A computed tomography (CT) scanner uses X-rays, a type of ionizing radiation, to acquire images, and make it a good tool for examining tissue composed of elements of a higher atomic number than the tissue surrounding them, such as bone and calcifications within the body, or of structures. MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire its images and is best suited for non-calcified tissue, though MRI images can also be acquired from bones and teeth as well as fossils.
CT may be enhanced by use of contrast agents containing elements of a higher atomic number than the surrounding flesh such as iodine or barium. Contrast agents for MRI have paramagnetic properties, e.g., gadolinium and manganese.
Both CT and MRI scanners are able to generate multiple two-dimensional cross-sections  of tissue and three-dimensional reconstructions. Unlike CT, which uses only medical x ray attenuation to produce image contrast, MRI has a long list of properties which may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to detect different features.
MRI can generate cross-sectional images in any plane. In the past, CT was limited to acquiring images in the axial plane. The scans are used to be called Computed Axial Tomography scans. However, the development of multi-detector CT scanners with near-isotropic resolution, allows the CT scanner to produce data which can be retrospectively reconstructed in any plane with minimal loss of image quality.
For purposes of tumor detection and identification in the brain, MRI is generally superior. However, in the case of solid tumors of the abdomen and chest, CT is often preferred due to less motion artifact. In addition, CT usually is more widely available, faster, less expensive, and may be less likely to require the person to be sedated or anesthetized.
MRI is also best suited for cases when a patient is to undergo the examination several times successively in the short term, because, unlike CT, it does not expose the patient to the hazards of ionizing radiation'

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