Nuclear magnetic resonance (NMR) vibration is a physical phenomenon. As an analytical method, it is widely used in physics, chemical biology and other fields. It was not used for medical clinical examination until 1973. In order to avoid confusion with radiological imaging in nuclear medicine, it is called magnetic resonance imaging (MR).
MR is a biological magnetic spin imaging technology, which uses the characteristics of nuclear spin motion to generate signals after being excited by radio frequency pulses in an external magnetic field, which are detected by a detector, input into a computer, processed and converted to display images on the screen.
The information provided by MR is not only greater than many other imaging technologies in medical images, but also different from the existing imaging technologies. Therefore, it has great potential advantages in disease diagnosis. It can directly make cross-sectional, sagittal, coronal and various inclined planes without artifacts in CT detection; No need to inject contrast agent; No ionizing radiation, no adverse effects on the body. MR is very effective in detecting common brain diseases such as intracerebral hematoma, extracerebral hematoma, brain tumor, intracranial aneurysm, arteriovenous malformation, cerebral ischemia, intraspinal tumor, syringomyelia, hydrocephalus, etc., and it is also very effective in diagnosing diseases such as lumbar disc herniation and primary liver cancer.
Mr. Wang also has some shortcomings. Its spatial resolution is not as good as that of CT, so patients with pacemakers or parts with some metal foreign bodies cannot be examined by MR, and it is expensive.
Magnetic vibration imaging is a tomography technology, which uses magnetic vibration phenomenon to obtain electromagnetic signals of human body and reconstruct human body information. 1946, Flex Bloch of Stanford University and Edward purcell of Harvard University independently discovered the phenomenon of nuclear magnetic resonance. Magnetic resonance imaging technology is based on this physical phenomenon. 1972, paul lauterbur developed a set of spatial coding methods for nuclear magnetic resonance signals, which can reconstruct human images.
Magnetic resonance imaging (MRI) has some similarities with other tomography techniques (such as CT). For example, they can display the spatial distribution of some physical quantities (such as density); It also has its own characteristics. Magnetic resonance imaging can obtain cross-sectional images, three-dimensional volume images and even four-dimensional images of spatial-spectral distribution in any direction.
Like PET and SPET, the magnetic vibration signal used for imaging comes directly from the object itself. It can also be said that magnetic vibration imaging is also an emission tomography. But unlike PET and SPET, magnetic resonance imaging can be imaged without injecting radioisotopes. This also makes the magnetic resonance imaging technology safer.
Many physical parameters of matter can be obtained from magnetic images, such as proton density, spin-lattice relaxation time T 1, spin-spin relaxation time T2, diffusion coefficient, magnetization coefficient and chemical shift. Compared with other imaging technologies (such as CT ultrasound PET, etc.). ), the magnetic resonance imaging methods are more diverse, the imaging principle is more complicated, and the information obtained is more abundant. Therefore, magnetic resonance imaging has become a hot research direction in the field of medical imaging.
The principle of nuclear magnetic resonance imaging: the nucleus is positively charged, and the nuclei of many elements, such as 1H, 19FT, 3 1P, are all spinning. Usually, the arrangement of nuclear spin axes is irregular, but when it is placed in an external magnetic field, the spatial orientation of nuclear spin changes from disorder to order. The magnetization vector of the spin system increases gradually from zero, and when the system reaches equilibrium, the magnetization reaches a stable value. If the nuclear spin system is acted by external force at this time, for example, a certain frequency of radio frequency excites the nuclear, it can cause * * * vibration effect. After the RF pulse stops, the nuclei excited by the spin system can not maintain this state, and will return to their original arrangement in the magnetic field, and at the same time release weak energy into radio signals. These signals can be detected and spatially resolved, and the distribution image of moving nuclei can be obtained. The process of nucleus returning from excited state to equilibrium arrangement state is called relaxation process. The time it takes is called relaxation time. There are two relaxation times, namely T 1 and T2, where T 1 is the spin-lattice or longitudinal relaxation time T2 and T2 is the spin-spin or transverse relaxation time.
The most commonly used nucleus of magnetic resonance is hydrogen proton (1H), because it has the strongest signal and is widely found in human tissues. The factors that affect the magnetic resonance image include: (a) the density of protons; (b) the length of relaxation time; (c) the flow of blood and cerebrospinal fluid; (d) paramagnetic materials; Protein. The characteristics of gray scale of magnetic vibration image are that the stronger the magnetic vibration signal, the greater the brightness, the weaker the magnetic vibration signal and the smaller the brightness, ranging from white, gray to black. The gray characteristics of magnetic resonance images of various tissues are as follows: adipose tissue and cancellous bone are white; Cerebrospinal fluid and bone marrow are white gray; Viscera and muscles are grayish white; Liquid, flowing at normal speed, blood is black; Bone cortex, air and air-containing lungs are all black.
Another characteristic of NMR vibration is that the flowing liquid does not produce signal, which is called flow effect or flow blank effect. So blood vessels are gray-white tubular structures, while blood is black and has no signal. This makes it easy for blood vessels to separate soft tissues. Normal spinal cord is surrounded by cerebrospinal fluid, which is black and has white dura mater set off by fat, which makes the spinal cord show white strong signal structure. Nuclear magnetic resonance (NMR) vibration has been applied to imaging diagnosis of various systems in the whole body. The best effect is the brain, its spinal cord, heart blood vessels, joint bones, soft tissues and pelvic cavity. For cardiovascular diseases, we can not only observe the anatomical changes of heart cavities, great vessels and valves, but also analyze ventricles for qualitative and semi-quantitative diagnosis. We can also make multiple sections with high spatial resolution to show the whole picture of heart and lesions and their relationship with surrounding structures, which is superior to other X-ray imaging, two-dimensional ultrasound, nuclides and CT examinations. In the diagnosis of encephalomyelitis, coronal, sagittal and transverse images can be made.
Purpose of examination: brain, spine and spinal cord diseases, five sense organs diseases, heart disease, mediastinal mass, bone and joint and muscle diseases, uterus, ovary, bladder, prostate, liver, kidney and pancreas diseases.
Advantages: 1 Nuclear magnetic resonance has no harm to human body;
2.MRI can obtain three-dimensional images of the brain and spinal cord, unlike CT, which may miss the diagnosis of diseases;
3. It can diagnose heart diseases, and CT scanning is slow and incompetent;
4. The examination of bladder, rectum, uterus, vagina, bones, joints and muscles is better than CT.
Disadvantages: 1 Like CT, MRI is also an imaging diagnosis. Many lesions are still difficult to be diagnosed by MRI alone, unlike endoscopy, which can obtain both imaging and pathological diagnosis.
2. The examination of lung is not superior to X-ray or CT, and the examination of liver, pancreas, adrenal gland and prostate is not superior to CT, but the cost is much higher;
3. Gastrointestinal lesions are not as good as endoscopy;
People with metal objects in their bodies should not receive MRI.
5. Critical patients can't do it.
6. Within 3 months of pregnancy
7. wear a pacemaker
Matters needing attention in nuclear magnetic resonance inspection
Because there is a very strong magnetic field in the MRI machine and MRI examination room, it is absolutely forbidden to do MRI examination for people with pacemakers, people who have left metal clips or metal stents after vascular surgery, or people who have undergone metal stent surgery such as coronary artery, esophagus, prostate and biliary tract. Otherwise, the metal will move due to the attraction of strong magnetic field, which may cause serious consequences and even endanger life. Generally, there are red or yellow eye-catching signs outside the MRI examination room of the hospital, indicating that MRI examination is absolutely forbidden.
If there are other metal foreign bodies that can't be taken out, such as metal internal fixings, artificial joints, metal dentures, brackets, silver clips, shrapnel and other metal residues, it is taboo to check them. When inspection is necessary, close observation should be made to prevent the metal from moving in a strong magnetic field during inspection, damaging adjacent large blood vessels and important tissues and causing serious consequences. If there is no special need, generally should not be nuclear magnetic resonance examination. Metal contraceptive rings and removable metal dentures must be removed before inspection.
Sometimes, the residual metallic iron ions in the body may affect the image quality and even the correct diagnosis.
Before entering the nuclear magnetic resonance examination room, remove the mobile phone, pager, magnetic card, watch, coin, key, lighter, metal belt, metal necklace, metal earrings, metal buttons and other metal ornaments or items. Otherwise, it may affect the uniformity of magnetic field, cause image interference and form artifacts, which is not conducive to the display of lesions; Moreover, due to the strong magnetic field, metal objects may be sucked into the nuclear magnetic vibration machine, thus damaging the very expensive nuclear magnetic vibration machine; In addition, mobile phones, pagers, magnetic cards, watches and other items may also be damaged by strong magnetic fields, causing unnecessary personal property losses.
In recent years, with the progress and development of science and technology, many orthopedic internal fixators, especially spinal internal fixators, have begun to be made of titanium alloy or titanium metal. Because titanium is not attracted by the magnetic field, it will not move in the magnetic field. Therefore, it is safe for patients with titanium internal fixator to undergo MRI vibration examination; Moreover, titanium metal will not interfere with the MRI image. This is very valuable for patients with spinal diseases who need spinal internal fixation surgery. However, the internal fixation of titanium alloy and titanium metal is expensive, which affects its popularization and application to some extent.
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Medical and medical imaging
References:
1. Medical imaging technology
Contributor:
Wtrecamel, yo motionless, waterone83, Xiao Wuhou, Dai Rui 725
This entry is mentioned in the following entries:
Heroin, amyotrophic lateral sclerosis, primary liver cancer
Interpretation of "MRI" in English-Chinese dictionaries (Source: Baidu Dictionary);
zeugmatography
Abbreviations.
1. magnetic resonance imaging medical magnetic resonance imaging
2. Machine readable information