title: A new method to evaluate image quality of CBCT images quantitatively without observers.
abstract: OBJECTIVE
  To develop an observer-free method for quantitatively evaluating the image quality of CBCT images by applying just-noticeable difference (JND).
  
  
  METHODS
  We used two test objects: (1) a Teflon (polytetrafluoroethylene) plate phantom attached to a dry human mandible; and (2) a block phantom consisting of a Teflon step phantom and an aluminium step phantom. These phantoms had holes with different depths. They were immersed in water and scanned with a CB MercuRay (Hitachi Medical Corporation, Tokyo, Japan) at tube voltages of 120 kV, 100 kV, 80 kV and 60 kV. Superimposed images of the phantoms with holes were used for evaluation. The number of detectable holes was used as an index of image quality. In detecting holes quantitatively, the threshold grey value (ΔG), which differentiated holes from the background, was calculated using a specific threshold (the JND), and we extracted the holes with grey values above ΔG. The indices obtained by this quantitative method (the extracted hole values) were compared with the observer evaluations (the observed hole values). In addition, the contrast-to-noise ratio (CNR) of the shallowest detectable holes and the deepest undetectable holes were measured to evaluate the contribution of CNR to detectability.
  
  
  RESULTS
  The results of this evaluation method corresponded almost exactly with the evaluations made by observers. The extracted hole values reflected the influence of different tube voltages. All extracted holes had an area with a CNR of ≥1.5.
  
  
  CONCLUSIONS
  This quantitative method of evaluating CBCT image quality may be more useful and less time-consuming than evaluation by observation.
authors.affiliation: 1 Department of Oral and Maxillofacial Radiology, Faculty of Dental Science, Kyushu University, Fukuoka, Japan.
authors.forename: Yohei
authors.initials: Y
authors.lastname: Takeshita
authors.affiliation: 2 Department of Oral and Maxillofacial Radiology, Kyushu University Hospital, Fukuoka, Japan.
authors.forename: Mayumi
authors.initials: M
authors.lastname: Shimizu
authors.affiliation: 1 Department of Oral and Maxillofacial Radiology, Faculty of Dental Science, Kyushu University, Fukuoka, Japan.
authors.forename: Kazutoshi
authors.initials: K
authors.lastname: Okamura
authors.affiliation: 3 Section of Image Diagnostics, Department of Diagnostics and General Care, Fukuoka Dental College, Fukuoka, Japan.
authors.forename: Shoko
authors.initials: S
authors.lastname: Yoshida
authors.affiliation: 1 Department of Oral and Maxillofacial Radiology, Faculty of Dental Science, Kyushu University, Fukuoka, Japan.
authors.forename: Warangkana
authors.initials: W
authors.lastname: Weerawanich
authors.affiliation: 4 Department of Radiological Technology, Faculty of Fukuoka Medical Technology, Teikyo University, Fukuoka, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Tokumori
authors.affiliation: 5 Oral Radiology Division, Faculty of Dentistry, Republic University, Montevideo, Uruguay.
authors.forename: Gainer R
authors.initials: GR
authors.lastname: Jasa
authors.affiliation: 1 Department of Oral and Maxillofacial Radiology, Faculty of Dental Science, Kyushu University, Fukuoka, Japan.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Yoshiura
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000592:standards* / Q000706:statistics & numerical data; D019047:Phantoms, Imaging
keywords: CBCT; computer-assisted; dental; digital radiography; image analysis; imaging; phantoms; radiography