Image analysis – Applications – Biomedical applications
Reexamination Certificate
2001-04-25
2004-11-02
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
Biomedical applications
C378S207000
Reexamination Certificate
active
06813374
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to computed tomography (CT) scanners, and more specifically to a method and apparatus to verify that a CT scanner meets its performance specifications.
Notation
For convenience of exposition, the following notations are used in the following specification:
AD Automatic Detection
CT Computed Tomography
FOV Field-of-view
IQP Image Quality Phantom
NSR Nutating Slice Reconstruction
ROI Region of Interest
SSP Slice Sensitivity Function (Profile)
MTF Modulation Transfer Function
z-axis Coordinate axis that coincides with axis of gantry rotation. The positive direction is the same as the direction of the gantry angular velocity vector.
x-axis Horizontal coordinate axis. The positive direction is to the right side of the gantry as viewed from the front face.
y-axis Vertical coordinate axis. The positive direction is upwards of the gantry is viewed from the front face. The x, y and z axes form a right handed system.
Axial slice Slice perpendicular to the z-axis
Sagittal slice Slice perpendicular to the x-axis
Coronal slice Slice perpendicular to the y-axis
BACKGROUND OF THE INVENTION
A CT scanner is a device used for manual or automatic discrimination of compositions, conditions or objects. In addition to traditional applications in medical imaging, non-medical applications for CT scanners are evolving. For example, baggage scanners using CT techniques have been proposed to search for contraband items such as explosives and narcotics in luggage at airports. Scanners for industrial testing have also been proposed.
One type of system using CT technology is a CT scanner of the third generation type, which typically includes an X-ray source and an X-ray detector system secured to diametrically opposite sides of an annular-shaped platform or disk. The disk is rotatably mounted within a gantry support so that in operation the disk continuously rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system can include a linear array of detectors disposed as a single row in the shape of a circular arc having a center of curvature at the focal spot of the X-ray source, i.e., the point within the X-ray source from which the X-rays emanate. Alternatively, the detector system can include a “two-dimensional” array of detectors disposed as multiple rows forming a cylindrical surface whose axis passes through the source. The X-ray source generates a fan-shaped beam (when used with a linear array of detectors) or cone-shaped beam (when used with a two-dimensional array of detectors) of X-rays that emanates from the focal spot, passes through an imaging field, and is received by the detectors. The CT scanner includes a predefined coordinate system, defined by mutually orthogonal X-, Y- and Z-axes, wherein the axes intersect and are all normal to one another at the center of rotation of the disk as the disk rotates about the rotation axis. This center of rotation is commonly referred to as the isocenter. The Z-axis is defined by the rotation axis of the scanner, and the X-and Y-axes intersect the Z-axis at the isocenter and are defined by and lie within the planar imaging field. The X, Y and Z-axes form a right handed system. The fan beam is thus defined as the volume of space existing between a point source, i.e., the focal spot, and the receiving surfaces of the detectors of the linear detector array exposed to the X-ray beam. In the case of the fan beam, because the dimension of the receiving surfaces of the linear array of detectors is relatively small in the Z-axis direction the fan beam is relatively thin in that direction. In a similar manner, the cone beam is defined as the volume of space existing between a point source, i.e., the focal spot, and the receiving surfaces of the detectors of the detector array exposed to the X-ray beam. Scanners have been developed for generating three dimensional images from the data acquired from a scan.
Each detector generates an output signal representative of the intensity of the X-rays incident on that detector during a sampling period. Since the X-rays are partially attenuated by all the mass in their path, the output signal generated by each detector is representative of the density of all the mass disposed in the imaging field between the X-ray source and that detector.
As the disk rotates, the detectors of the detector array are periodically sampled, and for each measuring interval each of the detectors in the detector array generates an output signal representative of the density of a portion of the object being scanned during that interval. The collection of all of the output signals generated by all the detectors of the detector array for any measuring interval is referred to as a “projection”, and the angular orientation of the disk (and the corresponding angular orientations of the X-ray source and the detector array) during generation of a projection is referred to as the “projection angle”. At each projection angle, the path of the X-rays from the focal spot to each detector, called a “ray”, increases in cross section from a point source to the receiving surface area of the detector, and thus is thought to magnify the density measurement because the receiving surface area of the detector area is larger than any cross sectional area of the object through which the ray passes.
As the disk rotates around the object being scanned, the scanner generates a plurality of projections at a corresponding plurality of projection angles. Using well known algorithms, a CT image of the object may be generated from all the projection data collected at each of the projection angles. Where the object is stationary during the scan (so called “constant axis” scanning), the CT image is representative of the density of a “two dimensional slice” of the object through which the X-ray beam has passed during the rotation of the disk through the various projection angles. When the object and rotating disk are moved relative to one another along the Z-axis (so called “helical” or “volumetric” scanning), a collection of data is acquired though a volumetric “slice” of the object through which the X-ray beam has passed during the rotation of the disk through the various projection angles. Multiple slices can be obtained in a “step-and-shoot” process, a mode of operation where successive “axial” slices are obtained from constant axis scans respectively at incremental positions of the gantry relative to the scanned object. Thus, step-and-shoot scanning is a mode of operation of the scanner in which projections for one axial slice or set of axial slices, are acquired without translating the object or patient and gantry during each axial scan. To get projections of another slice or set of slices, the patient and gantry are translated relative to one another before the next acquisition begins. There is no simultaneous imaging and translation as in helical scanning. Data from successive slices obtained from a step-and-shoot scan can be utilized to provide a volumetric image. The resolution of the CT image is determined in part by the width of the receiving surface area of each detector in the plane of the fan beam, the width of the detector being defined herein as the dimension measured in the same direction as the width of the fan beam, while the length of the detector is defined herein as the dimension measured in a direction normal to the fan beam parallel to the rotation or Z-axis of the scanner.
Image processing techniques are known in the art for generating an image of the target object, slice by slice. Each slice is viewed as being composed of a plurality of individual volume elements. Information regarding the different amounts of x-ray attenuation by the different volume elements within each slice is used to determine the density and position of the internal structures that make up the slice. Each volume element is characterized by a numerical value, referred to as the CT number, which represents its attenuat
Bechwati Ibrahim
Crawford Carl R.
Karimi Seemeen S.
Rozas David
Simanovsky Sergey
Analogic Corporation
Johns Andrew W.
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