MRI: A safe scanning procedure

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The Doctor Says
By Dr MILTON LUM

Magnetic resonance imaging is a safe way to scan a patient.

ONE of the medical advances that I saw developing right before my eyes was magnetic resonance imaging (MRI). When I was undergoing post specialty qualification training in Britain, I was privileged to witness the workings of MRI, which was then used in a research centre.

The improvements in technology and production, together with the decrease in pricing in the past two decades, have lead to increased availability of MRI to the general population in many countries.

MRI scans use magnetic fields and radio waves to produce images of different parts of the body. There is no exposure to radiation, unlike in radiography, which uses X-rays. Studies have not reported risks to humans from MRI. This means that an MRI scan is a safe procedure.

Scan me: Metallic objects must be removed from a patient’s body because magnetic fields are used in the MRI scanner.

Water is the main component of the human body, and is made up of hydrogen and oxygen atoms. At the centre of the hydrogen atom is a particle called a proton, which is very sensitive to magnetic fields.

The MRI scanner is composed of a large tube which contains several powerful magnets and a motorised surface on which the patient lies.

All the protons are pulled into the same direction when placed inside the magnetic field of the MRI scanner, in the same manner that a magnet pulls the needle of a compass. Radio signals sent by the MRI scanner pull the protons out of position. This leads to the transmission of a radio signal by each proton, thereby providing information about it.

Individual protons do not provide useful information, just like a single pixel on a television monitor is a only dot. As millions of pixels produce images, so do millions of protons produce detailed images of body parts. The images produced are stored in a computer system.

Preparations

Since magnetic fields are used in the MRI scanner, metallic objects must be removed from the patient’s body. This includes watches, belts, dentures, any ring on any part of the body, hearing aids, and even wigs, as some contain a trace amount of metal.

Most patients do not have to refrain from taking food, drink or medicines on the day of the procedure. The exception is a scan of the bile ducts, in which case, there should be abstinence from food and drink for about three hours prior to the scan.

Many patients are asked to wear a hospital gown during the investigation. If not asked to do so, he has to wear clothing without any metal parts, which include fasteners, zips, buttons or belts.

Procedure

The patient lies on a motorised surface, which moves in and out of the MRI scanner, which is tunnel-like and open at both ends. The magnets in the scanner are turned on and off during the procedure. This makes a loud sound, which can be reduced by the use of ear plugs or headphones. The transmitted radio signals from the protons are detected by a small receiving device placed behind or adjacent to the body part that is being scanned.

It is vital to keep the body part being scanned absolutely still during the procedure. This is to ensure that there is no blurring of the images taken.

The MRI scanner is controlled by a computer system in an adjacent room, so that it is further away from the magnetic fields produced by the scanner. The patient can communicate through an intercom with the operator of the computer system, who can see the patient at all times.

The patient will be moved out of the scanner when the procedure is completed.

Certain types of MRI scans require the injection of special dyes called contrast media, which ensure greater clarity and definition of images of certain body parts and blood vessels. Some patients experience a warm sensation with the injection of contrast medium. This is not abnormal and will resolve by itself rapidly.

The contrast media used in MRI scans are usually harmless and are excreted from the body in the urine. However, some patients may develop an allergic reaction to the contrast medium, causing complications; hence the importance of informing the radiologist or radiographer of any previous allergies.

The procedure can be claustrophobic for some patients. This can be addressed by requesting in advance for a sedative to be given prior to the procedure.

Anaesthesia is not usually required for MRI scans because the procedure is painless. However, it is used for babies and children because of their inability to keep very still during the procedure.

The duration of the procedure depends on the body part being scanned. The patient can return home after the procedure.

If a sedative is used for the procedure, prior arrangements should have been made for someone to drive the patient home. It is unsafe to drive, drink alcohol, or work for 24 hours after taking a sedative.

If contrast medium has been used, it would be advisable to drink plenty of water for 24 hours to facilitate the excretion of the dye from the body.

The many images taken will be stored in a computer system. The radiologist will study and analyse the images taken on a monitor screen from various angles, and then write a report for the doctor who ordered the MRI scan.

Uses

There are many uses of MRI scans. It is often used in the diagnosis and evaluation of conditions affecting certain organs such as:

> Brain and nervous system, e.g. stroke, infections like meningitis or encephalitis, dementia, Parkinson’s disease, tumours, injuries to the brain or spinal cord

> Heart, e.g. assessment of damage after a heart attack, abnormal valves or chambers

> Blood vessels, e.g. hardening of the walls of arteries (atherosclerosis), narrowing of blood vessels (stenosis), bulging of blood vessel due to weakness in its wall (aneurysm)

> Bones and joints, e.g. arthritis, bone infection (osteomyelitis), abnormalities and/or damage to tendons, ligaments, cartilage and muscles

> Conditions affecting the lungs, liver, kidneys or prostate

> Breasts, e.g. cancer.

Functional MRI is a technique that is used to provide information about brain function. Instead of taking one scan, repeated scans are taken to follow the movement of blood in the brain. It provides information about real time activity in various parts of the brain. The technique facilitates the planning of complex brain surgery.

Benefits and risks

MRI scans are safe for use in patients who may be susceptible to the effects of radiation, e.g. pregnant women, babies.

An MRI scan produces good quality images of organs like the brain, heart, and eyes; soft tissue structures like tendons, ligaments, and cartilage; and blood flow through certain organs and blood vessels.

There is no evidence to suggest that there are health risks from the use of magnetic waves in humans. There is also no evidence that MRI scans are harmful in pregnancy. However, as a general precaution, the use of MRI scans in the first trimester of pregnancy is not recommended.

Although the price of an MRI scanner has dropped over the years, it is still expensive. A single scanner costs about RM4mil or more, depending on the accessories purchased. This leads to pressure on private healthcare facilities to recover the expenditure incurred, which can lead to overservicing.

It has been estimated that there are more MRI scanners in the Klang Valley than in the whole of Singapore. Whether this is beneficial for the population is a matter for debate, which has to be dealt with in another setting.

As patients are required to be very still during the MRI scan, it is unsuitable for investigating babies, children, and tumours of the oral cavity. Claustrophobia can be a problem for some patients.

There have been reports of unsecured metal objects being pulled towards the MRI scanner when its magnetic fields are turned on, resulting in injury to the patient inside the scanner. These projectile accidents should not occur as long as there is strict adherence to the rules about the location and storage of metallic objects in the room in which the MRI scanner is located.

Category: Milton's Corner
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