The usage of NMR and EPR in medical researches

Physical meaning of electron paramagnetic resonance and nuclear magnetic resonance. Splitting of energy levels. Zeeman effect. Spin probe. Device and mechanism of spectrometer and introscopy. The usage of EPR specters, NMR in medico-biological researches.

Рубрика Медицина
Вид контрольная работа
Язык английский
Дата добавления 15.12.2015
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ASTANA MEDICAL UNIVERSITY

Department of “MEDICAL BIOPHYSICS AND THE BASICS OF LIFE SAFETY”

STUDENT'S

SELF-INDEPENDENT WORK

Theme: The usage of NMR and EPR in medical researches

Performed by: Atayeva Nazerke

Faculty: general medicine

Astana 2015

Contents:

Introduction

1. Physical meaning of EPR

2. Splitting of energy levels. Zeeman effect

3. Electronic splitting. Hyperfine splitting

4. Spectrometer of EPR. Device and mechanism of EPR

5. Spin probes

6. The usage of EPR specters in medico-biological researches

7. Physical meaning of NMR

8. Specters of NMR

9. NMR introscopy. The usage of NMR in medico-biological researches

Conclusion

References

nuclear resonance spin spectrometer

Introduction

When atoms are placed in a magnetic field, the spontaneous transitions between their sublevels of the same level are unlikely. However, such transitions occur is induced under the influence of an external electromagnetic field. A necessary condition is the coincidence of the frequency of the electromagnetic field with the frequency of the photon corresponding to the energy difference between the split sublevels. It is possible to observe the absorption of electromagnetic energy, which is called magnetic resonance. Depending on the type of particles - carriers magnetic moment - distinguish electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR). Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation. This energy is at a specific resonance frequency, which depends on the strength of the magnetic field and the magnetic properties of the isotope of the atoms; in practical applications, the frequency is similar to VHF and UHF television broadcasts (60-1000 MHz). NMR allows the observation of specific quantum mechanical magnetic properties of the atomic nucleus.

Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a technique for studying materials with unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of the spins of atomic nuclei. EPR spectroscopy is particularly useful for studying metal complexes or organic radicals. EPR was first observed in Kazan State University by Soviet physicist Yevgeny Zavoisky in 1944, and was developed independently at the same time by Brebis Bleaney at the University of Oxford.

1. Physical meaning of EPR

The essence of the phenomenon of electron paramagnetic resonance is as follows. If you place a free radical with a resultant angular momentum J in a magnetic field with an intensity B0, then J, non-zero magnetic field the degeneracy, and the interaction with the magnetic field arises 2J + 1 levels, the situation is described by the expression: W = gвB0M, (where M = J +, + J-1, ... -J) and is determined by the Zeeman interaction of the magnetic field with the magnetic moment J.

If now to the paramagnetic center to make an electromagnetic field with a frequency н, polarized in the plane perpendicular to the magnetic field vector B0, then it will cause magnetic dipole transitions, subject to the rules of selection ДM = 1. At concurrence of the energy of electron transition with a photon energy of the electromagnetic wave will be resonant absorption of microwave radiation. Thus, the resonance condition is determined by the ratio of the fundamental magnetic resonance hн = gвB0.

2. Splitting of energy levels. Zeeman effect

In the absence of an external magnetic field, the magnetic moments of the electrons are oriented randomly, and their energy does not differ from each other (E0). When an external magnetic field, the magnetic moments of the electrons are guided in the field depending on the magnitude of the spin magnetic moment, and their energy level is split into two. The interaction energy of the electron magnetic moment with the magnetic field is expressed by the equation:

E = ,

where ? - magnetic moment of the electron, H - intensity of the magnetic field. From the equation the proportionality factor implies that:

,

and the energy of electron interaction with an external magnetic field is

.

This equation describes the Zeeman effect, which can be expressed in the following words: the energy levels of electrons in a magnetic field, the field is split depending on the magnitude of the spin magnetic moment and magnetic field intensity.

3. Electronic splitting. Hyperfine splitting

Most applications, including biomedical, based on the analysis of a group of lines (and not only singlentyh) in the absorption spectrum of EPR. The presence in the EPR spectrum of groups loved ones conventionally called cleavage lines. There are two basic types of splitting of the EPR spectrum. The first - an electronic splitting - occurs when a molecule or atom has not one, but several electrons, causing the ESR. Second - hyperfine splitting - is observed in the interaction of electrons with the magnetic moment of the nucleus. According to classical concepts, an electron orbiting the nucleus, as well as any moving in a circular orbit of a charged particle has a magnetic dipole moment.

Similarly, in quantum mechanics, the orbital angular momentum of the electron creates a certain magnetic moment. The interaction of the magnetic moment with the magnetic moment of the nucleus (due to the nuclear spin) leads to the hyperfine splitting (ie, creates a hyperfine structure). But the electron also has a spin, which contributes to its magnetic moment. Therefore, the hyperfine splitting has even terms with zero orbital angular momentum. The distance between the sublevels of the hyperfine structure of the order of 1000 times less than that between the levels of the fine structure (the order of magnitude is essentially due to the ratio of the electron mass to the mass of the nucleus).

4. Spectrometer of EPR. Device and mechanism of EPR

The device EPR is much like device spectrophotometer for measuring the optical absorption in the visible and ultraviolet parts of the spectrum. The radiation source is an spectrometer in the klystron, representing a radio tube, giving a monochromatic radiation in the microwave range. Aperture Spectrophotometer radiospectrometer corresponds to the attenuator that allows to dose the power incident on the sample. Cuvette rf spectrometer with the sample stored in the special block, called a resonator. The resonator is a parallelepiped having a rectangular or cylindrical cavity in which is located an absorbing sample.

The dimensions of the cavity such that it forms a standing wave. Element is missing in the optical spectrometer is an electromagnet that creates a constant magnetic field necessary for splitting of the energy levels of the electrons. The radiation that has passed the sample measured in rf spectrometer and a spectrophotometer, falls on the detector, the detector signal is then amplified and recorded on a recorder or a computer. It should be noted another difference rf.

It consists in that the radiofrequency radiation is transmitted from the source to the sample and further to the detector by means of special tubes of rectangular cross section, called waveguides. Dimensions section waveguides defined wavelength of the transmitted radiation. This feature of the radio transmission of the waveguide and determines the fact that the registration of the EPR spectrum rf spectrometer used in a constant frequency of the radiation and the resonance condition is achieved by varying the magnetic field. Another important feature is the rf signal amplification through its modulation of high-frequency alternating field. As a result, the modulation signal is its differentiation and transformation of the absorption lines in its first derivative is a signal EPR.

5. Spin probes

Spin probe - individual paramagnetic chemical substances used for the study of various molecular systems using EPR spectroscopy. The nature of the changes in the EPR spectrum of these compounds makes it possible to obtain unique information about the interactions and dynamics of macromolecules and of the properties of different molecular systems. This method of investigation of molecular mobility and the various structural transformations in condensed media by electron paramagnetic resonance spectra of stable radicals (probes), are added to the test substance. If the stable radicals are chemically bonded to the particles under study environment, they are called markers and indicate the method of spin (or paramagnetic) tags.

As probes and tags mainly used nitroxides, which are stable over a wide temperature range (up to 100-200 _ C), capable of entering into chemical reaction with no loss of the paramagnetic properties, are readily soluble in aqueous and organic media. The high sensitivity allows EPR probes administered (liquid or vapor) in small amounts - from 0.001 to 0.01% by weight, which does not cause changes in the properties of the objects. The method of spin probes and labels are widely used especially for the study of synthetic polymers and biological objects. Thus it is possible to study the overall pattern of the dynamics of low-molecular polymers in the particles when the spin probes simulate the behavior of various additives (plasticizers, colorants, stabilizers, initiators); to receive information about changes in the molecular mobility of the chemical modifications and structural and physical transformations (aging, structuring, lamination, deformation); explore the binary and multicomponent systems (copolymers, filled and plasticized polymers, composites); study solutions of polymers, particularly of temperature and solvent effect on their behavior; to determine the rotational mobility of enzymes, structure and space.

Location of groups in the active site of the enzyme, protein conformation under different treatments, the rate of enzymatic catalysis; studying membrane preparations (for example, to determine the degree of ordering microviscosity and lipids in the membrane, to investigate the lipid-protein interactions, membrane fusion); study of the liquid crystal (the degree of order in molecular arrangement, phase transitions), DNA, RNA, polynucleotides (structural transformations under the influence of temperature and environment, interaction with ligands and DNA intercalating compounds). The method is also used in various fields of medicine to investigate the mechanism of drug action, the analysis of changes in the cells and tissues in various diseases, the determination of low concentrations of toxic and biologically active substances in the body, explore the mechanisms of action of viruses. Spin probe - individual paramagnetic chemical substances used for the study of various molecular systems using EPR spectroscopy.

The nature of the changes in the EPR spectrum of these compounds makes it possible to obtain unique information about the interactions and dynamics of macromolecules and of the properties of different molecular systems. This method of investigation of molecular mobility and the various structural transformations in condensed media by electron paramagnetic resonance spectra of stable radicals (probes), are added to the test substance. If the stable radicals are chemically bonded to the particles under study environment, they are called markers and indicate the method of spin (or paramagnetic) tags. As probes and tags mainly used nitroxides, which are stable over a wide temperature range (up to 100-200 _ C), capable of entering into chemical reaction with no loss of the paramagnetic properties, are readily soluble in aqueous and organic media.

The high sensitivity allows EPR probes administered (liquid or vapor) in small amounts - from 0.001 to 0.01% by weight, which does not cause changes in the properties of the objects. The method of spin probes and labels are widely used especially for the study of synthetic polymers and biological objects. Thus it is possible to study the overall pattern of the dynamics of low-molecular polymers in the particles when the spin probes simulate the behavior of various additives (plasticizers, colorants, stabilizers, initiators); to receive information about changes in the molecular mobility of the chemical modifications and structural and physical transformations (aging, structuring, lamination, deformation); explore the binary and multicomponent systems (copolymers, filled and plasticized polymers, composites); study solutions of polymers, particularly of temperature and solvent effect on their behavior; to determine the rotational mobility of enzymes, structure and space.

Location of groups in the active site of the enzyme, protein conformation under different treatments, the rate of enzymatic catalysis; studying membrane preparations (for example, to determine the degree of ordering microviscosity and lipids in the membrane, to investigate the lipid-protein interactions, membrane fusion); study of the liquid crystal (the degree of order in molecular arrangement, phase transitions), DNA, RNA, polynucleotides (structural transformations under the influence of temperature and environment, interaction with ligands and DNA intercalating compounds). The method is also used in various fields of medicine to investigate the mechanism of drug action, the analysis of changes in the cells and tissues in various diseases, the determination of low concentrations of toxic and biologically active substances in the body, explore the mechanisms of action of viruses.

6. The usage of EPR specters in medico-biological researches

EPR provides unique information about the paramagnetic centers. He clearly distinguishes between the impurity ions, is isomorphic to the lattice of the incoming microinclusions. This gives a complete information about this ion in the crystal: valence, coordination, local symmetry, hybridization of electrons, how many and in which structural position of the electron enters the orientation of the axes of the crystal field at the location of this ion, a complete characterization of the crystal field and detailed information on the chemical bond.

And, very importantly, the method allows to determine the concentration of paramagnetic centers in the regions with different crystal structure. With the help of EPR we were first investigated the mechanisms of action of ionizing (radioactive) radiation on living organisms. By studying the magnetic field, we found that living organisms are composed mainly of diamagnetic. Those. these substances do not absorb electromagnetic radiation of radio spectrum that is used in the ESR. Under the effect of radiation the formation of excited molecules, ions and free radicals, which have paramagnetic properties. As a result of the qualitative and quantitative study of the possibility of the EPR. ESR is widely used to study chemical processes in particular photosynthesis. Examine the activity of some carcinogenic substances.

7. Physical meaning of NMR

At the heart of the phenomenon of nuclear magnetic resonance are the magnetic properties of atomic nuclei composed of nucleons with half-integer spin 1/2, 3/2, 5/2 .... The nuclei with even mass and charge number (even-even-numbered kernel) do not have a magnetic moment, while for all other nuclear magnetic moment is not zero. Thus, the nuclei possess angular momentum J = hI, associated with a magnetic moment м ratio м=J, where h - Planck constant, I - spin quantum number, - gyromagnetic ratio.

Angular momentum and the magnetic moment of the nucleus are quantized, and the eigenvalues of the projection and the angular and magnetic moments in the z-axis is arbitrarily chosen coordinate system defined by the relation: JZ=hµI, where мI - magnetic quantum number of their own nuclear state, its values are determined by the spin quantum number of the nucleus мI = I, I-1, I-2, ..., -I. that is, the core may be in the 2I + 1 states.

It should be noted that in the absence of an external magnetic field, all the states of different мZ have the same energy, i.e. are degenerate. The degeneracy is removed in an external magnetic field, and the splitting of the relatively degenerate state is proportional to the external magnetic field and the magnetic moment of the state for the nucleus with spin quantum number I in an external magnetic field, there is a system of 2I + 1 energy levels - µZB0, , …, , µZB0 that is, nuclear magnetic resonance is of the same nature as the Zeeman effect splitting of the electron levels in a magnetic field.

8. Specters of NMR

The NMR spectra of two types of lines by their width. Spectra solids have a large width, and the scope of NMR called wide-line NMR. In liquids observed narrow lines, and it is called the high-resolution NMR. Features high resolution NMR method due to the fact that one kind of nucleus in different chemical environments at a predetermined constant field applied RF field absorb energy at different frequencies, due to varying degrees of shielding nuclei of the applied magnetic field. High-resolution NMR spectra usually consist of narrow, well-resolved lines (signals) corresponding to the magnetic cores in different chemical environments. Intensity (area) for recording spectra of signals proportional to the number of magnetic cores in each grouping, which enables the quantitative analysis of NMR spectra without prior calibration.

9. NMR introscopy. The usage of NMR in medico-biological researches

Nuclear magnetic resonance is called selective absorption of electromagnetic waves (read radio waves) substance (in this case the human body) in a magnetic field, which is possible due to the presence of nuclei with non-zero magnetic moment. In an external magnetic field, the protons and neutrons these nuclei as small magnets are oriented in a certain way, and for this reason, changing its energy state. The distance between these energy levels so small that the transitions between them can cause even radio waves.

Radio energy billions of times less than that of X-rays, so they can not cause any damage to the molecules. So, first, the absorption of radio waves. Then the radio waves emitted by nuclei and their transition to lower energy levels. And he and the other process can be fixed, studying the absorption and emission spectra of the nuclei. These spectra depend on many factors, and above all - the magnetic field. For spatial image in NMR tomography, in contrast to CT is not necessary in the mechanical scanning system, the source-detector (transmitter antenna and a receiver in the case of NMR). This problem is solved by changing the magnetic field strength at various points. After all, at the same time will change the frequency (wavelength) at which the signal transmission and reception. If we know the value of the field strength at a given point, we can definitely tie it to transmit and receive radio signals. Those. by creating an inhomogeneous magnetic field, the antenna can be adjusted on a strictly defined area of organ or tissue without its mechanical movement and take readings from these points, only changing the frequency of the wave reception.

The next stage - the processing of information from all scanned points and imaging. As a result, the computer processing of the information obtained images of organs and systems in "slices" of vascular structures in different planes, are formed three-dimensional structure of organs and tissues with high resolution.

What are the advantages of MRI?

The first advantage - the replacement of X-rays to radio waves. This eliminates restrictions on the troops surveyed (children, pregnant women) because removed the concept of radiation exposure to the patient and the doctor. In addition, there is no need for special measures to protect personnel and the environment from the X-ray.

The second advantage - sensitivity to certain vital isotopes, especially the hydrogen, one of the most common soft tissue elements. Thus image contrast is provided on the tomogram due to the difference in hydrogen concentrations in different parts of organs and tissues. In this study did not prevent the background from the bone, because the concentration of hydrogen in them even lower than in the surrounding tissues.

A third advantage lies in the sensitivity to various chemical bonds in different molecules, that increases the contrast of the picture.

The fourth advantage lies in the image of the vascular bed without contrast, and even the definition of the parameters of the blood flow.

The fifth advantage is greater today the resolution of the investigation - you can see objects as small as a fraction of a millimeter.

Finally, the sixth - MRI makes it easy to obtain not only the images of cross sections, but also longitudinal.

Of course, as any other technique, MRI has its drawbacks. These include:

1. The need for a high-intensity magnetic field that requires a lot of energy when the equipment and / or the use of expensive technologies for superconductivity. The good news is that in the literature there is no data on the negative health effects of high-power magnets.

2. The low, especially in comparison with X-ray, the sensitivity of MRI, which requires increasing the time radiography. This leads to a distortion of the image of the respiratory movements (especially lung reduces the effectiveness of the research, the study of the heart).

3. Inability to reliably detect stones, calcifications, some types of diseases of bone structures.

4. Inability to survey some patients, such as claustrophobia (fear of enclosed spaces), pacemakers, large metal implants. We should not forget that a relative contraindication to MRI - pregnancy. Well, pacemakers - a strict contraindication to the study.

However, progress does not stand still, and perhaps some of the shortcomings will soon be eliminated.

Conclusion

The history of science teaches us that each new physical phenomenon or a new method of runs hard way, starting at the opening of this phenomenon, and going through several phases. First, almost no one comes to mind about the possibility of even very remote, the application of this phenomenon in everyday life, in science or engineering. Then comes the phase of development, during which the results of experiments to convince all great practical significance of this phenomenon. Finally, it is the phase of rapid rise. New tools are in vogue, become highly productive, generate more sales and become the decisive factor in scientific and technological progress. Devices based on the long ago discovered the phenomenon, filled with physics, chemistry, medicine and industry.

The clearest example of the above somewhat simplified scheme of evolution is the phenomenon of magnetic resonance, open Zavoiskii in 1944 in the form of paramagnetic resonance and open independently by Bloch and Purcell in 1946 in the form of a resonance phenomenon of magnetic moments of atomic nuclei. The complex evolution of the NMR skeptics often pushed to pessimistic conclusions. They said that "NMR is dead" and that "NMR yourself completely exhausted." However, despite and in spite of these spells NMR continued to go forward and constantly prove their viability. Many times, this area of ??science turns to us a new, often quite unexpected quarter, and gave birth to a new direction. Recent revolutionizing inventions in the field of NMR, including the amazing methods for obtaining NMR - images show convincingly that the boundaries of the possible in NMR truly limitless. Remarkable benefits NMR - imaging, which will be highly appreciated by mankind and which are now powerful incentive rapid development of NMR - imaging and wide application in medicine, are at a very low hazard to human health inherent in this new method.

References:

1. Ремизов А.Н., Максина А.Г., Потапенко А.Я. Медицинская и биологическая физика. - Москва: Дрофа, 2003.

2. Хауссер К.Х., Кальбитцер Х.Р. ЯМР в медицине и биологии: структура молекул, томография, спектроскопия in-vivo. - Киев: Наукова думка, 1993.

3. Кузнецов А.Н. Метод спинового зонда. - Москва: Наука, 1976.

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