X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-11-22
2001-11-27
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S017000, C378S901000
Reexamination Certificate
active
06324246
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to computerized tomographic (CT) imaging, and specifically to multi-slice CT scanners having helical scan paths.
BACKGROUND OF THE INVENTION
Helical-path CT scanners are well known in the art. Generally, such scanners comprise an X-ray tube, mounted on an annular gantry, so as to rotate continuously about a subject being imaged. The subject lies on a table, which is translated continuously through the gantry simultaneously with the gantry's rotation, while X-ray detectors on the opposite side of the subject from the X-ray tube receive radiation transmitted through the subject. The axis of translation of the bed is generally parallel to the long axis of the subject's body, which is typically perpendicular to the plane of rotation of the gantry. Thus, the path of the X-ray tube relative to the subject generally describes a helix about this axis, and X-ray attenuation data received from the X-ray detectors similarly correspond to a series of helically-disposed “views” through the subject. In order to reconstruct planar cross-sectional image slices of the subject, attenuation data for each point in such a planar slice are derived by interpolation between data points in the original helical-path views.
Multi-slice helical-path scanners are similarly known in the art. For example, U.S. Pat. No. 5,485,493, which is incorporated herein by reference, describes a multiple detector ring spiral scanner with relatively adjustable helical paths, in which two or more adjacent, parallel slices are acquired along two or more parallel paths simultaneously or sequentially. Data corresponding to planar slices are derived by interpolating between data acquired along the two helical paths. Helical-path scanners in which more than two slices are acquired are also known in the art.
In some scanners, the long axis of the subject's body, along which direction the bed is translated, may be angled relative to the plane of rotation of the gantry, rather than being perpendicular to the axis, as in conventional scanners. This angling typically includes swiveling the bed about a vertical axis, tilting the gantry about a horizontal axis, or a combination of swiveling and tilting. Since the image views are similarly angled relative to the body axis, this angling function is frequently useful in resolving image features that may be difficult to observe in conventional, non-angled scanning. For example, bed swivel may be used in generating longitudinal image slices through the pancreas, and variable gantry tilt may be used to generate images of angled, sectional cuts through the disc spaces of the spine.
When the body axis is tilted, the scanning path of the X-ray tube relative to the axis no longer describes a simple, constant-pitch helix, but rather a more complex spiral figure. In this case, accurate interpolation between different points acquired along a helical path, for the purpose of reconstructing corrected planar image slices, becomes considerably more complicated. Improper selection of the points for interpolation can produce artifacts in the reconstructed image.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for accurate image reconstruction based on angled helical-scan CT data.
In one aspect of the present invention, angled helical scan data generated during different positions of the gantry (i.e., at different helical positions of the gantry) are combined to form a data set of a view for reconstruction of the image. Due to the geometry of the system this requires that data from non-corresponding elements be combined, where corresponding elements are defined as:
for 360 degree reconstruction, as elements having the same circumferential position; and
for 180 degree reconstruction, as elements having the same circumferential distance from an element corresponding to the center of rotation of the gantry, on opposite sides of that element.
In one aspect of the present invention, the helical-scan data are used to reconstruct planar corrected image slices.
In another aspect of the present invention, the CT data comprise multiple-slice CT data, acquired using a multi-row detector array.
In preferred embodiments of the present invention, a variable-angle multi-slice helical-scan CT scanner comprises an X-ray tube, mounted on an annular gantry, which rotates about a bed on which a subject lies, and a detector array. The X-ray tube irradiates the subject from multiple points along its helical trajectory. The detector array comprises one or more parallel rows of X-ray detector elements, each row having a long axis disposed in a generally circumferential direction with respect to the long axis of the subject's body. The detector elements receive radiation that has passed through the subject's body and generate signals responsive to attenuation of the X-rays. The bed is advanced through the gantry along a translation axis that is generally parallel to the long axis of the subject's body. The gantry tilts about a horizontal axis, and the bed swivels, relative to the gantry, about a vertical axis, so that the translation axis of the bed describes an acute angle relative to the axis of rotation of the gantry. The scanner thus performs an angled helical scan over at least a portion of the body.
For each view, i.e., each position of the X-ray tube relative to the body at which X-ray attenuation signals are received from the detector array, the detector array generates a matrix of attenuation signals. Each row in the signal matrix corresponds to a row of elements in the detector array. These signals are normalized and undergo a log operation, as is known in the art. Preferably, the resultant data are then interpolated to generate geometrically-corrected CT data, which are associated with planar slices through the body. These slices are generally perpendicular to the gantry rotation axis, and are therefore swiveled and/or tilted with respect to the long axis of the body. The corrected data in these planar slices are filtered and back-projected to reconstruct a three-dimensional CT image of the subject's body, using methods known in the art.
Alternatively, instead of interpolating the normalized, log data, the “raw” signals may first be interpolated before undergoing the log operations. Further alternatively, the data may be interpolated after the filtering or after the back-projection operation. It will be appreciated that the principles of the present invention may be applied in these cases, as well.
In preferred embodiments of the present invention, the geometrically-corrected CT data comprise effective attenuation values with respect to each of the planar slices. For each slice, these values are calculated for a plurality of effective detection points, geometrically fixed along a periphery of the slice. Each of the effective attenuation values corresponds to the approximate attenuation that would have been measured along a ray in the planar slice from the X-ray tube to the location of the effective detection point, at a given rotation angle of the tube about the gantry's axis of rotation The effective attenuation values for each planar slice are calculated for a plurality of rotation angles, preferably covering 360° of rotation about the axis (or more, depending on the helix angle). These values are filtered and back-projected using 360° CT image reconstruction, as is known in the art. Reconstruction using single slices requires at least two rotations and generally more, depending on the helix angle.
Alternatively, 180° reconstruction may be used, as described in an Israel Application filed on even date with the present application, entitled “On-Line Image Reconstruction in Helical CT Scanners” by Elscint Ltd., assignee of the present application, and incorporated herein by reference. In this case, the effective attenuation values for each planar slice are calculated for a plurality covering only about 180° of rotation.
Although the effective detection points are fixed in the
Bruce David V.
Fenster & Company Patent Attorneys Ltd.
Marconi Medical Systems Israel Ltd.
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