Abstrict A catheter probe flow meter and method of determining the volume
flow of a fluid through a conduit, at least one of the fluid and
the conduit having energy-scattering interfaces. The method in one
embodiment includes trasmitting at least two and receiving at least
one or transmitting at least one and receiving at least two sound
beams through the fluid, resolving the Doppler vectors into an average
velocity vector for each transmitting/receiving combination, and
resolving the resolved velocity vectors into their component parts.
Claims We claim:
1. A catheter transducer probe comprising
an elongate catheter housing having a bore therein;
first and second acoustical transducers which generate and receive
respective beams of sound, said transducers being mounted in a mutual
coaxial relationship to said housing;
electrical conductors extending through said bore and electrically
connected to said transducers;
first and second sound directing means comprising shaped acoustical
reflectors mounted to said housing and acoustically coupled to said
first and second transducers, respectively, so as to direct said
first and second beams outwardly from said housing at a predetermined
angle with respect to each other such that the Doppler vectors of
the reflected back portions of said sound beams are nonparallel.
2. A probe as claimed in claim 1 wherein said transducers are spaced
apart and coaxially mounted about the longitudinal axis of the housing
and said directing means are coaxially mounted therebetween.
3. A probe as claimed in claim 1 wherein said transducers are spaced
apart and said reflectors shaped as two back-to-back conical frustrum
portions joined at their enlarged bases.
4. A probe as claimed in claim 1 wherein said transducers are mounted
in mutual, juxtaposed, electrically insulated relationship and said
sound directing means are located outwardly thereof.
5. An acoustical transducer probe comprising:
an elongate housing;
first and second acoustical transducers which generate and receive
respective beams of sound, said transducers being mounted in a mutual
coaxial relationship to said housing;
a bore in said housing;
electrical conductors extending through said bore and electrically
connected to said transducers;
first and second sound directing means comprising shaped acoustical
reflectors mounted to said housing and acoustically coupled to said
first and second transducers, respectively, so as to direct said
first and second beams from said housing at a predetermined angle
with respect to each other such that the Doppler vectors of the
reflected back portions of said sound beams are nonparallel.
Description FIELD OF THE INVENTION
This invention relates generally to apparatus and methods for measuring
the flow of energy scattering interfaces within a fluid inside a
conduit and more particularly to ultrasonic pulse Doppler apparatus
and methods for measuring blood flow in a patient's circulatory
system.
DESCRIPTION OF THE PRIOR ART
Ultrasonic Doppler flowmeters for measuring the flow of particulate-containing
fluid in a conduit are well known. Examples of such devices adapted
for measurement of blood flow in a patient's circulatory system
are described in U.S. Pat. Nos. 3430625 (McLeod); 3888238 (Meindl
et al); 3901077 (McCarty et al); 3554030 (Peronneau); 3542014
(Peronneau); 3527115 (Bom); and 3827115 (Fahrbach). As disclosed
in these references, the flowmeter usually includes a catheter for
insertion into a blood vessel of the patient.
A disadvantage of these and other prior art devices is that the
accuracy of the velocity and lumen cross-sectional area measurements
obtained is dependent on the orientation of the catheter with respect
to the blood flow axis. All of these previous designs require a
fixed orientation or position to measure the size of the lumen or
the velocity.
A further disadvantage of the prior art is that either separate,
specialized function transducer arrangements are required to measure
both velocity and lumen area or sensitive and difficult-to-implement
measurement techniques must be used in order to measure velocity
and lumen area with a single, dual-purpose transducer arrangement.
A further disadvantage of the prior art catheters is their complexity
and the consequent difficulty of their manufacture.
SUMMARY OF THE INVENTION
These and other disadvantages of the prior art are overcome by
the apparatus and methods of the present invention. The present
method is independent of the orientation and position for the measurement
of volume flow.
One embodiment of the invention comprises a method for measuring
the velocity of a fluid flowing in a conduit wherein at least one
of the fluid and the conduit has energy-scattering interfaces. The
method comprises transmitting either at least two sound beams at
a predetermined angle with respect to each other through the fluid
and detecting at least some of the sound beams reflected by the
energy scattering interfaces with at least one receiver or transmitting
at least one sound beam and detecting the reflected sound beams
with at least two receivers. The corresponding Doppler vector for
at least two pairs of transmitted and detected sound beams is combined
into an average velocity vector and the two or more resolved average
velocity vectors are resolved into their component parts to obtain
the effective velocity vector.
A second embodiment of the invention relates to determining the
effective cross-sectional area of a conduit having a fluid flowing
therethrough wherein at least one of the conduit and the fluid has
energy-scattering interfaces. The method uses a means for transmitting
a sound beam and a means for detecting at least a component thereof
and comprises transmitting at least one sound beam per functional
set, and calculating the distance to the wall with Doppler range
gating techniques. These steps are repeated for a plurality of functional
sets and a geometrical shape is constructed. Then the effective
cross-sectional area of the constructed shape is calculated.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a block diagram of a flowmeter constructed according
to the present invention.
FIG. 2a is a side view, partially in section, of a first embodiment
of an intravenous catheter constructed according to the present
invention with a first embodiment of transducers 22.
FIG. 2b is a transverse cross-section view of the catheter in FIG.
2a taken along line A--A.
FIG. 2c is a longitudinal cross-section view of a second embodiment
of transducers 22.
FIG. 3a is a side view, partially in section, of the catheter of
FIG. 2a with a third embodiment of transducers 22.
FIG. 3b is a longitudinal cross-section view of a fourth embodiment
of transducers 22.
FIG. 4a is a perspective view of a fifth embodiment of transducers
22.
FIG. 4b is a longitudinal cross-section view of the embodiment
of transducers 22 illustrated in FIG. 4a.
FIG. 4c is a transverse cross-section view taken along the lines
A--A of the embodiment of transducers 22 illustrated in FIGS. 4a
and 4b.
FIG. 5a is a perspective view of a sixth embodiment of transducers
22.
FIG. 5b is a longitudinal cross-section view of the embodiment
of transducers 22 illustrated in FIG. 5a.
FIG. 5c is a top view of the embodiment of transducers 22 illustrated
in FIGS. 5a and 5b.
FIG. 6 is a side view, partially in section, of a second embodiment
of a catheter constructed according to the present invention having
mounted thereon a seventh embodiment of transducers 22.
FIG. 7 is a longitudinal cross-section view of an eighth embodiment
of transducers 22.
FIG. 8 is a longitudinal cross-section view of a ninth embodiment
of transducers 22.
FIG. 8a is a transverse cross-section view taken along the line
A--A of the embodiment of transducers 22 illustrated in FIG. 8.
FIG. 9 is a longitudinal cross-section view of a tenth embodiment
of transducers 22.
FIG. 9a is an end view of the embodiment of transducers 22 illustrated
in FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The mathematical starting point for determining flow is the Doppler
shift vector equation. This equation is as follows: ##EQU1## f=the
change in frequency f.sub.o =the base frequency
V=Vector velocity of the fluid
C=Velocity of sound in that media
T=unit vector representing the direction of the transmitted sound
beam
R=Unit vector representing the direction of the received sound
beam
(T-R)=Direction Vector of observed Doppler beam
The volume flow through any conduit is the vector dot product of
the average velocity vector and a cross-sectional area through which
that velocity flows. The present method and apparatus employ techniques
for velocity measurement and cross-sectional area determinations
which are independent of the orientation and position of the transducers.
In order to eliminate the orientation of the transducers from being
a factor in the determination of the flow velocity, in one embodiment
of the invention two sound beams are used. This is because in this
embodiment the field radiation pattern is not sufficiently uniform
to allow the use of only one transducer. These sound beams can either
be transmitted as two or more separate beams and received on one
or more receivers or received as two or more separate beams which
have been transmitted by one or more transmitters, such as by the
transducers described hereinbelow and shown in FIGS. 2 through 9.
Each transducer (or transducers) receives signals from the entire
cross-section of the conduit. A range gate is moved out in time
from the transducers in order to measure a selected region of the
conduit. Alternately, the signal received can be processed either
sequentially with a single movable gate or in parallel with multiple
gates or in combinations thereof. Each measurement is the representation
of the average velocity through that sampled region. All of the
discrete range-gated velocity measurements are summed into an average
velocity measurement by the use of a weighting function. This weighting
function is calculated to take into account the radar equation,
that is to account for transducer radiation pattern, attenuation
losses, and the scattering characteristics of the reflective interface.
Simultaneously or sequentially, the other transducer (or sets of
transducers) scans the conduit and similarly obtains an average
velocity measurement as seen by that transducer or transducers.
The rate of scan of the transducers must be sufficiently fast that
no appreciable change in velocity takes place during the measurement
period. Use of these two velocity vectors allows the resolution
of the effective velocity component along the axis of the probe
in this case. This is done by taking the sums and differences of
the vectors in manner consistent with vector algebra. It should
be noted that this effective velocity vector need not be parallel
to the probe or to the transducers. This effective velocity vector
would be an average velocity vector for the effective cross-sectional
area in question, as determined hereinbelow.
The next step in the measurement of flow is to measure an effective
diameter or cross-sectional area. One method employs the power returned
from a Doppler signal. This power is directly proportional to the
size of the sampled volume. If the total sampled region over which
this measurement takes place is greater than the cross-sectional
area of the conduit and includes the conduit in question, the total
uncalibrated cross-sectional area would be determined. This measurement
can be done as a single measurement or as a sum of separate measurements.
The uncalibrated area is a function of the probe angle to the conduit
wall. This uncalibrated area would also vary with different ultrasonic
transducers. To obtain the effective cross-sectional area of the
conduit, the uncalibrated area must be calibrated and this can be
done by the following method. The power returned from a known region
completely within the conduit is measured. In the preferred embodiment
this is accomplished by using very close range-gating techniques.
This technique simultaneously allows the calibration of each individual
transducer for gain and gives the power reflected for a known area
(volume) of the fluid. Dividing this new factor into the uncalibrated
power area mentioned above gives the effective cross-sectional area.
The effective velocity vector and the effective surface area need
not necessarily be orthogonal to each other. If they are not orthogonal,
the angle between the two vectors must be known so that the dot
product can be determined.
Since the effective cross-sectional area is known and calibrated
orthogonal to the transducers in the preferred embodiment and the
effective velocity vector is known parallel to the axis of the transducers,
the flow in the conduit is the product of these factors without
any other factors having to be known or calculated.
A second method for determining the cross-sectional area is to
range to the wall by the use of multiple sets of transducers. This
can be accomplished with two or more receivers and one or more transmitters
or one or more receivers with two or more transmitters per functional
set. In these cases, the low frequency Doppler signals, as returned
from the wall, can be used to determine the well position. The preferred
embodiment uses sets of transducers to allow the construction of
a radiation pattern orthogonal to the catheter. A surface plane
of known dimensions can be constructed from these vectors. Once
this effective cross-sectional area is known then this area can
be multiplied by the effective velocity vector to give the volume
flow through the conduit. A sufficient number of sets of transducers
must be used to sample the wall for any significant irregularities,
and for the distance to the wall from the sets of transducers. Construction
of one or more planes can now be made from the sets of transducers.
The angle that the beams were transmitted and received on are known
from the geometry of the probes. Since the wall has been sufficiently
illuminated, the vessel shape, position of the catheter and the
cross-section can all be determined by known algebraic and geometric
techniques.
The method of using two or more transducers allows the production
of a known radiated field. If a known uniform pattern of radiation
could be produced with one transmitter/receiver, it would be sufficient
to determine the volume flow in a manner to be described hereinafter.
A phased array is a multiple set of transducers generating or operating
within a fixed time relationship to simulate a complex transducer,
lens, reflector or field. Once the field has been produced, the
techniqes for measuring the effective velocity, cross-sectional
area and calibrations are as previously described.
The preferred embodiment is FIG. 2A and 2B. It is to be noted that
the methods described herein do not require that one of the exemplary
transducer arrangements be utilized. The only requirement for the
sound field is that the direction vectors must not be parallel.
The direction vector is defined as the vector quantity (T-R).
Several unique characteristics are embodied in the catheters. These
are that functional pairs of ultrasonic beams having non-parallel
Doppler direction vectors can be used for the measurement of velocity.
Within this constraint, as stated above, any arrangement of one
or more transmitters with two or more receivers would work as well
as one or more receivers with two or more transmitters. The transmitter/receiver
can be a single transducer or multiple transducer.
Another unique feature is a lens or prism to produce a selected
field. This lens or prism can be of any material or materials that
would refract or bend an ultrasonic signal either received or transmitted.
A third unique feature is the use of a reflector. A reflector is
any material from which ultrasonic signals would be reflected or
be directed as to give the desired angle or field pattern to the
beams. The shape of the reflector or reflectors can be varied to
give the desired field of radiation or reception. It should be noted
that combinations of these effects could be used.
Obviously, the aforementioned concepts can all be adapted extra-luminally.
Arrays of two or more transmitters and one or more receivers or
two or more receivers and one or more transmitters can be used.
A preferred embodiment uses three or more sets of transmitters and
one or more receivers or three or more receivers with one or more
transmitters. This is done to eliminate the possibility of a misalignment
between the conduit and the transducers and therefore possibly erroneous
results.
All functional combinations of transducers taken two at a time
must radiate in a field pattern which includes the conduit, and
the Doppler direction vectors must not be parallel. The techniques
for velocity measurement and cross-sectional area determination
are all as previously stated.
The single transducer case involves producing a known uniform ultrasonic
field to illuminate the total cross-section of the conduit. Since
the field pattern is defined, velocity measurements can be obtained.
The pulse length and sampling period are selected to produce a sampling
region normal to the direction of propagation of the sound beam.
Under these conditions, the velocity component is everywhere normal
to the sampling surface. Integration over this surface yields a
signal, the first moment of which is proportional to the flow. Calibration
is provided by the same technique as previously described.
For purposes of illustration, an embodiment of a flowmeter constructed
according to the present invention which is adapted for measuring
blood flow in a patient's circulatory system using an intravenous
catheter in the preferred embodiment will now be described.
Referring to FIG. 1 the flowmeter, which is generally denoted
10 comprises a catheter 20 having disposed therein first and second
transducers, denoted 22A and 22B and referred to collectively as
transducers 22 for transmitting and receiving ultrasonic sound
waves; generator 30 for pulsing transducers 22; receiver 40 for
receiving the signals produced by transducers 22; and data analyzer
50 for determining the blood flow velocity and an effective cross-sectional
area of the vessel lumen in the region of measurement.
Referring to FIGS. 2a and 2b, catheter 20 in a first embodiment
comprises a catheter housing 24 dimensioned for insertion into a
blood vessel of the patient. Transducers 22A and 22B are coaxially
disposed in catheter housing 24 on the longitudinal axis of catheter
20. As shown, catheter housing 24 may be tubular and have a blunt
tip 24A. A central bore 26 is provided which houses the electrical
conductors (not shown) used to connect transducers 22A and 22B with
generator 30 and receiver 40.
Referring to FIG. 6 catheter 20 in a second embodiment comprises
a support member 28 having a central bore 26' and a base 28A on
which transducers 22A and 22B are mounted.
Transducers 22A and 22B are configured and arranged such that first
and second beams of ultrasonic sound waves are radiated outwardly
at a predetermined angle with respect to each other such that the
Doppler vectors representing the sound beams are non-parallel.
Referring to FIGS. 2-9 transducers 22 may comprise a piezoelectric
transducer or transducers 110 having transmitting/receiving faces
112 of various configuration, and means 120 for directing the sound
waves outwardly from catheter 20 in either converging or diverging
directions. In the embodiment of transducers 22 illustrated in FIGS.
2a and 2b, transducers 110 are disposed in spaced substantially
coaxial relationship, with planar transducer faces 112 disposed
in opposed relationship, facing toward each other, and substantially
perpendicular to the axis of catheter 20. As shown in FIG. 2b, transducer
faces 112 may have a circular peripheral configuration, or any other
peripheral configuration desired. Sound directing means 120 comprises
a reflector 122 substantially coaxially disposed between transducers
110. Reflector 122 may have the shape of two back-to-back conical
frustrum portions 122A and 122B joined at their enlarged bases,
defining two oppositely facing reflecting surfaces 124A and 124B,
as shown. The sound beams radiated from transducer faces 112 are
thus reflected outwardly from catheter 20.
Reflector 122 may also be shaped such that the reflecting surfaces
124A and 124B of frustrum portions 122A and 122B are curved as viewed
in longitudinal cross-section, as shown in FIG. 2c, in order to
allow the production of a different ultrasonic field.
Referring to FIG. 3a, in which elements similar to those of the
embodiment of FIGS. 2a and 2b have been given the same reference
numbers, with primes attached, transducers 110' are substantially
coaxially disposed in juxtaposed, electrically insulated, relationship,
such that transducer faces 112 face in opposite directions away
from each other. The conical frustrum portions 122A' and 122B' of
reflector 122' are spaced, each disposed with the smaller base relatively
adjacent the corresponding transducer face, as shown. An embodiment
of reflector 122' having curved reflecting surfaces, as viewed in
longitudinal cross-section, is shown in FIG. 3b.
In the embodiment of transducers 22 illustrated in FIGS. 4a to
4c, a single, hollow, cylindrical transducer 114 is transversely
divided to form two electrically isolated coaxial portions 114A
and 114B, as shown. If a lead-titanate-zirconate (LTZ) piezoelectric
transducer is used, both the inner and outer walls thereof are silver
coated, and portions 114A and 114B are formed by breaking the silver
coating on one or both walls or by assembling two separate substantially
coaxially disposed shapes. It is to be noted that transducer 114
may have any desired configuration and need not be in the form of
a hollow cylinder.
Surrounding the outer cylindrical surface of transducer 114 is
a refracting lens or refractor 116 having the shape of two conical
frustrums with a common enlarged base, such that the peripheral
configuration defines a triangle, or prism, as viewed in longitudinal
cross-section. Refractor 116 is aligned with respect to transducer
114 such that the apex 118 of refractor 116 is in substantially
coplanar alignment with the dividing line 110 between transducer
portions 114A and 114B. The sound waves radiated outwardly from
transducer portions 114A and 114B are thus refracted in diverging
or converging directions depending upon the media and the diffractive
index of the material, for example as indicated by the arrows D.
Refractor 116 may have a laminate construction or utilize a variable
density composition to achieve a greater or lesser degree of diffraction.
The embodiment of transducers 22 illustrated in FIGS. 5a to 5c
includes elements similar to those of the embodiment of FIGS. 4a
to 4c and these elements have been given the same reference numbers,
with primes attached. Refractor 116' has a double conical frustrum
shape similar to that of refractor 116 except that the frustrum
portions are joined at their smaller bases. Refractor 116' is aligned
with respect to transducer 114' such that the line of joiner 118'
of the two frustrum portions of refractor 116' is in coplanar alignment
with the dividing line 119' between crystal portions 114A' and 114B'.
The sound waves radiated outwardly from transducer portions 114A'
and 114B' are thus refracted in converging or diverging directions,
depending upon the media and the refractive index of the material.
With reference to FIGS. 6 and 7 the desired sound beam propagation
patterns and directions may also be effected through use of transducers
22A and 22B comprising a piezoelectric transducer or transducers
70 having suitably configured and/or oriented transmitting/receiving
faces 72 rather than through use of distinct refracting or reflecting
sound directing means. FIGS. 6 and 7 illustrate exemplary embodiment
in which transducer 70 are conical, the outer conical surfaces of
which define transducers faces 72. The transducers 70 shown in FIG.
6 are so disposed with respect to each other that the respective
sound beams radiated therefrom converge, while the transducers 70
shown in FIG. 7 are so disposed that the respective sound beams
diverge.
Embodiments of transducers 22 which are advantageously adapted
for obtaining "real time" velocity profiles are shown
in FIGS. 8 and 9 and comprise first and second pairs of discrete
transducers 80A and 80B having transmitting/receiving faces 82 of
any desired peripheral configuration and mounted on supporting surfaces
84 which are inclined so as to provide the desired orientations
for the respective sound beams radiated from each pair of transducers
80A and 80B.
It is to be noted that the methods to be described hereinbelow
do not require that one of the exemplary transducer arrangements
described hereinabove be utilized.
Although the invention has been described in detail with respect
to exemplary embodiments thereof, it will be understood by those
of ordinary skill in the art that variations and modifications may
be effected within the scope and spirit of the invention. |