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.
Interventional magnetic resonance imaging
Because of the lack of harmful effects on the patient and the operator, MR is well suited for "interventional radiology", where the images produced by an MRI scanner are used to guide a minimally-invasive procedure intraoperatively and/or interactively. Interventional MRI can be used for a variety of specialized procedures. iMRI systems are often used for doing biopsies of lesions, resections of tumors, guiding thermal ablation of tissue, as well as other procedures. It is commonly used in neurosurgery where every millimeter of tissue spared in surgery can make a difference for patient recovery.