Definition
Definition
MRI allows the study of certain tissues of the body, by searching for the frequency, causing identical atomic nuclei (protons for example) to enter into resonance.
Generalities
Nuclear magnetic resonance imaging has advantages that conventional scanning or radiography does not have (it does not deliver X-rays), being harmless.
It is also more sensitive to brain lesions , giving better identification of lesions, and of the anatomy of the central nervous system.
However, it has some disadvantages:
- cost.
- time relatively long.
- The lack of identification of calcifications (mineral deposits in the body).
- The difficulties of distinguishing between a edema (liquid collection) and a tumor.
- The impossibility of use in the presence of a metal body (vascular clips, cardiac prostheses).
- Le waiting time.
History
Conceived by Édouard Mills Purcellet and Félix Bloch , who were awarded the 1952 Nobel Prize in Physics for it, the magnetic resonance phenomenon made it possible to obtain images from 1972 onwards, and in particular, medical applications in the 80s.
Classification
MRI-echo-planar :
This is a technique due to recent improvements in gradients, software and high-speed computer processors, which now make it possible to rapidly acquire magnetic resonance images of the brain. With the'MRI. echo planar (EPI), for specialists, rapid gradients are switched, at high speed, in order to generate information, which is useful for constituting a synthetic image.
En spin echo imaging routinely, brain images are usually obtained in 5 to 10 minutes. With this echo-planar MRI technique, all the information that allows an image to be obtained is acquired in 50 to 150 milliseconds. For the whole brain, this is obtained in one to two minutes. Of course, if a higher resolution is desired, the response is longer. This is how echo-planar MRI allows the number ofartifacts, which are related to the movement of the viscera, and to the movement of the patient. This technique makes it possible to obtain perfusion images, diffusion, functional MRI and a kinetic study. An artifact or artifact, is a phenomenon, or artificial structure, whose appearance is linked to the method used during an experiment, in a way fragments of radiological images, useless for understanding and unwanted.
Perfusion-weighted MRI and diffusion-weighted MRI allow for the acquisition of images of early brain lesions following an ischemic event (reduced blood flow to nerve tissue). These techniques can be useful for detecting an infarct as well as an ischemic penumbra.
Diffusion-weighted MRI is a highly sensitive technique that can detect acute strokes that have occurred within the past week. It can also reveal brain lesions or developing abscesses . These lesions appear as reduced diffusion, resulting in a hyperdense signal.
Functional MRI of the brain is an echo-planar MRI technique that allows the localization of one or more areas of the brain that are activated while the patient performs certain tasks. The activity of neurons leads to an increased flow of arterial blood to specific regions of the cerebral cortex . This results in a relatively slight modification of the balance between oxyhemoglobin and deoxyhemoglobin . This is how functional MRI has made it possible to differentiate the somatosensory cortex from the auditory cortex . Functional MRI is also used in neuroscience research. Indeed, it allows the localization of certain brain functions to be determined.
Medical exam
Technical
To undergo this examination, the patient must remove any metal objects (watch, bra, etc.) that could interfere with the procedure. Makeup and certain cosmetics are also prohibited, and dental fillings can cause distorted images.
Once the patient is lying on a table, he is slid into a tunnel open at both ends and located in the center of an intense electromagnetic field , which presents no danger to the patient.
The occurrence of a relatively loud sound indicates to him the emission of radio frequency waves and a microphone system allows him to stay in contact with a doctor.
Sometimes general anesthesia is necessary when the patient has psychological contraindications (phobia, hyperactivity), or when it is a child.
Injection of a product intended to improve the image, not containing iodine (without side effects such as allergies, but nevertheless contraindicated in pregnant women), but gadolinium , is sometimes necessary.
This examination, which lasts about half an hour, or even three quarters of an hour, requires no preparation, hospitalization or prior diet.
Once performed, the patient immediately leaves the radiology center where the examination took place.
Contraindications :
- The metal objects present in the body (vascular clip, pacemaker, etc.) and detected by a prior X-ray formally contraindicate MRI.
- The hip prostheses and the equipment used by the orthopedic surgeon, as well as the dental equipment usual are not a contraindication.
- Patient particularly corpulent.
Cause
Cause
Magnetic resonance imaging provides images of the spine , and the spinal cord contained within it, most often without the need for injection of contrast agent.
The results are better than for myelography.
This technique is particularly used in:
- La multiple sclerosis and other diseases, where there is a change in the nervous tissue of the brain and spinal cord.
- The attacks of the white matter due to a vascular problem (ischemia).
- The birth defects and central nervous system development problems.
- The vascular problems, in this case certain lesions are not visualized with the scanner (small hemorrhages).
- La herniated disc located in the dorsal spine.
- Spinal cord injuries (myopathy).
- The metastases (distant cancer from an initial focus).
- The you die.
- The affections of the myelin (fatty sheath surrounding the nerves in the white matter): demyelination.
- Abscesses of the brain.
- Pathologies secondary to operations on the spinal cord and the spine Overall, MRI allows us to explore the scar fibrosis (tissue changes due to healing) which may lead to further damage.
- Spinal cord malformations (syringomyelia).
- The lipomas (collections containing fat).