Keywords:
Radiation physics, Radioprotection / Radiation dose, Cone beam CT, CT, Dosimetry, Physics, Radiation safety, Quality assurance
Authors:
A. Schegerer1, U. Lechel1, M. Ritter2, G. Weisser2, C. Fink3, G. Brix1; 1Neuherberg/DE, 2Mannheim/DE, 3Celle/DE
DOI:
10.1594/ecr2013/C-1998
Methods and Materials
A CBCT system recently installed in the operating room of a German urological clinic was used for this study.
Four different protocols were used for different putative urological problems: a low-dose protocol,
a standard protocol for high-contrast objects,
a standard protocol for soft-contrast objects,
and a high-dose protocol.
The protocols differ in the total number of projections,
the dose per projection,
and the angulation step width.
In contrast to the 360°-rotation of MSCT,
the X-ray tube and detector of the CBCT rotate around the isocenter by ~230°.
The automatic dose modulation of the CBCT system that varies the exposure-time product and voltage was turned on.
For comparison,
the acquisition parameters of the MSCT scanner were modified in eight subsequent CT scans.
The voltage and exposure-time product were varied between 80 kV and 130 kV and 40 mAs and 120 mAs,
respectively.
The automatic dose modulation was turned on in some CT scans.
These variations in acquisition parameters resulted in CTDIvol values ranging from 1.5 mGy to 15 mGy.
Additionally,
different filter kernels and slice thicknesses were considered for image reconstruction.
The scan lengths for the examinations at the MSCT and CBCT were 18 cm.
The pixel sizes of the reconstructed images were 0.50 x 0.5 mm2 and 0.65 x 0.65 mm2 for the CBCT and MSCT,
respectively.
Local skin doses and organ doses were measured using thermoluminescent dosimeters (TLDs) placed at the surface and in the interior of an Alderson-Rando phantom representing the reference man with a body weight of 75 kg.
For the assessment of image quality,
image characteristics such as the contrast-to-noise ratio (CNR) were determined from images of the AAPM CT performance phantom using its sensitometry module.
To ensure identical absorption as in the Alderson-Rando phantom,
the AAPM phantom was placed between two shells of Perspex.
Different pairs of Perspex shells were used to adapt radiation absorption to radiation quality (s.
Fig.
1).