What is MRI?
MRI is a medical imaging technique that can produce detailed pictures of many parts of the body. An MRI scanner contains a large magnet with a tunnel through the middle where the patient lies on a movable bed. When the patient enters the magnet the hydrogen atoms in the water of their body tissues line up along the magnetic field. Radiofrequency pulses are then sent in, causing the atoms to ‘flip’ into another plane and then ‘relax’ back when the pulses are turned off. The repeated movement of these particles produces electrical signals which are picked up by a receiver, or coil, and then converted into an image by a computer. As tissues such as fat, muscles, and bone have different densities and water content, they produce different levels of signal. Similarly, diseased tissues will differ from healthy tissues and show up on the scan as brighter or darker than normal. Some scans also require use of a contrast dye which helps highlight certain tissues and body structures.
Uses of MRI
MRI is a widely used imaging method which is used to examine many parts of the body including bones, muscles, ligaments and tendons, the brain and nervous system, organs such as the kidneys and liver, as well as blood vessels, bile ducts and intestines. MRI can be used to diagnose disease, trauma or abnormality, assess the progress of a disease or treatment and to plan surgery.
Some of the most common scans and their indications are:
Brain: tumour, stroke, aneurysm, multiple sclerosis, congenital abnormality
Knee: damage to ACL, collateral ligaments, meniscus
Lumbar Spine: disc prolapse/herniation and nerve root impingement, spinal stenosis, cord and bony lesions
Shoulder: tears and damage to muscles and ligaments especially the rotator cuff, frequent dislocation and damage to shoulder joint
Breast: cancerous and non-cancerous lesions
MRCP: gallstones, bile duct obstruction, tumour
How to prepare for an MRI scan
Most MRI departments will send information before your scan telling you what to expect and whether special preparations are necessary. Some patients need to fast for a few hours before their MRI, or they may need to take a blood test to check kidney function. Before your scan you will need to fill in a questionnaire to confirm that it is safe to go ahead. You should alert the department staff if you think you may be pregnant, have had metal fragments in your body or eyes, or have any medical implants in the head or heart, including pacemaker, internal defibrillator, neurostimulator, aneurysm clips or coils or a mechanical shunt.
Before entering the scanner room, you should remove all mechanical, magnetic or metal objects from your clothes and body eg. jewellery, phone, watch, hairclips, money, bank cards. You may be asked to change into a hospital gown. An MRI scan can last from 10 minutes to an hour or more depending on the part of the body being examined. Some patients who require a contrast injection will have a small intravenous line known as a cannula inserted into their arm before the scan starts.
During the scan
Once in the scanner room you will lie on the scanner bed and equipment may be placed over the part of the body being scanned. You will be given an emergency buzzer in case you need to call the MRI staff during the scan. A set of earphones will protect your ears from the loud noises of the machine and let the staff talk to you directly.
If your scan is long, or you are feeling nervous you may be able to listen to music. Some departments will provide music, or you can ask if you can bring a CD or an i-pod.
During the MRI scan, the bed moves into the tunnel running through the middle of the machine. The part of the body being scanned is in the middle of the tunnel which stays open at both ends. The MRI machine makes a series of loud noises as it takes the pictures. Try to relax and lie still to avoid blurry images. In some procedures, you will be asked to hold your breath for a short period. If contrast dye is to be administered, it will be injected through a cannula or using a small needle.
Claustrophobia: Patients who know or suspect they suffer from claustrophobia (fear of enclosed spaces) should talk to the staff before their scan. For some scans the radiographer may be able to adapt your positioning, and s/he can talk to you through the headphones during the scan to provide reassurance. Some claustrophobic patients ask their doctor to prescribe a mild sedative to help them through the scan.
What are the risks of an MRI scan
MRI is a very safe and painless scanning method and uses no radiation. However, as a strong magnetic field is involved so it is very important to follow the safety guidelines outlined above. Loose metal or mechanical items must be removed before the scan, as they may fly into the machine and cause injury or equipment damage. Metal implants or fragments in the body such as bullets or shrapnel can be dislodged or heat up during the scan causing injury to the patient. Mechanical implants such as pacemakers, neurostimulators and insulin pumps may malfunction as a result of the magnetic field and place the patient at risk. It is therefore, very important to comply with the screening process undertaken by the MRI staff before entering a scanner room.
Pregnancy: If you are pregnant many departments will prefer to wait until the baby has been born or at least until the end of the first trimester before carrying out a scan. However, if your doctor deems that a scan is urgent, s/he will carefully discuss with you the risks and benefits involved. To date there is no record of harmful effects from MRI on the unborn baby, although there is a theoretical risk from the slight heating effects of the scan and from the very loud noises produced by the scanner.
Contrast reactions: Unlike the contrast used in CT scans the MRI dye does not contain iodine, and reactions following MRI contrast injection are rare. When they do occur they tend to be short-lived and mild – itching, pain or nausea. A rare condition called Nephrogenic Systemic Fibrosis (NSF) affecting the skin and organs has been associated with MRI injection in patients with severe kidney disease. MRI staff will screen patients to make sure they do not receive contrast if this is a risk.
After the scan
An MRI scan is sent to a radiologist or physician trained to interpret the images. The result is then forwarded to the doctor who requested the scan for you. Some doctors will ask you to provide CD with pictures of the scan. You can ask the MRI department how to arrange this.
Open MRI works on the same principles as normal MRI but the machine is designed so that there is more space around the body of the patient as they lie in the machine. In many cases, the sides of the tunnel are removed so the patient can turn their head and look out into the room. Open MRI is particularly suitable for claustrophobic patients, bariatric patients (high body mass), children, and elderly patients.
MRI of Children
MRI is generally considered very safe for children because it does not use ionising radiation. A parent or carer can generally accompany their child into the room during a scan, and young babies may sleep through the whole process. However, as the patient needs to stay still during the scan, older infants and children up to the age of 5 or 6 may require sedation beforehand. In this case, the child’s pulse and oxygen levels will normally be monitored by an accompanying doctor. Older children may be able to stay still during an MRI without sedation. Children can be prepared for the experience by careful reassurance and age-appropriate explanations of what will happen during an MRI.
Advantages and disadvantages of MRI
Advantages of MRI scanning include:
No use of ionising radiation so safer than CT for use in children, pregnant women, those requiring frequent scanning/monitoring
Excellent for showing soft tissues e.g. brain, spinal cord, internal organs, ligaments, cartilages and muscles
MRI contrast agent is less likely to trigger allergic reactions than CT contrast
Disadvantages of MRI scanning include:
MRI scanning is expensive and the scans are relatively lengthy, so availability may be limited
Unsuitable for patients with some metallic or mechanical implants e.g. aneurysm clips, internal defibrillators
Unsuitable for claustrophobic patients and those with a high body mass index or those unable to tolerate noise
Unsuitable for patients unable to lie still during the procedure
Contrast scans cannot be carried out on patients suffering from severe kidney disease