JP4406106B2 - X-ray CT system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention includes a cone or pyramid-shaped X-ray beam (hereinafter referred to as a cone beam) having a spread in the body axis direction of a subject, and a two-dimensional detector for receiving the X-ray beam. The present invention relates to an X-ray CT apparatus that can be used.
[0002]
[Prior art]
A scanning method of a conventional X-ray CT apparatus (hereinafter abbreviated as CT) used for imaging a tomographic plane of a subject will be briefly described.
(1) Two scanning methods (conventional scan and helical scan)
Two typical CT scanning methods will be described. The first scan method is a conventional scan shown in FIG.
This method is a scanning method in which the periphery of the target cross section (eg, cross section A) of the subject P is rotated once. In order to obtain images of a plurality of cross sections (for example, cross section A and cross section B), first, data is collected while rotating around the cross section A, and then a bed on which the subject P is placed or an X-ray focus and detection. Move the vessel to bring the section B to the plane of rotation. Thereafter, data is collected while rotating around the section B in the same manner as the section A. Therefore, in the conventional scanning method, when the imaging range is wide in the body axis direction (Z-axis direction) of the subject and when there are many target cross sections, the imaging time becomes long.
The second scan method is a helical scan shown in FIG. In this method, while the X-ray focal point and the detector are continuously rotated, data is collected by moving the bed in the body axis direction of the subject P in synchronization with the rotation. The locus of the X-ray focal point scans around the subject P in a spiral shape. According to this scanning method, a wide range can be scanned at high speed. Here, a coordinate system is defined as shown in FIG. The XY plane corresponds to the cross sections A and B scanned by the conventional scan, the Z-axis direction is the body axis direction of the subject P, and the direction referred to as the slice direction in the single slice CT described above.
[0003]
(2) Multi-slice CT
Now, in order to capture a wide range at a high speed with high definition, as shown in FIGS. 5A, 5B, and 5C, two, four, eight, or even more detector rows are arranged. A multi-slice CT system is proposed.
By the way, when performing a conventional scan in multi-slice CT, it is known to perform imaging by appropriately switching the number of detector rows to be used in accordance with a diagnosis request. For example, considering the case of CT equipped with a detector having 100 rows in the body axis direction, when examining a wide range like the whole lung field and diagnosing only the presence or absence of abnormal findings in group examinations, etc. In consideration of the possible breath-holding time, the imaging should be completed in as short a time as possible. For this purpose, imaging is performed using all the detector rows (100 rows). In addition, when it is limited to a specific narrow range and precise diagnosis of the range is required, X-ray irradiation is performed so as to limit the number of rows to, for example, 20 in the center of the detector row, and data is collected from the 20 rows. By doing so, shooting is performed.
Here, limiting the number of detector rows used for imaging in a narrow range in the body axis direction reduces the influence of artifacts caused by using a cone beam in addition to the purpose of preventing unnecessary exposure to the subject. There is also a purpose.
[0004]
That is, when reconstruction is performed based on projection data from a two-dimensional detector obtained using a cone beam, the projection data is perpendicular to the body axis at the center of the slice position as shown in FIG. Although it can be reconfigured, the projection data intersects as shown in FIG. 6B as the position is farther from the slice center in the body axis direction. Moreover, this angle increases in proportion to the deviation from the center.
Therefore, reconstruction in CT using a cone beam is a mathematically difficult problem, and reconstruction is performed by approximating something. However, it cannot be denied that accuracy deteriorates at a slice position far from the center, which causes artifacts. It is a factor that arises. Therefore, in the case of photographing in a narrow range in the body axis direction, the number of detector rows used is limited to reduce the influence of artifacts.
[Problems to be solved by the invention]
In order to capture a wide range with high definition and high speed, it is also known to perform helical scanning by the multi-slice CT as described above, and many patents have been filed. For example, JP-A-4-224736, “CT apparatus” Hiroshi Aratsuki, Shinjiro Nanbu (filed on Dec. 25, 1990), and the like.
When helical scanning is performed with such multi-slice CT, a new problem arises. That is, when the helical scan is performed with an X-ray beam having a cone angle set to a relatively wide width in the body axis direction and reconstructed, the image quality of the obtained tomographic image is lowered. This is caused by performing an interpolation calculation specific to the helical scan.
[0005]
That is, in the case of the helical scan, there is only one projection data on the desired slice position because the obtained projection data series draws a spiral trajectory in the body axis direction of the subject. Therefore, by performing an interpolation operation from a plurality of projection data close to the slice position and at the same angle as the projection angle or at the opposite angle, projection data on the slice position is generated for each projection angle. It is well known that a tomographic image is reconstructed from projection data.
When this interpolation operation is considered in multi-slice CT as shown in the above-mentioned known patent application, reconstruction is performed using projection data obtained from all detector rows included in a specified slice position. If there is a variation in mechanical arrangement and / or detection characteristics between the detection elements, it is considered that the variation adversely affects the image quality of the reconstructed tomographic image.
Therefore, the more detection elements that contribute to reconstruction, in other words, the greater the number of columns used in the body axis direction, the greater the effect of errors due to the above-described variations, leading to lower image quality. In addition, even if you notice this undesirable image quality after completion of imaging, it is necessary to redo the imaging to correct it, resulting in useless exposure for the subject and a long examination time. Is inherent.
[0006]
However, as far as we know, no patents or publicly known documents that focus on this degradation in image quality exist when performing helical scanning with multi-slice CT.
Accordingly, the present invention has been made in view of the above circumstances, and provides an X-ray CT apparatus that can suppress image quality degradation that may occur when performing helical scanning with multi-slice CT without the operator being aware of it. With the goal.
[0007]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the invention according to claim 1 is an X-ray generation unit that emits X-rays toward a subject, and a plurality of detections that obtain projection data by detecting the X-rays that have passed through the subject. When the detection means in which the elements are two-dimensionally arranged and the X-ray generation means scan the periphery of the subject, a normal scan that scans the subject without moving in the body axis direction, or a body Scan control means that can perform any one of helical scans that perform scanning with movement in the axial direction, scan selection means that selects one of the scan modes in this scan control means, and any desired number of slices and a slice number setting means capable of setting, the maximum number of slices that can be set arbitrarily by the number of slices setting means when the helical scan is selected by the scan selection means And it is characterized in that it comprises a number of slices limiting means for providing a limit to less number of slices than the maximum number of slices that can be set during the normal scan.
Next, in the invention described in claim 3, the X-ray generating means for emitting X-rays toward the subject and the plurality of detection elements for obtaining projection data by detecting the X-rays transmitted through the subject are two-dimensional. When the detection means and the X-ray generation means that are arranged in a scanning manner scan the periphery of the subject, a normal scan in which the subject is scanned without moving in the body axis direction, or in the body axis direction Scan control means capable of performing any one of helical scans that perform scanning with movement, a first standard slice value corresponding to the normal scan, and a second smaller than the first standard slice value corresponding to the helical scan Storage means for storing the standard slice numerical value in advance, scan selection means for selecting one of scan modes in the scan control means, and selection by this scan selection means By applying the first or second standard slices values stored in the storage means in accordance with a scan mode that is characterized in carrying out the scanning.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the X-ray CT apparatus according to the present invention will be described below. In the X-ray CT apparatus, an X-ray tube and an X-ray detector are combined into a single ROTATE / ROTATE type rotating around the subject P, and a large number of detection elements arranged in a ring shape are fixed. There are various types such as the STATIONARY / ROTATE type in which only the tube is rotated around the subject P, or the STATIONARY / STATIONARY type in which the electron beam is scanned on the target surrounding the subject P. Applicable. Here, it is described as ROTATE / ROTATE-TYPE, which currently occupies the mainstream.
In addition, in order to reconstruct one tomographic image, one set of projection data for about 360 degrees around the subject P, and projection data for about 21 degrees to 240 degrees by the half scan method. One set is required. Although the present invention can be applied to any of the methods, here, description will be made assuming that one tomographic image is reconstructed from the projection data set of about 360 degrees of the general former.
FIG. 1 shows a system configuration diagram of an X-ray CT apparatus to which this embodiment is applied.
As shown in FIG. 1, the X-ray CT apparatus 1 includes an input unit 2, a system control unit 3, a gantry / bed control unit 4, a bed moving unit 5, a precollimator control unit 6, and a precollimator C. An X-ray control device 7, a high voltage generator 8, an X-ray beam generation source 9, a detector 10 in which detection elements are two-dimensionally arranged, a rotating mount 11, a data collection unit 12, and an interpolation processing unit 13, an image reconstruction unit 14, and a display unit 15.
[0009]
The system control unit 3 receives a selection signal indicating whether it is a normal scan or a helical scan and a set slice number signal input using the input unit 2, and when the helical scan is selected, the slice thickness Among the helical scan conditions such as the rotation speed, the rotation speed, slice thickness, fan angle and the like are output to the gantry / bed control unit 4 as a gantry / bed control signal.
The system control unit 3 limits the maximum number of slices set by the input unit 2 to be smaller than the body axis direction width (number of columns) of the detector 10 when helical scan is selected as the scan mode. The number-of-slices limiting unit 3a is provided. Specifically, if the detector 10 has 100 columns in the body axis direction, the slice number limiting unit 3a limits the maximum number of columns that can be selected by the operator to 50 columns, for example, half. Here, the slice number restriction unit 3a restricts the number of columns of the pre-collimator C and the data collection unit 12 so that the detector row to be used is near the center. In this way, the spread angle of the cone beam from the central axis is suppressed, the amount of deviation from the center of the projection data is reduced, and the influence of artifacts is reduced.
Further, the system control unit 3 outputs an X-ray beam generation control signal for controlling the X-ray beam generation to the X-ray control device 7, and sends a detection control signal indicating the detection timing of the X-ray beam to the data collection unit 12. Output.
[0010]
Further, the system control unit 3 outputs a data collection control signal to the data collection unit 12 to collect data from the detection element array based on the number of slices set by the input unit 2, and performs interpolation control indicating an interpolation method. The signal is output to the interpolation processing unit 13.
The gantry / bed control section 4 rotates the rotating gantry 11 based on the gantry / bed control signal output by the system control section 3 and outputs a bed movement signal to the bed movement section 5.
The couch moving unit 5 obtains the movement amount of the couch T per rotation of the rotating gantry 11 based on the couch moving signal output by the gantry / couch controller 4 when, for example, a helical scan is selected. The bed T is moved by the amount of movement.
The pre-collimator control unit 6 controls the opening width in the body axis direction of the pre-collimator C provided near the irradiation port of the X-ray beam generation source 9.
The X-ray controller 7 controls the timing of high voltage generation by the high voltage generator 8 based on the X-ray beam generation control signal output by the system controller 3.
The high voltage generator 8 supplies a high voltage for exposing the X-ray beam from the X-ray beam generation source 9 to the X-ray beam generation source 9 in accordance with a control signal from the X-ray control unit 17. The X-ray beam generation source 9 emits a cone-shaped X-ray beam by the high voltage supplied from the high voltage generator 8.
[0011]
The rotating gantry 11 holds the X-ray beam generation source 9 and the detector 10. The rotating gantry 11 is rotated around a rotation axis passing through an intermediate point between the X-ray beam generation source 9 and the detector 10 by a gantry rotating mechanism (not shown).
The data collection unit 12 collects the X-ray beam (actually a detection signal) detected by the detector 10 in correspondence with the data collection control signal output by the system control unit 3. The interpolation processing unit 13 interpolates the X-ray beam at the target slice position based on the X-ray beam collected by the data collection unit 12.
The image reconstruction unit 14 reconstructs an image based on the X-ray beam interpolated by the interpolation processing unit 13. The display unit 15 displays the image reconstructed by the image reconstruction unit 14 on a monitor (not shown).
Next, the operation of the above embodiment will be described with reference to FIG. 1 and FIG. 2 showing the operation flowchart. The maximum number of columns that can be collected will be described below on the assumption that 100 columns.
First, the surgeon inputs various conditions for scanning, for example, a scan form of normal scan or helical scan, slice thickness, number of slices, slice pitch, scan range, and subject size. ……………… (S1)
Next, the system control unit 3 receives the input various conditions, and determines whether the input scan form is a helical scan. ……………………… (S2)
If it is determined in step S2 that the helical scan is performed, the limit value 50 sequence stored in the slice number limiting unit 3a is set as the maximum value that can be set. ... (S3)
Next, the surgeon sets the desired number of columns. ……………………………… (S4)
Compare whether the value set in step S4 is 50 columns or less (S5)
If the number of columns is 50 or less in the comparison in step S5, the opening of the pre-collimator C and the data collection control signal are controlled to match the number of columns set. ………………… (S6)
Start scanning. ………………………………………………………… (S7)
If the number of set columns exceeds 50 in step S5, the process returns to step S4, and the surgeon resets the desired number of columns.
[0012]
In step S2, if the scan form is a normal scan that is not helical, the operator sets the desired number of columns within a range that does not exceed 100 columns, which is the maximum number of columns that the detector has. ………………………………………………………… (S8)
The opening degree of the pre-collimator C and the data collection control signal are controlled so as to match the number of columns set in step S8. ………………………………… (S9)
Start scanning. ……………………………………………………… (S1)
Projection data obtained by scanning is input to the interpolation processing unit 13 via the data collecting unit 12, and a helical-specific interpolation operation is performed to generate projection data at a desired slice position. Thereafter, the interpolated projection data and the actual detected projection data are input to the image reconstruction unit 14 and a tomographic image is formed and displayed on the display unit 15 for diagnosis.
According to such an embodiment, when the operator selects a helical scan, the maximum number of slices that can be selected is automatically set to half of the maximum number of columns that the detector has, so that the cone beam spreads in the body axis direction. Since the angle is reduced, the number of rows of detection elements used in the body axis direction that contribute to each reconstruction is reduced, and the influence of errors caused by the detection elements is suppressed. Therefore, the deterioration of the image quality of the tomographic image obtained even in the helical scan can be suppressed.
[0013]
Further, as a matter of course, the pre-collimator is narrowed down so as to irradiate only the set number of slices, so that unnecessary exposure to the subject P can be prevented. Further, since it is possible to prevent re-shooting, it is possible to prevent unnecessary exposure and to prevent the inspection from taking a long time.
In addition, this invention is not limited to the said embodiment, In the range which does not deviate from the summary of invention, it can change and implement variously.
For example, the maximum number of slices that can be set during the helical scan is half the number of columns of the detector 10, but the present invention is not limited to this. Considering the image quality at the time of helical scanning, the image quality can be further improved by reducing the number of columns such as 1/3 or 1/4 of the number of detectors. This may be set in advance according to the required diagnostic form.
In the above embodiment, the method of suppressing the upper limit value of the number of slices that can be set is described. However, a standard slice value may be preset for each scan mode. For example, the default value is set to use 100 rows for normal scan, and use 30 rows for helical scan. In this method, when the scanning conditions are patterned according to the diagnostic site, the operator can obtain a tomographic image in which the deterioration of the image quality is suppressed without being particularly conscious of the scanning form, and is easy to use. .
[0014]
Furthermore, in the above-described embodiment, only the pre-collimator C is reduced according to the set number of slices. However, the post-collimator is further provided between the detector 10 and the subject P to further reduce the X-rays incident on the detector 10. If the X-ray CT apparatus is used, this post-collimator may be subject to opening degree control according to the set number of slices, similarly to the pre-collimator C.
[0015]
【The invention's effect】
According to the present invention described above, it is possible to achieve an excellent effect that image quality degradation that may occur when performing helical scanning with multi-slice CT can be suppressed without the operator being aware of it.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an X-ray CT apparatus according to an embodiment of the present invention.
FIG. 2 is a flowchart for explaining a processing flow in the embodiment of FIG. 1;
FIG. 3 is a conceptual diagram of conventional scanning and helical scanning.
FIG. 4 is a diagram for explaining a coordinate system for CT scanning.
FIG. 5 is a conceptual diagram of multi-slice CT.
FIG. 6 is a diagram showing an intersection angle of projection data at projection angles that are 180 degrees apart.
[Explanation of symbols]
2 ... Input unit 3 ... System control unit 3a ... Slice number limiting unit 9 ... X-ray beam generation source 10 ... Detector P ... Subject

Claims (3)

X-ray generation means that emits X-rays toward the subject, detection means in which a plurality of detection elements for obtaining projection data by detecting X-rays transmitted through the subject are two-dimensionally arranged, and When the X-ray generation means scans the periphery of the subject, either a normal scan in which the subject is scanned without moving in the body axis direction or a helical scan in which the scan is performed with movement in the body axis direction A scan control means capable of performing the above, a scan selection means for selecting one of the scan modes in the scan control means, a slice number setting means capable of arbitrarily setting the desired number of slices, and the scan selection means maximum Sula helical scan against the maximum number of slices that can be set arbitrarily by the number of slices setting means when selected, it may set the time of the normal scan X-ray CT apparatus characterized by comprising a number of slices limiting means for providing a limit to less number of slices than the scan number.     A pre-collimator that arbitrarily limits the opening width in the body axis direction of the X-rays that radiate the X-rays toward the subject; and the slice number limiting means includes at least the detector row to be used near the center. The X-ray CT apparatus according to claim 1, wherein the opening width of the pre-collimator is controlled. X-ray generation means that emits X-rays toward the subject, detection means in which a plurality of detection elements for obtaining projection data by detecting X-rays transmitted through the subject are two-dimensionally arranged, and When the X-ray generation means scans the periphery of the subject, either a normal scan in which the subject is scanned without moving in the body axis direction or a helical scan in which the scan is performed with movement in the body axis direction Scan control means capable of performing the above, storage means for preliminarily storing a first standard slice value corresponding to the normal scan and a second standard slice value smaller than the first standard slice value corresponding to the helical scan; A scan selection means for selecting one of the scan modes in the scan control means, and a scan mode selected in accordance with the scan mode selected by the scan selection means. By applying the first or second standard slices values stored in the storage means X-ray CT apparatus characterized by performing a scan.

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