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Description  |
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FIELD OF THE INVENTION
This invention relates to catheters and specifically to a catheter for
identifying and removing arteriosclerotic plaque deposits in blood
vessels.
BACKGROUND OF THE INVENTION
Arteriosclerotic plaque deposits which occlude and restrict the blood flow
of coronary arteries are a major cause of heart disease. Treatment of such
arteriosclerotic plaque has traditionally included open heart surgery or
angioplasty.
Presently there is growing interest in the use of laser irradiation to
remove arteriosclerotic plaque from occluded blood vessels, particularly
coronary arteries. Such laser irradiation is delivered to the occluded
vessel by means of a catheter comprising an optical fiber. Laser light is
passed through the optical fiber and onto the arteriosclerotic plaque. A
laser beam, thus delivered, has been found to remove arteriosclerotic
plaque resulting in patency of the blood vessel.
For example, Circulation, Volume 66, Supplement II (October, 1982) at page
368 describes a method wherein a fiberscope including quartz fiber was
inserted via the left subclavian artery to the aortailiac bifurcation in a
dog. The right iliac artery was then successfully photoirradiated by YAG
laser.
Circulation, Volumne 66, Supplement II (October, 1982) at page 366
describes a method using laser irradiation, delivered through a silicon
fiber to the right iliac artery of rabbits to remove arteriosclerotic
plaque.
The use of laser irradiation to remove arteriosclerotic plaque requires
means to accurately position the end of the optical fiber with respect to
the plaque which is to be removed. It also requires means to determine the
thickness and hardness of the arteriosclerotic plaque deposit so that
plaque removal may be maximized while arterial damage is minimized.
Methods for positioning the end of the optical fiber include X-ray
fluoroscopy and/or viewing through a flexible fiberoptic scope inserted
into the occluded artery. For example, U.S. Pat. No. 4,207,874 to Choy
describes a device using X-ray fluoroscopy and viewing through a
fiberoptic scope to locate obstructions in blood vessels and the like.
Laser irradiation is then transmitted through an optical fiber to vaporize
the obstruction.
X-ray fluoroscopy methods involve the positioning of the catheter tip by
the injecton of a radiopaque material into the occluded blood vessel and
viewing the X-ray shadow images of the artery and the catheter by a
fluoroscope. Such images do not generally give sufficient detail of the
occluded blood vessel to accurately and safely position the catheter tip
with respect to an arteriosclerotic plaque deposit. Further, X-ray
fluoroscopy methods also provide little information regarding thickness
and density of the plaque deposits.
Fiberoptic scopes have both illumination and viewing capabilities but
require blocking of the blood flow through the blood vessel followed by
flushing of the blood vessel with a clear liquid such as saline, until a
clear pathway is achieved. Viewing can then be done with the eye directly
or, for example, with a television camera and monitoring system.
There are several disadvantages associated with the fiberoptic scope
methods, including the fact that viewing cannot be done during laser
irradiation and that the blood flow must be stopped during viewing.
Further, viewing by a fiberoptic scope reveals little or no information
with regard to the thickness or density of the arteriosclerotic plaque
deposit. Also, fiberoptic scopes tend to be very expensive and generally
have only a limited life before costly repairs are required.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an apparatus
for determining the position and character of occlusions in blood vessels,
such as arteriosclerotic plaque, and for removing occlusions whose
position and character have been determined. The apparatus comprises a
catheter, a transmitting unit, a receiving unit and a laser.
The catheter comprises a catheter tube with an ultrasonic transducer
mounted at the distal tip of the catheter tube. An optical fiber is
disposed within the catheter tube and extends the length of the catheter
tube. The ultrasonic transducer is electrically connected to the
transmitting unit and the receiving unit by a pair of wires which are also
disposed in and extend the length of the catheter tube.
The catheter is inserted into and positioned within a blood vessel. The
ultrasonic transducer is activated by an electrical impulse generated by
the transmitting unit and delivered through the wires. The ultrasonic
transducer transmits an ultrasonic signal in response to the electrical
impulse. The ultrasonic transducer then receives ultrasonic echoes, i.e.,
ultrasonic signals reflected by the tissues surrounding the catheter tip,
and transmits electrical signals to the receiving unit in response to the
received ultrasonic echoes. The received electrical signals carry
information regarding the distance and character of the tissues which
reflect the ultrasonic signals and constitutes a "signature" of the
tissues. The presence of occlusions can be determined from the signature.
Occlusions whose position and character have been determined can be removed
by laser irradiation which is directed from the laser through the optical
fiber onto the occlusion.
In a preferred embodiment of the invention, the transmitting unit has two
modes. The first mode is a pulse-echo mode wherein a short electrical
impulse is generated and delivered to the ultrasonic transducer which
transmits an ultrasonic pulse signal in response. Ultrasonic echoes are
then received by the ultrasonic transducer which generates electrical
signals which are delivered to the receiving unit. These electrical
signals provide a signature of the surrounding tissue.
The second mode is a pulsed-doppler mode wherein the transmitting unit
generates electrical bursts which are delivered to the ultrasonic
transducer which transmits tone bursts in response. Ultrasonic echoes from
the tone bursts are received by the ultrasonic transducer which generates
electrical signals which are delivered to the receiving unit and are used
to determine the blood flow velocity at two selected distances from the
catheter tip. The tissue signature and the change in blood flow velocity
are used to determine the presence of occlusions.
In a particularly preferred embodiment of the invention, the ultrasonic
transducer is mounted at an angle from the longitudinal axis of the
catheter so that the transmitted ultrasonic signal is generally directed
toward the blood vessel wall. The catheter also comprises coupling optics
to direct laser irradiation, transmitted from the distal tip of the
optical fiber, in the direction of the transmitted ultrasonic signal.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the present invention will be
better understood by reference to the following detailed description when
considered in conjunction with the accompanying drawings wherein:
FIG. 1 is a schematic of an apparatus according to the invention including
an enlarged cross-sectional view of a preferred catheter;
FIG. 2 is a block diagram of a transmitter-receiver applicable to this
invention;
FIG. 3 is a block diagram of another transmitter-receiver applicable to the
invention;
FIG. 4 is a block diagram of another transmitter-receiver applicable to the
invention;
FIG. 5 is a cross-sectional view of another catheter according to the
invention;
FIG. 6 is an end view of a catheter having a multielement ultrasonic
transducer; and
FIG. 7 is a cross-sectional view of a catheter having a second lumen for
accommodating a guide wire.
DETAILED DESCRIPTION
The present invention is particularly applicable to removing
arteriosclerotic plaque deposits from blood vessels, especially coronary
arteries. With reference to FIG. 1, a preferred apparatus comprises a
catheter 6, a transmitter-receiver 7, and a laser 8.
The catheter 6 comprises an elongated flexible catheter tube 9 constructed
out of a suitable biologically non-reactive material, e.g., polyurethane.
The catheter tube 9 has a generally circular, transverse cross-section.
The preferred outer diameter of the catheter tube 9 varies according to the
application, i.e., according to the size of the blood vessel within which
the catheter is positioned. For example, a catheter having an outer
diameter of about 1 millimeter is presently preferred for identifying and
removing arteriosclerotic plaque deposits in coronary arteries.
The lumen of the catheter tube also has a generally circular transverse
cross-section. The diameter of the lumen is sufficiently large to enable
an optical fiber and a pair of wires to be disposed through the lumen and
is preferably sufficiently large to provide a space 10 between the optical
fiber and the catheter tube wall. Further, the catheter tube 9 comprises
holes 11 at its distal end which extend from the space 10 through the wall
of the catheter tube. The space 10 and holes 11 provide a passage for the
injection of a radiopaque material through the catheter into the blood
vessel which allows the rapid positioning of the distal tip of the
catheter in the approximate area of the suspected plaque deposit by X-ray
fluoroscopy. The same passage can also be used for the withdrawal of a
sample of the blood or fluid in the blood vessel following irradiation of
an arteriosclerotic plaque deposit.
An ultrasonic transducer 12 is mounted at the distal end of the catheter
tube 9 and is electrically connected on opposite sides to a pair of wires
13 and 14, which extend through the lumen of the catheter tube to the
transmitter-receiver 7. Ultrasonic transducers suitable for use with
catheters are known in the art. For example, see Circulation, Volume 56,
No. 1 (July, 1977) pages 18 to 25; IEEE Transactions on Sonics and
Ultrasonics, Volume SU-27, No. 6 (November, 1980) pages 277 to 286.
The ultrasonic transducer 12 is generally disc-shaped and has a
substantially flat face 16 and a generally circular opening 17 through its
center. The preferred outer diameter of the ultrasonic transducer is about
the same as the outer diameter of the catheter tube and likewise varies
according to the application in which the catheter is used. For removing
arteriosclerotic plaque from coronary arteries, a diameter of about 1
millimeter is presently preferred. The diameter of the opening 17 is
generally not critical but is sufficiently large to enable the distal end
of an optical fiber 18 to extend into and through the opening 17.
The thickness of the ultrasonic transducer is also not critical. For a
disc-shaped ultrasonic transducer having a diameter of about 1 millimeter,
a thickness of about 0.1 to about 0.3 millimeters is suitable.
The ultrasonic transducer 12 is made of a piezoelectric material,
preferably a piezoelectric ceramic crystal and generally has a response
frequency of from about 10 to about 30 MHz. The presently preferred
piezoelectric material comprises lead zirconate-titanate.
The ultrasonic transducer 12 is mounted at the distal end of the catheter
tube 9 so that the plane of the front face of the ultrasonic transducer is
at selected angle to the longitudinal axis of the catheter tube. In this
arrangement, the ultrasonic signals transmitted from the transducer are
directed toward the wall of the blood vessel rather than along the length
of the blood vessel. In other words, the included angle between the plane
of the face of the ultrasonic transducer and the longtitudinal axis of the
catheter body is less than 90.degree..
The ultrasonic transducer 12 may be attached to the distal end of the
catheter tube 11 by any suitable means, such as by epoxy cement 15.
The distal ends of wires 13 and 14 are electrically connected to opposite
sides of the ultrasonic transducer 12, by conventional means such as
soldering, ultrasonic bonding, cold welding and the like. Wires 13 and 14
can be made of any suitable metal, such as copper, stainless steel, or the
like. The wires are preferably clad with a non-conductive coating, e.g.,
nylon, to assure that electrical contact between the wires is prevented.
An optical fiber 18 is disposed within the lumen of the catheter tube 9.
The optical fiber has a generally circular transverse cross-section and
comprises a core 18a made of fused silica or its equivalent surrounded by
an optical clad 18b made of a material having a lower refractive index
than the core, as is well known in the art. Surrounding the optical fiber
18 is a protective buffer 19 which is generally made of a semi-rigid
plastic such as an epoxy resin or the like.
The optical fiber 18 has a diameter sufficiently large to transmit laser
irradiation of sufficient strength to vaporize arteriosclerotic plaque
desposits without a detrimental increase in the temperature of the optical
fiber. The presently preferred diameter is about the minimum diameter
which results in a safe increase in temperature during laser transmission
in order to minimize the overall diameter of the catheter. For removing
arteriosclerotic plaque from coronary arteries, an optical fiber having a
diameter in the range of from about 0.1 millimeters to about 0.5
millimeters is preferred.
At the distal end of the optical fiber 18, the protective sheath is removed
or cut back and the distal tip of the optical fiber 18 is rigidly mounted
within the central opening 17 of the ultrasonic transducer 12. Mounting
can be done by any suitable means, such as by epoxy cement. The diameter
of the central opening 17 of the ultrasonic transducer is sufficiently
larger than the diameter of the optical fiber to allow the distal tip of
the optical fiber to be mounted within the opening 17, and to provide a
sufficient gap between the ultrasonic transducer and the optical fiber to
prevent detrimental heating of the ultrasonic transducer as a result of
heat generated by the optical fiber during laser transmission.
In such an arrangement, wherein the tip of the optical fiber is mounted in
the opening of the transducer, laser irradiation is transmitted from the
center of the ultrasonic transducer. To direct the laser irradiation in
the same general direction as the transmitted ultrasonic signal, i.e.,
toward the wall of the blood vessel, coupling optics are used. A microlens
20 or its equivalent is mounted within the opening 17 in the transducer
adjacent the distal tip of the optical fiber 18 so that laser irradiation
emanating from the tip of the optical fiber will pass through the
microlens 20 which changes the direction of the laser irradiation to about
the center of the transmitted ultrasonic signal. Mounting of the microlens
may be by any suitable means, such as epoxy cement.
The transmitting and receiving units may be of conventional design. In a
preferred embodiment of the invention, the transmitting unit and receiving
unit are combined in a single transmitter-receiver 7.
The transmitter-receiver comprises two modes of operation and means for
switching between the two modes. The first mode is a pulse-echo mode which
activates the ultrasonic transducer to transmit an ultrasonic pulse signal
and senses the time delay and amplitude content of the ultrasonic echoes
resulting from the transmitted ultrasonic pulse signal for identification
of the tissue character at a selected distance from the catheter tip.
The second mode is a pulsed-doppler mode which activates the ultrasonic
transducer to transmit a tone burst and senses the phase and frequency of
echoes resulting from the transmitted tone burst for identifying the blood
flow velocity at two selectable distances from the catheter tip. The
pulsed-doppler mode is used to determine the presence of a restriction in
the blood vessel and the pulse-echo mode is used to identify the character
of the tissue causing the restriction.
In the pulse-echo mode, the transmitter-receiver generates short electrical
impulses at selected intervals which are delivered to the ultrasonic
transducer 12 through wires 13 and 14. The ultrasonic transducer emits an
ultrasonic signal in response to each electrical impulse. Ultrasonic
echoes, i.e., reflections of the transmitted ultrasonic signal, are
received by the transducer which then transmits electrical signals,
corresponding to the received echoes, back to the transmitter-receiver,
again through wires 13 and 14. The time delay and amplitude content of the
ultrasonic echoes are determined from the received electrical signals and
provide a "signature" of the reflecting tissue. Analysis of this signature
provides information related to the distance and character, e.g., density,
of the reflecting tissue.
With reference to FIG. 2, the transmitter-receiver comprises an oscillator
21 which controls the timing of the pulse-echo mode. A synchronous gating
logic 22 triggers a pulse generator 23 to produce a single short
electrical impulse at selected intervals, e.g., intervals in the 50 to 100
KHz range, for activating the ultrasonic transducer. The electrical
impulse is short, comprising as little as a single cycle of the oscillator
signal.
The short electrical impulse is amplified by a radio frequency power
amplifier 24 and is delivered by wires 13 and 14 to the ultrasonic
transducer 12. The ultrasonic transducer emits an ultrasonic pulse signal
in response to the short electrical impulse. The interval between
consecutive electrical impulses is sufficient to allow the ultrasonic
transducer to receive reflected ultrasonic echoes resulting from the
transmitted ultrasonic pulse signal and to transmit electrical signals to
the transmitter-receiver in response to those ultrasonic echoes before
transmitting another ultrasonic pulse signal.
Ultrasonic echoes, i.e., reflections of the transmitted ultrasonic signal,
occur at every boundary between materials differing in the velocity of the
propagation of sound, including boundaries between blood and
arteriosclerotic plaque deposits and between arteriosclerotic plaque
deposits and the wall of the blood vessel.
The ultrasonic echoes are received by the ultrasonic transducer which
transforms the ultrasonic echoes into electrical signals which are
transmitted to the trans- mitter-receiver through wires 13 and 14. The
electrical signals from the ultrasonic transducer are sensed and amplified
by a radio frequency receiving amplifier 26. The gain of the radio
frequency receiving amplifier 26, i.e., the increase in signal amplitude,
is controlled by a swept gain generator 27 which compensates for the
reduced signal amplitude of echoes returning from greater distances. The
amplified signal is then displayed or monitored by any suitable means,
e.g., an oscilloscope 25, and provides a "signature" of the reflecting
tissue.
To display the amplified signal on the oscilloscope 25, a transmission gate
28 is used as an oscilloscope trigger for indicating the onset of the
transmitted electrical pulse. Direct observation of the amplified signal
received from the ultrasonic transducer on the oscilloscope 25 provides
information related to the distance of the reflecting tissue, as
determined by the time on the oscilloscope trace, and the character of the
tissue interface as determined by the amplitude of reflection on the
oscilloscope trace.
The amplified signal can also be gated by a receiving gate 29 to provide a
signal which can be used by a spectrum analyzer for further identification
of the character or properties of the reflecting tissue. The receiving
gate 29 removes all portions of the signal except that portion
corresponding to a selected distance from the catheter tip. To accomplish
this, the receiving gate 29 is controlled by a first delay function 31
which activates the receiving gate 29 to pass a signal of preset width at
a selected time after the transmission gate 28 has signaled the onset of
the transmitted electrical impulse.
To determine the presence of arteriosclerotic plaque deposits in a blood
vessel from the tissue signature, a comparison is made between the
signature of the unknown tissue and previously obtained signatures for
which the character of the tissue, i.e., the presence or absence of
arteriosclerotic plaque, was determined by conventional means. That is,
preceding the analysis of an unknown tissue, tests are performed, for
example on laboratory animals, wherein the catheter is inserted into
non-occluded blood vessels, i.e., blood vessels having little or no
arteriosclerotic plaque, as well as blood vessels occluded to various
degrees by arteriosclerotic plaque, and tissue signatures are obtained
from those blood vessels. The character of the tissues, i.e., the amount
of arteriosclerotic plaque, of the "test" blood vessels is then determined
by visual inspection, e.g., surgery, or other conventional means and a
correlation between the presence and amount of arteriosclerotic plaque and
the tissue signatures is made. Once a correlation is made, the presence
and amount of plaque in an unknown blood vessel can then be determined
from the signature of that tissue.
In the pulsed-doppler mode, the transmitter-receiver generates electrical
bursts, i.e., electrical signals comprising multiple cycles, e.g., ten to
twenty cycles, of the oscillator signal which are delivered to the
ultrasonic transducer and results in the transmission of a tone burst,
i.e., a longer ultrasonic signal having a duration corresponding to the
duration of the electrical burst. Comparison of the phase and frequency of
the ultrasonic echoes resulting from a transmitted tone burst, at two or
more selected times following the transmission, with a signal from the
oscillator provides information which yields the velocity of the blood
flow at two distances from the transducer.
Again with reference to FIG. 2, the synchronous gating logic 22 of the
transmitter-receiver sends a signal to the transmission gate 28 to
transmit an electrical signal comprising a select number of cycles of the
oscillator signal at selected intervals, e.g., an interval in the 50-100
KHz range, the electrical signal is amplified by the radio frequency power
amplifier 24 and is then passed to the ultrasonic transducer 12 by wires
13 and 14. The ultrasonic transducer 12 receives the electrical signal and
transmits a corresponding ultrasonic signal, i.e., a tone burst in
response.
The transmitted ultrasonic signal is reflected by tissue interfaces
including moving interfaces in the blood stream, e.g., corpuscle-plasma
interfaces. Echoes are sensed by the ultrasonic transducer which transmits
an electrical signal to the transmitter-receiver in response to the
received echoes. The received electrical signal contains information
regarding the distance and velocity of tissues in the path of the sound
signals. The interval between the transmission of electrical signals from
the transmitter-receiver to the transducer is selected to allow the
transducer to receive the ultrasonic echoes resulting from the tone burst
and to transmit electrical signals to the transmitter-receiver in response
to the received echoes before transmitting another tone burst.
The electrical signal received from the ultrasonic transducer is amplified
by the radio frequency receiving amplifier 26 and is mixed by mixer 32
with the signal from the oscillator 21 to yield a signal comprising the
sum and difference of the frequencies of the received signal and the
oscillator signal.
The first delay function 31 and a second delay function 33 provide delayed
gating signals for first and second sample and hold amplifiers 34 and 36
for sensing the velocity at two distances from the ultrasonic transducer
as selected by first and second distance selectors 37 and 38, which are
adjustable receiver gates. First and second band pass filters 39 and 41
eliminate those portions of the combined signals corresponding to the
carrier signal, the sum of the combined signals and signals resulting from
stationary interfaces, leaving only the portion of the combined signals
resulting from motion of tissue interfaces at the two selected distances.
The outputs of the band pass filters are then used to drive headphones or
speakers 42 and 43 for providing audible signals. Alternatively, the
outputs from the band pass filters can be used to produce analog voltages
proportional to the respective velocities, which can be monitored and
recorded, by using frequency to voltage converters.
By monitoring signals received from two different distances from the
catheter tip, i.e., a first distance nearer the catheter tip and a second
distance farther from the catheter tip, the blood velocity can be
determined at those distances. These blood velocities are used to
determine the presence and extent of a restriction in the blood vessel. A
greater blood velocity, at the second distance as compared with that at
the first distance, indicates a restriction in the blood flow at that
second distance due to a narrowing of the blood vessel, e.g., due to the
formation of tributaries, or as a result of the occlusion of the blood
vessel by arteriosclerotic plaque. Conversely, a lower blood velocity at
the second distance indicates greater patency of the blood vessel,
possibly due to a broadening of the blood vessel or the absence of or at
least a lesser amount of arteriosclerotic plaque at the second distance as
compared with the first.
If desired, the direction of the blood flow can also be monitored. This can
be accomplished by providing a reference frequency to the mixer 32 which
is offset in frequency from oscillator 21. With reference to FIG. 3 the
reference frequency can be generated, for example, by reference oscillator
45. In such an arrangement, the output signal can be monitored, for
example, by metering devices 42a and 43a, each having an indicator. The
indicator would be displaced in one direction from a zero position for
motion toward the transducer and in the opposite direction for movement
away from the transducer. The amount of displacement would indicate the
velocity.
Alternatively, as shown in FIG. 4, by using dual channel circuitry from the
mixer stage through the output stage, i.e., dual mixer 46, dual sample and
hold amplifiers 47 and 48, and dual band pass filters 49 and 50, in
conjunction with a phase shifter 51 and phase quadrature signal processing
stage 52, direction sensing may also be provided, as is well known in the
art.
The transmitter-receiver comprises a switch 44 for rapidly switching
between the pulse-echo mode and the pulsed-doppler mode. Switching is
sufficiently rapid that the received signals from both modes can be
displayed "simultaneously". For example, the signals received in the
pulse-echo mode can be displayed on an oscilloscope while the signals
received in the pulsed-doppler mode can be used to drive a pair of
speakers. Switching between modes is preferably sufficiently rapid so
that, to an operator, each displayed signal appears continuous.
The combined information as to the position and extent of a blood vessel
restriction and the character or properties of the restricting tissue
provides an accurate determination of the presence, position and amount of
restriction caused by arteriosclerotic plaque deposits and the need for
its removal. Such removal can be effected immediately and with enhanced
precision by vaporizing the plaque with laser irradiation delivered
through the optical fiber.
Referring again to FIG. 1, the apparatus also comprises a laser 10 for
generating the laser irradiation which is delivered through the optical
fiber. Lasers for such applications are well known in the art. Suitable
lasers include argon-ion, neodymium-YAG, and carbon dioxide lasers.
This invention provides several unique advantages over similar systems
using a fiberoptic scope. For example, the presence of arteriosclerotic
plaque deposits in a blood vessel can be determined without the need to
stop the flow of blood through the blood vessel. Additionally, monitoring
of the plaque deposits need not be interrupted by laser transmission. A
catheter according to this invention can also be made sufficiently small
to be positioned in coronary arteries and thus presents a new non-surgical
treatment for heart disease due to occluded coronary arteries.
While the preceding description has been presented with reference to a
presently preferred embodiment of the invention, it is apparent that
changes and alterations in the above-described apparatus can be practiced
without departing from the scope of the invention. For example, the
ultrasonic transducer in the preferred embodiment described above is
annular, i.e., disc-shaped and having a central opening. This enables the
mounting of the distal tip of the optical fiber in the central opening.
Further, the ultrasonic transducer is preferably mounted at a selected
angle to the longitudinal axis of the catheter so that the transmitted
ultrasonic signals are directed toward the wall of the blood vessels.
However, other ultrasonic transducer shapes and mounting configurations
are equally suitable as is mounting the distal tip of the optical fiber to
one side of the ultrasonic transducer rather than through an opening in
the transducer. Further, coupling optics are not required if it is desired
to direct the laser irradiation in a direction generally along the length
of the blood vessel rather than toward the blood vessel wall.
As illustrative, FIG. 5 shows rectangular ultrasonic transducer 53 mounted
to the distal end of a catheter tube 54 with an optical fiber 56 extending
through the catheter tube and mounted so that its distal end is to one
side, e.g., below, the ultrasonic transducer. No coupling optics are used.
In another embodiment of the invention, the ultrasonic transducer may be
comprised of a plurality of elements, each element being capable of
independently transmitting and receiving ultrasonic signals. A preferred
multiple-element ultrasonic transducer is shown in FIG. 6. The elements 57
are positioned symmetrically about the longitudinal axis of the catheter
and provide a central opening 58 at the distal tip of the catheter. Each
element 57 has a generally flat face and is mounted on the distal end of
the catheter tube 59 at a select angle to the longitudinal axis of the
catheter tube 59. Each element is electrically connected by separate pairs
of wires 61 to the transmitter-receiver so that each element can be
activated independently. An optical fiber 62 is disposed through the lumen
of the catheter tube 59 and its distal tip is mounted in the central
opening 58.
Multiple-element ultrasonic transducers provide the advantage that tissue
signatures can be obtained in several radial directions without moving the
catheter. However, for small blood vessels, e.g., coronary arteries,
multiple-element ultrasonic transducers do not provide a significant
advantage over single-element ultrasonic transducers and hence are not
presently preferred.
If desired, the catheter tube can be designed to accommodate a guide wire
to assist in the positioning of the catheter within the blood vessel. Such
guide wires and their use are well known in the art. To accommodate a
guide wire, the catheter tube can have a lumen sufficiently large to
accommodate both the optical fiber and a guide wire and have an opening at
the distal tip of the catheter to allow the distal extension of the guide
wire. Alternatively, as shown in FIG. 7, the catheter tube 63 can comprise
a first lumen 64 through which the optical fiber 66 and wires 67 and 68
are disposed and a second lumen 69 through which a guide wire 71 extends
and is afforded lengthwise movement. Such a second lumen can also be used
for the injection of a radiopaque fluid or other material into the blood
vessel or the removal of fluid from the blood vessel.
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