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.
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. 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.
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.
- 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
- 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 contra-indication.
- 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.
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- 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.
- 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.
- 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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