A pragmatic approach to determine the optimal kVp in cone beam CT: balancing contrast-to-noise ratio and radiation dose.
OBJECTIVE
To determine the optimal kVp setting for a particular cone beam CT (CBCT) device by maximizing technical image quality at a fixed radiation dose.


METHODS
The 3D Accuitomo 170 (J. Morita Mfg. Corp., Kyoto, Japan) CBCT was used. The radiation dose as a function of kVp was measured in a cylindrical polymethyl methacrylate (PMMA) phantom using a small-volume ion chamber. Contrast-to-noise ratio (CNR) was measured using a PMMA phantom containing four materials (air, aluminium, polytetrafluoroethylene and low-density polyethylene), which was scanned using 180 combinations of kVp/mA, ranging from 60/1 to 90/8. The CNR was measured for each material using PMMA as background material. The pure effect of kVp and mAs on the CNR values was analysed. Using a polynomial fit for CNR as a function of mA for each kVp value, the optimal kVp was determined at five dose levels.


RESULTS
Absorbed doses ranged between 0.034 mGy mAs(-1) (14 × 10 cm, 60 kVp) and 0.108 mGy mAs(-1) (14 × 10 cm, 90 kVp). The relation between kVp and dose was quasilinear (R(2) > 0.99). The effect of mA and kVp on CNR could be modelled using a second-degree polynomial. At a fixed dose, there was a tendency for higher CNR values at increasing kVp values, especially at low dose levels. A dose reduction through mA was more efficient than an equivalent reduction through kVp in terms of image quality deterioration.


CONCLUSIONS
For the investigated CBCT model, the most optimal contrast at a fixed dose was found at the highest available kVp setting. There is great potential for dose reduction through mA with a minimal loss in image quality.
Department of Radiology, Faculty of Dentistry, Chulalongkorn University, Bangkok, Thailand.
R
R
Pauwels
O
O
Silkosessak
R
R
Jacobs
R
R
Bogaerts
H
H
Bosmans
S
S
Panmekiate
D000388:Air; D000465:Algorithms; D000535:Aluminum; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods; D019047:Phantoms, Imaging; D020959:Polyethylene; D019904:Polymethyl Methacrylate / Q000737:chemistry; D011138:Polytetrafluoroethylene; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods; D012542:Scattering, Radiation; D014965:X-Rays
D011138:Polytetrafluoroethylene; D020959:Polyethylene; D019904:Polymethyl Methacrylate; D000535:Aluminum
cone beam computed tomography; image contrast; optimization; radiation dose