What is an MRI scan?
MRI (magnetic resonance imaging) is a fairly new technique that has been used since
the beginning of the 1980s.
The MRI scan uses magnetic and radio waves, meaning that there is no exposure to
X-rays or any other damaging forms of radiation.
How does an MRI scanner work?
The patient lies inside a large, cylinder-shaped magnet. Radio waves 10,000 to 30,000
times stronger than the magnetic field of the earth are then sent through the body. This
affects the body's atoms, forcing the nuclei into a different position. As they move
back into place they send out radio waves of their own. The scanner picks up these
signals and a computer turns them into a picture. These pictures are based on the
location and strength of the incoming signals.
Our body consists mainly of water, and water contains hydrogen atoms. For this
reason, the nucleus of the hydrogen atom is often used to create an MRI scan in the
manner described above
What does an MRI scan show?
Using an MRI scanner, it is possible to make pictures of almost all the tissue in the
body. The tissue that has the least hydrogen atoms (such as bones) turns out dark,
while the tissue that has many hydrogen atoms (such as fatty tissue) looks much
brighter. By changing the timing of the radiowave pulses it is possible to gain
information about the different types of tissues that are present.
An MRI scan is also able to provide clear pictures of parts of the body that are
surrounded by bone tissue, so the technique is useful when examining the brain and
spinal cord.
Because the MRI scan gives very detailed pictures it is the best technique when it
comes to finding tumours (benign or malignant abnormal growths) in the brain. If a
tumour is present the scan can also be used to find out if it has spread into nearby
brain tissue.
The technique also allows us to focus on other details in the brain. For example, it
makes it possible to see the strands of abnormal tissue that occur if someone has
multiple sclerosis and it is possible to see changes occurring when there is bleeding in
the brain, or find out if the brain tissue has suffered lack of oxygen after a stroke.
The MRI scan is also able to show both the heart and the large blood vessels in the
surrounding tissue. This makes it possible to detect heart defects that have been
building up since birth, as well as changes in the thickness of the muscles around the
heart following a heart attack. The method can also be used to examine the joints,
spine and sometimes the soft parts of your body such as the liver, kidneys and spleen.
How does an MRI scan differ from a CT scan?
With an MRI scan it is possible to take pictures from almost every angle, whereas a
CT scan only shows pictures horizontally. There is no ionizing radiation (X-rays)
involved in producing an MRI scan. MRI scans are generally more detailed, too. The
difference between normal and abnormal tissue is often clearer on the MRI scan than
on the CT scan.
How is an MRI scan performed?
The scan is usually done as an outpatient procedure, which means that the patient can
go home after the test. During the scan it is important to lie completely still. For this
reason it might be necessary to give a child an anaesthetic before they are tested.
Since you are exposed to a powerful magnetic field during the MRI scan, it is
important not to wear jewellery or any other metal objects.
It is also important for the patient to inform medical staff if they use electrical
appliances, such as a hearing aid or pacemaker, or have any metal in their body such
as surgical clips, but orthopaedic metal-ware such as artificial hips or bone screws is
not normally a problem.
Is an MRI scan dangerous?
There are no known dangers or side effects connected to an MRI scan. The test is not
painful; you cannot feel it. Since radiation is not used, the procedure can be repeated
without problems. There is a small theoretical risk to the foetus in the first 12 weeks
of pregnancy, and therefore scans are not performed on pregnant women during this
time.
Because patients have to lie inside a large cylinder while the scans are being made
some people get claustrophobic during the test. Patients who are afraid this might
happen should talk to the doctor beforehand, who may give them some medication to
help them relax.
The machine also makes a banging noise while it is working, which might be
unpleasant.