U.S. patent application number 13/742107 was filed with the patent office on 2013-05-23 for spectroscopic method and system for assessing tissue temperature.
This patent application is currently assigned to Covidien LP. The applicant listed for this patent is Covidien LP. Invention is credited to Clark R. Baker, JR..
Application Number | 20130131671 13/742107 |
Document ID | / |
Family ID | 43781119 |
Filed Date | 2013-05-23 |
United States Patent
Application |
20130131671 |
Kind Code |
A1 |
Baker, JR.; Clark R. |
May 23, 2013 |
SPECTROSCOPIC METHOD AND SYSTEM FOR ASSESSING TISSUE
TEMPERATURE
Abstract
According to various embodiments, a medical system and method
for determining tissue temperature may include a spectroscopic
sensor. The spectroscopic sensors may be configured to provide
information about changes in water absorption profiles at one or
more absorption peaks. Such sensors may be incorporated into
ablation systems for tissue ablation. Temperature information may
be used to determine the scope, volume, and/or depth of the
ablation.
Inventors: |
Baker, JR.; Clark R.;
(Newman, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Covidien LP; |
Mansfield |
MA |
US |
|
|
Assignee: |
Covidien LP
Mansfield
MA
|
Family ID: |
43781119 |
Appl. No.: |
13/742107 |
Filed: |
January 15, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12569721 |
Sep 29, 2009 |
8376955 |
|
|
13742107 |
|
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Current U.S.
Class: |
606/41 |
Current CPC
Class: |
A61B 2017/00084
20130101; A61B 18/1815 20130101; A61B 2017/00057 20130101; A61B
5/01 20130101 |
Class at
Publication: |
606/41 |
International
Class: |
A61B 18/12 20060101
A61B018/12 |
Claims
1. A method, comprising: using a processor: receiving a signal from
a spectroscopic sensor, wherein a tissue temperature of a patient
may be generally determined from the signal; determining the tissue
temperature based at least in part on the signal; and determining
whether the tissue temperature exceeds a viability threshold.
2. The method of claim 1, wherein the method is performed prior to
an ablation being performed on the tissue.
3. The method of claim 1, wherein the method is performed during an
ablation being performed on the tissue.
4. The method of claim 1, wherein determining the tissue
temperature comprises determining a change in a magnitude, shape,
or position of at least one water absorption peak.
5. The method of claim 4, wherein determining the tissue
temperature comprises correlating the change to a tissue
temperature associated with the change.
6. The method of claim 1, wherein the viability threshold is
greater than about 43.degree. C.
7. The method of claim 1, wherein determining the tissue
temperature comprises determining the temperature at more than one
depth in the tissue.
8. The method of claim 1, wherein the determining the tissue
temperature comprises determining a temperature gradient in the
tissue.
9. The method of claim 1, comprising providing a user-perceivable
indication of the tissue temperature when the tissue temperature
exceeds the viability threshold.
10. A method, comprising: receiving spectrophotometric data
associated with a mean photon penetration depth in a tissue of a
patient from a spectrophotometric sensor; and determining one or
more temperatures of the tissue associated with the mean photon
penetration depth based at least in part upon the
spectrophotometric data.
11. The method of claim 10, comprising displaying an indication
related to the one or more tissue temperatures.
12. The method of claim 10, wherein determining the one or more
tissue temperatures comprising determining a change in a magnitude,
position, or shape of a water absorption peak.
13. The method of claim 12, wherein determining the one or more
tissue temperatures comprises correlating the change in the
magnitude, position, or shape of the water absorption peak to a
tissue temperature associated with the change in the magnitude,
position, or shape of the water absorption peak, respectively.
14. The method of claim 10, comprising: activating an energy source
configured to direct ablating energy into the tissue of the
patient; and determining whether the tissue temperature is
associated with heat from the ablating energy.
15. The method of claim 14, comprising assessing a scope of tissue
ablation based at least in part upon the tissue temperature.
16. The method of claim 15, wherein assessing the scope of tissue
ablation comprises determining a viability of the tissue or
determining a volume of ablated tissue.
17. A method, comprising: measuring one or more water absorption
peaks in a tissue of a patient using a spectrophotometric sensor
prior to an ablation procedure being performed on the tissue;
measuring the one or more water absorption peaks in the tissue
using the spectrophotometric sensor during the ablation procedure;
and determining a temperature of the tissue based at least in part
upon the measurements acquired prior to and during the ablation
procedure.
18. The method of claim 17, comprising: measuring the one or more
water absorption peaks in the tissue using the spectrophotometric
sensor subsequent to the ablation procedure; and determining the
temperature of the tissue based at least in part upon the
measurement acquired subsequent to the ablation procedure.
19. The method of claim 17, wherein determining the temperature of
the tissue comprises determining a change in a magnitude, position,
or shape of the one or more water absorption peaks.
20. The method of claim 17, comprising: receiving information
related to the tissue ablation procedure; and assessing the tissue
ablation based at least in part upon the tissue temperature.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 12/569,721, entitled "Spectroscopic Method and
System for Assessing Tissue Temperature," filed Sep. 29, 2009, the
disclosure of which is hereby incorporated by reference in its
entirety for all purposes.
BACKGROUND
[0002] The present disclosure relates generally to medical devices
and, more particularly, to the use of spectroscopy to monitor
changes in the temperature of water-bearing tissue.
[0003] This section is intended to introduce the reader to aspects
of the art that may be related to various aspects of the present
disclosure, which are described and/or claimed below. This
discussion is believed to be helpful in providing the reader with
background information to facilitate a better understanding of the
various aspects of the present disclosure. Accordingly, it should
be understood that these statements are to be read in this light,
and not as admissions of prior art.
[0004] Some forms of patient treatment involve removing unwanted
portions of tissue from the patient, for example by surgical
resection. However, for tissue areas that may be difficult to
access surgically or for very small areas of tissue, tissue
ablation may be more appropriate. Tissue ablation uses energy
directed at the tissue site of interest to heat the tissue to
temperatures that destroy the viability of the individual
components of the tissue cells. During tissue ablation, an unwanted
portion of a tissue, e.g., fibrous tissue, lesions, or
obstructions, may be destroyed. Ablation can be achieved by various
techniques, including the application of radio frequency energy,
microwave energy, lasers, and ultrasound. Generally, ablation
procedures involve ablating tissue that is surrounded by otherwise
healthy tissue that a clinician wishes to preserve. Accordingly,
better therapeutic outcomes may be achieved through precise
application of the ablating energy to the tissue.
[0005] The precision of the ablation may depend in part on the type
of energy applied, the skill of the clinician, and the
accessibility of the tissue in question. For example, ablation may
be complex if the target area is moving. During catheter ablation
to correct an abnormal heartbeat, the cardiac tissue in question is
typically in motion, which may affect the volume of tissue ablated.
Because the ablation may take place internally, as in the case of
cardiac ablation, assessment of the volume of the tissue necrosis
may be difficult. In addition, depending on the type of ablating
energy used, controlling the area of the ablation may be easier
than controlling the depth of the ablation. Accordingly, the depth
of the necrosis may vary from patient to patient.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Advantages of the disclosure may become apparent upon
reading the following detailed description and upon reference to
the drawings in which:
[0007] FIG. 1 is a graph of the absorption spectra for water for
two different temperature points;
[0008] FIG. 2 is a graph of estimated mean photon penetration depth
plotted against wavelength for a sample of 70% lean water
concentration and an emitter-detector spacing of 2.5 mm;
[0009] FIG. 3 is a graph of a simulated absorption spectrum of
water for an example tissue sample at 37.degree. C. and an
emitter-detector spacing of 2.5 mm;
[0010] FIG. 4 is a graph of a simulated absorption spectrum of
water for an example tissue sample at 50-60.degree. C. and an
emitter-detector spacing of 2.5 mm;
[0011] FIG. 5 is a graph of a simulated absorption spectrum of
water for an example tissue sample at 60-80.degree. C. and an
emitter-detector spacing of 2.5 mm;
[0012] FIG. 6 is a block diagram of a system for monitoring tissue
temperature according to an embodiment;
[0013] FIG. 7 is a side view of an example of a spectroscopic
sensor for acquiring information from the tissue according to an
embodiment;
[0014] FIG. 8 is a top view of an example of a spectroscopic sensor
with multiple detectors spaced apart from an emitter for acquiring
information from the tissue according to an embodiment; and
[0015] FIG. 9 is a block diagram of a method of monitoring tissue
temperature during ablation.
DETAILED DESCRIPTION
[0016] One or more specific embodiments of the present disclosure
will be described below. In an effort to provide a concise
description of these embodiments, not all features of an actual
implementation are described in the specification. It should be
appreciated that in the development of any such actual
implementation, as in any engineering or design project, numerous
implementation-specific decisions must be made to achieve the
developers' specific goals, such as compliance with system-related
and business-related constraints, which may vary from one
implementation to another. Moreover, it should be appreciated that
such a development effort might be complex and time consuming, but
would nevertheless be a routine undertaking of design, fabrication,
and manufacture for those of ordinary skill having the benefit of
this disclosure.
[0017] Provided herein are systems, sensors, and methods for
spectroscopic monitoring of tissue temperature. When such systems
are used in conjunction with a tissue ablation device, a medical
monitor may assess changes in spectrophotometric parameters to
determine the viability of a probed area of the tissue. Tissue
areas with water absorption profiles characteristic of particular
temperatures may be determined. Such systems may also be used to
determine the viability of the probed tissue, i.e., probed tissues
associated with temperatures above a certain threshold may be
considered nonviable. As a result, the efficacy of the ablation may
be determined. In other embodiments, the spectroscopic sensors as
provided may be used in conjunction with other types of medical
procedures that involve changing or monitoring tissue temperature,
such as hypothermic or hyperthermic treatments.
[0018] Monitoring the necrosis of ablated tissue may be complex,
particularly when using techniques that involve ablation of
internal tissue. As provided herein, spectroscopic sensing may be
used to noninvasively monitor tissue temperature at a number of
tissue depths. The temperature information may then be used to
determine the scope of the tissue ablation. Generally, ablated
tissue cells will have characteristically higher temperatures as a
result of the heat of ablation. During ablation, the tissue is
heated until the resultant higher temperature of the tissue causes
protein denaturation and other effects that lead to necrosis of the
tissue. The temperature changes may be monitored by
spectroscopically assessing changes in the shape, position and/or
magnitude of one or more water absorption peaks of the tissue.
Because wavelengths may be chosen that penetrate known depths of
the patient's tissue, temperature information may be collected for
relatively fine gradations of tissue depth that are otherwise
difficult to obtain. Such noninvasive monitoring may provide
information about the depth and/or volume of the tissue ablation
and may allow clinicians to more precisely determine whether
further ablating treatment may be needed. In addition, clinicians
may be able to determine the borders of any ablated tissue in
relation to the healthy tissue and may be able to match the borders
with previously acquired data (e.g., cardiac images or tumors) to
determine if the scope of the ablated tissue corresponds with the
size, location and/or shape of, for example, known obstructions or
tumors.
[0019] Sensors as provided may be applied to a patient's skin
and/or internal organs (e.g., as part of a catheter or other
inserted assembly) to monitor multiple absorption peaks of water,
for example in the red or near infrared spectrum. While other
potential absorbers may make up some percentage of the content of a
patient's tissue, many of these absorbers, such as lipid and
hemoglobin, do not change their absorption profiles significantly
with temperature. For this reason, the absorption of water or other
constituents whose absorption, as measured spectroscopically,
changes with temperature may provide more information that relates
to the tissue temperature. Accordingly, by monitoring changes in
the water absorption profile that occur with rising temperatures
for a single area of tissue (e.g., pre and post-ablation), a change
in temperature for the tissue area may be estimated. To account for
the variation in light scattering, a change may be measured against
a pre-ablation spectrum. In addition, such information may be
combined with a measured patient baseline temperature, either local
or systemic, to determine the extent of tissue temperature changes.
The near-infrared peaks of water shift and narrow with increasing
temperature due to increases in hydrogen bonding between water
molecules. FIG. 1 is a graph 10 of successive absorption peaks 12,
14, 16, and 18 (corresponding to peaks centered near approximately
975, 1180, 1450, and 1900, respectively). Shown are the shape and
position of the absorption peaks for water at 37.degree. C., shown
by data 20, and at 60.degree. C., shown by data 22, plotted against
the depth of tissue penetration. As illustrated, the water at
60.degree. C. is shown to have a different characteristic
absorption profile.
[0020] FIG. 2 is a graph 30 of estimated photon penetration depth
into tissue plotted against wavelength for light emitted into
tissue and detected by a detector spaced approximately 2.5 mm from
the emitter. In the depicted simulation, the tissue sample is
assumed to have 70% lean water concentration, which is
approximately the lean water concentration of typical tissue. As
shown, over a portion of the spectrum, the penetration depth varies
with wavelength. Accordingly, a particular wavelength is associated
with a particular penetration depth for a particular
emitter-detector spacing. By using a sensor or a combination of
sensors with different emitter-detector spacings as well as
different wavelengths, multiple spectroscopic temperature estimates
corresponding to multiple tissue depths may be combined to estimate
a thermal gradient that is predictive of the total volume of tissue
that has been rendered nonviable by ablation.
[0021] Simulations of the types of shifts seen at different tissue
temperatures are depicted in FIGS. 3-5, with all simulations
assuming a 2.5 mm separation between emitter and detector, with
both facing the same direction and embedded in tissue at or near
the heating source. For each exemplary simulated tissue spectrum,
multi-linear regressions were performed against experimentally
determined tissue component spectra of water, protein, and lipid,
plus the derivative of water absorption with respect to
temperature. These regressions were performed using data over the
spectral ranges of 945-1035 nm, 1127-1170 nm, and 1360-1570 nm,
corresponding to regions of tissue spectrum where water is the
dominant absorber and where water absorption changes significantly
with temperature. Mean photon penetration depths were determined to
be, respectively, 0.94 mm, 0.86 mm, and 0.52 mm for these spectral
ranges, and did not differ significantly between these examples.
However, in certain embodiments, depending ion the nature and
temperature of ablation, a significant decrease in tissue water
content may occur after ablation. A decrease in water content may
allow deeper penetration for a particular wavelength. Accordingly,
the mean photon penetration may increase over the course of the
ablation. Such effects may be accounted for in determining the
temperature gradient for a particular tissue sample. Similar
temperature estimates may be obtained via other methods of
comparing tissue spectra to features of tissue component spectra.
For example, regressions may be performed between derivatives, or
other mathematical functions, of tissue and component spectra.
[0022] Table I below shows the resulting temperatures estimated
from the multi-linear regressions corresponding to each example and
spectral range. As shown, the temperatures increase over the course
of the ablation.
TABLE-US-00001 Spectral Range Temp FIG. 1 Temp FIG. 2 Temp FIG. 3
945-1035 nm 36.83.degree. C. 49.45.degree. C. 58.45.degree. C.
1127-1170 nm 37.55.degree. C. 47.33.degree. C. 60.43.degree. C.
1360-1570 nm 37.52.degree. C. 60.64.degree. C. 82.16.degree. C.
[0023] The example shown in FIG. 3 is a graph 40 of absorption at
multiple mean photon penetration depths at a plurality of near
infrared water absorption peaks. The mean photon penetration depths
may be estimated by using a graph similar to FIG. 2 of mean photon
distribution for a particular emitter-detector spacing. Prior to
heating, tissue is simulated at 37.degree. C., assuming a tissue
composition of 66% water, 24% protein, and 10% lipid. Because this
corresponds to a pre-ablation tissue temperature, the temperature
at different depths is generally about 37.degree. C., corresponding
to normal body temperature, before any heating through ablation
occurs.
[0024] After ablation begins, the temperature of the tissue starts
to rise and the water absorption profile begins to show
characteristic shifting. FIG. 4 is a graph 50 of a simulation of
tissue heating during the course of ablation. At a depth of 0.5 mm,
the temperature was simulated to be 60.degree. C., changing
linearly to 50.degree. C. at a distance of 1.0 mm. This corresponds
with the temperature being highest closer to the ablating source
and lower the farther away from the source. The tissue is assumed
to have dried out during heating to a composition of 58% water, 30%
protein, and 12% lipid. As shown, the center of the water
absorption peak in the 1350-1600 nm range has shifted toward the
shorter wavelengths as a result of temperature change.
[0025] After the ablation is completed, the temperatures have
reached their highest point in the tissue and the affected region
is rendered nonviable. FIG. 5 is a graph 60 of temperatures reached
shortly after the completion of an ablating course of energy. At a
depth of 0.5 mm, the temperature was simulated to be 80.degree. C.,
changing linearly to 60.degree. C. at a distance of 1.0 mm. The
tissue is assumed to have dried out further due to continued
heating, to a composition of 40% water, 42% protein, and 17% lipid.
As shown, the center of the water absorption peak in the 1350-1600
nm range has shifted toward the shorter wavelengths as a result of
temperature change. A shifting and narrowing of the water
absorption peak in the 1350-1600 nm range may be observed.
[0026] FIG. 6 shows a system 70 that may be used for monitoring
temperature in conjunction with an ablation procedure. The system
70 includes a spectroscopic sensor 72 with a light emitter 74 and
detector 76 that may be of any suitable type. The emitter 74 may be
a broad spectrum emitter or may be configured to emit light of a
limited wavelength range or at select discrete wavelengths. In one
embodiment, the emitter 72 may include a filter wheel for tuning a
broad spectrum to a series of particular wavelengths. The emitter
74 may be one or more light emitting diodes adapted to transmit one
or more wavelengths of light in the red to infrared range, and the
detector 76 may be a photodetector configured to receive the
emitted light. In specific embodiments, the emitter 74 may be a
laser diode or a vertical cavity surface emitting laser (VCSEL).
The laser diode may be a tunable laser, such that a single diode
may be tuned to various wavelengths corresponding to a number of
absorption peaks of water. Depending on the particular arrangement
of the sensor 72, the emitter 74 may be associated with an optical
fiber for transmitting the emitted light into the tissue. The light
may be any suitable wavelength corresponding to the wavelengths
absorbed by water. For example, wavelengths between about 800 nm,
corresponding with far-red visible light, and about 1800 nm, in the
near infrared range, may be absorbed by water.
[0027] By way of example, FIG. 6 shows an ablation device 78 that
may be associated with the system 70. However, it should be
understood that ablation device 78 is merely illustrative of a
medical device that may be used in conjunction with a spectroscopic
sensor 72 for monitoring temperature and other devices may be
incorporated into the system 70 if appropriate. In certain
embodiments, the ablation device may be a microwave ablation device
78. In one embodiment, the sensor 72 is structurally associated
(e.g., is disposed on) the ablation device, for example the emitter
74 and the detector 76 are disposed on a catheter, such as a
cardiac catheter, or other implantable portion of the device. In
embodiments in which the ablation takes place on the surface of the
skin, the emitter and detector may be part of a housing or other
support structure for the ablation energy source.
[0028] An associated monitor 82 may receive signals, for example
from the spectroscopy sensor 72 through a sensor interface (e.g., a
sensor port or a wireless interface) and, in embodiments, from the
ablation device 78, to determine if the ablation has generated
sufficiently high tissue temperature to destroy the viability of
the tissue in the area of interest. The monitor 82 may include
appropriate processing circuitry for determining temperature
parameters, such as a microprocessor 92, which may be coupled to an
internal bus 94. Also connected to the bus may be a RAM memory 96
and a display 98. A time processing unit (TPU) 100 may provide
timing control signals to light drive circuitry 102, which controls
when the emitter 74 is activated, and, if multiple light sources
are used, the multiplexed timing for the different light sources.
TPU 100 may also control the gating-in of signals from the sensor
72 and amplifier 103 and a switching circuit 104. These signals are
sampled at the proper time, depending at least in part upon which
of multiple light sources is activated, if multiple light sources
are used. The received signal from the sensor 72 may be passed
through an amplifier 106, a low pass filter 108, and an
analog-to-digital converter 110. The digital data may then be
stored in a queued serial module (QSM) 112, for later downloading
to RAM 96 as QSM 112 fills up.
[0029] In an embodiment, based at least in part upon the received
signals corresponding to the water absorption peaks received by
detector 76 of the sensor 72, microprocessor 92 may calculate the
microcirculation parameters using various algorithms. In addition,
the microprocessor 92 may calculate tissue temperature. These
algorithms may employ certain coefficients, which may be
empirically determined, and may correspond to the wavelength of
light used. In addition, the algorithms may employ additional
correction coefficients. The algorithms and coefficients may be
stored in a ROM 116 or other suitable computer-readable storage
medium and accessed and operated according to microprocessor 92
instructions. In one embodiment, the correction coefficients may be
provided as a lookup table. In addition, the sensor 72 may include
certain data storage elements, such as an encoder 120, that may
encode information related to the characteristics of the sensor 72,
including information about the emitter 74 and the detector 76. The
information may be accessed by detector/decoder 122, located on the
monitor 82. Control inputs 124 may allow an operator to input
patient and/or sensor characteristics.
[0030] As noted, the sensor 72 may be incorporated into the
ablation device 78 or, may, in other embodiments, be a separate
device. FIGS. 7-8 show examples of configurations for the sensor
72. In FIG. 7, the emitter 74 is spaced apart from the detector 76
a particular distance that may be stored into the encoder 110, so
that the monitor may perform the analysis of the mean photon
penetration associated with a particular emitter-detector spacing.
The emitter 74 may be capable of emitting light of multiple
wavelengths. Depending on the wavelength, the mean photon
penetration depth may be shallower, as shown for path 130, or may
be deeper, as shown for path 132. By collecting data from different
depths, a temperature gradient through the tissue may be
determined.
[0031] In addition to using different wavelengths to acquire data
at different depths, a sensor 72 may also incorporate additional
detectors 76 with varied spacing around the emitter 74. As shown in
FIG. 8, a sensor 72 may incorporate detectors spaced apart
different distances (shown as distances d.sub.1, d.sub.2, d.sub.3,
and d.sub.4) from an emitter 74. If the distances correspond with
characteristic water absorption profiles and mean photon
penetration depths, then any change that occurs during an ablation
may be correlated to a empirically or mathematically-derived tissue
temperature.
[0032] FIG. 9 is a process flow diagram illustrating a method 134
for determining tissue temperature during an ablation procedure in
accordance with some embodiments. The method may be performed as an
automated procedure by a system, such as system 70. In addition,
certain steps of the method may be performed by a processor, or a
processor-based device such as a patient monitor 82 that includes
instructions for implementing certain steps of the method 134.
According to an embodiment, the method 134 begins with obtaining a
baseline, pre-ablation signal representative of one or more water
absorption peaks from detector 76 associated with the sensor 72
(block 136). The pre-ablation signal may be associated with normal
body temperatures. The ablation device 78 may be activated and the
sensor 72 may collect data during the ablation (block 138). The
sensor 72 may also collect post-ablation data (block 140). The
monitor 82 may perform analysis of the signals from the sensor 72
and calculation of the tissue temperature (block 142) based on the
signal obtained.
[0033] For example, in one embodiment, the tissue temperature may
be determined by examining changes in the water absorption peaks
over the course of the ablation. If the change in the temperature
is indicative of ablation (i.e., nonviability of the tissue), a
monitor 82 may determine that a successful ablation has occurred.
For example, tissue temperatures in excess of 43.degree. C.,
50.degree. C., 60.degree. C., or 80.degree. C. may be indicative of
ablation. In addition, such monitoring may include any appropriate
visual indication, such as a display of a temperature or
temperature versus depth, displayed on the monitor 82 or any
appropriate audio indication. For example, an increase of tissue
temperature above a predetermined viability threshold or outside of
a predetermined range may trigger an alarm or may trigger an
indication of ablation. Further, additional indications may include
text or other alerts to inform that the ablation was likely
successful.
[0034] While the invention may be susceptible to various
modifications and alternative forms, specific embodiments have been
shown by way of example in the drawings and will be described in
detail herein. However, it should be understood that the invention
is not intended to be limited to the particular forms disclosed.
Rather, the invention is to cover all modifications, equivalents
and alternatives falling within the spirit and scope of the
invention as defined by the following appended claims.
* * * * *