JPH0630927A - Image reconstruction method and apparatus for fast reverse projection computation of radiation ct - Google Patents

Abstract

PURPOSE:To realize a fast reverse projection image reconstruction method and apparatus which achieves an obtaining of more images in a short time without causing an increase in cost even when more images are obtained by scanning continuously. CONSTITUTION:A reverse projection computation of data is performed sequentially from the start of a scanning with a first reverse projection computing device 22 to store data of a collection data memory 21 into a first reverse projection memory 23 sequentially. A second reverse projection computing device 24 performs a reverse projection computation of data stored in the collection data memory 21 from the start of the scanning being delayed by 360 deg. to be stored into a second reverse projection data memory 25. The data from the reverse projection data memories 23 and 25 are subtracted with a sub tracter 26 thereby enabling the obtaining of reverse projection data at an arbitrary slice position.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reconstructing apparatus and method for performing high speed backprojection calculation of radiation CT.

[0002]

2. Description of the Related Art In CT, data is collected over 360 ° around an object and an image is reconstructed from these data to obtain and display a tomographic image of the object. This image reconstruction algorithm is roughly classified into a back projection method, a successive approximation method, and an analytical method.

The conventional backprojection method will be described below. In this backprojection operation, image data is normally obtained by calculating every 360 ° (image reconstruction for every 360 ° is called full recon, and the case of performing every 180 ° is called half recon). .

[0004]

When performing a helical scan, if an image pitch smaller than the distance traveled by the table is set for a scan of one second at a certain table speed, overlapping 360 ° data is created. Then, an image is obtained by a backprojection operation using it.

FIG. 6 is a diagram of data obtained by a helical scan in which a scan speed is 1 second and a table speed is 10 mm / s and an image pitch is 2.5 mm. In the figure, (a) is a diagram showing data obtained by continuous rotation and data at a slice position to be image-reconstructed. In this example, the scan time for one rotation is 1 second and the table speed is 10 mm / s,
The slice positions at 2.5 mm intervals are 90 ° intervals. The figure (b) shows the collected data at each slice position. That is, at the slice position 0 mm, the image data of the image No. 1 is obtained by the data set 1 in the range of 360 ° from 0 ° to 360 ° of the scan. When the slice position is 2.5 mm, the scan of (a) is 90 °.
Data from 0 to 450 ° in the range of 360 ° from 0 ° to 3
Image data of image No. 2 is obtained as a 60 ° data set 2. Similarly, the data up to the data set 5 is image-reconstructed to obtain image data of image No. 3, image No. 4, and image No. 5. In this way, five sets of data are obtained from the data obtained by two rotations of the helical scan, and five images are obtained by performing five backprojection operations.

Generally, the time required for one image reconstruction is t
When the image reconstruction is performed using the data of n rotations, the scan time is 1 second, the table speed is vmm / s, the pixel size is p, and the number N of images obtained is as follows.

N = (v / p) × (n−1) +1 Therefore, the time T required for image reconstruction is obtained by the following equation. T = Nt = {(v / p) × (n-1) +1} t v = 10 mm / s, p = 0.5, n = 11 rotations, t = 10
In seconds, T = {(10 / 0.5) × (11-1) +1} × 10 = 2010s = 33.5 min. That is, it takes 33.5 minutes to obtain a 3D image with slice data of 10 cm at the same image pitch as the pixel size, which is not practical. This is not limited to the helical scan, and the same applies to the case where the movement of the contrast agent is observed by continuous scans at the same position.

In order to reduce the required time, it is conceivable to install CPUs for backprojection calculation in parallel for each data set, but there is a limit to practical use due to increased cost. The present invention has been made in view of the above points, and an object thereof is to obtain many images in a short time while limiting an increase in cost even when many images are obtained by continuously scanning. It is to realize the high-speed backprojection image reconstruction method and its apparatus.

[0009]

According to a first aspect of the present invention for solving the above-mentioned problems, data obtained by continuous scanning is collected and stored in a pre-processed collected data memory, and this data is sequentially read to obtain the first data. Backprojection calculation is performed, then second backprojection calculation is performed on the data stored in the collected data memory from the start of data collection with a delay of one image reconstruction period, and then the first backprojection calculation is performed. Is obtained by subtracting the data of the result of the second backprojection operation from the data of the result of (1) to obtain the image data of the range of the required one image reconstruction period.

A second aspect of the present invention is a radiation CT image reconstructing apparatus for reconstructing an image by using a back projection method on data collected by continuous rotation. Collected data memory to be sequentially stored and a first backprojection operation to sequentially add the data read from the collected data memory are performed, and a scan stored in the collected data memory is delayed by one image reconstruction period. Backprojection computing means for reading data from a time point and performing a second backprojection computation, a first backprojection data memory for storing data of the first backprojection computation result, and the second backprojection computation. A second backprojection data memory that stores the resulting data, data read from the first backprojection data memory, and data read from the second backprojection data memory. Is characterized in that it comprises a subtracter for obtaining a difference between the over data.

[0011]

The data from the continuous scan is stored in the collected data memory from the scanning start time, the data is read from the scanning start time, the first backprojection calculation is performed by the backprojection calculation means, and the first backprojection data memory is stored. Then, after a delay of one image reconstruction period, the data stored in the collected data memory from the data collection start point is sequentially backprojected and stored in the second backprojection data memory. The image data of the second backprojection data memory is subtracted from the stored image of the first backprojection data memory by a subtractor,
Backprojection data at an arbitrary slice position is output.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An X-ray CT according to an embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram of an image reconstructing apparatus according to an embodiment of the present invention, and FIG. 2 is a block diagram of an X-ray CT using this image reconstructing apparatus.

In FIG. 2, reference numeral 1 denotes a gantry which is a component part of the X-ray CT, and causes the X-ray tube 2 and the detector 3 to operate around the subject according to various scanning methods. Four
Is a table on which the subject 5 is placed and sent into the gantry 1. Gantry 1 tilt, table 4
The movement of the table is controlled by the table / gantry controller 6.

Under the control of the X-ray tube drive generation control device 7, the X-ray tube 2 stops rotating and pauses the generation of X-rays. Detector 3
Rotates around the subject 5 under the control of the detector driving device 9. The X-rays transmitted through the subject 5 by the irradiation of the X-ray tube 2 are detected by the detector 3, and the data is collected by the data collecting device 10. The collected data is transferred to the image reconstruction device 11. The image reconstruction device 11 performs a backprojection operation on the input data to reconstruct an image. The reconstructed image is displayed on the display device 12 or stored in the data storage device 13.

The image data stored in the data storage device 13 is read out to form a 3D image suitable for mass examination by the 3D constituting device 14, and the projected image thereof is the display device 1.
It is displayed as 2. The system controller 15 controls the operation and timing of each of the above devices.

Next, an image reconstruction method by the back projection method performed by the image reconstruction apparatus 11 of the present embodiment will be described with reference to the block diagram of the image reconstruction apparatus 11 of FIG. In the figure, the same parts as those in FIG. 2 are designated by the same reference numerals. Reference numeral 21 is a collected data memory for storing the scan data collected by the data collecting apparatus 10 by a plurality of rotations as raw data.

Reference numeral 22 denotes a first backprojection computing unit for reading raw data collected by the data collecting apparatus 10 from the start of scanning from the collected data memory 21 and adding them one after another. The data is stored in the first backprojection data memory 23. To be done.

Of the data stored in the collected data memory 21, data 24 is delayed by 360 ° (in the case of full recon) from the data calculated by the first backprojection calculator 22 after the first rotation is completed. A second backprojection calculator that sequentially adds the result is stored in the second backprojection data memory 25. Reference numeral 26 is a subtractor to which the data stored in the first backprojection data memory 23 and the data stored in the second backprojection data memory 25 are input and which subtracts the latter from the former for each same scan angle. .

Next, the operation of the image reconstructing apparatus 11 configured as described above will be described with reference to the time chart of FIG. In FIG. 3, (a) is a diagram showing the timing of storage of raw data collected by the data collection device 10 by the scan and stored in the collected data memory 21, in which the scan is started at time t _{1 and then} at time t ₁ . _Second rotation at 2, time t
₃ indicates that it is in the third rotation.

The diagram (b) is calculated by the first backprojection calculator 22.
Image stored in the first backprojection data memory 23.
Data I₁In the figure showing the timing of the scan angle 0
In the first rotation from ° to 360 °, the raw data A
Data A₁In the second rotation from 360 ° to 720 °
Is image data A of raw data A and raw data B₁And B ₁
A that is sequentially added₁+ B₁At 720 ° or 1080 °
Is the raw image data A, B, C image data A₁, B₁, C
₁Image data A in which is sequentially added₁+ B₁+ C₁But
It is shown.

FIG. 5C is a diagram showing the timing of the image data I ₂ calculated by the second backprojection calculator 24 and stored in the second backprojection data memory 25. One rotation or 360 from the calculation start time t _1.
° A diagram showing the timing of the image data I ₂ to read the raw data A from the collected data memory 21 with a delay, perform the backprojection operation, and store it in the second backprojection data memory 25.
2 is calculated from the time t ₂ when the revolution of starting raw data A to the image data A _1, the image data A ₁ + B is added to the image data A ₁ while calculating from the time t ₃ the raw data B to the image data B _{1 It} shows the state where ₁ is calculated.

The image data I of FIG.
₁(C) Image data I of figure ₂Image reduced
Data I₃In the figure, from the data at each rotation angle,
A state where image data obtained by backprojection calculation can be obtained as it is
Is shown.

FIG. 4 shows the scan angle of each data shown in FIG.
Drawings based on degrees, (a) and (b) are shown in Figure 3.
Is the same as. (C) The figure shows the calculation of the first backprojection calculator 22.
Raw data A with a scan angle of 0 ° with a delay of 360 ° from the start
To calculate the image data A ₁Calculate and scan angle
Scan angle α from 360 °₂Image date
Type A₁+ B₁It shows the state of calculating.

In FIG. 4D, the subtractor 26 subtracts the image data I ₂ from the image data I ₁ to obtain the scan angle α.
Data I ₃ over 360 ° from ₂ to scan angle α ₁
Is output.

FIG. 5 is an explanatory diagram for obtaining the same image data by comparison with the conventional case shown in FIG. In the figure, (a) is the value of α ₁ in (b) of FIG.
The (b) diagram is a diagram in which α ₂ in the (c) diagram of FIG. 4 is 90 °. The output obtained from the subtracter 26 is the image data I ₃ shown in FIG. 6C, which is the same data as the image No. 2 consisting of the data set 2 shown in FIG. The back-projection calculation angle at the position of 2.5 mm is α ₁ = α ₂ + 360 ° =
It becomes 450 °.

As described above, according to this embodiment, 2
By using a backprojection calculator and two backprojection data memories, image data obtained by backprojecting the data obtained by scanning each rotation is always obtained at any time, and this reconstruction is performed. The time required for scanning is not related to the number of images, and the required time is determined by the number of rotations of the scan. That is, an extremely large number of image data in extremely small pixel units can be obtained with the same reconstruction time as a conventional normal scan, and using this, an image can be obtained by a three-dimensional helical scan with good image quality in a short time. You can Further, in the continuous rotation dynamic CT, the contrast effect of the contrast agent can be observed in real time.

The present invention is not limited to the above embodiment. In the embodiment, the description has been made with the full reconciliation of 360 °.
80 ° half-recon can be used. Although the case of two backprojection calculators has been described, one backprojection calculator may be used to perform time-division calculations and store them in the first and second backprojection data memories, respectively. Although the above description has been made for X-ray CT, it can be used for emission CT using a radioactive element such as an isotope.

[0028]

As described in detail above, according to the present invention, it becomes possible to obtain many images in a short time while limiting the number of backprojection calculation means and suppressing an increase in cost. The above effect is great.

[Brief description of drawings]

FIG. 1 is a block diagram of a configuration of an image reconstructing apparatus according to an exemplary embodiment of the present invention.

FIG. 2 is a block diagram of an X-ray CT using the image reconstructing apparatus of the present invention.

FIG. 3 is a time chart of backprojection calculation of the image reconstruction device.

FIG. 4 is a time chart of backprojection calculation based on the scan angle of the image reconstructing device.

FIG. 5 is a time chart of the operation when obtaining image data at a slice position of 2.5 mm.

FIG. 6 is a diagram of data in the case where the slice interval is 2.5 mm by the conventional X-ray CT helical scan.

[Explanation of symbols]

 11 Image Reconstructing Apparatus 21 Collected Data Memory 22 First Backprojection Calculator 23 First Backprojection Data Memory 24 Second Backprojection Calculator 25 Second Backprojection Data Memory 26 Subtractor

Claims (2)

[Claims] 1. Data obtained by continuous scanning is collected and stored in a collected data memory after preprocessing, the data is sequentially read out to perform a first backprojection operation, and then delayed by one image reconstruction period. Second backprojection calculation is performed on the data stored in the collected data memory from the start of data collection, and the data of the second backprojection calculation result is converted from the data of the first backprojection calculation result. An image reconstruction method for performing high-speed backprojection calculation of radiation CT, which is characterized by subtracting and obtaining image data within a required one image reconstruction period. 2. An image reconstruction apparatus for radiation CT, which performs image reconstruction on data collected by continuous rotation by using a backprojection method, in which acquisition data obtained by multi-rotation scan and subjected to preprocessing are sequentially stored. A data memory (21) and a first backprojection operation for sequentially adding the data read from the collected data memory (21) are performed, and further stored in the collected data memory (21) with a delay of one image reconstruction period. The data from the start of the current scan is read
Back projection calculation means (22, 24) for performing the back projection calculation, a first back projection data memory (23) for storing data of the first back projection calculation result, and a second back projection calculation memory for the second back projection calculation. Data read from a second backprojection data memory (25) storing the resulting data and the first backprojection data memory (23), and read from the second backprojection data memory (25) An image reconstructing device for performing a high-speed backprojection calculation of radiation CT, comprising a subtracter (26) for obtaining a difference from the generated data.