Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-06-24
2002-02-12
Kim, Robert H. (Department: 2882)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S363010, C378S004000
Reexamination Certificate
active
06346706
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to imaging systems and more particularly to an imaging system including at least two detectors, a first detector being a high resolution detector which detects energy and location of Compton scatters and a second detector being a residual energy detector.
Radionuclides are decaying substances which emit subatomic particles (e.g. beta particles, alpha particles, neutrons, positrons and/or photons). For example single photon emitters such as Technetium emit photons and perhaps also a charged particle wherein there is no angular correlation between multiple emitted photons. As another example, Fluorine-18 decays to Oxygen-17 by emitting a positron which has an energy between 0 and 600 keV with a mean energy of 250 keV.
Radionuclides 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. Hereinafter an exemplary radionuclide in an exemplary radiopharmaceutical used for imaging will be referred to as an imaging radionuclide.
It is known that specific radiopharmaceuticals become concentrated within certain organs. The process of concentrating often involves processes such as glucose metabolism, fatty acid metabolism and protein synthesis. Hereinafter, 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 a radiopharmaceutical becomes concentrated within an organ of interest and while the imaging radionuclides decay, the radionuclides emit subatomic particles including positrons and photons. Each of the particles and photons can be detected. Where the particles are positrons, the positrons travel very short distances (e.g., approximately 200 microns in the case of Flourine-18) before they encounter an electron and, when a positron encounters an electron, the positron is annihilated as the electron and positron unite and two photons are generated.
Photons are characterized by one feature which is pertinent to all medical imaging techniques which sense photons. Given a specific radionuclide, photons generated directly from decay or via decay followed by annihilation, have specific and known energy levels. For example, when a positron results from decay of Fluorine-18, annihilation of the positron always results in two photons, each of which has an energy of 511 keV. As other examples,
131
I generates photons having energies of 360 keV while
99m
T
c
generates photons having energies of 140 keV. In addition, particle annihilation events are characterized by an additional feature which is pertinent to medical imaging. This additional feature is that, upon an annihilation event, two photons are generated and the photons are directed in essentially opposite directions (i.e. the trajectories are separated by approximately 180°). There is approximately a ±0.25 degree variation from 180° in the photon trajectories related to the momentum of the electron-positron pair before annihilation.
In all photon imaging systems both photon energy and trajectory have to be determined. Photon energy is determined and compared to a range of expected energies associated with the particular radiopharmaceutical used during data generation. Where sensed energy is outside the expected range the detection is typically discarded. Where sensed energy is within the expected range, the detection is identified as valid, photon trajectory is determined and trajectories for all valid detections are combined to generate an image of the object of interest.
Three different imaging systems which are pertinent to the present invention include PET, collimated single photon imaging and Compton imaging systems, each of which is described separately below.
PET Systems
An exemplary PET system includes two oppositely facing cameras wherein the cameras are either scintillation cameras or solid state direct conversion detector (DCD) cameras.
An exemplary scintillation camera includes a plurality of detector units and a processor which, among other things, includes coincidence detection circuitry. An exemplary detector unit includes a two dimensional 6×6 matrix of bismuth germinate (BGO) scintillator crystals which are disposed in front of four photo multiplier tubes (PMTs). When a crystal absorbs a photon, the crystal generates light which is generally directed toward the PMTs. The PMTs absorb the light and each PMT produces an analog signal which arises sharply when a scintillation event occurs and then tails off exponentially with a time constant of approximately 300 nanoseconds. The relative magnitudes of the analog PMT signals are determined by the position in the 6×6 BGO matrix of the crystal which generates the light (i.e., where the scintillation event takes place), and the total magnitude of these signals is determined by the energy of the photon which causes an event.
For each total magnitude within a range of expected magnitudes corresponding to the imaging radionuclide, a set of acquisition circuits receives the PMT signals and determines x and y event coordinates within the BGO matrix thereby determining the crystal which absorbed the photon and the general x-y coordinate at which the absorption occurred on the face of the crystal. Each acquisition circuit also produces an event detection pulse (EDP) which indicates the exact moment at which a scintillation event took place.
The information regarding each valid event is assembled into a digital event data packet which indicates precisely when the event took place and the position of the BGO crystal which detected the event. Event data packets are conveyed to a coincidence detector which determines if any two events from the opposing detectors are in coincidence.
Coincidence is determined by a number of factors. First, the time markers in each event data packet must be within a specific time window of each other, and second, the locations indicated by the two event data packets must lie on a straight line which passes through the field of view of a scanner imaging area. Events which cannot be paired as coincidence events are discarded, but coincidence event pairs are located and recorded as coincidence data packets. Each coincidence data packet includes a pair of digital numbers which precisely identify the addresses of the two BGO crystals that detected the event. After an event pair has been identified, the source location of the pair can be identified along a straight line which passes through the locations of the events in the pair. After imaging data has been collected in this manner, a processor uses the collected information to generate a two or three dimensional image of the organ of interest.
DCDs may be based on pixilated semiconductor detectors such as Cadmium Telluride (CdTe) or Cadmium Zinc Telluride (CdZnTe) devices. Generally, each DCD includes an absorption member, a cathode, at least one anode, a potential biasing mechanism (i.e. voltage source) and a separate amplifier for each anode.
The absorption member is formed of a planar semiconductor material (e.g. CdTe or CdZnTe) which has oppositely facing cathode and anode surfaces. The dimension between the cathode and anode surfaces is an absorption member thickness. When photons are directed at the cathode surface, the photons penetrate the absorption member and each photon is absorbed at an absorption depth within the member thickness. When a photon interacts with the absorption member while being absorbed, the absorption member generates a plurality of electrons and holes.
The cathode is attached to and essentially covers the cathode surface and the anode is attached to the anode surface. The biasing mechanism is linked to the c
Clinthorne Neal H.
Rogers William L.
Hobden Pamela R.
Kim Robert H.
Quarles & Brady LLP
The Regents of the University of Michigan
LandOfFree
High resolution photon detector does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High resolution photon detector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High resolution photon detector will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2964953