X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2003-02-20
2004-09-21
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S019000, C378S901000
Reexamination Certificate
active
06795522
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2002-045494 filed Feb. 22, 2002.
BACKGROUND OF THE INVENTION
The present invention relates to a backprojection method and an X-ray CT (computed tomography) apparatus, and more particularly to a backprojection method and an X-ray CT apparatus by which backprojection processing can be simplified and sped up.
The current mainstream X-ray CT apparatus implements a filtered backprojection technique involving processes of data collection, preprocessing, filtering, backprojection processing, and post-processing to thereby reconstruct an image.
Conventional backprojection processing is disclosed in, for example, Japanese Patent Application Laid Open No. H8-187241 and U.S. Pat. No. 5,414,622.
In such backprojection processing, projection data D
0
(view, ch) obtained by a fan beam represented by a view angle view and a detector channel ch is subjected to a calculation for projecting the projection data D
0
(view, ch) onto coordinates (x, y) of a pixel constituting a reconstruction region to obtain backprojection pixel data D
2
(x, y), and the backprojection pixel data D
2
(x, y) for all views employed in image reconstruction are added to obtain backprojection data D
3
(x, y).
In the conventional backprojection processing, a calculation for obtaining the backprojection pixel data D
2
(x, y) from the projection data D
0
(view, ch) must be conducted for, for example, 512×512 pixels, in which the projection data D
0
(view, ch) line up along arc-shaped geometrical positions corresponding to an arc-like shape of the detector, and the backprojection pixel data D
2
(x, y) line up along geometrical positions on rectangular coordinates of a reconstruction region. This raises the problems that the processing is intricate and time-consuming.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention is to provide a backprojection method and an X-ray CT apparatus by which backprojection processing can be simplified and sped up.
The present invention, in accordance with its first aspect, provides a backprojection method characterized in comprising the steps of: obtaining axially projected data D
1
by projecting projection data D
0
(view, ch) obtained by a fan beam represented by a view angle view and a detector channel ch onto a straight projection axis; then, obtaining backprojection pixel data D
2
by projecting said axially projected data D
1
onto pixels constituting a reconstruction region; and obtaining backprojection data D
3
by adding the backprojection pixel data D
2
for all views employed in image reconstruction on a pixel-to-pixel basis.
In the backprojection method of the first aspect, instead of obtaining the backprojection pixel data D
2
directly from the projection data D
0
(view, ch), axially projected data D
1
(view, pt) is obtained from the projection data D
0
(view, ch), and then backprojection pixel data D
2
(x, y) is obtained from the axially projected data D
1
. The symbol pt represents a coordinate on the projection axis.
Although the calculation for obtaining the axially projected data D
1
(view, pt) lining up along geometrical positions on a straight projection axis from the projection data D
0
(view, ch) lining up along arc-shaped geometrical positions corresponding to an arc-like shape of the detector has a processing load per datum identical to that of a conventional calculation for obtaining backprojection pixel data D
2
(x, y) from the projection data D
0
(view, ch), the number of data is no more than about 8,000, for example, which is only {fraction (1/30)} of that of the conventional 512×512 pixels. On the other hand, although the calculation for obtaining the backprojection pixel data D
2
from the axially projected data D
1
requires calculations for 512×512 pixels as in the prior art, the calculation for obtaining the backprojection pixel data D
2
(x, y) lining up on rectangular coordinates from the axially projected data D
1
(view, pt) lining up along a straight line needs only simple processing involving mere sampling at a regular pitch and multiplication by a distance factor. Thus, as a whole, the backprojection processing can be simplified and sped up.
The present invention, in accordance with its second aspect, provides the backprojection method of the aforementioned configuration, characterized in that when a direction of a center axis of the fan beam at views=0° is represented by a y-direction and a direction orthogonal to the y-direction and parallel to a fan beam plane is represented by an x-direction, said projection axis is defined as a straight line passing through a center of reconstruction and parallel to the x-direction for a view angle range of −45°≦view<45° or a view angle range mainly including the range and also including its vicinity, and for a view angle range of 135≦view<225° or a view angle range mainly including the range and also including its vicinity; and said projection axis is defined as a straight line passing through the center of reconstruction and parallel to the y-direction for a view angle range of 45°≦view<135° or a view angle range mainly including the range and also including its vicinity, and for a view angle range of 225°≦view<315° or a view angle range mainly including the range and also including its vicinity.
Note that view=−45° and view=315° are separately expressed herein for convenience of representation, but they are the same and represent the same view in reality.
When data is projected onto a straight projection axis, accuracy increases as the angle formed between the projection direction line and the projection axis approaches 90°, and accuracy decreases as the angle approaches 0°.
In the backprojection method of the second aspect, since the angle formed between the projection direction line and the projection axis never falls below about 45°, reduction in accuracy is prevented.
The present invention, in accordance with its third aspect, provides the backprojection method of the aforementioned configuration, characterized in that one axially projected datum D
1
is obtained by interpolation calculation from a plurality of projection data D
0
.
The number of projection data D
0
and the positional intervals thereof at one view angle are determined by the detector. Specifically, the number of projection data D
0
is “the number of channels of the detector (e.g., 1,000)”, and the positional intervals of the projection data D
0
are “the channel pitch of the detector (e.g., 1 mm)”.
In the backprojection method of the third aspect, since one axially projected datum D
1
is obtained by interpolation calculation from a plurality of projection data D
0
, the number of the axially projected data D
1
(e.g., 3,500 per view angle) and the data intervals thereof (e.g., 1 mm) on the projection axis can be selected without limitation by the number and position intervals of the projection data D
0
.
The present invention, in accordance with its fourth aspect, provides the backprojection method of the aforementioned configuration, characterized in that addresses of the plurality of projection data D
0
and interpolation factors for obtaining the one axially projected datum D
1
are set in a table.
Although the addresses of the plurality of projection data D
0
and interpolation factors for obtaining the one axially projected datum D
1
may be calculated each time the one axially projected datum D
1
is to be obtained, the time of the calculation is an overhead.
In the backprojection method of the fourth aspect, this overhead is eliminated by calculating beforehand the addresses of the plurality of projection data D
0
and interpolation factors and setting them in a table.
The present invention, in accordance with its fifth aspect, provides the backprojection method of the aforementioned configuration, characterized in further comprising the steps of: obtaining one axially projected datum D
1
by interpolation calculation from a pluralit
Hagiwara Akira
Nishide Akihiko
Armstrong Teasdale LLP
Bruce David V.
GE Medical Systems Global Technology Company LLC
Horton Esq. Carl B.
Kiknadze Irakli
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