MRI vs. CT scan: What’s the difference?

When thinking of the difference between a CT scan and MRI, it can be tempting to distinguish them in terms of comfort. One is short, the other is long. One is quiet, the other is loud. But these tools are fundamentally different from each other beyond just the patient experience. And these differences are important to understand how and why they are used. 

CT (Computed Tomography) Scans and MRI (Magnetic Resonance Imaging) are both methods of diagnostic imaging that are used to diagnose and assess a variety of conditions, including cancer. 

A CT scan, sometimes called a CAT Scan, is a series of X-ray pictures taken around your body that are then composited into a 3D image. Unlike a typical, single X-ray where the image is 2D and internal structures are often overlapped and can be difficult to distinguish, a CT scan provides a cross-section of the body, allowing doctors to see individual internal structures in greater clarity. This allows doctors to identify blood clots, fractures and internal injuries, and tumors, among other things. 

MRI, on the other hand, uses a magnetic field and radio waves to produce images. The image MRI produces is, like a CT scan, 3D, though it will often be presented as a series of 2D images at different positions to allow doctors to better analyze the information they provide. The MRI machine’s magnetic field is produced by a very powerful magnet, which is always on and affects the entire room the machine is in. This is why you need to remove any metal jewelry before MRI, as the magnet may pull on it otherwise.       

Contrary to popular belief, MRI does not affect all metal types equally, and a metal implant may not disqualify you from receiving MRI. Your doctor will assess your implant to determine whether MRI is safe and suitable for you. Always disclose any metal in your body, including implants or fragments, to your doctor and MRI technologist before MRI.

MRI can be used to identify and assess many of the same things as a CT scan, so why might your doctor order one and not the other?

MRI is generally better at distinguishing between different types of tissue in the body, and a CT scan is generally better at distinguishing between the internal structures of the body. For example, an MRI can provide more detailed imaging of brain tumors, as it can distinguish between brain tissue and tumors more easily than a CT scan. A CT scan may be preferred for the initial detection of lung tumors, as it can better visualize small pulmonary nodules and internal structures. Getting one done doesn’t necessarily mean you won’t receive the other later down the line, even for the same condition.  

However, sometimes the reason you receive one and not the other is far simpler. Perhaps you have a metal implant that’s unsuitable for MRI, or you are sensitive to the iodinated contrast agents used for CT scans. While CT scans and MRI are different and have different strengths, the fact that they can be used for many of the same applications allows patients who may not be able to receive one to still get effective diagnostic imaging.

Eastern Radiologists offers CT and MRI scans. Talk to your doctor about which one is right for you and remember to ask for a referral to Eastern Radiologists.