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.

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.

Auto-focusing along with dynamic depth focusing (DDF) would be very valuable to inspect arbitrarily shaped parts when operating with wedges or with other coupling media to avoid the burden of computing and setting the correct focal laws while still getting the best possible resolution at all depths. This work proposes a three-step procedure to perform the auto-focusing function with DDF in real time. First, the part geometry is estimated by the first echo time-of-arrival following one of several possible strategies: pulse-echo, pitch-catch, or plane wave. These are analyzed with regard to their performances and acquisition time, giving closed formulae to get the coordinates of interface points. After a curve fitting and extrapolation process, a virtual array that operates in a homogeneous medium is computed, avoiding the complications of refraction at the interface and allowing operation with already known focusing hardware. This hardware is initialized with the set of focusing parameters adapted to the estimated probe- part geometry, and ensures that all received samples are in focus. Using a standard computer, the auto-focusing procedure currently takes about 2 s to perform. Experiments carried out under different conditions validate the proposed technique.

In this work, a novel procedure that considerably simplifies the fabrication process of ferroelectret-based multi-element array transducers is proposed and evaluated. Also, the potential of ferroelectrets being used as active material for aircoupled 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 plusmn1.8 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.

Ultrasound breast imaging is a promising alternative to conventional mammography because it does not expose women to harmful ionising radiation and it can successfully image dense breast tissue. However, conventional ultrasound imaging only provides morphological information with limited diagnostic value. Ultrasound computed tomography (USCT) uses energy in both transmission and reflection when imaging the breast to provide more diagnostically relevant quantitative tissue properties, but it is often based on time-of-flight tomography or similar ray approximations of the wave equation, resulting in reconstructed images with low resolution. Full-waveform inversion (FWI) is based on a more accurate approximation of wave-propagation phenomena and can consequently produce very high resolution images using frequencies below 1 megahertz. These low frequencies, however, are not available in most USCT acquisition systems, as they use transducers with central frequencies well above those required in FWI. To circumvent this problem, we designed, trained, and implemented a two-dimensional convolutional neural network to artificially generate missing low frequencies in USCT data. Our results show that FWI reconstructions using experiment data after the application of the proposed method successfully converged, showing good agreement with X-ray CT and reflection ultrasound-tomography images.

Phase Coherence Imaging (PCI) has been recently proposed as a robust method to improve the quality of ultrasound images. Based on a statistical analysis of the instantaneous phase of the aperture data, a Phase Coherence Factor (PCF) is computed for every sampling instant. When used to weight the beamformer output, side and grating lobe levels are reduced and lateral resolution is increased. In this work, its application for grating lobe artifacts reduction is further investigated. Dependence of grating lobes reduction level with signal bandwidth and the number of array elements is analyzed and detection of low amplitude echoes located into the grating lobe region is addressed. Experimental data obtained with a 2-D matrix sparse array are presented. In a second experiment, a standard tissue-mimic phantom and a sparse linear array are used to evaluate the PCI grating lobe reduction performance for medical images.