Helical (a.k.a. spiral) CT image acquisition was a major advance on the earlier stepwise ("step and shoot") method which led to faster image acquisition of a continuous data set 1.
Clinical applications
Helical image acquisition is used in the vast majority of CT applications in which a larger examination volume is acquired.
Technique
With helical CT, the patient is moved through a rotating x-ray beam and detector set. From the patient's perspective, 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 settings (e.g. standard head CT or cardiac CT, where helical acquisition can lead to artifacts or due to dose considerations).
The radiation dose administered during helical acquisition depends on the speed of the patient through the scanner, also known as the pitch.
Limitations
complex reconstruction
over-ranging (with a potential for a higher dose)
slight motion artefact (due to table motion)
Unable to process the form. Check for errors and try again.