Expanding the microbiologist toolbox via new far-red-emitting dyes suitable for bacterial imaging

IMPORTANCE

By harnessing the versatility of fluorescence microscopy and super-resolution imaging, bacteriologists explore critical aspects of bacterial physiology and resolve bacterial structures sized beyond the light diffraction limit. These techniques are based on fluorophores with profitable photochemical and tagging properties. The paucity of available far-red (FR)-emitting dyes for bacterial imaging strongly limits the multicolor choice of bacteriologists, hindering the possibility of labeling multiple structures in a single experiment. The set of FR fluorophores characterized in this study expands the palette of dyes useful for microbiologists, as they can be used for bacterial LIVE/DEAD staining and for tagging the membranes of viable Escherichia coli and Bacillus subtilis cells. The absence of toxicity makes these dyes suitable for live-cell imaging and allows monitoring of bacterial membrane biogenesis. Moreover, a newly synthesized FR-fluorophore can be employed for imaging bacterial membranes with stimulated emission depletion microscopy, a super-resolution technique capable of increasing the resolving power of conventional microscopes.

An image processing-based quantification of gram variability in Acinetobacter baumannii

Gram staining differentiates bacteria as gram-positive and gram-negative, depending on their cell wall constituents, and coloring cells in violet and pink, respectively. Sometimes, a subpopulation of the same bacterial species assumes different colors, ranging from pink to violet, for reasons that are not completely understood yet. We analyze conventional brightfield images and use an automated pipeline to count pink and violet cells. The ImageJ-based processing algorithm quantifies the gram variability in Acinetobacter baumannii ACICU in the stationary phase of growth with a percentage of 66% pink cells. Different bacterial strains and cell growth stages have been considered. RESEARCH HIGHLIGHTS: Gram staining differentiates bacteria into gram-positive (violet) and gram-negative (pink). Gram variability represents an inhomogeneous distribution of pink and violet cells within the same species. We developed an ImageJ-based pipeline for the quantification of Gram variability in Acinetobacter baumannii.

Growth Phase- and Desiccation-Dependent Acinetobacter baumannii Morphology: An Atomic Force Microscopy Investigation

Acinetobacter baumannii has emerged as a major bacterial pathogen during the past three decades. The majority of the A. baumannii infections occur in hospitals and are caused by strains endowed with high desiccation tolerance, which represents an essential feature for the adaptation to the nosocomial environment. This work aims at investigating the desiccation response of the multidrug-resistant A. baumannii strain ACICU as a function of the bacterial growth phase and oxygen availability, by correlating bacterial survival with shape alterations. The three-dimensional morphological analysis of bacteria was carried out by atomic force microscopy (AFM), following the evolution of bacterial shape descriptors, such as the area, volume, roughness of individual cell membranes, and the cell cluster roughness, which exhibited peculiar and distinctive behavior as a function of the growth conditions. AFM images of A. baumannii ACICU cells revealed the prevalence of the coccoid morphology at all growth stages, with a tendency to reduce their size in the stationary phase, accompanied by a higher survival rate to air-drying. Moreover, cells harvested from the logarithmic phase featured a larger volume and resulted to be more sensitive to desiccation compared to the cells harvested at later growth stages. In addition, oxygen deprivation caused a significant decrease in cellular size and was associated with the formation of pores in the cell membrane, accompanied by a relative reduction in culturability after desiccation. Morphological plasticity and multidrug resistance may contribute to desiccation tolerance and therefore to persistence in the hospital setting.

SSNOMBACTER: A collection of scattering-type scanning near-field optical microscopy and atomic force microscopy images of bacterial cells

BACKGROUND

In recent years, a variety of imaging techniques operating at nanoscale resolution have been reported. These techniques have the potential to enrich our understanding of bacterial species relevant to human health, such as antibiotic-resistant pathogens. However, owing to the novelty of these techniques, their use is still confined to addressing very particular applications, and their availability is limited owing to associated costs and required expertise. Among these, scattering-type scanning near field optical microscopy (s-SNOM) has been demonstrated as a powerful tool for exploring important optical properties at nanoscale resolution, depending only on the size of a sharp tip. Despite its huge potential to resolve aspects that cannot be tackled otherwise, the penetration of s-SNOM into the life sciences is still proceeding at a slow pace for the aforementioned reasons.

RESULTS

In this work we introduce SSNOMBACTER, a set of s-SNOM images collected on 15 bacterial species. These come accompanied by registered Atomic Force Microscopy images, which are useful for placing nanoscale optical information in a relevant topographic context.

CONCLUSIONS

The proposed dataset aims to augment the popularity of s-SNOM and for accelerating its penetration in life sciences. Furthermore, we consider this dataset to be useful for the development and benchmarking of image analysis tools dedicated to s-SNOM imaging, which are scarce, despite the high need. In this latter context we discuss a series of image processing and analysis applications where SSNOMBACTER could be of help.

STED nanoscopy of KK114-stained pathogenic bacteria

Super-resolution microscopy techniques can provide answers to still pending questions on prokaryotic organisms but are yet to be used at their full potential for this purpose. To address this, we evaluate the ability of the rhodamine-like KK114 dye to label various types of bacteria, to enable imaging of fine structural details with stimulated emission depletion microscopy (STED). We assessed fluorescent labeling with KK114 for eleven Gram-positive and Gram-negative bacterial species and observed that this contrast agent binds to their cell membranes. Significant differences in the labeling outputs were noticed across the tested bacterial species, but importantly, KK114-staining allowed the observation of subtle nanometric cell details in some cases. For example, a helix pattern resembling a cytoskeleton arrangement was detected in Bacillus subtilis. Furthermore, we found that KK114 easily penetrates the membrane of bacterial microorganism that lost their viability, which can be useful to discriminate between living and dead cells.

Geometrical-optics approach to measure the optical density of bacterial cultures using a LED-based photometer

We develop a suitable geometrical-optics approach and demonstrate that it is possible to measure the optical density (OD) of bacterial cultures using a light emitting diode (LED)-based photometer. We measure both attenuation and spot-size variation, and we compensate for diffraction and stray-light impairment related to the incoherent source and large detection area. The approach is validated for different concentrations of two bacterial species, Escherichia coli and Staphylococcus aureus, that present different shapes and clustering organization.

Geometrical-optics approach to increase the accuracy in LED-based photometers for point-of-care testing

A geometrical-optics approach is proposed to increase the accuracy in photometric measurements, using a point-of-care testing (POCT) LED-based sensor. Due to stray-light effects, the measurement accuracy depends on the dimension of the CMOS area, where the radiation is detected. We propose two image processing approaches and evaluate the influence of the sensor area. In addition, we demonstrate that with the same measurement, both absorption coefficient and refractive index can be determined, measuring the beam attenuation and the spot-size enlargement due to ray refraction.

Demonstration of PAM4-OCDM system with electrical amplitude-level pre-tuning and post-equalization for data centers applications

A PAM4-OCDM system with optical multi-/demultiplexing and electrical pre-/post-processing is proposed for short-reach applications. We experimentally demonstrate, for the first time, a power-efficient 4 OC x 10 GSymbol/s PAM4-OCDM system. The four PSK-OCs are simultaneously generated using a single light source and a passive multiport optical encoder and received by a single optical decoder and cascaded DSP. The effectiveness of the electrical-domain amplitude level pre-tuning and post-equalizer are demonstrated, considering different values of shot and beat noises.

All-optical Nyquist-OTDM to Nyquist-WDM conversion for high-granular flexible optical networks

We have proposed and experimentally demonstrated all-optical multiplexing (MUX)-format conversion from Nyquist optical time division multiplex (OTDM) to Nyquist wavelength division multiplex (WDM). The system is simply configured with a straight-type phase modulator (PM) driven by a sinusoidal wave and an optical Nyquist filter. In the theoretical investigation, it is proved that the single Nyquist signal is completely converted to Nyquist WDM signal, which consists of two half-baud-rate signals with different carrier frequencies. The theoretical modulation voltage for the phase modulator is slightly lower than Vπ: 0.913 Vπ, and it is experimentally verified. In the experimental demonstrations, 50 Gbaud to 25 Gbaud x 2 and 25 Gbaud to 12.5 Gbaud x 2 conversions are successfully demonstrated with quite low optical signal-to-noise ratio (OSNR) penalties. In addition, cascaded MUX-format conversion is also demonstrated; 50 Gbaud Nyquist signal is converted to four channels of 12.5 Gbaud Nyquist signals.

Spectrally-efficient all-optical OFDM by WSS and AWG

We report on the transmission experiment of seven 12.5-GHz spaced all optical-orthogonal frequency division multiplexed (AO-OFDM) subcarriers over a 35-km fiber link, using differential quadrature phase shift keying (DQPSK) modulation and direct detection. The system does not require chromatic dispersion compensation, optical time gating at the receiver (RX) or cyclic prefix (CP), achieving the maximum spectral efficiency. We use a wavelength selective switch (WSS) at the transmitter (TX) to allow subcarrier assignment flexibility and optimal filter shaping; an arrayed waveguide grating (AWG) AO-OFDM demultiplexer is used at the RX, to reduce the system cost and complexity.

A low-power photonic quantization approach using OFDM subcarrier spectral shifts

Photonic analog-to-digital conversion and optical quantization are demonstrated, based on the spectral shifts of orthogonal frequency division multiplexing subcarriers and a frequency-packed arrayed waveguide grating. The system is extremely low-energy consuming since the spectral shifts are small and generated by cross-phase modulation, using a linear-slope high-speed and low-jitter pulse train generated by a mode locked laser diode. The feasibility of a 2, 3 and 4-bit optical quantization scheme is demonstrated.

First demonstration of a scalable MDM/CDM optical access system

A novel hybrid all-optical mode-division multiplexing and code division multiplexing architecture for flexible and scalable access networks is presented. We successfully demonstrate, for the first time, an asynchronous on-off keying modulation, 2 mode x 4 code x 10 Gbps transmission over 42-km link, using a set of single-mode and two-mode fibers, without dispersion compensation. The four phase-shift keyed optical codes are generated at a single wavelength, by a multiport encoder/decoder, and we use an optical mode multiplexer/demultiplexer in the remote node and at the central office. We also experimentally evaluate the mode crosstalk tolerance considering different access span distances for the LP(01) and LP(11) modes.

Chromatic dispersion monitoring and adaptive compensation using pilot symbols in an 8 x 12.5 Gbit/s all-optical OFDM system

We propose and experimentally demonstrate a novel technique for chromatic dispersion (CD) monitoring and adaptive compensation in an 8 x 12.5 Gbit/s all-optical orthogonal frequency-division multiplexing (AO-OFDM) system by using two pilot symbols and a virtually imaged phased array (VIPA) for a tunable CD compensator. The two pilot symbols are added to the first and the last sub-channels of the OFDM signal, and their relative time delay is detected and used for CD estimation at the CD monitoring circuit. The monitored CD value is fed to VIPA for CD compensation. In the experiments, the relative time delay between the two pilot symbols was successfully observed, and the adaptive CD compensation drastically improved the bit-error-rate (BER) from over 10(-5) to under 10(-9). The estimated CD values showed less than 10 ps/nm difference from the values measured by a photonic dispersion analyzer, which is accurate enough since the AO-OFDM system can keep BER<10(-9) upto 20 ps/nm residual CD.

Tunable optical code converter using XPM and linear-slope pulse streams generated by FBGs

We propose for the first time (to the best of our knowledge) and demonstrate a tunable multiple optical code (OC) converter for flexible networks that allows dynamic code (bandwidth) allocation in a way similar to a tunable laser. With respect to other OC converters, this scheme can convert both specific and multiple OCs by changing only the amplifier gain. The proposed scheme uses cross-phase modulation (XPM) and two linear-slope control pulse streams generated by fiber Bragg gratings (FBGs) in the C-band. The OC converter can be used to avoid collisions in optical packet switching networks, and we analyze the corresponding packet loss probability performance.

Demonstration and performance investigation of all-optical OFDM systems based on arrayed waveguide gratings

We experimentally demonstrate an 8 x 12.5 Gbit/s all-optical orthogonal frequency-division multiplexing (AO-OFDM) system using arrayed waveguide gratings (AWGs), which perform discrete Fourier transform (DFT) and inverse DFT (IDFT) of a signal directly in the optical domain. The experimental results show that frequency orthogonality of OFDM sub-channels is degraded in the AWG due to the slab-diffraction effect. To restore the frequency orthogonality and improve the system performance, we propose and demonstrate a waveform reshaping scheme, that improve the bit-error-rate (BER) from 10(-4) to 10(-6). We also experimentally investigate the influence of frequency mismatch between the OFDM signal and AWG at the receiver. The measured BER shows a serious degradation from 10(-6) to 10(-4) in case of ± 1.88 GHz frequency mismatch. To keep the BER under 10(-5), the frequency mismatch should be smaller than ± 0.5 GHz ( ± 4% of the channel spacing).

What else can an AWG do?

The present paper aims to describe other functionalities for an arrayed waveguide grating (AWG)-based device, showing that this widely used configuration can be designed not only to frequency multiplex/demultiplex wavelength division multiplexing (WDM) signals, but also to perform the discrete Fourier transform (DFT) and the discrete fractional Fourier transform (DFrFT) of a signal, in all-optical orthogonal frequency division multiplexing (OFDM) systems. In addition 1 × N and N × N phased array switches architectures are described, as well as a new configuration to perform polarization diversity demultiplexing. Finally, a general approach, based on an analogy with the finite impulse response (FIR) filter approach, is presented to design optical modulators for any modulation format, using either phase modulators (PM) or electro-absorption modulators (EAM).

Noise suppression using optimum filtering of OCs generated by a multiport encoder/decoder

We propose a novel receiver configuration using an extreme narrow band-optical band pass filter (ENB-OBPF) to reduce the multiple access interference (MAI) and beat noises in an optical code division multiplexing (OCDM) transmission. We numerically and experimentally demonstrate an enhancement of the code detectability, that allows us to increase the number of users in a passive optical network (PON) from 4 to 8 without any forward error correction (FEC).

Demonstration of asynchronous, 40 Gbps x 4-user DPSK-OCDMA transmission using a multi-port encoder/decoder

We have developed a new 8-chip, 320 Gchip/s encoder/decoder with eight input/output ports, that can be used in 40-Gb/s PON networks. The device has been to multiplex four asynchronous 40 Gb/s users, using DPSK modulation. The transmission over 50 km has been successfully demonstrated for the first time.

Generalized fiber Fourier optics

A twofold generalization of the optical schemes that perform the discrete Fourier transform (DFT) is given: new passive planar architectures are presented where the 2 × 2 3 dB couplers are replaced by M × M hybrids, reducing the number of required connections and phase shifters. Furthermore, the planar implementation of the discrete fractional Fourier transform (DFrFT) is also described, with a waveguide grating router (WGR) configuration and a properly modified slab coupler.

High-security 2.5 Gbps, polarization multiplexed 256-ary OCDM using a single multi-port encoder/decoder

A block-ciphered (M-ary) optical code division multiplexing (OCDM) can provide larger security than a conventional OCDM system based on bit ciphering. We propose a polarization multiplexed (POL-MUX) M-ary OCDM system and demonstrated 2.5 Gbps, POL-MUX 256 ( = 16X16)-ary OCDM transmission using a single multi-port optical encoder/decoder (E/D). We show that this architecture reduces the number of required optical codes and enhances the system confidentiality.

Spectral coherent-state quantum cryptography

A novel implementation of quantum-noise optical cryptography is proposed, which is based on a simplified architecture that allows long-haul, high-speed transmission in a fiber optical network. By using a single multiport encoder/decoder and 16 phase shifters, this new approach can provide the same confidentiality as other implementations of Yuen's encryption protocol, which use a larger number of phase or polarization coherent states. Data confidentiality and error probability for authorized and unauthorized receivers are carefully analyzed.

Field trial of 160 Gbit/s DWDM-based optical packet switching and transmission

We demonstrated, for the first time, a field trial of 160 (16 lambda x 10) Gbit/s, fine granularity, DWDM-based optical packet switching and transmission by newly-developed burst-mode EDFAs and an optical packet switch prototype with multiple all-optical label processors. We achieved 64 km field transmission and switching of 160 (16 lambda x 10) Gbit/s DWDM-based optical packets encapsulating almost 10 Gbit/s IP packets with error-free operation (IP-packet-loss-rate <10(-6) and bit-error-rate <10(-9)).

Angular momentum dynamics of a paraxial beam in a uniaxial crystal

The conservation law governing the dynamics of the radiation angular momentum component along the optical axis (z axis) of a uniaxial crystal is derived from Maxwell's equations; the existence of this law is physically related to the rotational invariance of the crystal around the optical axis. Specializing the obtained general expression for the z component of the angular momentum flux to the case of a paraxial beam propagating along the optical axis, we find that the expression is the same as the corresponding one for a paraxial beam propagating in an isotropic medium of refractive index n(o) (ordinary refractive index of the crystal); besides, we show that the flux is conserved during propagation and that it decomposes into the sum of an intrinsic and an orbital contribution. Investigating their dynamics we demonstrate that they are coupled and, during propagation, an exchange between them exists. This exchange asymptotically exhibits a saturation process leading, for z--> infinity, the intrinsic part to vanish and the orbital one equates the total amount of angular momentum flux. As an example, the evolution of the intrinsic and the orbital contributions to the flux is investigated in the case of circularly polarized beams. Besides, the radiation angular momentum stored in the crystal is also investigated, in the paraxial regime, showing that it is simply given by the product of the total angular momentum flux by the time the radiation takes in passing through the crystal.

Circularly polarized beams and vortex generation in uniaxial media

We deduce the expressions for the two circularly polarized components of a paraxial beam propagating along the optical axis of a uniaxial crystal. We find that each of them is the sum of two contributions, the first being a free field and the second describing the interaction with the opposite component. Moreover, we expand both components as a superposition of vortices of any order, thus obtaining a complete physical picture of the interaction dynamics. Consequently, we argue that a left-hand circularly polarized incoming beam, endowed with a circular symmetric profile, gives rise, inside the crystal, to a right-hand circularly polarized vortex of order 2. The efficiency of this vortex generation is investigated by means of a power exchange analysis. The Gaussian case is fully discussed, showing the relevant features of the vortex generation.

Energy exchange between the Cartesian components of a paraxial beam in a uniaxial crystal

The evolution of the optical power associated with the Cartesian components of a paraxial beam propagating along the optical axis in a uniaxial crystal is investigated. The energy exchange is found to undergo a saturation that is due to both diffraction and coupling between the chi- and gamma-field components; for linearly polarized circularly symmetric input beams, the asymptotic power exchange always amounts to a quarter of the total power. The general results are applied to the case of astigmatic Gaussian beams, which admits of a fully analytical description. The case of finite length crystals is also considered.

Paraxial propagation along the optical axis of a uniaxial medium

An approach for describing paraxial propagation of light along the optical axis of a uniaxial medium is introduced. Contrary to previous theoretical schemes, our approach directly deals with the propagation of the whole optical field without resorting to the standard decomposition into ordinary and extraordinary parts, thus avoiding some related mathematical difficulties. A paraxial equation governing the field propagation has been derived, and its formal solution has been deduced. The structure of this solution allows us to think of the optical field in the crystal as the corresponding one propagating in vacuum "dressed" by the effect of anisotropy. This relationship is used to derive two analytical techniques for evaluating the propagated field. Starting from the formal solution, the closed-form expression of the anisotropic propagator is also derived. The proposed approach is used to predict the evolution of an astigmatic Gaussian beam through a calcite crystal, which has been also experimentally investigated. The agreement between theory and experiment is good.

Laguerre-Gauss and Bessel-Gauss beams in uniaxial crystals

A simple correspondence between the paraxial propagation formulas along the optical axis of a uniaxial crystal and inside an isotropic medium is found in the case of beams with linearly polarized circularly symmetric boundary distributions. The electric fields of the ordinary and the extraordinary beams are related to the corresponding expressions in a medium with refractive index n(o) and n(e)2/n(o), where n(o) and n(e) are the ordinary and the extraordinary refractive indexes, respectively. Closed-form expressions for Laguerre-Gauss and Bessel-Gauss beams propagating through an anisotropic crystal are given.

Stokes parameters of a Gaussian beam in a calcite crystal

We derive the analytical expression of the Stokes parameters corresponding to a Gaussian beam propagating along the optical axis of a uniaxial crystal, pointing the simultaneous effects of anisotropy and diffraction out. The theoretical results are compared with experimental measurements at the output of a calcite crystal, showing a good agreement.

Nonparaxial description of reflection and transmission at the interface between an isotropic medium and a uniaxial crystal

Angular spectra of reflected and transmitted fields, induced by an arbitrary electromagnetic beam passing through the planar interface between a homogeneous medium and a uniaxially anisotropic medium, are derived and related to the incident medium. By using these formulas, we obtain the expressions for paraxial and slightly nonparaxial fields. The reflected paraxial field is related to the incident one by means of Fresnel relations; the transmitted paraxial field is the superposition of an ordinary and an extraordinary beam, multiplied by the Fresnel coefficient. We find that the nonparaxial corrections, owing to the medium discontinuity, are larger than their free-propagation counterparts and that they are very simply related to the paraxial solutions of the incident beam. The case of two homogeneous media with different refractive indices is also discussed. The general expressions obtained are applied to the case of a nonparaxial Gaussian beam.

Propagation of cylindrically symmetric fields in uniaxial crystals

We investigate the paraxial propagation along the optical axis of a uniaxially anisotropic crystal of a general paraxial beam whose boundary Cartesian components possess cylindrical symmetry. This property allows us to obtain expressions whose dependence on the azimuth angle phi (in cylindrical coordinates) is fully described and very simple. We also find that the beam loses its boundary cylindrical symmetry during propagation, as a consequence of medium anisotropy. Further, these expressions elucidate the way in which the anisotropy changes the state of polarization. As an example, we discuss the case of a Gaussian beam focused into the crystal by a thin spherical lens.

Frequency decomposition and compounding of ultrasound medical images with wavelet packets

Ultrasound beams propagating in biological tissues undergo distortions due to local inhomogeneities of the acoustic parameters and the nonlinearity of the medium. The spectral analysis of the radio-frequency (RF) backscattered signals may yield important clinical information in the field of tissue characterization, as well as enhancing the detectability of tissue parenchymal diseases. In this paper, we propose a new tissue spectral imaging technique based on the wavelet packets (WP) decomposition. In a conventional ultrasound imaging system, the received echo-signals are generally decimated to generate a medical image, with a loss of information. With the proposed approach, all the RF data are processed to generate a set of frequency subband images. The ultrasound echo signals are simultaneously frequency decomposed and decimated, by using two quadrature mirror filters, followed by a dyadic subsampling. In addition, to enhance the lesion detectability and the image quality, we apply a nonlinear filter to reduce noise in each subband image. The proposed method requires simple additional signal processing and it can be implemented on any real-time imaging system. The frequency subband images, which are available simultaneously, can be either used in a multispectral display or summed up together to reduce speckle noise. To localize the different frequency response in the tissues, we propose a multifrequency display method where three different subband images, chosen among those available, are encoded as red, green, and blue intensities (RGB) to create a false-colored RGB image. According to the clinical application, different choices can evidence different spectral proprieties in the biological tissue under investigation. To enhance the lesion contrast in a grey-level image, one of the possible methods is the summation of the images obtained from narrow frequency subbands, according to the frequency compounding technique. We show that by adding the denoised subband images created with the WP decomposition, the contrast-to-noise ratio in two phantom images is largely increased.

Optimization of wide-band linear arrays

An optimization method is proposed for linear arrays to be used in ultrasound systems under wide-band operation. A fast algorithm, the threshold accepting, has been utilized to determine the element positions and weight coefficients of a linear array that generates a desired beam pattern. To reduce the computational burden in the optimization procedure, an efficient numerical routine for the beam pattern evaluation has been implemented. We address the optimization problem of both dense and sparse wide-band arrays. In the first case, the goal is to minimize the side-lobe energy by varying the element weights; we compare the optimized beam pattern with that obtained with classical shading functions, showing that better results can be achieved with a wide-band optimization. We also consider the optimization of the layout (positions and weights) of a sparse linear array to achieve a desired beam pattern with a fixed or minimum number of array elements. The comparison of the proposed method with a narrow-band optimization algorithm is presented, showing that better performances (about -7 dB further reduction of the side-lobe level) can be achieved with a wide-band sparse array optimization. Further numerical simulations are given, showing that the proposed method yields better results than wide-band sparse random arrays and periodic arrays with the same aperture width.

A novel approach to the aperture windowing in medical imaging

A new technique is proposed to improve the lateral resolution in the conventional B-mode imaging systems, which enables a simple array aperture windowing in the transmitting mode. Amplitude shaping is performed without modifying the transmitting voltage of the array elements, but only varying the excitation pulse length from one element to another. This method presents some attractive practical advantages, and the reduction of the sidelobe energy is comparable to that attainable with a conventional aperture windowing. Parametric plots are given, which transform an amplitude apodization into a 'time apodization' for any type of transducer array.

A new beamforming technique for ultrasonic imaging systems

We propose a simple, versatile and inexpensive beamforming method that performs the aperture windowing of an ultrasonic transducer array in the transmit mode, without modifying the driver voltage, but simply controlling the length of the electric pulse driving the array elements. A conversion formula has been determined that permits us to compute, for a desired emitted pulse amplitude, the corresponding driving pulse length to be applied. Any shading function can be implemented over any type of transducer array, using very low-cost hardware. Computer simulations and experimental measurements, with a 3.8 MHz convex array, confirm the effectiveness of this approach in enhancing the contrast resolution, since the off-axis intensity in the radiated beam pattern is largely reduced.

Diffractive variable beam splitter: optimal design

The analytical expression of the phase profile of the optimum diffractive beam splitter with an arbitrary power ratio between the two output beams is derived. The phase function is obtained by an analytical optimization procedure such that the diffraction efficiency of the resulting optical element is the highest for an actual device. Comparisons are presented with the efficiency of a diffractive beam splitter specified by a sawtooth phase function and with the pertinent theoretical upper bound for this type of element.

Dense and sparse 2-D array radiation patterns in lossy media

Two-dimensional (2-D) transducer arrays are potentially able to generate real-time volumetric images of internal organs of the human body, and much work has been done on the subject in recent years. A 2-D array with high resolution and low grating lobe level requires a prohibitively large number of elements for existing technology. A successful solution to reduce the number of elements, without sacrificing the above mentioned characteristics, is to select a limited number of elements in a random way or combining transmitting and receiving apertures with element spacing greater than one-half of a wavelength. In this work, the effect of the human body attenuation on the performances of these so-called sparse arrays is investigated. We analytically demonstrate that, for continuous wave excitation and under paraxial approximation, the medium losses can be modeled as a Gaussian weighting function, acting off-axis in the observation plane. The variance of this weighting function decreases with the covered distance. Radiation patterns computed with both this simple model and with a more exact expression, are presented for sparse and dense 2-D arrays under continuous and pulsed wave operation. Comparisons between the results obtained with and without attenuation also are shown.

Efficient transmit beamforming in pulse-echo ultrasonic imaging

A high performance ultrasound imaging system requires accurate control of the amplitude of the array elements, as well as of the time delays between them, both in the transmit and receive modes. In transmission, conventional array aperture windowing implies a different driving voltage for each element of the array, an expensive solution for systems with a large number of channels. In this paper, we present a simple, versatile, and inexpensive beamforming method that operates the aperture windowing in the transmit mode, simply controlling the lengths of the electric pulses driving the array elements. Computer simulations and experimental measurements are presented for different types of arrays. They confirm that the proposed beamforming technique improves the contrast resolution of the imaging system, reducing the off-axis intensity of the radiated field pattern. Moreover, the axial resolution is slightly enhanced, because the overall length of the transmitted ultrasonic pulse is reduced.

Spectral shifts of a partially coherent field after passing through a lens

A study of the spectral shifts of a well-known type of partially coherent field, namely, one formed by Gaussian Schell model beams, propagating beyond an optical system reveals that there are no shifts in the geometric-image plane, whereas the greatest (blue) shift occurs in the back focal plane. These results are relevant for spectroradiometric measurements.