Helical segment image reconstruction

Details Helical segment image reconstruction Helical segment image reconstruction

2001-11-21

2002-12-03

Dunn, Drew A. (Department: 2882)

X-ray or gamma ray systems or devices

Specific application

Computerized tomography

C378S004000, C378S901000, C382S131000

Reexamination Certificate

active

06490335

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
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 an algorithm, method and apparatus for using the same which increase the quality of resulting images by employing a new weighting algorithm.
In computerized tomography (CT) X-ray photon rays are directed through a region of interest (ROI within 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 ROI 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, as in other imaging techniques, the CT industry is constantly trying to identify ways to reduce the duration of acquisition periods without reducing the quality of the data acquired.
In addition, because huge amounts of data are acquired during an acquisition period and the processing methods for image reconstruction from the gathered data are relatively complex, a huge number of calculations are required to process data and reconstruct an image. Because of the huge number of required calculations, the time required to process collected data and reconstruct an image is appreciable. For this reason the CT industry is also constantly searching for new processing methods and algorithms which can speed up the reconstruction process.
Various CT system features and procedures have been developed to increase data acquisition speed and to speed up the reconstruction process. 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.
For example, referring to
FIG. 3
, an annular gantry opening
70
is illustrated with a patient slice
42
disposed (support table not illustrated) therein and with respect to a Cartesian coordinate system where the Z-axis is into the Figure and defines a transport axis. A source
10
is illustrated in first, second, third and fourth positions as
90
,
90
′,
90
″ and
90
′″, respectively. When in the first position, source
10
generates a fan beam
40
which includes a central ray Rc and additional rays diverging therefrom along fan angles, the maximum fan angle being G. The beam angle &bgr; is defined as the angle formed by central ray Rc with respect to the vertical Y-axis.
When in the fourth position, source
10
generates a fan beam
40
′″ which also includes a central ray (not illustrated) and rays diverging therefrom to form the fan beam. By rotating the source from the first to the fourth position in a clockwise direction data is collected at least once from every possible beam angle through slice
42
(i.e., the image plane). As known in the industry, data corresponding to every beam angle corresponding to a single image plane can be collected after a (&pgr;+2&Ggr;)2&pgr; rotation about the patient. Because less than an entire rotation about the image plane is required to acquire the imaging data these acquisition methods and systems are generally referred to as partial-scan methods and systems and, more specifically, where data is collected during a minimal gantry rotation, the methods and systems are 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.
In addition, because relatively less data has to be processed in the case of half-scan imaging methods and systems to generate an image, half-scan methods and systems also have the advantage of potentially reducing data processing and reconstruction times.
As a result of the fan beam geometry of the x-ray source and the detector array, a half scan contains certain redundant data. This redundant data requires that the half scan data set be weighted with a “half scan weighting” function so that the redundant data does not make a disproportionate contribution to the final image when incorporated with the non-redundant data. The weighting and reconstruction of images from a half scan data set are discussed in detail in “Optimal Short Scan Convolution Reconstruction for Fanbeam CT”, Dennis L. Parker, Medical Physics 9(2) March/April 1982.
While fan beams and half-scans have several advantages, often, during a diagnostics exercise a system user typically will not know the precise location within a patient of an object, cavity, etc. of interest to be imaged. For this reason, it is advantageous for a system user to be able to generate several cross sectional images in rapid succession by selecting different image/reconstruction planes. In these cases rapid data processing is extremely important to minimize delays between image generation so that a user does not lose her train of thought between image views.
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 an

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