[Assessment of the volume of the mastoid air cell system using digital image processing]

There has been a great deal of controversy concerning the development of pneumatization in the temporal bone. Many reports on the size of the mastoid air cell system have been discussed a planimetric X-ray method, however, there have been no techniques for direct volume measurement based on high-resolution computed tomography (CT). Discussion of the relationship between ear disease and pneumatization of the temporal bone requires three-dimensional measurement of the temporal bone in vivo. Recently, we developed such a technique by using digital image processing to measure the volume of the mastoid air cell system. With this technique, only the air cells and tympanic cavity, with a grey scale level similar to air outside the skull were easily selected on the CT films. Then, after image processing called "thresholding", only the areas and volumes of these extracted pneumatized parts were calculated. The volumes of the pneumatized parts of the temporal bone were calculated separately as partial volumes divided by several CT planes. The sum of the partial volumes was calculated as the total volume of the aerated cavity. This technique also made it possible to reconstruct a three-dimensional (3D) model of the air cell system for visual presentation. CT images of 43 normal temporal bones were analyzed. The average volume temporal bone pneumatization (including air cells and tympanic cavity) was about 6ml. There were no differences between pneumatization on the two sides in either sex. Comparisons of partial volumes of pneumatization in the temporal bone revealed that the volumes of portions including the tympanic cavity and its surrounding air cells and those of the antrum and its surrounding air cells were larger than those of other portions. 3D models of the air cell system revealed a variety of shapes. These findings indicated that this technique is not only useful for quantitative analyses but for easy subjective morphological analyses.