Simone Curletto

On the Optimization of the Transmitted Beam in Contrast-Enhanced Ultrasound Medical Imaging

The optimization of the ultrasound pulses transmitted by an echographic scanner is considered here in order to attain an insonification beam having a quasi-constant pressure profile over a large depth interval. An equalized pressure intensity allows a correct exploitation of the ultrasound contrast agent injected into the human body and, in turn, permits one to fully exploit its nonlinear response. A stochastic method of synthesis (based on the simulated annealing scheme) that is able to produce the desired transmission beam profile, acting simultaneously on the carrier frequencies of the acoustic pulses emitted by the arrays transducers and on the apodizing weights, has been developed and assessed. The results obtained by the joint optimization, in terms of acoustic pressure profile, sidelobe level, and main lobe shape, are reported, discussed, and compared with those obtained by the traditional emission strategy and by the exclusive optimization of the apodizing weights or the carrier frequencies. A significant improvement of the ultrasound beam generated by the joint optimization over those generated by the exclusive optimization is pointed out.

On the shaping of the main lobe in wide-band arrays

This paper describes a method to obtain a wide-band beam pattern whose main lobe has the desired shape and, at the same time, the level of the side lobes is acceptable. The aim is to reproduce a main lobe profile as close as possible to the desired one through the synthesis of the array-weighting window. A typical goal is to obtain, in wide-band conditions, a flat-top shaped beam similar to the ones produced by specific narrow-band windows. To achieve this result, an optimization process based on simulated annealing is developed and applied under different operating conditions, including two wide-band beam pattern definitions and different values of the fractional bandwidth. Although the shape of the resulting main lobe is not as good as the one obtained in narrow-band conditions (the reasons for this drawback are given), it is shown that the adoption of windows synthesized by the proposed method is more suitable than the direct application of narrow-band windows in wide-band conditions. In addition to providing better performances, it is shown that the windows synthesized by the proposed method also result in greater robustness to random perturbations of weight values, which are unavoidable in real systems.

Flattening the side-lobes of wide-band beam patterns [acoustic arrays]

Recently, simulated annealing has been proposed as a suitable method for optimizing the weighting window of a linear array working under wide-band conditions. This paper introduces and discusses a new energy function that, minimized by simulated annealing, produces a beam pattern showing a flat side-lobe profile over a large interval.

Interpolation of Medical Ultrasound Images From Coherent and Noncoherent Signals

Medical ultrasound imaging instrumentation typically performs image interpolation using the signals acquired after envelope extraction, i.e., noncoherent signals. This operation is completely satisfying when the Nyquist condition for spatial sampling is fulfilled. However, the maximum spatial frequency of a signal increases as a consequence of the envelope extraction, making it more difficult to fulfill the Nyquist condition. For this reason, this paper suggests the interpolation of signals before envelope extraction, i.e., the use of coherent data, and discusses the applicability of this procedure to actual ultrasound scanners. Emphasis is given to the linear-array imaging technique, and the adoption of windowed sinc functions as interpolation kernels with limited support is assessed. Moreover, the authors investigated the situation in which coherent signals are also undersampled, and perfect reconstruction is not at all possible. The investigation was carried out using a numerical study and a simulated imaging test, and it was supported by subjective and objective evaluations. It is concluded that, at a moderate undersampling level, the interpolation of coherent signals allows significantly better performance than the interpolation of noncoherent signals.

Extraction of 3D information from sonar image sequences

This paper describes a set of methods that make it possible to estimate the position of a feature inside a three-dimensional (3D) space by starting from a sequence of two-dimensional (2D) acoustic images of the seafloor acquired with a sonar system. Typical sonar imaging systems are able to generate just 2D images, and the acquisition of 3D information involves sharp increases in complexity and costs. The front-scan sonar proposed in this paper is a new equipment devoted to acquiring a 2D image of the seafloor to sail over, and allows one to collect a sequence of images showing a specific feature during the approach of the ship. This fact seems to make it possible to recover the 3D position of a feature by comparing the feature positions along the sequence of images acquired from different (known) ship positions. This opportunity is investigated in the paper, where it is shown that encouraging results have been obtained by a processing chain composed of some blocks devoted to low-level processing, feature extraction and analysis, a Kalman filter for robust feature tracking, and some ad hoc equations for depth estimation and averaging. A statistical error analysis demonstrated the great potential of the proposed system also if some inaccuracies affect the sonar measures and the knowledge of the ship position. This was also confirmed by several tests performed on both simulated and real sequences, obtaining satisfactory results on both the feature tracking and, above all, the estimation of the 3D position.

Signal processing strategy for the exploitation of ultrasound contrast agents by current medical instrumentation

A correct exploitation of an ultrasound contrast agent injected into the human body and the measurement of the nonlinear responses generated by the bubbles that form it need an acoustic insonification beam having a quasi-constant pressure profile over a large depth interval. In order to obtain this special beam, a stochastic method of synthesis (based on simulated annealing), aimed at the optimization of the weight coefficients to be applied to the array transducers, has been developed and assessed. The results obtained in terms of pressure profile and simulated echographic images are reported and discussed.

Ultrasound Pulse Optimization for the Measurement of the Contrast Agent Responses

Optimization of the ultrasound pulses transmitted by an echographic probe is here considered in order to attain an insonification beam having a quasi-constant pressure profile over a large depth interval. Equalized acoustic pressure values, in fact, allow a correct exploitation of an ultrasound contrast agent injected into the human body and, also, consent to measure the nonlinear responses generated by the bubbles that form it. A stochastic method of synthesis (based on simulated annealing) able to produce a desired transmission beam shape acting on the carrier frequencies of the acoustic pulses emitted by the arrays transducers, has been developed and assessed. The results obtained in terms of pressure profile and simulated echographic images are reported and discussed

Advanced generation of ultrasound beams for the measurement of the contrast agent responses

A correct exploitation of the contrast agent previously injected inside the human body and the measurement of nonlinear responses generated by the bubbles that composing it need to an acoustic beam pressure having a quasi-constant profile in a fixed range of depths. In order to obtain this objective, a synthesis stochastic method (based on simulated annealing) aimed to the optimization of the weight coefficients to be applied to the array sensors has been developed and performed. The results obtained in terms of pressure profile and simulated echographic images have been reported and discussed

Feature tracking and depth estimation in front-scan sonar image sequences

This paper describes a set of methods that make it possible to estimate the position of a feature inside a three-dimensional (3D) space by starting from a sequence of two-dimensional (2D) acoustic images of the seafloor acquired with a sonar system. The front-scan sonar devoted to generate a 2D image of the seafloor to sail over, and allows one to collect a sequence of images showing a specific feature during the approach of the ship. This fact seems to make it possible to recover the 3D position of a feature by comparing the feature position along the sequence of images acquired from different (known) ship positions. A feature extraction and analysis, a Kalman filter for robust feature tracking, and some ad hoc equations for depth estimation are proposed. Simulated image sequences demonstrated the great potential of the developed system, even though real sequences pointed out some inaccuracies due to errors concerning the knowledge of the ship position and the discrete image nature.

On the Interpolation of Echographic Images Exploiting Band-Pass Signals

This paper deals with the study of an innovative interpolation technique aimed at the generation of echographic images of a synthetic scene previously insonified. The traditional echographic equipments perform the interpolation process working on the data obtained after the envelope extraction (low- pass data). The proposed method carries out the interpolation process using the band-pass data before the envelope extraction, performing, in this way, a complex values interpolation. I. INTRODUCTION This work refers to the ultrasound imaging. Among all of the possible applications, here we consider the one referring to the generation of medical echographic images. The purposes of this work is the development of new band-pass interpolation techniques aimed to the composition of the echographic images, comparing the results with those obtained adopting the low-pass interpolation methods nowadays in use. The traditional echographic equipments perform the interpolation process on the image pixels, working, in this way, on the gray levels obtained extracting the envelope of the demodulated echoes received by the array and processed by the beamforming algorithm. The linear interpolation function is commonly adopted because it is less burdensome in terms of computational load and provides better results when the spatial sampling theorem is respected. Otherwise, the designed method interpolates directly the band-pass signals obtained by the beamforming process. It is useful to interpolate before the envelope extraction process, since this process causes loss of phase information, which is important to obtain a better image, especially in condition of strong undersampling. In particular, in this paper, we try to demonstrate that the interpolation process applied to the band-pass signals instead of the low-pass signals (after envelope extraction) allows to obtain better results in terms of reconstructed images quality also, as mentioned above, when the spatial sampling theorem is not respected. This characteristic makes it possible to acquire the echo signals in undersampling condition, and thus, reconstructing a fully spatial sampled image using the addressed interpolation method. Some tests regarding the adoption of interpolation functions more complex than the linear one have been carried out on both band-pass and low-pass signals. Even though the implementation of these functions produces an unavoidable computational load growth, the quality of the obtained images is better than those interpolated with traditionally linear functions. In particular, our attention has been