ELE-451: INTRODUCTION TO BIOMEDICAL ENGINEERING Dr. Atila YILMAZ Text Book: Webster J.G. Medical Instrumentation, Application and Design Contents: 1. Introduction 2. Bioelectricity 3. Medical Sensors and Their Principles 4. Biopotential Electrodes and Amplifiers 5. Electric Activities: ECG, EEG, EMG 6. Other Applications Currently recognized interdisciplinary fields that associate physics and engineering with medicine and biology: BEN = bioengineering, MPH = medical physics, MEN = medical engineering, BPH = biophysics, MEL = medical electronics, BEM = bioelectromagnetism, The coordinate origin represents the more theoretical sciences, such as biology and physics. As one moves away from the origin, the sciences become increasingly applied. Combining a pair of sciences from medical and technical fields yields interdisciplinary sciences such as medical engineering. It must be understood that the disciplines are actually multidimensional, and thus their twodimensional description is only suggestive. we have a special interest in those disciplines that combine engineering and physics with biology and medicine. These disciplines are briefly defined as follows I I I Biophysics and Biochemistry: The science that is concerned with the solution of biological problems in terms of the concepts of physics and chemistry. Bioengineering (Biomechanics and Bioelectronics): The application of engineering to the development of health care devices, analysis of biological systems, and manufacturing of products based on advances in this technology. This term is also frequently used to encompass both biomedical engineering and biochemical engineering (biotechnology). Biotechnology: The study of microbiological process technology. The main fields of application of biotechnology are agriculture, and food and drug production. Biomedical Engineers apply electrical, mechanical, chemical, optical and other engineering principles to understand, modify, or control biological systems, as well as design and manifucture products that can monitor physiologic functions and assist in the diognosis, and treatment of patients. I I I Medical electronics: A division of biomedical engineering concerned with electronic devices and methods in medicine. Medical physics: A science based upon physical problems in clinical medicine. Biomedical engineering: An engineering discipline concerned with the application of science and technology (devices and methods) to biology and medicine. I ) Measurements of fields (A) Bioelectricity (B) Bioelectromagnetism (C) Biomagnetism (Biomagnetism) Neural cells electroencephalography (EEG) electroneurography (ENG) electroretinography (ERG) magnetoencephalography (MEG) magnetoneurography (MNG) magnetoretinography (MRG) Muscle cells electrocardiography (ECG) electromyography (EMG) magnetocardiography (MCG) magnetomyography (MMG) Other tissue electro-oculography (EOG) magneto-oculography (MOG) electronystagmography (ENG) magnetonystagmography (MNG) magnetopneumogram magnetohepatogram Some Events in Biomedical Engineering I I I I I In 1903, Willem Einthoven devised the first electrocardiography to measure the electric activityof the heart The most significant innovation for clinical medicine was the development of X-rays in 1895, Roentgen,W.K) The English school of neurophysiology began when Richard Caton (British; 1842-1926) became interested in the recording technique of du BoisReymond and applied it to the measurement of the electric activity of the brains of rabbits and monkeys. The first report of his experiments, published in 1875 (Caton, 1875), is believed to constitute the discovery of the electroencephalogram (EEG). The founder of membrane theory was Julius Bernstein (German; 1839-1917), a pupil of Hermann von Helmholtz. Bernstein stated that the potential difference across the membrane was maintained by the difference in concentration of potassium ions on opposite sides of the membrane. In the early 1970s a major innovation was introduced into diagnostic imaging by Hounsfield. This innovation, x-ray computed tomography (CT), is recognized today as the most significant single event in medical imaging since the discovery of x rays. Stimulation experiment of Luigi Galvani (1786). He found that when the nerve and the muscle of a frog were simultaneously touched with a bimetallic arch of copper and zinc, a contraction of the muscle was produced d'Arsonval's great solenoid. (d'Arsonval, 1893). (Heating the tissues) The first experiment on magnetic stimulation of the nervous system. It was caused by the stimulating effect of the time varying magnetic field to the retina. He called this phenomenon "magnetophosphenes." 1.2. Physical Systems I Musculosceletal System I Respiratory System I Gastrointestinal System I Nervous System I Endocrine System I The Circulatory System I The Body as a Control System The development of medical devices benefited greatly from the following rapid surge of ‘technical finds’: Advances in Solid State Electronics I New Prosthetic Devices I Nuclear Medicine I Ultrasound on Sonar Technology I Material ‘Spare Part’ Tecnology I Advances in Materials I Computers I 1.3. Development of Biomedical Instrumentation In general, medicine is a multistep procedure performed by a physician, group of physician, or an institution and repeated until the symptoms dissapear Diognosis ICollecting data IAnalysis of data IDecision making Treatment IInstituted from the decision IRepeat Special features of medical instrumentation systems More complicated systems than non-medical systems I Living tissue and its function is involved I Accessibility of the information is limited I The measured signal is not always directly available and may be corrupted by noise I Physiological phenomena are seldom deterministic • intra- and intervidual variability • pathophysiological processes are not known I The basic objectives of any instrumentation system fall into one of the following major cathegories: measurements for Information gathering I Diognosis (detection, measuring, correction) I Evaluation (determining the ability of system) I Monitoring (obtain information about the state of sys.) I Control (automatically regulating the system) I Figure 1.3.1 (The Scientific Method) In the scientific method, a hypothesis is tested by experiment to determine its validity. Problem statement Review prior work State hypothesis Perform experiments Design further experiments More experiments necessary Analyze data Final conclusions Problem solved Figure 1.3.2 (Clinical Diagnosis) The physician obtains the history, examines the patient, performs tests to determine the diagnosis and prescribes treatment. Chief complaint Obtain history List the differential diagnosis Examination and tests Treatment and evaluation Select further tests Use data to narrow the diagnosis Final diagnosis More than one likely Only one likely Figure 1.3.3 (Feedback in Mesurement System) A typical measurement system uses sensors to measure the variable, has signal processing and display, and may provide feedback. Outputs Measurand Sensor Signal conditioning Feedback Effector Signal processing Data storage Data displays Data communication Functional components of the system • Object: measurand -- >signal--> information I • Sensor element I • Signal conditioner or processing I • Output recorder and display I • Control, Stimulus, Feedback I Generalized Medical Instrumentation System The sensor converts energy or information from the measurand to another form (usually electric). This signal is the processed and displayed so that humans can perceive the information. Elements and connections shown by dashed lines are optional for some applications Control And feedback Power source Sensor Measurand Primary Sensing element Calibration signal Radiation, electric current, or other applied energy Variable Conversion element Signal processing Output display Data storage Data transmission Perceptible output Measurand object that the system measures I physical quantity I electric voltage (ECG, EEG, ...), pressure, ... I property I expanse, severity, … I condition I heart rate, weakness, pregnancy… Measurand • generation of the measurand I generated by the physiological system spontaneously - EEG, ECG, blood pressure I produced by the subject voluntarily - muscle activity, deep ventilation I externally stimulated or activated - evoked potentials, paced potentials Measurand Accessibility of the measurand important issue because it determines the methods and devices to be used to record that quantity: - internal origin of the signals deep recordings, catheter recordings (EPS) - internal origin but appears on body surface electric potential on the skin - signal emanates from the body magnetic field (MCG, MEG) - information derived from a tissue sample blood sample - transmission or emission of applied external energy electric current (bio-impedance) Measurand Some medically important measurands biopotentials: EEG, ECG, ... I pressure: blood, intracranial, respiratory I flow: velocity and volumes, blood, respiratory gas I dimensions: (imaging) vessel diameters I displacement: velocity, acceleration, force I impedance: electric, mechanical airway resistance I temperature: body, internal I chemical concentration: pH, blood gases, glucose I Stimulator: in a wide meaning, methods and devices to feed external energy to the living system I to get a desired response (measured quantity) to the application of energy I Stimulator Stimulation of a living function electrical current - nerve conduction velocity - cardiac electrophysiological testing magnetic field - stimulus of the central nervous system light - visual evoked potentials mechanical oscillation - auditory evoked potentials Stimulation In some physiological measurements and medical imaging applications, external energy or quantity is needed to produce the desired information from the body: I physiological measurements bioimpedance measurements I medical imaging ionizing radiation (x-ray, gamma) excitation of nuclear magnets (MRI) ultrasound energy (Doppler) laser (imaging) Examples 1.3.2. Medical Measurement Constraints The general properties of medical parameters and condition of subject limit the practical choices available to designers I I I I I I I I I The measurement ranges of the parameters are quite low Audio frequency range or below (may contain dc and low frequencies) Safe level of the various type of energy is difficult to establish The heating of the tissue must be limited Equipment must be reliable, simple to operate, and capable of withstanding physical abuse Exposure to corrosive chemicals Electric shock hazard should be minimized Measuring in anyway endanger the life It should not require the subject to endure undue pain, disconfort or any other undesirable conditions Classification of the biomedical instruments 1. quantity that is sensed • pressure, flow, temperature ... • easy to compare devices used in measuring any quantity 2. principle of transducer • resistive, inductive, capacitive, ultrasonic,… • engineering approach 3. physiological system • cardiovascular, pulmonary, nervous, ... • medical approach, overlapping 4. clinical medicine specialities • pediatrics, cardiology, radiology, patient monitoring ... • valuable for medical personnel who are interested in specialized instruments Measurement 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 0 to 20 Fick, dye dilution Electrocardiography 0.5 to 4 mV 0.05 to 150 Skin electrodes Electroencephalography 5 to 300 µ V 0.5 to 150 Scalp electrodes Electromyography 0.1 to 5 mV 0 to 10000 Needle electrodes Electroretinography 0 to 900 µ V 0 to 50 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 0 to 2 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 Table 1. Sensor specifications for a blood pressure sensor are determined by a committee composed of individuals from academia, industry, hospitals, and government. Specification Value Pressure range –30 to +300 mmHg Overpressure without damage –400 to +4000 mmHg Maximum unbalance ±75 mmHg Linearity and hysteresis ± 2% of reading or ± 1 mmHg Risk current at 120 V 10 µA Defibrillator withstand 360 J into 50 Ω Table 2. Specification values for an electrocardiograph are agreed upon by a committee. Specification 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 MΩ Dc lead current 0.1 µΑ Return time after lead switch 1s Overload voltage without damage 5000 V Risk current at 120 V 10 µΑ Simplified electrocardiographic recording system Two possible interfering inputs are stray magnetic fields and capacitively coupled noise. Orientation of patient cables and changes in electrode-skin impedance are two possible modifying inputs. Z1 and Z2 represent the electrodeskin interface impedances. Electrodes vecg Z1 Z2 Zbody +Vcc 60-Hz ac magnetic field + Differential amplifier − Displacement currents Vcc vo desired input signal - the potential difference on the skin (Vecg) interfering inputs - line-frequency interference, - imbalance in electrode impedances - amplifier noise • modifying inputs - effect of orientation of the patients cables, - time-dependent change in electrode impedance - time-varying changes in amplifier 1.3.3. Some of the Characteristics of Medical Instruments It demonstrates the quality of the measurements; The following factors should be considered: Range: All the levels of imput amplitude and frequency (Exceeding the range) Amplitude 5 mV Time -5 mV Amplitude 1V (b) Time -1 V Dynamic Range Frequency Response: It is its variation in sensitivity over the frequency range of the measurement 1.0 Amplitude 0.1 0.05 Hz 150 Hz Frequency Figure. 1. Frequency response of the electrocardiograph. Figure 2.(a) Static-sensitivity curve that relates desired input xd to output y. Static sensitivity may be constant for only a limited range of inputs. (b) Static sensitivity: zero drift and sensitivity drift. Dotted lines indicate that zero drift and sensitivity drift can be negative. Figure (a) An input signal without dc offset. (b) An input signal with dc offset. Amplitude Time Amplitude (b) Dc offset Time Sensitivity: It determines how small a variation of a variable or parameter can reliably be measured (directly related to the resolution of the device Resolution: the minimum variation that can accurately be read X = ∑ X y = i n GM = n X 1 X 2 X 3 ⋅ ⋅ ⋅ X n ⎛ s CV = ⎜ ⎝X y = mxd + b ∑ (X s= ⎞ ⎟(100 % ) ⎠ b= Gd 1 + H hG ∑ (X i d 2 d d 2 d i −X ) )( − X Yi − Y − X (∑ y )(∑ x ) − (∑ x y )(∑ x ) n ∑ x − (∑ x ) 2 d d 2 n −1 ∑ (X r = i xd ) ∑ (Y 2 m= i ) −Y ) 2 n ∑ x d y − (∑ x d )(∑ y ) n ∑ x d2 − (∑ x d ) 2 Linearity: The sensitivity would be the same for all absolute levels of input x1 Linear system y1 (x1 + y2) and x2 Linear system (y1 + y2) Linear system and y2 Kx1 Ky1 Linear system Least-squares straight line (a) y (Output) B% of full scale Figure 1.4 (a) Basic definition of linearity for a system or element. The same linear system or element is shown four times for different inputs. (b) A graphical illustration of independent nonlinearity equals ±A% of the reading, or ±B% of full scale, whichever is greater (that is, whichever permits the larger error). A% of reading Overall tolerance band xd (Input) Point at which A% of reading = B% of full scale (b) Figure (a) Original waveform. (b) An interfering input may shift the baseline. (c) A modifying input may change the gain. (a) (c) (b) Accuracy: It is a measure of systemic error and calculated by accuracy=the true value-the measured value/the true value Precision: The precision of a measurement expresses the number of distinquishable alternatives from which a given result is selected Signal to Noise Ratio Stability Isolation Electrical Safety Figure Data points with (a) low precision, good accuracy and (b) high precision, low accuracy. (a) (b) Figure Data points with (a) low accuracy, high precision and (b) high accuracy, high precision. (a) (b) Figure 1.8 Design process for medical instruments Choice and design of instruments are affected by signal factors, and also by environmental, medical, and economic factors.