MRI vs CT Scan

One of the more common questions about MRI vs CT scan asked by patients is what's the difference between CT and MRI? Why should I have a CT instead of an MRI? Or why should I have an MRI instead of a CT?

These are very common questions and imaging professionals like us have the answers. The answers come from 2 other very important questions....

1. What part of the body does your doctor want to see?

AND

2. What's the reason for the exam?

CT Scan does not show you tendons and ligaments very well at all; at least not yet. MRI is the best choice for that. Tendons and ligaments around the shoulder and knee are best seen by the physics used in MRI. This is due to the density of the tissues that compose the tendons and ligaments.

Spinal cord is best seen by MRI for the same reason. The density of the spinal cord and the composition of it is such that MRI physics can show it to us much better than CT.

There are also reasons why CT is the exam of choice over MRI. It is the preferred modality for cancer, pneumonia, and abnormal chest x-rays. Bleeding in the brain, especially from injury, is better seen on CT than MRI. But a tumor in the brain is better seen on MRI. CT displays the inner auditory canals and is superior at visualization of the mastoid air cells.

If you've been in an accident, organs can get torn or damaged. CT shows organ tear and organ injury quickly and efficiently. Broken bones and vertebral bodies of the spine are better seen on CT but injury to the spinal cord itself is displayed on MRI far better than CT.

CT is far superior at visualizing the lungs and organs in the chest cavity between the lungs. MRI is not a good tool for visualizing the chest or lungs at all.


MRI vs CT scan depends on what needs to be visualized and the reason you need the test. Radiologists are the doctors that specialize in reading images of the body and therefore know which test is best for showing anatomy according to the reason for the exam. Very experienced CT and MRI technologists will also know from working with Radiologists.

The difference in the way the images are produced in MRI vs CT is the physics involved. CT scan uses an x-ray beam that slices through you like a knife carving a spiral ham.

MRI (magnetic resonance imaging) uses a magnetic field with radio frequencies introduced into it. When your body is placed inside the magnetic field, the molecules of water in your body (hydrogen molecules) will start to spin like a kid's top when he spins it. The top will begin to wobble as it slows down.

Your hydrogen molecules will start to wobble just like the top does at a certain rate of speed; the stronger the magnetic field, the faster they will wobble and the weaker the magnetic field, the slower they will wobble; it depends on the strength of the magnetic field.

Then a radio frequency is introduced into the magnetic field at the same rate of speed at which the "tops wobble" causing the wobbling tops and radio frequency to sing out together sharing the same signal frequency. (Thus the term resonance) That signal is used by the MRI computer to produce the image.


MRI is the one that has the longer shape to it like this.

CT scanners have more of a donut shape like this.

Click on CT vs MRI for further discussion and demonstration.



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