An underfluid ultrasound imaging catheter system includes a catheter having a distal end inserted into an underfluid structure, an ultrasonic transducer array mounted proximate the distal end of the catheter wherein the array has a row of individual transducer crystals, a lens mounted on the array for defocusing ultrasound beams in a direction perpendicular to an axis of the array so as to provide a volumetric field of view within which the underfluid features are imaged. Alternatively, the single row of transducer crystals is replaced by multiple rows of transducer crystals so as to provide a volumetric field of view. This imaging catheter system helps an operator see 3-dimensional images of an underfluid environment, such as the 3-dimensional images of fluid-filled cavities of heart, blood vessel, urinary bladder, etc. Features in such wide volumetric field of view can be imaged, measured, or intervened by an underfluid therapeutic device with an aid of the real-time image.