X-Ray CT Apparatus

Abstract

X-ray CT apparatus includes an x-ray data collecting device for collecting x-ray projection data transmitted by a subject positioned between an x-ray generating device and a multi-row x-ray detector, while rotating said x-ray generating device and said multi-row x-ray detector around a rotation center positioned in-between, an image reconstructing device for performing image reconstruction from the projection data collected from the x-ray data collecting device, an image display device for displaying a tomogram obtained by image reconstruction, and a scanning condition setting device for setting various scanning conditions of tomography scanning. The x-ray data collecting device is operable for variable-pitch helical scanning which x-ray projection data of the subject on a scanning table is collected by moving the scanning table while varying the speed relative to a scanning gantry in a z direction perendicular to an xy plane which is the rotating plane of the x-ray generating device and the two-dimensional x-ray area detector, and of which starting of the x-ray data collection and starting of the scanning table movement relative to the scanning gantry and/or stopping of the x-ray data collection and stopping of the scanning table movement relative to the scanning gantry are asynchronously executed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an x-ray CT apparatus in one mode for carrying out the invention.

FIG. 2 is a diagram illustrating an x-ray generating device (x-ray tube) and a multi-row x-ray detector as viewed on the xy plane.

FIG. 3 is a diagram illustrating an x-ray generating device (x-ray tube) and a multi-row x-ray detector as viewed on the xy plane.

FIG. 4 is a flow chart showing the flow of imaging a subject.

FIG. 5 is a flow chart outlining the operation of the x-ray CT apparatus pertaining to one mode for carrying out the invention.

FIG. 6 is a flow chart showing details of pre-treatments.

FIG. 7 is a flow chart showing details of three-dimensional image reconstruction processing.

FIG. 8 are conceptual diagrams showing a state of projecting lines on a reconstruction region in the x-ray transmitting direction.

FIG. 9 is a conceptual diagram showing a state of projecting lines on a reconstruction region in the x-ray transmitting direction.

FIG. 10 is a conceptual diagram showing lines projected on detector faces.

FIG. 11 is a conceptual diagram showing a state of projecting projection data Dr(view, x, y) on the reconstruction region.

FIG. 12 is a conceptual diagram showing back-projection pixel data D2 of pixels on the reconstruction region.

FIG. 13 is a diagram illustrating a state in which back-projection data D3 are obtained by subjecting the back-projection pixel data D2 to all-view addition pixel by pixel.

FIG. 14 is a conceptual diagram showing a state of projecting lines on a circular reconstruction region in the x-ray transmitting direction.

FIG. 15 is a diagram showing a scanning condition input screen for the x-ray CT apparatus.

FIG. 16 is a diagram illustrating the range in which helical scanning is possible.

FIG. 17 is a diagram showing a case of constant speed helical scanning.

FIG. 18 is a diagram showing a case of variable speed helical scanning.

FIG. 19 is a diagram showing a case in which the data collection line is inclined.

FIG. 20 is a flow chart of Implementation Example 1 of variable-pitch helical scanning.

FIG. 21 is a diagram showing the operation of Implementation Example 1 of variable-pitch helical scanning.

FIG. 22 is a flow chart of Implementation Example 2 of variable-pitch helical scanning.

FIG. 23 is a diagram showing the operation of Implementation Example 2 of variable-pitch helical scanning.

FIG. 24 is a diagram showing filter convolution of projection data in the z direction.

FIG. 25 is a diagram showing filter convolution of image space in the z direction.

FIG. 26 is a diagram showing processing of processing data view.

FIG. 27 is a table comparing the advantages and disadvantages of the method of convoluting the z-directional filter on projection data and the method of convoluting the z-directional filter on image space.

FIG. 28 is a diagram showing inconsistencies in the z-directional filter width of projection data.

FIG. 29 is a diagram showing an inconsistency-free image space z-directional filter.

FIG. 30 is a diagram showing projection data view weighting by one turn or more.

FIG. 31 is a table of projection data space z filter coefficients and image space z filter coefficients in variable-pitch helical scanning.

FIG. 32 is a diagram showing the operation of shuttle mode variable-pitch helical scanning.

FIG. 33 is a diagram showing the operation of variable-pitch helical scanning.

FIG. 34 is a diagram showing the positional relationship between the data collection line and the tomogram in conventional scanning (axial scanning) or cine-scanning.

FIG. 35 is a diagram showing the positional relationship between the data collection line and the tomogram in helical scanning.

FIG. 36 is a diagram showing the positional relationship among a view a and a view b opposing each other and a tomogram

FIG. 37 is a diagram showing the total imaging range and partial imaging ranges.

FIG. 38 is a diagram showing a range in which tomogram image reconstruction is possible in Implementation Example 1.

FIG. 39 is a diagram showing a range in which tomogram image reconstruction is possible in Implementation Example 2.

FIG. 40 is a diagram showing the relative actions of the x-ray data collection line and the subject by two-way variable-pitch helical scanning in the z direction (equivalent to 1.5 legs).

FIG. 41( a) is a diagram showing the time resolution at different points in two-way helical shuttle scanning.

FIG. 41( b) is a diagram showing the time resolution at different points in one-way helical shuttle scanning.

FIG. 42 is a diagram showing Example 1 of the relationship among the helical pitch, the number of turns of data used and the x-ray tube current of two-way variable-pitch helical scanning or helical shuttle scanning back and forth in the z direction.

FIG. 43 is a diagram showing Example 2 of the relationship among the helical pitch, the number of turns of data used and the x-ray tube current of two-way variable-pitch helical scanning or helical shuttle scanning back and forth in the z direction.

FIG. 44 is a diagram showing Example 3 of the relationship among the helical pitch, the number of turns of data used and the x-ray tube current of two-way variable-pitch helical scanning or helical shuttle scanning back and forth in the z direction.

FIG. 45 is a flow chart of an x-ray automatic exposure function which determines the x-ray tube current in consideration of the quantity of data to be used in image reconstruction.

Claims

1. An x-ray CT apparatus comprising: an x-ray data collecting device for collecting x-ray projection data transmitted by a subject positioned between an x-ray generating device and a multi-row x-ray detector, while rotating said x-ray generating device and said multi-row x-ray detector around a rotation center positioned in-between;an image reconstructing device for performing image reconstruction from the projection data collected from said x-ray data collecting device;an image display device for displaying a tomogram obtained by image reconstruction; anda scanning condition setting device for setting various scanning conditions of tomography scanning,wherein said x-ray data collecting device is operable for variable-pitch helical scanning in which x-ray projection data of the subject on a scanning table is collected by moving the scanning table while varying the speed relative to a scanning gantry in a z direction perpendicular to an xy plane which is the rotating plane of the x-ray generating device and the two-dimensional x-ray area detector, and of which starting of the x-ray data collection and starting of the scanning table movement relative to the scanning gantry and/or stopping of the x-ray data collection and stopping of the scanning table movement relative to the scanning gantry are asynchronously executed.

2. An x-ray CT apparatus according to claim 1, wherein said x-ray data collecting device is operable for said variable-pitch helical scanning of which starting the collection of x-ray data is executed after starting of the scanning table movement relative to the scanning gantry.

3. An x-ray CT apparatus according to claim 1, wherein said x-ray data collecting device is operable for said variable-pitch helical scanning of which stopping of the movement of the scanning table relative to the scanning gantry is executed after stopping of the x-ray data collection.

4. An x-ray CT apparatus according to claim 1, wherein said x-ray data collecting device is operable for said variable-pitch helical scanning of which starting of the movement of the scanning table relative to the scanning gantry is executed after starting of the x-ray data collection.

5. An x-ray CT apparatus according to claim 1, wherein said x-ray data collecting device is operable for said variable-pitch helical scanning of which stopping the collection of x-ray data is executed after stopping of the scanning table movement relative to the scanning gantry.

6. The x-ray CT apparatus according to claim 4, wherein said collection of x-ray data is executed by rotating the rotary unit of the scanning gantry during a period in which the scanning table and the scanning gantry are at halt relative to each other.

7. The x-ray CT apparatus according to claim 5, wherein said collection of x-ray data is executed by rotating the rotary unit of the scanning gantry during a period in which the scanning table and the scanning gantry are at halt relative to each other.

8. The x-ray CT apparatus according to claim 6, wherein view angle at which the rotary unit of the scanning gantry rotates to collect x-ray data during the period in which the scanning table and the scanning gantry are at halt relative to each other is not less than the fan angle+180 degrees.

9. The x-ray CT apparatus according to claim 7, wherein view angle at which the rotary unit of the scanning gantry rotates to collect x-ray data during the period in which the scanning table and the scanning gantry are at halt relative to each other is not less than the fan angle+180 degrees.

10. The x-ray CT apparatus according to claim 1, wherein said image reconstructing device is configured to perform image reconstruction of the whole imaging range in the same slice thickness.

11. The x-ray CT apparatus according to claim 1, wherein said image reconstructing device is configured to perform image reconstruction in the same slice thickness within a range of the number of ranges into which the whole imaging range is divided.

12. The x-ray CT apparatus according to claim 1, wherein said image reconstructing device is configured to control the slice thickness by performing filter convolution in the z direction (row direction).

13. The x-ray CT apparatus according to claim 1, wherein said image reconstructing device is configured to control the slice thickness by multiplying the projection data of each view by a weighting coefficient.

14. The x-ray CT apparatus according to claim 13, wherein said image reconstructing device is configured to use projection data of not less than 360 degrees as the projection data.

15. The x-ray CT apparatus according to claim 1, wherein said image reconstructing device is configured to control the slice thickness by weighted addition by multiplying image-reconstructed tomograms consecutive in the z direction by a weighting coefficient.

16. The x-ray CT apparatus according to claim 1, wherein said x-ray data collecting device includes the scanning gantry which performs variable-pitch helical scanning at an inclination to the xy plane.

17. The x-ray CT apparatus according to claim 1, wherein said x-ray data collecting device includes a planar x-ray detector or an x-ray detector combining a plurality of planar x-ray detectors.

18. The x-ray CT apparatus according to claim 1, wherein: said x-ray data collecting device is operable for measuring z-directional coordinate position of at least one view, andsaid reconstructing device is operable for reconstructing using a measured value of the z-directional coordinate position of at least one view or a predicted value of the z-directional coordinate position of at least one view.

19. The x-ray CT apparatus according to claim 1, wherein: said x-ray data collecting device is operable for consecutively repeating x-ray data collection in a certain range of z-directional coordinate positions.

20. A method comprising changing a helical pitch during z-direction velocity changes of a moving gantry to obtain substantially uniform image quality in a plurality of reconstructed images.