Abstrict A strobed blood flow meter provides periodic measurements of blood
flow velocity or volumetric blood flow over a cardiac cycle at reduced
average power consumption, which is advantageous for reducing battery
size, and extending device battery life, such as in an implantable
application. Continuous wave Doppler, pulsed Doppler, laser Doppler,
transit time, electromagnetic flow, and thermal dilution techniques
are included. Strobing provides higher level excitation during active
periods, which improves signal-to-noise ratio, and provides a low
power standby mode during an idle time between active periods. The
invention may be used for chronic or acute applications. Doppler
or other signals may be telemetered from an implanted portion of
the flow meter for further signal processing to extract velocity
or volumetric flow. Alternatively, such signal processing is also
implanted, such that the velocity signal can be telemetered to an
remote monitor.
Claims What is claimed is:
1. A method of estimating blood flow in a vessel over a period
of time by using measurement circuits, the method comprising the
steps of: (a) using the measurement circuits to obtain a sequence
of blood flow estimates providing samples used to reconstruct a
blood flow waveform indicative of blood flow, wherein more than
one sample is required to reconstruct the blood flow waveform; and
(b) automatically deactivating at least part of the measurement
circuits during at least part of the time an estimate is not being
obtained.
2. The method of claim 1 wherein steps (a) and (b) are performed
repeatedly sufficiently frequently such that the blood flow waveform
substantially represents the variable blood flow.
3. The method of claim 1 wherein the repeated performances of
steps (a) and (b) are periodic at regular time intervals.
4. The method of claim 1 wherein the repeated performances of
steps (a) and (b) are at irregular time intervals.
5. The method of claim 1 wherein power to at least a portion of
the deactivated measurement circuits is reduced or interrupted during
at least a portion of step (b).
6. The method of claim 1 wherein step (a) includes the step of
performing a continuous wave (CW) Doppler flow measurement.
7. The method of claim 6 wherein the step of performing a CW Doppler
flow measurement includes the step of obtaining a basebanded Doppler-shifted
signal as the blood flow estimate.
8. The method of claim 6 wherein the step of performing a CW Doppler
flow measurement includes the step of obtaining a blood flow output
signal value derived from a basebanded Doppler-shifted signal as
the blood flow estimate.
9. The method of claim 1 wherein step (a) includes the step of
performing a Doppler flow measurement.
10. The method of claim 9 wherein the step of performing a Doppler
flow measurement includes the step of obtaining a basebanded Doppler-shifted
signal as the blood flow estimate
11. The method of claim 9 wherein the step of performing a Doppler
flow measurement includes the step of obtaining a blood flow output
signal value derived from a basebanded Doppler-shifted signal as
the blood flow estimate.
12. The method of claim 1 wherein step (a) includes the-step of
performing a transit time flow measurement
13. The method of claim 1 wherein step (a) includes the step of
performing an electromagnetic flow measurement.
14. The method of claim 1 wherein step (a) includes the step of
performing a thermal dilution flow measurement
15. The method of claim 1 wherein step (a) includes the step of
performing a laser Doppler flow measurement.
16. The method of claim 1 wherein the period of time is a cardiac
cycle in a living organism.
17. An apparatus for repeatedly estimating blood flow in a vessel
over a period of time, the apparatus comprising: one or more transducers;
a source of an electrical excitation signal that is applied to at
least one of the one or more transducers for an active period sufficient
to obtain a blood flow estimate and is removed from the at least
one of the one or more transducers for an idle period until a subsequent
blood flow estimate; and
18. The apparatus of claim 17 further comprising a source of one
or more power control signals applied to the source of the excitation
signal such that the source of the excitation signal is powered
down or off for at least a portion of the idle period.
19. The apparatus of claim 17 further comprising an implantable
housing containing the source of the electrical excitation signal.
20. The apparatus of claim 19 further comprising a telemetry device,
contained within the housing, for transmitting from the implantable
apparatus one or more signals containing information indicative
of the blood flow estimate.
21. The apparatus of claim 10 further comprising a source of one
or more power control signals applied to the telemetry device such
that the telemetry device is powered down or off when not transmitting
signals from the implantable apparatus.
22. The apparatus of claim 17 wherein the period of time is a
cardiac cycle in a living organism.
23. A method of repeatedly estimating blood flow in a vessel over
a period of time, the method comprising the steps of: (a) illuminating
the blood vessel with ultrasonic energy for an active period sufficient
to obtain an estimate of blood flow; (b) receiving an ultrasonic
energy signal containing Doppler-shifted frequencies in response
to backscattering of the ultrasonic energy from step (a); (c) processing
the received ultrasonic energy signal to obtain a blood flow estimate;
(d) interrupting step (a) for an idle period until a subsequent
estimating of blood flow; and (e) repeating steps (a)-(d) over the
period of time.
24. The method of claim 23 wherein the ultrasonic energy in step
(a) is continuous wave ultrasonic energy.
25. The method of claim 23 wherein the active period in step (a)
is longer than the inverse of a system bandwidth.
26. The method of claim 23 wherein the active period in step (a)
is longer than a stabilization time.
27. The method of claim 23 wherein the active period in step (a)
is longer than a mean frequency estimation time.
28. The method of claim 23 wherein step (e) includes repeating
steps (a)-(d) at a strobing frequency greater than approximately
50 Hz.
29. A method of repeatedly estimating blood flow in a blood vessel
over a period of time, the method comprising the steps of: (a) powering
on electronic circuits for producing a blood flow estimate during
an active period; (b) illuminating the blood vessel with ultrasonic
energy during at least a portion of the active period; (c) receiving
an ultrasonic energy signal containing Doppler-shifted frequencies
in response to backscattering of the ultrasonic energy from step
(b); and (d) powering down or off the electronic circuits during
an idle period until a subsequent estimating of blood flow.
30. The method of claim 29 wherein step (a) comprises powering
on an amplifier.
31. The method of claim 29 wherein step (a) comprises powering
on a receiver.
32. The method of claim 29 wherein a strobing frequency, corresponding
to the inverse of a sum of the active and idle periods, is greater
than approximately 50 Hz.
33. An apparatus for obtaining a Doppler-shifted ultrasonic energy
signal for repeatedly estimating blood flow, the apparatus comprising:
a control circuit that repeatedly provides an electrical strobed
ultrasonic-frequency signal containing ultrasonic-frequency components
during an active period that is of sufficient duration to obtain
a blood flow estimate, and contains substantially no ultrasonic-frequency
components during an idle period between the active period and a
subsequent active period; an amplifier having an amplifier input,
which is electrically coupled to the control circuit for receiving
the strobed ultrasonic-frequency signal, and an amplifier output
providing an electrical strobed amplified ultrasonic-frequency signal
in response thereto; an ultrasound transducer having a transducer
electrical input, which is electrically coupled to the amplifier
output for receiving the amplified ultrasonic-frequency signal,
and having a transducer ultrasound output, for providing strobed
ultrasonic energy to the blood vessel in response to the amplified
ultrasonic-frequency signal, and having a transducer ultrasound
input, for receiving a reflected Doppler-shifted ultrasonic energy
signal from the blood vessel, and having a transducer electrical
output, for providing an electrical received Doppler-shifted signal
in response thereto; and a receiver having a receiver input electrically
coupled to the transducer electrical output for receiving the received
Doppler-shifted signal and having a receiver output for providing
a buffered Doppler-shifted signal in response thereto.
34. The apparatus of claim 33 wherein the ultrasound transducer
comprises: a transmit transducer, which is electrically coupled
to the amplifier output for receiving the amplified ultrasonic-frequency
signal and providing strobed ultrasonic energy to the blood vessel
in response thereto; and a receive transducer, for receiving the
reflected Doppler-shifted ultrasonic energy signal from the blood
vessel and providing the received Doppler-shifted signal in response
thereto.
35. The apparatus of claim 33 further comprising a mixer having
a mixer input electrically coupled to the receiver output for receiving
the buffered Doppler-shifted signal, and having an in-phase mixer
output providing in response thereto an in-phase signal having difference
and sum frequency components that are approximately equal to the
respective difference and sum of the frequencies of the ultrasonic-frequency
signal and the buffered Doppler-shifted signal.
36. The apparatus of claim 35 wherein the mixer includes a phase-shifted
mixer output providing in response to the buffered Doppler-shifted
signal a phase-shifted signal having difference and sum frequency
components that are approximately equal to the respective difference
and sum of the frequencies of the ultrasonic-frequency signal and
the buffered Doppler-shifted signal.
37. The apparatus of claim 36 further comprising a second low
pass filter having a second low pass filter input electrically coupled
to the phase-shifted mixer output for receiving the phase-shifted
signal, and having a second low pass filter output providing a basebanded
phase-shifted Doppler signal in response thereto.
38. The apparatus of claim 37 further comprising a telemetry circuit
coupled to the second low pass filter output for receiving, and
transmitting therefrom, the basebanded phase-shifted Doppler signal.
39. The apparatus of claim 38 further comprising a remote telemetry
device that is wirelessly coupled to the telemetry circuit for receiving
the basebanded phase-shifted Doppler signal that is transmitted
therefrom.
40. The apparatus of claim 37 further comprising a first low pass
filter having a first low pass filter input electrically coupled
to the in-phase mixer output for receiving the in-phase signal,
and having a first low pass filter output providing a basebanded
in-phase Doppler signal in response thereto.
41. The apparatus of claim 40 further comprising a signal processor
for providing a blood flow velocity signal in response to the basebanded
in-phase and phase-shifted Doppler signals.
42. The apparatus of claim 40 wherein the signal processor comprises:
a first zero crossing detector, electrically coupled for receiving
the basebanded in-phase Doppler signal, and providing a first zero
cross output in response thereto; a second zero crossing detector,
electrically coupled for receiving the basebanded phase-shifted
Doppler signal, and providing a second zero cross output in response
thereto; a quadrature decoder, electrically coupled for receiving
each of the first and second zero cross outputs, and providing in
response to each voltage transition thereof, a fixed duration voltage
pulse at one of a forward and reverse output nodes; and a differential
frequency-to-voltage converter, electrically coupled for receiving
the fixed duration voltage pulse at each of the forward and reverse
output nodes, and providing in differential response thereto, a
velocity output signal.
43. The apparatus of claim 35 further comprising a first low pass
filter having a first low pass filter input electrically coupled
to the in-phase mixer output for receiving the in-phase signal,
and having a first low pass filter output providing a basebanded
in-phase Doppler signal in response thereto.
44. The apparatus of claim 43 further comprising a telemetry circuit
coupled to the first low pass filter output for receiving, and transmitting
therefrom, the basebanded in-phase Doppler signal.
45. The apparatus of claim 44 further comprising a remote telemetry
device that is wirelessly coupled to the telemetry circuit for receiving
the basebanded in-phase Doppler signal that is transmitted therefrom.
46. The apparatus of claim 33 further comprising an impedance
matching network series-connected between the amplifier and the
transducer for improving power transfer therebetween.
Description FIELD OF THE INVENTION
[0001] This invention relates to estimation of fluid flow, and
more particularly to a chronic or acute measurement of blood flow
in a blood vessel.
BACKGROUND
[0002] There are many applications in clinical and research medicine
in which measurement or estimation of volumetric blood flow within
a blood vessel is desirable. One method of making such measurements
uses ultrasonic Doppler techniques to measure blood flow velocity
and thereby estimate volumetric blood flow. Velocity of an object
is often measured using the Doppler effect Single frequency ultrasonic
energy is transmitted into an area of tissue containing the blood
flow to be measured. This insonification of the area is typically
referred to as illumination Resulting ultrasonic energy is reflected,
or backscattered, from the illuminated area. Energy reflected from
moving targets, such as fluid and blood cells, will be shifted in
frequency from the illuminating frequency according to the well-known
Doppler effect The Doppler shifted frequency provides a measure
of the blood flow velocity.
[0003] In clinical and research applications, it is often necessary
to study blood flow for an extended period of time. Thus, in ambulatory
living organisms, such as animal or human subjects, there is a need
in the art to provide a battery-powered ultrasonic Doppler blood
flow meter for measuring blood flow velocity for an extended period
of time, allowing a human or animal patient freedom of movement
during the study and minimizing the need for supervision by the
clinician. There is also a need in the art to provide a small, low-power
ultrasonic Doppler blood flow meter that is suitable for implantation
in a human or animal subject. There is a further need in the art
to provide an implantable ultrasonic Doppler blood flow meter that
maintains adequate signal-to-noise (SNR) ratio for accurate velocity
estimation.
SUMMARY
[0004] The present invention includes a method and apparatus for
estimating blood flow or blood flow velocity in a blood vessel over
a period of time. According to the method, at least part of the
measurement circuits used to estimate blood flow are automatically
activated only during the time an estimate is being obtained. At
least part of the measurement circuits are automatically deactivated
during the time an estimate is not being obtained These steps are
performed repeatedly to provide a sequence of blood flow estimates
forming a blood flow waveform indicative of blood flow. More than
one estimate is typically required to obtain a waveform representative
of the blood flow.
[0005] The steps of activating and deactivating at least part of
the measurement circuits is repeatedly performed sufficiently frequently,
either periodically or at irregular intervals, such that the blood
flow waveform substantially represents the variable blood flow.
Power to at least a portion of the measurement circuits is reduced
or interrupted while the measurement circuits are deactivated.
[0006] Measurement of blood flow can be obtained through various
blood flow measurement techniques, including: continuous wave (CW)
Doppler flow measurement, pulsed Doppler flow measurement, laser
Doppler flow measurement, transit time flow measurement, thermal
dilution flow measurement, electromagnetic flow measurement, or
other suitable flow measurement technique.
[0007] In several embodiments, a basebanded Doppler-shifted signal
provides the blood flow estimate. In other embodiments, a blood
flow output signal is derived from the basebanded Doppler-shifted
signal and provided as the blood flow estimate.
[0008] Thus, the present invention provides a strobed blood flow
meter, such as an implantable strobed ultrasonic Doppler blood flow
meter, having reduced average power consumption, which is advantageous
for reducing battery size, extending battery life, and improving
signal-to-noise ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] In the drawings, like numerals describe substantially similar
components throughout the several views.
[0010] FIG. 1 is a block diagram of one embodiment of the invention.
[0011] FIG. 2 is a block diagram illustrating one embodiment of
the mixer of FIG. 1 in more detail.
[0012] FIG. 3 is a block diagram illustrating one embodiment of
the transducer of FIG. 1 in more detail
[0013] FIG. 4 is a block diagram illustrating one embodiment of
the control circuit of FIG. 1 in more detail.
[0014] FIG. 5A is a graph illustrating generally voltage vs. time
waveforms for one embodiment in which the invention is operated.
[0015] FIG. 5B is a graph illustrating generally a velocity vs.
time signal in operation of the embodiment of FIG. 5A, but on a
compressed time scale with respect to the illustration of FIG. 5A
[0016] FIG. 6 is a block diagram illustrating one embodiment of
the present invention in which certain components are turned off
during the idle period.
[0017] FIG. 7 is a block diagram illustrating another embodiment
of the present invention in which certain components are turned
off during the idle period.
[0018] FIG. 8 is a block diagram illustrating a further embodiment
of the present invention in which certain components are turned
off during the idle period.
[0019] FIG. 9 is a block diagram illustrating in more detail the
control circuit of FIG. 8 in more detail.
[0020] FIG. 10 is a block diagram illustrating an embodiment of
the present invention including an impedance matching network.
[0021] FIG. 11 is a block diagram illustrating an embodiment of
the present invention including a signal processor.
[0022] FIG. 12 is a block diagram illustrating one embodiment of
the signal processor of FIG. 11 in more detail.
[0023] FIG. 13 is a graph generally comparing the strobed continuous
wave and pulse Doppler ultrasonic frequency waveforms.
[0024] FIG. 14 is a block diagram illustrating one embodiment of
the present invention using transit time techniques of blood flow
velocity estimation.
[0025] FIG. 15 is an end view of the embodiment illustrated in
FIG. 14.
DETAILED DESCRIPTION
[0026] In the following detailed description, reference is made
to the accompanying drawings which form a part hereof, and in which
is shown by way of illustration specific embodiments in which the
invention may be practiced. These embodiments are described in sufficient
detail to enable those skilled in the art to practice the invention,
and it is to be understood that the embodiments may be combined,
or that other embodiments may be utilize and that structural, logical
and electrical changes may be made without departing from the spirit
and scope of the present invention. The following detailed description
is, therefore, not to be taken in a limiting sense, and the scope
of the present invention is defined by the appended claims and their
equivalents.
[0027] The present invention provides a strobed blood flow meter
useful for chronic or acute estimates of blood flow or blood flow
velocity and having reduced average power consumption, which has
advantages that include reducing battery size and extending battery
life. As discussed and defined herein, estimating volumetric blood
flow and blood flow velocity are understood as interchangeable concepts,
since estimates of volumetric blood flow are obtained from estimates
of blood flow velocity by multiplying blood flow velocity with a
known constant cross-sectional area of a blood vessel. When the
cross-sectional area of the blood vessel is unknown, a signal proportional
to estimates of blood flow can still be provided from estimates
of blood flow velocity since the cross-sectional area of the blood
vessel is assumed to be relatively constant.
[0028] As used herein, the term "strobing" is defined
as repeatedly estimating blood flow velocity during a period of
interest, as discussed below. In a living organism having a circulatory
system with a cardiac cycle, which is defined as the period between
successive heartbeats, the period of interest for strobing may be
one or more such cardiac cycles. However, it is also desirable to
repeatedly estimate blood flow velocity over a period of interest
when no cardiac cycle is present. For example, certain embodiments
of an artificial heart pump may be implemented without the periodic
pulsing associated with a heartbeat. In such systems, it may still
be desirable to repeatedly estimate blood flow velocity over some
other period of interest.
[0029] As will be described in detail below, the present invention
encompasses strobing or automatically activating certain portions
of the blood flow meter during an active period in order to obtain
an ultrasonic Doppler blood flow velocity estimate, and later automatically
deactivating these portions of the blood flow meter during an idle
time between such estates. As a result, average power consumption
is advantageously reduced Strobing according to the present invention
includes a wide variety of blood flow measurement techniques, including,
but not limited to: ultrasonic Doppler blood flow measurement, such
as both continuous wave (CW) and pulsed Doppler blood flow measurements;
transit time measurements; electromagnetic flow measurements; thermal
dilution measurements; and laser Doppler measurements, each of which
is described further below.
[0030] FIG. 1 is a block diagram illustrating one embodiment of
the present invention. In FIG. 1 strobed ultrasonic blood flow
meter 100 is capable of being implanted in a human or animal subject
for measurement of blood flow in blood vessel 105. Blood flow meter
100 comprises oscillator 110 which is a sine or square wave oscillator
operating at a carrier frequency in an ultrasonic region of the
frequency spectrum, typically in the 5-20 MHz range, though other
frequencies are also possible. The ultrasonic sine or square wave
output signal of oscillator 110 at node 115 is referred to as a
carrier signal. The carrier signal frequency at node 115 is in the
ultrasonic frequency range, and is electrically coupled to a control
circuit 120 at control circuit oscillator input 125. Control circuit
120 produces at control circuit output 130 a resulting electrical
strobed ultrasonic-frequency signal (shown as signal 145V in FIG.
5A) which is electrically coupled to amplifier input 135 of power
amplifier 140 through node 145. In response, amplifier 140 produces
a resulting electrical strobed amplified ultrasonic-frequency signal
at amplifier output 150 which is electrically coupled through node
165 to transducer electrical input 155 of transducer 160. In response,
transducer 160 provides, at transducer ultrasound output 170 ultrasonic
energy that is mechanically or acoustically coupled to tissue including
blood vessel 105. In this patent application, providing ultrasonic
energy, insonifying, and insonating, are all referred to generally
as illuminating.
[0031] Illumination of blood vessel 105 results in a reflected
Doppler-shifted ultrasound signal, also referred to as a backscattered
signal, that is received at transducer ultrasound input 175 and
converted by transducer 160 into a Doppler-shifted electrical signal
at transducer electrical output 180. The Doppler-shifted electrical
signal is electrically coupled through node 195 to receiver input
185 of receiver 190 which provides a buffered Doppler-shifted signal
in response thereto at receiver output 200.
[0032] Mixer 205 receives the buffered Doppler-shifted signal at
mixer input 210 through node 215. Mixer 205 also receives through
node 115 the carrier signal of oscillator 110 at mixer oscillator
input 220. Mixer 205 performs a demodulation function by quadrature
mixing, as described below, producing an in-phase (I) signal at
in-phase (I) output 225 and a phase-shifted (Q) signal, which is
90 degrees out of phase with respect to the I signal, at phase-shifted
(Q) output 230. The I and Q signals each have components that include
difference and sum frequency components that are approximately equal
to the respective difference and sum of the frequencies of the carrier
signal and the buffered Doppler-shifted signal. The I and Q signals
may also contain a carrier frequency component, also referred to
as carrier feedthrough.
[0033] The I signal is electrically coupled through node 235 to
a first low pass filter input 240 of first low pass filter 245.
First low pass filter 245 removes the carrier feedthrough and the
sum frequency components of the I signal, and provides the difference
frequency component at the first low pass filter output 250. The
difference frequency component at the first low pass filter output
250 is referred to as the basebanded in-phase Doppler signal or
the basebanded I Doppler signal. Similarly, the Q signal is electrically
coupled through node 255 to a second low pass filter input 260 of
second low pass filter 265. Second low pass filter 265 removes the
carrier feedthrough and the sum frequency components of the Q signal
and provides the difference frequency component at the second low
pass filter output 270. The difference frequency component at the
second low pass filter output 250 is referred to as the basebanded
phase-shifted Doppler signal, or the basebanded Q Doppler signal.
[0034] The basebanded I and Q Doppler signals are electrically
coupled through respective nodes 275 and 280 to respective inputs
of telemetry circuit 285. In one embodiment, the basebanded I and
Q Doppler signals are remodulated with a telemetry carrier frequency
for transmission to a remote telemetry device 282 such as an external
telemetry receiver. In another embodiment, as described below, an
analog velocity output signal is produced, which is encoded, such
as by pulse position modulation, for transmission to remote telemetry
device 282. Thus, telemetry circuit 285 allows transmission of the
signals corresponding to the basebanded I and Q Doppler signals
from implanted blood flow meter 100 to a remote telemetry device
282 for further precessing. In one embodiment, this further processing
includes velocity determination according to the well-known Doppler
equation, illustrated in Equation (1). 1 v = f d C 2 f c cos ( 1
)
[0035] In Equation (1): v is the blood flow velocity to be determined;
f.sub.d is the (basebanded) received Doppler shifted frequency reflected
from the blood flow, C is the speed of sound in the medium, e.g.
tissue; f.sub.c is the carrier frequency; and .theta. is the angle
formed by the velocity vector of the blood flow and the path along
which the illuminating ultrasonic energy is provided.
[0036] FIG. 2 is a block diagram illustrating one embodiment of
mixer 205 in more detail. In FIG. 2 mixer 205 includes quadrature
phase splitter 300 first multiplier 305 and second multiplier
310. Splitter 300 receives, through node 115 the carrier signal
at splitter input 315 and produces in response thereto a resulting
in-phase carrier signal at node 320 and a phase-shifted carrier
signal at node 325 that is phase-shifted by 90 degrees with respect
to the in-phase carrier signal. The in-phase carrier signal at node
320 and the phase-shifted carrier signal at node 325 are substantially
quadruture balanced, i.e. they are substantially matched in amplitude,
and have a phase difference which is very close to 90 degrees. The
buffered Doppler signal at node 215 is multiplied at first multiplier
305 by the in-phase carrier signal at node 320 to produce the I
signal at node 235. The buffered Doppler signal at node 215 is also
multiplied at second multiplier 31O by the phase shifted carrier
signal at node 325 to produce the Q signal at node 255.
[0037] FIG. 3 is a block diagram illustrating one embodiment of
transducer 160 in more detail, in relation to blood vessel 105.
In FIG. 3 transducer 160 includes ultrasound transmit transducer
330 and ultrasound receive transducer 335. Transmit and receiver
transducers 330 and 335 are preferably single piston piezoelectric
transducers, comprised of materials such as lead zirconate titanate
(PZT) crystal or composite materials. Other piezoelectric crystal,
ceramic, or polymer, or any other suitable transducer may also be
used.
[0038] Transmit transducer 330 receives the electrical strobed
amplified ultrasonic-frequency signal at input 155 and provides,
or launches, continuous wave (CW) ultrasonic energy at transducer
ultrasound output 170 for illumination of blood vessel 105. Illumination
of blood vessel 105 results in a reflected Doppler-shifted ultrasound
signal at transducer ultrasound input 175 that is received by receive
transducer 335 and converted into an electrical received Doppler-shifted
signal at transducer electrical output 180. In FIG. 3 separate
transmit and receive transducers 330 and 335 are used for simultaneously
illuminating and receiving CW Doppler ultrasound. However, it is
understood that a single transducer could also be used for sequentially
illuminating and receiving pulsed Doppler ultrasound, as described
below.
[0039] FIG. 4 is a block diagram illustrating one embodiment of
control circuit 120 in more detail. In FIG. 4 control circuit 120
includes sine wave to square wave converter 350 digital control
logic 355 and strobing switch 360. Converter 350 receives the carrier
signal at node 115 and provides to digital control logic 355 a square
wave clock signal at node 365 which can be divided down to lower
frequencies if desire. Converter 350 is omitted if oscillator 110
is a square wave, rather than a sine wave oscillator. Logic 355
provides a periodic strobing control signal at node 370 also available
at strobing control signal output 371 to control the conductance
of the carrier signal at node 115 through strobing twitch 360 to
control circuit output 130. However, the periodic strobing control
signal at node 370 could alternatively be provided at irregular
intervals. A resulting electrical strobed ultrasonic-frequency signal
is provided through node 145 for amplification by amplifier 140
and conversion into ultrasound energy by transducer 160.
[0040] FIG. 5A is a voltage vs. time graph illustrating generally
timing in one embodiment in which the present invention is operated.
FIG. 5A includes strobing control signal 370V at node 370 and the
strobed ultrasonic frequency signal 145V at node 145. A corresponding
velocity vs. time graph is illustrated in FIG. 5B, but with time
illustrated on a compressed time scale with respect to that in FIG.
5A. In FIG. 5A, strobing control signal 370 is a periodic control
signal having a corresponding strobing period, t.sub.strobe. The
strobing period is comprised of an active period, t.sub.on, and
an idle period, t.sub.off.
[0041] During the active period of the strobing control signal
370V, the carrier frequency signal at node 115 is conducted to node
145 through the strobing switch 360 as illustrated during the corresponding
portion of the strobed ultrasonic frequency signal 145V. During
the idle period of the strobing control signal 370V, the carrier
frequency signal at node 115 is isolated from node 145 by the strobing
switch 360 as illustrated during the corresponding portion of the
strobed ultrasonic frequency signal 145V. Blood vessel 105 is illuminated
during each active period of the strobing control signal 370V, as
illustrated in FIG. 5A. Velocity is determined near the end of each
active period of the strobing control signal 370V, such as at times
t.sub.1 t.sub.2 and t.sub.3 as illustrated in FIGS. 5A and 5B.
[0042] Blood velocity will vary depending on the size and physiological
location of the blood vessel 105 being measured. Blood velocity
will also vary as a function of time during the cardiac cycle, i.e.
during and between successive heartbeats. One embodiment of the
present invention uses a programmably adjustable strobing frequency,
which is the inverse of the strobing period. The strobing frequency
should be high enough to provide a representative estimated velocity
vs. time waveform both during the cardiac cycle and over many cardiac
cycles. For example, in most larger mammals, heart rate varies from
between 40 to 200 beats per minute. A strobing frequency of 50 Hz
respectively provides 75 and 15 estimated velocity data points for
each of these respective heart rates. For smaller mammals, such
as rats, heart rate may approach 400 beats per minute. Increasing
strobing frequency to 100 Hz would still allow 15 estimated velocity
data points for this case.
[0043] The particular strobing frequency may be selected to obtain
the desired time resolution of velocity estimates. The desired time
resolution of velocity estimates may in turn be selected to accommodate
the expected rate of change of blood flow velocity in the blood
vessel. The rate of change of the blood flow velocity is typically
higher for an arterial blood vessel 105 that is more proximal to
the heart than for an arterial blood vessel 105 that is more distal
from the heart or for a venal blood vessel 105. As set forth above,
t.sub.strobe will exceed t.sub.on. But the maximum value of t.sub.strobe
will depend on many factors, including whether an accurate reconstruction
of the velocity waveform is needed or whether the velocity estimates
are used only to determine blood flow, such that fewer estimates
per cardiac cycle may suffice.
[0044] In one embodiment, active period, t.sub.on, is minimized
to minimize average power consumption or to obtain other advantages,
as described below. However, the minimum active period is typically
longer than some combination of: a system bandwidth; a stabilization
time; and a mean-frequency estimation time.
[0045] The system bandwidth is defined as the inverse of the maximum
expected basebanded I and Q Doppler signal frequencies, which can
be calculated from the well-known Doppler equation for a particular
blood velocity.
[0046] The stabilization time is the time required to power up
and stabilize certain electronic circuits which are powered down
during the idle period. The required stabilization time may be dominated
by, for example, the filter time-constants of first and second low
pass filters 245 and 265 if these filters were powered down during
the idle period. In another example, the required stabilization
time may be dominated by the charging of a power supply output capacitor
from which power is supplied to those electronic circuits that were
turned off during the idle period. Separate control signals may
be provided to individual electronic circuits to tailor the time
that the circuits are powered to meet their individual stabilization
requirements. For example, first and second low pass filters 245
and 265 may be turned on prior to providing the electrical signal
to drive transducer 160 to accommodate longer stabilization time
requirements of first and second low pass filters 245 and 265.
[0047] The mean frequency estimation time is determined by the
number of samples of the basebanded I or Q Doppler signals at respective
nodes 275 and 280 that must be acquired to accurately estimate the
blood velocity for a particular velocity estimate. The mean frequency
estimation time depends, in turn, on the particular mean frequency
estimation technique used. In one embodiment, sophisticated digital
signal processing techniques are used to extract a relatively accurate
mean frequency estimate from as few as 8 of the samples. In another
embodiment, zero-cross detection techniques are used to provide
a root mean square (rms) reading of mean frequency from more than
100 samples.
[0048] The present invention uses strobed ultrasonic energy, which
advantageously reduces its average power consumption. This is particularly
important when power is drawn from a fixed resource, such as a battery,
which is implanted in vivo together with the electronics of blood
flow meter 100 and cannot be easily replaced. In such situations,
the reduced average power consumption of the present invention is
critical for extending battery life of blood flow meter 100. The
average power consumption of the present invention is illustrated
by Equation (2). 2 Power = P on t on + P off t off t strobe ( 2
)
[0049] In Equation (2), P.sub.on is the power consumption during
the active period and P.sub.off is the power consumption during
the idle period. As explained below, most of the electronics of
blood flow meter 100 are powered on during the active period, but
only a subset of these electronics are powered on during the idle
period For this reason, P.sub.on exceeds P.sub.off. Thus, as illustrated
in Equation (2), average power consumption is minimized by: reducing
the duration of the active period; and, increasing the strobing
period; and, decreasing both P.sub.on and P.sub.off, particularly
P.sub.on.
[0050] FIG. 6 is a block diagram illustrating one embodiment of
the present invention in which only amplifier 140 and telemetry
285 are turned off during the idle period. The strobing control
signal at node 370 is electrically coupled to switchably control
the conductances between each of amplifier 140 and telemetry 285
blocks and their respective power supplies. Transducer 160 typically
does not draw any bias current, but use of any transducer that does
draw bias current could similarly have its bias current switchably
controlled by strobing control signal 370. By leaving other blocks
powered during the idle period, stabilization time is reduced, as
described above. However, this embodiment does not minimize average
power consumption as much as other possible embodiments.
[0051] FIG. 7 is a block diagram illustrating another embodiment
of the present invention in which amplifier 140 receiver 190 mixer
205 fist and second low pass filters 245 and 265 and telemetry
285 are all turned off during the idle period. The strobing control
signal at node 370 is electrically coupled to switchably control,
either independently or in groups, the conductances between each
of amplifier 140 receiver 190 mixer 205 first and second low
pass filters 245 and 265 and telemetry 285 and their respective
power supplies. Since more components are powered down during the
idle period, this embodiment decreases average power consumption
further from that of FIG. 6 but stabilization time may be increased,
as explained above.
[0052] FIG. 8 is a block diagram illustrating another embodiment
of the present invention in which oscillator 110 amplifier 140
receiver 190 mixer 205 first and second low pass filters 245 and
265 and telemetry 285 are all turned off during the idle period.
The strobing control signal at node 370 is electrically coupled
to switchably control, either independently or in groups, the conductances
between each of oscillator 10 amplifier 140 receiver 190 mixer
205 first and second low pass filters 245 and 265 and telemetry
285 and their respective power supplies. FIG. 8 uses a control circuit
400 which is illustrated in more detail in FIG. 9. Since more components
are powered down during the idle period, this embodiment decreases
average power consumption further from that of FIGS. 6-7.
[0053] FIG. 9 is a block diagram illustrating in more detail the
control circuit 400 of FIG. 8. In FIG. 9 a separate timing generator
410 is provided for coupling a clock signal through node 365 to
digital control logic 355. As in the embodiments illustrated in
FIGS. 1 and 6-7 at least a portion of the digital control logic
remains powered during the idle period in the embodiment illustrated
in FIGS. 8-9. In the embodiment illustrated in FIGS. 8-9 the timing
generator 410 also remains powered during the idle period. Timing
generator 410 is capable of being operated at a lower frequency
than the ultrasonic frequencies of oscillator 110. Use of timing
generator 410 allows the higher frequency oscillator 110 to be powered
down during the idle period. This results in further average power
savings in some implementations of the present invention.
[0054] Thus, the invention described above in FIGS. 1-9 provides
a method of estimating the velocity of blood flow in a blood vessel.
At least part of the measurement circuits are automatically activated
only during the time an estimate is being obtained. At least part
of the measurement circuits are deactivated during the time an estimate
is not being obtained. These steps are performed repeatedly to provide
a sequence of blood flow estimates forming a blood flow waveform
indicative of blood flow. More than one estimate is required to
obtain the blood flow waveform.
[0055] According to one embodiment of the present invention, ultrasonic
energy is repeatedly applied to the blood flow in the blood vessel,
either periodically or at irregular time intervals over a period
of time, such as during all or a portion of one or more cardiac
cycles. A portion of the applied energy is reflected from the blood
flow to produce a reflected ultrasonic energy signal. The reflected
ultrasonic energy is received for further processing from which
blood flow velocity is measured. Electronic circuits are powered
off or down between the repeated applications of ultrasonic energy,
thereby allowing increased levels of illumination while maintaining
or reducing average power consumption.
[0056] As described above, one embodiment of the present invention
uses strobed ultrasonic energy, which advantageously reduces its
average power consumption because portions of the present invention
are powered off between strobing instances. This advantage, or a
portion thereof, may be traded for improved signal-to-noise ratio
(SNR), which is also a desirable characteristic for accurate measurement
of blood flow velocity. For example, transducer 160 is capable of
providing higher level illumination of blood vessel 105 than in
a conventional system, because strobed ultrasonic energy is used,
i.e. the higher level illumination is limited to a shorter duration.
Since blood vessel 105 is illuminated at a higher level, more reflected
energy is available for detection, thereby improving the SNR.
[0057] Similar signal processing improvements are also available,
for example, by using higher supply currents for shorter durations
in those other blocks that are capable of being powered down during
the idle period, such as receiver 190 mixer 205 and first and
second low pass filters 245 and 265. These signal processing improvements
obtained from higher current levels for shorter durations include
better noise performance and higher bandwidth These improvements
provided by the present invention are particularly advantageous
for the receiver 190 and mixer 205 blocks, which require bandwidths
capable of accommodating a Doppler-shifted signal centered around
the 5-20 MHz carrier frequency. Thus, the strobed ultrasonic blood
flow velocity measurements of the present invention offer considerable
advantages in addition to reduced average power consumption.
[0058] Trading off the average power savings of the strobed CW
Doppler system of present invention for higher power during the
active period is further illustrated by way Example 1 comparing
the present invention to a conventional CW Doppler system
EXAMPLE 1
[0059]
1 Conventional CW Doppler Strobed CW Doppler I.sub.avg = 2mA I.sub.avg
= 2mA t.sub.strobe = 20 ms (50 Hz strobing) t.sub.on = 2 ms t.sub.off
= 18 ms I.sub.idle = 500 .mu.A during t.sub.off I.sub.active = 15.5
mA during t.sub.on
[0060] Example 1 illustrates, for a 50 Hz strobing frequency and
(t.sub.on/t.sub.strobe)=10%, the strobed current can be as high
as 15.5 mA for an idle current of 500 .mu.A. Thus, in this example,
the current can be elevated by a factor of 7.75 in the strobed CW
Doppler system without increasing the average power consumption
over a conventional CW Doppler system.
[0061] FIG. 10 is a block diagram illustrating another embodiment
of the present invention in which an impedance matching network
450 has been interposed between amplifier output 150 and transducer
electrical input 155. Network 450 includes passive impedance matching
components to maximize power transfer between amplifier 140 and
transducer 160 at the carrier frequency, where amplifier 140 typically
presents an impedance that is unmatched to that of transducer 160.
[0062] The impedance matching network results in more efficient
power transfer at the output of network 450 at node 165B for the
strobed CW Doppler system of the present invention over a conventional
CW Doppler system, as illustrated in Example 2.
EXAMPLE 2
[0063]
2 Conventional CW Doppler Strobed CW Doppler Z.sub.out = 2000Q
at carrier frequency Z.sub.out = 400.OMEGA. at carrier frequency
Z.sub.tran = 20Q at carrier frequency Z.sub.tran = 20.OMEGA. at
carrier frequency Z.sub.network = 100 to 1 matching Z.sub.network
= 20 to 1 matching I.sub.amp = 1 mA peak I.sub.amp = 5 mA peak during
active period V.sub.amp = 4V.sub.p-p continuous V.sub.amp = 4 V.sub.p-p
during active period P.sub.amp = 1 mW P.sub.amp = 10 mW P.sub.transducer
< 1 mW P.sub.transducer = 10 mW V.sub.transducer = 0.25 V.sub.p-p
V.sub.transducer = 0.89 V.sub.p-p
[0064] In Example 2: Z.sub.out is the output impedance of amplifier
140 at amplifier output 150 at the ultrasonic carrier frequency;
Z.sub.tran is the impedance of transducer 160 at the carrier frequency;
Z.sub.network is the impedance matching ratio of network 450; I.sub.amp
is the peak output current of amplifier 140; V.sub.amp is the peak-to-peak
output voltage of amplifier 140; P.sub.amp is the power output of
amplifier 140; P.sub.transducer is the power input of transducer
160; and, V.sub.transducer is the peak-to-peak input voltage of
transducer 160.
[0065] In Example 2 the conventional CW Doppler system is operated
continuously, and the strobed CW Doppler system is operated at a
10% duty cycle (t.sub.on/t.sub.strobe) with the negligible current
during the idle period. As seen in Example 2 amplifier 140 and
network 450 of the strobed Doppler system of FIG. 10 allow higher
power output from amplifier 140 and a higher input voltage of transducer
160. This produces a higher level illumination, resulting in more
reflected ultrasonic energy, and thereby improving the SNR.
[0066] FIGS. 1-10 illustrate various embodiments of the present
invention in which the basebanded I and Q Doppler signals are telemetered
to other circuits for further processing to determine the blood
flow velocity estimate. In one embodiment, for example, the basebanded
I and Q Doppler signals are telemetered from an implanted portion
of the blood flow meter 100 to accompanying external circuits for
further processing. However, signal processing of the basebanded
I and Q Doppler signals can also be carried out within the implanted
blood flow meter 100.
[0067] FIG. 11 is a block diagram illustrating an embodiment of
the present invention in which a signal processor 470 is contained
within the implanted blood flow meter 100. In FIG. 11 signal processor
470 receives the basebanded I and Q Doppler signals at respective
nodes 275 and 280 and produces a blood flow output signal or velocity
output signal representing the estimated blood flow velocity. The
velocity output signal is electrically coupled through node 475
to telemetry 285 where it is transmitted from the implanted blood
flow meter 100 to an external receiver.
[0068] FIG. 12 is a block diagram illustrating one embodiment of
signal processor 470 that is particularly useful in applications
having a single-ended power supply, such as a battery in the implantable
blood flow meter 100 of the present invention. In FIG. 12 signal
processor 470 contains a voltage reference 500 which provides a
stable output bias voltage at node 505 to a first input of each
of first and second amplifiers 510 and 520 and first and second
zero cross detectors 530 and 540. First and second amplifiers 510
and 520 provide gain, or provide both gain and level-shifting. First
and second amplifiers may also be used to provide bandpass filtering.
A second input of first amplifier 510 receives the basebanded I
Doppler signal at node 275. A second input of second amplifier 520
receives the basebanded Q Doppler signal at node 280.
[0069] First amplifier 510 provides a buffered basebanded I Doppler
signal at node 545 to a second input of first zero cross detector
530. Second amplifier 520 provides a buffered basebanded Q Doppler
signal at node 550 to a second input of second zero cross detector
540. First and second zero cross detectors 530 and 540 provide first
and second zero cross outputs at respective nodes 555 and 560. The
first and second zero cross outputs at respective nodes 555 and
560 each change logic state in response to the voltage of respective
buffered I and Q Doppler signals passing through the bias voltage
at node 505. Each of the resulting pulsatile voltages waveforms
at the first and second zero cross outputs is approximately 90 degrees
out of phase with the other, and is at the basebanded Doppler frequency.
[0070] Quadrature encoder 565 receives the first and second zero
cross outputs at respective nodes 555 and 560. The 90 degree phase
difference between the voltage waveforms at nodes 555 and 560 make
it possible to determine their phase relationship at each logic
voltage transition of these voltage waveforms at nodes 555 and 560.
Quadrature encoder 565 contains logic circuitry for determining
the phase relationship between the first and second zero cross outputs
at nodes 555 and 560 and does so at each voltage transition at
each of nodes 555 and 560. In response to each such determination,
quadrature encoder 565 provides a fixed-duration voltage pulse to
only one of forward node 570 or reverse node 575.
[0071] Differential frequency-to-voltage converter 580 receives
voltage pulses at each of the respective forward and reverse nodes
570 and 575 and provides a resulting blood flow output signal such
as the analog velocity output signal at node 475. In one embodiment,
converter 580 provides charge integration of the fixed-duration
voltage pulses at each of the respective forward and reverse nodes
570 and 575 and provides the resulting blood flow output signal
in response thereto. The charge of the voltage pulses at the forward
node 570 incrementally increases the velocity output signal at node
475 and the charge of the voltage pulses at the reverse node 575
incrementally decreases the velocity output signal at node 475.
Converter 580 could also be implemented as an up-down counter providing
an output count representative of the velocity output signal. Voltage
pulses received at forward node 570 increment the output count,
and voltage pulses received at reverse node 575 decrement the output
count, or vice versa.
[0072] Thus, signal processor 470 is capable of providing, using
a single-ended power supply, an analog velocity output signal at
node 475 containing both magnitude and directional information of
blood flow velocity. The analog velocity output signal at node 475
can be repeatedly sampled to provide a sequence of blood flow estimates
forming a blood flow waveform indicative of blood flow. The analog
velocity output signal at node 475 or the samples derived therefrom
can be further processed and transmitted from the implanted blood
flow meter 100.
[0073] FIGS. 1-12 illustrate various bidirectional embodiments
of the present invention that are capable of determining the magnitude
and direction of blood flow velocity. If direction information is
not needed, a unidirectional embodiment of the present invention
could be used. In a unidirectional embodiment of the present invention,
one of the I or Q channels is omitted. In mixer 205 a quadrature
phase splitter 300 is omitted and only one of first and second multipliers
305 and 310 is needed. In signal processor 470 quadrature encoder
565 is replaced by a monostable oscillator (one-shot) providing
a fixed-duration pulse, and differential frequency-to-voltage converter
580 is replaced by a single-ended frequency-to-voltage converter.
[0074] The present invention has been described above with respect
to a particular embodiment of strobed ultrasonic Doppler blood flow
meter, i.e. a strobed continuous wave (CW) ultrasonic Doppler blood
flow meter, referred to as a strobed CW Doppler blood flow meter.
However, it is understood that the present invention is also broadly
applicable to any embodiment of a strobed ultrasonic Doppler blood
flow meter and its method of use.
[0075] For example, the invention encompasses the use of a strobed
ultrasonic pulsed Doppler blood flow meter, referred to as a strobed
pulsed Doppler blood flow meter. The strobed pulsed Doppler embodiment
also periodically illuminates a blood vessel by a transducer, but
each illumination comprises bursts of pulsatile (or pulse train)
ultrasonic-frequency energy. Each burst of ultrasonic-frequency
energy from a particular illumination is reflected, or backscattered,
from the blood flow and typically subsequently detected at the same
transducer. Samples of the resulting electrical signal, each corresponding
to a burst of pulsatile ultrasonic-frequency energy, are used to
estimate mean frequency. A resulting blood flow velocity estimate
is produced from the aggregation of mean frequency estimations within
a particular strobing.
[0076] FIG. 13 illustrates generally a comparison of the strobed
ultrasonic frequency signal waveforms used in each of the strobed
CW and strobed pulsed Doppler embodiments. In FIG. 13 the strobing
control signal 370V illustrates generally the active and idle periods
in relation to the strobing period. The CW embodiment provides an
ultrasonic frequency signal 145V continuously over the entire active
period or at least some portion thereof. The strobed pulsed Doppler
embodiment provides a pulsed ultrasonic frequency signal 600 that
typically contains more than one burst of pulsatile ultrasonic-frequency
energy over the active period or at least some portion thereof.
[0077] In fact, as illustrated in FIG. 13 the type of ultrasonic
energy signal used is not essential to the invention. Thus, both
of the above-described ultrasonic blood flow meters have characteristics
that include: repeatedly illuminating the blood vessel with ultrasonic
energy during a cardiac cycle; repeatedly receiving during the cardiac
cycle an ultrasonic energy signal, which contains Doppler-shifted
frequencies corresponding to a blood flow velocity estimate, reflected
from the blood flow; and, processing the received ultrasonic energy
signal to obtain the blood flow velocity estimate from the Doppler-shifted
frequencies contained therein.
[0078] In both species of strobed ultrasonic blood flow meters,
the ultrasonic energy is strobed repeatedly throughout the cardiac
cycle or other period of interest, with a strobing frequency which
is substantially lower than the ultrasonic energy frequency. In
one embodiment of the present invention, each strobing instance
corresponds to a resulting blood flow velocity estimate.
[0079] The above-described embodiments describe a blood flow meter
that estimates blood flow velocity by strobed Doppler measurements
of backscattered ultrasonic energy. However, the strobed blood flow
meter according to the present invention also includes other techniques
of estimating blood flow velocity, including, but not limited to:
transit time measurements, electromagnetic flow measurements, thermal
dilution measurements, and laser Doppler measurements, each of which
is described further below.
[0080] FIG. 14 is a generalized schematic illustration of one embodiment
of a transit time measurement of blood flow velocity that is encompassed
by the present invention. First and second transducers 650 and 655
respectively, are configured for ultrasonic communication therebetween
via an acoustic reflector 660. A first ultrasonic impulse 665 is
launched from first transducer 650 reflected from reflector 660
and received at second transducer 655. A second ultrasonic impulse
670 is launched from second transducer 655 reflected from reflector
660 and received at first transducer 650.
[0081] FIG. 14 illustrates the case where first impulse 665 has
a directional component in the same direction as the blood flow
in blood vessel 105 and second impulse 670 has a directional component
opposite the direction of blood flow in blood vessel 105. As a result,
a travel time of second impulse 670 from second transducer 655 to
first transducer 650 is longer than a travel time of first impulse
665 from first transducer 650 to second transducer 655. Blood flow
velocity is calculated from the difference in transit times of the
first and second impulses 665 and 670 respectively.
[0082] In this embodiment, the invention includes a control circuit
675 for providing a strobed ultrasonic frequency signal to each
of respective first and second amplifiers 680 and 685 through respective
nodes 690 and 695. Control circuit 675 optionally provides power
control signals to respective first and second receivers 700 and
705 through respective nodes 710 and 715. First and second amplifiers
680 and 685 respectively, provide an amplified strobed ultrasonic
frequency signal at respective nodes 720 and 725 to respective first
and second transducers 650 and 655 which provide the first and
second impulses 665 and 670 in response thereto.
[0083] First and second transducers 650 and 655 also receive respective
second and first impulses 670 and 665 as described above, and provide
resulting electrical signals to respective first and second receivers
700 and 705 through respective nodes 730 and 735. First and second
receivers 700 and 705 respectively, provide buffered electrical
signals to processing circuit 740 through respective nodes 745 and
750. Processing circuit 740 calculates blood flow velocity from
the difference in transit times of the first and second impulses
665 and 670 respectively, and provides through node 755 a signal
containing blood flow velocity information to telemetry device 760
for transmission to a remote telemetry device. Control circuit 675
optionally provides a power control signal to processing circuit
740 through node 765 for reducing or removing power from processing
circuit 740 between transit time estimates of blood flow velocity.
As described above, control circuit 675 may also optionally provide
a power control signal to telemetry device 760 to reduce or remove
power from telemetry device 760 when it is not transmitting a transit
time estimate of blood flow velocity.
[0084] FIG. 15 illustrates an end view of the configuration of
FIG. 14. In FIG. 15 first and second transducers 650 and 655 respectively,
and reflector 660 are arranged such that first and second impulses
665 and 670 respectively, each provide an insonification area 770
that includes the entire area of blood vessel 105 such that an
average estimate of blood flow over the area of blood vessel 105
is provided. The transit time estimate of blood flow velocity may
also be improved by averaging multiple transit time measurements
to provide a single estimate of blood flow velocity. In such an
embodiment, control circuit 675 reduces or removes power from other
circuits between each series of transit time measurements used to
provide a blood flow velocity estimate. A sequence of blood flow
estimates forms a waveform representative of blood flow over a period
of time.
[0085] The present invention also includes the use of electromagnetic
flow techniques to estimate blood flow velocity. In one embodiment
of this technique, first and second electrodes are disposed across
an interposed blood vessel such that the blood flow is in a direction
that is substantially orthogonal to a vector between the first and
second electrodes. A permanent magnet or electromagnet is used to
create a magnetic field through the blood vessel in a direction
that is substantially orthogonal to both the direction of blood
flow and the vector between the first and second electrodes. As
a result, ionized particles within the blood flow are deflected
toward one of the first and second electrodes, resulting in a voltage
difference therebetween that is proportional to the blood flow velocity.
The invention uses the above-described strobing technique to reduce
or remove power between blood flow estimates to circuits within
the blood flow meter, such as to the electromagnet, if any, or to
sensing and processing circuits that detect the voltage difference
between the first and second electrodes, or to telemetry circuits
that transmit electromagnetic flow estimates of blood flow velocity
to a remote telemetry device.
[0086] The present invention also includes the use of thermal dilution
techniques to estimate blood flow. In one embodiment of this technique,
a heater is used to pulsedly heat the blood, and the heated blood
pulse is detected by a temperature sensor located at a known distance
from the point of heating in the direction of the blood flow. Volumetric
blood flow is calculated from the time between the heating of the
blood pulse and the detection of the blood pulse. Several heated
blood pulses are typically introduced and detected to produce a
more accurate blood flow estimate.
[0087] In another embodiment of this technique, a single thermistor
is used for both heating and detection. A heated thermistor is introduced
into the blood vessel such that it is in thermal contact with the
blood flow, and cooling of the thermistor is effected by the blood
flow. Blood flow at a higher velocity cools the thermistor at a
higher rate than blood flow at a lower velocity. The energy delivered
to the thermistor to maintain the thermistor at a constant temperature
is proportional to blood flow velocity. Alternatively, the thermistor
can be heated to a known temperature, and the time required to cool
the thermistor to a second, lower temperature will be inversely
proportional to blood flow.
[0088] According to the present invention, measuring circuits in
the above-described thermal dilution embodiments are automatically
activated only during estimation of blood flow, and are powered
down or off between estimates of blood flow. A resulting volumetric
blood flow vs. time waveform constructed from the sequence of blood
flow estimates is thereby obtained at a reduced power consumption
by application of the strobing technique of the present invention.
[0089] The present invention also includes the use of laser Doppler
techniques to estimate blood flow. The blood flow is illuminated
with a coherent monochromatic light source signal. A resulting backscattered
Doppler-shifted light signal is received at an optical detector,
and demodulated such as by mixing with the monochromatic light source
signal. Blood flow velocity is estimated from a resulting basebanded
Doppler-shifted frequency of the received light signal. According
to the present invention, measuring circuits, optionally including
the monochromatic light source, are automatically activated only
during estimation of the blood flow velocity. These measuring circuits
are deactivated, i.e. powered down or off between estimates of blood
flow velocity. A resulting velocity vs. time waveform constructed
from the sequence of blood flow velocity is thereby obtained at
a reduced power consumption by application of the strobing technique
of the present invention.
[0090] Thus, the present invention provides an strobed blood flow
meter, such as an implantable strobed ultrasonic Doppler blood flow
meter, having reduced average power consumption, which is advantageous
for reducing battery size, improving signal-to-noise ratio, and
extending battery life.
[0091] It is to be understood that the above description is intended
to be illustrative, and not restrictive. Many other embodiments
will be apparent to those of skill in the art upon reviewing the
above description. The scope of the invention should, therefore,
be determined with reference to the appended claims, along with
the fill scope of equivalents to which such claims are entitled.
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