MRI or MRI
What is MRI?
There magnetic resonance (RM), also called nuclear magnetic resonance (NMR), it is an advanced imaging method, very expensive and with long execution times, which vary depending on the part of the body examined. It does not use ionizing radiation, but magnetic fields and radio waves. The principles of operation of magnetic resonance are extremely more complex than the other main imaging methods and will be treated here in a simplified way, in order to make them more understandable.
How does mangetic resonance work? Physical principles
MRI bases its functioning on the analysis of the hydrogen atoms of our body. Under normal conditions, all hydrogen atoms in the human body rotate with their central proton in a random fashion, generating a small magnetic field.
In magnetic resonance, using an external magnetic field, the magnetic field of the body's hydrogen atoms is influenced. By doing so, all atoms in random rotation assume a rotation in the same direction as the external magnetic field.
At this point, energy is supplied to the system, disturbing the magnetic field with radio frequencies. Once the body's exposure to radiofrequency is over, the protons thus stimulated release the acquired energy in the form of a electromagnetic signal, which after being captured by special detectors, forms the image.
The resulting image is in black and white. In resonance the tissues that are colored in white are called hyperintense, while the black tissues are hypointense. These characteristics do not depend on the density of the tissues, but on the time taken to return energy after the cessation of radiofrequency (T1) and after the cessation of exposure to the magnetic field (T2).
Based on the above times, each MRI consists of at least two complementary image sequences: T1-weighted sequences and T2-weighted sequences. The differences are related to the signal intensity of the tissues under examination:
- In T1 sequences, fluids (urine, cerebrospinal fluid) appear black, fat white, muscle gray, and inflammatory fluid black.
- In T2 sequences, fluids (urine, cerebrospinal fluid) appear white, fat light gray, muscles dark gray, and inflammatory fluid white.
The use of the two types of sequences allows to better differentiate the tissues under examination. To distinguish a T1 sequence from a T2 sequence, the Cephalorachidian fluid is observed: it will be black in T1 and white in T2.
In addition to these two basic sequences, others have been introduced through computerized processing. The most important example is the FLAIR sequence, a T2 sequence in which the white color of the cerebrospinal fluid is attenuated, while the inflammatory components remain white. It makes it much easier to identify central nervous system diseases. There are other types of sequences obtained in a similar way.
Although the use of several different types of sequences allows the resonance to visualize all the tissues in great detail, sometimes it is still necessary to use the contrast medium for greater definition. The first contrast medium introduced in resonance, still used today, was the gadolinium, a chemical element administered intravenously and which causes the vascular structures and organs to take on a strong hyperintensity. It is a chelated contrast medium, that is, administered in a form linked to substances that facilitate its elimination and reduce its toxicity.
Today there are also hepatospecific contrast agents, that is specific for the study of the liver, which are selectively picked up by hepatic cells. These are gadolinium or manganese-based substances, which make it possible to identify even very small tumor lesions in the liver, and which represent the first choice for the study of liver metastases, tumors or adenomas.
As previously mentioned, the contrast media used in resonance are bound to chelating substances and therefore are considered much less toxic and risky than the contrast media used in CT. The only contraindications to their use are therefore known allergy and severe renal failure, which can cause the appearance of nephrogenic systemic fibrosis as a complication. In a study involving 10608 patients who underwent MRI with contrast, adverse reactions were 32 (0.3%), of these only 3 reactions were severe. The most common adverse reactions were skin rash and urticaria.
The main uses of resonance are:
- The study of the musculoskeletal system. MRI is particularly indicated for joint problems affecting the back (cervical spine and lumbar sacral), shoulder, knee, ankle, the joints most frequently subjected to this examination. Tendon, muscle, ligament and meniscal injuries are recognized. In particular, the lesions of the menisci are not visible with any other method. These exams are essential in traumatology for an accurate diagnosis and preparation for surgery.
- The study of the central nervous system (brain and spinal cord). The brain and other parts of the head are also visible in CT, but with much lower resolution. Brain CT is mainly used in emergencies such as stroke due to its rapidity, but cerebral mangetic resonance (brain MRI) allows a thorough study even if in the face of longer times. It is the best alternative for visualizing brain tumors or other space-occupying masses. At the level of the spinal cord it is ideal for studying possible compressions of the nerves linked to herniated discs.
- In some cases it can be used as a diagnostic complement to ultrasound or mammography, for the study of the breast (mammary MRI), usually in patients at high risk, with a known familiarity of breast diseases or in the case in which the other methods provide clear results.
- It has increasing importance in the study of liver lesions (abdominal magnetic resonance or abdominal MRI), also thanks to hepato-specific contrast media. The study of the liver in resonance reaches such precision that in some cases of typical liver injury, the diagnosis can be made on the basis of the images alone, without further biopsy tests. MRI cholangiography is also performed to study the hepatic biliary tract, in case of stones or tumors that hinder the outflow of bile. Also useful for the study of the pancreas.
- It can be used for the study of the heart (cardiac magnetic resonance or MRI heart), especially together with other methods such as echocardiography and cardio-CT, in case of myocarditis, after heart attack, for congenital disorders or iron overloads
- MRA can be performed to study the body's vessels, to look for stenosis or aneurysms. However, the use of contrast media is necessary.
Functional mangetic resonance
Functional Magnetic Resonance (or fMRI) is a particular type of magnetic resonance> used for clinical and research purposes, which is used in neuroradiology to highlight which brain areas are activated during the execution of a certain task (such as thinking, reading, speak or move a hand). It requires a good dose of collaboration between the doctor and the patient, who will have to perform certain tasks in a coordinated way to carry out the procedure.
Multiparametric mangnetic resonance
It is a special type of magnetic resonance, which exploits the acquisition of dynamic sequences after administration of contrast medium, usually gadolinium. It is an innovative technique as it is functional imaging, similar to PET. In multiparametric resonance the absorption of the contrast medium by the tumor tissues is dynamically studied. This allows to have very high resolution images, and a very high sensitivity. There multiparameter resonance it is nowadays applied more and more in the diagnosis of prostate cancer, because it allows to diagnose smaller and smaller tumors that would escape conventional resonance. It is also useful in planning any interventions and in carrying out ever more precise biopsies.
Comparison with other radiological methods
- Compared to x-ray, resonance allows structures to be viewed with much greater precision, without exposure to ionizing radiation. However, since it is an expensive exam, it is never done in the first instance, unlike radiography, which is a first level exam.
- Compared to ultrasound, resonance allows you to view the structures in greater detail, it is not a method whose effectiveness depends on the operator's experience unlike ultrasound. However, as for radiography, ultrasound is also a first level method, as it is low cost, widely available and quickly executable. Both do not expose the body to ionizing radiation.
- Compared to CT, the precision and detail of the images is similar. However, the two methods have different applications: CT can be readily performed in an emergency, unlike MRI. However CT exposes to ionizing radiation, MRI does not. The resonance undoubtedly allows a better visualization of the musculotendinous apparatus (but not of the bone, better visualized in CT), of the brain and of the spinal cord. For these reasons, the two methods often need to be performed both as complementary.
Contraindications and limitations
Exist risks or contraindications in undergoing an MRI scan? The real contraindications to MRI are few. The major contraindication is the presence of a pacemaker, this is because the interaction of metal objects with the magnetic field of the resonance can be dangerous. For the same reason, other metal bodies such as splinters present in the body following accidents, ink from very old tattoos, joint prostheses or very old means of synthesis can contraindicate the examination.
The most modern prostheses, means of synthesis and mechanical heart valves are made of paramagnetic metal materials (titanium), which do not interact with magnetic fields and are considered safe. In the presence of any implanted prosthesis, clip or electrical or mechanical device, it is always advisable to report this to the radiologist responsible for carrying out the resonance. It should also be noted that the presence of these implants partially alters the image acquired in resonance, therefore the neighboring regions are not examinable due to the appearance of artifacts.
The execution of magnetic resonance with contrast agent it is contraindicated in patients with known allergy or in patients suffering from severe renal insufficiency. MRI is a time-consuming exam that requires the patient to remain still for a long time during image acquisition. Any movement can cause image artifacts that will therefore be of lower quality. There are no contraindications to the execution of magnetic resonance imaging in pregnant women. In pregnant patients the administration of contrast medium should be carefully evaluated, because the safety of gadolinium in pregnancy is not proven. However, if necessary for the health of the mother, the examination can be performed.
The apparatus in which the resonance is performed is a tube closed, this can be problematic in anxious or suffering people claustrophobia. However, there are devices available for the open magnetic resonance which solve this problem, even if they have a lower resolution than the classic closed magnetic resonance. In claustrophobic patients or in all those patients with problems that prevent adequate immobility during the examination, sedation can be proposed to ensure a better result.
Cost of MRI in the public and private sectors
How much does an MRI cost? Regardless of the examinations performed in hospitals or in affiliated structures, the cost of the MRI depends on the declared income: the cost will therefore be 36.15 euros for income less than 36.152 euros, 50 euros for income between 36.153 and 100.000 euros and finally 70 euros for incomes exceeding 100,000 euros.
As for the magnetic resonances performed in private clinics and centers, the cost is variable and is usually between 200 euros and 400 euros.
- Paramagnetic and hepatospecific contrast media Dr. F. Mastrapasqua UO Radiodiagnostic Hospital of Vaio
-Nephrogenic systemic fibrosis and gadolinium, AIFA 2008 bulletin
-Immediate Adverse Reactions to Gadolinium-Based MR Contrast Media: A Retrospective Analysis on 10,608 Examinations, Vincenza Granata, Marco Cascella, Roberta Fusco et al