Medical imaging
Because CT and MRI are sensitive to different tissue properties, the appearances of the images obtained with the two techniques differ markedly. In CT, X-rays must be blocked by some form of dense tissue to create an image, so the image quality when looking at soft tissues will be poor. In MRI, while any nucleus with a net nuclear spin can be used, the proton of the hydrogen atom remains the most widely used, especially in the clinical setting, because it is so ubiquitous and returns a large signal. This nucleus, present in water molecules, allows the excellent soft-tissue contrast achievable with MRI.
Myelography
Nowadays, MRI is used for most investigations previously performed by myelography. This has many advantages and does not require contrast fluid to be injected into the spine. However, a CT myelogram may be useful for patients who cannot undergo MRI (e.g., those with pacemakers or cochlear implants), or for those in whom MRI provides limited information (e.g., those with extensive metal in the spine).