Nuclear Magnetic Resonance (NMR)
Nuclear Magnetic Resonance (NMR)
NMR (Nuclear Magnetic Resonance) is an atomic-scale quantum magnetic physics. Nuclei with odd protons or neutrons have intrinsic properties: nuclear spins, spin angular momentum. Nuclear spins generate magnetic moments. The method of NMR observation of atoms is to place the sample under a strong magnetic field. Modern instruments usually use low temperature superconducting magnets. The magnetic field of the nuclear spin itself is rearranged under an applied magnetic field, and most nuclear spins will be in a low energy state. We additionally apply an electromagnetic field to interfere with the low-energy state of the nuclear spin-up to a high-energy state, and return to the equilibrium state will release the radio frequency, which is the NMR signal. Using such a process, we can conduct molecular scientific research, such as molecular structure, dynamics, and so on.
The history of nuclear magnetic resonance technology
In the 1930s, Isidore Rabbi discovered that the nuclei in the magnetic field were aligned in a positive or negative order along the direction of the magnetic field. After the radio waves were applied, the spin direction of the nuclei reversed. This is the earliest human understanding of the interaction between the atomic and magnetic fields and the applied RF field. Due to this study, Rabbi received the Nobel Prize in physics in 1944. In 1946, Felix Bloch and Edward Purcell discovered that the nucleus occurs when an atomic nucleus with an odd number of nucleons (including protons and neutrons) is placed in a magnetic field and a frequency field is applied at a specific frequency. Absorbing the energy of the radio frequency field, this is the people's initial understanding of the phenomenon of nuclear magnetic resonance. Both of them won the Nobel Prize for Physics in 1952 for this purpose.
People soon found practical use after discovering NMR phenomena. Chemists used the influence of molecular structure on the magnetic field around hydrogen atoms to develop nuclear magnetic resonance spectra for the analysis of molecular structures. Over time, NMR was used. Spectroscopic techniques have evolved from the original one-dimensional hydrogen spectrum to advanced spectrograms such as 13C spectroscopy and two-dimensional nuclear magnetic resonance spectroscopy. Nuclear magnetic resonance technology has become increasingly capable of analyzing molecular structures. After entering the 1990s, it has developed information relying on nuclear magnetic resonance. The technique of determining the tertiary structure of protein molecules makes it possible to accurately determine the molecular structure of the solution phase proteins.
On the other hand, medical scientists have discovered that hydrogen atoms in water molecules can produce nuclear magnetic resonance phenomena. This phenomenon can be used to obtain information on the distribution of water molecules in the human body, so as to accurately draw the internal structure of the human body. Based on this theory, in 1969, Damadian at the State Medical Center at the State University of New York at the University of New York at the State University of New York at Stony Brook, succeeded in distinguishing cancer cells from normal tissue cells by measuring the relaxation time of magnetic resonance. Campus physicist Paul Lauterbob developed MRI based on the phenomenon of nuclear magnetic resonance in 1973 and applied his equipment to successfully draw a picture of the internal structure of a living body. After Lauterbauer, MRI technology has become more mature and its range of applications has become more widespread. It has become a routine medical detection method and is widely used in the treatment and diagnosis of brain and spine disorders and cancers such as Parkinson's disease and multiple sclerosis. In 2003, Paul Lauterber and the University of Nottingham professor Peter Mansfield won the Nobel Prize in Physiology or Medicine for their contribution to MRI technology.
The principle of nuclear magnetic resonance
The NMR phenomenon originates from the precession of the spin angular momentum of the nuclei under the applied magnetic field.
According to the principles of quantum mechanics, the atomic nucleus, like electrons, also has a spin angular momentum. The specific numerical value of the spin angular momentum is determined by the spin quantum number of the nucleus. Experimental results show that different types of nuclear spin spin quantum numbers are also different:
1. Atomic nuclei with even numbers of protons and neutrons, with a spin quantum number of 0
2. Nuclei with an odd number of masses and a half spin number
3. Nuclei with an even number of masses, an odd number of protons and neutrons, and an integer spin number
Since the nucleus carries the charge, when the nucleus spins, a magnetic moment is generated by the spin. This magnetic moment is in the same direction as the spin direction of the nucleus, and the size is proportional to the spin angular momentum of the nucleus. The nucleus is placed in an applied magnetic field. If the magnetic moment of the nucleus differs from the direction of the applied magnetic field, the magnetic moment of the nucleus will rotate around the direction of the external magnetic field. This phenomenon is similar to the swing of the gyro during the rotation and is called precession. Precession has energy also has a certain frequency.
The frequency of the precession of the nucleus is determined by the strength of the applied magnetic field and the nature of the nucleus itself. That is to say, for a particular atom, the frequency of the precession of the spin of the nucleus in a certain intensity of an applied magnetic field is fixed.
The precession energy of a nucleus is related to the magnetic field, the magnetic moment of the nucleus, and the angle between the magnetic moment and the magnetic field. According to the principle of quantum mechanics, the angle between the nucleus magnetic moment and the applied magnetic field is not continuously distributed, but is from the nucleus. The magnetic quantum number determines that the direction of the magnetic moment of the atomic nucleus can only jump between these magnetic quantum numbers and cannot change smoothly, thus forming a series of energy levels. When an atomic nucleus receives energy input from another source in an external magnetic field, an energy level transition will occur, that is, the angle between the magnetic moment of the nucleus and the applied magnetic field will change. This energy level transition is the basis for acquiring nuclear magnetic resonance signals.
In order for the precession of an atomic nuclear spin to undergo an energy level transition, the energy required for the transition of the atomic nucleus needs to be supplied. This energy is usually provided by an externally applied RF field. According to the principle of physics, when the frequency of the applied RF field is the same as the frequency of spin precession of atomic nuclei, the energy of the RF field can be effectively absorbed by the atomic nucleus, which can help the energy level transition. Therefore, a particular atomic nucleus, in a given applied magnetic field, absorbs only the energy provided by the RF field at a specific frequency, thus forming a nuclear magnetic resonance signal.
Application of NMR
NMR technology
Nuclear magnetic resonance is operated by superconducting coils, and it needs to operate in a very low temperature working environment. The picture shows the addition of liquid nitrogen to the NMR instrument for cooling
NMR technology, or nuclear magnetic resonance spectroscopy, is a technique that applies the phenomenon of nuclear magnetic resonance to the determination of molecular structure. The NMR spectrum plays a very important role in the determination of the structure of organic molecules. Nuclear magnetic resonance spectroscopy, together with ultraviolet spectroscopy, infrared spectroscopy, and mass spectrometry, has been referred to by organic chemists as the “Four Genealogy”. At present, the study of nuclear magnetic resonance spectra mainly focuses on the atomic nuclei maps of 1H and 13C.
For an isolated nucleus, the same nucleus is only sensitive to the RF field at a particular frequency in an external magnetic field of the same intensity. However, atomic nuclei in the molecular structure, due to the influence of factors such as the distribution of electron clouds in the molecule, the actually perceived intensity of the external magnetic field tends to change to some extent, and atomic nuclei at different positions in the molecular structure, the applied magnetic field is felt. The intensity also varies. The influence of the electron cloud in this molecule on the external magnetic field strength will cause the nucleus at different positions in the molecule to be sensitive to the radio frequency field at different frequencies, resulting in the difference in nuclear magnetic resonance signals. The difference is through nuclear magnetic resonance. Analyze the basics of molecular structure. The distribution of chemical bonds and electron clouds near the nucleus is called the chemical environment of the atomic nucleus. The change in the frequency position of the nuclear magnetic resonance signal due to the influence of the chemical environment is called the chemical shift of the atomic nucleus.
Coupling constants are another important piece of information provided by nuclear magnetic resonance spectroscopy besides chemical shifts. Coupling refers to the interaction of the spin angular momentum of adjacent nuclei, which interacts with the spin angular momentum to change the nuclear spin in the external magnetic field. The distribution of energy levels in the middle precession causes splitting of the energy levels, which in turn causes changes in the shape of the signal peaks in the NMR spectrum. By analyzing the changes in these peak shapes, it is possible to infer the connections between atoms in the molecular structure. .
Finally, the signal strength is the third important information of the NMR spectrum. Atomic nuclei in the same chemical environment will show the same signal peak in the NMR spectrum. By analyzing the intensity of the signal peaks, the number of these nuclei can be known. The analysis of the structure provides important information. The characterization of signal peak intensities is the area under the curve of the signal peaks. This information is especially important for 1H-NMR spectra, and for the most common fully decoupled 13C-NMR spectra, due to the corresponding relationship between peak intensities and the number of atomic nuclei. Not significant, so the peak intensity is not very important.
The early NMR spectroscopy mainly focused on the hydrogen spectrum. This is because the 1H atom that can produce NMR signals has extremely high abundance in nature, and the NMR signal generated by it is very strong and easy to detect. With the development of the Fourier transform technique, the NMR apparatus can simultaneously emit radio frequency fields of different frequencies in a short period of time so that the sample can be repeatedly scanned to distinguish weak NMR signals from background noise. One can collect 13C NMR signals.
In recent years, people have developed two-dimensional nuclear magnetic resonance spectroscopy technology, which allows people to obtain more information about the molecular structure. At present, two-dimensional nuclear magnetic resonance spectroscopy has been able to resolve the spatial structure of protein molecules with smaller molecular weight.
MRI technology
Magnetic resonance imaging technology is the application of nuclear magnetic resonance in the medical field. The human body contains very rich water, different tissues, and different water contents. If the distribution information of these waters can be detected, a relatively complete image of the internal structure of the human body can be drawn. The magnetic resonance imaging technology is adopted. Identify the distribution of hydrogen atoms in water molecules to infer the distribution of water molecules in the human body, and then detect the internal structure of the human body.
Unlike NMR spectroscopy techniques used to identify molecular structures, MRI technology changes the strength of the applied magnetic field, rather than the frequency of the RF field. The MRI apparatus will provide two perpendicular gradient magnetic fields perpendicular to the direction of the main magnetic field. In this way, the distribution of the magnetic field in the human body will change with the spatial position. Each position will have a different intensity and direction. The magnetic field, so that hydrogen atoms located in different parts of the human body react to different RF field signals. By recording this reaction and calculating it, the information on the distribution of water molecules in space can be obtained, thereby obtaining the internal structure of the human body. image.
MRI technology can also be combined with X-ray tomography (CT) to provide important data for clinical diagnosis, physiology, and medical research.
MRI is a non-intrusive detection technology. Compared to X-ray fluoroscopy and radiography, MRI has no radiation effect on the human body. Compared to ultrasound detection technology, MRI is clearer and can display more details. Compared with other imaging techniques, MRI can not only show tangible solid lesions, but also can accurately determine brain, heart, liver and other functional responses. In the diagnosis of Parkinson's disease, Alzheimer's disease, cancer and other diseases, MRI technology has played a very important role.
Due to different principles, the contrast of CT on soft tissue imaging is not high, and the contrast of MRI on soft tissue imaging is much higher than CT. This makes MRI particularly suitable for brain tissue imaging. From the images acquired by MRI, the structural map of the brain neural network can be obtained through the DSI technique. In recent years, a series of papers have been published.
MRS technology
MRI detection is an extension of MRI technology in the field of geological exploration. Through the detection of water distribution information in the formation, it can be determined whether groundwater exists under a certain stratum, groundwater level height, water content in aquifer and porosity, etc. information.
At present, the NMR detection technology has become a supplementary measure for traditional drilling detection technology, and is applied to the prevention of geological disasters such as landslides, but compared with traditional drilling detection, the purchase, operation and maintenance costs of NMR detection equipment are very expensive. This severely limits the application of MRS technology in geological sciences.
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