JP5284429B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT apparatus, and more particularly to an X-ray CT apparatus capable of irradiating an X-ray beam having a certain width or spread in the direction of a subject body axis.

  X-rays emitted from an X-ray source are irradiated to the subject from multiple directions, X-rays transmitted through the subject are detected by an X-ray detector, and a tomogram of the subject is detected based on the detection result X-ray CT apparatuses for reconstructing an image are widely known.

  As such an X-ray CT apparatus, a “fan beam X-ray CT apparatus” that irradiates a fan-shaped X-ray beam from the X-ray source has been known. In this fan beam X-ray CT apparatus, for example, X-ray data transmitted through a subject is collected by a fan-shaped X-ray detector in which detection elements of about 1000 channels are arranged in a line. The X-ray data collection is performed while rotating the X-ray source and the X-ray detector around the subject. The data collected per revolution is about 1000 for each channel. Such an X-ray CT apparatus collects data for one cross section every rotation, and is sometimes called an “single slice” CT apparatus.

  Further, as an advanced form of the single slice X-ray CT apparatus, there is an X-ray detector in which detection elements are arranged two-dimensionally (M channels × N segments). In other words, this conforms to a form in which a plurality of X-ray detectors (about 1000 channels × 1 column) for the fan beam X-ray CT apparatus described above are stacked in N columns in the direction of the subject body. It is called an “X-ray CT apparatus”. The X-ray data collection method is the same as described above (Note that the X-ray CT apparatus further has a wider X-ray beam width in the body axis direction of the subject than the X-ray beam width of the multi-slice X-ray CT apparatus. A “cone beam X-ray CT apparatus” that irradiates a line beam is known).

  In such a single slice X-ray CT apparatus or a multi-slice X-ray CT apparatus, so-called “helical scan” can be performed. The helical scan is a method in which the X-ray source and the X-ray detector move relative to the body axis while rotating around the subject to obtain projection data. In other words, in this case, the X-ray source and the X-ray detector move substantially while drawing a spiral trajectory around the subject body in the axial direction. As a result, a wide range of tomographic images and the like can be obtained at once. It can be acquired.

Japanese Patent Laid-Open No. 11-9582

By the way, when a helical scan as described above is performed with a multi-slice X-ray CT apparatus and a tomographic image or the like is generated using a known so-called “Feldkamp reconstruction method”, the range in which the reconstruction is possible V is as shown in the hatched portion in FIG. That is, the general shape of the range V is a shape in which cones are attached to both ends of the cylinder, and the entire length (distance between the apexes of both cones) X2 is the length of the cylinder portion X1. When the helical scan distance, which is the distance that the X-ray tube 111 (or the top plate) moves, is X, as shown in FIG.
X <X1 <X2 (1)
(In FIG. 8, reference numeral 112 denotes an X-ray detector.)

  In such a case, for example, as shown in FIG. 9, a subject P1 (FOV is different in diameter) of the subject P of interest (a distance in a direction intersecting the body axis direction of the subject, hereinafter referred to as “FOV”) is different. When a helical scan is to be performed with respect to the subject PS (FOV is equal to “SS (<S)”) and the subject PS (with the helical scan distance X shown in FIG. It is possible to reconstruct a tomographic image or the like for the entire subject P1, but for the subject PS (FOV is equal to “SS (<S)”), the movement distance is the helical scan distance X shown in the figure. As a result, it is understood that the subject PS is forced to be exposed excessively. Incidentally, such a difference in FOV can be assumed to occur between an adult and a child, and even the same subject can be assumed to occur between its trunk and head. Note that the range in the body axis direction where both the subject P1 and PS are desired to be imaged is X1.

  In the case as shown in FIG. 9, it is preferable to set a more appropriate helical scan distance for the subject PS. However, conventionally, according to such a difference in FOV, the helical scan distance is set. There was no configuration to change the distance (in the above-mentioned single slice X-ray CT apparatus or two-row or four-row multi-slice X-ray CT apparatus, a “constant” helical is used regardless of the FOV due to the difference in the image reconstruction method. It wasn't necessary in the first place because the scanning distance was good.)

  Therefore, as described above, a child has a small FOV and can be photographed sufficiently with a smaller helical scanning distance. Even in such a case, photographing is performed at a helical scanning distance that can be applied to adults. As a result, such children are forced to use unnecessary exposure.

  This is not necessarily limited to the case of helical scanning, but may be a problem that can occur in all techniques that perform scanning using a cone beam.

  For example, a similar problem occurs in the case of a conventional scan that is known as a system in which a detector array is further expanded and a wide range of cone beams are used to collect a wide range of data in one scan.

By the way, in a helical scan using cone beam CT, a technique is disclosed in which streak artifacts based on cone angle discontinuity are prevented by associating the size of an imaging region with a helical pitch (Japanese Patent Laid-Open No. 11-239577). issue.). However, this technique only controls the helical pitch and not the scan range.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an X-ray CT apparatus that does not impose unnecessary exposure on a subject.

The present invention takes the following means in order to solve the above problems.
That is, an X-ray CT apparatus according to a first aspect of the present invention includes an X-ray source that generates X-rays extending in the body axis direction of the subject, and N rows (N is equal to or greater than 2) along the body axis direction of the subject. (Integer) A multi-channel X-ray detector that collects transmitted X-rays of the arrayed subject, and the X-ray source and the X-ray detector are arranged so as to face each other. Scanning means for performing a helical scan for collecting data, and reconstruction means for reconstructing an image from the collected data using a Feldkamp reconstruction method, in a direction orthogonal to the body axis direction of the subject Upon receiving the FOV indicating the distance, a range in which the corners at both ends of the region of interest in the body axis direction determined by the FOV is a ridge line determined by the spread angle of the X-ray and lies on the ridge line outside the region of interest. Control for controlling the scan range in the body axis direction The stage is provided, wherein if the FOV is reduced is characterized in that to shorten the scan range in accordance with the FOV.

1 is a schematic diagram illustrating a configuration example of an X-ray CT apparatus according to an embodiment of the present invention. It is a figure which shows the outline | summary of a helical scan, and the example of a setting of FOV, (A) is a body thickness direction, (B) is a figure which shows a body width direction. It is explanatory drawing which shows notionally the flow of the data in the X-ray CT apparatus shown in FIG. It is a flowchart which shows the flow of the helical scan distance automatic determination process according to the difference of FOV. It is a figure which shows the example of determination of the determined helical scan distance, Comprising: (a) is a figure which respectively shows the case where FOV is small (FOV = S), (b) is the case where FOV is large (FOV = L). . FIG. 10 is an explanatory diagram illustrating an example of controlling an X-ray exposure start time and an end time according to another embodiment of the present invention. It is a schematic explanatory drawing at the time of applying this invention to a conventional scan. It is a schematic diagram which shows the range which can be reconfigure | reconstructed when performing a helical scan and performing the Feldkamp reconstruction method with a multi-slice X-ray CT apparatus. It is a figure explaining the problem in a prior art example.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram illustrating a configuration example of an X-ray CT apparatus according to the first embodiment.

  In FIG. 1, the X-ray CT apparatus 1 includes a gantry 11 and a console 12. The gantry 11 is provided with a cavity portion 11a, and a subject P placed on a couch top (not shown) is introduced into the cavity portion 11a. An X-ray tube (X-ray source) 111 and an X-ray detector (X-ray detection means) 112 are arranged opposite to each other around the cavity 11a as shown in FIG. As shown in A, it is rotatable around the subject P. The X-ray tube 111 is connected to an X-ray generator 111a including a high voltage power source and the like, and the X-ray detector 112 has a two-dimensional (M channel × N) detection element composed of, for example, a scintillator and a photodiode. And is connected to a data collection unit 122 described later.

  The X-rays emitted from the X-ray tube 111 are exposed to the subject P in a cone shape as indicated by a broken line in FIG. 1, and the X-rays transmitted through the subject P are detected by X-rays. It is converted into an electric signal by the detection element of the instrument 112 and transmitted to the data collection unit 122.

  Note that the X-ray tube 111 in the present embodiment is provided with a collimator (not shown), and the thickness of the X-ray beam irradiated to the subject P is set in the body axis direction of the subject P (the paper surface in FIG. 1). It is possible to change the vertical direction (see FIG. 2A, in the case of FIG. 2A, the left-right direction on the paper surface). The axial thickness can be covered (“multi-slice” or “cone beam”).

  Further, in the X-ray CT apparatus in the present embodiment, the bed (top plate) on which the subject P is placed is configured to be relatively movable in the body axis direction of the subject P. As a result, the X-ray tube 111 and the X-ray detector 112 can perform the rotation operation indicated by the arrow A in FIG. 1 described above, and perform the rotation as shown in FIG. It is possible to move relatively in the body axis direction of the subject P while performing. That is, in this case, each of the X-ray tube 111 and the X-ray detector 112 moves in the body axis direction of the subject P while drawing a spiral trajectory around it (helical scan). ). Incidentally, this movement is performed by the “helical scan distance X” shown in FIG. 2A, but this embodiment is particularly characterized in a method for determining this distance X (described later). In FIG. 2A, the locus on the near side (here) of the subject P is indicated by a solid line, and the locus on the far side (behind) is indicated by a broken line. In addition, the situation shown in FIG. 2A conceptually shows an example of what the “helical scan” is, and the actual spiral trajectory is as shown in FIG. (The spiral trajectory usually has a finer pitch than FIG. 2A).

  Returning to FIG. 1, the console 12 has a control unit 121 that controls the rotation and movement of the bed (top plate), the X-ray tube 111 and the X-ray detector 112, and the apparatus user accesses the control unit 121. And an image display unit 12D for displaying a reconstructed tomographic image (axial image) and the like. Of these, specifically, a pointing device such as a mouse or a trackball can be employed as the input unit 127, and a CRT or the like can be employed as the image display unit 12D.

  The apparatus user issues a command to the control unit 121 via the input unit 127, and the control unit 121 receives the command and reconstructs a tomographic image or the like based on the X-ray information detected by the X-ray detector 112. (A command to be issued is issued to the reconstruction unit 125 described later), and this is displayed on the image display unit 12D.

  More specifically, the tomographic image or the like is reconstructed by data flow or processing as conceptually shown in FIG. It is displayed on the display unit 12D. In FIG. 3, first, the data collection unit 122 receives the electrical signal from the X-ray detector 112. In the data collecting unit 122, the electric signal as the X-ray information is converted into a digital signal by the A / D converter.

  The pre-processing unit 123 performs appropriate calibration processing such as sensitivity correction and X-ray intensity correction on the digital signal transmitted from the data collection unit 122 to obtain “projection data”.

  The memory 124 stores the projection data. The reconstruction unit 125 receives projection data from the memory 124, and based on the projection data, for example, in the body axis direction of the subject P by a three-dimensional image reconstruction algorithm represented by a method called the Feldkamp method. Reconstruction is performed for the X-ray absorption coefficient of a wide target area. The result is a three-dimensional distribution data set (hereinafter referred to as “voxel data set”). In the above description, the projection data is temporarily stored in the memory 124. However, in some cases, the projection data is sent directly from the preprocessing unit 123 to the reconstruction unit 125 without going through the memory 124. Also good.

  The voxel data set is directly or once stored in the storage device 12M and then sent to the data processing unit 126. In this data processing unit 126, a tomographic image of an arbitrary cross section, a transmission image from an arbitrary direction, a three-dimensional image, or the like is reconstructed based on the voxel data set. Finally, the reconstructed images and the like are displayed on the image display unit 12D. Note that which of the above images is displayed is based on an instruction from the apparatus user via the input unit 127. Further, the storage device 12D shown in FIG. 1 or FIG. 3 stores data (image data) relating to the tomographic image thus reconstructed, the projection data, the voxel data set, and the like according to circumstances. . As a specific configuration thereof, for example, a known hard disk may be employed.

  The configuration of the X-ray CT apparatus 1 shown in FIG. 1 and the like is merely an example. That is, in FIG. 1, the reconstruction unit 125 and the like are configured separately from the gantry 11 as the console 12, but the reconstruction unit 125 and the like may be installed in the gantry 11. Further, the data collection unit 122 may be provided on the gantry 11 and the pre-processing unit 123 and the subsequent units may be provided on the console 12, and the former may transmit electric signals from the former to the latter using a non-contact data transmission unit (not shown). In short, the present invention is not particularly limited to such a configuration.

  Below, the description regarding the effect about the X-ray CT apparatus 1 of this embodiment used as the said structural example is given along the flowchart shown in FIG. Since the present invention is characterized in that the above-described helical scan range is set in accordance with the FOV, this point will be described below.

  First, as shown in step S <b> 1 of FIG. 4, the FOV relating to the subject P to be imaged is input through the input unit 127. As shown in FIG. 2, the FOV is determined by the body thickness (the figure (A)) or the body width (the figure (B)) of the subject P. Usually, “body thickness <body width”. Because of this relationship, a scanogram as shown in FIG. 2B is obtained and determined by the body width. From this, between different subjects (for example, adults and children), even if they are the same region (for example, chest), the value of FOV will be different. Depending on the difference (for example, abdomen and head), the value of FOV will be different.

  In step S1 of FIG. 4, in addition to the input of the FOV value, input / setting for various other conditions such as an X-ray irradiation condition and the like are performed according to circumstances.

  When receiving the FOV value, the controller 121 determines a specific value of the helical scan distance X as shown in step S2 of FIG. This is, for example, as shown in FIG. 5, and the helical scan distance is such that the FOV values of the subjects P1 and P2 are “S” and “L”, respectively, as shown in FIGS. (X (P1)) and "X (P2) (> X (P1))" are set according to the difference from (> S) ".

  As described above, in the multi-slice X-ray CT apparatus, when a helical scan is performed and a tomographic image or the like is generated using the Feldkamp reconstruction method, FIGS. 5 (a) and 5 (b). (Also, as shown in FIGS. 8 and 9 referred to in the section of the prior art), the outline of the reconfigurable range V is such that a cone is attached to each end of the cylinder. Is. That is, as described above, the movement distance X, the cylinder length X1, and the entire range length X2 satisfy the relationship of the above expression (1), and in this case, X2 according to the required FOV. This is because it is possible to reconstruct a region as large as −X or X1−X, and the necessary helical scan distance X is different (in general, as the FOV increases, the helical (The scanning distance also increases.)

  In the present embodiment, as described above, according to the FOV = S or L (> S), the helical scan distance X (P1) or X (P2) (> X (P1)) The entire range (in the region to be imaged) of the subject P1 or P2 can be accommodated in the range V without excess or deficiency.

  Incidentally, in FIGS. 5A and 5B, the specific sizes of the range V are shown as V (P1) and V (P2). As can be seen from this figure, as a method for specifically determining the helical scan distance X, the spread angle θ of the X-ray beam (adjustable by changing the opening of the collimator) is input. Information regarding the position (coordinates) of the corner P1E or P2E that can be obtained from the FOV value may be used. That is, the X-ray tube 111 (or the bed) moves on the condition that the corner P1E or P2E (with respect to both right and left in the figure) is placed on the ridge line of the X-ray beam defined by the spread angle θ. If the distance to be determined is determined, this becomes the helical scan distances X (P1) and X (P2) with no excess or deficiency.

  After that, while moving the bed etc. by the determined helical scan distance X (P1) or X (P2) and rotating the X-ray tube 111 and the X-ray detector 112 around the subject P, Actual photographing may be started (step S3 in FIG. 4).

  As described above, in the X-ray CT apparatus according to the present embodiment, since the helical scan distance X is fixed as in the prior art, no extra exposure is imposed on a subject having a small FOV. (For example, referring to FIG. 5, there is a case where a helical scan distance X (P2) fixed to a larger FOV = L is applied to the subject P1 where FOV = S. Absent).

  In the above embodiment, it has been described that the helical scan is realized by moving the bed or the like, but the present invention is not limited to such a form. For example, even in a configuration in which the gantry moves while rotating the X-ray tube and the detector facing each other, the helical scan can naturally be realized. After all, in the present invention, in order to achieve the “movement” of the helical scan distance X, any configuration (such as the gantry 11 or the top plate) may be specifically configured to be movable. It is only necessary that the “relative movement” of the X-ray tube 111 and the X-ray detector 112 with respect to the specimen P is possible.

  As described above, the helical scan distance X determined in the above embodiment can be changed as appropriate depending on the magnitude of the spread angle θ of the X-ray beam. In other words, even if the helical scan distance X is prepared as a constant value, depending on the case, it is possible to change the spread angle θ of the X-ray beam, that is, to open the collimator. By adjusting, it may be possible to achieve the same effect (the setting of a range V with no excess or deficiency corresponding to a small or large subject).

  Furthermore, if the present invention is understood more broadly, even if the helical scan distance X is prepared as a constant value, the same effect as described above can be obtained by controlling the X-ray exposure start time and end time. You can see that it is achieved. That is, for example, as shown in FIG. 6, when the helical scan distance is uniquely defined as the maximum value “Xmax” so as to be able to cope with any subject (which becomes FOV) (that is, In a case where the reconfigurable range is uniquely defined as “Vmax”), when imaging a subject P3 that is too large in the range Vmax, the corner P3E is placed on the X-ray beam ridge line immediately after the start of movement. Up to the above, it is possible to make a configuration or action such that an X-ray beam is generated only when such a condition is satisfied without being exposed to X-rays (at the end (the right corner in the figure) The same applies to P3E).

  The present invention includes such forms within the scope thereof. In these cases, there is a concern that the present invention may slightly deviate from the meaning of “determining the distance”. However, as shown in FIG. 6, the X-ray exposure start time and end time In the form in which X-rays are exposed to the subject P3 only for a certain period, the “substantial” helical scan distance X (P3) is determined, so that there is no difference. In other words, even in the above case, it does not depart from the concept or technical idea of “determining the distance” according to the present invention.

  Furthermore, in the above description, the scan distance is determined based on the FOV value input through the input unit 127. However, in some cases, it may be configured to automatically detect the FOV value.

  In the first embodiment, the multi-slice X-ray CT apparatus using the helical scan method has been described. Next, the multi-slice X-ray CT apparatus using the conventional scan method as the second embodiment will be described. A case where the invention is applied will be described.

  As described above, this conventional scan method is arranged with X-ray detectors DET arranged in multiple rows (for example, 600 rows) in the body axis direction facing the X-ray tube 111 as shown in FIG. Then, multi-slice data is collected by rotating around the subject P in a state where a wide-angle cone beam (a cone angle and a fan angle are both wide angles) is emitted from the X-ray tube 111. The imaging region of the subject P covered with the cone beam BMI at this time is referred to as a scan range. At this time, if the imaging region of the subject P is a thick range (or a large diameter) in a range P1 (for example, an adult) and a small range PS (for example, a small item), the same scanning range is used. In the small range PS, unnecessary exposure is forced.

  Therefore, the present invention moves the collimator CM (X-ray beam stop) arranged near the lower part of the X-ray tube as shown by an arrow (reducing the interval) so that the cone beam width is reduced as shown by BM2 (so-called so-called BM2). Scan range control). As described in the first embodiment, this control is also performed by setting the size of the imaging region diameter (FOV) using the console input means and controlling the opening amount of the collimator. Used. In this case, the same effect as that of the first embodiment can be obtained.

  In addition, it goes without saying that various embodiments can be applied without departing from the spirit of the present invention.

P Subject 1 X-ray CT apparatus 11 Base 111 X-ray tube 112 X-ray detector 12 Console 121 Control unit 122 Data collection unit 123 Preprocessing unit 124 Memory 125 Reconfiguration unit 126 Data processing unit 127 Input unit 12D Image display unit 12M Storage device X, X (P1), X (P2), X (P3) Helical scan distance

Claims (4)

Collects X-ray sources that generate X-rays extending in the body axis direction of the subject, and transmitted X-rays of the subject arranged in N rows (N is an integer of 2 or more) along the body axis direction of the subject Multi-channel X-ray detection means, and scanning means for performing a helical scan for collecting transmission X-ray data of a subject by performing rotational movement in a state where the X-ray source and the X-ray detection means are arranged to face each other. Reconstructing means for reconstructing an image from the collected data using the Feldkamp reconstruction method ,
Upon receiving the FOV indicating the distance in the direction orthogonal to the body axis direction of the subject, the corners at both ends of the region of interest in the body axis direction determined by the FOV are ridge lines determined by the spread angle of the X-ray, and Control means is provided for controlling a range on the outer ridge line with respect to the region of interest as a scan range in the body axis direction, and when the FOV is reduced, the scan range is shortened according to the FOV. X-ray CT apparatus.   The X-ray CT apparatus according to claim 1, wherein the control unit controls a total movement amount of the subject placement top plate during the helical scan.   The X-ray CT apparatus according to claim 1, wherein the control unit controls the start and end positions of the X-ray exposure in the helical scan.   The X-ray CT apparatus according to claim 1, wherein the control unit controls a collimator opening for narrowing an X-ray beam width.

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