1. An ultrasonic diagnostic imaging system comprising: a two dimensional array transducer; a beamformer coupled to the array transducer which beamforms received echo signals; a controller, coupled to the array transducer, which controls the transducer to scan two image planes of a volumetric region; a display subsystem, coupled to the beamformer, which produces real-time images of the two image planes shown concurrently on a common display; and a user control, responsive to actuation by a user and coupled to the controller, which selects the orientation of the plane of one of the two images relative to the plane of the other image.