title: Normal Mastoid Air Cell System Geometry: Has Surface Area Been Overestimated?
abstract: OBJECTIVE
  The aim of this study was to emphasize the necessity of a standard in segmentation threshold and algorithm for measuring volume and surface area of mastoid air cell system (MACS).
  
  
  METHODS
  First, we obtained axial computed tomography scans of 54 normal temporal bones from 27 subjects. Then, we manipulated Hounsfield units (HU) image data in DICOM (digital imaging and communications in medicine) files directly using our program. The volume and surface area of MACS were computed and compared at segmentation thresholds (HU) from -700 to 0 at intervals of 50 using 2 algorithms; square pixel based (SP) algorithm and marching square (MS) algorithm.
  
  
  RESULTS
  No significant difference was found between the volumes computed by SP and MS algorithms at each segmentation threshold. The surface area computed by SP algorithm, however, was significantly larger than that by MS algorithm. We could minimize this significant difference through a modification of the SP algorithm. As the lower HU threshold value was set, the smaller volume was measured. The surface area showed a plateau at a threshold of approximately -200 HU. The segmentation threshold had greater influence on the measured volume of MACS than the algorithm did.
  
  
  CONCLUSIONS
  A standard method for measuring volume and surface area of MACS is thought to be necessary. We suggest that the MS algorithm and -200 HU of the threshold could be a standard in the measurement of volume and surface area of MACS.
authors.affiliation: Department of Otorhinolaryngology-Head and Neck Surgery, Ewha Womans University School of Medicine, Seoul, Korea.
authors.forename: Sung Wan
authors.initials: SW
authors.lastname: Byun
authors.affiliation: Department of Otorhinolaryngology-Head and Neck Surgery, Ewha Womans University School of Medicine, Seoul, Korea.
authors.forename: Seung-Sin
authors.initials: SS
authors.lastname: Lee
authors.affiliation: Department of Otorhinolaryngology-Head and Neck Surgery, Ewha Womans University School of Medicine, Seoul, Korea.
authors.forename: Jin Young
authors.initials: JY
authors.lastname: Park
authors.affiliation: Department of Radiology, Ewha Womans University School of Medicine, Seoul, Korea.
authors.forename: Jeong Hyun
authors.initials: JH
authors.lastname: Yoo
keywords: Algorithms; Image Processing, Computed-Assisted; Mastoid; Organ Size; Temporal Bone