nuclear magnetic resonance
nuclear magnetic resonance
Magnetic resonance imaging (NMRI), also known as magnetic resonance imaging (MRI),
The full name of magnetic resonance imaging (MRI) is nuclear magnetic resonance imaging (MRI), which is a physical process in which the spin level of nuclear with non-zero magnetic moment splits Zeeman under the action of external magnetic field, and * * vibrates to absorb radio frequency radiation of a certain frequency. Nuclear magnetic resonance spectrum is a branch of spectroscopy, its vibration frequency is in radio frequency band, and the corresponding transition is the transition of nuclear spin in nuclear Zeeman level.
Nuclear magnetic resonance (NMR) vibration is a physical phenomenon, that is, the nucleus in a static magnetic field is acted by another alternating magnetic field. Generally speaking, nuclear magnetic resonance refers to the technology of obtaining molecular structure and internal structure information of human body by using nuclear magnetic resonance phenomenon.
Not all nuclei can produce this phenomenon. Nuclei can produce nuclear magnetic resonance because they have nuclear spins. Nuclear spin produces magnetic moment, and when the nuclear magnetic moment is in a static external magnetic field, precession nucleus and energy level splitting are produced. Under the action of alternating magnetic field, the spin nucleus will absorb electromagnetic waves with a specific frequency and jump from a lower energy level to a higher energy level. This process is called nuclear magnetic resonance.
Magnetic resonance imaging (MRI) is also called magnetic resonance imaging technology. This is another great progress in medical imaging after CT. Since its application in 1980s, it has developed rapidly. The basic principle is that the human body is placed in a special magnetic field, and the hydrogen nucleus in the human body is excited by radio frequency pulses, so that the hydrogen nucleus vibrates and absorbs energy. After stopping the radio frequency pulse, the hydrogen nucleus sends out a radio signal at a specific frequency, and releases the absorbed energy, which is collected by a receiver in vitro and processed by an electronic computer to obtain an image, which is called nuclear magnetic resonance imaging.
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 (MRI).
Magnetic resonance imaging (MRI) 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 and input into a computer, and then processed and converted to display images on the screen.
The information provided by MRI is not only greater than many other imaging methods in medical images, but also different from the existing imaging methods. 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. MRI 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 effective in diagnosing diseases such as lumbar disc herniation and primary liver cancer.
Nuclear magnetic resonance also has some disadvantages. Its spatial resolution is not as good as that of CT, so for patients with pacemakers or some metal foreign bodies, MRI can not be used and it is more expensive.
The history of nuclear magnetic resonance technology
In the 1930 s, physicist isidor rabi found that the nuclei in the magnetic field would be arranged in parallel in the forward or reverse order along the direction of the magnetic field, and the spin direction of the nuclei would be reversed after radio waves were applied. This is the earliest understanding of the interaction between nucleus and magnetic field and external RF field. Because of this research, Rabbi won the 1944 Nobel Prize in Physics.
During the period of 1946, Bloch and purcell, two American scientists, found that when an odd number of nuclei (including protons and neutrons) are placed in a magnetic field and a radio frequency field with a specific frequency is applied, the energy of the radio frequency field will be absorbed by the nuclei, which is the preliminary understanding of the nuclear magnetic resonance phenomenon. Because of this, the two of them won the 1952 Nobel Prize in Physics.
People discovered the phenomenon of nuclear magnetic resonance, and it soon put into practical application. Chemists use the influence of molecular structure on the magnetic field around hydrogen atoms to develop nuclear magnetic resonance spectra for the analysis of molecular structure. With the passage of time, the nuclear magnetic resonance spectroscopy technology has been developing continuously, from the initial one-dimensional hydrogen spectrum to the advanced spectrum such as 13C spectrum and two-dimensional nuclear magnetic resonance spectrum. The ability of nuclear magnetic resonance technology to analyze molecular structure is getting stronger and stronger. After entering the 1990' s, people even developed the technology of determining the tertiary structure of protein molecule by the information of nuclear magnetic resonance vibration, which made it possible to accurately determine the molecular structure of protein in solution phase.
1946, purcell of Harvard University and Bloch of Stanford University announced that they had discovered nuclear magnetic resonance (NMR). Therefore, they won the 1952 Nobel Prize. Nuclear magnetic resonance (NMR) is a phenomenon that the nuclear magnetic moment is absorbed under the simultaneous action of a constant magnetic field and a high-frequency magnetic field (in the radio wave band). When certain conditions are met, * * * vibration will occur. Nuclear magnetic resonance (NMR) soon became a high-tech to explore and study the microstructure and properties of substances. At present, nuclear magnetic resonance has been widely used in physics, chemistry, materials science, life science and medicine.
The nucleus consists of protons and neutrons, both of which have inherent magnetic moments. It can be popularly understood that they behave like small magnetic needles in the magnetic field. Under the action of external magnetic field, the interaction between nuclear magnetic moment and magnetic field leads to energy level splitting, and the energy level difference is proportional to the strength of external magnetic field. If the alternating electromagnetic field corresponding to the energy level interval is added at the same time, it can cause the energy level transition of the nucleus and produce nuclear magnetic resonance. It can be seen that its basic principle is similar to the * * * vibration absorption phenomenon of atoms.
Early nuclear magnetic resonance was mainly used to study the structure and properties of nuclear, such as measuring nuclear magnetic moment, electric quadrupole distance and nuclear spin. Later, it was widely used in molecular composition and structure analysis, biological tissue and living tissue analysis, pathological analysis, medical diagnosis and nondestructive monitoring of products. For isolated hydrogen nuclei (protons), when the magnetic field is 1.4T, the vibration frequency of * * * is 59.6MHz, and the corresponding electromagnetic wave is a radio wave with a wavelength of 5m. But in the compound molecule, this * * * vibration frequency is also related to the chemical environment in which the hydrogen nucleus is located. Hydrogen nuclei in different chemical environments have different * * * vibration frequencies, which are called chemical shifts. This is caused by the shielding effect, inducing effect and * * * effect of the extranuclear electron cloud on the magnetic field. At the same time, due to the interaction of atoms between molecules, spin-coupled splitting will also occur. The molecular structure of compounds, especially organic compounds, can be inferred by chemical shift and cracking number. This is the spectral analysis of nuclear magnetic resonance. In 1970s, the appearance of pulsed Fourier transform nuclear magnetic resonance vibrometer made the application of C 13 spectrum increase day by day. The analysis of substance composition and structure by nuclear magnetic resonance has the advantages of high accuracy, less restrictions on samples and no damage to samples.
The earliest magnetic resonance imaging experiment was published by Lauterper in 1973, which immediately attracted widespread attention and entered the clinical application stage in just 10 years. There is a stable magnetic field and an alternating electromagnetic field acting on the sample. After removing the electromagnetic field, the excited nucleus can jump to a low energy level, radiate electromagnetic waves, and induce a voltage signal in the coil at the same time, which is called nuclear magnetic resonance signal. Due to the existence of a large number of water and hydrocarbons, there are a large number of hydrogen nuclei in human tissues. Generally speaking, the signal obtained by using hydrogen nuclei is more than 1000 times larger than other nuclei. The voltage signals of normal tissues and diseased tissues are different. Combined with CT technology, that is, computer tomography technology, we can get any cross-sectional image of human tissue, especially for the diagnosis of soft tissue lesions, showing its advantages, being very sensitive to the lesion site and the image is very clear.
In the research of magnetic resonance imaging, a frontier subject is functional magnetic resonance imaging, which studies the function and advanced thinking activities of the human brain. People have learned a lot about brain tissue, but little about how the brain works and why it has such advanced functions. Bell Laboratories began its research in this field on 1988, and the US government also designated the 1990s as the "Decade of the Brain". Nuclear magnetic resonance (NMR) can be used to directly observe the living body, and the tested object is conscious. It also has the advantages of no radiation damage, fast imaging speed, high temporal and spatial resolution (up to 100μm and tens of ms respectively), detection of various nuclides, selective chemical shift and so on. The Wisconsin Hospital in the United States has taken thousands of living images of the working brain, which is expected to unveil the mystery of the working brain in the near future.
If the frequency variables of nuclear magnetic resonance are increased to two or more, two-dimensional or multi-dimensional nuclear magnetic resonance can be realized, so that more information can be obtained than one-dimensional nuclear magnetic resonance. At present, the application of nuclear magnetic resonance imaging is limited to hydrogen nuclei, but from the point of view of practical application, other nuclei such as C 13, N 14, P3 1, S33, Na23 and I 127 are also required to perform nuclear magnetic resonance imaging. C 13 has entered the practical stage, but it still needs to be further expanded and deepened. The combination of nuclear magnetic resonance * * vibration with Mossbauer effect (absorption effect of γ -ray vibration without recoil * * *), electron spin * * vibration and other physical effects can obtain more valuable information, which is of great significance both in theory and in practical application. Nuclear magnetic resonance has a broad application prospect. With the breakthrough of pulse Fourier technology, the spectrum of C 13 has entered the application stage. It is reasonable to believe that the spectra of other nuclei should enter the application stage in the near future.
On the other hand, medical scientists have found that hydrogen atoms in water molecules can generate nuclear magnetic resonance (NMR), which can be used to obtain information about the distribution of water molecules in the human body, thus accurately drawing the internal structure of the human body. On the basis of this theory, Damadian, MD, Southern Medical Center of new york State University, successfully distinguished mouse cancer cells from normal tissue cells by measuring the relaxation time of nuclear magnetic resonance. Inspired by Damadi's new technology, physicist Paul Lauterper of State University of New York at Stony Brook developed an imaging technology (MRI) based on nuclear magnetic resonance (NMR) in 1973, and successfully drew an image of the internal structure of a living clam with his equipment. After Lauterper, magnetic resonance imaging technology has become more and more mature and widely used. It has become a routine medical detection method and is widely used in the treatment and diagnosis of brain and spinal cord diseases such as Parkinson's disease and multiple sclerosis, as well as cancer. In 2003, Paul Lauterper and Peter Mansfield, a professor at Nottingham University in England, won the Nobel Prize in Physiology or Medicine for their contributions to magnetic resonance imaging technology. The basic principle is that the human body is placed in a special magnetic field, and the hydrogen nucleus in the human body is excited by radio frequency pulses, so that the hydrogen nucleus vibrates and absorbs energy. After stopping the radio frequency pulse, the hydrogen nucleus sends out a radio signal at a specific frequency, and releases the absorbed energy, which is collected by a receiver in vitro and processed by an electronic computer to obtain an image, which is called nuclear magnetic resonance imaging.
The principle of nuclear magnetic resonance
The phenomenon of nuclear magnetic resonance vibration comes from the precession of nuclear spin angular momentum under the action of external magnetic field.
According to the principle of quantum mechanics, the nucleus, like electrons, also has spin angular momentum, and its specific value is determined by the spin quantum number of the nucleus. The experimental results show that the spin quantum numbers of different types of nuclei are also different:
A nucleus with even mass and proton number has a spin quantum number of 0.
Nuclei with odd mass and spin quantum numbers are semi-integers.
Nuclei with even mass and odd proton number have integer spin quantum numbers.
So far, people can only use nuclei with spin quantum number equal to 1/2. The commonly used nuclei are: 1H, 1 1B, 13C, 17O, 65438.
Because the nucleus is charged, when the nucleus spins, it will produce a magnetic moment, which is in the same direction as the nucleus, and its magnitude is proportional to the angular momentum of the nucleus. When the nucleus is placed in an external magnetic field, if the magnetic moment of the nucleus is different from the direction of the external magnetic field, the magnetic moment of the nucleus will rotate around the direction of the external magnetic field, which is similar to the swing of the rotating shaft of the gyro in the process of rotation, which is called precession. Precession has energy and a certain frequency.
The frequency of nuclear precession is determined by the strength of the external magnetic field and the nature of the nucleus itself, that is to say, for a specific atom, the frequency of nuclear precession is fixed under a certain external magnetic field strength.
The energy of nuclear precession is related to magnetic field, nuclear magnetic moment and the angle between magnetic moment and magnetic field. According to the principle of quantum mechanics, the angle between the nuclear magnetic moment and the external magnetic field is not continuously distributed, but determined by the nuclear magnetic quantum numbers, and the direction of the nuclear magnetic moment can only jump between these magnetic quantum numbers, but can not change smoothly, thus forming a series of energy levels. When the nucleus receives energy input from other sources in the external magnetic field, the energy level transition will occur, that is, the angle between the nuclear magnetic moment and the external magnetic field will change. This energy level transition is the basis of obtaining nuclear magnetic resonance signals.
In order to make the nuclear spin precession have energy level transition, it is necessary to provide the nuclear with the energy needed for the transition, which is usually provided by the external RF field. According to the principle of physics, when the frequency of the applied RF field is the same as that of the nuclear spin precession, the energy of the RF field can be effectively absorbed by the nuclear, which helps the energy level transition. Therefore, in a given external magnetic field, a specific nucleus only absorbs the energy provided by a certain frequency RF field, thus forming a nuclear magnetic resonance signal.
Application of NMR * * *
nuclear magnetic resonance technique
Nuclear magnetic resonance spectroscopy
NMR technology, namely nuclear magnetic resonance spectroscopy, is a technology that applies nuclear magnetic resonance phenomenon to determine molecular structure. Nuclear magnetic resonance spectrum plays a very important role in determining the structure of organic molecules. Nuclear magnetic resonance spectrum, ultraviolet spectrum, infrared spectrum and mass spectrum are called "four famous spectra" by organic chemists. At present, the study of NMR spectra mainly focuses on the spectra of 1H and 13C nuclei.
For isolated nuclei, the same nucleus is only sensitive to a certain frequency RF field in the same external magnetic field. However, due to the influence of factors such as the distribution of electron clouds in molecules, the actually felt external magnetic field intensity often changes to a certain extent, and the external magnetic field intensity felt by nuclei in different positions in the molecular structure is also different. The influence of the electron cloud in the molecule on the external magnetic field strength will make the nuclei in different positions in the molecule sensitive to different frequency RF fields, which will lead to the difference of nuclear magnetic resonance signals. This difference is produced by nuclear magnetic resonance. The distribution of chemical bonds and electron clouds near the nucleus is called the chemical environment of the nucleus, and the change of nuclear magnetic resonance signal frequency position caused by the influence of the chemical environment is called the chemical shift of the nucleus.
Coupling constant is another important information provided by NMR spectrum besides chemical shift. The so-called coupling refers to the interaction of spin angular momentum of adjacent nuclei, which will change the energy level distribution of nuclear spin precession in external magnetic field, lead to energy level splitting, and then lead to the change of signal peak shape in NMR spectrum. By analyzing the changes of these peak shapes, we can infer the connection relationship between atoms in the molecular structure.
Finally, the signal intensity is the third important information in NMR spectrum. Nuclei in the same chemical environment will show the same signal peak in NMR spectrum. By analyzing the signal peak intensity, we can know the number of these nuclei, thus providing important information for the analysis of molecular structure. The signal peak intensity is characterized by the area integral under the signal peak curve, which is particularly important for 1H-NMR spectrum, but not very important for 13C-NMR spectrum, because the corresponding relationship between peak intensity and nuclear number is not significant.
The early nuclear magnetic resonance spectrum mainly focused on the hydrogen spectrum, because the 1H atoms that can generate nuclear magnetic resonance signals are extremely abundant in nature, and the nuclear magnetic resonance signals generated by them are very strong and easy to detect. With the development of Fourier transform technology, nuclear magnetic resonance (NMR) oscillators can emit RF fields with different frequencies at the same time in a very short time, so that samples can be scanned repeatedly, thus distinguishing weak NMR vibration signals from background noise and enabling people to collect 13C NMR vibration signals.
In recent years, people have developed two-dimensional nuclear magnetic resonance spectroscopy technology, which enables people to obtain more information about molecular structure. At present, two-dimensional nuclear magnetic resonance spectroscopy has been able to analyze the spatial structure of small molecular weight protein molecules.
Magnetic resonance imaging technology
MRI
Magnetic resonance imaging technology is the application of magnetic resonance in medical field. The human body is rich in water, and the water content of different tissues is different. If we can detect the distribution information of water, we can draw a relatively complete picture of the internal structure of human body. Magnetic resonance imaging technology is a technology to infer the distribution of water molecules in human body by identifying the distribution of hydrogen atoms in water molecules, and then detect the internal structure of human body.
Unlike the nuclear magnetic resonance spectrum used to identify molecular structures, nuclear magnetic resonance imaging technology adapts to the intensity of external magnetic field, not the frequency of radio frequency field. Magnetic resonance imaging will provide two perpendicular gradient magnetic fields in the direction perpendicular to the main magnetic field, so that the distribution of the magnetic field in the human body will change with the change of spatial position, and each position will have a magnetic field with different intensity and direction, so that hydrogen atoms located in different parts of the human body will respond to different RF field signals. By recording and calculating this reaction, we can get the information of the spatial distribution of water molecules, and then get the image of the internal structure of the human body.
Magnetic resonance imaging can also be combined with X-ray tomography (CT) to provide important data for clinical diagnosis and physiological and medical research.
Magnetic resonance imaging (MRI) is a non-invasive detection technology. Compared with X-ray fluoroscopy and radiography, MRI has no radiation effect on human body. Compared with ultrasonic detection technology, MRI is clearer and can show more details. In addition, compared with other imaging techniques, MRI can not only show tangible solid lesions, but also accurately respond to functional reactions such as brain, heart and liver. Magnetic resonance imaging plays a very important role in the diagnosis of Parkinson's disease, Alzheimer's disease, cancer and other diseases.
MRS technology
Nuclear magnetic resonance vibration sounding
Nuclear magnetic resonance vibration detection is an extension of nuclear magnetic resonance technology in the field of geological exploration. By detecting the water distribution information in a certain stratum, we can determine whether there is groundwater under a certain stratum, the height of groundwater level, the water content and porosity of aquifer and other stratum structure information.
At present, nuclear magnetic resonance detection technology has become a supplementary means of traditional drilling detection technology, and has been applied to the prevention and control of geological disasters such as landslides. However, compared with traditional drilling exploration, the cost of purchasing, operating and maintaining nuclear magnetic resonance detection equipment is very high, which seriously limits the application of MRS technology in geological science.
Characteristics of nuclear magnetic resonance
① The frequency of * * * vibration depends on the electronic structure outside the nucleus and the configuration of the nuclear neighbors; ② The intensity of * * * vibration peak depends on the proportion of this configuration in the alloy; ③ The resolution of spectral lines is extremely high.
Advantages of magnetic resonance imaging
Compared with 190 1 year's ordinary X-ray and1year's computer tomography (CT), which won the Nobel Prize in medicine in 1979, the biggest advantage of magnetic resonance imaging is that it is a safe, rapid and accurate clinical diagnosis method that is harmless to human body. Today, at least 60 million cases worldwide are examined by magnetic resonance imaging technology every year. Specifically, there are the following points:
No radiation damage to human body;
Various parameters can be used for imaging, and various imaging parameters can provide rich diagnostic information, which makes medical diagnosis and study of metabolism and function in human body convenient and effective. For example, the T 1 value of hepatitis and liver cirrhosis is larger, and the T 1 value of liver cancer is larger. T 1 weighted images can be used to distinguish between benign and malignant liver tumors.
The required contour can be freely selected by adjusting the magnetic field. Images of parts that are inaccessible or difficult to access by other imaging technologies can be obtained. For intervertebral disc and spinal cord, sagittal, coronal and cross-sectional images can be made, and nerve roots, spinal cord and ganglia can be seen. Three-dimensional images of the brain and spinal cord can be obtained, unlike CT (which can only obtain cross-sectional views perpendicular to the human body's long axis), which may miss lesions;
Can diagnose heart diseases, CT scanning speed is slow and incompetent;
Excellent resolution for soft tissues. The examination of bladder, rectum, uterus, vagina, bones, joints, muscles and other parts is better than CT;
In principle, all nuclear elements with non-zero spin can be used for imaging, such as hydrogen (1H), carbon (13C), nitrogen (14N and 15N), phosphorus (3 1P) and so on.
Clinical significance: indications:
The pathological changes of nervous system, including tumor, infarction, hemorrhage, degeneration, congenital malformation, infection, etc., have almost become the means of diagnosis. In particular, the lesions of spinal cord and spine, such as tumor, atrophy, degeneration and traumatic intervertebral disc lesions, have become the first choice for examination.
Pathological changes of cardiac great vessels; Pulmonary mediastinal lesions.
Abdominal and pelvic organ examination; Biliary system and urinary system are obviously superior to CT.
For joint soft tissue lesions; It is very sensitive to aseptic necrosis of bone marrow and bone, and the lesion is found earlier than X-ray and CT.
Edit the difference between MRI and CT in this paragraph.
Computed tomography (CT) can accurately detect tiny density differences between different tissues on a cross-sectional anatomical plane, which is an ideal examination method for observing bone and joint and soft tissue diseases. In the diagnosis of arthritis, it is mainly used to check the spine, especially the sacroiliac joint. CT is superior to traditional X-ray examination in high resolution and axial imaging. Because of the high density resolution of CT, soft tissues, bones and joints can be seen clearly. In addition, CT can do axial scanning, and some joints that are difficult to distinguish on traditional X-ray films can be "exposed" on occlusal images. For example, because the articular surface of sacroiliac joint is naturally inclined and bent, overlapping with other tissues, although X-ray films of sacroiliac joint may meet the requirements in most cases, sometimes X-ray examination is difficult to find sacroiliac arthritis, so CT examination can be done for patients with problems.
Magnetic resonance imaging (MRI) is based on the interaction between radiation waves and hydrogen nuclei in a strong magnetic field. As soon as magnetic vibration came out, it soon became a useful imaging tool for diagnosing many diseases, including musculoskeletal system. Musculoskeletal system is most suitable for magnetic resonance imaging because of its large contrast range of tissue density. In the diagnosis of bone, joint and soft tissue diseases, magnetic resonance imaging (MRI) has imaging parameters several times that of CT and high soft tissue resolution, which makes its contrast to soft tissue significantly higher than that of CT. Magnetic resonance imaging (MRI) can obviously improve the imaging quality of various joints through its multi-directional planar imaging function, and display subtle results that cannot be distinguished by other imaging examinations such as nerves, tendons, ligaments, blood vessels and cartilage. The disadvantage of magnetic resonance imaging of bone and joint system is that the qualitative diagnosis of bone and soft tissue diseases is not specific and the imaging speed is slow. The patient's voluntary or involuntary activities will cause motion artifacts and affect the diagnosis.
X-ray, CT and MRI can be called troika. The organic combination of the three makes the current imaging examination not only expand the scope of examination, but also improve the diagnostic level.