A. A. Karabutov

Optoacoustic tomography utilizing focused transducers: The resolution study

The goal of this work was to investigate numerically the dependence of the resolution and the imaging window size, provided by a cylindrically focused wideband piezoelectric detector in optoacoustic tomography, on the detector dimensions and frequency bandwidth. Analytical expressions found by O’Neil for a continuous spherically focused ultrasonic radiator were found to fit the numerical data accurately to within a constant factor. The range of application and limitations of these expressions were investigated.

Bloch oscillations of an acoustic field in a layered structure

This study is devoted to experimentally achieving the phenomenon of periodic modulation of the acoustic-wave intensity, which is observed upon passage of an acoustic signal through a quasi-periodic structure and is the acoustic analog of the effect of Bloch oscillations (BO). Ultrasound at a frequency of 1 MHz is used, and a layered structure that consists of alternate plane-parallel glass and water layers serves as a super-lattice. In order to create an analog of the external electric field, the thickness of the water layers was changed inversely with respect to their ordinals. It is shown that the transmission spectrum of such a structure has the form of narrow equidistant peaks (an analog of the Wannier-Stark ladder), and the envelope of a transmitted signal undergoes periodic oscillations (analogous to BO). The experimental results are in good agreement with theoretical calculations performed by the transfer-matrix method.

Methods of optoacoustic diagnostics of biological tissues

Laser optoacoustic diagnostics is based on thermoelastic excitation of ultrasonic signals in a medium due to absorption of pulsed laser radiation. The pressure profile of such an ultrasonic (or optoacoustic) signal contains information on the distribution of heat sources in the medium. Hence, detection of optoacoustic signals reveals the distribution of absorbing inhomogeneities in the medium under investigation. Application of optoacoustic diagnostics in various fields of biology and medicine (e.g., for visualization of blood vessels and malignant tumors) has been much discussed in literature. Research in optoacoustic diagnostics is mainly concerned with the development of systems for optoacoustic signal detection, methods of signal processing, and algorithms for optoacoustic image reconstruction. The present paper reviews the latest achievements in this field of research and discusses its prospects.

Measurement of velocity distribution for longitudinal acoustic waves in welds by a laser optoacoustic technique

An optoacoustic technique for diagnostics of residual stress in metals is proposed. The theoretical part of the technique employs acoustoelastic relations establishing a linear relationship between the biaxial residual stress and the relative variation of the velocity of longitudinal ultrasonic waves. The experimental technique is based on laser excitation of nanosecond ultrasonic pulses at the surface of samples under investigation and their detection with a high time resolution. Distributions of the relative variation of longitudinal wave velocities due to the presence of residual stress in the samples are obtained.

Novel combined optoacoustic and laser-ultrasound transducer array system

A new multichannel integral system enabling simultaneous laser-optoacoutic and laser-ultrasonic and optical-acoustic images in real time has been proposed. The new potential of such a system and the advantages of both methods (large depth of penetration, high image contrast, and spatial resolution) are discussed in detail.

A laser optoacoustic method of inducing high-energy states and the investigation of phase transitions in metals at high pressures

Results are given of investigations of high-energy states and phase transitions, using lead as an example, under the effect of nanosecond laser pulse in a wide range of intensities. The metal surface being irradiated is mechanically confined by a transparent dielectric plate in order to realize conditions of highly effective generation of pressure while maintaining local thermodynamic equilibrium. The dynamics of thermodynamic state of metal are analyzed by the shape and amplitude of pressure pulse propagating from the surface being heated and by the variation of reflectivity of the metal surface being irradiated.The measurements of pressure pulse make possible the recording of phase transitions in lead: melting on the sample surfaces being irradiated at a pressure up to Pmax ≈ 0.1 GPa. It is demonstrated that the pressure amplitude prior to melting is proportional to the intensity of laser radiation, and that upon passing the melting threshold — to the absorbed energy of laser pulse. The instant of time, at which the “deformation” of the leading edge of pressure pulse begins, corresponds to the beginning of melting. When high-energy states are realized in the thin surface layer of metal, the density decreases significantly; this results in a significant decrease in both reflectivity and electrical conductivity.

Wideband focused transducer array for optoacoustic tomography

The calculation procedure of the parameters of a multielement transducer array for the optoacoustic tomography of biological objects with high spatial resolution values is proposed. A multielement transducer with given spatial resolution values in three dimensions has been developed based on the proposed procedure for the early detection of breast cancer. The transducer array consists of a set of 8 linear PVDF piezoelectric films located on a plane and a focusing cylindrical acoustic lens. A map of the transducer’s focal area and point spread function have been measured using the constructed transducer array. Spatial resolutions of the transducer array obtained experimentally are in agreement with their calculated values.

Application of laser generated ultrasonic pulses in diagnostics of residual stresses in welds

The laser optoacoustic method is applied for nondestructive evaluation of residual stresses in metals. Theoretical background establishes the acoustoelasticity relationships between applied biaxial stresses and deviations of the longitudinal wave velocity in solids. The experimental technique bases on laser thermoptical excitation of nanosecond ultrasonic pulses on the surface of samples under study and their backward mode piezodetection with a high temporal resolution. The samples, produced from stainless steel, had thickness from 1.67 mm to 8.87 mm. Welds are made up by means of electronic beam overheating of the samples with different intensities of the beam. Plane distributions of relative longitudinal wave velocity deviations, produced by stresses, were obtained.

Wide-band acoustic pulse detection in opto-acoustic tomography system

The optoacoustic tomography (OAT) utilizes acoustic waves induced by short laser pulses in tumor with preferential light absorption. Waveform of the opto-acoustic signal detected by a wide-band transducer array provides information about shape of tumor and its specificity. The 32-element acoustic array was employed in clinical prototype of laser optoacoustic imaging system (LOIS). Piezopolymer PVDF film was used for the array transducer fabrication. Sensitivity of the array and its transient directivity pattern are discussed in this paper. The array performance was tested in experiments with phantoms.

Optoacoustic study of laser-induced near-critical states of thin aluminum films

The results of the optoacoustic study of aluminum states within a temperature range of 3–14 kK and a pressure range of 0.1–4 kbar were considered. These high-energy states were achieved by heating a submicron metal film confined by a transparent dielectric via nanosecond laser pulses with a fluence of up to 11 J/cm2. The dynamics of the temperature, pressure, and reflectivity of aluminum was studied at a nanosecond time resolution.