U.S. patent application number 12/751999 was filed with the patent office on 2010-07-29 for apparatus and method for high resolution temperature measurement and for hyperthermia therapy.
Invention is credited to Harold Mukamal, Indu Saxena.
Application Number | 20100189157 12/751999 |
Document ID | / |
Family ID | 38225538 |
Filed Date | 2010-07-29 |
United States Patent
Application |
20100189157 |
Kind Code |
A1 |
Saxena; Indu ; et
al. |
July 29, 2010 |
APPARATUS AND METHOD FOR HIGH RESOLUTION TEMPERATURE MEASUREMENT
AND FOR HYPERTHERMIA THERAPY
Abstract
An apparatus and method for increasing the resolution of a
linear array of fiber Bragg gratings by applying a plastic coating
having a high CTE over the optical fiber. Apparatus and method for
determining the temperature of each of a succession of points along
a tissue portion during hyperthermia treatment includes an optical
fiber with a succession of closely spaced fiber Bragg gratings.
Each grating is responsive to a different wavelength and is
sensitive to ambient temperature to change that wavelength as a
function of temperature. A tunable laser operative continuously
over a range of wavelengths including those to which the gratings
respond is used to interrogate the gratings. Sensitivity-enhancing
coatings are used on the fibers and the lasers are tuned over very
short time cycles.
Inventors: |
Saxena; Indu; (Torrance,
CA) ; Mukamal; Harold; (Laguna Woods, CA) |
Correspondence
Address: |
LAWRENCE S. COHEN, ESQ.;LAW OFFICE OF LAWRENCE S. COHEN
10960 WILSHIRE BLVD, SUITE 1220
LOS ANGELES
CA
90024
US
|
Family ID: |
38225538 |
Appl. No.: |
12/751999 |
Filed: |
March 31, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11325907 |
Dec 30, 2005 |
7717618 |
|
|
12751999 |
|
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|
Current U.S.
Class: |
374/137 ;
374/E11.016; 374/E3.001 |
Current CPC
Class: |
A61N 5/025 20130101;
A61B 2017/00057 20130101; A61N 1/403 20130101; A61B 2017/00084
20130101; A61B 2017/00274 20130101; A61B 18/18 20130101; A61B
2018/00547 20130101 |
Class at
Publication: |
374/137 ;
374/E03.001; 374/E11.016 |
International
Class: |
G01K 11/32 20060101
G01K011/32; G01K 3/00 20060101 G01K003/00 |
Claims
1. A temperature profiling sensor for providing temperature
indications over a distance comprising; an optical fiber having a
plurality of linearly spaced apart FBGs formed therealong over a
selected length each of said FBGs having a different initial
periodicity wherein each FBG is responsive to incident light to
produce a reflected wavelength spectrum said wavelength spectra
being representative of the ambient temperature proximate to each
FBG; a non-conductive coating in sufficiently strong direct or
indirect contact with the optical fiber at least over the selected
length such that response to the temperature proximate to each FBG
by the coating is substantially transmitted as strain to the
selected length of the optical fiber and consequently to the FBGs,
said coating having a CTE greater than that of the optical fiber;
whereby expansion or contraction of the coating due to the
temperature exterior to the coating, local to each FBG, causes
mechanical strain to be transmitted to the FBGs and consequent
change in the reflected wavelength of each FBG as a function of the
temperature proximate thereto said mechanical strain being greater
than the thermal strain that would result alone from the optical
fiber being exposed to the same temperature thereby increasing the
sensitivity of the optical fiber to temperature changes.
2. The temperature profiling sensor of claim 1 wherein the coating
is selected to have a CTE of at least about
2.times.10.sup.-6/.degree. C.
3. The temperature profiling sensor of claim 1 wherein the coating
is selected to have a CTE sufficient to strain the FBGs to cause a
reflected wavelength shift per unit temperature (Kt) of at least
about 20 picometers/.degree. C.
4. The temperature profiling sensor of claim 1 further wherein the
FBGs have a separation distance no greater than about 10
micrometers.
5. The temperature profiling sensor of claim 1 wherein the
unstrained peak wavelength separation of wavelength adjacent FBGs
is greater than (Kt) (Tr)+X in which; Kt is a desired change in
reflected wavelength per unit temperature, Tr is an expected
maximum temperature range, and X is an expected wavelength change
induced by non-thermal effects.
6. The temperature profiling sensor of claim 5 in which Kt is at
least about 20 picometers/.degree. C.
7. The temperature profiling sensor of claim 6 in which Tr is a
range from about 20.degree. C. to about 55.degree. C.
8. The temperature profiling sensor of claim 1 wherein the FBGs
have a collective reflection spectrum from about 1220 nm to about
1680 nm.
9. The temperature profiling sensor of claim 1 wherein the FBGs
have individual lengths of about 5 mm or less.
10. The temperature profiling sensor of claim 1 further comprising
a light source connected to the optical fiber to provide scanning
incident light to the FBGs, the light source having a wavelength
range sufficient to obtain a reflection from each FBG.
11. The temperature profiling sensor of claim 3 wherein the FBGs
are constructed such that the reflected wavelength shift of at
least 20 picometers/.degree. C. occurs during exposure of the
temperature sensor to a temperature range from about 30.degree. C.
to about 55.degree. C.
12. The temperature profiling sensor of claim 1 wherein the coating
has a glass transition temperature effectively lower or higher than
the operating range of the sensor such that the properties of the
Young's modulus of the coating remain predictable and unchanged in
that temperature range
13. The temperature profiling sensor of claim 2 wherein the CTE of
the coating is at least about 4 times that of the fiber.
14. The temperature profiling sensor of claim 8 wherein the FBGs
are exposed to incident light having a range of about 1500 nm to
1600 nm from a light source having a scanning rate of at least 20
nm/second.
15. The temperature profiling sensor of claim 8 wherein the FBGs
have a collective reflection spectrum from about 1540 nm to about
1580 nm.
16. The temperature profiling sensor of claim 10 wherein the FBGs
have a collective reflection spectrum from about 1220 nm to about
1680 nm.
17. The temperature profiling sensor of claim 16 wherein the FBGs
have collective reflection spectrum from about 1540 nm to about
1580 nm.
18. The temperature profiling sensor of claim 16 wherein the
scanning light source has a wavelength range from about 1500 nm to
about 1600 nm with a scanning rate of at least about 20
nm/second.
19. The temperature profiling sensor of claim 1 wherein the coating
is a material of at least substantially polymer content.
20. The temperature profiling sensor of claim 19 wherein the
polymer is selected from the group consisting of; (a) polystyrene;
(b) polymethylmethacrylate; (c) polyethylene; (d)
polytetrachloride; (e) Polytetraflouroethylene; and (f)
combinations of (a), (b), (c), (d) and (e).
21. The temperature profiling sensor of claim 1 wherein the
separation of the specified characteristic peak wavelength of each
FGB is at least 0.5 nm.
22. The temperature profiling sensor of claim 21 further wherein
the reflection spectrum of any FBG has no sideband greater than 10%
of the peak intensity of the reflection spectrum.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of application Ser. No.
11/325,907 filed on Dec. 30, 2005 the content of which is
incorporated by reference herein.
FIELD OF THE INVENTION
[0002] This invention relates to temperature sensing using fiber
Bragg gratings in optical fibers.
[0003] This invention relates to fiber Bragg gratings in an optical
fiber for measuring temperature in a series of closely spaced
positions and more particularly to the closely spaced measurement
of temperature of tissue exposed to hyperthermia therapy.
[0004] The invention also relates to on-line temperature profiling
during hyperthermia therapy.
BACKGROUND
[0005] Sensing the value of environmental parameters such as
temperature and strain using an optical fiber with a plurality of
fiber Bragg gratings is known. For example in U.S. Pat. No.
4,996,419 there is disclosed for sensing strain a multitude of
separate longitudinally spaced Bragg sensing gratings of
substantially equal initial periodicity for all of the sensing
gratings, each of the sensing gratings being situated at a
different one of a multitude of separate locations.
[0006] During certain procedures, such as hyperthermia therapy a
surgeon needs to know the temperature profile in the tissue at and
near the area under treatment. A number of techniques and
technologies are in use or being investigated for temperature
sensing in conjunction with hyperthermia therapy. In one such
technique the temperature at different locations is determined by
inserting a cannula in the area under treatment, and by inserting a
single point temperature sensor into the cannula. The point
temperature sensor is moved within the cannula to different
locations for reading temperature. Such a procedure is laborious,
and of uncertain precision because of the need to relocate the
point sensor for each measurement at each of closely spaced
locations. Moreover, the temperature profile determined in this
manner is not totally reliable and is not obtained simultaneously
for the succession of locations. Another technique is the use of
several point sensors separately connected and placed.
[0007] The need for temperature profiling occurs, for example, in
cancer treatment when a tumor is to be heated during hyperthermia
treatment. In this case, the tumor is to be heated, for example, in
the range 41-45.degree. C. (up to 113.degree. F.), while the
surrounding tissue is to be maintained at lower temperatures to
avoid damaging healthy tissue.
[0008] Although hyperthermia therapy is under use and investigation
for cancer, it is understood to be useful for other treatments. One
such treatment is for BPH (benign prostatic hyperplasia).
[0009] As far as is known at present, different hyperthermia
therapies employ different temperature ranges. For example, in
investigational work with whole body hyperthermia a range of
40-42.degree. C. is employed. In hyperthermia therapy to sensitize
cancer cells to the effects of other therapies such as radiation
therapy, chemotherapy and biological therapies, localized heating
to temperature in the range of 41-45.degree. C. have been used.
Other techniques are used to achieve much higher temperature in
order to ablate the tissue being treated. These techniques have
been investigated in the brain, liver and prostate and require very
precise placement of the energy in the tissue that needs to be
ablated. In all types of hyperthermia therapy, temperature
monitoring is critical.
[0010] Body tissue temperatures as high as 45.degree. C.
(113.degree. F.) have been used in hyperthermia therapy. The
effectiveness of hyperthermia therapy is related to the temperature
achieved and other variables. In this regard it is important that
the desired temperature is reached, but not exceeded. To accomplish
this the temperature of the tumor or the area targeted for
treatment and surrounding tissue most be closely monitored.
Therefore accurate in vivo temperature monitoring is necessary, not
only at the point or area under treatment but also at adjacent
tissue. Also, for several of the heating methods, such as by
microwave radiation, sensor immunity to electromagnetic fields is
required. Consequently, it would be advantageous to provide a
temperature profiling system and method that can measure
temperature simultaneously (or nearly so) at a number of closely
spaced locations, and that can do so repeatedly over short time
intervals. Also, small changes in temperature should be made
available over time intervals to measure change in temperature
[0011] In certain hyperthermia applications the sensor must be able
to measure discrete points over a short distance and the measuring
points being as very close together. For example, the total
distance may be 5 cm, with 10 measuring points. Typical
hyperthermia treatment is applied to tissue areas spanning a length
of about 1 cm-5 cm. Consequently in order to measure temperature at
a plurality of point along such a distance very short fiber Bragg
gratings must be employed. Also, in some hyperthermia applications
it is desirable that each measuring point have a temperature
resolution of at least 0.1.degree. C. It is also desirable that the
several measuring points provide sufficient spatial resolution that
the temperature at one point does not overly influence the
temperature reading at adjacent points, even if the measuring
points are very close together. The invention in its various
aspects and according to its principles address these
requirements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic drawing, not to scale, of an optical
fiber having a linear array of FBGs and a plastic coating.
[0013] FIG. 2 is an end view of FIG. 1.
[0014] FIG. 3 is a schematic drawing, not to scale, of an optical
fiber having a linear array of FBGs and a plastic coating and an
area affected by use of an adhesion promoter between them.
[0015] FIG. 4 is an end view of FIG. 3.
[0016] FIG. 5 is a schematic view of a system for use in the
present invention.
[0017] FIG. 6 is a spectral plot of reflection spectra from FBGs
according to the present invention.
[0018] FIG. 7 is a temperature response plot 10 FBGs of a PMMA
coated 10 sensor array.
[0019] FIG. 8 is a comparative plot of temperature response of a
PMMA coated FBG and an identical uncoated FBG.
[0020] FIG. 9 is a comparative plot of temperature response of a
polyethylene coated FBG and an identical uncoated FBG.
[0021] FIG. 10 is a comparative plot of temperature response of a
Teflon coated FBG and an identical uncoated FBG.
[0022] FIG. 11 is a picture of a system in use for hyperthermia
therapy using the present invention.
[0023] FIG. 12 is a plot showing a reflection curve and a fitted
curve using the present invention.
[0024] FIG. 13 is a plot showing the variation of sensor resolution
with sampling rate.
DETAILED DESCRIPTION
[0025] In accordance with an aspect of this invention, a plurality
of longitudinally spaced fiber Bragg gratings (FBGs) is formed on
an optical fiber core. For purposes of this description the term
"optical fiber" is taken to mean an optical fiber core and
cladding, that is, the glass portion; unless the context requires a
different meaning. It is also understood that FBGs are generally in
the core portion. According to the invention, each of the plurality
of FBGs is constructed to respond to a different reflection
spectrum. Further, each FBG is operative to have a reflection
spectrum that varies in a manner responsive to the temperature that
is proximate to the FBG. Therefore, for a given state in which the
temperature at each FBG is to be determined, each FBG will reflect
at a different wavelength. Further, when the temperature changes
proximate each FBG, the reflection spectrum shifts in response to
the temperature changes imposed on the FBGs. Detection of the
wavelength shift can be converted to temperature change based on a
calibration. Since each FBG has a different reflection spectrum,
the instantaneous reflection and the shift due to a temperature
change, for each FBG can be discriminated from the others. In the
prior art an FBG will give a reflection shift on the order of 0.1
nm/.degree. C. due to change in temperature.
[0026] In the technology of FBGs, an FBG is defined as a periodic
or aperiodic perturbation of the effective absorption coefficient
and/or the effective refractive index of an optical waveguide. More
simply put, a Bragg Grating can reflect a predetermined narrow or
broad range of wavelengths of light incident on the grating, while
passing all other wavelengths of the light. An FBG of the type
under consideration here has an initial periodicity when it is
created (called "writing"). The reflected wavelength at that
initial periodicity at the time of fabrication of the FBG is
referred to herein as the "specified Bragg wavelength" or the
"specified characteristic peak wavelength". For purposes of this
description those terms are used as in the technology but limited
to the initial conditions at fabrication of the FBG. Those terms
are considered inapplicable to detected spectral plots that are
resolved sufficiently for the purposes of the invention because the
spectral plot is too indistinct to define a peak. As will be seen
below, in application of the present invention to obtain high
temperature resolution, it has been found that the spectral plot at
any given time does not exhibit a sufficiently distinct peak to
allow for providing as output the needed resolution. Consequently
such a reflection cannot be said to have a characteristic peak
until additional processing is carried out. That additional
processing will be described below.
[0027] A goal of the present invention is to provide a much greater
reflection shift per degree change in temperature, Kt, and as will
be seen, this can be accomplished with the present invention on the
order of 50 pm/.degree. C. By this means much smaller changes in
temperature can be measured.
[0028] It is understood under specified conditions the wavelength
separation of FBGs is determined by the formula Kt.times.Tr+X where
Kt is the desired change in reflected wavelength per unit
temperature, Tr is the expected or desired maximum temperature
range, and X is any expected wavelength change induced by
non-thermal effects. Consequently as the desired Kt goes up for a
given temperature, the wavelength separation must also increase.
And conversely, as Kt goes down, the wavelength separation can
decrease. The temperature range for these purposes is defined as
starting at the temperature of fabrication of the FBGs to the
maximum temperature to be measured.
[0029] This is accomplished by coating the optical fiber with a
non-conductive, preferably plastic, material having a coefficient
of thermal expansion (CTE) much greater than that of the optical
fiber.
[0030] It is a further goal of the present invention that by
forming the FBGs sufficiently close together and sufficiently small
enough to respond to the temperatures of tissue spaced over the
selected distance, a temperature profile can be produced without
the laborious repositioning necessitated by a point sensor
apparatus of the prior art, or with a plurality of separate point
sensors placed at selected places as for hyperthermia
applications.
[0031] In order to obtain accurate temperature readings from a
fiber, so constructed, a wavelength-tunable light source that tunes
over a wavelength range in a given time interval is used. A
sampling rate of the returned signal must then be applied
sufficient to provide the needed resolution data. In addition,
robust software algorithms are used to perform peak searches to
construct peak locations for the FBG reflection spectra at each
measurement cycle. The algorithms provide thresholding, binning,
curve smoothing and curve fitting as are all described below.
[0032] To this end, apparatus, in accordance with this invention,
includes a tunable laser, which changes the output wavelength as a
function of time over a selected wavelength range. The reflected
light from the FBG is incident on a photodetector, as the
wavelength is swept and thereby, by repeated cycles, makes a
wavelength change measurement as a function of time at the selected
sampling rate. The system is calibrated to a base
wavelength/temperature equivalent, thereby allowing each reflected
wavelength to be converted to a temperature measurement; and
repeated cycles to output change in temperature over time.
[0033] The present invention, in one aspect, provides a new and
improved system and method for measuring temperature at a plurality
of points by use of a plurality of FBGs linearly spaced on an
optical fiber.
[0034] In a particular aspect the system and method of the
invention provides improved temperature sensitivity, for example
responding to and allowing measurement of temperature changes at
least as small as 0.1.degree. C., and possibly as small as
0.02.degree. C. As will be explained in detail below this is
accomplished by surrounding the optical fiber with a coating that
has a CTE greater than that of the optical fiber.
[0035] In another particular aspect the system and method of the
invention provides much smaller spatial resolution than was
previously available, for example the space between adjacent FBGs
or the spatial resolution of the temperature measurement can be as
small as 1 mm. If the FBG can be constructed with a length of about
5 mm, then for a sensor having a total length of about 50 mm as
many as 10 sensor sites can be created.
[0036] In another aspect, the system and method of the invention is
for temperature measurements as part of apparatus for and in the
process of hyperthermia therapy.
[0037] In another aspect, it is recognized that the short FBGs
necessary for placing them over a short distance do not exhibit a
sufficiently precise peak wavelength for the accuracy needed for
very small temperature differences. Therefore a curve fitting and
smoothing procedure is applied.
[0038] For use in detecting and monitoring temperature changes as a
profile along a distance a plurality of FBGs can be spaced apart
along a selected length of optical fiber.
[0039] One aspect of the invention is a temperature sensor having a
plurality of fiber Bragg gratings spaced apart in the optical
fiber; in which increased sensitivity to temperature changes can be
achieved by coating the optical fiber with a material that has a
coefficient of thermal expansion higher than to that of the optical
fiber. This aspect is more particularly defined as using a
non-conductive coating so that it can be applied to usage where an
electromagnetic field is present whereby the non-conductive coating
does not respond to or cause a variation in the electromagnetic
field. Use of a non-conductive coating is particularly useful for
hyperthermia therapy in which microwave energy is used.
[0040] Another aspect of the invention is a temperature sensor as
described above in which the plurality of FBGs are interrogated in
timed cycles (scanning rate) in order to obtain changes in
temperature at each FBG location and thereby changes in the
temperature profile. In the context of hyperthermia therapy it is
necessary to detect very small changes in temperature at each
location of an FBG, such as at least 0.1.degree. C. It is also
important to obtain very accurate spatial resolution of temperature
measurement, that is, the number of measurement points, by making
the length of the FBGs as small as possible over the desired
distance for creating the temperature profile. The distances over
which temperature profiling for hyperthermia therapy is required
are very short ranging from about 2 cm to about 5 cm. This is
especially a problem where the number of measurement points
required may be as many as 10, over a 5 cm distance. In order to
place 10 FBGs in a distance of about 5 cm, each FBG must be very
short. However as FBGs are shortened, their reflection spectra at
any given set of parameters broadens from a sharp peak to a broad
"top hat" form along with considerable side lobes. Thus, when the
requirement is at once to obtain very precise spatial separation of
closely spaced measuring points coupled with the requirement for
very precise temperature sensitivity; a system is required that can
both obtain the required temperature sensitivity and spatial
discrimination and discriminate the data obtained from reflection
spectra that are imprecise.
[0041] In the preferred embodiments of the invention a resolution
of at least 0.1.degree. C. is desired. A plastic coating of the
optical fiber having a CTE of at least twice that of the optical
fiber enables this resolution. However, in the preferred
embodiment, an indistinct peak of the spectral plot of a reflected
FBG signal is caused by the short length of the FBG coupled with
the need to resolve the spectral plot to an accuracy of about
0.1.degree. C.
[0042] Where short FBGs such as about 5 mm or less are employed, in
order to detect and provide an output of the desired resolution it
is necessary to apply the desired sensitivity range or
specification, Kt, a sufficiently large sampling rate and a peak
detection algorithm.
[0043] The preferred coating CTE is at least about
2.times.10.sup.-6 and for hyperthermia applications, at least about
10.times.10.sup.-6.
[0044] Sampling rates of from about 5 kS/s to about 100 kS/s are
preferred (above this rate improvement of the resolution is
insignificant). This can be seen in FIG. 13.
Temperature Sensor
[0045] FIGS. 1 and 2 show a temperature sensor 10 having an optical
fiber 12 having a core 14 and a cladding 16 in which a series of 10
fiber Bragg gratings (FGBs) 18a, 18b, 18c, 18d, 18e, 18f, 18g, 18h,
18i, and 18j (shown schematically) are spaced apart in the fiber
core 14. The temperature sensor 10 has a non-conductive coating 22
in mechanical contact with the optical fiber 12.
[0046] The non-conductive coating 22 has a coefficient of thermal
expansion (CTE) substantially greater than that of the optical
fiber, in the range of at least about twice that of the optical
fiber. Preferably the CTE of the coating is at least about 10 times
that of the fiber. Further preferably, the CTE of the coating is in
the range of at least about 2.times.10.sup.-6/.degree. C. and up,
and for hyperthermia applications, preferably from at least about
10.times.10.sup.-6/.degree. C. For applications in an
electromagnetic field environment, the coating 22 is a
non-conductive material such as a plastic material formed around
the fiber. The coating will be applied over the optical fiber after
the FBGs have been formed. In the case of a polymeric
non-conductive plastic material, it will be applied and cured over
the fiber.
[0047] As will be seen, the temperature sensor 10 functions to
increase the temperature sensing sensitivity of the FBGs by means
of the coating expanding or contracting due to temperature change
and thereby straining the FBGs. The strain on the FBGs changes
their reflection spectra and detection of a peak wavelength of the
reflection spectra is used along with a correlation function to
determine the temperature or the change in temperature as related
to a base or series of prior measurement cycles.
[0048] Three factors influence the precision of temperature
measurement and the very small temperature resolution according to
the present invention. These are:
[0049] The thermo-optic effect by which the index of refraction of
the glass changes with temperature;
[0050] The change in length of the glass with change in
temperature, which acts upon the FBGs to shift the reflection
spectrum;
[0051] The strain effect on the FBGs caused by the much greater
expansion and contraction of the coating causing a shift in the
reflection spectrum.
[0052] The first two of these effects in a typical FBG allows on
the order of 0.1 nm change in reflected wavelength per 10.degree.
C. However in order to be able to detect temperature measurement on
the order of 0.1.degree. C., it is necessary to be able to read on
the order of 0.001 nm, that is 1 pm, change in the reflected
wavelength in the FBGs and the present invention provides this
level of resolution and detection of temperature change.
[0053] A number of factors make this possible. One of these factors
is the quality of the contact between the coating 22 and the
optical fiber 12. A more firm or stronger contact will result in
greater translation of the expansion or contraction of the coating
into strain of the optical fiber resulting in a greater shift of
the reflection spectra of the FBGs. In other words, for say
0.1.degree. C. change in temperature, the coating expands (or
contracts) more than would the optical fiber alone due to its
higher CTE, and to the extent that greater temperature response is
transmitted as more strain on the FBG, higher resolution is
possible.
[0054] FIGS. 3 and 4 show an alternative embodiment in which the
temperature sensor 10 has the FBGs 18a-18j as in the previous
example. In this case however an intermediate material 24 is in
place between the optical fiber 12 and the coating 22. The
intermediate material 24 is a material that will increase the
mechanical connection of the coating 22 and the optical fiber 12.
It could be an adhesive or an adhesion promoter or other material
that will bond to both the coating 18 and the fiber 12. Silane
adhesion promoters are especially applicable.
[0055] In operation the temperature sensor when exposed to
temperature variation will experience a degree of change in
thickness and length (expansion or contraction) of the coating that
will impose a radial and axial force (tension or compression) on
the fiber 12, seen as strain on the FBGs. The change in temperature
is also experienced in the fiber 12 itself resulting in a component
of the spectral shift of the FBGs; but the use of the high CTE
coating, by translating its higher thermal response to temperature
change to a mechanical force on the fiber, causes a much greater
effect on the FBGs. This much greater effect is defined herein as
thermal force strain amplification on the fiber 12. That increase
in strain per .degree. C. results in a greater wavelength shift per
.degree. C., consequently, greater sensitivity. The sensitivity to
temperature change in an optical fiber, in terms of wavelength
shift per .degree. C., Kt, without the benefit of the present
invention is typically in the range of 9-10 pm/.degree. C. With the
non-conductive plastic coating of the preferred embodiment of the
present invention, Kt is seen to increase to the level of 50-55
pm/.degree. C.
[0056] Consequently, the shift in the reflection spectra of the
FBGs is much greater than in the case in which the thermal effect
is only that of the optical fiber itself. Coatings that have been
used are polyethylene, polypropylene, polymethylmethacrylate,
polytetrachloride, polystyrene and polytetraflouroethylene (Teflon)
and polyamide.
[0057] With the described structure of the sensor, much greater
sensitivity to temperature change can be transmitted from the FBGs.
Temperature change as small as 0.1.degree. C. or even smaller; in
theory, as small as 0.02.degree. C. can be detected.
[0058] Non-conductive polymeric materials that have CTE levels 5 or
more times that of the optical fiber material are considered
suitable for the coating in hyperthermia therapy applications.
[0059] In most cases the coating's CTE falls off at temperature
over its glass transition temperature (T.sub.g). Consequently such
plastics should be used only in temperature ranges below the
T.sub.g. However some materials do not follow that condition, and
can be used even above their T.sub.g. The following materials have
sufficiently high CTEs in the order of tens to 100 times that of
fused silica that they are considered applicable for use in
constructing the sensor; polyvinylchloride (both below and above
its T.sub.g), polymethylmethacrylate (PMMA), polystyrene,
polytetrachloride, polyacrylonitrile, polyethylene, polypropylene,
polytetraflouroethylene and polyamide.
[0060] For medical hyperthermia therapy purposes heating to a
temperature range of 40-55.degree. C. is desired; except that for
ablation therapy higher temperatures are used. Use of a polymer
coating with a CTE in the above temperature range has been shown to
increase the temperature response of FBGs from the nominal value of
standard telecommunication grade fiber of 0.01 nm/.degree. C. to at
least 0.05 nm/.degree. C.
[0061] For purposes of the present invention it is preferable that
the FBG array have a collective reflection spectrum from about 1220
nm to about 1680 nm. In a case as illustrated in which the FBG
wavelength separation is as much as 3.5 nm, a collective reflection
spectrum of 35 nm is adequate, suggesting that a range of from 1540
nm to 1580 nm is adequate. In addition, the tunable laser scanning
range that is available is from 1500 nm to 1600 nm.
TESTS AND RESULTS
Test 1
[0062] A high sensitivity germanium doped single mode fiber that is
hydrogenated at a temperature of 80.degree. C. and pressure of 600
psi was used to write 10 FBGs to fabricate a 10 point temperature
sensor. The fiber parameters are:
[0063] Fiber (cladding) diameter=125 .mu.m, core diameter 9.8
.mu.m, numerical aperture 0.13, cutoff wavelength 1213 nm, index of
refraction (n) approximately 1.46. The fiber used was Fibercore
PS1500.
[0064] The FBGs were written into the fiber, each being 5 mm long
separated by 0.02 mm and corresponding to FBG wavelengths separated
by 3.5 nm, as follows:
TABLE-US-00001 Bragg Wavelength (nm) 1 1548 2 1551.5 3 1555 4
1558.5 5 1562 6 1565.5 7 1569 8 1572.5 9 1576 10 1579.5
[0065] This sensor array was dip coated in polymethylmethacrylate
(PMMA) (high molecular weight 18,226-5 from Sigma Aldrich)
dissolved in chloroform, to a thickness of about 0.9 mm, and cured
under high temperature in a vacuum oven.
[0066] In each case the sensor was placed inside a plastic tube of
3 mm inner diameter and immersed in a water bath. The bath
temperature was started at 65.degree. C. allowed to cool. The
temperature of the water was varied from 65.degree. C. to
25.degree. C. The temperature response was obtained at
intervals.
[0067] The reflection spectra of the 10-sensor array was measured
by a high speed tunable laser based detection system. The tunable
laser system for reading the reflection spectra is shown in FIG. 5
showing a sensor 10, a tunable laser 30, a wavelength selective
detector 32 and a specially programmed processor 34.
[0068] A spectral plot of the reflection spectra from one
measurement of this test is shown in FIG. 6. This spectral plot was
made with a scanning rate from the tunable laser light source 30 to
100 nm/s. The sampling rate of the detector 32 was 60 kS/s. This
spectral plot shows indistinct peaks when the interrogation is at a
high scanning rate over the wavelength range.
[0069] The 10 different values of d.lamda./dT (Kt) are for each 5
mm location of the FBG, corresponding to the slope of each FBG
response on the plot of FIG. 7. The baseline temperature
.lamda..sub.o is 0.degree. C. The Kt's are all approximately
(actually greater than) 50 pm/.degree. C. that is the response
required to obtain the 0.1.degree. C. temperature resolution.
[0070] The slope of each curve, which defines Kt is shown
below.
TABLE-US-00002 d.lamda./dT (nm/.degree. C.) .lamda..sub.o (nm)
0.05667 1545.5 0.07075 1548.0 0.06980 1550.9 0.06338 1554.2 0.05412
1557.9 0.05941 1560.4 0.05669 1563.8 0.05201 1566.8 0.04898 1569.9
0.4629 1573.3
[0071] In order to resolve the indistinct peaks, a set of peak
detection and calibration algorithms is employed. This is described
in detail below. After application of the algorithms, the spectral
plots will have a fitted curve from which a peak can be derived
(see FIG. 12).
[0072] A second sensor array was prepared in the same way but was
not coated.
[0073] A selected one of the FBGs in the coated and uncoated arrays
were tested for thermal response. The tunable laser was used as the
interrogating source. The reflected light was directed to the
detector and processed to determine the wavelength shift for each
specimen as the temperature was increased. The results are shown in
FIG. 8 in which line A shows the response from the specimen coated
with PMMA and line B shows the response for the uncoated specimen.
The response lines demonstrate that the PMMA coated specimen had a
temperature sensitivity on the order of 5 times that of the
uncoated specimen. That is, Kt for the uncoated sensor was 0.0104,
while for the coated sensor Kt was 0.0526
Test 2
[0074] In another test the coating was polyethylene. The two
specimens were prepared and the test conducted and the results
processed in the same way as for Test 1. The results are shown in
FIG. 9. The response lines demonstrate that the polyethylene coated
specimen had a temperature sensitivity Kt on the order of 1.57
times that of the uncoated specimen.
Test 3
[0075] In another test the coating was polytetraflouroethylene
(Teflon). The two specimens were prepared and the test conducted
and the results processed in the same way as for tests 1 and 2. The
results are shown in FIG. 10. The response lines demonstrate that
the Teflon coated specimen had temperature sensitivity on the order
of 2.8 times that of the uncoated specimen.
[0076] Detection System
[0077] FIG. 11 shows the system that embodies the system aspect of
the invention in the hyperthermia context in which a catheter has a
temperature sensor (optical fiber thermometer) comprising a series
of FBGs on an optical fiber. In considering this system description
in the hyperthermia context it is appreciated that the system can
be applied mutatis mutandis to other applications.
[0078] All surgical procedures incorporate some degree of risk, and
may cause physical, financial and even psychological distress to a
patient and the patient's family. This is particularly true in the
case of prostate problems because the results of surgery can
seriously affect a man's quality of life. Because of these risks,
less destructive and less invasive, but effective, alternatives
have been the subjects of extensive research and increased use.
Among these alternatives, microwave hyperthermia has been
clinically proven to be an effective, efficient and non-toxic
method for treating tumors. Unfortunately its use has been limited
by the lack of an effective tool to monitor and thereby control the
amount of heat distribution during microwave treatment of tumors.
This has been a problem both respecting the amount of heat and its
exact location and distribution in the human organ under treatment.
One problem in particular is the need to measure temperature and
temperature change over short distances and with very high
accuracy.
[0079] The present invention is a temperature sensor and an
associated system that can be used in-vivo in conjunction with
therapeutic heat application such as hyperthermia, including
microwave therapy. The invention is a temperature profiling sensor
that senses temperature and temperature change along a short
distance adjacent or within an animal organ; and that can do so
with a very high degree of sensitivity for temperature change.
[0080] The sensor can be implemented in one example to measure
multiple points along a 5 to 10 cm sensor length, with the ability
to pinpoint target areas within 0.5 cm. It can also be implemented
to monitor temperatures over a large range at each sensing point,
including the range desired for microwave hyperthermia of
35.degree. C. to 55.degree. C. with a 0.1.degree. C. temperature
resolution. These measurements can be accomplished with the
invention with no cross sensitivity to microwave self-heating. The
invention is implemented with a single mode or multi-mode optical
fiber with a specially defined coating as described above. It will
readily interface with clinical hyperthermia catheters.
[0081] In the particular application for hyperthermia the sensor
will provide an axial arranged array of FBGs, each being 0.5 cm
long that will provide high sensitivity (better than 0.5.degree.
C.), high accuracy (0.1.degree. C.) and accurate spatial resolution
(0.5 cm) in a single optical fiber. Distributed thermometry is
accomplished remotely, using an optoelectronic module that will
measure temperature induced wavelength shifts of the Bragg
wavelengths.
TABLE-US-00003 Thermal Response Change of 10 Sensor Array Upon
Coating with PMMA Bare Uncoated Sensor Coated Sensor Temperature
Response, Temperature Response, FBG Kt (pm/.degree. C.) Kt
(pm/.degree. C.) 1 11.0 41.4 2 10.3 41.8 3 9.4 40.2 4 9.3 39.8 5
9.2 40.4 6 9.7 38.9 7 9.6 39.2 8 10.6 42.3 9 11.9 38.3 10 8.8
25.8
[0082] As already noted, it is desirable to obtain temperature
changes at a level of at least 0.1.degree. C., and, as well,
temperature reading to an accuracy of at least 0.1.degree. C. in
FBGs that are used in an array over such a short distance that the
FBGs have to be quite short themselves. This is particularly the
case in hyperthermia treatment. However when attempting to resolve
the reflection spectra of such short FBGs to the precision that
will give such sensitivity, the peak of the reflection spectra is
too indistinct, as shown in FIG. 6.
[0083] The following described aspect of the invention is directed
at solving that problem. In this description, reference is made to
the system of FIG. 5.
[0084] A temperature sensor having sufficient resolution to detect
changes in temperature of 0.1.degree. C. or better is needed. One
such temperature sensor is that shown and described above.
[0085] The temperature sensor 10 is interrogated by a scanning
light source as exemplified by a tunable laser scanning light
source 30 having a scanning wavelength range sufficiently broad to
cause reflection from all the FBGs and operating at a selected or
otherwise set scanning rate.
[0086] Reflections from the FBGs are sent to the wavelength
selective detector 32 that operates at a sampling rate that is
selected or set, preferably by a specially programmed processor.
The detector converts the reflected light signals from the FBGs to
electrical changes varying in time that is converted to
wavelength.
[0087] The reflected lights signals are in a form as seen on FIG. 6
which is so finely resolved that there is no distinct peak. The
following described process is used to define peaks for the
reflected signals from each FBG. First it is necessary to have the
specified characteristic peak wavelength (as defined above) for
each FBG.
[0088] The processor is programmed with algorithms for detecting
changes and defining peaks in the FBG reflected spectra including
the steps of thresholding, binning, smoothing and least squares
fitting.
[0089] A major constraint is that the peaks must be spaced such
that the bins do not overlap. If the bins overlap, more than one
peak per channel could be detected, and incorrect shifts would be
observed and recorded. In addition if a peak moves out of its
predefined bin due to the applied temperature exceeding that
accounted for in the pre-defined bin, the reflected signal will no
longer be identified nor will its peak be detected to indicate the
temperature of the location at which it exists Hence the size of
the bin must be selected extremely carefully keeping all these
factors in mind.
[0090] One aspect that affects the binning size is accomplished by
selecting sufficient wavelength separation of the wavelength
adjacent FBGs, which in turn is a function of the expected
temperature range. In general if the desired Kt is lower, then the
minimum wavelength separation between FBGs can be lower or
conversely as the desired Kt is increased, the minimum wavelength
separation between FGBs must be higher.
[0091] For hyperthermia purposes having a temperature range of
about 38.degree. C. to about 45.degree. C., a Kt of 20 pm/.degree.
C., a wavelength separation of 0.5 nm is considered as being the
minimum. For procedures in which larger temperature ranges in
particular, higher temperatures are expected such as ablation
procedures, a wavelength separation of 3.5 nm is preferred assuming
a Kt of 50 pm/.degree. C. and a temperature range from room
temperature (25.degree. C.) to 100.degree. C.
The sampling rate ( data points / sec ) is defined by light source
scanning rate ( nm / sec ) desired temperature resolution ( deg C .
) .times. Kt ( nm / deg C . ) ##EQU00001##
[0092] In the system of FIG. 6 the sampling rate is applied to the
wavelength selective detector 32.
[0093] False (additional) peaks can also be detected due to
changing intensities of the side lobes of an FBG, where the side
lobe peaks may increase above the pre-defined intensity threshold
measurement during a measurement. This is remedied by implementing
a least squares polynomial fit to the raw data of the entire FBG's
spectral envelope for each reflection event. The peaks are fit to
at least a second order polynomial. A LABVIEW curve-fit program has
been used for this procedure.
[0094] FIG. 12 shows an exemplary fitted curve from an exemplary
spectral plot using the above procedure.
[0095] The foregoing Detailed Description of exemplary and
preferred embodiments is presented for purposes of illustration and
disclosure in accordance with the requirements of the law. It is
not intended to be exhaustive nor to limit the invention to the
precise form or forms described, but only to enable others skilled
in the art to understand how the invention may be suited for a
particular use or implementation. The possibility of modifications
and variations will be apparent to practitioners skilled in the
art. No limitation is intended by the description of exemplary
embodiments which may have included tolerances, feature dimensions,
specific operating conditions, engineering specifications, or the
like, and which may vary between implementations or with changes to
the state of the art, and no limitation should be implied
therefrom. This disclosure has been made with respect to the
current state of the art, but also contemplates advancements and
that adaptations in the future may take into consideration of those
advancements, namely in accordance with the then current state of
the art. It is intended that the scope of the invention be defined
by the Claims as written and equivalents as applicable. Reference
to a claim element in the singular is not intended to mean "one and
only one" unless explicitly so stated. Moreover, no element,
component, nor method or process step in this disclosure is
intended to be dedicated to the public regardless of whether the
element, component, or step is explicitly recited in the Claims. No
claim element herein is to be construed under the provisions of 35
U.S.C. Sec. 112, sixth paragraph, unless the element is expressly
recited using the phrase "means for . . . " and no method or
process step herein is to be construed under those provisions
unless the step, or steps, are expressly recited using the phrase
"comprising step(s) for . . . "
* * * * *