MRI and CT are very different and used for different needs and reasons; both are valuable and both have specific applications; they are not interchangeable and one is not a better test than the other for all things. The decision whether to use one or the other is based according to the density of the body tissue that needs to be seen. Softer tissues that have more water molecules or hydrogen atoms in them are better seen by MRI because of the physics used.
Body tissue that is composed of a greater number of hydrogen atoms are seen well using Magnetic Resonance Imaging . Thus, MR imaging shows us the spinal cord, and tendons and ligaments in the knee and shoulder and brain tissue well but not the skull bone.
Brain tissue that is dying from loss of blood supply and tumors of the brain are best seen on Magnetic Resonance Imaging. Neurological diseases of the brain, like Multiple Sclerosis, are detectable on MR imaging and present as plaque deposits on the brain surface.
This makes Magnetic Resonance Imaging the modality used to look for neurological conditions and abnormalities. It’s also used to look at bone marrow (inside the femur bone for example) but not the dense outer layer of bone; CT is used for that.
Magnetic Resonance Imaging is also used to evaluate internal organs and arteries in the neck and brain. IV contrast, different than what is used in CT, can also be used to fill the arteries for further evaluation when needed.
Magnetic Resonance Imaging does not use an x-ray beam; instead, you’re placed in a magnetic field instead of passing through a spinning X-ray beam like CT. This is why MR scanners often make people feel claustrophobic. The design is such that the magnetic field needs to surround the part being imaged for the best resolution.
The image on the left is a CT Brain image that shows bleeding (white areas) in the brain. The image on the right is an MR scan of the same brain. CT scan imaging is far superior than MR scan imaging at visualizing bleeding in the brain from a head injury or a ruptured artery.
This side by side comparison shows how both CT scan and MR imaging are often used to help in evaluation and diagnoses.