X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-11-04
2001-08-07
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S011000, C378S901000
Reexamination Certificate
active
06272199
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) apparatus.
2. Description of the Prior Art
A CT apparatus is described in U.S. Pat. No. 4,637,040 which has an X-ray source which is rotated around an examination subject for obtaining datasets (projections) from a number of different projection angles, the X-ray source having a focus which is moved back and forth between two end positions (alternating focus). A detector system, composed of a number of detector elements, receives the X-rays emanating from the alternating focus and attenuated by the examination subject. Successive, adjacent detector elements which are disposed at a grid element spacing from an adjacent detector element, each detector element having an aperture. The detector elements in the aperture emit output data, corresponding to the aforementioned attenuated X-rays, the number of output data items corresponding to the number of detector elements which participate in obtaining the data for a given projection. The output data are supplied to a computer which reconstructs an image of the examination subject, or a portion thereof, based on the output data.
In a CT apparatus such as this, besides the size of the focus of the X-ray source, the MTF (Modulation Transfer Function) of the reconstruction algorithm and the pixel size of the reconstructed image, the scanning frequency with which the projection set is obtained, and thus the grid element spacing between immediately adjacent detector elements, are important determinants of the achievable spatial resolution.
The movement of the focus between two end positions, that is, the use of an X-ray tube with an alternating focus, serves to enhance the spatial resolution, by increasing the scanning frequency, and is taught by the above cited U.S. Pat. No. 4,637,040.
In addition, U.S. Pat. No. 4,008,400 to increase the scanning frequency by obtaining complementary projection datasets by what is known as the &lgr;/4 shift. However, the &lgr;/4 shift presumes a CT-device with highly stable and precise mechanical relations and is also not very effective when the path radius at which the focus of the X-ray source moves around the subject is short.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a CT apparatus of the above type wherein it is possible to enhance the spatial resolution even given a short path radius of the focus of the X-ray source, without placing higher requirements on the mechanical stability of the CT apparatus.
This object is inventively achieved in a CT-device having an X-ray source which is moved around a subject for irradiating a subject from a number of projection angles, and a detector system for receiving the radiation emanating from the focus. The focus is moved back and forth between two end positions in the X-radiation source. The detector elements of the detector system are arranged in succession, each detector element being spaced from an adjacent detector element by a grid element spacing and each detector element having an aperture. The data represent the beam attenuation in the path of the X-rays to the respective detector element. The number of data items is twice the number of detector elements participating in capturing the projection. An electronic computing unit converts the output data into image reconstruction data. These data contain a number of data items per projection which is greater than twice the number of detector elements participating in the pick-up of the respective projection, preferably by a factor that is at least equal the ratio of the grid element spacing to the aperture size. The electronic computing unit reconstructs an image based on the image reconstruction data.
The invention makes use of the fact that the maximum spatial frequency that is contained in the output data corresponds to the detector aperture. This information is obtained but unused in conventional CT devices, since, due to the movement of the focus between two end positions, the number of output data items per projection corresponds to twice the number of detector elements participating in picking up the projection, and so the maximum spatial frequency corresponds to the grid element spacing. In the case of the invention, however, a conversion of the output data into image reconstruction data takes place, so that the maximum spatial frequency contained in the output data is used.
It is well known that in practice the maximum achievable spatial resolution usually does not reach the zero position that is predetermined by the aperture. Rather, 95% of the theoretical maximum can be expected in practice.
In a preferred embodiment of the invention, the output data represent fan projections, while the image reconstruction data represent parallel projections. The enhancement of the spatial resolution occurs in the course of the conversion of the output data representing the fan projections into data representing parallel projections, which serve for image reconstruction, this conversion preferably occurring by interpolation. This corresponds to the conversion into a new coordinate system by linking location information, that is, output data that have been collected in the capturing of various fan projections. In a further embodiment of the invention, in the interpolation signal portions with a spatial frequency greater than or equal to the reciprocal of the grid element spacing of the detector are substantially suppressed. One suitable interpolation function is described by the equation:
h
1
a
(&bgr;,&Dgr;&bgr;)=
ch
1
(&bgr;,&Dgr;&bgr;)+(1
−c
)
h
1
r
(&bgr;,&Dgr;&bgr;) (1)
wherein h
1
is the interpolation kernel of the linear interpolation, for which the following equation applies:
h
1
(
β
,
Δ
β
)
=
{
1
-
&LeftBracketingBar;
β
&RightBracketingBar;
Δ
β
&LeftBracketingBar;
β
&RightBracketingBar;
≤
Δβ
for
0
Δβ
<
&LeftBracketingBar;
β
&RightBracketingBar;
(
2
)
wherein h
r
1
is defined by
h
1
r
(
β
,
Δβ
)
=
{
1
3
(
1
+
1
-
3
(
β
Δβ
)
2
)
&LeftBracketingBar;
β
&RightBracketingBar;
≤
0.5
Δ
β
1
6
(
2
+
3
(
1
-
&LeftBracketingBar;
β
Δβ
&RightBracketingBar;
)
-
1
-
3
(
1
-
&LeftBracketingBar;
β
Δβ
&RightBracketingBar;
)
2
)
for
0.5
Δβ
<
&LeftBracketingBar;
β
&RightBracketingBar;
≤
1.5
Δβ
0
1.5
Δβ
<
&LeftBracketingBar;
β
&RightBracketingBar;
(
3
)
and wherein &bgr; is the fan channel angle and &Dgr;&bgr; is its increment.
REFERENCES:
patent: 4008400 (1977-02-01), Brunnett et al.
patent: 4637040 (1987-01-01), Sohval et al.
patent: 5361291 (1994-11-01), Toth et al.
patent: 5430785 (1995-07-01), Pfoh et al.
patent: 5625661 (1997-04-01), Oikawa
patent: 6047040 (2000-04-01), Hu et al.
Sembritzki Otto
Wallschlaeger Heinrich
Bruce David V.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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