Bakary Diarra

Design of Optimal 2-D Nongrid Sparse Arrays for Medical Ultrasound

Three-dimensional imaging with 2-D matrix probes is one of the most exciting recent ultrasound innovations. Unfortunately, the number of elements of a 2-D matrix probe is often very high, and reducing this number deteriorates the beam properties. In this paper, we propose a new sparse-array design technique with irregular element positioning, which significantly reduces the number of active elements as well as the grating-lobe level. In particular, we introduce a new cost function for optimizing the weighting coefficients of the elements and a new annealing-based algorithm to compute the lowest cost solutions. Numerical simulations show substantial improvements over standard sparse arrays.

2D matrix array optimization by simulated annealing for 3D hepatic imaging

In this paper we present a preliminary study of a 2D sparse array design technique. The probe is intended to be suitable for needle tacking during hepatic biopsy and therapy applications. It can also be used in other micro-tools or internal organs operations. The probe is composed of 1024 elements (64×16). Due to this huge number of elements, the sparse array technique is used to reasonably reduce this number and to make possible its connection to the recent scanners. The simulated annealing algorithm permits to optimize elements coefficients and their positions to have a better beam pattern. The features of the probe must satisfy the geometrical constraints imposed by the targeted applications as well as some users defined imaging characteristics. Several simulations are made to know the acoustical characteristics of the array and its convenience to needle detection operations. Combining sparse array and optimization algorithm we reduce the initial element number to 267 with good imaging features: the sidelobes level is maintained to - 40 dB, the lateral main lobe width at -6 dB is 1.1 mm and the elevation main lobe width is 4.4 mm.

Realistic acoustic simulation of 2-D probe elements in simulated annealing sparse array optimization

In this study, the benefits of integrating a realistic acoustic simulation in the 2-D non-grid sparse array design are investigated. A wideband shape sensitive (WSS) energy function is introduced. New degrees of freedom in 2-D arrays transducer optimization are thus made available. A related consequence is the capability of distinguishing probes configurations so far considered as performing identically. To clearly illustrate this, we compare the radiated patterns of two 5×5 elements array probes having elements with the same rectangular size and position, but perpendicular orientations. Under linear assumption, an ergonomic data management, here named Fixy method, enables the radiated pattern computation during the optimization process thanks to a 4.8 times speed-up. Finally two optimized probes are compared with an equivalent size full-array. One was optimized using the omnidirectional monochromatic model and the other one using the proposed WSS energy function. To consider the same acoustic radiation of the probe during the optimization as in reality yields promising results.

Non-grid based elements positioning for optimal 2D array beams

2D matrix probes present a real capability for three dimensional ultrasound imaging. The main difficulties to implement these probes come from the need of using a huge number of elements and from the grating lobes linked to the element periodical disposition and from the sidelobes arising when this number is reduced. In this study, a new design technique is proposed and shown capable of considerably reducing the probe element number as well as the grating lobes and sidelobes level. The grating lobes, in particular, are reduced by placing the elements without any grid consideration. The sidelobes and the number of active elements are optimized by the simulated annealing. Using the new approach we reduce the element number by 28% (from 235 to 169) and obtain a grating lobe reduction by -10 dB and -20 dB in lateral and elevation direction, respectively.

Variable-size elements in 2D sparse arrays for 3D medical ultrasound

The sparse array technique is an excellent approach to tackle the connection difficulties due to the huge number of elements of 2D matrix arrays. However, the reduction of the element number leads to significant performance reduction, especially in terms of energy loss. An efficient alternative to increase the array sensitivity is using elements wider than λ/2 while non-grid positioning strategies are adopted. To further decrease the unwanted lobes of the array beam, the variable-size elements array is proposed. This new configuration, combining the randomness of elements position and of their size, leads to excellent beam patterns. In addition, the main lobe width can be controlled to change the array spatial resolution.

Optimization of free-moving elements in 2D ultrasound sparse arrays

The non-grid sparse array technique is a promising approach to overcome the connection difficulties of 2D matrix arrays and to partially compensate the energy loss linked to the element number reduction. Being independent from the spatial sampling conditions, this method leads to a significant improvement of the beam pattern when combined to the simulated annealing algorithm. However, in the previous version of this method, the position of the elements cannot be changed during the optimization. In order to add a further degree of freedom and thus to improve the optimization performance, we propose a new strategy in which both element position and apodization can be modified. This new approach improves the sensitivity thanks to a better distribution of the elements on the array footprint.

Comparison of different optimized irregular sparse 2D ultrasound arrays

The role of 2D matrix arrays in the realization of real time 3D ultrasound imaging is crucial as the latter permit to acquire a complete volume. The lack of control systems for these arrays containing thousands of elements and the dimension of connection cables for such a number of elements are the main obstacles to the use of these technologies. The sparse array techniques present a solid candidate to solve these technological limitations but give rise to beam pattern deteriorations in terms of energy loss and unwanted lobes apparition. The irregular positioning of the array elements permits to drastically decrease the unwanted lobes while the energy loss may be compensated by increasing the element size. However, as elements get larger, the directivity decreases. To solve this directivity reduction, we propose to combine elements of different sizes in such a manner that a broad directivity is maintained using small elements whereas the energy loss may be compensated by wide ones. Additionally, an innovative simulated annealing based algorithm is used to refine the array beam profile when the active element number is set to 256. The results compared to those obtained by the basic non-grid array show significant improvement in the beam pattern.