showing a pair of fan beams (f1, f2) having a rotation phase θ when the interpolation method is used, FIG.
One pair of fan beams (f1, f2) in 4 (a)
The figure which shows the positional relationship of) typically.

FIG. 19 (a) is a fan beam f3 of a rotation phase θ in the case of using the counter beam interpolation method in a single CT.
And scan views showing scanning beams respective opposing beams fo constituting the fan beam f3, (b), the fan-beam f _b and its opposite beam f is detected by the center channel of the detector in FIG. 15 (a) _The figure which shows the positional relationship of b'typically.

FIG. 20 is a diagram showing the respective scanning beams f _a , f _b , and f _c of the fan beam f 3 having the divergence angle α and the opposing beams f _a ′,
f _b ', f _c' shows schematically.

FIG. 21 (a) is a diagram showing an X-ray detector in the case where the number of detector rows is one (single slice CT), and FIG. 21 (b) is in the case where the number of detector rows is two (multi-slice CT). The figure which shows an X-ray detector.

FIG. 22 (a) is a fan beam f with a rotation phase θ1 in the case of using the adjacent interpolation method in a multi-slice CT.
4 and f5 and fan beams f6 and f7 having a rotational phase θ2, (b) is a diagram schematically showing the positional relationship between the fan beams f4 and f5 in FIG. 22 (a), and (c) is The fan beam f in FIG.
The figure which shows the positional relationship of 6 and f7 typically.

FIG. 23 is a diagram schematically showing a state in which an X-ray focus, an X-ray detector, an X-ray fan beam, and an FOV are viewed from the Z-axis direction.

FIG. 24 is a diagram schematically showing each size of FOV.

FIG. 25 is a schematic view of a single slice CT viewed from a direction perpendicular to the Z axis.

FIG. 26 is a diagram schematically showing a fan beam for two slices in a single slice CT, a relationship between the fan beam and the slice, and a data loss region and a data overlapping region of those beams.

FIG. 27 is a diagram schematically showing a fan beam for four slices in single-slice CT, a relationship between the fan beam and the slice, and a data loss region and a data overlapping region of those beams.

28A and 28B show two detector rows (first row).
Schematic diagram of a multi-slice CT of a segment, a second segment) as viewed from a direction perpendicular to the Z axis.

FIG. 29 (a) shows s in multi-slice CT
The fan beam detected by eg1 is the upper side, and seg
2 is a diagram schematically showing an area where each fan beam passes through a target slice when the fan beam detected in 2 is on the lower side, (b) is a fan beam detected in seg2 in multi-slice CT Is the upper side,
The figure which shows typically the area | region which each fan beam passes in the case where the fan beam detected by seg1 is a lower side.

FIG. 30 is a diagram schematically showing a fan beam for four slices in multi-slice CT, a relationship between the fan beam and the slice, and a data loss region and a data overlapping region of these beams.

[Explanation of symbols]

 1 X-ray CT apparatus 2 Gantry 2a Diagnosis opening 3 Bed 3a Top plate 4 Stand / bed control unit 5 X-ray tube 6 X-ray detector 7 Collimator 8 Data acquisition unit 10 High-voltage generator 11 X-ray control unit 12 Projection data Correction / interpolation processing unit 13 Reconstruction processing unit 14 Display device 15 System control unit 16 Input unit

Claims (7)

[Claims] 1. A support means for supporting a subject, an X-ray tube for irradiating the subject with an X-ray beam, and a position opposed to the X-ray tube with the subject interposed therebetween. An X-ray detector, wherein the X-ray detector has a plurality of detection element rows each having a plurality of detection channels arranged along the body axis direction.
Of a helical scan in an X-ray CT apparatus that includes a dimensional detector and a gantry that rotatably supports the X-ray tube around the subject while keeping the X-ray tube facing the X-ray detector. In the projection data creating method, the X-ray tube is rotationally moved through the gantry, and one of the supporting means and the gantry is moved in the body axis direction of the subject while the X-ray tube is rotationally moved. By linearly moving along the X-ray tube, the object being helically scanned with the X-ray beam emitted from the X-ray tube; and the X-rays being scanned by the helical scan through the X-ray detector. Preprocessing the helical scan data obtained for each rotational phase of the tube, collecting as projection data, and the target slice position from the projection data And a step of creating interpolated projection data for each rotational phase, the step of creating interpolated projection data for each rotational phase is performed based on a pair of X-ray beams that are adjacent to each other across the target slice position. Creating a first interpolation data for each rotation phase of the target slice position by an interpolation process using the projection data of the pair, and X of the pair.
Creating a second interpolation data for each rotation phase of the target slice position by an interpolation process using a pair of projection data collected based on a pair of opposed beams opposite to a line beam; Of the helical scan and the second interpolation data and performing a simple averaging process for each rotation phase to create interpolation projection data for each rotation phase. How to make. 2. A support means for supporting a subject, an X-ray tube for irradiating the subject with an X-ray beam, and a position opposed to the X-ray tube with the subject interposed therebetween. An X-ray detector, wherein the X-ray detector has a plurality of detection element rows each having a plurality of detection channels arranged along the body axis direction.
Of a helical scan in an X-ray CT apparatus that includes a dimensional detector and a gantry that rotatably supports the X-ray tube around the subject while keeping the X-ray tube facing the X-ray detector. In the projection data creating method, the X-ray tube is rotationally moved through the gantry, and one of the supporting means and the gantry is moved in the body axis direction of the subject while the X-ray tube is rotationally moved. By linearly moving along the X-ray tube, the object being helically scanned with the X-ray beam emitted from the X-ray tube; and the X-rays being scanned by the helical scan through the X-ray detector. Preprocessing the helical scan data obtained for each rotational phase of the tube, collecting as projection data, and the target slice position from the projection data And a step of creating interpolated projection data for each rotational phase, the step of creating interpolated projection data for each rotational phase is performed based on a pair of X-ray beams that are adjacent to each other across the target slice position. Creating the first interpolation data of the rotation phase having the target slice position by the interpolation processing using the projection data of the pair, and the X of the pair.
Creating a second interpolation data of a rotation phase having a target slice position by an interpolation process using a pair of projection data collected based on a pair of opposed beams opposed to a line beam; A step of performing simple addition and averaging processing on the interpolated data and the second interpolated data to produce interpolated projection data of the certain rotation phase; the first interpolated data produced step, the second interpolated data produced step, and the interpolation A method of creating projection data for helical scan, comprising: repeatedly performing the projection data creating step while changing the rotation phase to create interpolation projection data for each rotation phase. 3. A support means for supporting the subject, an X-ray tube for irradiating the subject with an X-ray beam, and a position opposed to the X-ray tube with the subject interposed therebetween. An X-ray detector, wherein the X-ray detector has a plurality of detection element rows each having a plurality of detection channels arranged along the body axis direction.
Of a helical scan in an X-ray CT apparatus that includes a dimensional detector and a gantry that rotatably supports the X-ray tube around the subject while keeping the X-ray tube facing the X-ray detector. In the projection data creating method, the X-ray tube is rotationally moved through the gantry, and one of the supporting means and the gantry is moved in the body axis direction of the subject while the X-ray tube is rotationally moved. By linearly moving along the X-ray tube, the object being helically scanned with the X-ray beam emitted from the X-ray tube; and the X-rays being scanned by the helical scan through the X-ray detector. Preprocessing the helical scan data obtained for each rotational phase of the tube, collecting as projection data, and the target slice position from the projection data And a step of reconstructing an image of a target slice position based on the interpolation projection data for each rotation phase, and the interpolation projection for each rotation phase The step of creating data is performed by interpolation processing using projection data of a pair acquired based on X-ray beams of a pair adjacent to each other with the target slice position sandwiched between the first slice for each rotational phase of the target slice position. Creating interpolated data of the
Creating a second interpolation data for each rotation phase of the target slice position by an interpolation process using a pair of projection data collected based on a pair of opposed beams opposite to a line beam; Of the interpolated data and the second interpolated data are weighted according to the target slice position for each rotational phase, and the arithmetic mean is performed to create interpolated projection data for each rotational phase. A method for creating projection data for a helical scan, characterized in that 4. The interpolated projection data creation step is performed for the first interpolated data and the second interpolated data,
Which one of the X-ray tube of each beam in the pair of X-ray beams (original beam) that is the source of the first interpolation data and each of the beams of the opposite beam of the pair that is the source of the second interpolation data Weighting is performed in accordance with the positional relationship between one of the positions and the target slice position, and the weighted first interpolation data and the second interpolation data are added and averaged for each rotational phase. 4. The helical scan projection data creation method according to claim 3, which is a step of creating the interpolation projection data for each rotational phase. 5. The helical scan projection data creation method according to claim 4, wherein the interpolation projection data creation step is a step of creating the interpolation projection data by linearly changing a weight based on the following equation. . Interpolation projection data = (first interpolation projection data) × (1-
γ) + (second interpolation projection data) × γ where γ = min (x, y); x = | position of the X-ray tube of one of the original beams) −
(Target slice position) | / {(Number of detector rows) × Slice thickness × 1/2)}; y = | (Target slice position) − (X-ray tube position of the other beam of the original beam) ) | / {(Number of detector rows) × slice thickness × 1/2)} 6. The interpolation projection data creating step includes a step of determining a positional relationship between the position of the X-ray tube of each beam in the pair of X-ray beams and the target slice position and an original beam adjacent to the target slice position. 5. The projection data creation method for helical scan according to claim 4, which is a step of creating the interpolation projection data by changing at least a part of the weights non-linearly according to the rotation phase at the time of switching. 7. A support means for supporting the subject, an X-ray tube for irradiating the subject with an X-ray beam, and a position opposed to the X-ray tube with the subject interposed therebetween. An X-ray detector, wherein the X-ray detector has a plurality of detection element rows each having a plurality of detection channels arranged along the body axis direction.
An X-ray CT apparatus comprising a dimensional detector and a gantry that rotatably supports the X-ray tube around the subject while keeping the X-ray tube facing the X-ray detector. Rotating the X-ray tube through the X-ray tube, and linearly moving one of the supporting means and the gantry in the body axis direction of the subject while the X-ray tube is rotatingly moved. A helical scanning means for helically scanning the subject with an X-ray beam emitted from the X-ray tube, and a helical scan for each rotational phase of the X-ray tube via the X-ray detector. Data collection means for collecting the obtained helical scan data, and interpolation projection data for creating interpolation projection data for each rotational phase of the target slice position from the projection data. And an image reconstructing unit that reconstructs an image at the target slice position based on the interpolating projection data for each rotational phase of the target slice position. A first interpolation data for each rotation phase of the target slice position is created by an interpolation process using projection data of a pair acquired based on X-ray beams of a pair adjacent to each other across the slice position of Second interpolation for each rotational phase of the target slice position by interpolation processing using the interpolation data creation means and the projection data of the pair collected based on the opposed beams of the pair opposed to the X-ray beam of the pair. Second to create data
The interpolation data creating means and the data creating means for creating the interpolation projection data for each rotational phase by performing simple arithmetic mean processing of the first interpolation data and the second interpolation data for each rotational phase. An X-ray CT apparatus characterized in that