Biomedical Instrumentation, ,Measurement and Design

ELEC4623/ELEC9734: Semester 2, 2009

Dr Stephen RedmondSchool of Electrical Engineering & TelecommunicationsEmail: s redmond@unsw edu auEmail: s.redmond@unsw.edu.auPh: 9385 6101Rm: 458, ELECENG (G17)

Session 2, 2009 ELEC4623/ELEC9734 1

Course Outline

12 x 2 hour lectures5 x 3 hour laboratories (only three labs are assessed)

Laboratories (3 lab reports) 25%Assignment 10%Project seminar 10%three labs are assessed)

Project presentation in final weeks

Project seminar 10%Final examination 55%

Course SyllabusWeek Topic

1 Introduction & origin of biopotentials

2 Bioelectrodes and tissue equivalent circuits

3 Principles and operation of basic transducers and sensors3 Principles and operation of basic transducers and sensors

4 Characteristics of biological and instrumentation noise

5 Practical biopotential amplifier design and multilead ECG systems

6 Design testing and analysis of a high quality isolated biopotential amplifier6 Design, testing and analysis of a high quality isolated biopotential amplifier

7 Biological signal processing – filters

8 Statistical algorithms for automated signal detection and analysis

9 Circulatory system and the measurement of blood pressure and flow

10 The measurement of respiratory flows

11 Safety and performance standards (ASA, IEC and FDA) for medical instrumentation

12 Project presentations

13 Project presentations

Biomedical Instrumentation, Measurement and DesignELEC4623/ELEC9734/

Lecture 1Introduction to Biomedical Instrumentation and Physiological Measurement

Importance of physiological measurements in clinical medicine

Modern medicine is characterised by increasing application of scientific method to medicineMeasurement is the essence of the scientific methodMeasurement is the essence of the scientific methodPhysiological monitoring now multi-billion dollar industry (Hewlett Packard, Siemens, Philips and more)

Philips HealthcareClinical Specialties+ Anesthesiology

Home Healthcare+ Sleep Therapy

Mammography+ Analog Mammography + Digital Mammography + Computed Mammography

Mobile C-arms

Radiation Oncology+ Radiation Treatment Simulation + Radiation Treatment Planning

Resuscitationd l f b ll+ Anesthesiology

+ Cardiology + Oncology + Orthopedics + Surgery

Computed Tomography

+ Sleep Therapy + Home Respiratory + Lifeline Medical Alert + Remote Cardiac Services + HeartStart Home Defibrillator + Telehealth

+ C-arm with Flat Detector + C-arms with Image Intensifier

Monitoring+ Patient Monitors + Fetal and Maternal Monitors

+ Automated External Defibrillators + Advanced Life Support Products + AED Program Management + Data Management Solutions + Supplies & Accessories

SolutionsCo puted o og ap y+ Scanners + Workflow Solutions

Diagnostic ECG+ ECG Management + Holter Monitoring

Hospital Respiratory+ Noninvasive ventilation + Acute care ventilation + Patient Interfaces + Pre-hospital CPAP

+ Clinical Decision Support + Clinical Information Systems + Surveillance and Networking + Clinical Measurements + Remote Critical Care

N l M di i

Solutions+ Ambient Experience + BioShield for research + Cath Lab Experience + Discovery to Treatment + DoseWise radiation management + eICU Program g

+ PageWriter Cardiographs + Stress-Test Systems

Fluoroscopy+ Interventional Fluoroscopy + Diagnostic Fluoroscopy

Interventional X-ray+ Interventional Vascular Surgery + Interventional Cardiac Surgery + Electrophysiology + Interventional Cardiology + Interventional Radiology

I i l N di l

Nuclear Medicine+ SPECT/CT + PET/CT + Workflow Solutions

Preclinical Imaging+ MOSAIC HP

g+ Healthcare Consulting + Hospital-Acquired Infections + Remote services

Ultrasound+ Cardiology

Healthcare Informatics+ Cardiology Informatics + Enterprise Imaging Informatics + Applications & Workstations + Acute Care Informatics

ICU I f ti

+ Interventional Neuroradiology

Magnetic Resonance+ Systems + Elite Clinical Solutions + Options & Upgrades

+ MOSAIC HP + NanoSPECT/CT + IMALYTICS Workspace

Radiography+ Analog Radiography + Digital Radiography

+ Emergency Medicine + General Imaging + Regional Anesthesia + Vascular + Women’s Healthcare + Transducers

+ ICU Informatics + Digital Radiography + Computed Radiography + Mobile Radiography

General properties of medical instrumentation systems

Medical instruments monitor human tissue (loosely speaking!)Demand high level of safety, performance and qualityCost and complexityp y

Measurand is usually an accessible signal or quantity, e.g.Biopotentials (neural or neuromuscular)TemperaturePressureFlowDisplacementDisplacementImpedanceChemical composition

Examples of physiological measurands:

Electrocardiography (ECG)Electromyography (EMG)Electroencephalography (EEG)Electroencephalography (EEG)Phonocardiography (PCG)Blood pressureBlood flowBlood flowRespiratory pressures, flows and volumesBlood gases, PO2, PCO2Bl d HBlood pH

Essential elements of medical instruments

Great variety of methodsExternal electrodes (e.g. ECG)Application of external energy (e g X-ray ultrasound)Application of external energy (e.g. X ray, ultrasound)Collection of gas, liquid or tissue samples with analysis of chemical composition

Variety of signal content24 hour circadian rhythm (blood pressure) Bandwidth up to ~300 Hz (ECG) or 3-10 KHz (EMG and nerve potentials)Bi t ti l i V ( i lt) 0 300 HBiopotentials in μV (microvolt) range, pressures 0-300 mmHg

In design, must consider effects of Noise (ambient and generated)Temperature humidity pHTemperature, humidity, pHPerturbations caused by emotional or physical arousal

Sensors/Transducers

Conversion of physical quantities (e.g. flow, pressure) to electrical signals IdeallyIdeally

Minimally invasive or noninvasiveRespond only to measurand and not noiseProvide minimum interference to process being measuredp g

Sensing unit + converters e.g. in microphone: diaphragm (sensing element) responds to pressure by changing displacement, conversion of displacement to l t i l i l i t ll d i l telectrical signal requires externally powered conversion element

(such as strain gauge, bridge circuit)Microelectronic sensors revolutionise industry

Low costLow costSmall sizeGreater reliability and improved performance

Signal Conditioning

Transducers rarely provide a signal suitable directly for ADC (analog to digital conversion)

Must firstAmplify signal to appropriate level (fixed or variable gain)Filter signal with low pass, high pass, band pass or notch filtersFilter signal with low pass, high pass, band pass or notch filters

Signal processing in modern instruments is usually a combination of hardware and software

Analog filter in hardware: expensive in terms of cost and real estate, but necessary sometimesDigital filter in software: “free” and more flexible

Control and Feedback

May be required to adjust properties of the Sensor

PositionPositionContact force

ConditionerGainGainFilter properties

Display, storage and transmissionManual or automaticManual or automatic

Performance standards

Equipment from different manufacturers must meet certain performance criteriaStandards usually set by industry bodies:y y y

American Heart Association (AHA)American National Standards Institute (ANSI)Association for the Advancement of Medical Instrumentation (AAMI)(AAMI)International Electrotechnical Commission (IEC)

Certification comes from local bodies e.g.Federal Drug Administration (FDA USA)Federal Drug Administration (FDA USA)Therapeutic Goods Administration (TGA Australia)

Differences between USA and Europe standardsAustralia tends to follow EuropeAustralia tends to follow Europe

Electrical safety

Hospital equipment often used in presence of explosive anaesthetics such as etherMacroshock hazardsMacroshock hazards

defibrillation (~3000V)electrical faults (hot wire to ground)

Mi h k h dMicroshock hazards leakage currents (from direct cardiac electrical connections)

Rigorous safety standards apply to electromedical equipmentIEC601 series

IEC safety standards for ECG equipment

Patient leakage currents 10uA. Under single fault conditions < 50uA

Earth leakage currents500uA from mains to ground across insulation under normal conditionsnormal conditions

Enclosure current100uA from any part accessible to the operator or patientpatient

Isolation>3500 Vac between the patient and the mains inlet to the de icethe device

IEC Symbols for

A) body protected (BF) andA) body protected (BF) and B) cardiac protected (CF) equipment able to withstand defibrillation

Design example: 12 lead ECG

PCs often used as medical instrumentsLow costEase of programmingp g gNumber crunching capacity (DSP)Variety of third party products for support of functionsGraphical user interface (GUI)

Require an interface cardInternal

Work with desktops but not with notebooksSerial interface

RS-232 (old), USB or Firewire (IEEE1394)USB excellent choice for modern equipment

PC-ECG design example

PC-ECG design example

Defibrillation protection and RF filteringPrevents device from damage by high voltage during defibrillationBl k t i l di f (RF) i t fBlocks out wireless radio frequency (RF) interference

Driven right leg circuitReduces common mode noise (mains frequency) by ~50dB by driven right legdriven right leg

Shield driveReduces capacitance of cable shields thus improving common mode rejection ratio (CMRR) and high frequency performancemode rejection ratio (CMRR) and high frequency performance

PC-ECG design example

Differential amplifiersProvide high CMRR with tolerance to DC offsets (± 300mV)

High pass filter stageHigh pass filter stageRemoves baseline wander (fc = 0.5 Hz or 0.05 Hz)

Variable gain stageComputer controlled gain of 20 – 100Computer controlled gain of 20 100

Low pass filterBessel filter (or similar) for anti-aliasing (fc = 40, 100 or 150 Hz)Software controlSoftware control

PC-ECG design example

Analogue to digital converter (ADC)12 bit range, 8 channel

IsolationIsolationFor patient safety, isolates patient circuit from mains groundOptical method of isolating digital data and control lines Transformer isolation for powerTransformer isolation for power

PC bus interfaceFor communication with host PC and control of parameters (e.g. amplifier gain, filter cutoff)

Data display, printing and storageUsing normal PC resources

Data reduction and compression

12 lead ECG typically sampled at 500 or 1000 Hz10 second records require 80Kbytes storage

f h d h l h d d dLess of an issue these days with large hard drives and storage disks Still a big problem with long recordings (24 hour)Various data compression schemes available for ECG (such as turning point algorithm)

but quality may (will) be affected

Programming languages

MATLABExcellent for initial signal processing developmentC t C++ d f d l tCan generate C++ code for deployment

Visual BasicEase of programming for rapid development

C++Generally language of choice for final applicationObject orientedDSP libraries available

Database design

Relational databases commonly usedData and relationships between data are represented by a series of tablestablesEach column is known as a fieldEach row is known as a record

Data retrieval occurs by a programData retrieval occurs by a programStructured Query Language (SQL) is the most common language used

A sample relational database

Pat_code Pat_Name Pat_CityTable ‘patient’

0001 Jack Smith Brisbane

0002 John Doe Sydney

0003 Susan Smith Melbourne0003 Susan Smith Melbourne

Table ‘procedures’

Proc_code Proc_Date Proc_Description Proc_Blob0001 12/07/2005 12 lead ECG “binary data”

0002 05/06/2005 3 l d ECG “bi d t ”0002 05/06/2005 3 lead ECG “binary data”

0003 01/12/2004 Spirometry “binary data”

Automated interpretation of ECGs

Alternative to cardiologist (human) interpretationAccuracy comparable to cardiologists

fTwo formsRule-based decision logic (If-then)

Mimics decision process used by cardiologistC b d d b hCan be understood by humans

Statistical pattern recognition methodArtificial Neural Networks, Support Vector Machine, Regression Tree, and (many) moreand (many) moreMany alternatives these days with at least equal performance and fast trainingBlack box models (can’t see inside the box)

Automated interpretation of ECGs - Procedure

Preliminary signal processing/noise removalFind reference point (usually R wave)

f l d f ( lDetermine if valid waveform (e.g. cross correlation to some reference wave)Average all valid waveforms over some period (e.g. 10

d )seconds)Extract features from average waveformUse extracted features in model to determine an output (e.g. left bundle branch block)

Characteristic ECG beat with parameters measured in HP automated ECG interpretation

Reference

Adapted from Physiological Monitoring, Branko Celler, chapter 17 from “Health Informatics – an Overview”, edited by E. Hovenga, M Kidd B Cesnik Churchill Livingstone Melbourne 1996M. Kidd, B. Cesnik. Churchill Livingstone. Melbourne. 1996.

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