Note: Descriptions are shown in the official language in which they were submitted.
This inverltion is related to applicant's Canadian ~,at-
ent dpplicaLions Nos. 232,094, fi1ed July ~3, 1975 ~nd 28~,593
filed September 27, 1977O
The need for a non-invasive technique for measuring
the pressure in body cavities of animals or humans is recognized
as highly desi~able for continuous or in~ermittent moni-toring
of bod~r conditions. Such cavities as the cranium, vena cava,
bladder and others provide valuable and sometimes critical in-
formation for maintaining the well being or survival of an animal
or human. For example, it is known tha-t intracranial pressure
provides a valuable indication of well being for a variety of
clinical conditions, including shock trauma and hydrocephalus.
~ ccordingly, there is a recognized need for a pressure
sensor for continuous or intermittent monitoring of body condi-
tions. In particular, there is a need for a pressure sensor hav-
ing compensation for ambient pressure variations and low sensi-
tivity to temperature changes. Moreover, there is a need for a
sensor which can be calibrated ln iVO and which provides an
output which accurately re~lects the pressure in the animal or
human body.
The pressure sensor of the present invention is fully
implantable and contains output means, such as a radioactive
source and associated shielding, so that the pressure can be
read out non-invasively. In its preferred form, the pressure
sensor includes a housing containing a bellows in communication
with a body pressure sensing tambour placed in the body and ex- ~ -
posed to the pressure to be sensed. An ambient pressure sensing
tambour is also associated with the bellows for compensating for
ambient pressure variations. The housing is located ex-ternal to
the cavity being sensed and preferably situa-ted just under the
skin. The housing also con-tains the output means, which is
associated with the
''
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~6~i2
bellows.
The pressure acting upon the body pressure sensing
tambour causes the bellows to expand and contract. The movement
of the bellows causes the output means to provide an output
which is a function of the pressure such as by causing radiation
shielding to shield a radioactive source as a function of
the pressure sensed. The output is sensed from outside the
body by a receiver such as a conventional nuclear counter or
crystal detector instrument, in case of a radiation output.
The pressure sensor also includes means associated
with the bellows to enable ln vivo calibration of the pressure
sensor after implantation by establishing a preselected output
condition which is initially established during ln vitro
calibration. More specifically, and using a radioactive
source and associated radiation shielding as illustrative, a
stop is provided so that there is a preselected radiation
output which can be used for,calibration. -
The bellows is resilient, made of a material and
shape which has 100~ memory of position, and has a wall thickness
of less than 1 mil in order to provide an output which accurately
reflects the pressure being monitored. The bellows has a spring
constant substantially greater than the spring constant of
the body and ambient pressure sensing tambours, which offer
effectively no resistance to pressure changes, thereby making
the pressure sensor essentially insensitive to temperature
variations.
The pressure sensor is fully implantable and does
not require any energy source other than the radioactive ''
material, for e~:ample, contained in the device. Another major
advantage of the sensor is the elimination of leads or other
penetrations through the skin to provide power or transmit a ' ,
signal. With a long-lived radioisotope, such as promethium-
-3-
~$~
145, carbon-14, nickel-63, strontium-90, or americium-2~1,
the inventive pressure sensor can be fully implanted and
left in place for the life of the patient.
The pressure sensor furlctions accurately to within
several millimeters of water pressure and is unaffected by
variations in ambient pressure. Also, it is generally
insensitive to ambient temperature and can be calibrated in
vivo. Furthermore, the materials used to construct the
sensor are biologically inert and do not pose any health
ha7ard to the animal or human body or make the patient more
susceptible to mechanical trauma. The sensor unit is of
relatively small size and so does not produce unsightly bulging
when implanted subdermially.
Other features and advantages of the invention will be
apparent from the following description of the preferred
embodiments of the invention as shown in -the accompanying
drawings.
Figure 1 is a perspective view of a preferred
embodiment of the non-invasive pressure sensor of the invention
in an installed position for monitoring the pressure in an
intracranial cavity and communicating the monitored pressure
to the exterior of the body,
Figure 2 is a plan view of the apparatus of Figure l;
Figure 3 is a sectional view taken substantially along :
line 3-3 of Figure ~in the direction of the arrows;
Figure 4 is an enlarged sectional view of a portion
of the apparatus of Figure 3;
Figure 5 is an enlarged perspective view of the
mounting arrange:ments for the apparatus of Figure l;
Figure 6 is a perspective view of a portion of the
skull of a patie:nt prior to installation of the apparatus of
the invention; and
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Figure 7 is a sectional view taken substantially
along lines 7-7 of Figure ~ in the direction of the arrows.
Referring now to the drawings and to Figures 1 and 2 in
particular, there is shown the pressure sensor apparatus of
the invention with a housing des:ignated generally by the numeral
10, and a body pressure sensing means designated generally
by the numeral 11 and connected to the housing 10 for sensing
the pressure in a body portion such as a cavity. Ambient pressure
sensing means, designated generally by the numeral 12, is also
50nnected to the housing 10 and is responsive to ambient
pressure to compensate for changes in ambient pressure during
the operati~n o~ the apparatus. Although the pressure sensor
apparatus of the invention is shown in an installed position on
the head of a human body for non-invasively monitoring
intracranial cavity pressure and communicating it to the exterior,
it should be understood that this is only a preferred example
of the invention and that it is equally adaptable for monitoring
pressure in other areas of the body, both animal and human.
Therefore, while the description to follow will be directed
to the use of the invention for monitoring intracranial
cavity pressure, it should be understood that the invention
is equally applicable to monitoring pressure in other body
portions and cavities.
Referring now to Figure 3, and as specifically ~ ;
illustrative of the invention, the housing 10, which is preferably
formed of titanium, is of tubular shape having a side wall
13 defining an interior 1~. A first support member 16, also
preferably formed of titanium, has a central bore 17 positioned
within one end of the housing 10 in sealing relationship
with the housing side wall 13 by means of an epoxy resin or
the like. Preferably, an annular shoulder 13a is formed in
the housing side wall 13 for positioning the support member
-5-
. ~ .
-
~ Q ~ 216 in a precise location within the housing 10 as will be
explained hereinafter. The first support member central bore
17 is provided with a portion 17a of substantially enlarged
diameter defining a recess 20 which communicates with the
interior 14 of the housing 10.
The body pressure sensing means 11 includes a
fluid conduit 22 of deformable metallic material, preferably
titanium, which has been heat treated for deformability, one
end 22a of ~hich is arranged to be connected to the body
pressure sensing device or tambour 23 having an interior 24
which is arranged to be positioned within a body cavity such
as the intracranial cavity of Figure 1.
The tambour 23 is formed of a suitable elastomeric
- ~ ` material such as medical grade Silastic~rubber and is of a
substantially flat configuration including a neck portion 25
in the wall of which iq molded a helical spring 26, preferably
of stainless steel, for imparting rigidity to the neck portion
25. The neck portion 25 includes a central bore 25a which
communicates with the interior 24 of the tambour 23 and
which is arranged to receive the end 22a of the conduit 22
as shown in Figure 2. Preferably, a U-shaped clip 27 of
tantalum or the like is disposed within the interior 24 of
the tambour 23 for maintaining the side walls of the tambour
in spaced-apart relationship and to serve as a locating
means for the tambour with the use of X-rays. The tambour
23, the fluid conduit 22 and the communicating portions of
the housing interior 14, including the recess 20, are filled
with a pressure transmitting fluid through which the pressure ~-
sensed by the t~mbour 23 in the body cavity is transmitted
to the housiny interior 14.
The conduit 22 is press-fitted through the bore 17
with the end 22b of the conduit 22 extending through the bore 17
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-,: - . . . . ~ .
- . - .-: : -
ii5~
into the recess 20 as shown best in Figures 3 and 4. Sealing
engagement between the conduit 22 and bore 17 is obtaine~ by
means of epoxy resin or the like. Thus, the conduit end 22b
communicates with the recess 20 and with the housing interior
14.
The output means of the pressure sensor apparatus
preferably comprises a source 29 of radioactive material,
normally in the form of a shaped article, disposed within the
housing interior 14 together with associated radiation shielding
means designated generally by the numeral 31. Means are provided
in the housing interior 14 for resiliently urging the radioactive
material and radiation shielding into a shielding relationship.
More specifically, resilient means such as a bellows 32 having
an interior 33 is disposed within the housing interior 14, one
end 32a of which is mounted on a necked-down portion 34a of
a second support member 34, preferably formed of titanium,
suitably mounted in the other end of the housing 10 in sealing
engagement with the housing side wall 13 by means of an epoxy
resin or the like. The other end 32b of the bellows 32 is
closed as will be explained hereinafter.
The second support member 34 is provided with a
central bore 38 and the necked-down portion 34a is arranged
to support the bellows end 32a in a sealing relationship
therewith by means of an epoxy resin 41 or the like.
Bellows 32 is made of a resilient material, preferably
a metal such as gold-plated nickel and has essentially 100~
memory of position. The bellows has a wall thickness 32c which
is less than 1 mil and preferably less than about 0.5 mil. The
most preferred wall thickness range is between about 0.25 and
0.33 mil. The spring rate of the bellows is a function of the
wall thickness and the material from which the bellows is made.
For a gold-plated nickel bellows having a wall thickness of about
--7--
0.25 to 0.33 mils, the spring rate of the bellows is in the
ran~e of about 0.130 to 0.240 pound per inch with the preferred
spring rate being about 0.185 pound per inch. The bellows
has a length, typically about 1/2 inch, which varies according
to the pressure which the pressure sensor apparatus is
intended to monitor. By providing the bellows with a wall
thickness of less than l mil and a corresponding spring rate,
the bellows will typically travel (contract or expand) a distance
of about 50 to 60 mils when the body pressure sensing tambour
lQ 23 is exposed to pressure variations such as those normally
found in monitoring intracranial pressure. In this manner,
a relatively large movement (contraction or expansion) of bellows
32 is caused by the pressure being monitored, and therefore
the pressure sensor apparatus is capable of more accurately
monitoring pressure changes in the animal or human body than
conventional pressure sensors.
The central bore 38 of the second support member
34 includes a portion of enlarged diameter forming a recess 42 and
which is provided with an annular, inwardly directed flange 43.
The recess 42 receives the nipple g4 having a central passage
44a of an ambient pressure sensor or tambour 46 forming the
ambient pressure compensating means 12. The tambour 46 is
formed of a flexible material, preferably an elastomeric material
such as medical grade Silastic rubber, and includes an annular
peripheral portion 47 and a recessed central portion 48 defining
an interior 49 extending through the nipple central passage 44a.
A suitable adhesive such as a medical grade Silastic
adhesive, seals the nipple 44 in the recess 42 and the annular
flange 43 compr~esses the outer surface of the nipple 44 to
form a mechanical compression seal to securely retain the
nipple 44 in the recess 42.
A rigid metal tube 51, preferably formed of titanium,
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~ - , . . . . .. . .
is also sealed in nipple 44, such as by a Silastic adhesive,
and extends through nipple central passage 44a and the central
bore 3~ of the second support member 34 into the bellows
interior 33 to communicate the interior 49 of the tambour 46
with the bellows 32. The other end 51b of the tube 51 forms
a stop for the bellows end 32b.
In order to transmit the sensed ambient pressure to
the interior of the bellows 32, the interior of tambour 46, the
bellows interior 33 and the tube 51 are filled with a pressure
transmitting fluid, isolated by means of the bellows 32 from
the pressure transmitting fluid in the body pressure sensing
means 11. In the preferre~ embodiment, all of the exposed
metallic surfaces of the sensing apparatus of the invention
are coated with a suitable biocompatible material, such as a
medical grade Silastic adhesive. As shown in the drawings,
this Silastic adhesive 50 extends from the nipple 44 to tambour
46 to the joint between the metal conduit 22 and the neck
portion 25 of tambour 23.
In the illustrated embodiment, the radiation shielding
means 31 includes a first portion 53 of radiation shielding
material such as tantalum having a cup-shaped configuration.
The first portion 53 preferably includes an end plate 54,
typically in the form of a disc, and an annular side member
56 both mounted on the other end 32b of the bellows 32 in
closing relationship therewith as shown best in Figure 4.
The radiation shielding means first portion 53 is
mounted on an inwardly directed channel portion 57 adjacent
the last accordion pleat in the bellows 32, and a tubular sleeve
portion 58 extends axially outward therefrom which together
define an enclosure 59. The end plate 54 is adhesively secured
in the end of the enclosure by a suitable adhesive such as
an epoxy resin closing the end 32b o~ the bellows 32.
9~ ~'"
Similarly, side member 56 is adhesively secured by means of
an epoxy resin to sleeve portion 58.
The radiation shielding means also includes a second
portion 61 in the form of a tubular sleeve of radiation shielding
material, also preferably made of tantalum, which is press-
fitted or the like within the recess 20 of the first support
member 16. It can be seen that the second p~rtion 61 extends
throughout the depth of the recess 20 and has a forward end
edge portion 61a terminating flush with the end of the first
support member 16 abutting the housing side wall shoulder
13a. Thus, the second portion end 61a is precisely positioned
axially in the housing interior 14 adjacent the end edge
portion 56a of the first portion side member 56.
Radioactive source 29 is mounted on the end 32b of
the bellows 32 and is accommodated for guiding movement within
the radiation shield means second portion 61 disposed in the
recess 20. As will be appreciated, however, the radiation
shielding means rather than the radioactive source 29 may be
mounted on bellows 32, such an arrangement merely representing
an obvious reversal of cooperating parts. The radioactive
source 29, which is preferably of cylindrical shape, has an
outer diameter conforming generally to the inner diameter of
the bellows sleeve portion 58 and is adhesively secured within
the enclosure 59 defined by the tubular sleeve portion 58 by
means of a suitable adhesive such as an epoxy resin. The
end cap 6~ has a meniscus 62a formed by the adhesive material.
The blellows 32 resiliently urges the radioactive
source 29, together with the sleeve portion 58, in the direction
of the arrow I into the recess 20 with the edge portion 56a
of the radiation shielding means first portion side member
56 in adjacent cooperating relationship with the edge portion
61a of the tubu:Lar sleeve forming the radiation shield means
:.
-1 0- ,
.. - .... : : . :
- , ' . , ' ' -: ~
second portion 61 to establish a shielding relationship with
the radioactive source 29. The end cap 62 is therefore
disposed adjacent the outlet end 22b of the pressure transmitting
fluid conduit 22 as will be discussed in more detail hereinafter.
The outer diameter o~ the bellows sleeve portion
58 is selected to produce a loose-fitting relationship with
the inner surface of the sleeve ~Eorming the radiation shielding
means second portion 61 so that fluid introduced into the
recess 20 from the end 22b of conduit 22 may flow freely
therebetween and through a gap between the first and second
end edge portions 56a and 61a, respectively, to fill the interior
14 of the housing 10 on the outside of the bellows 32. It should
be understood that in the assembled apparatus of the invention
before installation in the body there is virtually no pressure
differential in the housing 10 between the pressure-transmitting
fluids on opposite side of the bellows 32. In this condition,
there is a gap as will be discussed in more detail hereinafter
between the adjacent end edge portions 56a and 61a of the
first and second portions 53 and 61, respectively. ~hen the
apparatus is installed in the body, the normal fluid pressure
in the body cavity slightly increases the pressure on the tambour
23 introducing additional pressure transmitting fluid into
the housing interior 14 on the outside of bellows 32, moving
the bellows in the direction of the arrow D, and thereby
increasing slightly the gap between the end edge portions
56a and 61a.
In the operation of the invention after installation,
an increase in body pressure is sensed in the body cavity by
the body pressure sensing device or tambour 23, the sensed
pressure is transmitted by the pressure transmitting fluid
flowing into the support member recess 20 through the end
22b of conduit 22 around the end cap 62 through the gap between
--11--
.
the edge portions 56a, 61a to move the bellows 32 together
with the radiation shielding means first portion 53 and the
radioactive source 29 in the direction indicated by the
arrow D in opposition to the urg:ing force exerted by the
bellows. During this movement, the radiation shielding means
'irst and second portions 53, 61 move apart increasing the
gap proportionally with the increase in cavity pressure thereby
modifying the shielding relationship between the shielding
means 31 and radioactive source 29 to expose more of the
radioactive source in accordance with the magnitude of the
cavity pressure. The radioactive output of the exposed
portion of the radioactive source 29 may then be sensed by a
receiver means (not shown) such as a conventional nuclear
counter or crystal detector disposed externally of the
housing 10 and the body.
The provision of the ambient pressure sensing means
12 permits the pressure sensor apparatus of the invention to
be responsive to pressure changes in the body cavity regardless
of ambient pressure changes. More specifically, ambient pressure
changes are imposed equally on both the ambient pressure sensing
means 12 and body pressure sensing means 11 whereby the sensing
apparatus of the invention responds to body cavity pressure
changes only.
The pressure sensor apparatus of the present invention
is essentially insensitive to ambient temperature variations
as well as temperature variations which may occur inside the
animal or human body whose pressure is being monitored.
More specifically, bellows 32 has a spring constant which is
orders of magnitude greater than the spring constant of pressure
sensing tambours 23 and 46 which offer effectively no resistance
to pressure changes. Accordingly, any volumetric changes in
the pressure transmitting fluid due to temperature variations
.
-12-
- : . . : ~ , :
s~
in the body or ambient temperature changes will act to distend
the body pressure sensing tambour 23 and ambient pressure
sPnsing tambour 46, respectively, instead of causing a movement
(contraction or expansion) of bellows 32. Therefore, temperature
variations whether external or internal to the cavity being
monitored do not affect or change the output of the pressure
sensor apparatus and therefore do not cause erroneous pressure
measurements.
A unique feature of the present invention is the
provision of means to enable in vivo calibration of the
pressure sensor apparatus after implantation by a preselected
output condition which is initially established during in vitro
calibration. More specifically, in accordance with the present
design, and using a radioactive source and associated radiation
shielding as illustrative, a stop is provided so that there
is a preselected output during calibration. This stop is provided
in the embodiment illustrated by the end 22b of fluid conduit
22. After the pressure sensor apparatus is implanted, the
surgeon can calibrate the instrument by pressing on the
ambient pressure compensating tambour 46 which will cause the
pressure transmittin~ fluid in the interior of tambour 46,
the bellows interior 33 and the tube 51 to move bellows 32
and radioactive source 29 in the direction of arrow I so that the
end cap 62 abuts against the end 22b of conduit 22. In this
extreme stop position, there is a fixed and repeatable amount
of radiation emitted from the pressure sensor apparatus.
Because the housing 10 in which the output means
of the pressure sensor apparatus is housed is implanted under
the scalp, for ~e~ample, the scalp or other body tissue
surrounding the housing will attenuate the radiation output
signal as a function of the scalp thickness. Accordingly,
in the laboratory before implantation, the radiation output
-13-
. . .
from the pressure sensor apparatus wi-th bellows 32 and radio-
active source 29 at the e~treme stop position is measured for a
range of simulated scalp thickness, for example 3 mil]imeter, 6
millimeter, and 9 millimeter of simulated scalp thickness. This
measurement is also made over a range of prcssures which corres-
ponds to the pressures which would normallybe encountered in the
particular body cavity being monitored. Then, a family of curves
is produced which correlate the radiation outpu-t with the pres-
sure being monitored for each scalp thickness. After the sensor
is implanted, the surgeon performs the in vivo calibration as
described above by pressing on the ambient pressure compensating
tambour 46 and forcing bellows 32 and radioactive source 29 to
the extreme stop position. The radiation count obtained will
fall on or near one of the family of curves. This curve is then
used in monitoring the pressure.
To insure a long life for the pressure sensing appara-
tus of the invention commensurate with body compatability, it
has been found that specific non-reactive fluids and elastomeric
materials eliminate such reactions. More specifically, the best
results that have been obtained are when the elastomeric material
of the various components are formed, in one example, from a
Silastic type of silicone rubber and the pressure transmitting
fluids are either castor oil or mineral oil between which there
is virtually no chemical or physical reaction thereby insuring
proper functioning of the pressure sensor apparatus throughout its
life. It has also been found that, when the pressure transmitting
fluid is a silicone oil, the outstanding results of the invention
are accomplished when the elastomeric materials are selected
from the group consisting of butyl, neoprene, Buna N (a -trade-
mark) and Viton A (a trademark) rubbers. It should be understoodhowever, that
- 14 -
. . .
other elastomeric ma~erials and fluids perform satisfactorily
but with less desirable results.
One major concern in selecting a fluid is the
osmotic pressure effects produced during implantation. In
order to eliminate these effects, it is preferred that a
simulated cerebrospinal fluid be used as the pressure transmitting
medium, and it may be used with all materials of construction
as it will be compatible with body fluids and will not leak
through the elastomeric materials as a consequence of osmotic
pressure.
In the use of the invention to monitor the fluid
pressure within an intracranial cavity and with reference
to Figures 5-7, the common practice is to provide a burr hole or
aperture 70 within the bony structure of skull 71 overlying
the intracranial cavity through which the metallic fluid
conduit 22 is inserted, the body pressure sensing device 23
being suitably disposed within the intracranial cavity. A
body pressure sensing means such as tambour 23 is normally
positioned subdurally. However, it should be appreciated
that the invention also contemplates positioning the body
pressure sensing means 11 epidurally in which case the shape
and size of the tambour will be appropriately changed. The
housing 10, together with the ambient pressure compensating
device 12, is mounted on the outer surface of the skull 71
under the scalp 72.
The apparatus of the invention includes means for
permanently mounting the housing 10 and ambient pressure
sensing means 12 subcutaneously on the outer surface of the
skull 71 in an inconspicuous, securely retained position.
More specifically, an elongated concave groove 73 is formed
within the outer surface of the skull 71 adjacent the burr
hole 70, and mounting means are provided for securing the
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. : ' :
housing 10 in a seated relationship within the groove 73.
The mounting means includes at least one, preferably two, tabs
74 arranged in longitudinally spaced relationship on the
housing 10 as shown best in Figure 5. Each of the tabs 74
includes an intermediate portion 75 of arcuate cross-sectional
shape for accommodating -the tubular housing 10 in underlying
engagement therewith. The tab portion 76 is secured to the
outer surface of the housing 10 by suitable means such as a
body compatible adhesive, welding or the like. The tabs 74
also include oppositely disposed end portions 77 and 7~
extending laterally outward of the housing 10 secured within
the intermediate portion 76.
Openings 79, 81 are provided in the tab end portions
77, 78 respectively for accommodating screw means such as
screws 82 extending therethrough in threaded engagement with
the underlying bone of the skull 71, and with the end portions
77, 78 in overlying engagement with the outer surface of the
skull 71, the tab intermediate portion 76 and housing 10 being
accommodated within the groove 73.
In the preferred embodiment, the tabs 74 are preferably
formed of a radiation shielding material such as tantalum.
One of the tabs 74 is positioned on the housing 10 with its
intermediate portion 76 extending throughout the path of
movement of the radioactive source 61 within the housing.
Thus, not only does the one tab 74 prevent downwardly directed
radiation into the body, but the radioactive output of source
29 is confined in a non-attenuating manner to the upward
direction to permit easy detection by an externally positioned
detection device.
The output of the radioactive source 29 need only
be of an extremely low order of magnitude, typically less
than 0.1 microcurie, a magnitude far less than that at which
-16-
.
s~
the adjacent body ~issue may be adversely affected. ~owever,
it should be characterized by an extremely precise and uniform
output rate which accurately reflects the changes in fluid
pressure within the body cavity throughout its range of
operation. The preferred radioisotopes used in the present
invention are promethium-145, carbon-14, nickel-63, strontium-
90 and americium-241, and, to obt:ain the proper radioactive
output from the source 29, it should be in the form of a
shaped article of highly homogeneous composition.
The radioactive source 29 typically comprises
promethium-145 chloride (PmC13), for example, uniformly
distributed and absorbed onto an inert carrier such as
diatomaceous earch the uniformly distributed throughout a
suitable binder such as an epoxy resin. Sources 29 of this
composition are extremely uniform regarding the concentration or
distribution of the radioisotope.
Although the invention has been described in terms
of a single preferred embodiment, nevertheless, changes and
modifications may be made within the scope of the invention.
For example, the pressure sensor apparatus as illustrated
provides an output which is a direct function of the pressure
being monitored since the output increases with increasing
body cavity pressure. However, as will be appreciated by
one of ordinary skill in the art, the sensor can also be
constructed so that the output is an indirect function of
the pressure by mounting the radioactive source and the associated
radiation shielding so that the radioactive source is increasingly
shielded b~ the radiation shielding as the pressure being
monitored increases. In this type of arrangement, the ln vivo
calibration is performed in the same manner as described
herein except that the output will be a predetermined maximum
output rather than a predetermined minimum output. Also,
-17-
while a radioactive source and associated radiation shielding
have been illustrated as the output means it will be appreciated
that other output means can be employed without altering the
basic characteristics of the invention. For example, the
output means could comprise a resonant L-C circuit having a
variable capacitor or inductor in which bellows 32 is mechanically
connected to the variable component to vary the value of
capacitance or inductance and hence the resonant frequency
of the L-C circuit in response to the pressure changes in
the body cavity being monitored. The output could then be
detected by a variable-frequency oscillator, for example, or
other means well known in the art. Accordingly, the invention
should not be limited by the specific embodiment illustrated
but only as defined in the appended claims.
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