Download Biomedical Instrumentation

University of Zagreb
Faculty of Electrical Engineering and
Computing
Biomedical Instrumentation
- Transducers and sensors
- Biopotential amplifiers
- Electromagnetic interference
prof.dr.sc. Ratko Magjarević
June 2012
Electrodes
• Bioelectrical potentials - recall:
bioelectrical potentials occur at the cell
membrane due to difference in
ions concentration (mostly Na +, K + and Cl-) in
intracellular fluid and in the extracellullar space
• Potential difference at the cellular membrane
may be in the range within 5mV and 100mV
• This potential difference is called the resting
potential
2
Electrodes
• Bioelectrical resting potentials:
– resting potential inside the cell is negative
comparing to the environment
– resting potential of nerve and muscle cells is
typically -70mV to -85mV
3
Electrodes
• Action potentials:
when the cell membrane is stimulated, there is
a sudden change in membrane conductance, first
for sodium ions (cell depolarization), and
then to potassium ions (repolarization)
• Negative potential inside the cell
reduces, such that short-term potential may become
positive
• Such a potential difference is called the action
potential
4
Electrodes
• How to access the cell and measure
bioelectrical potentials?
– a) individual cells
• thickness of the semi-permeable membrane approx.
10nm
• measurement of in vivo or in vitro
– b) groups of cells - tissue or organ
• access to tissue or organ - a non-invasive (bloodless) or
invasive measurements
• mutual influence of different tissues /organs
(potentials, impedance)
5
Electrodes
Electrode is an interface
• to connect the measurement devices and
measure bioelectrical potentials, electrode is
used as an interface, however..
The electrode is also a transducer
• exchange charge carriers :
– in electrical circuits, electrons are charge carriers
– in the body, ions are charge carriers
• connects to the surface of the body (skin, mucous
membranes) or on/in the organ inside the body
6
Electrodes
• Most of bioelectric potentials strive to measure
noninvasively, e.g. from the surface of the body, by
placing electrodes on the skin
• Electrical characteristics of different tissues
– specific conductivity (specific resistance)
– specific dielectric constant
• Characteristics of biological tissue are:
– nonlinearity (dependence on frequency and current density),
– inhomogenity (unequal material properties of the body)
– anisotropy (different properties in different dirrections, typically along
the fiber-cells)
7
Electrodes
• Using a model of the interface for
better understanding of the interface electrode -tissue
• Passive electrical characteristics of the skin - electrode
interface strive to express by ideal electric components
with intent parameters
– Resistance
– Capacity
• This model can be used for measurement electrodes in
limited frequency range
8
Equivalent circuit of the skinelectrode
Electrode
Skin
Virtual electrode
Biological
issue
Electrode – skin intarface and its simplified electrical
circuit
9
RP
Equivalent circuit of the skinelectrode
d
= ρ
A
µ - charge mobility
ρ =
CP
1
µ qn
A
= ε
d
q - charge
n - number of electrons in volume unit
Rp - resistance between the electrode and the well-conductive layer of tissue
(virtual electrode)
d – skin thickness
A - electrode surface
ρ - specific resistance
Cp - capacity between the electrode and virtual electrode
ε - dielectric constant of the skin
10
Equivalent circuit of the skinelectrode
11
Equivalent circuit of the skinelectrode
12
Nonlinearity of the electrode
interface
13
Metal-electrolyte potential
Potential
double-layer
Metal
Electrolyte
Electrolyte
Potential
Charge
Dissociation of water to H+
and OH- ions
Potential double-layer at the interface metal-electrolyte
14
Metal-electrolyte potential
Standard electrode potential relative to standard hydrogen electrode at 20°C
15
Metal-electrolyte potential
• If you plunge a metal in a solution of its salt, the
half cell potential E0 M appears, also the voltage
dependent on the concentration of metal ions in
solution:
RT
E0.5 M = E0 M 1 −
ln cM 1
nF
• If there is some other metal also immersed in a
solution of its own ions, its potential will be
E 0 .5 M
RT
ln cM 2
= E0 M 2 −
nF
16
Polarization voltage
• If these two solutions are separated with semi-permeable
membrane to allow passage of ions, and to avoid the
original combination of solutions, the potential difference between
the solutions can be measured according to the formula
E = E0.5 M 1 − E0 M 2
RT [cM 1 ]
= E0 M 1 − E 0 M 2 −
ln
nF [cM 2 ]
• Each electrode that comes in contact with the electrolyte will
have the potential of the expression above. This potential
is undesirable in the measurement of biological voltage
because when using high gain dc amplifier, it causes saturation
of the amplifier. To avoid saturation, amplifier with less gain in the
input is used and the next stages of amplification are
separated with condenser.
17
Dry electrodes
• Used to avoid the
appearance of polarization
voltage.
• The problem is in large input
impedance, which makes
them susceptible to
interference.
• Therefore, the electrode
itself incorporates an
amplifier designed to reduce
the high input resistance to
a small value and thus
reduce the impact of
interference.
18
Dry electrodes with integrated amplifier
Microelectrodes
• They are used to measure the biological
potential of the cells
19
Microelectrodes
• cell attached recording:
– pipette touching the
membrane and forming
a high-ohmic junction
(~ 1GOhm)
• whole cell recording:
– by suction through a
pipette the membrane
breaks - solution in the
pipette and inside of the
cells become uniform
20
Microelectrodes
Extracellular recording
21
Microelectrodes
Action potentials recorded extracellularly
22
Subcutaneous electrodes
• They are used, for example, to measure the voltage
on the individual muscle fibers or groups of neurons in
the brain
Subcutaneous needle electrodes a) monopolar,
b) bipolar c) wire d) electrode array d) cortical
23
Subcutaneous electrodes
• Example of
subcutaneous
electrodes used for
deep brain stimulation
• Example of electrode
implantation for deep
brain stimulation
24
Subcutaneous electrodes
25
Surface EEG electrodes
• EEG electrodes (passive, active)
• Conductive paste
and gel
26
Surface EEG electrodes
• 10-20 system - standards for placing EEG
electrodes
27
EEG recording
28
Surface ECG electrodes
• Adhesive:
• Suction (pump to
suck air)
29
Surface ECG electrodes
• Electrodes for extremities (hands and feet):
• EKG recodring
30
EMG electrodes
• Surface
• Subcutaneous
31
EMG recording
32
Surface electrodes - more examples
• EOG electrodes:
• Electrodes for
electrostimulation
33
Electrocardiograph
• The apparatus that records
the ECG is called the
electrocardiograph
• If a patient needs
continuous monitoring
during hospitalization, the
ECG in “monitored” on a
ECG monitor together with
other physiological
parameters
34
ECG Amplifier
• The most important part of any equipment recording
bioelectric potentials is the input amplifier
• Most important characteristics:
– Differential measurement (differentia or instrumetation
amplifier)
– High gain (input signal 50uV to 1 mV)
– High common mode rejection ratio (CMRR)
– Frequency range typically from 0,05 Hz to >=100 Hz
– Very high input impedance
– Low noise
35
ECG Input Signal
• Composite signal
– useful signal – ECG: amplitude span from 50 uV to 1
mV
– polarisation voltage (electrochemical contact
potential @ electrodes) – DC component, up to 300
mV
– interference – mains (50 Hz or 60 Hz), up to 100 mV
– interference voltages – defibrilator shock (n x 1000
V) or RF surgery equipment voltages
36
ECG Output
• multichannel output – 15 seconds time frame,
channels synchroneous
• Print out
– sensitivity: 2,5 mm/mV; 5 mm/mV; 10 mm/mV; 20
mm/mV
– pape speed: 10mm/s, 25 mm/s, 50mm/s, 100 mm/s
• Data storage
– different formats
– MIT BIH signal database (scientific)
– interoperability
37
ECG Equipment classification
• ECG recorders
– classification to the number of channels
•
•
•
•
•
1 channel ECG
3 channel ECG
6 channel ECG
12 channel ECG
> 12 channel ECG
38
Symplified ECG Block Diagram
Protection circuit
Lead Selector
Preamplifier
Calibration Circuit
(1mV step)
Isolation Circuit
Processing
(analog or digital)
Electrodes
Driver Amplifier +
Recorder - Printer
39
Amplifier Circuits - DC coupled
40
40
Common Mode Rejection Ratio
(CMRR)
•
•
•
•
The ratio of the differential gain over the common-mode gain
Expressed in decibels
Typically 100 – 120 dB for integrated instrumentation amplifiers
Function of frequency and source-impedance unbalance.
41
Measurement of CMRR
AD
H=
AZ
uizlD
AD =
uulD
∆
∆uizlZ
AZ =
uulZ
42
Amplifier Circuits - AC coupled
43
43
44
44
45
45
Auto-zero amplifiers
• Automatic nihilation of amplifier offset
voltage
∆u = u + − u −
u− = A2 ⋅ uizl = A2 ⋅ A1 ⋅ ∆u
uizl = A1 ⋅ ∆u
∆u = u+ − A2 A1 ⋅ ∆u
u+ = ∆u (1 + A1 A2 )
∆u =
∆u =
A1 ⋅ uoff
A1uoff
u+
=
=
1 + A1 A2 1 + A1 A2
A1 A2
uoff
A2
46
Isolation amplifiers
• Galvanic isolation of sensory and the measurement part of
the measurement system (attached to the patient) and the
processing and display part, usually powered by mains
• Floating principle of measurement of biopotentials
• The aim is also to protect the patient from the potentially
dangerous voltages or currents comming from the un-isolated
(mains powered part) of the system
Biopotential Isolation Amplifier
48
48
Principle of floating measurements
Ad
lim H =
=∞
∆u 2 i
u1i
za Z n → ∞
Isolation amplifiers
• Design of isolation amps:
– Optical coupling
•
•
•
•
•
Isolation voltage typ. 4 – 7 kV
fast
cheap
Nonlinear – digitazing of signals before the isolation gap
Noice high
– Electromagnetic coupling
• Isolation voltage up to 10 kV
• Resolution typ. 12 bit, max. 16 bit
• fg low, max 1 kHz
– Capacitive coupling
• Characteristics worse than other types, but cheapest
Otically coupled isolation amp
51
Linearisation
u2 = f
−1
 u1 
 
 R1 
u3
= f ' ( u2 )
R3
i2 = f ( u1 )
i2 ' = f ' ( u3 )
 −1  u1  
u3
= f '  f  
R3
 R1  

 −1  u1   u1
u3
= f '  f   =
R3
 R1   R1

u3 R3
f ( u1 ) = f ' ( u3 )
=
u1 R1
u1 = u3
Linearisation
EM coupled amps - AD 215
Capacitavly coupled isolation amps
ISO
124
The input signal is frequency modulated
fosc 500 kHz; Vizo = 2,4 kVef
Capacitively coupled isolation amps
56
Digital isolation amp principles
Preamplifier - Instrumentation Amplifier
58
ECG Shematic Diagram
Texas Instrument's ADS1298 family of fully integrated analog front ends (AFEs) created to make
much more portable ECG equipment possible
59
ECG Shematic Diagram
Single channel ECG
Source: A. Šantić, Biomedicinska elektronika, Školska knjiga, Zagreb
60
Signal and Noise Problems
http://medstat.med.utah.edu/kw/ecg/image_index/index.html
61
ECG Interference Sources
• Noise originating from sources
external to the patient
–
–
–
–
in most cases power-line
interference (50 or 60 Hz)
also Electrosurgical Unit (ESU)
interference - high-frequency
signals during operation
Electrostatic Sources
Electromagnetic Induction
• Artifacts originating from the
patient
• Artifacts originating from
patient-electrode contact
62
Artifacts in ECG
• Artifacts originating from the patient
– Movement Artifacts due to patient movement
– Baseline Wander - ECG waveform baseline drifting up and down slowly,
usually because of the patient breathing
– EMG Interference –
muscle contractions during
recording the ECG.
63
Artifacts in ECG
• Artifacts originating from patient-electrode contact
– electrodes not tightly coupled to the patient so there is a change in
electrode to skin impedance during the recording
64
Electromagnetic compatibility (EMC)
• the degree to which an electronic system is
able to function compatibly with other
electronic systems
• not susceptible to interference
• not produce interference
• Opposite: Electromagnetic interference (EMI)
65
Electromagnetic interference (EMI)
• disturbance that affects an electrical circuit due to
– electromagnetic radiation
– electromagnetic conduction (bellow ~50 MHz)
– electrostatic discharge
• emitted from a source
– external
– internal
• may interrupt, degrade or obstruct the performance
of an electronic circuit
66
Electromagnetic interference (EMI)
• Inductive coupling
– Inductive coupling occurs where the source and receiver
are separated by a short distance (typically less than a
wavelength). Strictly, "Inductive coupling" can be of two
kinds, electrical induction and magnetic induction. It is
common to refer to electrical induction as capacitive
coupling, and to magnetic induction as inductive coupling.
• Capacitive coupling
– Capacitive coupling occurs when a varying electrical field
exists between two adjacent conductors typically less than
a wavelength apart, inducing a change in voltage across
the gap.
67
Electromagnetic interference (EMI)
Conductive coupling
– Conductive coupling occurs when the coupling path
between the source and the receptor is formed by direct
contact with a conducting body, for example a
transmission line, wire, cable, PCB trace or metal
enclosure.
• Conduction modes
– Conducted noise is also characterised by the way it
appears on different conductors:
• Common-mode or common-impedance[1]) coupling: noise
appears in phase (in the same direction) on two conductors .
• Differential-mode coupling: noise appears out of phase (in
opposite directions) on two conductors.
68
Electromagnetic interference (EMI)
• Magnetic coupling
– Inductive coupling or magnetic coupling (MC) occurs when
a varying magnetic field exists between two parallel
conductors typically less than a wavelength apart, inducing
a change in voltage along the receiving conductor.
• Radiative coupling
– Radiative coupling or electromagnetic coupling occurs
when source and victim are separated by a large distance,
typically more than a wavelength. Source and victim act as
radio antennas: the source emits or radiates an
electromagnetic wave which propagates across the open
space in between and is picked up or received by the
victim.
69
AC Mains interference
• Capacitive
paths
70
ECG Amplifier with input guarding
71
ECG Amplifier with Right Leg Drive
• An operational amplifier which derives common mode
voltage, inverts it and returns to the patient through the right
leg. The patient is not grounded.
72
ECG input protection
73
Amplifier input protection
Protection of the amplifier input against high-voltage transients. a) The
connection diagram for voltage limiting elements with two optional resistors R
for current limitation
b) Typical current-voltage characteristic of voltage-limiting elements, Vb –
breakdown voltage c) anti-parallel connection of diodes d) anti-parallel
connection of Zener diodes e) gas-discharge tubes
74
Protection against overvoltage
12
600
600
600
10
500
500
500
8
400
400
400
6
300
300
300
V
V
V
kV
1 kV/µs
4
200
200
200
2
100
100
100
0
0
20
40
µs
UB = 24 V
60
0
0
1
2
0
µs
0
1
0
2
µs
L = 10 µH
tinjalica
0
1
2
µs
L = 10 µH
VDR
otpornik
Valni puls
obliciat
pojedinih
točaka višestepene
zaštite od
prenapona
Typical overvoltage
mains connector
and protection
with
gas discharge
tubes, VDR (voltage dependent resistor) and a surge diode
zaštitna
dioda
Protection of input against RF noise
x
10-15mH
100n
x
100n
0.5-1M8
y
3n3
RSO - filtar
y
3n3
Protection of common mode and differential noise at the mains connector of
the device
EMI to Medical Devices
Documented cases:
• a ventilator suddenly
changes its breath rate
• an electric powered
wheelchair suddenly
veers off course
• an apnea monitor fails
to alarm......
Casamento JP, Ruggera PS.Applying standardized electromagnetic compatibility testing methods for evaluating
radiofrequency interference with ventilators, Biomed Instrum Technol.
Witters D.M. and P. S. Ruggera, EMC of Powered Wheelchairs and Scooters, Proc. RESNA '94
77
Solving EMI at System Level
• EMI involves:
– the device itself
– the environment in which it is used
– anything that may come into that environment
• EMI - a systems problem requiring a systems
approach
• EMI solution requires involvement of
– the (medical) device industry,
– the EM source industry (power industry,
telecommunications industry....),
– the clinical user and patient
78
IEC Classification
•
•
•
International Electrotechnical
Commission (IEC)
classification of EM
environment
Conditions for the location
and power of local EM energy
sources (e.g., transmitters)
Table 1 indicates the general
classifications and the upper
range of radiated EM field
strength specified for each
environment.
79
Sources of EMI
• Radio broadcasting
• Television
• Public safety (police, fire, highway, forestry, and emergency services)
• Land transportation (taxis, trucks, buses, railroads)
• Amateur radio
• Cellular phones and paging systems
• Industrial, scientific, and medical
• Citizens' band (CB) radio
• Radar .....
80
Model of Interference
from: S. Hrabar, Analysis of EMI between mobile telephone and impllanted medical device, Ph.D.
thesis, 1999
81
81
Literature
• John G. Webster: Medical Instrumentation,
Chapter 5, Biopotential Electrodes; Chapter 6,
Biopotential Amplifiers
82