EEG technology

Danielle Morley
Flashcards by Danielle Morley, updated more than 1 year ago
Danielle Morley
Created by Danielle Morley over 7 years ago
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Description

Flashcards on EEG technology, created by Danielle Morley on 12/23/2013.

Resource summary

Question Answer
What is an electrode? Type of transducer that converts energy from flow of ions (brain tissue) to flow of electrons (electrode)
Ideally, what should an electrode do? Sense all ionic (brain activity) changes without participating in the flow of current
What are the properties of an electrode? 1. battery 2. capacitor 3. has a half cell potential 4. acts as a LFF 5. has a time constant
What are properties of polarized(non-reversible) electrodes vs non-polarized(reversible) electrodes? Polarized: large Epot, short TC. Non-polarized: small Epot, long TC
Which is more desirable... a polarized(non-reversible) electrode, or a non-polarized(reversible electrode? Why? Non-polarized because won't participate in flow of current, won't impede slow freqs. Polarized does, leading to signal distortion
What does it mean to say that an electrode has the property of a battery? Has an inherent Epot which provides R
What does it mean to say that an electrode has the property of a capacitor? Electric double layer generates a positive and negative side
What does it mean to say that an electrode has the property of a half cell potential? Has a D/C potential (+ve) that adds to Epot generated at EEI
What does it mean to say that an electrode has the property of a low frequency filter? is like a C and R in series--> impedes slow freqs more than fast
Do we want our electrodes to have a long or short time constant? What is an example of an electrode material that has this type of time constant? Long time constant (won't impede slow frequencies) ex: Ag AgCl, stainless steel
How can electrode properties be minimized? Choosing electrodes that are all made of the same metal, choosing metal based on lower chance of it participating in flow of current, prepping/scrubbing skin
What is impedence? Opposition to A/C flow by all components that oppose I flow ( resistors, transducers, capacitors, etc)
How do we test impedance? Apply weak A/C signal, compare R of each electrode to that of the ref and ground electrode
What is the desired level of electrode impedance for a routine recording? 0.1-5kΩ (100-5000Ω) and equal
Under what circumstances are impedance values of 0.1-10kΩ (100-10,000Ω) and equal acceptable Hostile environments such as ICU and NICU, head trauma, ECS recording, neonatal EEG
Are very low electrode impedances desirable? why or why not? No- can act as a shunt/ short circuit between electrodes
Are very high electrode impedances desirable? why or why not? No- when a high Z and a low Z electrode are connected, it creates an imbalance at the amplifier. This favours electrical interference
What is polarization? Function of the electrode when a voltage is applied to it. This favours I flow in one direction only --> causes distortion of A/C potentials)
How does an ohmmeter work? Why is/isn't this desirable for EEG? Applies small D/C potential to measure resistance across the electrodes. Not desirable because can cause polarization of electrodes which can electrocute patient and distort signals.
What is a similarity between ohmmeters and impedance meters? Both measure the opposition of current flow.
What are some differences between ohmmeters and impedance meters? IMs measure opposition to A/C flow, OMs measure opposition to D/C flow. Therefore IMs use small A/C potential, OMs use small D/C potential. IMs desirable for EEG, OMs not. OMs favour polarization, signal distortion, and electrocution of patient, IMs do not.
What is another name for the electrode electrolyte interface (EEI)? Electric double layer
What is an electrode potential? Potential difference measured when an electrode is placed in a conducting solution/ electrolyte
What is a half cell potential? Every metal (whether element, compound, or alloy) has its own potential when immersed in the same electrolye. This is compared against the standard electrode (hydrogen ion electrode) to find its relative voltage. This is its half cell potential. HCP is D/C potential, usually +'ve
What is more desirable... a small or large half cell potential, and why? Large. Why? small HCP will participate in flow of current and alter EEG recording.
Where does each of the following types of electrodes record from? -surface, subdermal, sphenoidals (invasive and non-invasive), nasopharyngeal, tympanic, indwelling (grids/strips/depth electrodes) surface: cortex from scalp, subdermal: cortex from under scalp, sphenoidals (invasive: inf lat TL from needle thru zygomatic arch @MN and non-invasive: ant TL from surface at MN), nasopharyngeal: ~2cm from ant mesial surface of TL from wire/ball thru nostril contacting nasopharynx, tympanic: 0.5-0.75cm from lat inf TL from wire/ball tip contacting tympanic membrane, indwelling (grids/strips/depth electrodes): from directly in/on brain
A: calib button pressed, max constant V applied to circuit, C completely unchanged, I and V at max (100%) B: C completely charged, I flow and Vr are at min (0%) C: calib button released, C supplies I flow in opposite direction. @ exact time of release, C is completely charged, I and Vr are max (100%) D: C again is completely unchanged, I and Vr are at min (0%)
What is calibration? Using a known Vin to measure ampl of EEG potentials and test amplifier and pen function. Signal used is a square wave pulse
What is damping? Name the 3 types of damping and describe what they mean and how to recognize them. Pressure of the pens on the paper. 1- overdamping: too much pressure -->rounding of the calibration pulse. 2- underdamping : too little pressure --> calibration pulse will have spiky overshoot. 3- critical damping: perfect pressure --> little or no (5-10%) rounding or overshoot of calibration pulse
What are baselines and what should they be? What are the 2 kinds of baselines? differentiate between the 2 kinds. Distances between pens, s/b equal, 18-20mm. 1-Mechanical baselines: space between pens w/o current flowing. 2- Electrical baselines: space between pens when current is flowing.
What components make up the calibration circuit? D/C circuit with a C and R in series and an alternate path for I to follow (controlled by a switch)
What does "pen alignment" refer to? What does incorrect pen alignment do to waveforms? What causes problems with pen alignment? Alignment of the pens on the vertical (time) axis. If incorrect, there will be errors in timing of events. Possible causes are paper being skewed/not aligned, or individual pens not being aligned
What is the standard paperspeed? What happens to brain activity when PPS is increased? decreased? Standard PPS: 30mm/sec. When PPS increased, BRAIN ACTIVITY DOESN'T CHANGE, but its frequency APPEARS to be slower. When PPS is decreased, BRAIN ACTIVITY DOESN'T CHANGE, but its frequency APPEARS to be faster
How would you calibrate/ verify the sensitivity on an analogue machine? Physically measure and verify that all deflections on each channel are equal at the same sensitivities, making sure that deflection (mm) = Vin (uV) / sensitivity (uV/mm)
Define time constant. **check against exam** Time it takes for a calibration signal to reach 33% of baseline amplitude
What is the equation to find impedance? z= √( ((Rˆ2) + ((Xc+Xi)ˆ2) ) where R= resistance, Xc= capacitive reactance (=1/2πfC), Xi= capacitive inductance (=2πfC)
High fidelity/ accuracy of signal reproduction depends on what? Having low, equal electrode Z and comparatively high amplifier/input EEG machine Z
What are each of the following electrode types made of? -surface, subdermal, sphenoidals (invasive and non-invasive), nasopharyngeal, tympanic, indwelling (grids/strips/depth electrodes) surface: Ag, AgCl, Pl, Au, Sn; subdermal platinum alloy or stainless steel needle; sphenoidals (invasive: platinum wire (inserted through canula) and non-invasive: see surface electrodes); nasopharyngeal: insulated Ag wire with Ag ball tip; tympanic: short Ag wire with Ag ball tip; indwelling (grids/strips/depth electrodes): Au or stainless steel (+/- embedded in silicone)
What is a time marker? How do you check this? Marker in top of page in analogue recording that indicates paperspeed. Places a square wave pulse every 1 sec. To check, measure distance between 2 square wave pulses, distance should be equal to paperspeed. (ie on PPS 30mm/sec, then distance between 2 pulses should be 30mm)
What is amplitude linearity and how would you check it? Proportional change in all pen deflections with change in Vin w/o changing sens. To check, use standard settings (7µV/mm, HFF 70Hz, LFF 0.5Hz), alter Vin from 50uV to 20uV to 10uV. *should see gradual decrease in amplitude of the calibration pulses
What will happen to the calibration pulse with changes to the HFF? Higher the HFF, pointier the peak will be. With decreases in HFF, peak of calibration pulse will get more and more rounded
What is step gain control and why do we do it? Process of obtaining pen deflections of 10mm by changing Vin both and sens settings. We do it because it accurately reflects changing cortical signals (Vin) and how we would respond to them with the machine (adjusting sens accordingly)
What is noise? Inherent electrical signals that are present in any sensitive system when there is no Vin
What causes noise? 2 things- 1: random fluctuations in I flowing through Rs and other components, 2: thermal agitation of electrons in resistive components when no I is flowing through them
How do you check noise levels? What should they be? Run for 5-10 sec using 1µV/mm, LFF 0.5Hz, HFF 70Hz, notch OFF. Check amplitude of pen deflections. Noise levels should not exceed 2uV
When does having acceptable noise levels become very important? Why is this? ECS recordings. By definition, ECS is the "lack of EEG activity over 2µV when recorded with interelectrode distances of at least 10cm and interelectrode impedances between 100Ω and 10000Ω". We need noise levels to be smaller than the brain activity that we are looking for.
What is a biological calibration (biocal) doing? Testing integrity of the amplifiers for each channel using an A/C source (ie the patient). It is run using an identical input for each channel (say Fp1-O1) and assessing that each amplifier treats this signal identically
What will the output of a biological calibration look like when machine is functioning correctly? Activity that varies with time that is identical in all channels (like a contaminated reference)
On an analogue machine, full calibration was to be done ____ ____. and partial calibration was to be done _____ _____ ______. Every month. Before every recording.
What effects does current have on living tissue? What factors does this depend on? Effects range from mild tingling sensation to muscle tetany to death. Depends on amount of I and where it flows. (think ohm's law, V=IR- amount of I depends on R and V applied)
Define macroshock and microshock. Macroshock: large amount of I that flows through a person's body. Microshock: small amount of I that flows directly through the heart
How much current is needed to flow through the body to cause the following: pain, muscle tetany, respiratory arrest, ventricular fibrillation (often fatal) Pain: 1mA, muscle tetany: 20mA, respiratory paralysis: 40mA, ventricular fibrillation: 100mA
How much current is needed to cause ventricular fibrillation when flowing through the heart? Which patients would be most at risk? 20µA (yes, MICRO amps!). Those at risk are electrically susceptible patients, including: neonates; those with indwelling catheters, IVs, ICPs, pacemakers, etc; those undergoing invasive procedures such as catheterization, angios, surgery, etc
What are some types of electrical hazards? Leakage current, ground loop, short circuit, broken chassis ground, sagging plugs
Give examples of electrically susceptible patients. Those with indwelling catheters, IVs, ICPs, pacemakers, etc; those undergoing invasive procedures such as catheterization, angios, surgery, etc
What is leakage current and how does this create an electrical hazard? Low values of electrical I that inherently flow from energized electrical parts of the metal instrument to the metal chassis. This normally runs through the ground... but if this pathway is broken and someone touches the machine and a grounded device, I will flow through them to the ground.
What causes leakage current? What is the main source of leakage current? Stray C and R. (Insulators aren't perfect so some I will flow through them). Main source is the power cord (stray C is increased+++ with use of extension cord!)
What should be done to ensure that injury does not occur as a result of leakage current? Regularly check levels of leakage I- levels should be below levels specified by CSA. Keep a record. DO NOT USE EXTENSION CORDS.
What are the acceptable levels of leakage current set by the CSA (Canadian Standards Association)? @ chassis: below 100µV, @ electrodes: below 50µV, @electrodes of an electrically susceptible patient: below 20µV
What is a ground loop and how does this occur? When I flows through patient as a result of patient being hooked up to machines using more than one ground.
How can ground loops be prevented? ONLY GROUND PT ONCE. Equipotential grounding. Equipment designed to limit amount of I flowing through it (ie EEG jackbox/headbox)
What danger is associated with a sagging plug? If plug is sagging, then the prongs are making poor contact, causing the circuit to weaken. If metal touches the metal of the sagging plug, the line current could be delivered directly to the patient's head (!)
A: live/hot wire B: neutral/cold wire C: chassis ground
What is the chassis ground? 3rd prong in a plug- safety device meant to provide a path of low R path to earth for I (in the form of leakage I, line I)
What causes a short-circuit to occur? Hot input wire makes contact with metal chassis of machine. This establishes a new "short" circuit and I flows back to chassis and chassis ground to earth.
What is the potential danger of a short circuit? When hot input wire makes contact with metal chassis of machine, this establishes a new "short" circuit and I flows back to chassis and chassis ground to earth. If chassis ground is broken and you (or your patient) touch the machine, current will flow through YOU to earth.
What is the potential danger of a broken chassis ground? This is a safety device that is meant to act as the path of least R for stray I to follow to earth... If this is broken then WE become the path of least resistance to earth
What safety measures are in place to prevent injury if a short circuit occurs? (usually a short circuit will cause large amounts of I to flow). Fuses/circuit breakers are in place and will blow with large amounts of I, thereby stopping flow of I
How can we prevent the dangers associated with a broken chassis ground? Have chassis ground check and logged regularly. NEVER use cheater plugs (3 to 2 prong plug adapter) as this acts as broken chassis ground. NEVER use extension cords. Keep machine out of pt's reach when possible.
When is the use of extension cords acceptable? Give reasons to support this. NEVER. Presents electrical safety issues because they... increase stray C thereby increasing leakage current, create a weak point in the circuit thereby giving a greater opportunity for a person to touch this and become the path of least resistance for I to ground
What is an electron? How does this differ from a proton or neutron? Small, negatively charged particle with almost no mass that orbits around the atom's nucleus. Protons and neutrons are larger particles that make up an atom's nucleus, that have a positive charge and no charge respectively
What is a Coulomb? Unit of charge (Q or q). SI defnition: "the charge transported by a constant current of one ampere in one second."
What is a conductor? How does this differ from an insulator? Conductor: material that promotes I flow in a circuit by easily giving up its valence electrons. An insulator does not easily give up its valence electrons and therefore opposes I flow in a circuit.
What is conductance? What is another way of stating this? The ease of electron flow through a conductor. Inverse of resistance.
What is current? Also give symbol and units. Movement of electrons from area of excess electrons (more electronegative) to an area deficient in electrons (more electropositive). Symbol: I. Unit: amperes, A=C/sec
Give name and description of the 2 types of current. D/C: direct current, I flow in one direction when a constant voltage is applied. A/C: alternating current, I flow that changes its direction very frequently at regular intervals, as determined by the frequency of the voltage applied
What is voltage? Also give symbol and units. Electrical force that causes electrons to flow as current in a circuit by providing a potential difference across a circuit. Symbol: V (or E). Unit: volts, V
What is resistance? Also give symbol and units. Opposition to D/C flow, affected by the following factors: size, length, material, and temperature of the sonductor. Symbol: R. Units: ohm, Ω
What is ohm's law? What does it state? What is the equation for this? Relationship between voltage, current, and resistance in an electrical circuit. It states that electric current is directly proportional to voltage and inversely proportional to the resistance. Equation is V=IR (can be restated as I=V/R or R=V/I)
What is a capacitor and what does it do? What basic components are needed to make a capacitor? Device used to store electrical charge. Also provides opposition to (A/C) current flow through capacitive reactance Xc. Made up of 2 conductive plates separated by an insulator (called the dielectric).
What is a resistor? Device used to introduce a known value of resistance into a circuit.
What is capacitive reactance? Also give symbol, units, and formula. Opposition to change in current (A/C flow) by a capacitor. Frequency dependant (ie less capacitive reactance for fast frequencies compared to slow). Symbol: Xc. Units: ohm, Ω. Formula: Xc=1/(2πfC)
What is capacitance? Also give symbol and units. Amount of charge a capacitor can hold on its plates when a voltage is applied to the circuit. Symbol: C. Units: farads, F
What factors determine the capacitance of a capacitor? thickness, permeability and area of the dielectric (insulator)
What is an inductor? What is an additional property of an inductor? Coiled wire that induces counter voltage in opposite direction to current flow in a circuit. Provides impedance/ opposition to A/C flow through inductive reactance.
What is inductive reactance? Also give symbol, units, and formula. Impedance to A/C flow by an inductor. Symbol: Xi. Units: ohm, Ω. Formula: Xi=2πfL
What is inductance? When is it produced? How can you increase inductance? Also give symbol and units. Property of an electrical circuit where a change in the electrical current through that circuit induces an electromotive force (emf) that oppose the change in I. If produced every time A/C changes direction, (--> more Xi with higher freqs of A/C). Can be increased by increasing #of coils or adding an iron (Fe) core. Symbol: L. Units: henry, H.
What is a circuit? By definition, what must it contain? Path for current to flow. Must have a complete closed loop with a voltage and a resistance.
What are the names of the 2 types of circuit layouts? What are the distinguishing characteristics? Series and parallel. Circuits in series have one path for current to flow, so current (I) is constant. Circuits in parallel have multiple paths for current to flow, so voltage (V) is constant.
Draw a circuit in series. Then give the equation that would determine Vt, Rt, and Ct in a circuit in series.
Draw a circuit in parallel. Then give the equation that would determine Vt, Rt, and Ct in a circuit in parallel.
What is a filter? Electronic device comprised of capacitors and resistors that discriminate against a particular band of frequencies.
Filters act by... Attenuating input signals by gradually decreasing amplitude. Shifting input signal peak in time (phase shift).
How does a resistor's filtering effect differ from a capacitor's? Resistors impede/filter all frequencies equally; capacitors have the filtering effect that impedes slow frequencies more than fast.
What is a frequency response curve? Graphical representation of the % of amplitude output of a band of frequencies based on a set of LFF and HFF settings.
What is bandwidth? Area under the curve of frequencies between the LFF and HFF settings that are relatively unfiltered.
What is turnover frequency? At what frequencies does this occur? Frequency at which 30% of the original signal is attenuated (GIVEN a roll-off of -3dB). Occurs at the frequencies that are being used as the LFF and HFF settings.
What is roll-off? What electric component produces this effect? What are the units? How much does a given roll-off attenuate the signal? Rate at which signal attenuation occurs. Is what makes filtering a gradual process. Function/process of the capacitor. Units: decibels, dB. % attenuated is 10x the roll-off (ie -3dB gives 30% signal attenuation at turnover frequencies, -2dB give 20% signal attenuation at turnover frequencies).
What is a low frequency filter and what does it do? What is another name for a LFF? Electronic device that attenuates slow frequencies and allows fast frequencies to pass and be seen at 100% amplitude output. Also called high pass filter.
What is the electrical construction of a LFF? Draw a simple diagram of this.
What is phase shift? Which electrical component does the property come from? Shift in original peak in time. Property of the capacitor.
What type of phase shift will be prodcued by a LFF? Is this enough to be noticeable on the EEG? At turnover frequency (LFF setting), peak is shifted 45 *degrees* or1/8th of its original duration to the LEFT (=EARLIER) in time. This IS enough to be noticeable on the EEG. (if LFF is at 1Hz, phase shift is 1/8th of 1 second= 125msec)
What is the formula to determine phase shift? mm of shift= input duration (s) x 45 degrees x PPS (mm/s)
What is a high frequency filter and what does it do? What is another name for a HFF? Electronic device that attenuates fast frequencies and allows slow frequencies to pass and be seen at 100% amplitude output. Also called low pass filter.
What is the electrical construction of a HFF? Draw a simple diagram of this.
What type of phase shift will be produced by a HFF? Is this enough to be noticeable on the EEG? At turnover frequency (HFF setting), peak is shifted 45 *degrees* or1/8th of its original duration to the RIGHT (=LATER) in time. This IS enough to be noticeable on the EEG.
What is a notch filter and what does it do? Which frequencies does this affect? What is another name for the notch filter? Electronic device that uses a combination of LFF and HFF to selectively attenuate very specific frequencies. Notch filters are used in EEG to reduce 60Hz A/C electrical noise by sharply eliminating 58-62Hz activity. Also called band-stop filter. (* eliminates 48-52Hz activity in places like Europe that use 50Hz A/C power)
When is/isn't use of the notch filter appropriate? Explain why. Notch filters are appropriate for harsh environments such as ICU where 60Hz often can not be eliminated through other means due to many wall powered machines in a small space. Notch filters should not be used routinely, as 60Hz artifact is a sign of poor electrode contact.
Give the equation used to determine LFF value from a given time constant. LFF= 1/2πTC
Give an equation that shows how LFF can be determined from resistance and capacitance values in the circuit. Then give an equation for time constant based on R and C values. LFF=1/2πRC and TC=RxC
Give the equation used to determine HFF value from a given time constant. HFF=1/2πTC
Give an equation that shows how HFF can be determined from resistance and capacitance values in the circuit. Then give an equation for time constant based on R and C values. HFF=1/2πRC and TC=RxC
How is digital filtering done? What are the names of the 3 main techniques used? Digital filtering is done using a computer program or algorhythm. Main techniques used are *FIR: finite impulse response (*most common), IIR: infinite impulse resposne, FFT fast fourier transform
What is an amplifier and what are 2 types? "Device that responds to a very small input voltage and delivers a larger output voltage that contains the essential waveform features of the original input." 2 types: power amplifier and differential amplifier.
What is a power amplifier? What is the gain of a power amplifier? Combination of electrical components designed to increase the size of the output voltage to a level that is enough to drive either the pens or a digital converter. Gain = Vi:Vo
What is the output voltage of a differential amplifier? Potential difference of 2 inputs
What is a differential amplifier? Why is this desired for EEG? Electronic device that attenuates/ discriminates (rejects) against in-phase (common/same) signals and amplifies out-of-phase (different) signals. This is desired mostly for the fact that it tends to cancel electrical and environmental artifacts.
What is CMRR? Common mode rejection ratio. How well differential amplifiers can cancel common signals.
What factors affect CMRR? How can we optimize the CMRR? Electrode impedance and amplifier affect CMRR. Can optimize by having low, equal electrode impedance and relatively high amplifier impedance.
According to guidelines, what is the minimum CMRR for the EEG machine? 10 000 : 1
How can we measure CMRR? Apply in-phase signal to both outputs. Input enough to produce a 10mm deflection. Apply out-of-phase signal and see how much Vin is needed to get the same 10mm deflection. ie: if it takes 500mV for in-phase and 50µV for out-of-phase, then 500mV(1000µv/1mV):50µV, =500 000µV:50µV =10 000:1
What is a transistor? What type of devices use transistors? FYI: name is derived from transfer resistor. Device composed of semiconductor material that amplifies a signal or opens or closes a circuit. It changes its resistance dependent on the current running through it (ie resistance is proportional to current). Transistors have become the key component of all digital circuits, including computers. Today's microprocessors contain tens of millions of microscopic transistors.
What is a semi-conductor? Give an example of a semi-conductor and an example of where semi-conductors are commonly used. Solid substance that has a conductivity between that of an insulator and that of most metals, either due to the addition of an impurity or because of temperature effects. Devices made of semiconductors, notably silicon, are essential components of most electronic circuits.
What are properties of a good conductor vs a good insulator? What are some materials that make good conductors? insulators? Conductor: easily gives up valence electrons and allows electrons from other molecules to move through them (current), low resistivity, high conductivity, usual metals or eletrolyte. Ex: copper, silver, aluminum, gold, electrolytes such as sweat. Insulator: holds on tightly to its valence electrons so as to not allow electrons from other molecules to pass through it (current), high resistivity, low conductivity. Ex: air, glass, plastic.
An in-phase signal of 4V is applied to both inputs. Using an out-of-phase signal, it takes 600µV to get an equal deflection. What is the CMRR? Show calculations. Is this adequate for EEG? Why or why not? 4V(1 000 000µV/1V):600µV..... =4 000 000µV:600µV.... =6666.7:1.... =not adequate for EEG because guidelines state that minimum CMRR is 10 000:1
An in-phase signal of 500mV is applied to both inputs. Using an out-of-phase signal, it takes 45µV to get an equal deflection. What is the CMRR? Show calculations. Is this adequate for EEG? Why or why not? 500mV(1000µV/1mV):45µV..... =500 000µV:45µV..... CMRR=11 111:1.... =adequate for EEG because guidelines state that minimum CMRR is 10 000:1
What is the ground electrode? Where is it typically placed? What is the purpose of the ground electrode? qq
What is an analogue EEG machine and how does it work? Pen and paper writing system consisting of galvanometers and amplifiers to record brain activity. Uses circuits made up of capacitors and resistors for controls.
What is a digital EEG machine and how does it work? Analogue digital converter with a PC to record activity.
What advantages does the digital have over analogue machines? Do analogue machines have any advantages over digital machines? Advantages of digital machines: offers flexibility of being able to change settings post-recording, machine portability, easier + faster storage and retrieval of records, ability to easily add channels. Advantages of analogue machines: give the truest representation of analogue waveforms without alterations that result from ADC
What is ADC? What does it involve? Analogue to digital conversion. Involves the conversion of continuous ongoing (A/C) brain activity (analogue) to a format that digital machines/computers can understand. (ie continuous analogue signal --> binary code- series of 1s and 0s)
Seeing as computers cannot digitize a continuous waveform, how is analogue to digital conversion done? Computers sample the voltage of a wave at small and equal tie intervals then connect the dots to make a continuous LOOKING signal.
What are the 2 key factors to ADC? What does each factor represent? 1: horizontal resolution- time axis= time, frequency (msec, Hz). 2: vertical resolution- amplitude/ voltage (µV).
What is horizontal resolution? Accuracy of analogue waveforms of differing frequencies when reproduced digitally.
What is the main factor that determines the horizontal resolution? Sampling rate.
Define sampling rate. #times/sec that an analogue signal is amplified for its voltage
What is dwell time? What is another name for this? Actual time between each sampling point (msec). AKA inter-sample-interval.
What happens to information recorded during the dwell time? Won't be recognized (ie information is lost).
Give formula that demonstrates relationship between sampling rate and dwell time. Include units. sampling rate (Hz) = 1sec/dwell time (msec)
What is aliasing? What causes this? Distortion of an analogue signal when it's reproduced digitally. Caused by having an inadequate sampling rate.
What aspects of a waveform can be affected by aliasing? Any- frequency, amplitude, morphology...
Which frequencies are most likely to be affected by aliasing? What kinds of changes can result? Faster frequencies- if bad, frequency and morphology can be completely altered and amplitude changes can also result.
What is the Nyquist frequnency? Minimum sampling rate needed to adequately reproduced the original waveform. Equal to 2x the fastest frequency recorded.
Is the Nyquist frequency adequate for EEG? Why or why not? NOT sufficient for EEG. Will give basic frequency information but will still have ++distortion of waveform morphology and amplitude at higher frequencies. Also, guidelines state that SR must be at least 3x the fastest recorded frequency (Nyquist is only 2x)
What is vertical resolution? Accuracy of the voltage of analogue signals when reproduced digitally.
What are the main factors that determine the quality of the vertical resolution? #BITS and the size of the dynamic range
What kind of process does the digital machine use to provide the vertical resolution? What does this mean? To provide vertical resolution, the vertical axis is divided into voltage increments through an all-or-nothing process. This means that values that fall between the BITs will be rounded down to the next lowest bit, which can cause distortion.
What are BITs? What does the number of BITs represent? Binary digits. Number of BITs represents the system's total number of voltage increments available.
Number of BITs is expressed as... 2^x where x is the number of BITs
According to guidelines, what is the minimum number of BITs? How many BITS does a good system have? Guideline minimum: 11 BITs. Good system has 12 BITs.
What is dynamic range? Preset voltage range over which the total number of available BITs are spread out.
What determines the size of the dynamic range? The maximum and minimum voltages to be recorded.
Vertical resolution is expressed as... BIT precsion
What is BIT precision? What does this represent? % of accuracy of the vertical resolution. This represents the % of the dynamic range that will fall between BITs. (=% of signal lost)
Give the equation for BIT precision. BIT precision=(1/2^x) x100
How do we determine the precision of machine? precision of machine= 100- %lost.... %lost= BIT precision= (1/2^x)x100
Using a machine with 11 bits, what is the precision of the actual machine? Show all calculations. BIT precision= (1/2^11)x100... BIT precision=0.048... machine precision= 100-%lost... machine precision= 100-0.048%... =99.952%... machine=99.5% precise
What is saturation? What causes this to occur? Portion of a waveform is not reproduced digitally, caused by a voltage of analogue signal that exceeds the dynamic range.
How do you know if saturation of the amplifiers has occurred? You will see flat channels on the EEG.
What is screen resolution and why is this important? Computer screen display made up of pixels (=tiny dots)... Inadequate screen/display resolution results in distortion.
According to guidelines, what is the minimum screen resolution? 2 pixels / vertical mm or 100 pixels / second
What is sample skewing? Distortion of the apparent timing of waveforms due to how the computer times its sampling points for each of the inputs.
What causes sample skewing to occur? Some machines do not sample all inputs for voltage at the same time... Usually will sample 8 inputs at once causing groups of 8 to be a few msec apart.
What modifications can cause sample skewing to become more apparent? Having many channels- think 128+... or when spreading out PPS to look at changes over msec.
The purpose of filters is to selectively reduce/eliminate certain frequencies by _________ ____ __________. reducing their amplitude
What is amplifier sensitivity/gain? What are the units? Refers to the dynamic range and the original amplification of the machine. Has NO units.
Amplifier sensitivity/gain is the ratio of _______ _______ to _______ _______. output voltage to input voltage
Give the equations/ratio for amplifier sensitivity/gain. Vout/Vin or Vout:Vin
What is display sensitivity/gain? What are the units? How collected data is viewed, changes to display sensitivity/gain will change the way voltage output looks on your screen. Units are µV/mm.
Display sensitivity/gain is the ratio of _______ _______ to _______ _______. Input voltage to output deflection
Give the equations/ratio for display sensitivity/gain. Vin/µVout or Vin:µVout or mm output x sensitivity(µV/mm)= µVin
Since computer screens vary in size, using standard paper speed of 30mm/sec, a second will not always be equal to 30mm. Give the formulas to find: a) frequency b) duration. Include units. a) Frequency (Hz)= 1sec measurement (mm)/ wave duration (mm) ..... b) Duration (msec)= 1sec measurement (mm)/ frequency (Hz)
What key component is needed for montage reformatting to be possible? system reference eletrode
What is the purpose of the system reference electrode? montage reformatting
What is a good indicator that there is a problem with the system reference electrode? artifact caused will be seen on all channels at the same time and will look the same in all channels
How is montage reformatting done with digital systems? Explain and give equations if helpful. Data from each electrode is stored separately so we can adjust settings later. Since we can't store information from a single electrode (because differential amplifiers require 2 inputs), all electrodes/inputs are referred to the system ref. Now when comparing 2 scalp electrodes, system ref will be common and therefore cancelled out.... Input1=F4-SR Input2=C4-SR ..... Channel 1= (F4-SR)-(C4-SR) where system ref cancels out
What is your amplifier sensitivity if your input voltage is 0.05µV and your output voltage is 25µV? Show work. amplifier sensitivity=output voltage:input voltage ...... =25µV:0.05µV ..... =500:1
If your display sensitivity is at 15µV/mm and a waveform is 450µV, how big (in mm) should the waveform appear on the screen? size(mm) x sensitivity(µV/mm)= original waveform voltage(µV/mm) ...... size(mm)=original waveform voltage(µV)/sens(µV/mm) ...... =450µV/ 15µV/mm ...... size of waveform on screen=30mm
What is the duration of (of each cycle) of a 15Hz rhythm (in msec)? Show calculations. Duration=1sec/frequency(Hz) ..... =1sec/15Hz ...... =0.066666sec x1000msec/sec ..... duration=66.67msec
If your display sensitivity is 20µV/mm and you have a 1cm high waveform, what is the voltage of the waveform (in µV)? Show calculations. height(mm) x sensitivity(µV/mm)= voltage(µV) ...... V(µV)= 10mm x 20µV ...... V=200µV
What is the frequency of a rhythm where the duration of each cycle is 140msec? Show calculations. 1000msec/140msec=7.14 .... freq=7.1Hz alternatively can be done another way, D(sec)=1/freq(Hz) ..... freq(Hz)= 1/D(sec).... =1/0.140sec... =7.14Hz
You have an 8bit system and your voltage increments are every 3.9µV. What is your dynamic range? Show calculations. DR= Vincr x bits.... bits=2^x=2^8=256... DR=3.9µVx256 DR=998.8µV
According to the Nyquist Theorem, what is the fastest waveform that you can accurately display if your sampling rate is 100Hz? Nyquist=2x the frequency... freq=100/2=50Hz
If you want to digitize a 15Hz waveform, what is the... a)minimum sampling rate needed to reproduce the waveform b)guideline minimum sampling rate c)ideal sampling rate a)minimum sampling rate to reproduce= Nyquist= 2x the waveform freq= 30Hz ..... b)guideline minimum sampling rate= 3x the HFF setting= 3x70Hz= 210Hz ..... c)ideal sampling rate 10-25x the frequency so 10x15Hz=150Hz to 25x15Hz=375Hz
Draw the frequency response curve for the following settings: LFF=5Hz, HFF=50Hz, notch filter off, assume roll-off of -3dB. What is the bandwidth? What will be the approx. %amplitude output of the following waveforms? a)100Hz b)60Hz c)20Hz d)5Hz e)1Hz f)0.1Hz
Draw the frequency response curve for the following settings: LFF=2Hz, HFF=70Hz, notch filter on, assume roll-off of -3dB. What is the bandwidth? Approximately what percentage of the amplitude of the following waveforms will be filtered out? a)100Hz b)60Hz c)20Hz d)5Hz e)1Hz f)0.1Hz
Draw the frequency response curve for the following settings: LFF=0.5Hz, HFF=70Hz, notch filter off, assume roll-off of -3dB. What is the bandwidth? What will be the approx. %amplitude output of the following waveforms? a)100Hz b)60Hz c)20Hz d)5Hz e)1Hz f)0.1Hz
Draw the frequency response curve for the following settings: LFF=0.5Hz, HFF=50Hz, notch filter on, assume roll-off of -2dB. What is the bandwidth? What will be the approx. %amplitude output of the following waveforms? a)100Hz b)60Hz c)20Hz d)5Hz e)1Hz f)0.1Hz
a)Roll-off= -2dB b)LFF setting= 3Hz c)HFF setting= 95Hz d)Notch= off e)Bandwidth= 3-95Hz
Resistors, capacitors and inductors all have a resistive function. Resistors oppose 1)_______ current in the form of 2)________ and the equation for this is 3)________. Capacitors oppose 4)_______ current in the form of 5)________ and the equation for this is 6)________. Inductors oppose 7)_______ current in the form of 8)________ and the equation for this is 9)________. 1)D/C 2)resistance 3)R=V/I 4)A/C 5)Capacitive reactance 6)Xc = 2πfC 7)A/C 8)Inductive reactance 9)Xl= 2πfL
A 60 uV wave at a sensitivity of 6 uV/mm will produce a deflection of (have an amplitude of)? A=V/S (where A=amplitude or deflection; V=voltage; S=sensitivity) A=60uV / 6uV/mm A=10mm
A 10mm deflection at a sensitivity setting of 20uV/mm will have a voltage of? V=AxS (where V=voltage; A=amplitude or deflection; S=sensitivity) V=10mm x 20uV/mm V=200uV
A spike measuring 15 mm peak-to-peak, is recorded at a sensitivity of 10uV/mm. What is the voltage of the spike? V = A x S (where V=voltage; A=amplitude or deflection; S=sensitivity) V = 15 mm x 10 uV/mm V = 150 uV
How much deflection will occur if a 100 uV spike is recorded at a sensitivity of 10 uV/mm? A = V/S (where A=amplitude or deflection; V=voltage; S=sensitivity) A = 100 uV / 10 uV/mm A = 10 mm
What is rise time? The amount of time it takes, in seconds, for a square wave to achieve 63% of its full amplitude.
Rise time is the opposite of...? time constant
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