O. Martinez

64 Elements two-dimensional piezoelectric array for 3D imaging

Ultrasound has a large potential on non-invasive inspection with main applications in medical imaging and non-destructive testing (NDT). The increasing interest in 3D imaging applications leads to investigate new solutions for two-dimensional (2D) ultrasonic arrays with an affordable number of electronic channels without resolution degradation. 2D segmented annular arrays (SAAs) are a good compromise between resolution--image quality--and number of electronically active channels. A 1-3 piezoelectric composites are used as basis material to manufacture the array transducers due to their low planar coupling and high electromechanical coupling coefficients. A 1.5 MHz SAA of 64 elements and 20 mm of diameter was designed, manufactured and tested. The design key point is the use of a flexible circuit with electrodes and tracks that define the array geometry. The piezocomposite was used as a monolithic support. Soft backing and one matching layer were used. The array elements have been tested electrically and acoustically showing good agreement with a KLM-based simulation model. Acoustical field measurements in water at different steering angles were made and compared with simulations performed with a model that uses an exact solution of the impulse response approach. Side lobes are important because the array geometry used was designed to work in metals for NDT purposes. Smaller array elements should be made for medical applications.

A small 2D ultrasonic array for NDT applications

A prototype of two-dimensional transducer array with reduced number of elements, based on segmented annular distribution is presented. The capability of this array to produce volumetric imaging is compared to the equivalent conventional 2D squared matrix array. The comparison between both apertures is made for the cases of the full-array emission/reception mode and SAFT mode. From the analysis it is deduced that the segmented annular arrays produce lower grating lobes than squared arrays, improving the image contrast. The fabrication process of a segmented annular array of 64 elements and the experimental work made with this array transducer is also presented in this paper.

The progressive focusing correction technique for ultrasound beamforming

This work presents a novel method for digital ultrasound beamforming based on programmable table look-ups, in which vectors containing coded focusing information are efficiently stored, achieving an information density of a fraction of bit per acquired sample. Timing errors at the foci are within half the period of a master clock of arbitrarily high frequency to improve imaging quality with low resource requirements. The technique is applicable with conventional as well as with DeltaSigma converters. The bit-width of the focusing code and the number of samples per focus can be defined to improve both memory size and F# with controlled timing errors. In the static mode, the number of samples per focus is fixed, and in the dynamic approach that figure grows progressively, taking advantage of the increasing depth of focus. Furthermore, the latter has the lowest memory requirements. The technique is well suited for research purposes as well as for real-world applications, offering a degree of freedom not available with other approaches. It allows, for example, modifying the sampling instants to phase aberration correction, beamforming in layered structures, etc. The described modular and scalable prototype has been built using low-cost field programmable gate arrays (FPGAs). Experimental measurements are in good agreement with the theoretically expected errors

Beam Steering with Segmented Annular Arrays

Two-dimensional (2-D) arrays of squared matrix have maximum periodicity in their main directions; consequently, they require half wavelength (lambda/2), interelement spacing to avoid grating lobes. This condition gives rise to well-known problems derived from the huge number of array elements arid from their small size. In contrast, 2-D arrays with curvilinear configuration produce lower grating lobes and, therefore, allow the element size to be increased beyond lambda/2. Using larger elements, these arrays have the advantage of reducing the number of elements arid of increasing the signal-to-noise ratio (SNR). In this paper, the beamforming properties of segmented annular phased arrays are theoretically analyzed and compared with the equivalent squared matrix array. In the first part, point-like elements are considered in order to facilitate the field analysis with respect to the array structure. Afterward, the effect of the element size on the steered beam properties also is presented. In the examples, it is shown that the segmented annular array has notably lower grating lobes than the equivalent squared matrix array and that it is possible to design segmented annular arrays with interelement distance higher than lambda whose beam characteristics are perfectly valid for volumetric imaging applications

A multirate scan conversion method

B-mode ultrasonic imaging requires that the acquired polar coordinate ultrasound data be converted to the Cartesian format used by digital monitors. Image quality depends on the interpolation algorithm used to this purpose. In this work a selective sampling technique, based on acquiring data at specific points of the scanned area together with a straightforward linear interpolation step, is proposed. Hardware complexity is avoided, because the interpolation task can be carried out by software in real time, concurrently with data acquisition. The performances of the proposed approach are analysed with regard to those provided by other algorithms and some implementation issues are addressed.

Application of digital signal processing techniques to synthetic aperture focusing technique images

Abstract Synthetic aperture focusing technique (SAFT) has become a popular alternative to transmission–reception focused arrays (TRFA) in order to reduce hardware complexity and cost associated to ultrasonic (UT) imaging systems. A shortcoming of SAFT processing is that it introduces artifacts that distort the images. However, as SAFT is sequential, efficient digital processing algorithms can be used to improve image quality. In this paper, several digital processing techniques are proposed including apodization, deconvolution, dynamic focusing, deflection and envelope extraction. A pipeline architecture which allows execution of these algorithms in parallel for real-time imaging is also proposed.

Reduction of grating lobes in SAFT images

In this work we present a new synthetic aperture technique, derived from the conventional SAFT, where only one element in emission and two in reception are used (2R-SAFT). This new technique suppresses the grating lobes of the conventional SAFT and allows obtaining images with a better contrast and signal-noise ratio, at the same time that minimizes the hardware requirements of the imaging systems and maintains a high frame rate.

A computational method to calculate the longitudinal wave evolution caused by interfaces between isotropic media

This paper presents a computational method to calculate the reflected and transmitted ultrasonic fields at interfaces of complex geometry. The method is performed in two steps. As first step, the velocity potential impulse response from an arbitrary aperture is determined at the interface using the Rayleigh integral and considering the reflection and transmission coefficients. In a second step, the simulated fields are calculated by applying the Rayleigh-Sommerfeld integral to the whole, extended interface. In order to validate the method, some experimental cases as, for instance, plane and cylindrical concave surfaces between two media (water-acrylic) were tested. The experimental ultrasonic fields are in good agreement with those provided by the model. Furthermore, in the work, the compromise between the accuracy of the method and the computation time is studied.

Beamforming with a reduced sampling rate

The beamforming process requires a high delay resolution to avoid the deteriorating effects of the delay quantization lobes on the image dynamic range and signal to noise ratio. Wideband transducers require delay resolutions in the order of 1/16 the signal period. If oversampling is used to achieve this timing resolution, a huge data volume has to be acquired and processed in real time. This is usually avoided by sampling just above the Nyquist rate and interpolating to achieve the required delay resolution. However this increases the hardware complexity. Baseband sampling has been alternatively proposed with sampling rates as low as the transducer frequency or even lower. This approach uses two A/D converters and processing chains for every channel, thus doubling the hardware requirements. Quadrature sampling can be used instead with a single A/D converter, but the sampling rate must be a multiple of four times the transducer frequency, decreasing the application flexibility. Furthermore, it produces relatively high errors in the detected envelope if wideband transducers are used. This work presents a new approach, the selective sampling technique (SST), which keeps the lowest sampling rate required by the imaging process or the signal bandwidth (whatever is larger) and, at the same time, provides a high delay resolution to keep the highest image dynamic range. The SST is based on a second order sampling process which, differently from the mentioned approaches, does not pose any constraints in the time interval between samples and produce lower errors in the detected envelope. The hardware requirements are low (a single A/D converter and processing chain for every transducer element), working at the lowest data rate compatible with the Nyquist criterion, thus reducing the data bandwidth. Furthermore, the sampling points can be also freely chosen, so that the SST simplify the usually required scan conversion process to a simple linear interpolation easily carried out by software in real-time.

P3R-3 Design and Characterization of Air Coupled Ultrasonic Transducers Based on MUMPs

This paper deals with the design and characterization of several capacitive micromachined ultrasonic transducer (cMUT) cells for the future design of an air-coupled transducer for non destructive testing (NDT). Each design was manufactured using the multi-user MEMS process (MUMPs). Special boundary conditions were used to obtain high efficiency: two opposite sides of the 150 mum square shaped membranes are free while the other sides are anchored to a fixed layer of the process. Using these designs a large displacement of the membrane and a higher surface contact is obtained in spite of their mechanical losses