Visualization and Quantification of Acoustic Beams in Echo-Graphic Real-Time Systems

For many years wires of small diameter, compared to the wavelengths, have been used to test echographic B-mode systems for medical diagnostic applications. The image generated on the display of a real-time linear scanning system by an array of such wires is analyzed. A two-dimensional mathematical interpretation of this particular image, which appears to be related to the acoustic beam of the multielement transducer employed, is given. F I. INTKODUCTION OR MANY years wires of small diameter, compared to the wavelengths, have been employed t o test echographic Bmode systems. A variety of test objects consisting of thin wires have been used and a standard “phantom” developed by the American Institute of Ultrasound in Medicine’s Standards Committee [ l ] is commercially available. Recently Hefner et al. [2] proposed a simple test-object consisting of a linear array of wires which generate an image on the display of a Bmode system, which appears t o be related to the pulsed acoustic beam of the particular transducer in use. Manuscript received October 22, 1981; revised August 17, 1982. The authors are with Consiglio Nazionale delle Ricerche, lstituto di Acustica “O.M. Corbino,” Via Cassia, 1216-00189 Rome, Italy. Independently of the above we recognized the validity of such a “phantom” to test real-time multielement systems [ 3 ] . In addition to the applications described by Hefner for single element B-scan systems, we pointed out the possibilities of such a “phantom” for a simple evaluation of the performances of the dynamic focusing techniques employed in the majority of commercial linear or sector scanning systems. in order to characterize multielement transducers [4 ] , [S] , [6] , [7] but a good quality optical system is required for this purpose. In fact the signal-noise ratio, i.e., the ratio between diffracted and undiffracted light, is not very good because the acoustic perturbation takes place for a short time and the repetition rate and the central frequency of the pulse are not very high. The kind of images obtained with this test object are comparable, as far as the quality is concerned, to that given by the Schlieren method but they are obtained without the aid of an optical system. In this paper we will analyze the images obtained by this “phantom” and we will give a two-dimensional mathematical interpretation of the acoustic beam image formation. We point out the similarity and the difference between the images generated by the “phantom” and the acoustic beam of the Schlieren images of the pulsed acoustic beam have been used 0018-9537/82/1100-0342$00.75