The Nobel Prize in Physics
Georges Charpak, a French citizen, was born on August 1, 1924 in Poland. Charpak's family moved from Poland to Paris when he was seven years old. During World War II Charpak served in the resistance and was imprisoned by Vichy authorities in 1943. In 1944 he was deported to the Nazi concentration camp at Dachau, where he remained until the camp was liberated in 1945. Charpak became a French citizen in 1946. He received a Ph.D. in 1955 from the College de France, Paris, where he worked in the laboratory of Frédéric Joliot-Curie. In 1959 he joined the staff of European Laboratory for Particle Physics at CERN in Geneva and in 1984 also became Joliot-Curie professor at the School of Advanced Studies in Physics and Chemistry, Paris. He was made a member of the French Academy of Science in 1985.
Georges Charpak, 1992 Nobel Prize laureate, is well known in Canada. Invited in October 1994 by the Scientific Service, he gave a sparkling lecture and was given a tour of Ottawa's premier laboratories and met with its leading scientists. When he returned in 1995, he was made an honorary doctor of the University of Ottawa. For the occasion, he sponsored a Canada-Israel research grant put together by the French embassy.
The staff using it at Dr. Gabriel Kalifa's ward at the Saint-Vincent-de-Paul hospital in Paris have dubbed it the "Charpak System". The revolutionary equipment, now being tested for clinical evaluation over a nine-month period, is the result of research conducted by the French Nobel prize laureate Georges Charpak. The project developed by a company called Biospace Radiologie means that patients will be subjected to one hundred times fewer x-rays than before. The system also increases the amount of information conveyed by the image and provides direct digital data acquisition.
In the early 80's, Lev Schektmann, a physicist who then worked at the Georges Charpak detector research department at CERN1 in Geneva, came up with the idea of a medical radiology application for the wire chamber, which is commonly used in nuclear physics. The originality of the system is largely based on the way the wires are set up in the chamber.
In 1983, Lev Schektmann's research work at the Nuclear Physics Facility in Siberia led to the set-up of a diagnostic machine at the Moscow Mother and Children's Hospital, which specialises in scanning women's pelvis. Two detectors using 256 wire chambers were set up to heighten resolution. In 1987, the new type of detector was installed in the Novossibirsk hospital for standard radiology examinations, and more particularly for examinations of the spinal column and the lungs. Since 1988, a new version of the detector, with 640 detection components, has been used as an experimental machine at the Budker Institute. An identical model is now being evaluated at the Saint-Vincent-de-Paul hospital.
Like a taut weave of fabric
The originality of the different machines stems from the proportional multi-wire chamber or "Charpak detector", whose design and construction earned the French physicist the Nobel Prize in 1992. The proportional multi-wire chamber is a gas (a mix of xenon and CO2) particle detector. From the outside the detector is a 50 cm wide aluminium box with a small manometer on top, which serves to maintain xenon pressure at 3 bars.
When the box is opened, an alignment of wires can be seen inside. They are made of copper. Measuring 10 microns in diameter, they are pulled tight like the weft of fabric on a loom. The axis of each wire faces the x-ray source, 1.30 m away; the wires measure 5 cm and are separated from one another by a distance of 1.2 mm. Cathodes are set up on each side of the wire layout. The connections to the first electronic level are under the chamber. Each wire is connected to an amplifier, a selection component and a counter. All the components are on 32 cards inserted into a built-in hood on the chamber. The chamber-electronic counting unit is supported by an arm with x-ray tube fitted onto its other end.
Two slots to reduce irradiation
A small piece of furniture, which is separate from the detector, holds the power input electronics, the control systems and the data processing unit of the information conveyed by the detector. The data is then transmitted to a PC-type microcomputer that can display and process the images. X-rays are still sent out by a standard system, which does, however, have an electronically controlled generator and an ordinary store-brought tube.
As the wire chamber is a linear detector which can record only one line at a time, the whole system has to scan the patient. The arm, with x-ray tube at one end and the detector at the other, is powered by an electronically controlled motor which moves the arm vertically. X-ray beam collimation slots have been fitted onto the arm to reduce patient irradiation radiation scattering effect. The equipment is computer-controlled and has a programme so the doctor can measure the distances between the objects on the image, the density of each point on the image and the average density of any fragment of the image. Other operations such as windowing, zooming, etc. are also possible.
One line every 30 milliseconds
A finely collimated x-ray beam scans one part of the patient's body during the examination. The detector records a line every 30 milliseconds and stores it in the computer memory bank. The line is then processed and displayed on the computer screen.
The speed at which the tube-detector unit moves determines the physical depth of the "slice" of anatomy being scanned, i.e. 0.6 mm on the new system now being built. Space resolution follows the other axis and is determined by the spaces between the wires in the detector. The wires, as was indicated earlier, are separated from one another by a distance of 1.2 mm, but an electronic device can halve the width of each scanning channel. So, with 320 wires, the equipment can supply 640 channels with a 6 mm scanning width.
The only drawback of the system is that it can only examine patients in a standing position. The drawback, which restricts the scanning of invalids for instance, could be removed with the construction of a multipurpose table now under study. The table should, among other things, be able to take into account the specific features of the detector, particularly, its potential sensitivity to vibrations.
One hundred times fewer X-rays
Georges Charpak has planned for a device which can increase space resolution (now at 0.6 x 0.6 mm) with a detector with a higher element density. The detector scanning speed which is now at 12 sec. for a 36 cm scanning area, could be increased in two different ways : with a one wire-chamber or with several wire chambers.
The main advantage of the system is that it delivers one hundred times fewer x-rays than existing radiology systems. The decrease of irradiation is due to a more efficient detector component and to the decrease of radiation scattered by the collimation system. The wire-chamber can actually count the photons entering the chamber, one by one. The incident photons interact with the xenon producing photo-electrons, which are detected by the wires when they are switched on. Each amplified occurrence is counted by the electronic unit.
The counting mechanism is more efficient than the standard film-screen unit that requires a greater number of photons to act on the emulsion and is simpler than radioscopy, which uses a detection chain where each stage generates its own artifacts.
Improved image quality
Scattered radiation is one of the main reasons why an x-ray image deteriorates and why irradiation needs to be increased to curtail deterioration. A collimation system lowers radiation. The x-ray beam has been scaled down to a 1 mm deep ray in front of the patient. Only the part of the body being scanned is irradiated. At the opening of the detector, a second collimator eliminates the scattered beams, which have veered from their initial path. Scattered radiation has been lowered to 1% of the main beam whereas, with no collimation, it skyrockets to 90%.
Besides lowering irradiation, eliminating scattered radiation provides improved data quality : weak contrast is no longer lost in background noise. At the end of the detector, each scanned point is coded into 64 000 levels of grey (16 bytes). So much data makes extremely weak contrast visible because the scattered radiation has been eliminated. Not to mention that weak contrast levels can be distinguished with a larger palette of greys because of the dynamics of the detector.
9 months and 250 patients
Counting of photons one by one also entails direct data digitalisation as soon as the detector supplies the data. When the detection is over, the image can be computer-processed where as other digital radiology systems entail digitisation either by video camera or processing by a laser read system. The data from the detector can also be easily built into standard digital radiology systems (reproduction, transmission, filing, etc.)
During the 9 months of clinical evaluation, 250 patients will be exposed, once to standard radiology, and once to the "Charpak system", each time they are examined. The amount of irradiation the patient was subjected to will be measured each time and Dr. Kalifa will compare the relevance of the two types of images as diagnostic tools. If the testing, sponsored by COGEMA, world leader in nuclear fuel cycle, is conclusive, production of the new radiology equipment will get rolling.
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