X-ray computed tomography
X-ray computerized tomography is a type of computed tomography and is defined as "tomography using x-ray transmission and a computer algorithm to reconstruct the image." The scans can be fairly quick, on the order of low minutes, and thus extremely valuable in urgent situations. Recent studies have shown that they deliver more ionizing radiation than once thought, making it a more risk-benefit decision of when to use them than nonionizing but slow magnetic resonance imaging or fast but less precise ultrasonography, or less precise two-dimensional X-rays. When urgency is not an issue, CT may give better quality than MRI for different studies (e.g., CT of the abdomen vs. MRI of the skull).
Newer types of X-ray computerized tomography
- Spiral CT. Spiral CT, also called helical CT is "computed tomography where there is continuous X-ray exposure to the patient while being transported in a spiral or helical pattern through the beam of irradiation. This provides improved three-dimensional contrast and spatial resolution compared to conventional computed tomography, where data is obtained and computed from individual sequential exposures". Examples include computed tomographic pulmonary angiography for detecting pulmonary embolism, computed tomographic cardiac angiography for detecting coronary heart disease, and computed tomographic colonography (virtual colonoscopy).
- Cone CT is uses a cone or pyramid-shaped beam of radiation.
- CT angiography, using injected contrast media, with multislice or spiral beams
The risk associated with a CT scan (the increased risk of cancer associated with the radiation doses) is extremely low for any one person. However, given the increasing number of CT scans being obtained, the increasing exposure to radiation in the population may be a public health issue in the future. 
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