WO2009027920A2

WO2009027920A2 - An ultrasound device for detecting presence or absence of cavitation events

Info

Publication number: WO2009027920A2
Application number: PCT/IB2008/053397
Authority: WO
Inventors: Christopher S. Hall; Anna T. Fernandez; Jan F. Suijver
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-08-31
Filing date: 2008-08-25
Publication date: 2009-03-05
Also published as: WO2009027920A3

Abstract

This invention relates to an ultrasound device for detecting presence or absence of cavitation events. An ultrasound transducer receives emitted and reflected ultrasound waves from tissues within a specific target volume and the received ultrasound waves are subsequently converted into a detection signal. A control unit then processes the detected signal for determining whether a cavitation event is detected. A liquid lens having an adjustable focal point is placed in front of the ultrasound transducer such that the received ultrasound waves go through the liquid lens prior to be received by the ultrasound transducer. The lens is controlled by the control unit such that in response to a detection of a cavitation event the focal position of the lens is adjusted substantially closer to the spatial location of the detected cavitation event.

Description

An ultrasound device for detecting presence or absence of cavitation events

FIELD OF THE INVENTION

The present invention relates to an ultrasound device and a method for detecting presence or absence of cavitation events.

BACKGROUND OF THE INVENTION

Adjustable liquid lens technology (FluidFocus Lens) has been disclosed in WO2003/069380, where it is possible allowing light to be focused through alterations in the physical boundaries (i.e. the meniscus) of a fluid filled cavity with specific refractive indices. A process known as electrowetting, wherein the fluid within the cavity is moved by the application of a voltage across conductive electrodes, accomplishes the movement of the surface of the fluid. This change in surface topology allows light to be refracted in such a way to alter the travel path to cause focusing.

Ultrasound propagates in a liquid medium; in fact, the human body is often referred to as a liquid incapable of supporting high frequency ultrasonic waves other than compression waves. In this sense, the waves are sensitive to distortion by differences in ultrasonic speed of propagation in bulk tissue, but also by abrupt changes in speed of sound at interfaces.

Passive cavitation devices (PCDs) are often used to detect the formation of unwanted gaseous microbubbles associated with highly focused ultrasound treatments. In some cases, the PCD is used to monitor formation of desired gaseous inclusions such as occur in the case of lithotripsy. These devices usually consist of a broadband ultrasound transducer with finite aperture and thereby limited spatial coverage. The passive cavitation device monitors the portion of the spectrum above the transmitted ultrasound's spectrum for the presence of large amplitude broadband "noise" associated with the collapse of a gaseous inclusion. Such devices have been proposed as safety monitoring techniques in high intensity focused ultrasound for thermal ablations, but also may be useful for determining the efficacy of rapid haemostasis (stopping blood flow) using ultrasound techniques. In current clinical practice, PCDs may play an important role in optimizing lithotripsy treatments. Passive cavitation devices are currently used as detection devices that merely detect the presence or absence of cavitation events. Their sensitivity usually varies inversely with the tissue volume being observed. In cases for high sensitivity, the field of view is usually quite narrow, thereby inadvertently missing some events outside the focal zone of the PCD. In addition, once a signal is detected, due to the low directivity, it is not possible to detect from which spatial location it occurs.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to overcome the above mentioned drawbacks by providing an ultrasound device and a method that enables detecting spatial location of cavitation events.

According to one aspect the present invention relates to ultrasound device for detecting presence or absence of cavitation events, comprising: an ultrasound transducer for receiving emitted and reflected ultrasound waves from at least one tissue within a specific target volume, the received ultrasound waves subsequently being converted into a detection signal, a control unit for processing the detection signal for determining whether a cavitation event is detected, wherein the ultrasound device further comprises a liquid lens having an adjustable focal point placed in front of the ultrasound transducer such that the received ultrasound waves go through the liquid lens prior to be received by the ultrasound transducer, the lens being controlled by the control unit such that in response to a detection of a cavitation event the focal position of the lens is adjusted substantially closer to the spatial location of the detected cavitation event. Accordingly, it allows the ultrasound transducer to control the position of the focus and the direction of propagation and thus, allows the transducer once it detects an event (or even prior to detection) to sweep a volume of tissue with specific degrees of focusing to increase sensitivity to detection of cavitation events. In addition, once the event is detected, subsequent events may be detected with greater spatial accuracy through movement (and adjustment of the f-number) of the ultrasound focus of the device, which could be a passive cavitation device (PCD). In contrast, a regular non-liquid lens would limit the transducer focus to a single spatial location and f-number. Thus, the liquid lens allows for spatial focusing control for the passive cavitation device during its use. In one embodiment, the length of the adjustable liquid lens is at least 1/2 of the width of the lens.

In one embodiment, the adjustable liquid lens has at least a first and a second fluid media not miscible with each another. In one embodiment, the separation between the first and second fluid media is a contact surface or meniscus which defines a boundary or interface there between. In one embodiment, one of the at least two fluid media is electrically conducting, and the other fluid medium is substantially non-electrically conducting, or electrically insulating. In one embodiment, electrodes are in contact with the electrically conducting fluid medium fluid media, the adjustment of the focal position by the control unit comprising adjusting the voltage across the electrodes, the adjustment of the voltage causing the conducting fluid medium to move and thus the focal position.

In one embodiment, the fluid media is selected from: - CC14,

Chlorobenzene, Decahydronaphtalene, Tetrahydronaphtalene, Phenylated silicone oil, - Water,

Methanol, Ethylene glycol, Perhydrofluorene, and a mix of two or more of the above mentioned fluid media. In one embodiment, the device further comprises a pre-amplifier coupled to the transducer, an attenuator coupled to the pre-amplifier, an amplifier coupled to the attenuator and a digitizer coupled to the amplifier.

In one embodiment, the ultrasound transducer acts further as the source of the ultrasound waves. According to another aspect, the present invention relates to a method of detecting presence or absence of cavitation events, comprising: a) placing a liquid lens having an adjustable focal position in front of an ultrasound transducer such that ultrasound waves emitted and reflected from at least one tissue within a target volume go through the liquid lens prior to be received by the ultrasound transducer, b) setting the focal position of the liquid lens to a first focal position having a focus at a first target volume, c) receiving emitted and reflected ultrasound waves from at least one tissue within the first target volume, the received ultrasound waves subsequently being converted into a first detection signal, d) processing the first detection signal for determining whether a cavitation event is detected at the first target volume, and in case a cavitation event is detected e) adjusting the focal position of the lens substantially closer to the spatial location of the first target volume, otherwise f) adjusting the focal position of the lens to a second target volume and repeating steps c) and d).

In one embodiment, subsequently after adjusting the focal position of the lens substantially closer to the spatial location within the first target volume the emitted and reflected ultrasound waves within the spatial location are received and processed resulting in a second detection signal, the step of adjusting the focal position of the lens substantially closer to the spatial location within the first target volume further including making at least one further adjusting within the first target volume until the detection signal reach maximum value.

According to yet another aspect, the present invention relates to a computer program product for instructing a processing unit to execute the method step of the above method when the product is run on a computer.

The aspects of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Figure 1 shows an embodiment of an ultrasound device according to the present invention for detecting presence or absence of cavitation events,

Figure 2a and 2b illustrate graphically the functioning of the device in Fig. 1, showing where the adjustable liquid lens is placed in front of the transducer, and Figure 3 shows a flowchart of a method of detecting presence or absence of cavitation events.

DESCRIPTION OF EMBODIMENTS Figure 1 shows an embodiment of an ultrasound device 100 according to the present invention for detecting presence or absence of cavitation events and comprises an ultrasound transducer 102 and a control unit 104, and a liquid lens 101 having an adjustable focal point placed in front of the ultrasound transducer 102. The ultrasound transducer 12 is adapted to receive emitted and reflected ultrasound waves from at least one tissue within a specific target volume, and the liquid lens is placed in front of the ultrasound transducer 102 such that the received ultrasound waves go through the liquid lens prior to be received by the ultrasound transducer 102. The device 100 may be a passive cavitation device (PCD) made from piezo-electric material, but also from pressure sensitive material such as mirror mounted interferometers, optical waveguides, and other devices capable of rendering an ultrasound signal into another processable form.

The ultrasound transducer 12 may as an example be a broadband ultrasound transducer 12 with finite aperture and thereby limited spatial coverage so that it monitors only a portion of the spectrum above the transmitted ultrasound's spectrum for the presence of large amplitude broadband "noise" associated with cavitation events. Accordingly, the broadband ultrasound transducer 12 monitors only a portion of the spectrum that is above a pre-defined threshold noise level.

In one embodiment, the ultrasound transducer 12 is a "receive only" transducer which will only "listen" for the event.

In another embodiment, the ultrasound transducer 12 is further adapted to emit ultrasound waves and thus to stimulate the cavitation event. Thus, the transducer is used to both stimulate the cavitation event and also to listen for it (in receive). The transducer can be any kind of piezo electric material (PZT, piezo-composites, PVDF, CMUTs, PMUTs). The size of the transducer is chosen to allow for optimal resolution and sensitivity to pick up cavitation events at a specific depth into the body / insonified material. The control unit 104 is adapted to process the out-coming detection signal from the ultrasound transducer 102 for determining whether a cavitation event is detected or not. If a cavitation event is detected the focal position of the adjustable liquid lens is adjusted substantially closer to the spatial location of the detected cavitation event, i.e. the tissue within the specific target volume. Such an adjustment has been disclosed in WO2003/069380, hereby incorporated as whole by reference, where light is allowed to be focused through alterations in the physical boundaries (i.e. the meniscus) of a fluid filled cavity with specific refractive indices. So-called electrowetting, wherein the fluid within the cavity is moved by the application of a voltage across conductive electrodes, accomplishes the movement of the surface of the fluid. This change in surface topology allows light to be refracted in such a way to alter the travel path to cause focusing.

In one embodiment, the adjustable liquid lens has at least two liquids not miscible with each another. Thus they always remain as separate fluid phases in the cavity. Advantageously, the speeds of sound in first and second fluid media are different from each other (i.e., acoustic waves propagate at a different velocity in fluid medium than they do in fluid medium) as the difference in the speed of sound leads to the refraction effect. Preferably, the separation between the first and second fluid media is a contact surface or meniscus which defines a boundary or interface between first and second fluid media and, without any solid part. In one embodiment, one of the two fluid media is electrically conducting, and the other fluid medium is substantially non-electrically conducting, or electrically insulating. In one embodiment, first fluid medium consists primarily of water. For example, it may be a salt solution, with ionic contents high enough to have an electrically polar behavior, or to be electrically conductive. In that case, first fluid medium may contain potassium and chloride ions, both with concentrations of 0.1 moll^"1, for example. Alternatively, it may be a mixture of water and ethyl alcohol with a substantial conductance due to the presence of ions such as sodium or potassium (for example with concentrations of 0.1 moll^"1). Second fluid medium, for example, may comprise silicone oil that is insensitive to electric fields. Table 1 below lists several exemplary fluids that may be employed as first or second fluid medium.

Table 1

Beneficially, a first electrode is provided in a housing (not shown) so as to be in contact with the one of the two fluid media that is electrically conducting. It may e.g. be assumed that the first fluid medium is an electrically conducting fluid medium, and the second fluid medium is the substantially non-electrically conducting fluid medium. However it should be understood that the first fluid medium could be the substantially non-electrically conducting fluid medium, and second fluid medium could be the electrically conducting fluid medium. In that case, first electrode would be arranged to be in contact with the second fluid medium. In one embodiment, the length of the adjustable liquid lens is at least 1/2 the width of the lens. However, by choosing suitable coatings, or make adjustments with the geometries, one can reduce this constraint (at the price of losing part of the working range of the lens).

In one embodiment, the device 100 further comprises pre-amplifier 106, an attenuator 107, an amplifier 103 and a digitizer 105.

The role of the pre-amplifier 106 is to amplify the signal before too much is lost due to impedance mismatch with cable and/or capacitance of the cable. The role of the attenuator 107 is to prevent that large signals out of the pre-amp (or even without the pre- amp) do not saturate the input of the amplifier 103, which amplifies the signal. The role of the digitizer 105 is clearly to convert the signal into a digital signal.

Figure 2a and 2b illustrate graphically the functioning of the device 100 in Fig. 1, showing where the adjustable liquid lens 101 is placed in front of the transducer 102. This placement allows the transducer 102, once it detects an event (or even prior to detection), to sweep a volume of tissue with specific degrees of focusing to increase sensitivity to detection of cavitation events. In addition, once the event is detected, subsequent events may be detected with greater spatial accuracy through movement (and adjust of the f-number) of the ultrasound focus of the PCD. In contrast, a regular (non- liquid lens) lens would limit the transducer focus to a single spatial location and f-number. Thus, the liquid lens allows for spatial focusing control for the passive cavitation device during its use. In one embodiment, this is done by means of adjusting the axial, lateral, and elevation focusing of the PCD. As depicted here, Fig. 2a depicts an initial focus of a localized volume 201 of the adjustable liquid lens where no cavitation event is detected, and Fig. 2b depicts a volume 200 where cavitation event is detected. In the scenario depicted in Fig. 2b, the geometry of the lens is adjusted to the volume closer to the spatial location 200 of the detected cavitation event.

According to the present invention, the control unit 104 in Fig. 1 is operated in accordance to an algorithm in a way that the control unit 104 instructs the adjustable liquid lens 101 to change its focal point to the spatial location of the event, namely to the spatial location 200. In this case, subsequent events may be detected and the lens 101 is adjusted each time to preferably maximize the received detection signal.

Figure 3 shows a flowchart of a method of detecting presence or absence of cavitation events.

In a first step (Sl) 300 said liquid lens having an adjustable focal position is placed in front of an ultrasound transducer such that ultrasound waves emitted and reflected from at least one tissue within a target volume go through the liquid lens prior to be received by the ultrasound transducer.

In a second step (S2) 301 the focal position of the liquid lens is set to a first focal position having a focus on at least one tissue within a first target volume. The emitted and reflected ultrasound waves from at least one tissue within the first target volume are then received (S3) and converted into a first detection signal. This signal is then processed for determining whether a cavitation event is detected at the first target volume.

If the processing of the signal results in that within this first target volume a cavitation event is detected, the focal position of the lens is adjusted substantially closer to the spatial location of the first target volume (S4) 304. By the term adjusting substantially closer is meant enhancing the degree of focusing for increasing the sensitivity of detection. After adjusting the focal position the ultrasound waves from the "new" or "more focused" spatial location within the first target volume are processed (S5) 305. In one embodiment, this adjustment within the first target volume is repeated until the signal resulted from the ultrasound waves reached maximum value. The situation could occur that "weak" cavitation event is detected within the first target volume, which could be interpreted in a way that a cavitation event is present within this first target volume. If this is the case, the first target volume is swept until the cavitation event becomes "stronger" or the resulting signal being detected reaches a maximum value. The displayed location will be given (S6) 307. If the result of the processing within said first target volume indicates however that none cavitation signal is detected, the focal position of the lens is set to a second target volume, where steps (S4) and (S5) are repeated within the second target volume. This means that within the second target volume, if a caviation event is detected the above mentioned steps are repeated, i.e. the second target volume is swept until the cavitation event becomes "stronger" or the resulting signal being detected reaches a maximum value. The displayed location will be given (S6) 307.

Again, if no cavitation event is detected for the second target volume, the focal position of the lens is set to a third target volume and said steps are repeated. Certain specific details of the disclosed embodiment are set forth for purposes of explanation rather than limitation, so as to provide a clear and thorough understanding of the present invention. However, it should be understood by those skilled in this art, that the present invention might be practiced in other embodiments that do not conform exactly to the details set forth herein, without departing significantly from the spirit and scope of this disclosure. Further, in this context, and for the purposes of brevity and clarity, detailed descriptions of well-known apparatuses, circuits and methodologies have been omitted so as to avoid unnecessary detail and possible confusion.

Reference signs are included in the claims, however the inclusion of the reference signs is only for clarity reasons and should not be construed as limiting the scope of the claims.

Claims

CLAIMS:

1. An ultrasound device (100) for detecting presence or absence of cavitation events, comprising: an ultrasound transducer (102) for receiving emitted and reflected ultrasound waves from at least one tissue within a specific target volume, the received ultrasound waves subsequently being converted into a detection signal, a control unit (104) for processing the detection signal for determining whether a cavitation event is detected, wherein the ultrasound device (100) further comprises a liquid lens (101) having an adjustable focal point placed in front of the ultrasound transducer (102) such that the received ultrasound waves go through the liquid lens prior to be received by the ultrasound transducer (102), the lens (101) being controlled by the control unit (104) such that in response to a detection of a cavitation event the focal position of the lens (101) is adjusted substantially closer to the spatial location of the detected cavitation event.

2. An ultrasound device according to claim 1, wherein the length of the adjustable liquid lens (101) is at least 1/2 of the width of the lens.

3. An ultrasound device according to claim 1, wherein the adjustable liquid lens (101) has at least a first and a second fluid media not miscible with each another.

4. An ultrasound device according to claim 3, wherein the separation between the first and second fluid media is a contact surface or meniscus which defines a boundary or interface there between.

5. An ultrasound device according to claim 3, wherein one of the at least two fluid media is electrically conducting, and the other fluid medium is substantially non- electrically conducting, or electrically insulating.

6. An ultrasound device according to claim 5, wherein electrodes are in contact with the electrically conducting fluid medium fluid media, the adjustment of the focal position by the control unit comprising adjusting the voltage across the electrodes, the adjustment of the voltage causing the conducting fluid medium to move and thus the focal position.

7. An ultrasound device according to claim 3, wherein the fluid media is selected from:

CCl₄, - Chlorobenzene,

Decahydronaphtalene,

Tetrahydronaphtalene,

Phenylated silicone oil,

Water, - Methanol,

Ethylene glycol,

Perhydrofluorene, and a mix of two or more of the above mentioned fluid media.

8. An ultrasound device according to claim 1, further comprising a pre-amp lifter

(106) coupled to the transducer, an attenuator (107) coupled to the pre-amplifϊer (106), an amplifier (103) coupled to the attenuator (107) and a digitizer (105) coupled to the amplifier (103).

9. An ultrasound device according to claim 1, wherein the ultrasound transducer

(102) acts further as the source of the ultrasound waves.

10. A method of detecting presence or absence of cavitation events, comprising: a) placing a liquid lens having an adjustable focal position in front of an ultrasound transducer such that ultrasound waves emitted and reflected from at least one tissue within a target volume go through the liquid lens prior to be received by the ultrasound transducer, b) setting the focal position of the liquid lens to a first focal position having a focus at a first target volume, c) receiving emitted and reflected ultrasound waves from at least one tissue within the first target volume, the received ultrasound waves subsequently being converted into a first detection signal, d) processing the first detection signal for determining whether a cavitation event is detected at the first target volume, and in case a cavitation event is detected e) adjusting the focal position of the lens substantially closer to the spatial location of the first target volume, otherwise f) adjusting the focal position of the lens to a second target volume and repeating steps c) and d).

11. A method according to claim 10, wherein subsequently after adjusting the focal position of the lens substantially closer to the spatial location within the first target volume the emitted and reflected ultrasound waves within the spatial location are received and processed resulting in a second detection signal, the step of adjusting the focal position of the lens substantially closer to the spatial location within the first target volume further including making at least one further adjusting within the first target volume until the detection signal reach maximum value.

12. A computer program product for instructing a processing unit to execute the method step of claim 10 when the product is run on a computer.