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Magnetic resonance imaging

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Magnetic resonance imaging (MRI) was first used in July 1977 and it took five hours of intensive work to produce what is, by today’s standards, a poor-quality image. MRI is now commonly used in most big hospitals

Magnetic resonance imaging is a non-invasive method of imaging which does not use ionising radiation; instead it exploits the property of nuclei in the body to behave like weak magnets. Any nucleus with an uneven number of protons and neutrons can be used, but in practice hydrogen is almost always used, since it is abundant in the body. The nuclei are normally arranged in a random fashion. However, when a strong magnet is applied, all the nuclei align themselves within the magnetic field.

During an MRI scan, a short burst of radio waves (called a pulsed radio frequency) is applied. As it commences, all the nuclei alter their alignment, and at the end of the pulsed radio wave, they return to the alignment within the magnet. As they do so, they emit a radio signal of their own, which is detected and amplified by the magnetic resonance imager.

The appearance of the scan can be altered, depending on the type of pathology being investigated, by manipulating the radio frequency pulse. The two types of image are termed T1 weighted and T2 weighted. In T1-weighted images, water shows up darker than most other material, so in the case of neuro-investigations, CSF appears darker than the brain and can therefore indicate gross anatomical detail of the brain and spinal cord. T2-weighted images show water and CSF as white and are extremely sensitive to increases in cerebral water, which commonly occurs in inflammatory or cancerous conditions.

Uses of MRI

The MRI is now the imaging of choice for neurological investigations, as it has a number of advantages over computed tomography scans; chief among these is the ability to take a scan in any plane, whereas the CT scan is limited to the transverse of axial plane. However, MRI does have some limitations, as it does not give a clear image of material containing low levels of hydrogen, such as bone, air and flowing blood; these appear dark on the scan. This means a CT scan provides a better image of calcified tissue and tumours within the brain.

MRI techniques can be manipulated to give an improved image in particular situations. For example, the hydrogen content of fat is very high and fat-containing structures appear very bright on the magnetic resonance image, but sequences that suppress fat signals can be used for imaging such structures as the orbit of the eye.

Magnetic resonance angiography (MRA) exploits that fact that hydrogen ions in flowing blood return a distinctive signal, and can be used to demonstrate the functioning of blood vessels, particularly within the cranium.

Functional MRI is a relatively new technique which uses the ability of magnetic resonance to distinguish between oxygenated and deoxygenated haemoglobin. This can indicate which parts of the brain are active and should be useful to show the brain working (‘thinking’) in different situations and pathologies.

Safety

MRI does not use ionising radiation and is relatively free from risk. However, the magnet is extremely powerful, so magnetic objects must not be taken near to the imager, as the magnet can pull such objects towards it at considerable speed. Therefore metal objects should not even be taken into the scanner room. Also, MRI cannot be used with people who have metal within them. This means those with pacemakers, some types of clip and foreign bodies cannot be scanned using an MR imager.

Many units are also reluctant to scan pregnant women, as the effect of powerful, albeit brief, magnetism on the placenta or foetus are unknown.

Nursing care

There is little by way of specific nursing care. However, patients do need to be prepared for the fact that they will be required to remain very still for several minutes. They will be placed inside the machine’s cylinder, which many people find very claustrophobic. Some newer machines do have small windows in them, while some open scanners have been produced. However, their scans are of poorer quality than scans produced by closed machines. Some patients may find visualisation techniques helpful to cope with the experience of being in the scanner, but others may need sedation or even general anaesthesia. Some may also find it helpful to visit the facilities before they are due for an MRI scan. Many scanning units give patients a ‘panic button’ to hold, which can reassure them, while many also allow them to listen to music of their choice.

 

Useful reading

Banerjee AK (2006) Radiology Made Easy. Cambridge: Cambridge University Press.

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