U.S. patent number 4,608,990 [Application Number 06/531,270] was granted by the patent office on 1986-09-02 for measuring skin perfusion.
Invention is credited to Virgil B. Elings.
United States Patent |
4,608,990 |
Elings |
September 2, 1986 |
Measuring skin perfusion
Abstract
A fluorometer and method of fluorometry for illuminating an area
containing fluorescent material which material fluoresces when
excited with light energy at first particular frequencies and for
collecting and measuring fluorescent light energy at second
particular frequencies produced at the area, including, a light
source for producing light energy including light energy at the
first particular frequencies and with the light source having a
wattage rating of no more than twenty (20) watts, pulsing the light
energy at a frequency of approximately at least 10 Hz, filtering
the pulsing light energy from the light source to pass
substantially only the light energy at the first particular
frequencies, directing the filtered pulsing light energy to the
area containing fluorescent material, collecting light energy
including fluorescent light energy at the second particular
frequencies produced at the area and directing the collected light
energy away from the area, filtering the collected light energy to
pass substantially only the collected light energy at the second
particular frequencies, detecting the filtered collected light
energy for producing a first signal in accordance with the filtered
collected light energy, and phase detecting the first signal for
producing a second signal in accordance with the first signal and
representative of the filtered collected light energy in phase with
the filtered pulsing light energy which second signal is
representative of the fluorescent light energy at the second
particular frequencies produced at the area.
Inventors: |
Elings; Virgil B. (Santa
Barbara, CA) |
Family
ID: |
24116963 |
Appl.
No.: |
06/531,270 |
Filed: |
September 12, 1983 |
Current U.S.
Class: |
600/317; 600/363;
600/504 |
Current CPC
Class: |
A61B
5/0059 (20130101); G01N 21/6428 (20130101); A61B
5/413 (20130101); A61B 5/0275 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61B 5/0275 (20060101); A61B
5/026 (20060101); G01N 21/64 (20060101); A61B
005/02 () |
Field of
Search: |
;128/666,633,654,691 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2531854 |
|
Feb 1984 |
|
FR |
|
1426319 |
|
Feb 1976 |
|
GB |
|
2039364 |
|
Aug 1980 |
|
GB |
|
Other References
Jacobs et al., "Determination of the Accuracies of Dye-Dilution and
Electromagnetic Flowmeter Methods of Measuring Blood Flow" J. of
Thoracic and Cardiovascular Surgery, v.58, No. 4, Oct. 1969, pp.
601-607..
|
Primary Examiner: Doll; John
Assistant Examiner: Johnson; Lance
Attorney, Agent or Firm: Schwartz; Charles H. Roston;
Ellsworth R.
Claims
I claim:
1. A method of measuring local blood flow in skin tissue using
fluorescent dye dilution, including the following steps:
injecting a fluorescent dye solution into the blood stream,
producing light energy from a light source including light energy
at first particular frequencies and with the light sources having a
wattage rating of no more than twenty (20) watts,
pulsing the light energy, filtering the pulsing light energy from
the light source to pass substantially only the light energy at the
first particular frequencies,
directing the filtered pulsing light energy to a portion of the
skin tissue containing fluorescent material,
collecting light energy including fluorescent light energy at
second particular frequencies produced at the portion of the local
area and directing the collected light energy away from the skin
tissue,
filtering the collected light energy to pass substantially only the
collected light energy at the second particular frequencies,
producing a first signal in accordance with the filtered collected
light energy,
producing a second signal in accordance with the first signal and
representative of the filtered collected light energy in phase with
the filtered pulsing light energy which second signal is
representative of the fluorescent light energy at the second
particular frequencies produced at the portion of the skin tissue,
and
using the magnitude of the second signal as a measure of the
relative blood perfusion in the skin tissue.
2. The method of claim 1 wherein the step of pulsing the light
energy provides pulsing at a frequency of approximately ten (10)
Hz.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a skin fluorometer perfusion
monitor and more specifically to a hand held skin fluorometer to
measure blood perfusion at a position exterior to the skin
tissue.
2. Description of the Prior Art
It is often desirable to provide for a measurement of the perfusion
of body tissue by the blood. Such a measurement is useful, for
example, to determine if a portion of the body is receiving an
adequate supply of blood such as after a surgical procedure. For
example, it would be desirable to monitor the perfusion in a body
extremity, such as a finger or hand, which was either partially or
completely severed and then reattached. The measurement of blood
perfusion is important so as to determine the likelihood of the
reattached member receiving an adequate blood supply to insure a
healthy healing of the reattached member. Another area in which the
monitoring of the perfusion of blood is useful is when skin flaps
have been attached such as for burn victims.
One method of measuring the blood perfusion is to measure the level
of fluorescense of a fluorescent dye, such as fluorescein, which is
carried by the bloodstream and diffuses into particular body tissue
through the blood perfusion into the tissue. The fluorescent dye
may be injected into the bloodstream and with the level of
fluorescense at some later time measured at the particular tissue
area. The crudest way of injecting the dye into the bloodstream is
to ingest the dye but this takes a considerable time before the dye
ultimately get into the blood.
A more common method is to inject the dye directly into the
bloodstream and then wait for about fifteen (15) minutes for the
dye to diffuse into the tissue of interest. A measurement of the
fluorescense of the tissue of interest, such as a reattached
finger, is then made and with a comparison made between the
fluorescence at the reattached finger and at a normal finger. It
would be useful to make this measurement on a regular periodic
basis, such as every hour, so that if a blockage of either the
artery or vein to the reattached area occurs, the blockage can be
detected within a short time and surgery can be performed to cure
the blockage before the reattached area dies because of a lack of
blood supply.
The general methods of measuring this perfusion into the blood
tissue, using fluorescense, has been of two (2) types. First, the
intensity of the fluorescense may be measured directly with the
eyes but such a qualitative measurement requires large doses of
fluorescent dye and such large doses cannot be repeated on a
periodic regular basis such as every hour.
Another method of measuring uses a fluorometer which has a long
optical fiber bundle and with this bundle carrying blue exciting
light to the skin through a first group of fibers in the bundle and
with a detection of the fluorescense with the remaining group of
fibers in the bundle. The prior art fluorometer normally includes a
high intensity steady state light source such as a one hundred
fifty (150) watt tungsten halogen light and with this light
filtered with a blue filter to give blue exciting light. The
fluorescent light is collected by the remaining fibers in the group
and is filtered by a yellow-green filter. The light energy passed
by the yellow-green filter is directed to a photomultiplier so as
to provide a measurement of the collected fluorescent light
energy.
The above described type of prior art device is relatively large,
expensive and is sensitive to ambient light. In particular,
sensitivity to ambient light is so critical that the device usually
includes a shield so as to prevent the entry of any ambient light
into the device when making the measurement of fluorescense. As an
alternative, the measurement may be made in a dark room but such a
measurement in a dark room is obviously difficult to perform.
SUMMARY OF THE INVENTION
The present invention provides for an improved fluorometer for
monitoring blood flow such as nutritive blood flow into body
tissue. The fluorometer of the present invention is inexpensive,
small and is not sensitive to ambient light. The fluorometer of the
present invention uses a solid state detector instead of the prior
art photo-multiplier and the use of the solid state detector
reduces the cost of the device and eliminates the necessity of a
high voltage power supply. One difficulty with the use of a solid
state detector is that a solid state detector with its associated
amplifier is much noiser electrically than a photomultiplier. The
electrical noise would generally be overcome by using a large light
source. However, in the present invention, the electrical noise is
overcome by the use of phase sensitive detection to reduce the
sensitivity to noise and as an additional advantage, the phase
sensitive detection makes the device of the present invention
insensitive to ambient light.
In order to provide for phase sensitive detection, the light source
must be modulated, either by pulsing the light source or by using a
mechanical chopper to interrupt the light beam. In order to
maintain the small size of the device so that the device may be
hand held, a mechanical chopper is not used and the present
invention incorporates a pulsing of the light source. In the prior
art it is generally considered best to use the largest light source
possible so as to provide for the maximum amount of exciting light.
However, a large light source necessitates a large filament and
large filaments have large rise times which are counter productive
to the proper pulsing of the light source. For example, as pulsed
electrical energy is supplied to the light source, the filament is
heated and because of the large rise time for large filaments, the
filament would appear to have a steady state output rather than a
pulsed light output. The proper pulsing of the light source is
compounded by the fact that gas filled halogen bulbs have very slow
decay times. It is preferable to use a halogen bulb because such a
bulb allows for a high filament temperature to provide for
sufficient light energy in the blue region.
In order to accomplish the pulsed light source and to overcome the
various problems described above, the present invention provides
for the use of a low wattage light source with a small filament and
the use of such a light source is generally the reverse of what is
provided for in the prior art. In particular, the size of the light
source of the present invention is defined in accordance with the
following criteria. In order to get reasonable noise rejection, the
phase sensitive detection should provide for an integration of the
detected flurosecent light over approximately ten (10) cycles of
light energy from the light source. The instrument response time
should not be too long and if a response time of one (1) second is
chosen, the light source is thereby pulsed at at least ten (10) Hz.
If, for example, a two (2) watt bulb is chosen, then the pulsing of
the bulb can occur between a range of between ten (10) to twenty
(20) Hz. The light bulb need not go all the way off but there
should be a sufficient flicker of at least fifty percent (50%) so
as to provide for a proper pulsed light output.
If a larger light source is chosen there is no substantial gain in
efficiency. Although a larger light source will provide for more
output light energy, the larger rise and decay times of the larger
bulb produce a smaller percent modulation. It has been determined
that the modulated light output is about the same independent of
the bulb size when the bulb size is larger than about six (6)
watts. For example, the bulb size of the present invention would
normally be limited to be at most twenty (20) watts which is
slightly more than one-tenth (1/10) the power that is normally used
in prior art fluorometers.
The use of the small wattage bulb for the fluorometer of the
present invention also provides for three (3) additional
advantages. First, a smaller wattage bulb means lower power
consumption and thereby less heat to be dissipated. This is of
considerable importance if the instrument is to be hand held.
Second, the smaller wattage bulb is smaller in size and therefore
allows for smaller packaging which is again important for a hand
held instrument. Lastly, the smaller wattage bulb has a small
filament and therefore it is easier to focus the light from the
smaller filament to a small region which is desirable with the
device of the present invention. Because of the use of the smaller
filament it is possible to get more usable modulated light to the
region of skin tissue of interest with the use of the small bulb
than with a big bulb.
The present invention also is directed to a hand held flurometer
which may use fiber optics to direct the modulated light to an area
of interest for excitating and with the fluorescent light collected
and carried by a light pipe or optical fibers to a solid state
detector. The various electronics for the pulsing of the light
source and performing the phase sensitive detection may be built
into the handle of the hand held device. In addition, the
measurement of the fluorescent level may be displayed on a visual
readout which is also part of the hand held device.
Alternatively, the various electronics and display may be
positioned within a shelf or table mounted cabinet and with the
light brought to the area to be measured and collected from that
area using long optical fibers in a bundle. Since the fibers
themselves tend to fluoresce, the fibers carrying the blue light
and the returning fluorescent light are not the same fibers and the
present invention uses a split bundle of fibers, half for the blue
exciting light and half for the collected fluorescent light.
With both the hand held and the fiber optic embodiments of the
present invention, a light mixer is used to position the source of
exciting light and the collection of fluorescent light to be at a
distance above the skin tissue. This insures that the entire
desired area of skin tissue is excited with light energy and with
the fluorescense detected from the same area.
BRIEF DESCRIPTION OF THE DRAWINGS
A clearer understanding of the invention will be had in reference
to the following descriptions and drawings wherein;
FIG. 1 illustrates a perspective view of a hand held skin
fluorometer perfusion monitor;
FIG. 2 is a block diagram of the various electrical components
which form the circuit for the fluorometer of FIG. 1.
FIG. 3 illustrates a long tip accessory for the fluorometer of FIG.
1 for providing measurement at an internal position;
FIG. 4 illustrates an alternative structure for providing and
collecting light energy for the hand held fluorometer of FIG.
1;
FIG. 5 illustrates a second embodiment of the invention including
long fiber optics to bring the exciting light to and the returning
light from the surface to be measured;
FIG. 6 illustrates the fiber optics of FIG. 5 within a monitoring
system for permanently monitoring small surfaces areas; and
FIGS. 7a and 7b illustrate an accessory item used to insure that
the same surface area is monitored for successive measurements.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a perspective view of a hand held fluorometer 10
constructed in accordance with the teachings of the present
invention. The hand held instrument 10 is essentially constructed
in three (3) sections. A first handle section 12 is used to contain
the various electronics 14 provided with the fluorometer 10.
Extending from the handle 12 is an intermediate section 16 which is
used to primarily contain the exciting light source. Finally, a
head section 18 contains the fluorescent detection portion of the
system and a visual display. A replaceable tip 20 may be positioned
at the light emitting and detecting end of the head 18.
As indicated above, all of the various electronics 14 for the
fluorometer may be contained in the handle portion 12. A power cord
22 supplies electrical power to the electronics and through the
electronics to the other portions of the fluorometer of the present
invention. In particular, pulsing electrical energy is supplied
from the electronics 14 to a tungsten-halogen lamp 24 and with the
halogen lamp providing a spectrum of light energy including
significant light energy in the blue region. A blue filter 26 is
positioned to pass essentially the blue exciting light from the
light source 24 to the end of a fiber optic bundle 28. The blue
exciting light is therefore passed through the fiber optic bundle
28 from the intermediate section 16 into the head section 18 and is
guided to the end of the head section 18 to be emitted as blue
emitting light at an end position 30 for the bundle. The bundle of
fiber optics 28 is actually formed in a circular path to surround a
solid light guide or pipe 32 so that the end emitting light portion
30 is formed as a cylinder. The light energy is then directed
toward the skin surface 35 to impinge on an area of the skin
surface defined by the opening in the replaceable tip 20.
The blue emitting light thereby excites any fluorescence in the
skin 35, due, for example, to the injection of a fluorescent dye
into the blood stream and with the fluorescent light returning to
the instrument through the solid light guide 32. Normally, if a
blue exciting light is used, the fluorescence will be in the
yellow-green region and so at the end of the light guide 32 a
yellow-green filter 34 is positioned to pass essentially only the
light in the yellow-green region to a detector 36. Part of the
output signal from the detector 36 represents the fluorescent
energy and this energy is measured by the electronics 14 and with
this measurement then provided by the electronics 14 to a digital
readout 38 located at the back of the head section 18. It is to be
appreciated that as an alternative to using power supplied through
the cord 22, such as from a separate power supply, the handle may
also be fitted with a battery pack, so that the entire unit may be
completely self-contained and portable.
As shown in FIG. 2, the electronics 14 may include the following
items as shown in the dotted portion. Power is supplied to the
electronics 14, such as through the cable 22 shown in FIG. 1 or
through the use of a portable power supply such as a battery pack
mounted in the handle 12. The power supply supplys power to all of
the components forming the electronics 14. In particular, an
adjustable power supply 50 supplies power through a switch 52 to
the halogen lamp 24. The switch 52 is controlled by a ten (10) Hz
oscillator 54 so that the output from the lamp 24 is a pulsing
output light at a ten (10) Hz rate.
The pulsing output light from the lamp 24 passes through the blue
filter 26 and is directed to an area of the skin such as with the
optical system shown in FIG. 1. The blue light causes the
fluorescent dye in the skin to fluoresce to produce pulsing light
energy in the yellow-green region representative of the
fluorescence. The light energy in the yellow-green region including
ambient light is passed through the yellow-green filter 34 to be
received by the detector 36 which is preferably a solid state photo
diode. The output from the photo diode is representative of the
yellow-green light. The light energy is coupled to a
current-to-voltage converter 56 to produce an output voltage
representative of all the yellow-green light energy. This output
voltage is then passed through a low pass filter 58 and with the
signal from the low pass filter 58 then amplified by an amplifier
60.
The signal from the amplifier 60 is then passed through a ten (10)
Hz band pass filter 62 and with both the signal from the filter 62
and the signal from the oscillator 54, coupled to a phase sensitive
detector 64. The detector 64 provides for an output signal
representative of the pulsing yellow-green light energy pulsing in
phase with the light source and this output signal is passed
through a low pass filter 66 and then is converted to a digital
signal by a digital-to-analog converter 68. The digital output from
the digital-to-analog converter 68 is then displayed as a numerical
readout representative of the fluorescent energy by the display
38.
The electronic system, as shown in FIG. 2, provides for an accurate
reading of the fluorescent energy produced at the skin surface and
with this reading not sensitive to ambient light. In particular,
the detector 36, such as the solid state photo diode, sees in
general three (3) sources of light. First, the solid state photo
diode detects the fluorescent light from the tissue and with this
fluorescent light pulsing at the same rate and in phase with the
exciting light from the lamp 24. Second, the photo diode 36 sees
light from any fluorescent room lights which would be pulsing at
one hundred and twenty (120) times per second. Third, the photo
diode sees light from incandescent room lights and/or sunlight,
both of which form essentially a steady background light. In order
to make an accurate measurement of the tissue fluorescence, without
having to darken the room, it is necessary to eliminate the light
from the second and third sources.
In the electronic circuit 14, the first stage after the detector 36
is the current to voltage converter 56 and the low pass filter 58.
This first stage passes the signals from all three sources but
converts a high percentage, such as ninety percent (90%), of the
signal from the second source into a DC level. The next stage of
the electronic circuit is the amplifier 60 and the band pass filter
62. This stage passes most of the signal from the first source but
removes most of the DC level which thereby eliminates most of the
signal from the second and third sources. However, since the
ambient light can be quite strong, the remaining background signal
may still be larger than the signal from the first source, which is
the fluorescent light from the tissue.
The signal passed by the first and second stages of the electronic
circuit 14, which consists of the pulsing fluorescent signal and
whatever background signal is left is then coupled to the phase
sensitive detector 64. The detector 64 passes essentially only the
pulsing fluorescent signal since the phase sensitive detector
compares the signal passed from the first and second stages with
the output from the ten (10) Hz oscillator 54 and passes only the
signal which is in phase with the signal from the oscillator 54.
The low pass filter 66 averages the output from the phase sensitive
detector 64 over a time equal to the time constant of the low pass
filter. Therefore, a signal in phase and at the same frequency as
the oscillator 54, such as the fluorescent signal from the tissue
gives a non zero value when passed through the low pass filter,
whereby signals such as a DC level or oscillating but not in phase
with the oscillator, gives a signal which, when averaged over a
period of time is zero.
In order to provide for a reasonable rejection of the background
signal, the low pass filter should average over several pulses of
the light source. However, it is desirable that the instrument
provide for a reading within a reasonable time response, so that
the operator may measure several areas of the skin in a short
period of time. In general, it is desirable that the instrument
provide for a measurement within a one (1) second time period.
Because of this, the time constant of the low pass filter at the
output of the phase sensitive detector 64 should be about one (1)
second. In order to provide for good background suppression, it is
desirable to average the signal over at least ten (10) pulses of
the light source, thereby requiring a light that can be pulsed at
ten (10) Hz or faster. This may only be accomplished if the bulb is
quite small so that its filament is small and therefore its rise
time is fast. In the present invention the preferred embodiment
uses a two (2) watt bulb and in general the bulb should not be
greater that approximately twenty (20) watts which is approximately
1/10th of the size of the bulbs normally used with
fluorometers.
It is desirable to calibrate the fluorometer, so that the output
reading at the digital display 38 is compared to an absolute scale.
This is particularly true since the output from bulb to bulb can
vary, the bulb can lose intensity with age and since the optics may
get dirty from handling. It would be best to provide for a method
of calibration which could be accomplished before each measurement
so as to provide for an accurate comparison between different
readings. In the prior art, instruments have been calibrated by
using a calibration solution of fluorescein in water. This type of
calibration is not satisfactory since fluorescein in water is not
stable for long periods so that the calibration solutions need to
be periodically renewed. Secondly, the solution may adhere to the
measurement probe which can cause a background fluorescence
independent of the fluorescence from the skin tissue. Lastly, the
use of a water solution is messy.
In the present invention, the calibration is provided by a solid
calibrator 40 shown in FIG. 1 formed from a plastic in which a
fluorescent material is added during manufacture. This material is
available from Rohm in Germany. The tip 20 of the fluorometer is
placed on the calibrator 40 just as it would be on the skin and the
adjustable power supply 50 is varied until the readout at the
display 38 gives the proper level. This level may be represented by
an arbitrary number such as 100 on the display. If desired, the
calibration level may be cross-calibrated in terms of a certain
concentration of fluorescein in water so that the output
measurement is representative of an actual concentration of
fluorescein as opposed to just an arbitrary measurement level which
may be repeated from measurement to measurement.
There may be times when it is necessary to provide for a
measurement either through a thick cast or on an internal organ
such as during surgery and it is therefore desirable to provide for
a long tip for the fluorometer of FIG. 1. This may be accomplished
by mounting a solid light guide of an appropriate length in the
removable tip 20 as shown in FIG. 3. In particular, a solid light
guide 70 may be mounted in the tip 20 so as to be adjacent the end
portion 30 of the fibers 28 and also adjacent the end of the light
guide 32. The solid light guide 70 would carry both the blue
exciting light and the collected fluorescent light and should be
large enough to cover the ring of fibers 28 emitting the blue light
at the end position 30 and also the light guide 32.
When the light guide 70, forming an extension, is used, the
instrument would be recalibrated, for example, through the use of
the solid calibrator 40. The measurement would then be made by
pressing the end of the light guide 70 against the surface 35 which
is to be measured. In the embodiment of the invention shown in FIG.
1, it is important that the end portion 30 of the fibers 28 and the
light pipe 32 be spaced away from the surface 35 to be measured.
This spacing may either be provided by air, as shown through the
use of the removable tip 20 or could be provided by a solid light
guide such as the light guide 70 shown in FIG. 3 or a shorter light
guide similar to that shown by the light guide 70. In either case,
the light guide would be of a size to cover both the fibers 28
carrying the blue light and the light guide 32 which collects the
fluorescent light. The air space or light guide forms a mixer since
this area mixes up the outgoing blue light and the returning yellow
light in this space.
The reason for the use of the air space or light guide forming the
mixer is to insure that the end of the light guide 32 which
collects the fluorescent light views the same area which has been
excited by the blue light. This provides for the maximum efficiency
in the collection of the fluorescent light. If the end portion 30
of the fibers 28 and the end of the light guide 32 were placed
directly on the skin 35 with no space, the blue light would excite
one area of the skin and the light pipe 32 would view another area
and would thereby see little fluorescent light. With the use of the
mixer, the blue light is directed to a particular area and the
fluorescent light is collected from the same area by the light pipe
32 thereby providing for a high efficiency in excitation and
collection.
If an air space is used to form the mixer, then the distance for
the spacing should be approximately of a size equal to the radius
of the light pipe 32. For example, measurements have been made of
the amount of collected fluorescent light versus the distance the
light pipe 32 is from the surface being measured. For a light pipe
having a diameter of one-fourth inch (1/4"), the amount of
collected light peaks when the light pipe is approximately
one-eighth inch (1/8") from the surface being measured. If a light
pipe, such as the light pipe 70, is used to provide for a mixer,
then the minimum length for this light pipe should be also about
equal to the radius of the collecting light pipe 32. However, once
the minimum length is provided any greater length may be
arbitrarily long since once the exciting and collecting light is
mixed the light pipe maintains this light energy within the
confined volume of the light pipe 70 and a greater length does not
result in any appreciable loss of light energy.
As as alternative to the use of the fiber optic and light pipe
system shown in FIG. 1, the head section 18 of the hand held
instrument may contain a lens systems as shown in FIG. 4. In
particular, a light source 20 such as a tungsten-halogen lamp may
direct light energy through a focusing lens 82 and through a blue
filter 84 to be focused in an area 86 at the surface of the tissue
35. Returning fluorescent energy would be passed through a lens 88
and a yellow-green filter 90 to be focused on a photo diode 92. The
remaining portion of the electronic system would be as shown in
FIG. 2. The light source 80 would be of low wattage, such as a two
(2) watt bulb, and with the light source pulsed and detected as
shown in FIG. 2.
One other preferred aspect of the present invention is in the use
of a particular type of solid state detector or photo diode.
Specifically, a gallium arsenide phosphide (GaAsP) detector is
preferred rather than a silicon photo diode, although silicon is
generally considered preferable for use as a detector. The reason
the GaAsP detector is preferred is that this type of detector is
not sensitive to wavelengths longer than seven hundred (700) nm and
therefore any infra-red light which passes through the filter is
not detected.
As a second embodiment of the invention, the light energy may be
brought to the surface to be measured and then collected using long
optical fibers. The electronics 14, the light source and detector
are positioned at a remote station and with the long fiber optic
system providing for the excitation and collection. Since the
optical fibers themselves tend to fluoresce the fibers carrying the
blue light and the returning fluorescent light should not be the
same fibers. This may be accomplished using a split bundle of
optical fibers, half for the blue light and half for the
fluorescent light as shown in FIG. 5.
As shown in FIG. 5, the blue light is transmitted to a first half
100 of a split optical fiber bundle and the fluorescent light is
collected by a second half 102 of the split bundle. The entire
bundle is shown joined at the position 104. If the fibers in the
two halves 100 and 102 are randomly mixed in the portion 104, then
it would be possible to position an end 106 of the bundle 104
adjacent the tissue 35. However, a mixer 108, which is formed as a
solid light pipe, may be used so as to eliminate the necessity of
providing for a randomly mixed bundle. When the mixer 108 is used,
all of the emitting fibers 100 can be on one side of the bundle in
the portion 104 and all of the collecting fibers 102 can be on the
other side. The mixer 108 insures that at the surface 35 to be
measured, the emitting and collecting fibers both view the same
area.
Since the mixer 108 is relatively short it does not provide any
appreciable fluorescence of its own. If all of the emitting fibers
are on one side and all of the collecting fibers are on the other
side, then the length of the mixer should be approximately the
radius of the bundle 104. If the emitting and collecting fibers are
randomly mixed within the portion 104, it is still possible to
increase the efficiency through the use of a mixer but in this
case, the mixer can be quite short, such as the radius of one of
the fibers in the bundle 104.
The fiber optic embodiment of the invention, as shown in FIG. 5,
may be particularly useful for continuous monitoring, since the end
of the fiber optic bundle may be attached to an area of the skin to
be continuously monitored and can be easily taped in position
similar to the taping of tubing now provided for in the
hospital.
The continuous monitoring provided for by the embodiment of FIG. 5,
allows for the measurement of local skin blood flow per unit volume
by injecting fluorescein in very small quantities into the dermus
and monitoring the washout of the fluorescein due to the blood flow
carrying it away. The quantities for example, may be one (1)
microgram of fluorescein diluted into a volume of water. With this
method it is possible to measure the blood flow per unit volume for
the tissue. In general, as with all single injection indicator
dilution, the concentration of the indicator decreases
exponentially where:
where
.tau.=Volume/Flow
so that flow/unit volume=1/.tau.
For good flow .tau. is about five (5) minutes, i.e., the flow/unit
volume is about 0.2 ml/sec.cc. This assumes that the fluorescein in
the tissue can diffuse freely into the blood vessels. This is true
for fluorescein since it does not attach to the tissue.
The standard way of using fluorescein is to measure tissue
perfusion by the injection of approximately one hundred (100) mg of
fluorescein to the central blood system and with a subsequent
measurement of the appearance of the fluorescein in the tissue. For
a newly transplanted piece of skin, for example, this provides a
measure of the blood flow into the skin (arterial flow) but it
would also be desirable to know if there is venous flow out of the
tissue. It is important to know that the venous flow is not
occluded since if the venous flow is occluded, this could be
corrected by surgery. By using the local injection technique
described above, a subsequent measurement may be made of the
washout of fluorescein. This provides for a direct indication of
the venous flow out of the tissue. If the venous flow is occluded
the washout is very slow. The amount of injected fluorescein is
very small, such as 1/100,000 times the normal injection, and this
method may be used not only to monitor washout but also may be used
for the patients who become nauseous from the use of large amounts
of fluorescein.
In order to monitor the washout, the structure shown in FIG. 5 is
used and with the end of the fibers taped to the tissue or attached
with a holder. A relatively small bundle of fibers is used and the
device is constructed to collect data, store that data and provide
an output display of the flow/volume. The use of the small pulsing
light source and the phase sensitive detection, as shown in FIG. 2,
provides for the detection of the fluorescense. The resultant
monitor therefore combines the fiber optic structure, as shown in
FIG. 5, with the small pulsing bulb and phase sensitive detection,
as shown in FIG. 2, to produce the resultant system shown in FIG.
6.
In the system of FIG. 6, the output from the electronics 14 is
coupled to a computer 110. The computer collects the data, stores
the collected data and then produces a fit of an exponential curve
to the data to determine flow/unit volume. The output flow/unit
volume may then be displayed by a display 112.
With both embodiments of the invention, it is important to provide
for a reproduction of measurements of the skin tissue at the same
area so as to produce an accurate time sequence of measurements
from the same area. This may be particularly important when using
the washout technique as described above. The present invention
therefore includes an accessory to insure that the measurement is
made on the skin at the same area each time. Specifically, the
present invention incorporates the use of donut shaped patches in
which the center hole is the same size as the tip of the
fluorometer and with these patches pasted on the skin in the
desired location. With the use of an individual patch or a series
or patches on the skin, measurements can be repeated at the same
spot or spots by simply placing the tip of the fluorometer in the
center of the donut.
Two (2) alternative donut structures are shown in FIGS. 7a and 7b.
In FIG. 7a a round donut 120 has a center opening 122 which is the
same size as the tip of the fluorometer. The donut may be
adhesively attached to the tissue 35 so that the measurement is at
the same position on the skin each time. This has been found to be
important since changing the position of the reading on the skin
even a small amount, can affect the magnitude of the reading.
As shown in FIG. 7b, the patch may have a different shape such as
the rectangular patch 124 of FIG. 7b. This rectangular patch 124 is
larger than the patch shown in FIG. 7a. This allows for indicia to
be applied to the patch so as to identify the particular patch.
Patch 124 also has an opening 126 to insure that the measurement is
made at the same position each time. In addition, the patch 124 may
include a transparent layer 128 so that the tip of the fluorometer
does not contact the skin during measurement.
The present invention therefore is directed to skin perfusion
fluorometry and in particular, to the use of a fluorometer which
incorporates an exciting light source of a small wattage, such as
less than twenty (20) watts, which can be pulsed at a frequency of
at least ten (10) Hz. The detection of the fluorescent light is
accomplished using phase sensitive detection so as to decrease
electronic noise and make the fluorometer impervious to ambient
light. The exciting and collected light from the tip of the
fluorometer is mixed so as to increase the efficiency of the
collected fluorescent light. In general the length of the mixer is
at least as large as the radius of the fiber optic bundle or
collecting light pipe. The present invention may provide for the
measurement of local wash out flow using a fluorescent dye dilution
technique and with a continuous monitoring of the fluorescense over
a period of time. In order to insure that the measurement is
provided at the same place on the skin, the present invention also
includes the use of patches to position the measurement at the same
point for repeated measurements.
Although the invention has been described with reference to
particular embodiments, it is to be appreciated that various
adaptations and modifications may be made and the invention is only
to be limited by the appended claims.
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