[Area and volume relationship of temporal bone air cells]

The area and volume of the air cell system of fifty normal temporal bones were investigated. The area on a plain X-ray, Sonnenkalb projection, was measured by a planimetric method and a rectangular dimensional method by Imai in 1978. The volumes of the temporal bone air cells on high-resolution CT were calculated separately as partial volumes divided in all CT planes. The sum of the partial volumes was calculated as the total volume of the air cells. The relationship between the area and the total volume of the air cell system was analyzed statistically. The ratio of the partial spatial volume, e.g. the tympanic cavity and the apical air space, to the total volume of the temporal bone air cells was also calculated. The results were as follows: 1) The relationship between the two measurements were as follows: planimetric method, Y = 0.901X - 2.26 (r = 0.885), and rectangular dimensional method, Y = 0.542X - 2.57 (r = 0.876). 2) The ratio of the volume of the tympanic cavity to the total volume of the air cell system was 6.74 +/- 4.07%. The larger the total volume, the smaller the ratio. 3) The ratio of the apical space was 7.09 +/- 4.98%, and the larger the total volume, the larger the ratio.