X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-08-16
2004-06-01
Bruce, David V (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S017000, C378S901000
Reexamination Certificate
active
06744844
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image producing method and X-ray CT (Computed Tomography) apparatus, and more particularly, to an image producing method and X-ray CT apparatus by which, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed.
FIG. 16
is a flow chart showing a prior art image producing method for producing an image based on data obtained by a helical scan employing a multi-row detector having more than one detector row.
In Step S
1
, preprocessing such as sensitivity correction is applied to the data.
In Step S
3
, multi-slice/helical interpolation processing is applied for calculating interpolated data from proximate data in an image reconstruction plane.
In Step S
4
, backprojection processing is applied to the interpolated data to produce an image.
FIG. 17
is a schematic diagram showing exemplary multi-slice/helical interpolation processing using a twin detector.
FIG. 17
expresses the twin detector as viewed from an X-ray tube at a view angle &pgr;/2.
The interpolated data D(&pgr;/2, i) at the view angle &pgr;/2 for a channel i is calculated by linear interpolation from data d
1
(&pgr;/2, i) at the view angle &pgr;/2 for the channel i in a first detector row (j=1) and data d
2
(&pgr;/2, i) at the view angle &pgr;/2 for the channel i in a second detector row (j=2).
The linear interpolation is used because the data d
1
(&pgr;/2, i) and data d
2
(&pgr;/2, i) lie on a line along the subject's body axis, assuming that the CT value varies linearly in the direction of the subject's body axis.
Since the scan plane is not tilted in
FIG. 17
, the position h
1
of the data d
1
(&pgr;/2, i) of the first detector row (j=1) relative to the axis of translation is equal to the position h
2
of the data d
2
(&pgr;/2, i) of the second detector row (j=2) relative to the axis of translation. That is, both the data d
1
(&pgr;/2, i) and d
2
(&pgr;/2, i) lie on a line along the subject's body axis, which satisfies the condition required for linear interpolation.
When the scan plane is tilted, however, the position h
1
of the data d
1
(&pgr;/2, i) relative to the axis of translation and the position h
2
of the data d
2
(&pgr;/2, i) relative to the axis of translation become unequal, as shown in FIG.
18
. That is, the data d
1
(&pgr;/2, i) and the data d
2
(&pgr;/2, i) no longer lie on a line along the subject's body axis. As a result, the condition required for linear interpolation is not satisfied, leading to artifacts.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention is to provide an image producing method and X-ray CT apparatus by which, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed.
In accordance with a first aspect, the present invention provides an image producing method for producing an image based on data collected by a helical scan employing a multi-row detector having more than one detector row with a scan plane tilted, characterized in comprising the step of: applying to the data tilt correcting processing for correcting view-to-view variation of the positions of channels in the detector rows relative to an axis of translation due to the tilt of the scan plane.
In the image producing method of the first aspect, tilt correcting processing is newly introduced for correcting view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane. Thus, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed.
In accordance with a second aspect, the present invention provides an image producing method characterized in comprising the steps of: applying preprocessing such as sensitivity correction to data collected by a helical scan employing a multi-row detector having more than one detector row with a scan plane tilted; next applying tilt correcting processing for correcting view-to-view variation of the positions of channels in the detector rows relative to an axis of translation due to the tilt of the scan plane; applying multi-slice/helical interpolation processing for calculating interpolated data from proximate data in an image reconstruction plane; and applying backprojection processing to the interpolated data to produce an image.
In the image producing method of the second aspect, the tilt correcting processing is applied before the multi-slice/helical interpolation processing for correcting view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane. Thus, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed using the same multi-slice/helical interpolation processing as conventionally used.
In accordance with a third aspect, the present invention provides the image producing method having the aforementioned configuration, characterized in that said tilt correcting processing includes data position shifting processing for shifting the positions of data arranged in a two-dimensional array along a channel index axis and a view index axis so that the view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of the scan plane is canceled out; data extracting processing for extracting a range of data in which data are completely present for all the views in the view direction from the shifted data array; dummy data appending processing for appending the extracted data with dummy data to adjust the data range; and data transforming processing for transforming the data into data enabling alignment of the channel positions through all views.
In the image producing method of the third aspect, the positions of data are first shifted so that the view-to-view variation of the positions of channels in the detector rows relative to the axis of translation due to the tilt of a scan plane is canceled out. However, the amount of data position shift is not limited to an integer multiple of the channel-to-channel distance. Accordingly, in the shifted data array, there occur concave and convex portions at the opposite ends of the channel index axis, and hence, a portion along the view direction in which data are completely present for all the views and a portion along the view direction that partially lacks data. The portion along the view direction in which data are completely present for all the views can be used as is, and a range of the data corresponding to the portion is extracted. On the other hand, using the portion along the view direction that partially lacks data may cause artifacts, and the portion is discarded. Missing data corresponding to the discarded portion are made up for with dummy data to adjust the data range. Since, as pointed out above, the amount of data position shift is not limited to an integer multiple of the channel-to-channel distance, the data positions in the resulting data array do not match the channel positions for all the views. Therefore, the data are transformed into data enabling alignment of the channel positions through all views by, for example, interpolation. Thus, when a helical scan is conducted employing a multi-row detector having more than one detector row with a scan plane tilted, an image can be obtained with artifacts suppressed using the same multi-slice/helical interpolation processing as conventionally used.
In accordance with a fourth aspect, the present invention provides the image producing method having the aforementioned configuration, characterized in that: said data position shifting processing shifts the positions of
Armstrong Teasdale LLP
Bruce David V
GE Medical Systems Global Technology Company LLC
Horton Esq. Carl B.
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