X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-01-19
2002-06-04
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S020000, C378S004000
Reexamination Certificate
active
06400789
ABSTRACT:
RELATED APPLICATION
This application is a US National filin of PCT Application PCT/IL98/00075, filed Feb. 12, 1998.
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 revolve continuously about a subject being imaged. The subject lies on a bed, which is translated continuously through the gantry simultaneously with the tube's revolution. An array of X-ray detectors on the opposite side of the subject from the X-ray tube receive radiation transmitted through the subject. The detectors generate signals proportional to the attenuated X-ray flux incident thereon. The signals are pre-processed to produce attenuation data, which are used in reconstructing orimages of the subject. In “third-generation” scanners, the array of detectors is mounted on the gantry so as to revolve along with the X-ray tube, whereas in “fourth-generation” scanners, the detectors are arrayed in a ring, which is generally stationary.
The axis of translation of the bed (conventionally the Z-axis) is generally parallel to the long axis of the subject's body, which is typically perpendicular to the plane of revolution of the tube. 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 detector array similarly correspond to a series of helically-disposed angular “views” through the subject.
In order to reconstruct a planar cross-sectional image slice of the subject at a desired axial position, based on the helical-scan views, effective attenuation values for each of a plurality of points around a circumference of such a planar slice are derived by interpolation between data received in the original helical-path views. For each of the plurality of points, the respective effective attenuation values thus correspond to the approximate attenuation along rays within the planar slice that pass through the point. For 360° reconstruction, as is known in the art, the plurality of points are distributed around the entire circumference of the slice. For 180° reconstruction, also known in the art, the points are distributed over an extent equal to half the circumference plus the “fan angle,” i.e., the angular field of view covered by the fan-shaped X-ray beam. (For convenience in the following discussion, we will refer to the total angular extent of all the views that are collectively used in the reconstruction of a complete planar slice as the “reconstruction angle,” typically 360° or 180°.) The interpolated effective attenuation values are filtered and back-projected to produce the cross-sectional image.
In general, to derive effective attenuation values for a given view angle and axial position, it is necessary to interpolate between actual data from at least two different views at that view angle, bracketing the axial position of the planar slice. The two views are acquired in successive scan segments, separated by the fall reconstruction angle. Because data from two different scan segments must typically be combined to reconstruct each planar image slice, data from the earlier of the two scans must be stored in buffer memory. Two full scans, comprising twice the total reconstruction angle, are thus needed to produce a single cross-sectional image. Reconstruction of each planar image slice lags behind acquisition of the data in the earlier of the two scans by at least the time it takes the tube to make a full scan around the subject. By comparison, in axial (non-helical) CT scanners, successive views may be processed in a “pipeline” data flow, so that the slice image is displayed with a minimal delay after completion of a single scan. The lag in helical scan reconstruction is particularly disadvantageous when CT imaging is used to track the progress of a physiological process, such as the flow of a contrast material.
Multi-slice helical-path scanners are 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 adjacent, parallel slices are acquired along two parallel paths simultaneously or sequentially. Data corresponding to planar slices are derived by interpolating between data acquired along the two helical paths.
U.S. Pat. No. 5,524,130, the disclosure of which is incorporated herein by reference describes a number of methods for utilizing a single detector ring scanner to provide successive axially spaced slices with reduced time between reconstruction of the slices. Some of these methods appear to utilize partial scan data from one scan to replace data from a second scan taken at a different time for reducing the reconstruction time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for on-line image reconstruction of helical-scan CT data.
In one 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 multi-slice helical-scan CT scanner comprises an X-ray tube, mounted to revolve on an annular gantry about a bed on which a subject lies, and a detector array. 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 X-ray tube thus describes a generally helical trajectory about the subject's body and irradiates the subject from multiple positions, or “views,” along this trajectory. The detector array preferably comprises two 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 at each of the views and generate signals responsive to attenuation of the X-rays.
Preferably, for 360° CT image reconstruction, the width of the detector array which is exposed to radiation, measured in a direction parallel to the translation axis, is substantially greater than the pitch of the helical trajectory. For 180° reconstruction, the width of the detector array which is exposed to radiation is substantially greater than half the pitch of the helical trajectory.
For each view, the two or more rows of the array simultaneously generate corresponding line attenuation signals. These signals are preprocessed, as is known in the art. The resultant line attenuation data are interpolated to generate geometrically-corrected effective attenuation values, which are associated with planar slices through the body that are generally perpendicular to the translation axis. The corrected effective attenuation values are filtered and back-projected to calculated CT values, which are used to update cross-sectional CT images substantially in real time.
In preferred embodiments of the present invention, an effective attenuation value is calculated for each of a plurality of points on a periphery of each planar slice by weighted interpolation between first and second measured attenuation values, taken from respective first and second line attenuation signals within a single, multiple-slice view. The values are preferably calculated by linear interpolation, but may alternatively be calculated using non-linear or other helix interpolation schemes known in the art. The first and second line attenuation signals are derived respectively from data received simultaneously from first and second rows of detector elements. Thus, there is no need for any additional buffer memory to store data from a preceding scan (although the data may be stored if desired), and a complete cross-sectional CT image may be acquired and immediately reconstructed within a time window correspon
Fenster & Company Patent Attorneys Ltd.
Hobden Pamela R.
Kim Robert H.
Philips Medical Systems Technologies Ltd.
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