A new beamforming method and hardware architecture for real time two way dynamic depth focusing.

The Total Focusing Method (TFM) yields a focused image in emission and in reception while Phased Array (PA) imaging provides Dynamic Depth Focusing (DDF) in reception only. Besides, most NDE applications have two propagation media, where refraction at the interface complicates time-of-flight (TOF) and focal law computations. This affects especially TFM, which must compute the TOFs from all elements to image pixels and use them to select the data for imaging. A new method with real-time Dynamic Depth Full Focusing (DDFF), in emission and reception, is proposed in this work. It is called Total Focusing Phased Array (TFPA) because it uses concepts of TFM and PA. Omnidirectional emissions are used to create a synthetic aperture as in TFM, while beamforming is carried out along scan lines as in PA, simplifying the delay calculation in the presence of interfaces and providing an efficient hardware implementation. Refraction at the interface between two media is eliminated by a Virtual Array (VA) that converts such scenario into a simple homogeneous medium. Propagation can be considered along scan lines from the virtual array at constant speed, as in homogeneous media. Strict dynamic focusing is performed in real-time, an important difference with other approaches that require iterative Fermat search to get the focal laws for every imaged point. With TFPA only 3 parameters per element and scan line are required to perform this task. Experiments are carried out to compare the three techniques, PA, TFM and TFPA. TFM and TFPA yield similar image quality, offering improved depth of field and resolution over PA. On the other hand, TFPA avoids most of the burden for computing TOFs and operates in real time with one or two media propagation.

Passive focusing techniques for piezoelectric air-coupled ultrasonic transducers.

This paper proposes a novel passive focusing system for Air-Coupled Ultrasonic (ACU) piezoelectric transducers which is inspired by the Newtonian-Cassegrain (NC) telescope concept. It consist of a primary spherical mirror with an output hole and a flat secondary mirror, normal to the propagation axis, that is the transducer surface itself. The device is modeled and acoustic field is calculated showing a collimated beam with a symmetrical focus. A prototype according to this design is built and tested with an ACU piezoelectric transducer with center frequency at 400 kHz, high-sensitivity, wideband and 25 mm diameter flat aperture. The acoustic field is measured and compared with calculations. The presented prototype exhibit a 1.5 mm focus width and a collimated beam up to 15 mm off the output hole. In addition, the performance of this novel design is compared, both theoretically and experimentally, with two techniques used before for electrostatic transducers: the Fresnel Zone Plate - FZP and the off-axis parabolic or spherical mirror. The proposed NC arrangement has a coaxial design, which eases the transducers positioning and use in many applications, and is less bulky than off-axis mirrors. Unlike in off-axis mirrors, it is now possible to use a spherical primary mirror with minimum aberrations. FZP provides a more compact solution and is easy to build, but presents some background noise due to interference of waves diffracted at out of focus regions. By contrast, off-axis parabolic mirrors provide a well defined focus and are free from background noise, although they are bulky and more difficult to build. Spherical mirrors are more easily built, but this yields a non symmetric beam and a poorly defined focus.

New method for real-time dynamic focusing through interfaces.

In nondestructive evaluation (NDE) a coupling medium (wedge) is frequently inserted between the array probe and the object being evaluated. In this situation, focal law computing is complicated by the refraction effects at the interface. Furthermore, there are not known techniques to perform dynamic focusing by hardware in these conditions. This work addresses these problems by following a two-step procedure. First, a virtual array that operates in a single medium with nearly equivalent time-of-flight to the foci is obtained. Then, simple hardware is proposed to perform dynamic focusing in real-time. It operates with arrays of any geometry as required by the virtual array in presence of arbitrarily shaped interfaces. The paper describes the theory and evaluates the timing errors of the approximations made. These errors are low enough to allow use of the new technique in most NDE and some specific medical applications. The new technique is validated by simulation and experimentally.

Phase coherence imaging of grained materials.

Ultrasound detection and evaluation of flaws in materials showing structural noise (austenitic steels, titanium alloys, composites, etc.) is difficult because of the low flaw-to-grain noise ratio. Much research has been performed looking for methods to improve flaw detection in grained materials. Many approaches require a cumbersome tuning process to select the correct parameter values or to use iterative techniques. In this work, the technique of phase coherence imaging is proposed to improve the flaw-to-grain noise ratio. The technique weights the output of a conventional beamformer with a coherence factor obtained from the aperture data phase dispersion. It can be simply implemented in real-time and it operates automatically, without needing any parameter adjustment. This paper presents the theoretical basis of phase coherence imaging to reduce grain noise, as well as experimental results that confirm the expected performance.

Phase coherence imaging.

A new method for grating and side lobes suppression in ultrasound images is presented. It is based on an analysis of the phase diversity at the aperture data. Two coherence factors, namely the phase coherence factor (PCF) and the sign coherence factor (SCF), are proposed to weight the coherent sum output. Different from other approaches, phase rather than amplitude information is used to perform the correction action. Besides achieving the main goal, the method obtains improvements in lateral resolution and SNR. Implementation of the SCF technique is quite straightforward, operating in realtime, and can be added to any virtually existing beamformer to improve the resolution, contrast, SNR, and dynamic range of the images. A programmable parameter allows adjusting the sensitivity of the method to out-of-phase signals, from zero to a strict coherence criterion. The theoretical basis for the 2 methods are given and their performances evaluated by simulation. Then, experiments are conducted to provide results that are in good agreement with those expected from theory and simulation.

Airborne ultrasonic phased arrays using ferroelectrets: a new fabrication approach.

In this work, a novel procedure that considerably simplifies the fabrication process of ferroelectret-based multielement array transducers is proposed and evaluated. Also, the potential of ferroelectrets being used as active material for air-coupled ultrasonic transducer design is demonstrated. The new construction method of multi-element transducers introduces 2 distinctive improvements. First, active ferroelectret material is not discretized into elements, and second, the need of structuring upper and/or lower electrodes in advance of the permanent polarization of the film is removed. The aperture discretization and the mechanical connection are achieved in one step using a through-thickness conductive tape. To validate the procedure, 2 linear array prototypes of 32 elements, with a pitch of 3.43 mm and a wide usable frequency range from 30 to 300 kHz, were built and evaluated using a commercial phased-array system. A low crosstalk among elements, below -30 dB, was measured by interferometry. Likewise, a homogeneous response of the array elements, with a maximum deviation of +/-1.8 dB, was obtained. Acoustic beam steering measurements were accomplished at different deflection angles using a calibrated microphone. The ultrasonic beam parameters, namely, lateral resolution, side lobe level, grating lobes, and focus depth, were congruent with theory. Acoustic images of a single reflector were obtained using one of the array elements as the receiver. Resulting images are also in accordance with numerical simulation, demonstrating the feasibility of using these arrays in pulse-echo mode. The proposed procedure simplifies the manufacturing of multidimensional arrays with arbitrary shape elements and not uniformly distributed. Furthermore, this concept can be extended to nonflat arrays as long as the transducer substrate conforms to a developable surface.

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.