What happens to the pitch during helical scanning if the table moves a distance shorter than the width of the beam?

In helical (or spiral) CT scanning, the pitch is defined as the ratio of the table movement per rotation to the width of the X-ray beam. When the table moves a distance that is shorter than the width of the beam, the resulting pitch will indeed be less than 1.

This occurs because if the table movement during one rotation is less than the width of the beam, the helical paths of adjacent slices will overlap. This means each segment of tissue being imaged will be scanned multiple times, resulting in a higher density of data being collected in that area. This can improve image quality due to increased data acquisition, but it also indicates that the pitch ratio drops below 1, reflecting that the table movement does not cover as much distance as the beam width during a complete rotation.

A pitch value less than 1 typically suggests that the coverage of the scanned area is more extensive and provides higher-quality imaging, although it can lead to longer scan times and increased radiation dose if not managed properly.