Apparatus and method for determining blood flow velocity
Apparatus and method comprising an ultrasound transmitter, for placement at a first location on the body of a subject, to emit an ultrasound pulse; an ultrasound receiver, for placement at a second location on the body, to detect an emitted ultrasound pulse; and a controller in communication with the transmitter and receiver. The controller causes an ultrasound pulse to be emitted by the transmitter; receives a measurement signal from the receiver; determines, based on the received measurement signal, a time of arrival at the receiver, T 1 s of a first part of the emitted ultrasound pulse; determines, based on the received measurement signal, a time of arrival at the receiver, T 2 , of a second part of the ultrasound pulse; and calculates, using T 1 and T 2 , a flow velocity of blood in a blood vessel between the first location and the second location.
1. An apparatus for measuring flow velocity of blood flowing in a blood vessel of a subject, the apparatus comprising:
an ultrasound transmitter, for placement at a first location on the body of a subject, and arranged to emit an ultrasound pulse;
an ultrasound receiver, for placement at a second location on the body of the subject, and arranged to detect an ultrasound pulse emitted by the ultrasound transmitter; and
a controller in communication with the ultrasound transmitter and the ultrasound receiver and arranged to:
cause an ultrasound pulse to be emitted by the ultrasound transmitter;
receive a measurement signal from the ultrasound receiver corresponding to the ultrasound pulse;
determine T 1 , based on the received measurement signal, wherein T 1 is a time of arrival at the ultrasound receiver of a first part of the emitted ultrasound pulse;
determine T 2 , based on the received measurement signal, wherein T 2 is a time of arrival at the ultrasound receiver of a second part of the emitted ultrasound pulse; and
calculate, using T 1 and T 2 , the flow velocity of the blood flowing in the blood vessel located between the first location and the second location, wherein the flow velocity is calculated from:
(
ToF
2
-
ToF
1
)
c
ToF
2
=
vc
(
c
+
v
)
≈
v
where ToF 1 is a time of flight of the first part of the emitted ultrasound pulse and is given by ToF 1 =T 1 −T 0 and
ToF
1
=
L
c
+
v
,
ToF 2 is a time of flight of the second part of the emitted ultrasound pulse and is given by ToF 2 =T 2 −T 0 and
ToF
2
=
L
c
,
L is the length of the ultrasound path between the ultrasound transmitter and the ultrasound receiver, v is the flow velocity, C is the speed of sound in soft tissue, and T 0 is a time at which the ultrasound pulse was emitted;
or is calculated from:
(
T
2
-
T
1
)
c
2
L
=
vc
(
c
+
v
)
≈
v
.
2. The apparatus of claim 1 , wherein the first part of the emitted ultrasound pulse corresponds to at least part of a first rising edge of an envelope of the received measurement signal and the second part of the emitted ultrasound pulse corresponds to at least part of a main peak of the envelope.
3. The apparatus of claim 1 , wherein determining T 1 comprises determining an earliest time at which the envelope of the received measurement signal equals a first predefined threshold amplitude and determining T 2 comprises determining the earliest time at which the envelope of the received measurement signal equals a second predefined threshold amplitude.
4. The apparatus of claim 3 , wherein the first predefined threshold amplitude corresponds to a first percentage of a maximum amplitude of the received measurement signal and the second predefined threshold amplitude corresponds to a second percentage of the maximum amplitude, wherein the second percentage of the maximum amplitude is larger than the first percentage of the maximum amplitude.
5. The apparatus of claim 1 , wherein the controller is arranged to:
cause the ultrasound pulse to be emitted by the ultrasound transmitter at regular intervals;
receive the measurement signal from the ultrasound receiver, determine T 1 and T 2 , and calculate the flow velocity, in respect of each emitted ultrasound pulse; and
generate a time-dependent flow velocity signal based on the calculated flow velocity values.
6. The apparatus of claim 5 , wherein the controller is further arranged to:
receive a heartbeat rhythm signal for the subject; and
filter the time-dependent flow velocity signal to extract variations synchronous with the received heartbeat rhythm signal using a band pass filter.
7. The apparatus of claim 5 , wherein the controller is further arranged to fit the time-dependent flow velocity signal to a model.
8. The apparatus of claim 5 , wherein the controller is further arranged to:
receive posture information for the subject; and
correct the flow velocity signal based on the received posture information.
9. The apparatus of claim 5 , wherein the controller is further arranged to calculate a stroke volume and/or a cardiac output of the subject, based on the flow velocity of the blood.
10. The apparatus of claim 1 , wherein one or more operational parameters of the ultrasound transmitter and/or the ultrasound receiver is automatically adjustable by the controller, and wherein the controller is further arranged to perform a calibration process comprising:
adjusting one or more operational parameters of the ultrasound transmitter and/or the ultrasound receiver;
measuring one or more attributes of the measurement signal received from the ultrasound receiver; and
selecting a value for each of the one or more operational parameters of the ultrasound transmitter and/or the ultrasound receiver based on the measured one or more attributes.
11. The apparatus of claim 10 , wherein the blood vessel comprises the descending aorta of the subject.
12. The apparatus of claim 1 , wherein each of the ultrasound transmitter and the ultrasound receiver comprises circuitry embedded in an adhesive patch for adhering to a surface of the subject.
13. The apparatus of claim 1 , the controller comprises:
a communications interface for enabling communication between the controller and ultrasound transmitter and between the controller and the ultrasound receiver.
14. The apparatus of claim 1 , wherein the blood vessel comprises the descending aorta of the subject.
15. The apparatus of claim 1 , wherein calculating the flow velocity of the blood flowing in the blood vessel comprises calculating, using T 1 and T 2 , the flow velocity of the blood in the blood vessel located between the ultrasound transmitter and the ultrasound receiver.
16. A method of measuring flow velocity of blood flowing in a blood vessel of a subject, the method comprising:
causing an ultrasound pulse to be emitted from an ultrasound transmitter at a first location on the body of the subject;
receiving a measurement signal from an ultrasound receiver at a second location on the body of the subject, the received signal corresponding to a detection by the ultrasound receiver of the ultrasound pulse;
determine T 1 , based on the received measurement signal, wherein T 1 is a time of arrival at the ultrasound receiver of a first part of the emitted ultrasound pulse;
determine T 2 , based on the received measurement signal, wherein T 2 is a time of arrival at the ultrasound receiver of a second part of the emitted ultrasound pulse; and
calculating, using T 1 and T 2 , the flow velocity of blood flowing in the blood vessel located between the first location and the second location, wherein the flow velocity is calculated from:
(
ToF
2
-
ToF
1
)
c
ToF
2
=
vc
(
c
+
v
)
≈
v
wherein ToF 1 is a time of flight of the first part of the emitted ultrasound pulse and is given by ToF 1 =T 1 −T 0 and
ToF
1
=
L
c
+
v
,
ToF 2 is a time of flight of the second part of the emitted ultrasound pulse and is given by ToF 2 =T 2 −T 0 and
ToF
2
=
L
c
,
L is the length of the ultrasound path between the ultrasound transmitter and the ultrasound receiver, v is the flow velocity, c is the speed of sound in soft tissue, and T 0 is a time at which the ultrasound pulse was emitted;
or is calculated from:
(
T
2
-
T
1
)
c
2
L
=
vc
(
c
+
v
)
≈
v
.
17. The method of claim 16 , wherein the blood vessel comprises the descending aorta of the subject.