X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-08-28
2003-12-09
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C250S363040, C378S004000
Reexamination Certificate
active
06661866
ABSTRACT:
BACKGROUND OF INVENTION
The field of the invention is medical imaging and more particularly medical imaging with a combined CT-PET system.
Throughout this specification, in the interest of simplifying this explanation, a clinical region of a patient to be imaged will be referred to generally as a “region of interest” and prior art and the invention will be described with respect to a hypothetical region 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 X-ray 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 a region 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 region 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. 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. Using various data collection and manipulation techniques CT data can be used to generate two and three dimensional images of the region 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 a region of interest. To this end, positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. The radionuclides most often employed in diagnostic imaging are fluorine-18, carbon-11, nitrogen-13 and oxygen-15. Radionuclides 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 administered, typically by injection, to 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 and tumors or, in the case of a vessel, that specific radiopharmaceuticals will not be absorbed by a vessel wall. Thus, to image a specific region of interest, a radiopharmaceutical known to accumulate either within the region of interest, within an organ that resides in the region of interest or within a fluid that passes through the region 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 a region of interest and while the radionuclides decay, the radionuclides 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 positron 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 a region 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. Each time an approximately 511 keV positron 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 coincidence events are recorded, each coincidence event is associated with a unique detector pair. After an acquisition period during which coincidence data is collected from every angle about an imaging area, recorded coincidence data can be used via any of several different well known procedures to construct images of radionuclide concentration in the region 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.
As is the case in virtually all imaging systems, one measure of the value of a PET system is throughput. To this end, in a radiology department the number of images generated is generally related to profitability with greater numbers of images translating into greater profitability. Thus, PET acquisition systems are generally designed to collect required imaging data rapidly. For this reason, one well accepted PET configuration is generally referred to as a full ring system which, as its label implies, includes a plurality of detector segments arranged to form an annular detector surface about an imaging area such that the system detects annihilation photons from many angles at a time. Hereinafter, for the purposes of this explanation a full ring detector system will be assumed unless indicated otherwise.
Each of the different imaging modalities typically has uses for which it is particularly advantageous. For example, CT systems that employ X-rays are useful for generating anatomical images of bone and the like while PET systems are useful for generating functional images corresponding to dynamic occurrences such as metabolism and the like.
For various reasons, in some diagnostic applications, it is advantageous to collect both CT and PET data corresponding to the same clinical region of interest. For instance, CT data may be used to compensate for inaccuracies in PET imaging data. To this end, one of the problems with PET imaging techniques is that gamma ray absorption and scatter by portions of the region of interest between the emitting radiopharmaceutical and a detector distort collected data and hence resultant images. One solution for compensating for gamma ray attenuation is to assume uniform positron attenuation throughout the region of interest. That is, the region of interest is assumed to be completely homogenous in terms of radiation attenuation with no distinction made for bone, soft tissue, lung, etc. This assumption enables attenuation estimates to be made based on the surface contour of the region of interest. Unfortunately, typical regions of interest do not cause uniform radiation attenuation and therefore a uniform attenuation assumption is generally inaccurate.
According to several methods, instead of assuming uniform attenuation characteristics throughout the region of interest, CT transmission data is collected for the entire region of interest and is used to accurately determine attenuation characteristics at every point throughout the region of interest. Thereafter, the PET emission data is corrected as a function of the CT attenuation map to generate more accurate PET images.
As another instance where it is advantageous to generate both CT and PET data for a region of interest, sometimes it is ad
Ganin Alexander
Limkeman Mark Kenneth
McDaniel David Leo
Senzig Robert Franklin
Stearns Charles William
Church Craig E.
GE Medical Systems Global Technology Company LLC
Horton Carl
Quarles & Brady LLP
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