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Diagnostic imaging
6th December 2019  •  Read 0 times

CT vs MRI- what’s the difference?

With so many diagnostic imaging tests available, patients often ask us to explain the differences between them. We have decided to start with the trusted CT scan and MRI.  

As a diagnostic healthcare professional, you often come across several questions on what modality to choose for a certain diagnosis and what the benefits of one modality are over another according to the clinical indications presented. 

What is the difference between a CT scan and an MRI? 

  • A CT scan uses X-rays to create detailed pictures of organs, bones, and other tissues. The patient lies on a table that moves through a scanning ring, which looks like a large doughnut. The data collected can be assembled to form three-dimensional images. The images reveal abnormalities in both bone and soft tissues, such as lung abnormalities, tumours in different organs, bone fractures or cancer staging.                           
  • An MRI also creates detailed pictures of areas inside the body, but it uses radio waves and a powerful magnet to generate the pictures, so this process is radiation free. The person also lies on a couch that moves into a tube (open at both ends) and produces loud noises during scanning. Similarly, these pictures can show the difference between normal and diseased tissue or characterise the nature of any lesion. 

What are the advantages of CT and MRI? 

With a CT scan, we can create an image of almost the entire body, from the head to the lower limbs, in a few seconds. CTs are incredibly useful for diagnosing and staging cancer, checking whether it has come back, and monitoring whether a treatment is working. It’s very effective for surveying the entire body to look for places where the cancer has spread, such as the lungs, liver, or bone. These are called metastases. 

Most of the time, CT is the first choice to stage cancer. We can also do special scans, such as CT cardiac scans which combined with a dye contrast injection can identify and quantify possible narrowing or blockage of the main coronary arteries. 

MRI is great to show some type of cancers, tumours or lesions that the CT scans can’t show. Some cancers, such as prostate cancer, uterine cancer, and certain liver cancers, are very hard to detect on a CT scan. Also, an MRI provides much more detailed images which can be used to characterize some lesions in much greater detail. Metastases to the bone and brain also show up better on an MRI, as do sports injuries. 


Both CT scans and MRIs pose some disadvantages and risks when used. These are based on the type of imaging as well as how the imaging is performed. 

CT scan disadvantages and risks include: 

  • The use of ionizing radiation.  
  •  A potential allergic reaction to the use of the contrast injection (should an injection be necessary for your scan). 

MRI disadvantages and risks include: 

  • For MRI, people who have trouble with claustrophobia or are unable to hold their breath, which may be required for certain abdominal imaging tests, may not be able to tolerate the procedure. 
  • Due to the powerful magnetic field some medical devices can be a contra-indication for your scan, such as certain pacemakers, and other medical devices. If you have a medical implant in your body, you may need to provide some information about the implant to the Radiographer performing your scan to check for MRI compatibility.  Newer medical devices are usually designed with this in mind, so they are safe inside an MRI but it’s always best to check. 
  • Potential allergic reaction to the use of the contrast injection (should an injection be necessary). 
  • Very loud noises although you will be provided with full ear protection. 

Choosing between an MRI and CT scan 

Your doctor will likely provide a recommendation based on your symptoms and explain whether an MRI or CT scan is more suitable for you. If you need a more detailed image of your soft tissue, ligaments, or organs, your doctor will commonly suggest an MRI. 

Such cases include:

  • Herniated disks 
  • Torn ligaments 
  • Soft tissue issues 
  • Muscle sprains 
  • Lesion/Tumor characterization  

Your doctor is likely to recommend a CT scan if you have experienced: 

  • Head trauma 
  • Lung pathology 
  • Cancer Staging  
  • Bone fracture 

It is important to remember that both CT scans and MRI scans are relatively low risk and both offer important information to help your doctor properly diagnose specific conditions. If your doctor sees something on your CT scan that they are unsure about, they may recommend an MRI for further evaluation. If you are unable to lie still or hold your breath (during some scans), the doctor may suggest that you have a CT scan as an alternative. If you have a cardiac pacemaker or other medical implant contraindicated for an MRI scan, you might have to have a CT scan instead. Rest assured that doctors will always choose your imaging scan based on the principle of whether the benefits of a test outweigh its risks. 

If you would like to discuss the topics in this blog, contact our friendly patient services team on 01442 331 900 or fill out our contact form here

Picture of Daniel Santos
About the author
Daniel Santos

Cross Sectional Radiographer

Qualified in 2010, I’m a very qualified Radiographer with nearly 10 years of experience shared between the NHS and the private sector. Having worked at NHS and the private sector allowed me to be trained in different environments and gather all my expertise in most of the Radiological modalities. My expertise covers most of the different Modalities as CT/MRI and x rays studies. I’ve specialised in Cardiac CT at OSD Healthcare which I am now fully trained in.  I have also undertaken a course in MRI Cardiac scans which I’m still in training for at OSD Healthcare. I believe in patient care. I dedicate a lot of my attention to patient care and listen to every individual patient as everyone is different and require different ways of dealing with. 

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