Radiant energy – Invisible radiant energy responsive electric signalling – Including a radiant energy responsive gas discharge device
Reexamination Certificate
1998-08-10
2001-03-27
Ham, Seungsook (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Including a radiant energy responsive gas discharge device
C250S374000, C250S375000
Reexamination Certificate
active
06207958
ABSTRACT:
TECHNICAL FIELD
The present invention is directed to x-ray digital radiography, including dual-energy imaging, computed tomography (CT), microtomography and x-ray microscopy; nuclear medicine, including quantitative autoradiography, single photon emission tomography (SPECT) and positron emission tomography (PET); and bio-optical imaging, including optical confocal microscopy and optical tomography. The invention is more particularly directed to novel gas detector media operating on gas-microstrip principles for use in these applications.
BACKGROUND OF THE INVENTION
The capture and detection of ionizing radiation in an efficient way, without significant loss or degradation of the image information, is of paramount significance in medical imaging.
Recent advances in medical detector technology make it possible for superior images to be produced by means of digital electronic techniques compared with classical film-screen techniques. In fact, considerable efforts are in progress to develop new methods of radiographic imaging that utilize recent advances in electronics and computer technology to improve diagnostic quality and to evolve new diagnositic modalities with reduced patient dose. These methods are generally known as digital radiography.
Specifically, digital radiography has many advantages over the conventional radiography such as expanded display of detector dynamic range, fast iamge acquisition and display, convenient storage, transmission and display of stored images without degradation, extended capabilities of data analysis and image processing and reduced patient dose.
Different detector technologies and beam geometries have been proposed for digital radiography, classified such as scintillator-photodiode, high-pressure gas filled detectors, scintillators-photomultiplier, kinestatic charge detector, proximity image intensifier/CCD, phosphor screen-photodiode and diode array.
Some of the disadvantages presently faced in the field relate to the relatively high initial cost of the digital radiographic systems, as well as to the limited detector resolution. The detector system should be ocnditioned by design criteria aimed at increasing spatial, temporatl resolution and contrast resolution, detective quantum efficiency (DQE) and the signal-to-noise ratio, while maintaining sufficient sampling rates. A careful design and development of the detecotr would be required to provide a beneficial impact on x-ray capture and their efficient conversion into charge carriers.
SUMMARY OF THE INVENTION
In the presetn invention gas microstrip detectors provide high spatial, contrast and energy resolution, resulting from fine collector size, double layer geometry, and high gain. A spatial reosltuion less than 25 microns may be achieved, with count rates higher than 10
7
particles/mm
2
.
One object of the invention is to provide a high spatial resolution, high contrast resolution, dual-energy gas microstrip detector for digital radiography which incrases image quality in digital x-ray radiography including dual energy imaging and provides superior detection of low contrast structures at equal or lower dose than film radiogrpahy.
Another object of the invention is to provide a high spatial resolution, high contrast resolution, dual-energy gas microstrip detector for digital radiography higher sensitivity and lower exposure times than film autoradiography, and which also provides imaging access at the cellular level.
According to the present invention, the applicability of gas-detection principles on both dual-energy detection, such as for chest radiography and mammography, and quantitative autoradiography may be employed in several detector embodiments. Overall, the gas-microstrip detection principles enhance dramatically the image quality of the digital dual-energy detector which may be applied to general-purpose digital radiography, computer assisted tomography (CT), microtomography and x-ray microscopy, including x-ray confocal microscopy.
Also, the same gas-detection principles offer notable advantages over film autoradiography by providing a higher sensitivity, much lower exposure times, as well as imaging access at the cellular level. Therefore, enhanced radioisotope imaging of either tissue samples, to determine radiopharmaceutical distribution at cellular levels, or of electrophoresis plates, for determining molecular weights, is enabled. In addition, the present invention is applicable to other areas of nuclear medicine, such as PET and SPECT.
In positron-emission tomography (PET) one has to efficiently detect 511 KeV annihilation gamma rays with optimal space and time resolutions. Traditionally, images obtained with PET show low contrast. Moreover, PET does not provide adequate counting statistics in an acceptable exposure time of approximately 30 minutes.
The most traditional approach to PET is to use a photomultiplier coupled to each NaI or BGO scintillation crystal. The scintillation light (emitted in the visible range) is detected by a plurality of photomultiplier tubes in order to cover a wide solid angle with a good resolution. This increases the complexity of the detector system and associated electronics. It is obvious that high costs associated with large numbers of photomultipliers increase the cost of PET multihead systems and significantly make their use prohibited.
In image detecting systems there is also a need for a low-cost, efficient and large sized imaging detector readout. It is known to provide detectors based upon indirect conversion of x-rays or gamma rays using scintillator crystals with photodiodes or photomultipliers. However, this technology is either expensive or results in mediocre images. It is known to use charge couple device (CCD) cameras, but these have a small active area that limits significantly the area of interrogation. Furthermore, a CCD camera offers a low quantum sink a low signal-to-noise ratio especially for low contrast applications.
It has also been known in other technologies to use several semi-conductor media such CdZnTe semi-conductors, amorphous selenium CdSe, or amorphous silicon. However, these materials create other problems. For instance, a CdZnTe detector, which has a high atomic number and density that results in a high quantum efficiency, unfortunately provides a poor collection efficiency. As a result, even if the detector is characterized by optimal detector quality parameters, no optimized contacts or impurities in the material result in an inferior performance. It is highly desirable that a detector exhibit linear current-voltage characteristics. This can be achieved only if the semi-conductor medium has high resistively and the contacts are ohmic. In contrast, non-ohmic contacts (blocking contacts) lead to the formation of Schottky diode detectors. These detectors exhibit nonlinear detector characteristics which are highly undesirable. Also, injecting contacts, act as an ohmic contact at low applied electric fields, but at a high applied electric fields the detected current is space-charge limited which is an undesirable effect. Generally, several contact studies based on electrolysis gold and evaporated metallic contacts gold, indium, zinc and platinum contacts) with the CdZnTe surface passivated or passivated have been studied. However, no optimal contact formulation has yet been found.
It is also well known that the high cost of a crystal scintillator-photomultiplier/microchannel plate system makes their use prohibitive in medical imaging application where a large number of detected elements are required. Specifically, these devices can offer average-to-high quantum efficiency and/or amplification, but also with a high or even prohibitive cost by virtue of the large number of photodetective elements required. Additionally, in some applications photomultipliers are prone to unwanted stabilities due to high applied voltages or too strong magnetic fields. Such devices are used for nuclear medicine, x-ray astronomy and scientific applications.
The detection principles of the present invention may also be employed fo
Ham Seungsook
Hanig Richard
Renner Kenner Greive Bobak Taylor & Weber
The University of Akron
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