Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI) is a safe and non-invasive scanning technique. Instead of using x rays, the MRI is based on nuclear magnetic resonance. In short, this means that all atoms have a nuclei that have their own resonant frequency. If you disturb them they sing like tuning forks. The different structural components of the brain have atoms with nuclei that have their own unique song. The MRI scan sends a high frequency alternating magnetic field through the brain, via electromagnets that surround the brain, thereby disturbing the various nuclei. The magnetic sensors in the scanner pick up the activity of the nuclei.

A computer then generates a two or three dimensional image of the brain. This detailed picture of brain structures (not function) helps physicians locate possible causes of seizures and identify areas that may generate seizures. No x-rays or radioactive material are used, therefore this procedure is not known to be harmful.

An MRI offers doctors the best chance of finding the source of seizures. Because epilepsy can arise from scar tissue in the brain, an MRI can show scar tissue and allow doctors to determine the nature of it. The images produced from the MRI are extremely precise. The information provided by MRI is valuable in the diagnosis and treatment of individuals with epilepsy and to determine whether surgery would be beneficial.


Before an MRI

• If you will be given a sedative, you should not eat or drink approximately 4 hours prior to the exam.
   
• Take all prescribed medication(s) as usual.
   
• Wear loose, comfortable clothing.
   
• Remove all metallic devices (watches, jewellery, hairpins, glasses, hearing aids, removable dental work) before the examination. Metallic devices can cause a bright or blank spot on the picture.
   
• Do not take any credit or bank cards with you: the scanner will erase the information recorded on the magnetic strip.
   
• It will be much easier for you to relax during the examination if you avoid caffeine drinks before the test.
   
• If you are claustrophobic (fear of being in closed-in spaces), be sure to inform your doctor before taking the test.
   

During an MRI

• You will be asked to provide the technologist about your medical history.
   
• You will lie down on a cushioned table that will slide towards the machine so the head is inside its circular opening. You will need to lie flat and still as you move through a narrow cylinder.
   
• You may have someone stay with you during the procedure because there are no known risks to those in the room with the machine.
   
• You will be able to speak to the technologist during the examination via an intercom.
   
• You should remain still, relax and breath normally as the images are taken. Any movement during this time will blur the images.
   
• You will not feel anything during the examination but may hear thumping sounds from the MRI equipment. These noises may be a little unsettling at times, but they are a normal part of the MRI procedure.
   
• In some cases, your physician may request an MRA (Magnetic Resonance Angiography) in addition to your MRI exam. This exam provides an analysis of your vascular system and major blood vessels. Usually, the MRI and MRA are completed in the same visit.
   
• The MRI procedure usually lasts anywhere from 30 to 90 minutes.
   

After an MRI

• There are no harmful side effects after the examination and you may return to your normal activities and diet immediately after the scan.
   
• If a sedative is administered, you will be monitored until the effects of the sedative subsides. You will then need a friend or relative to drive you home.
   
• The MRI scan will be reviewed by the radiologist who will send a report to your physician.
   
• After the physician receives the results of the test, they will confirm a diagnosis or prescribe further tests and treatments.
   

Personnel

MRI Technologist
• operates the MRI machines using giant magnets and radio waves to create an image;
• takes patients' medical histories;
• prepares patient for the MRI examination;
• prepares data for doctors to interpret.
   
Radiologist
• interprets images and sends report to the referring physician.
   
Doctor
• interprets the results.
   

Payment

An MRI examination is covered by the Ontario Health Insurance Plan (OHIP).
 

Equipment

The MRI machine is a cylindrical magnet in which the patient must lie still. They may feel claustrophobic. There are new MRI systems that are wider and shorter and do not fully enclose the patient. Some newer units are open on all sides, however the image quality may vary.
 

Additional Types of MRIs

A Functional MRI (fMRI) is a non-invasive technique that provides both an anatomical and functional view of the brain. Similar to the MRI, fMRI uses magnetic fields instead of x-rays to produce detailed pictures of the brain. This technique allows us to localize specific areas of brain function by imaging patients while they perform specific tasks. Therefore, functional MRI can identify regions of the brain that are active during cognitive, sensory, and other tasks by detecting changes in blood flow to particular areas of the brain. This information is often very useful to the neurosurgeon; it helps physicians identify the exact location of the source of the seizures.

The advantage of using an fMRI is that it can measure blood flow without using radioactive tracers. Instead, fMRI takes advantage of the fact that haemoglobin, an oxygen carrying molecule in the blood, contains an iron molecule which has magnetic properties. When a magnetic field is presented to the brain, the haemoglobin molecules line up, like tiny magnets. fMRI indicates the presence of brain activity because the haemoglobin molecules in areas of high brain activity lose some of the oxygen they are transporting. This makes the haemoglobin more magnetic, thereby responding more strongly to the magnetic field. The fMRI machine determines the relative activity of various areas of the brain by detecting changes in the magnetic response of haemoglobin.

Advantages of fMRI:

• It can look at discrete areas of brain activation
• The final image depicts more detail than CT scans
• It can measure fast-changing physiology better then the PET scan.

Magnetic Resonance Spectroscopy (MRS) gives information about the chemical and physiological information about certain structures in the brain.

The "open" MRI is a new design with an extra large opening. This allows more airflow and an open view around you during the examination. This open design is good for those who are claustrophobic because it allows you to feel less enclosed and confined, thus, minimizing anxiety. Because of the open view, this system provides enough room to image patients weighing over 500 pounds. In addition, the "open" MRI allows you to be accompanied throughout the examination by a family or staff member.
 

Important Considerations

• Because the strong magnetic field used for MRI will pull on any ferromagnetic metal object implanted in the body, you should notify the doctor or nurse prior to the examination if you have any of the following metal implants or objects:
  • aneurysm clips
  • artificial heart valve
  • bullets
  • cardiac pacemaker
  • intrauterine device (IUD)
  • eye/orbital prosthesis
  • hip or knee prosthesis
  • insulin pump implant
  • intracranial bypass graft clips
  • ear implants
  • shrapnel
  • sternal wire
  • sutures
  • tantalum mesh
  • neurostimulators (vagus nerve stimulation device).
     
• If you have ever been a metal worker, you may be required to have your eyes X-rayed before having the MRI scan.
   
• You should not have an MRI scan if you are, or suspect you may be pregnant.
   
• The strong static magnetic field, which interferes with the proper function of the usual life-support equipment, and the small bone of the magnets make it difficult or impossible to examine some critically ill patients.
   
• An MRI scan cannot be performed on uncooperative patients because it is essential that the patient lie still.
   
• An MRI scan cannot be carried out on claustrophobic patients. However, there are MRI systems with a more open design for patients who are claustrophobic. In this system, the individual can see all around them throughout the exam. These scanners increase comfort levels of patients, but are subject to more background interference in the images produced.
   
• Fillings in your teeth, dental braces, and permanent bridges may distort images of the facial area or brain, but will not cause harm to you or the MRI equipment.
   
• A small number of people have experienced skin irritation, swelling, or heating sensations at the site of any type of permanent colourings (tattooing, cosmetic applications such as eyeliner, lip-liner, lip colouring, etc.). The reason for this is that certain ferrous pigments used in tattoos and permanent colouring can interact with the electromagnetic fields used in the MRI procedure. The patient might feel some discomfort or a burning feeling on their skin. Large or very dark tattoos can cause "artifacts" or false shadows to appear on the image. Therefore the patient should inform the technologist, or doctor of any unusual sensations in area of the tattoo or permanent colouring during the examination.
   
• Individuals who are obese may be to big to be scanned.

To submit questions, comments, or suggestions please click here.

Last Modified: 07/17/2006 11:19:12 AM