Question

When a CT scanner is operating under tube current modulation how are dose and image quality affected bychanges in:
i. Pitch
ii. Tube rotation time
bearing in mind that the effective mAs (mAs/pitch) remains unchanged? In each case give an explanation for
what causes the observed changes.

Answer 1

Ans. -

Higher pitches resulted in higher tube currents in the ATCM in all scanners. The effective mAs was affected by pitch in all scanners except for Philips. The NRMSE of effective mAs was lower than that of tube current in all cases except for Toshiba with pitch 0.639.

Scanning parameters such as tube current and tube potential determine photon fluence and incident beam energy, which affect image quality.

The scanner was generally able to compensate with tube current to maintain a constant level of image noise when changes in rotation time, pitch and tube voltage were made. An increase in image noise was observed for scans with 80 kV.