The same nuclei of atoms in different environments in a molecule show different NMR signals. The difference between such an NMR signal and the signal of a standard substance makes it possible to determine the so-called chemical shift, which is due to the chemical structure of the substance under study. In NMR techniques, there are many opportunities to determine the chemical structure of substances, the conformations of molecules, the effects of mutual influence, and intramolecular transformations.
Physics NMR
The splitting of the energy levels of the nucleus with I = 1/2 in a magnetic field
The phenomenon of nuclear magnetic resonance is based on the magnetic properties of atomic nuclei, consisting of nucleons with half-integer spin 1/2, 3/2, 5/2 .... Nuclei with even mass and charge numbers (even-even nuclei) do not have a magnetic moment , while for all other nuclei the magnetic moment is nonzero.
Thus, the nuclei have an angular momentum related to the magnetic moment by the relation
,where is Planck's constant, is the spin quantum number, is the gyromagnetic ratio.
The angular momentum and magnetic moment of the nucleus are quantized and the eigenvalues of the projection and the angular and magnetic moments on the z-axis of an arbitrarily chosen coordinate system are determined by the relation
And ,where is the magnetic quantum number of the eigenstate of the nucleus, its values are determined by the spin quantum number of the nucleus
that is, the kernel can be in states.
So, for a proton (or another nucleus with I = 1/2- 13 C, 19 F, 31 P, etc.) can only be in two states
,such a core can be represented as a magnetic dipole, the z-component of which can be oriented parallel or antiparallel to the positive direction of the z-axis of an arbitrary coordinate system.
It should be noted that in the absence of an external magnetic field, all states with different states have the same energy, that is, they are degenerate. The degeneracy is removed in an external magnetic field, while the splitting with respect to the degenerate state is proportional to the magnitude of the external magnetic field and the magnetic moment of the state and for a nucleus with a spin quantum number I in an external magnetic field, a system of 2I+1 energy levels, that is, nuclear magnetic resonance has the same nature as the Zeeman effect of the splitting of electronic levels in a magnetic field.
In the simplest case, for a nucleus with spin c I = 1/2- for example, for a proton, splitting
and energy difference of spin states
Larmor frequencies of some atomic nuclei
The frequency for proton resonance is in the short wave range (wavelength about 7 m).
Application of NMR
Spectroscopy
Main article: NMR spectroscopy
Devices
The heart of the NMR spectrometer is a powerful magnet. In an experiment first put into practice by Purcell, a sample placed in a glass ampoule about 5 mm in diameter is placed between the poles of a strong electromagnet. Then the ampoule begins to rotate, and the magnetic field acting on it is gradually increased. A high-quality RF generator is used as a radiation source. Under the action of an increasing magnetic field, the nuclei to which the spectrometer is tuned begin to resonate. In this case, the shielded cores resonate at a frequency slightly lower than the nominal resonance frequency (and the device).
The energy absorption is recorded by an RF bridge and then recorded by a chart recorder. The frequency is increased until it reaches a certain limit, above which resonance is impossible.
Since the currents coming from the bridge are very small, they are not limited to taking one spectrum, but make several dozen passes. All received signals are summarized on the final graph, the quality of which depends on the signal-to-noise ratio of the instrument.
In this method, the sample is exposed to radio frequency radiation at a constant frequency while the strength of the magnetic field changes, hence it is also called the constant field (CW) method.
The traditional method of NMR spectroscopy has many disadvantages. First, it takes a lot of time to build each spectrum. Secondly, it is very picky about the absence of external interference, and as a rule, the resulting spectra have significant noise. Thirdly, it is unsuitable for creating high-frequency spectrometers (300, 400, 500 and more MHz). Therefore, in modern NMR instruments, the method of the so-called pulsed spectroscopy (PW) is used, based on the Fourier transform of the received signal. At present, all NMR spectrometers are built on the basis of powerful superconducting magnets with a constant magnetic field.
In contrast to the CW method, in the pulsed version, the excitation of nuclei is carried out not with a “constant wave”, but with the help of a short pulse, several microseconds long. The amplitudes of the frequency components of the pulse decrease with increasing distance from ν 0 . But since it is desirable that all nuclei be irradiated equally, it is necessary to use "hard pulses", that is, short pulses of high power. The pulse duration is chosen so that the frequency bandwidth is greater than the spectrum width by one or two orders of magnitude. Power reaches several watts.
As a result of pulsed spectroscopy, not an ordinary spectrum with visible resonance peaks is obtained, but an image of damped resonant oscillations, in which all signals from all resonating nuclei are mixed - the so-called "free induction decay" (FID, free induction decay). To transform this spectrum, mathematical methods are used, the so-called Fourier transform, according to which any function can be represented as the sum of a set of harmonic oscillations.
NMR spectra
Spectrum of 1 H 4-ethoxybenzaldehyde. In the weak field (singlet ~9.25 ppm) the signal of the proton of the aldehyde group, in the strong field (triplet ~1.85-2 ppm) - the proton of the methyl ethoxy group.
For qualitative analysis using NMR, spectral analysis is used, based on such remarkable properties of this method:
- the signals of the nuclei of atoms included in certain functional groups lie in strictly defined regions of the spectrum;
- the integral area limited by the peak is strictly proportional to the number of resonant atoms;
- nuclei lying through 1-4 bonds are capable of producing multiplet signals as a result of the so-called. splits on each other.
The position of the signal in the NMR spectra is characterized by their chemical shift relative to the reference signal. As the latter in 1 H and 13 C NMR, tetramethylsilane Si(CH 3) 4 is used. The unit of chemical shift is the parts per million (ppm) of the instrument frequency. If we take the TMS signal as 0, and consider the signal shift to a weak field as a positive chemical shift, then we will obtain the so-called δ scale. If the resonance of tetramethylsilane is equated to 10 ppm and reverse the signs, then the resulting scale will be the τ scale, which is practically not used at present. If the spectrum of a substance is too complicated to interpret, one can use quantum chemical methods for calculating screening constants and correlate the signals based on them.
NMR introscopy
The phenomenon of nuclear magnetic resonance can be used not only in physics and chemistry, but also in medicine: the human body is a combination of all the same organic and inorganic molecules.
To observe this phenomenon, an object is placed in a constant magnetic field and exposed to radio frequency and gradient magnetic fields. An alternating electromotive force (EMF) arises in the inductor surrounding the object under study, the amplitude-frequency spectrum of which and the time-transition characteristics carry information about the spatial density of resonating atomic nuclei, as well as about other parameters specific only for nuclear magnetic resonance. Computer processing of this information generates a three-dimensional image that characterizes the density of chemically equivalent nuclei, the relaxation times of nuclear magnetic resonance, the distribution of fluid flow rates, the diffusion of molecules, and the biochemical processes of metabolism in living tissues.
The essence of NMR introscopy (or magnetic resonance imaging) consists, in fact, in the implementation of a special kind of quantitative analysis of the amplitude of the nuclear magnetic resonance signal. In conventional NMR spectroscopy, the aim is to realize the best possible resolution of the spectral lines. To do this, the magnetic systems are adjusted in such a way as to create the best possible field uniformity within the sample. In the methods of NMR introscopy, on the contrary, the magnetic field is created obviously inhomogeneous. 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 some code for NMR signal amplitude gradations (brightness or color on the monitor screen), you can get a conditional image (
Nuclear magnetic resonance
Nuclear magnetic resonance (NMR) - resonant absorption or emission of electromagnetic energy by a substance containing nuclei with non-zero spin in an external magnetic field, at a frequency ν (called the NMR frequency), due to the reorientation of the magnetic moments of the nuclei. The phenomenon of nuclear magnetic resonance was discovered in 1938 by Isaac Raby in molecular beams, for which he was awarded the 1944 Nobel Prize. In 1946, Felix Bloch and Edward Mills Purcell obtained nuclear magnetic resonance in liquids and solids (1952 Nobel Prize). .
The same nuclei of atoms in different environments in a molecule show different NMR signals. The difference between such an NMR signal and the signal of a standard substance makes it possible to determine the so-called chemical shift, which is due to the chemical structure of the substance under study. In NMR techniques, there are many opportunities to determine the chemical structure of substances, the conformations of molecules, the effects of mutual influence, and intramolecular transformations.
Mathematical description Magnetic moment of the nucleus mu=y*l where l is the spin of the nucleus; y - constant bar Frequency at which NMR is observed
Chemical polarization of nuclei
When certain chemical reactions proceed in a magnetic field, the NMR spectra of the reaction products show either anomalously high absorption or radio emission. This fact indicates a nonequilibrium population of the nuclear Zeeman levels in the molecules of the reaction products. The overpopulation of the lower level is accompanied by anomalous absorption. Population inversion (the upper level is more populated than the lower one) results in radio emission. This phenomenon is called chemical polarization of nuclei
In NMR it is used to enhance nuclear magnetization Larmor frequencies of some atomic nuclei
|
core |
Larmor frequency in MHz at 0.5 Tesla |
Larmor frequency in MHz at 1 Tesla |
Larmor frequency in MHz at 7.05 Tesla |
|
1H( Hydrogen) |
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|
²D( Deuterium) |
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|
13 C ( Carbon) |
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23 Na( Sodium) |
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39 K ( Potassium) |
The frequency for proton resonance is in the range short waves(wavelength about 7 m) .
Application of NMR
Spectroscopy
NMR spectroscopy
Devices
The heart of the NMR spectrometer is a powerful magnet. In an experiment pioneered by Purcell, a sample placed in a glass ampoule about 5 mm in diameter is placed between the poles of a strong electromagnet. Then, to improve the uniformity of the magnetic field, the ampoule begins to rotate, and the magnetic field acting on it is gradually increased. A high-quality RF generator is used as a radiation source. Under the action of an increasing magnetic field, the nuclei to which the spectrometer is tuned begin to resonate. In this case, shielded nuclei resonate at a frequency slightly lower than nuclei without electron shells. The energy absorption is recorded by an RF bridge and then recorded by a chart recorder. The frequency is increased until it reaches a certain limit, above which resonance is impossible.
Since the currents coming from the bridge are very small, they are not limited to taking one spectrum, but make several dozen passes. All received signals are summarized on the final graph, the quality of which depends on the signal-to-noise ratio of the instrument.
In this method, the sample is exposed to radio frequency radiation of a constant frequency while the strength of the magnetic field changes, which is why it is also called the continuous irradiation method (CW, continous wave).
The traditional method of NMR spectroscopy has many disadvantages. First, it takes a lot of time to build each spectrum. Secondly, it is very picky about the absence of external interference, and as a rule, the resulting spectra have significant noise. Thirdly, it is unsuitable for creating high-frequency spectrometers (300, 400, 500 and more MHz). Therefore, in modern NMR instruments, the so-called pulsed spectroscopy (PW) method is used, based on the Fourier transform of the received signal. At present, all NMR spectrometers are built on the basis of powerful superconducting magnets with a constant magnetic field.
In contrast to the CW method, in the pulsed version, the excitation of nuclei is carried out not with a “constant wave”, but with the help of a short pulse, several microseconds long. The amplitudes of the frequency components of the pulse decrease with increasing distance from ν 0 . But since it is desirable that all nuclei be irradiated equally, it is necessary to use "hard pulses", that is, short pulses of high power. The pulse duration is chosen so that the frequency bandwidth is greater than the spectrum width by one or two orders of magnitude. Power reaches several thousand watts.
As a result of pulsed spectroscopy, not an ordinary spectrum with visible resonance peaks is obtained, but an image of damped resonant oscillations, in which all signals from all resonating nuclei are mixed - the so-called "free induction decay" (FID, free induction decay). To transform this spectrum, mathematical methods are used, the so-called Fourier transform, according to which any function can be represented as the sum of a set of harmonic oscillations.
NMR spectra
Spectrum of 1 H 4-ethoxybenzaldehyde. In the weak field (singlet ~9.25 ppm) the signal of the proton of the aldehyde group, in the strong field (triplet ~1.85-2 ppm) - the proton of the methyl ethoxy group.
For qualitative analysis using NMR, spectral analysis is used, based on such remarkable properties of this method:
the signals of the nuclei of atoms included in certain functional groups lie in strictly defined regions of the spectrum;
the integral area limited by the peak is strictly proportional to the number of resonant atoms;
nuclei lying through 1-4 bonds are capable of producing multiplet signals as a result of the so-called. splits on each other.
The position of the signal in the NMR spectra is characterized by their chemical shift relative to the reference signal. As the latter in 1 H and 13 C NMR, tetramethylsilane Si(CH 3) 4 (TMS) is used. The unit of chemical shift is the parts per million (ppm) of the instrument frequency. If we take the TMS signal as 0, and consider the signal shift to a weak field as a positive chemical shift, then we will obtain the so-called δ scale. If the resonance of tetramethylsilane is equated to 10 ppm and reverse the signs, then the resulting scale will be the τ scale, which is practically not used at present. If the spectrum of a substance is too complicated to interpret, one can use quantum chemical methods for calculating screening constants and correlate the signals based on them.
NMR introscopy
The phenomenon of nuclear magnetic resonance can be used not only in physics and chemistry, but also in medicine: the human body is a combination of all the same organic and inorganic molecules.
To observe this phenomenon, an object is placed in a constant magnetic field and exposed to radio frequency and gradient magnetic fields. An alternating electromotive force (EMF) arises in the inductor surrounding the object under study, the amplitude-frequency spectrum of which and the time-transition characteristics carry information about the spatial density of resonating atomic nuclei, as well as about other parameters specific only for nuclear magnetic resonance. Computer processing of this information forms a three-dimensional image that characterizes the density of chemically equivalent nuclei, the relaxation times of nuclear magnetic resonance, the distribution of fluid flow rates, the diffusion of molecules, and the biochemical processes of metabolism in living tissues.
The essence of NMR introscopy (or magnetic resonance imaging) consists, in fact, in the implementation of a special kind of quantitative analysis of the amplitude of the nuclear magnetic resonance signal. In conventional NMR spectroscopy, the aim is to realize the best possible resolution of the spectral lines. To do this, the magnetic systems are adjusted in such a way as to create the best possible field uniformity within the sample. In the methods of NMR introscopy, on the contrary, the magnetic field is created obviously inhomogeneous. 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 specifying some code for NMR signal amplitude gradations (brightness or color on the monitor screen), one can obtain a conditional image (tomogram) of sections of the object's internal structure.
NMR introscopy, NMR tomography were invented for the first time in the world in 1960 by V. A. Ivanov. The application for an invention (method and device) was rejected by an incompetent expert "... due to the apparent futility of the proposed solution", therefore, a copyright certificate for this was issued only after more than 10 years. Thus, it is officially recognized that the author of NMR imaging is not the team of the Nobel laureates listed below, but a Russian scientist. Despite this legal fact, the Nobel Prize was awarded for MRI tomography by no means to V. A. Ivanov.
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 moments of momentum (spins) changes. NMR occurs in samples placed in a strong constant magnetic field, while simultaneously exposing them to a weak alternating electromagnetic field of the radio frequency range (the lines of force of the alternating field must be perpendicular to the lines of force of the constant field). For hydrogen nuclei (protons) in a constant magnetic field with a strength of 10 4 oersted, resonance occurs at a radio wave frequency of 42.58 MHz. For other nuclei in magnetic fields of 103–104 oersted NMR is observed in the frequency range of 1–10 MHz. NMR is widely used in physics, chemistry and biochemistry to study the structure of solids and complex molecules. In medicine, using NMR with a resolution of 0.5–1 mm, a spatial image of the internal organs of a person is obtained.
Let's consider the phenomenon of NMR on the example of the simplest nucleus - hydrogen. The hydrogen nucleus is a proton, which has a certain value of its own mechanical moment of 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 rigidly tied to the direction of the spin vector. Therefore, the vector of the magnetic moment of the proton can be directed either “up” or “down”. Thus, the 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 as E ↓ . If the magnetic moment (spin) of the proton is directed in the same direction as the field, then the energy of the proton, denoted 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 relative to this field with spins directed “up” and “down”. Irradiation of a sample with electromagnetic waves with a frequency ω = (E ↓ − E )/ћ will cause a “massive” spin flip (magnetic moments) of protons, as a result of which all protons of the sample will be 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 only be observed in samples with a large number of nuclei (10 16) using special techniques and highly sensitive instruments.
NMR or in English NMR imaging is an abbreviation for the phrase "nuclear magnetic resonance". This method of research entered medical practice in the 80s of the last century. It is different from X-ray tomography. The radiation used in NMR includes the radio wave range with a wavelength from 1 to 300 m. By analogy with CT, nuclear magnetic tomography uses automatic control of computer scanning with processing of a layered image of the structure of internal organs.
What is the essence of MRI
NMR is based on strong magnetic fields, as well as radio waves, which make it possible to form an image of the human body from individual images (scans). This technique is necessary for emergency care for patients with injuries and brain damage, as well as for routine checks. NMRI is called the selective absorption of electromagnetic waves by a substance (the human body) that is in a magnetic field. This becomes possible in the presence of nuclei with a nonzero magnetic moment. First, radio waves are absorbed, then radio waves are emitted by the nuclei and they go to low energy levels. Both processes can be fixed in the study and absorption of nuclei. NMR creates a non-uniform magnetic field. It is only necessary to tune the transmitter antenna and receiver of the NMR tomograph to a strictly defined area of tissues or organs and take readings from points by changing the wave reception frequency.
When processing information from the scanned points, images of all organs and systems are obtained in various planes, in a cut, a high-resolution three-dimensional image of tissues and organs is formed. The technology of magnetic - nuclear tomography is very complex, it is based on the principle of resonant absorption of electromagnetic waves by atoms. A person is placed in an apparatus with a strong magnetic field. The molecules there turn in the direction of the magnetic field. Then an electric wave is scanned, the change in molecules is first recorded on a special matrix, and then transferred to a computer and all data is processed.
Applications of NMRI
NMR tomography has a fairly wide range of applications, so it is much more often used as an alternative to computed tomography. The list of diseases that can be detected using MRI is very voluminous.
- Brain.
Most often, such a study is used to scan the brain for injuries, tumors, dementia, epilepsy, and problems with the vessels of the brain.
- The cardiovascular system.
In the diagnosis of the heart and blood vessels, NMR complements methods such as angiography and CT.
MRI can detect cardiomyopathy, congenital heart disease, vascular changes, myocardial ischemia, dystrophy and tumors in the area of the heart and blood vessels.
- Musculoskeletal system.
NMR tomography is widely used in the diagnosis of problems with the musculoskeletal system. With this diagnostic method, ligaments, tendons and bone structures are very well differentiated.
- Internal organs.
In the study of the gastrointestinal tract and liver using nuclear magnetic resonance imaging, you can get complete information about the spleen, kidneys, liver, pancreas. If you additionally introduce a contrast agent, then it becomes possible to track the functional ability of these organs and their vascular system. And additional computer programs allow you to create images of the intestines, esophagus, biliary tract, bronchi.
Nuclear magnetic resonance imaging and MRI: is there a difference
Sometimes you can get confused in the names of MRI and MRI. Is there a difference between these two procedures? You can definitely answer no.
Initially, at the time of its discovery of magnetic resonance imaging, its name contained another word “nuclear”, which disappeared over time, leaving only the abbreviation MRI.
Nuclear magnetic resonance imaging is similar to an X-ray machine, however, the principle of operation and its capabilities are somewhat different. MRI helps to get a visual picture of the brain and spinal cord, other organs with soft tissues. With the help of tomography, it is possible to measure the speed of blood flow, the flow of cerebrospinal fluid and cerebrospinal fluid. It is also possible to consider how one or another part of the cerebral cortex is activated depending on human activity. The doctor during the study sees a three-dimensional image, which allows him to navigate in assessing the state of a person.
There are several research methods: angiography, perfusion, diffusion, spectroscopy. Nuclear magnetic resonance imaging is one of the best research methods, as it allows you to get a three-dimensional image of the state of organs and tissues, which means that the diagnosis will be established more accurately and the correct treatment will be chosen. NMR examination of the internal organs of a person is exactly images, not real tissues. Patterns appear on photosensitive film when x-rays are absorbed when an x-ray is taken.
The main advantages of NMR imaging
The advantages of NMR tomography over other research methods are many-sided and significant.
Cons of MRI
But of course, this method is not without its drawbacks.
- Big energy consumption. The operation of the chamber requires a lot of electricity and expensive technology for normal superconductivity. But magnets with high power do not have a negative impact on human health.
- Process duration. Nuclear magnetic resonance imaging is less sensitive than X-ray. Therefore, more time is required for transillumination. In addition, image distortion can occur due to respiratory movements, which distorts the data when conducting studies of the lungs and heart.
- In the presence of a disease such as claustrophobia, it is a contraindication for research using MRI. Also, it is impossible to diagnose using MRI tomography if there are large metal implants, pacemakers, artificial pacemakers. During pregnancy, diagnosis is carried out only in exceptional cases.
Every tiny object in the human body can be examined with NMR imaging. Only in some cases should the distribution of the concentration of chemical elements in the body be included. In order to make measurements more sensitive, a rather large number of signals should be accumulated and summed. In this case, a clear image of high quality is obtained, which adequately conveys reality. This is also related to the duration of a person's stay in the chamber for NMR imaging. You will have to lie still for a long time.
In conclusion, we can say that nuclear magnetic resonance imaging is a fairly safe and absolutely painless diagnostic method, which allows you to completely avoid exposure to x-rays. Computer programs allow you to process the resulting scans with the formation of virtual images. The limits of NMR are truly limitless.
Even now, this diagnostic method is a stimulus for its rapid development and wide application in medicine. The method is distinguished by its low harm to human health, but at the same time it allows you to carefully examine the structure of organs, both in a healthy person and in existing diseases.
