Scanning Laser Acoustic Microscope Research Articles

Glutathione As Biomarker for Neurodegenerative Diseases

Brain imaging is one of the main obstacles of modern disease diagnostics since it must allow accurate and quantitative measurements on a living organ that is placed in adults (in neonatal humans’ skull penetration is somewhat easier) inside a bone cage. Inside is the brain, the main function controller of our body that must be done in a non- invasive way on the living operation brain. Many spectroscopic methods are trying to overcome the difficult access to the brain: Photoacoustic microscopy, Confocal microscopy, Two-photon microscopy, Optical coherence tomography, Scanning Laser Acoustic Microscopy, Acoustic microscopy, Ultrasonography, positron tomography, fluorescence methods, photo caustic microscopy and multi (two) photon imaging spectroscopy. The situation is complex since the brain is wrapped in the blood brain barrier allowing only selected molecules to pass from the blood stream to and out of the brain. In this chapter, we will survey the current situation of brain diagnostics with the aid of the spectroscopic methods. Brain research is integrated in aging research as a major area of interest. Aging is in most cased coupled with the loss of brain function and dementia. The neurodegenerative diseases, although identified by Dr. Alzheimer and his collaborators more than a century ago, continue as the leading causes of mortality among the elderly. Brain research in trying to give hope to those people but unfortunately our understanding in this area is limited. Oxidative stress contributes to neurodegenerative diseases pathophysiology and progression. The target was to describe central and peripheral metabolites of redox metabolism and to describe correlations between glutathione status, age, and disease severity.

Resolution enhancement by tilted detection in the scanning laser acoustic microscope

We propose a variant for the optical method of detection currently used in the scanning laser acoustic microscope. The proposed method consists in tilting the stage that contains the immersed object under test and coverslip with respect to the direction of the probe beam. We present a mathematical analysis of the optical process of detection. Additionally, we include computer simulations that show that the imaging resolution in the optical detection process can be improved by using our method.

Cavitation asymmetry in silicon nitride by scanning laser acoustic microscopy

Scanning laser acoustic microscopy (SLAM) was used to visualize creep damage distribution in the gas-pressure-sintered silicon nitride after creep at 1300°C in 4-point bending. SLAM revealed asymmetrical distribution of creep damage beneath the tensile surface and in a narrow zone that spread continuously across the neutral axis toward the compression surface. Scanning and transmission electron microscopy confirmed the presence of cavities in tensile zone of bending bar. The combination of the homogeneous cavity development in the zone of tensile stresses and formation of the damage zone ahead of main crack was proposed to explain such cavity distribution. Current SLAM observation is a direct evidence of cavitation asymmetry in vitreous bonded ceramics. Such distribution supports the model of simultaneous presence of cavitation and non-cavitation creep mechanisms in silicon nitride and similar ceramics.

Comparative study of optical detection for scanning laser acoustic microscopy

Scanning laser acoustic microscopy (SLAM) provides a way of visualizing and measuring surface features of nanometric order by a combination of acoustical and optical means. The acousto-optical interface in SLAM is an acoustically transparent and optically reflecting surface embedded in the acoustic medium. Acoustical excitation causes a dynamic surface deformation which is picked up by a focused scanning laser beam which, after reflection, is modulated in its phase and angle of reflection. There are three basically different demodulation schemes reported in the literature: the knife-edge detector, the time-delay interferometric detector, and the reference-beam detector. (In addition, a new detector derived from the knife-edge detector, namely the pyramidal-mirror detector, is scheduled for separate treatment in this session.) This paper presents comparative studies of the above three basic methods of detection. The performance characteristics of each are analyzed in detail by a careful examination of the transfer function for each of the systems. Computer simulations for their operation provide a comprehensive picture of the strengths and weaknesses of each.

Scanning tomographic acoustic microscopy

This paper provides an overview of the design and development of the scanning tomographic acoustic microscopy (STAM). This research effort spans over a period of more than 12 years, which successfully elevated the acoustic microscopy from the traditional intensity-mapping mode to the level of holographic and tomographic imaging. The tomographic imaging capability of STAM was developed on the platform of the scanning laser acoustic microscope (SLAM), which operates in a coherent transmission mode with plane-wave illumination and scanning laser wavefield detection. The image formation techniques were based on the backward propagation method implemented in the plane-to-plane format. In this paper, the key elements of the design and development, including the modification of the data-acquisition hardware, implementation of image reconstruction algorithms for multiple-frequency and multiple-angle tomography, and the high-precision phase-correction and image registration techniques for the superposition of coherent sub-images, will be discussed. Results of full-scale experiments will also be included to demonstrate the capability of holographic and tomographic image formation in microscopic scale.

Glass–rubber phase transformation detected in polymers by means of ultrasonic waves

Ultrasonic attenuation and velocity measurements were carried out in order to study the kinetics of water sorption process in hydrogels characterized by strong structural changes occurring in the material. Hydrogel, or gel contains water, can exist in two forms: a solid glassy phase when it is dry, a rubber phase in equilibrium with water. Scanning laser acoustic microscope (SLAM) technique has been used to monitor the change in the ultrasonic attenuation, during water sorption in crosslinked poly(hydroxyethylmethacrylate) and poly-vinyl alcohol (PVA) hydrogels of different thickness at the frequencies of 10 and 30 MHz. The pulse-echo technique has been applied to the measurement of the longitudinal velocity and ultrasonic attenuation and to monitor the advancement of the swollen–unswollen fronts. During the hydrogel water sorption a peak in the ultrasonic attenuation and a decrease in the longitudinal velocity have been detected. The increment in the ultrasonic attenuation has been analysed in terms of reflections from the swollen–unswollen boundary, microvoids scattering and absorption of the ultrasonic waves due to the glass to rubber transformation.

Microscopy

Microscopy

The pyramidal-mirror detector for scanning laser acoustic microscopy

The pyramidal-mirror detector has important advantages over the knife-edge detector currently in use in scanning laser acoustic microscopy. A key element of this new detector is a four-sided pyramid with mirrored surfaces. In the operation of the microscope, the zero-order component of the light of the scanning laser beam that has been reflected from the microscope's coverslip is centered on the tip of the pyramid. Thus, each mirror face reflects one fourth of the light in this component. Making use of an arrangement of four photodetectors, one for each of the four faces of the pyramidal mirror, the light reflected from each face goes to a different photodetector and is processed separately from the light reflected from the other faces. This detector has an almost isotropic transfer function with no negative responses and fourfold symmetry around a null point in the center which coincides with the zero-frequency point in the spatial spectrum. This property makes it possible to detect spatial frequencies in all directions simultaneously with almost equal sensitivity. Oblique insonification, required for best operation with the knife-edge detector because of its region of negative response, is not preferred in the pyramidal-mirror detector and double side-band detection can be employed. Since there is no requirement that a negative response region be avoided, the spatial spectrum of this detector can be more than twice that of the knife-edge detector. The tighter the focusing of the scanning laser beam onto the coverslip, the higher the spatial frequencies detectable in the object. In fact, if the coverslip occupies a position in the very-near field of the object, evanescent-wave detection is possible. Low-frequency, highly penetrable ultrasound can be used and at the same time high spatial frequencies can be detected for obtaining high resolution. © 1999 John Wiley & Sons, Inc. Int J Imaging Syst Technol 10, 323–327, 1999

Reference-beam detection for scanning laser acoustic microscopy

Reference-beam detection is inherently superior to the knife-edge detection currently in use in scanning laser acoustic microscopy. This new detector makes use of a reference beam, retarded 90 degrees, which is mixed coherently in a photodiode with the acoustically obtained image-modulated beam. The new detector has an isotropic transfer function which is circularly symmetrical around its highest value, namely the zero-frequency point in the spatial spectrum. This property makes it possible to detect spatial frequencies in all directions simultaneously and with equal sensitivity and simplifies the associated electronics. It also makes possible the employment of acoustic evanescent-wave detection so that ultrasound of low temporal frequency can be used and at the same time high spatial frequencies can be detected for obtaining high resolution. Oblique insonification, required for best operation in the knife-edge detector, is thus not preferred in the reference-beam detector and the resultant Doppler shift in the detected frequency of the transmitted zero-order acoustic waves is avoided.

Tomographic scanning acoustic microscope system

In this paper, a new type of acoustic microscope system using the tomographic image processing technique of STAM (scanning tomographic acoustic microscope) and operation principles of the transmission-mode and detection technique of SAM (scanning acoustic microscope system) is proposed. Since SAM and scanning laser acoustic microscope system (SLAM) used in the past were operated at the reflection mode, these systems could not obtain high-resolution tomographic images. Although STAM could process tomographic images, due to the detection-using laser, problems such as complex detection hardware, alignment, and low S/N ratio occur. If the tomographic image processing technique is used for ultrasonic microscope systems, the axial resolution can be enhanced and it can help and work together with the existing systems to detect cracks that are difficult to find in the reflection mode. These experimental results will be used for various ultrasonic microscope tomographic image processing and ultrasonic measurements.

Ultrasonic characterization of water sorption in poly(2-hydroxyethyl methacrylate) hydrogels

A complex mechanism characterizes the water uptake kinetics in hydrogels, as a consequence of the strong structural changes occurring in the material during the sorption process. The water sorption involves the transformation of a glassy, moderately crosslinked polymer in a rubbery material. In this study, the changes in the ultrasonic attenuation and velocity in crosslinked poly(2-hydroxyethyl methacrylate) [poly(HEMA)] hydrogel films during water sorption are measured by scanning laser acoustic microscopy (SLAM) and a pulse-echo system. In particular, the pulse-echo technique provides additional valuable information, thanks to its capability for monitoring the position of the swollen/unswollen fronts during water sorption. The evolution of the attenuation observed by SLAM is analyzed in terms of reflections on macroscopic discontinuities and absorption mechanisms. Finally, the propagation of ultrasonic waves acts as a dynamic mechanical test of the material; and, therefore, the measured longitudinal velocity and ultrasonic attenuation are applied to the calculation of the storage bulk longitudinal modulus of the poly(HEMA) hydrogels during water sorption.

Characterization and imaging of SAW grooved transducer acoustic field by SLAM

Quantitative characterization of the surface acoustic field radiated forward, backward, and in orthogonal directions by the grooved transducer, implemented onto a glass substrate, is accomplished by a scanning laser acoustic microscope (SLAM) operating at 31 MHz. The surface acoustic wave (SAW) amplitude is measured as a function of the number of grooves and the position of the bulk acoustic wave (BAW) transducer. The number of the operating grooves efficiently contributing to the SAW response is estimated. Remarkable reflection of the SAW beam between two identical gratings is observed.

Automation and system integration of Scanning Tomographic Acoustic Microscope

The Scanning Tomographic Acoustic Microscope (STAM) is an instrument capable of performing subsurface imaging of microscopic specimens. Designed around the Scanning Laser Acoustic Microscope (SLAM), the STAM incorporates numerous hardware and software advances which allow automated imaging of the acoustic properties of thick specimens at high resolution. With these modifications the STAM is shown to be capable of image formation for tissue characterization and nondestructive evaluation (NDE) of microscopic specimens. This paper describes the design and operation of the STAM hardware, and software algorithms developed to permit high resolution imaging. Examples are provided to demonstrate the capability of multiple angle and multiple frequency tomographic reconstructions.

Acoustic Imaging of Defects in Flexible Food Packages

A study was conducted using a high-frequency acoustic imaging system: the scanning laser acoustic microscope (SLAM), operating at 100 MHz, to detect packaging defects to within the system's resolution limit of 20 μm. The purpose of the study was to assess the feasibility of high-frequency acoustic imaging to detect and classify channel defects that would have the potential for microbial contamination through visually undetected defects. The SLAM can characterize and image various materials and defects by exploiting the differences in acoustic (mechanical) transmission properties within different materials. Channel defects transverse to the heat-seal major axis were fabricated by sandwiching 10-, 16-, 25-, and 37-μm wire between two layers of either polyethylene or plastic retort-pouch laminate film which were then heat sealed. The wire was then pulled out, leaving a channel filled variously with saline solution, air, or both. The channel defects were then assessed using the SLAM and validated with confocal microscopy. The results indicate that the SLAM technology can readily detect channel defects as small as 10 μm, the smallest channel defects examined, which is one-half the imaging system stated resolution specification. This study has clearly demonstrated that acoustic microscopy can nondestructively image micrometer-scale channel defects in heat seals at and smaller than the SLAM's resolution limit.

Non-destructive characterization of hydrogels.

Hydrogels have been prepared by a freezing-thawing procedure and investigations made of the effect of both number of freezing-thawing cycles and different content of hyaluronic acid (HA) on the mechanical properties of the PVA-HA hydrogels using non-destructive testing. The bulk elastic modulus K of hydrogels has been determined by pulse-echo measurements. It is noted that hydrogel elastic properties improve with the number of the cycles in PVA-HA 100/0; on the other hand samples with a high HA (1,000,000 molecular weight) content, beyond the third cycle, seem to be unaffected by the number of cycles. A bulk elastic modulus fall-off is then observed in samples submitted to an additional overnight freezing between two subsequent cycles. K increases in hydrogels with the highest HA content, when samples undergo pulse-echo measurements soon after their preparation. When hydrogels reach equilibrium, after having been kept in deionized water for 12 h, K values are lower, showing a nearly constant behaviour with different PVA-HA ratios and cycles. Furthermore, by means of scanning laser acoustic microscopy (SLAM) defects have been detected in the hydrogels. In samples which have reached equilibrium, SLAM images show that these defects disappear in PVA-HA hydrogels.

A study of the water sorption kinetic of Poly(HEMA) hydrogels by SLAM and FT-IR measurements

The relative changes in the ultrasonic attenuation measured by scanning laser acoustic microscopy (SLAM) during water sorption in poly-hydroxy-ethil-methacrilate[Poly(HEMA)] are analyzed. Acoustical images performed at 10 and 30 MHz of Poly(HEMA) during water absorption show an ultrasonic attenuation increase followed by a decrease. This phenomenon is analyzed in terms of reflections, scattering, and absorption of the acoustic waves. However, the contribution of scattering and reflection may be neglected and the time dependence of acoustic attenuation is attributed to absorption of the acoustic waves occuring in accordance with the glass transition, although induced by a solvent and not by temperature. Fourier transform infrared spectrometry (FT-IR) measurements indicate that bound water is responsible for the glass-rubber transition. On the other hand, the longitudinal velocity changes are only affected by the variations of density and mechanical properties of the polymer during water sorption. The traditional weight gain data are compared with the attenuation changes observed by ultrasonic measurements.

Ultrasonic wave attenuation during water sorption in poly(2‐hydroxyethyl methacrylate) hydrogels

AbstractA scanning laser acoustic microscope (SLAM) was used to measure the acoustic attenuation during water sorption in a film of crosslinked poly(2‐hydroxyethyl methacrylate) [poly(HEMA)] hydrogel. The contribution of reflection, scattering, and absorption of acoustic waves to the measured attenuation is evaluated. The analysis of a model accounting only for the possible reflection of the acoustic waves at the swollen/unswollen boundaries indicates that the time dependence of the attenuation during water sorption cannot be explained simply by accounting for the presence of two additional interfaces. A predominant contribution of acoustic wave absorption during the glass transition, occurring at the two swollen/unswollen interfaces, is assumed. Also, microvoids cannot be excluded as contributors to craze growth in the glassy matrix at the penetrant/polymer interface.

Characterizing biological tissue using scanning laser acoustic microscopy

The scanning laser acoustic microscopy (SLAM) technique offers high resolution for determining the properties of individual layers. To introduce the SLAM technique as a method for measuring the speed of sound in tubular layered organs, and possibly distinguish between the layers by this measure, the authors chose the guinea-pig urethra, due to its relatively simple layered structure. To obtain more data of the acoustic properties of tissue, the attenuation coefficient was determined for the wall in toto.

Quantization bit rate reduction in scanning tomographic acoustic microscopy

The imaging capability of the scanning laser acoustic microscope has been enhanced by the addition of a quadrature detector for holographic imaging and rotational and frequency-varying hardware for tomographic reconstruction. Because of the expansion of data acquisition capability, the memory-intensive demands placed on the processing computer by these modifications require efficient methods of representing and storing the tomographic data while retaining the quality of image reconstruction. This paper reports the performance evaluation of low bit-rate data acquisition. Results show the feasibility of high-quality imaging at bit-rate savings at the level of 33%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Scanning tomographic acoustic microscopy

The scanning tomographic acoustic microscope (STAM) was proposed in 1982 as a method of improving the resolution capability of the scanning laser acoustic microscope (SLAM) based on the principles of tomography. By modifying the SLAM with a quadrature detector, tomographic projections that contain both the amplitude and phase information of the scattered wavefield can be acquired. Subsequently, multiple projections acquired with different incident waves are combined using the "back-and-forth" propagation algorithm to form the tomographic reconstruction. The first STAM reconstructions have been obtained to experimentally demonstrate the superior resolution capability of the STAM over the SLAM. In this paper, the implementation of the STAM is described, and experimental reconstructions of multiple-layer specimens are demonstrated.