This paper outlines the identification and accurate measurement of the essential anatomic elements of the temporal bone on a preoperative HRCT, which our report should include to facilitate the cochlear implantation procedure.

• Facial recess (FR) width:
  • measured in the axial plane: perpendicular to a line that runs from the posterior wall of the external auditory canal (EAC) to the anterolateral aspect of the facial nerve (FN);
  • normal width: if the distance between the EAC and the facial nerve exceeds 3 mm;
  • a narrow facial recess width is associated with inadequate access to the round window niche [3,4].
    

    Fig 1: Facial recess width measurement, axial view: line A is a line that passes through the posterior wall of the EAC; the measurement is made by tracing a line B from the anterolateral aspect of the FN perpendicular to line A
    

    Fig 2: Facial recess width measurement, axial view: normal width (> 3 mm)
    

    Fig 3: Facial recess width measurement, axial view: normal width (> 3 mm)
• Height of tegmen tympani:
  • measurement uses lateral semicircular canal (LSCC) as a landmark;
  • coronal plane: one horizontal line through the plane of the LSCC and a second vertical line that passes through the lowest point of the tegmen tympani and is perpendicular to the plane of the LSCC measures the lowest vertical height of the tegmen tympani;
  • normal height: lowest vertical height of the tegmen tympani above 3.5 mm;
  • a low vertical height of tegmen tympani adds complexity to mastoidectomy because of the increased risk of dural exposure [2,5].
    

    Fig 4: Height of tegmen tympani measurement, coronal view: line A is a horizontal line that passes through the plane of the LSCC; the measurement is made by tracing a vertical line B from the lowest point of the tegmen tympani perpendicular to line A to measure the lowest vertical height of the tegmen tympani
    

    Fig 5: Height of tegmen tympani measurement, coronal view: normal height (> 3.5 mm)
    

    Fig 6: Height of tegmen tympani measurement, coronal view: normal height (> 3.5 mm)
• Underpneumatized mastoid cells or sclerotic mastoid:
  • frequently associated with low-lying tegmen mastoid and anteriorly displaced sigmoid sinus;
  • low-lying tegmen mastoid can mislead the surgeon about the position of the antrum and the superior semicircular canal (SSCC); 
  • predisposing to intraoperative complications such as injury to the dural plate and other surrounding vital structures [6].
    

    Fig 7: Underpenumatized mastoid associated with anteriorly displaced sigmoid sinus
• Anteriorly displaced sigmoid sinus (ADSS):
  • frequently associated with underpneumatized mastoid;
  • increased risk of dural exposure or significant hemorrhage;
  • difficult access to round window niche;
  • measurement method, axial plane: line A, which goes across both the bony-cartilaginous junction and the tympanic annulus, delineating the posterior wall of EAC, and line B, parallel to line A, which passes through the anteromedial part of the FN and the basal turn of the cochlea. A measurement line C, from the most prominent point of the sigmoid sinus perpendicular to line B;
  • when the measurement value is ≤2.46 mm, there is an increased possibility that posterior tympanotomy may not be technically possible [7,8].
    

    Fig 8: Sigmoid sinus placement measurement, axial view: line A is a horizontal line that delineates the posterior wall of the EAC, and line B, traced parallel to line A, passes through the anteromedial part of the FN and the basal turn of the cochlea. The measurement is made by tracing line C from the most prominent point of the sigmoid sinus perpendicular to line B
    

    Fig 9: Anteriorly displaced sigmoid sinus, axial view ( ≤2.46 mm)
• Prominent mastoid emissary vein:
  • may complicate the positioning of the receiver-stimulator of the CI device;
  • increased risk of significant hemorrhage;
  • a mastoid emissary vein above 3 mm in diameter is defined as prominent or enlarged [2].
    

    Fig 10: Prominent mastoid emissary vein, diameter > 3 mm
    

    Fig 11: Prominent mastoid emissary vein, diameter > 3 mm
• High-riding jugular vein or giant jugular bulb:
  • defined by the presence of the jugular dome above the basal turn of the cochlea or the floor of the IAC;
  • a jugular bulb larger than 1 cm is considered a giant jugular bulb;
  • there is an increased risk for complications during mastoidectomy, such as dural exposure or significant hemorrhage, but preoperative awareness may reduce it [2,9].
    

    Fig 12: High-riding jugular vein, sagittal view: jugular dome above the basal turn of the cochlea
    

    Fig 13: High-riding jugular vein, sagittal view: jugular dome above the basal turn of the cochlea
    

    Fig 14: Giant jugular bulb: diameter > 1 cm