JP2013005949A - X-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce cone beam artifacts more than before.SOLUTION: In one embodiment, an X-ray CT apparatus includes an extraction processing part, a direction determination part, a correction amount adjustment part, and an image generation part. The extraction processing part extracts a bone region of a high X-ray absorption rate as a high absorption region from image data generated before main scanning. The direction determination part determines a straight line indicating the direction of the high absorption region as a direction line, on the basis of input information or the shape of the high absorption region. The correction amount adjustment part calculates the inclination angle of the advancing direction of a cone beam X-ray in the main scanning and the direction line, and adjusts the correction amount of correction processing of the cone beam artifact on the basis of the inclination angle. The image generation part detects the cone beam X-ray which a subject has transmitted in the main scanning, collects projection data on the basis of detection signals, and executes reconstruction processing and the correction processing based on the adjusted correction amount to the projection data to generate the image data.

Description

  Embodiments described herein relate generally to an X-ray CT apparatus (X-ray Computed Tomography: X-ray computed tomography apparatus).

  As an X-ray CT apparatus, an apparatus capable of generating volume data by reconstructing projection data collected by scanning a subject with cone beam X-rays is known. In computer tomography using a cone beam (hereinafter referred to as cone beam CT), cone beam reconstruction processing such as FDK (Feldkamp) reconstruction is used.

  In recent years, the number of X-ray detectors has increased, and the cone angle has increased accordingly. Due to the effect of the cone angle, cone beam artifacts appear prominently in the volume data generated based on cone beam reconstruction.

  Cone beam artifacts occur at various imaging sites. For example, in a sagittal cross-sectional image in which the spine is depicted, it is known that cone beam artifacts are generated from the vertebral body because of a steep CT value (Computed Tomography Number) gradient between the vertebral body and its periphery. Yes. For example, Patent Document 1 is known as a conventional technique for reducing this cone beam artifact.

  In Patent Literature 1, reference volume data is generated based on projection data collected by scanning with cone beam X-rays in imaging of the spine. Thereafter, the cone beam artifact from the vertebral body has a steep CT value gradient in the body axis (Z axis) direction, but based on the fact that the CT value gradient is gentle in the XY direction, the filtering process in the Z axis direction is performed. And XY plane filter processing are performed separately. Thereby, the cone beam artifact component included in the reference volume data is extracted, and the cone beam artifact is reduced by taking the difference between the cone beam artifact component and the reference volume data.

JP 2009-34478 A

  It is desirable that the cone beam artifact is as small as possible for accurate image diagnosis. For this reason, in the cone beam CT, a technique for further reducing cone beam artifacts than before has been demanded.

In one embodiment, the X-ray CT apparatus performs a main scan using cone beam X-rays, and includes an extraction processing unit, a direction determination unit, a correction amount adjustment unit, and an image generation unit.
In the bone region of the subject indicated by the image data generated by the detection of X-rays transmitted through the subject before the main scan, the extraction processing unit increases the bone region having a higher X-ray absorption rate than the adjacent bone region. Extract as an absorption region.
The direction determining unit determines a straight line indicating the direction of the high absorption region as a direction line based on the input information or the shape of the high absorption region.
The correction amount adjustment unit calculates a tilt angle between the representative straight line in the traveling direction of the cone beam X-ray in the main scan and the direction line, and adjusts the correction amount in the correction process of the cone beam artifact based on the tilt angle. .
The image generation unit detects cone beam X-rays that have passed through the subject as the main scan, collects projection data based on the detection signal, and based on the reconstruction process and the correction amount adjusted by the correction amount adjustment unit By applying correction processing to the projection data, image data of the subject is generated.

1 is a block diagram showing the overall configuration of an X-ray CT apparatus according to the present embodiment. The typical perspective view which shows an example of a structure of a X-ray detector, and a definition of a cone angle. The schematic diagram which shows the simulation result of the cone beam artifact when X-ray is irradiated in parallel with the long side of this rectangular area supposing that the skull base as a high absorption area | region is a rectangular area. The schematic diagram which shows the simulation result of the cone beam artifact when the rectangular area | region of a skull base is inclined 10 degrees from FIG. FIG. 3 is a schematic diagram of FIG. 3 in which the skull base region is a hatched region and the cone beam artifact region is a hatched region. FIG. 5 is a schematic diagram of FIG. 4 in which the skull base region is a hatched region and the cone beam artifact region is a hatched region. Explanatory drawing which shows the angle of the said center axis | shaft and high absorption area | region in case the center axis | shaft of the advancing direction of cone beam X-rays corresponds to the perpendicular direction. Explanatory drawing which shows the angle of the said center axis | shaft and high absorption area | region when the center axis | shaft of the advancing direction of cone beam X-rays is inclined from the perpendicular direction. The schematic diagram which shows an example of the scout image of a head. The schematic diagram which shows an example which extracted and displayed the high absorption area | region from the scout image of FIG. The schematic diagram which shows an example of the automatic calculation method of the direction line which shows the direction with respect to the high absorption area | region extracted as one area | region like FIG. The schematic diagram which shows another example which extracted and displayed the high absorption area | region from the scout image of FIG. The schematic diagram which shows an example of the automatic calculation method of the direction line which shows the direction with respect to the high absorption area | region extracted as several area | region like FIG. The schematic diagram which shows an example of a display in case a user sets the direction line which shows the direction of a high absorption area | region manually. The graph which shows an example of the correction coefficient according to an inclination angle. The flowchart which shows an example of the flow of operation | movement of the X-ray CT apparatus which concerns on this embodiment.

  Hereinafter, an embodiment of an X-ray CT apparatus and an X-ray computed tomography method will be described with reference to the accompanying drawings. In addition, in each figure, the same code | symbol is attached | subjected to the same element and the overlapping description is abbreviate | omitted.

(Configuration of this embodiment)
FIG. 1 is a block diagram showing a configuration of an X-ray CT apparatus 20 in the present embodiment. As shown in FIG. 1, the X-ray CT apparatus 20 includes a gantry 22, an X-ray tube 24, a rotating unit 28, a bed 32, an X-ray detector 36, a high voltage generator 40, and a rotation driving unit. 44, bed control unit 48, system bus 52, data acquisition system (hereinafter referred to as DAS) 56, reconstruction unit 60, extraction processing unit 64, direction determination unit 68, correction amount The adjustment unit 72, the correction unit 80, the storage unit 84, the input unit 88, the system control unit 92, and the display unit 96 are provided.

The gantry 22 rotatably supports an annular or disk-shaped rotating unit 28.
The bed 32 is inserted into an opening (not shown) provided at the center of the rotating unit 28, and the subject P is placed on the bed 32.

  In the rotating unit 28, the radiation port of the X-ray tube 24 and the X-ray detector 36 are arranged so as to face each other with the subject P in between.

  The X-ray detector 36 has a configuration in which multi-channel detection elements are arranged in an arc shape, and detects X-rays irradiated from the X-ray tube 24 and transmitted through the subject P.

  The rotation drive unit 44 drives the rotation unit 28 based on the drive control signal input from the system control unit 92, and moves the X-ray tube 24 and the X-ray detector 36 supported by the rotation unit 28 around the subject P. Rotate continuously with.

  The bed control unit 48 controls the position of the bed 32 based on the bed control signal input from the system control unit 92.

  The high voltage generator 40 is connected to a high voltage cable of the X-ray tube 24 via a slip ring (not shown). The high voltage generator 40 supplies a predetermined tube current and tube voltage to the X-ray tube 24 based on the X-ray control signal supplied from the system control unit 92.

  The input unit 88 includes input devices such as a keyboard, a mouse, a display panel, and a selection button, and the operator inputs information on the subject P and conditions for collecting projection data in the input unit 88 before collecting projection data. The reconstruction condition, the image display condition, etc. are set. The conditions set in this way are input to the system control unit 92.

  The system control unit 92 controls each unit of the X-ray CT apparatus 20 in accordance with various setting conditions input from the input unit 88. The system control unit 92 is connected to a control unit (not shown) of the X-ray tube 24 and controls imaging using the cone beam X-rays by the X-ray tube 24.

  The DAS 56 integrates an output from each detection element of the X-ray detector 36 with time, a multiplexer that takes the output of the integrator in a channel unit at high speed and serially, and an A that converts the output signal of the multiplexer into a digital signal. / D converter. The DAS 56 collects projection data reflecting the X-ray transmittance for each X-ray path detected by the X-ray detector 36 based on the data acquisition control signal input from the system control unit 92, and reconstructs this. To the unit 60 and the storage unit 84.

  The reconstruction unit 60 performs cone beam reconstruction on projection data collected on a plurality of slice planes of the subject P, and generates image data. The image data may be multi-slice image data or volume data, but here is assumed to be volume data as an example. The volume data is three-dimensional image data, that is, image data in a thick range. For example, each voxel (each pixel) has a CT value (pixel value) that defines its luminance level.

  In the bone region of the subject P indicated by the image data of the scout image obtained by the scout scan performed before the main scan, the extraction processing unit 64 selects a bone region having a higher X-ray absorption rate than the adjacent bone region as “high”. Extracted as “absorption region”.

  The direction determining unit 68 determines a straight line indicating the (longitudinal) direction of the high absorption region as a “direction line”.

  The correction amount adjustment unit 72 calculates a tilt angle between a representative straight line in the traveling direction of the cone beam X-ray in the main scan (for example, the central axis of the cone beam X-ray) and the direction line, and corrects the cone beam artifact. A correction coefficient in the correction process is determined based on the tilt angle.

  Based on the correction coefficient determined by the correction amount adjustment unit 72, the correction unit 80 performs cone beam artifact correction processing on the volume data reconstructed by the reconstruction unit 60.

  The storage unit 84 stores projection data collected for a plurality of slice planes of the subject P. The storage unit 84 stores the volume data generated by the reconstruction unit 60 and inputs the volume data to the display unit 96 in accordance with a command from the system control unit 92.

  The display unit 96 generates display data by adding incidental information such as patient information to the volume data, thereby displaying an X-ray CT image on a monitor (not shown).

  FIG. 2 is a schematic perspective view showing an example of the configuration of the X-ray detector 36 and the definition of the cone angle. The X-ray tube 24 can generate cone beam X-rays upon application of a high voltage from the high voltage generator 40 and supply of a filament current. The cone beam X-ray is an X-ray beam having a large cone angle, and generally has a quadrangular pyramid shape.

  As shown in FIG. 2, the cone beam X-ray generated from the X-ray focal point F of the X-ray tube 24 is detected by the X-ray detector 36. The X-ray detector 36 has a plurality of X-ray detection elements 36a that are densely distributed in both the channel direction and the column direction (in this example, the Z-axis direction of the apparatus coordinate system described later).

  In other words, the X-ray detector 36 has a plurality of X-ray detection element arrays 36r arranged along the column direction. Each X-ray detection element row 36r has a plurality of X-ray detection elements 36a arranged in the channel direction. As an example, the spread angle of the cone beam X-ray in the channel direction is defined as a fan angle α, and the spread angle of the cone beam X-ray in the column direction is defined as a cone angle β.

(Principle of this embodiment)
In general, since bone contains calcium, which is a metal, it has a higher X-ray absorption rate (a higher CT value) than other body tissues such as muscles and blood, so it is whiter than other body tissues in an X-ray CT image. Projected. For example, when the skull base portion of the skull is viewed in a two-dimensional cross-sectional image, the bones are denser than the surroundings, and the amount of bone per unit area is larger than the surroundings. That is, the gradient of the CT value (X-ray absorption rate) is large between adjacent bone regions.

  When cone beam reconstruction is performed, if there is a region where the gradient of the CT value is steep as described above, cone beam artifacts are likely to occur without compensating for the steep change during reconstruction. For example, in a head scan, a cone beam artifact is generated from the skull base portion because the skull base portion of the skull has more bone than the adjacent bone region.

  The present inventor pays attention to the relationship between the irradiation direction of cone beam X-rays to a region (high absorption region) having a higher X-ray absorption rate than the adjacent bone region, such as the skull base, and the size of the cone beam artifact. did.

  FIG. 3 is a simulation result of cone beam artifacts when X-rays are irradiated in parallel to the long side of the rectangular region, assuming that the skull base as the high absorption region is a rectangular region.

  FIG. 4 is a simulation result of cone beam artifacts when the rectangular region of the skull base is tilted by about 10 ° from FIG. In this case, the inclination angle between the X-ray traveling direction and the long side of the rectangular region is 10 °.

  3 and 4 show the X-ray detection element row 36r in the endmost row on the near side or the back side in FIG. 2 (the X-ray detection element blackened by hatching in FIG. 2 than the other X-ray detection element rows 36r). The detection result of column 36r) is assumed.

  FIG. 5 is a schematic diagram of FIG. 3 in which the skull base region is a hatched region and the cone beam artifact region is a hatched region.

  FIG. 6 is a schematic diagram of FIG. 4 in which the skull base region is a hatched region and the cone beam artifact region is a hatched region.

  As can be seen by comparing FIG. 3 and FIG. 5 with FIG. 4 and FIG. 6, when the high absorption region (the skull base) extends in parallel to the X-ray traveling direction, the cone beam artifact becomes strong. On the other hand, when the high absorption region is inclined with respect to the X-ray traveling direction as shown in FIGS. 4 and 6, the cone beam artifact is weakened.

  Thus, the inventor has clarified that the intensity of the cone beam artifact depends on the inclination angle between the high absorption region and the X-ray traveling direction. The position and shape of the high absorption region vary widely depending on the subject, and the angle of the cone beam X-ray traveling direction with respect to the high absorption region varies depending on the imaging. On the other hand, if a certain cone beam artifact correction process based on clinical data is performed regardless of the angle of the traveling direction of the cone beam X-ray with respect to the high absorption region as in the prior art, the cone beam artifact cannot be corrected. .

  The present inventor who paid attention to the above points is an epoch-making in that the correction amount is further adjusted by multiplying a correction coefficient corresponding to the inclination angle based on the inclination angle between the high absorption region and the X-ray traveling direction. Devised the right way. Thereby, cone beam artifacts can be further reduced as compared with the conventional case. Therefore, after describing an example of the definition of the tilt angle and the apparatus coordinate system, an example of the definition of the tilt angle between the high absorption region and the traveling direction of the cone beam X-ray will be described.

  Normally, the gantry 22 is in a state perpendicular to the bed 32 that moves horizontally (see FIG. 7 described later). That is, the plane formed by the X-rays radiated from the X-ray tube 24 to the X-ray detector 36 is in a state perpendicular to the moving axis of the bed 32 (usually the Z axis of the apparatus coordinate system). However, the gantry 22 can have a tilt angle φ with respect to the moving axis of the bed 32 according to the imaging method. That is, the plane formed by the X-rays radiated from the X-ray tube 24 to the X-ray detector 36 is not perpendicular to the moving axis of the bed 32 but tilts from the vertical plane with a tilt angle φ.

  Here, as an example, the X-axis, Y-axis, and Z-axis of the apparatus coordinate system of the X-ray CT apparatus 20 are defined as follows. It is assumed that the normal direction of the table surface of the bed 32 is the vertical direction and the Y-axis direction, and the bed 32 moves in the Z-axis direction perpendicular thereto. A direction orthogonal to the Z-axis direction and the Y-axis direction is taken as an X-axis direction.

  Further, the upward direction in the vertical direction is the Y-axis positive direction, and the insertion direction of the bed 32 into the gantry 22 is the Z-axis positive direction. In the following description, for simplification of the description, as an example, the subject P is set on the bed 32 so that the body axis (the direction in which the spine extends) matches the Z-axis direction.

  7 and 8 are explanatory diagrams showing the angle between the high absorption region and the traveling direction of the cone beam X-ray, and the difference between FIGS. 7 and 8 is only the traveling direction of the cone beam X-ray. That is, FIG. 7 shows a case where the central axis (thick arrow in FIG. 7) in the traveling direction of the cone beam X-rays coincides with the vertical direction, and corresponds to a case where the tilt angle φ is 0 °. FIG. 8 shows a case where a tilt angle φ is given, and the central axis (thick arrow in FIG. 8) in the traveling direction of the cone beam X-ray is tilted from the vertical direction which is the reference axis.

  Here, as an example, the tilt angle φ is an angle from the Y axis to the central axis of the cone beam X-ray with the clockwise direction as the positive direction, as shown in FIG. Further, since the tilt angle φ is an angle where two straight lines intersect, it is assumed to be in a range of −90 ° or more and 90 ° or less. In addition, the code | symbol 24a in FIG. 7, FIG. 8 is the bulb | ball 24a of the X-ray tube 24. FIG.

  Further, in FIGS. 7 and 8, as an example, it is simplified that the high absorption region is the skull base and the skull base region is a rectangular region. 7 and 8, the angle from the Y axis to the direction line (broken line in the figure) indicating the longitudinal direction of the high absorption region is defined as a deviation angle θ, with the counterclockwise direction being the positive direction. Since the deviation angle θ is also an angle at which two straight lines intersect, it is assumed that it is in a range of −90 ° to 90 °. Here, the direction line coincides with the direction of the long side because the high absorption region is rectangular here, but details of how to determine the direction line will be described later.

  In the example of FIG. 8, both the deviation angle θ and the tilt angle φ are positive values less than 90 °. In the present embodiment, as an example, the inclination angle γ between the central axis of the cone beam X-ray in the main scan and the direction line of the high absorption region is calculated by the following equation (1).

γ = φ + θ (1)
That is, when the tilt angle γ is represented by the sum of the tilt angle φ and the shift angle θ, both are represented by the angle from the same reference axis (the Y axis of the apparatus coordinate system) and the positive direction of the tilt angle φ (in this example, the clock Rotation) and the positive direction of the deviation angle θ (counterclockwise in this example) are desirably opposite directions.

  Here, since both the tilt angle φ and the deviation angle θ vary within a range of ± 90 °, the tilt angle γ varies within a range of ± 180 ° according to the equation (1). However, in practice, since the inclination angle γ is an angle where two straight lines intersect, it is easier to express the inclination angle γ within a range of ± 90 °. Therefore, in this embodiment, as an example, when the inclination angle γ calculated by the expression (1) is −180 ° or more and less than −90 °, 180 ° is added to the inclination angle γ, and the inclination angle γ is set to 0 ° to 90 °. Convert to a value in the ° range. Similarly, when the inclination angle γ calculated by the equation (1) is larger than 90 °, 180 ° is subtracted from the inclination angle γ to convert the inclination angle γ to a value in the range of −90 ° to 0 °.

  Next, in order to determine the deviation angle θ in the equation (1), a method for extracting a high absorption region and a method for determining a direction line indicating the direction will be described.

  FIG. 9 is a schematic diagram illustrating an example of a scout image of the head, in which a region having a higher X-ray transmittance is whiter and a region having a lower X-ray transmittance is blacker. FIG. 9 is a sagittal cross-sectional image in the human body coordinate system. Here, the sagittal section refers to the front-rear direction of the subject with the ventral side in front and the back side as the back, the Y-axis direction, and the vertical direction of the subject with the head in the spine extension direction and the foot in the bottom direction Z It is a YZ plane in a human body coordinate system in which an axial direction is set and a direction perpendicular to the Y axis and the Z axis is set as an X axis direction.

  FIG. 10 is a schematic diagram showing an example in which a high absorption region is extracted and identified from the scout image of FIG. FIG. 10 shows an example in which the high absorption region is extracted as a non-discrete region, and the high absorption region 150 is indicated by a hatched region.

  When extracting the high absorption region, the extraction processing unit 64 first generates an image in which the bone region is selectively enhanced from the image data of the scout image by a conventional image processing technique such as contrast enhancement or edge extraction. After this preprocessing, the extraction processing unit 64 extracts the high absorption region 150 by performing threshold processing on the pixel values of all the pixels of the image data of the scout image, for example. In one configuration example of the image data, a pixel having a higher CT value has a higher pixel value and is displayed whiter. Therefore, for example, a region having a pixel value equal to or larger than a predetermined value may be set as the high absorption region 150.

  Note that the method of extracting the high absorption region is not limited to the above threshold processing. For example, in the case of head photography, when it is known that at least the base of the skull is a high absorption region, scout images and template matching based on a standard human skeleton model including the shape and size of each bone The region of the skull base may be extracted to perform a high absorption region.

  FIG. 11 is a schematic diagram showing an example of a method for automatically calculating the direction line Ld indicating the direction of the high absorption region 150 extracted as one non-discrete region as shown in FIG. In this example, first, the longest straight line Lmax as a straight line that does not protrude from the high absorption region 150 is calculated. Next, the high absorption region 150 is divided into two by a straight line Lver perpendicular to the longest straight line Lmax so that the area of the high absorption region 150 is equally divided into two. FIG. 11A shows this state.

  Next, the position of the center of gravity W1 of one of the two high absorption regions 150 (the upper side in FIG. 11B) is calculated. Similarly, the position of the center of gravity W2 on the other side (the lower side in FIG. 11B) of the high absorption region 150 divided into two is calculated. FIG. 11B shows a state where the positions of the centroids W1 and W2 are obtained. Then, as shown in FIG. 11C, a straight line passing through the centroids W1 and W2 is defined as a direction line Ld.

  FIG. 12 is a schematic diagram showing another example in which a high absorption region is extracted from the scout image of FIG. 9 and identified and displayed. FIG. 12 shows an example in which the high absorption regions are extracted as two discrete regions, and the high absorption regions 152 and 154 are indicated by hatched regions. The extraction method of the high absorption regions 152 and 154 is the same as described above.

  FIG. 13 is a schematic diagram showing an example of a method for automatically calculating the direction line Ld ′ indicating the orientation of the high absorption regions 152 and 154 extracted as a plurality of discrete regions as shown in FIG. is there. In this example, first, the longest straight line Lmax ′ whose one end and the other end are within the high absorption regions 152 and 154 is calculated. A part of the longest straight line Lmax ′ may protrude from the high absorption regions 152 and 154.

  Next, the high absorption regions 152 and 154 are divided into two by a straight line Lver 'perpendicular to the longest straight line Lmax' so that the total area of the high absorption regions 152 and 154 is divided into two equal parts. In this example, the sum of the area above the high absorption region 154 divided by the straight line Lver ′ and the area of the high absorption region 152 is equal to the area below the high absorption region 154 divided by the straight line Lver ′. Become. FIG. 13A shows this state.

  Next, the position of the center of gravity W <b> 1 ′ of the summed area of the divided high absorption area 154 and the high absorption area 152 is calculated. Similarly, the position of the lower center of gravity W2 'divided by the straight line Lver' is calculated. FIG. 13B shows this state. Then, as shown in FIG. 13C, a straight line passing through the centroids W1 and W2 is defined as a direction line Ld ′.

  In addition, also when a high absorption area | region is extracted as three or more discrete area | regions, a direction line can be determined with the same method.

  FIG. 14 is a schematic diagram illustrating an example of a display when the user manually sets a direction line indicating the direction of the high absorption region 150. The display unit 96 identifies and displays the high absorption region 150 extracted from the scout image, for example, using different colors. Specifically, for example, the region inside the subject excluding the high absorption region 150 is displayed in white, the region outside the subject is displayed in black, and the high absorption region 150 is identified and displayed in a chromatic color such as red. In FIG. 14, the high absorption region 150 is indicated by a hatched region.

  Further, the display unit 96 superimposes and displays the longest straight line (thick line in the drawing) whose one end and the other end are in the high absorption region 150 in a color different from that of the high absorption region 150. Further, the display unit 96 sets the center of the superimposed line as the temporary rotation center 160.

  The user can freely rotate the superimposed line by an input operation from the input unit 88 while visually grasping the distribution of the superabsorbent region 150 identified and displayed in different colors in the entire head image. A direction line Ld indicating the longitudinal direction of the high absorption region 150 can be determined. At this time, the user can freely change the position of the rotation center 160 via the input unit 88 including a keyboard and a mouse.

  The above is an example of a method for determining the direction line indicating the direction of the superabsorbent region 150, but the method for determining the direction line may be another method.

  For example, the longest straight line having one end and the other end in the high absorption region 150 may be used as the direction line Ld.

  If the direction line Ld can be determined in this way, the angle between the direction line Ld and the Y-axis direction can be calculated as the deviation angle θ. Therefore, if the tilt angle φ is determined, the center of the cone beam X-ray is determined according to the above-described equation (1). The inclination angle γ between the axis and the direction line of the high absorption region can be calculated.

  Next, the correction amount of cone beam artifacts according to the inclination angle γ will be described. The cone beam artifact is corrected by, for example, generating false image component volume data obtained by extracting only the cone beam artifact component from the reference volume data, and subtracting the false image component volume data from the reference volume data.

  Therefore, for example, just before the false image component volume data is subtracted from the reference volume data, the CT value of all the voxels (all pixels) of the false image component volume data is multiplied by a correction coefficient corresponding to the inclination angle γ, thereby generating a cone beam artifact. The amount of correction can be adjusted. The volume data is, for example, data in which all voxels (all pixels) have CT values (corresponding to pixel values) indicating luminance levels.

  FIG. 15 is a graph showing an example of a correction coefficient corresponding to the inclination angle γ. As described with reference to FIGS. 3 to 6, the cone beam artifact becomes weaker as the longitudinal direction of the high absorption region is tilted from the traveling direction of the X-rays. Therefore, the tilt defined in the range of ± 90 ° as described above. It is desirable to reduce the correction amount as the absolute value of the angle γ increases.

  Therefore, in the example of FIG. 15, when the tilt angle is −90 ° or + 90 °, the correction coefficient is set to 1 and the correction amount is not substantially adjusted. When the tilt angle is 0 °, the cone beam artifact is considered to be the maximum. Therefore, the correction coefficient is set to a maximum value of 2, and the correction amount of the cone beam artifact is maximized.

  The correction coefficient in FIG. 15 changes linearly in each range from −90 ° to 0 ° and from −0 ° to 90 °, but this is only an example. The correction coefficient may be another value, and may be changed stepwise according to the inclination angle γ, instead of changing linearly as shown in FIG. The correction coefficient is obtained in advance by, for example, an experiment or simulation in which the tilt angle γ is changed, and is stored in advance in the correction amount adjustment unit 72.

(Description of operation of this embodiment)
FIG. 16 is a flowchart showing an example of the operation flow of the X-ray CT apparatus 20 of the present embodiment. Hereinafter, the operation of the X-ray CT apparatus 20 will be described in accordance with the step numbers of the flowcharts shown in the drawings while referring to the respective drawings as appropriate.

  [Step S1] The system control unit 92 (see FIG. 1) performs initial setting of the X-ray CT apparatus 20 based on input information that defines a part of the imaging conditions input to the input unit 88. Here, “part of imaging conditions” refers to conditions such as imaging methods such as cone beam CT and imaging regions such as the head and chest. In this initial setting, body posture information of the subject P with respect to the X axis, Y axis, and Z axis of the apparatus coordinate system, imaging conditions including the imaging region, and the like are set. Here, as an example, it is assumed that head scanning is set as the imaging condition included in the input information. Thereafter, the process proceeds to step S2.

  [Step S2] The system control unit 92 controls each unit of the X-ray CT apparatus 20 to execute a scout scan of the subject P. Here, as an example, a scout scan is performed using a normal narrow beam (cone angle is extremely small, for example, 3 °) generated by focusing with a collimator. However, a scout scan is performed using a cone beam X-ray. May be.

  The image data of the scout image of the sagittal section is generated by the reconstruction unit 60 by the scout scan (see FIG. 9) and stored in the storage unit 84. Thereafter, the process proceeds to step S3.

  [Step S3] The system control unit 92 sets shooting conditions for the main scan such as a tilt angle based on the scout image and information input to the input unit 88. Thereafter, the process proceeds to step S4.

  Here, for convenience, the shooting conditions for the main scan are set in the order of step S3, but this is only an example. The process of step S3 may be performed in parallel with the following steps S4 to S6, which is a process of calculating the inclination angle γ between the direction line Ld of the high absorption region and the central axis of the cone beam X-ray. You may perform after step S4-S6.

  [Step S4] The extraction processing unit 64 acquires the image data of the scout image from the storage unit 84, and generates image data that selectively enhances the bone region by image processing such as contrast enhancement and edge extraction as preprocessing. To do.

  After the preprocessing, the extraction processing unit 64 extracts a high absorption region by performing threshold processing on the pixel values of all the pixels of the image data of the scout image, for example (see FIGS. 10 and 12). The extraction processing unit 64 inputs data indicating the position and distribution of the extracted high absorption region to the direction determination unit 68 together with the image data of the scout image. Thereafter, the process proceeds to step S5.

  [Step S5] The direction determining unit 68 automatically generates a direction line Ld indicating the (longitudinal) direction of the high absorption region based on the image data of the scout image and the distribution (shape) of the extracted high absorption region. Calculate automatically. Since the automatic calculation method of the direction line Ld has been described with reference to FIGS.

The direction determining unit 68 displays an image in which the extracted high-absorption region is identified and displayed on the scout image as shown in FIG. 14 on the display unit 96. As described above, the ( A direction line Ld indicating the direction (in the longitudinal direction) may be determined.
Thereafter, the process proceeds to step S6.

  [Step S <b> 6] The correction amount adjustment unit 72 acquires the tilt angle φ as one imaging condition for the main scan from the system control unit 92. Further, the correction amount adjustment unit 72 acquires the image data of the scout image and the data of the direction line Ld from the direction determination unit 68.

  The correction amount adjustment unit 72 is based on the tilt angle φ, the data of the direction line Ld, and the inclination of the central axis in the traveling direction of the cone beam X-ray in the main scan and the direction line Ld based on the above-described equation (1). The angle γ is calculated as a value in the range of ± 90 °. The correction amount adjustment unit 72 determines a correction coefficient corresponding to the calculated inclination angle γ from the table data of correction coefficients for each inclination angle γ stored in advance (see FIG. 15).

  The correction amount of the cone beam artifact is adjusted with respect to the volume data generated in the main scan by the correction coefficient determined here. Thereafter, the process proceeds to step S7.

  [Step S7] The system control unit 92 determines whether or not the tilt angle φ has been changed after the calculation of the tilt angle γ in step S6. If not changed, the process proceeds to step S8. If changed, the process returns to step S6, and the process of step S6 is executed again based on the changed tilt angle φ.

  [Step S <b> 8] The system control unit 92 controls each unit of the X-ray CT apparatus 20 based on the imaging conditions of the main scan set up to step S <b> 7 and executes the main scan using cone beam X-rays.

  Thereby, the cone beam X-ray radiated from the X-ray tube 24 passes through the subject P and is detected by the detection element of the X-ray detector 36. This processing is performed while the rotation unit 28 is driven by the rotation drive unit 44 and the X-ray tube 24 and the X-ray detector 36 supported by the rotation unit 28 are continuously rotated around the subject P.

  The DAS 56 collects projection data reflecting the X-ray transmittance for each X-ray path sequentially detected by the X-ray detector 36, and sequentially inputs the projection data to the reconstruction unit 60 and the storage unit 84. Thereafter, when the X-ray irradiation and the collection of projection data are completed, the process proceeds to step S9.

  [Step S9] The reconstruction unit 60 performs cone beam reconstruction processing, such as FDK reconstruction, on the collected projection data to generate reference volume data. The reference volume data is not subjected to cone beam artifact correction but includes cone beam artifacts. The reconstruction unit 60 stores the reference volume data in the storage unit 84. Thereafter, the process proceeds to step S10.

  [Step S10] In step S10, false image volume data is generated by selectively extracting only cone beam artifact components from the reference volume data.

  Specifically, for example, the correction unit 80 acquires reference volume data from the storage unit 84 and performs high-pass filter processing on the reference volume data. When the body axis of the subject P (extension direction of the spine) is substantially matched with the Z-axis direction of the apparatus coordinate system as in this embodiment (see FIGS. 7 and 8), the cone beam artifact of the head is The gradient of the CT value becomes steep in the Z-axis direction of the coordinate system, and the gradient of the CT value becomes gentle in the X-axis direction and the Y-axis direction of the apparatus coordinate system.

  Therefore, the high-pass filter process here is a high-pass filter process in the Z-axis direction of the apparatus coordinate system, and for example, selectively transmits only the spatial frequency component having a spatial frequency equal to or higher than the first predetermined value.

  The volume data in which the CT value gradient component in the Z direction is emphasized by the high-pass filter is hereinafter referred to as first intermediate volume data. That is, the first intermediate volume data includes an edge component of cone beam artifacts in the Z-axis direction, an edge component between living tissues, noise, and the like. The correction unit 80 stores the first intermediate volume data in the storage unit 84.

  Next, the correction unit 80 performs threshold processing on the first intermediate volume data, and stores the first intermediate volume data after the threshold processing in the storage unit 84 as second intermediate volume data. This threshold processing uses the CT value of the cone beam artifact component as a threshold. The second intermediate volume data has a steep CT value gradient in the Z-axis direction and includes a component having a CT value of a cone beam artifact component.

  Next, the correction unit 80 performs low-pass filter processing on the second intermediate volume data, and stores the second intermediate volume data after this processing in the storage unit 84 as false image component volume data. Here, the low-pass filter process is a low-pass filter process related to the XY plane of the apparatus coordinate system. For example, only a two-dimensional Gaussian filter process or a spatial frequency component whose spatial frequency is equal to or smaller than a second predetermined value is selectively transmitted. Processing.

  By applying a low-pass filter process to the XY plane, a component having a steep CT value gradient in the XY plane contained in the second intermediate volume data is suppressed. That is, the false image component volume data generated in this way is obtained by enhancing the cone beam artifact component by suppressing the biological component. Thereafter, the process proceeds to step S11.

  [Step S11] The correction unit 80 converts the false image component volume data by multiplying the CT values (pixel values of all pixels) of all voxels of the false image component volume data by the correction coefficient determined in step S6. Then, the converted false image component volume data is stored in the storage unit 84. The absolute value of the correction coefficient becomes smaller as the absolute value of the inclination angle γ increases, because the cone beam artifact becomes weaker. That is, the correction amount of the cone beam artifact is adjusted according to the inclination angle γ. Thereafter, the process proceeds to step S12.

  [Step S12] The correction unit 80 subtracts the converted false image component volume data from the reference volume data, and stores it in the storage unit 84 as corrected volume data. Thereafter, the process proceeds to step S13.

  [Step S13] The system control unit 92 causes the monitor of the display unit 96 to display a desired cross section of the head of the subject P obtained from the correction volume data in response to an input to the input unit 88. The above is the description of the operation of the X-ray CT apparatus 20 of the present embodiment.

  As described above, in this embodiment, after extracting a high-absorption region such as a skull base that is a source of cone beam artifacts, an inclination angle γ between the direction line indicating the longitudinal direction and the X-ray traveling direction is calculated. Then, based on the fact that the cone beam artifact becomes weaker as the absolute value of the tilt angle γ is larger, by multiplying the false image component volume data by the correction coefficient corresponding to the tilt angle γ, the correction amount of the cone beam artifact is appropriately set. adjust.

  Therefore, cone beam artifacts can be appropriately corrected according to the position and shape of the high absorption region that varies depending on the subject and the traveling direction of the cone beam X-ray that changes depending on the tilt angle φ. For this reason, the cone beam artifact is almost removed from the captured image of the cone beam CT, and the image quality can be improved.

  In addition, since the calculation of the tilt angle γ can be automatically performed by the extraction processing unit 64, the direction determination unit 68, and the correction amount adjustment unit 72, the user is not burdened with operation.

  Furthermore, since the tilt angle γ is calculated again when the tilt angle φ is changed, it is possible to flexibly cope with a change in imaging conditions immediately before the start of the main scan.

  On the other hand, in the prior art, a constant cone beam artifact correction process based on clinical data is performed regardless of the angle of the traveling direction of the cone beam X-ray with respect to the high absorption region, so that the cone beam artifact can be completely corrected. There wasn't.

  That is, according to the embodiment described above, the cone beam artifact can be further reduced as compared with the conventional case.

(Supplementary items of the embodiment)
[1] In the above embodiment, an example has been described in which a high-absorption region is extracted using image data obtained by a scout scan, and a correction coefficient in the main scan is determined. The embodiment of the present invention is not limited to such an aspect. If the image data includes the same imaging region of the same subject P obtained before execution of the main scan, the correction coefficient is determined in the same manner using the image data obtained in the previous imaging sequence instead of the scout image. May be.

  [2] In the above embodiment, as an example, the example of reducing cone beam artifacts in head scanning has been described. The embodiment of the present invention is not limited to such an aspect, and can be applied to other imaging regions. Cone beam artifacts are also prominently generated in the spine vertebral body and the edge component of the linear portion of the lung (eg, vasculature). The present embodiment is applicable to projection data and volume data including cone beam artifact components, and the imaging region is not limited.

  [3] As an adjustment method of the correction amount of the cone beam artifact according to the inclination angle γ, an example has been described in which after the forgery component volume data is generated, the forgery component volume data multiplied by the correction coefficient is subtracted from the reference volume data. This is merely an example of a cone beam artifact correction method and a correction amount adjustment method, and other methods may be used for the correction method and the correction amount adjustment method.

  [4] The functions of the X-ray detector 36, the DAS 56, the reconstruction unit 60, the correction unit 80, and the system control unit 92 are an example of the image generation unit described in the claims.

  [5] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

20 X-ray CT device 22 Gantry 24 X-ray tube 28 Rotating unit 32 Couch 36 X-ray detector 40 High voltage generator 44 Rotating drive unit 48 Couch controlling unit 52 System bus 56 DAS
60 Reconstruction unit 64 Extraction processing unit 68 Direction determination unit 72 Correction amount adjustment unit 80 Correction unit 84 Storage unit 88 Input unit 92 System control unit 96 Display unit 150, 152, 154 High absorption region 160 Center of rotation Ld, Ld 'Direction line W1, W1 ′, W2, W2 ′ Center of gravity P Subject

Claims (6)

An X-ray CT apparatus that performs a main scan using cone beam X-rays,
In the bone region of the subject indicated by the image data generated by the detection of X-rays transmitted through the subject before the main scan, the bone region having a higher X-ray absorption rate than the adjacent bone region is highly absorbed. An extraction processing unit for extracting as an area;
Based on the input information or the shape of the high absorption region, a direction determination unit that determines a straight line indicating the direction of the high absorption region as a direction line;
A correction amount adjustment unit that calculates a tilt angle between a representative straight line in the traveling direction of the cone beam X-ray in the main scan and the direction line, and adjusts a correction amount in a cone beam artifact correction process based on the tilt angle. When,
As the main scan, the cone beam X-ray transmitted through the subject is detected, and projection data is collected based on the detection signal. Based on the reconstruction process and the correction amount adjusted by the correction amount adjustment unit An X-ray CT apparatus comprising: an image generation unit configured to generate image data of the subject by performing the correction process on the projection data. The X-ray CT apparatus according to claim 1,
The direction determining unit calculates the longest straight line that can be drawn in the high absorption region, then divides the high absorption region into two equal parts by a straight line perpendicular to the longest straight line, An X-ray CT apparatus characterized in that a straight line passing through the other center of gravity divided in half is used as the direction line. The X-ray CT apparatus according to claim 1,
The X-ray CT apparatus, wherein the correction amount adjustment unit adjusts the correction amount so that the correction amount with respect to the cone beam artifact increases as the absolute value of the tilt angle decreases. The X-ray CT apparatus according to claim 1,
The correction amount adjustment unit determines, as the correction amount, a correction coefficient that increases as the absolute value of the tilt angle decreases.
The image generation unit generates reference image data by performing the reconstruction process on the projection data, and then performs a high-pass filter process in the body axis direction of the subject and a plane orthogonal to the body axis direction. After generating false image image data in which the cone beam artifact is extracted by performing low pass filter processing on the reference image data, image data obtained by multiplying each pixel value of the false image image data by the correction coefficient, and An X-ray CT apparatus, wherein a difference from reference image data is generated as image data after the correction process. The X-ray CT apparatus according to claim 1,
When the tilt angle, which is the tilt between the traveling direction of the cone beam X-ray and the reference axis in the main scan, is changed after the tilt angle is calculated, the correction amount adjusting unit is based on the tilt angle after the change. An X-ray CT apparatus that recalculates the tilt angle by recalculating a representative straight line in the traveling direction of the cone beam X-ray. The X-ray CT apparatus according to claim 1,
The correction amount adjustment unit sets a straight line along a central axis of the cone beam X-ray in the main scan as the representative straight line,
The X-ray CT apparatus, wherein the extraction processing unit extracts the high absorption region by performing threshold processing on each pixel value of image data generated before the main scan.