Carlos Fritsch

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.

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 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.

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.

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

Protection circuits for ultrasound applications

A simple input protection circuit for ultrasound pulse-echo applications is described. Its performance is analyzed with regard to other widely used arrangements. Besides the primary function of showing high impedance during the transducer excitation time and a low impedance path to the amplifier in reception, issues of harmonic distortion, insertion losses, bandwidth, power dissipation, transient response, and noise are addressed. It is shown that the proposed circuit has many advantages, operating without any control signals or bias voltages. It is small and can be considered a good general-purpose protection circuit alternative.

Automatic dynamic depth focusing for NDT

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.

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.