1. A method for determining a parameter of blood flow in a blood vessel of interest, comprising the steps of: a) providing an array of sonic transducer elements, wherein the element spacing in the array is greater than, equal or less than a half wavelength of the sonic energy produced by the elements, wherein at least one element transmits sonic energy, and a portion of the elements receive sonic energy; b) directing sonic energy produced by the at least one element of the array into a volume of the subject's body having the blood vessel of interest, c) receiving echoes of the sonic energy from the volume of the subject's body having the blood vessel of interest; d) reporting the echoes to a processor electrically connected to the elements of the array, wherein the processor is programmed to i) Doppler process the echoes to determine radial velocity of the blood flowing in the blood vessel of interest; ii) determine a sum beam, an azimuth difference beam and an elevation difference beam from the echoes received from a portion of the blood vessel of interest; iii) modulate the directions of the transmitted and received sonic energy based upon the sum, azimuth difference and elevation difference beams in order to lock on to the highest Doppler energy calculated from echoes from the flow of blood in the blood vessel of interest, iv) calculate the three dimensional position of the highest Doppler energy from the blood flow in the vessel of interest; and v) calculate the parameter of blood flow in the blood vessel at the three dimensional position calculated in step (iv); and (e) displaying the parameter on a display monitor that is electrically connected to the processor.