WO2009001306A2

WO2009001306A2 - Acoustic device with a variable focal length

Info

Publication number: WO2009001306A2
Application number: PCT/IB2008/052541
Authority: WO
Inventors: Szabolcs Deladi; Jan F. Suijver
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-06-28
Filing date: 2008-06-25
Publication date: 2008-12-31
Also published as: WO2009001306A3

Abstract

The invention relates to an acoustic device comprising an acoustic lens with variable focal length and means (11) for directing incoming acoustic waves onto thelens, wherein the acoustic lens comprises a first fluid medium (1) and a second fluid medium (2) that are not misciblewitheach other, in which the acoustic waves (21) have different velocities and with a contact meniscus (3) between the two fluid media (1,2), wherein the 5 difference in viscosity of the two fluid media (1,2) is such that the contact meniscus is substantiallyunperturbed by the acoustic waves (21). The invention is based on the experimental observation that the main physical parameter whichdetermines the interaction between the acoustic waves (21) and the contact meniscus (3) are the values of the viscosity ofthe two fluid media (1,2). Therefore, the perturbation of the contact meniscus (3) between 10 the two fluid media (1,2) by the acousticwaves (21) is reduced considerably when the values ofthe viscosity of the two fluid media (1,2) are selected appropriately leading to an increased controlofthe functioning of the acoustic lens.

Description

Acoustic device with a variable focal length

FIELD OF THE INVENTION

The invention relates to an acoustic device comprising an acoustic lens with a variable focal length.

BACKGROUND OF THE INVENTION

Acoustic waves are useful in many scientific or technical fields, such as medical diagnosis, non-destructive control of mechanical parts and underwater imaging, etc. Acoustic waves allow diagnoses and controls which are complementary to optical observations, because acoustic waves can travel in media that are not transparent to electromagnetic waves. An example of a medical application of acoustic waves is the use of high intensity focused ultrasound (HIFU) in a highly precise medical procedure wherein high-intensity focused ultrasound is applied to heat and subsequently destroy pathogenic tissue rapidly. As an acoustic wave propagates through the tissue, part of it is absorbed and converted to heat. In HIFU therapy, ultrasound beams are focused on selected tissue, and due to the significant energy deposition at the focus, temperature within the tissue rises to 65° to 85°C, destroying the selected tissue by denaturazation followed by coagulation necrosis. Each "sonication" with the HIFU beam treats a precisely defined portion of the targeted tissue.

WO 2005/122139 discloses an acoustic device comprising an acoustic lens with a variable focal length. The acoustic lens comprises a curved boundary, for example a contact meniscus, between two liquids, typically immiscible, and means (for example using electrical or mechanical forces) to vary the shape of the boundary, which in turn varies the focal length of the lens. This publication also discloses that an acoustic wave generator, such as is disclosed in U.S. Patent 5,305,731, can optionally be incorporated into the acoustic device. The shape of the boundary between the two liquids may be perturbed by acoustic waves, specifically in the case that high intensity focused ultrasound is applied. A perturbation of the shape of the boundary by the acoustic waves leads to a loss of control of the focal length of the acoustic lens. SUMMARY OF THE INVENTION

It is an object of the invention to provide an acoustic device in which the influence of an acoustic wave on the shape of the boundary between two liquids is reduced. The invention is defined by the independent claims. Advantageous embodiments are defined by the dependent claims.

The object is achieved by the acoustic device according to the invention which comprises an acoustic lens with means for directing incoming acoustic waves onto the lens, wherein the acoustic lens comprises a first fluid medium and a second fluid medium that are not miscible with each other, in which the acoustic waves have different velocities and with a contact meniscus between the two fluid media, wherein the difference in viscosity of the two fluid media is such that the contact meniscus is substantially unperturbed by the acoustic waves. The invention is based on the experimental observation that the main physical parameter which determines the interaction between the acoustic waves and the boundary between the two immiscible fluid media, which is defined by the contact meniscus, is the value of the viscosity of the two fluid media. Therefore, the perturbation of the contact meniscus between the two fluid media by the acoustic waves is reduced considerably when the values of the viscosity of the two fluid media is selected appropriately leading to an increased control of the functioning of the acoustic lens.

In an embodiment of the acoustic device according to the invention the following condition is fulfilled:

15.5 - 0.5^» vl < v2 < 144 + 16 • vl with vl ... viscosity of the first fluid medium (1) v2 ... viscosity of the second fluid medium (2)

Experiments have shown that in this case the perturbation of the meniscus is reduced to an acceptable level enabling an improved control of the function of the lens.

In an embodiment of the acoustic device according to the invention the first fluid medium comprises water and the second fluid medium comprises a silicone oil mixture with a viscosity between 15 mm²/s and 160 mm²/s. In this way the difference in velocity of sound between the two fluid media is provided, because the velocity of sound in water is about 1,490 m/s and the velocity of sound in silicone oil is about 950 m/s to 1020 m/s, i.e. 1.5 times lower. Furthermore, because the viscosity of water is 1 mmVs, the perturbation of the contact meniscus by the acoustic waves is reduced considerably by the difference in viscosity between water and the silicon oil mixture. In a preferred embodiment the silicone oil mixture has a viscosity between 40 mm²/s and 50 mmVs. In an embodiment of the acoustic device according to the invention the first fluid medium comprises a mixture of water and glycerol and the second fluid medium comprises a silicone oil mixture with a viscosity between 5 mm²/s and 500 mm²/s. Also in this way the difference in velocity of sound between the two fluid media is provided. Furthermore, the application of a mixture of water and glycerol, instead of water alone, considerably increases the viscosity range of the silicon oil mixture which can be applied and in which the perturbation of the contact meniscus by the acoustic waves is reduced.

In an embodiment of the acoustic device according to the invention the first fluid medium comprises a mixture of water and glycerol with a viscosity between 15 mm²/s and 35 mm²/s and the second fluid medium comprises a silicone oil mixture.

In an embodiment of the acoustic device according to the invention the acoustic device operates under high intensity ultrasonic conditions. Experiments have shown that the contact meniscus of the acoustic device is extremely sensitive to being perturbed in the case that high intensity ultrasound conditions are applied. High intensity ultrasound conditions apply a power substantially higher than 1 Watt, typically between 10 Watt and 60 Watt.

In an embodiment of the acoustic device according to the invention the acoustic device further comprises means for applying a force directly onto at least part of one of the fluid media so as to selectively induce a displacement of at least part of the contact meniscus. In this way the focal length of the acoustic lens is variable and may be actively controlled by displacing at least part of the contact meniscus by applying a force directly on at least a part of one of the fluid media. Furthermore, because the movable parts in this acoustic lens are restricted to the contact meniscus only, resulting a relatively low mass, the active adjustment of the focal length of the acoustic lens may be varied very rapidly. In a further embodiment of the acoustic device according to the invention one of the first and second fluid media comprises a polar or electrically conductive liquid substance, and wherein the force applying means comprise an electrode arranged to apply an electric force onto at least part of the polar or electrically conductive liquid substance. Such means are adapted for electronically controlling the displacement of the contact meniscus. Variations of the focal length of the acoustic lens can thus be obtained. The electric force is applied advantageously on a part of the polar or electrically conductive liquid substance which is adjacent to the contact meniscus. Then the whole quantity of the polar or electrically conductive liquid substance may be reduced, allowing reductions in the mass and in the size of the device. The object is also achieved by a catheter comprising an acoustic device according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be further elucidated and described with reference to the drawings, in which:

Figs, la-c are schematic cross-sectional views of an acoustic device showing the effect of acoustic waves on the contact meniscus between two immiscible fluid media; and Figs. 2a-b are schematic cross-sectional views of an acoustic device according to an embodiment of the invention.

The Figures are not drawn to scale. In general, identical components are denoted by the same reference numerals in the Figures.

DETAILED DESCRIPTION OF EMBODIMENTS

Figs. Ia, Ib and Ic show a schematic cross-sectional view of an acoustic device showing the effect of acoustic waves on the contact meniscus between two immiscible fluid media. A first fluid medium 1 and a second fluid medium 2 are provided in a housing, in this example a cylinder housing. The first fluid medium 1 and the second fluid medium 2 are not miscible with each other resulting in a contact meniscus 3 forming the boundary between the first fluid medium 1 and the second fluid medium 2. An acoustic wave generator 11 provides for acoustic waves 21 first in the first fluid medium 1 which subsequently travel via the contact meniscus 3 through the second fluid medium 2. In a first experiment the first fluid medium 1 comprises water, which has a viscosity of 1 mm²/s, and the second fluid medium comprises a silicone oil mixture for which the viscosity could be set to a specific value, for example by mixing DMS-T05 and DMS-T25 which are two polydimethylsiloxane trimethylsiloxy terminated oils with a viscosity of 5 mm²/s and 500 mmVs, respectively. The acoustic wave generator 11 is, in this experiment, an 8 MHz high-intensity focused ultrasound transducer with a focal length of 70 mm, which can be operated up to 50 W. The contact meniscus 3 is, in this experiment located between 20 mm and 50 mm from the transducer. The viscosity of the silicon oil mixture was measured using a reometer. In Fig. Ia the silicon oil mixture has a viscosity of 500 mm²/s and the acoustic waves 21 originating from the acoustic wave generator 11 result in a first perturbation 3 a of the contact meniscus 3. In Fig. Ib the silicon oil mixture has a viscosity of 5 mmVs and the acoustic waves 21 originating from the acoustic wave generator 11 result in a second perturbation 3b of the contact meniscus 3. A silicon oil mixture with a viscosity of 45 mm²/s provides for an essentially unperturbed meniscus 3, as is schematically shown in Fig. 3c. Further experiments show that for a viscosity of the silicon oil mixture between 15 mm²/s and 160 mm²/s the influence of the acoustic waves 21 on the contact meniscus 3 is reduced considerably. This reduction is optimum for a viscosity of the silicon oil mixture between 40 mmVs and 50 mmVs.

In a second experiment the first fluid medium 1 comprises a mixture of water and glycerol and the second fluid medium again comprises the silicon oil mixture for which the viscosity can be set to a specific value. The results show that the viscosity range of the silicon oil mixture, for which the perturbance of the contact meniscus 3 by the acoustic waves 21 is reduced considerably, is enlarged compared to the experiment in which the first fluid medium 1 comprised water. If, for example, the first fluid medium 1 comprises a mixture of 33 vol.% water and 67 vol.% glycerol, which has a viscosity between 15 mm²/s and 35 mmVs, in this case for example 22 mm²/s, a silicon oil mixture with a viscosity between 5 IM²Zs and 500 mm²/s did not suffer from deformation of the contact meniscus 3 due to the acoustic waves 21. The effect of enlarged allowable viscosity range for the silicon oil mixture is partially due to increased attenuation of the acoustic waves 21 in the water/glycerol mixture compared to that of pure water in the previous example. Figs. 2a-b show schematic cross-sectional views of a catheter comprising an acoustic device 200 according to an embodiment of the invention. The catheter has a housing 9, which in this example is cylindrical, and comprises the acoustic wave generator 11 that in this case provides for high intensity ultrasound waves 21. The catheter furthermore comprises a container comprising the first fluid medium 1 and the second fluid medium 2 that are not miscible with each other. Thus they always remain as separate liquid phases in the container. The separation between the two fluidic media 1 ,2 is formed by the contact meniscus 3 which defines a boundary without any solid part. The velocity of the acoustic waves 21 in the first fluidic medium 1 is different from the velocity of the acoustic waves 21 in the second fluidic medium 2. A first electrode 5 and a second electrode 6 are provided in the acoustic device 200 and are separately powered by voltage sources 51, 61. The first and second electrodes 5,6 may be electrically insulated from the liquid media 1,2 by an insulation layer 13. In that case the first and second electrode 5,6 are coupled capacitively with the liquid media 1,2. In alternative embodiments, the first and second electrode 5,6 may be in contact with the liquid media 1,2. For example, the first and second electrode 5,6 comprise a metal coated with 5 to 10 micrometers of parylene-N, which forms the insulation layer 13, and, additionally, on the parylene-N layer, an amorphous fluoropolymer for switching with low hysteresis. The coupling of the acoustic waves of the wave generator 11 with the fluidic medium 1 is, in this example, provided for by an acoustic matching layer 12. Beneficially, a third electrode 99 is provided so as to be in contact with the one of the two fluid media 1,2 that is electrically conducting. For example, the second fluid medium 2 is an electrically conducting fluid medium and the first fluid medium 1 is the substantially non-electrically conducting fluid medium. However it should be understood that in another example the second fluid medium 2 is the substantially non-electrically conducting fluid medium and the first fluid medium 1 is the electrically conducting fluid medium. In that case, the third electrode 99 would be arranged to be in contact with the first fluid medium 1.

The first liquid medium 1 comprises water, in this example a salt solution with ionic contents high enough to have an electrically polar behavior, or to be electrically conductive. The first liquid medium 1 may contain potassium, sodium or chloride ions, both with concentrations of above 1 moll^"1, for example. Alternatively, it may be a mixture of water and ethyl alcohol. The second liquid medium 2 is in this example made of a silicone oil mixture, that is insensitive to electric fields and that has a viscosity range which provides for a contact meniscus that is essentially unperturbed by the acoustic waves 21. The contact angle of the contact meniscus 3 with the insulation layer 13 is adjusted by providing a first and a second voltage Vl, V2 on the first and the second electrodes 5,6 respectively. For a certain combination of voltages Vl and V2, the contact meniscus has a flat shape. The difference in the ultrasound velocity of the acoustic waves 21 in the two liquid media 1,2 causes a refraction of the acoustice waves 21 as is shown in Fig. 2a. If voltages V3 and V4 are applied on the first and the second electrode 5,6, respectively, that are different from the voltages Vl and V2, then the shape of the contact meniscus 3 changes to the one that is shown in Fig. 2b. The acoustic waves 21 are now refracted into a direction that is substantially different from that when voltages Vl and V2 were applied. In this way the first and the second electrodes 5,6 provide for a steering of the acoustic waves 21 into a direction where they are required, which is beneficial in the treatment of, for example, specific locations in tissue.

In another embodiment according to the invention the acoustic device does not comprise the electrodes 5,6,99 and, hence, comprises an acoustic lens whereby the refraction of the acoustic waves 21 by the contact meniscus is fixed. By choosing the right optical and acoustical properties for the two fluid media 1,2, the refraction of the acoustic waves 21 is changed and, hence, the focal length of the acoustic lens may be varied.

The invention is useful for all refracting of high intensity ultrasound. For example minimally invasive, catheter-based, ultrasound applications such as the high- intensity focused ultrasound required for the treatment of atrial fibrillation. Also potential uses for intravenous or intra-arterial clot dissolution benefit from this invention as do potential plaque detection, destruction and annealing schemes. The invention may also be used in a slightly modified form for intra-urinary tract prostate ablation.

In summary, the invention provides for an acoustic device comprising an acoustic lens with variable focal length and means for directing incoming acoustic waves onto the lens, wherein the acoustic lens comprises a first fluid medium and a second fluid medium that are not miscible with each other, in which the acoustic waves have different velocities and with a contact meniscus between the two fluid media, wherein the difference in viscosity of the two fluid media is such that the contact meniscus is substantially unperturbed by the acoustic waves. The invention is based on the experimental observation that the main physical parameter which determines the interaction between the acoustic waves and the contact meniscus are the values of the viscosity of the two fluid media. Therefore, the perturbation of the contact meniscus between the two fluid media by the acoustic waves is reduced considerably when the values of the viscosity of the two fluid media are selected appropriately leading to an increased control of the functioning of the acoustic lens.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. Acoustic device comprising an acoustic lens with means (11) for directing incoming acoustic waves onto the lens, wherein the acoustic lens comprises a first fluid medium (1) and a second fluid medium (2) that are not miscible with each other, in which the acoustic waves (21) have different velocities and with a contact meniscus (3) between the two fluid media (1,2), wherein the difference in viscosity of the two fluid media (1,2) is such that the contact meniscus is substantially unperturbed by the acoustic waves (21).

2. Acoustic device according to claim 1, wherein the following condition is fulfilled: 15.5 - 0.5^« vl < v2 < 144 + 16 • vl with vl ... viscosity of the first fluid medium (1) v2 ... viscosity of the second fluid medium (2)

3. Acoustic device as claimed in claim 1 or 2, in which the first fluid medium (1) comprises water and the second fluid medium (2) comprises a silicone oil mixture with a viscosity that is between 15 mm²/s and 160 mm²/s.

4. Acoustic device as claimed in claim 3, in which the viscosity of the silicone oil mixture is between 40 mm²/s and 50 mm²/s.

5. Acoustic device as claimed in claim 1 or 2, in which the first fluid medium (1) comprises a mixture of water and glycerol and the second fluid medium (2) comprises a silicone oil mixture with a viscosity between 5 mm²/s and 500 mm²/s.

6. Acoustic device as claimed in claim 1 or 2, in which the first fluid medium (1) comprises a mixture of water and glycerol with a viscosity between 15 mm²/s and 35 mm²/s and the second fluid medium (2) comprises a silicone oil mixture.

7. Acoustic device according to claim 1, wherein the acoustic device operates under high intensity ultrasonic conditions.

8. Acoustic device according to claim 1, wherein the acoustic device further comprises means (5,6) for applying a force directly onto at least part of one of the fluid media (1,2) so as to selectively induce a displacement of at least part of the contact meniscus (3).

9. Acoustic device according to claim 8, wherein one of the first and second fluid media (1,2) comprises a polar or electrically conductive liquid substance, and wherein the force applying means comprise an electrode (5,6) arranged to apply an electric force onto at least part of the polar or electrically conductive liquid substance.

10. Acoustic device according to claim 9, wherein the electrode (5,6) is arranged to apply the electric force on a part of the polar or electrically conductive liquid substance adjacent to the contact meniscus (3).

11. A catheter (200) comprising an acoustic device as claimed in any of the preceding claims.