title: Determination of the mastoid surface area and volume based on micro-CT scanning of human temporal bones. Geometrical parameters depend on scanning resolutions.
abstract: The mastoid air cell system (MACS) with its large complex of interconnected air cells reflects an enhanced surface area (SA) relative to its volume (V), which may indicate that the MACS is adapted to gas exchange and has a potential role in middle ear pressure regulation. These geometric parameters of the MACS have been studied by high resolution clinical CT scanning. However, the resolution of these scans is limited to a voxel size of around 0.6 mm in all dimensions, and so, the geometrical parameters are also limited. Small air cells may appear below the resolution and cannot be detected. Such air cells may contribute to a much higher SA, and thus, also the SA/V ratio. More accurate parameters are important for analysis of the function of the MACS including physiological modeling. Our aim was to determine the SA, V, and SA/V ratio in the MACS in human temporal bones at highest resolution by using micro-CT-scanning. Further, the influence of the resolution on these parameters was investigated by downsampling the data. Eight normally aerated temporal bones were scanned at the highest possible resolution (30-60 μm). The SA was determined using a triangular mesh fitted onto the segmented MACS. The V was determined by summing all the voxels containing air. Downsampling of the original data was applied four times by a factor of 2. The mean SA was 194 cm2, the mean V was 9 cm3, and the mean SA/V amounted to 22 cm-1(22 cm2/cm3). Decreasing the resolution resulted in a non-linear decrement of SA and SA/V, whereas V was mainly independent of the resolution. The current study found significantly higher SA and SA/V compared with previous studies using clinical CT scanning at lower resolutions. These findings indicate a role of the MACS different from that of the tympanum, though they are limited to only a smaller sample of temporal bones without knowledge about the disease history of the subjects. The current data on mastoid geometry seems important for a more accurate modeling of the middle ear physiology and future studies may include morphological investigations of the air cells with possible implications for their postnatal development.
authors.affiliation: Department of Otolaryngology, Head and Neck Surgery, Aalborg University Hospital, 9000 Aalborg, Denmark; Department of Biomedical Engineering (IMT), Linköping University Hospital, 58235 Linköping, Sweden; Center for Medical Image Science and Visualization (CMIV), Linköping University Hospital, 58235 Linköping, Sweden. Electronic address: olivier.cros@liu.se.
authors.forename: Olivier
authors.initials: O
authors.lastname: Cros
authors.affiliation: Department of Biomedical Engineering (IMT), Linköping University Hospital, 58235 Linköping, Sweden; Center for Medical Image Science and Visualization (CMIV), Linköping University Hospital, 58235 Linköping, Sweden. Electronic address: knutte@imt.liu.se.
authors.forename: Hans
authors.initials: H
authors.lastname: Knutsson
authors.affiliation: Department of Biomedical Engineering (IMT), Linköping University Hospital, 58235 Linköping, Sweden; Center for Medical Image Science and Visualization (CMIV), Linköping University Hospital, 58235 Linköping, Sweden. Electronic address: mats.andersson@liu.se.
authors.forename: Mats
authors.initials: M
authors.lastname: Andersson
authors.affiliation: Centre for X-ray Tomography, Department of Physics and Astronomy, University of Ghent, 9000 Ghent, Belgium. Electronic address: Elin.Pauwels@UGent.be.
authors.forename: Elin
authors.initials: E
authors.lastname: Pawels
authors.affiliation: Department of Biomedical Engineering (IMT), Linköping University Hospital, 58235 Linköping, Sweden; Center for Medical Image Science and Visualization (CMIV), Linköping University Hospital, 58235 Linköping, Sweden. Electronic address: magnus.borga@liu.se.
authors.forename: Magnus
authors.initials: M
authors.lastname: Borga
authors.affiliation: Department of Otolaryngology, Head and Neck Surgery, Aalborg University Hospital, 9000 Aalborg, Denmark; Department of Clinical Medicine, Aalborg University, 9000 Aalborg, Denmark. Electronic address: mlg@rn.dk.
authors.forename: Michael
authors.initials: M
authors.lastname: Gaihede
mesh_terms: D000388:Air; D002102:Cadaver; D004432:Ear, Middle / Q000000981:diagnostic imaging / Q000502:physiology; D006801:Humans; D008416:Mastoid / Q000000981:diagnostic imaging* / Q000502:physiology; D013701:Temporal Bone / Q000000981:diagnostic imaging* / Q000502:physiology; D014057:Tomography, X-Ray Computed; D055114:X-Ray Microtomography
keywords: Mastoid air cells; Medical imaging; Micro-CT; Surface area; Volume