Multimédia és egészségügy
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Transcript Multimédia és egészségügy
5th practice
Medical Informatics
Biomedical Signal Processing
TAMUS, Zoltán Ádám
[email protected]
The
electrical manifestation of the
contractile activity of the heart
The ECG can be easily recorded with
surface electrodes on the limbs or chest
(Einthoven’s triangle)
The waveshape is altered by
cardiovascular diseases
Einthoven’s
triangle
The
ECG is an electrical signal (function of
time)
The
SA node fires
Slow-moving
depolarization
(contraction) of
the atria
The P wave in
ECG
• 0.1-0.2 mV
• 60-80 ms
Propagation
delay at atriovenricular (AV)
node
Normally isoelectric segment
PQ segment
• 60-80 ms
His
bundle, the
bundle
branches, and
the Purkinje
system of
specialized
conduction
fibers
propagate the
stimulus to the
ventricles at
high rate
The
wave of
stimulus spreads
rapidly from the
apex of the heart
upwards
Rapid
depolarization
(contraction) of
ventricles
QRS wave
• 1 mV
• 80 ms
The
ventricular
muscle cells
possess a
relatively long
action potential
300-350 ms
Result: isoelectric segment
ST segment
• 100-120 ms
Repolarization
(relaxation) of
ventricles causes
slow T wave
• 0.1-0.3 mV
• 120-160 ms
Physiological
interference
• Respiration, squirms, coughs etc.
• Mother’s ECG appearing with the ECG of the
fetus
Electromagnetic
interference
• TV and radiostations, CRTs, computers,
fluorescent lamps
Result: noise
in the ECG signal
Signal
• Deterministic signal:
the value at a given instance of time may be computed
using a closed-form mathematical function of time or
Predicted from a knowledge of a few past values of the
signal
• Nondeterministic signal (random signal):
That does not meet the conditions of the deterministic
signal
The noise may be random, structured
(deterministic) and physiological
Random
noise
• ensemble average: 0
1 M
x (t1 ) lim
xk (t1 )
M M
k 1
Reduction: Moving Average (MA) filter
• general form:
N
y ( n) bk x (n k )
k 0
• bk:filter coefficients
Examples
of MA filters
• von Hann or Hanning filter:
1
y ( n) x( n) 2 x( n 1) x( n 2)
4
• 8-point average:
1 7
y ( n) x ( n k )
8 k 0
Rangaraj
M. Rangayyan: Biomedical
Signal Analysis, IEEE Press/Wiley, New
York, NY, 2002.
1. Observation
of noisy ECG signal
2. Filtering the noisy ECG signal by MA
filter
• von Hann or Hanning filter
• 8-point MA filter
sample files: http://zoltanadam.fw.hu
software: MS Excel