X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-08-16
2001-03-13
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Reexamination Certificate
active
06201849
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to computed tomography (CT) imaging and more particularly to three-dimensional CT imaging of helical cone beam scanning with improved efficiency and reduced image artifacts.
BACKGROUND OF THE INVENTION
FIG. 1
is a schematic axial view of a conventional third generation CT scanner
10
which includes an x-ray source
12
and an x-ray detector system
14
secured to diametrically opposite sides of an annular shaped disk
16
. The disk
16
is rotatably mounted within a gantry support (not shown), so that during a scan the disk
16
continuously rotates about a longitudinal z-axis while x-rays pass from the source
12
through an object, such as a patient
20
, positioned on a patient table
56
within the opening of the disk
16
. The z-axis is normal to the plane of the page in FIG.
1
and intersects the scanning plane at the mechanical center of rotation
18
of the disk
16
. The mechanical center of rotation
18
of the disk corresponds to the “isocenter” of the reconstructed image.
In one conventional system, the detector system
14
includes an array of individual detectors
22
disposed in a single row in a shape of an are having a center of curvature at the point
24
, referred to as the “focal spot,” where the radiation emanates from the x-ray source
12
. The source
12
and array of detectors
22
are positioned so that the x-ray paths between the source and each detector all lie in a “scanning plane” that is normal to the z-axis. Since the x-ray paths originate from what is substantially a point source and extend at different angles to the detectors, the diverging x-ray paths form a “fan beam”
26
that is incident on the detector array
14
in the form of one-dimensional linear projection. The x-rays incident on a single detector at a measuring instant during scan are commonly referred to as a “ray,” and each detector generates an output signal indicative of the intensity of its corresponding ray. The angle of a ray in space depends on the rotation angle of the disk and the location of the detector in the detector array. Since each ray is partially attenuated by all the mass in its path, the output signal generated by each detector is representative of the attenuation of all the mass disposed between that detector and the x-ray source, i.e., the attenuation of the mass lying in the detector's corresponding ray path. The x-ray intensity measured by each detector is converted by a logarithm function to represent a line integral of the object's density, i.e., the projection value of the object along the x-ray path.
The output signals generated by the x-ray detectors are normally processed by a signal processing portion (not shown) of the CT system. The signal processing portion generally includes a data acquisition system (DAS) which filters the output signals generated by the x-ray detectors to improve their signal-to-noise ratio (SNR). The output signals generated by the DAS during a measuring interval are commonly referred to as a “projection”, “projection profile”, or “view” and the angular orientation of the disk
16
, source
12
and detector system
14
corresponding to a particular projection profile is referred to as the “projection angle.”
If the detector array consists of N detectors, then N projection values are collected for each rotation angle. With the rays in a fan shape, these N projection values are collectively called a fan-beam projection profile of the object. The data of fan-beam projection profiles are often reordered or rebinned to become parallel-beam projection profiles. All rays in a parallel-beam profile have the same angle, called the parallel-beam projection view angle &phgr;. The image of the object can be reconstructed from parallel-beam projection profiles over a view angle range of 180°.
During a scan, the disk
16
rotates smoothly and continuously around the object being scanned, allowing the scanner
10
to generate a set of projections at a corresponding set of projection angles. In a conventional scan, the patient remains at the constant z-axis position during the scan. When obtaining multiple scans, the patient is stepped along the longitudinal z-axis between scans. These processes are commonly referred to as “step-and-shoot” scanning or “constant-z-axis” (CZA) scanning. Using well-known algorithms, such as the inverse Radon transform, a tomogram may be generated from a set of projections that all share the same scanning plane normal to the z-axis. This common scanning plane is typically referred to as the “slice plane.”
A tomogram is a representation of the density of a two-dimensional slice along the slice plane of the object being scanned. The process of generating a tomogram from the projections is commonly referred to as “reconstruction,” since the tomogram may be thought of as being reconstructed from the projection data. The reconstruction process can include several steps including reordering to form parallel-beam data from the fan-beam data, convolution to deblur the data, and back projection in which image data for each image pixel is generated from the projection data. In CZA scanning, for a particular image slice, all the projections share a common scanning plane, so these projections may be applied directly for convolution and to the back projector for generation of a tomogram.
The step-and-shoot CZA scanning approach can be a slow process. During this time consuming approach, the patient can be exposed to high amounts of x-ray radiation. Also, as the scanning table is moved between each scan, patient motion can result, causing motion and misregistration artifacts which result in reduced image quality.
Several approaches have been developed to decrease the time required to obtain a full scan of an object. One of these approaches is helical or spiral scanning in which the object being scanned is translated along the z-axis while the disk
16
with source
12
and linear detector array
14
are rotated about the patient. In helical scanning, the projections are normally acquired such that the position z is linearly related to the view angle. This form of helical scanning is commonly referred to as constant-speed-helical (CSH) scanning.
FIG. 2A
illustrates the data collected during a conventional CZA scan, and
FIG. 2B
illustrates the data collected during a CSH scan. As shown in
FIG. 2A
, if the x-ray source
12
and the detector system
14
are rotated about the object
20
while the object remains at a fixed z-axis location, the scanning planes associated with all the projections collected by the detector system
14
will all lie in a common slice plane
50
. As shown in
FIG. 2B
, if the object
20
is continuously translated in the direction of the z-axis while the disk is rotated about the object
20
, none of the scanning planes will be coplanar. Rather, the scanning plane associated with each projection will lie at a unique position along the z-axis at a locus point on a helical set of loci.
FIG. 2B
illustrates the z-axis coordinate of the scanning planes corresponding to helical projection angles in the interval (0, 10&pgr;).
In CZA scanning, all the projections share a common scanning plane, so these projections may be applied to the back projector after convolution to generate a tomogram. In CSH scanning however, each projection has a unique scanning plane located at a unique z-axis coordinate, so CSH projections may not be applied to a back projector. However, the data collected during a CSH scan can be interpolated in various fashions to generate a set of interpolated projections that do all share a common scanning plane extending normal to the z-axis. Each interpolated projection, for example, may be generated by combining two projections taken at equivalent projection angles and at different z-axis positions. These interpolated projections may be treated as CZA data and applied after convolution to a back projector to generate a tomogram.
CSH scanning requires some form of interpolation to generate a tomogram, and tomograms g
Analogic Corporation
Bruce David V.
McDermott & Will & Emery
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