United States Patent (19)
`Bullara
`
`(54) IMPLANTABLE PRESSURE TRANSDUCER
`Leo A. Bullara, Glendora, Calif.
`(75
`Inventor:
`73) Assignee:
`Huntington Institute of Applied
`Medical Research, Pasadena, Calif.
`(21) Appl. No.: 689,169
`22 Filed:
`May 24, 1976
`51) Int. C.’................................................ A61B5/00
`52 U.S. C. ...................................... 128/2 P; 73/718;
`73/729; 128/2.05 E
`58) Field of Search ............... 128/2.1 A, 2 P, 2.05 P,
`128/2.05 E; 73/398 R,398 C,399, 410;325/67,
`118; 324/40
`
`(56)
`
`3,034,356
`3,135,914
`3,943,915
`3,958,558
`4,022,190
`4,026,276
`4,027,661
`
`References Cited
`U.S. PATENT DOCUMENTS
`5/1962 Bieganski et al. .................... 128/2P
`6/1964 Callan et al. ........................... 324/40
`3/1976 Severson .............................. 128/2 P
`5/1976 Dunphy et al. ...................... 128/2 P
`5/1977 Meyer ............
`... 128/2A
`5/1977 Chubbuck.
`... 128/2 P
`6/1977 Lyon et al. .......................... 128/2A
`OTHER PUBLICATIONS
`Atkinson, J. R., et al., Journ, of Neurosurgery, vol. 27,
`No. 5, 1967, pp. 428-432.
`Medical Engineering, Ch. 15, C. R. Ray, Yearbook Pub
`lishing, Chicago, 1974, p. 156.
`
`11)
`45)
`
`4,127,110
`Nov. 28, 1978
`
`Primary Examiner-Kyle L. Howell
`Attorney, Agent, or Firm-Christie, Parker & Hale
`57
`ABSTRACT
`A wireless, surgically implantable pressure transducer
`for measuring pressure of fluid or tissue in a body cham
`ber such as a brain ventricle of a patient suffering hy
`drocephalus or a head injury. The transducer includes
`an inductor and a capacitor connected in parallel to
`form a resonant L-C circuit. One of these reactive com
`ponents is variable, and a bellows (or similar pressure
`sensitive force-summing device) is mechanically con
`nected to the variable component to vary the value of
`capacitance or inductance and hence the resonant fre
`quency of the L-C circuit in response to pressure
`changes of the fluid in which the bellows is immersed.
`The transducer is electromagnetically coupled to an
`external source of variable-frequency energy such as a
`grid-dip oscillator, enabling external detection of the
`transducer resonant frequency which is a measure of the
`fluid pressure being sensed. An antenna coil is induc
`tively coupled to the transducer inductor, and is posi
`tioned in the transducer to be just beneath the skin when
`the housing is implanted to provide efficient coupling of
`the external oscillator and internal resonant circuit. A
`second bellows or analogous device may be coupled to
`a reference-pressure side of the transducer to provide
`improved compensation of variations in ambient tem
`perature and atmospheric pressure.
`13 Claims, 12 Drawing Figures
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`Abbott
`Exhibit 1017
`Page 001
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`

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`U.S. Patent Nov. 28, 1978
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`Page 002
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`U.S. Patent
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`Exhibit 1017
`Page 003
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`

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`U.S. Patent Nov. 28, 1978
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`Abbott
`Exhibit 1017
`Page 004
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`

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`Abbott
`Exhibit 1017
`Page 005
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`Exhibit 1017
`Page 008
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`

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`U.S. Patent Nov. 28, 1978
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`

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`1.
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`4,127,110
`2
`form is also suitable for mounting on hydrocephalus
`shunt apparatus as often used in treating and controlling
`this disease.
`The transducer is also suitable for implantation else
`where in the body or in other systems, and is believed
`useful in any application where a very small wireless
`device is needed to measure fluid or tissue pressure. For
`example, the transducer is believed useful for either
`short- or long-term monitoring of abnormal intracranial
`pressure in headinjury patients, or for post-surgical
`monitoring of braintumor victims to detect possible
`recurrence of the tumor. When such monitoring is no
`longer needed, the implanted transducer is removed by
`a simple re-opening and closure of the overlying scalp
`tissue.
`
`10
`
`15
`
`20
`
`IMPLANTABLE PRESSURE TRANSDUCER
`BACKGROUND OF THE INVENTION
`Hydrocephalus is a brain condition in which cerebro
`spinal fluid accumulates at abnormally high pressure in
`ventricles or chambers within the brain. The ventricles
`expand in response to the pressure exerted by the fluid,
`and surrounding brain tissue is compressed between the
`ventricles and the skull. Hydrocephalus usually occurs
`in babies or young children, and, if unchecked, results in
`brain damage, enlargement and deformation of the
`head, and eventual death.
`Modern medical methods are effective in arresting
`many cases of hydrocephalus, but it is often desirable to
`monitor pressure of the cerebrospinal fluid over an
`extended period to detect relapse and to determine
`longrange effectiveness of treatment. In the past, this
`measurement has been made by surgically implanting a
`miniature but generally conventional transducer such as
`a straingage-bridge pressure pickup. This technique
`requires that wiring be conducted from the implanted
`transducer to external instrumentation which provides
`excitation voltage to the bridge and detects bridge
`unbalance voltage signals indicative of pressure. Alter
`25
`natively, non-electrical manometric measurement meth
`ods may be used, but these techniques require installa
`tion of a conduit extending from the interior of the brain
`ventricle through the skull and scalp to external mea
`surement equipment.
`30
`The primary disadvantage of these known techniques
`is that they involve conducting an electrical cable or
`fluid tube through the skull and scalp to enable direct
`electrical or mechanical connection between the inte
`rior of the brain ventricle and external equipment. This
`35
`connection is disturbing and uncomfortable for the pa
`tient, and the danger of infection of tissue surrounding
`the cable or tube (and the risk of infection spread result
`ing in meningitis, ventriculitis, brain abscess or septice
`mia) requires constant supervision and usually full-time
`hospitalization of the patient. There is accordingly a
`need for a measuring device which does not require
`direct electrical or mechanical connection from the
`brain to external equipment, and which permits the
`patient to be ambulatory after the device is installed.
`45
`Connection-free implantable transducers have been
`previously proposed, and they typically function by
`external detection of the resonant frequency of a reso
`nant circuit in the implanted device. For example, the
`prior art includes a biological pressure transducer for
`sensing pressure in the gastrointestinal tract and having
`a resonant circuit with a pressure-controlled inductor.
`Wireless systems are also used for sensing EEG or ECG
`voltages, the implantable part of the system using an
`electrically variable capacitor in a resonant circuit. A
`55
`wireless resonant-circuit transducer has also been used
`for measuring intraocular pressure, the transducer using
`a pair of variably spaced Archimedean-spiral coils
`mounted on pressure-sensitive diaphragms.
`The transducer of this invention operates in wireless
`fashion similar to the instruments described above, but
`provides improved performance and lower drift in im
`plantation applications involving placement in body
`cavities such as brain ventricles, bladder, or heart cham
`bers were only a very small transducer can be tolerated.
`65
`The transducer is disclosed below in a specific form
`suitable for intracranial implantation to monitor pres
`sure of cerebrospinal fluid in a brain ventricle. This
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`The invention herein disclosed is an improvement of
`subject matter disclosed in copending U.S. patent appli
`cation Ser. No. 506,217 filed Sept. 16, 1974 by applicant
`and others, now U.S. Pat. No. 3,958,558.
`SUMMARY OF THE INVENTION
`Briefly stated, this invention relates to improvements
`in transducers which are preferably of an implantable
`type having a resonant L-C circuit with capacitor and
`inductor elements, one of which is variable and driven
`by a mechanical force sensor such as a bellows or simi
`lar pressure-sensing means. The circuit has a variable
`resonant frequency related to the magnitude of force
`imposed on the sensor, and this resonant frequency can
`be determined in wireless fashion by an external interro
`gating device such as a variable-frequency grid-dip
`oscillator. In the forms disclosed below, the transducer
`is intended for measuring fluid in a body cavity such as
`cerebrospinal fluid in a brain ventricle, but the trans
`ducer is not limited to this particular application.
`In one form, the transducer inductor element includes
`a coil wound on a hollow tubular mandrel, and a ferrite
`core positioned within the mandrel and driven by the
`force sensor. A coupling coil is wound adjacent the
`inductor coil, and is electrically connected to a re
`motely positioned antenna coil disposed in a portion of
`a housing for the transducer which is close to the pa
`tient's skin when the transducer is used as an implant
`able device. Preferably, the coupling and antenna coils
`have the same number of turns, but the antenna coil is
`larger in diameter than the coupling coil for efficient
`inductive coupling with an external antenna associated
`with the interrogating equipment.
`The transducer may also include a pressure-sensing
`variable-volume means such as a second bellows which
`is in fluid connection with a sealed volume within the
`transducer to define a reference pressure for the trans
`ducer. The purpose of the second bellows is to reduce
`or eliminate transducer errors arising from unwanted
`response to variations in barometric pressure or ambient
`temperature. In one form the two bellows are substan
`tially identical, and are rigidly connected together by a
`shaft or similar means. In another form, the two bellows
`are not mechanically connected, but are of unequal
`diameter and stiffness, the second bellows having the
`larger diameter and lower stiffness.
`These improvements enable the overall transducer
`system to have improved sensitivity and better coupling
`with external equipment, and a significant reduction in
`sensitivity to environmental variations is achieved.
`
`50
`
`Abbott
`Exhibit 1017
`Page 010
`
`

`

`5
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`O
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`3
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is an exploded view of a portion of a pressure
`transducer according to the invention;
`FIG. 2 is a side elevation, partly in section, of the
`assembled transducer;
`FIG. 3 is a side elevation of the transducer mounted
`on a plug for intracranial installation;
`FIG. 4 is a block diagram of external electronic
`equipment used with the transducer; and
`FIG. 5 is a side sectional elevation of a first alterna
`tive transducer according to the invention and using a
`variable inductor.
`FIG. 6 is a side elevation, partly in section, of a sec
`ond alternative transducer subassembly using an an
`15
`tenna coil;
`FIG. 7 is a sectional elevation of the second alterna
`tive transducer subassembly mounted to measure epidu
`ral pressure;
`FIG. 8 is a view similar to FIG. 7, but including a
`20
`ventricular cannula.
`FIG. 9 is a view similar to FIG. 7, but showing the
`transducer subassembly mounted for positioning in a
`ventricle;
`FIG. 10 is a schematic diagram of the measurement
`25
`system;
`FIG. 11 is a sectional elevation of a double-coupled
`bellows version of the transducer; and
`FIG. 12 is a sectional elevation of another style of
`double-bellows transducer.
`30
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`Referring to FIGS. 1-2, a pressure transducer 10
`according to the invention includes a housing 11 which
`35
`is preferably a hollow cylindrical cup of cast epoxy
`resin as sold under the trademark "Hysol'. In a typical
`form, the housing is 0.550-inch long, and has outside
`and inside diameters of 0.275 and 0.260-inch respec
`tively.
`40
`A hollow cylindrical ferromagnetic core 12 is sized to
`make a loose fit within housing 11, and the core is typi
`cally 0.500-inch long, with outside and inside diameters
`of 0.250 and 0.125-inch respectively. The core is prefer
`ably made from a sintered ferrite material such as sold
`45
`by Indiana General Division of Electronic Memories
`Magnetics Corporation as "Q-2 Ferramic' material.
`A coil 13 (FIG. 2) is formed by helically wrapping
`about 10 turns of a conductor such as 0.020-inch-diame
`ter gold wire around the outside surface of core 12.
`Core 12 and coil 13 form an inductor 14 for the trans
`ducer as shown in the electrical schematic in FIG. 4.
`A hollow cylindrical sleeve 17 (FIGS. 1-2) of a non
`ferrous material such as brass forms a fixed electrode of
`a coaxial variable capacitor 18 (FIG. 4) in transducer
`55
`10. The sleeve is sized to make a snug slip fit within core
`12, and is typically 0.400-inch long with outside and
`inside diameters of 0.124 and 0.100-inch respectively.
`A rod or piston 20, having a thin, integrally formed
`and radially extending flange 21 at one end, is also made
`60
`from a non-ferrous material such as brass. A portion of
`the outside of the rod is covered with a thin dielectric
`coating 22 of a material such as tantalum pentoxide. The
`piston fits into sleeve 17 in piston-cylinder fashion, and
`forms a movable element or electrode of coaxial vari
`65
`able capacitor 18. The piston has an overall length of
`about 0.500-inch, and the piston and flange have diame
`ters of about 0.095 and 0.020-inch respectively.
`
`4,127,110
`4.
`A generally cylindrical bellows 24 (FIG. 2) provides
`a force-summing surface for transducer 10, the bellows
`varying in length according to the pressure of fluid in
`which the transducer is immersed. A typical and suit
`able bellows is sold by Servometer Corporation as a
`Type SK4681. The bellows is made of an electrically
`conductive material which is preferably gold-plated
`nickel. The ends of the bellows are open, and each end
`defines an axially extending shell or flange 25.
`To assemble the transducer, capacitor sleeve 17 is
`cemented within core 12, the left ends (as viewed in
`FIG. 2) of these components being flush. The left end of
`coil 13 is drawn around the end of core 12 and soldered
`into electrical contact with the sleeve. Flange 25 at the
`left end of bellows 24 is then slipped over the right end
`of core 12 and cemented in place. The right end of coil
`13 is soldered or otherwise bonded into electrical
`contact with the bellows flange as shown in FIG. 2.
`Capacitor piston 20 is then fitted through the bellows
`into sleeve 17, and flange 21 of the piston is secured
`within flange 25 at the right end of the bellows, the
`attachment being made with an electrically conductive
`cement such as a conductive silver-epoxy or gold-epoxy
`adhesive. The capacitor piston is thus electrically con
`nected to the right end of coil 13 through bellows 24.
`Sealing of the transducer is completed by placing an
`annular body 27 of epoxy resin or a similar sealant be
`tween the right end of housing 11 and the bellows.
`The transducer interior is hermetically sealed from
`the outside environment so fluid cannot seep into the
`bellows or variable coaxial capacitor. Preferably, the
`transducer is evacuated prior to final sealing, and is
`back-filled with dry nitrogen. Back-filling is normally
`done at one atmosphere of pressure to provide a trans
`ducer which functions as a 'sealed gage pressure' mea
`suring device, but other pressures may be used if a refer
`ence pressure other than one atmosphere is preferred.
`Preferably, housing 11 is sheathed in a covering 29 of
`a biologically compatible material such as plastic sold
`under the trademark "Silastic'. In a preferred embodi
`ment, covering 29 is extended to form a loose balloon
`like enclosure 30 around bellows 24, and enclosure 30 is
`filled with distilled water 31, or preferably with a fluid
`which approximates the composition of the fluid being
`monitored (such as Elliot's 'B' solution when cerebro
`spinal fluid is being monitored) to provide a correct
`ionic balance on both sides of the enclosure. Pressure of
`the fluid being monitored is transmitted through enclo
`sure 30 and water 31 to actuate bellows 24, but the
`enclosure and water form a chemical and mechanical
`buffer preventing tissue encroachment which could
`interfere with free compression and extension of the
`bellows.
`When used as an intracranial implant in a brain ventri
`cle or in brain tissue, transducer 10 is preferably
`mounted on a flanged plug 33 of a material such as
`'Silastic' plastic. Surgical installation of this equipment
`involves generally the same procedures used in install
`ing hydrocephalus shunts or pressure absorbers, these
`procedures being briefly discussed in U.S. Pat. No.
`3,583,387-Garner and Bullara titled "Pressure Absorb
`ing Appliance for Treating Hydrocephalus'.
`The values of inductance and capacitance of the
`parallelconnected inductor and capacitor of transducer
`10 can be computed and pre-determined using known
`engineering formulae. Circuits having nominal resonant
`frequencies in the range of about 30 to perhaps 100
`megaHertz are believed best suited for biological appli
`
`50
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`Abbott
`Exhibit 1017
`Page 011
`
`

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`10
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`15
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`25
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`20
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`4,127,110
`6
`5
`0.005-inch-diameter copper or gold insulated wire. The
`cations. Higher frequencies (e.g., 200 mHz) have some
`ends of the coil are fed through a pair of longitudinal
`advantages, but low "Q's' typically experienced in
`slots 44 at one end of sleeve 41 for connection to a
`tissue at these frequencies tend to obscure the accurate
`miniature fixed capacitor 46 mounted on a wall 47
`external detection of resonance of the transducer L-C
`which closes one end of the sleeve. The coil and capaci
`circuit.
`tor are preferably "potted' in a medical-grade paraffin
`Pressure range of the transducer is determined pri
`marily by the mechanical performance of bellows 24,
`(not shown).
`A bellows 48 (generally corresponding to bellows 24
`and these displacement-versus-pressure characteristics
`described above) is fitted over and secured to the open
`can also be calculated by known engineering formulae.
`end of sleeve 41. A solid cylindrical ferrite core 49 is
`Typical units which have been tested had an operating
`positioned within sleeve 41 to form an inductor with
`pressure range of 0 to 1000 millimeters of water (gage),
`coil 43. A stiff metal shaft 50 (preferably a length of
`and the transducer L-C circuit has been designed to
`stainlesssteel tubing of about 0.009-inch outside diame
`have a zero-pressure resonant frequency of about 82
`ter as used in hypodermic needles) is secured to the core
`mHz. As the fluid pressure is increased, bellows 24
`and extends therefrom through a central opening 51 in
`contracts to drive capacitor piston 20 into sleeve 17,
`the closed end of bellows 48. During assembly of the
`thereby increasing the capacity of the coaxial capacitor
`transducer, the "zero' position of the core is adjusted to
`and decreasing the resonant frequency of the circuit. A
`provide a desired inductance of the coil and core, and
`change in resonant frequency of about 20 mHz is typi
`shaft 50 is then permanently secured to the bellows to
`cally obtained in driving the transducer from zero to
`support the core.
`full-scale pressure.
`A cup-shaped housing 53 made of medical-grade
`In use, the installed transducer is irradiated with elec
`acrylic plastic is slipped over and secured to sleeve 41.
`tromagnetic energy transmitted through the body and
`An enclosure 54 is fitted over and sealed to the open end
`generated by an external variable-frequency oscillator.
`of housing 53, and this enclosure is preferably a mem
`Some of this radio-frequency energy is absorbed (and
`brane of "Silastic' plastic material. The space between
`also reflected or retransmitted) by the resonant circuit,
`the outer surface of the bellows and the inner surfaces
`depending on how close the incident frequency is to the
`of the membrane and housing is filled with distilled
`resonance frequency of the circuit. The frequency of
`water or a fluid compatible with the characteristics of
`the external oscillator is varied or swept until resonance
`the fluid being monitored as described above.
`of the transducer L-C circuit is externally detected.
`The dimensions of housing 53 are about 0.165-inch
`This resonant frequency is in turn indicative of the
`diameter by 0.445-inch length, and a very compact
`internal fluid pressure being sensed by the transducer.
`assembly is provided which is suitable for implantation.
`A simple and accurate way to detect internal trans
`A nominal resonant frequency of about 80 mHz is pro
`ducer resonance with an external circuit involves use of
`vided by using a capacitor of 5 picofarads and an induc
`a griddip oscillator 35 (FIG. 4) which shows a sharp
`tance of about 0.8 microhenries. Installation and use of
`drop or "valley” in grid current when the resonant
`transducer 40 corresponds to the procedures discussed
`point of the "receiving' circuit is swept through by the
`above with respect to transducer 10.
`"transmitting' oscillator. The oscillator is preferably
`A more sensitive embodiment of the invention in
`used in conjunction with a conventional electronic fre
`cludes a transducer subassembly 68 in FIG. 6, and is
`quency counter which provides a direct visual readout
`characterized by the use of an antenna coil 70 con
`of frequency at the resonant point.
`nected to a coupling coil 71 which is inductively cou
`External phase-sensitive equipment can also be used
`pled to an inductor coil 72 wound on an open-ended
`to detect the characteristic and marked phase shift
`mandrel or hollow sleeve 73. A solid cylindrical ferrite
`which occurs when the resonant circuit receives energy
`core 74 is movably positioned within sleeve 73 to form
`at its resonant frequency. Other external detection sys
`a variable inductor with coil 72. A capacitor 75 is con
`tems are discussed in the literature, such as IEEE
`45
`nected across inductor coil 72 to provide a variable
`Transactions on Bio-Medical Engineering, Volume
`resonant-frequency L-C circuit.
`BME 14, No. 2, pages 74-83, April 1967, and the refer
`A pressure-sensitive bellows 77 is secured and sealed
`ences therein cited.
`over an open end of sleeve 73, and a shaft 78 is secured
`Prior to installation, the transducer is calibrated by
`immersing it in a fluid (e.g., Elliot's 'B' solution) having
`to the closed distal end of the bellows and to core 74.
`50
`Motion of the bellows in response to pressure changes is
`characteristics similar to the biological fluid or tissue to
`thus transmitted to the core to vary the inductance and
`be eventually monitored. The pressure of the test fluid is
`the resonant frequency of the L-C circuit as already
`then varied under controlled conditions while the reso
`nant frequency of the transducer is tracked as described
`described.
`An acrylic-plastic shield sleeve 79 may be fitted over
`above to develop a pressure-versus-frequency calibra
`and secured to the bellows end of sleeve 73 to extend
`tion curve.
`over the side surfaces of the bellows to prevent tissue
`The transducer of this invention can also be made
`interference with the bellows. Holes (not shown) may
`with a variable-reactance element which is a coaxial
`be formed in the sidewall of the shield sleeve to insure
`variable inductor connected across a fixed capacitor, or
`both the capacitive and inductive components can be
`free fluid circulation around the bellows. The distal end
`60
`of the shield sleeve is open, and the inner wall of the
`variable under control of the pressure-sensitive bellows.
`shield sleeve is radially spaced from the bellows side
`A variable-inductor embodiment of the invention is
`wall to insure free movement of the bellows responsive
`shown as a transducer 40 in FIG. 5.
`to pressure changes.
`Transducer 40 includes a cup-shaped hollow cylindri
`The L-C circuit components with bellows 77 and
`cal coil-supporting sleeve 41 which is preferably made
`65
`of polytetrafluorethylene plastic or a medical-grade
`sleeve 73 (and optionally sleeve 79, depending on the
`application) form transducer subassembly 68 which can
`acrylic plastic. The sleeve has an annular recess 42 in
`be mounted in several ways depending upon the geome
`which is wound an inductive coil 43 of say 12 turns of
`
`30
`
`35
`
`55
`
`Abbott
`Exhibit 1017
`Page 012
`
`

`

`5
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`10
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`15
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`4,127,110
`8
`7
`henries and 2 picofarads, providing a nominal resonant
`try of the space in which fluid pressure is to be mea
`frequency of about 140 mHz.
`sured. In an epidural configuration 81 as shown in FIG.
`The transducer assembly and housing is surgically
`7, the transducer subassembly is seated in a hollow
`installed as discussed above to be fully implanted with
`dome-shaped housing 83 which includes a base 84 and a
`out need for wired or tubing connections to external
`cover 85.
`equipment. When used to measure intracranial pres
`Base 84 is a flat circular plate of an implantable mate
`sures, a small hole is formed through the skull, and the
`rial such as acrylic plastic, and an opening 86 is formed
`bellows 'shank' of the transducer is fitted into the skull
`in the center of the plate. A tubular extension 87 is
`opening. Base 84 and domed cover 85 are seated against
`integrally formed with the plate around opening 86, and
`the skull, and the overlying scalp is then sutured to
`the extension defines an inwardly extending annular
`close the incision and seal the transducer assembly
`flange 88 against which hollow sleeve 73 of the trans
`under the skin. Interrogation of the transducer to detect
`ducer subassembly is seated. An annular groove 90 is
`the resonant frequency is made with a grid-dip oscilla
`formed in the sidewall of base 84, and antenna coil 70 is
`tor or other sensing devices such as a pulse generator
`wound in this groove. The ends of the antenna coil are
`and phase-shift detector.
`fed through a shallow clearance groove 91 in the base to
`The antenna and coupling coils in effect form an
`extend over the base to the transducer subassembly into
`inductive link between the transducer subassembly and
`connection with coupling coil 71.
`an external transmitting-antenna coil of a grid-dip oscil
`Cover 85 is generally circular in planform, and is
`lator (illustrated schematically in FIG. 10) used as al
`inwardly concave to receive the transducer subassem
`20
`ready described to sense the resonant frequency of the
`bly. An annular shoulder 92 is formed in the undersur
`L-C circuit which is in turn indicative of the fluid pres
`face of the cover, and base 84 is seated against this
`sure being sensed. The relatively large diameter of an
`shoulder. The cover, base, and transducer subassembly
`tenna coil 70 provides good inductive coupling with the
`are preferably cemented together, and an epoxy coating
`transmitting-antenna coil of the external grid-dip oscil
`is preferably provided along the entire undersurface of
`25
`lator, and the small diameter of coupling coil 71 insures
`the base and dome to resist fluid leakage from the space
`good coupling with inductor coil 72. The result is a
`in which the transducer is immersed. Cover 85 is prefer
`significant increase in overall system sensitivity and
`ably formed from acrylic plastic.
`"Q' or sharpness of the resonant frequency of the L-C
`A thin and flexible cap-shaped diaphragm 95 of "Si
`lastic' plastic or a comparable sheet material surrounds
`circuit.
`30
`The epidural configuration just described is readily
`the transducer bellows, and is secured in place by a
`modified to measure intraventricular fluid pressure by
`clamping ring 96 cemented to the housing base. The
`adding a hollow elongated cannula 105 as shown in
`space between the bellows and diaphragm is filled with
`Elliot's 'B' solution or a comparable fluid as explained
`FIG. 8. One end 106 of the cannula is fitted over and
`secured to a tubular extension 107 of a hollow annular
`above.
`adapter plate 108 cemented and sealed to the floor of
`The interior of the transducer assembly is vented
`through the open inner end of hollow sleeve 73 to be in
`base 84 and cover 85. Holes 110 in the distal end of the
`cannula permit cerebrospinal fluid in the ventricle to fill
`communication with the sealed interior volume of hous
`the interior of the cannula, the fluid pressure thus being
`ing 83. A relatively large reference-pressure volume is
`sensed by bellows 77 of transducer subassembly 68.
`thus provided for the transducer. A small threaded hole
`An equally important feature of the antenna and cou
`99 may be formed through base 84 to vent this volume
`pling coils is that positioning of the transducer subas
`during assembly, and the opening is subsequently closed
`sembly within the body can be varied without affecting
`with a screw 100 which is sealed with cement with the
`performance of the transducer system. For example, it
`reference pressure stabilized at say one atmosphere.
`may be desirable to position the transducer subassembly
`In a typical configuration, inductor coil 72 is about 12
`45
`deeply into a brain ventricle at a significant spacing
`turns of 40-gage enameled copper wire wound in a
`from the scalp. The antenna coil, however, remains
`single layer on a mandrel outside diameter of 0.109 inch.
`positioned immediately below the scalp for good cou
`Coupling coil 71 is two turns of 32-gage enameled cop
`pling to the grid-dip oscillator, and the wires connect
`per wire wound on the same mandrel diameter immedi
`ing the antenna and coupling coils are simply length
`ately adjacent the end of the inductor coil closest to the
`50
`ened as necessary to accommodate the spacing of the
`bellows. This arrangement positions the coupling coil
`transducer subassembly from base 84 and cover 85.
`around movable ferrite core 74 throughout the full
`An example of this arrangement is shown in FIG. 9
`range of movement of the core, whereby the inductance
`which depicts a transducer subassembly 68 arranged to
`of the coupling coil is substantially unaffected by core
`measure pressure directly in a cavity remote from the
`movement. The core extends only partially into induc
`55
`skin. A tubular boss 115 is formed on a modified housing
`tor coil 72 because a significant change in inductance is
`base 116 having a central opening 117. An extension
`desired to insure an easily measurable shift in resonant
`tube 118 is secured to boss 115 and a generally cylindri
`frequency of the L-C circuit in response to bellows
`cal transducer housing shell 120 is secured at the distal
`movement arising from pressure changes.
`end of the extension tube. The outer surface of sleeve 73
`Antenna coil 70 is matched to coupling coil 71 with
`of the transducer subassembly is cemented in sealed
`respect to number of turns, and is hence typically two
`fashion to the inner surface of housing shell 120. Elon
`turns of the same 32-gage wire used to form the cou
`gated wires 122 extend through extension tube 118 to
`pling coil. The antenna coil, has a significantly larger
`connect coupling coil 71 with antenna coil 70 which is
`diameter than that of the coupling coil, and the diameter
`wrapped around the periphery of base 116. This ar
`of the base of groove 90 (forming a mandrel for the
`65
`rangement is advantageous in that fluid pressure is mea
`antenna coil) is typically about 0.778 inch. In a unit
`sured in the area of interest, and the measurement is
`which has been tested, the nominal inductance and
`unaffected by the position of the patient's body (i.e., the
`capacitance values of the L-C circuit were 0.65 micro
`
`35