An ultrasonic diagnostic imaging system scans a plurality of planar slices in a volumetric region which are parallel to each other. Following detection of the image data of the slices the slice data is combined by projecting the data in the elevation dimension to produce a “thick slice” image. Combining may be by means of an averaging or maximum intensity detection or weighting process or by raycasting in the elevation dimension in a volumetric rendering process. Thick slice images are displayed at a high frame rate of display by combining a newly acquired slice with slices previously acquired from different elevational planes which were used in a previous combination. A new thick slice image may be produced each time at least one of the slice images is updated by a newly acquired slice. Frame rate is further improved by multiline acquisition of the slices.