Sequential beamforming for synthetic aperture imaging

Synthetic aperture sequential beamforming (SASB) is a novel technique which allows to implement synthetic aperture beamforming on a system with a restricted complexity, and without storing RF-data. The objective is to improve lateral resolution and obtain a more depth independent resolution compared to conventional ultrasound imaging. SASB is a two-stage procedure using two separate beamformers. The initial step is to construct and store a set of B-mode image lines using a single focal point in both transmit and receive. The focal points are considered virtual sources and virtual receivers making up a virtual array. The second stage applies the focused image lines from the first stage as input data, and take advantage of the virtual array in the delay and sum beamforming. The size of the virtual array is dynamically expanded and the image is dynamically focused in both transmit and receive and a range independent lateral resolution is obtained. The SASB method has been investigated using simulations in Field II and by off-line processing of data acquired with a commercial scanner. The lateral resolution increases with a decreasing F#. Grating lobes appear if F# ⩽ 2 for a linear array with λ-pitch. The performance of SASB with the virtual source at 20 mm and F# = 1.5 is compared with conventional dynamic receive focusing (DRF). The axial resolution is the same for the two methods. For the lateral resolution there is improvement in FWHM of at least a factor of 2 and the improvement at −40 dB is at least a factor of 3. With SASB the resolution is almost constant throughout the range. For DRF the FWHM increases almost linearly with range and the resolution at −40 dB is fluctuating with range. The theoretical potential improvement in SNR of SASB over DRF has been estimated. An improvement is attained at the entire range, and at a depth of 80 mm the improvement is 8 dB.     

110 °C range athermalization of wavefront coding infrared imaging systems

110 °C range athermalization is significant but difficult for designing infrared imaging systems. Our wavefront coding athermalized infrared imaging system adopts an optical phase mask with less manufacturing errors and a decoding method based on shrinkage function. The qualitative experiments prove that our wavefront coding athermalized infrared imaging system has three prominent merits: (1) working well over a temperature range of 110 °C; (2) extending the focal depth up to 15.2 times; (3) achieving a decoded image being approximate to its corresponding in-focus infrared image, with a mean structural similarity index (MSSIM) value greater than 0.85.     

Remote and high precision step height measurement with an optical fiber multiplexing interferometric system

An optical fiber multiplexing low coherence and high coherence interferometric system, which includes a Fizeau interferometer as the sensing element and a Michelson interferometer as the demodulating element, is designed for remote and high precision step height measurement. The Fizeau interferometer is placed in the remote field for sensing the measurand, while the Michelson interferometer which works in both modes of low coherence interferometry and high coherence interferometry is employed for demodulating the measurand. The range of the step height is determined by the low coherence interferometry and the value of it is measured precisely by the high coherence interferometry. High precision has been obtained by searching precisely the peak of the low coherence interferogram symmetrically from two sides of the low coherence interferogram and stabilizing the Michelson interferometer with a feedback loop. The maximum step height that could be measured is 6 mm while the measurement resolution is less than 1 nm. The standard deviation of 10 times measurement results of a step height of 1 mm configurated with two gauge blocks is 0.5 nm.     

The characterization of an ultrasound spherical phased array for the ablation of deep-seated tissue

In this paper we have developed and evaluated a spherical phased array ultrasound applicator for deep tissue ablation. The 90-element prototype array has a 21 cm aperture and an 18 cm radius of curvature with a 5 cm wide central hole. Annular distribution with circular elements is used to reduce the number of elements. The array is constructed with piezoelectric (PZT-8) circular planar elements that are 1.4 cm in diameter and 0.2-cm thick. With the water-muscle propagation path, the array offers an effective beam focusing depth of at least 8 cm in the muscle layer. Simulation results show that the array provides good beam focusing and steering capability over a cylindrical volume of approximately π × 1 × 1 × 4 cm³ (up to 10 mm off center over ranges from 15 cm to 19 cm). We also present its beam focusing and steering capability in deep tissue through a series of ex vivo experiments by measuring discoloration areas after sonications. The ex vivo experiments show a similar focal range as that found in the simulations.     

Calibration for stereo vision system based on phase matching and bundle adjustment algorithm

Calibration for stereo vision system plays an important role in the field of machine vision applications. The existing accurate calibration methods are usually carried out by capturing a high-accuracy calibration target with the same size as the measurement view. In in-situ 3D measurement and in large field of view measurement, the extrinsic parameters of the system usually need to be calibrated in real-time. Furthermore, the large high-accuracy calibration target in the field is a big challenge for manufacturing. Therefore, an accurate and rapid calibration method in the in-situ measurement is needed. In this paper, a novel calibration method for stereo vision system is proposed based on phase-based matching method and the bundle adjustment algorithm. As the camera is usually mechanically locked once adjusted appropriately after calibrated in lab, the intrinsic parameters are usually stable. We emphasize on the extrinsic parameters calibration in the measurement field. Firstly, the matching method based on heterodyne multi-frequency phase-shifting technique is applied to find thousands of pairs of corresponding points between images of two cameras. The large amount of pairs of corresponding points can help improve the accuracy of the calibration. Then the method of bundle adjustment in photogrammetry is used to optimize the extrinsic parameters and the 3D coordinates of the measured objects. Finally, the quantity traceability is carried out to transform the optimized extrinsic parameters from the 3D metric coordinate system into Euclid coordinate system to obtain the ultimate optimal extrinsic parameters. Experiment results show that the procedure of calibration takes less than 3 s. And, based on the stereo vision system calibrated by the proposed method, the measurement RMS (Root Mean Square) error can reach 0.025 mm when measuring the calibrated gauge with nominal length of 999.576 mm.     

Real-time sensing and three-dimensional display of far outdoor scenes based on asymmetric integral imaging

A new asymmetric integral imaging (AII) system for real-time pickup and three-dimensional (3-D) display of far outdoor scenes based on dynamic-pixel-mapping (DPM) is proposed. DPM is a digital process to transform the elemental images captured with a lens array into the perspective-variant object images (POIs) whose structures are matched with those of display lenses, where the orders of pixels in each POI are reversely mapped, and then capture a set of virtual elemental images (EIs) at the specific depth planes from the back-propagated POIs. This DPM enables an asymmetrical use of pickup and display lens arrays, allowing the long-ranged pickup of far outdoor scenes and their resolution-enhanced 3-D reconstruction. Experiments with a pair of pickup and display lens arrays whose pitches and focal lengths are given by 7.5 mm, 30 mm and 1.2 mm, 8 mm, respectively, show that the effective pickup-range and resolution of the proposed system have been increased up to 6 m and 1600×1600 pixels, respectively, from 0.064 m and 480×480 pixels of the conventional systems employing the same pickup and display lens arrays. In addition, experiments with an implemented test bed confirms that the proposed system can provide real-time 3-D images in 25 frames per second.     

Single image rectification of thermal images for geometric studies in façade inspections

This work presents a photogrammetric technique that provides geometric and thermal information about building façades. It uses low cost and portable scale bars, specially designed for thermal imaging, and processing software based on single image rectification. Image rectification corrects the original photo displacement due to the projection and perspective, and radial distortions introduced by the lens of the camera.The technique is tested by comparing laser scanning and thermal data. Seven segments of different orientation and length are selected for the measurement. Accuracy tests show errors between 44 mm and 151 mm. Precision values range between 22 mm and 61 mm for a maximum length of 7259 mm. The accuracy and precision results obtained for the technique open the possibility of extending its use to building inspection tasks.     

Performance analysis of a swept-source optical coherence tomography system with a quadrature interferometer and optical amplification

A performance analysis of signal to noise ratio for an optical coherence tomography system with quadrature detection and a semiconductor optical amplifier in the sample arm is discussed. The results are compared and discussed in relation to a conventional OCT system (without optical amplification). An increase of the signal to noise ratio up to 14 dB at a depth of 0.5 mm is obtained compared to the system without the optical amplifier. Overall, an improvement was demonstrated for signal coming from deeper regions within the samples. Arterial plaque from a myocardial infarction-prone Watanabe heritable hyperlipidemic (WHHLMI) rabbit is visualized and characterized using this system. Improvement of signal to noise ratio increases the penetration depth possible for OCT images, from 1 mm to 2 mm within the vessel wall of an artery. Preliminary results show that vulnerable plaque with fibrous cap, macrophage accumulations and calcification in the arterial tissue are measurable with this OCT system.     

Accuracy and efficiency of an infrared based positioning and tracking system for patient set-up and monitoring in image guided radiotherapy

An infrared based positioning and tracking (IPT) system was introduced and its accuracy and efficiency for patient setup and monitoring were tested for daily radiotherapy treatment. The IPT system consists of a pair of floor mounted infrared stereoscopic cameras, passive infrared markers and tools used for acquiring localization information as well as a custom controlled software which can perform the positioning and tracking functions. The evaluation of IPT system characteristics was conducted based on the AAPM 147 task report. Experiments on spatial drift and reproducibility as well as static and dynamic localization accuracy were carried out to test the efficiency of the IPT system. Measurements of known translational (up to 55.0 mm) set-up errors in three dimensions have been performed on a calibration phantom. The accuracy of positioning was evaluated on an anthropomorphic phantom with five markers attached to the surface; the precision of the tracking ability was investigated through a sinusoidal motion platform. For the monitoring of the respiration, three volunteers contributed to the breathing testing in real time. The spatial drift of the IPT system was 0.65 mm within 60 min to be stable. The reproducibility of position variations were between 0.01 and 0.04 mm. The standard deviation of static marker localization was 0.26 mm. The repositioning accuracy was 0.19 mm, 0.29 mm, and 0.53 mm in the left/right (L/R), superior/inferior (S/I) and anterior/posterior (A/P) directions, respectively. The measured dynamic accuracy was 0.57 mm and discrepancies measured for the respiratory motion tracking was better than 1 mm. The overall positioning accuracy of the IPT system was within 2 mm. In conclusion, the IPT system is an accurate and effective tool for assisting patient positioning in the treatment room. The characteristics of the IPT system can successfully meet the needs for real time external marker tracking and patient positioning as well as respiration monitoring during image guided radiotherapy treatments.     

Micro-force measurement of drag on a small flat plate in the presence of a corona discharge

The design of a micro-force sensor suitable for the measurement of corona drag and other low velocity drag studies in a small laboratory wind tunnel facility is described. Example drag data are given for dc corona discharge generated by sharp parallel electrodes mounted on a microscope glass slide with discharge parallel to the air flow. The arrangement simulates two-dimensional flow over a flat plate useful for theoretical analysis. Measurements of free stream wind velocities in the range 0–210 cm/s with attendant drag down to 10⁻⁷ N can be detected in this facility depending on the calibration. The force sensor utilizes two strain gages mounted on a 0.127 mm stainless steel “feeler gage” in a cantilever arrangement. A bridge circuit provides sensitivities in the range 40–250 N/mV using a gravitational calibration technique. Anomalous effects from suspension wires and the interaction of electrostatic forces with the surroundings are discussed.     

About ageing and fading of Cr-39 PADC track detectors used as air radon concentration measurement devices

PADC detectors are widely used as air radon concentration measurement devices and the typical procedure that a Radon Service Laboratory uses to manage the detectors provides a calibration phase followed by the field measurement. The calibration is performed in a reference radon concentration atmosphere, using high radon concentration values in order to achieve typical exposure values of few MBq h m^?3 with an exposure time of few days. On the other hand the field measurement is characterized by long term exposures lasting up to six months and by radon concentrations that are quite lower than the ones used in the calibration.The aim of this study is to check whether the calibration procedure is actually representative of a field measurement, or, in other words, whether and how much ageing or fading can affect the calibration factor.We found that the ageing and fading effect can produce a decrease in the detector sensitivity leading to an underestimation of the radon concentration up to 40% for exposure lasting few months. An important issue is that both ageing and fading can be ascribed to a decrease in the track etching velocity V_t.In the paper we also provide an algorithm to compensate for the sensitivity variation due to fading/ageing effect.     

Quantitative determination of subsurface defects in a reference specimen made of Plexiglas by means of lock-in and pulse phase infrared thermography

Lock-in and pulse phase infrared thermography measurement techniques have been exploited for quantitative assessment of subsurface defects in a reference specimen made of Plexiglas. Radiometric thermal images were post-processed using a contrast approach in the frequency domain, allowing defect depth to be resolved with a combined standard uncertainty of about 5% for thicknesses up to 3.6 mm. Conversely, significant radial heat diffusion next to the boundary of the discontinuities made accurate sizing of deeper subsurface defects more difficult, resulting in a combined standard uncertainty of about 17% for a 10 mm diameter flat-bottomed hole of 3.6 mm deep. The obtained results demonstrate the potentiality of active thermography as a fast, powerful contactless NDE measurement tool.     

Focal length measurement based on Hartmann–Shack principle

A novel means of focal length measurement is proposed based upon Hartmann–Shack principle of wavefront detection. Mathematical approaches of focal length measurement are deduced without the necessity of knowing the position of lens principle plane. In experiment, laser sources with wavelength of 633 nm and 780 nm are separately used and focal length of three pieces of doublets is measured under two wavelengths mentioned above. As were shown by the results, this method of focal length measurement could not only accurately measure lens focal length, but also evaluate chromatic aberration of them. Since the intensity of light is detected by CCD, optical characteristics of lens beyond the range of visible light are also measurable when response range of CCD allows. This new means of focal length measurement is characterized by high accuracy, fast measuring and easy to setup, thereby, it is applicable in wide fields of area.     

Ultrasonic imaging using air-coupled P(VDF/TrFE) transducers at 2 MHz

A reflection non-contact ultrasonic microscope system working both in amplitude and phase difference modes at 2 MHz has been developed using an air-coupled concave transducer made of piezoelectric polymer films of poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)]. The transducer is composed of three 95 μm-thick P(VDF/TrFE) films stacked together, each of which is activated electrically in parallel by a driving source. The transducer has a wide aperture angle of 140° and a focal length of 10 mm. The measured two-way transducer insertion loss is 80 dB at 1.83 MHz. Despite 20 dB higher insertion loss than that estimated from Mason’s equivalent circuit, we have obtained clear amplitude acoustic images of a coin with transverse resolution of 150 μm, and clear phase difference acoustic images of the rough surface of a paper currency bill with depth resolution of sub-micrometer. Using two planar transducers of P(VDF/TrFE), we have also successfully measured in through-transmission mode the sound velocity and absorption of a 3 mm-thick silicone-rubber plate. The present study proves that, owing to its low acoustic impedance and flexibility, P(VDF/TrFE) piezoelectric film is very useful for high frequency acoustic imaging in air in the MHz range.     

Effects of high power ultrasonic vibration on temperature distribution of workpiece in dry creep feed up grinding

Temperature history and distribution of steel workpiece (X20Cr13) was measured by a high tech infrared camera under ultrasonic assisted dry creep feed up grinding. For this purpose, a special experimental setup was designed and fabricated to vibrate only workpiece along two directions by a high power ultrasonic transducer. In this study, ultrasonic effects with respect to grinding parameters including depth of cut (a_e), feed speed (v_w), and cutting speed (v_s) has been investigated. The results indicate that the ultrasonic vibration has considerable effect on reduction of temperature, depth of thermal damage of workpiece and width of temperature contours. Maximum temperature reduction of 25.91% was reported at condition of v_s = 15 m/s, v_w = 500 mm/min, a_e = 0.4 mm in the presence of ultrasonic vibration.     

Synthesis of Ag-deionized water nanofluids using multi-beam laser ablation in liquids

Multi-pulse laser ablation of silver in deionized water was studied. The laser beams were arranged in a cross-beam configuration. In our experiments, two single-mode, Q-switched Nd-Yag lasers operating at 1064 nm, pulse duration of 5.5 ns and 10 Hz rep rate were used. The laser fluence of the second beam was 0.265 J/cm² for all tests. Two levels of the laser fluences were used for the ablating beam: 0.09 and 0.265 J/cm² (11,014 and 33,042 J/cm² at the focal point, respectively). The silver target was at 50 mm from the cell window and 10 mm deep. The second beam was aligned parallelly with the silver target and focused at 2 mm in front of the focal point of the ablating beam. For all cases, the delay time between the ablating beam and the cross-beam was 40 μs. In general, the ablated particles were almost all spherical. For fluence of 0.09 J/cm ² and single-beam approach, the mean particle size was about 29 nm. The majority of the particles, however, were in 19–35 nm range and there were some big ones as large as 50–60 nm in size. For double-beam approach, the particles were smaller with the average size of about 18 nm and the majority of the particles were in 9–21 nm range with few big one as large as 40 nm. For the beam fluence of 0.265 J/cm² and single-beam configuration, the particle sizes were smaller, the mean particles size was about 18 nm and the majority of the particles were in the range of 10–22 nm with some big one as large as 40 nm. For double-beam approach, the mean particle size was larger (24.2 nm) and the majority of the particle were distributed from 14 to 35 nm with some big particles can be found with sizes as big as 70 nm. Preliminary measurements of the thermal conductivity and viscosity of the produced samples showed that the thermal conductivity increased about 3–5% and the viscosity increased 3.7% above the base fluid viscosity even with the particle volume concentration as low as 0.01%.     

High-speed non-intrusive measurements of fuel velocity fields at high-pressure injectors

Using a single high-speed camera and a frequency modulated laser, a novel approach is presented for fast velocity field measurements in unsteady spray flows. The velocity range is from zero up to several 100 m/s, which requires a high measurement rate and a large dynamic. Typically, flow measurements require to seed tracer particles to the fluid. A paradigm shift to seeding-free measurements is presented. The light scattered at the phase boundaries of the fluid droplets is evaluated. In order to validate the high-speed measurement system, a detailed uncertainty analysis is performed by means of measurements as well as simulations. Thereby, variations of the scattered light intensity, which are based on the high temporal velocity gradients, are found to be the main contribution to the uncertainty. The eventually measurement results, obtained at a measurement rate of 500 kHz, exhibit spray velocities ranging from 0 m/s up to 400 m/s in less than 1 ms, and the detection of unsteady and irregular flow phenomena with a characteristic time of several μs is achieved. This demonstrates the high measurement rate, the high temporal resolution and the large measurement range of the proposed high-speed measurement system.     

Aircraft noise-monitoring according to ISO 20906. Evaluation of uncertainty derived from the classification and identification of aircraft noise events

Noise monitoring continues to be one of the most important tools in noise management around airports, since noise pollution is a serious problem for the surrounding communities. The monitoring units must be reliable and precise in order to ensure the quality of the results provided. As a previous step to achieve this, it is necessary to make an estimation of the uncertainty of the results, taking into account the contribution of every single element in the measurement chain. Among other contributions to uncertainty as listed in ISO 20906, the events marking system has an influence on the measurement results on two different levels: the first one, derives from the human factors affecting the event detection while the second one derives from the error rates of the classification–identification chain. In this paper, the focus is set on the latter, which has been called identification uncertainty. A model has been defined for its estimation and a methodology of application has been described. The results have been calculated for some specific situations in order to clarify the methodology and to have some estimations of the value of this contribution to uncertainty. Just for a location very close to the airport (aircraft events range over 20 dB), this contribution to uncertainty can be up to 0.5 dB when simple detection techniques are used, and can be reduced to approximately 0.1 dB when radar tracking is used to enhance the identification task. In more complex acoustic environments, the identification uncertainty can increase up to 5 dB (or more) in the absence of accurate classification/identification tools.     

Multispectral pyrometry for surface temperature measurement of oxidized Zircaloy claddings

Non-contact temperature measurement in a nuclear reactor is still a huge challenge because of the numerous constraints to consider, such as the high temperature, the steam atmosphere, and irradiation. A device is currently developed at CEA to study the nuclear fuel claddings behavior during a Loss-of-Coolant Accident. As a first step of development, we designed and tested an optical pyrometry procedure to measure the surface temperature of nuclear fuel claddings without any contact, under air, in the temperature range 700–850 °C. The temperature of Zircaloy-4 cladding samples was retrieved at various temperature levels. We used Multispectral Radiation Thermometry with the hypothesis of a constant emissivity profile in the spectral ranges 1–1.3 µm and 1.45–1.6 µm. To allow for comparisons, a reference temperature was provided by a thermocouple welded on the cladding surface. Because of thermal losses induced by the presence of the thermocouple, a heat transfer simulation was also performed to estimate the bias. We found a good agreement between the pyrometry measurement and the temperature reference, validating the constant emissivity profile hypothesis used in the MRT estimation. The expanded measurement uncertainty (k = 2) of the temperature obtained by the pyrometry method was ±4 °C, for temperatures between 700 and 850 °C. Emissivity values, between 0.86 and 0.91 were obtained.     

Spatially variant regularization of lateral displacement measurement using variance

The purpose of this work is to confirm the effectiveness of our proposed spatially variant displacement component-dependent regularization for our previously developed ultrasonic two-dimensional (2D) displacement vector measurement methods, i.e., 2D cross-spectrum phase gradient method (CSPGM), 2D autocorrelation method (AM), and 2D Doppler method (DM). Generally, the measurement accuracy of lateral displacement spatially varies and the accuracy is lower than that of axial displacement that is accurate enough. This inaccurate measurement causes an instability in a 2D shear modulus reconstruction. Thus, the spatially variant lateral displacement regularization using the lateral displacement variance will be effective in obtaining an accurate lateral strain measurement and a stable shear modulus reconstruction than a conventional spatially uniform regularization. The effectiveness is verified through agar phantom experiments. The agar phantom [60 mm (height) × 100 mm (lateral width) × 40 mm (elevational width)] that has, at a depth of 10 mm, a circular cylindrical inclusion (dia. = 10 mm) of a higher shear modulus (2.95 and 1.43 × 10⁶ N/m², i.e., relative shear modulus, 2.06) is compressed in the axial direction from the upper surface of the phantom using a commercial linear array type transducer that has a nominal frequency of 7.5-MHz. Because a contrast-to-noise ratio (CNR) expresses the detectability of the inhomogeneous region in the lateral strain image and further has almost the same sense as that of signal-to-noise ratio (SNR) for strain measurement, the obtained results show that the proposed spatially variant lateral displacement regularization yields a more accurate lateral strain measurement as well as a higher detectability in the lateral strain image (e.g., CNRs and SNRs for 2D CSPGM, 2.36 vs 2.27 and 1.74 vs 1.71, respectively). Furthermore, the spatially variant lateral displacement regularization yields a more stable and more accurate 2D shear modulus reconstruction than the uniform regularization (however, for the regularized relative shear modulus reconstructions, slightly accurate, e.g., for 2D CSPGM, 1.51 vs 1.50). These results indicate that the spatially variant displacement component-dependent regularization will enable the 2D shear modulus reconstruction to be used as practical diagnostic and monitoring tools for the effectiveness of various noninvasive therapy techniques of soft tissue diseases (e.g., breast, liver cancers). Application of the regularization to the elevational displacement will also increase the stability of a three-dimensional (3D) reconstruction.