Deriving Hounsfield units using grey levels in cone beam computed tomography

Abstract

Objectives: an in vitro study was performed to investigate the relationship between grey levels in dental cone beam CT (CBCT) and Hounsfield units (HU) in CBCT scanners.

Methods: a phantom containing 8 different materials of known composition and density was imaged with 11 different dental CBCT scanners and 2 medical CT scanners. The phantom was scanned under three conditions: phantom alone and phantom in a small and large water container. The reconstructed data were exported as Digital Imaging and Communications in Medicine (DICOM) and analysed with On Demand 3D(R) by Cybermed, Seoul, Korea. The relationship between grey levels and linear attenuation coefficients was investigated.

Results: it was demonstrated that a linear relationship between the grey levels and the attenuation coefficients of each of the materials exists at some "effective" energy. From the linear regression equation of the reference materials, attenuation coefficients were obtained for each of the materials and CT numbers in HU were derived using the standard equation.

Conclusions: HU can be derived from the grey levels in dental CBCT scanners using linear attenuation coefficients as an intermediate step.

Figure 1

Three-dimensional eight-tissue radiographic phantom

Figure 2

NewTom VG three-dimensional dental phantom only, 110 kVp 1.0 mA, 63 effective keV using seven reference materials. ♦, Series 1

Figure 3

NewTom VG three-dimensional dental phantom only, 110 kVp 1.0 mA, 63 effective keV using seven reference materials. y = 0.0001734x + 0.2844124; R² = 0.9999033

Figure 4

NewTom VG 3D dental phantom only, 110 kVp, 1.0 mA, 63 effective keV using three reference materials. ♦, Series 1

Figure 5

NewTom VG 3D dental phantom only 110 kVp, 1.0 mA, 63 effective keV using three reference materials. y = 0.0001725x + 0.2854034; R² = 0.9999996