ecg signal delineation and compression
DESCRIPTION
T-61.181 Biomedical Signal Processing. ECG Signal Delineation And Compression. Chapters 6.2.6 – 6.3 18th November. Outline. ECG signal delineation Definition (What) Clinical and biophysical background (Why) Delineation as a signal processing (How) ECG signal compression - PowerPoint PPT PresentationTRANSCRIPT
ECG Signal Delineation And Compression
Chapters 6.2.6 – 6.3
18th November
T-61.181 Biomedical Signal Processing
Outline
I. ECG signal delineation Definition (What) Clinical and biophysical background (Why) Delineation as a signal processing (How)
II. ECG signal compression General approach to data compression ECG signal compression
(Intrabeat/Interbeat/Interlead)
III. Summary
Delineation - Overview• Aim – Automatically decide/find onsets and
offsets for every wave (P, QRS, and T) from ECG signal (PQRST-complex)
• Note! Experts (Cardiologist) use manual/visual approach
Why?
• Why – Clinically relevant parameters such as time intervals between waves, duration of each wave or composite wave forms, peak amplitudes etc. can be derived
• To understand this look how ECG signal is generated
What Are We Measuring?
• ECG gives (clinical) information from generation and propagation of electric signals in the heart.
• Abnormalities related to generation (arrhythmia) and propagation (ischemia, infarct etc.) can be seen in ECG-signal
• Also localization of abnormality is possible (12 lead systems and BSM)
Clinically Relevant Parameters
• PR interval SA ventricles
• QT interval ventricular fibrillation
• ST segment ischemia
• QRS duration Bundle brand block depolarization
Signal Processing Approach to Delineation (How)
• Clinical importance should now be clear
• Delineation can also be done manually by experts (cardiologist) expensive and time consuming. We want to do delineation automatically (signal processing)
• No analytical solution performance has to be evaluated with annotated databases
Building Onset/Offset Detector
Many algorithms simulate cardiologist manual delineation (ground truth) process:
Experts look 1) where the slope reduce to flat line 2) respect maximum upward, downward slope
Simulate this: define the boundary according to relative slope reduction with respect maximum slope LPD approach
Low-Pass Differentiated (LPD)
• Signal is 1) low-pass filtered i.e. high frequency noise is removed (attenuated) and 2) differentiated dv/dt
• New signal is proportional to slope
• Operations can be done using only one FIR filter :
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LPD cont.
• Each wave has a unique frequency band thus different low-pass (LP) filtering (impulse) responses are needed for each wave (P, QRS, and T)
• Design cut-off frequencies using Power Spectral Density (PSD)
• Differentiation amplifies (high freq.) noise and thus LP filtering is required
LPD cont..
• Waves w={P,QRS,T} are segmented from the i:th heart beat.
• Using initial and final extreme points thresholds for can be derived
oteherwise
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LPD cont...
• Constants are control the boundary detection they can be learnt from annotated database
• Search backwards from initial extreme point. When threshold is crossed onset has been detected
• Search forward from last extreme point and when threshold is crossed offset is detected.
General Data Compression
• The idea is represent the signal/information with fewer bits
• Any signal that contains some redundancy can be compressed
• Types of compression: lossless and lossy compression
• In lossy compression preserve those features which carry (clinical) information
ECG Data Compression
1) Amount of data is increasing: databases, number of ECG leads, sampling rate, amplitude resolution etc.
2) ECG signal transmission
3) Telemetry
ECG Data Compression
• Redundancy in ECG data: 1) Intrabeat 2) Interbeat, and 3) Interlead
• Sampling rate, number of bits, signal bandwidth, noise level and number of leads influence the outcome of compression
• Waveforms are clinically important (preserve them) whereas isoelectric segments are not (so) relevant
Intrabeat Lossless Compression
• Not efficient – has mainly historical value
• Sample is predicted as a linear combination of past samples and only prediction error is stored (smaller magnitude):
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pnxanxanx
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Intrabeat Lossy CompressionDirect Method
• Basic idea: Subsample the signal using parse sampling for flat segments and dense sampling for waves:
(n,x(n)), n=0,...,N-1 (nk,x(nk)), k=0,...,K-1
Example AZTEC
• Last sampled time point is in n0
• Increment time (n) As long as signal in within certain amplitude limits (flat)
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Intrabeat Lossy Compression Transform Based Methods
• Signal is represented as an expansion of basis functions:
• Only coefficients need to be restored
• Requirement: Partition of signal is needed (QRS-detectors)
• Method provides noise reduction
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1
Interbeat Lossy Compression
• Heart beats are almost identical (requires QRS detection, fiducial point)
• Subtract average beat and code residuals (linear prediction or transform)
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Interlead Compression
• Multilead (e.g. 12-lead) systems measure same event from different angles redundancy
• Extend direct and transform based method to multilead environment– Extended AZTEC– Transform concatenated signals
12
2
1
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Summary - part I
• Delineation = automatically detect waves and their on- and offsets (What)
• Clinically important parameters are obtained (Why)
• Design algorithm that looks relative slope reduction (How)
• LPD-method – Differentiate low-pass filtered signal
Summary - part II
• Compression = remove redundancy: intrabeat, interbeat, and interlead
• Why – Large amount of data, transmission and telemetry
• Lossless (historical) and lossy compression• Notice which features are lost (isoelectric
segments don’t carry any clinical information)
Summary - part II cont.
• Intrabeat 1) direct and 2) transform based methods– 1) Subsample signal with non-uniform way
– 2) Use basis function (save only weights)
• Interbeat subtract average beat and code residuals (linear prediction or transform-coding)
• Interlead extend intrabeat methods to multilead environment