Sleep 2008 Electronicsv.3


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  • analog Of, relating to, or being a device in which data are represented by continuously variable, measurable, physical quantities, such as length, width, voltage, or pressure. Or a circuit or device having an output that is proportional to the input; "analogue device" Digital-Of or relating to a device that can read, write, or store information that is represented in numerical form
  • Resting potential- the cell is polarized in a way in which the cell is negative inside and positive outside. When the polarized cell receives a stimulus, depolarization takes place. This stimulus causes the cell membrane to suddenly become permeable to Na+ ions and they rush into the cell in an attempt to lessen their concentration gradient. In the area immediately affected by the impulse the cell becomes more positively charged. The switching of sodium ions continues all along the surface of the neuron, causing a change in net electrical charge which moves along the length of the cell. Volume conduction of electrical events in the body. The tissues of the body can and do conduct electricity. The body and its parts are 3-dimensional structures and therefore have volume. Electrical currents spread (are conducted) throughout this volume, thus it is correct to speak of "volume conduction" of electricity in tissue. Because of the conductivity of tissue, at rest the volume conductor formed by the body is of equal potential (isopotential) at all points. When a dipole is formed, current flows until isopotentiality is reached.
  • The tissue lying between the generating cells and the recording electrode through which electrical current must flow forms an electrical volume conductor. The volume conductor greatly modifies the amplitude and morphology of the cortical signal before it reaches the recording electrodes. The epidermis is the thin outer layer of the skin. The epidermis itself is made up of three sub-layers: status corneum (horny layer) This layer contains continually shedding, dead keratinocytes (the primary cell type of the epidermis). The keratin, a protein formed from the dead cells, protects the skin from harmful substances. keratinocytes (squamous cells) This layer contains living keratinocytes (squamous cells), which help provide the skin with what it needs to protect the rest of the body. basal layer The basal layer is the inner layer of the epidermis, containing basal cells. Basal cells continually divide, forming new keratinocytes and replacing the old ones that are shed from the skin's surface. The epidermis also contains melanocytes, which are cells that produce melanin (skin pigment).
  • In polysomnography we measure voltages of waveforms and therefore calibration becomes necessary. We use a known voltage e.g. 50uV and derive a desired output. Inputting a voltage creates a square wave of which the output is modified by settings e.g. Gain and filter. Another setting that relates to output is the sensitivity setting. This relates amplifier output voltage to pen deflection. For example a common sensitivity setting is 7uV/mm. This is to say for every 7uV we should see a 1mm deflection. If I use a 50uV cal signal I therefore get a deflection of 7.14mm 50 / 7=7.14 We also use calibration to verify the integrity of each channel by showing that like channels amplify and filter the same signal in the same way. Digital systems use cursors which give you the voltage eliminating the need for the calculations. In order to verify that such as system is working correctly though you would use an external signal generator and verify output to input is correct. Calibrations should be performed at the beginning and end of the study.
  • Discrimination is the ability of the amplifier to reveal differences in electrical potential between amplifier electrode inputs 1 & 2 while rejecting potentials which are common to the inputs.
  • The reason we use differential amplifiers is to eliminate potentials from sources other that the bio-electrical potentials and transduced signals that we are trying to record. The environment we record in often contains voltage interference from various sources also our bodies produce signals that we do not wish to record. When both inputs receive these signals they are rejected and are not a problem. They are rejected as common mode signal. Another type of amplifier that may be in your system is the single ended amplifier. This is an amplifier that is used merely to further increase the voltage of the signals recorded. They do not have filters or discrimination. A differential amplifier is used to amplify only the difference between two selected inputs. CMRR is the process of two like signals canceling each other out. If you reference an electrode to itself you would get no signal this happens when one has a salt bridge.
  • So the polarity of the output depends on the polarity of the input signal and to which terminal it is applied.
  • In polysomnography we use the terms “AC” and “DC” to describe the characteristics of various recorded voltages. We also use the terms “AC” and “DC” to describe the type of amplifier used to record these two types of signals.
  • An AC amp. Can not do a linear scale because it has a time constant and always goes back to baseline.
  • If I where to solve the above equation I must know my deflection and the voltage. where I is the current in amperes , V is the potential difference between two points of interest in volts , and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), and is called the resistance . where I is the current in amperes , V is the potential difference between two points of interest in volts , and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), and is called the resistance . I=E/R Ohm’s Law Current is directly proportional to applied voltage and inversely proportional to resistance. The basic unit of current (I) is the ampere (A). Movement of electrons The basic unit of electromotive force (E) is the volt (V). The basic unit of resistance is the ohm (R). I = E/R D= V/S (polysomnography) Voltage (E) = Input voltage (V) Resistance (R) = Sensitivity (S) Current (I) = Pen Deflection (D) Sensitivity = Voltage/Deflection
  • If I would rather solve for voltage I would again determine my deflection and multiply by the sensitivity.
  • For those of you math challenged there is a simple way of remembering how to solve for a variable. Draw a circle divide the circle in half horizontally. Then divide the lower half vertically. Now put a V in the top half of the circle. In lower half put a D in the first section and S in the other. Now when solving for a variable cover that letter. Then solve by multiplying values on the same level or dividing values on diff. levels upper over lower. V over D & S Sensitivity is the input voltage ( µ V) required to produce a 1 millimeter (mm) of pen deflection at the output.
  • Electromotive force (EMF) measured in volts (V) is the strength of the interaction of positive and negative electrical charges and reflects the potential or tendency for charges to attract or repel. Voltage by definition is the maximum work that can be extracted as one unit of positive charge moves from one point to another. A potential difference between two points of one volt will require one joule of work to move a positive charge of one coulomb from the point of lower potential to the point of higher potential. Current is the movement of electrons through a conductor connected to two points when there is a potential difference of charge between the two points. Resistance is the opposite of conductance, resistance reflects the forces working against current flow. Resistance to AC current is called “impedance” whereas the term “resistance” is for DC current, although a resistor reduces the flow equally in DC or AC current and is not affected by the frequency of alternating current.
  • Delta 0.5 – 2 Hz duration
  • Time constant = inductance / resistance Inductance is the ability of a conductor to induce voltage in itself, measured in henrys (H) Resistance is the opposition to current flow, measured in ohms; resistance = volts / current
  • It is important to know what causes the 60 Hz so leaving on the notch filter is counter to a good study. Poorly applied leads-high impedance Pt. or machine or building not grounded Ungrounded equipment near the recording equipment.
  • This is determined by the Nyquist Theorem which gives us the Nyquist Rate. The Nyquist Rate is 2 times the high frequency filter. We wish to sample at a rate higher then the Nyquist Rate because we will get a more accurate display of waveform morphology. Sampling rate does effect data storage the higher the rate the more data collected. So setting each channels sample rate rather than using the highest high frequency filter within the system may be more practical.
  • Each bit is a power of two
  • The range of voltages that the channel can display. This may be changed by changing the sensitivity setting. If you are going beyond the dynamic range you would need to lower the sensitivity by increasing the sensitivity setting. Say from 10 uV/mm to 20 uV/mm.
  • Sleep 2008 Electronicsv.3

    1. 1. Back to Basics Electronics & Filters In PSG Systems Will Eckhardt , BS,RPSGT, CRT
    2. 2. Conflicts <ul><li>I work for Respironics, Inc. </li></ul><ul><li>I do not believe this has any influence on the content of this presentation. </li></ul>
    3. 3. What Will Be Answered in this Presentation <ul><li>Where do EEG potentials originate? </li></ul><ul><li>We record various physiological signals what means of processing do we utilize ? </li></ul><ul><li>What are the analog components of signal processing? </li></ul><ul><li>What are the digital components of signal processing? </li></ul><ul><li>An understanding some basic underlying principles of the technology by looking at the analog signal output. </li></ul>
    4. 4. Pyramidal Neuron EEG is derived from thousands of synchronized pyramidal cell postsynaptic potentials. Volume Conduction is the process of current flow through the tissues between the electrical generator and the electrode.
    5. 5. Amplitude of the Recorded Potentials <ul><li>Depends on: </li></ul><ul><ul><li>Intensity of the Electrical Source </li></ul></ul><ul><ul><li>Distance of Source to Recording Electrodes </li></ul></ul><ul><ul><li>Spatial Orientation </li></ul></ul><ul><ul><ul><li>Orientation of the electrical generator to the electrode </li></ul></ul></ul><ul><ul><li>Electrical Resistance </li></ul></ul><ul><ul><ul><li>Scalp-electrode interface </li></ul></ul></ul><ul><ul><ul><li>Recording electrodes </li></ul></ul></ul>
    6. 6.
    7. 7. Electrode Placement <ul><li>Exploring electrode </li></ul><ul><li>Reference electrode </li></ul><ul><li>Referential derivation </li></ul><ul><li>Bipolar derivation </li></ul>
    8. 8. Components of Polysomnography Equipment and their relationship to signal processing.
    9. 9. Path of Signals from the Patient to the Tracing
    10. 10. Recording Electrodes <ul><li>Electrode types </li></ul><ul><ul><li>Gold, Silver Chloride, Tin, Platinum </li></ul></ul><ul><li>Placement </li></ul><ul><ul><li>10-20 system </li></ul></ul><ul><li>Application </li></ul><ul><ul><li>Electrode impedance </li></ul></ul>
    11. 11. Headbox <ul><li>The headbox is an intermediate connection conducting the electrical signal from the electrode to the amplifier. </li></ul><ul><li>Enables multi-sensor output to one low impedance cable. </li></ul><ul><li>The cable then connects to: </li></ul><ul><ul><li>electrode selector panel (analog) </li></ul></ul><ul><ul><li>amplifier </li></ul></ul><ul><li>Note: </li></ul><ul><ul><li>the headbox may include the systems amplifiers. </li></ul></ul>
    12. 12. Wave Form - Input & Output <ul><li>Voltage input to output using a known input. </li></ul><ul><ul><li>50 µV </li></ul></ul><ul><ul><li>Square wave </li></ul></ul><ul><ul><ul><li>Gain setting (amount of amplification) </li></ul></ul></ul><ul><ul><ul><li>Filters (high, low, notch) </li></ul></ul></ul><ul><ul><ul><li>Sensitivity (amplifier output voltage to pen deflection) </li></ul></ul></ul><ul><li>Verify signal integrity </li></ul><ul><ul><li>Like channels give the same output </li></ul></ul>
    13. 13. Types of Calibration <ul><li>Machine Cal </li></ul><ul><ul><li>All channels respond the same </li></ul></ul><ul><li>Montage Cal </li></ul><ul><ul><li>Channels respond as set </li></ul></ul><ul><li>Bio-Cal </li></ul><ul><ul><li>Channels respond appropriately for the maneuver/sensor </li></ul></ul><ul><ul><li>Should be performed at the beginning & end of the study. </li></ul></ul>
    14. 14. Amplifiers <ul><li>Function: Signal modification </li></ul><ul><ul><li>Amplification ( ↑ voltage of potential to drive pens or card) </li></ul></ul><ul><ul><li>Discrimination (reveal differences in inputs 1 & 2) </li></ul></ul><ul><li>Inputs: Input 1 (G1) – Input 2 (G2) </li></ul><ul><ul><li>G1 exploring </li></ul></ul><ul><ul><li>G2 reference </li></ul></ul><ul><li>Types: AC & DC Amplifiers </li></ul><ul><li>Digital and Analog Amplifiers </li></ul>
    15. 15. Differential Amplifier <ul><li>Amplifies only the difference in voltage between two inputs. </li></ul><ul><li>Does not amplify equal voltages. </li></ul><ul><li>Has the ability to reject external interference. </li></ul><ul><li>The amount that an amplifier will reject a common mode signal is called “common mode rejection ratio (CMRR). </li></ul><ul><li>10,000/1- Common mode/output voltage </li></ul>Identical signals
    16. 16. Signal Polarity <ul><li>Designed such that (-) voltage to input G1 of a differential amplifier [with respect to G2] will result in a upward deflection. </li></ul><ul><li>When G1 becomes more (+) than G2 this will result in a downward deflection. </li></ul>
    17. 17. AC Amplifier <ul><li>This type of amplifier is used for signals that vary frequently from positive to negative. </li></ul><ul><li>Examples: EEG, EMG, EOG </li></ul>
    18. 18. DC Amplifier <ul><li>This type of amplifier is used for signals that slowly increase or decrease. </li></ul><ul><li>Use DC amplifiers: </li></ul><ul><ul><li>Used for signals i.e. oximetry, respiration, temp, PH. </li></ul></ul><ul><ul><li>Used to generate an output that is linear and can scale (i.e. 0-100%). </li></ul></ul>
    19. 19. Display Features <ul><li>Controls that change the output from the amplifiers. </li></ul><ul><ul><li>Amplification </li></ul></ul><ul><ul><li>Filters </li></ul></ul><ul><ul><li>Sampling rate </li></ul></ul><ul><ul><li>Monitor settings </li></ul></ul><ul><ul><li>Analog features </li></ul></ul>
    20. 20. Amplification <ul><li>Increases voltage difference between inputs. </li></ul><ul><li>Small potentials </li></ul><ul><ul><li>Voltage to drive pens (not often anymore) </li></ul></ul><ul><ul><li>Voltage for analog to digital converter (ADC) </li></ul></ul><ul><li>Boost voltage of biological signal µV to V </li></ul><ul><li>Characterized by: </li></ul><ul><ul><li>Gain </li></ul></ul><ul><ul><li>Sensitivity </li></ul></ul>
    21. 21. Gain <ul><li>Gain is a ratio: amplifier output voltage to input voltage. </li></ul><ul><ul><li>Output of 10 V for input 10 µV = gain of 1 million </li></ul></ul><ul><li>Gain is the degree of magnification of the amplifier signal. </li></ul><ul><li>We do not directly measured gain. </li></ul>
    22. 22. Sensitivity <ul><li>Relates to Ohm’s Law I=E/R or in PSG D=V/S </li></ul><ul><ul><li>Current (I) = Pen Deflection (D </li></ul></ul><ul><ul><li>Voltage (E) = Input voltage (V) </li></ul></ul><ul><ul><li>Resistance (R) = Sensitivity (S) </li></ul></ul><ul><li>Sensitivity is a ratio: of input voltage to deflection produced. Sensitivity = Voltage/Deflection </li></ul><ul><li>Sensitivity is a measure of the response of an instrument to an incoming signal. </li></ul><ul><ul><li>A sensitive instrument will produce a large response to a small incoming signal. </li></ul></ul><ul><ul><li>A low sensitivity setting produces a large signal change (deflection). </li></ul></ul><ul><ul><ul><li>µV/mm (voltage per unit of pen deflection) </li></ul></ul></ul><ul><ul><ul><li>Exam may express as µV/cm </li></ul></ul></ul>
    23. 23. Video Sensitivity Sensitivity 2 2008_05_04_10_30_38.avi
    24. 24. Computing the Voltage <ul><li>Measure the amplitude of the wave </li></ul><ul><ul><li>Peak to trough </li></ul></ul><ul><li>The vertical distance of the rise or deflection (in mm) is multiplied by the sensitivity of the amplifier. </li></ul><ul><ul><li>Analog sensitivity is on a dial </li></ul></ul><ul><li>Gives us the voltage of the wave. </li></ul>
    25. 25. Sensitivity Settings <ul><li>The higher the sensitivity setting the smaller the wave (deflection). </li></ul><ul><li>V/S=D </li></ul><ul><li>The sensitivity setting affects how the wave is displayed, it does not change the actual voltage. </li></ul>
    26. 26.
    27. 27. Sensitivity Equations <ul><li>S=V/D 5=50 µV/10mm </li></ul><ul><li>V=S x D 50µV=5 x 10mm </li></ul><ul><li>D=V/S 10mm=50µV/5 </li></ul>Settings: Adult 5-7 µV/mm Children 10 µV/mm
    28. 28. Filters <ul><li>Filters give us the ability to focus on only the signal frequencies that we wish to see. </li></ul><ul><ul><li>Bandwidth </li></ul></ul><ul><li>They attenuate unwanted signals. </li></ul><ul><li>Each channel can be optimized, using filters, to allow only signals in the desired frequency range. </li></ul>
    29. 29. Bandwidth
    30. 30. Low Frequency Filter (LFF) <ul><li>The low frequency filter is also known as the “High Pass” filter. </li></ul><ul><li>This means that the LFF allows higher frequencies to pass unchanged while lower frequencies are attenuated. </li></ul>
    31. 31. Low Frequency Filtering on EEG Channels Note: Increasing the filter setting attenuates slow waves.
    32. 32. Low Frequency Filters <ul><li>Examples of typical LFF settings by derivation: </li></ul><ul><li>EEG = 0.3 Hz (C4-C3-O1-O2) </li></ul><ul><li>EOG = 0.3 Hz (LOC-ROC) </li></ul><ul><li>EMG = 10 Hz (Chin, Leg, Intercostals) </li></ul><ul><li>EKG = 0.3 Hz </li></ul><ul><ul><li>The AASM Manual for the Scoring of Sleep and Associated Events </li></ul></ul><ul><ul><li>Rules, Terminology and Technical Specifications, 2007 </li></ul></ul>
    33. 33. High Frequency Filter (HFF) <ul><li>The high frequency filter is also known as the “Low Pass” filter. </li></ul><ul><li>This filter lets slower waves through and attenuates higher frequencies. </li></ul><ul><li>Examples </li></ul><ul><ul><li>HFF eliminates muscle artifact or external electrical artifact in the EEG channels. </li></ul></ul><ul><ul><li>Caution: This filter can also attenuate desired high frequencies such as arousals </li></ul></ul>
    34. 34. High Frequency Filters Note: Increasing the filter setting allows more fast waves to be seen.
    35. 35. High Frequency Filters <ul><li>EEG = 35 Hz </li></ul><ul><li>E0G = 35 Hz </li></ul><ul><li>EMG = 100 Hz </li></ul><ul><li>ECG = 70 Hz </li></ul><ul><ul><li>The AASM Manual for the Scoring of Sleep and Associated Events </li></ul></ul><ul><ul><li>Rules, Terminology and Technical Specifications, 2007 </li></ul></ul>Examples of typical HFF settings by derivation:
    36. 36. <ul><li>The time constant is a measure of how a signal is displayed and is determined by the voltage of the signal and the low and high frequency filters used. </li></ul><ul><ul><li>Time constant = time (seconds) for the signal to change a specified amount. </li></ul></ul><ul><ul><li>Two time constants come into play, the rise time constant and the fall time constant . </li></ul></ul>Time Constants
    37. 37. Time Constants <ul><li>Fall time constant = the time it takes (in seconds) for a square wave to decay to 37% of its maximum amplitude. </li></ul><ul><ul><li>The fall time constant is usually what the term “time constant” refers to in polysomnography. </li></ul></ul><ul><ul><li>The LFF determines the fall time constant. </li></ul></ul><ul><ul><li>↑ the LFF setting will ↓ the time constant. </li></ul></ul><ul><li>Rise time constant = the time it takes (in seconds) for the square wave to reach 63% of its maximum amplitude. </li></ul><ul><ul><li>The HFF determines the rise time constant. </li></ul></ul>
    38. 38. Time Constants Time Constant Fall time constant = time for the signal to decay from its peak to 37%
    39. 39. Video Time Constant Time Constant 3 2008_05_04_14_40_05.avi
    40. 40. LFF - Fall Time Constant <ul><li>LFF (HZ) TC (sec) </li></ul>0.1 0.3 1 5 1 0.4 0.12 0.05
    41. 41. 60 Hertz / Notch Filter <ul><li>60 Hz artifact is a high frequency artifact caused by: </li></ul><ul><ul><li>Poor impedance </li></ul></ul><ul><ul><li>Interference from surrounding electrical equipment </li></ul></ul><ul><ul><li>Poor application of electrodes </li></ul></ul><ul><li>Notch filters attenuate specific frequencies. </li></ul><ul><li>Do not use unless you must! </li></ul>
    42. 42. Sampling Rate Digital Systems <ul><li>The shape of the wave form on the display is determined by how frequently the signal is sampled. </li></ul><ul><ul><li>The higher the sampling rate, the more frequently the signal is sampled. </li></ul></ul><ul><ul><li>The more frequent the sampling, the more accurate the shape. </li></ul></ul><ul><li>The minimum acceptable sampling rate is 2.5 times greater than the highest high frequency filter. </li></ul><ul><li>Nyquist theorem: </li></ul><ul><ul><li>2x the fastest freq. = The Nyquist Rate </li></ul></ul>
    43. 43. sampling rate Low sampling rate Input signal Low sampling rate High sampling rate
    44. 44. Samplling Rates <ul><li>Signal Desirable Minimal </li></ul><ul><li>EEG 500 Hz 200 Hz </li></ul><ul><li>EOG 500 Hz 200 Hz </li></ul><ul><li>EMG 500 Hz 200 Hz </li></ul><ul><li>ECG 500 Hz 200 Hz </li></ul><ul><li>Airflow/Pr 100 Hz 25 Hz </li></ul><ul><li>Oximetry 25 Hz 10 Hz </li></ul><ul><li>Effort 100 Hz 25 Hz </li></ul>
    45. 45. Waveform Display Digital Systems <ul><li>The resolution capabilities: </li></ul><ul><ul><li>Computer -bits </li></ul></ul><ul><ul><li>Monitor – Resolution </li></ul></ul><ul><li>bits – refer to the digital amplitude resolution by the ADC. </li></ul><ul><ul><li>E.g. 8-bit ADC has 2 8 or 256 amplitude levels </li></ul></ul><ul><li>The resolution of the monitor is expressed by the number of pixels, or points of light. </li></ul><ul><ul><li>The higher the number of pixels, the greater the resolution of the monitor. </li></ul></ul><ul><ul><li>The number of pixels is stated for the horizontal and vertical axis of the monitor (must be > :1600 x 1200). </li></ul></ul><ul><ul><li>The AASM Manual for the Scoring of Sleep and Associated Events </li></ul></ul><ul><ul><li>Rules, Terminology and Technical Specifications, 2007 </li></ul></ul>
    46. 46. Dynamic Range <ul><li>Analog & Digital systems </li></ul>
    47. 47. Troubleshooting from the Patient to the Tracing
    48. 48. Questions?
    49. 49. References <ul><ul><li>EEG Primer Basic Principles of Digital and Analog EEG, B Fisch </li></ul></ul><ul><ul><li>Fundamentals of EEG Technology Basic Concepts and Methods, F Tyner, J Knott </li></ul></ul><ul><ul><li>Principles and Practice of Sleep Medicine, M Kryger, T Roth, W Dement </li></ul></ul><ul><ul><li>Sleep Disorders Medicine Basic Science, Technical Considerations, and Clinical Aspects, S Chokroverty </li></ul></ul><ul><ul><li>Sleep Medicine, T Lee-Chiong, M Sateia, M Carskadon </li></ul></ul><ul><ul><li>The AASM Manual for the Scoring of Sleep and Associated Events - Rules, Terminology and Technical Specifications, 2007 </li></ul></ul>