BME 353E – Biomedical Instrumentation & Measurements

BME 353E – Biomedical Instrumentation & Measurements
Instrumentation Basics

BME 353 - Biomedical Instrumentation and Measurement
Rehearsals Why the computer related medical technologies will experience a high rate of development in the next decade? What are the major contributing technologies for home-care? What are the industries where the biomedical engineers can work and what will be their responsibilities? What type of medical knowledge will be needed mostly by biomedical engineers? 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Block diagram of a generalized instrumentation system
5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Questions to Answer for Measurement
Why measure? What to measure? How to measure? How to establish conditions for measurement? How to verify the data? How to convert data into information? How to present the results? How to interpret the results? 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

A simplified measurement system
A typical measurement system uses sensors to measure the variable, has signal processing and display, and may provide feedback. 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

What the Physician Wants?
O2 and nutrient concentrations in the cells; why? Not possible in a direct way Blood flow and changes in the volume; how it is related to the first choice? Cardiac output = stroke volume x heart rate Not easy to do Blood pressure; what is pressure and how it is related to flow and O2 concentrations? Direct - requires invasive methods to be used Indirect - noninvasive, but full waveform is not easy to obtain Electrocardiogram and heart rate – noninvasive; what is the electrocardiogram and what is its physiological relevance? 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Rehearsals What is metabolism? What is homeostasis? Why a clinician is an essential element for an effective clinical instrument? Why the physician’s primary interest is in determining “O2 and nutrient concentrations in the cells”? 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Medically important measurands
Biopotentials Pressure Flow Dimensions (imaging) Displacement (velocity, acceleration and force) Impedance Temperature Chemical concentrations 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Alternative operational modes
Direct-indirect modes Sampling and continuous modes Generating and modulating sensors Analog and digital modes Real-time and delayed-time modes 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Analog and digital signals
Time Amplitude Amplitude Time Analog signals can have any amplitude value at any time Digital signals have a limited number of amplitude values 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Continuous and discrete-time signals
Amplitude Amplitude Time Continuous signals have values at every instant of time Discrete-time signals are sampled periodically and do not provide values between these sampling times Questions: What is the basic reason in choosing analog or digital signals for a medical diagnosis? How we convert analog signals into digital ones? 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Example to sampled data
Laboratory test Typical value Hemoglobin 13.5 to 18 g/dL Hematocrit 40 to 54% Erythrocyte count 4.6 to 6.2  106/ L Leukocyte count 4500 to 11000/ L Differential count Neutrophil 35 to 71% Band 0 to 6% Lymphocyte 1 to 10% Monocyte 1 to 10% Eosinophil 0 to 4% Basophil 0 to 2% Complete blood count for a male subject. 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Origins of common biological signal
Type of signal Name of organ in Latin Type of presenting prefix+ name + suffix Electro Mechano Pnemo Cardio Encepha Myo Gram Graph Graphy 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Voltage and freq. ranges of some common biopot. signals

Medical measurement constraints
Range Frequency, Hz Method Blood flow 1 to 300 mL/s 0 to 20 Electromagnetic or ultrasonic Blood pressure 0 to 400 mmHg 0 to 50 Cuff or strain gage Cardiac output 4 to 25 L/min Fick, dye dilution Electrocardiography 0.5 to 4 mV 0.05 to 150 Skin electrodes Electroencephalography 5 to 300  V 0.5 to 150 Scalp electrodes Electromyography 0.1 to 5 mV 0 to 10000 Needle electrodes Electroretinography 0 to 900  V Contact lens electrodes pH 3 to 13 pH units 0 to 1 pH electrode pCO2 40 to 100 mmHg 0 to 2 pCO2 electrode pO2 30 to 100 mmHg pO2 electrode Pneumotachography 0 to 600 L/min 0 to 40 Pneumotachometer Respiratory rate 2 to 50 breaths/min 0.1 to 10 Impedance Temperature 32 to 40 °C 0 to 0.1 Thermistor 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Setting sensor specifications
Value Pressure range –30 to +300 mmHg Overpressure without damage –400 to mmHg Maximum unbalance ±75 mmHg Linearity and hysteresis ± 2% of reading or ± 1 mmHg Risk current at 120 V 10 A Defibrillator withstand 360 J into 50  Sensor specifications for a blood pressure sensor are determined by a committee composed of individuals from academia, industry, hospitals, and government. 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Specifications for ECG
Value Input signal dynamic range ±5 mV Dc offset voltage ±300 mV Slew rate 320 mV/s Frequency response 0.05 to 150 Hz Input impedance at 10 Hz 2.5 M Dc lead current 0.1 A Return time after lead switch 1 s Overload voltage without damage 5000 V Risk current at 120 V 10 A Specification values for an electrocardiograph are agreed upon by a committee. 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Classification of biomedical instruments
Quantity sensed: pressure, flow, temperature etc. Principle of transduction: resistive, inductive, capacitive, ultrasonic or electrochemical Organ system studied: cardiovascular, pulmonary, nervous, and endocrine systems. Clinical medical specialties: pediatrics, obstetrics, cardiology, or radiology. 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Interfering and modifying inputs
An interfering input may shift the baseline Original waveform A modifying input may change the gain 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Simplified Electrocardiographic recording system

Compensation Techniques
Inherent insensitivity Negative feedback Signal filtering Opposing inputs 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Negative feedback y + Gd xd - Hf 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Signal filtering Signals without noise are uncorrupted
Interference superimposed on signals causes error. Frequency filters can be used to reduce noise and interference 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Opposing inputs Differential amplifier: v0 = Gd(vA- vB) DC cancellation (bucking) An input signal with dc offset An input signal without dc offset 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Generalized Static Characteristics
Error Accuracy Tolerance Precision and reproducibility Resolution Statistical control Static sensitivity Zero drift Sensitivity drift Linearity Input ranges Input impedance 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Tolerance Maximum deviation allowed from the conventional true value. It is not possible to built a perfect system or make an exact measurement. All devices deviate from their ideal (design) characteristics and all measurements include uncertainties (doubts). Hence, all devices include tolerances in their specifications. If the instrument is used for high-precision applications, the design tolerances must be small. However, if a low degree of accuracy is acceptable, it is not economical to use expensive sensors and precise sensing components. 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Static sensitivity Sensor signal Measurand Sensor signal Measurand A low-sensitivity sensor has low gain A high sensitivity sensor has high gain 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Static sensitivity constant over a limited range

Zero and sensitivity drifts

Linearity Output Input Output Input A nonlinear system does not fit a straight line A linear system fits the equation y = mx + b. 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Calibration for linearity
Output Input Output Input The one-point calibration may miss nonlinearity The two-point calibration may also miss nonlinearity Measuring instruments should be calibrated against a standard that has an accuracy 3 to 10 times better than the desired calibration accuracy 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Hysteresis A hysteresis loop. The output curve obtained when increasing the measurand is different from the output obtained when decreasing the measurand. Question: Why we have hysteresis in responses? 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Independent nonlinearity

Input ranges An input signal which exceeds the dynamic range The resulting amplified signal is saturated at 1 V 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Input impedance System Xd1 : effort variable Output Xd2 : flow variable 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Frequency range Frequency response of the electrocardiograph 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Time delay Log scale w K x(t) 1 t y(t) Log scale w 1 td t 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Engineering Design Imagine Deliver Criticize Realize Contemplate Imagine: Tahayyül Contemplate: Tasavvur Realize: Tahakkuk Criticize: Tenkit Deliver: Teslim Design is the innovative process of identifying needs and then devising a product to fill those needs. It is a problem solving using the existing knowledge and technology: integration of knowledge. It considers alternative solutions for selecting the optimal solution with a fixed goal or specifications in mind. 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Design criteria Initial instrument design Signal factors Prototype tests Environmental factors Final instrument design Measurand Medical factors FDA, BMD approval Economical factors Production 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Design criteria Specificity Signal-to-noise ratio Stability Temperature Humidity Pressure Acceleration Shock Vibration Radiation Power requirements Mounting size, shape Initial instrument design Sensitivity Range Differential or absolute input Input impedance Transient and frequency response Accuracy Linearity Reliability Signal factors Prototype tests Environmental factors Final instrument design Measurand Invasive or non-invasive Tissue-transducer interface requirements Material toxicity Electrical safety Radiation and heat dissipation Patient discomfort Cost Availability Warranty Consumable requirements Compatibility with existing equipment Medical factors FDA, BMD approval Economical factors Production 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Signal factors Sensitivity Range Differential or absolute input Input impedance Transient and frequency response Accuracy Linearity Reliability 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Environmental factors
Specificity Signal-to-noise ratio Stability Temperature Humidity Pressure Acceleration Shock Vibration Radiation Power requirements Mounting size, shape 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Medical factors Invasive or non-invasive Tissue-transducer interface requirements Material toxicity Electrical safety Radiation and heat dissipation Patient discomfort 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Economic factors Cost Availability Warranty Consumable requirements Compatibility with existing equipment 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Rehearsals What is the function of a sensor? What is the difference between sensor and transducer? Why do we need a calibration signal? Why we need an external power source? What are the frequently used external power sources? What are the means for displaying the output? 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

Measurand 5 Ekim 2015 BME Biomedical Instrumentation and Measurement

BME 353E – Biomedical Instrumentation & Measurements

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