nuclear magnetic resonance
nuclear magnetic resonance (NMR
) is a physical process in which the spin level of a nucleus with a non-zero magnetic moment undergoes Zeeman splitting under the action of an external magnetic field, and the * * vibration absorbs a certain frequency of radio frequency radiation. Nuclear magnetic resonance spectroscopy is a branch of spectroscopy, and its vibration frequency is in the radio frequency band, and the corresponding transition is the transition of nuclear spin at the nuclear Zeeman level.
nuclear magnetic resonance * * * is a physical phenomenon that the nucleus in a static magnetic field is subjected to another alternating magnetic field. Usually, what people call 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, and nuclei can produce nuclear magnetic resonance because they have nuclear spins. The nuclear spin produces a magnetic moment, and when the nuclear magnetic moment is in a static external magnetic field, it produces precession nuclei and energy level splitting. 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. It is another great progress in medical imaging after CT. Since its application in the 198s, it has developed at an extremely rapid speed. Its 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, which causes the hydrogen nucleus to vibrate and absorb energy. After stopping the radio frequency pulse, the hydrogen nucleus sends out radio signals at a specific frequency, and releases the absorbed energy, which is collected by the receiver in vitro and processed by the electronic computer to obtain images, which is called nuclear magnetic resonance imaging.
nuclear magnetic resonance (NMR) vibration is a physical phenomenon, which is widely used in physics, chemical biology and other fields as an analytical method. It was not used in medical clinical testing until 1973. In order to avoid confusion with radiological imaging in nuclear medicine, it is called magnetic resonance imaging (MR).
MR is a bio-magnetic spin imaging technology, which uses the characteristics of nuclear spin motion, generates signals after being excited by radio frequency pulses in an external magnetic field, detects them with a detector, inputs them into a computer, and displays images on the screen after processing and conversion.
the information provided by p>MR is not only greater than many other imaging methods in medical imaging, but also different from the existing imaging methods, so it has great potential advantages in the diagnosis of diseases. It can directly make tomography images of cross section, sagittal plane, coronal plane 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 and hydrocephalus, and it is also effective in diagnosing lumbar disc herniation, primary liver cancer and other diseases.
MR 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.
the history of nuclear magnetic resonance technology
in 193s, physicist isidor rabi discovered that the nuclei in a magnetic field would be arranged in parallel in a forward or reverse order along the direction of the magnetic field, and after radio waves were applied, the spin direction of the nuclei would be reversed. This is the earliest understanding of the interaction between nucleus and magnetic field and external RF field. Because of this research, Rabbi won the Nobel Prize in Physics in 1944.
in p>1946, two American scientists, Bloch and purcell, discovered 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 nucleus, which is the initial understanding of the phenomenon of nuclear magnetic resonance. For this reason, the two of them won the 1952 Nobel Prize in Physics.
people discovered the phenomenon of nuclear magnetic resonance (NMR), and it soon came into practical use. Chemists used the influence of molecular structure on the magnetic field around hydrogen atoms to develop NMR spectra, which were used to analyze molecular structures. With the passage of time, NMR spectroscopy technology continued to develop, from the initial one-dimensional hydrogen spectrum to 13C spectrum, two-dimensional NMR spectra and other advanced spectra, nuclear magnetic resonance. Since the 199s, people have even developed a technology to determine the tertiary structure of protein molecule by nuclear magnetic resonance information, which makes it possible to accurately determine the molecular structure of protein in solution phase.
In p>1946, purcell of Harvard University and Bloch of Stanford University announced that they had discovered nuclear magnetic resonance (NMR). They won the Nobel Prize in 1952. Nuclear magnetic resonance (NMR) is the phenomenon of absorption of nuclear magnetic moment under the simultaneous action of constant magnetic field and high frequency magnetic field (in radio wave band). When certain conditions are met, it will produce * * * vibration. Nuclear magnetic resonance (NMR) soon became a high-tech to explore and study the microstructure and properties of matter. At present, nuclear magnetic resonance has been widely used in physics, chemistry, material 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 an 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.
In the early days, nuclear magnetic resonance (NMR) was mainly used to study nuclear structure and properties, such as measuring nuclear magnetic moment, electric quadrupole distance and nuclear spin, etc. Later, it was widely used in molecular composition and structure analysis, biological tissue and living tissue analysis, pathological analysis, medical diagnosis, product nondestructive monitoring and so on. For isolated hydrogen nuclei (i.e. 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 compound molecules, this * * * vibration frequency is also related to the chemical environment in which hydrogen nuclei are located. Hydrogen nuclei in different chemical environments have different * * * vibration frequencies, which is called chemical shift. This is caused by the shielding effect, induced 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-coupling splitting will also occur. The molecular structure of compounds, especially organic compounds, can be inferred by using chemical shifts and cleavage numbers. This is the spectrum analysis of nuclear magnetic resonance. In 197s, the pulsed Fourier transform nuclear magnetic resonance oscillator appeared, which made the application of C13 spectrum increase day by day. The analysis of material composition and structure by nuclear magnetic resonance has the advantages of high accuracy, less restrictions on samples and no damage to samples.
The earliest MRI experiment was published by Lauterper in 1973, which immediately attracted widespread attention and entered the clinical application stage in just 1 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 at the same time induce a voltage signal in the coil, which is called a nuclear magnetic resonance signal. There are a lot of hydrogen nuclei in human tissues due to the existence of a lot of water and hydrocarbons. Generally, the signal obtained by using hydrogen nuclei is more than 1 times larger than that of other nuclei. The voltage signals of normal tissues and diseased tissues are different. Combined with CT technology, that is, computerized tomography technology, we can get any cross-sectional images of human tissues, especially for the diagnosis of soft tissue lesions, which shows its advantages, and it is very sensitive to the diseased parts and the images are 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 already know a lot about brain tissue, but little about how the brain works and why it has such advanced functions. Bell laboratory started the research in this field in 1988, and the US government also designated the 199s as the "Decade of the Brain". Nuclear magnetic resonance (NMR) can be used to directly observe living organisms, and the tested objects are conscious. It also has the advantages of no radiation damage, fast imaging speed, high temporal and spatial resolution (up to 1μm 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 live images of the working human brain, which is expected to reveal the mystery of the working human brain in the near future.
if the frequency variable of the nuclear magnetic resonance * * * is increased to two or more, two-dimensional or multi-dimensional nuclear magnetic resonance * * * can be realized, thus obtaining more information than one-dimensional nuclear magnetic resonance * * *. At present, the application of nuclear magnetic resonance imaging is limited to hydrogen nuclei, but from the needs of practical application, it is also required that other nuclei such as C13, N14, P31, S33, Na23 and I127 can be imaged by nuclear magnetic resonance imaging. C13 has entered the practical stage, but it still needs to be further expanded and deepened. The combination of nuclear magnetic resonance * * vibration with other physical effects such as Mossbauer effect (absorption effect of γ-ray without recoil * * * vibration) and electron spin * * * vibration can obtain more valuable information, which is of great significance both in theory and in practical application. Nuclear magnetic resonance (NMR) has a wide application prospect. With the breakthrough of pulse Fourier technology, the C13 spectrum 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) vibration, which can be used to obtain the information about the distribution of water molecules in the human body, so as to accurately draw the internal structure of the human body. On the basis of this theory, in 1969, Damadian, M.D., Southern Medical Center of new york State University, successfully distinguished mouse cancer cells from normal tissue cells by measuring the relaxation time of NMR vibration. 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 the phenomenon of nuclear magnetic resonance in 1973, and successfully drew an image of the internal structure of a living clam with his equipment. After Lauterper, MRI technology has become more and more mature and widely used, and it has become a routine medical detection method, which is widely used in the treatment and diagnosis of brain and spinal diseases such as Parkinson's disease and multiple sclerosis, as well as cancer. In 23, Paul Lauterper and Peter Mansfield, a professor at the University of Nottingham, UK, won the Nobel Prize in Physiology or Medicine for their contributions to the magnetic resonance imaging technology. Its 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, which causes the hydrogen nucleus to vibrate and absorb energy. After stopping the radio frequency pulse, the hydrogen nucleus sends out radio signals at a specific frequency, and releases the absorbed energy, which is collected by the receiver in vitro and processed by the electronic computer to obtain images, which is called nuclear magnetic resonance imaging.
the principle of nuclear magnetic resonance
the phenomenon of nuclear magnetic resonance comes from the precession of the spin angular momentum of the nucleus under the action of an external magnetic field.
according to the principle of quantum mechanics, nuclei, like electrons, also have spin angular momentum, and the specific value of spin angular momentum is determined by the spin quantum number of nuclei. The experimental results show that the spin quantum numbers of different types of nuclei are also different:
nuclei with even mass and proton numbers, nuclei with odd mass numbers, and nuclei with semi-integer spin quantum numbers
nuclei with even mass numbers, Up to now, only nuclei with odd proton numbers have spin quantum numbers equal to 1/2, and their nuclear magnetic resonance signals can be used by people. The nuclei that are often used by people are: 1H, 11B, 13C, 17O, 19F and 31P
Because the nuclei carry charges, when the nuclei spin, they will generate a magnetic moment by spin. 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 during the rotation, and 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 in a certain strength of the external magnetic field.
the energy of nuclear precession is related to the magnetic field, the nuclear magnetic moment and the angle between the magnetic moment and the 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 is determined by the nuclear magnetic quantum number, and the direction of the nuclear magnetic moment can only jump between these magnetic quantum numbers, but can't 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 an 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, providing assistance for the energy level transition. Therefore, in a given external magnetic field, a specific nucleus only absorbs the energy provided by a radio frequency field with a specific frequency, thus forming a nuclear magnetic * * * vibration signal.
Application of nuclear magnetic * * * vibration
NMR technology
NMR spectroscopy
NMR technology is nuclear magnetic * * * vibration spectrum technology, which is to combine nuclear magnetic * * * For the determination of organic molecular structure, nuclear magnetic resonance spectrum plays a very important role. Together with ultraviolet spectrum, infrared spectrum and mass spectrum, nuclear magnetic resonance spectrum is called "four famous spectra" by organic chemists. At present, the study of nuclear magnetic resonance spectra mainly focuses on the spectra of 1H and 13C nuclei.
for isolated nuclei, the same kind of nuclei 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 the molecule, the actually felt external magnetic field intensity often changes to a certain extent, and the external magnetic field intensity felt by the nuclei in different positions in the molecular structure is also different. The influence of electron clouds in the molecule on the external magnetic field intensity will cause the nuclei in different positions in the molecule to be sensitive to radio frequency fields with different frequencies, thus leading to the difference of nuclear magnetic resonance signals. This difference is through nuclear magnetic resonance.