Experimental Investigations on the Effects of Ocular Tissues upon the Ultrasonic Waves

ABSTRACT

The purpose of this report is to investigate experimentally how the corea, sclera, crystalline lens and focusing lens influence the ultrasound intensity and beam width and to find characteristic differences due to the transducer parameters; frequency and size. A time-amplitude ultrasonic diagnostic apparatus, Aloka Model SSD-2D was used with transcucers of 10 MHz and 5 MHz, 10 mm and 5 mm diameters. The metal ball, 1 mm in diameter, was served as the echo-reflecting target which was immersed in water and moved horizontally under the transducer with a micrometer screw. The distances between the transducer and the metal ball varied from 10 mm to 50 mm in 10 mm step. The amplitudes curves of the echoes so obtained were represented by relative values. In water control experiment, the nearer the metal ball from the transducer, the more irregular the curves, with multiple amplitudes maxima and minima. These irregular curves became homogeneous as the transducer moved away from the target. The multiple peak curves were more marked when measured with transducers of higher frequency and larger diameter. At the end of the near field only a single peak was found in the axial portion of the sound field. In the far field, the beam width reduced slightly due to the divergence of the beam accompanied by slight attenuation of the echo amplitudes. When the sound beam passed through the cornea and sclera, the form of the curves remained almost unchanged, but the amplitudes decreased slightly due to the absorption of the sound energy. Scleral tissue absorbed the energy more strongly then the corneal tissue. When the crystalline lens was placed under the transducer, divergence of the beam and themoderate absorption took place. This effect was more pronounced with the use of transducers of higher frequency and smaller diameter. When a focusing lens, 25 mm focal length, was attached to the transducer surface, sound beam converged to a narrow zone, followed by later scattering. These results suggest that the transducer should be selected as to the frequency and diameter according to the site of the lesion suspected and the nature of the pathology.