Issuance of ultrasound pulses to a volume and receiving echo data is followed by estimating, based on the received data, center frequency subvolume-by-subvolume. Distinguishing between heart and lung tissue occurs based on a result of the estimating, and may include automatically identifying a spatial boundary (332) between the heart and lung tissue (324, 328), or a user display of center frequencies that allows for visual distinguishing. The issuance can include issuing, ray line by ray line, pair-wise identical, and/or pair-wise mutually inverted, ultrasound pulses. Center frequency calculations may be made for incremental sampling locations of respective imaging depth along each of the A-lines generated from echo data of the rays. The distinguishing might entail averaging center frequencies for locations along an A-line, and applying a central frequency threshold to the average. The leftmost of the qualifying A-lines, i.e., that meet the threshold, may determine the spatial boundary in the current imaging plane.
