Conventional Measurement Methods Research Articles

Comprehensive characterization of nano-pore system for Chang 7 shale oil reservoir in Ordos Basin

Detailed study on the pore structure of shale oil reservoir is significantly for the exploration and development, and the conventional single pore structure measurement method cannot accurately describe the pore structure characteristics of the shale oil reservoir. In this paper, the Field Emission Scanning Electron Microscope (FESEM), low-pressure nitrogen adsorption (LP-N2A) and mercury injection porosimetry (MIP) techniques are used to comprehensive evaluate the pore structure of Chang 7 shale oil reservoir. The FESEM results show that inter pores, inner pores, organic pores and micro-cracks are developed in Chang 7 shale oil reservoir, and the pore structure can be divided into two groups from the LP-N2A and MIP. A new pore structure comprehensive evaluation method was promoted according to the connection points from the pore sizes distribution curves of LP-N2A and MIP. With this comprehensive analysis of the pore size distribution, the pore size distribution of various shale samples feature as triple-peak pattern. Due to the heterogeneity of the shale oil samples, the corresponding pore apertures of the connection points are various, and the overall pore size distribution of shale oil reservoir samples can also be divided into two types. In Group I, the size distributions exhibited a bimodal feature in a narrow range from 1.71 to 100 nm. The trimodal feature of size distributions was captured in Group II with the pore diameter ranges from 1.71 to 1426.8 nm. Group I features smaller sorting coefficient and good pore connectivity. However, the trimodal corresponds to the complex pore structure and larger sorting coefficient for Group II.

Haptics in Teleoperated Medical Interventions: Force Measurement, Haptic Interfaces and Their Influence on User's Performance.

Haptics in teleoperated medical interventions enables measurement and transfer of force information to the operator during robot-environment interaction. This paper provides an overview of the current research in this domain and guidelines for future investigations. We review current technologies in force measurement and haptic devices as well as their experimental evaluation and influence on user's performance. Force sensing is moving away from the conventional proximal measurement methods to distal sensing and contact-less methods. Wearable devices that deliver haptic feedback on different body parts are increasingly playing an important role. Performance and accuracy improvement are the widely reported benefits of haptic feedback, while there is a debate on its effect on task completion time and exerted force. With the surge of new ideas, there is a need for better and more systematic validation of the new sensing and feedback technology, through better user studies and novel methods like validated benchmarks and new taxonomies. This review investigates haptics from sensing to interfaces within the context of user's performance and the validation procedures to highlight salient advances. It provides guidelines to future developments and highlights the shortcomings in the field.

Comparison between conventional human energy measurement and physical human energy measurement methods in wetland rice production

Measurement of human energy expenditure during crop production helps in the optimization of production operations and costs by identifying steps which that can benefit from the use of appropriate mechanization technologies. This study measures human energy expenditure associated with all 6 major rice (Oryza sativa L.) cultivation operations using two measurement methods-i.e. conventional human energy expenditure method and direct measurement with a Garmin forerunner 35 body media. The aim of this study was to provide a detailed comparison of these two methods and document the human energy costs in a manner that will identify steps to be taken to help optimize agricultural practices. Results (mean + 95%CL) revealed that the total human energy expenditure obtained through the conventional method was 25.5% higher (33.3 ± 1 versus 26.6 ± 1.3) in transplanting and 26.1% higher (30.3 ± 1.9 versus 24.0 ± 2.1) than the human energy expenditure recorded using the Garmin method in broadcast seeding method.Similarly, during the harvesting operation, the conventional measurement and Garmin measurement methods differed significantly, with the conventional method the human energy expenditure was 89.9% higher (3.2 ± 0.4 versus 1.68 ± 0.2) in the fields using the transplanting and 88.7% higher (3.3 ± 0.5 versus 1.8 ± 0.3) in the fields using the broadcast seeding than the human energy expenditure recorded using the Garmin method. When using Garmin method, the human energy expenditure in the case of using the midsize combine harvester was 13.49% lesser (592.4 ± 67.2 versus 522.0 ± 75.1) than the case of using conventional one.Results based on heart rate also indicated that operations such as tillage were less intensive (72 ± 3.3 bpm) compared with operations such as chemicals spraying (135 ± 4 bpm). Although we did not have a criterion measure available to determine which method was the most accurate, the Garmin measurement gives an estimate of actual physical human energy expended in performing a specific task with consider all conditions and thus more information to aid in identifying critical operations that could be optimized and mechanized.

Kernel crack characteristics for X-ray computed microtomography (μCT) and their relationship with the breakage rate of maize varieties

The most significant problem of maize grain mechanical harvesting quality in China at present is the high grain breakage rate (BR). BR is often the key characteristic that is measured to select hybrids desirable for mechanical grain harvesting. However, conventional BR evaluation and measurement methods have challenges and limitations. Microstructural crack parameters evaluation of maize kernel is of great importance to BR. In this connection, X-ray computed microtomography (μ-CT) has proven to be a quite useful method for the assessment of microstructure, as it provides important microstructural parameters, such as object volume, surface, surface/volume ratio, number of closed pores, and others. X-ray computed microtomography is a non-destructive technique that enables the reuse of samples already measured and also yields bidimensional (2D) cross-sectional images of the sample as well as volume rendering. In this paper, six different maize hybrid genotypes are used as materials, and the BR of the maize kernels of each variety is tested in the field mechanical grain harvesting, and the BR is used as an index for evaluating the breakage resistance of the variety. The crack characteristic parameters of kernel were detected by X-ray micro-computed tomography, and the relationship between the BR and the kernel crack characteristics was analyzed by stepwise regression analysis. Establishing a relationship between crack characteristic parameters and BR of maize is vital for judging breakage resistance. The results of stepwise multiple linear regression (MLR) showed that the crack characteristics of the object surface, number of closed pores, surface of closed pores, and closed porosity percent were significantly correlated to the BR of field mechanical grain harvesting, with the standard partial regression coefficients of −0.998, −0.988, −0.999, and −0.998, respectively. The R2 of this model was 0.999. Results validation showed that the Stepwise MLR Model could well predict the BR of maize based on these four variables.

Soil resistivity modeling for temperature rise calculating of HVDC deep-well earth electrode

Deep-well DC earthing electrode occupied much smaller area than the traditional one, which greatly reduce the difficulty of site selection. However, due to the extremely high current density at the end of linear earthing electrode, temperature rise has become an important factor restricting the development of deep-well earthing technology. In order to evaluate the heating situation of deep-well earthing electrode in an accurate way, this paper carried out the field temperature rise test of deep-well earthing electrode, and the error between the temperature rise result calculated by the wide-area soil resistivity model obtained by conventional measurement methods are analyzed. It was found that the local soil resistivity near the deep-well earthing electrode determines the temperature rise characteristic. Based on the fitting results of the apparent resistivity logging, a local soil resistivity modeling method suitable for deep-well earthing electrode temperature rise calculation is proposed, and the deep-well earthing electrode temperature rise calculation model considering temperature characteristics of water-saturated underground medium and local soil model is established. The calculation result of this model is used to compare with the field measured data of the temperature rise in actual projects, and the maximum temperature error is found to be 5.5 °C. The trend of electrode temperature rise is in good agreement with the test results, which explains the special phenomenon of high temperature rise in low resistance layer. Project cases prove the effectiveness and accuracy of the soil modeling and temperature rise simulation method proposed in this paper.

Determination of some physicomechanical and biochemical parameters of hazelnut (Corylus avellana L.) cultivars

Hazelnut is one of the most popular nuts consumed by people; it has different cultivars in Turkey. The aim of the current study was to characterize some physicomechanical characteristics, shape features, color, and biochemical properties of 6 standard and 3 local hazelnut cultivars grown in Turkey. The shape and size properties of the samples were determined using image processing techniques as an alternative to conventional measurement methods. Additionally, principal component analysis (PCA) was used to classify the hazelnut samples in terms of the biochemical parameters of the hazelnut cultivars. According to the findings, the highest crude oil (63.25%) and lowest protein contents (13.63%) were observed in the Kalınkara cultivar. Oleic and linoleic acids were the major fatty acids for all hazelnut samples. While local Devedişi and standard Çakıldak cultivars had the highest oleic acid levels, the highest linoleic acid level was observed for the Dağ fındığı cultivar. The cultivars of Foşa had the highest Zn and Mn, while the highest Cu was found in the Tombul cultivar. The greatest surface and projected areas were calculated for the Kara fındık and Dağ fındığı samples, while the greatest hardness value was measured for the Devedişi cultivar. PCA revealed some positive and negative correlations between the physicomechanical and biochemical parameters. The present analyses revealed significant correlations between hardness and internal shell b* values and between Cu content and internal L*. Such correlations should be taken into consideration in food processing applications and machine design for these hazelnut cultivars.

Pavement Macrotexture Determination Using Multi-View Smartphone Images

Pavement macrotexture contributes greatly to road surface friction, which in turn plays a vital role in reducing road accidents. Conventional methods for macrotexture measurement are either expensive, time-consuming, or of poor repeatability. Based on multi-view smartphone images collected in situ, this paper develops and evaluates an affordable and convenient alternative approach for pavement macrotexture measurement. Photogrammetric computer vision techniques are first applied to create high resolution point clouds of the pavement. Analytics are then developed to determine the macrotexture metric: mean profile depth by using the image-based point clouds. Experiments are carried out with 790 images over 25 spots on three state routes and six spots at an Indiana Department of Transportation test site. We demonstrate multi-view smartphone images can yield results comparable to the ones from the conventional laser texture scanner. It is expected that the developed approach can be adopted for large scale operational uses.

True Origin of Gate Ringing in Superjunction MOSFETs: Device View

As superjunction devices are scaled down to smaller dimensions, the gate ringing becomes more prominent in dynamic switching. The exact origin of superjunction MOSFET's gate ringing has not been so far identified as the conventional three-terminal measurement method cannot capture the dynamic behavior of the device, in particular the redistribution of charge between the different internal capacitive components in the superjunction structure. In this article, it is found that the gate ringing is highly related to the input capacitance. Specifically, by employing a TCAD model with a split gate method, the gate-to-source (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> ) and gate-to-drain (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> ) current are investigated during the dynamic transitions. The gate ringing is highly dependent on sum of the input capacitances (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> + C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> ) for the turn-on. In the case of the turn-off, however, the gate ringing is affected by the ratio of the input capacitances (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GD</sub> /C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> ) and a lower CGS is desirable for a low gate oscillation.

Nanoelectronic Characterization: Using Near-Field Microwave Microscopy for Nanotechnological Research

The optical characterization of nanoscale objects is challenging due to the Abbe diffraction limit. This limit constrains the achievable spatial resolution of a conventional microscope using visible light to a 200 nm . 200 nm. The current era of nanotechnological research has been enabled by a host of experimental techniques that circumvent the diffraction limit. Since the Abbe limit scales proportional to the wavelength of electromagnetic radiation, the diffraction limit acts even more unfavorably on longer wavelengths. At 10 GHz (l = 30 mm), the best achievable resolution is around 15 mm. Thus, while traditional microwave metrology enables precision broadband measurements, conventional measurement methods are insufficient to address the host of novel and exciting phenomena that arise in nanoscale systems.

Collaborative Study to Validate Purity Determination by 1H Quantitative NMR Spectroscopy by Using Internal Calibration Methodology.

NMR spectroscopy has recently been utilized to determine the absolute amounts of organic molecules with metrological traceability since signal intensity is directly proportional to the number of each nucleus in a molecule. The NMR methodology that uses hydrogen nucleus (1H) to quantify chemicals is called quantitative 1H-NMR (1H qNMR). The quantitative method using 1H qNMR for determining the purity or content of chemicals has been adopted into some compendial guidelines and official standards. However, there are still few reports in the literature regarding validation of 1H qNMR methodology. Here, we coordinated an international collaborative study to validate a 1H qNMR based on the use of an internal calibration methodology. Thirteen laboratories participated in this study, and the purities of three samples were individually measured using 1H qNMR method. The three samples were all certified via conventional primary methods of measurement, such as butyl p-hydroxybenzoate Japanese Pharmacopeia (JP) reference standard certified by mass balance; benzoic acid certified reference material (CRM) certified by coulometric titration; fludioxonil CRM certified by a combination of freezing point depression method and 1H qNMR. For each sample, 1H qNMR experiments were optimized before quantitative analysis. The results showed that the measured values of each sample were equivalent to the corresponding reference labeled value. Furthermore, assessment of these 1H qNMR data using the normalized error, En-value, concluded that statistically 1H qNMR has the competence to obtain the same quantification performance and accuracy as the conventional primary methods of measurement.

Lightness prediction of mirror display by amending CIECAM02 to include specular reflection.

This paper investigates if the color appearance model accurately predicts the lightness of a display whose surface features specular reflection such as a mirror display. Eight observers participate in psychophysical experiments and the results are compared with model prediction value. It is found that the model overestimates the lightness under the average surround condition where there is an intense specular reflection on the display surface. Based on the measurement results, we figured out that the conventional measurement method cannot measure practical specular reflection value. Thus we propose a new measurement method and modify the background parameter to include the specular reflection value. The proposed CIECAM02 considering specular reflection improves the coefficient of variation (CV) value under average surround condition from 20.56 to 12.40 while maintaining CVs under dim and dark surround conditions. In conclusion, we found that people perceive the specular reflection image as a background.

Experimental Verification of Objective Visual Fatigue Measurement Based on Accurate Pupil Detection of Infrared Eye Image and Multi-Feature Analysis

As the use of electronic displays increases rapidly, visual fatigue problems are also increasing. The subjective evaluation methods used for visual fatigue measurement have individual difference problems, while objective methods based on bio-signal measurement have problems regarding motion artifacts. Conventional eye image analysis-based visual fatigue measurement methods do not accurately characterize the complex changes in the appearance of the eye. To solve this problem, in this paper, an objective visual fatigue measurement method based on infrared eye image analysis is proposed. For accurate pupil detection, a convolutional neural network-based semantic segmentation method was used. Three features are calculated based on the pupil detection results: (1) pupil accommodation speed, (2) blink frequency, and (3) eye-closed duration. In order to verify the calculated features, differences in fatigue caused by changes in content color components such as gamma, color temperature, and brightness were compared with a reference video. The pupil detection accuracy was confirmed to be 96.63% based on the mean intersection over union. In addition, it was confirmed that all three features showed significant differences from the reference group; thus, it was verified that the proposed analysis method can be used for the objective measurement of visual fatigue.

Indirect Method for Measuring Absolute Acoustic Nonlinearity Parameter Using Surface Acoustic Waves with a Fully Non-Contact Laser-Ultrasonic Technique

This paper proposes an indirect method to measure absolute acoustic nonlinearity parameters using surface acoustic waves by employing a fully non-contact laser-ultrasonic technique. For this purpose, the relationship between the ratio of relative acoustic nonlinearity parameters measured using the proposed method in two different materials (a test material and a reference material) and the ratio of absolute acoustic nonlinearity parameters in these two materials was theoretically derived. Using this relationship, when the absolute nonlinearity parameter of the reference material is known, the absolute nonlinearity parameter of the test material can be obtained using the ratio of the measured relative parameters of the two materials. For experimental verification, aluminum and copper specimens were used as reference and test materials, respectively. The relative acoustic nonlinearity parameters of the two materials were measured from surface waves generated and received using lasers. Additionally, the absolute parameters of aluminum and copper were measured using a conventional direct measurement method, with the former being used as a reference value and the latter being used for comparison with the estimation result. The absolute parameter of copper estimated by the proposed method showed good agreement with the directly measured result.

A new monitoring method for overlying strata failure height in Neogene laterite caused by underground coal mining

Determination of the overlying strata failure height, that is, water flowing fractured zone height (WFFZH), is critical to underwater coal mining, especially in the fragile geo-environment areas. Due to the property discrepancy of rock and soil, WFFZH penetrating the clay layer in the Jurassic coalfield cannot be accurately determined by conventional measurement method, a new formation micro-scanner image (FMI) method was presented and verified in the paper. Taking the N1208 coalface in the Shennan mining area, Shaanxi, China, as a case study, FMI based field test for monitoring WFFZH was carried out firstly. The scanning results show that subtle changes of drilling sidewall cracks could be presented intuitively in FMI scanning image, and the WFFZH was be determined as 145.15 m. Secondly, the whole evolutionary process of WFFZH is simulated using UDEC, whose maximum height of WFFZ is calculated as 147.0 m. Comparison results between on-site measurement and numerical simulation suggest that FMI technology is applicable to monitor the WFFZH in laterite layer. Thirdly, in order to illustrate the discrepancies in WFFZH developed in laterite and bedrock, a comparative numerical simulation tests results show that laterite layer has an inhibitory effect on WFFZH.

Core- and pore-scale investigation on the migration and plugging of polymer microspheres in a heterogeneous porous media

Polymer microspheres (PMs) can be a very promising, cost-effective in-depth fluid diversion strategy in the development of heterogeneous reservoir. The key to success for the PMs application is to fully understand the PMs-migration and -plugging mechanisms through the confined space. The plugging rate is widely used to evaluate the performance of PMs-plugging. But it always presents a large value using the conventional measurement method even PMs cannot transport through the porous media. The impact factors of PMs-migration and-plugging in previous work based the conventional plugging rate may not be reliable in the reservoir. In this study, three kinds of polymer microspheres (PMs-1, PMs-2, PMs-3) with different particle sizes were synthesized, and their matching relationship with cores and impact factors of plugging were investigated using a new proposed measurement method. Flowing experiment at core- and pore-scale were performed to reveal PMs-migration and -plugging mechanisms. The synthesized PMs had a shape of the regular spheres with initial sizes of 9.8 μm, 6.2 μm and 3.1 μm respectively. They can swell from 3.44 to 2.68 times. The optimal matching factors between PMs sizes and pore sizes were obtained by the redefined plugging strength, which were 1.0–1.2 for PMs-2 and 0.5–0.8 for PMs-3. In the range of the optimal matching factors, PMs-plugging and -migration is affected by PMs sizes rather than the PMs concentration and injection rate. Two plugging patterns of direct-plugging by one single PM and bridge-plugging by multi-PMs were observed in the micromodel. Large PMs was tendency to the direct-plugging, which has much stronger plugging efficiency than the bridge-plugging of multiple small PMs.

Failure analysis of cyclic damage to a railway rail – A case study

Various damage to the railway rail has a significant impact on the operation safety and life of the railway infrastructure. The paper focuses on failure analysis of rare cyclic damage to the surface of a railway rail loaded with normal operation. Straining of the rail at the point of contact with a railway wheel, due to vertical forces, causes permanent deformation of the rail surface. The research is focused on a non-traditional case of cyclic damage to rails on a continuous track section of considerable length (21,725 m), on the analysis of the cause of damage and on the analysis of effectiveness of the damage removal method. The methodology used consists of a visual field study using conventional measurement methods, supported by a non-destructive on-site inspection to identify the severity of deficiencies in railway safety; and to measure the effectiveness of operator maintenance policies that need to be improved to identify and correct infrastructure extensions. In total, the section was analyzed that showed 5725 defects, which is an unconventional rarity in terms of the number of defects and has not been scientifically investigated yet.

A Novel Mainstream Capnometer System for Non-invasive Positive Pressure Ventilation.

Capnometry is a method to measure carbon dioxide (CO2) in exhaled gas and it has been used to monitor patient respiratory status. CO2 monitoring is also used for patients receiving non-invasive positive pressure ventilation (NPPV) therapy during mechanical ventilation. Ventilators actively dilute exhaled gas during non-invasive ventilation. In order to accurately measure end-tidal CO2, an adequate amount of expired gas needs to be filled in a CO2 measurement cell before expiratory positive airway pressure (EPAP) gas from the ventilator arrives to the cell. This is the reason why it is difficult to measure CO2 stably during non-invasive ventilation using the conventional CO2 measurement method. Therefore, we developed NPPV cap-ONE mask, which accurately measures CO2 in exhaled gas during non-invasive ventilation. In this study, we evaluated the basic performance of the NPPV cap-ONE mask system. The NPPV cap-ONE mask system could accurately measure CO2 in exhaled gas comparing to the conventional device in this study.

DEVELOPMENT OF AN ALGORITHM TO DETERMINE THE LIQUID SPREADING AREA OF AIRLAID NONWOVEN FABRICS BY IMAGE PROCESSING METHOD

Today, many quality control processes that are controlled by Image Processing (IP) technique has achieved state-of-the-art performance for all technology field. The image processing technique, which provides as accurate objective evaluation as the long and tiring traditional measurement methods, is nowadays widely used in textile industry. In this study, an alternative image processing technique has been proposed against the conventional wickability measurement method of nonwoven fabrics especially used in the hygiene products. An average liquid spread area ratio of airlaid nonwoven fabric was calculated with the developed algorithm and the accuracy of the proposed technology was demonstrated.

Assessment of two POC technologies for CD4 count in Morocco

BackgroundIn the era of “test and treat strategy”, CD4 testing remains an important tool for monitoring HIV-infected individuals. Since conventional methods of CD4 count measurement are costly and cumbersome, POC CD4 counting technique are more affordable and practical for countries with limited resources. Before introducing such methods in Morocco, we decided to assess their reliability.MethodsIn this study 92 blood samples from HIV-infected patients, were tested by PIMA and FACSPresto to derive CD4 count. Flow cytometry using FacsCalibur, was used as reference method for CD4 count comparison. Linear regression, Bland–Altman analysis were performed to assess correlation and agreement between these POC methods and the reference method. In addition, sensitivity and specificity, positive predictive value (PPV), negative predictive value (NPV) and misclassification percentage at 350 and 200 CD4 count thresholds; were also determined. Finally, because FACSPresto can also measure hemoglobin (Hb) concentration, 52 samples were used to compare FACSPresto against an automated hematology analyzer.ResultsThe coefficient of determination R2 was 0.93 for both methods. Bland–Altman analysis displayed a mean bias of − 32.3 and − 8.1 cells/µl for PIMA and FACSPresto, respectively. Moreover, with a threshold of 350 CD4 count, PIMA displayed a sensitivity, specificity, PPV, NPV, were 88.57%, 94.12%, 91.18%, 92.31%; respectively. FACSPresto showed 88.23%, 96.23%, 93.75% and 92.73%; respectively. Furthermore, the upward misclassification percentage was 8.57 and 5.88%, for PIMA and FACSPresto, respectively; whereas the downward misclassification percentage was 7.84% and 7.54%; respectively. With 200 cells/µl threshold, PIMA had a sensitivity, specificity, PPV and NPV of 83.33%, 98.53%, 93.75% and 95.71%, respectively. Regarding FACSPresto, sensitivity, specificity, PPV and NPV was 82.35%, 98.57%, 88.57% and 95.83%; respectively. Upward misclassification percentage was 5.56% and 5.88%, for PIMA and FACSPresto, respectively; whereas downward misclassification percentage was 4.41% and 4.29%; respectively. Finally, the hemoglobin measurement evaluation displayed an R2 of 0.80 and a mean bias of − 0.12 with a LOA between − 1.75 and 1.51.ConclusionWhen compared to the reference method, PIMA and FACSPresto have shown good performance, for CD4 counting. The introduction of such POC technology will speed up the uptake of patients in the continuum of HIV care, in our country.

Development of Stiffness Measurement Program Using Color Mapping in Shear Wave Elastography.

Shear wave elastography with ultrasound is a noninvasive method used for measuring stiffness in the human body. Shear wave elastography can be used for accurately and quantitatively measuring stiffness. However, its disadvantage is that the stiffness value can vary significantly because the region of interest (ROI) setting depends on the diagnostic operator. In this study, a stiffness measurement program using color mapping in shear wave elastography was developed to address the above-mentioned disadvantage. Color map and color ratios were obtained and evaluated for major lower limb muscles (i.e., biceps femoris, medial gastrocnemius, rectus femoris, and tibialis anterior) at active voluntary contraction. According to the result, when the developed program was used, a small standard deviation compared to the conventional stiffness measurement method, such as kilopascal or meter per second unit using ROIs, was measured in all cases. In conclusion, our results demonstrate that the stiffness measurement method using our program is expected to improve reliability in shear wave elastography ultrasound imaging.