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While there are a couple of transfer function design approaches for CT and MRI (magnetic resonance imaging) data, direct volume rendering of ultrasound data still relies on manual adjustment of an inflexible piecewise linear opacity transfer function (OTF) on a trial-and-error basis. The main challenge of automatically designing an OTF for visualization of sonographic data is the low signal-to-noise ratio in combination with real time data acquisition at frame rates up to 25 volumes per second. In this paper, we present an efficient solution of this task. Our approach is based on the evaluation of tube cores, i.e., collections of voxels gathered by traversing the volume in rendering directions. We use information about the probable position of an interface between tissues of different echogenicity to adaptively design an OTF in a multiplicative way. We show the appropriateness of our approach by examples, deliberately on data sets of moderate quality arising frequently in clinical settings.