Valeriy G. Andreev

Detection of ultrawide-band ultrasound pulses in optoacoustic tomography

A laser optoacoustic imaging system (LOIS) uses time-resolved detection of laser-induced pressure profiles in tissue in order to reconstruct images of the tissue based on distribution of acoustic sources. Laser illumination with short pulses generates distribution of acoustic sources that accurately replicates the distribution of absorbed optical energy. The complex spatial profile of heterogeneous distribution of acoustic sources can be represented in the frequency domain by a wide spectrum of ultrasound ranging from tens of kilohertz to tens of megahertz. Therefore, LOIS requires a unique acoustic detector operating simultaneously within a wide range of ultrasonic frequencies. Physical principles of an array of ultrawide-band ultrasonic transducers used in LOIS designed for imaging tumors in the depth of tissue are described. The performance characteristics of the transducer array were modeled and compared with experiments performed in gel phantoms resembling optical and acoustic properties of human tissue with small tumors. The amplitude and the spectrum of laser-induced ultrasound pulses were measured in order to determine the transducer sensitivity and the level of thermal noises within the entire ultrasonic band of detection. Spatial resolution of optoacoustic images obtained with an array of piezoelectric transducers and its transient directivity pattern within the field of view are described. The detector design considerations essential for obtaining high-quality optoacoustic images are presented.

Image Reconstruction in 3D Optoacoustic Tomography System with Hemispherical Transducer Array

Image reconstruction algorithm for three-dimensional laser optoacoustic imaging system (LOIS) is proposed and tested in computer-simulating experiments. It was assumed that acoustic transducers were evenly distributed with 3-degree interval along polar and azimuthal angles on the surface of a 100-mm diameter hemisphere, all optoacoustic sources were located inside the hemisphere of a slightly smaller diameter. At the first stage of calculations, the initial data in a form of spherical surface integrals were converted into the plane surface integrals. We deduced an approximate analytical formula for this conversion. At the second stage, the three-dimensional Radon transform algorithm was applied for reconstruction of optoacoustic sources. Three- dimensional images of computer-simulated spherical objects were generated. Quality of reconstructed images was evaluated with the following four criteria: The noise level on the entire tomogram, the step-transfer function, the loss-of-contrast function and the contrast-dimension relation. These quality criteria may be employed to characterize any tomography systems regardless of the type of technology employed. Image analysis demonstrated that the artifact level associated with data conversion from spherical into planar coordinates did not exceed 10%. A 1-mm spatial resolution could be obtained with the proposed algorithm, provided the signal-to-noise ratio equals approximately 3 on the tomogram. Very small (0.5-mm diameter) and small (3-mm diameter) spherical tumors could be revealed on optoacoustic tomograms if their contrast equals at least 6 and 0.3, respectively.

Inverse Radon transform for optoacoustic imaging

Mathematical model of image reconstruction for two-dimensional optoacoustic imaging system is described. It was assumed that receiving transducers are uniformly distributed along the perimeter of a 60 mm radius ring with 2.1 mm gaps between transducer centers and initial data were known with 0.1 mm increments. The algorithm of radial back projection with convolution was used for optoacoustic image reconstruction. The convolution was evaluated with modified Shepp-Logan (MSL) and rectangular (RECT) space spectrum windows. Linear interpolation was applied for calculation of the convolution at the intermediate space points. The following four criteria were employed for estimation of resulting image quality: noise level on entire tomogram, a jump transfer function, loss contrast function and the contrast-dimension reflation. Theoretical expressions for these parameters were derived and used for optimization of the proposed algorithm. Two-dimensional images of computer simulated spherical objects were reconstructed. It was shown that 0.1 mm spatial resolution could be obtained provided the signal-to-noise ratio equals approximately 3 at the tomogram. A very small (0.2 mm diameter) tumor and a small (2-mm diameter) tumor could be clearly revealed at the tomogram if their optical absorption contrast equals at least 2 and 0.1 respectively.

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.

Nonlinear moduli estimation for rubber-like media with local inhomogeneities elastography

Static shear deformations of a plane-parallel layer of rubber-like material created simultaneously with the uniaxial compression are considered. The layer is fixed between the rigid plates. Displacement of one plate relative to the other resulted in shear strain of the layer. This strain could reach 0.6 of the layer thickness. At such strain effects due to the cubic nonlinearity arise. It is shown that measuring the dependence of the shear stress on the shear strain along one axis at different compression along the perpendicular axis one could determine nonlinear Landau parameters. The measurements were performed in two layers of polymeric material plastisol of 7 mm thickness with a rectangular base 8.9x8.9 cm, mounted between three aluminum plates. The upper plate was loaded with masses ranging from 0 to 25 kg and was fixed in each series of the stress-strain measurements. The values of the Landau coefficient A were measured in layers with different value of linear shear modulus.

In vivo measurements of muscle elasticity applying shear waves excited with focused ultrasound

The common algorithm of shear waves excitation for diagnostical ultrasonic devices was modified for measurements in muscles. We measured the speed of shear waves, excited by a focused ultrasound at a frequency of 5 MHz in the muscles of the volunteers. Siemens Acuson S2000 was used for in vivo measurements. The suggested algorithm was tested on the muscle mimicking phantoms. The values of shear wave velocities in the same areas of studied phantoms at the same angles measured with Siemens Acuson S2000 system corresponded to the values obtained by Verasonics, where the region of shear wave excitation had a form of “blade” of thickness less than 0.5 mm, length and width of 1.5-2 mm. Due to this form of the region, the excited shear wave has propagated codirectional with the long side of the ultrasonic medical probe. Thus, the direction of propagation of the shear wave with respect to the phantom fibers, became dependent on the position of the probe. [The reported study was funded by RFBR and Moscow city Gove...

Nonstationary fluid dynamics in inhomogeneous electric field

Summary form only given. Formation of voids (micro-pores) in a vicinity of a pin electrode in a highly non-uniform electric field applied within a few nanoseconds was proposed recently as a main mechanism of electrical breakdown of liquid dielectrics1. We solve the problem of fluid dynamics when voltage pulse duration of 7 ns with a peak value of 300 kV was applied to a spherical electrode - plane setup. Motion of liquid (water) is described by the system of equations of hydrodynamics written in general terms in the spherical coordinates and the Tait equation of state. Electrostrictive force is taken in a form proposed for polar dielectrics2.Action of electrostrictive forces results in the formation of a shock wave, which propagates from the spherical electrode. The speed of the shock front reaches 3 Mach numbers. The rarefaction region is formed before the shock front, duration and peak value of which increases in a few nanoseconds. At some point, the negative pressure reaches a value at which liquid is ruptured and microcavity can be formed. The size of the cavities is determined by the rate a shock front growth. It is demonstrated that the size of cavities is sufficient to accelerate electrons to the energies of ionization of molecules of water vapor in the cavity.

Thermoacoustic Detector for Registration of Powerful Terahertz Pulses

The feasibility of application of a thermoacoustic detector (TAD) for registration of high-power pulse radiation at frequencies 0.55, 0.68, and 0.87 THz is studied. Transformation of the electromagnetic wave into an acoustic wave is produced in a structure consisting of a quartz substrate with a 10 nm chromium film deposited on one of its surfaces, which is in contact with a layer of the immersion liquid. It is shown that for a pulse of microsecond duration (3-10 μs), the signal waveform detected by the TAD is adequately matched to the derivative of the profile of the terahertz pulse. For the measurement of electromagnetic radiation in the frequency range 0.5-0.9 THz, it is possible to employ a simplified design of the TAD in which the transparent quartz substrate is in contact with a layer of water or ethanol.

Transient cavitation induced by periodic sequence of tone bursts

Development of cavitation in liquid under a periodical ultrasonic exposure was studied. The cavitation development in a regime of rarely repeating bursts was practically the same as in undisturbed liquid. The time of stable cavitation development was two times shorter when the series of ultrasonic bursts with small pause duration (1-3 sec) between bursts was applied compared to the case of undisturbed liquid. The results are explained by the finite time of bubble concentration evolution after the ultrasonic exposure.

Optoacoustic probe for prostate cancer diagnostics

The optoacoustic probe for prostate cancer detection was developed and tested. The 10‐ns pulses of the YAG:Nd laser were delivered by an optical fiber with a turning mirror at its tip. A fiber tip was placed above an ultrasonic array which was employed for the detection of acoustic transients excited inside prostate tissue. The increased infrared light absorption inside prostate tumors resulted in acoustic pulses with enhanced peak pressure providing 200%–300% optoacoustic contrast. The transducer array and the optical fiber were wrapped inside a 20‐mm diameter thin cylindrical shell filled with ultrasonic gel transparent for infrared radiation. Each acoustic transducer was made of 0.05‐mm thick PVDF film with dimensions of 1 mm×12 mm. The frequency bandwidth of transducer array provided 0.3‐mm axial in‐depth resolution. The lateral resolution is defined by the array length and was estimated as 0.8‐mm for 32‐element array with 1‐mm gap between transducers. Transducer sensitivity of 0.05 mV/Pa allowed the ...