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What are MRI's? - We explain all

By Adam Shelverton
Senior Rehabilitation Consultant

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?

Radio waves 10,000 to 30,000 times stronger than the magnetic field of the earth are sent through the body affecting the body's atoms, forcing their nuclei into different positions.

As they move back into place they send out radio waves of their own which the scanner picks up and a computer turns them into an image.

What does an MRI scan show?

Using an MRI scanner, it is possible to make pictures of almost all the tissues 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.

An MRI scan is also able to provide clear pictures of parts of the body that are encased by bone, so the technique is useful when examining the brain and spinal cord. Because the MRI scan gives very detailed pictures it is the best investigatory technique to use when it comes to finding tumours (benign or malignant abnormal growths) especially within the brain. If a tumour is present the scan can be used to find out if it has also spread into surrounding brain tissue.

The powerful magnetic field during the MRI scan, means it is important not to wear jewellery or have any internal metal objects eg  a Pacemaker

There are no known dangers or side effects connected to an MRI scan and the test is not painful. Since radiation is not used, the procedure can be repeated without any problems. However there is a small theoretical risk to a foetus in the first 12 weeks of pregnancy, and therefore scans are not performed on pregnant women during this time.

MRI's V's X-Rays

MRI's provide images in cross sections and beacuse they are able to show multiple structures they are generally preferred over x-rays due to their increased level of detail, providing an increased level of detail is required of course.

X-rays are still preferred to image fractures; which in part is down to their speed, but also the portability of the X-ray machines. This mean patients that are bed bound or on intensive care that cannot be moved can still be X-rayed in situ.

There are no portable MRI machines.

What are the drawback of MRI's

  • MRI's due to their detail take a long time to complete (between 15-45 mins) so as a result only one body part is generally scanned at a time eg. Knee
  • MRI's require the subjects to be very still during the scan. This can be difficult given the length of time they take and if subjects are in pain or are young - sedation is sometimes required to help facilitate this. If subjects move during the scanning process it can mean imaging is not as clear and defined as it could be and the results are then difficult to ascertain.
  • MRI's are VERY noisy and often ear plugs are supplied to protect people's ears. However, most new scanners allow patients to listen to music during their scans.
  • MRI's can feel very claustrophobic, which is not surprising given the limited space in the scanner. Some open scanners have been developed to address this but they are sadly few and far between.

Your contact

Your contact

Adam Shelverton

Adam Shelverton

Rehabilitation Manager

Tel: +44 (113) 2906321