Patent Description:
In conventional ultrasound imaging, ultrasonic acoustic signals are directed towards a region of interest and the corresponding reflected echo signals are detected. Characteristics of the echo signals such as their amplitude, phase shift, Doppler shift, power etc. are analyzed and quantified into pixel data that are used to create an image of or representation of flow. With conventional single transducer ultrasound imaging, the received ultrasound echo signals are in the same frequency range as the transmitted ultrasound signals.

Another approach to performing ultrasound imaging is to apply ultrasonic acoustic signals to the region of interest at one frequency and to capture and analyze the received echo signals at another frequency such as at one or more harmonics of the transmitted ultrasound signals. Typically, the harmonics have a frequency that is <NUM>-<NUM> times that of the transmitted signals. One specific use of harmonic imaging is imaging tissue with contrast agents. Contrast agents are generally fluid or lipid encapsulated microbubbles that are sized to resonate at a particular transmitted ultrasound frequency. Exposure to ultrasound in the body at the resonant frequency of the microbubbles, causes the bubbles to rupture and produce non-linear ultrasound echoes having a much higher frequency than the applied ultrasound. For example, non-linear microbubbles can be designed to resonate at <NUM>-<NUM> but produce echo signals in the range of <NUM>-<NUM>. The high frequency echo signals allow detailed images of tissue structures to be produced and studied such as the micro-vasculature surrounding tumors in clinical or pre-clinical settings.

The most conventional way of performing dual frequency imaging is with a mechanically scanned, single element transducer having confocal low and high frequency transducer elements. While such transducers work well, faster scanning can be performed with transducer arrays that can be electronically steered. Such transducers generally have a low frequency transducer array and a high frequency transducer array that are aligned with each other. One problem with dual frequency transducers is aligning the low and high frequency arrays. In a <NUM> high frequency phased array, the element size (e.g. <NUM>/<NUM> A or less) is about <NUM> microns. At <NUM>, the element size is approximately <NUM> microns. For comparison, a typical human hair is approximately <NUM> microns in diameter. The procedure required to align the arrays often requires making micro-adjustments to the position of the high and low frequency transducers on a wet bench and then adhering them together when the best match is found. This is both time consuming and costly. The technology discussed herein relates to an improved dual frequency transducer that is easier and/or less costly to manufacture.

<CIT> teaches a dual frequency ultrasound transducer having a low frequency ultrasound array positioned behind a high frequency ultrasound array, and a dampening material for attenuating high frequency signals positioned between the high frequency and low frequency arrays.

<CIT> teaches a semiconductor device having alignment markings on a layer of the device for use in aligning the layer with other layers of the device.

<CIT> discloses an ultrasound transducer that includes alignment markers on a bottom surface of a piezoelectric layer for use in correctly orienting the piezoelectric layer with respect to a flex circuit.

According to a first aspect, the invention provides a dual frequency transducer comprising: a high frequency (HF) transducer comprising a sheet of PZT material including a number of HF transducer elements, one or more matching layers, a lens positioned in front of the HF transducer elements and a support frame on a back side of the PZT material that supports one or more flex circuits with signal traces that connect to the HF transducer elements; an intermediate layer that is configured to absorb HF ultrasound signals and positioned behind the HF transducer elements; and a low frequency (LF) transducer; characterised in that the dual frequency transducer comprises one or more fiducials placed at known positions with respect to the HF transducer elements; and the LF transducer includes a sheet of PZT material including a number of LF transducer elements, one or more matching layers and a flex circuit with signal traces that connect to the LF transducer elements, wherein the LF transducer includes one or more alignment tabs with an alignment feature that aligns the LF transducer with one of the one or more fiducials.

The support frame may include an alignment post that fits with a cooperating slot or keyway on the intermediate layer to align the intermediate layer with the support frame and the high frequency transducer elements. One or more fiducials on the intermediate layer may be positioned at locations referenced to the high frequency transducer elements to align the low frequency transducer to the high frequency transducer elements.

According to a second embodiment, the invention provides a dual frequency transducer comprising: a high frequency (HF) transducer comprising a sheet of PZT material including a number of HF transducer elements; a backing material that is constructed to absorb HF ultrasound signals and pass low frequency (LF) ultrasound signals; and a low frequency (LF) transducer; characterised in that the dual frequency transducer comprises one or more fiducials placed at known positions with respect to the HF transducer elements; and the LF transducer includes a number of LF transducer elements including one or more alignment tabs that are positioned using the one or more fiducials to align the LF transducer elements with the HF transducer elements.

The technology shown in the figures and described below relates to a dual frequency ultrasound transducer. Components of a dual frequency transducer are shown for the purpose of explaining how to make and use the disclosed technology. It will be appreciated that the illustrations are not necessarily drawn to scale.

<FIG> is a top view of a piezoelectric layer <NUM> for a high frequency ultrasound transducer in accordance with one embodiment of the disclosed technology. In one embodiment, the layer <NUM> has an outer frame <NUM> that surrounds a sheet <NUM> of PZT, single crystal piezoelectric or other known piezoelectric material. In one embodiment, the frame <NUM> is a pre-machined, non-conductive alumina ceramic with a center cut out area that is slightly larger than the outer dimensions of the sheet <NUM> of piezoelectric material. In other embodiments, the frame <NUM> can be made of a conductive material such as molybdenum or graphite. The frame <NUM> preferably has a coefficient of thermal expansion that is similar to that of the piezoelectric material so that the piezoelectric material doesn't crack during manufacture, handling or use. An adhesive <NUM> such as EPO-TEK <NUM> epoxy fills a gap between the frame and the piezoelectric material. The piezoelectric material <NUM> is diced with a patterning laser or a dicing saw to create a number (e.g. <NUM>, <NUM> or fewer or a larger number) of piezoelectric elements (not separately shown). By surrounding the sheet <NUM> of piezoelectric material with the frame <NUM>, the kerf cuts that define the individual transducer elements can extend across the entire width of the sheet of piezoelectric material.

If the frame <NUM> is made of a non-conductive material, a cut-out area or notch <NUM> on one or both sides of the frame is filled with a conductive epoxy or can include a number of conductive vias to provide an electrical path from a front surface of the frame to a rear surface of the frame. More details about the construction of an ultrasound transducer with a surrounding frame can be found in commonly-owned <CIT> and International Publication No.<CIT>.

<FIG> shows a piezoelectric layer <NUM> of a low frequency ultrasound transducer that in one embodiment, is constructed in the same manner as the piezoelectric layer of the high frequency transducer shown in <FIG>. An alumina frame <NUM> surrounds a sheet <NUM> of piezoelectric material. The frame <NUM> has a central opening that is sized to form a gap between the edges of the opening and the edges of the piezoelectric sheet <NUM>. The gap is filled with a non-conductive epoxy <NUM>. The low frequency array has fewer transducer elements than the high frequency transducer. In one embodiment, the low frequency transducer has <NUM> individually addressable transducer elements but could have fewer or more elements. A pair of cut-out areas or notches <NUM> on either side of the frame <NUM> can be filled with a conductive epoxy or can include conductive vies through the frame to provide an electrical path from a front surface of the low frequency array to the rear surface of the frame.

<FIG> is an isometric view of a conductive support frame <NUM> that is to be secured to a rear surface of the piezoelectric layer <NUM> of the high frequency transducer. The frame <NUM> supports one or more flex circuits (not shown) having signal traces therein that are electrically connected to corresponding electrodes on each of the high frequency transducer elements. In addition, the support frame <NUM> provides electromagnetic shielding for the rear surface of the transducer elements. A common ground electrode on the front surface of the piezoelectric layer <NUM> is connected to a ground plane in the flex circuits via a conductive path that includes the conductive support frame <NUM>. In one embodiment, the support frame <NUM> is secured to the piezoelectric layer <NUM> with a conductive epoxy. Also shown in <FIG> are one or more matching layers and a lens that are positioned ahead of (e.g. in front of) the piezoelectric layer <NUM>.

<FIG> shows one flex circuit <NUM> attached to the support frame <NUM>. In some embodiments, the high frequency transducer includes two flex circuits with the signal traces of one flex circuit electrically coupled to the even numbered transducer elements and the signal traces of the other flex circuit electrically connected to the odd numbered transducer elements. Electrical connections are made from the individual transducer elements on the rear surface of the piezoelectric layer to the metal signal traces in the flex circuits. Connections between the transducer elements and the metal
signal traces in the flex circuits can be made using the techniques described in <CIT> or <CIT>. After the connections are made, an open back side of the support frame is filled with an epoxy material <NUM> such as EPO-TEK <NUM>. In one embodiment, the height of the epoxy <NUM> in the frame extends above the rim of the support frame <NUM>. As shown in <FIG>, the epoxy <NUM> is molded or laser machined to form a pair of alignment posts 112a, 112b and a recess area <NUM> that is behind the high frequency transducer elements. In one embodiment, the depth of the recess area <NUM> is about <NUM>. The alignment posts 112a, 112b are configured to align an intermediate layer over the high frequency transducer as will be described below. A pair of fiducials 120a, 120b are marked with a laser on each alignment post at a known distance from the high frequency transducer elements. The fiducials 120a, 120b serve as a reference so that other components of the dual frequency transducer can be placed at known positions with respect to the high frequency transducer elements.

<FIG> illustrates one embodiment of an intermediate layer <NUM> that is placed behind the high frequency transducer. In one embodiment, the intermediate layer <NUM> is made of silicone powder-doped EPO-TEK <NUM> epoxy. The size of the silicone powder is in the nanometer range such that it highly attenuates high frequency (HF) ultrasound and passes low frequency (LF) ultrasound. The intermediate layer is sized to absorb high frequency ultrasound to a specified level, such as <NUM> dB, but to pass low frequency signals transmitted from, or to be received by, the low frequency transducer. The top of the intermediate layer <NUM> includes a pair of slots or keyways 152a, 152b that cooperate with the alignment posts 112a, 112b on the back side of the frame <NUM> to align the intermediate layer <NUM> at a known position with respect to transducer elements of the high frequency transducer. In one embodiment, the intermediate layer <NUM> is secured to the frame <NUM> using the same epoxy that fills the back side of the frame to avoid acoustic discontinuities at the bond line.

Once the intermediate layer is secured to the frame <NUM>, additional fiducials 154a, 154b can be placed on the intermediate layer as measured from the fiducials 120a, 120b that are marked on the support frame (i.e. on the filler epoxy at the back side of the support frame).

<FIG> shows a cross section of the high frequency transducer with the intermediate layer <NUM> secured to the back of the support frame <NUM>. As shown, the high frequency transducer comprises a lens <NUM> designed to focus the high frequency ultrasound signals at a specific depth. Typical lens materials include polymethylpentene (tradename TPX), cross-linked polystyrene (tradename Rexolite) or polybenzimidazone (tradename Celezole). However, other non-attenuating plastic materials could also be used. Behind the lens are one or more quarter wave matching layers <NUM> that match the impedance of the piezoelectric layer <NUM> to the impedance of the lens <NUM>. The one or more matching layers are generally formed by adding particles to an epoxy to adjust its acoustic impedance. A metallic (e.g. gold or gold+chromium) common ground electrode is deposited on the front surface of the piezoelectric layer <NUM> is electrically coupled to the conductive support frame <NUM> through the slots or vias in the frame <NUM> surrounding the PZT material. An applied metal coating on the rear or proximal surface of the piezoelectric layer is formed into conductive paths from individual transducer elements to the signal traces in the flex circuit. The layer of epoxy <NUM> covers the proximal side of the transducer elements and fills in the rear side of the support frame. In one embodiment, the recess <NUM> formed in the epoxy behind the high frequency transducer array is sized such that the intermediate layer is larger in the elevation and azimuthal dimensions of the transducer than the size of the transducer elements so that the intermediate layer extends over the ends of high frequency transducer elements.

<FIG> illustrate a pair fiducials 154a, 154b that are marked on a top surface of the intermediate layer <NUM>. The fiducials 154a, 154b are measured with respect the fiducials 112a, 112b that are marked on the epoxy in the support frame. The fiducials 154a, 154b are marked on the intermediate layer once the intermediate layer is secured to the support frame behind the high frequency transducer. In the embodiment shown, the fiducials 112a, 112b, 154a, 154b are crosses made with a laser.

<FIG> shows an embodiment of a piezoelectric layer <NUM> for the low frequency transducer. In the embodiment shown, the piezoelectric layer <NUM> also includes an alumina frame surrounding a sheet of PZT or other piezoelectric material. In one embodiment, the sheet of PZT is diced into a number of columns of triangular pillars as a <NUM>-<NUM> composite as best shown in <FIG>. In the example shown, each column 72a, 72b, 72c of triangular elements <NUM> is treated as a single transducer element. In <FIG>, only <NUM> columns of transducer elements are shown for purposes of illustration. In one embodiment, the low frequency transducer includes <NUM> columns of triangular elements. However, the low frequency transducer could include fewer or a greater number of columns as desired. Furthermore, each transducer element in the low frequency transducer may have other shapes such a small squares or rectangular-shaped elements.

<FIG> is an isometric view of a low frequency transducer in accordance with an embodiment of the disclosed technology. The low frequency transducer includes the piezoelectric layer <NUM>, one or more matching layers <NUM> positioned ahead of (e.g. in front of) the piezoelectric layer <NUM> and a backing layer <NUM> positioned behind the transducer elements. A flex circuit <NUM> includes signal traces that are electrically connected to individual low frequency transducer elements. The low frequency transducer also includes a pair of alignment tabs 162a, 162b. In one embodiment, the alignment tabs are Kapton ™ sheets inserted into an epoxy matching layer at a position away from the position of the low frequency transducer elements so as not to interfere with the operation of the low frequency transducer. Other materials for the alignment tabs could also be used such as alumina. Holes 164a, 164b are placed in the alignment tabs at a known distance from the low frequency transducer elements. The holes are placed over a fiducial on the high frequency transducer in order to precisely align the low frequency transducer elements with respect to the high frequency transducer elements.

In one embodiment, the holes 164a, 164b in the alignment tabs 162a, 162b are placed over the fiducials on the intermediate layer. If the alignment tabs are long enough, the holes in the alignment tabs could be placed over the fiducials marked on the support frame in order to align the low frequency transducer with respect to the high frequency transducer.

<FIG> is a cross-sectional view of a low frequency transducer in accordance with one embodiment of the disclosed technology. The transducer includes the piezoelectric layer <NUM> and two matching layers <NUM> positioned in front of the piezoelectric layer. The matching layers match the impedance of the low frequency piezoelectric elements to the impedance of the intermediate layer to which the low frequency transducer is secured. A backing layer <NUM> is positioned behind the low frequency transducer elements. As shown in <FIG>, the alignment tabs 162a, 162b are part of a matching layer ahead of the low frequency transducer elements. The holes 164a, 164b in the alignment tabs are positioned at a known distance from the low transducer elements as can be seen in <FIG>. A flex circuit <NUM> extends out from under the backing layer where connections are made to the transducer elements.

<FIG> show one way of connecting the signal and ground traces of a flex circuit to the corresponding electrodes in the low frequency array. As shown in <FIG>, the flex circuit <NUM> includes a number of signal traces <NUM> and a ground trace <NUM>. The ground trace <NUM> is electrically connected to the ground electrode on the front of the low frequency array with a conductive epoxy <NUM> joined to conductive epoxy <NUM> in the notches <NUM> in the frame <NUM> or to vias in the frame. As shown in <FIG>, the exposed ends of the signal traces <NUM> can be adhesively secured to the transducer elements with a conductive epoxy. Because the transducer elements of the low frequency transducer are relatively large, they can be easily aligned signal traces <NUM> in the flex circuit <NUM>.

<FIG> is a cross sectional view of the dual frequency transducer in accordance with one embodiment of the disclosed technology. The low frequency transducer elements <NUM> are directly aligned with the high frequency transducer elements <NUM> by placing the holes on the alignment tabs 162a, 162b over the fiducials on the intermediate layer. The holes in the alignment tabs are placed at a known location with respect to the low frequency transducer elements and the fiducials are precisely placed at a known location with respect to the high frequency transducer elements. Therefore, placing the holes over the fiducials on the intermediate layer precisely positions the low frequency transducer elements with respect to the high frequency transducer elements without having to align the transducer on a wet bench.

<FIG> shows one embodiment of a transducer housing <NUM> surrounding the dual frequency transducer array. In the embodiment shown the housing includes separate cables <NUM>, <NUM> connecting the low and high frequency transducers. In one embodiment, the low frequency transducer can be used alone to image tissue with low frequency ultrasound (e.g. in a frequency range of <NUM>-<NUM>). Similarly, the high frequency transducer can be used to image tissue with high frequency ultrasound (e.g. in a frequency range of <NUM>-<NUM>). Alternatively, both transducers can be used to perform imaging such as harmonic or contrast agent imaging where a region of interest is insonified with signals from the low frequency transducer and ultrasound echo signals at harmonics of the excitation frequency are detected with the high frequency transducer.

<FIG> are some sample beam plots of a low frequency transducer operating in a plane wave imaging mode. In one embodiment, a <NUM>-element low frequency transducer is positioned behind a <NUM> element high frequency transducer. Electrical matching using <NUM>:<NUM> mini-transformers bring the impedance of the low frequency transducer elements from about <NUM> ohms to about <NUM> +<NUM>- <NUM> ohms with a phase of minus <NUM> +<NUM>- <NUM> degrees. The single low frequency element peak-peak pressures measured at <NUM>. from the lens were increased from <NUM> kPaN without matching to <NUM> kPa/V with matching in the <NUM>-140V range with a variation of about +<NUM>°. <NUM> across the elements. In plane wave imaging, pressure is <NUM> times higher than for a single element. Focusing is observed in the elevation plane at <NUM> with a 6dB beamwidth of <NUM> (<FIG>,<FIG>). Past the focus, the pressure drops as the wavefront diverges (<FIG>). In the azimuth direction, the beam is relatively uniform (without apodization), with a variation of 2dB relative to the maximum on the edges (<FIG>,<FIG>). Beam steering is achieved for +<NUM>- <NUM> degree angles (<FIG>) allowing for planewave compounding.

Claim 1:
A dual frequency transducer comprising:
a high frequency transducer comprising a sheet of PZT material (<NUM>) including a number of high frequency transducer elements, one or more matching layers (<NUM>), a lens (<NUM>) positioned in front of the high frequency transducer elements and a support frame (<NUM>) on a back side of the PZT material (<NUM>) that supports one or more flex circuits (<NUM>) with signal traces that connect to the high frequency transducer elements;
an intermediate layer (<NUM>) that is configured to absorb high frequency ultrasound signals and positioned behind the high frequency transducer elements; and
a low frequency transducer;
characterised in that the dual frequency transducer comprises one or more fiducials (120a, 120b) placed at known positions with respect to the high frequency transducer elements; and
the low frequency transducer includes a sheet of PZT material including a number of low frequency transducer elements, one or more matching layers (<NUM>) and a flex circuit (<NUM>) with signal traces that connect to the low frequency transducer elements, wherein the low frequency transducer includes one or more alignment tabs (162a, 162b) with an alignment feature that aligns the low frequency transducer with one of the one or more fiducials (120a, 120b).