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Magnetic Resonance Imaging (MRI)

Written by Chris Heywood
Cons Physiotherapist

The Acronym MRI stands for 'Magnetic Resonance Imaging'. This technique was first used in 1977 in America to visualise the structures of the body but wasn’t more widely available until the mid to late 80’s. 

What is an MRI Scanner?


To  make a magnetic field strong enough to produce an image, the MRI scanners themselves are historically very big and need you to lie down in a tunnel. Many people are worried about the enclosed tunnel but most find it ok. For those with significant claustrophobia, there are now ‘open’ scanners but their availability is less and you may have to travel some distance to find them. Some facilities may also be able to offer a light sedation to help you through the scan.

The MRI scan will take longer than conventional X-Rays lasting anywhere from around 20 minutes to 2 hours depending on the type and size of scan you need. You will feel comfortably warm when you are in the tunnel and you will hear a repeated loud banging. This is simply the large magnets working and you will receive some form of ear protection but this varies from unit to unit. Some may also play music for you during the scan.

The pictures that are produced are very different from the more regular X-Rays that you may have had before. Due to the computer aided technology, several 'variations' of MRI scan can be taken ranging from different angles, different depths and even different colours/shading for highlighting fluid such as inflammation.

Because the MRI uses harmless magnetic fields instead or X-Rays, there are no known side effects making it very safe. The only group of patients who cannot be scanned are those with metal in their body but this will be screened for before hand. You may also find that most units will not scan pregnant ladies unless there are extreme circumstances.

Is It a Single Picture?

The radiographer can take images from different angles depending on what they are investigating. The obvious angles are from the front (Coronal plane), the side (Sagittal plane) and the top/bottom (Transverse plane). The clever part is that unlike an X-Ray, the MRI scan captures its pictures in slices. This can be demonstrated by the scan in figure 2, showing a torso, looking at it from front to back (note the kidneys on each side). The vertical lines represent the slices that are being scanned in this sequence. 

Note that you can faintly see the vertebral column and discs underneath the vertical slice lines.
The way to picture this is to look at figure 3 and imagine you dropped the 'sagittal plane' (dark blue, front to back) from above, many times from the left side of the body, all the way over to the right side, with 2mm gaps. If you then looked at these slices side on, this is what the scan will show you. This can be repeated from any of the planes. The most commonly used in relation to the spine are the Sagittal and Axial, front to back, and up to down respectively.

How does it work?

The way in which the scanners work make the unique imagery possible. In simple terms, your body is made up of billions of small particles that are all spinning in different directions about their central axis. Their anatomical make up affects the way they are electrically charged (protons and neutrons) and this in turn characterises the way they spin. The MRI scanner emits its energy as a Radio Frequency (RF) that specifically targets hydrogen ions in our body that are positively charged.

This is done by a  large and extremely powerful electro-magnet that run along the length of the tube in which you lie. The magnets used today are between 10,000 to 40,000 more powerful than the earths magnetic pull (gravity). This forces a small proportion of your hydrogen ions to spin in alignment with this magnetic pull where they  become increasingly energised. A combination of lesser magnets rapidly switch on and off (this represents the Resonance part of MRI) to directly concentrate the energy to the area being investigated. 

When all of these magnets are turned off, the hydrogen atoms start to spin back into their normal pattern and when fully returned will emit bits of energy. This energy release is then picked up by the scanner as mathematical data. This is done extremely quickly, repeated hundreds of times, and is the reason for the loud banging which is synonymous within the MRI scan. The way in which the mathematical data is analysed by the computer allows the final pictures on the films to acquire different attributes dependant on what structures are being analysed and for what reason.
The common variations are types T1 and T2 as shown in figure 4 respectively, which are extracts from a sequence from an identical spine. A T2 type scan will express any areas of fluid such as intervertebral disc fluid, spinal cord fluid or inflammation as a white bias depending on its volume whereas a T1 will not. This scan selection allows a greater diagnostic potential by allowing the radiologist or surgeon to look more closely at different anatomical details. The bottom two  discs in figure 4 are darker on the T2 images because they are degenerate, containing minimal fluid in contrast to their neighbours above.

MRI is without doubt a significant aid in the diagnosis and subsequent management of your spinal pain.

****This is a vital piece of information that you should read if you are due to have an MRI scan****
There are groups of professionals who discourage the use of MRI in all but potential fusion clients due to the potential negative psychological impact of the results. By this, we mean that it is extremely likely that there will be 'changes' of varying types noted on the radiology report. These are often described in complex terminology and can make even the most simple problem sound like a Hollywood storyline. Some argue that this sets up certain personality traits to 'fear' (see yellow flags) their back further and can reinforce more negatives than positives. Although there may be some mileage in this in a minority of clients we believe that where medically justified, an MRI reported in a sensible and informative environment is more helpful than harmful.

You must remember that contrary to popular belief, (some more than others!) none of us are perfect and all of us will 'wear out' to some extent. This does not mean however that all reported finds are adverse in nature. In fact most, if not all of the changes are likely to have nothing to do with your pain and never will. It just means you are normal! It is the skill of your surgeon to justify which, if any, are most likely to be the cause of your complaint and which are not.
We do not claim any rights over the linked sights at all and have nothing to do with their content or subsequent editing. They are simply links that at the time of build offered very good educational material that we would like our clients to be aware of.

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