1. A catheter apparatus, comprising: an elongated body having proximal and distal ends; an ultrasonic transducer linear phased array mounted proximate the distal end of the elongated body, the array transmitting ultrasound beams and receiving resultant echoes, the array comprising a row of individual transducer segments aligned generally in an azimuthal dimension; a lens mounted on the array for defocusing the ultrasound beams such that at least some of the beams are directed laterally outward in an elevational dimension which is perpendicular to the azimuthal dimension so as to provide a volumetric field of view within which flow rates can be measured and features imaged; and an electrical conductor, disposed in the elongated body, electrically connecting the transducer to a control circuitry external of the elongated body.