BLOOD FLOW METER

INTRODUCTION

• Blood flow is the one of the important physiologicalparameter 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.

Need for blood flowmeter

• Inspection for block in blood flow.

• Testing artificial blood vessels during organ transplantation.

• During Fistula creation in dialysis.

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

Types of Blood Flow Meter

• Most widely used blood flow meters are:

Electromagnetic blood flow meter

Ultrasonic blood flow meters

Laser Doppler blood flow meter

ELECTROMAGNETIC BLOOD FLOWMETER

ELECTROMAGNETIC BLOODFLOWMETER

• Measures instantaneous pulsatile flow of blood

• Works based on the principle of electromagnetic induction

• The voltage induced in a conductor moving in a magneticfieldis proportional to the velocity of the conductor

• The conductive blood is the moving conductor

Principle of EM blood flow meter.

ELECTROMAGNETIC BLOODFLOWMETER

• A permanent magnet or electromagnet positioned around the bloodvessel generates a magnetic field perpendicular to the direction of theflow of the blood.

• Voltage induced in the moving blood column is measured with stationaryelectrodes located on opposite sides of the blood vessel andperpendicular 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.

• The Induced emf

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

Fig: Typical flow transducer

• The electromagnetic flow-transducer is a tube of non-magneticmaterial to ensure that the magnetic flux does not bypass theflowing liquid and go into the walls of the tube.

Ultrasonic blood flowmeter

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.

Fig : ultra sonic flow meter

(a)Doppler-shift flow-velocitymeters

• 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 directionof the flow velocity.

Doppler-shift flow-velocitymeters

5 MHzexciter

Rx Tx

R Famplifierdetector

LPFAudio

amplifier

Zero crossing detector

LPF

Blood vessel

Fig : block diagram

output

(b)Range gated pulsed Doppler flowmeter

• Baker(1970) stated that recording blood flow using Dopplershift 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 andvelocity of blood stream can measure together.

Receiverlimiter

Phase detector LPF Sample and amplifier

Power amp Pulse ampSample

pulse gen.LPF

Freq divisionMaster Oscillator

(4.5-5.5 MHz)

Rx element

Tx element

AudibleDoppleroutput

Fig : block diagram of pulsed doppler flow meter.

(c) Flow measurement by DopplerImaging

• 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.

Flow measurement by DopplerImaging

Figs : Flow measurement by Doppler Imaging

Flow measurement by DopplerImaging

• Using this recording of mean blood flow, peak flow, reversepeak flow etc. of cardiac cycle are possible.

• This method is also helpful for taking measurements from brain , which is difficult to access.

Laser Doppler blood flowmeter

Fig : schematic diagram of laser doppler blood flow meter