Credit Valley Hospital Department

Diagnostic Imaging - Magnetic resonance imaging (MRI)

Diagnostic Imaging

MAGNETIC RESONANCE IMAGING - MRI

Magnetic resonance imaging (MRI) is a painless, non-invasive diagnostic imaging technology that uses a strong magnetic field and radio frequency (RF) waves to produce images. The scanner creates a magnetic field and sends RF pulses through the body tissues. The tissue response is then measured and an image is created from this information. During the scan loud noises up to 93 dB can be produced, therefore patients must wear ear plugs or a head set to protect their hearing.

MRI scans can provide information about the brain, spine, joints, organs and vessels. MRI can also illustrate differences in tissues that may not be visible with other imaging modalities. Some MR examinations require an injection of Gadolinium to visualize structures more clearly. Gadolinium has a paramagnetic property that makes it appear bright within a magnetic field. It can be helpful in demonstrating vascular structures within tissues.

The Credit Valley Hospital MRI department is equipped with two Philips 1.5 tesla MRI scanners.

Booking MRI Appointments:

In order to help us provide efficient MRI services please follow the following guidelines.

1. All MRI requisitions must be fully completed with the following:

  • patient’s full name and date of birth,
  • area of interest to be scanned
  • any previous relevant imaging studies (date and facility)

2. All MRI requests must include a completed MRI Information & Screening Form. It is important that the patient both completes and signs this form. All previous surgical information must be recorded on the form. Operative reports may be requested to ensure patient safety prior to the booking of an appointment. An MRI may be contraindicated if the patient has surgically implanted devices.

3. Any patients with a history of exposure to metal fragments in or around the eyes, or exposure to metal dust in the workplace (i.e. welders, metal workers), will require a pre MRI orbital x-ray to ensure there are no metallic foreign objects within the orbit.

4. If a patient suffers from claustrophobia and requires sedation prior to the MRI, the ordering physician must arrange a prescription for the patient prior to their MRI exam. The MRI department does not provide sedation. If a patient is prescribed sedation they must be accompanied home by another adult.

5. All MR Breast Imaging requests require an MR Breast Questionnaire as well as the MRI Information & Screening Form.

MRI Policies:

Post-op Policy
All MRI requests that are within 6-8 weeks of surgery must be approved by the radiologist. Most surgeries are MRI safe but must all be listed on the screening form. There are some contraindicated implanted devices that should not be placed within a magnetic field. To ensure patient safety all implanted devices should be listed on the screening form including the type and model of implant.

Breast Feeding Policy
Patient who require an injection of gadolinium are advised to express and discard breast milk for 24 hours after the examination.

Pregnancy Policy
There have been no reported adverse effects demonstrated in pregnant women or the fetus; however the FDA guidelines suggest that MRI be used during pregnancy only when there are clear medical indications. Our policy is to avoid an MRI prior to the 2nd trimester.

Trans-Dermal Medication Patches
All trans-dermal medication patches should be removed prior to the MRI exam. It is important that the ordering physician is aware that the medication patch is being removed so they can instruct their patient when to reapply it.

MRI Safety:

Contraindicated Objects & Implanted Devices

  • cardiac pacemaker
  • cardiac defibrillator
  • neurostimulator
  • pacing wires
  • implanted insulin pump
  • Swan-Ganz catheters
  • implanted infusion pumps
  • shrapnel in a vital organ
  • ferromagnetic foreign body in the eye
  • eye implants (require operative report)
  • intravascular stents, coils, valves (require type and model)
  • ear implants (require operative report)
  • aneurysm clips

Relative Contraindications

  • arterial lines, (-the transducer must be removed)
  • surgical staples, (-ideal to remove)
  • previous surgery, (-must be 6-8wks post-op)
  • tattoos, (-may experience a heating effect on the skin due to iron oxide)
  • penile prosthesis, (-most compatible, indicate type and model)

Items That Would Need to Be Removed

  • hearing aids
  • purses/wallets
  • eye prosthesis
  • coins
  • any implant held in by a magnet
  • trans-dermal medication patches
  • prosthetic limb
  • metal zippers
  • hair pins/clips
  • jewelry/eye glasses
  • wigs
  • dentures

Any metallic object can become a projectile within a magnetic field, posing a serious risk of injury to the patient and the technologists. To prevent any possible injuries we ask that all patients change into a hospital gown and place all belongings and valuables in a locker. The key can accompany the patient into the room.

Information for MRI Patients

Registration:


On the day of your exam, please arrive at the Diagnostic Imaging Registration desk on time. In order to allow time for registration, the appointment time you are given is 30 minutes prior to your actual scan time. Please be sure to provide a valid Ontario Health Card.

If you have any relevant x-rays or scans from an imaging facility other than Credit Valley Imaging Associates/CVIA please be sure to bring them with you. These films or scans will be reviewed and then returned.

Preparation:

After the registration you will be brought to the MRI department. There is a patient information video available for viewing.
An MRI technologist will explain the procedure to you and confirm the information provided on the screening form. It is advised that if you plan on taking sedation that it is taken after the technologist has reviewed your screening form and you have given consent for the exam. If you plan on taking sedation, please be sure to have an adult accompany you home.

Once the procedure has been explained you will be instructed to change into a hospital gown. Lockers are available to lock up your belongings and valuables. When it is time for your exam a technologist will escort you into the room.

Preparation for Specific Exams

Abdominal MRI NPO 4hrs prior ( nothing to eat or drink)
medications can be taken with a small amount of water
Pelvic some exams require a bowel prep, (ask your doctor)
Brain no prep required
Spine/Extremity no prep required

During the Exam:

After the exam has been explained to you the technologist will provide you with hearing protection (either ear plugs or a headset). This protection will help reduce the loud thumping sounds which are produced during the scan.

The MR scanner is a large tube shaped magnet with a padded table that moves into the centre of the magnet. The body part being examined will determine if you go into the scanner head or feet first, as well as how far you go in. In most cases equipment called surface coils will be placed on the area we are scanning.

During the exam the technologist will remain in contact with you thru a two-way intercom. There is an emergency call bell if you need to get their attention.

It is very important that you remain relaxed and very still during the exam. Movement can cause non-diagnostic images and they will need to be repeated. In some abdominal exams we may ask you to hold your breath for up to 20secs while the pictures are acquired.

Most exams will take between 30-45 minutes, however there are some that can take up to an hour. Please allow for enough time in your day to accommodate your MRI exam.

Some exams require an injection of contrast media called gadolinium. It is a colorless fluid that is injected into a vein in your arm. Gadolinium is very safe and the risks will be discussed with you by the technologist. If contrast is required it is important that you remain very still as the images before the injection must match the images obtained after the injection.

Once your MRI is completed it will be reviewed by a radiologist and the report will be available to your ordering physician within about one week.

Frequently Asked Questions:

How does the MRI scanner work?
Your body is composed of small particles called atoms. Most of the body is composed of Hydrogen atoms that under normal circumstances spin around at random. However, when you are placed within a strong magnetic field, the hydrogen atoms line up and spin in the same direction as the magnetic field. When a radio frequency wave is transmitted through the tissues in the body the Hydrogen atoms produce a signal. These signals are measured to produce an image.

What causes the noise in the scanner?
The noise that the scanner creates is the electrical current rising within the wires of the gradient magnet. The current in the wires are opposing the main magnet field; the stronger the field the louder the gradient noise.

Will it hurt?
No. You will not be able to feel the magnetic field.

What is the difference between MRI and CT?
Both MRI and CT create cross-sectional images of the body. The main difference is that MRI uses a large magnet and radio waves to produce images where as a CT scanner uses ionizing radiation.

Can you scan my entire body while I am in there?
No. The MR scanner can scan almost any part of the body but each scan is limited to a specific area. It can take from 30-60 minutes to scan each area.

Why is my whole body in the scanner if you are only scanning my head?
The area of the scanner that creates the images is located in the centre of the magnet and is called the isocentre. Therefore, in order to scan your head most of your upper body will be in the scanner. The same is true when imaging the spine and upper extremities.

Does the MRI table have a weight and size limit?
Yes, the table weight limit is 159kg with a maximum width of 60cm. For optimal images it is necessary for the area being examined to be within the magnets isocentre which is located directly in the centre of the scanner. For patient specific questions please contact our MRI bookings department.