JP2009207701A - X-ray ct system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT system can materialize compression having high compressibility even when there is a large difference between a projection data and a projection data of the same channel obtained by a preceding projection angle. <P>SOLUTION: The X-ray CT system includes: an X-ray source; an X-ray detector arranged opposite to the X-ray source and having a plurality of X-ray detection elements detecting X-rays; a scanner which is mounted with the X-ray source and the X-ray detector and rotates in the periphery of a reagent; an image reconstituting device for reconstituting the tomographic image of the reagent, based on projection data being transmission X-ray dosages from various projection angles, which are detected by the X-ray detection elements; an image display for displaying the tomographic image reconstituted by the image reconstituting device; and a storage device for storing the projection data and the tomographic image. The X-ray CT system also includes: a projection data rearranging device for rearranging the projection databased on a prescribed condition; a data compression device for compressing the rearranged projection data and preserving the data in the storage device; and a data restoring device for restoring the compressed projection data. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus, and more particularly to a technique for compressing projection data.

  The X-ray CT apparatus reconstructs and displays a tomographic image in a subject based on projection data from various angles acquired by imaging, and provides for image diagnosis. Its configuration consists of an X-ray source that irradiates the subject with X-rays, an X-ray detector that detects the X-ray dose that has passed through the subject as projection data, a scanner that carries both and rotates around the subject, It consists of a device for reconstructing a tomographic image based on the detected projection data from various angles and a display device for displaying the tomographic image.

  In recent years, X-ray CT systems have made it possible to collect a wide range of data in a short period of time by increasing the number of X-ray detectors and increasing the scanner rotation speed. On the other hand, since the amount of projection data generated per examination has become enormous, it may take a long time to write and read data to and from the storage device while squeezing the capacity of the storage device that stores the projection data. It has become.

  As a means for coping with such problems, for example, a data compression technique as described in Patent Document 1 is used.

Japanese Patent Laid-Open No. 2004-159152 discloses a difference between the projection data of each projection angle and the projection data of the same channel at the previous projection angle with respect to the collection of projection data detected by the X-ray CT apparatus. And a method of compressing projection data by adopting original data when the difference value exceeds a predetermined threshold value is disclosed.

  However, the technique of Patent Document 1 is effective when the collection of projection data is projection data with little difference from the projection data of the same channel at the previous projection angle, but otherwise the effect is large. Absent.

  An object of the present invention is to provide an X-ray CT apparatus that realizes compression processing with a high compression rate.

  An X-ray CT apparatus according to the present invention includes an X-ray source that irradiates a subject with X-rays, and a plurality of X-ray detection elements that are arranged to face the X-ray source and detect X-rays that have passed through the subject An X-ray detector, a scanner that mounts the X-ray source and the X-ray detector and rotates around the subject, and a projected X-ray dose from various projection angles detected by the X-ray detection element An image reconstruction device for reconstructing a tomographic image of a subject based on data, an image display device for displaying a tomographic image reconstructed by the image reconstruction device, a storage device for storing the projection data and the tomographic image; A projection data rearrangement device that rearranges the projection data based on a predetermined condition, a data compression device that compresses the rearranged projection data and stores the data in the storage device, and a compression Data restoration device for restoring the projection data received And a device.

  ADVANTAGE OF THE INVENTION According to this invention, the X-ray CT apparatus which implement | achieves the compression process with a high compression rate can be provided.

  An X-ray CT apparatus to which the present invention is applied will be described with reference to the drawings.

  FIG. 1 is an overall configuration diagram of an X-ray CT apparatus 1 to which the present invention is applied. This apparatus includes a scan gantry unit 100 and an operation console 120.

  The scan gantry unit 100 includes an X-ray tube 101, a rotating disk 102, a collimator 103, an X-ray detector 106, a data collection device 107, a bed 105, a gantry control device 108, a bed control device 109, An X-ray control device 110. The X-ray tube 101 is an apparatus that irradiates a subject placed on a bed 105 with X-rays. The collimator 103 is a device that controls the irradiation range of X-rays emitted from the X-ray tube 101. The X-ray detector 106 is a device that detects the X-rays that are disposed to face the X-ray tube 101 and pass through the subject. The rotating disk 102 includes an opening 104 through which a subject placed on a bed 105 enters, and is equipped with an X-ray tube 101 and an X-ray detector 106, and rotates around the subject. The X-ray detector 106 includes a plurality (eg, 1000) of detection elements in the rotation direction (also referred to as channel direction) of the rotating disk 102. Further, the X-ray detector 106 may have a plurality of detection elements arranged in one row, and this row may be arranged in multiple rows (for example, 64 rows) in the rotation axis direction (also referred to as slice direction) of the rotating disk 102. good. The data collection device 107 is a device that converts the X-rays detected by the X-ray detector 106 into a predetermined signal. The gantry control device 108 is a device that controls the rotation of the rotary disk 102. The bed control device 109 is a device that controls the vertical movement of the bed 105. The X-ray control device 110 is a device that controls the output to the X-ray tube 101.

  The console 120 includes an input device 121, an image arithmetic device 122, a display device 125, a storage device 123, and a system control device 124. The input device 121 is a device for inputting a subject's name, examination date and time, imaging conditions, and the like, specifically a keyboard, a pointing device, and the like. The image calculation device 122 is a device that performs calculation processing on measurement data sent from the data collection device 107 and performs CT image reconstruction. Specifically, a CPU (Central Processing Unit) that executes calculation processing or a dedicated processing device It is an arithmetic circuit. The display device 125 is a device that displays the CT image created by the image calculation device 122, and specifically, is a CRT (Cathode Ray Tube), a liquid crystal display, or the like. The storage device 123 is a device that stores data collected by the data collection device 107 and image data of a CT image created by the image calculation device 122, and is specifically an HD (Hard Disk) or the like. The system control device 124 is a device that controls these devices, the gantry control device 108, the bed control device 109, and the X-ray control device 110.

  The X-ray tube 101 is controlled by the X-ray control device 110, and emits X-rays based on imaging conditions (X-ray tube voltage, X-ray tube current, etc.) input from the input device 121. The X-ray detector 106 has a plurality of X-ray detection elements arranged in the circumferential direction of the rotating disk 102 (for example, 1000), or two-dimensionally in the circumferential direction of the rotating disk 102 and the rotational axis direction of the rotating disk 102. The X-rays irradiated from the X-ray tube 101 and transmitted through the subject are detected by each detection element. The rotating disk 102 is controlled by the gantry control device 108 and rotates based on imaging conditions (scanning speed, etc.) input from the input device 121. The bed 105 is controlled by the bed control device 109 and operates based on the imaging conditions (such as a helical pitch) input from the input device 121.

  Projection data from various directions (for example, 1000 directions) is collected by the data collection device 107 by irradiating and detecting X-rays while rotating the rotating disk 102 around the subject. The amount of projection data collected when rotating the rotating disk 102 is, for example, 1000 detection elements of the X-ray detector 106, 64 columns, and 1000 projections per rotation If the dynamic range of the detection element is 20 bits, the capacity will be 160MB. The projection data collected by the data collection device 107 is sent to the image calculation device 122. The image calculation device 122 reconstructs the projection data into a CT image. The reconstructed CT image is displayed on the display device 125, and is stored in the storage device 123 as image data together with the imaging conditions.

  The relationship between the subject and the projection data will be described with reference to FIG. Fig. 2 (a) and Fig. 2 (c) show cross sections of the subjects, and Fig. 2 (b) and Fig. 2 (d) show the projection data for one rotation obtained when each subject is CT scanned. It was. 2 (a) and 2 (c), the horizontal direction is the x direction and the vertical direction is the y direction. 2 (b) and 2 (d), the horizontal direction is the channel direction of the detector element of the X-ray detector 106, the vertical direction is the projection angle direction, and the channel direction is from 1 to n rows in the projection angle direction. Has 1 to p lines. In addition, the arrangement of data in the channel direction is a projection data row, and the arrangement of data in the projection angle direction is a projection data string. The projection data dealt with here is a subtraction of the detection signal when there is a subject from the detection signal when there is no subject, and it becomes a high signal at the portion where X-rays are absorbed, and almost all X-rays are absorbed. In the part that is not performed (air part), the signal is substantially zero.

  In FIG. 2 (a), a cylindrical object 200 is arranged at the center of rotation of the rotating disk 102, and in FIG. A specimen 200 is arranged.

  In the projection data obtained by CT scanning of Fig. 2 (a), as shown in Fig. 2 (b), data corresponding to the X-ray dose absorbed by the subject 200 appears in the center of the channel direction, and other A data distribution (see the lower part of FIG. 2B) that is substantially zero in the channel No. 1 is shown in all projection data rows. When the data distribution of all the projection data rows is the same as shown in FIG. 2 (b), the difference value becomes zero when the difference calculation is performed between the projection data rows. Therefore, the data distribution of all projection data rows is stored only by storing in the storage device 123 information indicating that the data distribution of any projection data row is the same as the data distribution of other projection data rows. Become. By storing such data, the amount of data stored in the storage device 123 can be greatly reduced, that is, data compression can be performed.

  On the other hand, in the case of FIG. 2 (c), the position of the channel indicating the data corresponding to the X-ray dose absorbed by the subject 200 differs depending on the projection data row, and the curve as shown in FIG. Will be drawn. When the difference calculation is performed between the projection data rows for the projection data as shown in FIG. 2 (d), the difference value becomes zero at the end in the channel direction, and data compression is possible. However, since the difference value does not become zero in the channel range corresponding to the curve in FIG. 2 (d), the compression ratio of the projection data as a whole is greatly reduced compared to the case of FIG. 2 (b).

  Therefore, in the present invention, prior to performing data compression processing on projection data, a projection data rearrangement device that rearranges projection data based on a predetermined condition so as to further increase the data compression rate, and rearranged projections A data compression device that performs compression processing on data is provided. The projection data rearrangement device and the data compression device may be provided in the image calculation device 122 or the system control device 124, or may be provided separately from these. The actual cross-section of the subject is more complex than those shown in FIGS. 2 (a) and 2 (c), and tissues with various X-ray absorption rates are distributed at various positions. The projection data also has a plurality of overlapping curves as shown in FIG. 2 (d).

  A specific procedure for compressing and storing projection data according to the present invention will be described with reference to a processing flowchart shown in FIG.

(Step S301)
The system controller 124 detects X-rays transmitted through the subject by the X-ray detector 106 by irradiating the subject from the X-ray tube 101 while rotating the rotating plate 102, and various types Projection data from the projection angle is acquired.

(Step S302)
The projection data rearrangement apparatus rearranges the projection data acquired in step S301 based on a predetermined condition. The rearrangement procedure will be described later.

(Step S303)
The data compression apparatus calculates a difference between the projection data rows of the projection data rearranged in step S302, and calculates a difference projection data row that is a sequence of difference values between the projection data rows. The difference calculation between projection data rows may be a difference calculation between adjacent projection data rows in the projection angle direction, or may be a difference calculation for a specific projection data row (for example, the first projection data row).

(Step S304)
Based on the result of comparing the difference value calculated in step S303 with a predetermined threshold value, the data compression apparatus determines the difference value so that the data length of the difference value is shorter than the data length of the projection data before the difference. Data compression processing is performed by determining the data arrangement. For example, it is determined whether or not the difference value is substantially zero, and the data compression process is performed by recording the range of the data group in which the difference value that is zero continues in each difference projection data row. The recording of the range of the data group is performed, for example, by recording the number of data groups, the start position of the data group, and the length of the data group. A more specific data array will be described later.

(Step S305)
The data compression apparatus stores the data compressed in step S304 in the storage device 123.

  Next, the procedure for rearranging the projection data in step S302 will be described with reference to the processing flowchart shown in FIG. 4 and the supplementary explanatory diagram of FIG.

ここで、b_iは投影データ行の中のiチャンネルのデータであり、nはX線検出器106のチャンネル数である。
(数1)を計算した結果、cの値が整数でなければ、小数点以下を四捨五入し、重心位置cを整数とする。
全投影データ行に対し算出した重心位置501-(1)、501-(2)、…、501-(N-1)、501-(N)を図5(a)中に例示した。 (Step S401)
The projection data rearrangement device calculates the centroid position c using, for example, (Equation 1) based on the value in each projection data row.
(Equation 1)

Here, b _i is i channel data in the projection data row, and n is the number of channels of the X-ray detector 106.
As a result of calculating (Equation 1), if the value of c is not an integer, the decimal point is rounded off and the center of gravity position c is set as an integer.
FIG. 5A illustrates the center-of-gravity positions 501- (1), 501- (2),..., 501- (N-1), 501- (N) calculated for all projection data rows.

(Step S402)
The projection data rearrangement apparatus rearranges each projection data row so that the barycentric position calculated in step S401 is aligned with the same column position. FIG. 5B shows the rearrangement so that the barycentric positions of all the projection data rows are located at a specific column position 502.

(Step S403)
In the projection data rearranged in step S402, there is a portion where no data exists at the end in the channel direction. Therefore, the projection data rearrangement apparatus performs end data processing so that data is filled in a location where no data exists.
An example of the edge data processing will be described with reference to FIG. In FIG. 5 (c), the projection data rearrangement apparatus fills places 503a and 503b where no data exists with zero values. Since the data in the vicinity of the locations 503a and 503b where no data exists is data obtained by detecting X-rays transmitted through the air portion, the data is substantially zero. Therefore, even if the difference calculation is performed in step S303 after the locations 503a and 503b where no data exists are filled with zero values, incorrect data is not calculated. Since the process of filling with the zero value is a simple process, there is an advantage that the load on the arithmetic unit is not increased.

In portions 503a and 503b where no data exists in FIG. 5 (c), the data in the first column of each projection data row can be copied to 503a and the data in the nth column can be copied to 503b as non-zero data. good.
Another example of the edge data processing will be described with reference to FIG. In FIG. 5 (d), the projection data rearrangement apparatus moves one end data 504 to a location 505 where no data exists. In many cases, the end data 504 is data obtained by detecting X-rays transmitted through the air portion, and the vicinity of the portion 505 where no data exists is also substantially zero, so the difference calculation is performed in step S303. Will not calculate incorrect data. Further, compared to the case of FIG. 5 (c), there is an advantage that the data amount is not increased.

  Two examples of the compressed data array obtained by the above processing procedure are shown in FIG. All the data arrays shown in FIG. 6 are differential projection data rows.

  First, an example of the data array shown in FIG. 6 (a) will be described. This data array includes a shift amount, a flag, and difference data for each column.

  The shift amount is a numerical value indicating the amount by which each projection data row is shifted in the channel direction in the rearrangement process in step S302. The center of gravity calculated in step S401 may be used. Since the maximum shift amount is about the number of detection elements, for example, if the number of detection elements is 1000 channels, 10 bits are required to represent the shift amount.

  A flag and difference data are treated as a pair. The flag indicates the content represented by the difference data. For example, when the data length of the flag is 1 bit, the flag can have the following meaning.

When the flag is 0: The difference value is a specified number of bits, for example, a value that can be represented by 5 bits. When the flag is 1: The difference value is the number of bits that represents the projection data, for example, a value represented by 20 bits. If the amount of shortening of the data length indicating the difference data is larger than the amount of extension of the data length caused by the addition, the data can be compressed by using the data array shown in FIG. 6 (a). For example, if the number of detection elements is 1000 channels and the dynamic range of the detection elements is 20 bits, the data length of the projection data row before compression is 2500 bytes (= 20 × 1000/8). On the other hand, assuming that the number of channels that can be represented by 5 bits is 250 channels, the data length after compression is about 2200 bytes (≒ (10 + 1 × 1000 + 5 × 250 + 20 × (1000−250)) / 8) Therefore, the data can be compressed to about 90%.

  For example, the data length of the flag can be 2 bits, and the following meanings can be given.

When the flag is 00: The difference value is a specified number of bits, for example, 1 bit. When the flag is 01: The difference value is a specified number of bits, for example, 5 bits. When the flag is 10. The difference value is a value that can be expressed in a specified number of bits, for example, 10 bits. When the flag is 11, the number of bits in which the difference value represents the projection data, for example, a value expressed in 20 bits. Also in this case, the channel corresponding to each flag. Assuming that the number is 250 channels, the data length after compression is about 1400 bytes (≒ (10 + 2 x 1000 + 1 x 250 + 5 x 250 + 10 x 250 + 20 x 250) / 8), which can be compressed to about 60% data Become.

  The number of bits of the flag and the meaning of each flag may be set according to the distribution of the projection data. Further, the compressed data can be restored by taking a procedure reverse to the compression procedure using the shift amount, the flag, the difference data of each column, the number of bits of the flag, and the meaning of each flag.

  Next, an example of the data array shown in FIG. 6 (b) will be described. This data array includes a shift amount, the number of data groups below the threshold, the position of the data group, the length of the data group, and other data. The shift amount is the same as in the example of FIG.

  The number of data groups below the threshold indicates the number of data groups in which the threshold value determined in advance is compared with the difference value calculated in step S303 and data whose difference value is substantially zero is continuously arranged. It is a numerical value. For example, if the arrangement of the difference value data calculated in step S303 is “000121001320000” and the threshold value is 0, there are three places where 0 is continuously arranged, so the number of data groups is 3. . In the example of FIG.

  The position of the data group and the length of the data group are paired and are arranged by the number of data groups. The position of the data group is a numerical value indicating the start position of each data group. If the previous data arrangement is “000121001320000”, the position of the data group 1 is 1, the data group 2 is 7, and the data group 3 Becomes 12. The length of the data group is a numerical value representing the length of each data group. If the previous data arrangement is “000121001320000”, the length of the data group 1 is 3, the data group 2 is 2, and the data group 3 becomes 4. In the example of FIG. 6, a pair of the position of the data group 1 and the length of the data group 1 and a pair of the position of the data group 2 and the length of the data group 2 are arranged.

  Other data is a sequence of data that is not a data group below a threshold. If the previous data arrangement is “000121001320000”, it becomes “121132”.

  Even in such a data arrangement, data compression is possible, and the compressed data can be restored by including the reverse procedure. Note that FIG. 6 shows an example of data arrangement, and this arrangement is not limited.

  Next, another example of the procedure for rearranging the projection data in step S302 will be described with reference to the processing flowchart shown in FIG. In this procedure, rearrangement for shifting the projection data row to a position where the data length becomes the shortest when the differential projection data row is compressed is performed. However, the first projection data row is not shifted.

(Step S701)
The projection data rearrangement apparatus sets initial values of the projection data row length Lcomp and the channel direction shift amount S. For example, the initial value of Lcomp is set to lb × n as the length of the projection data row when compression processing is not performed, and the initial value of S is set to −n / 2. Here, 1b is the dynamic range of the detection element of the X-ray detector 106, and n is the number of channels of the X-ray detector 106. If n is an odd number, S is set to-(n + 1) / 2.

(Step S702)
The projection data rearrangement apparatus shifts the projection data row by S in the channel direction, calculates a difference projection data row from the previous projection data row, and calculates a difference projection data row. For example, if the projection data row is the second projection data row, the second projection data row is S-shifted in the channel direction, and then the difference calculation with the first projection data row is performed.

(Step S703)
The projection data rearrangement apparatus calculates the length L of the compressed data when the differential projection data row is compressed. The length L of the compressed data is calculated based on the data array as shown in FIG.

(Step S704)
The projection data rearrangement apparatus compares L and Lcomp. If L is small, the process proceeds to step S705, and if not, the process proceeds to step S706.

(Step S705)
The projection data rearrangement apparatus sets Lcomp to L and the optimum shift amount Sopt to S.

(Step S706)
The projection data rearrangement apparatus adds 1 to the channel direction shift amount S and updates it.

(Step S707)
The projection data rearrangement apparatus determines whether or not the channel direction shift amount S exceeds the shiftable range. If it exceeds the shiftable range, the process proceeds to step S708; otherwise, the process returns to step S702. For example, a comparison between S and n / 2 may be used as the determination criterion.

(Step S708)
The projection data rearrangement apparatus shifts the projection data row by Sopt in the channel direction.

(Step S709)
The projection data rearrangement apparatus determines whether or not rearrangement of all the projection data rows has been completed. If completed, the process proceeds to step S403; otherwise, the process proceeds to step S710.

(Step S710)
The projection data rearrangement device advances the projection data row to the next row, and returns to step S701.

(Step S403)
Since the processing content of this step has already been described, it is omitted here.

  According to the above processing procedure, the projection data row can be shifted to a position where the data length becomes the shortest when the differential projection data row is compressed, so that the compression rate can be improved.

  From the foregoing description of the embodiments of the present invention, it is evident that the objects of the invention have been achieved. While the present invention has been described and illustrated in detail, they are intended for purposes of illustration and illustration only and are not intended to be limiting.

The figure which shows the whole block diagram of the X-ray CT apparatus concerning this invention. The figure explaining the relationship between a subject and projection data. The figure explaining the whole processing flow of this invention. The figure explaining the 1st processing flow of step S302 in FIG. FIG. 5 is a diagram for supplementarily explaining the processing flow of FIG. FIG. 4 is a diagram for explaining an example of a data array obtained by the processing flow of FIG. The figure explaining the 2nd processing flow of step S302 in FIG.

Explanation of symbols

  1 X-ray CT system, 100 scan gantry unit, 101 X-ray tube, 102 rotating disk, 103 collimator, 104 opening, 105 bed, 106 X-ray detector, 107 data collection device, 108 gantry control device, 109 bed control device , 110 X-ray control device, 120 console, 121 input device, 122 image processing device, 123 storage device, 124 system control device, 125 display device, 200 subject, 501 center of gravity position of projection data row, 502 specific column position , 503 location where no data exists, 504 edge data on one side, location where 505 data does not exist

Claims (6)

An X-ray source that irradiates the subject with X-rays, an X-ray detector that includes a plurality of X-ray detection elements that are arranged opposite to the X-ray source and detect the X-rays that have passed through the subject, and the X-ray source A tomographic image of the subject based on projection data which is a transmitted X-ray dose from various projection angles detected by the X-ray detection element and a scanner mounted with the X-ray detector and rotating around the subject. In an X-ray CT apparatus comprising: an image reconstruction device to be configured; an image display device that displays a tomogram reconstructed by the image reconstruction device; and a storage device that accumulates the projection data and the tomogram.
A projection data rearrangement device for rearranging the projection data based on a predetermined condition;
A data compression device for compressing the rearranged projection data and storing it in the storage device;
An X-ray CT apparatus further comprising: a data restoration device that restores the compressed projection data. The X-ray CT apparatus according to claim 1,
The data compression device calculates a difference between projection data from each projection angle and projection data from a predetermined projection angle,
Determining whether the difference value is less than or equal to a predetermined threshold;
An X-ray CT apparatus, wherein the data array of the difference values is determined so that the data length of the difference values is shorter than the data length before the difference based on the determination result. The X-ray CT apparatus according to claim 2,
The projection data rearrangement device calculates a gravity center position of projection data from each projection angle,
An X-ray CT apparatus characterized by rearranging projection data from each projection angle by aligning the position of the center of gravity. The X-ray CT apparatus according to claim 2,
The projection data rearrangement device calculates a shift amount that maximizes a compression rate when the data compression device compresses data,
An X-ray CT apparatus, wherein projection data from each projection angle is rearranged by the shift amount. The X-ray CT apparatus according to claim 3, wherein
The X-ray CT apparatus, wherein the projection data rearrangement apparatus generates rearrangement projection data by satisfying a zero value in a projection data area not filled with data. The X-ray CT apparatus according to claim 3, wherein
The X-ray CT apparatus is characterized in that the projection data rearrangement apparatus generates rearrangement projection data by moving the other end data to one of the projection data areas not filled with data.

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