Width of the extended facial recess: a numerical study of ultrahigh-resolution computed tomography and its implications in minimally invasive otologic surgery.

BACKGROUND: Methods of minimally invasive computer-assisted otologic surgery lag behind other fields. The reason seems to be the extremely small dimensions of the corridors between important structures in the temporal bone and the fact that these structures are encased in bony frameworks, are obscured before drilling, and are not movable. The extended facial recess is a surgical pass to the tympanic cavity. It is bounded medially by the facial nerve and laterally by the tympanic annulus, and varies among individuals. For computer-assisted, minimally invasive temporal bone surgery, high-resolution definition is critically important. AIMS: To determine the width of the extended facial recess and evaluate the computerized findings as a pre- and intraoperative aid to otologic surgery planning. METHODS: Bilateral temporal bone high-resolution computed tomographic images of 100 male and 100 female patients were measured twice at five levels (caudal to cephalic), first using a window-independent algorithm (extended facial recess, full-width at half-maximum), implemented in a computed tomographic image-processing workstation, and then manually with calipers on the same axial computed tomographic images. RESULTS: As expected, the extended facial recess, full-width at half-maximum method yielded the widest values superiorly (4.15 +/- 0.41 mm in the female patients and 4.32 +/- 0.54 mm in the male patients). From this level down, the extended facial recess, full-width at half-maximum method yielded values that tapered gradually to 2.50 +/- 0.56 mm in the female patients and 2.42 +/- 0.46 mm in the male patients at the most interior level. The manual method (extended facial recess, computed tomographic images) yielded a significantly higher value than that obtained with the objective, window-independent method at all levels, and at some levels was higher by as much as one-third. At Level 2, which corresponded roughly to the round window, the extended facial recess was 4.00 +/- 0.65 in the female study group and 4.11 +/- 0.67 mm in the male study group. CONCLUSION: Image processing methods such as extended facial recess, full-width at half-maximum method might lead to fine tuning and thus improvement of computer-assisted otologic surgery. Before clinical application and complete dependence on these automated methods during otologic surgery, their reliability should be further validated.

Volume of mastoid pneumatization: three-dimensional reconstruction with ultrahigh-resolution computed tomography.

The volume of the mastoid air cell system was measured in 69 patients with normal middle ears. All patients underwent axial ultrahigh-resolution computed tomography. Mastoid pneumatization was marked on each axial slice, and 3-dimensional reconstruction was performed. The volumes were measured with a volumetric algorithm. A polyethylene tubing phantom with a density similar to that of bone on computed tomography was devised. The polyethylene tubing was tied in a particular fashion so as to create interconnecting air spaces with a known volume. The phantom was scanned with the imaging parameters used for scanning the temporal bone. The air in the tubing was marked, and 3-dimensional reconstruction for the marked phantom air was performed. The volume of the interconnecting air spaces was measured and found to be identical to its known volume, thereby verifying the accuracy of the method used. The mean mastoid volume was 6.61 cm3. The smallest volume measured was 1.3 cm3, and the largest was 12.7 cm3. The importance of this technique lies in its high accuracy, ease of use, and ability to directly correlate mastoid size and clinical findings.