A Study of Near Field Ultrasonic Beam Patterns from a Pulsed Linear Array

A major advantage of pulse-echo imaging in the near-field of an acoustic array is the improved lateral resolution that focusing makes possible. Usually, equivalence is tacitly assumed between a lateral beam dimension for continuous wave excitation and a lateral pulse dimension for the same geometry. Although distinctions certainly exist between the lateral dimensions in these two cases, it is convenient in dealing with pulsed ultrasound to speak of a “beam” or “beams” of ultrasonic energy. By analogy to the continuous wave case, such a beam may be defined as the envelope of a pulse dimension. This dimension may be specified by some absolute or r.m.s. pressure magnitude criterion (e.g., the distance to a point at which the absolute pressure is down “n” dB from the maximum value in the field).