The effect of vertical centering and scout direction on automatic tube voltage selection in chest CT: a preliminary phantom study on two different CT equipments

Touko Kaasalainen; Teemu Mäkelä; Mika Kortesniemi

doi:10.1016/j.ejro.2018.12.001

The effect of vertical centering and scout direction on automatic tube voltage selection in chest CT: a preliminary phantom study on two different CT equipments

Eur J Radiol Open. 2018 Dec 17:6:24-32. doi: 10.1016/j.ejro.2018.12.001. eCollection 2019.

Authors

Touko Kaasalainen^{1

2}, Teemu Mäkelä^{1

2}, Mika Kortesniemi^{1

2}

Affiliations

¹ HUS Medical Imaging Center, Helsinki University Central Hospital, Finland.
² Department of Physics, University of Helsinki, Finland.

Abstract

Purpose: To determine the effect of patient's vertical off-centering and scout direction on the function of automatic tube voltage selection (ATVS) and tube current modulation (TCM) in chest computed tomography (CT).

Methods: Chest phantom was scanned with Siemens and GE CT systems using three clinical chest CT protocols exploiting ATVS and a fixed 120 kVp chest protocol. The scans were performed at five vertical positions of the phantom (-6 to +6 cm from the scanner isocenter). The effects of scout direction (posterior-to-anterior, anterior-to-posterior, and lateral) and vertical off-centering on the function of ATVS and TCM were studied by examining changes in selected voltage, radiation dose (volume CT dose index, CTDI_vol), and image noise and contrast.

Results: Both scout direction and vertical off-centering affected ATVS. The effect differed between the vendors for the studied geometry, demonstrating differences in technical approaches. The greatest observed increase in CTDI_vol due to off-centering was 91%. Anterior-to-posterior scout produced highest doses at the uppermost table position, whereas posterior-to-anterior scout produced highest doses at the lowermost table position. Dose varied least using lateral scouts. Vertical off-centering impacted image noise and contrast due to the combined effect of ATVS, TCM, structural noise, and bowtie filters.

Conclusions: Patient vertical off-centering and scout direction affected substantially the CTDI_vol and image quality in chest CT examinations. Vertical off-centering caused variation also in the selected tube voltage. The function of ATVS and TCM methods differ significantly between the CT vendors, resulting in differences in CTDI_vol and image noise characteristics.

Keywords: Automatic tube voltage selection; Image quality; Patient centering; Tube current modulation.