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Magnetic Resonance Imaging (MRI) Study Principle

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Magnetic resonance imaging (MRI examination)

MRI (magnetic resonance imaging) is an imaging technique using a force magnet and radio waves, with the help of which an image of organs (brain, cerebral vessels, spine, spinal cord, joints, heart, abdominal and pelvic organs, etc.), etc.). This technique of magnetic resonance imaging is based on the initial position that tiny particles exist in the tissues of the body, the direction of movement of which can be changed in a magnetic field. During the MRI scan, the patient does not receive radiation exposure (radiation exposure).

The advent of MRI scanning has had as revolutionary an impact on neurological and neurosurgical practice as CT did in the 1970s.

An apparatus for diagnostics of magnetic resonance imaging (MRI studies).

Most often, magnetic resonance imaging machines use a magnetic field, the strength of which is 0.15 T (Tesla). Each Tesla is equal to 10,000 G (for comparison: the strength of the Earth's magnetic field is 0.6 G). The radiofrequency field in MRI is not static, it rotates in a projection directly to the body at a speed equal to the frequency of radio waves.

Our patients are invited to undergo an MRI scan using a device with a magnetic field of 3.0 T (Tesla). It is also possible to conduct MRI with intravenous contrast (Omniscan contrast) to increase the visual difference between healthy tissue and tumor. Weight restriction (for a patient with a large weight) during magnetic resonance imaging - up to 200 kg.

 

Magnetic resonance imaging procedure

During the MRI procedure, the patient should wear clothes that do not contain metal fasteners, belts, or buttons. If there are metal implants in the patient's body (clips for aneurysms, joint prostheses, plates and screws for osteosynthesis, pacemakers, and neurostimulators), MRI is also not possible and not safe. In the presence of metal, the resulting image will be blurred and there is a risk of displacement (migration) of the implants in the body. Recently, medical device manufacturers have begun to produce the above-mentioned components implanted into the patient's body using special alloys that are not affected by the magnetic field of the MRI machine. The manufacturer specifies the product's compatibility with the MRI diagnostic procedure separately in the accompanying documentation.

The patient is placed on the sliding table of the MRI machine, which then automatically moves with it to the center of the scanner to start the study.

Some types of MRI require the use of a special drug - contrast. The contrast agent is administered to the patient through a vein in the elbow or hand. Contrast in MRI, accumulating in altered tissues, enhances the resulting image. This gives the most detailed picture of the state of the organ or tissue being examined for the radiologist, who will describe the images obtained on the MRI.

Intravenous Gadolinium Contrast is a non-ionic paramagnetic contrast agent for magnetic resonance imaging. The paramagnetic properties of gadolinium determine the contrast enhancement during MRI. Intravenous administration of the drug (contrast) causes an increase in the signal from the tissue sites of only those organs that have dysfunction as a result of damage in any pathological process. This provides more informational content of the obtained images in comparison with the MRI data of the same organ without the use of contrast enhancement.

The radiologist and his assistant during the MRI procedure are in a separate room, observing through the glass the operation of the device and the patient's condition. The duration of an MRI study takes from 30 to 60 minutes, depending on the volume of the planned study in a given patient. More about some types of magnetic resonance imaging by department and organ:

Magnetic resonance imaging of the brain and cerebral vessels (MRI angiography) on a side section.
Magnetic resonance imaging of the brain and cerebral vessels (MRI angiography) on a side section.

 

Preparing the patient for the magnetic resonance imaging procedure

Before the scheduled examination, the patient is asked not to eat or drink fluids 4-6 hours before the start of the MRI procedure.

It is necessary to clarify in advance whether the patient sent for an MRI examination has a fear of a closed space (claustrophobia). In such a case, he will need to take a sedative in advance, which will cause slight drowsiness and relieve anxiety. There is also an "open" type of MRI machine ("open MRI", "open-loop MRI"), in which there are free spaces on the sides, which does not scare the patient so much. The MRI unit installed in the radiology department has a large space with an aperture of 70 cm, which leads to a decrease in the number of patients who need preliminary sedation during the examination.

Before performing an MRI procedure, the patient should be excluded:

  • the presence of clips for aneurysm of the arteries of the brain
  • the presence of intravascular stents
  • the presence of artificial metal heart valves
  • cardiac defibrillators and pacemakers
  • neurostimulators of the brain and spinal cord
  • cochlear implants in the inner ear
  • kidney disease, dialysis (risk of toxic effects from intravenous contrast)
  • the presence of artificial joints
  • the presence of metal fragments in the body after accidents, explosions, injuries, etc.

The MRI device installed in the department of radiation diagnostics has a strong magnetic field - 3 Tesla, therefore, the room where the device is installed should be avoided from:

  • pens, pocket knives, spectacles with metal frame;
  • metal jewelry, watches, bank credit cards and passes, hearing aids;
  • metal hairpins, safety pins, clasps and zippers;
  • removable dentures (removed before the tomography).

 

Tolerability of the magnetic resonance imaging procedure

The MRI procedure itself does not cause any physical pain or discomfort in the patient. In the case when the patient cannot lie still in the MRI machine (children) or is nervous during the examination, a short-acting sedative drug can be used to help him calm down and relax during the procedure. The movements of the patient's body during the MRI scan will lubricate the images received by the apparatus, introducing errors in the diagnosis.

Magnetic resonance imaging of the lumbosacral spine, side view of the spine.

The surface of the MRI scanner table may be too cool and hard for some patients, so they can be covered during the scan with a blanket or a pillow under their head (if this does not interfere with the main examination).

An intercom built into the MRI machine allows the patient to communicate during the diagnostic procedure with the medical staff located in the next room. Some MRI tomographs also have built-in video monitors and headphones, which allows the patient to be distracted while he is in the tomograph during the scanning process.

After the completion of the MRI procedure, the patient can continue their normal life. He will not have any restriction in diet, physical activity, except for those cases when during the MRI scan he was sedated.

 

Indications for magnetic resonance imaging

MRI allows specialists to obtain an image of the patient's organs and tissues (brain, cerebral vessels, spine, joints, abdominal and pelvic organs, heart, bronchi, etc.) to assess their functional state or identify organic changes that have occurred in them.

MRI is performed in patients in the following cases:

  • as an alternative to CT angiography to avoid radiation exposure to the patient's body
  • to clarify the already available data obtained during previous radiography or computed tomography
  • diagnostics of pathological growth of tissues of some organs and tissues
  • assessment of the state of blood flow (veins, arteries, sinuses, heart)
  • assessment of the condition of the lymph nodes
  • evaluation of organs in three-dimensional reconstruction from different angles
  • detection of the stage of cancer - the spread of cells from the tumor to other parts of the body (metastases)
  • determining the stage of cancer development allows you to choose an adequate method of treatment and make a prognosis in the future condition of a patient with oncology

In the case of the norm, the examined person on MRI will not reveal any pathological changes. The obtained data of MRI results in the presence of visible changes are further analyzed by the patient's attending physician, who develops a plan for the necessary additional examination and treatment.

MRI of the hip joint (acetabulum, articular cartilage) with coxarthrosis.

 

Potential risks and complications during magnetic resonance imaging

The MRI machine does not use X-rays in its work, unlike CT tomographs. Until now, there have not been any references and reports in studies from doctors about the possible side effects of an MRI tomograph on the body as a result of its use. Most types of contrast or contrast media contain gadolinium. Intravenous contrast is safe for MRI. Allergic reactions to an MRI drug administered through a vein are extremely rare in patients. But gadolinium can be harmful to patients with impaired kidney function, through which it is excreted from the body, or in patients on dialysis to cleanse the blood plasma. If the patient has renal insufficiency, caution should be exercised in prescribing such studies of the chest organs on an MRI machine with contrast enhancement. The strong magnetic field generated by the MRI machine can damage the operation of a pacemaker or other type of electronic implant in the patient's body. Also exposed to the magnetic field are metal fragments and plates in the body, which can move and shift. Carrying out an examination of organs on an MRI machine to a patient may not be beneficial for several reasons:

  • the high cost of MRI examination (in comparison with CT tomography)
  • the long duration of the MRI scan process (in comparison with CT tomography)
  • high sensitivity to the movement of internal organs and the whole-body during MRI tomography (in comparison with CT tomography)