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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Tomograph. History of invention and production
Directory / The history of technology, technology, objects around us Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI) or magnetic resonance imaging (MRI), is the main medical imaging tool used in radiology for detailed visualization of human internal structures and organs. The tomograph provides good contrast between the various soft tissues of the body, making it particularly useful in brain, muscle, heart, and cancer diagnostics compared to other medical imaging modalities such as X-ray computed tomography (CT) or radiography. Unlike a CT scanner or a traditional X-ray machine, an MRI scanner does not use ionizing radiation. Instead, he uses powerful magnetic fields to even out the magnetization of some of the atoms in the body, and then uses radio frequency fields to systematically change the direction of that magnetization. This leads to the appearance of a rotating magnetic field registered by the scanner and allows you to build an image of the scanned area of the body. The magnetic resonance imaging scanner uses a relatively new technology. The first images from tomographs were published in 1973, and the first cross-sectional image of a live mouse was published in January 1974. The first human studies were published in 1977. For comparison, the first human X-ray was taken in 1895.
Among the diagnostic methods that have appeared in recent years, the so-called intrascopic methods, X-ray computed tomography, nuclear magnetic resonance (NMR) tomography and NMR spectroscopy, as well as positron emission tomography (PET) are especially informative, according to medical scientists. When a suspicious area or organ is illuminated with a laser pulse, the spectral response - a sort of optical signature - of the cancerous tissue differs markedly from that of normal tissue. Computed tomography is the best-known example of three-dimensional imaging today. Conventional methods, even with a very good X-ray tube and ultra-sensitive film, give a fuzzy and very "noisy" image, moreover, only two-dimensional, so correctly interpreting it is a separate science. “Diagnostic methods have made an unprecedented leap in recent years,” says Academician Ternovoy, “thanks to computer technology. About 20 years ago, an X-ray computed tomograph was created, and it became possible to study the structure of the human brain without opening the skull. And the current equipment has such properties that you can directly observe, for example, a beating heart.Therefore, traditional, invasive diagnostics ("invasion" means "penetration") is gradually becoming a thing of the past.For example, with the help of a magnetic resonance imaging scanner, the internal organs are visible in action even without the introduction of contrast agents that "outline their contours. ... The principle of its action is based on two trivial facts: firstly, the human body consists mainly of water, and its molecules form chemical bonds with proteins and other structures that are different in different tissues; secondly, the water molecule is a dipole. In the body, these dipoles are oriented, of course, at random and, moreover, rotate. But if a person is briefly placed in a magnetic field (quite strong, but not so strong as to pose a health hazard), all water molecules turn "face" in the direction of his lines of force. Then a special radio frequency is applied - it gives the dipoles additional energy and deflects them from the orientation given by the magnetic field at one angle or another. Actually, the whole point is that the angles are different, their size depends on the internal structure of the organ or tissue, and also - which is especially important - on the presence of pathologies. The external radio pulse is given only for a moment, but it is enough. Then the water molecules return to their previous position, lining up again in the magnetic field. At the same time, they dump excess energy - special coils register it (even if it is very small!). The received data is sent to the computer, where it is processed ... " Unlike traditional X-ray methods, tomography is a volumetric reconstruction of internal organs based on numerical data that are characteristics of the physical properties of tissues. On an MRI tomograph, for example, a three-dimensional image of the fetus can be obtained. The doctor can examine the smallest details, transform the image in any way, it can also be easily compressed, archived, transmitted over communication channels to participate in teleconcilia, etc. When examining on an X-ray tomograph, the patient lies on the table so that the part of the body, the image of which is required to be obtained, would be within the circular hole in the frame of the tomograph. In the upper part of the frame, there is usually an x-ray source and a collimator - a device that converts a divergent beam of rays into a thin directed stream. At the bottom of the frame is a line of X-ray detectors, as if replacing the film. If necessary, the doctor may preliminarily introduce a chemical into the patient's body, which improves the visual contrast between the organ under study and the surrounding tissues. When the X-ray source is turned on, the rays as thin as a pencil shine through the body and the data recorded by the detector are transmitted to the computer. As the frame rotates around the patient, this process is repeated many times, and each time the data from the detectors, corresponding to a set of different positions, is processed by the computer.
Thanks to a mathematical algorithm based on the Radon transform known in classical integral geometry, a set of numerical readings of the detectors turns into a picture on the screen. A nuclear magnetic resonance tomography (NMR tomograph) is usually a tube containing a long cylindrical magnet and windings in which a current is excited corresponding to the sent and received radio frequency signals. Strictly speaking, magnetic resonance is a purely quantum phenomenon, and for its explanation it is necessary to use standard quantum mechanical concepts. The essence of the phenomenon is that a strong constant magnetic field created by a cylindrical magnet builds randomly oriented spins of the nuclei of hydrogen atoms in the patient's body along a single direction, just as iron filings line up along invisible field lines near a magnet. When a specially excited - probing - radio frequency pulse passes through the camera-tube of the tomograph, the magnetic field of the pulse, although weak, nevertheless slightly deviates the aligned spins from the given direction for some time, and they begin to oscillate, as they say, to precess, around the direction strong field of a permanent magnet, like a spinning top that is gently nudged. At the same time, the nuclei of atoms resonate, that is, they also emit a weak radio signal that can be detected by sensitive detectors. When the probing RF pulse is turned off, the spins return to an ordered state and the signal generated by the nuclei decays. By the time of this decay and other characteristics of the signal processed by the computer, one can judge the chemical composition and biological properties of tissues. For each point of the image on the screen, data from resonating hydrogen nuclei (protons) in the internal organ under study are collected and averaged, and each value obtained is assigned its own color. As a result, regions with different proton densities and, accordingly, inhomogeneous tissues are marked with different colors. Unlike an X-ray examination, the NMR method is absolutely harmless and guarantees a much better contrast between different types of tissues, which makes it easy to distinguish between healthy and diseased areas. NMR tomography is especially successfully used in the diagnosis of pathologies of the central nervous system and the musculoskeletal system, as well as for the recognition of tumors against the background of healthy tissues. However, NMR tomography is gaining new positions. A promising method for diagnosing the lungs using MRI tomography, for example, was developed in Germany. It was presented at the exhibition "Expo-2000" in Hannover and was highly appreciated by specialists and the press. For the diagnosis of lung diseases, German doctors take twenty-one million x-rays every year. However, these images are not contrast enough, and x-rays are harmful to the body. Another thing is MRI tomography. In many diseases that occur with respiratory failure, such as asthma or emphysema, the NMR tomograph gives an insufficiently clear image - due to the slight density of the lung tissue. And such important for the diagnosis of a light substance, like oxygen and nitrogen, does not register at all. So researchers are trying to improve lung imaging by having patients inhale harmless gases as a contrast agent. Polarized rare gases are especially promising. Tests have shown that saturating the lung with them allows you to get a clear image. The better magnetization of polarized inert gases in comparison with hydrogen facilitates the work of the tomograph. Thus, doctors can not only diagnose asthma, cystic fibrosis and other lung diseases at an early stage, but also additionally check the effectiveness of treatment. In Germany, the foundations of the new method were laid by Ernst Wilhelm Otten and Werner Geil from the Institute of Physics at the University of Mainz. Otten and Gail chose helium-3 as a contrast agent for their experiments. In their opinion, xenon is not very suitable here, since it is absorbed by the blood and has a narcotic effect on patients. And so, using an MRI scanner and polarized helium-3 as a contrast agent, Otten and Geil, together with the radiologist from Mainz, Manfred Thelen, and experts from the German Cancer Research Center in Heidelberg, finally obtained a clear image of the air distribution in the lung. The new method in an experiment with one thirty-year-old subject made it possible to ascertain the signs of already old pulmonary emphysema. And this despite the fact that although the person smoked, she felt completely healthy and did not complain about her lungs. Another example is the use of an NMR scanner to diagnose an infarction instead of a cardiac catheter. Examination of the heart using ECG, ultrasound and radiation exposure to radioactive isotopes does not always lead to satisfactory results. In such cases, diagnosis is often indicated using a cardiac catheter, which is inserted into the heart through the blood vessels. This is a serious burden for the body of the subject, and many patients prefer the new, most modern, harmless to humans magnetic fields to the traditional method: the heart is “shown through” by a nuclear magnetic resonance tomograph. Previous models of NMR tomographs, due to too long measurement periods, did not give clear enough images (the heart beats constantly, and the “long exposure” image is blurry). The latest devices, improved hardware and software allow you to take fairly clear pictures of the heart in between heartbeats. "The accuracy is now clearly higher than with previous non-invasive methods," explains Eike Nagel from the German Heart Center in Berlin. "Using the technique, the number of examinations with a cardiac catheter can be reduced by at least 20 percent." And according to optimists - half. As a comprehensive diagnostic instrument, the MRI imager spatially depicts the heart and large arteries, measures blood supply parameters and recognizes dead tissue. A gentle high-tech method is suitable for both prevention and treatment of heart patients. MRI tomography saves heart patients from unnecessary stress. Using this method, it is possible to predict whether the expansion of the vessel or the operation on the anastomosis promises success at all. This was shown by scientists from Northwestern University in Chicago in their clinical study. It is very important that the new technique can protect many young patients from dangerous interventions. The strong magnetic fields to which the subjects are exposed are practically harmless - at least according to modern science. Alternative methods, for example, computed and positron emission tomography, work, on the contrary, with substances that are unsafe for the body - x-rays and radioactive isotopes. A kind of boom is experiencing tomographic prevention of cardiovascular diseases in the capital of Taiwan, Taipei. A special examination center has recently opened there, where an approximately half-hour examination of the heart and blood vessels with an NMR tomograph costs a thousand dollars, while video glasses and pleasant music help patients relax ... Author: Musskiy S.A.
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