X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-10-17
2001-11-27
Dunn, Drew (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Reexamination Certificate
active
06324245
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application includes subject matter related to U.S. patent application entitled A Method and Apparatus for Simplifying the Correction of Image Inaccuracies Caused by Processing of Masked Cone Beam Projection Data, filed simultaneously herewith by the present inventor.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to a cone beam computed tomography (CT) imaging system, and more specifically to a method and apparatus for identifying and correcting image inaccuracies caused by simplified processing of masked cone beam projection data.
2. Description of the Prior Art
Recently a system employing cone beam geometry has been developed for three-dimensional (3D) computed tomography (CT) imaging that includes a cone beam x-ray source and a 2D area detector. An object to be imaged is scanned, preferably over a 360° angular range and along its entire length, by any one of various methods wherein the position of the area detector is fixed relative to the source, and relative rotational and translational movement between the source and object provides the scanning (irradiation of the object by radiation energy). The cone beam approach for 3D CT has the potential to achieve 3D imaging in both medical and industrial applications with improved speed, as well as improved dose utilization when compared with conventional 3D CT apparatus (i.e., a stack of slices approach obtained using parallel or fan beam x-rays).
As a result of the relative movement of the cone beam source to a plurality of source positions (i.e., “views”) along the scan path, the detector acquires a corresponding plurality of sequential sets of cone beam projection data (also referred to herein as cone beam data or projection data), each set of cone beam data being representative of x-ray attenuation caused by the object at a respective one of the source positions.
As well known, and fully described for example in the present inventor's U.S. Pat. No. 5,257,183 entitled METHOD AND APPARATUS FOR CONVERTING CONE BEAM X-RAY PROJECTION DATA TO PLANAR INTEGRAL AND RECONSTRUCTING A THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY (CT) IMAGE OF AN OBJECT issued Oct. 26, 1993, incorporated herein by reference, image reconstruction processing generally begins by calculating Radon derivative data from the acquired cone beam data. The Radon derivative data is typically determined by calculating line integral derivatives for a plurality of line segments L drawn in the acquired cone beam data. In the embodiment described in detail in the U.S. Pat. No. 5,257,183, Radon space driven conversion of the derivative data is used to develop an exact image reconstruction of a region-of-interest (ROI) in the object. Calculation of the line integral derivative data is a relatively complex and computationally time consuming task.
A cone beam data masking technique is known which improves the efficiency of the calculation of the derivative data in such Radon space driven reconstruction processing, as described in the present inventor's U.S. Pat. No. 5,504,792 entitled METHOD AND SYSTEM FOR MASKING CONE BEAM PROJECTION DATA GENERATED FROM EITHER A REGION OF INTEREST HELICAL SCAN OR A HELICAL SCAN, issued Apr. 2, 1996, also incorporated herein by reference. The masking technique facilitates efficient 3D CT imaging when only the ROI in the object is to be imaged, as is normally the case. In the preferred embodiment described therein, a scanning trajectory is provided about the object, the trajectory including first and second scanning circles positioned proximate the top and bottom edges, respectively, of the ROI, and a spiral scanning path connected therebetween. The scanning trajectory is then sampled at a plurality of source positions where cone beam energy is emitted toward the ROI. After passing through the ROI, the residual energy at each of the source positions is acquired on an area detector as a given one of a plurality of sets of cone beam data. Each set of the cone beam data is then masked so as to remove a portion of the cone beam data that is outside a given sub-section of a projection of the ROI in the object and to retain cone beam projection data that is within the given sub-section. The shape of each mask for a given set of cone beam data is determined by a projection onto the detector of the scan path which is above and below the source position which acquired the given set of cone beam data. The masked (i.e., retained) cone beam data is then processed so as to develop line integral derivative reconstruction data. An exact image of the ROI is developed by combining the reconstruction data from the various source positions which intersect a common integration plane. Hence, the masks are commonly referred to as “data-combination” masks.
Although the use of the data combination masks significantly reduces the processing required in Radon driven approaches, calculation of the line integral derivative data is still a relatively complex task and computationally time consuming. One known technique for developing the line integral derivative reconstruction data in such a Radon space driven reconstruction processing approach, is to use linograms. Although the linogram technique provides for a much faster and more simplified processing of the masked data for developing the line integral derivative reconstruction data, its creates less than exact, i.e., quasi-exact, reconstructed images.
Data-combination masks can also be used to improve the efficiency of the calculation of the derivative data in detector data driven techniques, such as those using Filtered BackProjection (FBP) techniques. A “simplified” FBP technique is described in the present inventor's U.S. Pat. No. 5,881,123 entitled SIMPLIFIED CONE BEAM IMAGE RECONSTRUCTION USING 3D BACKPROJECTION, issued Mar. 9, 1999, also incorporated herein by reference. This simplified technique reconstructs the image using 2D approximation data sets formed by ramp filtering of the masked cone beam data. The filtering is carried out in the direction of the projection of a line drawn tangent to the scan path at the source position that acquired that set of cone beam data. Although this technique is also less complex than the prior techniques, the reconstructed image is also quasi-exact.
Accordingly, the present inventor's U.S. Pat. No. 6,084,937 entitled ADAPTIVE MASK BOUNDARY CORRECTION IN A CONE BEAM IMAGING SYSTEM, issued Jul. 4, 2000, and also incorporated herein by reference, describes a technique for computing 2D correction data which, when combined with the ramp filtered 2D approximation data sets, is intended to yield an exact image reconstruction. The 2D correction data basically comprises a point spread function representative of image reconstruction processing for each point on the detector which intersects the boundary of the data-combination mask.
Although this technique, as well as the technique of the forenoted U.S. Pat. No. 5,504,792 are intended to yield and exact image reconstruction, the present inventor has realized that such techniques are in fact also quasi-exact. More specifically, in an exemplary filtered backprojection (FBP) image reconstruction, on a detector let u and v be the Cartesian coordinate axes of the detector with the v axis coinciding with the rotation axis, and let L(&thgr;,s) denote the line on which projection/backprojection is carried out, where (&thgr;−&pgr;/2) is the angle line L(&thgr;,s) makes with the u axis, and s is the displacement of the line from the origin. Filtering of the cone beam image consists, either explicitly or implicitly, of the combined operation D
t
H, where D
t
is the differentiation (spatial) operation in the projected scan path direction t, and H is a shorthand notation for ∫B(&thgr;)D
s
(&thgr;)P(&thgr;)d&thgr;. P(&thgr;) is the 2D projection operation (line integration) at angle &thgr;, D
s
(&thgr;) is the 1D differentiation operation with respect to s for the projection at angle &thgr;, and B(&thgr;) the 2D backprojection operatio
Dunn Drew
Paschburg Donald B.
Siemens Corporation Research, Inc.
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