Abstract:
An imaging guidewire can include one or more optical fibers communicating light along the guidewire. At or near its distal end, one or more blazed or other Fiber Bragg Gratings (FBGs) can direct light to a photoacoustic transducer material that provides ultrasonic imaging energy. Returned ultrasound can be sensed by an FBG sensor. A responsive signal can be optically communicated to the proximal end of the guidewire, and processed such as to develop a 2D or 3D image. In an example, the guidewire outer diameter can be small enough such that an intravascular catheter can be passed over the guidewire. To minimize the size of the guidewire, an ultrasound-to-acoustic transducer that is relatively insensitive to the polarization of the optical sensing signal can be used. The ultrasound-to-optical transducer can be manufactured so that it is relatively insensitive to the polarization of the optical sensing signal.

Description:
CLAIM OF PRIORITY 
     This patent application is a continuation application of U.S. patent application Ser. No. 12/571,724, filed on Oct. 1, 2009, now U.S. Pat. No. 8,560,048, issued on Oct. 15, 2013, which claims the benefit of priority of Michael J. Eberle et al. U.S. Provisional Patent Application Ser. No. 61/102,216, which was filed on Oct. 2, 2009, the entire content of each being incorporated herein by reference in its entirety, and the benefit of priority of each is claimed herein. 
    
    
     BACKGROUND 
     Bates et al. U.S. Pat. No. 7,245,789 is incorporated by reference herein in its entirety, including its discussion of systems and methods for minimally-invasive optical-acoustic imaging. It discusses, among other things, an imaging guidewire that includes one or more optical fibers communicating light along the guidewire. At or near a distal end of the guidewire, light is directed to a photoacoustic transducer material that provides ultrasonic imaging energy. Returned ultrasound energy is sensed by an ultrasound-to-optical transducer. A responsive signal is optically communicated to the proximal end of the guidewire, such that it can be processed to develop a 2D or 3D image. 
     Overview 
     Among other things, the present applicant has recognized the desirability of reducing or minimizing the size of a system like that described in Bates. However, the present applicant has recognized that reducing the size of the ultrasound-to-optical transducer can also change the mechanical acoustic resonance of the transducer at some ultrasound frequencies and can also result in a generally smaller output signal. Additionally, the transducer response can become dependent on the polarization of the optical signal since it is responsive to acoustic impedances in the direction of polarization. This can result in undesirable variations in the output signal conditional on the polarization direction due to different resonances in different lateral directions, for example. One approach to reduce or eliminate this effect would be to control polarization of the optical signal. However, such polarization is difficult to control or predict in a cost-effective manner. Accordingly, the present applicant has recognized, among other things, that it can be advantageous to provide an ultrasound-to-optical transducer that is relatively insensitive to optical signal polarization. 
     Example 1 can include an ultrasound-to-optical transducer, including an optical fiber, an ultrasound-absorptive backing configured for substantially absorbing ultrasound energy that passes beyond the optical fiber to reach the backing, and a Fiber Bragg Grating (FBG) interferometer configured for receiving the ultrasound energy and capable of modulating an optical sensing signal, in response to the ultrasound energy, substantially independent of a polarization angle of the optical sensing signal. In this example, the optical fiber can include an optical fiber diameter that is less than half of a wavelength of target ultrasound energy to be received by the transducer. 
     In Example 2, the subject matter of Example 1 can optionally comprise an elongated intravascular imaging assembly, comprising a proximal external portion comprising an optical signal interface and a distal portion sized and shaped and configured for internal intravascular acoustic imaging. In this example, the imaging assembly can comprise an elongated support, extending substantially from the proximal portion of the imaging assembly to the distal portion of the imaging assembly, the support configured to provide adequate length, rigidity, and flexibility to permit intravascular introduction and steering of a distal portion of the imaging assembly to a location of interest, the optical fiber, extending longitudinally and affixed along the support substantially from the proximal portion of the imaging assembly substantially to the distal portion of the imaging assembly, a transducer, located at or near a distal portion of the imaging assembly and configured as at least one of the ultrasound-to-optical transducer or an optical-to-ultrasound transducer; and the backing, located in a backing region between the transducer and the elongated support, the backing configured to attenuate ultrasound energy that reaches the backing region by at least 90%. 
     In Example 3, the subject matter of any one of Examples 1 or 2 can optionally be configured so that the assembly comprises a plurality of the optical fibers, extending longitudinally and affixed along the support, the fibers arranged circumferentially about a central longitudinal axis of the support, and a plurality of the transducers, wherein the transducer includes a photoacoustic material located at a peripheral portion of the optical fiber that is away from the backing. 
     In Example 4, the subject matter of any one of Examples 1-3 can optionally be configured such that the optical fiber diameter is less than sixty micrometers. 
     In Example 5, the subject matter of any one of Examples 1-4 can optionally be configured so that the backing comprises microballoons containing a gas. 
     In Example 6, the subject matter of any one of Examples 1-5 can optionally be configured so that the microballoons comprise about 30% to about 50% of the volume of the backing. 
     In Example 7, the subject matter of any one of Examples 1-6 can optionally be configured so that a thickness of the backing is between about 25 micrometers and about 100 micrometers. 
     In Example 8, the subject matter of any one of Examples 1-7 can optionally comprise the transducer being configured such that a peak-to-peak amplitude of an oscillating output optical bias signal modulated by the transducer varies by no more than one decibel when the polarization angle of the optical sensing signal is varied by 90 degrees. 
     In Example 9, the subject matter of any one of Examples 1-8 can optionally be configured such that a total birefringence induced on an optical signal with a wavelength of about 1550 nanometers transmitted within the optical fiber is less than (3×10 −6 ). 
     Example 10 can include (or can be combined with the subject matter of any one of Examples 1-9 to include) fabricating an ultrasound-to-optical transducer, the fabricating comprising providing an optical fiber that includes a diameter that is less than half of a wavelength of target ultrasound energy to be received by the transducer, creating a Fiber Bragg Grating (FBG) interferometer configured for receiving the ultrasound energy and capable of modulating an optical sensing signal substantially independent of a polarization of the optical sensing signal, associating an ultrasound-absorptive backing with the optical fiber, and configuring the backing for substantially absorbing ultrasound energy that passes beyond the optical fiber to reach the backing. 
     In Example 11, the subject matter of any one of Examples 1-10 can optionally comprise associating the backing comprising microballoons containing a gas with the fiber. 
     In Example 12, the subject matter of any one of Examples 1-11 can optionally comprise providing the backing comprising the microballoons that comprise about 30% to about 50% of the volume of the backing. 
     In Example 13, the subject matter of any one of Examples 1-12 can optionally comprise inserting the backing into a backing region between a central support structure and the optical fiber, wherein both the support structure and the fiber extend longitudinally, the backing region being bounded laterally by a tubular sheath. 
     In Example 14, the subject matter of any one of Examples 1-13 can optionally comprise conforming the backing to the external surface of a central support structure, wherein both the support structure and the fiber extend longitudinally. 
     In Example 15, the subject matter of any one of Examples 1-14 can optionally further comprise conforming a tubular sheath to a surface of the backing that is away from the support structure. 
     Example 16 can include (or can be combined with the subject matter of any one of Examples 1-15 to include) receiving ultrasound energy with an optical fiber, the optical fiber including a diameter that is less than half of a wavelength of the ultrasound energy, and using the ultrasound energy to modulate an optical sensing signal substantially independent of a polarization of the optical sensing signal. 
     In Example 17, the subject matter of any one of Examples 1-16 can optionally comprise using the ultrasound energy to modulate the optical sensing signal substantially independent of the polarization of the optical sensing signal comprises generating a modulated output optical signal with a peak-to-peak amplitude that varies by no more than one decibel when a polarization angle of the optical sensing signal is varied by 90 degrees. 
     In Example 18, the subject matter of any one of Examples 1-17 can optionally comprise using an ultrasound-absorptive backing to substantially attenuate ultrasound energy that passes beyond the optical fiber to reach the backing. 
     In Example 19, the subject matter of any one of Examples 1-18 can optionally comprise using the ultrasound-absorptive backing to attenuate ultrasound energy that passes beyond the optical fiber to reach the backing by at least 90%. 
     In Example 20, the subject matter of any one of Examples 16-19 can optionally comprise forming an image of a region within a living body using information from a modulated output optical signal. 
     These examples can be combined with each other in any permutation or combination and or with other subject matter disclosed herein. This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1  is a cross-sectional side view illustrating generally, by way of example, but not by way of limitation, an example of an FBG strain sensor in an optical fiber. 
         FIG. 2  is a cross-sectional side view illustrating generally, by way of example, but not by way of limitation, an example of an FBG grating interferometer sensor. 
         FIG. 3  is a cross-sectional schematic diagram illustrating generally an example of an acousto-optic transducer. 
         FIG. 4  is a cross-sectional schematic diagram illustrating generally another example of an FBG strain sensor in an optical fiber. 
         FIG. 5  is a schematic diagram that illustrates generally an example of a side view of a distal portion of a guidewire. 
         FIG. 6  is a schematic diagram that illustrates generally an example of a cross-sectional side view of a distal portion of a guidewire. 
         FIG. 7  is a schematic diagram that illustrates generally an example of a cross-sectional end view of a proximal portion of a guidewire. 
         FIG. 8  is a schematic diagram that illustrates generally an example of a cross-sectional end view of a distal portion of a guidewire. 
         FIG. 9  is a cross-sectional end view that illustrates generally an example of a guidewire assembly at a location of a transducing window. 
     
    
    
     DETAILED DESCRIPTION 
     1. Examples of Fiber Bragg Grating Strain Sensors 
       FIG. 1  is a cross-sectional side view illustrating generally, by way of example, but not by way of limitation, an example of a strain-detecting or pressure-detecting acoustic-to-optical FBG sensor  100  in an optical fiber  105 . FBG sensor  100  senses acoustic energy received from a nearby area to be imaged, and transduces the received acoustic energy into an optical signal within optical fiber  105 . In the example of  FIG. 1 , FBG sensor  100  can include Bragg gratings  110 A-B in an optical fiber core  115 , such as surrounded by an optical fiber cladding  120 . Bragg gratings  110 A-B can be separated by a strain or pressure sensing region  125 , which, in an example, can be about a millimeter in length. This example can sense strain or pressure such as by detecting a variation in length of the optical path between these gratings  110 A-B. 
     A Fiber Bragg Grating can be conceptualized as a periodic change in the optical refractive index of a portion of the optical fiber core  115 . Light of specific wavelengths traveling down such a portion of core  115  will be reflected; the period (distance)  130  of the periodic change in the optical index determines the particular wavelengths of light that will be reflected. The degree of optical index change and the length  135  of the grating determine the ratio of light reflected to that transmitted through the grating  110 A-B. 
       FIG. 2  is a cross-sectional side view illustrating generally, by way of example, but not by way of limitation, an operative example of an interferometric FBG sensor  100 . The example of  FIG. 2  can include two FBG mirrors  110 A-B, which can be both partially reflective such as for a specific range of wavelengths of light passing through fiber core  115 . Generally, the reflectivity of each FBG will be substantially similar, but can differ for particular implementations. This interferometric arrangement of FBGs  110 A-B can be capable of discerning the “optical distance” between FBGs  110 A-B with extreme sensitivity. The “optical distance” can be a function of the effective refractive index of the material of fiber core  115  as well as the length  125  between FBGs  110 A-B. Thus, a change in the refractive index can induce a change in optical path length, even though the physical distance  125  between FBGs  110 A-B has not substantially changed. 
     An interferometer, such as FBG sensor  100 , can be conceptualized as a device that measures the interference between light reflected from each of the partially reflective FBGs  110 A-B. When the optical path length between the FBG mirrors  110 A-B is an exact integer multiple of the wavelength of the optical signal in the optical fiber core  115 , then the light that passes through the FBG sensor  100  will be a maximum and the light reflected will be a minimum, so the optical signal is substantially fully transmitted through the FBG sensor  100 . This addition or subtraction can be conceptualized as interference. The occurrence of full transmission or minimum reflection can be called a “null” and occurs at a precise wavelength of light for a given optical path length. Measuring the wavelength at which this null occurs can yield an indication of the length of the optical path between the two partially reflective FBGs  110 A-B. In such a manner, an interferometer, such as FBG sensor  100 , can sense a small change in distance, such as a change in the optical distance  125  between FBGs  110 A-B resulting from received ultrasound or other received acoustic energy. This arrangement can be thought of as a special case of the FBG Fabry-Perot interferometer, sometimes more particularly described as an Etalon, because the distance  125  between the FBGs  110 A-B is substantially fixed. 
     The sensitivity of an interferometer, such as FBG sensor  100 , can depend in part on the steepness of the “skirt” of the null in the frequency response. The steepness of the skirt can be increased by increasing the reflectivity of the FBGs  100 A-B, which also increases the “finesse” of the interferometer. The present applicant has recognized, among other things, that increasing the finesse or steepness of the skirt of FBG sensor  100  can increase the sensitivity of the FBG sensor  100  to the reflected ultrasound signals within a particular wavelength range but can decrease the dynamic range. As such, keeping the wavelength of the optical sensing signal within the wavelength range can be advantageous. In an example, a closed-loop system can monitor a representative wavelength (e.g., the center wavelength of the skirt of the filtering FBG sensor  100 ) and can adjust the wavelength of an optical output laser to remain substantially close to the center of the skirt of the filter characteristic of the FBG sensor  100  as forces external to the optical fiber  105 , such as bending and stress, cause the center wavelength of the skirt of the filter characteristic of the FBG sensor  100  to shift. 
     In an example, such as illustrated in  FIG. 2 , the interferometric FBG sensor  100  can cause interference between that portion of the optical beam that is reflected off the first partially reflective FBG  110 A with that reflected from the second partially reflective FBG  110 B. The wavelength of light where an interferometric null will occur can be very sensitive to the “optical distance” between the two FBGs  110 A-B. The interferometric FBG sensor  100  of  FIG. 2  can provide another very practical advantage. In this example, the two optical paths along the fiber core  115  are the same, except for the sensing region between FBGs  110 A-B. This shared optical path ensures that any optical changes in the shared portion of optical fiber  105  will have substantially no effect upon the interferometric signal; only the change in the sensing region  125  between FBGs  110 A- 110 B is sensed. However, the present applicant has recognized, among other things, that this sensing can be affected by birefringence of the optical fiber within FBG sensor  100 . 
     Optical birefringence is a measure of the difference in refractive index of an optical medium for light of different polarizations. The polarization of light can be defined as the orientation of the electric vector of the electromagnetic light wave. Birefringence between two at least partially reflective FBGs, such FBGs  110 A-B, can cause different beams of light with different polarizations to effectively travel slightly different optical path lengths between the FBGs  110 A-B. Therefore, a combination of birefringence between the at least partially reflective FBGs  110 A-B and a shift in the polarization of the optical signal within the optical fiber core  115  can cause a shift in the exact wavelength of the null of the FBG sensor  100 . When light is split between different polarization states, the light will be reflected or transmitted from different parts of the skirt of the null, which can lead to fading of the optically transduced signal. There are many possible states of polarization, such as linear, circular, and elliptical, but the worst signal fading generally occurs when the optical signal is split equally between linear polarization states that are orthogonally aligned. With this in mind, the present applicant has recognized, among other things, that birefringence should be reduced or otherwise addressed, if possible. 
     The present applicant has also recognized that there can be two main sources of optical birefringence within the FBG sensor  100 . The first source is the intrinsic birefringence of the optical fiber  105 . The intrinsic birefringence is determined mostly during the manufacturing of the optical fiber  105  and is generally a function of the level of geometric symmetry, the uniformity of dopant distribution, and the level of stress in the fiber core  115  during the drawing of the fiber core  115 . 
     The second source of optical birefringence in the FBG sensor  100  is the process of writing the FBGs  110 A and  110 B. The present applicant has also recognized that the birefringence induced by the writing of FBGs  110 A and  110 B can be reduced such as by controlling one or more aspects of the writing process, such as the polarization of the writing laser, the laser pulse energy, the writing exposure time, the amount of hydrogen or deuterium in the fiber during writing, or any combination thereof. 
     2. Examples of FBG Acoustic-to-Optical Transducers 
     In an example, an FBG sensor  100  senses pressure or strain such as generated by ultrasound or other acoustic energy received from a nearby imaging region to be visualized and, in response, modulates an optical sensing signal in an optical fiber. Increasing the sensitivity of the FBG sensor  100  can provide improved imaging. A first example of increasing sensitivity is to increase the amount of strain induced in the FBG sensor  100  for a given dynamic pressure provided by the acoustic energy. A second example is to increase the modulation of the optical signal for a given change in strain of the FBG sensor  100 . Any combination of the techniques of these first and second examples can also be used. 
     One technique of increasing the strain induced in the FBG sensor  100  is to configure the physical attributes of the FBG sensor  100  such as to increase the degree of strain for a given externally-applied acoustic field. In an example, the FBG sensor  100  can be shaped so as to increase the strain for a given applied acoustic pressure field.  FIG. 3  is a cross-sectional schematic diagram illustrating such an example in which the FBG sensor  100  is shaped such that it mechanically resonates at or near the frequency of the acoustic energy received from the nearby imaging region, thereby resulting in increased strain. In the example of  FIG. 3 , all or a portion of the strain sensing region between FBGs  110 A-B is selected to provide a thickness  300  that promotes such mechanical resonance of the received acoustic energy, thereby increasing the resulting strain sensed by FBG sensor  100 . In an example, such as illustrated in  FIG. 3 , this can be accomplished by grinding or otherwise removing a portion of fiber cladding  120 , such that the remaining thickness of fiber core  115  or fiber cladding  120  between opposing planar (or other) surfaces is selected to mechanically resonate at or near the frequency of the acoustic energy received from the nearby imaging region. 
     In an example, mechanical resonance can be obtained by making the thickness  300  of the strain sensing region substantially the same thickness as ½ the acoustic wavelength (or an odd integer multiple thereof) in the material(s) of FBG sensor  100  at the acoustic center frequency of the desired acoustic frequency band received from the imaging region. In other examples, such as for other materials, the thickness  300  can be selected to match a different proportion of the acoustic wavelength that obtains the desired mechanical resonance for that material. Calculations indicate that obtaining such mechanical resonance can increase the strain sensitivity by a factor of 2 or more over that of a sensor that is not constructed to obtain such mechanical resonance. 
     In a third example, a coating  305  can be applied to the FBG sensor  100  such as to increase the acoustic pressure as seen by the FBG sensor  100  over a band of acoustic frequencies, such as for improving its sensitivity over that band. The difference between the mechanical characteristics of water (or tissue and/or blood, which is mostly comprised of water) and glass material of the optical fiber  105  carrying the FBG sensor  100  is typically so significant that only a small amount of acoustic energy “enters” the FBG sensor  100  and thereby causes strain; the remaining energy is reflected back into the biological or other material being imaged. For a particular range of acoustic frequencies, one or more coatings  305  of specific thickness  310  or mechanical properties (e.g., the particular mechanical impedance) of the coating material can be used to dramatically reduce such attenuation due to the different mechanical characteristics. An example can use quarter wave matching, providing a coating  305  of a thickness  310  that is approximately equal to one quarter of the acoustic signal wavelength received from the region being imaged. Using such matching, the sensitivity of the FBG sensor  100 , over a given band of acoustic frequencies of interest, is expected to increase by about a factor of 2. 
     However, for a mechanically resonant transducer, reducing the thickness of fiber cladding  120 , and therefore reducing the total size of the transducer, is limited by the wavelength of the acoustic energy to be received. In a fourth example, the thickness  300  of the strain sensing region is substantially less than half of the wavelength of the target acoustic energy.  FIG. 4  is a cross-sectional schematic diagram illustrating such an example. In this example, the thickness  300  of the strain sensing region can be less than 60 micrometers. As the acoustic width of the transducer is reduced, the cross-sectional area of the transducer is reduced, and the total strain due to acoustic pressure on the transducer is reduced. Therefore, a smaller thickness  300  of the strain sensing region results in a smaller received acoustic signal, leading to reduced receive sensitivity of the FBG sensor  100 . This reduced receive sensitivity of the FBG sensor  100  can be improved by placing a backing  440  between the strain sensing region of the optical fiber  105  and a core wire  450  that can be configured to provide support, rigidity, and flexibility for the assembly. The backing  440  can include a high acoustic impedance material such as a metal or ceramic. In an example, the backing  440  can be separated from the optical fiber such as by an acoustically thin (e.g., less than a quarter wavelength thick) tubular sheath  460 . The coating  305  can also be configured, in an example, for acoustic impedance matching. 
     In an example, the backing  440  can be configured to reflect acoustic energy back to the strain sensing region, such as in a resonant manner, to increase the total amount of acoustic energy received by the strain sensing region and thereby increase the strain on the region. In this example, however, the receive sensitivity of FBG sensor  100  may depend on the polarization of the optical sensing signal because the backing  440  does not enhance the acoustic signal in all directions. The receive sensitivity generally is highest when the optical sensing signal is polarized parallel to an axis extending through the backing  440  and the fiber core  115  or an axis extending through the core wire  450  and the fiber core  115 . The receive sensitivity is generally lowest when the angle of the polarization of the optical sensing signal is parallel to the outline of the interface between the backing  440  and the sheath  460 , because the backing does not enhance the signal for this polarization orientation. The variable sensitivity in this example results at least partially from acoustic reflections from the backing  440  and from the core wire  450 . The present applicant has recognized, among other things, that sensitivity to polarization of the optical sensing signal should be reduced or eliminated, if possible, in an example. 
     In an example, the backing  440  can be configured to absorb the acoustic energy that penetrates completely through the optical fiber to reach the backing  440 . In this example, the backing  440  can be configured both to not reflect any acoustic energy back to FBG sensor  100  and also to inhibit or prevent the core wire  450  from reflecting any acoustic energy back to FBG sensor  100 . The sheath  460  in this example can be configured to not reflect any substantial amount of acoustic energy. In an illustrative example, the sheath  460  can comprise a layer of UV-curable polyester such as with a thickness of about 10 micrometers or less. The backing  440  in this illustrative example can comprise microballoons filled with gas or microballoons filled with gas mixed into a polymer matrix. In an example of a device configured for imaging within a coronary vasculature, the room for the backing  440  is very limited, and therefore it can be advantageous if the backing  440  is relatively highly acoustically absorbent with a relatively small thickness. In an example in which acoustic energy that penetrates completely through the optical fiber to reach the backing  440  is substantially absorbed by a relatively small thickness of the backing  440 , microballoons can comprise 30% to 50% of the volume of the backing  440 . With such a polymer-microballoon mixture, the backing  440  can have a thickness of as little as 50 micrometers and can be configured to be capable of attenuating acoustic energy with a frequency of 20 megahertz by 17 decibels to 23 decibels per millimeter of thickness of the backing  440 . In an example, the sensitivity of FBG sensor  100  is relatively independent of the polarization of the output optical beam within FBG sensor  100 . In an example, a peak-to-peak amplitude of the oscillating output optical sensing signal varies by no more than one decibel as the polarization of the output optical sensing signal is rotated 90 degrees. In an example, the backing  440  attenuates ultrasound energy by at least 90%. 
     The above examples can be combined with each other or can include more or fewer elements than are recited in the examples and can still function as described in the examples. For example, an ultrasound-to-optical transducer that is relatively insensitive to the polarization of the optical sensing signal can include an optical fiber, a backing configured to absorb ultrasound energy that goes through the optical fiber to reach the backing, and an FBG interferometer configured to modulate an optical sensing signal in response to the ultrasound energy. The optical fiber can include a thickness of the optical fiber that is less than half the wavelength of the ultrasound energy that is sensed by the transducer. 
     3. Examples of Guidewire Design 
       FIG. 5  is a schematic diagram that illustrates generally an example of a side view of a distal portion  500  of an imaging guidewire  505  or other elongated catheter (in an example, the guidewire  505  is sized and shaped and is of flexibility and rigidity such that it is capable of being used for introducing and/or guiding a catheter or other medical instrument, e.g., over the guidewire  505  within a living body). In this example, the distal portion  500  of the imaging guidewire  505  includes one or more imaging windows  510 A,  510 B, . . . ,  510 N located slightly or considerably proximal to a distal tip  515  of the guidewire  505 . Each imaging window  510  includes one or more acoustic-to-optical FBG sensors  100 . In an example, the different imaging windows  510 A,  510 B, . . . ,  510 N are designed for different optical wavelengths, such that particular individual windows can be addressed by changing the optical wavelength being communicated through fiber core  115 . 
       FIG. 6  is a schematic diagram that illustrates generally an example of a cross-sectional side view of a distal portion  600  of a guidewire  605 . In this example, the guidewire  605  can include a solid metal or other core wire  450  that can taper down in diameter (e.g., from an outer diameter of about 0.011 inches) at a suitable distance  615  (e.g., about 50 cm) from the distal tip  620 , to which the tapered-down distal end of core wire  450  can be attached. In this example, optical fibers  625  can be distributed around the outer circumference of the guidewire core  450 , and can be attached to the distal tip  620 . In this example, the optical fibers  625  can be at least partially embedded in a binder material (e.g., UV curable acrylate polymer) that bonds the optical fibers  625  to the guidewire core wire  450  or the distal tip  620 . The binder material may also contribute to the torsion response of the resulting guidewire assembly  605 . In an example, the optical fibers  625  and binder material can be overcoated with a polymer or other coating  630 , such as for providing abrasion resistance, optical fiber protection, or friction control, or a combination thereof. At least one metallic or other bulkhead  640  can be provided along the tapered portion of the core wire  450 . In an example, the optical fibers  625  and binder  635  can be attached to a proximal side of the bulkhead  640  such as near its circumferential perimeter. A distal side of the bulkhead  640  can be attached, such as near its circumferential perimeter, to a coil winding  610  that can extend further, in the distal direction, to a ball or other distal tip  620  of the guidewire  605 . In this example, the composite structure of the distal region  600  of the guidewire  605  can provide, among other things, flexibility and rotational stiffness, such as for allowing the guidewire  605  to be maneuvered to an imaging region of interest such as within a vascular or any other lumen. 
       FIG. 7  is a schematic diagram that illustrates generally an example of a cross-sectional end view of a proximal portion  700  of guidewire  605 , which can include core wire  450 , optical fibers  625 , binder material  635 , and outer coating  630 . In this example, but not by way of limitation, the diameter of the core  450  can be about 11/1000 inch, the diameter of the optical fibers  625  can be about (1.25)/1000 inch, and the optional outer coating  1030  can be about (0.25)/1000 inch thick. 
       FIG. 8  is a schematic diagram that illustrates generally an example of a cross-sectional end view of the distal portion  600  of the guidewire  605 , e.g., adjacent to the distal tip  620 . In this example, but not by way of limitation, the diameter of core wire  450  has tapered down to about 1.5/1000 inch, circumferentially surrounded by a void  800  of about the same outer diameter (e.g., about 11/1000 inch) as the core wire  450  near the proximal end  700  of the guidewire  605 . In this example, the optical fibers  625  can be circumferentially disposed in the binder material  635  around the void  800 . Binder material  635  can provide structural support. Optical fibers  625  can be optionally overlaid with the outer coating  630 . 
     4. Examples of Acoustic Transducer Construction 
     In an example, before one or more acoustic transducers are fabricated, the guidewire  605  can be assembled.  FIG. 9  is a cross-sectional end view illustrating an example of a structure of such a guidewire assembly such as at a location of a transducing window  510 . An example of such assembling can include placing the tubular sheath  460  on the core wire  450  such as at the locations selected for transducing, inserting the backing  440  into a gap between the core wire  450  and the tubular sheath  460 , and binding the optical fibers  625  to the sheath  460  and the distal tip  620  or bulkhead  640 . The coating  305 , which, in an example, can include the outer coating  630 , can then optionally be layered over the optical fibers  625 . In another example of such assembling, the backing  440  can be formed to the surface of the core wire  450  such as at the locations selected for transducing, and the tubular sheath  460  can be formed to the exposed surface of the backing, such as by heat-shrinking, for example. In an example, the optical fibers  625  can each have a diameter of less than half of the wavelength of the acoustic energy that the acoustic transducers are designed to sense, although the diameter of each of the optical fibers  625  can be larger than this in other examples. 
     After the guidewire  605  has been assembled, the FBGs can be added to one or more portions of the optical fibers  625 , such as within the transducer windows  510 . In an example, an FBG can be created using an optical process in which a portion of the optical fiber  625  is exposed to a carefully controlled pattern of ultraviolet (UV) radiation that defines the Bragg grating. Then, a photoacoustic material or other desired overlayer can be deposited or otherwise added in the transducer windows  510  such as over the Bragg grating. Thus, in this example, the FBGs can be advantageously constructed after the optical fibers  625  have been mechanically assembled into the guidewire assembly  605 . 
     An FBG writing laser can be used to expose the desired portion of the optical fiber  625  to a carefully controlled pattern of UV radiation to define the Bragg grating. The FBG writing laser can be operated so as to reduce the amount of birefringence caused by the FBGs. This will reduce the dependence of the FBG sensor  100  on the polarization of the optical sensing signal that is modulated by the received acoustic energy. In an illustrative example, at least one of hydrogen or deuterium can first be optionally infused into the optical fiber core  115 . Conditions for diffusion of the gas into the fiber can be 150 atm pressure at room temperature for 1 to 10 days. Then, the optical fiber  625  can be exposed to the writing laser, such as for a time period of between about 30 seconds and about 10 minutes. In this example, the writing laser can have a pulse energy of between about 0.1 millijoules (mJ) and about 10 millijoules and can have a polarization angle that is substantially parallel to the longitudinal axis of the optical fiber core  115 . 
     In an illustrative example, such as in which hydrogen is not infused into the optical fiber core  115 , the optical fiber  625  can be exposed to the writing laser such as for a time period of between about 30 seconds and about 10 minutes. In this example, the writing laser can have a pulse energy of between about 0.1 millijoules and about 10 millijoules and a polarization angle that is substantially perpendicular to the longitudinal axis of the optical fiber core  115 . In either this example or the above example in which hydrogen or deuterium is first optionally infused into the optical fiber core  115 , desired portions of all layers or coverings over the fiber cladding  120  can optionally be removed before the optical fiber  625  is exposed to the writing laser. 
     In an example, the writing conditions can be controlled so that the FBG sensor  100  is relatively insensitive to the polarization of the output optical signal. This can include, for example, reducing the speed of writing by lowering the intensity of the UV lamp in order to reduce heating biases. In an example, the shift in the center wavelength of the skirt of the FBG sensor  100  as the polarization of the output optical sensing signal is rotated 90 degrees is less that half of the Full Width Half Maximum of the skirt of the FBG sensor  100 . In another example, a total birefringence induced on an optical signal with a wavelength of about 1550 nanometers transmitted within the optical fiber  625  is less than (3×10 −6 ). 
     Additional Notes 
     In this document, the term “minimally-invasive” refers to techniques that are less invasive than conventional surgery; the term “minimally-invasive” is not intended to be restricted to the least-invasive technique possible. 
     Although certain of the above examples have been described with respect to intravascular imaging (e.g., for viewing or identifying vulnerable plaque), the present systems, devices, and methods are also applicable to imaging any other body part. For example, the guidewire or other elongated body as discussed above can be inserted into a biopsy needle, laparoscopic device, or any other lumen or cavity such as for performing imaging. Moreover, such imaging need not involve insertion of an elongate body into a lumen, for example, an imaging apparatus can be wrapped around a portion of a region to be imaged. 
     In an example, the present systems, devices, and methods can be used to process the Doppler shift in acoustic frequency to image or measure blood flow. The operation can be similar to that described above, however, this would increase the length of the transmitted acoustic signal, and can use Doppler signal processing in the image processing portion of the control electronics. The transmitted acoustic signal can be lengthened, in an example, such as by repeatedly pulsing the transmit optical energy at the same rate as the desired acoustic frequency. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown and described. However, the present inventors also contemplate examples in which only those elements shown and described are provided. 
     All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, the code may be tangibly stored on one or more volatile or non-volatile computer-readable media during execution or at other times. These computer-readable media may include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.