Magnetic resonance imaging (MRI) - a method of obtaining tomographic medical images for the study of internal organs and tissues using the phenomenon of nuclear magnetic resonance. The method is based on measuring the electromagnetic response of atomic nuclei in a strong constant magnetic field in response to their excitation by a particular combination of electromagnetic waves. In MRI such nuclei are nuclei of hydrogen atoms, which are present in great quantities in the human body as part of water and other substances.
MRI does not use X-rays or ionizing radiation, which distinguishes it from computed tomography (CT) and positron emission tomography. Compared to CT scans, MRI procedures often take longer and are noisier, and usually require the subject to be in a narrow tunnel. Also, people with certain medical implants or other non-removable metal inside the body may not be able to safely undergo an MRI.
History
The founding year of magnetic resonance imaging (MRI) is thought to be 1973, when chemistry professor Paul Lauterbur published an article in Nature, "Imaging by induced local interaction; examples based on magnetic resonance." Later Peter Mansfield improved the mathematical algorithms for image acquisition. In 2003, both researchers were awarded the Nobel Prize in Physiology or Medicine for their discoveries concerning the MRI method. However, the awarding of this prize was accompanied by a scandal, as has been the case on several occasions, over the authorship of the discovery.
Raymond Damadyan, an American scientist of Armenian origin, one of the first researchers of MRI principles, a patent holder of the MRI and the creator of the first commercial MRI scanner, also made a famous contribution to the creation of magnetic resonance imaging. In 1971, he published his idea called "Tumor Detection by Nuclear Magnetic Resonance." It is reported that he invented the MRI device itself. In addition, back in 1960, the USSR inventor V.A. Ivanov sent an invention application to the Committee on Inventions and Discoveries, which, according to specialists, in the early 2000's outlined the principles of the MRI method in detail. However, the certificate of authorship "Method of determining the internal structure of material objects" № 1112266 for this application with preservation of the priority date of its filing was issued to V.A. Ivanov only in 1984.
The phenomenon of nuclear magnetic resonance (NMR) used in the MRI method has been known since 1938. Originally the term NMR tomography was used, which after the Chernobyl accident in 1986 was replaced by MRI due to the development of radiophobia in humans. The new name dropped the reference to the "nuclear" origin of the method, which allowed it to enter everyday medical practice, but the original name is still used.
CT scans can visualize the brain, spinal cord and other internal organs with high quality. Modern MRI technologies make it possible to non-invasively (without interference) study the functioning of organs - to measure the speed of blood flow, cerebrospinal fluid flow, to determine the level of diffusion in tissues, to see the activation of the cerebral cortex during the functioning of organs for which a given cortical area is responsible (functional magnetic resonance imaging - fMRI).
Method
The nuclear magnetic resonance method allows studying the human body based on the saturation of body tissues with hydrogen and peculiarities of their magnetic properties associated with being surrounded by different atoms and molecules. The hydrogen nucleus consists of a single proton, which has a spin and changes its spatial orientation in a powerful magnetic field, as well as under the influence of additional fields, called gradient fields, and external radiofrequency pulses supplied at a proton-specific resonance frequency in a given magnetic field. Based on the parameters of the proton (spins) and their vector directions, which can only be in two opposite phases, as well as their binding to the proton's magnetic moment, it is possible to determine in which tissues a particular hydrogen atom is located. Sometimes MR contrasts[en] based on gadolinium or iron oxides can also be used.
If a proton is placed in an external magnetic field, its magnetic moment will be either co-directed or oppositely directed to the magnetic field, and in the second case its energy will be higher. When electromagnetic radiation of a certain frequency is applied to the studied area, some of the protons will change their magnetic moment to the opposite one, and then return to their original position. At the same time the tomograph's data collection system registers energy release during the relaxation of pre-excited protons.
The first tomographs had a magnetic field induction of 0.005 Tesla, and the quality of images obtained with them was low. Modern tomographs have powerful sources of strong magnetic field. Both electromagnets (usually up to 1-3 Tesla, in some cases up to 9.4 Tesla) and permanent magnets (up to 0.7 Tesla) are used as such sources. In this case, since the field must be very strong, superconducting electromagnets operating in liquid helium are used, and permanent magnets are suitable only for very powerful, neodymium. The magnetic resonance "response" of tissues in MRI scanners with permanent magnets is weaker than that of electromagnets, so the field of application of permanent magnets is limited. However, permanent magnets can be a so-called "open" configuration, which allows for studies in motion, in a standing position, as well as physician access to the patient during the study and carrying out manipulations (diagnostic, therapeutic) under MRI control - the so-called interventional MRI.
To locate the signal in space, in addition to the permanent magnet in the MRI scanner, which can be an electromagnet or a permanent magnet, gradient coils are used, which add a gradient magnetic disturbance to the overall homogeneous magnetic field. This provides localization of the nuclear magnetic resonance signal and accurate correlation of the investigated area and the obtained data. The action of the gradient, which provides a choice of slice, provides selective excitation of protons precisely in the desired region. The power and speed of gradient amplifiers is one of the most important indicators of magnetic resonance imaging. The speed, resolution and signal-to-noise ratio largely depend on them.
Modern technology and the introduction of computer technology have led to the emergence of such a method as virtual endoscopy, which allows three-dimensional modeling of structures visualized by CT or MRI. This method is informative when endoscopic examination is impossible, for example, in case of severe pathology of cardiovascular and respiratory systems. Virtual endoscopy method is used in angiology, oncology, urology and other fields of medicine.
The results of the study are stored in the medical institution in DICOM format and can be transmitted to the patient or used to study the dynamics of treatment.
Before and during the MRI procedure
Before the scan, you must remove all metal objects and check for tattoos and medicated patches. MRI scans usually last up to 20-30 minutes, but may last longer. In particular, abdominal scans take longer than brain scans.
Since MRI scans produce loud noise, ear protection (earplugs or headphones) is mandatory. For some types of studies, an intravenous injection of a contrasting agent is used.
Before scheduling an MRI scan, patients are advised to find out: what information the scan will provide and how it will affect their treatment strategy, whether there are contraindications to MRI, whether contrast will be used and for what purpose. Before starting the procedure: how long the scan will last, where the call button is located, and how the staff can be approached during the scan.
MR diffusion
MR diffusion is a technique for determining the movement of intracellular water molecules in tissues.
Diffusion-weighted imaging
Diffusion-weighted tomography is a technique of magnetic resonance imaging based on recording the rate of movement of radio-labeled protons. It allows characterizing the preservation of cell membranes and the state of intercellular spaces. The initial and most effective application is in the diagnosis of acute ischemic cerebral circulation disorder in the acute and acute stages. It is now actively used in the diagnosis of oncological diseases.
MR perfusion
A method for evaluating the flow of blood through body tissues.
In particular, there are special characteristics indicating velocity and volume blood flow, permeability of vessel walls, activity of venous outflow, as well as other parameters that allow to differentiate healthy and pathologically altered tissues:
Blood flow through brain tissue
Blood flow through liver tissue
The method allows you to determine the degree of ischemia of the brain and other organs.
MR spectroscopy
Magnetic resonance spectroscopy (MRS) is a technique that helps determine biochemical changes in tissues in various diseases by the concentration of certain metabolites. MR spectra reflect the relative content of biologically active substances in a certain tissue area, which characterizes metabolic processes. Metabolic disorders usually occur before clinical manifestations of the disease, so based on MR spectroscopy data it is possible to diagnose diseases at earlier stages of development.
Types of MR spectroscopy:
MR spectroscopy of internal organs (in vivo)
MR spectroscopy of biological fluids (in vitro)
MR angiography
Main article: Magnetic resonance angiography
Magnetic resonance angiography (MRA) is a method of imaging the lumen of blood vessels using a magnetic resonance imager. The method allows assessing both anatomical and functional features of blood flow. MRA is based on distinguishing the signal from the moving protons (blood) from the surrounding immobile tissues, which allows obtaining images of vessels without using any contrast agents - non-contrast angiography (phase-contrast MRA and time-of-flight MRA). Special contrast agents based on paramagnetics (gadolinium) are used to obtain clearer images.
Functional MRI
Functional MRI (fMRI) is a method of mapping the cerebral cortex that allows the individual location and features of brain areas responsible for movement, speech, vision, memory, and other functions to be determined individually for each patient. The essence of the method is that when certain parts of the brain work, blood flow in them increases. During the FMRI process, the patient is asked to perform certain tasks, the areas of the brain with increased blood flow are registered, and their image is superimposed on the regular brain MRI.
Spine MRI with verticalization (axial loading)
The technique for examining the lumbosacral spine is an MRI with verticalization. The procedure is based on the traditional MRI examination of the spine in prone position followed by vertical positioning (lifting) of the patient with the MRI table and magnet. In doing so, gravity begins to act on the spine, and the adjacent vertebrae can shift relative to each other and the herniated disc becomes more pronounced. Also, this method of examination is used by neurosurgeons to determine the level of instability of the spine in order to provide the most reliable fixation. The only place in Russia where this examination has been performed so far.
Temperature measurement with MRI
MRI thermometry is a technique based on obtaining resonance from the hydrogen protons of the object under study. The difference in resonance frequencies gives information about the absolute temperature of tissues. The frequency of the emitted radio waves changes with the heating or cooling of the tissues under study.
This technique increases the informative value of MRI examinations and allows increasing the effectiveness of therapeutic procedures based on selective tissue heating. Local tissue heating is used in the treatment of tumors of different origin.
Electromagnetic compatibility with medical equipment
The combination of the intense magnetic field used in MRI scans and the intense radiofrequency field places extreme demands on the medical equipment used during studies. It must be of a special design and may have additional restrictions on use near the MRI unit.
Contraindications
There are both relative contraindications, in which the study is possible under certain conditions, and absolute contraindications, in which the study is inadmissible:
Absolute contraindications
Cardiac pacemaker installed (changes in the magnetic field can mimic the heart rhythm)
ferromagnetic or electronic middle ear implants
Large metal implants, ferromagnetic splinters
ferromagnetic Ilizarov devices.
Relative contraindications
insulin pumps
nerve stimulators
non-ferromagnetic inner ear implants
prosthetic heart valves (in high-field, suspected dysfunction)
Blood clips (except for cerebral vessels)
decompensated heart failure
The first trimester of pregnancy (not enough evidence has been collected so far to prove the absence of teratogenic effects of magnetic fields, but this method is preferable to x-ray or CT scanning)
claustrophobia (panic attacks while in the tunnel may prevent the examination)
need for physiological monitoring
patient inadequacy
Severe/extremely severe condition of the patient
Tattoos done with metallic dyes (can cause burns)
Dentures and braces, as field heterogeneity artifacts are possible.
Widely used in prosthetics, titanium is not ferromagnetic and is practically safe for MRI; the exception is the presence of tattoos made with dyes based on titanium compounds (for example, based on titanium dioxide).
An additional contraindication for MRI is the presence of cochlear implants - prosthetic inner ear devices. MRI is contraindicated for some types of inner ear prostheses because the cochlear implant has metal parts that contain ferromagnetic materials.
If the MRI is performed with contrast, the following contraindications are added:
Hemolytic anemia;
Individual intolerance to the components that make up the contrast agent;
Chronic renal insufficiency, as in this case the contrast may be retained in the body;
Pregnancy at any stage of pregnancy, as the contrast penetrates the placental barrier, and its effect on the fetus is poorly understood.
