The thickness of the temporal squama and its implication for transcranial sonography

The difficult transtemporal ultrasound window is a relatively frequent occurrence. The authors assessed if the thickness of the temporal bone squama as measured in the "bone window" of the head computerized tomography (CT) scan can predict the transtemporal acoustic window.

The authors retrospectively reviewed the head CTs on their bone window setting of patients in which nonimaging transcranial Dopplers (TCDs) had been performed. The thickness of the temporal squama in its thinnest portion was measured. The temporal TCD windows were graded in three classes: class 1 as good, class 2 when only a partial study is possible, and class 3 as an impossible ultrasonic window. In a case-control design, for every patient with any class 2 and 3 TCD temporal window, a patient with a class 1 window was randomly included from the same time period.

Fifty-five temporal bones (56%) were class 1, 17 (17%) were class 2, and 27 (27%) were class 3. Bone thicknesses (in mm, mean +/- SD) were greater in those with poorer windows: class 1 = 2.67 +/- 0.70, class 2 = 4.06 +/- 0.56, and class 3 = 5.04 +/- 1.06, P < or = .0001 by Cusick's nonparametric test of trend. Temporal squama thickness of > or = 5 mm portends 86% sensitivity, 90% specificity, 70% positive predictive value, and a positive likelihood ratio of 8.6 for a class 3 transtemporal ultrasound window.

Measurement of temporal bone thickness on the bone window setting of the head CT scan may be useful in identifying patients who are poor candidates for transcranial ultrasound.