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Pitch (P) is a term used in helical CT. It has two terminologies depending on whether single slice or multislice CT scanners are used 1-3.
Single slice CT (SSCT)
The term detector pitch is used and is defined as table distance traveled in one 360° gantry rotation divided by beam collimation 2.
For example, if the table traveled 5 mm in one rotation and the beam collimation was 5 mm then pitch equals 5 mm / 5 mm = 1.0.
Choice of pitch affects both image quality and patient dose 2:
- P = 1.0: x-ray beams are contiguous for adjacent rotations
- P >1.0: x-ray beams are not contiguous for adjacent rotations; i.e. there are gaps in the x-ray helix, but the full volume is still irradiated, only with fewer projections per rotation
- P <1.0: there is x-ray beam overlap; i.e. a volume of tissue is irradiated more than once per scan
Thus a pitch >1.0 results in decreased patient dose but also decreased image quality (fewer projections are obtained, resulting in lower SNR). A pitch of <1.0 results in better image quality, but a higher patient dose.
Multislice CT (MSCT)
Beam pitch is defined as table distance traveled in one 360° gantry rotation divided by the total thickness of all simultaneously acquired slices 3.
- 1. Allisy-Roberts P, Williams J. Farr's Physics for Medical Imaging. Saunders Limited. (2007) ISBN:0702028444. Read it at Google Books - Find it at Amazon
- 2. Silverman PM, Kalender WA, Hazle JD. Common terminology for single and multislice helical CT. AJR Am J Roentgenol. 2001;176 (5): 1135-6. doi:10.2214/ajr.176.5.1761135 - Pubmed citation