Effects of adaptive section collimation on patient radiation dose in multisection spiral CT

To evaluate the potential effectiveness of adaptive collimation in reducing computed tomographic (CT) radiation dose owing to z-overscanning by using dose measurements and Monte Carlo (MC) dose simulations.

Institutional review board approval was not necessary. Dose profiles were measured with thermoluminescent dosimeters in CT dose index phantoms and in an Alderson-Rando phantom without and with adaptive section collimation for spiral cardiac and chest CT protocols and were compared with the MC simulated dose profiles. Additional dose measurements were performed with an ionization chamber for scan ranges of 5-50 cm and pitch factors of 0.5-1.5.

The measured and simulated dose profiles agreed to within 3%. By using adaptive section collimation, a substantial dose reduction of up to 10% was achieved for cardiac and chest CT when measurements were performed free in air and of 7% on average when measurements were performed in phantoms. For scan ranges smaller than 12 cm, ionization chamber measurements and simulations indicated a dose reduction of up to 38%.

Adaptive section collimation allows substantial reduction of unnecessary exposure owing to z-overscanning in spiral CT. It can be combined in synergy with other means of dose reduction, such as spectral optimization and automatic exposure control.