1. A method of reducing motion artifacts generated during Doppler imaging using at least one rotating transducer, the method comprising: inserting an imaging device into a blood vessel of a patient, the imaging device coupled to a processor, the imaging device comprising at least one transducer disposed in a lumen defined in a sheath, the at least one transducer configured and arranged for transforming applied electrical signals to acoustic signals; transmitting the acoustic signals along a plurality of radial scan lines by the at least one transducer as the at least one transducer rotates about a longitudinal axis of the sheath; receiving reflected echo signals by the at least one transducer along a plurality of radial scan lines, the echo signals corresponding to the transmitted acoustic signals; passing at least some of the received echo signals through a first signal processing channel to generate a first resolution tissue structural image; passing at least some of the received echo signals through a second signal processing channel to generate a blood-flow image that comprises at least one of a blood-flow velocity image, a blood-flow energy image, or a second resolution structural energy image; combining the first resolution tissue structural image with the blood-flow image to generate a composite image comprising a plurality of pixels; and determining, for each pixel of the composite image, whether to use a corresponding pixel of the first resolution tissue structural image or a corresponding pixel of the blood-flow image to generate the pixel of the composite image.