13 Apr, 2022

CT vs MRI Scans: What are the differences, use cases, and risks?

Diagnostic imaging allows medical professionals to see exactly what is going on inside your body, and helps them gain valuable information when compiling a diagnosis and treatment plan.

There are several different types of diagnostic scans, all with their own benefits. The different methods we offer (e.g. CT, MRI, Ultrasound) are complementary to each other, and no one type of scan is better than another. Our consultant radiologists, who are specialists in medical imaging, will guide you towards the best scan for your needs from a medical perspective.

However, before you go ahead and book your scan, you might be wondering what the differences are between MRI and CT scans, and which option could best for you. This guide will help you to understand the main features, benefits and risks of each scan type.

🩺 When you book your scan with us, a dedicated clinician will assess your booking and medical history to make sure you receive the correct scan for your requirements.

What is an MRI scan, and how does it work?

MRI stands for magnetic resonance imaging, and uses strong magnets and a radiofrequency current to generate images of the inside of your body. It is best suited to scanning the soft tissues of the body, such as tendons and ligaments, the spinal cord, and blood vessels, as well as internal organs, bones and joints.

MRI scans look for abnormalities, inflammation, disease and tumours, and a special type of MRI called Magnetic Resonance Angioplasty (MRA) can be used to assess the health of blood vessels.

An MRI scanner is shaped like a cylinder, with a flat motorised bed that moves the patient into the scanning machine. Open MRI scanners are available, which have magnets above and below the patient, rather than all around them, which can help alleviate feelings of claustrophobia.

📌 Learn more about MRI technology, processes and uses in our complete guide to MRI scans

What is a CT scan, and how does it work?

CT stands for computerised tomography, and uses a large X-ray machine to capture images of the inside of the body. Modern multidetector CT scans can compile a series of 2D images into 3D images, while standard X-ray scans only produce 2D images.

A CT scanner is shaped like a doughnut, with an X-ray source on one side, and detectors on the other. It also has a flat bed, which moves through the scanner.

The X-ray beam circles around you during the scan, and the detectors capture the strength of the rays which have passed through your body. Different bodily tissues allow varying levels of X-rays to pass through them, and the scanning machine uses this variation to compile an image.

Iodine-based contrast materials may be required for vascular enhancement, to image the vessels of the body, and to delineate differences between the tissues.

CT scans are most commonly used for body imaging, including the brain, neck, chest, abdomen and pelvis. Indications can include suspected cancer, suspected or confirmed stroke, clots to the pulmonary artery (pulmonary embolism), and aortic aneurysm. CT scanning is a major modality used to assess trauma of the spine and body in general.

What are the differences between MRI and CT scan processes?

  • Noise: As the magnets and radiofrequency currents are switched on and off during an MRI scan, there can be a series of loud clanging and buzzing noises. CT scans are much quieter and faster.
  • Radiation: An MRI scan does not involve any ionising radiation, while a CT scan does, in the form of X-rays. The radiation exposure for a CT scan is carefully considered to limit the risk, however, CT scans are not recommended for pregnant women due to the potential impacts of radiation exposure on an unborn baby.
  • Speed: MRI scans can take from 15 minutes up to 90 minutes, whereas a whole-body CT scan generally takes less than 1 minute. In emergency scenarios, a CT scan may be the best method to understand the urgent care required as quickly as possible. Also, patients who are unable to lie still for a longer MRI scan may be better suited to a fast CT scan.
  • Contrast: MRI scans don’t always require a contrast agent, but when they do, a Gadolinium-based contrast dye is used. Meanwhile, CT scans require contrast agents more commonly and use iodine-based dyes.
  • Claustrophobia: A traditional MRI scanner resembles a large cylinder, with a motorised flat bed that moves the patient in and out as required. This can cause anxiety and claustrophobic feelings in some cases, although some modern MRI scanners have open sides. Meanwhile, CT scanners are more of a ring than a tunnel, which means a patient is not fully surrounded at any one time, which does not cause as much anxiety.
  • Detail: MRI scans tend to capture more detailed images of soft tissue, ligaments and organs than CT scans do. CT scans are more commonly used for assessing fractures, scanning larger areas quickly, or pinpointing and staging cancerous tumours.

📌 For further information about MRI scans and claustrophobia, visit our guide.

What are the different preparations for MRI and CT scans?

An MRI scan does not normally include any prior preparation, other than removing metal objects such as jewellery, hearing aids or belts. You might be asked to change into a hospital gown, depending on the type of scan you’re having and the clothes you wear to your appointment. You will not usually be asked to refrain from eating, drinking or taking medications prior to your scan.

For a CT scan, you might be required to fast (not eat or drink) for a couple of hours before the scan appointment. You will also be asked to remove any metal objects, not due to a magnetic field, but because they may affect the quality of the images obtained. As with an MRI scan, it might be necessary to change into a hospital gown for a CT procedure.

What are the risks?

CT scan risks:

  • CT scans do use ionising radiation, which can be harmful in high doses. The exposure during your scan is low, and our expert clinicians carefully consider patients’ suitability for a CT scan.
  • CT scans often require an iodine-based contrast agent, which helps define certain body parts in the scan images. The contrast agent can be injected into the bloodstream via an IV, swallowed in liquid form, or administered rectally (via enema) depending on the purpose of the scan. Iodine contrast agents can cause mild side effects such as nausea and vomiting. In rare cases, more serious reactions including swelling, skin rash and convulsions can occur, but radiology departments are equipped to recognise and treat more serious side effects. Blood-work to assess kidney function is also required beforehand to check suitability.
  • While CT scans can be valuable tools for assessing the treatment and progress of cancer, it is not recommended to repeat them regularly due to the radiation exposure involved.

MRI scan risks:

  • MRI scans are generally safe and non-invasive, with no known side effects. They do not use ionising radiation, so can be repeated with no harmful effects. However, despite having no known dangers, MRI scans are not recommended for pregnant women during their first trimester.
  • As MRI scans use very strong magnetic fields, they can interact with metal in the body. This includes shrapnel, medical devices (such as pacemakers and hearing aids) and even some tattoo inks. When you book an MRI scan, you’ll be asked about your medical history and a safety form will be required. This helps our expert clinicians to assess whether you are a suitable candidate for an MRI scan.
  • Some MRI scans do require contrast agents, but unlike for CT scans, they are gadolinium based rather than iodine based. There are fewer side effects to gadolinium, and adverse reactions are rarer. However, people with kidney disease are at higher risk of complications, so blood-work assessing your kidney function is usually required beforehand.

Should you get an MRI scan or a CT scan?

A key difference between an MRI scan and a CT scan is the radiation involved for CT, whereas MRI does not use any radiation. This means CT scans are more heavily regulated, and can only be carried out where it is deemed medically justifiable. Medical justifications include:

  • Where the body part you’re concerned about is best visualised in a CT scan, for example the lungs
  • Where other scans have been inconclusive
  • Where an MRI scan would take too long to complete
  • To assess fractures
  • To assess internal bleeding
  • To assess cancer stage or the response to treatment

Our expert clinical team assess every CT scan booking we receive, and speak to every prospective patient for a 1-1 pre-scan consultation to understand their medical requirements. Using their expertise, they will refer you for the appropriate scan (CT, MRI or ultrasound) depending on your clinical need and indication.

📌 Next Steps:

Book your CT scan at a convenient location near you

Book an MRI scan quickly and easily

Visit our news page for further information about scan types, symptoms, and support

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