Patent Description:
<CIT> discloses an ultrasonic transducer system comprising a lens having a concave surface. A middle portion of the lens has a thickness equal to a ¼ wavelength of the second harmonic or greater of the ultrasound signal transmitted by the transducer.

<CIT> discloses an ultrasound transducer stack having a matching layer comprising a matrix material loaded with a plurality of particles.

<CIT> discloses an ultrasound transducer formed of a plurality of layers stacked together. The plurality of layers comprises a piezoelectric layer and a dielectric layer connected to one another.

<CIT> discloses an ultrasonic wave probe having a concave surface.

<CIT> teaches an ultrasound therapy probe comprising an ultrasound transducer and a housing in which the transducer is disposed. The housing has a curved surface configured to function as an acoustic lens, with a center of the of the curved surface having a thickness of approximately a full wavelength of the acoustic beam generated by the transducer. In a first aspect, the present invention provides an ultrasound transducer stack comprising: a transducer layer configured to transmit ultrasound energy at a center frequency; and a lens layer secured to the transducer layer, wherein at least a portion of the lens layer has a concave curved recess with a length measured in an elevation direction of the transducer stack; wherein a center portion of the concave curved recess has a length that is less than the length of the concave curved recess and is defined between a midpoint of the concave curved recess and points outwardly therefrom in the elevation direction such that the center portion has an average thickness measured in an axial direction of the transducer stack that is substantially equal to one of <NUM>, <NUM>, <NUM> or <NUM> multiples of a <NUM>/<NUM> wavelength of the center frequency of the transducer layer characterised in that a first matching layer is disposed between the transducer layer and the lens layer; and a second matching layer is disposed between the lens layer and the first matching layer, wherein the second matching layer comprises cyanoacrylate, and wherein the first matching layer and the second matching layer each have a thickness of approximately <NUM>/<NUM>-wavelength of the center frequency of the ultrasound energy from the transducer layer.

In a second aspect, the present invention provides a method of constructing an ultrasound transducer stack, comprising: fabricating an acoustic lens layer having a center curved section and two flat side sections, wherein the center curved section comprises a concave curved recess with a length measured in an elevation direction of the transducer stack, wherein fabricating the curved section includes fabricating a center portion having a midpoint and two endpoints such that the center portion has a first thickness at the midpoint and a second thickness at each of the two endpoints, wherein the center portion has a length less than the length of the concave curved recess and is defined between the midpoint of the concave curved recess and points outwardly therefrom in the elevation direction, and wherein an average of the first thickness and the second thickness is substantially equal to one of <NUM>, <NUM>, <NUM> or <NUM> multiples of a <NUM>/<NUM> wavelength of the center frequency of the ultrasound transducer; characterised by bonding the lens layer to a first matching layer operationally coupled to a transducer layer; and using a second matching layer to bond the lens layer to the first matching layer such that the second matching layer is disposed between the lens layer and the first matching layer, wherein the second matching layer comprises cyanoacrylate, and wherein the first matching layer and the second matching layer each have a thickness of approximately <NUM>/<NUM>-wavelength of the center frequency of the ultrasound energy from the transducer layer.

The invention may be more completely understood in consideration of the accompanying drawings, which are incorporated in and constitute a part of this specification, and together with the description, serve to illustrate the disclosed technology.

Ultrasonic transducers provide a means for converting electrical energy into acoustic energy and vice versa. When the electrical energy is in the form of a radio frequency (RF) signal, a transducer can produce ultrasonic signals with the same frequency characteristics as the driving electrical RF signal. Conventional clinical ultrasound transducers are typically operated at center frequencies ranging from less than <NUM> Megahertz (MHz) to about <NUM>. Ultrasound in the frequency spectrum of <NUM>-<NUM> generally provides a means of imaging biological tissue with a resolution ranging from several millimeters to generally greater than <NUM> microns and at depths from a few millimeters to greater than <NUM> centimeters.

In contrast, high frequency ultrasonic (HFUS) transducers are generally ultrasonic transducers with center frequencies above <NUM> and ranging to over <NUM> (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>). HFUS transducers provide higher resolution than transducers that operate at lower frequencies (e.g., less than <NUM>. ) while limiting a maximum depth of penetration. As a result, HFUS transducers can image biological tissue at depths ranging from, for example, a fraction of a millimeter (e.g., <NUM>, <NUM>, <NUM>) to <NUM> or greater (e.g., <NUM>) with resolutions ranging, for example, from about <NUM> microns to about <NUM> microns.

For transducers operating at frequencies less than <NUM>, for example, a wide variety of lens materials are available to produce convex lenses that are substantially acoustically impedance-matched to a medium (e.g., tissue in a subject) to be imaged. Acoustic energy received at these transducers is typically almost completely transmitted through the lens material to be received by the transducer, with almost no energy reflected back into the medium, and thus no multipath artifacts are created. In addition, one skilled in the art will understand that a well-designed transducer, having a well matched lens material will not exhibit multiple reflections within the lens itself. In the case of HFUS transducers, however, very few materials are suitable for constructing acoustic lenses due to significantly higher acoustic attenuation. As those of ordinary skill in the art will appreciate, acoustic attenuation in polymers tends to increase exponentially with frequency. Accordingly, an acoustic attenuation of ultrasound energy at <NUM> in a polymer can be an order of magnitude (e.g., <NUM> times greater, <NUM> times greater, <NUM> times greater) than an acoustic attenuation of ultrasound energy of <NUM> and below in the same polymer.

There can be many challenges associated with fabricating HFUS transducers that do not arise when working with traditional clinical ultrasonic transducers that operate at frequencies below about <NUM>. Those of ordinary skill in the art will appreciate that structures (e.g., transducer layers, matching layers, lenses) associated with an ultrasound transducer generally scale in a manner that is inversely proportional to an operating frequency of the transducer. For example, a <NUM> transducer will have structures about <NUM> times smaller than a <NUM> transducer. In many cases, materials or techniques used with lower frequency transducers (e.g., less than about <NUM>. ) cannot be scaled down to sizes and/or shapes suitable for use in HFUS transducers. Accordingly, new technologies may need to be developed or adapted in the fabrication of HFUS transducers. In other cases, entirely new requirements exist when dealing with the higher radio frequency electronic and acoustic signals associated with HFUS transducers.

Conventional HFUS transducers typically include hard plastic acoustic lenses shaped and/or formed into concave lenses in order to focus an elevation dimension of the transducer. Suitable HFUS lens materials may include, for example, polymethylpentene (e.g., TPX®), cross-linked polystyrene (e.g., Rexolite®), and polybenzimidazole (e.g., Celazole®), all of which have relatively low attenuation at frequencies greater than about <NUM>. Acoustic lenses made from materials suited for HFUS use, however, may also have acoustic impedances significantly or substantially different (e.g., <NUM>% different, <NUM>% different, <NUM>% different) from an acoustic impedance of a subject to be imaged. The resulting acoustic impedance mismatch (e.g., a difference of <NUM> MRayl, <NUM> MRayl, <NUM> MRayl, <NUM> MRayl, <NUM> MRayls) between the lens and the subject can cause multipath imaging artifacts when ultrasound energy is transmitted from the transducer and received at the transducer to form an ultrasound image. An acoustic impedance mismatch at the front of the lens with respect to the coupling medium or the subject can also result in intra-lens reflections and/or lens reverberation artifacts that can degrade the axial resolution of the ultrasound transducer.

The multipath or multi-bounce artifacts can cause a ghost image of bright specular reflectors appearing an equal depth below the true image of the specular reflector. A skin line of a subject, for example, may be imaged at a depth of <NUM> in the image and cause a multipath artifact at a depth of <NUM>. Those of ordinary skill in the art will appreciate that such an artifact may be produced when ultrasonic energy emitted from the transducer strikes a strong specular reflector (e.g., a skin line of a subject) roughly normal to the path of the ultrasound. A portion (e.g., <NUM>%, <NUM>%) of the emitted ultrasonic energy may be reflected back from the specular reflector toward the transducer lens, whereupon a second reflection may occur if the lens is not substantially acoustically matched to the transmission medium (e.g., gel, water). The second reflection may then propagate back to the specular reflector a second time, where again, a specular reflection occurs and acoustic energy is once again received by the transducer. A cascade of such reflections can cause a series of multipath artifacts to appear in an ultrasound image. Such partial reflections can occur repeatedly until no significant energy remains in the reflections. One approach to mitigating imaging artifacts may include positioning an acoustic matching layer on an outer surface of an acoustic lens. Lenses having matching layers on their outer surfaces, however, can be very difficult to fabricate and, in many cases, are impractical for use with ultrasound transducers that operate at higher frequencies (e.g., greater than about <NUM>.

Lens reverberation artifacts caused by, for example, intra-lens multiple reflections can be similar to the multipath artifacts described above. Intra-lens reflections, however, occur entirely within the lens material and may be caused by an acoustic mismatch between the outer surface of the lens and the acoustic coupling medium or the subject being imaged. A partial echo is produced at the front face of the lens as the acoustic pulse exits the transducer and enters the subject. This echo can then reverberate between any internal acoustic mismatch in the transducer acoustic stack, such as the back surface of the lens for example. As those of ordinary skill in the art will appreciate, every effort will be made to acoustically match the back surface of the lens to the acoustic stack of the transducer, typically through the use of some form of acoustic matching layer. However, due to the low attenuation of HFUS lens materials, even a small reflection from the back surface/stack interface can give rise to a lens reverb artifact. The effect of the lens reverb artifact is to effectively lengthen the pulse of the transducer as each reverb echo become part of the main transducer pulse and thus any echoes received by the transducer.

<FIG> is a schematic view of a prior art transducer <NUM> that illustrates one example of the intra-lens reflections and reverberation artifacts described above. The transducer <NUM> includes a transducer layer <NUM>, a matching layer <NUM> and an acoustic lens <NUM> having a lower surface <NUM> and a thickness T. The transducer <NUM> transmits and receives ultrasound energy (e.g., high frequency ultrasound of <NUM> or greater) through a skin line <NUM> of a subject <NUM> (e.g., a human patient, an animal). The transducer layer <NUM> is configured to transmit a primary ultrasound signal S into the subject <NUM> and receives ultrasound echoes S', which are used to form an ultrasound image.

First, second and third reflections R1, R1' and R1" illustrate one example of the multipath artifacts described above. The skin line <NUM> reflects a portion (e.g., <NUM>%, <NUM>%, <NUM>%) of the signal S thereby forming the first reflection R1. The first reflection R1 propagates back toward the transducer layer <NUM>, which reflects a portion (e.g., <NUM>%, <NUM>%, <NUM>%) of the first reflection R1 back toward the subject thereby forming the second reflection R1'. The skin line <NUM> reflects a portion of the second reflection R1' back toward the transducer layer <NUM> thereby forming the third reflection R1". The transducer layer <NUM> receives the echoes S' along with portions of the first reflection R1 and third reflection R1", all of which are combined by an image processor (not shown) to form an ultrasound image. As those of ordinary skill in the art will appreciate, the reflections R1 and R1" can cause undesirable artifacts in the ultrasound image.

First, second and third reflections R2, R2' and R2" illustrate one example of the intra-lens reverberation artifacts described above. The lower surface <NUM> of the lens <NUM> reflects a portion (e.g., <NUM>%, <NUM>%, <NUM>%) of the signal S thereby forming the first reflection R2. The first reflection R2 propagates back toward the transducer layer <NUM>, which reflects a portion (e.g., <NUM>%, <NUM>%, <NUM>%) of the first reflection R2 back toward the subject thereby forming the second reflection R2'. The lower surface <NUM> of the lens <NUM> reflects a portion of the second reflection R2' back toward the transducer layer <NUM> thereby forming the third reflection R2". The transducer layer <NUM> receives a combination of the echoes S' along with portions of the first reflection R2 and the third reflection R2" to form an ultrasound image. The reflections R2 and R2" can cause undesirable artifacts in the ultrasound image. In many instances, reflections similar to R1, R1", R2 and R2" can cause artifacts in the same ultrasound image, which can significantly reduce image quality.

<FIG> is a schematic side view of a transducer <NUM> configured in accordance with one or more embodiments of the disclosed technology. The transducer <NUM> includes a lens <NUM> having a curved surface <NUM> and a center portion <NUM>. The center portion <NUM> has an average thickness approximately equal to <NUM>/<NUM>-wavelength, <NUM>/<NUM>-wavelength, <NUM>/<NUM>-wavelength or <NUM>/<NUM>-wavelength of the center frequency of the transducer <NUM>. A signal S2 is transmitted into the subject <NUM>. The skin line <NUM> reflects a portion of the signal S2 to form a first reflection R3, and the curved portion <NUM> reflects a portion of the signal S2 to form second reflections R4. In contrast to reflections R1 and R2 discussed above, the first reflection R3 and the second reflections R4 are not specular reflections and thus do not travel back to the transducer <NUM>. Accordingly the lens <NUM> can significantly reduce artifacts in a HFUS image, such as the intra-lens and multipath reflections discussed above with reference to <FIG>.

The disclosed technology can provide a reduction of multipath artifacts (e.g., intra-lens reverberation artifacts, external multi-bounce artifacts) in HFUS transducers described above. In one aspect of the present disclosure, an ultrasound transducer includes an acoustical lens in which a center portion of the lens (e.g., the thinnest part of the concave shape of the lens between two end portions of the lens) has a thickness of about a fractional portion of a wavelength of the transducer center frequency. The lens center portion has an average thickness approximately equal to <NUM>/<NUM>-wavelength, <NUM>/<NUM>-wavelength, <NUM>/<NUM>-wavelength or <NUM>/<NUM>-wavelength of the transducer center frequency (e.g., <NUM>. Incorporating the lens described above onto an ultrasound transducer results in the central portion of the lens effectively adding an additional matching layer (e.g., a quarter wavelength matching layer) to the front of the transducer. The disclosed technology therefore provides a lens having reduced acoustic reflectivity to normal incident plane waves, thus mitigating multipath acoustic artifacts in the image, and reducing intra lens reverb artifacts as well. In some embodiments, for example, the disclosed technology can increase the transmission coefficient of an HFUS transducer lens from <NUM>% to about <NUM>%. Stated differently, the disclosed technology can reduce the reflection coefficient of an HFUS transducer lens from <NUM>% to less than between <NUM>% and <NUM>% or less, thereby significantly increasing sensitivity of the HFUS transducer (e.g., an increase between 1dB and <NUM>.

In the disclosed technology, an ultrasound transducer stack includes a transducer layer and a lens layer. The transducer layer is configured to transmit ultrasound energy at a center frequency (e.g., <NUM>. or higher). The lens layer has an upper surface underlying the transducer layer. At least a portion of the lens layer has a concave curvature in a direction normal to an axial direction of the transducer. A center portion of the lens layer has an average thickness that is substantially equal to <NUM>, <NUM>, <NUM> or <NUM> multiples of a <NUM>/<NUM> wavelength of the center frequency of the transducer layer. A matching layer is disposed between the lens layer and the transducer layer. The matching layer is attached to the lens layer by another matching layer that comprises cyanoacrylate. In some embodiments, the lens layer has an acoustic impedance substantially different (e.g., <NUM>% different, <NUM>% different, <NUM>% different) than an acoustic impedance of water.

In yet another embodiment of the disclosed technology, an ultrasound system includes an ultrasound imaging system coupled to an ultrasound transducer probe. The ultrasound transducer probe is configured to transmit ultrasound toward a subject and receive ultrasound energy from the subject. The transducer probe includes a lens layer and one or more transducer elements configured to operate at a center frequency (e.g., between about <NUM> and about <NUM>). A portion of the lens layer has a concave curvature in a direction normal to an axial direction of the transducer. A center portion of the concave curvature has an average thickness substantially equal to (e.g., within about <NUM>%, within about <NUM>%, within about <NUM>%) <NUM>, <NUM>, <NUM> or <NUM> multiples of a <NUM>/<NUM> wavelength of the center frequency of the one or more transducer elements. In some embodiments, a reflection coefficient of the lens layer is less than about <NUM>%. In some embodiments, the reflection coefficient is between, for example, about <NUM>% and <NUM>%.

In the disclosed technology, a method of constructing an ultrasound transducer includes fabricating an acoustic lens layer and attaching or bonding the lens layer to a first matching layer operationally coupled to a transducer layer. The lens layer is fabricated to have a center curved section and two flat side sections. Fabricating the curved section includes fabricating a center portion having a midpoint and two endpoints such that the center portion has a first thickness at the midpoint and a second thickness at each of the two endpoints. An average of the first thickness and the second thickness is substantially equal to (e.g., within about <NUM>%, within about <NUM>%, within about <NUM>%) <NUM>/<NUM>-wavelength, <NUM>/<NUM> wavelength, <NUM>/<NUM>-wavelength or <NUM>/<NUM> wavelength of the center frequency (e.g., between about <NUM> and about <NUM>) of the ultrasound transducer. The method further includes bonding or attaching a second matching layer to the lens layer with the first matching layer such that the second matching layer is positioned between the first matching layer and the transducer layer. In some embodiments, the lens layer has a speed of sound significantly different (e.g., <NUM>% different, <NUM>% different) than a speed of sound in water.

In another embodiment of the disclosed technology, an ultrasound transducer stack includes a transducer layer comprising one or more ultrasound transducer elements configured to operate at a center frequency of <NUM> or greater (e.g., between about <NUM> and about <NUM>). The transducer stack further includes an acoustic lens having a rear surface attached to a matching layer operationally coupled to the transducer layer. A front surface of the acoustic lens includes two flat side sections and a center curved section extending therebetween in an elevation direction relative to the transducer stack. A first thickness of the center curved section in an axial direction relative to the transducer stack is less than <NUM>, <NUM>, <NUM> or <NUM> multiples of <NUM>/<NUM>-wavelength of the center frequency. The thickness of the center curved section increases outwardly a first distance in the elevation direction to an endpoint having a second thickness in the axial direction that is greater than <NUM>, <NUM>, <NUM> or <NUM> multiples of <NUM>/<NUM>-wavelength of the center frequency such that the average thickness in the axial direction of the center curved section between the midpoint and the endpoint is substantially an odd multiple of <NUM>/<NUM>-wavelength of the center frequency. In some embodiments, a length of the center curved section is twice the first distance. In some embodiments, the length of the center curved section is about <NUM>% or less of a total length of the transducer stack in the elevation direction. In some embodiments, the first thickness is between about <NUM>% and <NUM>% of the said <NUM>, <NUM>, <NUM> or <NUM> multiples of the <NUM>/<NUM>-wavelength of the center frequency, and the second thickness is between about <NUM>% and <NUM>% of the <NUM>, <NUM>, <NUM> or <NUM> multiples of the <NUM>/<NUM>-wavelength of the center frequency.

<FIG> is a schematic view of an ultrasound system <NUM> configured in accordance with an embodiment of the disclosed technology. The ultrasound system <NUM> includes an ultrasound probe <NUM> coupled to an image processing system <NUM> via a link <NUM> (e.g., a wire, a wireless connection). The probe <NUM> includes a transducer <NUM> (e.g., an HFUS stack). The transducer <NUM> can transmit ultrasound energy (e.g., HFUS energy) into a subject and receive at least a portion of the reflected ultrasound energy from the subject. The received ultrasound energy can be converted into a corresponding electrical signal and transmitted electrically to the image processing system <NUM>, which can form one or more ultrasound images based on the received ultrasound energy.

<FIG> is a cross section schematic view of an ultrasound transducer stack <NUM> (e.g., the transducer <NUM> of <FIG>) configured in accordance with one or more embodiments of the disclosed technology. The transducer stack <NUM> includes an acoustic lens <NUM>, a first matching layer <NUM>, a second matching layer <NUM>, a third matching layer <NUM> and a transducer layer <NUM> (e.g., a piezoelectric transducer layer, a PMUT layer, a CMUT layer). The first matching layer <NUM> comprises cyanoacrylate having a <NUM>/<NUM>-wavelength thickness and can be configured to bond or otherwise attach a front surface of the second matching layer <NUM> to a rear surface <NUM> of the lens <NUM>. A rear surface of the matching layer <NUM> is bonded or otherwise attached to a front surface of the third matching layer <NUM>. A rear surface of the third matching layer <NUM> is attached to a front surface of the transducer layer <NUM>. A centerline <NUM> extends along an axial direction (i.e., along the y-axis shown in <FIG>) of the transducer stack <NUM>. In the illustrated embodiment, the transducer stack <NUM> includes a three matching layers-the first matching layer <NUM>, the second matching layer <NUM> and the third matching layer <NUM>. In some embodiments, however, the transducer stack <NUM> may include one or more additional matching layers as disclosed, for example, in <CIT>. Other embodiments of the transducer stack <NUM> may not include the third matching layer <NUM>.

The lens <NUM> includes a curved section <NUM> that has a concave curvature (e.g., cylindrical, parabolic or hyperbolic curvature) in an elevation direction (i.e., along the x-axis shown in <FIG>) of the transducer stack <NUM>. The curved section <NUM> is bounded by side section <NUM> (identified individually as a first side section 224a and a second side section 224b). The curved section <NUM> has a curved outer surface <NUM> and the flat side portions <NUM> have outer surfaces <NUM> (identified individually as a first outer surface 229a and a second outer surface 229b). The curved section <NUM> includes a center portion <NUM> centered at the centerline <NUM>. As discussed in more detail with reference to <FIG>, the center portion <NUM> has a first thickness T<NUM> at a midpoint and a second thickness T<NUM> at two endpoints. The center portion <NUM> has a length L (e.g., less than <NUM>, <NUM>, <NUM>, <NUM>, greater than <NUM>) in the elevation direction of the transducer. In some embodiments, the length L can extend between about <NUM>% and <NUM>% of the length of the transducer in the elevation direction. In some embodiments, the length L and a radius of curvature of the center portion <NUM> can be determined by the focal number (e.g., F2, F5, F8, F10) of the lens and the focal depth of the transducer. As those of ordinary skill in the art will appreciate, the focal number of the lens is proportional to a ratio of the focal depth of the transducer and a length of the curved section <NUM> of the lens.

The lens <NUM> can comprise, for example, polymethylpentene, cross-linked polystyrene and/or polybenzimidazole. In other embodiments, however, the lens <NUM> can comprise any suitable material (e.g., metals, such as aluminum or stainless steel, or ceramic materials, such as PZT or alumina) having a speed of sound higher than a speed of sound of a medium being imaged (e.g., water, tissue in a subject). Moreover, in some embodiments, the first thickness T<NUM> of the center portion <NUM> may be slightly less than <NUM>, <NUM>, <NUM> or <NUM> multiples of <NUM>/<NUM> of the wavelength (e.g., between approximately <NUM>% and <NUM>% of <NUM>, <NUM>, <NUM> or <NUM> multiples of the1/<NUM> wavelength thickness) of a center frequency (e.g., <NUM> or greater) of the transducer layer <NUM>. Correspondingly, the second thickness T<NUM> may be slightly more than <NUM>, <NUM>, <NUM> or <NUM> multiples of <NUM>/<NUM> of the wavelength (e.g., between approximately <NUM>% and <NUM>% of <NUM>, <NUM>, <NUM> or <NUM> multiples of the <NUM>/<NUM> wavelength thickness) of the center frequency. The center portion <NUM> of the curved section <NUM> therefore has a substantially average thickness of approximately <NUM>, <NUM>, <NUM> or <NUM> multiples of <NUM>/<NUM> of the wavelength (within a +/- <NUM>% of <NUM>, <NUM>, <NUM> or <NUM> multiples of <NUM>/<NUM> wavelength). Fabricating the center portion <NUM> to have an average thickness substantially equal to a fractional wavelength of the center frequency of the transducer layer <NUM> can provide an improved acoustic match to a subject being imaged and therefore can significantly reduce multipath reflections compared to an acoustic lens having an arbitrary thickness.

<FIG> is an enlarged view of a portion P of <FIG> showing the center portion <NUM> in more detail. The center portion <NUM> has a midpoint <NUM> and extends between a first endpoint 234a and second endpoint 234b in the elevation direction. The midpoint <NUM> is spaced apart from each of the first and second midpoints 234a and 234b a distance D in the elevation direction (i.e., one-half the length L). The thickness of the center portion <NUM> in the axial direction increases outwardly from T<NUM> at the midpoint <NUM> to the thickness T<NUM> at each of the first and second midpoints 234a and 234b. The average thickness of the center portion <NUM> is substantially equal to <NUM>, <NUM>, <NUM> or <NUM> multiples of a <NUM>/<NUM>-wavelength of the center frequency of the transducer layer <NUM> (<FIG>). Further, at intermediate points 236a and 236b, the center portion <NUM> has a thickness T<NUM> generally corresponding to the average thickness of the center portion <NUM> between midpoint <NUM> and each of the first and second midpoints 234a and 234b.

In some embodiments, however, the center portion <NUM> can be configured to have an average %-wavelength thickness to provide adequate dielectric strength to meet desired medical electrical safety standards. In other embodiments, the center portion <NUM> may have an average thickness less than <NUM>/<NUM> wavelength. In some embodiments, for example, the center portion <NUM> can be fabricated to have an average thickness of the <NUM>/<NUM> of the wavelength of an operational center frequency (e.g., <NUM>, <NUM>, <NUM>) of the transducer layer <NUM>. According to the invention, the average thickness of the center portion <NUM> is any of <NUM>, <NUM>, <NUM> or <NUM> multiples of <NUM>/<NUM> of the wavelength of the operational center frequency of the transducer layer <NUM> (<FIG>). Those of ordinary skill in the art will appreciate, for example, that for broadband ultrasound transducers, a <NUM>/<NUM> wavelength lens thickness will generally perform better than a <NUM>/<NUM> wavelength lens thickness, and increasing odd multiples of <NUM>/<NUM>-wavelength generally perform progressively worse. In contrast, narrowband transducers (e.g., CW Doppler transducers) can have acoustic lenses with increasing odd multiples of the <NUM>/<NUM>-wavelength without a significant reduction in performance.

Fabricating the center portion <NUM> to have of an average thickness corresponding generally to a fractional portion (e.g., <NUM>/<NUM>, <NUM>/<NUM>) of the wavelength can, in addition to minimizing multi-path artifacts, acoustically enhance a central part of the elevation dimension (i.e., along the x-axis of <FIG>) of the transducer layer <NUM> (<FIG>), thereby providing a desirable boost to a normal component of the elevation beam. This can be viewed as achieving the equivalent of mild apodization of the elevation beam by enhancing the central part of the beam relative to the edges, as opposed to attenuating the edges relative to the center of the beam. The apodization of the elevation beam can lead to a reduction in sidelobes in the elevation beam.

Claim 1:
An ultrasound transducer stack (<NUM>, <NUM>) comprising:
a transducer layer (<NUM>) configured to transmit ultrasound energy at a center frequency; and
a lens layer (<NUM>) secured to the transducer layer, wherein at least a portion of the lens layer has a concave curved recess (<NUM>) with a length measured in an elevation direction of the transducer stack;
wherein a center portion (<NUM>) of the concave curved recess has a length that is less than the length of the concave curved recess and is defined between a midpoint (<NUM>) of the concave curved recess and points outwardly therefrom in the elevation direction such that the center portion has an average thickness measured in an axial direction of the transducer stack that is substantially equal to one of <NUM>, <NUM>, <NUM> or <NUM> multiples of a <NUM>/<NUM> wavelength of the center frequency of the transducer layer;
characterised in that a first matching layer (<NUM>) is disposed between the transducer layer (<NUM>) and the lens layer (<NUM>); and
a second matching layer (<NUM>) is disposed between the lens layer (<NUM>) and the first matching layer,
wherein the second matching layer comprises cyanoacrylate, and
wherein the first matching layer and the second matching layer each have a thickness of approximately <NUM>/<NUM>-wavelength of the center frequency of the ultrasound energy from the transducer layer.