How is an MRI performed? MRI – what is this procedure, indications, contraindications. What is MRI

Was nominated in 1973.

The MR tomograph consists of:

• magnetic gradients;
• main magnet;
• radio pulse receiver;

Let's only consider Quality and speed

1. ultra low: less than 0.1 T;
2. low-floor: in the range from 0.1 to 0.5 T;
3. average: from 0.5 to 1.0 T;
4. high-field: 1.0 - 2.0 T 1.5 Tesla;
5. ultra high: from 2.0 T and above 3.0 T.

• permanent;
• resistive electrical;

• field 0.2 - 0.3 T;
• economical to operate

open type tomographs MRI claustrophobia.
weighing more than 120 kg

• a magnetic field from 0.2 to 0.4 T;

• design features:
• field 0.35 - 4 T.

• high-field;
• creation based on them open type tomographs.

• high price;
• magnetic field alignment

The principle of operation of an MRI tomograph

• modulates them into impulses;
• computer
  • centrally manage the entire system;

The idea of ​​​​forming an image of human internal organs using nuclear magnetic resonance was nominated in 1973.
In 2003, Paul Christian Lauterbur from the University of Illinois (USA) and Peter Mansfield from the University of Nottingham (UK) received Nobel Prize in Physiology or Medicine for the invention of the MRI scanner.

The MR tomograph consists of:

• magnetic gradients;
• main magnet;
• data collection and processing systems;
• generator (transmitter) of radio pulses;
• radio pulse receiver;
• power supply and cooling systems.

Let's only consider general principles of the structure of MR tomographs, since frequent updating of the model range makes it meaningless to consider the design features of a particular device. Quality and speed obtaining an output image, determined by the signal in the receiving coil of the tomograph, depends on the magnetic induction (magnet strength).

According to the strength of the magnetic field, tomographs are divided into:

1. ultra low: less than 0.1 T;
2. low-floor: in the range from 0.1 to 0.5 T;
3. average: from 0.5 to 1.0 T;
4. high-field: 1.0 - 2.0 T, a typical high-field tomograph 1.5 Tesla;
5. ultra high: from 2.0 T and above, the most common tomograph models 3.0 T.

Magnets in MRI scanners are classified as:

• permanent;
• resistive electrical;
• superconducting electrical.

Characteristics of class 1 permanent magnets:

• consist of ferromagnetic alloys;
• field 0.2 - 0.3 T;
• economical to operate, since they do not require electricity and cooling;
• magnetic field orientation - vertical;

The advantage of permanent magnets and open type tomographs based on them, it is possible to conduct MRI for patients suffering from seizures claustrophobia.
Cost-effectiveness, simplicity and ability to accommodate patients with claustrophobia and weighing more than 120 kg contributed to the growth in demand for open type MRI scanners with permanent magnets.

Characteristics of class 2 resistive electromagnets:

• resistive electric magnet design:
  • solenoid made of copper or iron wire;
  • water cooling is used;
• a magnetic field from 0.2 to 0.4 T;
• the field is oriented along the solenoid hole;
• modern models of MR tomographs based on resistive electromagnets are of the open type.

Characteristics of superconducting electromagnets of classes 3, 4 and 5:

• design features:
  • solenoid made of niobium - titanium alloy;
  • cooled with liquid helium to - 269 gr. Celsius (4K) at which it goes into a superconducting state;
• field 0.35 - 4 T.

Advantages of superconducting magnets:

• high-field;
• creation based on them open type tomographs.

Disadvantages of high-field MR tomographs:

• high price;
• use of liquid helium for cooling;
• need for additional magnetic field alignment to obtain a high-quality image.

The principle of operation of an MRI tomograph

• the transmitting coil generates waves of resonant frequency and modulates them into impulses;
• a receiving coil representing a highly sensitive antenna located perpendicular to the direction of the main field (X-Y plane) transmits the received signal to the ADC;
• analog-to-digital converter (ADC) sends data digitally to the operator's computer for image reconstruction;
• computer, in addition to obtaining images from a tomograph, allows you to:
  • centrally manage the entire system;
  • process, record and print images;
  • perform fast Fourier transform.

Magnetic resonance imaging. Magnetic resonance imaging (MRI) has acquired great importance in modern radiation diagnostics. MRI provides valuable diagnostic information about physical and chemical parameters that allow one to judge the nature and morphological structure of the organs and tissues being studied. In addition, the image can be obtained in any plane. The main components of an MRI scanner are a power magnet, a radio transmitter, a radio frequency receiving coil, and a computer. Most magnets have a magnetic field parallel to the long axis of the human body. The strength of a magnetic field is measured in teslas (T). For clinical MRI, fields of 0.02 -3 Tesla are used.

When a patient is placed in a strong magnetic field, all of the body's small proton magnets (hydrogen nuclei) turn in the direction of the external field (like a compass needle aligned with the Earth's magnetic field). In addition, the magnetic axes of each proton begin to rotate (precess) around the direction of the external magnetic field. When radio waves of the same frequency as the rotation frequency of the protons (Larmor frequency) are passed through the patient's body, the magnetic field of the radio waves causes the magnetic moments of all protons to rotate clockwise. This phenomenon is called magnetic resonance.

Resonance refers to synchronous oscillations, and in order to change the orientation of magnetic protons, the magnetic fields of protons and radio waves must resonate, i.e. have the same frequency.

A net magnetic moment is created in the patient's tissues: the tissues are magnetized and their magnetism is oriented exactly parallel to the external magnetic field. Magnetism is proportional to the number of protons per unit volume of tissue. The sheer number of protons (hydrogen nuclei) contained in most tissues means that the magnetic moment is large enough to induce an electrical current in the external receiving coil. This induced electrical current "MR signal" is used to reconstruct the image.

In the interval between the transfer of impulses, the protons undergo two different relaxation processes T1 and T2. Relaxation is a consequence of the gradual disappearance of magnetization caused by small differences in the strength of local magnetic fields. T2 relaxation – loss of magnetism. T1 relaxation is the time of magnetism recovery. The shorter T1, the faster magnetism is restored.

Table 1 - Dependence of the MR signal on the tissue being studied

The very high information content of MRI is due to a number of its advantages.

Particularly high tissue contrast, based not on density, but on several parameters depending on a number of physicochemical properties of tissues, and thanks to this visualization of changes that are not differentiated by ultrasound and CT.

The ability to control contrast, making it dependent on one or another parameter. By varying the contrast, you can highlight some fabrics and details and suppress the image of others. Due to this, MRI, for example, made it possible for the first time to visualize all soft tissue elements of joints without contrast.

The absence of bone artifacts, which often overlap soft tissue contrasts on CT, allows visualization of lesions in the spinal and basal parts of the brain without interference.

Multiplanarity – the ability to image in any plane.

MRI also has functional applications, for example, imaging regurgitation in valvular heart disease in cinema mode or the dynamics of movements in joints.

MRI shows blood flow without artificial contrast. Special angioprograms with two-dimensional or three-dimensional data acquisition provide images of blood flow with excellent contrast. Contrast agents for MRI. The contrast resolution of an MP image can be significantly improved by various contrast agents. Depending on their magnetic properties, MR contrast agents are divided into paramagnetic and supermagnetic.

Paramagnetic contrast agents. Atoms with one or more unpaired electrons have paramagnetic properties. These are magnetic ions of gadolinium, chromium, nickel, iron, and manganese. The most widely used clinical compounds are gadolinium compounds.

The contrasting effect of gadolinium is due to a shortening of the T1 and T2 relaxation times. At low doses, the effect on T1 predominates, increasing signal intensity. At high doses, the effect on T2 predominates with a decrease in signal intensity. The most widely used paramagnetic extracellular MR contrast agents are:

Magnevist (gadopentate dimeglumine).

Dotarem (gadoterate meglumine).

Omniscan (gadodiamide).

Prohans (gadoteridol).

Superparamagnetic contrast agents. Superparamagnetic iron oxide – magnetite. Its dominant effect is a shortening of T2 relaxation. As the dose increases, the signal intensity decreases.

As in CT scanning, oral contrast agents are used in abdominal examinations to differentiate between the intestine and normal or pathological tissue.

Magnetite (Fe 3 O 4) – used in studies of the gastrointestinal tract. This is a superparamagnetic substance with a predominant effect on T2 relaxation. Acts as a negative contrast agent, i.e. reduces signal intensity.

Disadvantages of MRI:

Calcifications are poorly displayed

Long image times, together with artifacts from respiratory and other movements, limit the use of MRI in the diagnosis of diseases of the chest and abdominal cavities.

Harmfulness. There is no ionizing radiation or radiation hazards with MRI. For the vast majority of patients, the method does not pose any danger.

MRI is contraindicated:

Patients with an installed pacemaker or with intraorbital, intracranial and intravertebral ferromagnetic foreign bodies and with vascular clips made of ferromagnetic materials (absolute contraindication).

Intensive care patients due to the impact of the magnetic fields of the MRI scanner on life support systems.

Patients with claustrophobia (approximately 1%); although it is often inferior to sedatives (Relanium).

Women in the first third of pregnancy.

Magnetic resonance imaging, or MRI for short, is a modern, safe and effective diagnostic method that allows specialists to accurately determine disease, pathology, injury or other disorders in the functioning of organs of the human body. Simply put, MRI is a scan, but with a different principle of operation, unlike radiography and CT.

Magnetic resonance imaging has a number of advantages over other diagnostic methods, as well as indications and contraindications for its use. A preliminary interpretation of the study results is carried out by a radiologist after the procedure. A more accurate and specific explanation of the MRI results is made by the doctor, taking into account the anamnesis and clinical picture.

Operating principle and advantages over other diagnostic methods

The operating principle of an MRI scanner is based on the characteristics of the magnetic field and the magnetic properties of body tissue. Thanks to the interaction of nuclear magnetic resonance and the nuclei of hydrogen atoms, during the examination a layer-by-layer image of the organs of the human body is displayed on the computer screen. In this way, it is possible not only to differentiate some organs and tissues from others, but also to record the presence of even minor disorders, tumor and inflammatory processes.

The operating principle of MRI allows you to accurately assess the condition of soft tissues, cartilage, brain, organs, spinal discs, ligaments - those structures that are largely composed of fluid. At the same time, MRI is used less in medicine if it is necessary to study bones or tissues of the lungs, intestines, stomach - structures in which the water content is minimal.

Closed-type tomography apparatus

Due to the way MRI works, there are a number of advantages of this type of research over others:

• As a result of the examination, it is possible to obtain a detailed image. Therefore, this technique is considered the most effective for the early detection of tumors and foci of inflammation, the study of disorders of the central nervous system, musculoskeletal system, abdominal and pelvic organs, brain, spine, joints, and blood vessels.
• Magnetic tomography allows diagnostics to be made in areas where CT is not effective due to the overlap of the examined area with bone tissue or due to the insensitivity of CT to changes in tissue density.
• There is no ionizing radiation to the patient during the procedure.
• It is possible to obtain not only an image of the structure of tissues, but also MRI indications of their functioning. For example, the speed of blood flow, cerebrospinal fluid flow and brain activity are recorded using functional magnetic resonance imaging.
• Possibility of conducting contrast MRI. The contrast agent increases the diagnostic potential of the procedure.
• Open MRIs allow patients with fear of closed spaces to undergo examinations.

Another advantage is that errors are virtually eliminated when making a diagnosis. If the patient is concerned with the question: “Can MRI be wrong?”, then the answer is a little ambiguous. On the one hand, this procedure is one of the most accurate diagnostic methods. On the other hand, errors can occur at the stage of deciphering the results and making a diagnosis by the doctor.

Classification of modern magnetic tomographs

Most patients are wary of magnetic tomography machines because they do not know what to expect during the procedure and are afraid that they will become ill in a confined space. For other people, a standard test is not available due to their weight (more than 150 kg), the presence of psychological disorders or childhood.

However, not everyone knows that modern scientists-technologists have long solved these problems by developing different types of tomographs:

• Closed type scanner;
• Open type MRI scanner.

Most medical institutions have standard closed-type MRI machines, that is, those where the patient is in a “tunnel” during the examination. Such equipment is considered the most reliable, since the magnetic field strength in them is quite high.

But some clinics install open MRI. Such devices are considered not so reliable due to the low magnetic field strength. But every year technologies are improving, and an open-type tomograph can no longer be classified as less informative or insufficiently powerful. Moreover, such a device has the following advantages:

1. The design of the tomograph does not require a sliding table, which makes it possible to examine patients with significant body weight.
2. During the examination, the patient is not in a confined space. This can significantly reduce psychological discomfort, eliminate panic attacks and claustrophobia.
3. For some injuries, specific fixation of the limbs makes it impossible to place the patient in a closed-type tomograph. Therefore, open types of MRI are the only way to diagnose possible injuries to internal organs and the brain.

The ability to examine a patient using an open or closed tomograph significantly expands the capabilities of doctors in complex or unusual cases.

Indications for the procedure

Why is MRI done, and in what situations will this research method be effective? As already noted, magnetic tomography allows for the diagnosis of a wide range of diseases and conditions. All types of MRI studies and indications for their implementation can be classified depending on the organs/systems examined:

• : impaired circulation in the brain, suspicion of tumor lesions, monitoring the state of the brain after surgery, monitoring possible relapses of tumor processes, suspicion of the presence of foci of inflammation, epilepsy, lesions due to arterial hypertension, head injury.
• Temporomandibular joints: diagnosis of the condition of the joint discs, assessment of the effectiveness of surgical treatment, malocclusion, preparation for orthodontic treatment.
• Eyes: suspicion of a tumor, injury, inflammatory processes, diagnosis of the condition of the lacrimal glands after injury.
• Area of ​​the nose, mouth: sinusitis, preparatory manipulations before plastic surgery.
• : various degenerative changes in the structure of the spine (for example, osteochondrosis), pinched nerve roots, congenital pathologies, injuries and assessment of the effectiveness of treatment after injuries, suspicion of tumor processes, osteoporosis.
• Bones and joints: bones, soft tissues, joints - injuries (including sports), age-related changes, inflammatory processes, suspected tumors, muscle and tendon injuries, rheumatoid arthritis.
• : pathology of internal organs.
• : adenoma, prostate cancer, assessment of the spread of tumor lesions, preoperative preparation, assessment of the condition of the bladder, ureters, rectum, ovaries, scrotum, uterine fibroids, anomalies of the pelvic organs.

Also, if necessary, examination of the vessels of the brain, neck, and thoracic region is carried out; arteries, veins, thyroid gland. If the presence of tumor lesions or metastases is suspected, the patient's entire body can be examined.

Also, indications for MRI may be a heart attack, defect or coronary heart disease.

Contraindications to the procedure

Many patients are concerned about whether there are contraindications to MRI. Of course, such limitations exist for tomography, as for any other medical procedure.

The entire list of contraindications to MRI can be divided into absolute and relative. Absolute ones include the presence of a metal foreign body, a prosthesis or electromagnetic implant, or a pacemaker. If an MRI with contrast is performed, renal failure and allergy to the contrast agent.

The presence of these factors makes the procedure absolutely impossible. Relative contraindications mean conditions or circumstances that may pass/change over time, and the examination becomes possible.

Relative contraindications:

1. First 3 months.
2. Mental problems, schizophrenia, claustrophobia, panic states.
3. Severe illnesses in the stage of decompensation.
4. The patient has tattoos that were made using dyes based on metal compounds.
5. Severe pain, as a result of which the person cannot remain completely still.
6. State of intoxication - alcohol or drugs.

Is the patient’s childhood a contraindication and can MRI be performed on children, and if so, at what age? Experts answer these questions that childhood is not an obstacle to conducting research. That is, MRI is done even on newborn babies. However, with small children there is another problem - it is very difficult to force them to remain still. Especially for a long time, especially in a confined space. There are several solutions to this problem, for example, a preliminary conversation with the child or the use of anesthesia. MRI examinations under anesthesia are also performed on adults in cases where the procedure is absolutely necessary, but the person suffers from claustrophobia or panic attacks.

Preparatory activities

General preparation for MRI is an important stage of the study that cannot be ignored. The success of the procedure and the accuracy of the results depend on how accurately the patient follows the recommendations of specialists.

Preparation for the study begins with a mandatory consultation with a therapist. The doctor will clarify your medical history, conduct an external examination, clarify the issue of contraindications, tell you in detail how an MRI is done, and give directions for examining specific problem areas.

Preparing for an MRI also includes assessing your own condition. The patient must be prepared to be in a closed, noisy space for some time. If a person assumes that he may begin to panic, he should enlist the support of a loved one in advance. A relative or spouse will also help you get home after the procedure if the patient is given sedatives to calm you before the examination. MRI under anesthesia also requires the presence of a loved one who will take the patient home after the examination.

MRI preparation includes removing (from yourself and from clothing) all metal objects - pins, piercings, earrings and other jewelry, removable implants and dentures, hairpins, underwear with metal inserts, etc.

Before the procedure, you need to go to the toilet, you should not drink alcohol or drugs. Is it possible to eat before an MRI or take regular medications? Yes, if there is an examination of the brain, joints, eyes, nasopharynx or spine.

Some types of tomographic examinations require special preparation for MRI.

For example, before examining the pelvic organs, you need to urinate 3 hours before the procedure and not do it again. 60 minutes before the session, drink half a liter of plain water, so the bladder will be half full, which is required for correct diagnosis. The night before, you need to completely cleanse the intestines with an enema or laxative.

MRI of the abdominal organs is done only on an empty stomach, so the question of whether you can eat before the procedure is not appropriate in this case. Exceptions are situations when the session cannot be held in the morning. In this case, it is permissible to have a very light breakfast. Cleansing the intestines the day before and taking antispasmodics 30 minutes before the session is very advisable.

Preparing children for magnetic tomography examination

Physically, children are prepared for the procedure in the same way as adults. If the child is already at an age when he understands what is wanted from him and obeys his parents (6-7 years old), you need to tell him how to prepare for an MRI on his own. If necessary, help.

Preparing a child for an MRI of the brain using an open-type device

Psychological preparation of the child is a necessary preliminary stage. You need to tell your child why an MRI is done, what awaits him during this procedure, what sensations may arise, and how to suppress negative thoughts and fears. You also need to warn the child about how long it takes to do an MRI and that during this time he should be as motionless as possible.

If parents see that the child is psychologically unprepared, feels severe fear, or there are other associated factors (severe pain, epilepsy, seizures), deep sedation or superficial anesthesia will probably have to be used.

How does a magnetic resonance imaging session work?

To ensure that no unexpected or unpleasant surprises occur during the examination session, the patient needs to have a rough idea of ​​how an MRI is performed. The standard procedure includes the following steps:

1. The patient is asked to undress and remove all foreign objects from the body, including a wig, dentures and hearing aids, jewelry, etc. The doctor will give you a disposable cape for your change.
2. The patient takes a horizontal position on a special sliding table. Then the table slides into the apparatus tunnel. With modern tomographs, variations of this stage are possible. For example, in the case of using an open-type tomograph or a device requiring a sitting position.
3. How long an MRI takes depends on the type of examination. On average - from 20 to 120 minutes. All this time, the patient must maintain absolute immobility of the area of ​​the body being examined.
4. During a tomography session, the patient hears noise or buzzing, and may feel a slight vibration. To make it easier to be in a confined space, it is better to close your eyes and relax as much as possible.

After the end of the session, the patient may be asked to wait for some time to make sure that everything was successful, the data obtained is sufficient and no additional manipulations are required. After this, personal belongings and clothes are returned to the patient - the magnetic resonance imaging session is over.

Special attention is required to specify how the MRI procedure takes place in the case of the use of anesthesia or contrast agents.

Features of MRI in patients under anesthesia

MRI under anesthesia can be of two types:

• Deep sedation using modern tranquilizer drugs. Helps significantly calm the patient, relieve anxiety, and stop panic attacks.
• Anesthesia, which is given through intravenous injection or inhalation. This method may require additional ventilation and the connection of vital signs monitoring devices.

Typically, the effect of anesthesia wears off within 30-60 minutes after the end of the study session. Before anesthesia, you should not eat for 9 hours, and for children under 6 years old - 6 hours. You can only drink clean water and tea in small portions. Stop taking liquids 2 hours before the procedure.

After anesthesia, you can only leave the clinic with an accompanying person; independent driving is strictly prohibited.

Magnetic resonance imaging with contrast

Injector for administering contrast agent during the examination

What is MRI with contrast? This is the same procedure as a standard MRI, only to increase the information content of the procedure, a safe, non-toxic substance is injected into the patient’s vein. In most cases, this is necessary when diagnosing tumor lesions. In this way, it is possible to conduct the most comprehensive study, studying in detail the size of the tumor, its structure and extent of spread.

However, a tumor is not the only reason for this type of procedure. There are a number of indications for contrast-enhanced examinations.

Contraindications: pregnancy, lactation, allergies (very rare cases).

The patient does not experience any consequences or adverse reactions after a tomography session with contrast.

Results of magnetic resonance examination

What the MRI shows, that is, the examination results, will be ready within 1 or 2 days. If everything is normal in the body, then the results will show that all organs and tissues of the body are in their places, have standard sizes, shape, structure, density. Magnetic resonance imaging will also show that there are no malignant or benign tumors, bleeding, blood clots, inflammatory or infectious processes in the body.

Radiologists make a conclusion on an MRI study

If the doctor discovers any violations, this will be reflected in the conclusion and medical history.

Let's sum it up

MRI is the most modern, one of the most accurate and safe non-invasive methods for studying the human body. A magnetic tomography session is absolutely painless and is suitable for examining even small children. What an MRI can show helps a doctor diagnose any health problem or confirm the absence of one.

One of the most effective methods of medical research is MRI or magnetic resonance imaging, which allows you to obtain the most accurate information about the anatomical features of the patient’s body, metabolic processes, physiology of tissues and internal organs. With its advent, detailed examination of the brain became possible to diagnose diseases and degenerative lesions. The ability to determine the localization of the process and the extent of damage that has occurred becomes the main advantage of this procedure in identifying neoplasms and studying blood vessels.

What is MRI

Magnetic resonance imaging is a unique opportunity to obtain high-precision layer-by-layer images of the area under study. The procedure is carried out using a special device, the effect of which on the human body is to stimulate radio waves, create a strong magnetic field and register the response electromagnetic radiation of the body. The result of the process is the construction of an image by processing the incoming signal on a computer.

What is a magnetic resonance imaging scanner? This is a device that allows you to achieve effective diagnostics, identify changes in the functioning of the body and produce high-precision visualization of the organs being studied, which significantly exceeds the results of other methods (X-ray, CT, ultrasound). This procedure makes it possible to identify oncology and a number of other diseases and dangerous pathologies, measure the speed of blood flow and movement of cerebrospinal fluid, etc.

The operation of the device is based on the principle of NMR with subsequent processing of the obtained information by special programs. The MRI unit creates a strong magnetic field. An important factor explaining the principle of operation of the device is the presence of protons in the human body (in the chemical sense, this is the nucleus of a hydrogen atom). A magnetic resonance imaging scanner allows you to maintain a stable state of magnetism in the patient’s body when placed in a force field. The device produces:

stimulation of the body using radio waves, promoting a change in the stationary orientation of charged particles;

stopping radio waves and recording electromagnetic radiation from the body;

processing the received signal and converting it into an image.

The resulting image is not a photograph of the department or organ being examined. The technician receives a high-quality, detailed image of the radio signals emitted by the patient's body. MRI diagnostics is completely superior to the computed tomography method, since in this case the procedure does not use ionizing radiation, but uses electromagnetic waves that are safe for the human body.

History of creation and operating principle of MRI

The year of creation of this method is considered to be 1973, and one of the founding fathers of magnetic resonance imaging is Paul Lauterbur. He published an article in one of the magazines that described in detail the phenomenon of visualizing structures and organs using magnetic and radio waves.

This is not the only scientist involved in the discovery of MRI - back in 1946, Felix Bloch and Richard Purcell, working at Harvard, studied a physical phenomenon based on the properties inherent in atomic nuclei (primary absorption of the received energy and its subsequent re-emission. i.e. . selection with transition to the initial state). For this research, scientists received the Nobel Prize (1952).

The discovery of Bloch and Purcell became a kind of impetus for the development of the NMR theory. The unusual phenomenon was studied by both chemists and physicists. The demonstration of the first CT scanner, which included a series of tests, occurred in 1972. The result of the study was the discovery of a fundamentally new diagnostic method that allows detailed visualization of the most important structures of the body.

Further, Lauterbur partially formulated the principle of operation of the MRI apparatus - the scientist’s work formed the basis for research carried out to this day. In particular, the article contained the following statements:

Three-dimensional projections of objects are obtained from the NMR spectra of water protons from the examined structures, organs, etc.

Particular attention was paid to the surveillance of malignant neoplasms. Experiments conducted by Lauterbur showed that they are significantly different from healthy cells. The difference lies in the characteristics of the received signal.

In the 70s of the 20th century, a new era in the development of MRI diagnostics began. At this time, Richard Ernst proposed magnetic resonance imaging using a special method - coding (both frequency and phase). It is this method of visualizing the areas of interest that doctors use today. In 1980, a photograph was demonstrated that took about 5 minutes to obtain. After just six years, the display duration was reduced to five seconds. At the same time, the picture quality remained unchanged.

In 1988, the angiography method was also improved, making it possible to display the patient’s blood flow without additional injection of drugs into the blood that act as contrast.

The development of MRI was a new milestone in modern medicine. This procedure is used in the diagnosis of diseases:

spine;

joints;

brain (brain and spinal cord);

pituitary gland;

internal organs;

mammary glands, etc.

The capabilities of the open method make it possible to detect diseases in the early stages and identify pathologies that require timely treatment or immediate surgical intervention. Tomography, carried out using modern equipment, makes it possible to obtain an accurate image of organs, examined structures and tissues, as well as:

collect the necessary information about the circulation of cerebrospinal fluid;

determine the level of activation of areas of the cerebral cortex;

monitor gas exchange in tissues.

The MRI method compares favorably with other diagnostic methods:

It does not involve exposure using surgical instruments.

Magnetic resonance imaging is safe and highly effective.

This procedure is relatively widely available and is in demand when studying the most complex cases that require detailed visualization of changes occurring in the body.

The video below demonstrates the main stages of the functioning of a modern tomograph:

How MRI works (video)

Operating principle of a magnetic resonance scanner (MRI)

How is the procedure done? A person is placed in a special narrow tunnel, in which he must be in a horizontal position. In the pipe it is exposed to the strong magnetic field of the device. The study lasts from 15 to 20 minutes.

Afterwards, the patient is given an image. It is created using the NMR method - a physical phenomenon of nuclear magnetic resonance associated with the properties of protons. Using a radio frequency pulse, radiation is generated in the electromagnetic field created by the device, which is converted into a signal. It is then registered and processed by a computer program.

Each slice examined and displayed on the screen as an image has its own thickness. The display method under consideration is similar to the technology of removing everything that is located above and below the layer. In this case, individual elements of volume and plane - parts of the slice and structural components of the resulting magnetic resonance image - play a large role.

Since the human body is 90% water, the protons of the hydrogen atoms are stimulated. This method of exposure allows you to look into the body and diagnose serious diseases without physical intervention.

MRI machine design

The modern equipment under consideration consists of the following parts:

magnet;

coils;

a device that generates radio pulses;

Faraday cage;

power supply;

cooling system;

systems used to process incoming data.

Magnet

Creates a stable field characterized by uniformity and high intensity. It is by the latter indicator that the power of the device is assessed. Let us remind you that the quality of the resulting image and the speed of the procedure depend on it.

Depending on the voltage, all devices are divided into the following groups:

Low-field - entry-level equipment, open, field strength< 0.5 Tл.

Mid-field - indicators from 0.5-1 T.

High-field - characterized by high research speed and a clear image even when the patient moves during the examination. The magnetic field strength of these installations is 1-2 Tesla.

Ultra-high field - more than 2 Tesla. Used for research purposes.

The following types of magnets used are also distinguished:

Permanent - made from alloys with ferromagnetic properties. The advantage of such elements is that they do not need to be cooled, since they do not require energy to maintain a uniform field. Among the disadvantages are the large weight of the system used and low tension. Also, such magnets are sensitive to temperature changes.

Superconducting - a coil made of a special alloy. Large currents can pass through it. The result of such a device is the creation of a strong magnetic field. An addition to the design is a cooling system. The disadvantages of this type are increased consumption of liquid helium with low energy consumption, high operating costs of the device, and mandatory shielding. There is also a high risk of coolant being ejected from the cryostat when superconducting properties are lost.

• Resistive electromagnets do not require the use of special cooling systems and are capable of creating a relatively homogeneous field for complex research. Disadvantage - heavy weight (about 5 tons, increases during the shielding process)

The principle of operation of the coil in MRI

These elements are designed to increase the uniformity of the magnetic field. By passing current through themselves, they adjust the characteristics, compensating for the lack of homogeneity. Such parts are either placed directly in liquid helium or do not require cooling.

The effect of gradient coils is to create a clear image by localizing the signal and maintaining an exact match between the data obtained during the procedure and the area being examined by the doctor.

The power and speed of action of the parts are of great importance - the resolution of the device, the noise level in relation to the signal and the speed of action depend on these indicators.

Transmitter in MRI: operating principle of an element in a tomograph system

This device generates radio frequency oscillations and pulses (rectangular and complex shapes). Such a transformation makes it possible to achieve excitation of the nuclei and influence the contrast of the image displayed on the image. The signal from the element is applied to a switch, which in turn acts on a coil, generating an RF magnetic field that affects the spin system.

Receiver

It is a signal amplifier with high sensitivity and low noise level, which operates at ultra-high frequencies. The recorded response undergoes a change - conversion from MHz to kHz (from high frequencies to low frequencies).

Spare parts for tomographs

Recording sensors, which are located around the patient’s organ under study, are also responsible for obtaining an accurate, detailed image. This procedure is absolutely safe: having emitted the transmitted energy, the protons return to their previous state.

Recording sensors, which are located around the patient’s organ under study, are also responsible for obtaining an accurate, detailed image. This procedure is absolutely safe: having emitted the transmitted energy, the protons return to their previous state. To improve image quality and provide greater image detail, the patient may be injected with a gadolinium-based contrast agent, which does not cause adverse reactions. A special drug is placed in a syringe or injector, which automatically calculates the dosage and injection rate. The supply of the product is completely synchronized with the scanning progress.

The quality of the examination depends not only on the strength of the magnetic field, but also on the coil used, the use of contrast agent, diagnostic features and the experience of the specialist performing the tomography.

The advantages of such a procedure:

the ability to obtain the most accurate image of the organ being examined;

improving the quality of diagnostics;
safety for the patient.

Tomographs differ in the strength of the field they create and the “openness” of the magnet. The greater the field power, the faster the scanning procedure and the higher the quality of the resulting three-dimensional image.

Open MRI machines are C-shaped and are the best option for examining people suffering from severe claustrophobia. They were created to carry out additional procedures inside a magnet. This type of installation is much weaker than closed tomographs.

An MRI examination is one of the most effective and safe diagnostic methods and the most informative method for a detailed examination of the spinal cord and brain, spine, abdominal and pelvic organs.

In 1973, American chemist Paul Lauterbur published a paper in Nature entitled “Imaging by Induced Local Interaction; examples based on magnetic resonance." Later, British physicist Peter Mansfield would propose a more advanced mathematical model for obtaining images of a whole organism, and in 2003 the researchers would receive the Nobel Prize for the discovery of the MRI method in medicine.

The American scientist Raymond Damadian, the father of the first commercial MRI machine and the author of the work “Tumor Detection Using Nuclear Magnetic Resonance,” published in 1971, will also make a significant contribution to the creation of modern magnetic resonance imaging.

But in fairness, it is worth noting that long before Western researchers, in 1960, the Soviet scientist Vladislav Ivanov had already outlined in detail the principles of MRI, however, he received an author’s certificate only in 1984... Let’s leave the debate about authorship and finally look at it in general outlines the operating principle of a magnetic resonance imaging scanner.

There are a lot of hydrogen atoms in our bodies, and the nucleus of each hydrogen atom is one proton, which can be represented as a small magnet that exists due to the presence of a non-zero spin on the proton. The fact that the nucleus of a hydrogen atom (proton) has a spin means that it seems to rotate around its axis. It is known that the hydrogen nucleus has a positive electric charge, and the charge rotating with the outer surface of the nucleus is like a small coil with current. It turns out that each nucleus of a hydrogen atom is a miniature source of a magnetic field.

If now many nuclei of hydrogen atoms (protons) are placed in an external magnetic field, they will begin to try to orient themselves along this magnetic field like compass needles. However, in the process of such reorientation, the nuclei will begin to precess (as the axis of a gyroscope precesses when trying to tilt it), because the magnetic moment of each nucleus turns out to be associated with the mechanical moment of the nucleus, with the presence of the spin mentioned above.

Let's say a hydrogen nucleus is placed in an external magnetic field with an induction of 1 Tesla. The precession frequency in this case will be 42.58 MHz (this is the so-called Larmor frequency for a given nucleus and for a given magnetic field induction). And if we now exert an additional influence on this nucleus with an electromagnetic wave with a frequency of 42.58 MHz, the phenomenon of nuclear magnetic resonance will arise, that is, the amplitude of the precession will increase, since the vector of the general magnetization of the nucleus will become larger.

And there are billions of billions of billions of such nuclei in our bodies, capable of precessing and falling into resonance. But since in normal everyday life the magnetic moments of all hydrogen nuclei and other substances in our body interact with each other, the total magnetic moment of the entire body is zero.

By acting on protons with radio waves, they obtain a resonant amplification of the oscillations (an increase in the precession amplitudes) of these protons, and after the end of the external influence, the protons tend to return to their original equilibrium states, and then they themselves emit photons of radio waves.

Thus, in an MRI machine, the human body (or some other body or object under study) periodically turns into either a set of radio receivers or a set of radio transmitters. By examining section by section of the body in this way, the device builds a spatial picture of the distribution of hydrogen atoms in the body. And the higher the magnetic field strength of the tomograph, the more hydrogen atoms associated with other atoms located nearby can be examined (the higher the resolution of the magnetic resonance tomograph).

Modern medical tomographs contain liquid helium cooled sources of external magnetic fields. Some open-type tomographs are used for this purpose.

The optimal magnetic field induction in an MRI machine today is 1.5 Tesla, which allows one to obtain fairly high-quality images of many parts of the body. With an induction of less than 1 Tesla, it will not be possible to take a high-quality image (sufficiently high resolution), for example, of the pelvis or abdominal cavity, however, such weak fields are also suitable for obtaining ordinary MRI images of the head and joints.

For correct spatial orientation, in addition to a constant magnetic field, a magnetic resonance imaging scanner also uses gradient coils, which create an additional gradient disturbance in a uniform magnetic field. As a result, the strongest resonant signal is localized more accurately in a particular slice. The power and operating parameters of gradient coils are the most significant indicators in MRI; the resolution and performance of the tomograph depend on them.