The present invention relates to an X-ray CT (Computed Tomography) imaging method, subject center position detection method and X-ray CT apparatus, and more particularly to an X-ray CT imaging method, subject center position detection method and X-ray CT apparatus which can detect whether the center position of a subject is significantly offset from a scan center.
In X-ray CT imaging, a human operator adjusts the height of a top plate for laying a subject by visual measurement so that the center position of the subject coincides with a scan center (a center of rotation of an X-ray tube) with minimum offset.
However, the adjustment by the operator""s visual measurement sometimes makes the center position of the subject significantly offset from the scan center, which results in poor homogeneity in the CT value of a subject""s image. Moreover, overrange may occur near the scan center, when the scan center falls outside the subject.
It is therefore an object of the present invention to provide an X-ray CT imaging method, subject center position detection method and X-ray CT apparatus which can detect whether the center position of a subject is significantly offset from a scan center.
In accordance with a first aspect of the present invention, there is provided an X-ray CT imaging method for performing a scan by an X-ray tube rotating around a subject, comprising the steps of: detecting a subject center position based on a projection profile acquired by imaging the subject; and giving a warning when a calculated distance between the subject center position and a scan center exceeds a predefined range.
The X-ray CT imaging method of the first aspect displays a warning on a display screen or gives a vocal prompt to a human operator to adjust the position when the subject center position is offset from the scan center by an amount exceeding a predefined range (e.g., 50 cm). Thus, the method can detect whether the center position of a subject is significantly offset from a scan center.
In accordance with a second aspect, there is provided a subject center position detection method comprising the steps of: acquiring a projection profile by imaging a subject from a direction orthogonal to a plane containing a body axis of the subject by an X-ray CT apparatus for performing a scan by an X-ray tube rotating around the subject; defining an X-ray detector channel index corresponding to a calculated centroid of the projection profile as a centroid channel index; examining a projection value in the projection profile toward a direction along a first direction of the subject from the centroid channel index; defining a channel index having a projection value no more than a predetermined projection value threshold Pth as a first threshold channel index; examining on the other hand a projection value in the projection profile toward a direction along a second direction of the subject opposite to the first direction from the centroid channel index; defining a channel index having a projection value no more than the predetermined projection value threshold Pth as a second threshold channel index; defining an intermediate value between the first and second threshold channel indices as a subject position channel index; and obtaining a subject center position from the subject position channel index.
A projection profile acquired by imaging a subject lying on a top plate from a direction orthogonal to a plane containing the subject""s body axis contains a projection of the top plate as well as a projection of the subject. Thus, the centroid of the projection profile is shifted toward the top plate relative to a subject center position. Therefore, the subject center position should be determined on a side opposite to the top plate with respect to the centroid of the projection profile.
The subject center position detection method of the second aspect introduces xe2x80x9cprojection value threshold Pthxe2x80x9d as a parameter, searches channels having respective values no more than the projection value threshold Pth in two directions, calculates an intermediate of the retrieved two channels, and determines the subject center position on a side opposite to the top plate with respect to the centroid of the projection profile based on the intermediate. Thus, the subject center position can be accurately determined with the effect of the top plate eliminated.
In the subject center position detection method of the above configuration, the projection value threshold Pth may be selected empirically as, for example, xc2xd of a projection value corresponding to the centroid channel index.
Moreover, in the subject center position detection method of the above configuration, the subject center position can be determined more accurately by first removing profiles other than the subject""s profile from the projection profile, and then calculating the centroid.
In accordance with a fifth aspect, there is provided an X-ray CT apparatus for performing a scan by an X-ray tube rotating around a subject, comprising: projection profile acquisition means for acquiring a projection profile by imaging the subject; subject position detection means for detecting a subject center position based on the acquired projection profile; and warning displaying means for displaying a warning when the distance between the subject center position and a scan center exceeds a predefined range.
The X-ray CT apparatus of the fifth aspect can suitably implement the X-ray CT imaging method as described regarding the first aspect.
In accordance with a sixth aspect, there is provided an X-ray CT apparatus for performing a scan by an X-ray tube rotating around a subject, comprising subject center position detection means configured to: define an X-ray detector channel index corresponding to a calculated centroid of a projection profile acquired by imaging a subject from a direction orthogonal to a plane containing a body axis of the subject as a centroid channel index; examine a projection value in the projection profile toward a direction along a first direction of the subject from the centroid channel index; define a channel index having a projection value no more than a predetermined projection value threshold Pth as a first threshold channel index; examine on the other hand a projection value in the projection profile toward a direction along a second direction of the subject opposite to the first direction from the centroid channel index; define a channel index having a projection value no more than the predetermined projection value threshold Pth as a second threshold channel index; define an intermediate value between the first and second threshold channel indices as a subject position channel index; and obtain a subject center position from the subject position channel index.
The X-ray CT apparatus of the sixth aspect can suitably implement the subject center position detection method as described regarding the second aspect.
In the X-ray CT apparatus of the above configuration, the projection value threshold Pth may be selected empirically as, for example, xc2xd of a projection value corresponding to the centroid channel index.
Moreover, in the X-ray CT apparatus of the above configuration, the subject center position can be determined more accurately by first removing profiles other than the subject""s profile from the projection profile, and then calculating the centroid.
To remove profiles other than the subject""s profile from the projection profile, a one-dimensional minimum value filter, followed by a one-dimensional maximum value filter, for example, may be applied to the projection file.
The X-ray CT imaging method, subject center position detection method and X-ray CT apparatus of the present invention can detect whether the center position of a subject is offset from a scan center, and can notify an operator of the result of the detection. Thus, even when the subject center position is significantly offset from the scan center, the position of the subject is adjusted and imaging is repeated, whereby an X-ray image with good quality can be obtained.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.