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Magnetic Resonance Imaging
Every human cell contains protons. When a patient is placed within a magnetic field, the protons in his/her cells
align like small magnets. Then radiofrequency pulses producing an electromagnetic field are transmitted in a
plane perpendicular to the magnet, resulting in the protons becoming excited and moving out of their original
position. When the protons return to their baseline state, ie relax, energy is produced which can be transduced
and, in turn, be translated into images.
Magnetic resonance imaging (MRI) scanning can discriminate between body substances based on their physical
properties; for example, differences between water- and fat-containing tissues. MRI scanning is therefore
particularly useful at providing highly detailed images of soft tissues. MRI scanning can also provide images in
various planes without movement of the patient.
Types
On an MRI scan some tissues appear to be brighter or darker than other tissues. Darkness depends on the
density of protons in that area - an increased density being associated with a darker area. Relaxation times for
protons can vary and two times are commonly measured - known as T1 and T2. White matter is brighter than
grey matter in T1-weighted images and darker than grey matter in T2-weighted images.
T1 - T1 and T2 are technical terms applied to the time taken for proton relaxation. T1 and T2 provide
different intensities of images and each has its own advantages and disadvantages.
T2 - in these scans, fat, water and fluid are bright; therefore, T2-weighted imaging is ideal to pick up
tissue oedema.
T2 - used in functional MRI scanning (see 'Functional MRI', below).
Diffusion MRI - measures the diffusion of water into tissues - eg, to study areas of demyelination.
Diffusion-weighted imaging - this also works on the principles of diffusion MRI and allows the
detection of areas where diffusion has become restricted - most commonly, in the setting of a
stroke. [1] Therefore, stroke can be detected within minutes from the onset of ischaemia.
Magnetic resonance angiogram (MRA) and venogram - MRI can be used to look for abnormalities
in arteries and veins, such as stenosis or aneurysm formation. Some of the most common MRAs
used are looking for renal artery stenosis and vertebral artery dissection.
Functional MRI - this is a fairly recent technique. It allows visualisation of neural activity in the brain by
detecting areas of increased blood flow. [2]
Common indications for magnetic resonance imaging scanning
Central nervous system - stroke, demyelinating disorders and tumours.
Musculoskeletal joint MRI scans are now widely used and MRI can detect minute ligament tears. [3]
Imaging arteries and veins.
More novel uses of MRI:
MRI is being used for cardiac investigations and there is growing interest in using MRI to
detect atherosclerotic plaques in coronary arteries.
Open MRI scanners are available for patients who are claustrophobic or have severe anxiety. Many of these are
not available on the NHS routinely. Sensitivity and specificity of MRI scans are enhanced with contrast agents.
Various different types of MRI contrast agents are available depending on the site of imaging and the nature of any
lesion. Gadolinium is one contrast agent for example.
Advantages of magnetic resonance imaging scanning
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Harmless to the patient - no radiation is involved (unlike computed tomography (CT) scanning and
conventional radiology).
Excellent detail makes it similar, and even superior to, CT scanning in some situations.
MRI contrast agent used is normally gadolinium which is less allergenic than iodine-based contrast
agents used in CT scanning.
Disadvantages of magnetic resonance imaging scanning
Limited availability - although this is rapidly improving.
It is a lengthy procedure - eg, a pituitary gland MRI scan can take up to 30 minutes.
In MRI scanning of the chest and abdomen the patient must lie still for long periods, which can prove
difficult. Therefore, CT scanning is preferred in these situations.
MRI scanning cannot be performed in the presence of foreign bodies or metallic implants - eg,
pacemakers, aneurysm clips and some cardiac stents (even if distant from the site of the image).
However, stainless steel objects, such as those in hip prostheses, may be OK.
It is relatively expensive compared with other forms of imaging.
It may not be available 'out of hours'.
Interference from foreign metallic bodies and cardiac pacemakers in MRI scanning
This varies somewhat according to the type of implant and will usually be checked by the MRI department by
contacting the manufacturers. It is important to bear in mind that there are no trials comparing MRI scans in
patients with or without implants - these could possibly be too risky to perform. However, there is some evidence
available in patients where the benefits of having the MRI scan outweighed the risks.
Cardiac pacemakers - this is an absolute contra-indication to an MRI scan. The reason for this is that
putting the pacemaker in a magnetic field temporarily stops it from working and this can be potentially
fatal for the patient; for example, if the patient then has a bradycardic episode.
Intracranial aneurysm clips - again, patient safety is the issue. The aneurysm clips can be dislodged
from their site by the strong magnetic field of the MRI scanner. Again, this is an absolute contraindication.
Coronary artery stents - on the whole these are thought mostly to be safe, but this must be checked
with the manufacturer.
Metal pieces in a welder's eyes can result in visual loss as the metal fragment is dislodged in the
strong magnetic field.
Similarly, metal objects in the room may be attracted to the magnet - eg, a pair of scissors or a metal
watch. This could potentially lead to a dangerous situation.
What patients need to know about magnetic resonance imaging
scanning
There is the need to lie in a cylindrical tube (which is the magnet).
The cylindrical tube is very tight and can make you feel claustrophobic. However, there are a few open
scanners available.
A contrast agent may need to be given.
The scan can take up to one hour depending on how much of the body is being scanned.
Whilst having the scan there is a drilling noise which is very loud. The drilling noise is essentially the
magnetic field being switched on and off at a high frequency. The MRI department will usually provide
earplugs and possibly music in the background.
Further reading & references
1. Wardlaw JM, Keir SL, Seymour J, et al; What is the best imaging strategy for acute stroke? Health Technol Assess. 2004
Jan;8(1):iii, ix-x, 1-180.
2. Xu XJ, Zhang MM, Shang DS, et al; Cortical language activation in aphasia: a functional MRI study. Chin Med J (Engl). 2004
Jul;117(7):1011-6.
3. Brooks S, Morgan M; Accuracy of clinical diagnosis in knee arthroscopy. Ann R Coll Surg Engl. 2002 Jul;84(4):265-8.
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Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical
conditions. EMIS has used all reasonable care in compiling the information but makes no warranty as to its
accuracy. Consult a doctor or other healthcare professional for diagnosis and treatment of medical conditions.
For details see our conditions.
Original Author:
Dr Gurvinder Rull
Current Version:
Dr Gurvinder Rull
Peer Reviewer:
Dr Helen Huins
Document ID:
631 (v24)
Last Checked:
25/01/2013
Next Review:
24/01/2018
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