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Fused silica an optical material of choice for high energy laser applications offers a wide range of advantages in the UV and NIR wavelength range. Increasing laser performance (especially with critical fluences) require detailed information on laser induced damage threshold of the optical material itself, and not just the coating. Therefore, S-on-1 measurements according to ISO 21254-2 were commissioned from two different, experienced, and well-respected laboratories. The resulting LIDT (laser induced damage threshold) values are discussed in the context of current publications and differences are worked out. The measurements focus on the ns-pulse regime at 1064 and 355nm. The results differ up to a factor of 10 between different laboratories. In comparison to the literature study, we noticed a spread of measured values around a factor of 20. Non-binding recommendations of the standard as well as the optical setup have significant influence on the obtained LIDT values. However, depending on the optical design of the application, one or the other optical setup with the corresponding LIDT value can be relevant. In this respect there is no such thing as a single, generally applicable value. Nevertheless, a standard should enable material comparisons under sufficiently standardized conditions.

In this paper the results of development and investigation on textile-based radar absorbing materials for protection against battlefield radar are presented. This research has been carried out within the Project “Adaptive Camouflage for the Soldier” (ACAMSII) which has received funding from the European Union’s Preparatory Action for Defence Research—PADR programme under grant agreement No 800871. To develop the fabrics with microwave shielding and absorbing properties samples of woven fabrics were coated with compositions containing inherently conducting polymers (ICPs), carbon-based formulations or their mixtures. For coating screen printing and knife-over-roll techniques were applied, as our aim was to develop the fabrics coated with conductive layer only on the back side of camouflage pattern printed fabric, that it could be integrated in the military camouflage clothing system. In the radar threat evaluation, as a part of ACAMSII project, it was pointed up that a major threat to dismounted soldiers are battlefield radars commonly operating within X and Ku-bands. Consequently, the investigation of reflection and transmission properties of developed textile fabrics was performed in a frequency range of 6–18 GHz, which cover the defined frequencies relevant to the application. It was found that shielding effectiveness (SE) as well as absorption properties depend not only on the amount and type of conductive paste topped on the fabric, but also resides in the construction parameters of fabrics and their finishing before coating.

In recent decades, the use of ultra-high performance concrete (UHPC) has been widely accepted by the construction industry for buildings and bridges. The exceptional properties of UHPC on strength (18 ksi 35 ksi) and durability (freeze-thaw resistance, abrasion resistance, chloride ion penetration resistance) have made it a popular construction material for durable and sustainable civil infrastructure systems. The objective of this paper is to investigate the shortterm mechanical strength development of UHPC specimens using a noncontact synthetic aperture radar (SAR) imaging sensor. UHPC cubes and cylinders were designed and manufactured for nondestructive strength monitoring and kinematic and rheological characterization. Change in moisture content and distribution inside UHPC cylinders was monitored by a laboratory 10-GHz SAR imaging sensor inside a microwave anechoic chamber at UMass Lowell. Hydraulic permeability of UHPC specimens was measured by their bulk electrical resistivity using a concrete resistivity meter (ASTM C1876). The rate of water uptake (absorption or sorptivity) was characterized by an apparatus used to measure the water absorption rate of both the concrete surface and interior concrete (ASTM C1585). Early stage shrinkage behavior of UHPC specimens during the first seven days was also measured using a shrinkage cone. Level of cement hydration in UHPC specimens was quantified by the loss of free water inside UHPC and remotely measured by the SAR imaging sensor. Mechanical strength development in UHPC specimens was monitored by following ASTM C109/C109M. From our preliminary result, it is found that change in SAR amplitude and amplitude distribution can be correlated to the level of strength development.

Traditional origami patterns can be applied to pre-strained polystyrene (PSPS) sheets to create precisely fabricated three-dimensional shapes through self-folding. The basis of these folded shapes are the tessellated repeat units, comprising only a few faces and folds. Subtle modifications of the geometry of the patterns allows generation of many final shapes based on the same fundamental repeat unit. These subtle changes allow for variations in attributes like packing density or curvature. Further, self-folded PSPS sheets represent a novel kind of engineering material in which the mechanical properties depend on the fold pattern and the extent of folding . After folding, the adjacent hinges and faces of these structures differ from each other in thickness, temperature history, and orientation relative to loading directions. We seek to characterize the mechanical properties of self-folded, periodic structures to gain a better understanding of how to design and utilize them in engineering applications. Miura-ori patterns will be applied to PSPS sheets, which self-fold in response to infrared light absorption. Folded samples with a range of face sizes and pattern angles will be subjected to compressive testing. Modification of geometric parameters, along with exposure time to the infrared light, has a significant effect on orientation of faces relative to the direction of loading, which will allow control over the final shape and mechanical properties.

Shearography is an optical measurement technique based on the interference between two laterally displaced images of the region of interest. Due to its quasi-equal-path configuration, shearography is robust enough for applications outside the laboratory. In fact, shearography has been successfully used for nondestructive testing in composite materials mainly in the aerospace, boating and, more recently, in the oil and gas industries. There are several demands in the oil and gas industry for nondestructive testing on subsea metallic elements coated with composite materials to prevent or mitigate the effects of corrosion. The present work deals with the underwater shearography for nondestructive testing in metal parts coated with composite materials. The paper begins with considerations about the propagation of light in underwater environments and a discussion on the elements that disturb the measurements with shearography such as changes in wavelength, absorption, dispersion, distortions caused by refraction in the air-glass-water interfaces among others. The different loading possibilities to reveal defects in the tested elements are analyzed: by pressure loading, thermal loading by radiation and thermal loading by induction. Exploratory experiments carried out inside water tanks involving the three loading techniques are presented and discussed. The results are compared and the combinations that led to the best results are emphasized. Finally, future works that involve a fully immersed system for underwater shearogaphy are briefly announced.

In this study, terahertz non-destructive testing technology was used to insulation materials. The terahertz wave nondestructive testing was carried out to test the thickness of the layer, the air hole in the insulation layer and the defect of adiabatic structure. The refraction and absorption coefficients of terahertz wave in asbestos-filled nitrile rubber, aramid fiber-filled ethylene propylene diene monomer(EPDM) and silica-filled polyurethane rubber were studied. Two terahertz wave scanning imaging results were compared. Terahertz wave has a good penetrating ability to the insulation material and can be used to test the insulation layer thickness. At the same time, quality defects such as stratification and porosity of samples with a thickness of 2 mm can be identified. It is proved that terahertz technology can be applied to the detection of insulation materials and internal defects.

Hard X-rays tomography enables us to determine the shape and inner microstructure of a wide variety of solid states and their combinations. Biological samples, and in particular soft tissues from animals and humans, can be encapsulated in suitable containers to avoid deformations during the rotation and radiograph acquisition. The quantitative characterization of soft silicone objects, however, is challenging, because their shape depends on the orientation with respect to the gravitation force. Solid supports may help, but have some impact on the more or less complex-shaped silicone part. In medicine, silicone implants and devices play a major role, since the mechanical properties can be easily tailored. Using the nanotom® m (phoenix|x-ray, GE Sensing & Inspection Technologies GmbH, Wunstorf, Germany), we measured the X-ray absorption of silicone with pre-defined softness. The density resolution of the CT-system was insufficient to establish a density-stiffness relationship. In addition, the shape of a silicone part of a soft intraoral device was measured for three angles with respect to the gravitation force, for which the bending was obvious. These successful tomographic measurements will become the basis of finite element modeling to extract the mechanical properties including their local variations.

We investigated several curved vane baffle designs coated with specular black paint as an inexpensive manufacturing alternative to traditional diffuse baffles vanes to reduce stray light and heat due to absorption in a standard Cassegrain telescope design configuration. The heat absorption is a very large problem in infrared systems and the specular designs solve this part of the problem. Our study involved simulating two different types of baffle systems, diffuse and specular painted vanes on the main barrel baffle. The first type of baffle consisted of evenly and non-evenly spaced diffusely black coated straight planar vanes on the main barrel baffle and a second type using specular black paint on curved vanes. TracePro, a stray light simulation software from Lambda Research Corporation, was used to simulate and compare each of the nine baffle systems for stray light rejection. The diffuse black painted straight vane baffle design was used as the baseline design to compare results to the other eight designs. In all designs except for the baseline design, TracePro's local downhill simplex optimization method was used to optimize each vane curvature and spacing in the main barrel baffle to reject incoming stray light. These curved vanes were designed to reject stray light back out of the main barrel baffle rather than to be absorbed by diffuse black paint.

The proposed method relates to spectroscopic ones and is based on the measurement of the intensity of the absorbed irradiation when it passes through the blood-containing organ of a patient. This way of non-invasive determination of glucose contents in human blood was proposed far ago but the reason of its absence in the market is the non-solved problem of the irradiation part definition which is absorbed just by glucose but not by the other absorbing components of the human organ. This problem solution includes measurements of irradiation absorption at numerous wavelengths of irradiation and application of specific mathematical apparatus which we propose to apply for this method. The mathematical calculations include the solution of linear equations system with light intensity parts which are absorbed by different absorbing components as variables. The number of equations id equal to the amount of absorbing components and the number of wavelengths at which the measurements must be performed. The calculations allowing to transfer the part of absorbed light into glucose concentration also are listed.

While most of the conventional acoustic liner which comprised of a micro-perforated panel (MPP) backed by a honeycomb structure shows good acoustic performance at high or medium frequency range, there is still very challenging for them at low-frequency range. Difference from MPP where resonance frequency of a MPP based liner is much depended on the backing space and shifting the absorption peak of the liner to low-frequency range generally makes the structure bulky and down-grades the absorbing quality, Helmholtz resonators shows easy frequency-tuning ability and outstanding absorption at low-frequency. In this research, by carefully design, the proposed designs of acoustic liner based on multiple Helmholtz resonators (MHR) show very good absorption in a broadband at low-frequency. First, a super unit-cell which is an annulus disk comprised of three segments or Helmholtz resonators is numerically studied to tune the working frequency band into the desired region, to form a broadband working region and to have the supercells absorbing potential. Afterward, the proposed liner constructed by patterning the supercells along the central axis of the liner is studied on absorption by using both commercial code (COMSOL) and theoretical analysis. The results show high absorption (>80%) at low-frequency broadband (400Hz to 650Hz) and good agreement between numerical and theoretical analysis. The proposed design and analysis can be served as effective model and tool for design and constructing advanced acoustic liners.

Opto-acoustic imaging involves using light to produce sound waves for visualizing blood in biological tissue. By using two different optical wavelengths, diagnostic images of blood oxygen saturation can be generated using endogenous optical contrast, without injection of any external contrast agent and without using any ionizing radiation. The technology has been used in recent clinical studies for diagnosis of breast cancer to help distinguish benign from malignant lesions, potentially reducing the need for biopsy through improved diagnostic imaging accuracy. To enable this application, techniques that can accurately map and effectively display small differences in oxygen saturation are necessary. We analyze the ability of an opto-acoustic imaging system to display a colorized parametric map for oxygen saturation of blood using biologically-relevant opto-acoustic phantoms. The relationship between colorized image output and known oxygen saturation values is examined. To mimic breast tissue, a material with closely matching properties for optical absorption, optical scattering, acoustic attenuation and speed of sound is used. The phantoms include two vessels filled with whole blood at oxygen saturation levels determined using a gold-standard sensor-based approach. A flow system with gas-mixer and membrane oxygenator adjusts the oxygen saturation of each vessel independently. Data is collected with an investigational Imagio breast imaging system. An opto- acoustic relative color map is generated using a novel statistical mapping approach. In addition, we propose a technique to characterize the ability to distinguish small differences in oxygen saturation as the oxygenation level is varied. When applied to the phantom, with reference vessel at 99% saturation, hematocrit of 42% and depth of 1.5cm, a contrast distinction threshold was reached when an adjusted vessel achieved a difference of approximately 4.6% saturation compared to the reference.

X-ray testing is based on the attenuation of X-rays when passing through matter. Image detectors acquire the X-ray information which is defined by the local penetrated wall thickness of the tested sample. By X-ray absorption in the detector and following read-out and digitization steps a digital image is generated. As detectors a radiographic film and film digitization, a storage phosphor imaging plate and a special Laser scanner (Computer Radiography - CR) or a digital detector array (DDA) can be used. The digital image in the computer can then be further analyzed using many types of image processing. In the presented work the automated evaluation of wall thickness profiles are investigated using a test steel pipe with 9 different wall thicknesses and various X-ray voltages and different filter materials at the tube port and intermediate between object and detector. In this way the influence of different radiation qualities on the accuracy of the automated wall thickness evaluation depending on the penetrated wall thickness of the steel pipe was investigated.

Due to the light scattering and absorption, underwater images were blurred such as fog, uneven illumination, overexposure or lack of light. We proposed an image enhancement algorithm based on granular computing to enhance underwater optical image in this paper. First, the illumination information of underwater image was extracted. Then, we dividing the illumination information into granularity of different sizes from wide to thin by calculated the effectiveness indicators. Finally, by calculating their value and compensation for each granularity, we obtained the adaptive enhancement image. The simulation and experiment results verify the effectiveness of the algorithm.

In the recent years, various membrane-type acoustic metamaterials were developed for low frequency sound absorption. However, a membrane absorber usually requires a large back cavity to achieve low frequency sound absorption and on the other hand, in order to guaranty a multiple peaks absorption decorated membrane resonators or membrane with multiple magnetic negative stiffness cell shall be considered. This research proposes a new concept of membrane-type metamaterial which can achieve multiple peaks and broadband absorption at low frequencies. The basic concept behind the design of the elementary cell is associated to the vibro-acoustic behavior of the structure. In fact, the maximum sound absorption is related to the symmetrical mode of the membrane, so playing with the geometry, the mass and the stiffness of the membrane the eigenfrequencies can be tuned easily in the prescribed frequency range. At same time local increase of strain energy around geometrical discontinuity or around discontinuity associated to the material properties may lead a gain in sound absorption. A mono-layer membrane structure is presented where the geometrical shape and material properties distribution in terms of density and stiffness in the elementary cell are optimized in order to manipulate the vibro-acoustic properties and maximize the absorption at required frequencies. To optimize the geometry and the vibro-acoustic properties of the proposed metamaterial, finite element simulation were carried out. The numerical model was then validated using experimental measurements. A preliminary prototype was tested into an impedance tube test ring and the normal sound absorption was measured following the transfer function approach and compared with the numerical results

Composite elements are recently commonly used in many industrial branches like aviation, marine or civil engineering. One of the problems that can influence on material durability are contaminations (e.g. water, oil, dust) that can be introduced into elements structure during manufacturing process or can diffuse into its internal part during exploitation. Such intrusions locally change material characteristics, affect its durability and can be a damage origin significantly decreasing mechanical properties of a composite object. Due to this it is important to have a non-destructive method to inspect an internal structure of elements just after manufacturing or in intervals during its exploitation. One of the techniques is THz spectroscopy that can be applied for non-conductive materials. This method can be used for identification of discontinuities or material structural disintegrations that results in changes of absorption, refractive index or scattering of THz waves propagating throughout the material structure. The paper presents an application of THz spectroscopy for detection and localization of different contaminations (like water, oil or dust) that were introduced into composite material during its manufacturing process or were an effect of exploitation faults. During analysis the limitations of proposed method will be determined.

Phantoms are crucial for developing photoacoustic imaging systems and for training practitioners. Advances in 3D printing technology have allowed for the generation of detailed moulds for tissue-mimicking materials that represent anatomically realistic tissue structures such as blood vessels. Here, we present methods to generate phantoms for photoacoustic and ultrasound imaging based on patient-specific anatomy and mineral oil based compounds as tissue-mimicking materials. Moulds were created using a 3D printer with fused deposition modelling. Optical and acoustic properties were independently tuned to match different soft tissue types using additives: inorganic dyes for optical absorption, TiO2 particles for optical scattering, paraffin wax for acoustic attenuation, and solid glass spheres for acoustic backscattering. Melted mineral oil compounds with additives were poured into the 3D printed moulds to fabricate different anatomical structures. Optical absorption and reduced scattering coefficients across the wavelength range of 400 to 1600 nm were measured using a spectrophotometer with an integrating sphere, and inverse adding-doubling. The acoustic attenuation and speed-of-sound were measured in reflection mode using a 10 MHz transducer. Three phantoms were created to represent nerves and adjacent blood vessels, a human placenta obtained after caesarean section, and a human heart based on an MRI image volume. Co-registered multi-wavelength photoacoustic and ultrasound images were acquired with a system that comprised a clinical ultrasound imaging scanner, an optical parametric oscillator, and linear-array ultrasound imaging probes. We conclude that mineral oil based compounds can be well suited to create anatomically-realistic phantoms for photoacoustic and ultrasound imaging using 3D printed moulds.

The installation and debugging of optical circuit structure in the application of carbon monoxide distributed laser gas analysis and measurement, there are difficult key technical problems. Based on the three-component expansion theory, multi-multiple expander structure with expansion ratio of 4, 5, 6 and 7 is adopted in the absorption chamber to enhance the adaptability of the installation environment of the gas analysis and measurement device. According to the basic theory of aberration, the optimal design of multi-multiple beam expander structure is carried out. By using image quality evaluation method, the difference of image quality under different magnifications is analyzed. The results show that the optical quality of the optical system with the expanded beam structure is the best when the expansion ratio is 5-7.

Photoacoustic (PA) imaging forms an image based on optical absorption contrasts with ultrasound (US) resolution. In contrast, US imaging is based on acoustic backscattering to provide structural information. In this study, we develop a miniature all-optical probe for high-resolution PA-US dual-modality imaging over a large imaging depth range. The probe employs three individual optical fibers (F1-F3) to achieve optical generation and detection of acoustic waves for both PA and US modalities. To offer wide-angle laser illumination, fiber F1 with a large numerical aperture (NA) is used for PA excitation. On the other hand, wide-angle US waves are generated by laser illumination on an optically absorbing composite film which is coated on the end face of fiber F2. Both the excited PA and backscattered US waves are detected by a Fabry-Perot cavity on the tip of fiber F3 for wide-angle acoustic detection. The wide angular features of the three optical fibers make large-NA synthetic aperture focusing technique possible and thus high-resolution PA and US imaging. The probe diameter is less than 2 mm. Over a depth range of 4 mm, lateral resolutions of PA and US imaging are 104−154 μm and 64−112 μm, respectively, and axial resolutions of PA and US imaging are 72−117 μm and 31−67 μm, respectively. To show the imaging capability of the probe, phantom imaging with both PA and US contrasts is demonstrated. The results show that the probe has potential for endoscopic and intravascular imaging applications that require PA and US contrast with high resolution.

The main task of a step-and-scan system is to transfer an image as present on a mask into a pattern on a wafer. The mask holds a magnified version (typical 4x) of the desired pattern. Figure 9.1 illustrates the principle of such a system. A source generates EUV light. This light is radiated into a so-called illuminator and reshaped to illuminate the pattern on the mask. Note that the illuminator is projected in a curved shape in order to match the shape of the entrance slit shape of the projection optics box (POB). The reason for this shape is explained in Chapter 5. The light reflected from the mask is collected by the POB, and a demagnified image is projected on the wafer. Note that current high-end commercial lithography tools scan the mask while the wafer moves synchronously with the mask. This way, relatively small projection optics can be used to expose a large area on the wafer. Because of the strong absorption of EUV light in air, the core parts of the system are enclosed in a vacuum vessel. This puts several constraints on the system design. The component on the lower left in Fig. 9.1 is the 13.5nm EUV light source. More details on these topics will be given in section 5.3.

Many nontransparent materials used in optics and optoelectronics devices show frequency-dependent complex electrical permittivity in the visible domain. The complex refractive index of bulk material is an important parameter that determines the optical behavior of a given material and in many cases any nanophotonic structure that might be fashioned from such a material. The complex refractive index for materials in the visible domain is given by the square root of the electrical permittivity [Eq. (8.36)]. The complex refractive index constitutes an important simulation parameter in optics and photonics. The imaginary parts of the refractive index and the electrical permittivity are associated with the absorption and the conductivity of the material, respectively. It is also important to understand that the implementation of FDTD depends on the actual relations among various components of the electric and magnetic fields, which in turn are determined by the nature of material properties. In this chapter we develop a simple dispersive algorithm that can be used when metal nanostructures need to be simulated in the visible domain.

Fatigue limit estimation using infrared thermography has recently received attention as a method for reducing the time required for product design. In this study, the applicability of a method based on mean temperature and dissipated energy measurements was experimentally investigated on a titanium alloy; fatigue plate specimens were fabricated from the titanium alloy Ti-6Al-4V ELI. The fatigue limit of these specimens obtained from conventional fatigue testing was found to be 620 MPa. The estimated fatigue limit obtained from mean temperature measurements was found to be 600 MPa, although estimating the fatigue limit using dissipated energy measurements was difficult because little significant change in dissipated energy values with the stress amplitude was observed. These tendencies are probably attributed to the crystal structure displaying different deformation properties and high vibration absorption properties. The resonance components from the fatigue testing instruments (noise components) were calculated from the frequency analysis of the time-series temperature fluctuation data measured by infrared thermography. The increase in the dissipated energy values (with the noise components subtracted) against the stress amplitude changed at a certain stress amplitude and the fatigue limit could be estimated to be 565 MPa. Therefore, the relative error between the fatigue limit value obtained from conventional fatigue testing and the estimated values was within 10%. The fatigue limit could be estimated more accurately by considering the influence of different deformation properties between tensile and compressive loading due to the crystal structure differences in the dissipated energy measurement.

The large diffusion of structural parts made of carbon fibres reinforced polymers (CFRP) in the aerospace and automotive sectors has highlighted the importance of developing hybrid multifunctional materials characterised by improved mechanical properties and coupled with non-structural features. Indeed, while due to their high specific strength and light weight, composite systems are characterised by very high mechanical properties in the in-plane direction, their intrinsic layered structure makes them very susceptible to low-velocity impacts resulting in Barely Visible Impact Damage (BVID) that can lead to the critical failure of primarily structures. Based on these premises, the development of a multifunctional hybrid system can overcome this drawback by tackling this issue from two different points of view, enhancing the total reliability of light-weight composite parts in order to improve fuel efficiency and optimise the footprint of new generation aero-structures. Indeed, by including an additional metallic phase within the structure of a traditional laminate it is possible to develop a smart multifunctional system in which the hybrid phase acts simultaneously as a reinforcement to enhance the out-of-plane properties of the material and as an intelligent embedded sensor system able to communicate information about the health status of the part and detect impact events or critical loads. This work is focused on the design, manufacturing and testing of a hybrid CFRP (H-CFRP) in which the hybridisation is obtained by including an array of Shape Memory Alloys (SMA) or Copper wires within the laminate. The electrical properties of the hybrid network is exploited to design a smart sensing system which can be interrogated to monitor the load distribution on the part and detect critical solicitations in critical points. The low-power system, controlled by an Arduino microcontroller, is able to monitor the integrity status of the part using each wire as a linear probe to scan complex structures at a certain frequency, measuring the local change in the electrical resistance from which it is possible to build a map of the stress distribution. The position of the metallic network along the laminate’s thickness was determined by analysing the response of different configurations of hybrid samples subjected to Low Velocity Impacts (LVI) in order to optimise the design of the H-CFRP and enhance the energy absorption. Using the same Arduinocontrolled Multiplex the smart wires array was exploited as heat source to scan the sample inner structure and monitoring the variation of the superficial apparent thermal variation with an Infra-Red (IR) Camera, a simulated delaminated area was detected.

The spread of smart structures has recorded a significant increase during the last decades. Nowadays these solutions are applied in various fields such as aerospace, automotive and civil constructions. This kind of structures was born in the past in order to cope with the high vibrations that every lightweight structure has to face. In order to reduce weight designers usually decide to use very thin and lightweight structures. In the automotive field, for example, a reduced fuel consumption is obtained employing lightweight materials. However, in general a worsening of the vibroacoustic comfort is obtained with undesired vibrations that can be really annoying for passengers and dangerous for the structure itself. This work presents an innovative smart plate that is able to actively vary its dynamic properties, by means of an IMSC control logic, in order to improve the acoustic performances. An investigation about the system response in the high frequency range allowed to assess the behavior in terms of absorption, reflection coefficient and transmission loss.

International audience; A Helmholtz resonator is a passive acoustic resonator classically used to control a single frequency resulting from the cavity volume and the resonator neck size. The aim of the proposed study is to present a new concept and strategy allowing real-time tunability of the Helmholtz resonator in order to enhance acoustic absorption performances at low frequencies (< 500 Hz). The proposed concept consists in replacing the resonator rigid front plate by an electroactive polymer (EAP) membrane. The first proposed strategy consists on a change in the mechanical properties of the membrane resulting from the applied electric field. This induces a resonance frequency shift. A second strategy is based on a well-located spring, which could direct the membrane deformation following the axis of the resonator to obtain a cavity volume variation. Both strategies allow variation of theresonance frequency of the device. Experimental measurements are performed to determine the potential of this concept for improvement of low-frequency performances of the acoustic devices.

Recent years have seen rapid development of hybrid optical-acoustic imaging modalities with broad applications in research and clinical imaging, including photoacoustic tomography (PAT), photoacoustic microscopy, and ultrasound-modulated optical tomography. Tissue-mimicking phantoms are an important tool for objectively and quantitatively simulating in vivo imaging system performance. However, no standard tissue phantoms exist for such systems. One major challenge is the development of tissue-mimicking materials (TMMs) that are both highly stable and possess biologically realistic properties. To address this need, we have explored the use of various formulations of PVC plastisol (PVCP) based on varying mixtures of several liquid plasticizers. We developed a custom PVCP formulation with optical absorption and scattering coefficients, speed of sound, and acoustic attenuation that are tunable and tissue-relevant. This TMM can simulate different tissue compositions and offers greater mechanical strength than hydrogels. Optical properties of PVCP samples with varying composition were characterized using integrating sphere spectrophotometry and the inverse adding-doubling method. Acoustic properties were determined using a broadband pulse-transmission technique. To demonstrate the utility of this bimodal TMM, we constructed an image quality phantom designed to enable quantitative evaluation of PAT spatial resolution. The phantom was imaged using a custom combined PAT-ultrasound imaging system. Results indicated that this more biologically realistic TMM produced performance trends not captured in simpler liquid phantoms. In the future, this TMM may be broadly utilized for performance evaluation of optical, acoustic, and hybrid optical-acoustic imaging systems.

Photoacoustic imaging is a fast growing in vivo imaging modality combining the optical absorption contrast and high spatial ultrasonic resolution. Using this technique deeper tissue penetration greater than optical mean free path (~1 mm in skin) is possible. Low resolution with deep penetration depth is possible utilizing acoustic focusing and high resolution with shallow imaging depth can be possible by optical focusing using different photoacoustic microscopy (PAM) systems. Here, we present a combined/switchable optical resolution and acoustic resolution photoacoustic microscopy (OR-AR-PAM) system capable of both high resolution and low resolution deep tissue imaging on the same sample. Lateral resolution of 3.9 μm using optical focusing and lateral resolution of 57 μm using acoustic focusing was successfully demonstrated using the combined system. MOE (Min. of Education, S’pore) Published version

First, we will provide some recent advances on low-frequency phononic and acoustic insulators based on metasurfaces. We will introduce a new concept of acoustic absorption based on the coiling-up space design which exhibits extreme acoustic properties. A new acoustic absorber operating at 125Hz with deep-subwavelength thickness (~ λ/230) acting as an “acoustic sink” is designed and its functionalities are discussed. Second, as we are dealing with how to reduce or to absorb low-frequency sound/noise, we will explore a novel idea on the use in a positive manner these noises as a source of energy. We theoretically and numerically show the properties of a novel acoustic energy harvester resulting from the confinement and conversion of low-frequency acoustic/noise disturbances to electrical energy. We indeed present preliminary results showing a metamaterial structure producing an electrical power density of 0.54μw/cm3 for one confined frequency, which presents a very good value compared the others acoustic energy harvesting approaches.

The concept of the coiling up space, based on which artificial structures could exhibit extreme acoustic properties, such as high refractive index, double negativity, near-zero index, etc., have been investigated intensively recently due to the fascinating underlying physics and diverse potential applications [1-3]. One of the most important functionality is the ability to shrink bulky structures into deep sub-wavelength scale. It is therefore intuitive to prospect that the concept of coiling up space, if could be extended into the perforated system, will benefit to significantly reduce the total thickness while keeping total absorption. Conventional acoustic absorbers require a structure with a thickness comparable to the working wavelength, resulting major obstacles in real applications in low frequency range. We present a metasurface-based perfect absorber capable of achieving the total absorption of acoustic wave in extremely low frequency region. The metasurface possessing a deep sub-wavelength thickness down to a feature size of ~ lambda/223 is composed of a perforated plate and a coiled coplanar air chamber. Simulations based on fully coupled acoustic with thermodynamic equations and theoretical impedance analysis are utilized to reveal the underlying physics and the acoustic performances, showing an excellent agreement. Our realization should have high impact on amount of applications due to the extremely thin thickness, easy fabrication and high efficiency of the proposed structure. References 1. Z. Liang and J. Li, Phys. Rev. Lett. 108, 114301 (2012). 2. Y. Li, B. Liang, X. Tao, X. F. Zhu, X. Y. Zou, and J. C. Cheng, Appl. Phys. Lett. 101, 233508 (2012). 3. Y. Xie, W. Wang, H. Chen, A. Konneker, B. I. Popa, and S. A. Cummer, Nat. Commun. 5, 5553 (2014).

A gamma-ray spectrometer for radioactive waste sorting is presented. The equipment is based on a new “thin-walled” xenon gamma-ray detector with sensitive volume of 4 liters and a digital electronics unit. Use of the thin wall (0.5 mm of stainless steel covered with fiberglass) provides lower absorption of gamma-rays by the detector’s walls and expansion of the energy range of radiation being registered. The digital electronics unit makes it possible to use the equipment in unfavorable field conditions such as high levels of acoustic influence.

A testing method has been set up to evaluate the attenuation performance of an absorption coat to 8mm waves, which is based on a set of detecting system included by an 8mm wave emitter, a millimeter power meter, a point to point collimator and a reflecting plate. The power meter was aimed at the 8 mm wave emitter along the reflection optical path instead of the direction observation between incident and reflected millimeter wave. Some Al, Fe and aluminum alloy sample plates were made and painted by the dope which was complexed with chopped carbon fibers. A naked metal plate was first used to adjust the transmission path of the millimeter wave. Then the power meter was adjusted to phase locking after preheating, and the millimeter wave power was sampled as the background value. Then the other painted plates were tested under the same conditions. When the concentration of chopped carbon fibers is 0.5mg/ml and the thickness of the absorption coat is 0.5mm, the attenuation percentages of Al, Fe and aluminum alloy painted plates respectively is 54.29%, 58.31% and 41.12%. By the result, the reflection testing method may be widely used to measure the reflection capacity or attenuation performance of various surfaces to millimeter waves.

Presented here is a new metamaterial panel based on multi-frequency vibration absorbers for broadband vibration absorption. The proposed metamaterial panel consists of a uniform isotropic panel and small two-mass spring-mass-damper subsystem many locations along the panel to act as multi-frequency vibration absorbers. The existence of two stopbands is demonstrated using a model based on averaging material properties over a cell length and a model based on finite element modeling and the Bloch-Floquet theory for periodic structures. For a finite metamaterial panel, because these two idealized models can not be used for finite panels and/or elastic waves having short wavelengths, a finite-element method is used for detailed modeling and analysis. The concepts of negative effective stiffness is explained in detail. For an incoming wave with a frequency in one of the two stopbands, the absorbers are excited to vibrate in their optical modes to create shear forces to straighten the panel and stop the wave propagation. For an incoming wave with a frequency outside of but between the two stopbands, it can be efficiently damped out by the damper with these mass of each absorber. Hence, the two stopbands are connected in to a wide stopband. Numerical examples validate the concept and show that the structures boundary conditions do not have significant influence on the absorption of high-frequency waves. However, for absorption of low-frequency waves, the structures boundary conditions and resonance frequencies and the location and spatial distribution of absorbers need to be considered in design, and it is better to use heavier masses for absorbers.

Micro-fluidic devices are widely used today in the areas of medical diagnostics and drug research, as well as for applications within the process, electronics and chemical industry. Microliters of fluids or single cell to cell interactions can be conveniently analyzed with such devices using fluorescence imaging, phase contrast microscopy or spectroscopic techniques. Typical micro-fluidic devices consist of a thermoplastic base component with chambers and channels covered by a hermetic fluid and gas tight sealed lid component. Both components are usually from the same or similar thermoplastic material. Different mechanical, adhesive or thermal joining processes can be used to assemble base component and lid. Today, laser beam welding shows the potential to become a novel manufacturing opportunity for midsize and large scale production of micro-fluidic devices resulting in excellent processing quality by localized heat input and low thermal stress to the device during processing. For laser welding, optical absorption of the resin and laser wavelength has to be matched for proper joining. This paper will focus on a new approach to prepare micro-fluidic channels in such devices using a thermal transfer printing process, where an optical absorbing layer absorbs the laser energy. Advantages of this process will be discussed in combination with laser welding of optical transparent micro-fluidic devices.

The paper presents the results of modeling of the aerosol optical characteristics taking into account the effect of relative humidity of air. Two algorithms were applied: humidification by the Hanel formula with the same parameter of condensation activity for all particles and introducing the dependence of aerosol hygroscopic properties on the particle size. The results of calculations are compared with the data of field measurements.

Carbon fiber composite (CFC) materials have been used for many structural applications for decades. Their electromagnetic properties are also of great interest and are being quantified by recent research. This research explores shielding effectiveness, antenna design, conductivity, reflection, and absorption properties. The work in this paper specifically characterizes the radar cross section (RCS) of CFC structures. Various CFC planar samples were created using a wet layup method and vacuum bagging techniques. These samples were then placed in an anechoic chamber and their RCS values were measured at normal incidence. These measured values were compared to those of aluminum samples made into the same shape as the CFC samples. All of the measurements were made over 7 - 12 GHz frequency range. The RCS of the CFC samples show some interesting results. The fiber direction in the CFC samples had great influence on the RCS. Theories and reasoning for the results are presented and discussed.

Water, in its various phases, in any environment other than desert (hot or cold) conditions, is the single most destructive element that causes deterioration of materials and failure of building assemblies. It is the key element present in the formation of mold and fungi that lead to indoor air quality problems. Water is the primary element that needs to be managed in buildings to ensure human comfort, health and safety. Under the right thermodynamic conditions the detection of moisture in its various states is possible through the use of infrared thermography for a large variety of building assemblies and materials. The difficulty is that moisture is transient and mobile from one environment to another via air movement, vapor pressure or phase change. Building materials and enclosures provide both repositories and barriers to this moisture movement. In real life steady state conditions do not exist for moisture within building materials and enclosures. Thus the detection of moisture is in a constant state of transition. Sometimes you will see it and sometimes you will not. Understanding the limitations at the time of inspection will go a long way to mitigating unsatisfied clients or difficult litigation. Moisture detection can be observed by IRT via three physical mechanisms; latent heat absorption or release during phase change; a change in conductive heat transfer; and a change in thermal capacitance. Complicating the three methodologies is the factor of variable temperature differentials and variable mass air flow on, through and around surfaces being inspected. Building enclosures come in variable assembly types and are designed to perform differently in different environmental regions. Sources for moisture accumulation will vary for different environmental conditions. Detection methodologies will change for each assembly type in different ambient environments. This paper will look at the issue of the methodologies for detection of the presence of moisture and determination of the various sources from which it accumulates in building assemblies. The end objective for IRT based moisture detection inspections is not to just identify that moisture is present but to determine its extent and source. Accurate assessment of the source(s) and root cause of the moisture is critical to the development of a permanent solution to the problem.

A%675$&7Fibre Metal Lam inate (FML), a metal sandwiche d hybrid composite material is well-known for its enhanced impact properties and better damage tolerance and it has been successfully implemented in diverse engineering applications in aviation industry. With heterogeneous constituents, the stacking sequence of FML is believe to play a cri tical role to govern its overall energy absorption capability by means of controlli ng delamination of metal composite interface and plastic deformation of metal layers. As a precursor, low velocity impact experiments were conducted on interply configured transparent plastic plates in orde r to extract the significance of stacking sequence and realize the characteristics of each layer through nake d eye which is not possible in FML due to opacity of metal layer. The stack configuration constitute hard acrylic (brittle) and soft polycarbonate (ductile) plates analogous to composite (brittle) and metal (ductile) layers on FML laminate and the impact ev ent is performed on either hard or soft facing sides separately. Hard side samples resemble more protective than soft side impact sample, with large peak resistant force and expose smaller damage growth in all experimented cases. Keywords: Transparent plastics, Fibre Metal Lamina te, Low velocity impact, Impact face 1. IntroductionIn engineering applications, structures made of heterogeneous composition fulfill majority of anticipation in terms of robustness, reliability and durability. The investigation of bi-material structure often provides a fundame ntal understanding of role played by the individual material constituents and also pave the way to enhance and optimize its influence. Wear resistant structures, coated substrates, sandwich panels, and laminated composites are some of the

Thermally-induced deformations of steering mirrors reflecting 100 J/10 Hz laser pulses in vacuum have been analyzed. This deformation is caused by the thermal stress arisen due to parasitic absorption of 1 kW square-shaped flat-top laser beam in the dielectric multi-layer structure. Deformation depends on amount of absorbed power and geometry of the mirror as well as on the heat removal scheme. In our calculations, the following percentages of absorption of the incident power have been used: 1%, 0.5% and 0.1%. The absorbed power has been considered to be much higher than that expected in reality to assess the worst case scenario. Rectangular and circular mirrors made of zerodur (low thermal expansion glass) were considered for these simulations. The effect of coating layers on induced deformations has been neglected. Induced deformation of the mirror surface can significantly degrade the quality of the laser beam in the beam delivery system. Therefore, the proper design of the cooling scheme for the mirror in order to minimize the deformations is needed. Three possible cooling schemes of the mirror have been investigated. The first one takes advantage of a radiation cooling of the mirror and a copper heatsink fixed to the rear face of the mirror, the second scheme is based on additional heat conduction provided by flexible copper wires connected to the mirror holder, and the last scheme combines two above mentioned methods.

In this paper, we present a new technology to enhance the sensitivity by absorbing part of the light at the entry of the Z-scan system. Compared to the top-hat Z-scan, the curves of the modified top-hat Z-scan for the nonlinear refraction show higher peak-to-valley values, which mean higher sensitivity with our modified method. Nonlinear refraction of CS2 is investigated using this technique with 19ps pulses at wavelength of 532 nm.

Purpose: Compare of the efficiency of 1470nm laser and 1470nm laser heat head for tissue cutting in vitro porcine kidney tissue . Method: We designed a laser heat head that convert laser energy into thermal energy by the absorbing materials. Fresh kidney tissue was harvested from a porcine and then placed on a turntable with constant speed . The same power of 1470nm laser and 1470nm laser heat head was used to cutting tissue, respectively .The cutting results and the range of thermal damage was compared after cutting . Result: Compared with 1470nm laser, 1470nm laser heat head's cutting traces is more smooth and the thermal damage area is very regular ,so it has smaller damage to deep tissue . Conclusion: The efficiency of laser heat head for tissue cutting was better. This study indicate that we might be able to make laser which the tissue have a low absorption coefficient about it to obtain good results for tissue cutting through the laser point heat source.

Photoacoustic tomography (PAT) is a maturing imaging technique which combines optical excitation and acoustic detection to enable deep tissue sensing for biomedical applications. Optical absorption provides biochemical specificity and high optical contrast while ultrasonic detection provides high spatial resolution and penetration depth. These characteristics make PAT highly suitable as an approach for vascular imaging. However, standard testing methods are needed in order to characterize and compare the performance of these systems. Tissue-mimicking phantoms are commonly used as standard test samples for imaging system development and evaluation due to their repeatable fabrication and tunable properties. The multi-domain mechanism behind PAT necessitates development of phantoms that accurately mimic both acoustic and optical properties of tissues. While a wide variety of materials have been used in the literature, from gelatin and agar hydrogels to silicone, published data indicates that poly(vinyl chloride) plastisol (PVCP) is a promising candidate material for simulating tissue optical and acoustic properties while also providing superior longevity and stability. Critical acoustic properties of PVCP phantoms, including sound velocity and attenuation, were measured using acoustic transmission measurements at multiple frequencies relevant to typical PAT systems. Optical absorption and scattering coefficients of PVCP gels with and without biologically relevant absorbers and scatterers were measured over wavelengths from 500 to 1100 nm. A custom PAT system was developed to assess image contrast in PVCP phantoms containing fluid channels filled with absorbing dye. PVCP demonstrates strong potential as the basis of high-fidelity polymer phantoms for developing and evaluating PAT systems for vascular imaging applications.

To reduce the noise emitted by commercial aircraft turbofan engines, the inlet and aft nacelle ducts are lined with acoustic absorbing structures called acoustic liners. Traditionally, these structures consist of a perforated facesheet bonded on top of a honeycomb core. These traditional perforate over honeycomb core (POHC) liners create an absorption spectra where the maximum absorption occurs at a frequency that is dictated by the depth of the honeycomb core; which acts as a quarter-wave resonator. Recent advances in turbofan engine design have increased the need for thin acoustic liners that are effective at low frequencies. One design that has been developed uses an acoustic metamaterial architecture to improve the low frequency absorption. Specifically, the liner consists of an array of Helmholtz resonators separated by quarter-wave volumes to create a dual-resonance acoustic liner. While previous work investigated the acoustic behavior under normal incidence, this paper outlines the modeling and predicted transmission loss and absorption of a dual-resonance acoustic metamaterial when subjected to grazing incidence sound.

Optoacoustic sensing is a hybrid technique that combines the advantages of high sensing depth of ultrasound with contrast of optical absorption. In this study a miniature optoacoustic probe that can characterize the target properties located at the distal end of a catheter is investigated. The probe includes an optical fiber to illuminate the target with the pulsed laser light and a hydrophone to detect the generated optoacoustic signal. The probe is designed for the forwardsensing and therefore the acoustic signal propagates along the tube before being detected. Due to the circular geometry, the waves inside the tube are highly complex. A three dimensional numerical simulation is performed to model the optoacoustic wave generation and propagation inside the water filled cylindrical tubes. The effect of the boundary condition, tube diameter and target size on the detected signal is systematically evaluated. A prototype of the probe is made and tested for detecting an absorbing target inside a 2mm diameter tube submerged in water. The preliminary experimental results corresponding to the simulation is acquired. Although many different medical applications for this miniature probe may exist, our main focus is on detecting the occlusion inside the ventricular shunts. These catheters are used to divert the excess cerebrospinal fluid to the absorption site and regulate inter cranial pressure of hydrocephalous patients. Unfortunately the malfunction rate of these catheters due to blockage is very high. This sensing tool could locate the occluding tissue non-invasively and can potentially characterize the occlusion composites by scanning at different wavelengths of the light.

During an MR procedure, the patient absorbs a portion of the transmitted RF energy, which may result in tissue heating and other adverse effects, such as alterations in visual, auditory and neural functions. The Specific Absorption Rate (SAR), in W/kg, is the RF power absorbed per unit mass of tissue and is one of the most important parameters related with thermal effects and acts as a guideline for MRI safety. Strict limits to the SAR levels are imposed by patient safety international regulations (CEI - EN 60601 - 2 - 33) and SAR measurements are required in order to verify its respect. The recommended methods for mean SAR measurement are quite problematic and often require a maintenance man intervention and long stop machine. For example, in the CEI recommended pulse energy method, the presence of a maintenance man is required in order to correctly connect the required instrumentation; furthermore, the procedure is complex and requires remarkable processing and calculus. Simpler are the calorimetric methods, also if in this case long acquisition times are required in order to have significant temperature variations and accurate heat capacity knowledge (CEI - EN 60601 - 2 - 33). The phase transition method is a new method to measure SAR in MRI which has the advantages to be very simple and to overcome all the typical calorimetric method problems. It does not require in gantry temperature measurements, any specific heat or heat capacity knowledge, but only mass and time measurement. Nevertheless, in order to consider this method it is necessary to verify that all deposited SAR power can be considered acquired and measured. In this paper considerations about this aspect are conducted.

Moisture ingression creates huge concerns on fiber reinforced composites because of the reduction in mechanical strengths with the moisture absorption and additional weight gains on the aircraft. If the appropriate techniques are used to predict the moisture ingression in the laminate composite structures, it will be of an immense advantage to detect moisture contents and their effects on the properties of the composites. Mechanisms of moisture influence in the composite structures also need to be thoroughly understood for a better design of new structural composites. This research study aims at investigating the effectiveness of different non-destructive inspection (NDI) techniques to detect moisture absorption. Carbon and glass fiber reinforced composite panels were prepared and used during the moisture ingression tests. Different NDI techniques, including ultrasonic through transmission and pulse echo, phase array, bondmaster and digital tap hammer were chosen to determine the moisture ingression levels. Throughout the tests, initial and final weights of the composite panels were determined using the high accuracy balances. NDI scanning parameters were kept constant for the tests. The effectiveness of each technique was determined and their results were discussed in detail.

Photoacoustic imaging has been recently developed for biomedical imaging. This imaging technique is based on the photoacoustic effect, which includes a process involving the absorption of photons, the subsequent thermal expansion, and propagation of photoacoustic waves. The propagation of photoacoustic waves has been modeled by using linear acoustic theories although the generated photoacoustic waves are naturally shock waves. In this work, the propagation of photoacoustic shock waves are studied by using nonlinear acoustic wave solutions based on Hugoniot’s shock relation combining Earnshaw solution with Poisson solution. The non-linear solution is compared with the existing linear solution using the propagating waveforms for spherical wave. The simulation results show a discrepancy between the two solutions.

Ultrasound-induced inertial cavitation is a mechanical process used for site-localized therapies such as non-invasive surgery. Initiating cavitation in tissue requires very high intensity focused ultrasound (HIFU) and low-frequencies. Hence, some applications like thrombolysis require targeted contrast agents to reduce peak intensities and the potential for secondary effects. A new type of theranostic nanoemulsion has been developed as a combined ultrasound (US)/photoacoustic(PA) agent for molecular imaging and therapy. It includes a nanoscale emulsion core encapsulated with a layer of gold nanospheres at the water/ oil interface. Its optical absorption exhibits a spectrum broadened up to 1100 nm, opening the possibility that 1064 nm light can excite cavitation nuclei. If optically-excited nuclei are produced at the same time that a low-frequency US wave is at peak negative pressure, then highly localized therapies based on acoustic cavitation may be enabled at very low US pressures. We have demonstrated this concept using a low-cost, low energy, portable 1064 nm fiber laser in conjunction with a 1.24 MHz US transducer for simultaneous laser/US excitation of nanoemulsions. Active cavitation detection from backscattered signals indicated that cavitation can be initiated at very low acoustic pressures (less than 1 MPa) when laser excitation coincides with the rarefaction phase of the acoustic wave, and that no cavitation is produced when light is delivered during the compressive phase. US can sustain cavitation activity during long acoustic bursts and stimulate diffusion of the emulsion, thus increasing treatment speed. An in vitro clot model has been used to demonstrate combined US and laser excitation of the nanoemulsion for efficient thrombolysis.

We will report newly developed polymer-based phantom for photoacoustic (PA) imaging systems. Phantoms are important for performance evaluation and calibration of new modalities; however, there is no established method for making phantoms with no long-term change. We have developed skin mimicking phantoms simulating both optical and acoustic properties (i.e. optical scattering and absorption coefficients, and sound velocity). Furthermore, the phantoms are able to give accurate simulation of blood vessels by Inkjet-printing. Newly developed phantoms are consisted of castor oil included acrylic block copolymer and we can fabricate 0.8mm or less thick sheets and pile them using their self-adhesiveness.

A novel methodology is presented to mimic diffuse reflectance spectra of arbitrary biological tissues in the visible and near-infrared ranges. The prerequisite for this method is that the spectral information of basic components is sufficient to mimic an arbitrary tissue. Using a sterile disposable fiber optic probe the diffuse reflectance spectrum of a tissue (either in vivo or ex vivo) is measured, which forms the target spectrum. With the same type of fiber probe, a wide variety of basic components (ingredients) has been previously measured and all together forms a spectral database. The validity of the principle is demonstrated by mimicking an arbitrary mixture of components. The method can be applied with different kinds of materials, e.g. gelatins, waxes and silicones, thus providing the possibility of mimicking the mechanical properties of target tissues as well. The algorithm can be extended from single point contact spectral measurement to contactless multi-and hyper-spectral camera acquisition. It can be applied to produce portable and durable tissue-like phantoms that provides consistent results over time for calibration, demonstration, comparison of instruments or other such tasks. They are also more readily available than living tissue or a cadaver and are not so limited by ease of handling and legislation; hence they are highly useful when developing new devices. © 2014 SPIE.

Photoacoustic computed tomography (PACT) holds great promise for transcranial brain imaging. However, the strong reflection, scattering and attenuation of acoustic waves in the skull present significant challenges to developing this method. We report on a systematic computer-simulation study of transcranial brain imaging using PACT. The goal of this study was to identify an effective imaging system design that can be translated for clinical use. The propagation of photoacoustic waves through a model skull was studied by use of an elastic finite-difference time-domain (FDTD) method. The acoustic radiation pattern from a photoacoustic source just beneath the skull was observed with a ring transducer array that was level with the source. The observed radiation pattern was found to contain stronger contributions from waves that were converted to shear waves in skull than longitudinal waves that did not undergo mode conversion. Images reconstructed from the pressure data that contain shear wave components possess better resolution than images reconstructed from the data that only contain the longitudinal wave signals. These observations revealed that the detection system should be designed to capture photoacoustic signals that travel through the skull in the form of shear waves as well as in the form of longitudinal waves. A preliminary investigation on the effect of the presence of absorption in the skull is also reported. This study provides an insight into the wave phenomena in transcranial PACT imaging, as well as a concrete detection design strategy that mitigates the degraded resolution of reconstructed images.

In vivo optical imaging of biological tissue not only requires the development of new theoretical models and experimental procedures, but also the design and construction of realistic tissue-mimicking phantoms. However, most of the phantoms available currently in literature or the market, have either simple geometrical shapes (cubes, slabs, cylinders) or when realistic in shape they use homogeneous approximations of the tissue or animal under investigation. The goal of this study is to develop a non-homogeneous realistic phantom that matches the anatomical geometry and optical characteristics of the mouse head in the visible and near-infrared spectral range. The fabrication of the phantom consisted of three stages. Initially, anatomical information extracted from either mouse head atlases or structural imaging modalities (MRI, XCT) was used to design a digital phantom comprising of the three main layers of the mouse head; the brain, skull and skin. Based on that, initial prototypes were manufactured by using accurate 3D printing, allowing complex objects to be built layer by layer with sub-millimeter resolution. During the second stage the fabrication of individual molds was performed by embedding the prototypes into a rubber-like silicone mixture. In the final stage the detailed phantom was constructed by loading the molds with epoxy resin of controlled optical properties. The optical properties of the resin were regulated by using appropriate quantities of India ink and intralipid. The final phantom consisted of 3 layers, each one with different absorption and scattering coefficient (μ a ,μ s ) to simulate the region of the mouse brain, skull and skin.