X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-11-09
2003-02-25
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000, C382S131000
Reexamination Certificate
active
06526117
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
The field of the invention is gated computerized tomography (CT) imaging and more specifically methods and apparatus that reduce the amount of misalignment between parallel two dimensional images in a working CT image set.
Many different types of medical imaging systems have been developed that are used for different purposes. Perhaps the most common type of imaging system category includes X-ray systems wherein radiation is directed across a portion of a patient to be imaged and toward a detector panel. An exemplary X-ray detector panel includes a CsI scintillator coupled with an amorphous silicon array. With radiation directed toward a region of a patient to be images (i.e., a region of interest), the region of interest blocks some of the radiation and some of the radiation passes through the region and is collected by the panel. The amount of radiation that passes through the region along the trajectory of a given radiation ray depends upon the type of tissue along the trajectory. Thus, a tumor may block more radiation than flesh and bone may block more radiation than a tumor and so on. Hence X-ray system can be used to collect a “projection” through a patient.
While useful, simple X-ray systems have many limitations. One important limitation to X-ray imaging systems is that such systems, as described above, only provide side projections through a region and cannot be used to generate other useful images such as “slice” images (i.e., images perpendicular to projection images) through a region of interest. For instance, an exemplary useful slice image may include a slice image through a patient's heart.
Another type of imaging system that is useful in generating slice images is generally referred to as a computerized tomography (CT) system. An exemplary CT system includes a radiation source and a radiation detector mounted on opposite sides of an imaging area where the imaging area is centered along a translation or Z-axis. The source generates radiation that is collimated into a beam including a plurality of radiation rays directed along trajectories generally across the imaging area. A line detector may be positioned perpendicular to the Z-axis to collect slice image data during a data acquisition period.
During an acquisition period a region of interest is positioned within the imaging area and, with the radiation source turned on, the region of interest blocks some of the radiation and some of the radiation passes through the region and is collected by the line detector. As in X-ray systems, the amount of radiation that passes through the region of interest along the trajectory of a given radiation ray is dependent upon the type of tissue along the trajectory. In CT systems the source and line detector are rotated about the region of interest within a rotation plane through the region of interest so that radiation “projections” can be collected for a large number of angles about the region. By combining the projections corresponding to a slice through the region of interest using a filtering and back projecting technique, a two-dimensional tomographic or axial image (i.e., a slice image) of the slice is generated.
While some diagnostic techniques only require one or a small number of slice images, many techniques require a large number of parallel CT slice images. For example, some techniques require examination of many parallel images to identify where an arterial blockage begins and ends and the nature of the blockage there between. As another example, many techniques reformat two dimensional data into, in effect, three dimensional volumetric images, that can be sliced and diced in several different directions so that various image planes can be employed. For instance, where two dimensional data is acquired for transverse or cross sectional slices through a three dimensional region of interest (e.g., through a patient's torso), the data may be reformatted to generate sagital (i.e., the side plane passing through the long axis of the body) or coronal (i.e., the frontal plane passing through the long axis of the body) images through the region of interest.
In order to generate several slice images rapidly, CT detectors are typically configured having several parallel detector rows such that, during a single rotation about the imaging area, each detector row collects data that can subsequently be used to generate a separate CT slice image.
While increasing the number of detector rows reduces acquisition time, detector elements are relatively expensive and thus more rows translates into a more costly overall system. As a balance between cost and speed, most multi-row detectors include less than 10 detector rows. Hereinafter it will be assumed that an exemplary detector includes eight detector rows.
Where a detector includes eight rows and more than eight slice images are required, several different acquisition periods are typically used to acquire the necessary slice image data. For instance, assume that 80 slice images (an admittedly small number but sufficient for exemplary purposes) through a ROI are required. In this case, the ROI may be divided into ten separate sub-volumes, each of the ten sub-volumes corresponding to a separate eight of the 80 required slice images. Thereafter, ten separate acquisition periods may be used to collect the sets of slice image data corresponding to the ten sub-volumes, data corresponding to eight separate slice images collected during each of the ten separate acquisition periods.
It has been found that, for large volumes or ROIs, data necessary to generate many parallel thin slice images can be acquired most rapidly by helically collecting the data. To this end, while the source and detector are rotated about the imaging area, a patient bed is translated there through so that the radiation fan beam sweeps a helical path through the ROI. After helical data is collected, the data is converted to slice image data by any of several different weighting and filtering processes and thereafter the slice image data is backprojected to form the viewable image.
In the case of helically acquired and stored raw data, the data can be used to construct virtually any number of slice images through a corresponding ROI. For instance, despite using a detector having eight rows of elements to collect helical data, the data may be processed to generate 16, 20, 500 or even thousands of separate slice images or, indeed, may be interpolated to generate a 3-D volumetric image, if desired.
In most imaging systems that generate still images, it is important that, to the extent possible, during data acquisition, the structure being imaged remain completely still. Even slight structure movement during acquisition can cause image artifacts in, and substantially reduce the diagnostic value of, resulting images. For this reason, during acquisition periods, patients are typically instructed to maintain the region of interest within the imaging area as still as possible by, for instance, holding the patient's breadth.
Despite a patient's attempts to control movement, certain anatomical structures cannot be held still and continue movement during acquisition periods. For instance, a patient's heart beats continually during data acquisition cycles and the beating movement complicates the process of acquiring diagnostic quality data.
In the case of the heart, fortunately, the beating cycle is repetitive and there are certain cycle phases during which the heart muscle is relatively at rest. As well known in the art, during a diastolic phase of the beating cycle when the heart is filling with blood, the heart is relatively at rest and movement is minimal. Thus, by restricting data acquisition periods to the diastolic phases of the heart beating cycle, relatively movement-free data can be acquired and used to generate CT slice images.
To this end, the industry has developed cardiac gated CT imaging syste
Knoplioch Jerome F.
Okerlund Darin R.
Srinivas Sankar V.
Dunn Drew A.
GE Medical Systems Global Technology Company LLC
Horton Carl
Kiknadze Irakli
Quarles & Brady LLP
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