1. An ultrasonic diagnostic imaging system for a qualitative or quantitative analysis of tissue perfusion, the imaging system comprising: an array transducer configured to transmit and receive scanlines over a volumetric region containing tissue which may have a perfusion defect; a beamformer, coupled to the array transducer, configured to produce image data of a plurality of elevationally distinct, parallel slices of the volumetric region; a dual port slice memory, coupled to the beamformer, configured to store slice image data comprising a first plurality of parallel slices acquired at a first time interval and a second plurality of parallel slices at a second time interval; a write address controller, coupled to a first port of the dual port slice memory, configured to control writing of slice image data comprising the second plurality of parallel slices into a second location in the dual port slice memory; a read address controller, coupled to a second port of the dual port slice memory, configured to control reading of slice image data comprising the first plurality of parallel slices from a first location in the dual port slice memory while the slice image data comprising the second plurality of parallel slices is being written into the second location; a combiner, coupled to the dual port slice memory, configured to combine slice image data comprising the first plurality of parallel slices and the second plurality of parallel slices in the elevation direction; a perfusion detector configured to operate on the combined slice image data to qualitatively depict perfusion by a concentration of microbubbles in tissue or to produce a quantified perfusion parameter for a region of tissue; and a display responsive to the perfusion detector configured to display at least one of a tissue image with perfusion depicted by color or brightness, or a quantified perfusion parameter for a region of tissue.