Patent Application
20150173625
The disclosed technology generally relates to the fields of ultrasonic transducers and medical diagnostic imaging. More specifically, the disclosed technology relates to high frequency ultrasonic transducer stacks configured for use in photoacoustic imaging.
The following patents are also incorporated by reference herein in their entireties: U.S. Pat. No. 7,052,460, titled “SYSTEM FOR PRODUCING AN ULTRASOUND IMAGE USING LINE-BASED IMAGE RECONSTRUCTION,” and filed Dec. 15, 2003; U.S. Pat. No. 7,255,648, titled “HIGH FREQUENCY, HIGH FRAME-RATE ULTRASOUND IMAGING SYSTEM,” and filed Oct. 10, 2003; U.S. Pat. No. 7,230,368, titled “ARRAYED ULTRASOUND TRANSDUCER,” and filed Apr. 20, 2005; U.S. Pat. No. 7,808,156, titled “ULTRASONIC MATCHING LAYER AND TRANSDUCER,” and filed Mar. 2, 2006; U.S. Pat. No. 7,901,358, titled “HIGH FREQUENCY ARRAY ULTRASOUND SYSTEM,” and filed Nov. 2, 2006; and U.S. Pat. No. 8,316,518, titled “METHODS FOR MANUFACTURING ULTRASOUND TRANSDUCERS AND OTHER COMPONENTS,” and filed Sep. 18, 2009.
Ultrasonic transducers convert electrical energy into acoustic energy and vice versa. When the electrical energy is in the form of a radio frequency (RF) signal, a properly designed transducer can produce ultrasonic signals having the same or similar frequency characteristics as the driving electrical RF signal. Diagnostic ultrasound has traditionally been used at center frequencies ranging from less than 1 MHz to about 10 MHz. One skilled in the art will appreciate that this frequency spectrum provides a capability to image biological tissue with a resolution ranging from, for example, several millimeters to greater than 300 microns, and at depths ranging, for example, from a millimeter to several centimeters.
High frequency ultrasonic (HFUS) transducers generally include ultrasonic transducers having center frequencies above 15 MHz and ranging to over 60 MHz. HFUS transducers can provide higher resolution while limiting the maximum depth of penetration, and as such, provide a means of imaging biological tissue from a depth of a fraction of a mm to over 3 cm with resolutions in the 20 um to 300 um range. There are many challenges associated with fabricating high frequency ultrasonic transducers that do not arise when working with traditional clinical ultrasonic transducers that operate at frequencies below about 10 MHz. One skilled in the art will appreciate that structures generally scale down according to the inverse of the frequency, so that a 50 MHz transducer will have structures about 10 times smaller than a 5 MHz transducer. In some cases, materials or techniques cannot be scaled down to the required size or shape, or in doing so they lose their function and new technologies must be developed or adapted to allow high frequency ultrasonic transducers to be realized. In other cases, entirely new requirements exist when dealing with the higher radio frequency electronic and acoustic signals associated with HFUS transducers.
Photoacoustic imaging is a modified form of ultrasound imaging that is based on the photoacoustic effect in which the absorption of electromagnetic energy (e.g., infrared light, visible light, ultraviolet light, radio-frequency waves, etc.) generates acoustic waves. In photoacoustic imaging, light pulses are transmitted into biological tissues, and a portion of the transmitted light energy is absorbed by tissues in a subject and converted into heat. The resulting heat can cause transient thermoelastic expansion, which can generate ultrasound waves. The generated ultrasonic waves are detected by ultrasonic transducers, which convert the received ultrasound waves into electrical signals used to form images.
One limitation of current photoacoustic systems is noise or artifacts in images formed using HFUS signals. Some of these artifacts are caused by transmitted laser light that is reflected by the skin of a subject back toward an HFUS transducer. The reflected light can be absorbed by one or more layers of the HFUS transducer and cause a secondary photoacoustic signal. The secondary photoacoustic signal shows up as an artifact in the photoacoustic image and, in many cases, can be stronger than the photoacoustic signals generated by light absorbed into the subject.
One approach to reduce secondary photoacoustic artifacts is to form several tomographic images by obtaining image data by rotating a transducer around a line normal to and located in the imaging plane. The resulting set of collected data taken at varied angles about the normal to the imaging plane can be combined through tomographic techniques reduce or eliminate non-coherent signals (e.g., noise, artifacts, etc.) between the angled data sets, thus forming images having little or no secondary artifact. As one skilled in the art will appreciate, however, a tomographic approach requires a subject to remain still for several seconds or more, and even then may take longer to acquire a single image. As a result, tomographic photoacoustic systems may not be practical in clinical or preclinical applications in which holding a subject still may not possible or desirable. In addition, observation of some anatomical functions, pharmacokinetics, or other dynamics may not be possible with the frame rate limitations inherent in multi-look approaches like tomography.
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.
The technology disclosed herein generally relates to high frequency ultrasound transducers. In one aspect, a high frequency ultrasound transducer includes an acoustically penetrable, optically-reflective lens. The lens can be configured to have very low acoustic losses and sufficient acoustic lensing capability while exhibiting high reflectivity in an optical wavelength region of interest (e.g., 680-970 nanometers) while having low optical absorption in the same region. In one embodiment of this aspect, the optical reflectivity of the lens may be Lambertian (i.e., diffusively reflective). In some embodiments, a diffuse reflection may take place not only at the surface of the lens, but in a gradient extending into the surface of the lens, thus exhibiting a characteristic lying between truly opaque and having an opacity of less than 100%. A gradient based diffuse reflectivity can reduce or eliminate secondary photoacoustic artifacts as a result of reflected light. Moreover, in addition to opacity (i.e., a reduction of light transmission), it may be desirable to control a mechanism that prevents optical transmission such that light is reflected but not absorbed within the lens. Absorbed light will generally give rise to a photoacoustic effect that may lead to an artifact. Since it can be challenging to find a material that is 100% reflective with no significant acoustic absorption coefficient, other strategies may be employed to mitigate undesirable absorption artifacts in the lens. Accordingly, in one embodiment, a lens material (e.g., polymethylpentene) doped with reflective particles (e.g., titanium dioxide particles) can exhibit diffuse reflectivity with very low absorption while maintaining excellent acoustic transmission characteristics at high acoustic frequencies. In another embodiment, an optically-reflective coating (e.g., sputtered aluminum) can be applied to a surface (e.g., an underside surface) of an acoustic lens (e.g., a thermo set cross-linked polystyrene lens), thus preventing optical absorption of photons on the transducer stack behind the lens. In some embodiments, the lens includes between 90-95% of the matrix material and between 5-10% of the optically reflective material.
In another aspect of the present disclosure, an ultrasound transducer stack may include an optically reflective acoustic matching layer positioned behind (e.g., under) an acoustics lens. In one embodiment, the acoustic matching layer is configured to be at least partially opaque for the wavelengths used in the photoacoustic array. There are few materials suited for making HFUS acoustic matching layers that also have a high reflectivity in the optical wavelength range. An acoustic matching layer comprising, for example, a titanium dioxide powdered-loaded matrix may be suitable for use in HFUS arrays where a low to medium acoustic impedance (approximately 3 to 4 MR) is desired. In one embodiment, a matching layer may comprise an epoxy or glue doped with titanium dioxide at a ratio of 1:0.35 by weight (e.g., 1 g epoxy for 0.35 g of TiO2). In other embodiments, for example, a hafnium dioxide and titanium dioxide powder mix may be suitable for use in HFUS arrays, where a medium acoustic impedance (e.g., between about 4 MR to about 6 MR) may be desired. Matching layers can be made opaque at relatively thin matching layer thicknesses (e.g., 25 microns or less). An opaque acoustic matching layer can reduce and/or mitigate secondary photoacoustic effects arising within the acoustic stack, even if the lens is optically transparent or only partially opaque.
In yet another aspect of the present disclosure, an external optically-reflective layer may be positioned in front of (e.g., on top of) an optically-transparent or highly-translucent acoustic lens. As discussed above, very few acoustic lens materials are optically reflective and acoustically transparent at frequencies associated with HFUS (e.g., 15 MHz or greater). If, however, the optically reflective layer at the front of the ultrasound stack is an acoustic matching layer, the acoustic losses may be disregarded, allowing for a larger selection of materials. Furthermore, those of ordinary skill in the art will appreciate that an acoustic lens material may be selected to provide as close an acoustic impedance match as possible to a medium to be imaged (e.g., tissue or water). A close impedance match may, for example, avoid unwanted multi-path reverberations between the acoustic lens and the acoustic objects in the field of the array.
In one embodiment of this aspect, an acoustic lens having a higher than typical acoustic impedance (e.g., between about 3 MR and about 5 MR) may be selected for use with the transducer stack to facilitate selection of the external optically-reflective layer. The external optically-reflective matching layer can be positioned in front of the lens and selected to have an acoustic impedance that is, for example, approximately the geometric mean of the lens and the tissue (e.g., less than 3 MR, between about 2 MR and about 3 MR, etc.). The external matching layer can be configured and/or selected to have excellent optical reflectance (e.g., greater than or equal to 50%, greater than or equal to 90%, etc.) and a thickness on the order of a fraction of an ultrasound wavelength (e.g., ¼ wave thick, ¾ wave thick, etc.) at frequencies associated with HFUS. Accordingly, in this embodiment, the external matching layer can be selected based on optical properties with less emphasis or consideration of acoustic losses. Correspondingly, the acoustic lens can be configured and/or selected based on acoustic lensing and attenuation characteristics, with less emphasis on optical characteristics of the acoustic lens.
Polybenzimidazole (hereinafter “PBI”) is one material that may be used to fabricate an acoustic lens having an optically reflective acoustic matching layer attached to the curved front of the lens. The external matching layer may comprise, for example a low acoustic impedance polymer (e.g., an optically transparent epoxy) doped with a light but highly optically reflective particle (e.g. TiO2). This embodiment therefore requires no special consideration to the optical properties of acoustic layers behind the lens, as all optical energy is reflected from the front of the lens. In addition, an acoustic lens can be selected to have a relatively high speed of sound allowing the acoustic lens to have a relatively shallow curvature, thus mitigating an undesirable groove found on the face of conventional HFUS transducers. This property is generally useful for a matching layer placed in front of a higher speed of sound lens material (e.g., FBI) whether the external matching layer is optically reflective or not.
A laser system 112 is coupled to the optical fibers 109 and configured to produce electromagnetic (EM) energy (e.g., non-ionizing EM radiation, infrared light, visible light, ultraviolet light, etc.) An ultrasound system 114 is coupled (via, e.g., a wire, a wireless link, etc.) to the transducer 110 and is configured to generate high-frequency ultrasound (e.g., ultrasound energy having a center frequency of 15 MHz or greater). The ultrasound system 114 is also configured to receive high-frequency ultrasound echoes from the transducer 110. A computer 116 can receive the ultrasound signals (e.g., scan converted ultrasound signals) from the ultrasound system 114 and form one or more ultrasound images that can be presented to an operator via a display 118. One or more embodiments of the system 100 can include embodiments described in the applicants' co-pending U.S. patent application Ser. No. 13/695,275, which is incorporated by reference herein in its entirety.
In operation, the optical fibers 109 can transmit and direct laser light pulses (e.g., light pulses having wavelengths between approximately 680 nm and 970 nm) from the laser system 112 toward the one or more tissue structures (e.g., a heart, one or more blood vessels, a kidney, a uterus, a prostate, etc.) in and/or at the target 102. As those of ordinary skill in the art will appreciate, at least a portion of the laser light can be absorbed by the one or more tissue structures and converted into heat. The converted heat can cause a thermoelastic expansion in the tissue and a corresponding emission of acoustic energy (e.g., ultrasound energy). The transducer 110 receives the resulting ultrasound echoes from the target 102 and converts them into ultrasound signals. The computer 116 can include a memory and/or one or more processors configured to process the ultrasound signals and form one or more ultrasound images.
The transducer layer 260 can comprise any suitable transducer material capable of transmitting and/or receiving high frequency ultrasound [e.g., piezoelectric transducers (e.g., lithium niobate transducers), capacitive micromachined ultrasound transducers (CMUTs), piezoelectric micromachined ultrasound transducers (PMUTs), etc.]. The transducer layer 260 can comprise one transducer (e.g., a single element transducer) or a plurality of transducers (e.g., a one-dimensional array of transducer elements and/or a multi-dimensional array of transducer elements). In some embodiments, the transducer layer 260 can comprise one or more additional transducer layers (not shown). The transducer layer 260 is configured to transmit and receive ultrasound energy at frequencies greater than 15 MHz. In one embodiment, the transducer layer 260 may comprise a transducer described in, for example, U.S. Pat. No. 7,230,368 and U.S. patent application Ser. No. 11/109,986 which are incorporated by reference herein in their entireties.
The backing layer 270 underlies the transducer layer 260, and can be configured to absorb rear-propagated acoustic energy and/or thermal energy produced by the transducer 210. Suitable backing layers are described in U.S. Pat. No. 7,750,536 and U.S. patent application Ser. No. 11/366,953 which are incorporated by reference herein in their entireties. In some embodiments (not shown), one or more layers (e.g., a dematching layer) can be disposed between the transducer layer 260 and the backing layer 270.
In the embodiment illustrated in
In one aspect of the disclosed technology (described in more detail below with reference to
As shown in
The matching layers 230-250 can be configured to provide and/or improve an impedance match between the lens layer 220 and the transducer layer 260. As those of ordinary skill in the art will appreciate, the transducer layer 260 may have, for example a relatively high acoustic impedance (e.g., greater than 10 MR) while the lens layer 220 may have an acoustical impedance (e.g., 1.5-2.5 MR) relatively similar to a subject being imaged (e.g., the target 102 of
In one aspect of the present technology, one or more of the matching layers 230-250 can comprise a composite material that includes a matrix material (e.g., a polymer) and a plurality of first and second particles. In some embodiments, for example, the first particles may comprise a first material having a first density, and the second particles may comprise a second material having a second density less than the first density. The composite material may be formed by adding the first particles in a first amount to the matrix material until a desired density and/or acoustical impedance of the composite material is achieved. The second particles may be selected based on, for example, such that the second density of the second particles is substantially similar and/or identical to the desired density of the composite material. The second particles may be therefore be added to the composite material in a second amount until a desired consistency, homogeneity, viscosity, and/or thixotropic index of the composite material achieved. Because the second density is substantially similar to the desired density of the composite material, the second particles can be added without significantly altering the density and, thus, the acoustical impedance of the composite material. In another aspect of the present technology, as described in, for example, U.S. Pat. No. 7,750,536, the first particles can include micron-sized particles and the second particles can include nano-sized particles. In yet another aspect of the present technology, as described in detail below with reference to
The first particles 326 can comprise an optically reflective material (e.g., TiO2, a white pigment, etc.) that, within a range of concentration (e.g., between about 1% and about 20%), is also substantially acoustically transparent at high frequencies. The first particles 326 can have a diameter significantly small to allow, for example, multiple grain heights along the z-direction of the lens layer 326. In some embodiments, for example, the diameter may be less than 5 microns or between about 2 and 3 microns. In other embodiments, however, the first particles 326 may have any suitable diameter. Further, the first particles 326 may comprise a material having a density substantially similar to the density of the matrix material 324 such that the composite material 322 has a density (and thus, an acoustical impedance) substantially similar to the matrix material 324.
The first particles 326 may be doped or otherwise loaded into the matrix material 324 in a first amount (e.g., a volumetric ratio of 5%, 10%, 20%, 30%, 40%, etc.) to achieve a desired reflectance (e.g., greater than 90% at EM wavelengths between about 680 nm and 970 nm within the thickness of the lens) of the composite material 322, while remaining substantially acoustically transparent at high frequencies. Moreover, in the illustrated embodiment of
As those of ordinary skill in the art will appreciate, a transducer configured for use with low frequency ultrasound (e.g., 10 MHz or less) can include a relatively thick acoustic lens (e.g., 250 microns or greater) having sufficient opacity to resist the secondary photoacoustic effects described above. On the contrary, a transducer configured for use with high frequency ultrasound (e.g., 15 MHz or greater) may require an acoustic lens having a relatively low thickness (e.g., 100 microns or less) and attenuation. Acoustic lenses suitable for use with high-frequency ultrasound are typically formed as optically-transparent films that allow virtually all incoming light to pass therethrough. As noted above, light entering the transducer (e.g., the transducer 210 of
The first particles 446 can comprise, for example, a first optically-reflective powder (e.g., hafnium oxide) selected to have a high density much higher than the density of the composite material 442 which has an acoustic impedance between about 4.0 MR and about 7 MR). The second particles 448 can comprise a second optically-reflective powder (e.g., TiO2, a white powder, a white pigment, and/or any suitable optically reflective material) having a density substantially similar to the desired density of the composite material. The second particles 448 can thus be added relatively freely without significantly changing the density of the composite, allowing a designer to vary the viscosity and reflectance somewhat independently from the acoustic impedance (which is a product of the density and speed of sound). In some embodiments, the first particles 446 and the second particles 448 may comprise the same material. In one embodiment, for example, the first particles 446 can have a first diameter ranging from about 2.0 microns to about 6 microns, and the second particles 448 can have a second diameter ranging from about 0.5 microns to about 0.9 microns. In some embodiments, however, either the first particles 446 or the second particles 448 may have diameters substantially less than 1.0 micron (e.g., nano-sized particles). Moreover, in the embodiment illustrated in
The first particles 446 can be loaded into the matrix material 442 in a first amount (e.g., 60% by weight) and the second particles 448 can be loaded into the matrix material 442 in a second amount (e.g., 10% by weight) to achieve a desired reflectance (e.g., greater than 90% at EM wavelengths between about 680 nm and 970 nm) and/or desired acoustic impedance of the composite material 442. Both sets of particles may be implemented, for example, because the first particles 446 (e.g., Hafnium Oxide) may have a desirable density that can increase or decrease an acoustical impedance of the composited material 442, and may be at least partially optical reflective when loaded into the matrix material 444. However, the resulting composite material with only the matrix material 444 and the first particles 446 may not be sufficient to achieve a desired reflectance (e.g., greater than 90%). Adding the second particles 448 to the matrix material 444 with the first particles 446 can result in the composite material 442 having the desired reflectance without significantly affecting the acoustical performance of the matching layer 440. A composite layer 442 having a sufficient high reflectance can provide at least an advantage of preventing, reducing and/or mitigating secondary photoacoustic effects in high frequency ultrasound transducers, as discussed above in reference to the lens layer 320 of
In some embodiments, as shown in
From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.
This application claims the benefit of U.S. Provisional Patent Application 61/919,163 filed on Dec. 20, 2013, which is incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61919163 | Dec 2013 | US |
