U.S. patent application number 17/825038 was filed with the patent office on 2022-09-08 for tip reflection reduction for shape-sensing optical fiber.
The applicant listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Raymond CHAN, Robert MANZKE, Gert Wim 'T HOOFT, Martinus Bernardus VAN DER MARK.
Application Number | 20220283373 17/825038 |
Document ID | / |
Family ID | 1000006359322 |
Filed Date | 2022-09-08 |
United States Patent
Application |
20220283373 |
Kind Code |
A1 |
VAN DER MARK; Martinus Bernardus ;
et al. |
September 8, 2022 |
TIP REFLECTION REDUCTION FOR SHAPE-SENSING OPTICAL FIBER
Abstract
A method for end-reflection reduction of an optical
shape-sensing fiber is described. The includes: providing a tip
portion having a length dimension (d); connecting the tip portion
to an end portion of an optical fiber configured for optical
shape-sensing, the tip portion being indexed matched to the optical
fiber; and adjusting the absorption properties of the tip portion
using back reflections as feedback to provide an absorption length
for light traveling in the optical fiber to reduce the back
reflections.
Inventors: |
VAN DER MARK; Martinus
Bernardus; (BEST, NL) ; CHAN; Raymond; (SAN
DIEGO, CA) ; MANZKE; Robert; (BONEBUTTEL, DE)
; 'T HOOFT; Gert Wim; (EINDHOVEN, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
EIDHOVEN |
|
NL |
|
|
Family ID: |
1000006359322 |
Appl. No.: |
17/825038 |
Filed: |
May 26, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13981933 |
Nov 15, 2013 |
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PCT/IB2012/050328 |
Jan 24, 2012 |
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17825038 |
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61470058 |
Mar 31, 2011 |
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61437039 |
Jan 28, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/243 20130101;
A61B 1/0017 20130101; A61B 34/20 20160201; A61B 2562/0266 20130101;
A61B 2034/2061 20160201; G02B 6/24 20130101; A61B 5/065 20130101;
G02B 6/262 20130101 |
International
Class: |
G02B 6/24 20060101
G02B006/24; A61B 5/06 20060101 A61B005/06; G02B 6/26 20060101
G02B006/26; A61B 1/00 20060101 A61B001/00; A61B 34/20 20060101
A61B034/20 |
Claims
1. A method for reducing end-reflection of an optical shape-sensing
fiber, the method comprising: providing a tip portion having a
length dimension; connecting the tip portion to an end portion of
an optical fiber configured for optical shape-sensing, the tip
portion being indexed matched to the optical fiber; and adjusting
the absorption properties of the tip portion using back reflections
as feedback to provide an absorption length for light traveling in
the optical fiber to reduce the back reflections.
2. The method as recited in claim 1, the tip portion comprising an
absorption length having at least one material on interior and
exterior portions of the absorption length that is configured to
absorb and scatter light within the length dimension, wherein the
absorption length for light traveling in the optical fiber is less
than the length dimension.
3. The method as recited in claim 1, wherein the tip portion
comprises a length dimension that is less than about 5 mm.
4. The method as recited in claim 1, wherein the tip portion
comprises a media having light absorbing dopants or particulates
and the step of adjusting the absorption properties comprises
adjusting a concentration of dopants or particulates.
5. The method as recited in claim 4, wherein the light absorbing
dopants or particulates comprise one or more of graphite,
nanotubes, buckyballs, a metal complex or particles, cations,
anions, a modified metal material fabricated by femtosecond etching
lasers for surface modification of a metallic substrate and a
dye.
6. The method as recited in claim 4, wherein the light absorbing
dopants or particulates comprise a metal having a relative
permittivity of about 2 at a plasmon frequency.
7. The method as recited in claim 4, wherein the light absorbing
dopants or particulates have a minimum reflection extinction
coefficient (k.sub.min) for a given length dimension of the tip
portion.
8. The method as recited in claim 4, wherein the tip portion
comprises a material with an index of refraction matched to that of
the optical fiber and a minimum reflection extinction coefficient
(k.sub.min) for a given length dimension of the tip portion.
9. The method as recited in claim 1, further comprising providing
the tip portion with a distal end surface comprising a light
dispersive shape or finish.
10. A method for reducing end-reflection of an optical
shape-sensing fiber, the method comprising: providing a shape
sensing enabled medical device having at least one optical fiber;
providing a console configured to receive optical signals from the
at least one optical fiber and interpret the optical signals to
determine a shape of the medical device; and providing a tip
portion coupled to a distal end portion of the at least one optical
fiber, the tip portion comprising: an absorption length having at
least one material on interior and exterior portions of the
absorption length that is configured to absorb and scatter light
within the length dimension; and a surface opposite the end portion
having a shape comprising ridges or a slant that is configured to
reduce back reflections.
11. The method of claim 10, wherein the tip portion comprises a
media having light absorbing dopants or particulates.
12. The system as recited in claim 10, wherein the light absorbing
dopants or particulates comprise a metal having a relative
permittivity of about 2 at a plasmon frequency.
13. The method as recited in claim 10, wherein the light absorbing
dopants or particulates have a minimum reflection extinction
coefficient (kmin) for a given length dimension.
14. The method as recited in claim 10, wherein the tip portion
comprises a material with an index of refraction matched to that of
the optical fiber and a minimum reflection extinction coefficient
(kmin) for a given length dimension.
Description
CROSS-REFERENCE TO PRIOR APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 13/981,933 filed Nov. 15, 2013, which is the U.S. National
Phase application under 35 U.S.C. .sctn. 371 of International
Application No. PCT/IB2012/050328, filed Jan. 24, 2012, which
claims the benefit of U.S. Provisional Patent Application No.
61/470,058 filed Mar. 31, 2011 and U.S. Provisional Patent
Application No. 61/437,039 filed Jan. 28, 2011. These applications
are hereby incorporated by reference herein.
TECHNICAL FIELD
[0002] This disclosure relates to medical instruments and more
particularly to shape sensing optical fibers in medical
applications that include reduced tip reflections.
BACKGROUND
[0003] The ability to accurately sense the tip and shape of a
medical instrument or device plays a key role in interventional
guidance. A technology based on optical shape sensing for
localization is known. However, the manner of attachment of the
fiber to an instrument plays a role in how tracking and/or shape
sensing is performed. For example, if an optical fiber is attached
along the length of a catheter and terminates abruptly at the tip
of the distal end, backscattering due to Fresnel reflections at the
tip can interfere with the desired optical signal.
[0004] Coupling the tip of the fiber with index matching gel to
dissipate light out into the surrounding tissue medium can help to
reduce Fresnel reflection effects at the fiber tip. However, while
this approach is workable in a laboratory, it can complicate
manufacturing of medical grade products.
[0005] Shape sensing based on fiber optics may be based on fiber
optic Bragg grating sensors. A fiber optic Bragg grating (FBG) is a
short segment of optical fiber that reflects particular wavelengths
of light and transmits all others. This is achieved by adding a
periodic variation of the refractive index in the fiber core, which
generates a wavelength-specific dielectric mirror. A fiber Bragg
grating can therefore be used as an inline optical filter to block
certain wavelengths, or as a wavelength-specific reflector.
[0006] A fundamental principle behind the operation of a fiber
Bragg grating is Fresnel reflection at each of the interfaces where
the refractive index is changing. For some wavelengths, the
reflected light of the various periods is in phase so that
constructive interference exists for reflection and, consequently,
destructive interference for transmission. The Bragg wavelength is
sensitive to strain as well as to temperature. This means that
Bragg gratings can be used as sensing elements in fiber optical
sensors. In an FBG sensor, the measurand (e.g., strain) causes a
shift in the Bragg wavelength.
[0007] One advantage of this technique is that various sensor
elements can be distributed over the length of a fiber.
Incorporating three or more cores with various sensors (gauges)
along the length of a fiber that is embedded in a structure permits
a three dimensional form of such a structure to be precisely
determined, typically with better than 1 mm accuracy. Along the
length of the fiber, at various positions, a multitude of FBG
sensors can be located (e.g., 3 or more fiber sensing cores). From
the strain measurement of each FBG, the curvature of the structure
can be inferred at that position. From the multitude of measured
positions, the total three-dimensional form is determined.
[0008] As an alternative to fiber-optic Bragg gratings, the
inherent backscatter in conventional optical fiber can be
exploited. One such approach is to use Rayleigh scatter in standard
single-mode communications fiber. Rayleigh scatter occurs as a
result of random fluctuations of the index of refraction in the
fiber core. These random fluctuations can be modeled as a Bragg
grating with a random variation of amplitude and phase along the
grating length. By using this effect in three or more cores running
within a single length of multi-core fiber, the 3D shape and
dynamics of the surface of interest can be followed.
[0009] When an optical fiber is attached along the length of a
catheter, it terminates abruptly at the tip of the distal end. In
this configuration, backscattering due to reflections at the tip
interface can interfere with a desired optical grating or
backscattering signal. In practice, this results in a signal on the
detector with extra noise and compromises the dynamic range of the
detector, amplifier and sampling accuracy of the system. In
particular, for a certain level of reflection the phase noise of
the laser may become limiting.
[0010] Coupling the tip of the fiber with index matching gel to
dissipate light out into the surrounding tissue medium can reduce
back-reflection effects. As mentioned, this approach can complicate
manufacturing of medical grade products. Furthermore, it is only a
partial solution since the optimal refractive index of the coupling
gel depends on the tissue that the instrument operates within
(e.g., air versus blood).
[0011] Fiber loops at the tip of the catheter have been employed so
that the distal fiber end is far away from the catheter tip. The
fiber loops can even be in the laboratory instrument outside the
patient. Then, the end portion can be dealt with appropriately,
since the surrounding environment is known and there is space to
put the distal end in any size, shape or form of matching medium.
The looping solution has two practical challenges. First, the loop
itself will have to be very tight, but nonetheless should keep the
same single-mode character of the whole fiber waveguide. Second,
effectively, the catheter has to be made with twice the amount of
fiber for a return path.
SUMMARY
[0012] In accordance with the present principles, a tip
configuration for a medical instrument is provided which provides
light absorption and index matching. The tip configuration is
compact and may be biocompatible.
[0013] A method for end-reflection reduction of an optical
shape-sensing fiber includes providing a tip portion having a
length dimension (d); connecting the tip portion to an end portion
of an optical fiber configured for optical shape-sensing, the tip
portion being indexed matched to the optical fiber; and adjusting
the absorption properties of the tip portion using back reflections
as feedback to provide an absorption length for light traveling in
the optical fiber to reduce the back reflections.
[0014] A method for end-reflection reduction of an optical
shape-sensing fiber includes providing a shape sensing enabled
medical device having at least one optical fiber; providing a
console configured to receive optical signals from the at least one
optical fiber and interpret the optical signals to determine a
shape of the medical device; and providing a tip portion coupled to
a distal end portion of the at least one optical fiber, the tip
portion comprising: an absorption length having at least one
material on interior and exterior portions of the absorption length
that is configured to absorb and scatter light within the length
dimension; and a surface (S) opposite the end portion having a
shape comprising ridges or a slant that is configured to reduce
back reflections.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] These and other objects, features and advantages of the
present disclosure will become apparent from the following detailed
description of illustrative embodiments thereof, which is to be
read in connection with the accompanying drawings.
[0016] This disclosure will present in detail the following
description of preferred embodiments with reference to the
following figures wherein:
[0017] FIG. 1 is a tip configuration for providing a beneficial end
condition to an optical fiber in accordance with the present
principles;
[0018] FIG. 2 is a plot of total reflection versus extinction
coefficient (k) for a single mode fiber (e.g., SMF28e) at a 1550 nm
wavelength and an index of refraction n.sub.0 of 1.468 for
determining a minimum reflection extinction coefficient (k.sub.min)
in accordance with the present principles;
[0019] FIG. 3 is a block/flow diagram showing a shape sensing
system which employs a tip configuration in accordance with one
embodiment;
[0020] FIG. 4 is another illustrative tip configuration for an
optical fiber in accordance with the present principles; and
[0021] FIG. 5 is a flow diagram showing a method for end-reflection
reduction of an optical shape-sensing fiber in accordance with an
illustrative embodiment.
DETAILED DESCRIPTION
[0022] In accordance with the present principles, a tip
configuration for a medical instrument or other fiber optic system
is provided which provides light absorption and index matching. The
tip configuration is compact and is preferably formed in a manner
that permits biocompatible use in vivo (e.g., glass doped with a
strong absorber, colloidal polymer nanospheres or mineral
impurities which absorb light, in combination with termination tip
coating which is protective and biocompatible, a polymer sheath
such as PEBAX, PTFE, silicone, etc.). The ability to accurately
sense the tip and shape of a medical instrument or device plays a
role in interventional guidance to make use of fiber optic shape
sensing and localization. The tip of an instrument in accordance
with the present principles incorporates fiber optic shape sensing
and localization, has light absorbing properties and index-matches
the optical fiber.
[0023] In one embodiment, a tip configuration which has both light
absorbing and index matching properties provides low light
scattering and is small enough to avoid compromising shape sensing
even toward the end of the medical instrument (e.g., a guide wire
or catheter) that incorporates fiber optic shape sensing. Material
deposited at the tip has absorption that is sufficiently large at
an interrogation wavelength, and both its refractive index and its
absorption are set such that an index mismatch is sufficiently low,
in particular at the temperature of use, e.g., 37.degree. C. for
medical practice. An interface between the fiber and absorber tip
may be slanted to minimize reflection in the case where imperfect
refractive index matching occurs. An optical shape (S) and finish
of the distal end of the tip portion determine the direction of
back reflected light.
[0024] It also should be understood that the present invention will
be described in terms of medical instruments; however, the
teachings of the present invention are much broader and are
applicable to any fiber optic instruments. In some embodiments, the
present principles are employed in tracking or analyzing complex
biological or mechanical systems. In particular, the present
principles are applicable to internal tracking procedures of
biological systems, procedures in all areas of the body such as the
lungs, gastro-intestinal tract, excretory organs, blood vessels,
etc. The elements depicted in the FIGS. may be implemented in
various combinations of hardware and software and provide functions
which may be combined in a single element or multiple elements.
[0025] The functions of the various elements shown in the FIGS. can
be provided through the use of dedicated hardware as well as
hardware capable of executing software in association with
appropriate software. When provided by a processor, the functions
can be provided by a single dedicated processor, by a single shared
processor, or by a plurality of individual processors, some of
which can be shared. Moreover, explicit use of the term "processor"
or "controller" should not be construed to refer exclusively to
hardware capable of executing software, and can implicitly include,
without limitation, digital signal processor ("DSP") hardware,
read-only memory ("ROM") for storing software, random access memory
("RAM"), non-volatile storage, etc.
[0026] Moreover, all statements herein reciting principles,
aspects, and embodiments of the invention, as well as specific
examples thereof, are intended to encompass both structural and
functional equivalents thereof. Additionally, it is intended that
such equivalents include both currently known equivalents as well
as equivalents developed in the future (i.e., any elements
developed that perform the same function, regardless of structure).
Thus, for example, it will be appreciated by those skilled in the
art that the block diagrams presented herein represent conceptual
views of illustrative system components and/or circuitry embodying
the principles of the invention. Similarly, it will be appreciated
that any flow charts, flow diagrams and the like represent various
processes which may be substantially represented in computer
readable storage media and so executed by a computer or processor,
whether or not such computer or processor is explicitly shown.
[0027] Furthermore, embodiments of the present invention can take
the form of a computer program product accessible from a
computer-usable or computer-readable storage medium providing
program code for use by or in connection with a computer or any
instruction execution system. For the purposes of this description,
a computer-usable or computer readable storage medium can be any
apparatus that may include, store, communicate, propagate, or
transport the program for use by or in connection with the
instruction execution system, apparatus, or device. The medium can
be an electronic, magnetic, optical, electromagnetic, infrared, or
semiconductor system (or apparatus or device) or a propagation
medium. Examples of a computer-readable medium include a
semiconductor or solid state memory, magnetic tape, a removable
computer diskette, a random access memory (RAM), a read-only memory
(ROM), a rigid magnetic disk and an optical disk. Current examples
of optical disks include compact disk-read only memory (CD-ROM),
compact disk-read/write (CD-R/W), Blu-Ray.TM. and DVD.
[0028] Referring now to the drawings in which like numerals
represent the same or similar elements and initially to FIG. 1, a
tip configuration 100 includes a tip portion 102 connected to an
optical fiber 104, which may be shape sensing. The optical fiber
104 may include a distal end portion of a medical device having
shape-sensing capabilities. The medical device may include, e.g., a
catheter, a guide wire, an endoscope, a probe, a robot, an
electrode, a filter device, a balloon device, or other medical
component, etc. The tip portion 102 is formed from a material that
is both light absorbing and index matching with optical fiber 104
and surrounding materials. In one embodiment, the tip portion 102
is low light scattering. Further, it is preferable that the tip
portion 102 be small so that it does not compromise shape sensing
or miniaturization of medical instrument design, and shape sensing
may be provided down to the end of the medical instrument that the
shape sensing optical fiber 104 is part of
[0029] The tip portion 102 includes a light absorption property
that is sufficiently large at an interrogation wavelength employed
for shape sensing. In this way, the light traveling down the
optical fiber 104 is not reflected back and/or is attenuated to
prevent negative effects in shape-sensing. In one embodiment, both
a refractive index and absorption of the tip portion 102 provide a
low index mismatch between the tip portion 102 and the optical
fiber 104, in particular, at a temperature of usage, e.g.,
37.degree. C. for medical practice. The tip portion 102 includes
light absorbing/scattering materials 108 (e.g., dopants and/or
particulates) to reduce back reflections.
[0030] The tip portion 102 may include a glue or other adhesive, a
composite containing an absorbent (e.g., dye, pigment, colloidal or
other particulate doping material), an antireflection coated volume
or semiconductor, a fiber segment spliced onto the end portion of
the fiber, a ferrule or other mechanical connection or other
material which can provide the desired properties. The tip portion
102 is preferably made of materials that permit use in biological
structures or is sufficiently encased in a biocompatible material.
Such materials may include biocompatible polymers, such as, e.g.,
PET, PTFE, PEBAX, silicone, MMA, or other similar materials.
[0031] For example, the light absorbing/scattering materials 108
may include a metal, where at a plasmon frequency, the material's
relative permittivity, .epsilon..sub.r, is approximately 2
(.epsilon..sub.r=n.sup.2-k.sup.2, .epsilon..sub.i=2nk), where n and
k are the real and imaginary parts of the index of refraction of
the material. The real part of the refractive index n indicates the
phase speed and the imaginary part k indicates the amount of
absorption loss when the electromagnetic wave propagates through
the material. k is called the extinction coefficient. The light
absorbing/scattering materials 108 in the tip portion 102 may
include carbon based materials, such as, graphite, nanotubes,
buckyballs, etc., a metal complex (e.g., Ruthenium based) or a
photonic crystalline nanostructure. The light absorbing/scattering
materials 108 may include metal particles, cations, anions, or
molecules that absorb light of the wavelength desired dispersed in
a matrix (e.g., of glass or other material).
[0032] In other embodiments, the light absorbing/scattering
materials 108 may be provided using a modified metal material. Such
materials may be fabricated by, e.g., femtosecond etching lasers
for surface modification of a metallic substrate (e.g., blacken
metals for absorption at particular wavelengths of light). These
materials may be dispersed in a media to absorb fiber light. In
another example, infrared dye 1110 (e.g., organic dye-metal complex
of the (substituted) aminium chemical family) may be employed.
[0033] The tip portion 102 may include material that is
biocompatible. The tip is preferably small, e.g., its length d is 5
mm or less, preferably 2 mm or less, and more preferably 1 mm or
less. A tip diameter is preferably sub-millimeter, preferably with
a same diameter as the optical shape sensing fiber tether. In one
embodiment, an interface 106 between optical fiber 104 and tip
portion 102 could be slanted, ridged or otherwise geometrically
formed to minimize reflection in case of imperfect refractive index
matching. The interface 106 may include a ferrule connection having
highly polished and shaped contact surfaces to reduce
back-reflections.
[0034] In one embodiment, a distal end surface of the tip portion
102 is configured to have a favorable shape S to reflect, defocus
or scatter any remaining light. This may include ridges, a slant,
or other light scattering features. The shape S can form a complex
maze-like structure that wraps upon itself to increase the
effective path length traversed by light in the cores while keeping
the overall footprint of the tip portion small. The optical shape S
and finish of the distal end portion of the tip portion 102
determine the direction of any back reflected light. The present
embodiments may simultaneously suppress the reflections from the
tip portion 102 of multi-core optical fibers, which may include,
e.g., 3 to 7 cores or single mode fibers (SMF).
[0035] The tip portion 102 controls a reflection limit. For Fiber
Bragg Grating (FBG) optical fibers, an integral reflection over 1
mm is approximately R.sub.lmm=10.sup.-4, where the integral
reflection for Rayleigh scattering over the same length is
approximately 10.sup.-6. A matching medium (tip portion 102) may
have a size on the order of 1 mm or less, so an absorption length
l.sub.abs should be less than that. This determines the minimum
absorption coefficient a and imaginary part of the refractive index
k at the interrogation wavelength .lamda..sub.vac:
l abs = .lamda. vac 4 .times. .pi. .times. k = 1 .alpha. ' = 1
.alpha. .times. ln .times. 10 ##EQU00001##
[0036] As an example, realistic values for pure water at 1550 nm
are n=1.311 and k=1.35.times.10.sup.-4, so l.sub.abs=0.914 mm. The
intensity loss due to a double pass through the absorbing tip
portion 102 of length d is:
[0037] I=I.sub.0e.sup.-2d/l.sup.abs, where I.sub.0 is the initial
intensity. The reflection at the interface 106 of between optical
fiber 104 and tip portion 102 due to index mismatch is:
R fa = ( n - n 0 ) 2 + k 2 ( n + n 0 ) 2 + k 2 , ##EQU00002##
where n and k are the real and imaginary part of the refractive
index of the tip fiber material.
[0038] For a single mode fiber (e.g., SMF28e), the refractive index
at 1550 nm is n.sub.0=1.468. The reflection at the absorber distal
end (R.sub.ae) depends on any further material, such as tissue,
water, or worst case it could be air (n=1), in which case, we
have:
R ae = ( n - 1 ) 2 + k 2 ( n + 1 ) 2 + k 2 . ##EQU00003##
[0039] The curvature or roughness of this interface 106 will also
determine any potential reflection of light back into the original
guided optical fiber mode. This will introduce an extra factor S
that accounts for surface shape of the distal end of tip portion
102. For example, diffraction at the end of optical fiber 104 will
give a diverging beam of width:
tan .times. .theta. ' = a ' z = .lamda. vac .pi. .times. na .
##EQU00004##
For a single-mode fused silica optical fiber, a numerical aperture
is NA=sin .theta.=n sin .theta.'=0.14 (e.g., for a Corning.TM.
SMF-28e). The Mode Field Diameter of this fiber is 2a=10.5 micron
at 1550 nm. For distance z from the fiber end equal to the absorber
length d=1 mm, we can calculate the diameter of the diffracted spot
to be 2a'=0.13 mm, that is 154 times the original area, and for a
round trip this would potentially reduce the back reflection by an
extra factor of 4 so that S= 1/600.
[0040] From the construction of optical fiber connectors, it is
known that their back reflection can be reduced by proper polishing
(reduced scattering), in particular at an angle of 8.degree.. This
angle happens to correspond to the NA=0.14, but is larger than the
internal aperture angle of 5.6.degree., and hence the reflected
light cannot find its way back into the guided mode.
[0041] The same optical fiber considered above (Corning.TM.
SMF-28e) has attenuation that is less than 0.4 dB/km, hence
.alpha.=0.04 km.sup.-1, l.sub.abs=10.86 km and
k=1.14.times.10.sup.-11, with: I=I.sub.010.sup.-.alpha.d.
[0042] Assuming that interference effects are insignificant, the
total reflection at the distal tip would be:
[0043] R.sub.tot=.left brkt-bot.(R.sub.fa+(1-R)).sub.fa.right
brkt-bot.R.sub.aeSe.sup.-2d/l.sup.abs where R.sub.fa is the
reflection at interface 106 and R.sub.ae is the reflection from the
distal end.
[0044] An optimum solution fulfills the condition n=n.sub.0, and if
we choose S=1 (e.g., the least favorable choice as it is obtained
by giving an end facet a radius of curvature equal to the length of
the absorber), we find:
R tot = ( n 0 - 1 ) 2 + k 2 ( n 0 + 1 ) 2 + k 2 + ( 2 .times. n 0 )
2 ( n 0 + 1 ) 2 + k 2 .times. k 2 ( 2 .times. n 0 ) 2 + k 2 .times.
e - 8 .times. .pi. .times. kd / .lamda. vac ##EQU00005##
[0045] From this equation, we can deduce a minimum reflection value
(k.sub.min) given a desired total minimum reflection value
R.sub.min for a given length of the absorber d. k.sub.min is an
optimum value for the extinction coefficient k, given a certain
length d of the tip, such that the reflection is minimal.
[0046] Referring to FIG. 2, a plot of total reflection versus k for
a silica fiber at a 1550 nm wavelength and an index of refraction
n.sub.0 of 1.468 is shown. For FIG. 2, we find that for a silica
fiber at a wavelength of 1550 nm that there is a lower bound to the
reflection of R.sub.min=1.12.times.10.sup.-7 for k=0.0009=k.sub.min
if we allow d=1 mm. In accordance with the present principles, one
embodiment may employ a highly absorptive dopant diffused into the
optical fiber material with k.sub.dopant>>k.sub.min and apply
the dopant in sufficient concentration in that material. Another
embodiment may include providing a material that naturally has
n=n.sub.0 and k=k.sub.min under a given set of conditions.
[0047] For example, a tip portion 102 may include graphite
nanoparticles in a hydrogel or other fluid solution. In another
example, a laser processed metal material may be employed for
perfectly absorbing light at specific wavelengths using femtosecond
etching lasers for surface modification of a metallic substrate or
employing flakes or particles or such processed metal in a matrix
material. In yet another embodiment, particular conditions may be
determined and a material selected that provides n=n.sub.0 and
k=k.sub.min for interrogation wavelengths.
[0048] Referring to FIG. 3, a system 200 for configuring a tip
portion 229 and employing an optical fiber 226 with the tip portion
229 is illustratively shown. System 200 may include a workstation
or console 212 from which a procedure is supervised and/or managed.
Workstation 212 preferably includes one or more processors 214 and
memory 216 for storing programs and applications. Memory 216 may
store an optical sensing module 215 configured to interpret optical
feedback signals from a shape sensing medical device 202 or shape
sensing system 204. Optical sensing module 215 is configured to use
the optical signal feedback (and any other feedback, e.g.,
electromagnetic (EM) tracking) to reconstruct deformations,
deflections and other changes associated with a shape sensing
medical device 202 and/or its surrounding region. The shape sensing
medical device 202 may include a catheter, a guidewire, a probe, an
endoscope, a robot, an electrode, a filter device, a balloon
device, or other medical component, etc.
[0049] The shape sensing system 204 on shape sensing medical device
202 includes one or more optical fibers 226 which are coupled to
the shape sensing medical device 202 in a set pattern or patterns.
The optical fibers 226 connect to the workstation 212 through
cabling 227. The cabling 227 may include fiber optics, electrical
connections, other instrumentation, etc., as needed. On a distal
end of the optical fibers 226, the tip portion 229 may be attached
or applied. The tip portion 229 may include a prefabricated
material attached to a distal end of the optical fiber 226. The
pre-fabricated material may include a material applied to the tip
portion 229, may include a tip portion attached or spliced to the
optical fiber 226, may include a tip portion dynamically altered
using optical feedback to obtain desired properties, etc.
[0050] In one embodiment, the prefabricated material of the tip
portion 229 may be attached to the distal end portion of the fiber
by a splice or other attachment mechanism. The interface between
the fiber and the tip portion 229 may be slanted or otherwise
geometrically angled to reduce back reflection. The prefabricated
material may include light absorbing dopants or dyes as described
above. In a particularly useful embodiment, the distal fiber end
may be attached to an inside wall of the shape sensing medical
device 202. In one embodiment, the wall of the shape sensing
medical device 202 may function as the tip portion 229.
[0051] In another embodiment, a material may be applied to the tip
portion 229. In this embodiment, a liquid, adhesive or gel may be
applied to the distal end of the optical fiber 226. The liquid or
gel may be monitored using the optical sensing module 215. The
liquid or gel may be altered by adding dopants or other materials
until desired characteristics are achieved. The material may be
permitted to solidify, if needed, to ensure the maintenance of the
desired characteristics.
[0052] Referring to FIG. 4, another embodiment includes a tip
portion 304 which forms an absorption cavity 308. The absorption
cavity 308 fits over or is formed over an end portion of an optical
fiber 302, which may be shape sensing. A configuration and material
of the absorption cavity 308 may be optimized or adjusted
dynamically based upon tip reflection measurements or attached
based upon use scenarios. The absorption cavity 308 may include
some of the following illustrative configurations. In one
embodiment, the absorption cavity 308 includes a sleeve that fits
over the end portion of the fiber and may include index matching
gel 312 in the absorption cavity 308. In another embodiment, the
absorption cavity 308 includes a liquid or gel and accepts dopants
or particulates. The concentration of the dopants or particulates
may be adjusted based upon reflective measurements. The liquid or
gel is then preferably solidified. In another embodiment, the
absorption cavity 308 may include a ferrule connection 310 with an
end portion of the fiber. The ferrule connection 310 selected
preferably provides the lowest amount of back-reflection (e.g.,
polished contact surfaces between the fiber and the tip portion
with no air gaps therebetween).
[0053] Fiber optic connectors can have several different ferrule
shapes or finishes, usually referred to as polishes. The polish on
a fiber connector ferrule determines the amount of back reflection.
Back reflection is a measure of the light reflected off the
polished end of a fiber connector usually measured in negative dB.
The ferrule connections may include an air gap, physically
contacted surfaces, an angled physical contact interface, index
matching gel, etc.
[0054] In another embodiment, the tip portion 304 may include
multiple absorbing fiber optic cores, one for each of the optical
cores within a shape sensing optical fiber 302. This approach may
be more costly and challenging to fabricate due to the need to
align the cores between the shape sensing body and tip portions.
However, an advantage is that crosstalk between optical cores can
be further minimized with such a design.
[0055] Referring again to FIG. 3, workstation 212 may include a
display 218 for viewing internal images of a subject (patient) if
an imaging system 210 is employed. Imaging system 210 may include a
magnetic resonance imaging (MRI) system, a fluoroscopy system, a
computed tomography (CT) system, etc. Display 218 may also permit a
user to interact with the workstation 212 and its components and
functions, or any other element within the system 200. This is
further facilitated by an interface 220 which may include a
keyboard, mouse, a joystick, a haptic device, or any other
peripheral or control to permit user feedback from and interaction
with the workstation 212.
[0056] System 200 may include an EM tracking or similar position or
orientation sensing system 217 which may be integrated with the
workstation 212 or be a separate system. Workstation 212 includes
an optical source 206 to provide optical fibers with light. An
optical interrogation unit or module 208 is employed to detect
light returning from all fibers. This permits the determination of
strains or other parameters, which will be used to interpret the
shape, orientation, or other characteristics, sensed by the shape
sensing medical device 202, which may be interventional. The light
signals will be employed as feedback to make adjustments to access
errors and to calibrate the shape sensing medical device 202 or
system 200.
[0057] Shape sensing medical device 202 may include one or more
optical fibers 226 with tip portions 229. The tip portions 229
prevent or reduce back reflections to enable shape-sensing down to
the distal end of the fiber. The fiber ends will now be
well-defined and can be exploited for shape-sensing. Optical
interrogation module 208 works with optical sensing module 215
(e.g., shape determination program) to permit a determination of a
location and orientation of the tip portion 229 as well as shape of
the instrument or shape sensing medical device 202. The present
principles enable real-time characterization of any elongated
instrument based on minimally invasive interventions, in which the
shape of the device is needed for improving the accuracy of
navigation or targeting.
[0058] Referring to FIG. 5, a method for end-reflection reduction
of an optical shape-sensing fiber is illustratively shown. In block
502, a tip portion including an overall length is provided. The
overall length is preferably less than about 5 mm.
[0059] In block 506, the tip portion is coupled to an end portion
of an optical fiber. The fiber may be configured for optical
shape-sensing or other applications. The tip portion is refractive
index matched to the optical fiber.
[0060] In block 514, in one embodiment, the absorption properties
of the tip portion are adjusted using back reflections as feedback
to provide an absorption length for light traveling in the optical
fiber to be less than the length dimension of the tip portion. The
tip portion may include a media including light absorbing dopants
or particulates. The adjustment of the absorption properties may
include adjusting a concentration of dopants or particulates, in
block 516.
[0061] The light absorbing dopants or particulates may include
graphite, nanotubes, buckyballs, a metal complex or particles,
cations, anions, a modified metal material fabricated by
femtosecond etching lasers for surface modification of a metallic
substrate and a dye. The light absorbing dopants or particulates
may include a metal having a relative permittivity of about 2 at a
plasmon frequency. The light absorbing dopants or particulates may
have a minimum reflection extinction coefficient (k.sub.min) for a
given length. The tip portion may include a material with an index
of refraction matched to that of the optical fiber and a minimum
reflection extinction coefficient (k.sub.min) for a given
length.
[0062] In block 518, a distal end shape and/or finish (S) is
provided or selected to further reduce back reflections. In block
520, an operative procedure is carried out employing the tip
portion to reduce back reflection to further distinguish an end of
the optical fiber and/or the end portion of the medical device.
[0063] In interpreting the appended claims, it should be understood
that: [0064] a) the word "comprising" does not exclude the presence
of other elements or acts than those listed in a given claim;
[0065] b) the word "a" or "an" preceding an element does not
exclude the presence of a plurality of such elements; [0066] c) any
reference signs in the claims do not limit their scope; [0067] d)
several "means" may be represented by the same item or hardware or
software implemented structure or function; and [0068] e) no
specific sequence of acts is intended to be required unless
specifically indicated.
[0069] Having described preferred embodiments for tip reflection
reduction for shape-sensing optical fiber (which are intended to be
illustrative and not limiting), it is noted that modifications and
variations can be made by persons skilled in the art in light of
the above teachings. It is therefore to be understood that changes
may be made in the particular embodiments of the disclosure
disclosed which are within the scope of the embodiments disclosed
herein as outlined by the appended claims. Having thus described
the details and particularity required by the patent laws, what is
claimed and desired protected by Letters Patent is set forth in the
appended claims.
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