Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
2000-02-08
2003-04-22
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S36100C
Reexamination Certificate
active
06552347
ABSTRACT:
The subject of this invention is a new process for detecting and analyzing the interaction of gamma and X-rays, particularly at high energies (in practice called hard X-rays) with an object or a patient (target under study further on).
It includes a detector which starts up this process, and a radiographic system, which reconstitutes in real time the detected image of an object or an X-rayed element, the incidental flux of which is made up of gamma photons or hard X-rays, thus activating the detector.
The generation of gamma rays and hard X-rays, typically with an energy of more than some 100 kiloelectronvolts (keV), is nowadays widely developed in order to study the internal structure of material objects or, in the sphere of radiotherapy, notably to treat cancers. However, the difficulty in obtaining images of quality that can be correctly interpreted is important, especially in comparison with traditional radiographical techniques using low energies.
These difficulties are not only inherent in the lower absorption coefficient of the gamma rays, they also come from the different phenomena of interaction that come into play. Indeed, when a beam of x- or gamma rays penetrates matter, there occurs a phenomenon of attenuation of energy of the incidental photons following the interaction between the photons and the matter they pass through (i.e., pass-through radiation).
This interaction can occur either with an electron of matter or with one of the nuclei of the atoms that make up this matter. One type of interaction that can be distinguished with electrons is the Compton effect, i.e. the ejection of the electron that was the object of the interaction, and the creation of a scattered photon; another is the photoelectric effect, i.e. the ejection of an orbital electron under the action of the energy transferred by the incidental photon. The photoelectric effect is dominant for energies of incidental photons of up to around 100 keV. Beyond this threshold, the photoelectronic effect diminishes in favor of the Compton effect.
The interaction with the nuclei leads to the creation of electron-positron pairs, this effect becoming dominating over the Compton effect beyond energies of the order of 5-10 MeV depending on the atomic number of the matter passed through.
The low absorption coefficient added to the fact that its value, specific for each atomic element, becomes very similar for these elements as soon as the energy of the incidental photons reaches about 1 MeV, leads to a significant reduction in contrast, limits the efficiency of the detectors of these rays integrated within the radiographic installations, and seriously limits the quality of the images that are captured.
Moreover, highly energetic electrons and high energy secondary photons interact with the experimental volume, making it necessary to use stronger barriers against these secondary effects, constituting as many sources of particles affecting the detected signal, notably its contrast and limiting the resolution in terms of position within the resulting image.
In industrial radiography, high energy X-rays are used to X-ray relatively dense objects, for example in order to carry out non-destructive testing of steel constructions, to perform tests on soldering, etc. X-ray imaging also provides information on the internal structure of the object.
This type of X-rays, situated in the same range of energy, are also used in radiotherapy for the treatment of malignant tumors. In this case, this radiation is used in order to modify the biological structure of living tissues and also to destroy them. The X-ray image of the irradiated patient helps the operator to improve the positioning of the patient and to better regulate the collimation diaphragms in order to achieve a better quality of treatment and especially to reduce the risks of lesions of healthy tissues by reducing as much as possible the dose delivered whilst checking the position.
Radiographic imaging using X-rays of such energies, i.e. higher than 500 keV, would appear to be difficult to perfect, taking into account the weak effective cross-section of the interaction of the photons with matter. In fact, images with weak contrast are generally obtained, taking into account the weak dependence of the attenuation coefficient on the photon with the atomic number, so that the regions constituted by different elements are distinguished with difficulty. In order to improve the quality of the images, it would therefore appear to be necessary to acquire a large sample of photons detected at the level of the detector. Thus, two possibilities present themselves. The first is to increase the dose of radiation administered to the object. The second is to optimize the efficiency of the detection of photons at the level of the detector.
If the first of these possibilities is entirely acceptable in the sphere of analysis of static objects, especially at the industrial level, this is certainly not the case in the context of radiotherapeutic treatment for which the dose administered is strictly controlled for obvious safety reasons.
Another important factor that reduces the quality of radiotherapeutic images when high energy photons are used is the large fraction of photons that come not from the principal source of photons, but which are secondary photons created by interactions such as the Compton effect or Bremsstrahlung, occurring as much in the matter surrounding the irradiated object, in the irradiated object itself, in the collimation elements, or even in the detector itself.
Finally, another factor contributing to the difficulty in obtaining images of good quality relates to the weak effective cross-section of the interaction of high energy photons, for which only a small fraction of the photons passing through the object to be analyzed contribute to the image.
One of the objectives of the invention is to optimize the efficiency of the detection of primary photons by the detector. To do this, it aims to selectively detect only the part of the spectrum of primary or direct photons by the use of a threshold, and thus to minimize the contribution of secondary or scattered photons to the signal.
A gamma ray or X-ray detector is placed behind the object or the patient to be treated. For industrial radiography, this detector should be able to provide an image of the internal structure of the irradiated part of the object with a maximum of precision and contrast. For radiotherapy, it must provide an image of sufficient quality with a minimum dose of radiation.
Among the best known detectors are those that use portable films made of an X-ray sensitive film sandwiched between two metallic plates. Despite a good efficiency at low energy and a good spatial resolution, this kind of system has a mediocre contrast.
Moreover, and above all, the use of these films makes it impossible to obtain images in real time or nearly real time, which is becoming more and more necessary in radiotherapy.
It therefore became necessary to develop an electronic imaging device for testing, of the type better known as “Electronic Portal Imaging Devices” (EPID), especially on-line, able to deliver an image in real time at high contrast and using a weak dose administered to the patient.
Systems were proposed such as a video system with a mirror and phosphorescent screens. This system consists of a metallic plate coated with fluorescent phosphorus, the screen being visualized by a video camera using a mirror at an angle of 45 degrees. The interaction of the X-rays with the metal plate creates high energy electrons by photoelectric effects, the Compton effect and the creation of pairs, and induces fluorescence inside the screen.
Although this type of system develops a good spatial resolution, it has only a weak contrast and a high dose is often necessary to obtain a readable image. Moreover, these systems have a tendency to age relatively rapidly, are bulky and costly in production
Another device that was developed was an ionization liquid chamber. The ionization chamber consists of a matrix of wires compose
Bio-Scan S.A.
Bugnion S.A.
Gagliardi Albert
Hannaher Constantine
Moetteli John
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