Magnetic resonance imaging (MRI) of the brain

MRI of the brain is currently the leading noninvasive method of intravital imaging of the brain structure. Synonyms MRI - nuclear magnetic resonance imaging and magnetic resonance imaging. The spatial resolution of the MRI method is 1-2 mm, it can be increased by contrasting with gadolinium.

[1], [2], [3], [4]

Purpose of MRI of the brain

The purpose of the MRI of the brain is to identify and determine the shape, size and location of various brain lesions [posttraumatic, atrophic, ischemic (day after day) and hemorrhagic (from the first hours) stroke, demyelinating processes, meningiomas and glial tumors], displacement brain structures, the manifestation of cerebral edema, the state of liquor-containing spaces to exclude possible "organic" causes of psychopathological symptoms. MRI is also performed to diagnose lesions of the brain and spine.

Indications for MRI of the brain

• Diagnosis of brain lesions.
• Differential diagnosis of neuroinfections with non-infectious brain lesions.
• Monitoring the effectiveness of treatment of neuroinfections.

Indication for the study in a psychiatric clinic:

• suspicion of the presence of atrophic, degenerative or demyelinating process, epileptic focus, stroke, brain tumor.

Preparation for the study of MRI of the brain

Before the MRI, the patient is informed about the procedure, its painlessness and the absence of irradiation, unless a radioactive contrast medium is used. When contrasting MRI of a patient, it is necessary to warn that after the introduction of a contrast medium, a feeling of heat and tidal flare, a headache, a taste of metal in the mouth, nausea or vomiting may occur.

The patient should be dressed in comfortable light clothing, all metal objects in the field of the tomograph should be removed. With motor anxiety, anxiety, and also suffering claustrophobia, the patient is prescribed sedatives, since during the study he must remain immobile.

The doctor should receive written consent from the patient or his relatives for the study, and also must find out and note in the medical history the patient's intolerance to iodine (seafood) and contrast agents. In case of allergic reactions to iodine, it is necessary to prophylaxisally prescribe antihistamines or to cancel the introduction of contrast medium.

Method for the study of MRI of the brain

The study is carried out on a table, which is then inserted into the cylindrical space of the scanner, in the position lying on the back.

The examining physician changes the frequency of the radio waves emitted by the scanner and adjusts the quality of the image using a computer.

The information about the slices is stored digitally in the computer, displayed on the display and reported to the medical history in the form of a photograph.

The method is based on the physical phenomenon of nuclear magnetic resonance. The nuclei of many atoms, in particular the nucleus of the hydrogen atom (proton), have a magnetic moment that is associated with their spin rotation. Such nuclei can be considered miniaturized elementary magnets. In a constant magnetic field, the spin can be located in the direction or opposite to the magnetic lines of force, in these two cases the energy of the nucleus is different.

When an external radio-frequency pulsed magnetic field is acted upon with certain parameters that cause magnetic resonance, the total magnetic field of the object created by elementary magnets changes and then decays to zero as a result of reorientation of spins during the longitudinal relaxation time (Tj), and also due to violation of the coherence of individual spins under the influence of the environment during the transverse relaxation time (T2).

These changes register special sensors, while the value of the resulting magnetic signal corresponds to the local concentration of the nuclei, and from the values of T1 and T2, it is possible to judge which chemical structures they are included in. With the help of computer processing, the picture of the distribution of the corresponding nuclei on the "slices" or in the volume of the brain is reproduced.

When using magnets that create high levels of magnetic field strength, the signal can be subjected to spectral analysis with the separation of components associated with atoms not only of hydrogen, but also of phosphorus (for example, to study the distribution of adenosine triphosphate metabolism), carbon and fluorine. Since the exposure time (time resolution) is also reduced (up to several seconds and even 100 ms), it is possible to study metabolic changes in various types of intellectual activity. Such a modification of the method, called "nuclear magnetic resonance spectroscopy", or "functional MRI", allows not only visualize the structure, but also explore some of the functions of the brain.

Contraindications MRI of the brain

• pregnancy;
• presence on the body or in the body of a patient of foreign metal and, especially, ferromagnetic objects, as well as electronic devices (in particular, watches, ornaments, metal braces on vessels, fragments), since the influence of a strong magnetic field can cause them to shift, heat or exit (so, categorically it is contraindicated to carry out MRI to patients with a worn or implanted pacemaker).

[5], [6], [7], [8]

Interpretation of MRI results

MRI examines the state of brain structures according to their shape, size and tissue density. It should be taken into account that MRI reflects the density of tissues depending on the water content in them, and therefore, first of all, such lesions as swelling-swelling of the brain (ONGM), demyelinating diseases, tumors are revealed.

Since the maximum concentration of protons is associated with water (intercellular fluid) and with lipids forming myelin sheaths of nerve fibers, the MRI method clearly delineates the gray and white matter of the brain, visualizes spaces filled with fluid (cerebral vesicles, edema, cystic formations), allows diagnose atrophic and demyelinating processes, neoplasms, and also to obtain volumetric distributions of a number of compounds (choline, lactate).

Factors affecting the outcome

A certain limitation of the MRI method (especially when using equipment that provides a relatively low level of magnetic field intensity 0.12-0.15 T) is the duration of exposure, which can reach 10-15 min, when the patient must maintain a fixed posture (which is not always possible with study of children, elderly and restless mentally ill). In these cases it is possible to use muscle relaxants or anesthesia [the use of anti-anxiety drugs (tranquilizers, anxiolytics) may not be sufficient to relieve the motor anxiety of patients], necessarily taking into account the ratio of the diagnostic information of the study and the risk of possible complications from the use of drugs of these groups.

[9], [10], [11], [12], [13], [14], [15]

Complications

The absence of ionizing radiation makes the MRI method highly safe, which determined its widespread use. Complications of the MRI method are not described. As side effects, there was some improvement in cerebral blood flow in 10-15% of patients (which is associated with a change in the rheological properties of blood under the influence of a magnetic field).

When performing a contrast MRI in a patient, allergic reactions to a contrast agent in the form of a feeling of heat, headache, a taste of metal in the mouth, nausea, or vomiting are possible. After completing a long-term study in a horizontal position, the patient may have orthostatic hypotension.

[16], [17], [18], [19]

Alternative methods

In the absence of equipment for MRI, the best alternative is to conduct a CT scan taking into account the characteristics and limitations of the method.

[20], [21], [22], [23]