U.S. patent application number 12/615774 was filed with the patent office on 2010-03-04 for apparatus for and method of producing an ultrasonic signal.
Invention is credited to Ran Yaron.
Application Number | 20100057066 12/615774 |
Document ID | / |
Family ID | 23339698 |
Filed Date | 2010-03-04 |
United States Patent
Application |
20100057066 |
Kind Code |
A1 |
Yaron; Ran |
March 4, 2010 |
APPARATUS FOR AND METHOD OF PRODUCING AN ULTRASONIC SIGNAL
Abstract
An ultrasonic actuator comprises a miniature ultrasonic
transducer powered by electromagnetic radiation. The ultrasonic
transducer comprises a housing defining a chamber. A liquid mass
oscillates within the chamber at a frequency within the ultrasonic
range. To cause this oscillation, a source of electromagnetic
radiation energizes the liquid mass by exposing a portion of the
liquid mass to electromagnetic radiation. The source of
electromagnetic radiation thus drives the liquid mass at a
frequency within the ultrasonic range.
Inventors: |
Yaron; Ran; (Boulder,
CO) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
23339698 |
Appl. No.: |
12/615774 |
Filed: |
November 10, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11210215 |
Aug 23, 2005 |
7615048 |
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12615774 |
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10328380 |
Dec 19, 2002 |
6949094 |
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11210215 |
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60341952 |
Dec 19, 2001 |
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Current U.S.
Class: |
606/21 |
Current CPC
Class: |
A61B 18/02 20130101;
A61B 2018/0262 20130101; A61B 2018/0212 20130101 |
Class at
Publication: |
606/21 |
International
Class: |
A61B 18/02 20060101
A61B018/02 |
Claims
1-45. (canceled)
46. An ultrasonic actuator comprising: a housing defining a
chamber; a liquid mass positioned to oscillate within the chamber
at a frequency within the ultrasonic range; and a source of
electromagnetic radiation energizing the liquid mass by exposing a
portion of the liquid mass to electromagnetic radiation, the source
of electromagnetic radiation driving the liquid mass at said
frequency.
47. An ultrasonic actuator as in claim 46, wherein the liquid mass
is disposed between a pair of gas springs.
48. An ultrasonic actuator as in claim 46 additionally comprising:
an elongated body defined between a proximal end and a distal end;
and a waveguide for conducting electromagnetic energy, the
waveguide extending from the proximal end of the elongated body to
the distal end and cooperating with a source of electromagnetic
radiation to provide electromagnetic radiation to drive the liquid
mass.
49. The ultrasonic actuator as in claim 46, whereas the housing is
configured to generator ultrasonic wave propagations.
50. The ultrasonic actuator as in claim 49, wherein the housing
cooperates with a ultrasonic receiver, which is part of an imaging
system.
51. The ultrasonic actuator of claim 46 in combination with a
medical apparatus having an elongated body.
52. A method of producing a ultrasonic signal comprising the steps
of: (a) providing a liquid mass within a chamber of a housing; (b)
exposing a portion of the liquid mass to electromagnetic radiation;
(c) vaporizing at least a portion of the liquid mass to propel the
liquid mass in a first direction; (d) redirecting the liquid mass
in a second direction that is generally opposite the first
direction; (e) repeating at least steps (b) through (d) to cause
the liquid mass to oscillate at an ultrasonic frequency; and (f)
propagating the ultrasonic frequency along a propagation path.
Description
RELATED APPLICATION
[0001] The present application is based upon and claims priority
under 35 U.S.C. .sctn.119(e) to U.S. Provisional Application No.
60/341,952, filed Dec. 19, 2001, entitled LASER REFRIGERATOR, and
U.S. patent application Ser. No. 10/328,380, filed Dec. 19, 2002
entitled MINIATURE REFRIGERATION SYSTEM FOR CRYOTHERMAL ABLATION
CATHETER; and U.S. patent application Ser. No. 11/210,215, filed
Aug. 23, 2005 entitled ENGINE WITH LIQUID PISTON, which are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present application relates to cryothermal ablation
catheters, as well as to miniature refrigerators and miniature
engines for producing mechanical force or effecting mechanical
work.
[0004] 2. Description of the Related Art
[0005] Ablation catheters are commonly used to treat arrhythmias by
destroying or disrupting cardiac tissue associated with the source
of the arrhythmias or their conductive pathways. At present, most
ablation procedures are performed using an ablation catheter in
which radio frequency (RF) current is passed through tissue
contacting the catheter tip to create lesions by means of
hyperthermia. Use of such RF current involves risk of char and
coagulum formation, particularly if the lesions created are more
extensive than focal lesions, such as may be required for
circumferential lesions in the pulmonary veins. Formation of char
and coagulum is ordinarily caused by poor tissue contact with the
catheter tip and creates an undesirable risk of thromboembolic
stroke. Other risks of RF ablation include possible unroofing of
the endothelium or producing pulmonary vein contraction.
[0006] Cryothermal ablation solves many of the problems associated
with RF ablation. Destruction of tissue by freezing leaves the
connective tissue matrix intact. Lesions are created by rupturing
cell membranes, and damaged cells are replaced by fibrotic tissue.
There is no formation of char or coagulum, and thus the risk of
thromboembolic stroke is low. Additionally, as the tissue is
cooled, the catheter tip adheres to the tissue which provides
improved stability.
[0007] Although cryothermal ablation provides many advantages over
RF ablation, it has proven difficult to implement. Cryocatheters
today typically comprise a cooling system that provides cooling
power by pumping a vaporized refrigerant through a lumen in the
catheter to a Joule-Thomson expander located at the catheter distal
end. The length of the catheter (often 1 meter or longer) and the
small diameter of the lumen within the catheter (often less than 1
mm in diameter) limit the flow rate through the Joule-Thomson
expander. Additionally, the pressure of the vapor returning through
the catheter must be held under 1 atmosphere to meet FDA
requirements, which further limits the flow rate through the
Joule-Thomson expander. The cooling power of the system
consequently is limited to about 2 Watts, which limits the depth of
tissue ablation to about 4 mm.
[0008] Accordingly, there is a need in the art for a cryocatheter
that does not require transport of refrigerant along the length of
the catheter so as to permit increased refrigerant flow rates. At a
more fundamental level, there is a need for a miniature engine that
can be adapted to, among other things, drive a miniature
refrigeration system in the tip of a cryocatheter.
SUMMARY OF THE INVENTION
[0009] The preferred embodiment of the present invention overcomes
disadvantages of conventional cryocatheters by housing a miniature
refrigerator in the tip of a cryocatheter and powering the
refrigerator using electromagnetic radiation delivered through a
waveguide. A summary of a preferred embodiment is provided followed
by a summary of inventive aspects.
[0010] The preferred miniature refrigerator avoids the need to
transport refrigerant through the length of the cryocatheter by
housing the refrigerant circulation system entirely in the tip of
the catheter. It utilizes a unique engine that is a revolutionary
breakthrough in miniaturization. The preferred engine harnesses the
power of a laser by converting electromagnetic energy into
mechanical work. It provides an enormous gain in delivered power
per unit volume compared to other power sources such as electric
motors. Conversion of optical energy to mechanical energy is
accomplished by directing laser energy through a gas spring onto a
free surface of a liquid mass to non-uniformity heat the liquid
mass. The heating is very rapid (e.g. <100 nsec) such that the
portion of liquid exposed to radiation quickly reaches its
superheat limit and explosively boils. The explosion propels the
remaining portion of the liquid (which functions as a piston) to
adiabatically compress refrigerant in a compression chamber. The
compression results in a pressure increase, thus providing a
restoring force which pushes the liquid towards its original
position and against the gas spring, which allows the original
position to be overshot. At the point of maximum displacement, the
laser is fired again and the cycle repeats. The inertia of the
liquid and the compression of the vapor cause the device to
function as an oscillator which possess a natural frequency. The
energy lost during each oscillation is replenished by tuning the
repetition rate of the laser pulses to the natural frequency. By
firing the laser at (or just after) the point of maximum
displacement, resonant operation is established, and the
oscillations will persist.
[0011] Inventive aspects associated with the embodiments described
herein are abundant. In one such inventive aspect, a cryo-medical
apparatus comprises an elongated body defined between a proximal
end and a distal end. A closed-cycle miniature refrigeration unit,
which includes a compressor and at least a first heat exchanger, is
disposed at the distal end. A waveguide for conducting
electromagnetic energy extends from the proximal end of the
elongated body to the distal end. The waveguide provides
electromagnetic radiation to drive the compressor.
[0012] In another aspect of the invention, a cryo-medical system
comprises a cryo-apparatus which includes an elongated body defined
between a proximal end and a distal end. A closed-cycle miniature
refrigeration unit, which includes a compressor in at least a first
heat exchanger, is disposed at the distal end. A waveguide
extending from the proximal end of the catheter body to the distal
end conducts electromagnetic energy to drive the compressor. A
coupler coupled the source of electromagnetic radiation to the
waveguide.
[0013] A further aspect of the invention is a closed-cycle
miniature refrigeration system comprising a compressor having a
housing defining at least one chamber. A liquid piston is
positioned to reciprocate within the chamber. A source of
electromagnetic radiation energizes the liquid piston by exposing a
portion of the liquid piston to electromagnetic radiation. The
source of electromagnetic radiation drives the liquid piston to
reciprocate within the chamber such that the liquid piston
compresses a working fluid. A heat exchanger is in communication
with the compressor.
[0014] Yet another aspect of the invention is a medical apparatus
having an elongated body defined between a proximal end and a
distal end. An engine is disposed within the elongated body and
preferably at the distal end of the elongated body. The engine
includes a housing defining a chamber and a liquid mass position
within the chamber. A waveguide extends from the proximal end of
the elongated body to the distal end and conducts electromagnetic
radiation such that the liquid mass is heated non-uniformly.
[0015] An additional aspect of the invention is an engine
comprising a housing defining a chamber. A liquid mass is
positioned to oscillate within the chamber at a frequency. A source
of electromagnetic radiation energizes the liquid mass by exposing
a portion of a liquid mass to the radiation. The radiation causes
the liquid mass to be driven at the frequency of oscillation.
Preferably, the frequency of oscillation is a natural frequency of
the liquid mass in the housing.
[0016] In yet another aspect of the invention, an engine comprises
a housing defining a chamber. A liquid mass is disposed within the
chamber. The source of electromagnetic radiation energizes the
liquid mass by exposing a portion of the liquid mass to the
electromagnetic radiation. A gas spring is disposed within the
chamber and within a propagation path of the electromagnetic
radiation.
[0017] A further aspect of the invention is an engine which
comprises a housing defining a chamber which includes first and
second end sections and an intermediate section. A liquid mass is
disposed within the chamber. Each of the first and second end
sections of the chamber is formed of a material having a low
affinity for the liquid of the liquid mass, and the intermediate
section is formed of the material having a higher affinity for the
liquid of the liquid mass. A source of electromagnetic radiation
heats a portion of the liquid mass.
[0018] In an additional aspect of the invention, a method of
oscillating a liquid mass within a housing comprises converting a
portion of the liquid mass to a gas phase to propel the remainder
of the liquid mass within the housing. A substantial portion of the
gas phase portion is reconverted back to the liquid phase, and the
converting and reconverting are repeated to cause the liquid mass
to oscillate.
[0019] A further aspect of the invention is a method comprising
oscillating a liquid mass within a housing by converting
electromagnetic energy into mechanical work and heat. The
oscillations are stabilized by removing heat such that the
oscillations reach steady state.
[0020] Another aspect of the present invention involves an engine
comprising a housing having chamber wall that defines a chamber
within the housing. A liquid piston is disposed within the chamber
and has at least one free surface not in contact with the chamber
wall. A source of laser energy is positioned to directly heat the
free surface of the liquid piston. The engine also includes a gas
spring and a spring mechanism. The gas spring is disposed within
the chamber adjacent the free surface of the liquid piston and
within the propagation path of the laser energy. The spring
mechanism is also positioned within the housing and is arranged to
exert pressure on another surface of the liquid piston. Preferably,
the gas spring and spring mechanism are symmetrically disposed
relative to the liquid piston.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] These and other features, aspects and advantages of the
present invention will be further understood with reference to
preferred embodiments, which are illustrated in the accompanying
drawings. The illustrated embodiments are merely exemplary and are
not intended to limit of the scope of the present invention. The
drawings of the illustrated embodiments comprise 24 figures.
[0022] FIG. 1 is schematic illustration of a cryothermal ablation
system including a cryocatheter, which is configured in accordance
with a preferred embodiment of the present invention.
[0023] FIG. 2 is an enlarged sectional schematic view of a distal
end of the cryocatheter.
[0024] FIG. 2A is a cross-section of the cryocatheter taken along
line 2A-2A of FIG. 2.
[0025] FIG. 3 is a block diagram illustrating the components of a
closed loop refrigeration system disposed at the distal end of the
cryocatheter.
[0026] FIG. 4A is an enlarged perspective view of a heat exchanger
of the refrigeration system that is configured in accordance with a
preferred mode of the refrigeration system.
[0027] FIG. 4B is an exploded perspective view of etched foils of
the heat exchanger of FIG. 4A in an unformed, pre-assembled
state.
[0028] FIG. 5A is an enlarged perspective view of another heat
exchanger of the refrigeration system that can be used in the place
of the heat exchanger illustrated in FIG. 4A. In particular, FIG.
5A illustrates a stacked etched-disk heat exchanger configured in
accordance with another preferred mode of the refrigeration
system.
[0029] FIG. 5B is a partially exploded, cross-sectional view of the
heat exchanger of FIG. 5A taken along line 5B-5B. For illustration
purposes only, FIG. 5B shows two disks of the stacked as spaced
apart from the body of the stack to illustrate the cross-section of
an individual disk and to illustrate the structural identicalness
between the disks in the stack.
[0030] FIG. 5C is a cross-sectional view of the heat exchanger of
FIG. 5A taken along line 5C-5C and illustrates an annular face of a
disk in the stack.
[0031] FIG. 6A is a schematic view of the distal end of the
cryocatheter and schematically illustrates a compressor engine of
the refrigeration system.
[0032] FIG. 6B is a schematic cross-sectional view of the distal
end of the cryocatheter taken along line 6B-6B of FIG. 6A and
schematically illustrates the construction of the catheter proximal
of the compressor engine. Only those lumens associated with the
compressor engine have been illustrated.
[0033] FIG. 6C is a schematic cross-sectional view of the distal
end of the cryocatheter taken along line 6C-6C of FIG. 6A and
illustrates the construction a central part of a housing of the
compressor engine.
[0034] FIG. 6D is a schematic cross-sectional view of the distal
end of the cryocatheter taken along line 6D-6D of FIG. 6A and
illustrates the construction of a valve mechanism of the compressor
that is configured in accordance with a preferred mode of the
refrigeration system.
[0035] FIG. 7A is a sectional view that is similar to that of FIG.
6D and illustrates the construction of another valve mechanism that
can be used with the compressor.
[0036] FIG. 7B is a cross-sectional view of the valve mechanism
taken along line 7B-7B of FIG. 7A.
[0037] FIG. 7C is an enlarged cross-sectional view of a jet valve
to illustrates a variation of a valve design for the valve
mechanism illustrated in FIGS. 7A and 7B.
[0038] FIG. 7D is an enlarged cross-sectional view of a vortex
valve to illustrate another variation of a valve design for the
valve mechanism illustrated in FIGS. 7A and 7B.
[0039] FIG. 8 is a schematic illustration of the engine that is
used with the compressor in the refrigeration system and that is
constructed in accordance with a preferred embodiment of the
present invention.
[0040] FIGS. 9A through 9D are schematic sectional views of the
compressor engine of FIG. 8 shown at four different stages of an
operation cycle.
[0041] FIG. 10A is a schematic illustration of a compressor engine
configured in accordance with another embodiment of the present
invention.
[0042] FIG. 10B is a cross section of the compressor engine of FIG.
10A taken along line 10B-10B.
[0043] FIG. 10C is a plan view of a distal plate of the compression
engine of FIG. 10A as viewed in the direction of section 10C-10C,
and illustrates a valve mechanism that regulates fluid flow into
and out of the compressor engine.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] A miniature engine is particularly well suited to function
as a compressor in conjunction with a miniature refrigeration
system and, in particular, in conjunction with a miniature
refrigeration system employed with a cryoablation catheter. The
illustrated preferred embodiments are thus of a cryoablation
system. The miniature engine, however, can be used in a variety of
other applications including, for example, but without limitation,
micro-actuators (e.g. linear actuators), micro-pumps (e.g., for
drug delivery), micro-acoustical generator (e.g., an ultrasound
transducer), micro-injectors (e.g., for inkjet and fuel injection
applications), and optical switches. Additionally, the described
miniature refrigeration system can be used in other applications as
well, such as, for example, but without limitation, in the cooling
of high power density electronics, solid-state lasers, IR sensors
and similar devices. The following description of the preferred
embodiments thus represents only one possible application of the
engine described herein in a biomedical application relating to
cryoablation of cardiac tissues for the treatment of
arrhythmias.
Cryoablation System
[0045] As seen in FIG. 1, the cryothermal ablation system 100
includes a source of electromagnetic radiation 102, a cryocatheter
104, and a source of fluid coolant 106. The cryocatheter 104 has a
proximal end 108 and a distal end 110 and includes a miniature,
closed-loop refrigeration system 112 disposed within the catheter
104. In the illustrated embodiment, the refrigeration system is
disposed at a distal end 110 of the catheter 104. A waveguide 114
within the catheter 104 couples the source of electromagnetic
radiation 102 to the refrigeration system 112 so as to power the
refrigeration system 112. The coolant source 106 provides coolant
to the refrigeration system 112 to remove heat build-up from
components of the system 112 and from the refrigerant circulating
just at the distal end of the catheter 104.
[0046] In the illustrated embodiment, a laser provides laser light
to power the refrigeration system 112. Other sources of
electromagnetic radiation, however, can be used in other
embodiments and applications. For example, electric discharge,
microwave or X-ray radiation can be used to power the refrigeration
system.
[0047] The source of coolant 106 preferably provides a cooled or
chilled liquid coolant (e.g., saline), which flows through at least
the distal end 110 of the catheter 104 when the catheter 104 is
connected to the coolant source 106. The coolant source 106 can
either forms a closed-loop cooling system with the cryocatheter 104
or an open-loop cooling system with the cryocatheter 104. In the
illustrated embodiment, a closed-loop system is formed with the
coolant circulating between the catheter 104 and the coolant source
106. For this purpose, the coolant source 106 preferably comprises
a heat exchanger in order to control the temperature of the coolant
entering the catheter 104. The coolant, however, need not be
chilled to temperatures required for cryoablation. The coolant
rather functions to remove heat from components of the miniature
refrigeration system 112, as will be described in greater
detail.
[0048] The ablation system 100 can also include a controller 116
that controls the operation of the laser 102 and the coolant supply
106 in response to manual control, as well as possibly in response
to one or more feedback signals from various sensors and monitors
used in combination with or integrated into the cryoablation system
100. For example, various thermocouples and mapping electrodes can
be incorporated onto the distal end 110 of the cryocatheter 106, as
known in the art, in order to provide temperature information, to
assess ablation efficacy, and to locate foci of arrhythmia prior to
ablation.
Cryocatheter
[0049] As seen in FIG. 1, the cryocatheter 104 includes a handle
120 having a proximal end 122 and a distal end 124, and is
configured to be comfortably held by a practitioner during a
treatment procedure involving cryoablation. A plurality of
conduits, conductors, and wires extend from the proximal end of the
handle 120 for connection to the laser 102, the coolant supply 106
and the controller 116. In the illustrated embodiment, a plurality
of conduits 126 connect the coolant supply 106 to the catheter
handle 120, a wiring harness 128 connects the handle 120 to the
controller 116, and an optical coupler 130 couples the waveguide
114 to the laser 102. These conduits, conductors, waveguide and
wires extend through the handle 120 to the handle's distal end
124.
[0050] An elongated, flexible catheter body or shaft 132 extends
from the distal end 124 of the handle 120. The catheter 104
preferably has a sufficient length to be introduced into the heart
(e.g., into the left atrium or right ventricle) through a
percutaneous translumenal procedure. Moreover, for certain
applications, the cryocatheter 104 can be designed to access the
left atrium in a transeptal procedure. Accordingly, the distal
portion of the catheter body 132 is preferably flexible; however,
the proximal portion of the catheter body 132 can be more rigid, as
known in the art.
[0051] In the illustrated embodiment, the catheter 104 includes a
deflectable distal segment 134 in order to steer the catheter and
to position the cryoablation element(s) of the catheter 104. For
this purpose, the catheter 104 includes one or more pull wires that
extends through the catheter body 132 from the handle 120 and that
are affixed to one or more locations at the distal segment 134. The
pull wire or wires are connected at their proximal ends to a manual
controller 136, such as a thumb lever. The thumb lever 136 when
moved tightens the pull wires to deflect the distal segment 134 of
the catheter body 132, as well known in the art. In this manner,
the practitioner can steer the catheter 104 through the vascular
structure and introduce the catheter into one of the heart chambers
(e.g., into the left atrium with the aid of a transeptal
sheath).
[0052] Other positioning mechanism, however, can be used with the
catheter 104, either as an alternative to or in addition to the
pull wire steering mechanism. For example, the catheter can be
slidably coupled with a guidewire and, for this purpose, can
include a guidewire lumen that extends at least a substantial
length of the catheter. The catheter can be slidably coupled to the
guidewire externally of the patient's body in a "back-loading"
technique after the distal end of the guidewire is first positioned
at the target site. Other guidewire tracking designs may also be
suitable substitutes, such as, for example, catheter devices known
as "rapid exchange" or "monorail" variations wherein the guidewire
is only housed within a lumen of the catheter in a distal region of
the catheter. Additionally, the catheter (or guidewire) can be
guided to and positioned at the target site by the use of
sub-selective sheaths for advancing the guidewire and/or catheter
into the desired location and position within the heart.
[0053] The waveguide 114 extends through the catheter body 132 from
its proximal end to the distal segment 134 of the catheter and
cooperates with the refrigeration system 112. The waveguide 114 in
the illustrated embodiment comprises an optical fiber that has a
core and a cladding; however, other types of waveguides (both solid
and hollow) can be used in other applications. Advantageously, the
optical fiber 114 carries optical energy the length of the catheter
104 with minimal attenuation.
[0054] The catheter body 132 houses the waveguide 114 and includes
a plurality of lumens. Each lumen extends from a proximal port at
the proximal end of the catheter body to a distal port located at
or near the distal end of the catheter. Some or all of the lumens
can be arranged in a side-by-side relationship, in a coaxial
relationship or in any of a wide variety of configurations that
will be readily apparent to one of ordinary skill in the art.
[0055] With reference to FIGS. 2 and 2A, a first lumen 138 in the
illustrated embodiment delivers coolant to the distal end 110 and a
second lumen 140 returns the coolant to the coolant source 106.
Neither of these lumens 138 140 need to be significantly insulated
because the coolant supplied and returned through these lumens has
a temperature well above that capable of damaging tissue or blood
cells. A third lumen 142 houses the pull wire.
[0056] Additional lumens can be provided for additional purposes.
For example, an additional lumen can be arranged adjacent to or
coaxially about the waveguide 114 and connected to a supply of
cooling fluid, such as gas, in order to cool the waveguide. In
addition to or in the alternative to such cooling fluid, the first
and second lumens 138, 140, which carry the liquid coolant, can be
used to cool the waveguide. Further lumens can carry electrical
wires or other conductors that are connected to sensors (e.g.,
thermocouples, thermisters, mapping electrodes, ultrasound imaging
transducers, etc.) disposed at the distal end 110 of the catheter
104. A lumen(s) can also be provided to provide an inflation medium
in variations of the cryocatheter that include one or more
inflatable balloons. Aspiration, irrigation, and perfusion lumens
can similarly be incorporated into the catheter body. Accordingly,
in addition to the illustrated lumens, one or more additional
lumens or conduits can be provided for additional connections to
the distal end 110 of the catheter 104.
[0057] The catheter body 132 accordingly includes a number of
internal components housed within the internal structure of the
body and can also include various layers over the internal
structure. Any of a variety of different polymeric materials, which
are known by those of skill in the art to be suitable for catheter
body manufacture, can be used to form the catheter body 132. For
example, the body 132 may be formed out of polymers such as
polyethylene, PEBAX (Atochem, France), polyimide, polyether
etherketone, and the like. Additionally, the catheter body 132 can
also includes a biocompatible, leak-proof outer jacket formed of
any of a variety of materials, such as, for example, but without
limitation, nylon, PEBAX, Teflon, or other suitable plastic or
polymer materials, as well known to those skilled in the art. The
catheter preferably is made in accordance with known manufacturing
techniques.
[0058] The catheter body 132 also preferably has sufficient
structural integrity, or "stiffness," to permit the catheter 104 to
be pushed through the vasculature to target site without buckling
or undesirable bending of the body 132. It is also desirable,
however, for the body 132 to be fairly flexible near its distal end
110, so that the distal segment 134 of the catheter 104 can be
navigated through tortuous blood vessel networks. Thus, in one
preferred embodiment, the body 132 of the catheter 104 is formed
from a polymer such as polyethylene or PEBAX made to have variable
stiffness along its length, with the proximal portion of the body
being less flexible than the distal portion of the body.
Advantageously, a body of this construction enables a user to more
easily insert the tubular body into vascular networks.
Additionally, the catheter body, or at least certain sections
thereof, can be include reinforcing braid or coil incorporated into
its wall structure. The reinforcing can be formed of metal or of
various polymers.
[0059] As seen in FIG. 2, the distal end 110 of the illustrated
catheter 104 has a heat transfer element 144 with a blunt contact
tip. The distal end 110, however, can have other configurations.
For example, the distal end 110 can have a shaped tip design, such
as, for example a loop design that can be expanded or manipulated
as known by those of ordinary skill in the art. A shaped stylet can
also be used with the catheter to vary the shape of the distal end
110 of the catheter in order to hold a particular shape during an
ablation procedure to ablate a desired pattern (e.g., arcuate or
circular), or for steering or positioning purposes. Additionally,
the distal end 110 can include a plurality of heat transfer
elements that the refrigeration system 112 cools.
Refrigeration System
[0060] With reference to FIGS. 2 and 3, the closed-cycle
refrigeration system 112 includes at least a compressor engine 150
and a heat exchanger to cool the heat transfer element(s) 144 at
the distal end 110 of the catheter 104. In the illustrated
embodiment, the refrigeration system also comprises a second heat
exchanger, an expander 152, and preferably a third heat exchanger.
One of the heat exchangers functions as a condenser 154, another
heat exchanger functions as an evaporator or boiler 156, and the
third heat exchanger functions as a counter-flow heat exchanger
158. A refrigerant circulates through the closed-cycle system.
[0061] The compressor engine 150 draws in saturated refrigerant
into the compressor 150 from an inlet side of the compressor engine
150. The compressor engine 150 compresses the vapor isentropically
to a superheated vapor, which then flows to the condenser 154 on an
outlet side of the compressor engine 150. The refrigerant vapor
then enters a condenser 154, where heat is removed at constant
pressure until the fluid becomes a saturated liquid. The liquid
then passes through the high temperature side of the counter-flow
heat exchanger 158 and into the expander 152. The liquid expands
adiabatically in order to bring the fluid to a lower pressure. The
liquid refrigerant thence passes through the evaporator 156 at a
constant pressure. Heat flows into the evaporator 156 from the heat
transfer element(s) 144 (FIG. 2) and vaporizes the fluid to the
saturated-vapor state for reentry into the compressor 150. In
particular, the liquid absorbs heat from an inner surface of the
heat transfer element 144, thereby cooling the outer surface and
vaporizing the liquid refrigerant within the evaporator 156. The
fluid enters the low temperature side of the counter-flow heat
exchanger 158 before it enters the compressor 150. The counter-flow
heat exchanger 158 is used to cool further the liquid refrigerant
before it enters the expander 152 and to heat the vapor before it
returns to the compressor 150.
[0062] The condenser 154 receives coolant from the catheter coolant
delivery lumen 138 and discharges it to the compressor 150 for
cooling purposes or returns it to the coolant return lumen 140.
Alternatively, the condenser 154 can receive coolant from the
compressor 150.
[0063] The expander 152 can include one or more valves, orifices,
capillary tubes or similar types of flow restrictions. In one
preferred mode, the expander 152 is a Joule-Thomson expansion
device.
[0064] Each of the heat exchangers--the condenser 154, the
evaporator 156, the counter-flow heat exchanger 158--can be
manufactured as miniature structures with high surface area using
photoetching technology as taught by U.S. Pat. No. 5,935,424, the
disclosure of which is hereby incorporated by reference. An example
of a suitable heat exchanger structure for the counter-flow heat
exchanger 158 is illustrated in FIGS. 4A and 4B while, in FIGS. 5A
through 5C, an example of the condenser 154 is illustrated. The
size, shape and relative scale of the illustrated heat exchangers
are only by way of examples, and the heat exchangers can be
configured and constructed to meet specific design requirements,
e.g., to fit within a distal end of a catheter having an overall
outer diameter of 3 mm. Additionally, either of the illustrated
heat exchanger structures can be used to form the condenser 154,
the evaporator 156 or the counter-flow heat exchanger 158. The
illustrated heat exchanger structures also allow for the
integration of two or more of the refrigerator's heat exchangers
154, 156, 158 into a single structural unit (e.g., the stacked disk
structure described below can be configured so as to form both the
condenser and the counter-flow heat exchanger).
[0065] With reference to FIGS. 4A and 4B, the counter-flow heat
exchanger 158 is formed by at least two foil sheets 160, 162. A
discharge from the condenser 154 is schematically illustrated as
connecting to an inlet 164 of a microchannel 166 that is etched
onto the first sheet 160. The second sheet 162 has a similar
microchannel 168 etched onto it, with an inlet 170 and an outlet
172. The inlet 170 communicates with the evaporator 156 and the
outlet 172 communicates with the inlet side of the compressor
engine 150. The microchannels 166, 168 preferably are etched only
halfway through the respective foil sheets 160, 162. The foil
sheets 160, 162 can be joined (e.g., diffusion bonded) together in
the orientation shown. The assembly then can be rolled, as shown in
FIG. 4A, to construct a cylindrical heat exchanger.
[0066] FIGS. 5A through 5C illustrate a variation of the
counter-flow heat exchanger 158. In this embodiment, the heat
exchanger 158 comprises a plurality of stacked disks 174.
Preferably, at least most of the disks 174 have the same
configuration, and end caps (not shown) close the end disks in the
stack. Each disk 174 includes a plurality of annular ribs 176 that
are concentrically arranged, as best seen in FIG. 5C. A plurality
of openings 178 are disposed between each pair of adjacent ribs
176. When the disks 174 are stacked and joined together, as seen in
FIG. 5B, the stacked assembly forms four annular flow channels
180a, 180b, 180c, 180d. In each flow channel 180a-d, the fluid
flows through the disk openings 178 and then into an annular space
defined between adjacent disk ribs 176 (which ribs 176 may be of
the same disk 174 or of the adjacent disk 174 depending upon the
flow direction). The ribs 176 and the openings 178 preferably are
formed on and through the disk 174, respectively, by photo-etching,
laser-drilling, EDM (electrical discharge machining) and/or similar
processes.
[0067] When this heat exchanger structure is used as the condenser
154, the inner channel 180a preferably carries incoming coolant
from the coolant lumen 138, and the adjacent channel 180b delivers
high pressure refrigerant from the compressor 150. The next channel
180c carries the returning coolant, which is delivered either to
the compressor 150 for cooling purposes or to the coolant return
lumen 140. The outermost channel 180d returns low pressure vapor to
the compressor 150. Of course, where this heat exchanger structure
is used for other purposes, for example, as the counter-flow heat
exchanger, the disk stack can define fewer channels (e.g., two
channels).
[0068] While the structure of the evaporator 156 can take the form
of either of the heat exchanger embodiments just described, its
structure preferably corresponds to the configuration of the heat
transfer element(s) 144 that contacts the targeted tissue during a
cryoablation procedure. The evaporator 156 thus is preferably
configured to maximize contact between the microchannels that form
the evaporator 156 (and through which refrigerant passes) and the
inner surface or surfaces of the heat transfer element(s) 144.
[0069] With reference to FIG. 6A, the compressor engine 150 of the
system 112 includes a housing 182 that defines a chamber 184 and a
liquid piston 186 that reciprocates within the chamber 184. In the
illustrated embodiment, the chamber 184 has a cylindrical shape;
however, other shapes are practicable. While the engine 150 can be
employed on larger scales, the inside diameter of the cylindrical
chamber 184 for its application in a catheter is preferably between
about 50 .mu.m and 5 mm, more preferably less than about 2 mm, and
most preferably generally not greater than 1 mm. The small diameter
cylinder 184 also provides a capillary action to help maintain the
integrity of the liquid piston 186 during operation.
[0070] The cylinder chamber 184 has sufficient length to
accommodate the piston 186 and to provide for its reciprocation in
the chamber 184. The cylinder chamber length preferably provides
the piston 186 with a sufficient stroke for the compressor engine
150 to compress and pump an amount of refrigerant necessary to cool
the heat transfer element(s) 144 to a desired temperature (e.g., to
-100.degree. C.) and to actuate the valves of the compressor. For
the present application in a cryocatheter, the length of the engine
chamber 184 is preferably less than 10 mm, and more preferably less
than about 5 mm.
[0071] The housing 182 preferably is constructed to cause the
liquid piston 186 to migrate toward a generally central position
within the chamber 184 when the engine is not operating.
Accordingly, different parts of the housing walls preferably
exhibit different affinities for the liquid of the liquid piston
186. In the illustrated embodiment, the housing comprises at least
three parts that define the cylinder chamber 184: a central part
188 formed by a tube having high affinity for the liquid of the
liquid piston 186; a proximal part 190 formed by a tube having low
affinity for the liquid; and a distal part 192 formed by a tube
also having a low affinity for the liquid. Either the material of
the tubes or coatings on the tubes can have the desired affinities
for the liquid.
[0072] The proximal part 190 and the distal part 192 are preferably
made of thermally insulating material with an inner surface having
a low affinity for the liquid, resulting in close to adiabatic
compression and expansion of the vapor in those chambers. One
suitable material is polytetrafluoroethylene (PTFE), available
commercially as Teflon.TM. from E.I. du Pont and Nemours and
Company. The central part 188, in addition to having a high
affinity for the liquid, preferably is made of a thermally
conductive material, such as, for example, copper.
[0073] The ends of the cylinder 184 also preferably have low
affinities for the liquid of the liquid piston 186. The proximal
end of the cylinder 184 preferably is closed by the distal end of
the optical fiber 114 or by a lens (e.g., a collimating lens) or an
intermediate transmitter that directs the laser light into the
chamber 184 through the proximal end. In the illustrated
embodiment, the distal end of the optical fiber 114 seals against
the housing 182 at the proximal end thereof. The housing can
additionally comprise a window element. In this variation of the
housing construction, the window element would be in communication
with the optical fiber and would seal the proximal end of the
chamber.
[0074] In the illustrated embodiment, a distal disk or plate 194
closes the distal end of the chamber 184. The distal plate 194
includes a valve mechanism 196 that selectively permits the
refrigerant to flow into and out of the chamber 184. FIGS. 6A and
6D illustrates a compressor engine 150 in which one-way check
valves 198a, 198b serve as the inlet and outlet to the chamber 184.
A suction valve 198a permits refrigerant to flow from the
evaporator 156 into the distal space of the chamber 184, and a
discharge valve 198b permits refrigerant to flow from the distal
space toward the condenser 154. Neither valve 198a, 198b, however,
permits flow in an opposite direction. The valve plate 194
preferably is formed of a superelastic, shape memory material, such
as Ni--Ti alloy, available commercially as Nitinol.TM.. The valves
198a, 198b are etched in the desired configurations. The surface of
the distal plate 194 that faces into the chamber preferably is
coated with a material that has a low affinity for the liquid of
the liquid piston 186.
[0075] FIGS. 7A and 7B illustrate another form of a one-way or
check valve that employ no moving parts. Through well known
principles of fluid dynamics, either a jet valve (FIG. 7C) or a
vortex valve (FIG. 7D) can also provide only one-way flow from and
to the compressor.
[0076] As best seen in FIG. 8, which illustrates the engine in
isolation, the resulting affinity of the liquid piston 186 to
central part 188 of the cylinder creates spaces 200, 202 on the
proximal and distal sides of the liquid piston 186, respectively.
One or more gases occupy the proximal space. The gas can be
substantially pure vapor of the fluid used for the piston or can be
a different fluid. By selecting the type of gas present in the
proximal space, the gas spring can have a linear (or close thereto)
spring constant or a non-linear spring constant. In the illustrated
embodiment, such gas or gases preferably include air and/or a vapor
form of the liquid that forms the liquid piston. The laser light
passes from the fiber optic 114 through gas and into the liquid
rather than directly from the fiber optic 114 into the liquid.
Consequently, it is preferable that the gas or vapor be
substantially transparent to the laser radiation.
[0077] The volume of the proximal space 200 is on the same order of
magnitude as the volume of the liquid piston 186. In the
illustrated embodiment, the proximal space 200 has a diameter of
about 1 mm and a length of about 1 mm.
[0078] In the illustrated embodiment, the distal space 202
functions as a variable-volume compression chamber that increases
and decreases in volume as the piston 186 reciprocates within the
chamber 184. The valve mechanism 196 regulates refrigerant flow
into and out of the distal space 202.
[0079] The gas-filled proximal space 200 functions as a gas spring.
A gas spring is also formed by the combination of the distal space
202 and the condenser 154 and the evaporator 156 that communicate
with the distal space 202. The inertia of the liquid piston 186 and
the compression of the gas springs 200, 202 constitute the typical
components of an oscillator: the system 150 posses a well-defined
natural frequency and is therefore capable of operating at
resonance if excited at the right frequency. Consequently, in the
present application, the liquid piston can be conceptually modeled
as a mass disposed between a pair of springs. This system thus will
have a natural frequency (f.sub.n), which can be approximated by
equation 1:
f n = 1 2 .pi. ( P o L liq .rho. ) ( L gas L gas 1 L gas 2 ) [ 1 ]
##EQU00001##
[0080] where:
[0081] P.sub.o is the system average pressure;
[0082] L.sub.liq is the length of the liquid piston 186;
[0083] .rho. is the density of the liquid of the liquid piston
186;
[0084] L.sub.gas is the combined length of the two gas springs 200,
202;
[0085] L.sub.gas1 is the length of proximal gas spring 200; and
[0086] L.sub.gas2 is the length of the distal gas spring 202.
[0087] While in the illustrated embodiment, the distal gas spring
202 is disposed on the distal side of the piston 186, other types
of spring mechanism can also be used. For example, as illustrated,
an elastic diaphragm (see FIGS. 10A and 10B and associated
description provided below) can replace the distal gas spring of
the present embodiment.
[0088] As seen in FIG. 8, the housing 182 also includes a cooling
jacket 204 to cool at least the central part 188 of the housing
182. In the illustrated embodiment, the cooling jacket 204 includes
a one or more microchannels cut into the central part 188,
preferably using an etching technique. The cooling jacket 204
receives coolant (e.g., saline) either from the catheter coolant
delivery lumen 138 or from the condenser 154 and returns it to the
coolant return lumen 140. To further facilitate removal of heat
from the engine 150, the central part 188 of the housing 182
preferably is formed of a material having a relatively high heat
transfer coefficient.
[0089] Laser light energy pulses are delivered via the optical
fiber 114 to the free surface at the proximal end of the liquid
piston 186. For this purpose, the waveguide can either include: (1)
a focusing lens that focuses the light beam to a diameter
substantially matching the diameter of the chamber 184 at a
location near (but distal of) the proximal end of the chamber 184;
or (2) a collimating lens that aligns the beam emitted from the
distal end of the optical fiber 114, which has a core diameter
substantially equal to the diameter of the chamber 184. In this
manner, the laser energy is directed to heat generally the entire
area of the free surface that faces the optical fiber 114.
[0090] The liquid absorbs sufficient laser energy to superheat
(instantly vaporize) the liquid to a depth of at least 0.1 of the
wavelength of the laser light. The absorption characteristics of
the liquid material and the high energy density of the laser are
such that the absorption results in rapid formation of a
superheated layer which converts liquid into gas. As the liquid is
vaporized, the liquid beneath is exposed to the laser light and the
superheated layer effectively migrated further into the liquid
piston 186 (like the sparks of a burning fuse migrating along the
length of the fuse). The migration of the superheated layer is
extremely fast such that the vaporized portion of the piston 186
rapidly increases the pressure within the proximal space in a
manner akin to an explosion. While vaporization is rapid, the
duration of vaporization is limited by the duration of the laser
pulse. Accordingly, only a small fraction of the liquid piston 186
is vaporized by any given laser pulse. The vaporized portion
preferably represents between about 0.05% and 5% of the liquid
piston 186 by volume, and more preferably between about 0.1% and 1%
by volume. The remaining portion of the piston 186 (still in liquid
phase) is sufficiently long to serve as a piston and to perform
mechanical work (e.g., compress the fluid in the distal space).
Typically, the length of the vaporized portion of the liquid piston
186 may be greater than 50 .mu.m, more preferably between 0.5 and 5
mm, and most preferably about 1 mm.
[0091] Similarly, the liquid piston 186 should have a diameter
sufficient to perform its function. In general, the greater the
diameter of the liquid piston 186, the more power can be produced
by the engine. At some point, however, increasing the diameter of
the liquid piston 186 will lead to loss of capillary action,
depending upon the surface tension of the liquid and the affinity
of the central part 188 therefor, leading to loss of the liquid
piston's integrity during operation. The liquid piston 186
preferably behaves generally as a "plug flow" with a defined
boundary layer around its periphery. The thickness of the boundary
layer will depend upon the liquid's density and viscosity and upon
the system's frequency, as understood from the following
equation:
.lamda. = 2 .mu. .omega. .rho. [ 2 ] ##EQU00002##
[0092] where:
[0093] .lamda.=thickness of boundary layer
[0094] .mu.=viscosity of the liquid
[0095] .omega.=2.pi. times the system's frequency (e.g., the
natural frequency (see Equation [1]))
[0096] .rho.=density of the liquid
[0097] The boundary layer in the illustrated embodiment has a
thickness .lamda. on the order of fractions of microns.
Consequently, the piston 186 oscillates generally as a mass
plug.
[0098] In the illustrated embodiment, where the distal space
directly communicates with the heat exchangers of the refrigeration
system 112, the liquid of the liquid piston 186 preferably is the
same refrigerant used in the refrigeration system. In one preferred
mode, the refrigerant liquid comprises R-134a, having the chemical
formula C.sub.2H.sub.2F.sub.2, a critical temperature of
101.2.degree. C. and an estimated practicable superheat limit of
about 64.degree. C. (based upon 90% of the critical temperature in
Kelvin), as compared to its normal boiling point of -26.5.degree.
C. under standard conditions. In another preferred mode, the
refrigerant liquid comprises R-12, having the chemical formula
CCl.sub.2F.sub.2, a critical temperature of 112.degree. C. and an
estimated practicable superheat limit of about 74.degree. C. (based
upon 90% of the critical temperature in Kelvin), as compared to its
normal boiling point of -30.degree. C. under standard conditions.
Additionally, the refrigerant can comprise a mixture of fluids such
as R-134a, R-23, R1-4, and cryogenic fluids such as helium,
hydrogen, neon, nitrogen, and argon. The refrigerant mixture allow
the refrigerator to reach temperature as low as 70.degree. K as
taught in U.S. Pat. No. 5,579,654, entitled CRYOSTAT REFRIGERATION
SYSTEM USING MIXED REFRIGERANTS IN A CLOSED VAPOR COMPRESSOR CYCLE
HAVING A FIXED FLOW RESTRICTION, which disclosure is hereby
incorporated by reference.
[0099] A dye preferably is added to the liquid to increase
absorption of the input optical energy. To facilitate high
absorption at 1.064 .mu.m for use with a Nd:YAG laser, one of the
following near infrared (NIR) dyes can be added to the liquid:
SDA8080 and DSB6592, both available commercially from H.W. Sands
Corp., of Jupiter, Florida. The concentration of the dye in the
refrigerant can be tailored to match the required optical density
(e.g., 50 .mu.m optical density).
[0100] The laser 102, which supplies the energy to drive the
compressor engine 150, produces short pulses having a duration of
less than 100 nanoseconds, and preferably about 50 nanoseconds to
ensure rapid formation of the superheated layer and resulting gas
bubble. The frequency of the laser pulses substantially matches the
natural frequency of the liquid piston 186, which is, in turn, a
product of the speed of explosive vaporization and the size and
mass of the liquid piston and gas springs, as described above. Heat
introduced into the oscillating system by the laser is removed by
the cooling jacket described above.
[0101] In the illustrated embodiment, the laser 102 is a Q-switched
solid state Nd:YAG laser that outputs 35 Watts of optical power at
a wavelength of 1.064 .mu.m. Although the preferred embodiment
utilizes a solid-state Nd:YAG laser, other types of lasers can be
used, including laser diodes and gas lasers. The Nd:YAG laser
provides pulses at a repetition rate of about 20 kHz to oscillate
the liquid piston 186 at its natural frequency. The energy provided
per pulse preferably is about 1.75 millijoules. The energy density
preferably is sufficient to vaporize during a single pulse
substantially the entire area of proximal liquid surface
(approximately 0.8 mm.sup.2) to a depth 50 .mu.M, starting from a
liquid temperature around ambient (e.g., body temperature:
37.degree. C.). When using a Nd:YAG laser, the vaporized layer is
preferably between 10 nm and 100 .mu.m, and more preferably between
1 .mu.m and 50 .mu.m.
[0102] The explosion pushes the liquid piston 186 distally. The
liquid piston 186 rebounds, moves proximally, rebounds again, and
then is driven distally again by re-firing the laser 102. With
correct dimensional design and operational conditions (laser pulses
synchronized with piston oscillation), undesired losses due to
vapor-liquid heat and mass transfer through the liquid-vapor
surfaces can be minimized and engine efficiency maximize.
[0103] If the laser pulses are all at the same energy, the
oscillations amplitude will start small and within few oscillations
(about 5 to 10) will reach steady state level. The exact number of
oscillations to full amplitude is also influenced by heat removal
characteristics and other thermophysical characteristics of the
system. In the preferred embodiment, the first pulse has higher
(from 2 to 5 times greater) energy then the following pulses, which
helps the system reach full scale oscillations quicker.
[0104] The operation cycle of the engine running at steady state
can be further understood by examining four sequential snapshots
during the operation cycle. With reference to FIG. 9A, the liquid
piston 186 is disposed at a generally central location within the
chamber 184 and is moving proximally at this point in the cycle for
reasons that will be soon apparent. The suction valve 198a in the
distal plate 194 opens as the piston 186 moves proximally. This
movement of the piston 186 also draws refrigerant vapor into the
distal space 202 from the evaporator 156.
[0105] As seen in FIG. 9B, the laser 102 is fired when the liquid
piston 186 reaches it maximal displacement in the proximal
direction. The laser light, which is delivered by the optical fiber
114, passes through the proximal vapor space 200 and is absorbed in
the proximal free surface of the liquid piston 186, which heats the
liquid non-uniformly (i.e., the electromagnetic radiation
superheats a layer of the liquid without significantly heating the
adjacent portion of the liquid mass). The heating of the liquid is
too fast to allow normal boiling and about 50 .mu.m on the surface
is vaporized by heating to the liquid superheat limit. In the
illustrated embodiment, the vaporized layer preferably represents
about 1% the liquid piston volume. Within one microsecond the
superheated layer causes vaporization to create a large pressure
rise in the proximal space. The explosive bubble following
superheating thus provides a propulsive force to move the
unvaporized remainder of the liquid piston 186.
[0106] Under the action of the high pressure in the proximal space
200, the liquid piston 186 starts moving distally, as seen in FIG.
9C. During the piston's distal travel (as well as during its
proximal travel), the liquid mass exhibits a plug flow profile with
a defined boundary layer around the perimeter, as noted above.
Cohesive forces (e.g., viscosity), as well as its cooled
temperature, tend to keep the liquid piston 186 as one continuous
unit that generally moves as a monolith, thereby acting similar to
a solid piston.
[0107] Distal movement of the piston 186 compresses the refrigerant
vapor in the distal space 202 adiabatically (similar to a
conventional positive displacement vapor compressor). The increased
pressure in the distal chamber 202 closes the suction valve 198a
and opens the discharge valve 198b. At least part of the kinetic
energy of the moving piston 186 is returned to the piston 186 by
elastic expansion of the distal gas spring 202, causing the liquid
piston to move in the proximal direction. The resultant restoring
force helps to push the liquid piston 186 toward its original
position.
[0108] Additionally, once the piston 186 has reached the point of
its maximum displacement distally, as shown in FIG. 9D, the piston
186 moves proximally. The work of expansion of the proximal chamber
200 and the condensation of refrigerant vapor on the wall of the
cooled central part 188 of the housing 182 causes a pressure
decrease which also has the consequence of imparting velocity to
(i.e., draws) the liquid piston 186 in the proximal direction. Due
to the inertia of the liquid piston 186, however, the original
position of the piston is overshot and the piston 186 moves toward
its maximum displacement in the proximal direction. The laser 102
once again is fired and the cycle repeats.
[0109] In the preferred embodiment, heat is actively removed from
the engine 150 to maintain the body of the liquid piston 186 below
its boiling point and to allow the explosively vaporized portion of
the fluid to return to the liquid state, serving as a reusable fuel
for continued operation. As noted above, the central part 188 of
the housing 182 is preferably formed from a material that is a good
conductor of heat, so as to provide a heat sink. The heat sink is
constructed to have a large surface area and is preferably further
cooled by a coolant (e.g., saline) that flows in or about the
central part 188. The coolant readily removes heat from the heat
sink by forced convection. With water microchannels, forced
convection can remove heat at 800 W/cm.sup.2, permitting continual
operation at high power. The cooling system removes both the heat
generated by the laser beam and the heat carried by the refrigerant
pumped through the refrigeration system. According to The Second
Law of Thermodynamics, the refrigerator rejected heat is at least
its refrigeration power multiplied by the Carnot ratio of its
operating temperatures. Stable pressure oscillations are achieved
when the total heat from the laser beam and from the refrigerator
(heat pump) section is balanced by the heat drawn out of the engine
by the coolant flow. The coolant flow of the illustrated embodiment
is capable of removing over 100 Watts of heat. The heat transfer
element 144 at the catheter distal end 110 can consequently reach
-100.degree. C. and can produce cardiac tissue necrosis to a depth
of greater than 10 mm.
[0110] The combined duration of the laser pulse and the explosive
boiling is less than 2% of each cycle, and more preferably between
0.01% and 1% of each cycle. With the solid-state Nd:YAG laser in
the illustrated embodiment, the period of the cycle is on the order
of 50 microseconds, while the combined duration of the laser pulse
and the explosive boiling (which occurs during the laser pulse) is
in the order of 200 nanoseconds. The relatively long time period of
the cycle, in comparison to the pulse duration, permits the system
to react and recover before the next laser pulse is delivered.
[0111] Accordingly, in the present engine, non-uniform heating is
by radiation onto a free surface of the liquid. Vapor spaces on
each side of the liquid mass function as gas springs to provide a
restoring force, which enables the liquid mass to enter a regime of
steady state oscillations. The engine can power a compressor
enormously faster and much smaller (e.g., 2-3 orders of magnitude
faster and smaller) than a conventional compressor and has
significantly more (e.g., ten times more) refrigeration power of a
comparable Joule-Thomson expander that is commonly used in
cryocatheter today. The compressor engine thus gives the present
cryocatheter a greater cryoablation capacity (killing depth) than
that of conventional cardiac ablation catheters.
[0112] While the illustrated embodiment is a cryocatheter, it is
understood that the present refrigeration system (or engine
thereof) can be incorporated into other types of medical apparatus
as well. For example, the present cryocooler (i.e., refrigeration
system) can be incorporated into a handheld surgical ablation probe
that is substantially rigid and can be used to directly ablate
cardiac tissue during trans-thoracic or minimally invasive surgery.
The probe can include a deflectable tip for enhanced
maneuverability and precise placement of the heat transfer element.
The engine can also be used as a miniature ultrasound source for
medical imaging and treatment. The laser bubble technology enables
an ultrasound actuator that is significantly smaller than
conventional piezoelectric ultrasound transducers. As a result,
imaging catheters with the present engine can be used to provide
deeper views of tissue through blood vessels.
[0113] In the refrigeration cycle, the piston requires significant
displacement in order to actuate the valves and to move refrigerant
through the system. The piston can have a significantly smaller
displacement when the engine is used as an ultrasound transducer
and can be oscillated at a frequency within the ultrasound range
(for example, at 10 MHz to 40 MHz). Importantly, the amplitude of
oscillation of the piston roughly matches the size of the cylinder,
and accordingly, the engine can be made much smaller than a
conventional piezoelectric transducer where the amplitude of
oscillation represents only a 0.2% volume change of the total
transducer volume. For example, the engine may be 100 times smaller
than the piezoelectric transducer used for similar imaging
purposes.
[0114] Another embodiment of the engine compressor is illustrated
in FIGS. 10A and 10B. Where appropriate, like reference numbers
with a suffix "a" have been used to indicate like parts of the two
embodiments for ease of understanding. The foregoing description
thus should be understood to apply equally to like parts of the
present embodiment.
[0115] A diaphragm 300 distal of the liquid piston 186a separates
the liquid piston 186a from the refrigerant. This diaphragm 300 can
be made of a composite of silicon rubber and NiTi flexure. A
similar microdiaphragm is FDA approved and is currently used in
other implantable devices.
[0116] Movement of the piston 186a causes the diaphragm 300 to flex
proximally and distally to increase and decrease, respectively, the
volume in a compressor chamber 302 that is located on the distal
side of the diaphragm 300. Distal movement of the diaphragm 300
adiabatically compresses the refrigerant within the compressor
chamber 302, which is discharged from the compressor chamber 300
through the discharge valve 304. Proximal movement of the diaphragm
300 draws refrigerant vapor into the pump chamber 302 through the
suction valve 306.
[0117] Water-based liquids can absorb and remove about five times
more heat than a comparable amount of refrigerant, such as R-134a.
Therefore, a variation of the embodiment illustrated in FIG. 10A
can utilize dyed saline as the liquid piston of the engine.
[0118] Although this invention has been disclosed in the context of
certain preferred embodiments and examples, it will be understood
by those skilled in the art that the present invention extends
beyond the specifically disclosed embodiments to other alternative
embodiments and/or uses of the invention and obvious modifications
and equivalents thereof. In particular, while the present engine
has been described in the context of particularly preferred
embodiments, the skilled artisan will appreciate, in view of the
present disclosure, that certain advantages, features and aspects
of the engine may be realized in a variety of other applications,
many of which have been noted above. For example, while
particularly useful for small-scale applications (e.g., chamber
volumes of less than 2 cm.sup.3), such as the illustrated medical
application, the skilled artisan can readily adopt the principles
and advantages described herein to a variety of other applications,
including larger scale devices. Additionally, it is contemplated
that various aspects and features of the invention described can be
practiced separately, combined together, or substituted for one
another, and that a variety of combination and subcombinations of
the features and aspects can be made and still fall within the
scope of the invention. Thus, it is intended that the scope of the
present invention herein disclosed should not be limited by the
particular disclosed embodiments described above, but should be
determined only by a fair reading of the claims that follow.
* * * * *