Devices and methods for obtaining a real-time, three-dimensional image of a body part, particularly a blood vessel. A catheter has a chamber in its tip. The chamber contains an ultrasound transducer and reflector which generally face each other and rotate about a common axis. The transducer element on the transducer and the reflective face on the reflector are both tilted off-axis. The difference in angular velocity generally creates a phase shift between the transducer and the reflective face. The phase shift allows the transducer and the reflective face to actively scan a three-dimensional volume that is generally bounded interiorly by a hyperboloid and exteriorly by the effective range of the ultrasound beam. The transducer and reflector may rotate at constant speeds or nonconstant speeds as well in the same direction or in opposite directions.