Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1998-11-19
2001-05-29
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S363020, C250S370090, C250S363070, C250S369000, C600S436000
Reexamination Certificate
active
06239438
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to medical imaging generally and more specifically to a method for acquiring several data acquisition sets during an acquisition period wherein at least one of the acquisitions is a high resolution acquisition and other acquisitions have relatively lesser resolution but are useful for various purposes in addition to generating an image.
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 (
18
F), carbon-11 (
11
C), nitrogen-13 (
13
N), and oxygen-15 (
15
O). Radio nuclides are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances such as glucose or carbon dioxide. One common use for radiopharmaceuticals is in the medical imaging field.
To use a radiopharmaceutical in 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. The process of concentrating often involves processes such as glucose metabolism, fatty acid metabolism and protein synthesis. Hereinafter, 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.
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 camera includes 12,000 detectors which are arranged to form an annular gantry about an imaging area. Each time a 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 the three dimensional image of the organ of interest.
To compress annihilation data somewhat, instead of separately storing an indication of each detected annihilation, a typical PET processor simply stores a separate counter for each possible “meaningful” detector pair. What is meant by the term “meaningful” is that there are certain theoretically possible detector pairs which will almost certainly never provide a true annihilation indication. For example, because an annihilation typically sends gamma rays in opposite directions and annihilation points are within an imaging area, it is essentially impossible for two adjacent detectors to provide a true annihilation indication. Similarly, other detectors which are disposed in the same general area as a first detector cannot provide a true annihilation indication along with the first detector. Thus, proximate detectors are not meaningful pairs and the processor does not provide a counter for these pairs.
In addition, the number of meaningful detector pairs is also limited by the fact that certain detector pairs are positioned such that the organ of interest, and any radiopharmaceutical accumulated therein, is not within the space therebetween. In this case, once again, the detector pair cannot provide a true annihilation indication. Thus, while theoretically it is possible to have approximately 144 million detector pairs where there are 12,000 detectors, instead of providing 144 million counters and memory required to store 144 million annihilation counts, the meaningful number of detector pairs and hence processor counters, can be reduced to approximately 25 million. Hereinafter, it will be assumed that memory required to store data corresponding to a single acquisition consists of 25 megabytes (Mb), one byte for each meaningful detector pair.
Given a specific radiopharmaceutical (i.e. a substance characterized by a known positron emission rate), acquisition period duration is primarily dependent upon required image quality. Thus, in cases where poor quality images are acceptable, fewer annihilation events have to be detected to create an image and acquisition period duration can be reduced. However, where high quality images are acceptable, a large number of annihilations must be detected and the acquisition period is typically relatively long. Where an image is to be used for medical diagnostics, usually high quality is extremely important and therefore acquisition periods tend to be relatively long. For the purposes of this explanation it will be assumed that, given a specific radiopharmaceutical, annihilation detections are required over a twenty minute period.
During an acquisition period there are several sources of annihilation detection error. Two of the more prominent sources of detection error are referred to as “dead time” and “randoms”. The phenomenon known as dead time occurs when two gamma rays impact a single detector at essentially the same time so that the total absorbed energy far exceeds 511 keV or so that, while a first of the rays is being processed by the detector, a second of the rays is ignored by the detector. In these cases, either one or two annihilations are not recognized and an error occurs.
The phenomenon known as randoms occurs when gamma rays from two different annihilations are detected by two detectors at essentially the same time. For example, assuming two gamma rays are detected at the same time from two different annihilations, the coincidence circuitry cannot determine which detection correspond to a first annihilation and which detection correspond to a second annihilation. The two annihilations that randomly occur at the same time are recorded at a third unrelated location.
Error due to both dead time and randoms should be corrected to provide the highest quality image. To this end, typically, the processor counts dead time errors and randoms and, at the end of an acquisition period and prior to reconstructing an image from the collected data, corrects acquired data. Other common sources of error include attenuation of the gamma rays by different portions of a patient's body and detector gain nonuniformities.
While long acquisition periods (e.g. 20 minutes) are required to generate data needed to reconstruct high resolution and high quality images, long acquisition periods have a number of shortcomings. Fi
Cabou Christan G.
Gagliardi Albert
General Electric Company
Hannaher Constantine
Quarles & Brady LLP
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