Full-sensitivity depth-resolved measurements of displacement fields inside weakly scattering materials using wavelength scanning interferometry

Optical interferometers are widely used in the measurement of micro- and nanoscale surface topography. However, their accuracy and resolution can be seriously affected by environmental noise. We present a multi-mode interferometric measurement system based on wavelength modulation and active vibration resistance. This supports two measurement modes: wavelength-scanning interferometry, which is suitable for structured surfaces, and wavelength-tuning interferometry, which is suitable for smooth continuous surfaces. In addition, the system can measure the optical path difference of the current position in real time, which is convenient for making system adjustments and improving the measurement accuracy. The proposed system was used to measure 1.806 µm and 43.2 nm step height standards. Under different degrees of vibration, the measured heights in the two modes agreed well with the calibrated values.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping is a rapidly expanding field in the geosciences because of the wealth of spatial information it provides on the petrogenesis of igneous, metamorphic and sedimentary rocks and ore mineral formation. The technique has also opened many new applications in forensics, archaeology and the imaging of trace metals in biological tissues. Instrumentation advances in both LA and ICP-MS systems now permit precise isotopic analysis with laser spot sizes of <10 μm and sub-ppm detections limits. In particular LA-quadrupole (Q)-ICP-MS systems facilitate mapping of large numbers of elements across nearly the entire mass range of the periodic table. A range of key applications in petrogenesis studies and in the fields of environmental and biological sciences is reviewed. Furthermore, technical considerations including a practical guide for setting up LA-ICP-MS multi-element mapping experiments and the latest innovations in dedicated data reduction and image processing packages for the visualization, interrogation and extraction of quantitative data from LA-ICP-MS maps are discussed, with particular emphasis placed on quadrupole systems. A key issue is imaging artefacts (spectral skew) caused by interaction between the laser repetition rate and the total sweep cycle time, particularly when coupling a sequential Q-ICP-MS analyser to modern low dispersion (fast-washout) LA cells. Running at high repetition rates is recommended for low-dispersion cells as it enables faster scanning while minimizing temporal variations in signal intensity caused by pulsing of the laser. Advances in LA-ICP-MS instrumentation and methodologies (data processing, visualization and extraction) have enabled simultaneous acquisition of compositional and U-Pb age information at high-spatial resolution which has great potential in geochronological studies. Formation histories of complex polyphase accessory minerals such as zircon, titanite or monazite, where discrete age domains are often linked with specific rock-forming processes and their physical conditions (i.e. U-Pb petrochronology), are now directly accessible. Moreover, fine-scale processes affecting the U-Pb systematics of accessory minerals can be constrained when LA-ICP-MS mapping is combined with other textural (e.g. CL or Raman) imaging techniques. U-Pb analysis of materials with variable initial Pb concentrations and/or heterogeneous genetic origin such as carbonates also benefit from such a mapping approach as areas with sufficient spread in 238U/204Pb ratio (μ) can be targeted, while simultaneous imaging of diagnostic trace elements allows identification and exclusion of zones affected by alteration or detrital contamination. The quasi-simultaneous detection offered by LA-time-of-flight (TOF)-ICP-MS allows rapid multi-element analysis of very fast transient signals (i.e. laser ablation imaging in low dispersion laser ablation cells). The approach is ideally suited to 2D and 3D imaging of biological and geological materials and will likely replace LA-Q-ICP-MS for such applications once more routinely available.

Photoacoustic (PA) imaging is an emerging technology that could map the functional contrasts in deep biological tissues in high resolution by “listening” to the laser induced thermoelastic waves. Almost all of the current studies in PA imaging are focused on the intensity of the PA signals as an indication of the optical absorbance of the biological tissues. Our group has for the first time demonstrated that the frequency domain power distribution of the broadband PA signals encode the texture information within the regions-of-interest (ROI). Following the similar method of ultrasound spectral analysis (USSA), photoacoustic spectrum analysis (PASA) could evaluate the relative concentrations and, more importantly, the dimensions of microstructures of the optically absorbing materials in biological tissues, including lipid, collagen, water and hemoglobin. By providing valuable insights into tissue pathology, PASA should benefit basic research and clinical management of many diseases, and may help achieve eventual “noninvasive biopsy”. In this work, taking advantage of the optical absorption contrasts contributed by lipid and hemoglobin at 1200-nm and 532-nm wavelengths respectively, we investigated the capability of PASA in identifying histological changes corresponding to fat accumulation livers through the study on ex vivo and in situ mouse models. The PA signals from the mouse livers were acquired using our PA and US dual-modality imaging system, and analyzed in the frequency domain. After quantifying the power spectrum by fitting it to a first order model, three spectral parameters, including the intercept, the midband fit and the slope, were extracted and used to differentiate fatty livers from normal livers. The comparison between the PASA parameters from the normal and the fatty livers supports our hypotheses that PASA can quantitatively identify the microstructure changes in liver tissues for differentiating normal and fatty livers.

A modification of a dual-beam illumination arrangement for electronic speckle pattern interferometry (ESPI) that leads to a twofold increase in the sensitivity to in-plane displacements is described. For each of the two illumination beams, the scattered light is observed along the direction of illumination, and these two components of the scattered light are then combined. The error in perspective caused by oblique observation can be almost corrected for by using two right-angle prisms in front of the object being studied. Experimental results including phasestepping are presented and compared with the results predicted by theory.

The probability distribution of aperture averaged signal intensity from free-space laser optical systems depends on the optical turbulence along the optical path. For most current free-space laser systems, random fluctuations of signal intensity are assumed to be statistically homogeneous and isotropic. Moreover, it is assumed that probability distributions are generally log-normal and that Kolmogorov -5/3 wave number turbulence power law represents the signal intensity data. In this paper, the Kolmogorov model is investigated for an optical path that traverses a complex non-uniform topography. Experimental research is conducted to determine the characteristic behavior of high frequency (2000 Hz) signal intensity data collected over a 2.33 km optical path at the Army Research Laboratory (ARL) Atmospheric Laser Optics Testbed (A_LOT) Facility. Results focus mainly on calculated power spectra and frequency distributions. In addition, a model is developed to calculate optical turbulence intensity (C2/n) as a function of receiving (and transmitting) aperture diameter, log-amplitude variance, and path length. Here, initial comparisons of calculated to measured C2/n data are favorable. It is anticipated that this kind of signal data analysis will benefit laser communication systems development and testing at the ARL.

The principle and instrumental structure of dispersion Hadamard transform spectral imager were briefly described in the present paper, and the disadvantages of the imager both in dislocation of spatial and spectral information and in spectral resolution limited by the width of Hadamard encoding mask were pointed out. A new instrumental principle and design of spatially modulated interference Hadamard transform spectral imager was proposed. A lateral shearing interferometer was used to acquire interference signals of all the Hadamard encoding information at different optical path difference. Then the methods of Fourier transform and Hadamard transform for interferogram were performed to acquire the spectra of objectives. Theory analysis of this imager demonstrated that the modulation of interferogram would not be affected by some factors such as the form and size of Hadamard encoding mask, and the spectral resolution would not be influenced by the size of Hadamard encoding mask. Furthermore, such technique not only effectively eradicated the dislocation of spatial information and spectral information existing in dispersion Hadamard transform spectral imager, but also made it convenient to image with high-throughput, high spatial resolution and high spectral resolution.

P258] Optical absorbance properties of PVA-GTA Fricke gel dosimeters irradiated with 6 MV and 15 MV X-rays

High-resolution optical coherence tomography (OCT) is of critical importance to disease diagnosis because it is capable of providing detailed microstructural information of the biological tissues. However, a compromise usually has to be made between its spatial resolutions and sensitivity due to the suboptimal spectral response of the system components, such as the linear camera, the dispersion grating, and the focusing lenses, etc. In this study, we demonstrate an OCT system that achieves both high spatial resolutions and enhanced sensitivity through utilizing a spectrally encoded source. The system achieves a lateral resolution of 3.1 μm and an axial resolution of 2.3 μm in air; when with a simple dispersive prism placed in the infinity space of the sample arm optics, the illumination beam on the sample is transformed into a line source with a visual angle of 10.3 mrad. Such an extended source technique allows a ~4 times larger maximum permissible exposure (MPE) than its point source counterpart, which thus improves the system sensitivity by ~6dB. In addition, the dispersive prism can be conveniently switched to a reflector. Such flexibility helps increase the penetration depth of the system without increasing the complexity of the current point source devices. We conducted experiments to characterize the system’s imaging capability using the human fingertip in vivo and the swine eye optic never disc ex vivo. The higher penetration depth of such a system over the conventional point source OCT system is also demonstrated in these two tissues.

Background: Magnetic resonance imaging (MRI) is an excellent noninvasive technique to evaluate the female pelvis due to its high spatial resolution, excellent tissue contrast, and multiplanar imaging capability. Radiologists often evaluate a wide range of pelvic masses during routine clinical practice. Objective: Aim and objective of the study is to evaluate the importance of MRI in the pelvic masses and to characterize the MRI findings in female pelvic masses. Materials and Methods: We prospectively performed MRI in our institute in 58 patients which detected pelvic pathologies or pelvic masses on clinical examination and/or ultrasonography. These lesions were examined for size, shape, signal intensity, character, and postcontrast enhancement. Secondary signs such as ascites, lymphadenopathy, and invasion of the adjacent organs were observed. Results: MRI was significantly superior to ultrasound in the evaluation of the tumor extension, myometrium invasion, detection of lymph nodes, and parametrical involvement anorectal carcinoma. Overall accuracy for detection of lesion by MRI was 97%, and sensitivity for the diagnosis of malignancy was 94%. Conclusion: MRI is an excellent investigation to evaluate the female pelvic masses due to its high spatial resolution, excellent tissue contrast, and multiplanar imaging capability. It is a very good modality for the tumor staging and follow-up of the cases. Certain key imaging features of uterine or adnexal pathologies on MRI are helpful in the specific diagnosis or narrow down the differential diagnosis. Characterization of uterine and ovarian tumors helps in the surgical planning.

CHAPTER 4 - SOURCES OF CONTRAST AND PULSE SEQUENCES

Sinusoidal wavelength-scanning (SWS) interferometry is unique in that a time-varying interference signal contains phase-modulation amplitude Zb due to the SWS besides a conventional phase &agr;. A rough value of an optical path difference (OPD) longer than a wavelength is obtained from the amplitude Zb, which is useful for shape measurements of rough surfaces. By combing the two values of Zb and &agr;, an OPD longer than a wavelength can be measured with a high accuracy of the order of nanometer, which is useful for shape measurements of optical surfaces. As light sources of SWS interferometers a tunable laser diode with an external cavity is used and a superluminescent diode (SLD) is used with an acousto-optic tunable filter. A shape of a metal surface, a step shape of an optical surface, and a thin-film shape of optical surfaces are measured.

The truncation artifact in magnetic resonance (MR) images is a line of abnormal signal intensity that occurs parallel to an interface between tissues of markedly different signal intensity. In order to demonstrate the truncation artifact in sagittal images of the canine spinal cord and the effect of changing spatial resolution, we conducted an experimental in vitro study. A section of fixed canine spinal cord was imaged using a 1.5T magnet. Spatial resolution was increased by increasing the acquisition matrix and reconstruction matrix, producing series of T2-weighted (T2w) images with the following pixel sizes: A, 1.6 (vertical) × 2.2 mm2 (horizontal); B, 1.2 × 1.7 mm2 ; C, 0.8 × 1.1 mm2 ; D, 0.4 × 0. 6 mm2 . Plots of mean pixel value across the cord showed variations in signal intensity compatible with truncation artifact, which appeared as a single, wide central hyperintense zone in low-resolution images and as multiple narrower zones in high spatial resolution images. Even in images obtained using the highest spatial resolution available for the MR system, the edge of the spinal cord was not accurately defined and the central canal was not visible. The experiment was repeated using an unfixed spinal cord specimen with focal compression applied to mimic a pathologic lesion. Slight hyperintensity was observed within the spinal cord at the site of compression although the cord was normal histologically. Results of this study suggest that caution should be applied when interpreting hyperintensity affecting the spinal cord in T2w sagittal images of clinical patients because of the possibility that the abnormal signal could represent a truncation artifact.

This study aimed to compare the imaging characteristics and diagnostic utility of Time-Resolved Imaging of Contrast Kinetics (TRICKS) Magnetic Resonance Angiography (MRA) and Computed Tomography Angiography (CTA) for assessing intracranial and extracranial vascular structures in veterinary patients. This retrospective methods comparison study included nine client-owned dogs and one cat with head and neck tumors, all of which underwent both CTA and TRICKS MRA. A qualitative assessment of major intracranial and extracranial vessels, as well as tumorassociated vessels-including feeding and draining vessels-was performed. In addition, the signal intensity (SI) of the vessels was quantitatively measured. Both imaging techniques provided similar SI measurements, although CTA demonstrated significantly higher SI in the basilar artery. CTA also offered higher visibility and clearer delineation of certain arteries and veins, with a significantly shorter acquisition time. However, TRICKS MRA demonstrated comparable or superior performance in visualizing venous structures and effectively identified tumor-related vessels, showing similar diagnostic performance to CTA in detecting feeding and draining vessels. Additionally, TRICKS exhibited a distinct advantage in differentiating vessels from surrounding bone, effectively reducing bone-related artifacts that can interfere with vascular delineation in CTA. TRICKS MRA is a valuable imaging modality for vascular assessment, particularly for evaluating venous structures and tumor-associated circulation. While CTA remains superior for arterial imaging due to its higher spatial resolution and shorter scan time, TRICKS MRA enables dynamic vascular assessment with reduced dependency on contrast injection timing. These findings suggest that TRICKS MRA could serve as a complementary imaging modality in veterinary neuro-oncology, especially for surgical and radiation therapy planning.

Radioactive waste management agencies all around the world are focusing their efforts to provide a reliable technical solution to the Deep Geological Disposal (DGD) concept, in order to store high level (HLW) and intermediate level long lived (IL-LLW) radioactive waste. The protection and isolation of radionucleides is guaranteed by a set of barriers. Bentonite/sand blocks -in proportion of 40/60 on dry mass of the mixture- constitute the main core of the sealing barrier in the French concept run by the French National Radioactive Waste Management Agency (Andra). Its high swelling potential (&gt;2MPa) during hydration, its low hydraulic permeability (&lt;10-11 m/s) and its gas entry pressure (&lt;2MPa) make it ideal to seal the DGD galleries excavated inside the host rock used as geological barrier. Bentonite/sand blocks are placed at unsaturated state during construction. Afterwards, water migration –in both liquid and vapour states- from the saturated host rock to the blocks will induce the humidification of the blocks until hydraulic equilibrium is reached, hypothetically reaching the saturation state. As this natural process will take hundreds of years, a multitude of experimental devices have been reported at laboratory [3, 5] and real scale [4] to understand the evolution of water transfer in bentonite mixtures and support the predictions of numerical models [1]. In laboratory experiments, samples made of a compacted bentonite mixture are usually placed inside a rigid cell connected to a water source. The infiltration of water is not measured directly but deduced by relative humidity gradients measured along the specimen height.&#x0D; Numerical models and experimental results have not yet reached accurate predictions of how long it takes for bentonite mixtures to hydrate until complete saturation or how the unsaturated hydraulic permeability evolves. In this context, there is a motivation to study this phenomena by advanced techniques able to capture, image and quantify water transfer through porous media. Thus, an experimental campaign based on Magnetic Resonance Imaging (MRI) and 3D X-Ray micro computed tomography (µCT) is proposed to analyze water infiltration in bentonite/sand blocks. MRI is extensively used in health sciences to create images where the contrast is given by the water’s hydrogen proton content in the different tissues of the body. For geological media, where water content is much smaller than in biological tissues and relaxation times of the porewater are shorter -from 0.1 to 2 ms for compacted bentonite/sand [2]- the experimental requirements are different and less documented. On the other hand, µCT scanning provides high spatial resolution images based on the contrast between the X-Ray absorption properties of the constitutive phases observed. Its temporal variations, in a bentonite/sand mixture, might be the result of concurrent phenomena, namely, porosity and saturation changes together with heterogenous motions induced by hydromechanical couplings -the swelling of bentonite solids when they absorb water-.&#x0D; On going MRI and µCT experimental campaigns on a PEEK cell aim at proving the suitability of these techniques to track the evolution of water infiltration in compacted bentonite/sand mixtures, characterizing the water front and the water content along the specimen in function of time. At this stage, preliminary MRI tests have shown the potential of Single Point Imaging (SPI) sequence to quantify, with one-dimensional images, the water content along the bentonite/sand specimen based on a linear correlation with its signal intensity. Preliminary X-Ray radiography tests (Figure 1) run on a cylindric cell of PMMA filled with small blocks of 12 mm in diameter and 10 mm high, prove that X-Ray radiography image analysis gives access to observable and quantifiable increase of absorption associated to the measured decrease in grey level at different transversal sections of the sample. This evolution is linked to the increase of apparent density of the solids -that indeed are absorbing the water-, as described by the Beer-Lambert equation. Even though apparent density increase is goverened by the augmentation of water content, changes in porosity due to the hydromechanical coupling along the sample cannot be neglected. Thus, observing the liquid phase evolution (hydraulics) by MRI, as well as the evolution of the solid/liquid phase (hydromechanics) by 3D-µCT imaging, will potentially provide complementary insight into the mechanisms governing water infiltration in bentonite/sand mixtures.

Frequency‐wavenumber integration is used to compute time‐domain sonic waveforms at any point in a stratified medium. The source is either a point force, or a point dislocation, at any depth. The medium consists of welded homogeneous layers with all 21 elastic constants varying independently from layer to layer. Currently, a free‐surface boundary condition at the top of the medium and a radiation condition at the bottom is used, but these boundary conditions can be easily changed. Although final results are in the time domain, the basic calculations are carried out in the frequency domain so anelastic effects are incorporated by use of complex elastic constants. (Usually, instead of directly specifying the elastic constants in each layer, Schoenberg algorithms were used to “fracture” a layer or to “construct” a layer from many thin microlayers of simpler material. These algorithms have a group property [Schoenberg and Muir, 1988] which makes it easy to generate fractured laminates.) For elastic materials with azimuthal anisotropy, two wavenumber integrations are required. If the medium is many wavelengths thick, and the source and receiver(s) are many wavelengths apart, then several hours of Cray CPU time are needed to generate a complete response. [Work supported by Geo‐Pacific Corporation.]