An acoustic array is helical or twisted about or around an azimuth axis. For example, one end of the array has an emitting face at 0 degrees, but the other end has an emitting face at 20 degrees. The elements in between gradually transition between the different rotations. Different apertures of the twisted array may be used to scan different generally radial diverging planes for three-dimensional imaging. The different amount of relative rotation associated with each aperture cause angular elevation spacing of the planes. For use in a catheter, a single row of elements may be used to scan electrically a volume.
