Spinal Imaging

With Iona Collins

Here is an article about the common types of imaging available to us. I hope you find this interesting and useful!

X Ray

X Rays can look at the bones of our joints and give us information about their alignment as well. There are many X Ray machines in the UK, making them easily accessible for getting basic information. In the spine, X Ray is very useful, since the pictures can be taken when we are standing up or sitting down- this gives us useful information about spinal alignment when the spine is loaded. A lying down X Ray tends to be clearer, since we keep still better when we lie down, but the pictures are not quite as useful, since the spine is not being loaded. X Rays are not good for us, since the X Ray causes DNA damage which can ultimately cause cancer and X Rays can only show us calcified structures, such as bone. We can’t see structures such as the shock-absorbing discs, nerves, tendons, ligaments, muscles on X Ray, although sometimes, we can see the aorta if we have atherosclerosis, where plaque has built up in our arteries and hardened over time (‘Athere’ is Greek for gruel and ‘skleros’ is Greek for hard). X Rays are quick to perform, so the waiting time for an X Ray in the NHS is very short, usually a matter of days at most. An independently-funded X Ray would cost around £60.

CT

CT, or “Cat Scan” as it is often called, is an X Ray taken many, many times, so that a computer can create a three dimensional image of ourselves. The CT scan shows a lot of detail for our bony anatomy, but unlike X Ray, CT can also give some information about soft tissue structures as well. So, CT scans can be used to look at the abdomen, for example. CT is also used to look at the spine and is especially useful for computer-aided navigation when performing complex spinal operations. CT scans take longer than X Rays to acquire and there are fewer CT scanners than there X Ray machines. An NHS CT scan usually involves a wait of a few weeks, with an independently-funded CT scan costing around £350 – CT and MRI scans can be ‘bought’ via GP or private specialist referrals in places which don’t tend to advertise themselves, such as Swansea University, for example, where the prices tend to be very competitive.

How Harmful are X Rays?

The sun irradiates us every day and we can also receive irradiation from the ground (limestone foundations can emit radon gas). Together, this is called background radiation, which is mostly unavoidable. When we have medical images which include X Rays, though, we are absorbing additional radiation, which increases our lifetime risk of developing cancer. Our baseline risk of dying from cancer is around 1 in 5. An additional dose of 1 milliSievert (mSv) increases our lifetime risk of cancer by 1 in 20,000 and equates to three and a half months of background radiation. A great website call-ed www.xrayrisk.com allows you to enter the study that you are considering having done, then it esti-mates the additional risk of developing cancer as a result of the scan in question. As an example, an X Ray of my lumbar spine will increase my lifetime risk of cancer by 1 in 7,000, while a CT of my lumbar spine will increase my lifetime risk of cancer by approximately 1 in 2,000. If I wanted a bone density scan (DEXA) to see whether I may have low bone density or osteoporosis, my increased risk of cancer from the scan is neglibly low at around one in a million.

Ultrasound

Ultrasound is useful for looking at the opposite of X Rays ie. everything except bone. Ultrasound is used mostly for investigating soft tissue problems, such as rotator cuff pain in the shoulder. It is also useful for assessing blood flow, since ultrasound is a ‘real time’ image, or a movie, in contrast to most other types of imaging assessment that we use. This means that the ultrasound is very dependent on the interpretation of the ultrasonographer, since only snapshots, like screenshots, are saved in order to support the ultra-sonographer’s report. In the spine, ultrasound is not used routinely, since the bones which make up the spine are too close together to allow ultrasound assessment of the discs and nerves. There are no known risks associated with ultrasound, which uses radio waves rather than harmful X Rays. An NHS ultrasound does not usually involve an extended waiting time and an independently-funded ultrasound scan can cost around £100.

MRI

MRI is a relatively new addition to the imaging repertoire. Various places around the world claim to have developed MRI in the 1970s but it appears that a Russian and American researcher independently discovered magnetic resonance imaging, with a British researcher adapting the technique so that an MRI scan could be performed more quickly. Prof Mansfield in Nottingham received a Nobel prize for his contribution to the development on MRI in 2003. MRI is risk-free, as far as we know, since it involves a strong magnet and radio waves but no X Rays. The magnets range from relatively weak at 0.5 Teslas, through to very strong magnets used for research, up to 9 Teslas. In South Wales, the magnet strength is between 1.5 Tesla and 3 Tesla. The main problem with the magnets relates to metallic objects which have already been placed inside us, either deliberately, such as a pacemaker, or accidentally, such as a metal fragment lodged in the eye. The magnet can move the metallic objects inside us or heat them up, causing local damage. The stronger the magnet, the more likely that the metallic objects would be affected. For this reason, pacemakers are nowadays inserted as ‘MRI compatible’ devices, whereas originally, they were not. When an MRI cannot be performed safely due to metallic objects, then we usually resort to CT instead.

Most MRI scans involve lying inside a tube and keeping still for around twenty minutes – some people can find this experience quite claustrophobic, while others find the time to be quite relaxing! There is an upright and open MRI scanner in Cardiff (European Scanning Centre), which has a 0.5 Tesla magnet, meaning that the scanner is less noisy but still provides clear images. An upright scanner tends not to be quite as clear as a lying down conventional scanner, though, since we are more likely to move when we stand up for the scan. When MRI first became available, the scans were very expensive, with some units charging £700 for an MRI of the lumbar spine, for example. The prices have now reduced to around £200 for a lumbar spine MRI, although there is still a fairly wide range of prices between MRI units. An NHS scan typically involves an eight week queue for a routine scan.

In conclusion, as a spinal surgeon, virtually everybody considering having an operation will have an MRI and sometimes a CT as well. Ultrasound unfortunately does not tend to be useful in the spine due to the bones surrounding the nerves and ligaments blocking the sound waves. We try to limit the use of X Rays since the X Ray irradiation that we absorb stays with us for life.

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