[Effect of collimation and pitch on radiation exposure and image quality in spiral CT of the thorax].

Purpose: The aim of the study was to optimize the scan parameters for spiral-CT of the thorax.

Methods: Using a Somatom AR-SP (Siemens AG, Erlangen) CT-unit, four different scanning protocols (collimation/table feed) were compared: A (5/10), B (10/10), C (10/15), D (10/20). High contrast resolution in the xy-plane and in the z-direction were investigated with special phantoms. Noise levels were measured using a water phantom, the radiation exposure of lung parenchyma was investigated using an Alderson-Rande-Phantom. An anthropoid chest phantom was scanned to compare the image quality of the four protocols.

Results: Protocol A showed the best resolution in the z-direction; the resolution in the xy-plane was equal for all protocols. Image noise in the center of the water phantom was 58 HU for protocol A and 40 HU for the other three. The following equivalent organ dose were measured for lung parenchyma: A 14 mSv, B 29 mSv, C 20 mSv, D 14 mSv.

Conclusion: Scanning protocols with small collimation and a pitch of 2 are characterised by low radiation exposure and excellent resolution in the z-direction. These parameters seem to be most suitable for lung scanning.