Helical CT image acquisition
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Helical (a.k.a. spiral) CT image acquisition was a major advance on the earlier stepwise ("stop and shoot") method.
With helical CT, the patient is moved through a rotating x-ray beam and detector set. From the perspective of the patient, the x-ray beam from the CT traces a helical path. The helical path results in a three-dimensional data set, which can then be reconstructed into sequential images for an image stack.
Helical CT allows a scan to be performed in a single breath-hold.
Most modern CT protocols use helical acquisition due to its speed and because it reduces misregistration from patient movement or breathing. Sequential scanning (step-and-shoot) acquisition is still used in some situations (e.g. helical acquisition can lead to artifacts on head CT).
The radiation dose administered during helical acquisition depends on the speed of the patient through the scanner, also known as the pitch.