M. Parrilla

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.

A digital envelope detection filter for real-time operation

A time-domain method to extract the envelope of an amplitude modulated signal at high speed is presented. This method, the envelope detection filter (EDF), is based on a nonlinear function of two consecutive samples of the input sequence. In spite of its simplicity, EDF provides a quite good approximation to the analytical signal magnitude for a wide range of signals and conditions. The paper presents the basis of the technique and analyzes the sources of error and its sensitivity to noise and signal bandwidth. Results of experiments with simulated signals are in good agreement with the theoretical model. Furthermore, EDF allows a straightforward implementation using a pipeline of two registers and a memory look-up table. The hardware implementation of EDP has been tested at a 10 MS/s, although current technology could achieve several times this throughput rate. Results obtained with EDF for real data of nondestructive testing experiments, are compared with those provided by the full precision analytical signal magnitude obtained by a software Hilbert transform. In all cases, no significant differences are appreciated between the analytical signal magnitude and the EDF output, which is computed in much less time.

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

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.

Grating-lobes reduction by application of Phase Coherence Factors

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

Dynamic focusing through arbitrary geometry interfaces

This paper introduces the fast focal law calculator (FFLC), a Newton-Raphson based algorithm that performs such task accurately at high speed. It is especially well suited for dynamic focusing through arbitrary geometry interfaces, where other algorithms are order of magnitude slower. In spite of the high speed of the FFLC, errors are kept very small, typically within a few tens of picoseconds. Besides a short background theory, the paper compares the results of the FFLC with regard to exact solutions (for planar interfaces) and those based on search algorithms. Field simulations are performed to assess the correctness of the method. Also, experiments are carried out with a curved interface showing the advantages of the FFLC for dynamic focusing to improve the image quality and the flaw detection and evaluation capabilities.

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.

New ultrasound imaging techniques with phase coherence processing.

This work addresses three key subjects to the image quality with phased arrays: timing accuracy, beamforming strategy and post-processing for increased resolution and suppression of grating and side lobes. Timing accuracy is achieved by defining a modular and scalable architecture which guarantees low timing errors, whatever is the system size. The proposed beamforming methodology follows the progressive focusing correction technique, which keeps low focusing errors, provides a high information density and has a simple implementation for real-time imaging in modular architectures. Then, phase coherence imaging is defined to suppress grating and sidelobe indications, simultaneously increasing the lateral resolution.

P2D-4 A Front-End Ultrasound Array Processor Based on LVDS Analog-to-Digital Converters

This work presents a beamformer based on standard LVDS 12-bit analog-to-digital converters, which deliver the samples serially over a differential pair. The bit stream is synchronized and converted to a sequence of words in a FPGA. Beamforming is then carried out following the progressive focusing correction technique described in a previous work. The low pin count of serial converters and the FPGA ball grid array package allowed a high level of board integration. A scalable architecture allows many different configurations. Some of the possible applications are: phased array, linear scan, multi-channel ultrasound systems, SAFT imaging and TOFD techniques. The configuration is changed by software or just installing more modules in a back-plane. This work describes the system architecture, analog and digital preprocessing and assesses the performance with experimental images from a test block and tissue phantoms