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 3DH 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.

Objective: To present a clinical study demonstrating a method to derive Hounsfield units from grey levels in cone beam CT (CBCT). Methods: An acrylic intraoral reference object with aluminium, outer bone equivalent material (cortical bone), inner bone equivalent material (trabecular bone), polymethlymethacrylate and water equivalent material was used. Patients were asked if they would be willing to have an acrylic bite plate with the reference object placed in their mouth during a routine CBCT scan. There were 31 scans taken on the Asahi Alphard 3030 (Belmont Takara, Kyoto, Japan) and 30 scans taken on the Planmeca ProMax 3D (Planmeca, Helsinki, Finland) CBCT. Linear regression between the grey levels of the reference materials and their linear attenuation coefficients was performed for various photon energies. The energy with the highest regression coefficient was chosen as the effective energy. The attenuation coefficients for the five materials at the effective energy were scaled as Hounsfield units using the standard Hounsfield units equation and compared to those derived from the measured grey levels of the materials using the regression equation. Results: In general, there was a satisfactory linear relation between the grey levels and the attenuation coefficients. This made it possible to calculate Hounsfield units from the measured grey levels. Uncertainty in determining effective energies resulted in unrealistic effective energies and significant variability of calculated CT numbers. Linear regression from grey levels directly to Hounsfield units at specified energies resulted in greater consistency. Conclusions:The clinical application of a method for deriving Hounsfield units from grey levels in CBCT was demonstrated.

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We appreciate the reader's interest in our research paper "Deriving Hounsfield units using grey levels in cone beam computed tomography" in the September 2010 issue of DMFR. 1 We have subsequently applied this method in a clinical situation and the results will be submitted shortly for publication by the same authors. This should help to explain some of the issues raised in this letter to the editor.We are aware of the paper by Bryant J A et al 2 and we will attempt to address some of the issues regarding non-uniformity in the flat panel detector and the issue of noise raised in this paper in a future article.At this time we are unable to scan the device in an i-Cat CBCT machine as we do not have one of these scanners in our institution. We have several other CBCT devices but not this particular manufacturer's device.