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.
Prostate cancer screening
Transrectal ultrasonography (TRUS) has the advantage of being fast and minimally invasive, and better than MRI for the evaluation of superficial tumor. It also gives details about the layers of the rectal wall, accurate and useful for staging primary rectal cancer. While MRI is better in visualization of locally advanced and stenosing cancers, for staging perirectal lymph nodes, both TRUS and MRI are capable. TRUS has a small field of view, but 3D TRUS can improve the diagnosis of anorectal diseases.