Diagnosis of osteoarthritis: magnetic resonance imaging

Magnetic resonance imaging (MRI) in recent years has become one of the leading methods of non-invasive diagnosis of osteoarthritis. Since the 70s, when the principles of magnetic resonance (MP) were first used to study the human body, to this day this method of medical imaging has radically changed and continues to develop rapidly.

Technical equipment, software are improving, imaging techniques are developing, MP-contrast preparations are being developed. This allows you to constantly find new areas of application of MRI. If initially its use was limited only to studies of the central nervous system, now MRI is successfully used in almost all areas of medicine.

In 1946, a group of researchers from Stanford and Harvard Universities independently discovered the phenomenon, which was called nuclear magnetic resonance (NMR). The essence of it was that the nuclei of some atoms, being in a magnetic field, under the influence of an external electromagnetic field are able to absorb energy, and then emit it in the form of a radio signal. For this discovery F. Bloch and E. Parmel in 1952 were awarded the Nobel Prize. A new phenomenon soon learned how to use for spectral analysis of biological structures (NMR spectroscopy). In 1973, Paul Rautenburg demonstrated for the first time the possibility of obtaining an image using NMR signals. Thus, NMR tomography appeared. The first NMR tomograms of the internal organs of a living person were demonstrated in 1982 at the International Congress of Radiologists in Paris.

Two explanations should be given. Despite the fact that the method is based on the phenomenon of NMR, it is called magnetic resonance (MP), omitting the word "nuclear". This is done so that patients do not have an idea about the radioactivity associated with the decay of atomic nuclei. And the second circumstance: MP-tomographs are not accidentally "tuned" to protons, i.e. On the nucleus of hydrogen. This element in the tissues is very much, and its nuclei have the greatest magnetic moment among all atomic nuclei, which causes a sufficiently high level of the MR signal.

If in 1983 there were only a few devices around the world suitable for clinical research, by the beginning of 1996 there were about 10,000 tomographs in the world. Every year, 1000 new instruments are introduced into practice. More than 90% of the fleet of MP-tomographs are models with superconducting magnets (0.5-1.5 T). It is interesting to note that if in the mid-1980s the manufacturers of MP scanners were guided by the principle "the higher the field, the better", focusing on models with a field of 1.5 T and higher, then by the end of the 1980s it is clear that in most applications they do not have significant advantages over models with medium field strength. Therefore, the main manufacturers of MP-tomographs (General Electric, Siemens, Philips, Toshi-ba, Picker, Brucker, etc.) are currently paying great attention to the production of models with an average and even low field, which differ from high-field systems in compactness and economy with satisfactory image quality and significantly lower cost. High-floor systems are used primarily in research centers for conducting MR spectroscopy.

[1], [2], [3], [4], [5], [6], [7]

The principle of the MRI method

The main components of the MP-tomograph are: ultra-strong magnet, radio transmitter, receiving radio frequency coil, computer and control panel. Most devices have a magnetic field with a magnetic moment parallel to the long axis of the human body. The strength of the magnetic field is measured in Tesla (T). For clinical MRI use fields with a force of 0.2-1.5 T.

When a patient is placed in a strong magnetic field, all the protons that are magnetic dipoles unfold in the direction of the external field (like a compass needle, which is guided by the Earth's magnetic field). In addition, the magnetic axes of each proton begin to rotate around the direction of the external magnetic field. This specific rotational motion is called a process, and its frequency is a resonant frequency. When a short electromagnetic radio frequency pulse is transmitted through the patient's body, the magnetic field of the radio waves causes the magnetic moments of all protons to rotate around the magnetic moment of the external field. In order for this to happen, it is necessary that the frequency of the radio waves be equal to the resonant frequency of the protons. This phenomenon is called magnetic resonance. To change the orientation of magnetic protons, the magnetic fields of protons and radio waves must resonate, i.e. Have the same frequency.

A total magnetic moment is created in the patient's tissues: the tissues are magnetized and their magnetism is oriented strictly parallel to the external magnetic field. Magnetism is proportional to the number of protons per unit volume of tissue. The huge number of protons (hydrogen nuclei) contained in most tissues causes the fact that the pure magnetic moment is large enough to induce an electric current in the receiving coil located outside the patient. These induced MP signals are used to reconstruct the MR image.

The process of transition of the electrons of the nucleus from the excited state to the equilibrium state is called a spin-lattice relaxation process or longitudinal relaxation. It is characterized by a T1-spin-lattice relaxation time-the time necessary to transfer 63% of the nuclei to an equilibrium state after they are excited by a 90 ° pulse. T2 is also a spin-spin relaxation time.

There are a number of ways to obtain MP-tomograms. Their difference lies in the order and nature of the generation of radio frequency pulses, methods for analyzing MP signals. The most common are two methods: spin-lattice and spin-echo. For the spin-lattice, the relaxation time T1 is mainly analyzed. Various tissues (gray and white matter of the brain, cerebrospinal fluid, tumor tissue, cartilage, muscles, etc.) have protons with different relaxation times T1. With the duration of T1, the intensity of the MP signal is related: the shorter the T1, the more intense the MR signal and the lighter the image space appears on the TV monitor. Fat tissue on the MP-tomogram is white, followed by the intensity of the MP signal in descending order are the brain and spinal cord, dense internal organs, vascular walls and muscles. Air, bones and calcifications practically do not give an MP signal and therefore are displayed in black. These relationships of relaxation time T1 create the prerequisites for visualization of normal and altered tissues on MR tomograms.

In another method of MP-tomography, called spin-echo, a series of radio-frequency pulses are sent to the patient turning the precessing protons 90 °. After stopping the pulses, the response MP signals are recorded. However, the intensity of the response signal is differently related to the duration of T2: the shorter T2, the weaker the signal and, consequently, the brightness of the screen of the TV monitor is lower. Thus, the final picture of MRI in method T2 is opposite to that of T1 (as negative to positive).

On MP-tomograms, soft tissues are displayed better than on computer tomograms: muscles, fat layers, cartilage, vessels. On some devices, one can obtain a picture of the vessels without introducing a contrast agent (MP-angiography). Due to the low water content in the bone tissue, the latter does not create a shielding effect, as in X-ray computed tomography, i.e. Does not interfere with the image, for example, the spinal cord, intervertebral discs, etc. Of course, the hydrogen nuclei are contained not only in water, but in bone tissue they are fixed in very large molecules and dense structures and do not interfere with MRI.

Advantages and disadvantages of MRI

The main advantages of MRI are non-invasiveness, harmlessness (absence of radiation load), three-dimensional nature of image acquisition, natural contrast from moving blood, absence of artifacts from bone tissue, high soft tissue differentiation, possibility of MP-spectroscopy for in vivo intravital study of tissue metabolism . MPT allows you to obtain an image of thin layers of the human body in any section - in the frontal, sagittal, axial and oblique planes. It is possible to reconstruct volumetric images of organs, to synchronize the reception of tomograms with electrocardiogram teeth.

The main shortcomings usually include a sufficiently long time to obtain images (usually minutes), which leads to the appearance of artifacts from respiratory movements (this especially reduces the effectiveness of lung examination), arrhythmias (in the study of the heart), the inability to reliably detect stones, calcifications, some types of pathology of bone structures, the high cost of equipment and its operation, special requirements for the premises in which the instruments are located (screening from interference), the impossibility of examining I am sick with claustrophobia, artificial pacemakers, large metal implants from non-medical metals.

[8], [9], [10], [11], [12], [13], [14], [15]

Contrast substances for MRI

At the beginning of MRI use, it was believed that the natural contrast between different tissues eliminates the need for contrast agents. Soon it was discovered that the difference in signals between different tissues, i.e. The contrast of the MR image can be significantly improved by contrast media. When the first MP contrast medium (containing paramagnetic gadolinium ions) became commercially available, the diagnostic information of MRI increased significantly. The essence of the MR-contrast agent is to change the magnetic parameters of the protons of tissues and organs, i.e. Change the relaxation time (TR) of T1 and T2 protons. To date, there are several classifications of MP-contrast agents (or rather, contrast agents - CA).

By the predominant effect on the relaxation time of the MR-Cadel at:

• T1-KA, which shorten T1 and thereby increase the intensity of the MP signal of the tissues. They are also called positive SC.
• T2-KA, which shorten T2, reducing the intensity of the MR signal. This is a negative SC.

Depending on the magnetic properties of the MR-SC are divided into paramagnetic and superparamagnetic:

[16], [17], [18], [19], [20]

Paramagnetic contrast media

Paramagnetic properties are possessed by atoms with one or more unpaired electrons. These are magnetic ions of gadolinium (Gd), chromium, nickel, iron, and also manganese. Gadolinium compounds were most widely used clinically. The contrasting effect of gadolinium is due to the shortening of the relaxation time T1 and T2. In low doses, the influence on T1, which increases the intensity of the signal, predominates. In high doses, the effect on T2 predominates with a decrease in signal intensity. Paramagnetics are now most widely used in clinical diagnostic practice.

Superparamagnetic contrast media

The dominant effect of superparamagnetic iron oxide is the shortening of T2 relaxation. As the dose is raised, the intensity of the signal decreases. To this group of spacecraft can be attributed and ferromagnetic satellites, which include ferromagnetic iron oxides structurally similar to magnetite ferrite (Fe ²⁺ OFe ₂ ³⁺ 0 ₃ ).

The following classification is based on the pharmacokinetics of the CA (Sergeev, V.V., Isoavt., 1995):

• extracellular (tissue-specific);
• gastrointestinal;
• organotropic (tissue-specific);
• macromolecular, which are used to determine the vascular space.

In Ukraine, four MR-CAs are known, which are extracellular water-soluble paramagnetic SCs, of which gadodiamide and gadopentetic acid are widely used. The remaining SC groups (2-4) undergo a stage of clinical trials abroad.

Extracellular water-soluble MP-CA

International name	Chemical formula	Structure
Gadopentetic acid	Gadolinium dimeglumina diethylenetriaminepentaacetate ((NMG) 2Gd-DTPA)	Linear, ionic
Acid gadoterovaya	(NMG) Gd-DOTA	Cyclic, ionic
Gadodiamide	Gadolinium diethylenetriaminepentaacetate-bis-methylamide (Gd-DTPA-BMA)	Linear, non-ionic
Gadoteridol	Gd-HP-D03A	Cyclic, non-ionic

Extracellular spacecraft is administered intravenously, 98% of them are excreted by the kidneys, do not penetrate the blood-brain barrier, have low toxicity, belong to the paramagnetic group.

Contraindications to MRI

Absolute contraindications include the conditions under which the study is life-threatening patients. For example, the presence of implants, which are activated by electronic, magnetic or mechanical means, is primarily artificial pacemakers. The impact of RF radiation from the MR scanner may interfere with the functioning of the stimulator operating in the query system, since changes in magnetic fields can mimic cardiac activity. The magnetic attraction can also cause the stimulator to move in the nest and move the electrodes. In addition, the magnetic field creates obstacles for the operation of the ferromagnetic or electronic implants of the middle ear. The presence of artificial heart valves represents a danger and is an absolute contraindication only when examined on high-field MR scanners, and also if the valve is clinically assumed to be damaged. The presence of small metal surgical implants (haemostatic clips) in the central nervous system also refers to absolute contraindications to the study, since their displacement due to magnetic attraction threatens to bleed. Their presence in other parts of the body is less of a threat, since after treatment, fibrosis and encapsulation of the clamps help keep them in a stable state. However, in addition to the potential danger, the presence of metal implants with magnetic properties in any case causes artifacts that create difficulties for interpreting the results of the study.

Contraindications to MRI

Absolute:	Relative:
Pacemakers	Other stimulants (insulin pumps, nerve stimulators)
Ferromagnetic or electronic implants of the middle ear	Non-ferromagnetic implants of the inner ear, prosthetic heart valves (in high fields, with suspected dysfunction)
Hemostatic clamps of cerebral vessels	Hemostatic clips of other localization, decompensated heart failure, pregnancy, claustrophobia, the need for physiological monitoring

To relative contraindications, in addition to the above, also include decompensated heart failure, the need for physiological monitoring (mechanical ventilation, electric infusion pumps). Claustrophobia is an obstacle to research in 1-4% of cases. It can be overcome, on the one hand, using devices with open magnets, on the other - a detailed explanation of the apparatus and the course of the survey. Evidence of the damaging effect of MRI on the embryo or fetus is not obtained, but it is recommended to avoid MRI in the first trimester of pregnancy. The use of MRI during pregnancy is indicated in cases where other non-ionizing methods of diagnostic imaging do not provide satisfactory information. An MRI scan requires more patient involvement than computed tomography, since the movements of the patient during the study significantly affect the image quality, so the study of patients with acute pathology, impaired consciousness, spastic conditions, dementia, and children is often difficult.

[21], [22], [23], [24], [25], [26]