title: Deriving Hounsfield units using grey levels in cone beam CT: a clinical application.
abstract: 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.
authors: Lackland Airforce Base, San Antonio, TX, USA.
authors: T E
authors: TE
authors: Reeves
authors: P
authors: P
authors: Mah
authors: W D
authors: WD
authors: McDavid
mesh_terms: D000535:Aluminum; D003188:Composite Resins; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation* / Q000379:methods; D004867:Equipment Design; D006801:Humans; D016014:Linear Models; D019047:Phantoms, Imaging; D019904:Polymethyl Methacrylate; D011856:Radiographic Image Enhancement; D019252:Radiography, Dental, Digital / Q000295:instrumentation* / Q000379:methods; D011873:Radiology Information Systems
chemical_list: D003188:Composite Resins; D019904:Polymethyl Methacrylate; D000535:Aluminum