Detectability of calcifications in breast tissues by the ultrasonic echo method

Abstract

The tumour processes in breasts involve calcifications which can be detected using ultrasonic methods. The aim of the present paper is to determine the minimum size of these calcifications which can be detected by the ultrasonic echo method. The models of the calcification which was assumed in this paper are a rigid and an elastic sphere onto which a plane wave is incident. Such density and longitudinal wave velocity were assumed here as are characteristic of the skull bone, and it is for these values that the far field form function f_ ∞(ka) was determined for different values of the Poisson’s ratio. On the basis of these calcification models, the detectability of the calcification by the echo method was evaluated, showing that, when a typical ultrasonograph at a frequency of 3 MHz is used, sphere-shaped calcifications with radii from 4 μm to 52 μm, depending on the depth at which they occur, give signals at the level of the electronic noise of the ultrasonograph. Experimental research has shown that the detectability by the echo method is restricted by breast tissue heterogeneities which cause the interfering background to occur. The level of these interference signals was determined at a frequency of 3 MHz. At 4 cm depth this level was higher by 31 dB than the electronic noise level. From these results, the present author determined the radii of calcifications detectable by the echo method at a level higher by 20 dB (ten times higher) than the level of the tissue interference signal. The radii are 0.05, 0.15 and 1 mm long at the respective depths of 2, 4 and 6 cm. Their dependence on the depth results mainly from the wave attenuation in tissues, which increases as the depth at which a given calcification is, grows. A linear receiver should be used for calcification detection.