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Limbs are paired body appendages in animals. They are separate from the trunk/head and are equipped with special muscular strength. Limbs are responsible for support, locomotion, grasping, touching, collecting and processing information. In order for the system to work efficiently, it must be fed with useful nutrients, energy and blood. The last are the vessels. These are elastic tubes that are placed throughout the body and move blood around the body. Arteries, arterioles, capillaries, venules, and veins are subject to occasional clogging, blockage, or inflammation. To track unwanted processes, doctors recommend undergoing an MRI. What do you need to know about the scanned area, how exactly is the procedure carried out and how to properly prepare for the study?
General characteristics of the study area
Limbs are appendages of the body that are responsible for motor function. In humans, these include the arms and legs. The functionality of the limbs is possible not only due to muscles, bones, several types of tissues, but also blood vessels. These are elastic tubular formations that permeate all parts of the body and are responsible for moving blood around the body. The scheme of blood flow looks like this: blood enters the organs and tissues through arteries, arterioles, capillaries, and from them it goes to the heart through venules and veins.
Blood vessels are located throughout the body, but each of them differs in tissue composition, diameter and functional features. Let us analyze in more detail the main types of elastic tubes. Arteries are vessels through which blood moves away from the heart. They are characterized by thick walls with muscle, collagen and elastic fibers. Arteries are maximally plastic and can expand / narrow depending on the amount of blood. The blood circulating through the arteries is saturated with oxygen. The only exception is the pulmonary artery, which carries venous blood.
In the extremities, blood circulates through the following arteries: digital, radial, brachial, axillary, dorsal digital, arcuate, anterior / posterior tibial, peroneal, popliteal, femoral.
Arterioles are small arteries that precede the capillaries. The vascular wall of an arteriole consists of a muscle fiber. This allows the vessel to change the size of the lumen and to resist. Capillaries are the smallest blood vessels. There are about 40 billion of them in the human body. Through the capillary walls, oxygen and nutrients pass from the blood into the cells, and carbon dioxide and other waste products pass from the cells into the blood.
Venules are small blood vessels that are responsible for the circulation of blood with an abundance of waste products and a lack of oxygen from the capillaries to the vein. Veins are vessels that carry blood to the heart. The walls of the veins are much thinner than the walls of the arteries, they have fewer elastic elements and muscle fibers.
In the extremities, blood circulates through the following veins: palmar digital, ulnar, lateral subcutaneous, median, medial subcutaneous, axillary, dorsal digital, anterior / posterior tibial, lesser saphenous, popliteal, femoral, greater subcutaneous.
The principle of operation of a magnetic resonance tomograph
MRI uses the response of fluid-filled tissues to a magnetic field. What does it look like from a practical point of view? The human body is largely composed of liquid. One of the constituents of water is the elementary particles protons. If you place a proton in a strong magnetic field, it will emit radio waves. Each wave is unique because it has its own frequency and intensity. These characteristics depend on the location of the proton. For example, the radio wave coming from the shoulder joint will be significantly different from the one that comes from the lower extremities.
How to get this magnetic field? The tomograph itself serves as the source of the magnetic field. Additionally, it is able to capture, process and transform the information received into a three-dimensional image of the scanned area. The tomograph is a whole software and hardware complex, each part of which is responsible for a specific set of commands and tasks.
Most often, a specific affected area is diagnosed, and not all parts of the body – the lower extremities, shoulder / elbow / knee / ankle joint, hands or wrist. Scanning of the whole body is necessary for “check-up” (a comprehensive check of all body systems) or multiple oncological pathologies.
Magnetic resonance angiography
This is one type of MRI that is suitable for scanning blood vessels. Diagnostics is carried out on tomographs, the power indicators of which are above 0,3 T. The advantage of the method lies in its informativeness. The medical staff will have access to both anatomical and functional features of the blood flow, which will facilitate the diagnosis and development of therapy. MRA is based on the difference in the signal of moving protons from surrounding motionless tissues.
To improve the differentiation of blood vessels and increase the information content of the method, contrast agents are used. These are special preparations based on gadolinium, which are injected into the bloodstream before the start of the scan. Gadolinium gradually spreads throughout the body, enhances the magnetic field (since it has a positive magnetic susceptibility) and improves the visualization of blood vessels in the final image.
The decision to conduct a contrast scan is made by the doctor. Before the study, the patient should take allergy tests to exclude individual intolerance to the drug.
Indications/contraindications for MRI
Scanning is necessary to confirm such diagnoses:
- decreased sensitivity of the limb, discoloration (blanching or blue);
- numbness, pain, partial depletion of muscle strength;
- gangrene (necrosis of tissues of the human body, which is accompanied by decay);
- mechanical damage to blood vessels;
- the presence of cancerous growths that develop inside the vessel or compress it;
- pathology of the veins / arteries (for example, arterial obstruction or venous insufficiency);
- aneurysm (protrusion of the arterial or venous wall due to thinning or stretching).
Contraindications to the procedure are divided into several groups – absolute, relative and specific. Let’s analyze each group in more detail. The absolute ones include the presence of prostheses, stimulants, implants and devices, which include metallized substances. They can affect the magnetic field, distorting the final result, so MRI is unacceptable.
The decision to conduct a diagnosis with relative contraindications is made by the attending physician. These include:
- pregnancy (in the early stages of pregnancy, the baby is advised to protect it from any impact, including the magnetic field);
- hemostatic clips;
- claustrophobia, inappropriate behavior of the patient or mental deviations (the person simply will not be able to follow the instructions of the laboratory assistant and disrupt the course of the study);
- temporary deterioration of the condition (fever or persistent vomiting can also cause an MRI to be postponed or refused. Be sure to tell your doctor about this);
- tattoos (provided that the ink contains metallic elements);
- braces and dentures.
For contrast MRI, a separate group of specific contraindications is distinguished. The introduction of gadolinium is prohibited during pregnancy, regardless of the age / condition of the mother or child. The contrast agent crosses the placental barrier, and its effect on the fetus is still poorly understood. Doctors advise not to risk the baby’s health and choose a more gentle diagnostic method. Also, the introduction of the drug is unacceptable in case of individual intolerance and violations of the functionality of the kidneys. In the first case, the patient will develop a specific allergic reaction, in the second, there will be difficulties with the introduction of gadolinium from the body. Another contraindication to contrast tomography is hemolytic anemia (a rare blood disease).
Preparation and methodology
Before scanning, it is necessary to remove all metal objects and change into loose clothing (the patient brings it on his own or receives it from a medical institution). The laboratory assistant studies the necessary documentation, preliminary diagnosis, anamnesis and the specifics of the scan. After familiarization, he sets up the device, conducts a short briefing and helps the patient to sit on the retractable table of the tomograph. The duration of the scan depends on the area to be scanned. Diagnosis of the lower extremities will take more time than the study of the elbow joint. In general, the scan lasts from 20 to 45 minutes, but it is better to check with the laboratory assistant for information.
The MRI makes a loud noise. In most diagnostic centers, the patient is given earplugs or special headphones that will provide maximum peace and comfort. If the medical documentation mentions contrast, then the laboratory assistant injects gadolinium intradermally into the scanned area.
The contrast agent may cause side effects. Among the possible consequences are nausea, vomiting, fever, dizziness, general malaise. Be sure to notify the technician or treating physician if the condition worsens after the scan.
Once all preparations are completed, the medical officer goes to an adjacent room to perform the scan. From this moment on, communication between the patient and the laboratory assistant is carried out through the signal button (located next to the tomograph) and the speakerphone built into the room. The patient can stop the diagnosis at any time by pressing the alarm button.
As soon as the device is started, the scanning ring will begin to rotate around the scanned area, creating a magnetic field and fixing outgoing radio waves. Information about the intensity and frequency of radio waves enters the computer. Through complex mathematical problems, it is transformed into a layered, and then a three-dimensional image of the limb. The picture is displayed on the screen, printed or transmitted to the patient in electronic format. At the end of the scan, the laboratory assistant returns to the office, learns about the patient’s well-being and asks to wait for the results outside the door. Usually, they take from 20 to 60 minutes to process, depending on the workload of the medical staff. The lab technician reviews the scans, makes a preliminary diagnosis, and redirects the patient back to the primary care physician for more information.
Magnetic resonance imaging is one of the safest and most informative diagnostic methods. The equipment captures even the slightest deviations from the norm and provides a detailed image of the affected area. After the MRI, the patient can immediately start his usual activities, no specific adaptation is required. The magnetic field does not affect the body’s performance in any way, and the risk of side effects is extremely small. The main thing is to undergo an MRI only as prescribed by a doctor and be healthy!