X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-02-16
2002-07-02
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S017000
Reexamination Certificate
active
06415012
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray CT (Computed Tomography) apparatus. More particularly, in a multi-slice X-ray CT apparatus that uses a plurality of rows of detectors for detecting an X-ray image formed by helical scanning of the surrounding of a subject to be examined, this invention relates to a technique for achieving a data interpolation and an image reconstruction based on data by helical scanning of the subject by tilting a patient couch or a gantry.
2. Description of the Background Art
There have so far been proposed X-ray CT apparatuses using a helical scanning system. The X-ray CT apparatus based on the helical scanning system collects tomographic image data of a subject to be examined
12
by moving a patient couch to a body axial direction (hereinafter to be referred to as a Z-axial direction) of the subject
12
in synchronism with a continuous rotation of both an X-ray focus
13
and a detector
11
, as shown in FIG.
1
. In other words, in the helical scanning system, X-ray CT apparatus moves the patient couch to a body axial direction of the subject
12
through a center of the rotation of the X-ray focus
13
and the detector
11
while rotating these units. Accordingly, it can be understood that the X-ray focus
13
and the detector
11
take a spiral locus around the subject
12
. On the other hand,
FIG. 2
is a view for explaining a conventional scanning system for collecting data by moving the patient couch for each rotation of the X-ray focus and the detector. As compared with the conventional scanning system, the helical scanning system achieves a scanning at a higher speed over a wider range.
The X-ray CT apparatus based on the helical scanning system is further broadly divided into two kinds, that is, a single-slice CT apparatus and a multi-slice CT apparatus, based on a structure of the detector.
The first single-slice CT apparatus has an X-ray beam generation source for irradiating fan-shaped X-ray beams (hereinafter to be referred to as fan beams), and a detector having M channels (for example 1,000 channels) arrayed in a fan shape or in a linear shape in one row. This single-slice CT apparatus has the X-ray beam generation source and the detector rotated around the subject, and collects M data (for example of 1,000 data) in one rotation. Data collection in one time is called one view.
The second multi-slice CT apparatus has an X-ray beam generation source for irradiating conical X-ray beams (hereinafter to be referred to as cone beams), and a two-dimensional detector having detectors arrayed in a Z-axis (body axis) direction in a plurality of rows, each detector having an arcuate array of M-channel detectors (M channels times N rows).
FIGS. 3A
,
3
B and
3
C show detectors, each detector having two rows, four rows and eight rows, respectively. The multi-slice CT apparatus rotates the X-ray beam generation source (X-ray focus)
13
and the detector
32
around the subject, and collects M times N data in one rotation. Accordingly, as compared with the first single-slice CT apparatus, it is possible to scan over a wide range in higher precision and at a higher speed.
In the coordinates of scanning in
FIG. 4
, the Z-axis (body axial direction) coincides with a slice direction in which the slicing proceeds.
FIG. 5
is a view for showing the scanning of the multi-slice CT apparatus as observed from a Z-axial direction. In the drawing, a reference numeral
51
within a circle represents an effective field of view diameter FOV (Field of View). A reference numeral
52
placed between the X-ray focus
13
and the center of the FOV represents a distance between the X-ray focus and the rotation center, FCD (Focus Rotation Center Distance). A reference numeral
53
represents a fan angle.
FIG. 6
is a view of a four-row multi-slice CT as observed from a direction perpendicular to the Z-axis including the Z-axis. A beam thickness
61
in the Z-axial direction, when X-rays incident from the X-ray focus
13
to the detector element
32
has passed through the rotation center (that is, FCD
52
), is expressed as a reference slice thickness T. In
FIG. 6
, a central slice exists between the second-row detector and the third-row detector. A couch travel distance in one rotation is called a helical pitch. A helical pitch P (as represented by
62
) in the multi-slice CT becomes a product of the number of detector rows N times the reference slice thickness T.
Next, an outline of an image reconstruction processing in the helical scanning system will be explained. In the following explanation, the subject
12
having only an arrow signal around the rotation is considered as shown in FIG.
7
.
(1) Projection Data Collection Processing
First, as shown in
FIG. 8A
, projection data collected by the detector at each view of the helical scanning is collected for all angles. The projection data is corrected by taking into consideration the sensitivity of the detector, the X-ray intensity and various other physical factors. The data after the correction is called raw data.
(2) Helical Interpolation Processing
Second, in the case of the helical scanning, interpolation is conducted based on the raw data in a Z-axial direction, to generate interpolated data on a desired slice surface. This interpolation is called a helical interpolation. This processing is carried out, as only data of one view is collected on the targeted slice surface according to the helical scanning. The interpolation processing will be explained in detail later.
(3) Convolution Processing
Third, as shown in
FIG. 8B
, the interpolated data for the respective angles are subjected to convolution calculation of a reconstructing function (a filter function).
FIGS. 10A
,
10
B,
10
C and
10
D show examples of shapes of filters. These filter shapes are selected according to the characteristics of the image data to be obtained. The convoluted data after the calculation exhibits a shape with a decay on the surrounding for an actually existing signal.
(4) Back Projection and Fan Beam Reconstruction Processing
Fourth, the convoluted data is added to all the pixels which are arrayed along the path of an X-ray beam at the time of data collection.
FIG. 9
shows a result of the back projection calculation at a certain angle. When this back projection is repeated for the convoluted data at necessary angles according to the beam shape, only the original signal remains, and desired image data is fan-beam reconstructed.
An interpolation method in the case of carrying out a helical scanning in the multi-slice CT apparatus will be explained next. As such an interpolation method, there exists an adjacent interpolation method as disclosed in Japanese Laid-open Publication Hei 4-224736.
FIG. 15
shows a conceptional diagram of the adjacent interpolation method for the case where the helical pitch is 4 in the four-row multi-slice CT. According to this adjacent interpolation method, real data or opposite data corresponding to the real data at two adjacent points in a Z-axial direction (slice direction) at a target slicing position, are used for linear interpolation with an inverse ratio of a distance between the target slicing location
151
and the sampling position. In this case, the real data is equivalent to the raw data. This adjacent interpolation method is a method employed by extensively applying a 360-degree interpolation method used for the single-slice CT apparatus. As shown in
FIG. 11
, according to the 360-degree interpolation method, real data
152
and
153
of two views which are in same phase with each other at the nearest positions and sandwiching a target slice plane
151
, are used for linear interpolation with an inverse ratio of a distance between the slice plane and the sampling position. This processing is repeated for all the necessary phases.
Further, in Japanese Laid-open Publication Hei 9-234195, there is disclosed a filter interpolation method for performing an addition of weighted multi-point data.
FIG. 16
shows a conceptional view
Suzuki Tatsuro
Taguchi Katsuyuki
Bruce David V.
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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