Magnetic Resonance Imaging (MRI)

August 8, 2011

Magnetic Resonance Imaging (MRI) is a safe and non-invasive scanning technique. Instead of using X-rays, MRI is based on nuclear magnetic resonance. This means all atoms have nuclei with 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 surrounding 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 brain functions) helps physicians locate possible causes of seizures and identify areas that may generate seizures. No X-rays or radioactive materials are used. As such, this procedure is not known to be harmful.

MRI offers doctors the best chance of finding the source of seizures. Because epilepsy can arise from scar tissue in the brain, MRI can show scar tissue and allow doctors to determine the nature of it. The images produced from 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 MRIExpand Before an MRI Section

  • If you will be taking a sedative for the test, you should not eat or drink approximately four hours prior to the exam.
  • Take all prescribed medication(s) as usual.
  • Wear loose, comfortable clothing.
  • Remove all metallic devices (e.g. watches, jewellery, hairpins, glasses, hearing aids, removable dental work) before the examination. (See Important Considerations at the bottom of the page.) 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.
  • Avoid caffeine drinks to make it easier for you to relax during the examination.
  • If you are claustrophobic (afraid of being in enclosed spaces), be sure to inform your doctor before taking the test.

During an MRIExpand During an MRI Section

  • The technologist will ask you to provide your medical history.
    • You will lie down on a cushioned table that slides towards the machine so your head is inside its circular opening.
  • You will need to lie flat and still as you move through a narrow cylinder.
  • You should remain still, relax and breathe normally as the images are taken. Any movement during this time will blur the images.
  • 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 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 MRIExpand After an MRI Section

If you have taken a sedative for the test, you will be monitored until the effects of the sedative subside. Ask a friend or relative to drive you home from the test. There are no harmful side effects after the examination. You may return to your normal activities and diet immediately after the scan.

The radiologist will review the MRI scan and send a report to your physician. After the physician receives the results of the test, s/he will confirm a diagnosis or prescribe further tests and treatments.

PersonnelExpand Personnel Section

MRI technologists

  • operate the MRI machines using giant magnets and radio waves to create an image
  • take down patients’ medical histories
  • prepare patients for the MRI examination
  • prepare data for doctors to interpret


  • interpret images
  • send reports to the referring physician


  • interpret the results

PaymentExpand Payment Section

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

EquipmentExpand Equipment Section

The MRI machine is a cylindrical magnet in which the patient must lie still. For patients that 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, but the image quality may vary.

Additional Types of MRIs

Functional MRI (fMRI)Expand Functional MRI (fMRI) Section

An fMRI is a non-invasive technique that provides both an anatomical and functional view of the brain. Similar to the MRI, the 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.

An fMRI can identify regions of the brain that are active during cognitive, sensory, and other tasks by detecting changes in the flow of blood 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.

One benefit of using an fMRI is it can still measure blood flow without using radioactive tracers. Instead, the fMRI takes advantage of an iron molecule with magnetic properties contained in hemoglobin, an oxygen-carrying molecule in the blood. When a magnetic field is presented to the brain, the hemoglobin molecules line up like tiny magnets.

An fMRI indicates the presence of brain activity because hemoglobin molecules in areas of high activity lose some of the oxygen they are transporting. This makes the hemoglobin 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 hemoglobin.

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)Expand Magnetic Resonance Spectroscopy (MRS) Section

The MRS gives information about the chemical and physiological information about certain structures in the brain.

Open MRIExpand Open MRI Section

The “open” MRI is a new design with an extra large opening. This allows more airflow and an open view around the patient during the examination, an open design that minimizes anxiety for those who are claustrophobic. These scanners increase comfort levels of patients, but are subject to more background interference in the images produced.

Because of the open view, this system provides enough room to image patients weighing more than 500 pounds (about 227 kilograms). In addition, the “open” MRI allows a family or staff member to accompany patients throughout the examination.

Important Considerations

The strong magnetic field used for MRI will pull on any ferromagnetic metal object implanted in the body. If you have ever been a metal worker, you may be required to have your eyes X-rayed before having the MRI scan. 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.

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,
  • ear implants,
  • eye/orbital prosthesis,
  • hip or knee prosthesis,
  • insulin pump implant,
  • intracranial bypass graft clips,
  • intrauterine device (IUD),
  • neurostimulators (vagus nerve stimulation device),
  • shrapnel,
  • sternal wire,
  • sutures, or
  • tantalum mesh.

A small number of people have experienced skin irritation, swelling, discomfort or burning/heating sensations at the site of any type of permanent colourings (tattooing, cosmetic applications such as eyeliner, lip-liner, lip colouring, etc.). Certain ferrous pigments used in tattoos and permanent colouring can interact with the electromagnetic fields used in the MRI procedure. Large or very dark tattoos can cause “artefacts” or false shadows to appear on the image. Inform the technologist or doctor of any unusual sensations in area of the tattoo or permanent colouring during the examination.

Patients may not have an MRI scan if they are:

  • pregnant (whether suspected or confirmed)
  • critically ill (The strong static magnetic field interferes with the proper function of the usual life-support equipment, which may make it difficult or impossible to examine some critically ill patients.
  • uncooperative (It is essential that the patient lie still.)

Consider “open” MRI systems for:

  • claustrophobic patients
  • obese patients who may not fit in the scanner

Click here to learn about other types of diagnostic tests.

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