WO2016168620A1

WO2016168620A1 - Ultrasound wide-angle transducer

Info

Publication number: WO2016168620A1
Application number: PCT/US2016/027781
Authority: WO
Inventors: David Vilkomerson
Original assignee: Dvx, Llc
Priority date: 2015-04-17
Filing date: 2016-04-15
Publication date: 2016-10-20

Abstract

Methods and systems of wide-angle ultrasound transducers that receive or transmit over a wide angular range are disclosed. In one embodiment, the transducer includes an aperture that includes multiple piezoelectric segments sequentially arranged based on a quasi- random sequence that has high autocorrelation at zero-shift and low autocorrelation at non¬ zero shifts, such as one of the known Barker sequences. The transducer may be in the shape of a cylinder or in a strip, and multiple piezoelectric segments may be arranged in various sequential arrangements such that the autocorrelation of the aperture of the transducer has high autocorrelation at zero-shift and low autocorrelation at non-zero shifts. Methods of making and using the wide-angle transducer in an ultrasound imaging system are also disclosed.

Description

TITLE: ULTRASOUND WIDE-ANGLE TRANSDUCER

RELATED APPLICATIONS AND CLAIM OF PRIORITY

[0001] This patent document claims priority to U.S. Provisional Patent Application No. 62/149, 131 filed April 17, 2015, the disclosure of which is incorporated herein by reference in full.

BACKGROUND

[0002] This patent document relates to an ultrasound transducer for ultrasound imaging, and particularly to an ultrasound wide-angle transducer.

[0003] Transducers in ultrasound imaging normally can receive an ultrasound beam only from a certain range of angles. For example, there are systems for guiding

interventional devices with ultrasound imaging that function by producing a signal when the ultrasound beam of the imaging system strikes a sensitive portion of the interventional device, its ultrasound receiver. Therefore, it is desirable for such a receiver to be

omnidirectional so as to respond to a wide range of angles of impinging ultrasound beams. Such omnidirectional transducers can also be useful for transmitting over a large range of angles. One such application would be to guide surgical tools with the body. However, a transducer that is required for responding over a large angular range will often intercept only a small portion of the ultrasound energy in the beam, and thus be of limited sensitivity.

[0004] The present invention describes devices and methods that address some or all of the issues described above. SUMMARY

[0005] In one embodiment, an ultrasound wide-angle transducer for use in conjunction with an ultrasound imaging system may include an aperture that has multiple piezoelectric segments placed in a sequential arrangement. Each of the segments has a width and a positive or negative polarity, and the sequential arrangement of the multiple piezoelectric segments is such that the autocorrelation of the aperture has a largest value at zero shift and has one or more side lobes of smaller value at non-zero shifts. In one embodiment, the values of one or more side lobes of the autocorrelation of the aperture are 50% or smaller than the largest value of the autocorrelation. In one embodiment, the spatial- pulse sequence derived from the multiple piezoelectric segments in the aperture corresponds to a Barker sequence, for example, a Barker-13, Barker-11, Barker-9, Barker-7 and Barker- 5 sequence, etc. In one embodiment, the multiple piezoelectric segments form a shape of a cylinder. In another embodiment, the multiple piezoelectric segments form a flat shape defining a horizontal plane and having a uniform thickness.

[0006] In one embodiment, a method is provided for fabricating an ultrasound wide- angle transducer that contains multiple piezoelectric segments in the aperture placed in a sequential arrangement, such that the autocorrelation of the aperture has a largest value at zero shift and that has one or more side lobes of smaller value at non-zero shifts. The fabrication method may include (i) coating a metalized tubular device with piezoelectric layer; (ii) electroding a first segment in the selected sequential arrangement on the coated tubular device; (iii) poling the first segment in a selected polarity perpendicular to a surface of the coated tubular device; (iv) repeating the steps (ii) and (iii) for all remaining segments in the selected sequential arrangement on the coated tubular device; and (v) connecting all the electrodes for all the segments on the coated tubular device. [0007] Various embodiments are provided for using the ultrasound wide-angle cylindrical transducer in conjunction with an ultrasound imaging system. In one

embodiment, a method for using an ultrasound imaging system for locating a tubular device in a medium may include accommodating a tubular device in a central opening of an aperture of a cylindrical transducer, locating at least a portion of the tubular device and at least a portion of the transducer into a medium, operating the ultrasound imaging system at a selected operating frequency, and recording ultrasound signals received at the transducer. In one embodiment, the aperture of the cylindrical transducer has multiple piezoelectric bands placed in a sequential arrangement such that the autocorrelation of the aperture has a largest value at zero shift and has one or more side lobes of smaller value at non-zero shifts, where each of the piezoelectric bands has a negative or positive polarity. In one

embodiment, the sequential arrangement of multiple bands can correspond to a Barker sequence, such as Barker- 13 sequence. In one embodiment, the tubular device can be a needle or a trocar. In one embodiment, each of the multiple piezoelectric bands in the aperture is one or more unit widths, the unit width being half of a wavelength of the selected operating frequency, such as in the range of 1 to 50MHz.

[0008] In another embodiment, a method for using an ultrasound imaging system for locating a tubular device in a medium may include accommodating a tubular device in a central opening of an aperture of a cylindrical transducer described above, may include locating the tubular device and the transducer in a medium, exciting an ultrasound beam at the transducer at a selected operating frequency, and receiving ultrasound signals at the ultrasound imaging system.

[0009] In another embodiment, an ultrasound wide-angle transducer for use in conjunction with an ultrasound imaging system may include a substrate and a piezoelectric layer sandwiched between two electrode layers and disposed on the substrate. The piezoelectric layer may include multiple strips disposed in a sequential arrangement such that a spatial-pulse sequence derived from the sequential arrangement has an autocorrelation having a largest value at zero shift and having one or more side lobes of smaller value at non-zero shifts. The spatial-pulse sequence is derived from the sequential arrangement of the multiple strips of particular order, width and polarity, and may correspond to a Barker sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 illustrates a wide-angle transducer according to one embodiment.

[0011] FIG. 2 illustrates the spatial-pulse sequence of the embodiment in FIG. 1.

[0012] FIG. 3 A illustrates the autocorrelation of aperture of the embodiment in FIG.

1.

[0013] FIG. 3B illustrates the autocorrelation of aperture of a single-band transducer.

[0014] FIGs. 4A-4C illustrate the autocorrelation of aperture corresponding to various Barker sequences of spatial pulses.

[0015] FIG. 5 illustrates a method of making a cylindrical transducer according to one embodiment.

[0016] FIG. 6 illustrates methods of operating an ultrasonic imaging system according to various embodiments.

[0017] FIG. 7 illustrates a strip transducer according to one embodiment.

[0018] FIG. 8 illustrates a method of making a strip transducer according to one embodiment. DETAILED DESCRIPTION

[0019] As used in this document, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. As used in this document, the term "comprising" means "including, but not limited to."

[0020] The term "cylinder" or "cylindrical shape" or "cylindrical transducer" refers to a transducer having the surface traced by a straight line moving parallel to a fixed straight line and intersecting a fixed planar closed curve. The closed curve may be of various shapes, such as circular, oval or other curved shapes.

[0021] The terms "tubular device" refers to a device in the shape of a tube, the cross section of which may be of any shape such as circle, oval or other curved shapes.

[0022] The term "substrate" refers to a material that provides support in the making of a transducer. A substrate can be of any material suitable for the application, such as metal, plastic, or other non-metal or metalized materials.

[0023] The term "spatial-pulse sequence" refers to a sequence of elements in an aperture that includes multiple transducer segments of a particular order, width and polarity.

[0024] The term "autocorrelation" of an aperture refers to the autocorrelation function of the spatial-pulse sequence of the aperture.

[0025] The term "electroding" refers to placing an electrode on the piezoelectric material so the piezoelectric material can be poled or voltages can be applied to or signals can be received from.

[0026] The term "poling" refers to applying a voltage to the electrode of the piezoelectric material so that the piezoelectric material can be polarized.

[0027] One way of achieving a wide-angular-range receiver that can be useful for interventional ultrasound imaging device is to use a single narrow cylindrical band around a tubular device, where the width of the band is less than the wavelength of ultrasound used. The response of such band receiver can be represented by

R(0) = { [sin( sin0 w/ ]/( sin0w/ )}² x C (1) where Θ is the angle of the impinging ultrasound beam perpendicular to the band, w is the width of the band, λ the wavelength of the ultrasound in the beam, and C is a constant. This equation makes clear that the band must be less wide than a wavelength for significant response at higher angles. For example, if it is desired that the receiver be at least half as sensitive at 60° as at zero degrees, the width must be such that w < 1/2 x λ, i.e. less than half a wavelength. While such single-band receiver responds over a large angular range, it will intercept only a small portion of the ultrasound energy in the beam, and thus be of limited sensitivity.

[0028] Ultrasound beams, when used in diagnostic imaging systems, are always several wavelengths across, even at their narrowest "waist," i.e. their focal point. The width of the waist of a focused ultrasound beam is 2.44 x f-number, where the f-number is the distance of the focus point from the transducer divided by the diameter of the transducer. The usual f-number for a diagnostic ultrasound probe is greater than 3, thus, for an f/3 imaging system the energy in the beam is distributed over -7.5 wavelengths at focus, and even more away from focus. To intercept a greater proportion of energy in the ultrasound beam over a higher angular range, in one embodiment, the aperture of a transducer may include multiple segments sequentially arranged to achieve a wide ultrasonic angular spectrum. [0029] The far-field (Fraunhoffer) diffraction pattern of an aperture is directly proportional to the Fourier Transform of the aperture illumination. See J.W. Goodman, Introduction to Fourier Optics, McGraw-Hill, 1968, page 61. This can be understood as the distribution in energy in the far field results from the combination of plane waves arising from corresponding frequency components of a Fourier decomposition of the ultrasound amplitude on the aperture, with the angle of the plane wave related to the frequency component of the Fourier analysis of the aperture by sinG = f λ. The angular spectrum of plane waves to which the aperture responds or transmits can thus be treated as a range of frequencies, with a wider the range of frequencies corresponding to a greater range of angles of response. In particular, the problem of receiving (or sending) energy over a broad angular range can be considered to be that of synthesizing an aperture that has a broad Fourier transform.

[0030] Whereas it is proven that the autocorrelation of a function is equal to its Fourier power spectrum (the Wiener-Khinchin theorem), for a transducer receiver or transmitter that is sensitive over a range of angles, the desired receiver or transmitter aperture should have an autocorrelation function that is peaked at zero and very low elsewhere, which corresponds a Fourier power spectrum that extends over the desired range of angles.

[0031] In one embodiment, the aperture of a transducer includes multiple piezoelectric segments of positive and negative polarity disposed in a sequential arrangement, each of the piezoelectric segments having a width and polarity. The autocorrelation of the aperture in the transducer can be calculated as the autocorrelation function of a spatial-pulse sequence. The spatial-pulse sequence refers to a sequential arrangement of the plurality of segments in the aperture, on the basis of the order, the width and the polarity of each segment. The autocorrelation at zero-spatial shift means the total value as each element of the autocorrelation is multiplied by itself.

[0032] In one embodiment, the goal of the design is to place the multiple piezoelectric segments in the aperture in a sequential arrangement such that the

autocorrelation of the aperture achieves high value at zero shift and low at other spatial shifts, approaching an "impulse" function at the origin. There may be various sequential arrangements that can achieve such an autocorrelation characteristic. For example, white noise, being perfectly random, has an autocorrelation function which is large at zero shift, i.e. the total noise energy as each element of the noise is multiplied by itself, but almost zero everywhere else, as the random components sum to near zero. Practically, a special quasi- noise sequence that is known and that has a high autocorrelation at zero shift and low elsewhere is the Barker code or Barker sequence (see

https://en.wikipedia.org/wiki/Barker_code), of which only 9 Barker sequences are known and the longest sequence has a length of 13.

[0033] With reference to FIG. 1, according to one embodiment, a transducer that may achieve a high value of autocorrelation function at zero shift and low at other shifts uses an aperture that corresponds to a Barker sequence of length 13 (Barker- 13). The transducer may include an aperture that includes multiple cylindrical piezoelectric bands 202-208 that are sequentially arranged. Each of the plurality of bands has a width of one or more unit widths, and is poled (polarized) to produce either positive or negative voltages in response to an impinging ultrasound beam. In one embodiment, the unit width is λ/2, where λ is the wavelength of the operation frequency of the transducer. Each of the bands 205, 202, 206, 203, 207, 204 and 208 may have the width of 5, 2, 2, 1, 1, 1 and 1 unit width(s), respectively. And the polarity of each of the bands changes alternatively, i.e. the polarity of bands 205, 202, 206, 203, 207, 204 and 208 may be +, -, +, -, +, - and +, respectively. This sequential arrangement of the multiple piezoelectric bands is configured such that the spatial-pulse sequence derived from the sequential arrangement has an autocorrelation function that has its largest value at or near zero shift and one or more side lobes of lower values at non-zero shifts.

[0034] The spatial-pulse sequence derived from the sequential arrangement of the multiple bands in FIG. 1 is further explained. With reference to FIG. 2, the spatial-pulse sequence of the transducer in the embodiment in FIG. 1 has a sequence of pulses, 102-108, each corresponding to one of the multiple bands 202-208 in FIG. 1. For example, the first spatial-pulse 105 corresponds to the first band 205 in FIG. 1, i.e. the width of pulse 105 corresponds to the width of band 205, the polarity of pulse 105 corresponds to the polarity of band 205, and the order of pulses corresponds to the order of multiple bands. Likewise, the pulses 102, 106, 103, 107, 104 and 108 of FIG. 2 correspond to the bands 202, 206, 203, 207, 204 and 208, respectively. Consequently, the total length of the spatial-pulse sequence equals the sum of the widths of all of the plurality of bands in unit width, which is in this case, 13.

[0035] With further reference to FIG. 2, the spatial-pulse sequence that is derived from the sequential arrangement of the multiple bands corresponds to a Barker- 13 sequence, i.e. [+1 +1 +1 +1 +1 -1 -1 +1 +1 -1 +1 -1 +1], with a total of 13 values. Each of the groups in the Barker-13 sequence corresponds to a spatial-pulse in the spatial-pulse sequence, which corresponds to one of the multiple bands in the transducer. For example, the first group of the sequence (+1 +1 +1 +1 +1) corresponds to the spatial-pulse 105, which is derived from the band 205 that has a width of 5 unit widths. The second group (-1 -1) corresponds to the pulse 102 and the band 202 having a width of 2 unit widths, and so on. [0036] With reference to FIG. 3 A, the autocorrelation of the aperture, i.e. the autocorrelation function of the spatial-pulse sequence derived from the sequential arrangement of multiple piezoelectric bands in the embodiment of the transducer in FIG. 1 is shown. As can be seen, and as expected of a Barker sequence, the autocorrelation of the aperture has the largest component 302 at or about zero shift (all spatial frequencies equal) plus some "spurs" at (1/13)², or .006, at other spatial frequencies. That is, the side lobes of the autocorrelation, 303-314 have much lower values than the value at zero shift. In one embodiment, a slight shift of the largest component about zero, in particular, within a small window of unit width (i.e. +/- half the unit width), while leaving the autocorrelation at other shifts outside the small window with smaller values, would still be able to achieve a wide angular range.

[0037] In comparison to FIG. 3 A, FIG. 3B illustrates the autocorrelation of a single- band transducer. When the unit width of both single-band transducer and multi-band transducer is λ/2, for example, a comparison of FIG. 3 A and FIG. 3B shows that the transducer's angular spectrum would be the same as a single band, but 13 times its size (compare 302 and 320). This characteristic of the transducer would allow such a multi-band transducer to have the response with angles the same as the single narrow band but 13 times higher in value, while having small irregularities at some angles corresponding to the additional "spur" frequencies arising from the small autocorrelation bumps 303, 304, etc. In one embodiment, Eq. (1) can be used to determine what unit width "w" should be to attain the desired angular response.

[0038] In other embodiments, various sequential arrangements of the piezoelectric segments in the aperture, and variations of the number of piezoelectric segments and their polarities may be possible. For example, the piezoelectric segments and their polarities may be configured such that the spatial-pulse sequence, which can be derived from the aperture in the manner described above, corresponds to other Barker sequences. In one embodiment, the sequential arrangement of the multiple piezoelectric segments can be such that the spatial-pulse sequence derived therefrom corresponds a Barker-5 sequence, i.e. [+1 +1 +1 -1 +1]. Alternatively, the sequential arrangement of the multiple piezoelectric segments can correspond to a Barker-7 sequence: [+1 +1 +1 -1 -1 +1 -1], or a Barker-11 sequence: [+1 +1 +1 -1 -1 -1 +1 -1 -1 +1 -1].

[0039] With reference to FIGs. 4A-4C, when the sequential arrangement of multiple segments in the aperture is made to correspond to Barker-5, Barker-7 and Barker- 11 sequences, the autocorrelation of the aperture for each arrangement is shown in 330, 331 and 332, respectively. All of these autocorrelations will have a largest value at or about zero shift and lower value autocorrelation values at other spatial shifts. In other embodiments, other Barker sequences or later developed sequences that can achieve an autocorrelation having a largest value at or about zero shift and lower value side lobes at other spatial shifts may also be used. In one embodiment, the lower value side lobes may be no larger than 50% of the largest value at or about zero spatial shifts.

[0040] Returning to FIG. 1, the plurality of piezoelectric segments can be configured in various ways. In one embodiment, the plurality of segments can be configured to form a shape of a cylinder that has a central opening 210 for accommodating a tubular device, such as a needle or trocar. In an ultrasound imaging system, the needle or the trocar, together with all or a portion of the transducer can be put into a patient's body.

[0041] With reference to FIG. 5, a method of making a cylindrical transducer is illustrated according to one embodiment. The method may include metalizing a tubular device 401, coating metalized tubular device with copolymer 402, electroding a band in a spatial-pulse sequence on the coated device 403, poling the band in the desired polarity 404, and continue repeating the steps 403 and 404 for each band in the transducer. The spatial- pulse sequence is derived from the sequential arrangement of multiple bands in the aperture in the embodiment of FIG. 1. In other embodiments, the spatial-pulse may be derived from other sequential arrangement of multiple bands in the aperture, of which the autocorrelation has a largest value at zero spatial shift and smaller values at all other non-zero shifts. Once all of the bands in the spatial-pulse sequence are electroded and poled 405, the method further includes connecting all of the electrodes for all of the bands in the spatial-pulse sequence 406.

[0042] Various methods may be used for each of the steps in FIG. 5. In one embodiment, the tubular device may be made of piezoelectric material. In another embodiment, piezoplastic, which can be flexible and easy to make, may also be used.

Metalizing the tubular device 401 may include spin coating, dipping or electro-spraying. Alternatively and/or additionally, the tubular device may be a piezoelectric or piezoplastic sleeve in the shape of a hollow cylinder for accommodating an instrument such as a needle. When the tubular device is made of piezoplastic, the coating 402 may include dipping the tubular device with a metallic coating into dissolved piezoplastic material to achieve a uniform layer of thickness such that it is sensitive to the desired frequency, which depends upon the material of the device. For example, a hard-backed transducer will resonate at the frequency where the layer thickness corresponds to a quarter- wave-thick layer of the piezoelectric material. There are also methods of calculating the ideal thickness material for the general case, such as a computer program called PiezoCAD™. In another embodiment, the tubular device, such as a needle, may be made of metal. In that case the coating 402 can be directly applied to the needle without metalizing 401. [0043] Electroding the piezoelectric material 403 generally includes placing a metallic layer in intimate or close contact with the piezoelectric material. This can be done by sputtering on a layer of metal, painting by spray or other means the electrode by using metallic paint, electroless plating of a metal, evaporating a metallic layer onto the piezoelectric etc., as is known and practiced in the art. The electrodes are parallel to the surface of the device on which the wide-angle transducer will be placed, whereas the positive and negative polarization desired is perpendicular to the device surface for maximum coupling to the desired range of angles of ultrasound wavefronts.

[0044] Poling the piezoelectric material 404 can use various known techniques. An example of poling piezoelements in different directions is reported by Cannata et al., Development of a Flexible Implantable Sensor for Postoperative Monitoring of Blood, Journal of Ultrasound in Medicine 2012, vol 31, page 1795. In one embodiment, piezoplastics are particularly suited to building such structures. With further reference to FIG. 5, in one embodiment, electroding 403 and poling (polarizing) 404 may be repeated for each band of the plurality of bands in the transducer, using the method reported by Cannata et al., sequentially. After electroding and poling the first band, the next adjacent band can be electroded and then polarized with the proper polarity as the first band or nearby band is held at ground, to avoid depolarization. In one embodiment, by using the method disclosed, each of the plurality of piezoelectric bands of the transducer will be of uniform thickness around the circumference of the opening of the cylindrical transducer.

[0045] In one embodiment, the step of connecting all electrodes for all of the bands 406 may include disposing on the electroded and polarized bands by a sheet electroded or sputtered onto the band electrodes, and the top sheet electrode and the device metal layer may be connected to receive or transmit ultrasound signals. [0046] With reference to FIG. 6, methods of operating an ultrasound imaging system using the transducer disclosed in this document are further explained. In one embodiment, a method 500 for using the transducer to receive ultrasound waves includes accommodating a tubular device in the transducer 501 such as passing a needle or a trocar through the opening of the cylindrical transducer. The method may also include locating the tubular device and transducer into a patient's body 502 and operating the ultrasound imaging system at a frequency 503. When the ultrasound beam strikes the transducer, the transducer's signal is carried from the transducer to outside the body, the system may determine the location of the transducer based on the time the signal is received and known beam direction. Thus the method further includes recording the ultrasound signals emitted by the ultrasound imaging system and received at the transducer 504. In one embodiment, the method of ultrasonic position indicating described in the U.S. Patent No. 5,161,536 to Vilkomerson et. al. can be used.

[0047] With further reference to FIG. 6, the transducer can also be used as an emitter in an ultrasound imaging system. The method 510 for using the transducer may include accommodating a tubular device in the transducer 505, locating the tubular device and transducer into a patient's body 505, exciting the ultrasound beam at the transducer at an operating frequency 507, and receiving ultrasound signals at ultrasound imaging system 508. In receiving the ultrasound signals, the system scans the space where the device is expected until it detects the emitted signal, allowing computation of the device location. In one embodiment, the method described by Guo, X et al in Active Ultrasound Pattern Injection System (AUSPIS) for Interventional Tool Guidance, PLOS/one, vol 9, issue 10 October 2014 can be used.

[0048] In one embodiment, optimal performance tradeoff in ultrasound imaging may be achieved if the width of each of the plurality of piezoelectric segments is configured to be at a value relative to the wavelength of the ultrasound wave based on the operating frequency. In one embodiment, the operating frequency of the transducer can be in the range of 1 and 50 MHz depending upon the preferred ultrasound frequency for the interventional procedure. Alternatively and/or additionally, the unit width of the plurality of piezoelectric segments is the wavelength of the preferred operating frequency of the transducer or a fraction thereof, such as in the range of 1 to 0.05, of the wavelength of the operating frequency. The shorter the unit width relative to the wavelength is the closer the Fourier transform approaches the ideal impulse function, but the lower the sensitivity is. In one embodiment, a tradeoff unit width may be selected as half of the wavelength, i.e. λ/2.

[0049] In one embodiment, the operating frequency and the unit width of the segment may depend on the particular interventional device application. For example, for shallow tissue depths, a high frequency or short wavelengths may be used. In another example, for deep scanning within the body, lower frequency with longer wavelength that requires a larger unit width may be used. In both scenarios, the unit width of the segment may be half of the wavelength of the operating frequency.

[0050] Whereas an omnidirectional wide-angle transducer can be configured in a cylindrical shape, other variations may be possible. In one embodiment, the multiple piezoelectric segments do not need to be fully wrapped around to form the cylindrical shape, or the plane intersecting the cylindrical shape can yield an open curve, while leaving a gap. In another embodiment, each of the multiple segments can even be of a shape of a half cylinder, i.e. the plane intersecting the cylindrical shape can yield a half circle, for example. While the all-around sensors may help to guide devices to locate the transducer (because it does not matter how the transducer is rotated with respect to the device), the open structure can still be practical for use in an ultrasound imaging system. For example, interventional devices, such as trocars and to a lesser extent needles, may not be rotated after they enter the body so a partial coverage of the device may be all that is required. The method of operating the ultrasound imaging system may further include rotating the transducer such that the piezoelectric segments are faced in the direction of the skin, where the interrogating ultrasound transducer is located.

[0051] With reference to FIG. 7, in another embodiment, the plurality of piezoelectric segments of the transducer may be in the shape of strips such that the transducer forms a flat shape forming a horizontal plane and having a uniform thickness. In FIG. 7, a spatial sequential arrangement of plurality of segments or strips is shown to include seven strips sequentially arranged in the order of 601, 610, 602, 611, 603, 612 and 604, such that the spatial-pulse sequence that can be derived from such sequential arrangement corresponds to the Barker- 13 sequence. In one embodiment, the transducer may include a substrate layer 606 and a copolymer piezoplastic layer 605 sandwiched between two electrode layers 607.

[0052] With reference to FIG. 8, a fabrication process of the strip transducer is described. In one embodiment, the process may include metalizing a substrate 701, such as depositing a metal layer or spraying the substrate with a metallic material. The process may further include coating the metalized substrate with copolymer 702, electroding a strip 703 and poling the strip 704, for which the width and polarity of the strip may derive a spatial- pulse sequence that corresponds to the Barker-13 sequence. Electroding and poling processes are similar to those described in the fabrication of cylindrical transducer in this document. The process further includes repeating the electroding and poling steps 703, 704 for each strip in the spatial-pulse sequence until all the strips are electroded and poled. Like the process for fabricating the cylindrical transducer, in electroding each of the strips, the previous adjacent strip that has been electroded can be held at ground to avoid

depolarization. Finally, the fabrication process includes connecting all electrodes for all strips in the spatial-pulse sequence 706, such as by disposing a metal layer.

[0053] The strip transducer may function as either a receiver or an emitter, and can be used in the same manner as the cylindrical transducer disclosed in this document. In one embodiment, a method for operating an ultrasound imaging system using such a transducer as a receiver may include locating the transducer into a patient's body, operating the ultrasound imaging system at a frequency and recording the ultrasound signals emitted by the ultrasound imaging system and received at the transducer. In one embodiment, the method of ultrasonic position indicating described in the U.S. Patent No. 5, 161,536 can be used.

[0054] In another embodiment, a method for operating an ultrasound imaging system using the transducer as an emitter may include locating the transducer into a patient's body, exciting an ultrasound beam at the transducer at an operating frequency, and receiving ultrasound signals at ultrasound imaging system. In receiving the ultrasound signals, the system scans the space where the device is expected until it detects the emitted signal, allowing computation of the device location. In one embodiment, the method described by Guo, X et al in Active Ultrasound Pattern Injection System (AUSPIS) for Interventional Tool Guidance, PLOS/one, vol 9, issue 10 October 2014 can be used.

[0055] Similar to the cylindrical transducer, in one embodiment, the unit width that comprises each of the plurality of piezoelectric strips may be configured to be half of the wavelength of the ultrasound beam of the ultrasound imaging system. In another embodiment, the unit width can be in the range of 1 to 0.05 wavelengths of the preferred operating frequency of the transducer. In one embodiment, the operating frequency of the ultrasound imaging system can be in the range of 1 and 50 MHz depending upon the preferred ultrasound frequency for the interventional procedure. For example, a higher frequency may be used for shallow tissue depths, or a lower frequency with longer wavelength may be used for deep scanning.

[0056] In some alternative embodiments, poling the multiple piezoelectric segments in the aperture of the transducer (for example, refer to 404 in FIG. 5 and 704 in FIG. 8) may not result in the preferred polarities that could achieve the autocorrelation characteristics we would like to have for the aperture, i.e. an autocorrelation that has the largest value at or about zero shift and smaller values at other shifts. For example, the polarities of all of the piezoelectric segments may be the same or even randomly poled. These incorrect polarities can be compensated when the transducer in being used in the ultrasound imaging system. The polarity compensation can be done in various ways depending on whether the transducer is used as an emitting transducer or a receiving transducer.

[0057] In one embodiment, for example, with reference to FIG. 6, the method of using the wide-angle transducer as a receiver 500 may additionally include identifying the polarity of each of the multiple piezoelectric segments and adding a 0-degree or 180-degree phase shift accordingly (e.g. "+" and "-" alternate if the multiple piezoelectric segments are poled in the same direction) to the received signal at each of the multiple piezoelectric segments. Reversing the phase of a signal can be done by changing a positive voltage to a negative voltage by an inverter circuit or a transformer or other known methods. Similarly, the method of using the wide-angle transducer as a transmitter 501 may alternatively include a step of driving each of the multiple piezoelectric segments with the required positive or negative voltage, to achieve the same result as shown in FIG. 1 or FIG. 7, i.e. the autocorrelation of the aperture has the largest value at zero shift and lower values at other shifts.

[0058] If a transducer is fabricated such that polarity compensation is required in utilizing the transducer in order to achieve the desired autocorreclation characteristics for the aperture, the electrodes for all of the multiple piezoelectric segments cannot be connected, but instead at least the connection to the individual segment that has the incorrect polarity must be separate to allow compensation by voltage driving or phase shifting on an individual segment basis. Alternatively and/or additionally, the method of using the transducer may include detecting the polarization of each segment. This can be done by driving each of the multiple piezoelectric segments separately and determining if the initial signal received by a test hydrophone is positive when the initial voltage on that segment is positive ("+" poled) or negative ( "-" poled), as is known in the art.

[0059] The above-disclosed features and functions, as well as alternatives, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be made by those skilled in the art, each of which is also intended to be encompassed by the disclosed embodiments.

Claims

1. A transducer for use in conjunction with an ultrasound imaging system, comprising: an aperture comprising a plurality of piezoelectric segments each having a width and a positive or negative polarity;

wherein the plurality of piezoelectric segments in the aperture are disposed in a sequential arrangement so that an autocorrelation of the aperture has a largest value at zero shift and has one or more side lobes of smaller value at non-zero shifts.

2. The transducer of claim 1, wherein the plurality of piezoelectric segments are configured to form a shape of a cylinder, wherein the cylinder defines a central opening for accommodating a tubular device.

3. The transducer of claim 1, wherein the plurality of piezoelectric segments are configured to form a flat shape defining a horizontal plane and having a uniform thickness.

4. The transducer of claim 1, wherein the values of one or more side lobes of the autocorrelation of the aperture are 50% or smaller than the largest value of the

autocorrelation.

5. The transducer of claim 1, wherein a spatial -pulse sequence derived from the plurality of piezoelectric segments in the aperture corresponds to a Barker sequence.

6. The transducer of claim 5, wherein the spatial-pulse sequence derived from the plurality of piezoelectric segments in the aperture corresponds to a Barker- 13 sequence.

7. The transducer of claim 1, wherein the plurality of piezoelectric segments are configured to emit ultrasound waves at an operating frequency.

8 The transducer of claim 1, wherein the plurality of piezoelectric segments are configured to receive ultrasound waves at an operating frequency.

9. The transducer of claim 2, wherein the tubular device is a needle or a trocar.

10. The transducer of claim 1, wherein the width of each of the plurality of piezoelectric segments is one or more unit widths, the unit width being a fraction of a wavelength of a selected operating frequency of the transducer.

11. The transducer of claim 10, wherein the operating frequency of the transducer is in the range from 1 to 50 MHz.

12. A method of fabricating a transducer for use in conjunction with an ultrasound imaging system, wherein the transducer comprises an aperture comprising a plurality of segments disposed in a selected sequential arrangement, and wherein each of the plurality of segments has a width and a positive or negative polarity, the method comprising:

(i) coating a metalized tubular device with piezoelectric layer;

(ii) electroding a first segment in the selected sequential arrangement on the coated tubular device; (iii) poling the first segment in a selected polarity perpendicular to a surface of the coated tubular device;

(iv) repeating the steps (ii) and (iii) for all remaining segments in the selected sequential arrangement on the coated tubular device; and

(v) connecting all the electrodes for all the segments on the coated tubular device; wherein the selected sequential arrangement is such that an autocorrelation of the aperture has a largest value at zero shift and that has one or more side lobes of smaller value at non-zero shifts.

13. The method of claim 12, wherein a spatial-pulse sequence derived from the plurality of segments in the aperture corresponds to a Barker sequence.

14. The method of claim 13, wherein the spatial-pulse sequence derived from the plurality of segments in the aperture corresponds to a Barker- 13 sequence.

15. The method of claim 12, wherein the metalized tubular device is a needle or a trocar.

16. The method of claim 12 further comprising metalizing the tubular device, wherein the tubular device is made of a piezoelectric material.

17. The method of claim 12, wherein the width of each of the plurality of segments is one or more unit widths, the unit width being a fraction of a wavelength of a selected operating frequency of the transducer.

18. A method for using an ultrasound imaging system for locating a tubular device in a medium, comprising:

accommodating a tubular device in a central opening of a cylindrical transducer, wherein the cylindrical transducer comprises an aperture comprising a plurality of piezoelectric bands disposed in a sequential arrangement and forming the central opening of the cylindrical transducer;

locating at least a portion of the tubular device and at least a portion of the transducer into a medium;

operating the ultrasound imaging system at a selected operating frequency; and recording, by the ultrasound imaging system, ultrasound signals received at the transducer;

wherein:

each of the plurality of piezoelectric bands has a width and has a positive or negative polarity, and

the sequential arrangement of the plurality of piezoelectric bands in the aperture is such that an autocorrelation of the aperture has a largest value at zero shift and has one or more side lobes of smaller value at non-zero shifts.

19. The method of claim 18, wherein a spatial -pulse sequence derived from the plurality of piezoelectric bands in the aperture corresponds to a Barker- 13 sequence, a Barker- 11 sequence, a Barker-9 sequence, a Barker-7 sequence or a Barker-5 sequence.

20. The method of claim 18, wherein the tubular device is a needle or a trocar.

21. The method of claim 18, wherein the width of each of the plurality of piezoelectric bands in the aperture is one or more unit widths, the unit width being half of a wavelength of the selected operating frequency.

22. The method of claim 20, wherein the selected operating frequency is in the range of 1 and 50 MHz.

23. A method for using an ultrasound imaging system for locating a tubular device in a medium, comprising:

accommodating a tubular device in an opening of a cylindrical transducer, wherein the cylindrical transducer comprises an aperture comprising a plurality of piezoelectric bands disposed in a sequential arrangement and forming the opening of the cylindrical transducer;

locating the tubular device and the transducer in a medium;

exciting an ultrasound beam at the transducer at a selected operating frequency; and receiving ultrasound signals at the ultrasound imaging system;

wherein:

24. A transducer for use in conjunction with an ultrasound imaging system, comprising: a substrate; and

a piezoelectric layer sandwiched between two electrode layers and disposed on the substrate;

wherein:

the piezoelectric layer comprises a plurality of strips disposed in a sequential arrangement, wherein each strip has a width and has a positive or negative polarity, and

the sequential arrangement of the plurality of strips is such that a spatial- pulse sequence derived from the sequential arrangement has an autocorrelation having a largest value at zero shift and having one or more side lobes of smaller value at non-zero shifts.

25. A method for using an ultrasound imaging system, comprising:

locating a transducer in a medium, wherein the transducer comprises an aperture comprising a plurality of piezoelectric segments disposed in a sequential arrangement, each piezoelectric segment having a width and a polarity;

operating the ultrasound imaging system at a selected operating frequency;

applying a selected signal phase-shift at one or more of the plurality of the piezoelectric segments so that an autocorrelation of the aperture has a largest value at zero shift and has one or more side lobes of smaller value at non-zero shifts; and

recording, by the ultrasound imaging system, ultrasound signals received at the transducer.

26. A method for using an ultrasound imaging system, comprising: locating a transducer in a medium, wherein the transducer comprises an aperture comprising a plurality of piezoelectric segments disposed in a sequential arrangement, each piezoelectric segment having a width and a polarity;

identifying the polarity of each of the plurality of piezoelectric segments;

exciting ultrasound beams at the transducer by driving a voltage at one or more of the plurality of piezoelectric segments and driving an opposite-shift voltage at the remaining of the plurality of piezoelectric segments so that an autocorrelation of the aperture has a largest value at zero shift and has one or more side lobes of smaller value at non-zero shifts; and

receiving ultrasound signals from the ultrasound beams.