State-of-the art of acousto-optic sensing and imaging of turbid media

Acousto-optic (AO) is an emerging hybrid technique for measuring optical contrast in turbid media using coherent light and ultrasound (US). A turbid object is illuminated with a coherent light source leading to speckle formation in the remitted light. With the use of US, a small volume is selected,which is commonly referred to as the “tagging” volume. This volume acts as a source of modulated light, where modulation might involve phase and intensity change. The tagging volume is created by focusing ultrasound for good lateral resolution; the axial resolution is accomplished by making either the US frequency, amplitude, or phase time-dependent. Typical resolutions are in the order of 1 mm. We will concentrate on the progress in the field since 2003. Different schemes will be discussed to detect the modulated photons based on speckle detection, heterodyne detection, photorefractive crystal (PRC) assisted detection, and spectral hole burning (SHB) as well as Fabry-Perot interferometers. The SHB and Fabry-Perot interferometer techniques are insensitive to speckle decorrelation and therefore suitable for in vivo imaging. However, heterodyne and PRC methods also have potential for in vivo measurements. Besides measuring optical properties such as scattering and absorption, AO can be applied in fluorescence and elastography applications

Bioinspired low-frequency material characterisation

New-coded signals, transmitted by high-sensitivity broadband transducers in the 40–200 kHz range, allow subwavelength material discrimination and thickness determination of polypropylene, polyvinylchloride, and brass samples. Frequency domain spectra enable simultaneous measurement of material properties including longitudinal sound velocity and the attenuation constant as well as thickness measurements. Laboratory test measurements agree well with model results, with sound velocity prediction errors of less than 1%, and thickness discrimination of at least wavelength/15. The resolution of these measurements has only been matched in the past through methods that utilise higher frequencies. The ability to obtain the same resolution using low frequencies has many advantages, particularly when dealing with highly attenuating materials. This approach differs significantly from past biomimetic approaches where actual or simulated animal signals have been used and consequently has the potential for application in a range of fields where both improved penetration and high resolution are required, such as nondestructive testing and evaluation, geophysics, and medical physics

Investigating ultrasound–light interaction in scattering media

Significance: Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulation’s “tagging efficiency.” Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature. Aim: The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process. Approach: We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component. Results: Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4  mm⁻¹ for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other. Conclusion: Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques

Acoustical Ranging Techniques in Embedded Wireless Sensor Networked Devices

Location sensing provides endless opportunities for a wide range of applications in GPS-obstructed environments; where, typically, there is a need for higher degree of accuracy. In this article, we focus on robust range estimation, an important prerequisite for fine-grained localization. Motivated by the promise of acoustic in delivering high ranging accuracy, we present the design, implementation and evaluation of acoustic (both ultrasound and audible) ranging systems.We distill the limitations of acoustic ranging; and present efficient signal designs and detection algorithms to overcome the challenges of coverage, range, accuracy/resolution, tolerance to Doppler’s effect, and audible intensity. We evaluate our proposed techniques experimentally on TWEET, a low-power platform purpose-built for acoustic ranging applications. Our experiments demonstrate an operational range of 20 m (outdoor) and an average accuracy 2 cm in the ultrasound domain. Finally, we present the design of an audible-range acoustic tracking service that encompasses the benefits of a near-inaudible acoustic broadband chirp and approximately two times increase in Doppler tolerance to achieve better performance

The detection of ultrasound using fiber-optic sensors

Ultrasound is a valuable tool for the detection of damage in structures and the characterization of material properties. Its detection is conventionally done by piezoelectric transducers, however fiber-optic sensors can operate over a greater range of frequencies and also yield information on the direction of wave propagation. The interaction between fiber sensors and ultrasound both demonstrates the integrating features of intrinsic fiber-optic sensors and presents new opportunities in ultrasonic detection, offering enormous diversity in polar and frequency response. This paper summarizes the interaction mechanisms between ultrasound and fiber sensors and confirms their functional flexibility. We use these results to demonstrate the practical use of these sensors to detect and locate damage in a sample

Ultrasound Methods for Biodiesel Production and Analysis

Ultrasonic techniques have been widely used in biodiesel production, since the acoustic cavitation is a phenomenon capable of accelerating potentially the transesterification reactions. The equipment employed in such approach was simply equipment available in any regular laboratory of chemistry. Further developments introduced the ultrasound as an important tool to produce biodiesel. The main advantage is increasing the conversion of esters at reduced reaction times, with significantly lower production costs. As a method for characterization and analysis of materials, ultrasound has been used since several decades ago. However, ultrasonic analytical methods based on metrological principles are fairly recent investigated. Using ultrasound as physical principle to interrogate biodiesel is a promising field of research, with some remarkable outcomes produced so far. The aim of this chapter is to demonstrate advances of using ultrasonic techniques in production and characterization of biodiesel, as well as an appraisal of the current technology status, and provide insights into future developments

Coded Signals for High Frequency Ultrasound Imaging

Degeneration of articular cartilage is known as a serious and painful knee disease seriously affecting people in all ages. The disease also marks the presence of osteoarthritis which is a complex musculoskeletal disorder. A successful assessment of the degeneration status is of great importance for estimating osteoarthritis progression, and thereby beneficial for implementing clinical treatments. Ultrasound has played a vital role in imaging the articular cartilage since it is capable of providing distinct information of important cartilage structures. However, various types of noise in ultrasound signals (e.g. clutter noise) are known to limit the quality of ultrasound images, especially at high frequencies where wave attenuation becomes severe. The possibility for improving the signal to noise ratio (SNR) by using coded signals is therefore the motivation behind this thesis, with the main objective is to investigate suitable codes and compression methods for cartilage imaging. The main focus of this thesis has been put on coded ultrasound signals and related signal processing methods. Transducers made from two different piezoelectric materials (PZT and PVDF) are used to image a thick cartilage sample. For each transducer, three different waveforms (Ricker wavelet, Gaussian chirped, and a 13-bit Barker) are used to excite the ultrasonic transducers. Two different wave compression methods (Matched filtering and Wiener filtering) are also explored to decode the signals received by transducers. Ahead of processing the received signals, a time calibration was used to compensate for sample tilting, yielding an improved precision in the phase/time delay. A maximum method and a center of mass method were used for calibration. The results from the experimental work show that both Chirp coded signals and Barker coded signals work well in improving the SNR, and that both transducers are able to produce high quality images of the cartilage sample. For the situations using coded excitation signals, however, the PZT transducer has high requirement for excitation repetition frequency because of its built-in delay line. Different time calibration methods have their own applicable conditions. Matched filter and Wiener filter both perform well for decoding, but the “noise” parameter in the Wiener filter has to be adjusted carefully to produce reasonable results

Photoacoustic Doppler axial flow measurement of homogenous media using structured illumination

We propose time and frequency domain methods for homogenous flow measurement based on the photoacoustic Doppler effect. Excited by spatially modulated laser pulses, the flowing medium induces a Doppler frequency shift in the received photoacoustic signals. The frequency shift is proportional to the component of the flow speed projected onto the acoustic beam axis. These methods do not rely on particle heterogeneity in the medium. A red-ink phantom flowing in a tube immersed in water was used to validate the methods in both frequency and time domains