3B-4 Statistically Motivated Design of Input Signals for Ultrasonic Array Imaging

Modern array systems allow for excitation of separate elements using arbitrary waveforms. This is utilized in pulse compression and coded excitation techniques to improve the imaging performance. Such techniques are however somewhat inflexible since they use predefined excitation schemes. This paper presents a new more flexible method for optimizing the input signals to an ultrasonic array in such way that the scattering strengths at arbitrarily chosen control points in the insonified object can be estimated with as low error as possible, measured with a mean squared error criteria. The statistically motivated method is based on a linear model of the array imaging system where both prior information regarding the scattering strengths and measurement errors are taken into account. The input signals are found by using genetic optimization and are constrained to have finite duration and bounds on the maximum amplitudes. Different constellations of control points yield different excitation schemes. The design approach finds multiple selective focal laws when choosing relatively well separated control points and when the control points are closely spaced, the resulting excitations results in more diffuse fields, reminiscent to those resulting from coded excitation. Because of the flexibility in choosing the control points, the design method will be useful when developing transmission schemes aiming at fast imaging of large image areas using few transmissions