Magnetic nuclear resonance in medicine. Nuclear magnetic resonance
MRI got its start as a tomographic imaging technique that produces images of the NMR signal from thin sections passing through the human body. MRI has evolved from a tomographic imaging technique to a volumetric imaging technique. The method has established itself as extremely informative, and being relatively young, it is constantly evolving, opening up new opportunities.
Magnetic resonance imaging (MRI)
Magnetic resonance imaging (MRI) is an imaging technique mainly used in medical settings to obtain high quality images of the organs of the human body. The method is based on the principles of nuclear magnetic resonance (NMR), a spectroscopy technique used by scientists to obtain information about the chemical and physical properties molecules. But despite its foundation, the method spread under the name of magnetic resonance imaging - MRI, and not nuclear magnetic resonance imaging - NMRI, and the reason for this was the negative associations with the word "nuclear" that arose in connection with the tragic accident at Chernobyl nuclear power plant in 1986. At that time, the term NMR tomography was replaced by MRI, so the indication of the “nuclear” origin of the method disappeared from the new term, which allowed it to seamlessly integrate into everyday medical practice. But despite this original name - MRI, also takes place.
History of the development of MRI
In 1946, Felix Bloch of Stanford University and Edward Purcell of Harvard University independently discovered the phenomenon of nuclear magnetic resonance. In 1952, both were awarded the Nobel Prize in Physics "for the development of new methods for accurate nuclear magnetic measurements and related discoveries." Between 1950 and 1970, NMR was developed and used for chemical and physical molecular analysis. In 1972, the first computed tomography (CT) scanner based on x-rays. This date was a milestone in the history of MRI, as it showed that medical institutions were willing to spend large sums of money on imaging equipment.
The founding year of magnetic resonance imaging is considered to be 1973, when Paul Lauterbur, professor of chemistry and radiology at Stony Brook University of New York, published an article in the journal Nature “Creating an image using induced local interaction; examples based on magnetic resonance” in which three-dimensional images of objects obtained from the spectra of proton magnetic resonance of water from these objects were presented. This work formed the basis of the method of magnetic resonance imaging (MRI). Later, Dr. Peter Mansfield improved the mathematical algorithms for image acquisition. Both were awarded the 2003 Nobel Prize in Physiology or Medicine for their decisive contributions to the invention and development of magnetic resonance imaging.
In 1975, Richard Ernst proposed magnetic resonance imaging using phase and frequency encoding, a technique that is currently used in MRI. In 1980, Edelstein and co-workers demonstrated the mapping of the human body using this method. It took approximately 5 minutes to acquire one image. By 1986, the display time had been reduced to 5 seconds without any significant loss in quality. In the same year, an NMR microscope was created that made it possible to achieve a resolution of 10 mm on 1 cm samples. In 1988, Dumoulin improved MRI angiography, which made it possible to display flowing blood without the use of contrast agents. In 1989, a planar tomography method was introduced that allowed images to be captured at video frequencies (30 ms). Many clinicians thought that this method would find application in dynamic MRI of the joints, but instead, it was used to image areas of the brain responsible for thinking and motor activity. In 1991, Richard Ernst was awarded the Nobel Prize in Chemistry for his achievements in pulsed NMR and MRI. In 1994, New York researchers state university at Stony Brock and Princeton University demonstrated hyperpolarized 129Xe gas imaging for respiration studies. Raymond Damadian, one of the first researchers of the principles of MRI, the holder of a patent for MRI and the creator of the first commercial MRI scanner, also made a well-known contribution to the creation of magnetic resonance imaging.
The first tomographs for examining the human body appeared in clinics in 1980-1981, and today tomography has become a whole area of medicine. Magnetic resonance imaging (MRI) is one of the most effective modern diagnostic tools that allows you to visualize with high quality brain, spinal cord and other internal organs. Modern MRI techniques make it possible to non-invasively study the function of organs - to measure the speed of blood flow, the flow of cerebrospinal fluid, to determine the level of diffusion in tissues, to see the activation of the cerebral cortex during the functioning of the organs for which this area of the cortex is responsible (functional MRI). According to many scientists, it was the advent of CT and MRI that served as an impetus for the unprecedented progress of modern medicine in recent years.
Magnetic resonance imaging(nuclear magnetic resonance imaging, MRI, NMR, NMR, MRI) - non-radiological method of research internal organs and human tissues. X-rays are not used here, which makes this method safe for most people.How the study is done
MRI technology rather complicated: the effect of resonant absorption of electromagnetic waves by atoms is used. A person is placed in a magnetic field created by the apparatus. Molecules in the body at the same time unfold according to the direction of the magnetic field. After that, a radio wave is scanned. The change in the state of the molecules is recorded on a special matrix and transmitted to a computer, where the received data is processed. Unlike computed tomography, MRI allows you to get an image of the pathological process in different planes.
Magnetic resonance imaging in my own way appearance looks like a computer. The study is carried out in the same way as a CT scan. The table gradually moves along the scanner. MRI requires more time than CT and usually takes at least 1 hour (diagnosis of one section of the spine takes 20-30 minutes).
The method has been named magnetic resonance imaging, rather than nuclear magnetic resonance imaging (NMRI) due to negative associations with the word "nuclear" in the late 1970s. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopy technique used by scientists to obtain data on the chemical and physical properties of molecules. MRI got its start as a tomographic imaging technique that produces images of the NMR signal from thin sections passing through the human body. MRI has evolved from a tomographic imaging technique to a volumetric imaging technique.
The method is especially effective for studying dynamic processes(for example, the state of blood flow and the results of its violation) in organs and tissues.
Benefits of Magnetic Resonance Imaging
Now about harm magnetic field nothing is known. However, most scientists believe that in conditions where there is no data on its complete safety, pregnant women should not be subjected to such studies. For these reasons, as well as due to the high cost and low availability of equipment, computed tomography and MRI are prescribed according to strict indications in cases of a controversial diagnosis or the ineffectiveness of other research methods. MRI can also not be performed in those people whose body contains various metal structures - artificial joints, pacemakers, defibrillators, orthopedic structures that hold bones, etc.
Like other research methods, computed and magnetic resonance imaging prescribed only by a doctor. Not all medical institutions conduct these studies, so if necessary, try to contact the diagnostic center.
MRI - magnetic resonance imaging - is a modern, safe(without ionizing radiation) and reliable method of radiation diagnostics. MRI is a unique and practically unparalleled study for diagnosing diseases of the central nervous system, spine, musculo-articular system and a number of internal organs.
Special preparation for the study is not required, except for the examination of the pelvic organs, when a full bladder is required. During the examination, the patient is placed in a horizontal position in a narrow tunnel (pipe) with a strong magnetic field for approximately 15 to 20 minutes, depending on the type of examination. The patient must maintain complete immobility of the anatomical area under study. The MRI procedure is painless, but accompanied by a lot of noise. Headphones will be provided to reduce discomfort.
Psychological discomfort due to being in a confined space is also possible. Accompanying persons may be in the MRI room (magnetic resonance imaging) with the patient, provided that they have no contraindications to being in a magnetic field and after signing an informed consent for each person in the field of magnetic radiation.
Magnetic resonance imaging - MRI - before and after.
Before conducting an MRI examination, it is necessary to fill out a questionnaire that allows you to identify the presence of contraindications to the procedure. Contraindications for MRI examination are: the patient has pacemakers (heart pacemakers), hearing aids and implants of unknown origin; inadequate behavior of the patient (psychomotor agitation, panic attack), state of alcoholic or drug intoxication, claustrophobia (fear and severe discomfort when being in confined spaces), inability to remain still during the entire study (for example, due to severe pain or inappropriate behavior), the need for constant monitoring vital signs (ECG, blood pressure, respiratory rate) and ongoing resuscitation (eg artificial respiration).
If there is a history surgeries and foreign bodies(implants), a certificate for the implanted material or a certificate from the attending physician who performed the surgery (implantation) on the safety of conducting an MRI study with this material is required. Information for female patients: menstruation, the presence of an intrauterine device, as well as breastfeeding are not contraindications for the study. Pregnancy is considered as a relative contraindication, in connection with which the conclusion of a gynecologist is required on the possibility of conducting an MRI study. The final decision to refuse an MRI examination to a patient is made immediately before the examination by the MRI radiologist on duty.
Due to the presence of a strong magnetic field it is prohibited to transport wheelchairs for bedridden patients, wheelchairs, assistive devices for movement (crutches, canes, frames) containing metal components into the MRI room. Personal belongings, jewelry and valuables, clothes containing metal and electromagnetic devices are not allowed in the MRI scanning room and can be left in a safe in the MRI control room.
Magnetic resonance imaging is harmless!
The patient needs to be aware that magnetic resonance imaging, as a study, has certain diagnostic limits, as well as possible limited sensitivity and specificity in the diagnosis of pathological processes. In this regard, as well as if there are doubts about the appropriateness of the study, it is recommended to consult with your doctor or MRI doctor. The decision to conduct an MRI study and choose an anatomical area of study is made by the patient on the basis of a referral from the attending physician or on his own initiative. Before conducting an MRI study, the patient independently indicates the anatomical area of the study in writing, thereby confirming the need to study this area. After the MRI examination, claims are not accepted, and the payment for the MRI examination is not refundable.
In some cases, there is diagnostic need for MRI studies with intravenous contrast enhancement. These studies are carried out only in the direction of the attending physician or MRI doctor. The introduction of a contrast agent contains a minimal risk of adverse reactions. The patient will be asked to fill out an additional questionnaire - a sheet of informed consent for intravenous administration of a contrast agent. Contraindications to internal contrast enhancement are pregnancy, breastfeeding, previously identified hypersensitivity to drugs of this group, as well as renal failure.
For increase diagnostic efficiency MRI studies patients are advised to bring with them data from previous MRI studies, other methods of radiation, laboratory or functional diagnostics, as well as outpatient cards or referrals from the attending physicians indicating the area and purpose of the study.
Our center is equipped with a Magnetom Harmony magnetic resonance tomograph from Siemens
Our center conducts MRI examinations of the brain (head), spine, joints and the whole body. Our clinic has a Magnetic Resonance Tomograph based on the use of a superconducting magnet with a field strength of 1.0 T.
Gentle magnet design (only 160cm including sheath) and anterior-frontal patient access for patient comfort, greatly reducing the problem of claustrophobia.
A set of high-performance gradients (20 mT/m at 50 T/m/s, 30 mT/m at 75 T/m/s and 30 mT/m at 125 T/m/s in each of the x, y, z axes) ), circular-polarized technology of multi-element RF coils combined into a single virtual array for their panoramic use, and the latest unique pulse sequences in their clinically oriented variations (TrueFisp, VIBE, HASTE, EPI, PSIF-Diffusion, etc.) for all kinds of routine and high-speed examinations both with and without breath holding (neuro: head and spine, orthopedics, abdominal, angiographic and cardiological examinations), but also proton spectroscopy, functional studies of the brain, etc.
Scanner with technology Maestro class which allows to ensure the intelligence and expertise of MRI (magnetic resonance imaging) examinations (Inline processing and correction of displacements in the process of collecting 1D, 2D, 3D PACE data) and additionally increase the speed of data collection using iPAT technology up to 2-3 times. As a result, Maestro Сlass expands the capabilities of existing applications and opens up new ones.
The use of lasers in medicine.
The laser is used in medicine as a scalpel that cuts tissue without mechanical contact. Deep-lying tissues are not affected, the risk of infection is excluded, the incisions are bloodless. diffuse laser radiation accelerates wound healing by about 2 times. In ophthalmic surgery - operations without opening the eyeball and anesthesia - the finest perforations are obtained at the points of radiation focusing.
Used:
o Puncture with a laser beam in coronary heart disease
o To destroy stones in the kidneys and gallbladder, due to the high energy density of the pulsed laser, a shock wave is created that destroys stones
o Photoradiation effects on cancer cells in oncology. The impact of the laser on the tumor leads to a photochemical reaction involving hematoporphyrin and the death of cancer cells. Healthy cells do not absorb hematoporphyrin.
o Endoscopic intervention - heating of biological tissue due to the absorption of laser radiation energy.
o When healing wounds and ulcers.
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13. Electron paramagnetic resonance. EPR in medicine.
For an atom placed in a magnetic field, spontaneous transitions between sublevels of the same level are unlikely. Such transitions are carried out induced under the influence of an external electromagnetic field. 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. In this case, one can observe the absorption of the energy of the electromagnetic field, which is called electromagnetic resonance. The biomedical application of EPR is to detect and study free radicals and, in this regard, to trace changes in primary and secondary products radiation injury. Spin probes are used - paramagnetic particles that are non-covalently bonded to molecules. A change in the EPR spectrum of spin probes provides information about the state of the surrounding molecules. Held big studies biological objects by the EPR method.
NMR is selective absorption electromagnetic waves certain frequency substance in a constant magnetic field, due to the magnetic reorientation of the magnetic moments of the nuclei. NMR can be observed when the condition is met only for free atomic nuclei. In spectral NMR, two types of lines are distinguished according to their width. The spectra of solids have a large width, and this area of application of NMR is called broad line NMR. Narrow lines are observed in liquids, and this is called NMR high definition.
An interesting possibility for medicine can be provided by determining the parameters of the NMR spectrum at many points in the sample.
NMR - introscopy allows you to distinguish between bones, blood vessels, normal tissues and tissues with malignant pathology. NMR - introscopy allows you to distinguish images of soft tissues. NMR is referred to as radiospectroscopy.
Magnetic resonance imaging (MRI) is one of the modern methods radiation diagnostics, which allows non-invasively obtaining images of the internal structures of the human body.
The method was called magnetic resonance imaging rather than nuclear magnetic resonance imaging (NMRI) due to negative associations with the word "nuclear" in the late 1970s. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopy technique used by scientists to obtain data on the chemical and physical properties of molecules.
MRI got its start as a tomographic imaging technique that produces images of the NMR signal from thin sections passing through the human body. MRI has evolved from a tomographic imaging technique to a volumetric imaging technique.
Benefits of MRI
The most important advantage of MRI compared to other imaging modalities is:
the absence of ionizing radiation and, as a consequence, the effects of carcinogenesis and mutagenesis, the risk of which is associated (albeit to a very small extent) with exposure to x-rays.
MRI allows examination in any planes, taking into account the anatomical features of the patient's body, and, if necessary, obtaining three-dimensional images for an accurate assessment of the relative position of various structures.
MRI has a high soft tissue contrast and allows to identify and characterize pathological processes that develop in various organs and tissues of the human body.
MRI is the only non-invasive diagnostic method with high sensitivity and specificity in detecting edema and bone tissue infiltration.
The development of MR spectroscopy and diffusion MRI, as well as the creation of new organotropic contrast agents is the basis for the development of “molecular imaging” and allows for histochemical studies in vivo.
MRI better visualizes some structures of the brain and spinal cord, as well as other nervous structures, in this regard, it is more often used to diagnose injuries, tumor formations of the nervous system, as well as in oncology, when it is necessary to determine the presence and extent of the tumor process.
Physical basis of MRI
MRI is based on the phenomenon nuclear magnetic resonance opened in 1946. physicists F. Bloch and E. Purcell ( Nobel Prize in Physics, 1952). The essence of this phenomenon is the ability of the nuclei of some elements under the influence of a static magnetic field to receive the energy of a radio frequency pulse. In 1973 American scientist P. Lauterbur proposed to supplement the phenomenon of nuclear magnetic resonance with the imposition of gradient magnetic fields for the spatial localization of the signal. Using the image reconstruction protocol used at the time for computed tomography (CT), he was able to obtain the first MRI scan. In subsequent years, MRI has undergone whole line qualitative transformations, becoming at present the most complex and diverse method of radiation diagnostics. The principle of MRI makes it possible to receive a signal from any nuclei in the human body, but the evaluation of the distribution of protons that make up bioorganic compounds has the greatest clinical significance, which determines the high soft tissue contrast of the method, i.e. examine internal organs.
Theoretically, any atoms containing an odd number of protons and/or neutrons have magnetic properties. Being in a magnetic field, they are guided along its lines. When an external alternating electromagnetic field is applied, the atoms, which are actually dipoles, line up along new lines of the electromagnetic field. When rearranged along new lines of force, the nuclei generate an electromagnetic signal that can be registered by a receiving coil.
In the phase of the disappearance of the magnetic field, the dipole nuclei return to their original position, while the rate of return to their original position is determined by two time constants, T1 and T2:
T1 is the longitudinal (spin-lattice) time, which reflects the rate of energy loss by excited nuclei
T2 is the transverse relaxation time, which depends on the rate at which the excited nuclei exchange energy with each other
The signal received from the tissues depends on the number of protons (proton density) and the values of T1 and T2. The pulse sequences used in MRI are designed to make better use of tissue differences in T1 and T2 in order to create maximum contrast between normal and pathological tissues.
MRI allows you to obtain a large number of types of images using pulse sequences with different time characteristics of electromagnetic pulses.
Pulse intervals are built in such a way as to emphasize the differences in T1 and T2 more strongly. The most commonly used sequences "inversion recovery" (IR) and "spin echo" (SE), which depend on the proton density.
The main technical parameter that determines the diagnostic capabilities of MRI, is magnetic field strength, measured in T(tesla). High-field tomographs (from 1 to 3 T) allow for the widest range of studies of all areas of the human body, including functional studies, angiography, and fast tomography. Tomographs of this level are high-tech complexes, requiring constant technical control and large financial costs.
Against, low-field tomographs are usually economical, compact and less technically and operationally demanding. However, the possibilities of visualizing small structures on low-field tomographs are limited by a lower spatial resolution, and the range of examined anatomical regions is mainly limited to the brain, spinal cord, and large joints.
Examination of one anatomical region by MRI includes execution of several so-called pulse sequences. Various pulse sequences allow obtaining specific characteristics of human tissues, evaluating the relative content of liquid, fat, protein structures or paramagnetic elements (iron, copper, manganese, etc.).
Standard MRI protocols include T1-weighted images (sensitive to the presence of fat or blood) and T2-weighted images (sensitive to edema and infiltration) in two or three planes.
Structures containing virtually no protons(cortical bone, calcifications, fibrocartilaginous tissue), as well as arterial blood flow, have low signal intensity on both T1- and T2-weighted images.
Time of the study usually ranges from 20 to 40 minutes, depending on the anatomical region and the clinical situation.
Accuracy of diagnosis and characterization of hypervascular processes(tumors, inflammation, vascular malformations) can be significantly increased when using intravenous contrast enhancement. Many pathological processes (for example, small brain tumors) are often not detected without intravenous contrast.
Rare earth metal became the basis for the creation of MR-contrast preparations gadolinium (drug - magnevist). In its pure form, this metal is highly toxic, but in the form of a chelate it becomes practically safe (including no nephrotoxicity). Adverse reactions are extremely rare (less than 1% of cases) and usually have a mild severity (nausea, headache, burning at the injection site, paresthesia, dizziness, rash). In renal failure, the frequency of side effects does not increase.
The introduction of MR-contrast agents during pregnancy is not recommended, since the rate of clearance from the amniotic fluid is unknown.
Other classes of contrast agents for MRI have been developed, including - organ-specific and intravascular.
Limitations and disadvantages of MRI
Long duration of the study (from 20 to 40 minutes)
prerequisite quality imaging is a calm and immobile state of the patient, which determines the need for sedation in restless patients or the use of analgesics in patients with severe pain
the need for the patient to stay in an uncomfortable, non-physiological position with some special styling (for example, when examining the shoulder joint in large patients)
fear of closed spaces (claustrophobia) can be an insurmountable obstacle to examination
technical limitations associated with the load on the tomography table when examining overweight patients (usually more than 130 kg).
the restriction to the examination may be the waist circumference, which is incompatible with the diameter of the tunnel of the tomograph (with the exception of the examination on open-type tomographs with a low magnetic field strength)
the impossibility of reliable detection of calcifications, assessment of the mineral structure of bone tissue (flat bones, cortical plate)
does not allow detailed characterization of the lung parenchyma (in this area it is inferior to the capabilities of CT)
to a much greater extent than with CT, there are artifacts from movement (the quality of tomograms can be sharply reduced due to artifacts from the movement of the patient - breathing, heartbeat, pulsation of blood vessels, involuntary movements) and metal objects (fixed inside the body or in clothing ), as well as from incorrect settings of the tomograph
the distribution and implementation of this research technique is significantly limited due to the high cost of the equipment itself (tomograph, RF coils, software, workstations, etc.) and its maintenance
The main contraindications for MRI (magnetic resonance imaging) are:
absolute:
the presence of artificial pacemakers
the presence of large metal implants, fragments
the presence of metal brackets, clips on blood vessels
artificial heart valves
artificial joints
patient weight over 160 kg
!!! The presence of metal teeth, gold threads, and other suture and fastening material is not a contraindication to MRI - the study is not, although the image quality is reduced.
relative:
claustrophobia - fear of closed spaces
epilepsy, schizophrenia
pregnancy (first trimester)
extremely serious condition of the patient
inability for the patient to remain still during the examination
In most cases, special preparation for an MRI examination is not required., but when examining the heart and its vessels, chest hair should be shaved. When researching pelvic organs(bladder, prostate) you need to come with a full bladder.Research abdominal organs are carried out on an empty stomach.
!!! No metal objects should be brought into the room of the MRI scanner, as they can be attracted by the magnetic field at high speed, injure the patient or medical personnel and disable the scanner for a long time.
Magnetic resonance imaging (MRI)- a method for obtaining tomographic medical images for the study of internal organs and tissues using the phenomenon of nuclear magnetic resonance. Peter Mansfield and Paul Lauterbur received the 2003 Nobel Prize in Medicine for their invention of MRI.
Initially, this method was called nuclear magnetic resonance imaging (NMR tomography). But then, in order not to frighten the public, zombified by radiophobia, they removed the mention of the "nuclear" origin of the method, especially since ionizing radiation is not used in this method.
Nuclear magnetic resonance
Nuclear magnetic resonance is realized on nuclei with nonzero spins. The most interesting for medicine are the nuclei of hydrogen (1 H), carbon (13 C), sodium (23 Na) and phosphorus (31 P), since they are all present in the human body. It has the most (63%) hydrogen atoms, which are found in fat and water, which are the most in the human body. For these reasons, modern MRI scanners are most often "tuned" to hydrogen nuclei - protons.
In the absence of an external field, the spins and magnetic moments of protons are randomly oriented (Fig. 8a). If a proton is placed in an external magnetic field, then its magnetic moment will either be co-directed or opposite to the magnetic field (Fig. 8b), and in the second case, its energy will be higher.
A particle with a spin placed in a magnetic field of strength B can absorb a photon with a frequency ν that depends on its gyromagnetic ratio γ.
For hydrogen, γ = 42.58 MHz/T.
A particle can undergo a transition between two energy states by absorbing a photon. Particle on the bottom energy level absorbs a photon and finds itself at the highest energy level. The energy of a given photon must exactly match the difference between the two states. The energy of a proton, E, is related to its frequency, ν, through Planck's constant (h = 6.626·10 -34 J·s).
In NMR, the quantity ν is called the resonant or Larmor frequency. ν = γB and E = hν, therefore, in order to cause a transition between two spin states, a photon must have an energy
When the photon's energy matches the difference between the two spin states, energy absorption occurs. The intensity of the constant magnetic field and the frequency of the radio frequency magnetic field must strictly correspond to each other (resonance). In NMR experiments, the frequency of a photon corresponds to the radio frequency (RF) range. In clinical MRI, for hydrogen imaging, ν is typically between 15 and 80 MHz.
At room temperature the number of protons with spins at the lower energy level slightly exceeds their number at the upper level. The signal in NMR spectroscopy is proportional to the difference in level populations. The number of excess protons is proportional to B 0 . This difference in a field of 0.5 T is only 3 protons per million, in a field of 1.5 T it is 9 protons per million. However, the total number of excess protons in 0.02 ml of water in a field of 1.5 T is 6.02·10 15 . The stronger the magnetic field, the better the image.
In the state of equilibrium, the net magnetization vector is parallel to the direction of the applied magnetic field B 0 and is called the equilibrium magnetization M 0 . In this state, the Z-component of the magnetization M Z is equal to M 0 . M Z is also called longitudinal magnetization. AT this case, there is no transverse (M X or M Y) magnetization. By sending an RF pulse at the Larmor frequency, one can rotate the net magnetization vector in a plane perpendicular to the Z axis, in this case X-Y planes.
T1 Relaxation
After the termination of the RF pulse, the total magnetization vector will be restored along the Z-axis, emitting RF waves. The time constant describing how M Z returns to its equilibrium value is called the spin-lattice relaxation time (T 1 ).
M Z \u003d M 0 (1 - e -t / T 1 )
T1 relaxation occurs in a volume containing protons. However, the bonds of protons in molecules are not the same. These bonds are different for each tissue. One 1H atom may be very strongly bonded, as in adipose tissue, while another atom may be weaker bonded, such as in water. Strongly bound protons release energy much faster than weakly bound protons. Each tissue releases energy at a different rate, which is why MRI has such good contrast resolution.
T2 Relaxation
T1 relaxation describes processes occurring in the Z direction, while T2 relaxation describes processes in the X-Y plane.
Immediately after exposure to the RF pulse, the total magnetization vector (now called transverse magnetization) begins to rotate in the X-Y plane around the Z axis. All vectors have the same direction because they are in phase. However, they do not retain this state. The net magnetization vector begins to shift out of phase (out of phase) due to the fact that each spin packet experiences a magnetic field slightly different from the magnetic field experienced by other packets and rotates at its own Larmor frequency. At first, the number of out-of-phase vectors will be small, but rapidly increasing until the moment when the phase coherence disappears: there will be no vector that coincides in direction with another. The total magnetization in the XY plane tends to zero, and then the longitudinal magnetization increases until M 0 is along Z.
 Rice. 9. Recession of magnetic induction |
The time constant describing the behavior of the transverse magnetization, M XY , is called the spin-spin relaxation time, T 2 . T2 relaxation is called spin-spin relaxation because it describes the interactions between protons in their immediate environment (molecules). T2 relaxation is a damped process, meaning high phase coherence at the beginning of the process, but decreasing rapidly until the coherence disappears completely at the end. The signal is strong at the beginning, but quickly weakens due to T2 relaxation. The signal is called the decline in magnetic induction (FID - Free Induction Decay) (Fig. 9).
M XY \u003d M XYo e -t / T 2
T 2 is always less than T 1 .
The rate of phase shift is different for each tissue. Dephasing in adipose tissue is faster than in water. One more note about T2 relaxation: it is much faster than T1 relaxation. T2 relaxation occurs in tens of milliseconds, while T1 relaxation can be as long as seconds.
For illustration, Table 1 shows the times T 1 and T 2 for various tissues.
Table 1
| fabrics | T 1 (ms), 1.5 T | T2 (ms) |
|---|---|---|
| BRAIN | ||
| Gray matter | 921 | 101 |
| white matter | 787 | 92 |
| Tumors | 1073 | 121 |
| Edema | 1090 | 113 |
| BREAST | ||
| fibrous tissue | 868 | 49 |
| Adipose tissue | 259 | 84 |
| Tumors | 976 | 80 |
| Carcinoma | 923 | 94 |
| LIVER | ||
| normal tissue | 493 | 43 |
| Tumors | 905 | 84 |
| Cirrhosis of the liver | 438 | 45 |
| MUSCLE | ||
| normal tissue | 868 | 47 |
| Tumors | 1083 | 87 |
| Carcinoma | 1046 | 82 |
| Edema | 1488 | 67 |
Magnetic resonance imaging device
 Rice. 10. MRI scheme |
The scheme of the magnetic resonance tomograph is shown in fig. 10. The MRI consists of a magnet, gradient coils and RF coils.
Permanent magnet
MRI scanners use powerful magnets. The quality and speed of image acquisition depends on the magnitude of the field strength. Modern MRI scanners use either permanent or superconducting magnets. Permanent magnets are cheap and easy to use, but do not allow you to create magnetic fields with a strength greater than 0.7 T. Most magnetic resonance imaging scanners are models with superconducting magnets (0.5 - 1.5 T). Tomographs with a superstrong field (above 3.0 T) are very expensive to operate. On MRI scanners with a field below 1 T, high-quality tomography of internal organs cannot be done, since the power of such devices is too low to obtain high-resolution images. On tomographs with magnetic field strength< 1 Тл можно проводить
только исследования головы, позвоночника и суставов.
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gradient coils
Gradient coils are located inside the magnet. Gradient coils allow you to create additional magnetic fields that are superimposed on the main magnetic field B 0 . There are 3 sets of coils. Each set can produce a magnetic field in a specific direction: Z, X, or Y. For example, when current is applied in the Z gradient, a uniform field ramp is created in the Z direction (along the long axis of the body). At the center of the magnet, the field has a strength B 0 , and the resonant frequency is ν 0 , but at a distance ΔZ the field changes by ΔB, and the resonant frequency changes accordingly (Fig. 11). By adding a gradient magnetic disturbance to the general homogeneous magnetic field, the localization of the NMR signal is provided. The action of the gradient, which ensures the selection of the cut, ensures the selective excitation of protons precisely in the desired region. The speed, signal-to-noise ratio, and resolution of the tomograph depend on the power and speed of the coils.
RF Coils
RF coils create a field B 1 that rotates the net magnetization in a pulse train. They also register transverse magnetization as it precesses in the XY plane. RF coils come in three main categories: transmit and receive, receive only, transmit only. RF coils serve as emitters of B 1 fields and receivers of RF energy from the object under study.
Signal encoding
When the patient is in a uniform magnetic field B 0 , all protons from head to toes align along B 0 . They all rotate at the Larmor frequency. If an RF excitation pulse is generated to transfer the magnetization vector to the X-Y plane, all protons react and a response signal occurs, but there is no localization of the signal source.
Slice-encoding gradient
When the Z-gradient is enabled, an additional magnetic field G Z is generated in this direction, superimposed on B 0 . A stronger field means a higher Larmor frequency. Along the entire slope of the gradient, the field B is different and, therefore, the protons rotate at different frequencies. Now, if we generate an RF pulse with a frequency of ν + Δν, only the protons in the thin section will react, because they are the only ones spinning at the same frequency. The response signal will be only from protons from this slice. Thus, the signal source is localized along the Z axis. The protons in this slice rotate with the same frequency and have the same phase. There are a huge number of protons in the slice, and the localization of sources along the X and Y axes is unknown. Therefore, further coding is required to accurately determine the direct source of the signal.
 Rice. 12. |
Phase encoding gradient
To further encode protons, the gradient G Y is switched on for a very short time. During this time, an additional gradient magnetic field is created in the Y direction. In this case, the protons will have slightly different rotational speeds. They no longer rotate in phase. The phase difference will accumulate. When the G Y gradient is off, the protons in the slice will rotate at the same frequency but have a different phase. This is called phase encoding.
Frequency encoding gradient
For left-right encoding, a third gradient G X is included. Protons on the left side rotate at a lower frequency than those on the right. They accumulate additional phase shift due to frequency differences, but the already acquired phase difference obtained by encoding the phase of the gradient in the previous step is preserved.
Thus, magnetic field gradients are used to localize the source of the signals that are received by the coil.
- G Z gradient selects the axial slice.
- G Y gradient creates rows with different phases.
- G X gradient forms columns with different frequencies.
In one step, phase encoding is performed for only one line. To scan an entire slice, the entire slice, phase, and frequency encoding process must be repeated several times.
In this way small volumes (voxels) are created. Each voxel has a unique combination of frequency and phase (Figure 12). The number of protons in each voxel determines the amplitude of the RF wave. The received signal coming from various areas of the body contains a complex combination of frequencies, phases and amplitudes.
Pulse sequences
On fig. 13 shows a diagram of the simplest sequence. First, the cut-selective gradient (1) (Gss) is turned on. Simultaneously with it, a 90 0 RF cutoff selection pulse (2) is generated, which "flips" the total magnetization into the X-Y plane. The phase encoding gradient (3) (Gpe) is then turned on to perform the first phase encoding step. After that, a frequency-coding or reading gradient (4) (Gro) is applied, during which the free induction decay signal (5) (FID) is recorded. The pulse sequence is typically repeated 128 or 256 times to collect all the necessary data for imaging. The time between repetitions of a sequence is called the repetition time (TR). With each iteration of the sequence, the magnitude of the phase-coding gradient changes. However, in this case, the signal (FID) was extremely weak, so the resulting image was poor. A spin echo sequence is used to increase the signal strength.
Spin echo sequence
After applying a 90 0 excitation pulse, the total magnetization is in the X-Y plane. The phase shift begins immediately due to T2 relaxation. It is because of this dephasing that the signal drops sharply. Ideally, it is necessary to maintain phase coherence, which provides the best signal. To do this, a short time after the 90 0 RF pulse, a 180 0 pulse is applied. 180 0 impulse causes rephasing of spins. When all spins are re-phased, the signal becomes high again and the image quality is much higher.
On fig. 14 shows a diagram of the spin echo pulse sequence.
Rice. 14. Diagram of the spin-echo pulse sequence
First, the slice-selective gradient (1) (G SS ) is turned on. A 90º RF pulse is applied at the same time. The phase encoding gradient (3) (Gpe) is then turned on to perform the first phase encoding step. Gss (4) is turned on again during the 180º rephasing pulse (5), so the same protons that were excited by the 90º pulse are affected. After that, a frequency encoding or reading gradient (6) (Gro) is applied, during which the signal (7) is received.
TR (Repeat time). The complete process must be repeated several times. TR is the time between two 90º excitation pulses. TE (Echo Time). This is the time between the 90º excitation pulse and the echo.
Image Contrast
During NMR scanning, two relaxation processes T1 and T2 occur simultaneously. And
T1 >> T2. Image contrast strongly depends on these processes and on how fully each of them manifests itself at the selected scanning time parameters TR and TE. Consider obtaining a contrast image on the example of a brain scan.
T1 contrast
Rice. 15. a) spin-spin relaxation and b) spin-lattice relaxation in various fabrics brain
We choose the following scan parameters: TR = 600 ms and TE = 10 ms. That is, T1 relaxation takes 600 ms, and T2 relaxation takes only
5 ms (TE/2). As can be seen from fig. 15a after 5 ms, the phase shift is small and does not differ much in different tissues. Image contrast is therefore very weakly dependent on T2 relaxation. As for T1 relaxation, after 600 ms, the fat is almost completely relaxed, but CSF needs some more time.
(Fig. 15b). This means that the contribution from the CSF to the overall signal will be negligible. The image contrast becomes dependent on the T1 relaxation process. The image is "T1 weighted" because the contrast is more dependent on the T1 relaxation process. In the resulting image, CSF will be dark, adipose tissue will be bright, and gray matter intensity will be somewhere in between.
T2 contrast
Rice. 16. a) spin-spin relaxation and b) spin-lattice relaxation in various brain tissues
Now let's set the following parameters: TR = 3000 ms and TE = 120 ms, i.e. T2 relaxation to occur in 60 ms. As follows from Fig. 16b, almost all tissues underwent complete T1 relaxation. Here, TE is the dominant factor for image contrast. The image is "weighted by T2". In the image, CSF will be bright, while other fabrics will have different shades of gray.
Proton density contrast
There is another type of image contrast called proton density (PD).
Let's set the following parameters: TR = 2000 ms and TE 10 ms. Thus, as in the first case, T2 relaxation makes an insignificant contribution to the image contrast. With TR = 2000 ms, the total magnetization of most tissues will recover along the Z-axis. Image contrast in PD images is independent of either T2 or T1 relaxation. The received signal depends entirely on the amount of protons in the tissue: a small amount of protons means a low signal and a dark image, while a large number of them produces a strong signal and a bright image.
 Rice. 17. |
All images have combinations of T1 and T2 contrasts. The contrast depends only on how long T2 relaxation is allowed to occur. In spin echo (SE) sequences, the times TR and TE are most important for image contrast.
On fig. 17 shows schematically how TR and TE are related in terms of image contrast in the SE sequence. Short TR and short TE give T1 weighted contrast. Long TR and short TE give PD contrast. A long TR and a long TE result in a T2-weighted contrast.
Rice. 18. Images with different contrasts: T1 weighted, proton density and T2 weighted. Note the differences in tissue signal intensity. CSF is dark on T1, gray on PD, and bright on T2.
 Rice. 19. Magnetic resonance tomograph |
MRI is good at visualizing soft tissue, while CT is better at visualizing bony structures. Nerves, muscles, ligaments, and tendons are seen much more clearly on MRI than on CT. In addition, the magnetic resonance method is indispensable for examining the brain and spinal cord. In the brain, MRI can distinguish between white and gray matter. Due to the high accuracy and clarity of the images obtained, magnetic resonance imaging is successfully used in the diagnosis of inflammatory, infectious, oncological diseases, in the study of joints, all parts of the spine, mammary glands, heart, abdominal organs, small pelvis, blood vessels. Modern MRI techniques make it possible to study the function of organs - to measure the speed of blood flow, the flow of cerebrospinal fluid, to observe the structure and activation of various parts of the cerebral cortex.