Flat ended steel wires, backscattering targets for calibrating over a large dynamic range.

A series of flat ended stainless steel wires was constructed and experimentally evaluated as point targets giving a calibrated backscattering over a large range (up to 72 dB) for ultrasound frequencies in the range 2 to 10 MHz. Over a range of 36 dB, theory was strictly followed (within +/- 1 dB), giving proportionality with the square of the frequency and the fourth power of the diameter. In this range, the relative scattering strength of targets does not depend on the frequency content of the ultrasound pulse. A deviation of maximal 5 dB at the low side is tentatively ascribed to a failure of simple theory for target diameters smaller than half a wavelength. For large diameters, backscattering is sensitive to orientation of the target: a numerical relation between misalignment and desired accuracy is proposed. A replication test for targets with a diameter of 0.24 mm gave equal scattering strengths within 0.8 dB. It is proposed to apply the targets for calibration of the dynamic range of a scanner and as point targets to establish resolution in three perpendicular directions, axial, lateral and elevational.