Electroacoustic response at the focal point of a focused transducer as a function of the acoustical properties of the lens (original) (raw)
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Effect of acoustical properties of a lens on the pulse-echo response of a single element transducer
IEEE Ultrasonics Symposium, 2004, 2004
The purpose of this study is to compare two methods for calculating the electroacoustic response of a transducer at the focal point through an acoustic focusing lens. The theoretical conditions depend on the degrees of freedom taken into account as well as on the boundary conditions for the propagation. The most widely used conditions, i.e. longitudinal vibration and rigid baffle boundary condition are considered here. The theoretical approach consists in calculating the electroacoustic response using each electroacoustic model, both of these results being then propagated with the Rayleigh integral for a focused source. The first section consists in a modeling of the electroacoustic response based either on the modified classical KLM electrical equivalent circuit (1D) or on the finite element method (FEM) ATILA (axisymmetric 2D). Both models are applied to an axisymmetric configuration, and a classical propagation operation is used to obtain the response at the focal point. The second part deals with the experimental verification of the modeling methods. Measurements on a series of 10 MHz focused transducers are compared to theoretical curves, and results are discussed as a function of geometry and properties of the acoustical lens.
Effect of Radial Displacement of Lens on Response of Focused Ultrasonic Transducer
Japanese Journal of Applied Physics, 2007
A lens-focused single-element transducer designed for high-resolution medical imaging requires a high ratio of radius of curvature to source radius. Therefore, classical models neglecting the radial contribution may not be accurate. The objective of this study is to evaluate the contribution of radial displacement to the pressure response of the transducer, both in terms of focal spot and pulse response characteristics. To achieve this objective, two finite element method calculations (FEM) were performed (commercial ATILA Ò software), namely those for free and clamped radial displacements. A propagation code adapted to an axisymmetric transducer geometry was implemented to compare the radiated fields, and FEM results for the transducer surface were used as inputs to obtain the radiated fields. Subsequently, the differences between the results of the two calculations results were determined. However, it was demonstrated that the radial displacements slightly affect the propagated field and can therefore be neglected in realistic transducer designs. Moreover, the effects of lens acoustic properties were studied for realistic configurations in terms of resolution and sensitivity to obtain an optimal ultrasound image quality.
Journal of Applied Physics, 2014
ABSTRACT Prediction and measurement of the acoustic field emitted from a high intensity focused ultrasound (HIFU) is essential for the accurate ultrasonic treatment. In this study, the acoustic field generated from a strongly focused HIFU transmitter was characterized by a combined experiment and simulation method. The spheroidal beam equation (SBE) was utilized to describe the nonlinear sound propagation. The curve of the source pressure amplitude versus voltage excitation was determined by fitting the measured ratio of the second harmonic to the fundamental component of the focal waveform to the simulation result; finally, the acoustic pressure field generated by the strongly focused HIFU transmitter was predicted by using the SBE model. A commercial fiber optic probe hydrophone was utilized to measure the acoustic pressure field generated from a 1.1 MHz HIFU transmitter with a large half aperture angle of 30°. The maximum measured peak-to-peak pressure was up to 72 MPa. The validity of this combined approach was confirmed by the comparison between the measured results and the calculated ones. The results indicate that the current approach might be useful to describe the HIFU field. The results also suggest that this method is not valid for low excitations owing to low sensitivity of the second harmonic.
Finite Element Modeling and Wave Propagation Analysis for Lens-Less Line Focus Acoustic Microscopy
International Journal of Acoustics and Vibrations, 2017
A finite element method for simulation of lens-less line focus acoustic microscopy is proposed in this paper to nondestructively evaluate the leaky surface wave (LSW) velocity. The defocusing measurement model is established, in which the geometrical focusing radius will be 20 mm. The piezoelectric polyvinylidene fluoride film is selected as the active element. The excitation is a standard black Harris wavelet signal with a centre frequency of 5 MHz. Simulations of measurements on typical bulk materials (Al) are carried out. Then, the time-resolved wave signal series are acquired when the defocusing distance varies continuously. The LSW velocity will be easily determined by the examination of the slope of the LSW's arrival time versus the defocusing position. The LSWs' propagating path will be analyzed geometrically in time-domain. Meanwhile, the LSWs' velocities are also extracted by applying the specially developed digital signal processing algorithm to the defocusing experimental data, which is called V (f, z) analysis method based on two-dimensional fast Fourier transform. Finally, the relationship between the time-resolved method and the V (f, z) technique is discussed, in which the interpretation of the formation of surface waves and the description of its analysing methods will be given.
The Journal of the Acoustical Society of America, 2008
Acoustic characterization of high intensity focused ultrasound ͑HIFU͒ fields is important both for the accurate prediction of ultrasound induced bioeffects in tissues and for the development of regulatory standards for clinical HIFU devices. In this paper, a method to determine HIFU field parameters at and around the focus is proposed. Nonlinear pressure waveforms were measured and modeled in water and in a tissue-mimicking gel phantom for a 2 MHz transducer with an aperture and focal length of 4.4 cm. Measurements were performed with a fiber optic probe hydrophone at intensity levels up to 24 000 W / cm 2 . The inputs to a Khokhlov-Zabolotskaya-Kuznetsov-type numerical model were determined based on experimental low amplitude beam plots. Strongly asymmetric waveforms with peak positive pressures up to 80 MPa and peak negative pressures up to 15 MPa were obtained both numerically and experimentally. Numerical simulations and experimental measurements agreed well; however, when steep shocks were present in the waveform at focal intensity levels higher than 6000 W / cm 2 , lower values of the peak positive pressure were observed in the measured waveforms. This underrepresentation was attributed mainly to the limited hydrophone bandwidth of 100 MHz. It is shown that a combination of measurements and modeling is necessary to enable accurate characterization of HIFU fields.
Journal of Sound and Vibration
Conventional excitation techniques typically use an impact hammer, piezoelectric actuator, or mechanical shaker excitation for experimental modal testing. However, the use of these devices may be challenging if accurate high-frequency dynamic measurements on small or lightweight structural parts have to be performed. To overcome these problems, the highfrequency radiation force generated by focused ultrasonic transducers (FUTs) can be used. This approach has shown potential to be used as a non-contact method for modal excitation of small-sized or flexible structures such as MEMS devices, small turbine blades, integral blade rotors (IBR), and biological structures. However, the sound radiation in the air of these ultrasonic transducers and the resulting radiation force imparted onto a structure is not well understood and critically crucial for performing accurate modal analysis and system identification. In this research, the technical development of ultrasound radiation pressure mapping and simulation is presented. Starting from the calibrated sound pressure fields generated by the spherically FUT, driven by amplitude modulated signals, the dynamic focused ultrasound radiation force is modeled and estimated. The acoustic pressure field of a FUT operating in the air is measured and used for validating the accuracy of a new numerical boundary element method (BEM) model in predicting the direct acoustic force generated in the high-frequency range (i.e., 300 e400 kHz). The results show that an excellent agreement is found regarding both the pressure profile and amplitude. Pressure fields up to 1200 Pa can be generated as the transducer is driven at 400 kHz. Experiments also prove that the FUT is capable of creating a focal spot size of nearly 3 mm in diameter. To finish, the FUT's dynamic focused ultrasound radiation force is quantified and could be used to quantify a force-response relationship for experimental modal analysis purposes.
Ultrasonic Imaging with an Acoustic Lens
IEEE Transactions on Sonics and Ultrasonics, 1977
Absrracr-An acoustic system is discussed, which is composed of a planar array of ultrasonic transducers placed before a convergent acoustic lens. For pulseecho imaging, the array elements are used one at a time, for both the transmission of pulses and the ensuing reception of their echoes. It is shown that such a system constitutes a sector scanner with the sector apex at the focal point of the lens. An acoustic lens is described which, with a linear array at 2.25 MHz, will produce a sector scan having a 60" field of view and approximately 4-mm lateral resolution, over a depth o f focus in excess of 15 cm. Sensitivity is optimum on axis and varies no more than 8 dB over the entire sector. This design is equally applicable to imaging with a two-dimensional array, in which case the region covered by high resolution electronic scanning is a pyramidal volume whose apex is at the focal point of the lens. E I. INTRODUCTION LECTRONICALLY scanned ultrasonic imaging has been discussed and developed largely in relation to two basic approaches: 1) a linear array of transducers, excited one at a time, to produce an ultrasonic beam that, in effect, moves laterally and produces a rectilinear scan, in which the field of view is as wide as the array [ l ] ; 2) a phased array in which the elements are all excited in particular phase relationships to steer the beam and generate a sector scan formed of beams which all originate at the array surface [3] , [4]. This paper presents analytical and experimental results on a third approach similar to that proposed by Maginness et al. [2] : an acoustic imaging system composed of an array of ultrasonic transducers and a convergent lens. Although the analysis is identical to that of optical lens imaging, the conclusions may be unfamiliar because the concept of beam formation as applied to acoustics appears to lack a counterpart in optics. sonic transducers placed before a positive acoustic lens, and used independently one-at-a-time for both transmission and reception. It will be shown that such a system constitutes a sector scanner with the sector apex at the focal point of the lens. A linear transducer array in this system may be identical to the linear transmit-receive arrays already used in rectilinear scanning. The array can also be two-dimensional, and, with transmit-receive electronics in matrix form [2], can produce beams that are electronically steerable anywhere within a pyramidal volume whose apex is at the back focal point of the lens. This technique represents a way to achieve threedimensional high resolution imaging by a direct and practical extension of present electronic methods. The acoustic system is composed of a planar array of ultra
Contribution of radial displacement to lens-focused transducer response
Japanese Journal of Applied Physics
A lens-focused single-element transducer designed for high-resolution medical imaging requires a high ratio of radius of curvature to source radius. Therefore, classical models neglecting the radial contribution may not be accurate. The objective of this study is to evaluate the contribution of radial displacement to the pressure response of the transducer, both in terms of focal spot and pulse response characteristics. To achieve this objective, two finite element method calculations (FEM) were performed (commercial ATILA software), namely those for free and clamped radial displacements. A propagation code adapted to an axisymmetric transducer geometry was implemented to compare the radiated fields, and FEM results for the transducer surface were used as inputs to obtain the radiated fields. Subsequently, the differences between the results of the two calculations results were determined. However, it was demonstrated that the radial displacements slightly affect the propagated fiel...
Ultrasound in Medicine & Biology, 2013
With the popularity of ultrasound therapy in clinics, characterization of the acoustic field is important not only to the tolerability and efficiency of ablation, but also for treatment planning. A quantitative method was introduced to assess the intensity distribution of a focused ultrasound beam using a hydrophone and an infrared camera with no prior knowledge of the acoustic and thermal parameters of the absorber or the configuration of the array elements. This method was evaluated in both theoretical simulations and experimental measurements. A three-layer model was developed to calculate the acoustic field in the absorber, the absorbed acoustic energy during the sonication and the consequent temperature elevation. Experiments were carried out to measure the acoustic pressure with the hydrophone and the temperature elevation with the infrared camera. The percentage differences between the derived results and the simulation are ,4.1% for on-axis intensity and ,21.1% for 26-dB beam width at heating times up to 360 ms in the focal region of three phased-array ultrasound transducers using two different absorbers. The proposed method is an easy, quick and reliable approach to calibrating focused ultrasound transducers with satisfactory accuracy. (