<Li> <P> MR Conditional </P> </Li> <Li> <P> MR Unsafe </P> </Li> <P> The CT scan was introduced in the 1970s and quickly became one of the most widely used methods of imaging . A CT scan can be performed in under a second and produce rapid results for clinicians, with its ease of use leading to an increase in CT scans performed in the United States from 3 million in 1980 to 62 million in 2007 . Clinicians oftentimes take multiple scans, with 30% of individuals undergoing at least 3 scans in one study of CT scan usage . CT scans can expose patients to levels of radiation 100 - 500 times higher than traditional x-rays, with higher radiation doses producing better resolution imaging . While easy to use, increases in CT scan use, especially in asymptomatic patients, is a topic of concern since patients are exposed to significantly high levels of radiation . </P> <P> In PET scans, imaging does not rely on intrinsic biological processes, but relies on a foreign substance injected into the blood stream traveling to the brain . Patients are injected with radioisotopes that are metabolized in the brain and emit positrons to produce a visualization of brain activity . The amount of radiation a patient is exposed to in a PET scan is relatively small, comparable to the amount of environmental radiation an individual is exposed to across a year . PET radioisotopes have limited exposure time in the body as they commonly have very short half - lives (~ 2 hours) and decay rapidly . Currently, fMRI is a preferred method of imaging brain activity compared to PET, since it does not involve radiation, has a higher temporal resolution than PET, and is more readily available in most medical settings . </P>

A technique for studying brain structure that uses high-resolution x-rays is known as