Pitch (CT)

Pitch (P) is a term used in helical CT. It has two terminologies depending on whether single slice or multislice CT scanners are used .

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 .

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 :

• 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 .