X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-09-25
2004-03-02
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000, C378S020000, C378S011000, C378S145000, C378S147000, C378S195000, C250S363030, C250S363100
Reexamination Certificate
active
06700949
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The field of the invention is medical imaging and more particularly collimator apparatus to be used in combined imaging modality systems and still more particularly retractable PET collimator apparatus for use in combined CT-PET systems.
Throughout this specification, in the interest of simplifying this explanation, an organ to be imaged will be referred to generally as an “organ of interest” and prior art and the invention will be described with respect to a hypothetical organ of interest. In addition, the phrase “translation axis” will be used to refer to an axis along which a patient is translated through an imaging system during data acquisition.
The medical imaging industry has developed many different types of imaging systems that are useful for diagnostic purposes. Two of the more widely used systems include computerized tomography (CT) systems and positron emission tomography (PET) systems.
In CT systems, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “CT imaging plane.” The x-ray beam passes through an organ of interest, such as the torso of a patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the organ of interest and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
Third generation CT systems include a base support for supporting the CT source and detector for rotation about the translation axis. To accommodate system tilt and reduce the overall system height and width dimensions, the source and detector are typically mounted axially along the translation axis with respect to the base support via a slip ring that provides power to the source and detector and also provides a data bus for transferring collected data to an image processor and archive.
In third generation CT systems the source and detector are rotated on the base support within the imaging plane and around the organ of interest so that the angle at which the x-ray beam intersects the organ constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. Using various data collection and manipulation techniques CT data can be used to generate two and three dimensional images of the organ of interest.
Unlike CT systems that rely on an external X-ray source to generate image data, PET systems rely on an energy source that resides within an organ of interest. To this end, positrons are positively charged electrons which are emitted by radio nuclides that have been prepared using a cyclotron or other device. The radio nuclides most often employed in diagnostic imaging are fluorine-18, carbon-11, nitrogen-13 and oxygen-15. Radio nuclides are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances such as glucose or carbon dioxide.
To use a radiopharmaceutical in PET imaging, the radiopharmaceutical is injected into a patient and accumulates in an organ, vessel or the like, which is to be imaged. It is known that specific radiopharmaceuticals become concentrated within certain organs or, in the case of a vessel, that specific radiopharmaceuticals will not be absorbed by a vessel wall. Thus, to image a specific organ or interest, a radiopharmaceutical known to accumulate either within the organ of interest or within a fluid that passes through the organ of interest can be selected. The process of concentrating often involves processes such as glucose metabolism, fatty acid metabolism and protein synthesis.
After the radiopharmaceutical becomes concentrated within an organ of interest and while the radio nuclides decay, the radio nuclides emit positrons. The positrons travel a very short distance before they encounter an electron and, when the positron encounters an electron, the positron is annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to medical imaging and particularly to medical imaging using photon emission tomography (PET). First, each gamma ray has an energy of essentially 511 keV upon annihilation. Second, the two gamma rays are directed in substantially opposite directions.
In PET imaging, if the general locations of annihilations can be identified in three dimensions, a three dimensional image of an organ of interest can be reconstructed for observation. To detect annihilation locations, a PET camera is employed. An exemplary PET camera includes a plurality of detectors and a processor which, among other things, includes coincidence detection circuitry. For the purposes of this explanation it will be assumed that a PET camera includes detectors that are arranged to form an annular gantry about a PET imaging area. Each time an approximatly 511 keV photon impacts a detector, the detector generates an electronic signal or pulse which is provided to the processor coincidence circuitry.
The coincidence circuitry identifies essentially simultaneous pulse pairs which correspond to detectors which are essentially on opposite sides of the imaging area. Thus, a simultaneous pulse pair indicates that an annihilation has occurred on a straight line between an associated pair of detectors. Over an acquisition period of a few minutes millions of annihilations are recorded, each annihilation associated with a unique detector pair. After an acquisition period, recorded annihilation data can be used via any of several different well known back projection procedures to construct images of the organ of interest.
In the case of PET systems, PET data can be collected simultaneously from a volume within an object of interest so that a 3D image can be generated. While there are several advantages to generating 3D images, many diagnostic requirements do not require such complex images and in these cases two dimensional “slice” images are sufficient.
Where 2D images will suffice, 2D images are preferred as the time required to acquire data needed to generate two dimensional images is less than that required to acquire data to generate three dimensional images. In addition to increasing system throughput (i.e., the number of imaging sessions that can be completed within a day), faster acquisition times increase patient comfort (i.e., reduce time during which patient must remain still) and, because the duration over which a patient must remain still is minimized, often result in images having reduces artifacts (i.e., the likelihood of patient movement is reduced as the acquisition time is shortened). In addition to reducing acquisition time, 2D data processing algorithms are simpler than 3D algorithms and processing procedures are therefore expedited.
In order to increase system versatility many conventional PET systems are capable of both 2D and 3D data acquisition. To this end a collimator is provided that is capable of restricting photons that pass through to a PET detector to within a series of parallel and adjacent planes. When 2D acquisition is required the collimator is positioned between the object of interest and the PET detector. When 3D acquisition is required the collimator is removed from between the object and detector.
In most PET systems that include a collimator, a collimator support is attached to the annular PET gantry axially along the translation axis. Thus, during 2D data acquisition the collimator is positioned within the gantry and during 3D acquisition the collimator is displaced outside the gantry and supported by the collimator support adjacent the gan
Maki Michael Dale
Schaefer Thomas Robert
Susami Larry
GE Medical Systems Global Technology LLC
Horton Carl
Kim Robert H.
Quarles & Brady LLP
Wang George
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