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on the principles of time-reversed acoustics (TRA). Focused ultrasound fields obtained in the experiments with the TRA focusing system (TRA FS), which ...
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IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,

vol. 57, no. 4,

April

2010

Time-Reversal Acoustic Focusing System as a Virtual Random Phased Array Armen Sarvazyan, Laurent Fillinger, and Leonid R. Gavrilov Abstract—This paper compares the performance of two different systems for dynamic focusing of ultrasonic waves: conventional 2-D phased arrays (PA) and a focusing system based on the principles of time-reversed acoustics (TRA). Focused ultrasound fields obtained in the experiments with the TRA focusing system (TRA FS), which employs a liquid-filled reverberator with 4 piezotransducers attached to its wall, are compared with the focused fields obtained by mathematical simulation of PAs comprised from several tens to several hundreds of elements distributed randomly on the array surface. The experimental and simulated focusing systems had the same aperture and operated at a frequency centered about 600 kHz. Experimental results demonstrated that the TRA FS with a small number of channels can produce complex focused patterns and can steer them with efficiency comparable to that of a PA with hundreds of elements. It is shown that the TRA FS can be realized using an extremely simple means, such as a reverberator made of a water-filled plastic bottle with just a few piezotransducers attached to its walls.

I. Introduction

T

he focusing of ultrasonic waves is a fundamental feature of most medical applications of ultrasound. There are two main approaches to ultrasound focusing: geometrical and electronic. Geometrical focusing is related to the use of concave piezoceramic transducers manufactured as a part of a spherical shell [1], [2] or acoustic lenses [3], [4]. Such systems are simple, inexpensive, and easy to make, but their principal disadvantage is that they have a fixed focal distance and cannot steer the focus along or off the axis. Electronic focusing is based on the use of a phased array (PA) [5] consisting of many separate elements. Fed from separate generators and power amplifiers, separate element configurations allow controllable change of the phase relationships over the array aperture, creating a wave front with any desired shape. Such arrays permit steering the focus both along and off the axis of the array (which is especially important in medical imaging) and generate several foci simultaneously, therefore shortening the time of the treatment procedure in therapeutic applications. The price for such flexibility in PAs is the complexity of their structure. An-

Manuscript received December 15, 2008; accepted December 13, 2009. L. Gavrilov is grateful for support from INTAS (05-1000008-7841) and RFBR (09-02-00066). A. Sarvazyan and L. Fillinger are with Artann Laboratories, West Trenton, NJ (e-mail: [email protected]). L. Gavrilov is with N. N. Andreev Acoustics Institute, Moscow, Russia. Digital Object Identifier 10.1109/TUFFC.2010.1486 0885–3010/$25.00

other drawback of PAs, especially in ultrasound therapy, is the occurrence of grating lobes in the acoustical field that may cause damage to tissues located outside of the main focus or foci. It was shown that essential improvement of the quality of the intensity distribution of the ultrasonic field focused by a 2-D array, especially in the case of steering the focus or foci, could be achieved if the elements are randomly distributed over the array aperture [6], [7]. An alternative technique of focusing ultrasonic waves based on principles of time-reversed acoustics (TRA) has been developed in the last decade by M. Fink and associates [8], [9]. The TRA technique of focusing ultrasonic waves is based on the reversibility of acoustic propagation, which implies that the time-reversed version of an incident pressure field naturally refocuses on its source. In contrast to conventional ultrasound focusing techniques, TRA focusing allows concentrating acoustic energy not only in space but also in time. That is, the duration of the high-intensity ultrasonic pulse at the focal point of the TRA focusing system (TRA FS) can be several orders of magnitude shorter than that of the transmitted signal. Most importantly, efficient focusing of acoustic waves and 3-D steering of the focal region can be achieved by a TRA FS comprised of a minimal number of (or even just one) transducers [10]–[13]. For the sake of comparison with conventional PAs, the TRA focusing of ultrasound can be presented as a two-stage process. The first stage includes multiple reflections and random scattering of an acoustical signal in a reverberator. As a result, a continuous pattern of the signal amplitudes and phases is formed on the reverberator’s radiating surface, which in the second stage of the process acts as a virtual PA focusing ultrasound at the target region. Hence, we assume in this work that the TRA FS can be modeled as a random PA with a large number of elements. This idea was originally introduced in [14]. The effects of signal duration, bandwidth, and the number of radiating channels on focusing have been investigated in [13] and [15]. The main goal of this paper is to analyze the applicability and limitations of modeling a TRA FS as a virtual random PA by comparing the performance of the TRA FS with the focusing capability of a PA when dealing with multiple foci and steering. By the terms “quality of focusing,” “better focusing ability,” etc., we shall mean the sharpness of the focus, a lower level of ultrasound energy outside the focus, a lower level or complete absence of side lobes, and the ability of a focusing system to generate multiple foci or a focal region of a predetermined shape.

© 2010 IEEE

sarvazyan et al.: time-reversal acoustic focusing system

Fig. 1. (a) Schematic of the time-reversed acoustics (TRA) experimental setup, (b) reverberator with glued transducers used in the experiments.

II. Materials and Methods A. TRA Focusing Experiments A schematic of the experimental setup for the TRA experiments is presented in Fig. 1(a). Focusing was done using a reverberator partially immersed in a water tank with the walls covered by a sound-absorbing lining. The reverberator was excited by transducers glued on its surface and driven by a TRA electronic unit allowing independent transmission of arbitrary signals through each channel (up to 4 channels were used). A needle hydrophone mounted on a 3-D positioning system was used for the TRA focusing and for the assessment of the resulting ultrasonic field. Both the TRA electronic unit and the positioning system were connected to a personal computer and interfaced through Matlab. Fig. 1(b) shows a TRA reverberator used in the experiments: a water filled 0.5L plastic bottle with four piezoceramic disks of 10  mm diameter and resonance frequency of 600 kHz glued to its wall. The 60-mm-diameter bottom of the bottle immersed in the water tank acted as a radiating surface. The hydrophone, placed at the target position for focusing, recorded the response to the linear sweep emitted by the radiating channel. The cross correlation of the emitted sweep with its response is the impulse response from the radiating channel to the location of the hydrophone. This impulse response was computed and time reversed. An example of time-reversed impulse response is shown in Fig. 2(a). The same procedure was repeated for each radiating channel leading to a set of time-reversed impulse responses. Their simultaneous emission generates an acoustic field focused at the hydrophone location [see Fig. 2(b) and (c)]. Once the time-reversed impulse responses were recorded, the hydrophone, which initially served as a beacon for the TRA focusing, was used to scan and measure the acoustic field distribution. Because time reversal is a linear process, complex focal patterns can be decomposed into a set of separate focal regions [16]. The emission of the superposition of the signals necessary for focusing at each of those regions leads to the focusing of the complex multiple-foci pattern.

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Fig. 2. (a) Example of signal emitted from a channel, (b) focused signal, (c) time-space diagram of the focused ultrasound intensity along a line in the focal plane.

B. Computer Modeling of Phased Array Focusing The acoustic fields produced by 2-D PAs were computed using the pseudo-inverse method proposed in [5] and [17]. The detailed description of the application of this method for calculation of the acoustic fields and the phases of the signals applied to the array of elements required to generate and steer multiple foci is given in [6], [7], and [18]. Numerical experiments were carried out and the comparison of the ability of 2-D random arrays with geometric characteristics similar to the considered TRA reverberator to generate several foci and to steer them off the array axis were completed. For computer modeling, the selected arrays consisted of elements distributed randomly over the aperture. All arrays had a flat circular shape 60  mm in diameter, equal to the aperture of the TRA FS reverberator. The ultrasound frequency was 600 kHz, which is about the central frequency of the TRA focused pulse. The intensity distributions were computed in the focal plane, i.e., at the distance z = 60 mm from the center of the array, in two orthogonal directions from 0 to ± 35 mm. The elements of the array were disks of 3 mm in diameter. III. Results The following figures are contour plots representing normalized peak intensity distributions measured in the focal plane. In the case of TRA focusing, the peak intensity at each point of the focal plane is reached at different times. If a snapshot of the intensity distribution is taken only at the time of focusing, it could lead to underestimating the level of side lobes. Therefore, the spatial distributions plotted in Fig. 3 represent the maximum intensity reached over time at a given location. The focal plane was chosen at a distance 6 cm from the radiating surface. Level lines correspond to 10, 30, 50, 70, and 90% of maximum intensity. In the case of the PA, the emission is continuous, as opposed to pulsed mode with TRA. In the latter case, the presented distribution corresponds to the peak intensity, i.e., the spatial distribution of the maximum intensity

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Fig. 4. Peak intensity distribution in the focal plane for a 4-foci pattern; (a)–(c) phased arrays, (d) time-reversal acoustic focusing system. The axes are marked in millimeters.

TABLE I. Evolution of the Intensity on Focus for the TimeReversal Acoustic Focusing System. Fig. 3. Peak intensity distribution in the focal plane for the focal spot with increasing number of channels; (a) phased array, (b) time-reversal acoustic focusing system. The axes are marked in millimeters.

Number of channels Relative intensity

reached over time. In all cases, the presented spatial windows are the same for both focusing modalities to allow a comparison as directly as possible.

multiple foci pattern (4 foci at the corners of a square with 5-mm sides), as is shown in Fig. 4(a)–(c). In the multiple-foci case, a 64-element PA (which produced a perfect single focal spot on the axis of the array, see Fig. 3) generates many side lobes, some reaching up to 30% of maximum intensity. However, the TRA FS generates a well-defined complex focal pattern [Fig. 4(d)]. The focal pattern generated by TRA FS is not as sharp as the 128 element PA; the four foci are not as well separated and the contour line at 10% of peak intensity encircles the entire focal region. However, as noted previously, the theoretically obtained PA-generated patterns represent an ideal situation, whereas the patterns for the TRA focusing represent experimental data affected by various sources of noise. Therefore, it may be assumed that the TRA FS focusing quality with 4 channels is comparable to that obtained by a PA with over a hundred channels.

A. Number of Channels Fig. 3 shows what influence the number of radiating channels has on the formation of the focal spot for the TRA FS with a reverberator [as illustrated in Fig. 1(b)] and for random PAs with differing number of elements. With a PA, a few dozens of channels are required to generate a focal spot located on the axis of the array with no side lobes. In contrast, a single channel leads to satisfactory focusing with TRA FS. Additional channels produce insignificant improvement in the focusing ability of the TRA FS. The focal spot obtained with the PA (1.46 mm at the half intensity level) is smaller than that obtained with the TRA FS (between 1.65 and 2.41 mm at the half intensity level). However, it should be kept in mind that the patterns for TRA focusing represent experimental data affected by various factors, such as water quality, hydrophone size (about 1 mm), transducer and electronics dynamic range limitations, and the inherent noise that occurs in the electronic circuits. On the contrary, the PAgenerated patterns are obtained theoretically, representing an ideal case. Therefore, it may be assumed that the focusing quality of TRA FS and PA are comparable. For the TRA FS, the dependence of the peak intensity as a function of the number of channels is reported in Table I. The time-reversed impulse responses of the individual channels were scaled to make use of the full dynamic range of the TRA electronics. This channel-dependent scaling accounts for the deviation from quadratic behavior [19]. B. Multiple Foci Obviously, generation of a multiple foci ultrasonic field should require more elements of a PA than in the case of a single focus located on the axis of the PA. Indeed, over a hundred elements are needed for the PA to generate a

1 1.0

2 2.4

3 5.9

4 13.4

C. Steering Fig. 5 presents data on comparison of the ability of 4-channel TRA FS and PAs with 64 and 128 channels to steer the four-foci pattern. The four-foci pattern is steered by 10, 20, and 30 or 40 mm, corresponding to steering angles of 9.5°, 18.4°, and 26.6° or 33.7°, respectively. The performances of PAs degrade as the focal pattern is steered off the axis. At more than 20 mm away from the axis, a 64-channel PA loses its ability to focus acoustic waves. In the case of the 128-channel PA, the side lobes reached 30% of maximum intensity with 30  mm of steering. Further steering away from the axis caused the ultrasound energy in the side lobes to become greater than that in the intended focal region. In the case of TRA FS, even with a steering of 40 mm, the significant peak intensity remained in the vicinity of the focal pattern. For both PA and TRA focusing, steering of the focus was accompanied by a concurrent reduction in intensity of focused ultrasound. The intensity drops as the focus is steered away from the axis. With the 128-channel PA, the peak intensity at 30  mm steering distance is 69.1% of what it was without steering,

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Fig. 7. Influence of the absorbing lining on time-reversal acoustic focusing: peak intensity distribution of a 20 mm steered 4-foci pattern measured with lining (a) and without (b).

Fig. 5. Peak intensity distribution in the focal plane for steered 4-foci pattern; (a) and (b) phased arrays, (c) time-reversal acoustic focusing system. The axes are marked in millimeters.

surement of the four foci pattern with 20  mm steering distance performed in the water tank with absorbing lining. Panel (b) of the figure shows the result of the similar measurement performed without the absorbing lining. In that case, the time-reversal process took advantage of the reverberation occurring in the water tank, leading to a more concentrated field with sharper focus (focus size between 1.01 and 1.86 mm at half intensity). IV. Discussion

Fig. 6. Peak intensity distribution in the focal plane for 128- and 256-channel random phased arrays and for 4-channel time-reversal acoustic focusing system.

and with the 4-channel TRA FS, the peak intensity at 40 mm steering was 75.8% of what it was without steering. Fig. 6 presents additional simulations of the focal area steered by 35  mm for a PA composed of 128 randomly placed elements and for another PA with 256 randomly placed elements. Doubling the number of elements allowed significant reduction of the side-lobes. None of the sidelobes reached 30% of the maximum intensity, though some foci remained unresolved. With a larger steering distance of 40 mm, the TRA FS produced a pattern in which the four foci were resolved, though they were not as sharp and their relative amplitudes were not equal. D. The Influence of an External Reverberator Fig. 7 illustrates the influence of the absorbing lining covering the walls of the water tank. Panel (a) shows mea-

We assumed in this work that the TRA FS can be modeled as a random PA with a large number of elements. Therefore, in the described experiments, we analyzed the applicability and limitations of such a model and evaluated how close the performance of a TRA FS is to the quality of focusing by PAs. Experimental results demonstrated that the TRA FS with a small number of channels (1 to 4) can produce complex focused patterns and can steer them with efficiency comparable to that of a PA with hundreds of elements. Increasing the number of channels in the TRA FS may be important for increasing the intensity of ultrasound in the focal region rather than for improving the focusing ability of the system. It is demonstrated that the TRA FS can be realized using a simple reverberator made of a water-filled plastic bottle with just a few piezotransducers attached to its walls. One of the advantages of water-filled reverberators, especially for biomedical applications, is good acoustical matching with soft tissues. In both PA and TRA FS, the formation of a focal region is a result of emitting an appropriate wavefront from the radiating surface. A TRA FS with a small number of radiation channels generates the necessary complex vibration pattern on the radiating surface, acting as a virtual PA. The complex pattern is produced by interference of acoustic waves resulting from multiple reflections of the

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signals radiated by the transducers attached to the reverberator. It seems natural to think that adding more channels in the TRA FS should improve focusing ability of the system by producing more complex and better adjusted vibration patterns on the reverberator radiating surface. However, the spatial distribution improvement and corresponding improvement of focusing quality offered by additional channels seemed to saturate quickly (see Fig. 3). The experimental data showed that even a four-channel TRA FS can produce sufficiently complex vibration patterns for efficient focusing of ultrasound. Indeed, there is a physical limitation on the complexity of the pattern that can be generated on the radiating surface. The motion of two points cannot be independent if they are closer than the spatial correlation distance, which is of the order of the wavelength, λ [14], [15]. Once the limit of the complexity of the pattern that can be generated on the radiating surface has been reached, additional channels only lead to an increase of the intensity of ultrasound in the focal region without improving further the sharpness or the overall quality of the focusing. Let us make a rough estimate of the number of elements that a PA of the considered aperture should have to provide an acoustic field without significant secondary maxima of the intensity even at large steering of the focus (foci) as demonstrated with the TRA FS (Figs. 5 and 6). It is well known that the side lobes in the field induced by an array will be eliminated if the center-to-center distance between the elements is equal to the half of wavelength [20]. That means that for an array with a 60-mm aperture (radiating surface of 2800   mm2) and the wavelength of ultrasound of 2.5 mm, corresponding to the frequency of 600 kHz, the number of elements should be about 1800. Our simulations made for arrays of other configurations have confirmed that the arrays with the center-to-center distance between the elements equal to the half of wavelength provided steering of the foci at least at a distance of 40  mm without appearance of grating lobes in the acoustic field [21]. It is possible to estimate in a similar way the maximum number of the virtual acoustic sources that can be formed on the radiating surface of the reverberator in the TRA FS. Evidently, it will be the same as for a PA of same aperture (60 mm). Consequently, it is possible to assume that TRA FS can be modeled as a random PA with large number of elements (at least, over 1000). This figure will be greater for greater apertures of the reverberator and at higher frequencies (smaller wavelengths) of ultrasound. The proposed analogy between PA and TRA FS only holds when the TRA focusing is performed without an external reverberator. When focusing is performed inside a reverberating medium, e.g., in a water tank with reflective walls or in a skull, the effective aperture of the TRA FS becomes virtually infinite. Indeed, the impulse response from a radiating element to the point of focusing involves contributions from both the TRA FS reverberator and the medium in which focusing is performed. In that case, the focal region results from a converging wave front coming

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not only from the radiating surface but potentially from every direction. This corresponds to an infinite aperture (sources surrounding the focus) producing a focal region that is spherical rather than elliptical and results in a sharper focus. This is illustrated in Fig. 7, which compared results of focusing with TRA FS in the water tank with and without absorbing lining: the localization of the steered pattern is improved when using the reflections in the water tank and the focal spots are sharper. However, when it comes to generation of a high-intensity focused ultrasound field, a PA has a clear advantage because of the huge number of channels and the continuous nature of the focused signal, whereas a TRA FS suffers from the short duration of the focused field. Even though the quality of focusing by the TRA FS quickly saturates with increasing number of channels, many channels are necessary to achieve higher intensity, which is critical in applications like ultrasonic therapy. V. Conclusions We proposed an analogy between a TRA FS and a random PA. According to the developed analogy, the radiating surface of the TRA FS reverberator acts as a virtual PA. Even a four-channel TRA FS allows formation of a sufficiently complex vibration pattern on the radiating surface, providing focusing of acoustic waves. Additional channels insignificantly increase the complexity of the vibration pattern that can be produced on the radiating surface and therefore insignificantly improve the focusing ability of the TRA FS. Ultimately, the number of portions on the radiating surface which can independently contribute to the focusing ability of TRA FS is limited by the correlation length, which is close to the half-wavelength of ultrasound. This limit can actually be reached using just a few channels of the TRA FS, allowing it to steer the focus without generating detrimental side lobes. As shown experimentally in this work, a four-channel TRA FS outperforms a 256-channel random PA in terms of steering ability. An estimated number of channels of a PA which could provide ultrasound focusing and steering ability comparable to that of a TRA FS with just a few channels, such as that tested in this study, is over a thousand. Acknowledgment The authors acknowledge insightful discussion with A. Sutin, M. Fink, and M. Tanter. References [1] H. O’Neil, “Theory of focusing radiators,” J. Acoust. Soc. Am., vol. 21, pp. 516–526, Sep. 1949. [2] G. Kossoff, “Analysis of focusing action of spherically curved transducers,” Ultrasound Med. Biol., vol. 5, no. 4, pp. 359–365, 1979. [3] W. J. Fry and F. Dunn, “Ultrasound: Analysis and experimental methods in biological research,” in Physical Techniques in Biological

sarvazyan et al.: time-reversal acoustic focusing system Research, W. L. Nastuk, Ed. New York: Academic Press, 1962, ch. 4, pp. 261–394. [4] R. J. Lalonde, A. Worthington, and J. W. Hunt, “Field conjugate acoustic lenses for ultrasound hyperthermia,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 40, no. 5, pp. 1592–1602, 1993. [5] E. S. Ebbini and C. A. Cain, “A spherical-section ultrasound phased array applicator for deep localized hyperthermia,” IEEE Trans. Biomed. Eng., vol. 38, no. 7, pp. 634–643, Jul. 1991. [6] L. R. Gavrilov and J. W. Hand, “A theoretical assessment of the relative performance of spherical phased arrays for ultrasound surgery and therapy,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 47, no. 1, pp. 125–139, Jan. 2000. [7] S. A. Goss, L. Frizzell, J. T. Kouzmanoff, J. M. Barich, and J. M. Yang, “Sparse random ultrasound phased array for focal surgery,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 43, no. 6, pp. 1111–1121, Nov. 1996. [8] M. Fink, G. Montaldo, and M. Tanter, “Time reversal acoustics in biomedical engineering,” Annu. Rev. Biomed. Eng., vol. 5, pp. 465–497, 2003. [9] M. Fink, “Time reversed acoustics,” Sci. Am., vol. 281, pp. 91–97, Nov. 1999. [10] C. Draeger and M. Fink, “One-channel time-reversal in chaotic cavities: Theoretical limits,” J. Acoust. Soc. Am., vol. 105, no. 2, pp. 611–617, Feb. 1999. [11] C. Draeger, J.-C. Aime, and M. Fink, “One-channel time-reversal in chaotic cavities: Experimental results,” J. Acoust. Soc. Am., vol. 105, no. 2, pp. 618–625, Feb. 1999. [12] A. Sutin and A. Sarvazyan, “Spatial and temporal concentrating of ultrasound energy in complex systems by single transmitter using time reversal principles,” in Proc. 5th World Congr. on Ultrasonics, Paris, France, 2003, pp. 863–866. [13] N. Quieffin, S. Catheline, R. K. Ing, and M. Fink, “Real-time focusing using an ultrasonic one channel time-reversal mirror coupled to a solid cavity,” J. Acoust. Soc. Am., vol. 115, no. 1, pp. 1955–1960, May. 2004. [14] N. Quieffin, S. Catheline, R. K. Ing, and M. Fink,, “2D pseudo-array using an ultrasonic one channel time-reversal mirror,” in Proc. IEEE Ultrasonics Symposium, vol. 1, 23–27 Aug. 2004, pp. 801–804. [15] G. Montaldo, D. Palacio, M. Tanter, and M. Fink, “Building threedimensional images using a time-reversal chaotic cavity,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 52, no. 9, pp. 1489– 1497, Sept. 2005. [16] B. K. Choi, A. Sutin, and A. Sarvazyan, “Formation of desired waveform and focus structure by time reversal acoustic focusing system,” in Proc. 2006 IEEE Int. Ultrasonics Symp., Vancouver, Canada, 2006, pp. 2177–2181. [17] E. S. Ebbini and C. A. Cain, “Multiple-focus ultrasound phased array pattern synthesis: Optimal driving signal distributions for hyperthermia,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 36, no. 5, pp. 540–548, Sep. 1989. [18] L. R. Gavrilov, “Two-dimensional phased arrays for surgical application: Multiple foci generation and scanning,” Acoust. Phys., vol. 49, no. 5, pp. 508–516, 2003. [19] Y. Sinelnikov, A. Sutin, A. Vedernikov, and A. Sarvazyan, “Time reversal acoustic focusing with a catheter balloon,” Ultrasound Med. Biol., vol. 36, no. 1, pp. 86–94, 2010. [20] M. Skolnik, Introduction to Radar Systems. New York, NY: McGrawHill, 1962. [21] A. Sarvazyan, L. Fillinger, and L. Gavrilov, “A comparative study of systems used for dynamic focusing of ultrasound,” Acoust. Phys., vol. 55, no. 4–5, pp. 630–637, 2009.

817 Armen Sarvazyan received the Ph.D. degree in biophysics in 1969 from the Institute of Biophysics of the USSR Academy of Science and the D.Sc. degree in bioacoustics in 1983 from the same institute. In 1992, he organized the Biomolecular Acoustics Laboratory at Rutgers University, New Brunswick, NJ, and in 1995, he founded Artann Laboratories, a research company with the mission of early-stage development and validation of original technologies and devices. Currently, he is Chief Scientist of Artann Laboratories and Honorary Foreign Professor of Physics Faculty, Moscow State University, Russia. His current research interests include medical imaging, biomechanics, nondestructive testing, and time-reversal acoustics. He developed a new modality of medical imaging called mechanical (or tactile) imaging, based on the reconstruction of internal tissue structures using surface stress data, and another medical diagnostic technology, shear wave elasticity imaging, based on the use of the radiation force of a focused ultrasonic beam for remotely testing tissue elasticity. He holds over 90 international patents in the fields of ultrasonics and biomechanics, has edited 6 books, and has published more than 200 research papers.

Laurent Fillinger was born in September 1981 in Thann, France. In 2003, he received an M.S. degree in vibrations from the École Nationale Supérieure d’Ingénieur du Mans, France, and an M.S. degree in acoustics from the Université du Maine, France. He received his Ph.D. degree in acoustics from the Université du Maine, France, in 2006. He is now a postdoctoral fellow at Artann Laboratories, West Trenton, NJ, where he works on biomedical and industrial applications of timereversal acoustics, and at Stevens Institute of Technology, Hoboken, NJ, where he works on passive acoustic detection of scuba divers.

Leonid R. Gavrilov was born in Perm, Russia, on May 25, 1938. He graduated from the Leningrad (now St. Petersburg) Electrotechnical Institute, Department of Electronic Engineering, in 1961. He received the Ph.D. degree in engineering for a dissertation related to the development of methods of measurements distributions of the size and number of gas bubbles-cavitation nuclei in liquids from the Central Institute of Turbo-Machines, Leningrad, in 1966. He also received the D.Sc. degree from the N. N. Andreev Acoustics Institute, Moscow, for his dissertation, “Investigations of the effects of focused ultrasound on biological structures for application in medicine and physiology” in 1982. Since 1967, he has been with the N. N. Andreev Acoustical Institute studying initially the acoustical properties of gas bubbles in liquids and, since 1969, the application of focused ultrasound in medicine and physiology. Since 1980, he has been Head of the Laboratory of Medical Acoustics there. From 1993 to 1994, he was an Honorary Senior Research Fellow in the Queen’s University of Belfast, UK, and, from 1995 to 1998, he was a Senior Research Officer in the Radiological Sciences Unit, Hammersmith Hospital, Imperial College, London. Now he is working at the N. N. Andreev Acoustics Institute as a Chief Research Scientist. He is Co-Chair of the Section on Medical Acoustics of the Russian Acoustical Society. His current research interests are in the application of focused ultrasound for stimulation of neural structures, therapeutic application of ultrasound (in particular, in the development of high-intensity phased arrays for surgery), and biological effects and safety aspects of ultrasound.