Sergey Tsysar

A multi-element interstitial ultrasound applicator for the thermal therapy of brain tumors

Interstitial thermal therapy is a minimally invasive treatment modality that has been used clinically for ablating both primary and secondary brain tumors. Here a multi-element interstitial ultrasound applicator is described that allows for increased spatial control during thermal ablation of tumors as compared to existing clinical devices. The device consists of an array of 56 ultrasound elements operating at 6 MHz, oriented on the seven faces of a 3.2 mm flexible catheter. The device was first characterized using the acoustic holography method to examine the functioning of the array. Then experiments were performed to measure heating in tissue-mimicking gel phantoms and ex vivo tissue samples using magnetic resonance imaging-based thermometry. Experimental measurements were compared with results obtained using numerical simulations. Last, simulations were performed to study the feasibility of using the device for thermal ablation in the brain. Experimental results show that the device can be used to induce a temperature rise of greater than 20 °C in ex vivo tissue samples and numerical simulations further demonstrate that tumors with diameters of greater than 30-mm could potentially be treated.

Acoustic holography as a metrological tool for characterizing medical ultrasound sources and fields

Acoustic holography is a powerful technique for characterizing ultrasound sources and the fields they radiate, with the ability to quantify source vibrations and reduce the number of required measurements. These capabilities are increasingly appealing for meeting measurement standards in medical ultrasound; however, associated uncertainties have not been investigated systematically. Here errors associated with holographic representations of a linear, continuous-wave ultrasound field are studied. To facilitate the analysis, error metrics are defined explicitly, and a detailed description of a holography formulation based on the Rayleigh integral is provided. Errors are evaluated both for simulations of a typical therapeutic ultrasound source and for physical experiments with three different ultrasound sources. Simulated experiments explore sampling errors introduced by the use of a finite number of measurements, geometric uncertainties in the actual positions of acquired measurements, and uncertainties in the properties of the propagation medium. Results demonstrate the theoretical feasibility of keeping errors less than about 1%. Typical errors in physical experiments were somewhat larger, on the order of a few percent; comparison with simulations provides specific guidelines for improving the experimental implementation to reduce these errors. Overall, results suggest that holography can be implemented successfully as a metrological tool with small, quantifiable errors.

Measuring the radiation force of megahertz ultrasound acting on a solid spherical scatterer

The paper considers the problem of precise measurement of the acoustic radiation force of an ultrasonic beam on targets in the form of solid spherical scatterers. Using known analytic relations, a numerical model is developed to perform calculations for different sizes of spherical scatterers and arbitrary frequencies of the incident acoustic wave. A novel method is proposed for measuring the radiation force, which is based on the principle of acoustic echolocation. The radiation force is measured experimentally in a wide range of incident wave intensities using two chosen methods differing in the way the location of the target is controlled.

Characterization of cylindrical ultrasonic transducers using acoustic holography

We present the results of studying the vibrational velocity distribution over the surface of cylindrical ultrasound transducers by acoustic holography. We describe two approaches for acoustic holography: the spatial spectrum method and the Rayleigh integral method. In the case of cylindrical sources the spectral method has a specific feature in comparison to the case of quasi-plane sources: small-scale spectrum components having the form of evanescent (nonpropagating) waves near the source, turn into propagating waves at a certain distance from the source. The use of such a mixed type of waves makes it possible to increase the holographic resolution. To conduct holography of cylindrical sources by the Rayleigh integral method, a modification consisting in the superimposing of boundaries on the integration region is proposed. We present the results of numerical simulation and physical experiments on holography of small cylindrical piezoelectric transducers. We demonstrate that the proposed methods of holography make it possible to recover the vibration structure of source surfaces up to order of the wavelength scales.

Ultrasonic holography of 3D objects

The holographic approach used here relies on the principle of a time-reversal mirror and the Rayleigh integral. An ultrasonic beam consisting of long tone bursts is directed at a target object and the resulting acoustic field is measured at a large number of points surrounding the object. A computer-controlled positioning system is used to scan a small broadband hydrophone across a grid of measurement points in a single surface near the target. Object reconstruction is then accomplished numerically by back-propagating the acoustic field from measurement locations to a 3D region representing the object. Theoretically, the accuracy and the optimal parameters of the method were studied by modeling forward and backward propagation from a point scatterer. Experimentally set of 3-mm diameter plastic beads was investigated. Ultrasound frequencies from 1 to 1.5 MHz were considered, while hologram measurements were collected with grid spacings between 0.3 and 0.4 mm. Both theory and measurements show that the spatial resolution of 3D ultrasonic holography is limited by diffraction effects. Discrete scatterers larger than a wavelength are well-resolved. Using this 3D ultrasonic holography method, it is possible to reconstruct the position and shape of objects or collections of objects that do not involve a significant amount of multiple scattering. Because the spatial resolution of the method has a typical diffraction limit on the order of a wavelength, improved spatial resolution can be achieved with higher frequencies.

Experimental implementation of a synthesized two-dimensional phased array for transcranial imaging with aberration correction

Ultrasound (US) imaging of brain structures is a challenging, but highly promising diagnostic technology in medical ultrasound. Recent advances in transcranial US therapy suggest the potential to implement diagnostic US at higher frequencies, ideally for full brain imaging. In this work. we present experimental results of ultrasound imaging of spherical and tubular scatterers placed behind a skull phantom. The phantom was produced from a casting compound with acoustic properties matching those of skull. Phantom shape was defined from CT data of a human skull and 3D printing of a mold. A two-dimensional ultrasound array was simulated by mechanical translation of the focal spot of a broadband single-element 2 MHz transducer over the phantom surface. This synthesized array mimicked a 2D flexible phased array placed on the top of the patients head. A pulse-echo technique was used for reconstructing the thickness of the skull phantom and detecting backscattered signals from the test objects. Transcranial imag...

Vortex beams and radiation torque for kidney stone management

Our team previously developed an instrument to reposition kidney stones with acoustic radiation force. In a clinical trial, the technology was used to transcutaneously facilitate passage of small stones and to relieve pain by dislodging obstructing large stones. Acoustic trapping and manipulation of kidney stones in water has recently been investigated using both single element and sector arrays in the range of 0.3–1.5 MHz. Experimental holographic reconstruction of the transducer surface velocity confirmed the proper operation of each transducer. Human stones approximately 5 mm, as well as glass and aluminum beads, were placed on a flat tissue phantom in a water bath. During exposure, stones were drawn to the beam axis, and then controllably translated along the surface in any direction transverse to the beam. The phase between sector elements could be used to control the vortex size, as well as rate and direction of rotation of the trapped object. The trapping effect was disrupted at increased transduce...

Experimental study of transmission of a pulsed focused beam through a skull phantom in nonlinear regime

In the paper the use of receiving and radiating system, which allows to determine the parameters of bone by nonlinear pulse-echo technique and to image of brain structures through the skull bones, was proposed. Accuracy of the skull bone characterization is due to higher measured harmonic and is significantly better than in linear case. In the experimental part focused piezoelectric transducer with diameter 100 mm, focal distance 100 mm, the frequency of 1.092 MHz was used. It was shown that skull bone profiling can be performed with the use of 3rd harmonic since 1st harmonic can be used for visualization of the underlying objects. The use of wideband systems for both skull profiling and brain visualization is restricted by skull attenuation and resulting low effective sensitivity.

Diffraction effects accompanying focused ultrasonic pulse propagation in a medium with a thermal inhomogeneity

Propagation of a probing pulse through a thermal inhomogeneity produced in the medium as a result of heating by a focused ultrasonic beam is considered. The wave equation is used to numerically calculate the waveform of the pulse arriving at the receiver whose focus coincides with that of the radiator. Calculations are performed for a pulse focused at the center of the inhomogeneity and crossing it in the transverse direction. The results of numerical simulation are compared with experimental data. Conditions under which the diffraction effects should be taken into account in calculating the pulse delay at the receiver are formulated.

Acoustic radiation force acting on a spherical scatterer in water: Measurements and simulation

Acoustic radiation force (ARF) is successfully used in a recently proposed ultrasonic technology for kidney stone propulsion. The planning of the treatment requires calibration of ARF for stones of different dimensions and locations. However, such calibration remains a problem. Here, a method of measuring ARF acting on a mm-sized spherical object positioned on the axis of a focused ultrasound beam is proposed and tested. Acoustic field was generated by a single-element 1.072 MHz transducer of 100 mm aperture and 70 mm focal length positioned at the bottom of the water tank. Measurements were performed for nylon and glass spherical scatterers with diameters from 2 to 4 mm located at different distances from the source along the vertical beam axis. For each scatterer, the source power was gradually decreased until the scatterer started to fall down from the trap. At this threshold power of the source, the value of ARF was determined as a difference between the gravity and buoyancy forces acting on the scatterer. At other source power outputs, the value of ARF was linearly scaled. ARF was also calculated from pressure distributions reconstructed from acoustic hologram of the source and physical parameters of the scatterers. Experimental and theoretical results were found in a good agreement. It was shown that the most effective pushing was observed at distances where the beam was slightly wider than the scatterer. [Work supported by the stipend of the President of Russia (SP-2621.2016.4), RFBR №18-32-00659, and NIH P01 DK43881.]Acoustic radiation force (ARF) is successfully used in a recently proposed ultrasonic technology for kidney stone propulsion. The planning of the treatment requires calibration of ARF for stones of different dimensions and locations. However, such calibration remains a problem. Here, a method of measuring ARF acting on a mm-sized spherical object positioned on the axis of a focused ultrasound beam is proposed and tested. Acoustic field was generated by a single-element 1.072 MHz transducer of 100 mm aperture and 70 mm focal length positioned at the bottom of the water tank. Measurements were performed for nylon and glass spherical scatterers with diameters from 2 to 4 mm located at different distances from the source along the vertical beam axis. For each scatterer, the source power was gradually decreased until the scatterer started to fall down from the trap. At this threshold power of the source, the value of ARF was determined as a difference between the gravity and buoyancy forces acting on the scatt...