Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-06-04
2001-10-02
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S369000
Reexamination Certificate
active
06297490
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to digital scintillation cameras, and more particularly to a method and apparatus for determining which photodetector in a planar array of photodetectors has received the most light from a scintillation event in a digital electronic scintillation camera.
BACKGROUND OF THE INVENTION
In the human body, increased metabolic activity is associated with an increase in emitted radiation. In the field of nuclear medicine, increased metabolic activity within a patient is detected using a radiation detector such as a scintillation camera.
Scintillation cameras are well known in the art, and are used for medical diagnostics. A patient ingests, or inhales or is injected with a small quantity of a radioactive isotope. The radioactive isotope emits photons that are detected by a scintillation medium in the scintillation camera. The scintillation medium is commonly a sodium iodide crystal or other suitable crystal. The scintillation medium emits a small flash or scintillation of light, in response to stimulating radiation, such as from a patient. The intensity of the scintillation of light is proportional to the energy of the stimulating photon, such as a gamma photon. Note that the relationship between the intensity of the scintillation of light and the gamma photon is not entirely linear.
A conventional scintillation camera such as a gamma camera includes a detector which converts into electrical signals gamma rays emitted from a patient after radioisotope has been administered to the patient. The detector includes a scintillator and photomultiplier tubes. The gamma rays are directed to the scintillator which absorbs the radiation and produces, in response, a very small flash of light. An array of photodetectors, which are placed in optical communication with the scintillation crystal, converts these flashes into electrical signals which are subsequently processed. The processing enables the camera to produce an image of the distribution of the radioisotope within the patient.
Gamma radiation is emitted in all directions and it is necessary to collimate the radiation before the radiation impinges on the crystal scintillator. This is accomplished by a collimator which is a sheet of absorbing material, usually lead, perforated by relatively narrow channels. The collimator is detachably secured to the detector head, allowing the collimator to be changed to enable the detector head to be used with the different energies of isotope to suit particular characteristics of the patient study. A collimator may vary considerably in weight to match the isotope or study type.
The determination of the centre photodetector in a scintillation camera is an important function if the intensity signals are to be processed digitally. Since the majority of scintillation cameras at the present use digital processing naturally they are various methods developed to achieve this purpose. Examples are U.S. Pat. No. 5,508,524 and U.S. Pat. No. 5,446,287. It is of great importance to execute this task as fast as possible since the time required to execute the task will to a great extent determine the dead time and therefore the overall throughput or maximum count rate of the system.
Thus, there is a need for an apparatus and a method for sorting intensity values received from a plurality of photodetector multiplying tubes (PMTs) according to their magnitude as well as to determine the identity of the PMT with the maximum intensity value.
SUMMARY OF THE INVENTION
There is an object of the present invention to provide an improved method and apparatus for determining which photodetector in a planar array of photodetectors has received the most light from a scintillation event in a digital electronic scintillation camera.
The photo multiplier tube (PMT) having the highest intensity value, is considered to be the centre photodetector which is nearest to the point of scintillation. It is a further object of the present invention to provide the intensity value and the identity of the centre PMT having the maximum intensity value to generate a data packet used for further processing.
According to a further aspect of the present invention, there is provided a method for determining a centre photo multiplying tube (PMT) from a plurality of PMTs, each PMT for receiving scintillation light and generating an analog signal, the PMTs being arranged in a planar array and outputting a set of PMT analog signals indicative of a scintillation event received from a target area. The method comprises the steps of converting the set of PMT analog signals into a set of PMT logical signals; recording in a look up table the set of PMT logical signals; dividing the plurality of PMTs into groups; forming pairs of logical signals within said groups; successively comparing paired logical signals of individual groups and outputting a local PMT highest intensity value for each of the groups so as to obtain a subset of local PMT highest intensity values; and repeating the comparison for the subset of local PMT highest intensity values until a PMT maximum intensity value representing the centre PMT is obtained.
According to a further aspect of the present invention, there is provided an apparatus for determining a centre PMT, from a plurality of PMTs. Each PMT receives scintillation light and generates an analog signal. The PMTs are arranged in a planar array generate a set of PMT analog signals indicative of the scintillation event received from a target area. The apparatus comprises means for converting the set of PMT analog signals into a set of PMT logical signals; a look up table for storing the set of PMT logical signals; means for dividing the set of PMT logical signals into groups; first comparison means for successively receiving groups of logical signals and combining them into pairs, comparing the paired signals and outputting a local PMT highest intensity value for each individual group so as to obtain a subset of local PMT highest intensity values; second comparison means for receiving the subset of local PMT highest intensity values, combining the subset of local PMT highest intensity values into pairs, comparing the values of the pairforming signals and outputting the highest intensity values, until a PMT maximum intensity value representing said centre PMT is found.
Advantageously, the hardware solution of the present invention provides for a reduced dead time since it allows for more than one comparison to be performed simultaneously. In addition, the use of a look up table the determination of the PMT identity is accurate and requires less hardware than the prior art methods using tagging or counting.
REFERENCES:
patent: 3721824 (1973-03-01), Bristol
patent: 5293044 (1994-03-01), Klingenbeck-Regn et al.
patent: 5309357 (1994-05-01), Stark et al.
patent: 5446287 (1995-08-01), Schreck et al.
patent: 5508524 (1996-04-01), Goldberg et al.
patent: 5519224 (1996-05-01), Mattern
patent: 5576546 (1996-11-01), Gagnon
Schreck Zoltan
Stark Iain
152 Research Inc.
Lee John R.
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