Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-07-03
2002-10-22
Walberg, Teresa (Department: 3742)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06470206
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an addition tomographic image producing method and an X-ray CT apparatus, and more particularly to an addition tomographic image producing method and an X-ray CT apparatus which, when an addition tomographic image is produced by adding a plurality of tomographic images, can produce an addition tomographic image not adversely affected by the partial volume effect even when the slice thicknesses of the tomographic images are different.
1. First Example (Prior Art)
FIG. 1
is a flow chart showing a first example of a conventional addition tomographic image producing process in a conventional X-ray CT apparatus comprising a multi X-ray detector having a plurality of detector rows.
In Step ST
61
, an image index counter j is initialized to “1”.
In Step ST
62
, row data Ij is read from a j-th detector row in the multi X-ray detector.
In Step ST
63
, an offset correction is performed. Specifically, an offset is subtracted from the row data Ij of the j-th detector row. For brevity of notation, the row data from which the offset has been subtracted will also be designated as
In Step ST
64
, a detector sensitivity correction is performed. Specifically, the row data Ij of the j-th detector row is divided by an entering X-ray dose Tj·Io, wherein Tj is the slice thickness of the j-th detector row, and Io is the entering X-ray dose per unit thickness.
In Step ST
65
, a first correction is applied to the row data Ij/(Tj·Io) subjected to the detector sensitivity correction. The first correction is a reference correction. For brevity of notation, the row data subjected to the first correction will also be designated as Ij/(Tj·Io).
In Step ST
66
, if the value of a certain channel in the row data Ij/(Tj·Io) subjected to the first correction is not more than a predefined threshold, then the value is replaced by the average value of one or more adjacent channels. This processing will be referred to as Log filtering hereinbelow.
In Step ST
67
, logarithm processing is performed. Specifically, −Log{Ij/(Tj·Io)} is calculated.
In Step ST
68
, a second correction is applied to the row data subjected to the logarithm processing. The second correction is at least one of a beam hardening correction, a water correction, a body motion correction and a helical correction.
In Step ST
69
, filtering and a back projection operation are applied to the row data subjected to the second correction to obtain a CT image Gj.
In Steps ST
70
and ST
71
, Steps ST
62
-ST
69
are repeated for j=2-J. Thus, CT images GI-GJ are reconstructed.
In Step ST
72
, the CT images GI-GJ are simply added to obtain an addition CT image G.
2. Second Example (Prior Art)
FIG. 2
is a flow chart showing a second example of a conventional addition tomographic image producing process in a conventional X-ray CT apparatus comprising a multi X-ray detector having a plurality of detector rows.
In Step ST
81
, an image index counter j is initialized to “1”.
In Step ST
82
, row data Ij is read from a j-th detector row in the multi X-ray detector.
In Step ST
83
, an offset correction is performed. Specifically, an offset is subtracted from the row data Ij of the j-th detector row. For brevity of notation, the row data from which the offset has been subtracted will also be designated as Ij.
In Step ST
84
, a detector sensitivity correction is performed. Specifically, the row data Ij of the j-th detector row is divided by an entering X-ray dose Tj·Io.
In Step ST
85
, a first correction is applied to the row data Ij/(Tj·Io) subjected to the detector sensitivity correction. The first correction is a reference correction. For brevity of notation, the row data subjected to the first correction will also be designated as Ij/(Tj·Io).
In Step ST
86
, Log filtering is performed.
In Step ST
87
, logarithm processing is performed. Specifically, −Log{Ij/(Tj·Io)} is calculated.
In Step ST
88
, a second correction is applied to the row data subjected to the logarithm processing. The second correction is at least one of a beam hardening correction, a water correction, a body motion correction and a helical correction.
In Steps ST
89
and ST
90
, Steps ST
82
-ST
88
are repeated for j=2-J. Thus, row data I′
1
-I′J subjected to the second correction are obtained.
In Step ST
91
, the row data I′
1
-I′J subjected to the second correction are simply added to obtain addition data I, that is:
I=&Sgr;I′j
In Step ST
92
, filtering and a back projection operation are applied to the addition data I to obtain an addition CT image G.
In the first and second examples of the conventional technique, when the slice thicknesses of the detector rows are equal, there is no significant problem in obtaining the addition CT image G.
However, when the slice thicknesses are different, an adverse effect of the partial volume appears on the addition CT image G. Specifically, the different slice thicknesses produce different partial volume effects, resulting in different CT values for the same substance. In other words, discontinuity arises in the CT values of the same substance. Accordingly, the CT values of an object which should appear to be uniform appear to partially vary on the addition CT image G.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an addition tomographic image producing method and an X-ray CT apparatus which, when an addition tomographic image is produced by adding a plurality of tomographic images, can produce an addition tomographic image not adversely affected by the partial volume effect even when the slice thicknesses of the tomographic images are different.
In accordance with a first aspect, the present invention provides an addition tomographic image producing method comprising the steps of: applying an offset correction and a sensitivity correction to data Ij (j=
1
-J) from which respective tomographic images Gj can be reconstructed; generating addition data by performing a weighted addition on the data Ij with a weight Tj/&Sgr;Tj corresponding to the slice thickness Tj; applying logarithm processing to the addition data; and performing reconstruction on the addition data to produce an addition tomographic image G.
According to the addition tomographic image producing method in the first aspect, the addition data is generated by performing a weighted addition on the data Kj subjected to the sensitivity correction with a weight Tj/&Sgr;Tj corresponding to the slice thickness Tj. Specifically, when the addition data is represented as K,
K
=&Sgr;{(
Tj/&Sgr;Tj
)·
Kj
As explained further later, K is equivalent to sensitivity-corrected data of one slice having a slice thickness of &Sgr;Tj. Therefore, no adverse effect of the partial volume will appear on the addition tomographic image G produced by adding a plurality of tomographic images Gj having different slice thicknesses.
In accordance with a second aspect, the present invention provides the addition tomographic image producing method of the aforementioned configuration, wherein the tomographic image Gj is an X-ray CT image, and the data Ij is row data.
According to the addition tomographic image producing method in the second aspect, the addition data K is generated by performing a weighted addition on the row data Kj subjected to the sensitivity correction with a weight Tj/&Sgr;Tj corresponding to the slice thickness Tj. Specifically,
K
=&Sgr;{(
Tj/&Sgr;Tj
)·
Kj
As described later, K is equivalent to sensitivity-corrected row data of one slice having a thickness of &Sgr;Tj . Therefore, no adverse effect of the partial volume will appear on the addition tomographic image G produced by adding a plurality of X-ray CT images Gj having different slice thicknesses.
In accordance with a third aspect, the present invention provides the addition tomographic image producing method of aforementioned configuration, wherein the row data is that obtained by each detector row in a multi X-ray detector.
According to
Nishide Akihiko
Nukui Masatake
GE Medical Systems Global Technology Company LLC
Moonray Kojima
Robinson Daniel
Walberg Teresa
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