X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-11-09
2003-02-18
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S019000, C378S901000
Reexamination Certificate
active
06522714
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to multi-slice helical computerized tomography and more particularly to a tomography algorithm, method and apparatus which reduces the data acquisition time and data processing time required to generate an image while maintaining high image quality.
In computerized tomography (CT) X-ray photon rays are directed through a patient toward a detector. Attenuated rays are detected by the detector, the amount of attenuation indicative of the make up (e.g. bone, flesh, air pocket, etc.) of the patient through which the rays traversed. The attenuation data is then processed and back-projected according to a reconstruction algorithm to generate an image of the patient's internal anatomy. Generally, the “back-projection” is performed in software but, as the name implies, is akin to physically projecting rays from many different angles within an image plane through the image plane, the values of rays passing through the same image voxels being combined in some manner to have a combined effect on the voxel in the resulting image. Hereinafter the data corresponding to rays which are back-projected will be referred to as back-projection rays.
During data acquisition, if a patient moves, artifacts can occur in the resulting image which often render images useless or difficult to use for diagnostics purposes. For this and other reasons the CT industry is constantly trying to identify ways to reduce the duration of acquisition periods without reducing the quality of the data acquired.
Various CT system features and procedures have been developed to increase data acquisition speed. Some of the more popular features and procedures including fan beam acquisition, simultaneous multiple slice acquisition, helical scanning and half-scanning. In fan beam acquisition the source is collimated into a thin fan beam which is directed at a detector on a side opposite a patient. In this manner, a complete fan beam projection data set is instantaneously generated for a beam angle defined by a central ray of the source fan beam. The source and detector are rotated about an image plane to collect data from all (e.g., typically 360 degrees) beam angles. Thereafter the collected data is used to reconstruct an image in the image plane. Thus, fan beam acquisition reduces acquisition period duration.
With respect to half-scanning, assuming a patient remains still during a data acquisition period, conjugate data acquisitions (i.e., data acquired along the same path from opposite directions) should be identical. In addition, using a fan beam, at least one ray can be directed through an image plane from every possible beam angle without having to perform a complete rotation about the patient. To this end, as known in the industry, data corresponding to every beam angle associated with a single image plane can be collected after a (&pgr;+2&ggr;)/2&pgr; rotation about the patient where &ggr; is the fan beam angle. Because less than an entire rotation about a patient is required to acquire data corresponding to a slice image, these acquisition methods and systems are generally referred to as half-scan methods and systems. Thus, half-scan acquisition has been employed to reduce acquisition period duration in conjunction with single row detectors.
Single slice detectors, fan beams and half-scans can be used to generate data in several different parallel image planes which, after data acquisition, can be used by a processor to generate an image anywhere between the image planes through interpolation/extrapolation procedures known in the art. For example, assume that during two data acquisition periods first and second data sets were acquired which correspond to first and second parallel acquisition planes, respectively, the planes separated by 0.25 inches. If a user selects an image plane for reconstructing an image which resides between the first and second acquisition planes, interpolation between data in the first and second sets can be used to estimate values of data corresponding to the selected image plane. For instance, assume that, among other rays, during the acquisition periods a first ray and a second ray were used to generate data in the first and second sets, respectively, and that the first and second rays were parallel (i.e. had the same beam and fan angles). In this case, by interpolating between the data acquired from the first and second rays generates an estimated value corresponding to a hypothetical back-projection ray which is parallel to the first and second rays and which is within the image plane. By performing such interpolation to generate back-projection rays for every beam and fan angle through the image plane a complete data set corresponding to the image plane is generated.
While such systems work, unfortunately, the acquisition time required to generate data corresponding to many image planes is excessive and inevitable patient movement often causes image artifacts.
One way to speed up data acquisition corresponding to several image planes is by employing a multi-row detector with a fan beam. In multi-row detector systems, a relatively thick fan beam is collimated and directed through a patient at a multi-row detector, each detector row in effect gathering data for a separate “slice” of the thick fan beam along the Z or translation axis perpendicular to a fan beam width. Despite each detector row having a thickness, in these systems it is assumed that the detected signals in each row correspond to a plane centered within the row as projected onto the isocenter Z. Hereinafter the central plane through a row will be referred to as a row center.
After data acquisition an interface enables a system user to select an image plane from within the area corresponding to the collected data. The selected image plane is between the row centers of at least two adjacent detector rows. After image plane selection, a processor interpolates between data corresponding to adjacent rows to generate back-projection rays corresponding to the selected image plane. When another image corresponding to a different image plane is desired, after selecting the plane, the processor again identifies an acquired data subset for interpolation, additional processing and back-projection. Thus, multi-row detector systems further reduce data acquisition period duration where several image planes may be selected for reconstruction.
One limitation with multi-row detectors is that, during a single acquisition period, data can only be collected which corresponds to the detector thickness. To collect additional data corresponding to a greater patient volume or region of interest (ROI), after one acquisition period corresponding to a first volume, the patient has to be moved along a translation axis until a second volume which is adjacent the first volume is between the source and detector. Thereafter a second acquisition process has to be performed. Similarly, to collect additional data corresponding to a third volume the patient has to be transported to another relative location with respect to the source and detector. Required translation without acquisition necessarily prolong the acquisition period and the additional acquisition time and aligning processes inevitably result in relative discomfort, additional patient movements and undesirable image artifacts.
Helical scanning systems have been developed so that data can be collected during a single acquisition period without halting patient translation during the acquisition period. In a helical scanning system, the source and detector array are mounted on opposing surfaces of an annular gantry and are rotated there around as a patient is transported at constant speed through the gantry. The X-ray beam sweeps a helical path through the patient, hence the nomenclature “helical scanning system”. Data acquisition can be sped up by increasing operating pitch (i.e., table translation speed relative
Li Jianying
Metz Stephen
Simoni Piero U.
Toth Thomas
Wang Sharon
Bruce David V.
GE Medical Systems Global Technologies Company LLC
Horton Carl
Quarles & Brady LLP
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