Giovanni Cardone

Design of wide-band arrays for low side-lobe level beam patterns by simulated annealing

We address the window design problem of linear arrays to be used in wide-band beamforming systems. During last decades, stochastic optimization algorithms have proven to be very efficient in optimal beamforming synthesis, but most research interest has focused on linear and two-dimensional arrays under narrow-band excitation. In the present paper, we introduce a general approach, based on the simulated annealing optimization algorithm, to design uniformly spaced wide-band arrays that generate low side-lobe level beam patterns. The effectiveness of different beam pattern formulations is evaluated within the proposed framework; we also consider three different methods to control the side-lobe level, yielding different array performances. Moreover, we investigate the influence of the frequency bandwidth on the optimized beam patterns.

Optimization of wide-band linear arrays

An optimization method is proposed for linear arrays to be used in ultrasound systems under wide-band operation. A fast algorithm, the threshold accepting, has been utilized to determine the element positions and weight coefficients of a linear array that generates a desired beam pattern. To reduce the computational burden in the optimization procedure, an efficient numerical routine for the beam pattern evaluation has been implemented. We address the optimization problem of both dense and sparse wideband arrays. In the first case, the goal is to minimize the side-lobe energy by varying the element weights; we compare the optimized beam pattern with that obtained with classical shading functions, showing that better results can be achieved with a wide-band optimization. We also consider the optimization of the layout (positions and weights) of a sparse linear array to achieve a desired beam pattern with a fixed or minimum number of array elements. The comparison of the proposed method with a narrow-band optimization algorithm is presented, showing that better performances (about -7 dB further reduction of the side-lobe level) can be achieved with a wide-band sparse array optimization. Further numerical simulations are given, showing that the proposed method yields better results than wideband sparse random arrays and periodic arrays with the same aperture width.

Efficient transmit beamforming in pulse-echo ultrasonic imaging

A high performance ultrasound imaging system requires accurate control of the amplitude of the array elements, as well as of the time delays between them, both in the transmit and receive modes. In transmission, conventional array aperture windowing implies a different driving voltage for each element of the array, an expensive solution for systems with a large number of channels. In this paper, we present a simple, versatile, and inexpensive beamforming method that operates the aperture windowing in the transmit mode, simply controlling the lengths of the electric pulses driving the array elements. Computer simulations and experimental measurements are presented for different types of arrays. They confirm that the proposed beamforming technique improves the contrast resolution of the imaging system, reducing the off-axis intensity of the radiated field pattern. Moreover, the axial resolution is slightly enhanced, because the overall length of the transmitted ultrasonic pulse is reduced.

Spectral changes in Gaussian-cavity lasers

Spectral changes occurring to laser radiation originated from amplified stimulated emission are evaluated as a function of the number of transits performed inside a Gaussian optical cavity. It is shown that the spectral shifts on the optical axis are quite different from those already established for Gaussian Schell-model beams and that pulsed lasers deliver pulses whose peak frequency slightly changes from the beginning to the end of the pulse.

A new beamforming technique for ultrasonic imaging systems

We propose a simple, versatile and inexpensive beamforming method that performs the aperture windowing of an ultrasonic transducer array in the transmit mode, without modifying the driver voltage, but simply controlling the length of the electric pulse driving the array elements. A conversion formula has been determined that permits us to compute, for a desired emitted pulse amplitude, the corresponding driving pulse length to be applied. Any shading function can be implemented over any type of transducer array, using very low-cost hardware. Computer simulations and experimental measurements, with a 3.8 MHz convex array, confirm the effectiveness of this approach in enhancing the contrast resolution, since the off-axis intensity in the radiated beam pattern is largely reduced.

Fast iterative deconvolution technique for echographic imaging

Many deconvolution techniques have been proposed in literature based on the knowledge of the Point Spread Function or on its estimation from the observed image. In this paper, we propose an alternative approach, which performs a local inversion through an iterative technique. The proposed iterative deconvolution combines accuracy with fast execution and it is well suited for fast hardware implementation. A discussion on the convergence of the algorithm is also presented in the paper. A novel approach to the deconvolution in medical imaging is proposed. Although its efficiency has been demonstrated in the work only for improving lateral resolution, it can easily be applied to full 2-dimensional deconvolution. The proposed technique has local characteristics and can operate on a limited number of data at a time with great advantage for memory storage requirements; further, it is well suited for a fast hardware implementation because only multiplications and summations are used in the algorithm. The feasibility of an alternative technique for the medical image deconvolution is analyzed theoretically; experimental results are also presented. The results are compared with those obtained by a conventional Fourier-based method.