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BACKGROUND OF THE INVENTION
A variety of optical measuring systems must be calibrated prior to use. One
example of such a system is an oximetry system which must provide accurate
in vivo measurements of mixed venous oxygen saturation. A system of this
type includes an optical thermodilution catheter, a light source for
supplying light at selected wavelengths to the optical catheter, and an
optical measuring device for measuring the intensity of the light signal
received from the optical catheter. Any of these components can introduce
variations into the system, and thus, the entire system must be
calibrated.
The oximetry system can be calibrated using in vivo techniques. According
to this approach, an oximeter catheter is inserted into the vascular
system of a patient, and light intensity measurements of the blood are
made. A blood sample is also drawn from the patient, and the degree of
oxygen saturation of the sample is accurately measured in a laboratory.
These two measurements are then appropriately correlated to calibrate the
oximetry system. This technique requires that a blood sample be drawn, and
this can introduce error due to sampling techniques and due to time lags,
such as the time lag between the sample time and the completion of the
laboratory analysis of the sample. In addition, this technique is very
time consuming.
The optical system can also be calibrated using in vitro techniques. One
such technique requires that a reference element be brought into optical
contact with the end of an optical catheter while the catheter is clamped
in position. The clamping of the optical catheter, particularly in the
region of the balloon, may damage the balloon. In addition, a mechanism
for moving the reference element is required. This technique also requires
that the reference element be compliant at the surface which contacts the
end of the catheter. Such a device is shown in Shaw et al U.S. Pat. No.
4,322,164.
It is also known to employ an optically open system as shown in Polyani et
al U.S. Pat. No. 4,050,450. However, being optically open, this system is
not immune to ambient light.
SUMMARY OF THE INVENTION
This invention overcomes these disadvantages by providing a calibration
reference apparatus which is immune to ambient light and which does not
require that the end face of the optical catheter or other light guide be
in contact with the operative surface of a calibration element. As a
consequence, the mechanism of the prior art for moving the calibration
element into contact with the end face of the light guide is eliminated.
Also, the mechanical clamping device for clamping of the light guide is
not required, and the calibration element need not be constructed of
compliant material.
With this invention, the calibration element has a surface defining a
cavity having an opening at one end, with the opening being sized to
receive the end portion of the light guide. To provide immunity to ambient
light and to prevent the escape of light from the cavity, the cavity is
preferably, essentially optically closed, except for such opening. If
desired, the cavity may have other openings, provided such openings do not
permit the transmission of significant light through such openings.
Means is provided for releasably positioning the end portion of the light
guide in the cavity, with the end face of the light guide spaced from the
surface of the cavity opposite the opening to define a gap. Accordingly,
the light guide can direct light at least at one wavelength from the end
face thereof across the gap and against the surface of the cavity.
The calibration element and the gap are adapted to return a known ratio of
the light at such one wavelength which is directed into the gap from the
end face of the light guide. Accordingly, contact between the calibration
element and the end face of the light guide are not required. The light
returned is returned to the light guide for transmission proximally along
the light guide to a measuring device which measures the intensity of the
light returned. This information is utilized in calibration of the light
guide and the other components of the system.
The surface which defines the cavity need not be compliant and is
preferably rigid. This surface is preferably symmetrical in a direction
which will permit the end portion of the light guide to be inserted into
the cavity in any angular orientation without affecting the ratio of the
light which is returned. A preferred configuration is part spherical.
The optical properties of the calibration element must be known and be
repeatable from element to element in production. This is necessary so
that the calibration element will do its part to return the known ratio of
light at the wavelength or wavelengths of interest back to the end face of
the light guide. The optical properties of the calibration element should
be homogeneous so that the ratio of light returned is not affected by the
relative angular orientation of the calibration element and the end
portion of the light guide.
Preferably, certain optical characteristics of the calibration element
mimic the substance with which the light guide is adapted for use. For
example, in the case of an oximetry system, the calibration element
preferably has light returning properties which mimic blood. More
specifically, the calibration element preferably has light-scattering,
absorption and reflection properties which, in the aggregate (but not
necessarily individually), mimic blood. In order to provide the
calibration element with the desired light-scattering properties, the
calibration element preferably includes a plurality of light-scattering
particles distributed in a matrix.
It is preferred that essentially none of the light directed into the gap
from the end face of the light guide be allowed to escape from the
calibration reference apparatus, except back through the light guide. This
can be accomplished, for example, by constructing the calibration element
sufficiently thick and/or opaque to the wavelengths of interest so that
essentially none of this light is transmitted completely through the
calibration element. Alternatively, the calibration element may be at
least partially received in an essentially opaque optical barrier element.
In either case, the surface of the cavity surrounds the gap to at least
assist in essentially preventing transmission of light at the wavelength
of interest directed into the gap from the end face of the light guide
radially of the gap to outside of the calibration reference device.
One important function of the positioning means for the light guide is that
the positioning means establishes the size of the gap, and the size of the
gap should be repeatable so that the light-attenuating effects of the gap
will be repeatable. Although it is not necessary that the gap size be
completely identical from unit to unit, the gap size should be repeatable
within some reasonable tolerances. To meet these requirements in a simple,
inexpensive construction, the position means can advantageously include a
portion of the surface which defines the cavity, with such surface portion
being adapted to form a friction fit with the end portion of the light
guide. To assist in providing repeatability, this surface is preferably
rigid so that it will allow the light guide to enter the cavity a
predetermined amount.
The calibration reference apparatus can be of simple and inexpensive
construction and be disposable. For example, the calibration reference
apparatus may take the form of a calibration cup which comprises an
elongated peripheral tubular wall open at one end and an end wall closing
the other end of the tubular wall. The tubular wall and the end wall can
be integrally molded. With this construction, the end wall defines the
cavity, and the cavity is curved and opens toward the open end of the
tubular wall. The calibration cup is adapted to receive the light guide
through the tubular wall and in the cavity. Thus, the end wall provides
the calibration reference element for use in calibrating the light guide
and the associated components.
The invention, together with additional features and advantages thereof,
may best be understood by reference to the following description, taken in
connection with the accompanying illustrative drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a somewhat schematic illustration of one form of calibration
reference apparatus constructed in accordance with the teachings of this
invention and an optical oximeter catheter system.
FIG. 2 is a fragmentary side elevational view of a distal end portion of
the optical catheter.
FIG. 3 is an end elevational view of the catheter.
FIG. 4 is a sectional view taken on an axial plane through the calibration
reference apparatus.
FIG. 5 is an end elevational view of the calibration reference apparatus.
FIG. 6 is a sectional view similar to FIG. 4, with the end portion of the
optical catheter being positioned in the calibration reference apparatus.
FIG. 7 is an isometric view of an optical barrier.
FIG. 8 is an isometric view of the calibration element with portions broken
away and of the optical barrier element.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a rigid calibration reference apparatus 11 being used with an
oximeter catheter system 13. The oximeter catheter system 13 is
conventional and may comprise, for example, an optical oximeter catheter
15, a light source 17 and a measuring and processing apparatus 19. The
oximeter catheter 15 comprises a catheter body 21 and a balloon 23 and
constitutes a light guide in that it includes a sending light conductor 25
(FIG. 3) and a receiving light conductor 27 retained within a passage of
the catheter body by an elongated element 29. The catheter 15 has a distal
end portion 31 and terminates in a distal end face 33. Although various
constructions are possible, in this embodiment, the catheter body 21 is
cylindrical and has a cylindrical peripheral surface 35 which is joined to
the end face 33 by a curved surface 37 so that the diameter of the end
face 33 is slightly less than the diameter of the peripheral surface 35.
For example, the diameters of the peripheral surface 35 and the end face
33 may be 0.105 inch and 0.093 inch, respectively. In this embodiment, the
end face 33 is planar and is perpendicular to the axis of the peripheral
surface 35.
In use, the catheter 15 is inserted into the pulmonary artery using known
techniques, and light from the light source 17 is transmitted along the
sending light conductor 25, which may be an optical fiber, to the end face
33 where it impinges upon the blood in the vein. The blood scatters,
reflects and absorbs some of the light from the light conductor 25 and
returns a portion of the light along the receiving light conductor 27 to
the measuring and processing apparatus 19. By comparing the intensities of
light returned by the blood at two or more wavelengths to the apparatus
19, the oxygen saturation of the venous blood can be determined in
accordance with known techniques. For this purpose, the light source 17
may transmit light at a selected wavelength or wavelengths depending upon
the algorithm being employed.
If the system 13 were used without calibration, the catheter 15, the light
source 17 and/or the apparatus 19 may introduce variables into the system
which would prevent an accurate determination of oxygen saturation.
Accordingly, prior to use of the apparatus 13, it is calibrated using the
calibration reference apparatus 11.
As shown in FIG. 4, the calibration reference apparatus 11 is in the form
of an integrally molded calibration cup which comprises an elongated,
peripheral, tubular wall 39 having an opening 41 at its proximal end and a
curved, rigid end wall 43 closing the other end of the tubular wall. The
end wall 43 constitutes a calibration element, and it has a rigid,
imperforate hemispherical surface 45 defining a hemispherical cavity 47
coaxial with the tubular wall 39. The cavity 47 has an opening 48 at the
proximal end of the cavity which faces toward the opening 41. Except for
the opening 48, the cavity 47 is closed. The surface 45 blends smoothly
into a very short surface extension 49 which is joined to an elongated,
inner cylindrical surface 51 by a conical guide surface 53. The tubular
wall 39 is stiffened by four axially extending, external wings or ribs 55,
and a proximal region of the tubular wall 39 is flared radially outwardly
in a conical lead-in section 57 for protection of the balloon 23.
In the embodiment illustrated, the calibration reference apparatus 11
comprises a plurality of light-scattering particles distributed in a
matrix of plastic material, and the plastic material includes a dye.
Although many rigid, non-toxic, and sterilizable materials may be
utilized, polyethylene 306 is currently preferred for the matrix.
The light-scattering particles may be, for example, oxides, carbonates and
sulfates. However, titanium dioxide, particles are preferred for use with
polyethylene. Although particle size can vary, in the preferred range of
particle sizes, at least 99 percent of the particles will pass a 325 mesh
screen.
Various non-toxic dyes may be used. The dye is used primarily for light
absorption and as a secondary light scatterer. In the illustrated
embodiment, FDC Red Lake No. 3 dye is utilized.
These ingredients may be mixed in various proportions depending upon the
results desired. Thus, to increase light scattering, a greater percentage
of light-scattering particles should be used. Similarly, to increase light
absorption, the percent of dye should be increased. In the illustrated
embodiment, the apparatus 11 consists of 0.17 percent by weight of
titanium dioxide, 0.5 percent by weight FDC Red Lake No. 3 dye with the
remainder being polyethylene 306. The ingredients of the apparatus 11 are
mixed homogeneously so that the surface 45 and the end wall 43 will have
homogeneous optical properties and be repeatable in production so that
when a large number of the calibration apparatuses 11 are molded, each of
the end walls and associated surfaces 45 will have substantially the same
reflection, absorption, and scattering properties. The preferred
ingredients and proportions stated above provide light-scattering,
absorption and reflection properties which, in the aggregate, mimic blood.
The surface finish of the surface 45 is carefully controlled so that it
will be the same in production from calibration element to calibration
element. The surface 45 may have various degrees of smoothness and may be,
for example, smooth, rough or matted. In this embodiment, the surface 45
is very smooth and has a 20-micron surface finish.
In use of the apparatus 11, the end portion 31 of the catheter 15 is
inserted through the opening 41 and is guided by the tubular wall 39 and
the conical surface 53 into the cavity 47. The diameter of the cavity 47
is slightly smaller than the diameter of the peripheral surface 35.
Because the cavity 47 is of progressively reducing cross-sectional area as
it extends distally, the end portion 31 of the catheter 15 can be forced
into the cavity 47 for only a short distance as shown in FIG. 6.
Specifically, the outer or proximal regions or portions of the surface 45
form a friction fit with the very distal tip of the end portion 31. Thus,
this portion of the surface 45 constitutes means for releasably
positioning the end portion 31 of the catheter 15 in the cavity 47. In
this position, the end face 33 is spaced from a region of the surface 45
to form a gap 59. This outer portion of the surface 45 can also be
considered as a stop for arresting further inward movement of the end
portion 31 into the cavity 47. By sizing of the surface 45 and the cavity
47 with respect to the end portion 31, the axial dimension of the gap 59
can be predicted with sufficient accuracy to provide adequate calibration
of the system 13.
With the components in the position of FIG. 6, light from the light source
17 is directed through the sending light conductor 25 to the end face 33.
The light can then be directed at the wavelength of interest from the end
face thereof across the gap 59 and against the surface 45. ratio of the
light which is directed into the gap from the end face 33. The intensity
of the light returned at two or more wavelengths is measured by the
apparatus 19 and compared with the known ideal rates. Adjustments are then
made in the apparatus 19 to obtain calibration of the apparatus 11.
In the form shown in FIG. 4, the calibration reference apparatus is assumed
to be of sufficient thickness so that essentially none of the light
directed into the gap 50 from the end face 33 of the catheter 15 is
transmitted completely through the end wall 43. This same effect can be
obtained by utilizing an end wall 43 which would transmit more light than
is desirable out of the gap 53. When using an end wall 43 of this latter
type, it is preferred to use an opaque optical barrier element, such as
the barrier element 61, FIG. 7. Although the barrier element 61 can be of
various different constructions, in the form illustrated, it is integrally
molded of plastic material and includes a relatively wide receptacle 63
which includes a peripheral wall 65 with tapered ends, an end wall 67, and
a flange 69 around an opening 71. The opening 71 is widened at a central
region 73, and the receptacle 63 may be curved to facilitate loading it in
a curved groove of a catheter package (not shown). The peripheral wall 65
and the end wall 67 are opaque, and preferably the interior and exterior
surfaces of these surfaces are white to maximize reflection.
The optical barrier element 61 can be used with a calibration reference
apparatus 11a as shown in FIG. 8. The calibration reference apparatus 11a
is identical to the calibration reference apparatus 11, except that it
would transmit a greater percentage of light through the end wall 43 at
the wavelengths of interest.
In use, the apparatus 11a is inserted into the opening 71 at the central
region 73 to place the end wall 43 in contact with the end wall 67. The
interior surface of the end wall 67 may have a rounded, shallow cavity for
receiving, or partly receiving, the end wall 43. Thus, an optical barrier
around the end wall 43 is provided by the optical barrier 61. By making
the opening 71 relatively long, additional widened regions, such as the
region 73 can be provided, if desired, so that a single optical barrier
element 61 can be used for two or more of the calibration reference
apparatuses.
Although exemplary embodiments of the invention have been shown and
described, many changes, modifications and substitutions may be made by
one having ordinary skill in the art without necessarily departing from
the spirit and scope of this invention.
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