This ppt provides basic idea about different blood flow measurement systems. It is helpful in biomedical instrumentation and nursing field.

1. BLOOD FLOW METER
Prepared by Abhijith Prabha , L6A ,ECE Department.

2. INTRODUCTION
• Blood flow is the one of the important physiological
parameter and the most difficult to measure accurately.
• The average velocities of blood flow vary over a wide range
depending on diameter of blood vessel.
• There are many techniques for measuring the blood flow and
velocity.
• They are categorized into
1.invasive(surgical).
2.non invasive(through the skin).

3. Need for blood flowmeter
• Inspection for block in blood flow.
• Testing artificial blood vessels during organ transplantation.
• During Fistula creation in dialysis.

4. Typical values of blood flow
Type of blood
vessels
Total cross-section
area
Blood velocity in
cm/s
Aorta 3–5 cm2 40 cm/s
Capillaries 4500–6000 cm2 0.03 cm/s
Vena cava inferior
and superior
14 cm2 15 cm/s

5. Introduction
• Most widely used blood flow meters are:
Electromagnetic blood flow meter
Ultrasonic blood flow meters
NMR blood flow meter
Laser Doppler blood flow meter

6. ELECTROMAGNETIC BLOOD
FLOWMETER

7. ELECTROMAGNETIC BLOOD FLOWMETER
• Measures instantaneous pulsatile flow of blood
• Works based on the principle of electromagnetic induction
• The voltage induced in a conductor moving in a magnetic
field is proportional to the velocity of the conductor
• The conductive blood is the moving conductor

8. Principle of EM blood flow meter.

9. ELECTROMAGNETIC BLOOD FLOWMETER
• A permanent magnet or electromagnet positioned around the blood
vessel generates a magnetic field perpendicular to the direction of the
flow of the blood.
• Voltage induced in the moving blood column is measured with stationary
electrodes located on opposite sides of the blood vessel and
perpendicular to the direction of the magnetic field.
• This method requires that the blood vessel be exposed so that the flow
head or the measuring probe can be put across it.

10. • The Induced emf
• Where
• B = magnetic flux density, T
• L = length between electrodes, m
• u = instantaneous velocity of blood, m/s

11. Fig: Typical flow transducer
• The electromagnetic flow-transducer is a tube of non-magnetic
material to ensure that the magnetic flux does not bypass the
flowing liquid and go into the walls of the tube.

12. Types of EM Flowmeters
• basically, all modern flowmeters consist of a
generator of AC, a probe assembly , a series of
capacitance coupled amplifiers, a demodulator, a DC
amplifier and a suitable recording device.
• Basing shape of the energizing current waveform for
the electromagnet 2 types of EM Flowmeters are :
A. Sine wave flometer.
B. Square wave flowmeter.

13. Sine wave Flowmeters
• Probe magnet is energized with a sine wave and the induced
voltage will also be sinusoidal.
• Since the flow of blood acts as a secondary terminal of a
transformer w.r.t probe magnet, an additional artifact voltage
induced is called transformer voltage.
• This voltage is 900 out of phase with the original signal
corresponding to flow of blood .
• A method for eliminating transformer voltage by using a gated
amplifier(amplify signal only during flow induced voltage is
maximum).
• This type of instrument is known as ‘gated sine wave
flowmeter’ ,

14. Square wave flowmeter
• Probe magnet is energized with a square wave and induced
voltage is also square wave.
• It is easier to control magnitude and wave shape of energizing
current.
• Separation of transformer voltage is easy .
• For the measurement action square wave is amplitude
modulated by variation in blood flow.

15. Block diagram of square wave flowmeter

16. Block diagram
Transducer
• consist of an electromagnet, a pair of electrodes.
• Electrodes may be in contact with either flowing blood or
outer surface of the blood vessel
Preamplifier
• The induced voltage pick up by the electrodes, is given to a
low noise differential amplifier through a capacitive coupling
• Must have a very high CMRR and input impedance

17. Block diagram
 Gating circuit
• It helps to remove spurious voltages generated during magnet
current reversal
• The gating action is controlled by the circuit which provides
an excitation current to the electromagnet
 Band pass filter
• It is an active RC band pass amplifier , which selectively pass
through it the amplified square wave signal
• Peak response is kept for 400Hz

18. Block diagram
 Detector
• A phase sensitive detector is used to recover the signal
• It also helps in the rejection of interfering voltages at
frequencies below the carrier frequency
Low pass filter and output stage
• Demodulated signal is given to an RC LPF , which provides a
uniform frequency response and a linear phase shift
• Followed by an integrator provide output corresponding to the
mean flow

19. Block diagram
Magnet current drive
• The square wave input to the power amplifier stage which
supplies current to the electromagnet is fed from a free
running multivibrator
Zero flow reference line
• it establish the signal corresponding to zero-flow before
measurement.

20. Ultrasonic blood flowmeter

21. Ultrasonic blood flowmeter
• An ultrasonic flow meter is a type of flow meter that
measures the velocity of a fluid with ultrasound to calculate
volume flow.
• Using ultrasonic transducers, the flow meter can measure the
average velocity along the path of an emitted beam of
ultrasound.
• By averaging the difference in measured transit time between
the pulses of ultrasound propagating into and against the
direction of the flow or by measuring the frequency shift from
the Doppler effect.

22. Fig : ultra sonic flow meter

23. (a)Doppler-shift flow-velocity meters
• It is an non invasive method.
• It is based on the analysis of echo signals from
erythrocytes(RBCs) in blood.
• The incident ultrasound is scattered by the blood cells and
scattered wave is received by the second receiver.
• The frequency shift of the scattered wave gives idea about
velocity of scatterers.
• The Doppler frequency shift is a measure of size and direction
of the flow velocity.

24. Doppler-shift flow-velocity meters
5 MHz
exciter
Rx Tx
R F
amplifierdetector
LPF
Audio
amplifier
Zero
crossing
detector
LPF
Blood vessel
Fig : block diagram
output

25. (b)Range gated pulsed Doppler flow meter
• Baker(1970) stated that recording blood flow using Doppler
shift are sometimes misleading and inaccurate.
• These difficulties can be overcome , if the ultrasonic source is
pulsed.
• If the returning signal is range gated ,then the diameter and
velocity of blood stream can measure together.

26. Receiver
limiter
Phase detector LPF Sample and amplifier
Power amp Pulse amp
Sample
pulse gen.
LPF
Freq division
Master Oscillator
(4.5-5.5 MHz)
Rx element
Tx element
Audible
Doppler
output
Fig : block diagram of pulsed doppler flow meter.

27. (c) Flow measurement by Doppler Imaging
• Doppler ultrasound is not only used for measurement of the
absolute value of blood velocity and volume, but it also helps
to visualize blood flow.
• The probe this imaging equipment is mechanically coupled to
position resolver, which gives electrical output.
• Imaging is done by moving the probe through the skin, and
developing a 3D image using a computer.
• Thus, it is possible to construct anterior-posterior , lateral and
cross-sectional scans of blood vessel.

28. Flow measurement by Doppler Imaging
Figs : Flow measurement by Doppler Imaging

29. Flow measurement by Doppler Imaging
• Using this recording of mean blood flow, peak flow, reverse
peak flow etc. of cardiac cycle are possible.
• This method is also helpful for taking measurements from
brain , which is difficult to access.

30. NMR Blood Flowmeter

31. NMR Blood Flowmeter
• Non invasive method by Nuclear magnetic resonance
principle.
• For flow measurement , behaviour of the 2 Hydrogen atoms
of water is studied.(Since blood is approximately 83% water).
• Due to magnetic moment of the H atom , the nucleus behaves
as a micro miniature magnet which can affected by externally
applied magnetic field.
• the hydrogen nuclei orient themselves to produce a net
parallel alignment to a steady magnetic field.

32. NMR Blood Flowmeter
• Orientation of H atoms will be change according to the status
of blood flow.
• This orientation will results in the magnetization of H atoms.
• Therefore, magnitude of magnetization can be related to
either velocity of blood or flow rate.
• A crossed coil configuration is used to detect the level of
magnetization in body.
• The voltage induced in coil(NMR signal) is proportional to
velocity and area of vessel carrying blood.
• NMR flow meters are limited in their application to the
measurement in limbs.

33. Laser Doppler blood flowmeter

34. Fig : schematic diagram of laser doppler blood flow meter

35. THE END