Nuclear magnetic resonance (NMR) is a nuclear spectroscopy that is widely used in all physical sciences and industry. In NMR for probing the intrinsic spin properties of atomic nuclei a large magnet is used. Like any spectroscopy, it uses electromagnetic radiation (radio frequency waves in the VHF range) to create a transition between energy levels (resonance). In chemistry, NMR helps determine the structure of small molecules. Nuclear magnetic resonance in medicine has found application in magnetic resonance imaging (MRI).
Opening
NMR was discovered in 1946 by Harvard University scientists Purcell, Pound, and Torrey, and Bloch, Hansen, and Packard at Stanford. They noticed that the 1 H and 31 P nuclei (proton and phosphorus-31) are able to absorb radio frequency energy when exposed to a magnetic field, the strength of which is specific to each atom. When absorbed, they began to resonate, each element at its own frequency. This observation allowed for a detailed analysis of the structure of the molecule. Since then, NMR has found application in kinetic and structural studies of solids, liquids and gases, resulting in the award of 6 Nobel Prizes.
Spin and magnetic properties
The nucleus consists of elementary particles called neutrons and protons. They have their own angular momentum, called spin. Like electrons, the spin of a nucleus can be described by quantum numbers I and in a magnetic field m. Atomic nuclei with an even number of protons and neutrons have zero spin, and all others have non-zero spin. In addition, molecules with non-zero spin have a magnetic moment μ = γ I, where γ is the gyromagnetic ratio, the constant of proportionality between the magnetic dipole moment and the angular one, which is different for each atom.
The magnetic moment of the nucleus causes it to behave like a tiny magnet. In the absence of an external magnetic field, each magnet is oriented randomly. During an NMR experiment, the sample is placed in an external magnetic field B0, which causes low-energy bar magnets to align in the B0 direction and high-energy bar magnets in the opposite direction. In this case, a change in the orientation of the spin of the magnets occurs. To understand this rather abstract concept, one must consider the energy levels of a nucleus during an NMR experiment.
Energy levels
To flip the spin, an integer number of quanta is required. For any m there are 2m + 1 energy levels. For a spin 1/2 nucleus there are only 2 - a low one, occupied by spins aligned with B0, and a high one, occupied by spins aligned against B0. Each energy level is defined by the expression E = -mℏγB 0, where m is the magnetic quantum number, in this case +/- 1/2. The energy levels for m > 1/2, known as quadrupole nuclei, are more complex.
The energy difference between the levels is equal to: ΔE = ℏγB 0, where ℏ is Planck’s constant.
As can be seen, the strength of the magnetic field is of great importance, since in its absence the levels degenerate.
Energy transitions
For nuclear magnetic resonance to occur, a spin flip between energy levels must occur. The energy difference between the two states corresponds to the energy of electromagnetic radiation, which causes the nuclei to change their energy levels. For most NMR spectrometers B 0 is of order 1 Tesla (T), and γ is of order 10 7. Therefore, the required electromagnetic radiation is of the order of 10 7 Hz. The energy of a photon is represented by the formula E = hν. Therefore, the frequency required for absorption is: ν= γB 0 /2π.
Nuclear shielding
The physics of NMR is based on the concept of nuclear shielding, which allows the structure of matter to be determined. Each atom is surrounded by electrons that orbit the nucleus and act on its magnetic field, which in turn causes small changes in energy levels. This is called shielding. Nuclei that experience different magnetic fields associated with local electronic interactions are called nonequivalent. Changing energy levels to spin flip requires a different frequency, which creates a new peak in the NMR spectrum. Screening allows structural determination of molecules by analyzing the NMR signal using Fourier transform. The result is a spectrum consisting of a set of peaks, each corresponding to a different chemical environment. The peak area is directly proportional to the number of nuclei. Detailed structure information is extracted by NMR interactions, changing the spectrum in different ways.
Relaxation
Relaxation refers to the phenomenon of nuclei returning to their thermodynamically states that are stable after excitation to higher energy levels. This releases the energy absorbed during the transition from a lower level to a higher one. This is a rather complex process that takes place over different time frames. The two most common types of relaxation are spin-lattice and spin-spin.
To understand relaxation, it is necessary to consider the entire pattern. If the nuclei are placed in an external magnetic field, they will create volume magnetization along the Z axis. Their spins are also coherent and allow the signal to be detected. NMR shifts bulk magnetization from the Z axis to the XY plane, where it appears.
Spin-lattice relaxation is characterized by the time T 1 required to restore 37% of the volume magnetization along the Z axis. The more efficient the relaxation process, the lower T 1 . In solids, since the movement between molecules is limited, the relaxation time is long. Measurements are usually carried out using pulsed methods.
Spin-spin relaxation is characterized by the loss of mutual coherence time T 2 . It may be less than or equal to T1.
Nuclear magnetic resonance and its applications
The two main areas in which NMR has proven extremely important are medicine and chemistry, but new applications are being developed every day.
Nuclear magnetic resonance imaging, more commonly known as magnetic resonance imaging (MRI), is important medical diagnostic tool, used to study the functions and structure of the human body. It allows you to obtain detailed images of any organ, especially soft tissues, in all possible planes. Used in the fields of cardiovascular, neurological, musculoskeletal and oncology imaging. Unlike alternative computer imaging, magnetic resonance imaging does not use ionizing radiation and is therefore completely safe.
MRI can detect subtle changes that occur over time. NMR imaging can be used to identify structural abnormalities that occur during the course of the disease, how they influence subsequent development, and how their progression correlates with the mental and emotional aspects of the disorder. Because MRI does not visualize bone well, it produces excellent images of the intracranial and intravertebral content.
Principles of using nuclear magnetic resonance in diagnostics
During an MRI procedure, the patient lies inside a massive, hollow cylindrical magnet and is exposed to a powerful, sustained magnetic field. Different atoms in the scanned part of the body resonate at different field frequencies. MRI is used primarily to detect vibrations of hydrogen atoms, which contain a spinning proton nucleus that has a small magnetic field. In MRI, a background magnetic field lines up all the hydrogen atoms in the tissue. A second magnetic field, oriented differently from the background field, switches on and off many times per second. At a certain frequency, the atoms resonate and line up with a second field. When it turns off, the atoms bounce back, aligning with the background. This creates a signal that can be received and converted into an image.
Tissues with a large amount of hydrogen, which is present in the human body as part of water, create a bright image, and with little or no hydrogen content (for example, bones) they look dark. The brightness of the MRI is enhanced by a contrast agent such as gadodiamide, which patients take before the procedure. Although these agents can improve image quality, the sensitivity of the procedure remains relatively limited. Methods are being developed to increase the sensitivity of MRI. The most promising is the use of parahydrogen, a form of hydrogen with unique molecular spin properties that is very sensitive to magnetic fields.
Improvements in the characteristics of the magnetic fields used in MRI have led to the development of highly sensitive imaging techniques such as diffusion and functional MRI, which are designed to image very specific tissue properties. Additionally, a unique form of MRI technology called magnetic resonance angiography is used to image the movement of blood. It allows you to visualize arteries and veins without the need for needles, catheters or contrast agents. As with MRI, these techniques have helped revolutionize biomedical research and diagnostics.
Advanced computer technology has allowed radiologists to create three-dimensional holograms from digital sections obtained by MRI scanners, which are used to determine the exact location of damage. Tomography is especially valuable in examining the brain and spinal cord, as well as pelvic organs such as the bladder and cancellous bone. The method can quickly and clearly accurately determine the extent of tumor damage and assess the potential damage from a stroke, allowing doctors to prescribe appropriate treatment in a timely manner. MRI has largely replaced arthrography, the need to inject contrast material into a joint to visualize cartilage or ligament damage, and myelography, the injection of contrast material into the spinal canal to visualize spinal cord or intervertebral disc abnormalities.
Application in chemistry
Many laboratories today use nuclear magnetic resonance to determine the structures of important chemical and biological compounds. In NMR spectra, different peaks provide information about the specific chemical environment and bonds between atoms. Most common The isotopes used to detect magnetic resonance signals are 1 H and 13 C, but many others are suitable, such as 2 H, 3 He, 15 N, 19 F, etc.
Modern NMR spectroscopy has found wide application in biomolecular systems and plays an important role in structural biology. With the development of methodology and tools, NMR has become one of the most powerful and versatile spectroscopic methods for the analysis of biomacromolecules, which allows the characterization of them and their complexes up to 100 kDa in size. Together with X-ray crystallography this is one of the two leading technologies for determining their structure at the atomic level. In addition, NMR provides unique and important information about protein function, which plays a critical role in drug development. Some of the uses NMR spectroscopy are given below.
- This is the only method for determining the atomic structure of biomacromolecules in aqueous solutions at close to physiological conditions or membrane-mimicking environments.
- Molecular dynamics. This is the most powerful method for quantitative determination of dynamic properties of biomacromolecules.
- Protein folding. NMR spectroscopy is the most powerful tool for determining the residual structures of unfolded proteins and folding mediators.
- Ionization state. The method is effective in determining the chemical properties of functional groups in biomacromolecules, such as ionization states of ionizable groups of active sites of enzymes.
- Nuclear magnetic resonance allows the study of weak functional interactions between macrobiomolecules (for example, with dissociation constants in the micromolar and millimolar ranges), which cannot be done using other methods.
- Protein hydration. NMR is a tool for detecting internal water and its interactions with biomacromolecules.
- This is unique direct interaction detection method hydrogen bonds.
- Screening and drug development. In particular, nuclear magnetic resonance is particularly useful in identifying drugs and determining the conformations of compounds associated with enzymes, receptors and other proteins.
- Native membrane protein. Solid-state NMR has the potential determination of atomic structures of membrane protein domains in the environment of the native membrane, including with bound ligands.
- Metabolic analysis.
- Chemical analysis. Chemical identification and conformational analysis of synthetic and natural chemicals.
- Materials Science. A powerful tool in the study of polymer chemistry and physics.
Other Applications
Nuclear magnetic resonance and its applications are not limited to medicine and chemistry. The method has proven to be very useful in other fields such as climate testing, petroleum industry, process control, Earth field NMR and magnetometers. Non-destructive testing saves on expensive biological samples, which can be reused if more testing is needed. Nuclear magnetic resonance in geology is used to measure the porosity of rocks and the permeability of underground fluids. Magnetometers are used to measure various magnetic fields.
Nuclear magnetic resonance spectroscopy is one of the most common and very sensitive methods for determining the structure of organic compounds, allowing one to obtain information not only about the qualitative and quantitative composition, but also the location of atoms relative to each other. Various NMR techniques have many possibilities for determining the chemical structure of substances, confirmation states of molecules, effects of mutual influence, and intramolecular transformations.
The nuclear magnetic resonance method has a number of distinctive features: in contrast to optical molecular spectra, the absorption of electromagnetic radiation by a substance occurs in a strong uniform external magnetic field. Moreover, to conduct an NMR study, the experiment must meet a number of conditions reflecting the general principles of NMR spectroscopy:
1) recording NMR spectra is possible only for atomic nuclei with their own magnetic moment or so-called magnetic nuclei, in which the number of protons and neutrons is such that the mass number of isotope nuclei is odd. All nuclei with an odd mass number have spin I, the value of which is 1/2. So for nuclei 1 H, 13 C, l 5 N, 19 F, 31 R the spin value is equal to 1/2, for nuclei 7 Li, 23 Na, 39 K and 4 l R the spin is equal to 3/2. Nuclei with an even mass number either have no spin at all if the nuclear charge is even, or have integer spin values if the charge is odd. Only those nuclei whose spin is I 0 can produce an NMR spectrum.
The presence of spin is associated with the circulation of atomic charge around the nucleus, therefore, a magnetic moment arises μ . A rotating charge (for example, a proton) with angular momentum J creates a magnetic moment μ=γ*J . The angular nuclear momentum J and the magnetic moment μ arising during rotation can be represented as vectors. Their constant ratio is called the gyromagnetic ratio γ. It is this constant that determines the resonant frequency of the core (Fig. 1.1).
Figure 1.1 - A rotating charge with an angular moment J creates a magnetic moment μ=γ*J.
2) the NMR method examines the absorption or emission of energy under unusual conditions of spectrum formation: in contrast to other spectral methods. The NMR spectrum is recorded from a substance located in a strong uniform magnetic field. Such nuclei in an external field have different potential energy values depending on several possible (quantized) orientation angles of the vector μ relative to the external magnetic field strength vector H 0 . In the absence of an external magnetic field, the magnetic moments or spins of nuclei do not have a specific orientation. If magnetic nuclei with spin 1/2 are placed in a magnetic field, then some of the nuclear spins will be parallel to the magnetic field lines, and the other part will be antiparallel. These two orientations are no longer energetically equivalent and the spins are said to be distributed at two energy levels.
Spins with a magnetic moment oriented along the +1/2 field are designated by the symbol | α >, with an orientation antiparallel to the external field -1/2 - symbol | β > (Fig. 1.2) .
Figure 1.2 - Formation of energy levels when an external field H 0 is applied.
1.2.1 NMR spectroscopy on 1 H nuclei. Parameters of PMR spectra.
To decipher the data of 1H NMR spectra and assign signals, the main characteristics of the spectra are used: chemical shift, spin-spin interaction constant, integrated signal intensity, signal width [57].
A) Chemical shift (C.C). H.S. scale Chemical shift is the distance between this signal and the signal of the reference substance, expressed in parts per million of the external field strength.
Tetramethylsilane [TMS, Si(CH 3) 4], containing 12 structurally equivalent, highly shielded protons, is most often used as a standard for measuring the chemical shifts of protons.
B) Spin-spin interaction constant. In high-resolution NMR spectra, signal splitting is observed. This splitting or fine structure in high-resolution spectra results from spin-spin interactions between magnetic nuclei. This phenomenon, along with the chemical shift, serves as the most important source of information about the structure of complex organic molecules and the distribution of the electron cloud in them. It does not depend on H0, but depends on the electronic structure of the molecule. The signal of a magnetic nucleus interacting with another magnetic nucleus is split into several lines depending on the number of spin states, i.e. depends on the spins of nuclei I.
The distance between these lines characterizes the spin-spin coupling energy between nuclei and is called the spin-spin coupling constant n J, where n-the number of bonds that separate interacting nuclei.
There are direct constants J HH, geminal constants 2 J HH , vicinal constants 3 J HH and some long-range constants 4 J HH , 5 J HH .
- geminal constants 2 J HH can be both positive and negative and occupy the range from -30 Hz to +40 Hz.
The vicinal constants 3 J HH occupy the range 0 20 Hz; they are almost always positive. It has been established that vicinal interaction in saturated systems very strongly depends on the angle between carbon-hydrogen bonds, that is, on the dihedral angle - (Fig. 1.3).
Figure 1.3 - Dihedral angle φ between carbon-hydrogen bonds.
Long-range spin-spin interaction (4 J HH , 5 J HH ) - interaction of two nuclei separated by four or more bonds; the constants of such interaction are usually from 0 to +3 Hz.
Table 1.1 – Spin-spin interaction constants
B) Integrated signal intensity. The area of the signals is proportional to the number of magnetic nuclei resonating at a given field strength, so that the ratio of the areas of the signals gives the relative number of protons of each structural variety and is called the integrated signal intensity. Modern spectrometers use special integrators, the readings of which are recorded in the form of a curve, the height of the steps of which is proportional to the area of the corresponding signals.
D) Width of lines. To characterize the width of lines, it is customary to measure the width at a distance of half the height from the zero line of the spectrum. The experimentally observed line width consists of the natural line width, which depends on the structure and mobility, and the broadening due to instrumental reasons
The usual line width in PMR is 0.1-0.3 Hz, but it can increase due to the overlap of adjacent transitions, which do not exactly coincide, but are not resolved as separate lines. Broadening is possible in the presence of nuclei with a spin greater than 1/2 and chemical exchange.
1.2.2 Application of 1 H NMR data to determine the structure of organic molecules.
When solving a number of problems of structural analysis, in addition to tables of empirical values, Kh.S. It may be useful to quantify the effects of neighboring substituents on Ch.S. according to the rule of additivity of effective screening contributions. In this case, substituents that are no more than 2-3 bonds distant from a given proton are usually taken into account, and the calculation is made using the formula:
δ=δ 0 +ε i *δ i (3)
where δ 0 is the chemical shift of protons of the standard group;
δi is the contribution of screening by the substituent.
1.3 NMR spectroscopy 13 C. Obtaining and modes of recording spectra.
The first reports of the observation of 13 C NMR appeared in 1957, but the transformation of 13 C NMR spectroscopy into a practically used method of analytical research began much later.
Magnetic resonance 13 C and 1 H have much in common, but there are also significant differences. The most common carbon isotope 12 C has I=0. The 13 C isotope has I=1/2, but its natural content is 1.1%. This is along with the fact that the gyromagnetic ratio of 13 C nuclei is 1/4 of the gyromagnetic ratio for protons. Which reduces the sensitivity of the method in experiments on observing 13 C NMR by 6000 times compared to 1 H nuclei.
a) without suppressing spin-spin interaction with protons. 13 C NMR spectra obtained in the absence of complete suppression of spin-spin resonance with protons were called high-resolution spectra. These spectra contain complete information about the 13 C - 1 H constants. In relatively simple molecules, both types of constants - direct and long-range - are found quite simply. So 1 J (C-H) is 125 - 250 Hz, however, spin-spin interaction can also occur with more distant protons with constants less than 20 Hz.
b) complete suppression of spin-spin interaction with protons. The first major progress in the field of 13 C NMR spectroscopy is associated with the use of complete suppression of spin-spin interaction with protons. The use of complete suppression of spin-spin interaction with protons leads to the merging of multiplets with the formation of singlet lines if there are no other magnetic nuclei in the molecule, such as 19 F and 31 P.
c) incomplete suppression of spin-spin interaction with protons. However, using the mode of complete decoupling from protons has its drawbacks. Since all carbon signals are now in the form of singlets, all information about the spin-spin interaction constants 13 C- 1 H is lost. A method is proposed that makes it possible to partially restore information about the direct spin-spin interaction constants 13 C- 1 H and at the same time retain more part of the benefits of broadband decoupling. In this case, splittings will appear in the spectra due to the direct constants of the spin-spin interaction 13 C - 1 H. This procedure makes it possible to detect signals from unprotonated carbon atoms, since the latter do not have protons directly associated with 13 C and appear in the spectra with incomplete decoupling from protons as singlets.
d) modulation of the CH interaction constant, JMODCH spectrum. A traditional problem in 13C NMR spectroscopy is determining the number of protons associated with each carbon atom, i.e., the degree of protonation of the carbon atom. Partial suppression by protons makes it possible to resolve the carbon signal from multiplicity caused by long-range spin-spin interaction constants and obtain signal splitting due to direct 13 C-1 H coupling constants. However, in the case of strongly coupled spin systems AB and the overlap of multiplets in the OFFR mode makes unambiguous resolution of signals difficult.
Nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance (NMR) – resonant absorption of electromagnetic waves by atomic nuclei, which occurs when the orientation of the vectors of their own angular momentum (spins) changes. NMR occurs in samples placed in a strong constant magnetic field while simultaneously being exposed to a weak alternating electromagnetic field in the radio frequency range (the alternating field lines must be perpendicular to the constant field lines). For hydrogen nuclei (protons) in a constant magnetic field of 10 4 oersteds, resonance occurs at a radio wave frequency of 42.58 MHz. For other nuclei in magnetic fields of 10 3 –10 4 oersted NMR is observed in the frequency range 1–10 MHz. NMR is widely used in physics, chemistry and biochemistry to study the structure of solids and complex molecules. In medicine, NMR is used to obtain a spatial image of human internal organs with a resolution of 0.5–1 mm.
Let us consider the phenomenon of NMR using the example of the simplest nucleus – hydrogen. The hydrogen nucleus is a proton, which has a certain value of its own mechanical angular momentum (spin). In accordance with quantum mechanics, the proton spin vector can have only two mutually opposite directions in space, conventionally denoted by the words “up” and “down”. The proton also has a magnetic moment, the direction of the vector of which is strictly tied to the direction of the spin vector. Therefore, the vector of the proton’s magnetic moment can be directed either “up” or “down”. Thus, a proton can be represented as a microscopic magnet with two possible orientations in space. If you place a proton in an external constant magnetic field, then the energy of the proton in this field will depend on where its magnetic moment is directed. The energy of a proton will be greater if its magnetic moment (and spin) is directed in the direction opposite to the field. Let's denote this energy E ↓. If the magnetic moment (spin) of a proton is directed in the same direction as the field, then the proton energy, denoted by E, will be less (E<
E ↓).
Пусть протон оказался именно в этом последнем состоянии. Если теперь протону
добавить энергию Δ Е =
E ↓ − E ,
то он сможет скачком перейти в состояние с большей энергией, в котором его
спин будет направлен против поля. Добавить энергию протону можно, “облучая”
его квантами электромагнитных волн с частотой ω, определяемой соотношением
ΔЕ = ћω.
Let's move from a single proton to a macroscopic sample of hydrogen containing a large number of protons. The situation will look like this. In the sample, due to the averaging of random spin orientations, approximately equal numbers of protons, when a constant external magnetic field is applied, will appear with spins directed “up” and “down” relative to this field. Irradiation of a sample with electromagnetic waves with a frequency ω = (E ↓ − E )/ћ will cause a “massive” flip of the spins (magnetic moments) of protons, as a result of which all protons of the sample will find themselves in a state with spins directed against the field. Such a massive change in the orientation of protons will be accompanied by a sharp (resonant) absorption of quanta (and energy) of the irradiating electromagnetic field. This is NMR. NMR can be observed only in samples with a large number of nuclei (10 16), using special techniques and highly sensitive instruments.
The term “magnetic resonance” refers to the selective (resonant) absorption of the energy of an alternating electromagnetic field by the electronic or nuclear subsystem of a substance exposed to a constant magnetic field. The absorption mechanism is associated with quantum transitions in these subsystems between discrete energy levels that arise in the presence of a magnetic field.
Magnetic resonances are usually divided into five types: 1) cyclotron resonance (CR); 2) electron paramagnetic resonance (EPR); 3) nuclear magnetic resonance (NMR); 4) electron ferromagnetic resonance; 5) electronic antiferromagnetic resonance.
Cyclotron resonance. During CR, selective absorption of electromagnetic field energy is observed in semiconductors and metals located in a constant magnetic field, caused by quantum transitions of electrons between Landau energy levels. The quasi-continuous energy spectrum of conduction electrons in an external magnetic field is split into such equidistant levels.
The essence of the physical mechanism of CR can be understood within the framework of classical theory. A free electron moves in a constant magnetic field (directed along the axis) along a spiral trajectory around magnetic induction lines with a cyclotron frequency
where and are the magnitude of the charge and the effective mass of the electron, respectively. Let us now turn on a radio frequency field with a frequency and a vector perpendicular to (for example, along the axis). If the electron has a suitable phase of its movement along the spiral, then since the frequency of its rotation coincides with the frequency of the external field, it will accelerate and the spiral will expand. Accelerating an electron means increasing its energy, which occurs due to its transfer from the radio frequency field. Thus, resonant absorption is possible if the following conditions are met:
the frequency of the external electromagnetic field, the energy of which is absorbed, must coincide with the cyclotron frequency of electrons;
the electric field strength vector of an electromagnetic wave must have a component normal to the direction of the constant magnetic field;
the average free travel time of electrons in the crystal must exceed the period of cyclotron oscillations.
The CR method is used to determine the effective mass of carriers in semiconductors. From the half-width of the CR line, one can determine the characteristic scattering times, and thereby determine the carrier mobility. Based on the line area, the concentration of charge carriers in the sample can be determined.
Electron paramagnetic resonance. The EPR phenomenon consists of the resonant absorption of electromagnetic field energy in paramagnetic samples placed in a constant magnetic field normal to the magnetic vector of the electromagnetic field. The physical essence of the phenomenon is as follows.
The magnetic moment of an atom having unpaired electrons is determined by expression (5.35). In a magnetic field, the energy levels of an atom, due to the interaction of the magnetic moment with the magnetic field, are split into sublevels with energy
where is the magnetic quantum number of the atom and takes the value
From (5.52) it is clear that the number of sublevels is equal to , and the distance between sublevels is
Transitions of atoms from low to higher levels can occur under the influence of an external electromagnetic field. According to quantum mechanical selection rules, allowed transitions are those in which the magnetic quantum number changes by one, that is. Consequently, the energy quantum of such a field must be equal to the distance between the sublevels
Relationship (5.55) is the EPR condition. An alternating magnetic field of a resonant frequency will with equal probability cause transitions from lower magnetic sublevels to upper ones (absorption) and vice versa (emission). In a state of thermodynamic equilibrium, the relationship between the populations of two neighboring levels is determined by Boltzmann's law
From (5.56) it is clear that states with lower energy have a higher population (). Therefore, the number of atoms absorbing quanta of the electromagnetic field, under these conditions, will prevail over the number of emitting atoms; As a result, the system will absorb the energy of the electromagnetic field, which leads to an increase. However, due to interaction with the lattice, the absorbed energy is transferred in the form of heat to the lattice, and usually so quickly that at the frequencies used the ratio differs very little from its equilibrium value (5.56).
EPR frequencies can be determined from (5.55). Substituting the value and counting (purely spin moment), we obtain for the resonant frequency
From (5.57) it is clear that in fields from up to 1 T the resonant frequencies lie in the Hz range, that is, in the radio frequency and microwave regions.
The resonance condition (5.55) applies to isolated atoms having magnetic moments. However, it remains valid for a system of atoms if the interaction between magnetic moments is negligible. Such a system is a paramagnetic crystal, in which magnetic atoms are located at large distances from one another.
The EPR phenomenon was predicted in 1923. Ya.G. Dorfman and experimentally discovered in 1944. E.K. Zavoisky. Currently, EPR is used as one of the most powerful methods for studying solids. Based on the interpretation of EPR spectra, information is obtained about defects, impurities in solids and electronic structure, about the mechanisms of chemical reactions, etc. Paramagnetic amplifiers and generators are built on the ESR phenomenon.
Nuclear magnetic resonance. Heavy elementary particles are protons and neutrons (nucleons), and, therefore, atomic nuclei built from them have their own magnetic moments, which serve as a source of nuclear magnetism. The role of the elementary magnetic moment, by analogy with the electron, is played here by the Bohr nuclear magneton
The atomic nucleus has a magnetic moment
where is the -factor of the nucleus, is the spin number of the nucleus, which takes half-integer and integer values:
0, 1/2, 1, 3/2, 2, ... . (5.60)
Projection of the nuclear magnetic moment onto the axis z arbitrarily chosen coordinate system is determined by the relation
Here, the magnetic quantum number, when known, takes the following values:
In the absence of an external magnetic field, all states with different ones have the same energy, therefore, they are degenerate. An atomic nucleus with a non-zero magnetic moment, placed in an external constant magnetic field, experiences spatial quantization, and its -fold degenerate level splits into a Zeeman multiplet, the levels of which have energies
If after this the nucleus is exposed to an alternating field, the energy quantum of which is equal to the distance between the levels (5.63)
then a resonant absorption of energy by atomic nuclei occurs, which is called nuclear paramagnetic resonance or simply nuclear magnetic resonance.
Due to the fact that it is much smaller, the NMR resonance frequency is noticeably lower than the EPR frequency. Thus, NMR in fields of the order of 1 T is observed in the radio frequency region.
NMR as a method for studying nuclei, atoms and molecules has received various applications in physics, chemistry, biology, medicine, technology, in particular, for measuring the strength of magnetic fields.
The traditional NMR spectroscopy method has many disadvantages. First, it requires a large amount of time to construct each spectrum. Secondly, it is very demanding on the absence of external interference, and, as a rule, the resulting spectra have significant noise. Thirdly, it is unsuitable for creating high-frequency spectrometers. Therefore, modern NMR instruments use the method of so-called pulse spectroscopy, based on Fourier transforms of the received signal.
Currently, all NMR spectrometers are built on the basis of powerful superconducting magnets with a constant magnetic field.
The essence of NMR introscopy (or magnetic resonance imaging) is the implementation of a special kind of quantitative analysis of the amplitude of the nuclear magnetic resonance signal. In NMR introscopy methods, the magnetic field is created to be obviously non-uniform. Then there is reason to expect that the frequency of nuclear magnetic resonance at each point of the sample has its own value, different from the values in other parts. By setting any code for gradations of the amplitude of NMR signals (brightness or color on the monitor screen), you can obtain a conventional image (tomogram) of sections of the internal structure of the object.
Ferro- and antiferromagnetic resonance. The physical essence of ferromagnetic resonance is that under the influence of an external magnetic field that magnetizes the ferromagnet to saturation, the total magnetic moment of the sample begins to precess around this field with a Larmor frequency that depends on the field. If a high-frequency electromagnetic field is applied to such a sample, perpendicular to , and its frequency is changed, then resonant absorption of the field energy occurs. Absorption in this case is several orders of magnitude higher than with paramagnetic resonance, because the magnetic susceptibility, and, consequently, the magnetic saturation moment in them is much higher than that of paramagnetic materials.
Features of resonance phenomena in ferro - and antiferromagnets are determined primarily by the fact that in such substances they deal not with isolated atoms or relatively weakly interacting ions of ordinary paramagnetic bodies, but with a complex system of strongly interacting electrons. The exchange (electrostatic) interaction creates a large resultant magnetization, and with it a large internal magnetic field, which significantly changes the resonance conditions (5.55).
Ferromagnetic resonance differs from EPR in that the energy absorption in this case is many orders of magnitude stronger and the resonance condition (the relationship between the resonant frequency of the alternating field and the magnitude of the constant magnetic field) significantly depends on the shape of the samples.
Many microwave devices are based on the phenomenon of ferromagnetic resonance: resonant valves and filters, paramagnetic amplifiers, power limiters and delay lines.
Antiferromagnetic resonance ( electronic magnetic resonance V antiferromagnets) – the phenomenon of a relatively large selective response of the magnetic system of an antiferromagnet to the influence of an electromagnetic field with a frequency (10-1000 GHz) close to the natural frequencies of the precession of the magnetization vectors of the magnetic sublattices of the system. This phenomenon is accompanied by strong absorption of electromagnetic field energy.
From a quantum point of view, a antiferromagnetic resonance can be considered as a resonant transformation of electromagnetic field photons into magnons with a wave vector.
To observe a antiferromagnetic resonance radio spectrometers are used, similar to those used to study ESR, but allowing measurements to be carried out at high (up to 1000 GHz) frequencies and in strong (up to 1 MG) magnetic fields. The most promising spectrometers are those in which it is not the magnetic field that is scanned, but the frequency. Optical detection methods have become widespread antiferromagnetic resonance.
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General information
Phenomenon nuclear magnetic resonance (NMR) was discovered in 1938 by Rabbi Isaac. The phenomenon is based on the presence of magnetic properties in the nuclei of atoms. It was only in 2003 that a method was invented to use this phenomenon for diagnostic purposes in medicine. For the invention, its authors received the Nobel Prize. In spectroscopy, the body being studied ( that is, the patient's body) is placed in an electromagnetic field and irradiated with radio waves. This is a completely safe method ( unlike, for example, computed tomography), which has a very high degree of resolution and sensitivity.Application in economics and science
1. In chemistry and physics to identify the substances participating in the reaction, as well as the final results of the reactions,2. In pharmacology for the production of drugs,
3. In agriculture, to determine the chemical composition of grain and readiness for sowing ( very useful in breeding new species),
4. In medicine - for diagnostics. A very informative method for diagnosing diseases of the spine, especially intervertebral discs. Makes it possible to detect even the smallest violations of disk integrity. Detects cancer tumors in the early stages of formation.
The essence of the method
The nuclear magnetic resonance method is based on the fact that at the moment when the body is in a specially tuned very strong magnetic field ( 10,000 times stronger than our planet's magnetic field), water molecules present in all cells of the body form chains located parallel to the direction of the magnetic field.If you suddenly change the direction of the field, the water molecule releases a particle of electricity. It is these charges that are detected by the device’s sensors and analyzed by a computer. Based on the intensity of water concentration in the cells, the computer creates a model of the organ or part of the body that is being studied.
At the exit, the doctor has a monochrome image on which you can see thin sections of the organ in great detail. In terms of information content, this method significantly exceeds computed tomography. Sometimes even more details about the organ being examined are given than is needed for diagnosis.
Types of magnetic resonance spectroscopy
- Biological fluids,
- Internal organs.
The magnetic resonance perfusion technique makes it possible to monitor the movement of blood through the tissues of the liver and brain.
Today in medicine the name is more widely used MRI (Magnetic resonance imaging ), since the mention of a nuclear reaction in the title frightens patients.
Indications
1. Brain diseases2. Studies of the functions of parts of the brain,
3. Joint diseases,
4. Spinal cord diseases,
5. Diseases of the internal organs of the abdominal cavity,
6. Diseases of the urinary and reproductive system,
7. Diseases of the mediastinum and heart,
8. Vascular diseases.
Contraindications
Absolute contraindications:1. Pacemaker,
2. Electronic or ferromagnetic middle ear prostheses,
3. Ferromagnetic Ilizarov apparatuses,
4. Large metal internal prostheses,
5. Hemostatic clamps of cerebral vessels.
Relative contraindications:
1.
Nervous system stimulants,
2.
Insulin pumps,
3.
Other types of internal ear prostheses,
4.
Prosthetic heart valves,
5.
Hemostatic clamps on other organs,
6.
Pregnancy ( it is necessary to obtain a gynecologist's opinion),
7.
Heart failure in the stage of decompensation,
8.
Claustrophobia ( fear of confined spaces).
Preparing for the study
Special preparation is required only for those patients who are undergoing examination of internal organs ( genitourinary and digestive tract): You should not eat food five hours before the procedure.If the head is being examined, the fair sex is advised to remove makeup, since the substances contained in cosmetics ( for example, in eye shadow), may affect the results. All metal jewelry should be removed.
Sometimes medical staff will check a patient using a portable metal detector.
How is the research conducted?
Before starting the study, each patient fills out a questionnaire to help identify contraindications. The device is a wide tube into which the patient is placed in a horizontal position. The patient must remain completely still, otherwise the image will not be clear enough. The inside of the pipe is not dark and there is fresh ventilation, so the conditions for the procedure are quite comfortable. Some installations produce a noticeable hum, then the person being examined wears noise-absorbing headphones.
The duration of the examination can range from 15 minutes to 60 minutes.
Some medical centers allow a relative or accompanying person to be with the patient in the room where the study is being conducted ( if it has no contraindications).
In some medical centers, an anesthesiologist administers sedatives. In this case, the procedure is much easier to tolerate, especially for patients suffering from claustrophobia, small children or patients who, for some reason, find it difficult to remain still. The patient falls into a state of therapeutic sleep and comes out of it rested and invigorated. The drugs used are quickly eliminated from the body and are safe for the patient.
The examination result is ready within 30 minutes after the end of the procedure. The result is issued in the form of a DVD, a doctor’s report and photographs.
Use of contrast agent in NMR
Most often, the procedure takes place without the use of contrast. However, in some cases it is necessary ( for vascular research). In this case, the contrast agent is infused intravenously using a catheter. The procedure is similar to any intravenous injection. For this type of research, special substances are used - paramagnets. These are weak magnetic substances, the particles of which, being in an external magnetic field, are magnetized parallel to the field lines.Contraindications to the use of contrast media:
- Pregnancy,
- Individual intolerance to the components of the contrast agent, previously identified.
Vascular examination (magnetic resonance angiography)
Using this method, you can monitor both the state of the circulatory network and the movement of blood through the vessels.Despite the fact that the method makes it possible to “see” the vessels without a contrast agent, with its use the image is more clear.
Special 4-D installations make it possible to monitor the movement of blood in almost real time.
Indications:
- Congenital heart defects,
- Aneurysm, dissection,
- Vessel stenosis,
Brain research
This is a brain test that does not use radioactive beams. The method allows you to see the bones of the skull, but you can examine the soft tissues in more detail. An excellent diagnostic method in neurosurgery, as well as neurology. Makes it possible to detect the consequences of old bruises and concussions, strokes, as well as neoplasms. It is usually prescribed for migraine-like conditions of unknown etiology, impaired consciousness, neoplasms, hematomas, and lack of coordination.
Brain MRI examines:
- main vessels of the neck,
- blood vessels supplying the brain
- brain tissue,
- orbits of the eye sockets,
- deeper parts of the brain ( cerebellum, pineal gland, pituitary gland, oblongata and intermediate divisions).
Functional NMR
This diagnosis is based on the fact that when any part of the brain responsible for a certain function is activated, blood circulation in that area increases.The person being examined is given various tasks, and during their execution, blood circulation in different parts of the brain is recorded. The data obtained during the experiments are compared with the tomogram obtained during the rest period.
Spine examination
This method is excellent for studying nerve endings, muscles, bone marrow and ligaments, as well as intervertebral discs. But in cases of spinal fractures or the need to examine bone structures, it is somewhat inferior to computed tomography. You can examine the entire spine, or you can only examine the area of concern: the cervical, thoracic, lumbosacral, and also separately the coccyx. Thus, when examining the cervical spine, pathologies of blood vessels and vertebrae can be detected that affect the blood supply to the brain.
When examining the lumbar region, intervertebral hernias, bone and cartilage spikes, as well as pinched nerves can be detected.
Indications:
- Changes in the shape of intervertebral discs, including hernias,
- Back and spine injuries
- Osteochondrosis, dystrophic and inflammatory processes in the bones,
- Neoplasms.
Spinal cord examination
It is carried out simultaneously with a spinal examination.Indications:
- The likelihood of spinal cord neoplasms, focal lesions,
- To control the filling of the spinal cord cavities with cerebrospinal fluid,
- Spinal cord cysts,
- To monitor recovery after surgery,
- If there is a risk of spinal cord disease.
Joint examination
This research method is very effective for studying the condition of the soft tissues that make up the joint.Used for diagnostics:
- Chronic arthritis,
- Tendon, muscle and ligament injuries ( especially often used in sports medicine),
- Perelomov,
- Neoplasms of soft tissue and bones,
- Damage not detected by other diagnostic methods.
- Examination of the hip joints for osteomyelitis, necrosis of the femoral head, stress fracture, septic arthritis,
- Examination of knee joints for stress fractures, violation of the integrity of some internal components ( meniscus, cartilage),
- Examination of the shoulder joint for dislocations, pinched nerves, rupture of the joint capsule,
- Examination of the wrist joint in cases of instability, multiple fractures, entrapment of the median nerve, and ligament damage.
Examination of the temporomandibular joint
Prescribed to determine the causes of dysfunction in the joint. This study most fully reveals the condition of cartilage and muscles and makes it possible to detect dislocations. It is also used before orthodontic or orthopedic surgeries.Indications:
- Impaired mobility of the lower jaw,
- Clicking sounds when opening and closing the mouth,
- Pain in the temple when opening and closing the mouth,
- Pain when palpating the masticatory muscles,
- Pain in the muscles of the neck and head.
Examination of the internal organs of the abdominal cavity
An examination of the pancreas and liver is prescribed for:- Non-infectious jaundice,
- Probability of liver neoplasm, degeneration, abscess, cysts, with cirrhosis,
- To monitor the progress of treatment,
- For traumatic ruptures,
- Stones in the gall bladder or bile ducts,
- Pancreatitis of any form,
- Probability of neoplasms,
- Ischemia of parenchymal organs.
A kidney examination is prescribed when:
- Suspicion of a neoplasm,
- Diseases of organs and tissues located near the kidneys,
- The likelihood of disruption of the formation of urinary organs,
- If it is impossible to perform excretory urography.
Research for diseases of the reproductive system
Pelvic examinations are prescribed for:- The likelihood of a neoplasm of the uterus, bladder, prostate,
- Injuries,
- Pelvic neoplasms to identify metastases,
- Pain in the sacral area,
- Vesiculitis,
- To examine the condition of the lymph nodes.
It is not advisable to urinate an hour before the test, as the image will be more informative if the bladder is somewhat full.
Study during pregnancy
Despite the fact that this research method is much safer than x-rays or computed tomography, it is strictly not allowed to be used in the first trimester of pregnancy. In the second and third trimesters, the method is prescribed only for health reasons. The danger of the procedure for the body of a pregnant woman is that during the procedure some tissues are heated, which can cause undesirable changes in the formation of the fetus.
But the use of a contrast agent during pregnancy is strictly prohibited at any stage of gestation.
Precautionary measures
1. Some NMR installations are designed as a closed tube. People who suffer from a fear of enclosed spaces may experience an attack. Therefore, it is better to inquire in advance about how the procedure will go. There are open type installations. They are a room similar to an X-ray room, but such installations are rare.2.
It is prohibited to enter the room where the device is located with metal objects and electronic devices ( e.g. watches, jewelry, keys), since in a powerful electromagnetic field, electronic devices may break, and small metal objects will fly apart. At the same time, not entirely correct survey data will be obtained.
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