The magnetic resonance (MRI) is the neuroimaging technique most used in neurosciences because of its many advantages, the main ones being that it is a non-invasive technique and it is the magnetic resonance technique with higher spatial resolution.
Being a non-invasive technique, it is not necessary to open any wound to perform it and is also painless. Its spatial resolution allows to identify structures to the millimeter, also has a good temporal resolution, inferior to the second, although this one is not as good as the one of other techniques, like the electroencephalography (EEG).
Its high spatial resolution allows to investigate aspects and morphological characteristics at the tissue level. Like metabolism, blood volume or hemodynamics.
This technique is considered harmless, that is to say, it does not produce any damage in the body of the person to whom it is performed, therefore it is also painless. Although the participant must be introduced in a magnetic field, this does not pose a risk to the individual, since this field is very small, usually equal to or less than 3 teslas (3 T).
But not all are advantages, MRI is a difficult technique to perform and analyze, so professionals must perform a previous training. In addition, quite expensive facilities and machinery are needed, therefore, it has a high spatial and economic cost.
Being such a complex technique, a multidisciplinary team is needed to use it. This team usually includes a physicist, someone who knows physiopathology (such as a neuroradiologist) and someone who designs the experiments, for example, a neuropsychologist.
This article will explain above the physical basis of magnetic resonance, but will focus mainly on the psychophysiological basis and practical information for those who have to perform an MRI.
Psychophysiological bases of magnetic resonance
Brain functioning is based on the exchange of information through chemical and electrical synapses.
In order to carry out this activity, it is necessary that it be consumed, and the energy consumption is carried out by means of a complex metabolic process which, in short, results in an increase of a substance denominated Adenosine triphosphate , Better known as ATP, which is the energy source that the brain uses to function.
ATP is made from the oxidation of glucose , Therefore, for the brain to function requires oxygen and glucose. To give you an idea, a brain at rest consumes 60% of all the glucose we consume, approximately 120 g. So if the supply of glucose or oxygen was interrupted the brain would suffer damage.
These substances reach the neurons that require them through blood perfusion, through the capillary beds. Therefore, increased brain activity, increased need for glucose and oxygen, and increased localized cerebral blood flow.
So to check which zone of the brain is active, we can look at the consumption of oxygen or glucose, the increase of regional brain flow and changes in cerebral blood volume.
The type of indicator to be used will depend on multiple factors, among which are the characteristics of the task to be performed.
Several studies have found that when brain stimulation proceeds long, the first changes to be observed are glucose and oxygen, then there is an increase in regional brain flow, and if stimulation continues, there will be an increase Of total brain volume (Clarke & Sokoloff, 1994; Gross, Sposito, Pettersen, Panton, & Fenstermacher, 1987; Klein, Kuschinsky, Schrock, & Vetterlein, 1986).
Oxygen is transported through the cerebral blood vessels attached to hemoglobin. When hemoglobin contains oxygen it is called oxyhemoglobin and when it is left without it, deoxyhemoglobin. So when the cerebral activation begins, there is a localized increase in oxyhemoglobin and a decrease in deoxyhemoglobin.
This balance produces a magnetic change in the brain that is reflected in the MR images.
As is known, intravascular oxygen is transported bound to hemoglobin. When this protein is full of oxygen, it is called oxyhemoglobin and when it is released it is transformed into deoxyhemoglobin.
During cerebral activation there will be a locoregional increase of arterial and capillary oxyhemoglobin; however, the concentration of deoxyhemoglobin will decrease due, as explained above, to the decrease in tissue oxygen transport.
This drop in concentration of deoxyhemoglobin, due to its paramagnetic property, will cause a signal increase in the MR images.
In summary, MRI is based on identifying hemodynamic changes of oxygen in the blood through the BOLD effect, although blood flow levels can also be inferred indirectly through methods such as imaging and perfusion and ASL ( Arterial spin labeling ).
Mechanism of BOLD Effect
The most commonly used MRI technique today is the one based on the BOLD effect. This technique allows the identification of hemodynamic changes due to the magnetic changes produced in hemoglobin (Hb).
This effect is quite complex, but I will try to explain it in the simplest possible way.
If you want to know more I advise you to see the following presentation:
The first to describe this effect were Ogawa and his team. These researchers realized that when Hb contains no oxygen, deoxyhemoglobin, it is Paramagnetic (Attracts magnetic fields), but when fully oxygenated (oxyHb) changes and becomes Diamagnetic (Repulses magnetic fields) (Ogawa, et al., 1992).
When there is a greater presence of deoxyhemoglobin the local magnetic field is altered and the nuclei need less time to return to their original position, so there is a smaller signal T2, and conversely, the more oxyHb the slower the recovery of nuclei And less signal T2 is received.
In summary, the detection of brain activity with the mechanism of the BOLD effect occurs as follows:
- The brain activity of a particular area increases.
- Activated neurons require oxygen, for energy, that they acquire from the neurons around them.
- The area around the active neurons loses oxygen, therefore, at the onset, deoxyhemoglobin increases and T2 decreases.
- At the end of the time (6-7s) the area recovers and increases oxyHb, so T2 increases (between 2 and 3% using magnetic fields of 1.5 T).
Functional Magnetic Resonance
Thanks to the BOLD effect, functional magnetic resonance imaging (MRI) can be performed. Functional magnetic resonance differs from magnetic resonance imaging to dry, in which, in the first, the participant performs an exercise while performing an MRI, so that their brain activity can be measured when performing a function and not just at rest .
The exercises consist of two parts, during the first the participant performs the task and then is allowed to rest during the rest time. The MRI analysis is performed by comparing Voxel To voxel the images received during the accomplishment of the task and in the time of rest.
Therefore, this technique allows to relate the functional activity with the cerebral anatomy with a high precision, something that does not happen with other techniques like the EEG or the magnetoencefalografía.
Although MRI is a fairly accurate technique, it measures brain activity indirectly and there are multiple factors that can interfere with the data obtained and modify the results, whether internal to the patient or external, such as the characteristics of the magnetic field or the postprocessing.
Practical information
This section will explain some information that may be of interest if you have to participate in an MRI study, either patient or healthy control.
MRs can be performed on almost any part of the body, the most common being the abdomen, the cervical, the thorax, the cerebral or cranial, the heart, the lumbar and the pelvic. Here the brain will be explained as it is the closest to my field of study.
How is the test carried out?
MR studies should be carried out in specialized centers and with the necessary facilities, such as hospitals, radiology centers or laboratories.
The first step is dressing properly, you must remove all the metal things you do not interfere with MRI.
He will then be asked to lie on a horizontal surface that is inserted into a kind of tunnel, which is the scanner. Some studies require that you tumble in a certain way, but, usually, it is usually on the top.
While performing the MRI will not be alone, the doctor or the person controlling the machine will be placed in a protected room protected from the magnetic field that usually has a window to see everything that happens in the MRI room. This room also has monitors where the person in charge can observe if everything is going well while performing the MRI.
The test lasts between 30 and 60 minutes, although it can last longer, especially if it is an MRI, in which you will have to perform the exercises that will indicate to you while MRI collects your brain activity.
How to prepare for the test?
When you are told that your MRI test should be done, your doctor should make sure that you do not have any metallic devices in your body that may interfere with MRI such as the following:
- Artificial heart valves.
- Clips for cerebral aneurysm.
- Defibrillator or cardiac pacemaker.
- Implants in the inner ear (cochlear).
- Nephropathy or dialysis.
- Artificial joints recently laid.
- Vascular stents.
In addition, you should tell the doctor if you have worked with metal since you may need a study to examine if there are metal particles in the eyes or nostrils, for example.
You should also notify your doctor if you suffer claustrophobia (Fear of enclosed spaces), because if possible, your doctor will advise you to perform an open MRI, which is more separated from the body. If it is not possible and you are very anxious you may be prescribed anti-anxiety or sleeping pills.
The day of the test should not consume food or drink before the test, approximately 4 or 6 hours before.
You should try to bring the minimum of metallic things to the studio (jewelry, watches, mobile, money, credit card...) as these may interfere with MRI. If you take them you will have to leave them all outside the room where the RM machine is located.
What does it feel like?
The MRI exam is completely painless, but can be a little annoying or uncomfortable.
In the first place, it can provoke anxiety by having to lie so long in an enclosed space. In addition, the machine should be as still as possible because if it can not cause errors in the images. If you are unable to be still so long you may be given some medication to relax you.
Secondly, the machine produces a series of continuous noises that can be annoying, to reduce the sound can take a plug in the ears, always consulting it with your doctor before.
The machine has an intercom with which you can communicate with the person in charge of the examination, so if you feel anything that seems abnormal you can consult it.
You do not need to stay in the hospital, after the test you can go home, eat if you want and do your normal life.
What is it for?
MRI is used, along with other tests or evidence, to make a diagnosis and to evaluate the condition of a person suffering from a disease.
The information to be obtained depends on where the resonance is to be performed. Brain magnetic resonance imaging is useful for detecting brain signs characteristic of the following conditions:
- Congenital brain anomaly
- Bleeding in the brain (subarachnoid or intracranial haemorrhage)
- Infection of the brain
- Brain tumors
- Hormonal disorders (such as acromegaly, galactorrhea and Cushing's syndrome)
- Multiple sclerosis
- Stroke
In addition, it may also be useful to determine the cause of conditions such as:
- Muscle weakness or numbness and tingling
- Changes in thinking or behavior
- Hearing loss
- Headaches when some other symptoms or signs are present
- Difficulty speaking
- Vision problems
- Dementia
Do you have risks?
Magnetic resonance imaging uses magnetic fields and, unlike radiation, has not yet been found in any study that causes any type of damage.
Contrast MR studies, which require dyeing, are usually performed with gadolinium. This dye is very safe and allergic reactions rarely occur, although it can be harmful for people with kidney problems. Therefore, if you have any kidney problems, you should tell your doctor before doing the study.
Magnetic MRI can be dangerous if the person wears metal artifacts such as cardiac pacemakers and implants, as these may not work as well as before.
In addition, a study must be carried out if there is a risk of having metal chips inside your body, as the magnetic field can cause them to move and cause organic or tissue damage.
References
- Álvarez, J., Ríos, M., Hernández, J., Bargalló, N., & Calvo-Merino, B. (2008). Magnetic Resonance I: Functional Magnetic Resonance. In F. Maestú, M. Ríos, & R. Cabestrero, Cognitive processes and techniques (Pages 27-64). [Links]
- Clarke, D., & Sokoloff, L. (1994). Circulation and energy metabolism of the brain. In G. Siegel, & B. Agranoff, Basic Neurochemistry (Pages 645-680). New York: Raven.
- Gross, P., Sposito, N., Pettersen, S., Panton, D., & Fenstermacher, J. (1987). Topography of capillary density, glucose metabolism, and microvascular function within the inferior rat colliculus. J Cereb Blood Flow Metab , 154-160.
- Klein, B., Kuschinsky, W., Schrock, H., & Vetterlein, F. (1986). Interdependence of local capillary density, blood flow, and metabolism in rat brains. Am J Physiol , H1333-H1340.
- Levy, J. (October 22, 2014). Head MRI . Obtained from MedlinePlus.
- Levy, J. (October 22, 2014). MRI . Obtained from MedlinePlus.
- Ogawa, S., Tank, D., Menon, R., Ellermann, J., Kim, S., & Merkle, H. (1992). Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging. Proc Natl Acad Sci U.S.A. , 5951-5955.
- Puigcerver, P. (s.f.). Fundamentals of Magnetic Resonance. Valencia, Comunidad Valenciana, Spain. Retrieved on June 8, 2016.