Magnetic Resonance Imaging (MRI for short), is just one of many different diagnostic tests used by doctors on a daily basis in hospitals to diagnose illness. Our specialist imaging centres in London carry out around 6000 examinations each year, a mere drop in the ocean of the circa 3 million MRI scans performed each year across the UK.
As with all hospital tests, it is perfectly natural for the patient to wonder whether the test they are undergoing is safe. This is a sentiment also shared by the referring doctor, who is ultimately responsible for always considering the risks vs the benefits of any investigations they are referring their patient for. It is for this reason that most diagnostic tests require a referral from a qualified doctor. If a benefit cannot be proven, then the test should not be carried out (whether or not the test is deemed ‘safe’).
Diagnostic Imaging and the question of ‘radiation’
MRI is a form of diagnostic medical imaging, also known as a radiological examination. These terms encompass many types of ‘test’ which all share a common purpose – to create an image of the tissues inside the human body in order to demonstrate structures and reveal any pathology.
The means by which that image is created varies greatly but it can be said that the vast majority of all radiological tests use a form of electromagnetic radiation in order to create a meaningful image.
Now…..using the word ‘radiation’ can immediately inspire thoughts of danger and hazard! However, without going into too much physics (no one wants that), the spectrum of electromagnetic radiation simply describes different types of naturally occurring waves, all with different levels of energy. This spectrum includes everything from low energy radio waves, light and sound, through to higher energy waves known as X-rays and Gamma Rays.
The types of radiation that occur in different parts of the spectrum have different uses and dangers, which depend on their energy. Low energy waves have no effect on the human body, whereas very high energy waves can interact with the cells in human tissues and potentially damage them.
MRI and Radio Waves
Like all other radiological examinations, MRI uses a form of electromagnetic radiation to create pictures. Specifically, MRI uses a strong magnetic field and radio waves in order to obtain a ‘signal’ from the human body, much like an antenna sending out a signal to be picked up by a radio. Radio waves are classed as the lowest energy form of electromagnetic radiation and for this reason, exposure to dangerous radiation from an MRI scanner is never a concern. In fact, the human body experiences both strong magnetic fields and radio waves on a constant basis, with the Earth’s magnetic field and invisible radio waves for radio and television constantly surrounding us.
It is for this reason that MRI is deemed to be a perfectly safe test for children, pregnant women and for taking images of areas of the body that would normally be at higher risk of damage from high energy (or ionising) radiation (such as the breast, eye and thyroid gland).
One slightly undesirable side effect from the production of radio waves in an MRI scanner is vibration within the scanner itself. Whilst the patient will not feel this vibration in most cases, there is another giveaway – MRI scanners are known to be noisy machines! The noise produced by an MRI scanner when it is working is perfectly normal and should never be a cause for concern. The level of the noise is not even dangerously loud. However, because MRI scans can sometimes be long, we will always provide some ear protection, so that there is no risk imposed to the patient’s hearing.
If it is safe, why does the radiographer go through a safety questionnaire with me before my scan?
Whilst radio waves and the strong magnetic field produced by an MRI scanner are technically safe, staff using this type of scanner do still need to be aware of what they are taking in to the magnetic field. As we are all aware, magnets can cause certain types of metal to move. If this happened unexpectedly in an MRI scanner, this could cause injury to the patient, or anyone else in the MRI room. For this reason, anyone entering an MRI scanning room will always be taken through a safety procedure before going in, whether they have been in an MRI scanner before or not. This safety procedure involves asking the person about any metal that may be implanted in their body, any operations they may have had recently involving any implants or devices, or any accidents they may have had where metal may have entered the body. Using a detailed questionnaire, the radiographer caring for the patient can be absolutely certain that they know what they are taking into the magnetic field, and whether it is safe or not for that patient to enter.
Now this is not to say that all metal in the human body prevents people from having an MRI scan. In fact, there are very few situations where a patient may be prevented from having a scan. Many patients we see will have metal dental fillings, fixed braces and even perhaps joint replacements and bits of metal holding broken bones together. None of these things pose a safety risk. However, it is still important that the radiographer is aware of everything, so that they can assess the risk and alter their technique if necessary, ensuring total safety and also ensuring that the quality of the scan is as high as it can be (metal causes large artefacts on MRI images).
One type of medical implant which continues to cause some difficulty are cardiac pacemakers. Many departments are unable to scan these devices, due to the potential risks imposed on the function of the device from the strong magnetic field. However, even these are now less of a contraindication than they once were, and a few departments, including our Chenies Mews Imaging Centre, have now devised safe processes in which patients with cardiac pacemakers can have an MRI scan safely, without any unnecessary risk.
MRI and Contrast Dyes
The use of dyes in medical imaging is extremely common. Contrast dyes are typically injected into a vein in the hand or arm, and are used to enhance the appearance of certain structures within the body. They are not always essential, but for certain conditions, they are needed in order to make an accurate diagnosis. They are only ever be used if certain information is required which cannot otherwise be obtained using non-contrast imaging techniques.
The particular type of contrast dye used in an MRI scan is different to other, x-ray based tests. In MRI, the contrast used contains a naturally occurring substance called Gadolinium. Gadolinium is generally considered to be very safe, and allergic-type reactions to gadolinium are very rare indeed. The vast majority of patients will feel nothing after receiving an injection of contrast and only a very small number of people will have any adverse effects.
As with all injections, whatever the substance being injected, doctors and radiology staff must always be aware of the risks vs the benefits of performing the injection. A contrast dye injection is only ever used if it is absolutely necessary and certain questions within the MRI safety form are asked specifically so that staff are aware of any possible reason why contrast dyes should not be used.
If you would like to read more about Gadolinium contrast, you may like to read our associated article here.
An MRI scan is a painless and safe procedure and most patients find it manageable with the correct amount of support from the radiographers. MRI scans do not expose the body to any dangerous radiation, meaning that it is a good test for anyone who may be vulnerable to the effect of ionising (high energy) radiation. Extensive research is constantly being carried out into whether the magnetic fields and radio waves used during MRI scans pose a risk to the body, but no evidence has ever been found, meaning MRI scans are one of the safest medical procedures currently available.