Instrumentation Flashcards
How many classes of medical devices are there?
4
Class I
Class IIa
Class IIb
Class III
What is the most common class for a medical device?
Class I
Which type of med device is used to maintain control of vital organs?
Class III
Which classes of medical device are commonly installed within the human body?
Class IIa
Class IIb
Class III
Give an example of a Class I device
Thermometer, ultrasound gel…
Give an example of a Class IIa device
Diagnostic software, ultrasound equipment, blood pressure,
Give an example of a Class IIb device
Equipment used for intraoperative monitoring, blood bags, gauze, contact lenses, hearing aids
Give an example of a Class III device
Defibrillator, Pacemake, Vagal Nerve Stimulator
What determines the class of device?
Invasive vs non-invasive
Length of time spent in-vivo: I - not implanted, IIa - 60mins - 30 days, IIb - 30+ days, III - longterm
What is an active medical device?
An active medical device is any medical device that relies on an external source of energy to function, rather than being powered solely by the body or gravity. These devices require electrical or other forms of energy (e.g., mechanical, thermal, or chemical) to operate, excluding energy generated directly by the human body. e.g. pacemaker, infusion pumps, usually IIa
What is a passive medical device?
Passive medical devices (like bandages or stents) do not require an external power source to perform their intended medical function.
4 Rules of Medical Device Classification
Rule 1: Do they touch the patient?
Rule 2 + 3: Do they Store, Channel or modify the biological or chemical composition?
Rule 4: In contact with broken skin?
Bonus: Invasive? Active Device?
If a software device provides info to make decisions, what class of medical device is it?
Class IIa
If a device is responsible for controlling or monitoring an implantable device, what class of medical device is it?
Class III
Probability vs Consequence
(name the 4 levels)
Severity: mild, major, critical, fatal
Is standardisation or specific safety tests designed by regulatory bodies? (pick one)
Standardisation
Standards exists so manufacturers can: (2)
Identify potential hazards
Estimate and evaluate associated risks
Risk classifications (3)
Insignificant/negligible
Investigate risk reduction
Unacceptable
What is resolution?
Smallest differential value that can be measured, useful for images & signals, the devices ability to detect change for a given input
Risk Process (6)
Analysis > Evaluation > Risk Control > Residual Risk > Risk Management Report <> Post-Production
What needs to be considered in the evaluation of a new device? (4)
Accuracy, precision, reliability, reproducibility
Does sensitivity determine the ability to identify True Positive (TP) or True Negative?
True Positive
Does specificity determine the ability to identify True Positive (TP) or True Negative (TN)?
True Negative (TN)
What is used to screen for a disease? (Sensitivity or specificity)
A test with the highest sensitivity
What is used to confirm a disease?
The test with the highest specificity
What are the physiological effects of current flowing through body to the ground? (4)
- Muscle cramps
- Respiratory arrest
- Ventricular fibrillation
- physical burns, tissue damage
What is the severity of the injury dependent on for current flowing through body to the ground? (6)
- Amount of current
- Duration of shock
- Pathway of current (does it pass through the heart?)
- DC or AC
- Spatial distribution of current throughout body
- Skin resistance
What can reduce resistance of skin? (3)
- IV catheters (fluid lower resistsnce to skin)
- if skin is wet
- if there is a wound that has broken the skin (resistance goes down to 100-1000 ohms)
(Normal skin resistance is high - 15,000 t0 1,000,000 ohms at 50/60 Hz)
Macroshock vs Microshock
Macroshock: Large current (>100mA), small fraction of current flows through heart, risk of VF small, no direct conductive path to heart
Microshock: Direct conductive path to heart, path is insulated (except at heart), VF at 80-600 microA
Decontamination (x3)
- Cleaning
- Disinfecting
- Sterilisation
What is cleaning (in reference to the decontamination process)?
Physical removal of foreign material (dirt/blood) - water + detergents + mechanical action
What is disinfection (in reference to the decontamination process)?
Inactivates non-sporing infectious micro-organisms (thermal or chemical means)
Achieved by using chemical disinfectants (alcohol/hydrogen peroxide)
What is sterilisation (in reference to the decontamination process)?
Removing or killing all viable organisms (e.g. use of steam)
What is electromagnetism?
Branch of physics that deals with electromagnetic forced between electrically charged particles
Why is electromagnetism important in physiology?
Every physiological process is produced and/or measured by an electromagnetic phenomena
What are Maxwell’s equations? (4)
- Gauss’ Law of electricity
- Gauss’ law of magnetism
- Faraday’s Law
- Ampère’s Law
What is a field?
Spatial distribution of a vector or scalar quantity
E.g. temperature map - scalar field
Ocean currents map - vector field
What is the fundamental unit of electric charge?
1 Coulomb (= 6.28 x 10^28 electrons)
In positive charges, do arrows point towards or away?
Away (source)
In negative charges, do arrows point towards or away
Towards (sink)
Do fields get stronger or weaker with increasing distance from charge?
Weaker
With 2 opposing charges, when the field lines exiting one charge = field lines entering the other charge, what is the net charge?
0
Is the gravitational potential energy greater on the earths surface or 100km above the Earth’s surce?
100km above the earth’s surface
What is Potential Difference / Voltage?
The potential energy required to move between electric field potentials
Why is voltage hard to calculate? (3)
- Requires knowledge of all charge sizes and location (in a given situation)
- Calculate potential due to each charge individually (at a given location)
- Add resulting potential from each charge to get total
What is ground?
Some object/point that acts as a reference electric potential
Measure all voltage relative to this reference
(Actual potential does not matter, earth ground = 0V, GND = less noise)
What is the ground point on a toaster?
GND pin on plug
What us the ground point on a laptop?
The chassis
What would happen if an item that requires electricity to run was not grounded?
Voltage too high. Damage.
What is a battery?
A continuous source of electric charge.
Produces a chemical reaction that separates charge.
This creates an electric field, E.
What determines the voltage of a battery?
The chemical composition
What is a conductor?
A material that allows electricity to flow through it
Give examples of a good electrical conductor and state why.
- Gold, platinum, silver, copper
- Not as many collisions
- A good conductor will not use charge before it reaches the endpoint
Which terminal on a battery is GND?
Negative terminal
What is the voltage on the positive terminal of a 1.5V battery?
1.5V
Does the voltage on a conductor change over time?
No. Voltage on a conductor is constant.
What does it mean for a battery to stop working?
The chemical reactions have been used up.
What is resistance?
Electrical friction
Resistance dissipates energy
What is conventional current?
Flow of + charge
How to achieve 6V from 4 equal batteries?
Place 4x1.5V batteries in series
Kirchoff’s Loop Rule
In a loop condition, the number of voltage rises is equal to the number of voltage drops
Kirchoff’s Current Rule
In a loop condition, total current entering a junction is equal to the amount exiting
Current prefers to flow through the path of least resistance
What is a capacitor?
An electrical device that has the ability to store charge.
A capacitor will store electric charge when a voltage is applied to the terminals.
What is a medical device? (7)
An instrument/apparatus intended by manufacturer to be used alone/in combination for human beings for the purpose of:
- diagnosis
- prevention
- monitoring
- prediction
- prognosis
- treatment
- alleviation of disease
What is a non-invasive device? Give examples
- On skin/indirect contact
- ECG, EEG, Ultrasound, pulmonary function test
What is an invasive device? Give examples?
- Within the body
- Pacemake, loop recorder, deep brain electrodes, intraoperative monitoring
List a relevant physiological measurement parameter for cardio, neuro, vascular, respiratory
- Blood pressure (mmHg): vascular, cardio
- Action potential (V): neuro
- Velocity (m/sec): vascular, neuro
- Area (m^2): vascular, cardio
- Volume (L): respiratory
- Frequency: neuro
What is frequency? (Hz)
The number of oscillations in a signal
What is artefact in a recording signal?
Noise. Unwanted frequencies within the bio electric signals
An unintended/unwanted distortion, noise, or error in the signal that does not originate from the actual source being measured.
Artefacts can interfere with the quality of a recording leading to misleading or false data.
What causes artefact in a medical recording?
- Patient movement
- Muscle contractions
- Poor electrode contact
- External electrical interference
Electrodes
Transform/transduce ionic currents into electric voltage
An electrode is a conductor used to make contact with a non-metallic conductor (e.g. people)
Transducer
Anything that converts energy into another form of
What is the purpose of a piezoelectric crystal in a transducer?
Compression & converts energy from sound to electric
What is the purpose of a spirometer in a transducer?
Uses a turbine to generate a signal
What is the purpose of electrolyte gel in electrodes?
Electrolyte works as a medium to carry over the ionic current within the body towards the electrode
What is the difference between a low pass and a high pass filter?
Low pass filter uses a resistor next to the input voltage.
High pass filter uses a capacitor next to the input voltage.
What is another purpose of an amplifier?
Filter signal
What is a common use of an amplifier in medical devices?
As an op-amp
Sampling Rate
Number of samples retrieved per second (fs)
What is reproducibility?
An indication of the difference between successive measurement under identical measuring conditions
What is specificity?
1 - false negative (specificity of 1 = no false negatives)
Law of Energy Conversion
Energy In = Energy Out
What is the energy conversion in spirometry?
Mechanical → Electrical
What is the energy conversion in pulse oximetry?
Light → Electrical
What is the energy conversion in ultrasound?
Electrical → Mechanical → Electrical
What is electricity? (in terms of electrons)
The flow of free electrons
Ohm’s Law
V = IR
Conductors
Allow the flow of electrons
Insulators
Restrict the flow of electrons
What determines the quality of conductancy?
Valency of the atom (# electrons in outer shell)
What determines the quality of an insulator?
Resistivity (material vs flow of electrons within it)
What determines how we perceive light? (2)
Wavelength and frequency of the light wave
At what speed do photons travel?
Speed of light
2.9 x 10^6 m/s
How is light energy produced?
If ligh interacts with an electron, it elevates to a higher orbit and for a brief periods.
When it returns to its original state, it will emit a photon with energy (E)
Heat
Energy transferred from one body to another as a result of the temperature difference
How can heat energy travel? (3)
Conduction, convection, radiation
Mechanical energy
Sum of potential and kinetic energy
Why are most medical devices battery powered?
In case of a power cut
What is the difference between AC and DC?
AC: constantly changes direction, power factor varies (0-1), f = 50 Hz
DC: one direction, power factor constant (1), no f
Is AC or DC safer to use?
DC - > AC has a higher risk of elec. shock
What device converts DC to AC?
Invertor
What device converts AC to DC?
Rectifier
What chemical reaction takes place in an anode?
Oxidation (loss of electrons, anode = positive)
What chemical reaction takes place in an cathode?
Reduction (gain of electrons -redox reaction, cathode = negative)
What is the use of a voltaic cell / galvanic reaction? What is different between the anode and cathode in this reaction?
An electrochemical cell that converts the chemical energy of spontaneous redox reactions into electrical energy.
anode = negative, cathode = positive
Resistor
Device which regulates the resistance of a current.
Capacitor
Device that can store energy in an electric field.
Inductor
Device that stores energy in a magnetic field
I.e. capacitor in electric field = inductor in magnetic field
Diode
Device that allows a current to flow in one direction only.
What is the purpose of earthing a system?
Provides a safe path for electricity in the event of a short circuit
Acts as a reference point
What is cathodal stimulation?
anions (-) are discharged into the body ascurrent flows from the cathode (-),throughthe tissue, and back to the anode (+).
What is anodal stimulation?
anodal stimulation, cations (+) are discharged into the body as current flows from the anode (+), through the tissue, and back to the cathode (-).
Why would cathodal stimulation be used over anodal stimulation?
cathodal stimulation excites those with a direction component parallel to its surface whereas elements perpendicular to the electrode surface are preferentially excited by anodal stimulation
What happens to the cell during anodal or cathodal stimulation?
Cell becomes polarized (one side is more positive and the other side is more negative.)
Impedance
Impedance is the opposition to alternating current presented by the combined effect of both resistance and reactance.
What is the purpose of a voltage divider?
A voltage divider is a simple circuit which turns a large voltage into a smaller one. Using just two series resistors and an input voltage, we can create an output voltage that is a fraction of the input.
commonly used to produce scaled voltages to serve as reference, sensing, or bias values
Semiconductors. Give examples of elements which are good semiconductors.
Has properties of insulators and conductors
Examples: Silicon, Germanium, Selenium
What does doping do to semi-conductors?
Makes them more like conductors
What are the 2 types of doping
P-type
N-type
How does P-type doping work?
Semi-conductor bonds with atom with 3 valence electrons, forms 3 bonds and has one hole which cannot form a bond as it is 3 valence electrons to the 4 in the semi-conductor
How does N-type doping work?
Semi-conductor bonds with atom with 5 valence electrons, forms 4 bonds and has one extra electron which cannot form a bond as it is 5 valence electrons to the 4 in the semi-conductor
What device in an electrical system using the principle of the P-N junction?
Diode
What happens with the electrons in a P-N junction?
Extra electrons in N-type semi-conductor transfer to P-type semi-conductor to fill holes
Are p-types and n-type semi-conductors positive or negative?
Neutral
Photodiode
Light sensitive semi-conductor that produces current when it absorbs photons
LED
Semi-conductor that emits a light when a voltage is applied
What is an energy gap in a semi-conductor?
The region between highest level of completely filled valence band and the lowest level of allowed conduction band is very wide. This is called energy gap, denoted by Eg and is about 3 eV to 7 eV in case of insulators. Determines colour in LED
Explain how a pulse oximeter works.
SpO2 is measured by measuring the amount of light transmitted through the tissue
By using two LEDs at two at different wavelength, the SpO2 level is calculated using a photodiode
Beer’s Law
the concentration of a given solute in a solvent is determined by the amount of light that is absorbed by the solute at a specific wavelength.
Lambert’s Law
equal parts in the same absorbing medium absorb equal fractions of the light that enters them.
Beer-Lambert Law
The measured absorbance for a single compound is directly proportional to the concentration of the compound and the length of the light path through the sample
Name 4 methods for measuring temperature.
- Fluid/Glass thermometer
- Thermocouple
- Thermistor
- Radiation and optical system
Thermistor
Semiconductor that has a resistance that varies with temperature.
Sensitive to small changes in temperature.
Bridge Circuit
- Convert a change in the impedance of a sensor into a change in an output voltage
- Strain gauge, is incorporated into the bridge circuit in order to produce the output voltage proportional to the change of resistance
Most common = Wheatstone bridge, used to measure resistance of unknown resistor. This change in resistance can additionally be converted to voltage changes
Longitudinal Wave
(+ example)
a class of waves in which the particles of the disturbed medium are displaced in a direction that is parallel to the direction of propagation of the wave.
e.g. sound, mechanical
Transverse Wave
(+ example)
a class of waves in which the particles of the disturbed medium are displaced in a direction that is perpendicular to the direction of propagation of the wave.
e.g. ocean, EM radiation
What 2 properties is the speed of sound dependent on?
Temperature
Medium
What is ultrasound frequency?
Frequencies above human audible range
Human audible range frequency
20 Hz – 20 kHz
Explain the movement of a particle in a medium that travels via longitudinal wave. (3)
E.g. Hit table
- Pressure waves travel through the material (from left to right)-
- Energy is passed from particle to particle
- Particles oscillate – however net movement is zero
What is the net movement of a particle moving in a longitudinal wave? Why?
0
Energy is transferred through the medium as the wave moves, causing particles to bump into one another. However, the particles do not have a net movement in the direction of the wave; they return to their original positions after passing energy
Is the speed of sound faster in water or biological tissue?
Biological tissue
Water: 1482 m/s
Biological tissue: 1540
Why does sound travel faster in denser substances?
Particles move through, bumping off each other more easily as there are more particles closer together
What are the 2 types of ultrasound frequency? Give examples of each.
Destructive (0.8 – 2MHz): Cancer therapy, Kidney stone therapy
Non-Destructive (2 – 18MHz): US Imaging
Piezoelectric effect
The ability of certain materials to generate an electric charge in response to applied mechanical stress.
Name a material used for piezoelectricity
- quartz
- zirconium titanate
- modern ceramics
Piezoelectric transmission vs receiving
Transmission: sends out pulses of large frequencies (1-20 MHz)
Piezo crystal deflects due to voltage change, Sound waves travel from crystal through adjacent material
Receiving: Sound waves
arrive at crystal, Sound waves cause crystal to deflect, receives pulses of small voltage (1mV)
Piezoelectric transducer
An electronic device that converts energy from one form to another
(transmission: elec -> sound (mech),
reception: sound (mech) -> elec)
What are the different modes of ultrasound?
A-Mode (Amplitude of Echo Reflection vs. Time)
B-mode (Brightness)
US Transducer Array
M-mode( Motion - B-mode over time)
Doppler US
How does A-mode ultrasound work?
Used to measure distance between tissue planes
How does B-mode ultrasound work?
The brighter the image, the more intense and focused the echo
an oscillating sound beam is emitted, passing through the eye and imaging a slice of tissue, the echoes of which are represented as a multitude of dots that together form an image on the screen
How does a US transducer array work?
A lens composed of n-transducers (in layers) focuses the us energy in a beam shape.
1. Transducers produce pulse
2. Reflections return at different times
3. Pixels are plotted based on a) time and b) strength of echo and
c) position of transducer
What effect does a low frequency US have on the resolution and penetration of the result?
Less resolution
More penetration
What effect does a high frequency US have on the resolution and penetration of the result?
High resolution
Less penetration
What is artefact in an US image?
US image that does not match actual anatomy
What type of artefacts can occur in an US image?
Posterior Shadowing
Posterior Enhancement
Reverberation
How does posterior shadowing happen?
Caused by tissue that is either
Highly reflective – most of the energy gets reflected back
Highly absorbative – most of the energy get converted to heat in tissue
E.g. Bone, gallstones, air . Reduce by adjusting angle of US probe
How does posterior enhancement happen?
Caused by
Lack of sound attenuation through a fluid-filled structure.
Sound energy passes through structure unimpeded (black) until other side where it reflects strongly off the tissue beneath (Bright zone).
Reduce by adjusting angle of US probe
How does reverberation happen?
Occurs when US beam encounters 2 strong parallel reflectors
Multiple parallel echoes result from back-and-forth travel of US between 2 reflecting surfaces
e.g. metal clip
What are biosensors?
- detect biological signals
- convert biosignals to output signal (V)
e.g. electrodes placed on the body, other transducers
Name 3 types of measurable electrical signals.
- voltage (pressure)
- current (flow)
- total flow of charge (Coulombs - in F)
What is the ideal resistance?
An ideal biosensor has infinite resistance
Name 3 types of observable electrical properties that require electrical actuation to measure.
- Resistance
- Capacitance
- Inductance
What is electrical actuation? Give an example.
An electric actuator is a device that can create movement of a load, or an action requiring a force such as clamping, using an electric motor to create the necessary force.
e.g. current flow
What is the most common method of measuring voltage in. the body?
Electrodes
- place at least 2 and measure potential difference between them
What part of the body could be considered the battery of the body?
Cell membrane
How is potential difference created across the cell membrane?
Due to active transport of ions across the cell membrane there is a large difference in intraocular and extra cellular concentrations, which creates a potential difference across the membrane.
What is the difference between intracellular and extracellular fluid?
Intracellular: inside cell, main function of intracellular fluid is to help with the transport of gases, nutrients, and other molecules. Intracellular fluid is also important for intracellular communication and cell signalling.
Extracellular: outside cell, found in blood plasma and in the interstitial space between cells.
Resting membrane potentials of skeletal muscle, cardiac muscle, neurons?
Skeletal muscle: ~ -85 to -90mV
Cardiac muscle: ~ -85 to -90mV
Neurons:~ -70mV
How is an action potential generated from a cell?
If cells are depolarised to threshold an action potential is generated.
What are the 5 steps in the conduction of an action potential?
- Stimulus
- Depolarisation
- Repolarization
- Hyperpolarization
- Resting state
In the rapid depolarization phase for generating an action potential, what happens and what ions are involved?
- Influx of Na2+ in neurons, muscles, and cardiomyocytes
- Influx of Ca2+ extends plateau phase in cardiomyocytes
Opening of voltage gates channels in - SA and AV nodes and most smooth muscle cells
What ions are involve in the repolarization phase for generating an action potental?
Efflux of K+ ions in neurons
How are APs detected and measured on the surface of the skin?
- APs propogate from multiple positions
- Detected by electrodes on the skin
What type of current flow has a large contribution to the EEG potentials generated by the brain?
post synaptic current flows in dendrites
Propgation of APs generates —– current. (what type)
Ionic
What type of electrodes are used in biomedical measurements?
Ag/AgCl electrodes (electrodes coated in Ag/AgCl gel)
What solution are the electrodes placed in in order to set up an ionic exchange?
- saturated solution of
sodium chloride. - when the electrode comes into contact with a solution that contains chloride ions, an exchange takes place between the electrode and the solution.
How is potential generated on an electrode used in biosensing?
The electrode develops a potential which is dependent on the chloride concentration in the electrolyte
What are the uses and benefits of electrolyte gel? (3)
- Enables transfer of charge from skin to electrode
- Increases contact with skin (which isn’t perfectly flat) thereby reducing contact impedance
- Allows minor movement of electrode without large changes in the electrical properties of the contact
How do cardiac pacemaker cells work?
Pacemaker cells provide the generation of spontaneous action potentials within the myocardium.
Since APs can easily propagate from cardiomyocyte-to- cardiomyocyte through gap junctions, these pacemaker cells regulate how often a wave of APs propagate through the myocardium; stimulating contraction of the cardiomyocytes.
Pacemaker Potential
a slow spontaneous depolarisation exhibited by pacemaker cells during diastole
develops as a result of hyperpolarisation which are activated when the pacemaker cell is hyperpolarised
What happens at the threshold for a pacemaker cell during depolarisation?
At threshold (~ -40mV) an AP is generated. This action potential propagates throughout the myocardium and gives rise to systole.
What contributes to the slow depolarisation of the pacemaker cells?
Influx of Na2+ and Ca2+
What are the only cells that can generate spontaneous APs? Why?
Pacemaker Cells
Unstable Resting Membrane Potential
In what type of cell does an AP last the longest?
Cardiomyocyte (~250 ms)
What creates the spike at the start of an AP?
rapid activation and rapid inactivation of voltage-gated Na+ channels
Polarisable electrode.
Uses?
Current flowing through the electrode will charge the electrode-electrolyte interface causing “polarisation.”
Measuring
Non-polarisable electrode.
Uses?
Potential will not change from its
equilibrium potential (even with large current) through the electrode.
Measuring and Pacing/Stimulation
Dipole moment (3)
Pair of equal and opposite charges separated by a distance.
Detected by electrode pairs on the body surface.
Amplified and displayed.
What initiates ventricular depolarisation?
Activation of bundle of His, bundle branches and Purkinje system
Why is the QRS complex larger than the P wave?
Muscle mass of the ventricles is much larger than that of the atria
What happens in the PR interval?
AV nodal conduction
What plane of the heart do the augmented leads measure? (aVL, aVR, aVF)
frontal plane
view through the middle of the heart from top to bottom
What plane of the heart do the precordial limb leads measure? (V1-V6I)
horizontal plane
transverse view through the middle of the heart, dividing it into upper and lower portions
Does the direction of electrical activity swing towards or away from areas of damage/necrosis?
From areas of damage or necrosis and toward areas of hypertrophy
How many layers in the cortex?
6
What type of neurons are most prevalent?
Pyramidal
What contributes to the summation of neural potentials in one direction?
Pyramidal neurons, alligned in parrallel
What are Excitatory Post-synaptic potentials (EPSPs)?
Depolarisations that bring the membrane towards threshold
What are Inhibatory Post-synaptic potentials (IPSPs)?
Hyperpolarisations that move the membrane potential
farther from threshold
Where do EPSPs occur?
dendrites & cell body
What effect does EPSPs not having a threshold have? (3)
- graded response
- no refractory period
- summation
What is responsible for generating the electrical potentials recorded on scalp surface?
low- frequency, summed potentials (EPSPs and IPSPs) form pyramidal neurons
Where are electrical dipoles created? (2)
Between the soma and apical dendrites
What are the 4 prerequisites for EEG?
- Perpendicular Neurons - Neurons must generate most of their electrical signals along a specific axis, oriented perpendicular to the scalp.
- Parallel Neuronal Dendrites - Neuronal dendrites must be aligned in parallel so that their field potentials summate to create a signal that is detectable at a distance.
- Synchronous Activity - Activity must have a synchronous structure.
- ++ or – Electrical activity produced by each neuron needs to have the same electrical sign.
What type of electrodes are used to measure bioactivity?
Metal plate electrodes with reference electrodes or bipolar linkages
What determines the amplitude of recorded EEG potentials? (4)
- Intensity of the electrical source
- Distance from the recording electrodes
- Spatial orientation
- Electrical properties of the structures between the source and the recording electrode.
How are electrical impulses from the cells measured?
Arises from changing ionic concentrations and the resulting charge in the extracellular space
What are nodes of Ranvier?
gaps between myelin
What is lost in MS disease? What is the adverse effect of this?
myelin sheaths
adversely affects nerve impulse transmission
What is bipolar configuration?
two recording electrodes and a reference electrode
What are the different measurements that can be done during polysomnography? (6)
- ECG
- EEG
- EMG
- EOG
- Respiratory effort sensors
- Respiratory airflow sensors
Explain the difference between sEMG and iEMG.
sEMG: EMG recorded on skin surface, provides information about global muscle function, particularly onset of muscle contraction
iEMG: EMG recorded within the muscle using needles or wire electrodes, used to examine motor units (more selective than sEMG)
Electromyography (EMG)
extracellular potential of an active muscle
What is corneoretinal potential? Why does it happen?
electrical potential between the front and the back of the eye
The retina (site of photoreceptors) has a resting potential of ~ -40mV
The cornea secretes Na + and K + into the cornea and excrete negatively charged ions - creates a potential of approximately ~ +30 - +40mV.
How long does it take for a RBC to circulate through the body?
30-45 secs
Briefly explain how energy conversion in a pulse oximeter works.
Light that passes through the tissue to a photodiode detector placed on the other side
is converted into a voltage,
which is then filtered, amplified and digitized.
What is key in the design of a good pressure biosensor?
- Small dimensions
- High degree of biocompatibility
What part of the body might benefit from using a strain gauge?
Diaphragm
What contributes to the change in resistance of a conductor? (3)
- Length: The resistance of a conductor is proportional to its length so stretching increases the resistance
- Cross-sectional area: As the conductor is stretched, its cross-sectional area is reduced, which also increases the resistance
- Piezoresistive effect: The inherent resistivity of some materials increases when it is stretched
What device can be used to measure the change in resistance of a sensor?
Wheatstone Bridge
Briefly explain the benefits of a Wheatstone Bridge? (2)
- Small changes in the resistance of the sensor to be converted to an output voltage that is proportional to the change in resistance.
- Can be filtered, amplified and digitized
Give an example of a physiological application of Piezoresistive pressure sensors.
Venous Occlusion Plethysmography (assess general vascular function of patient)
What are Kortokoff sounds?
The sounds heard during manual (ausculatory) blood pressure taking which indicate systolic and diastolic pressure points.
How do capacitive pressure sensors measure pressure?
By detecting changes in electrical capacitance e.g. caused by the movement of a diaphragm
Inductive transducers measure changes in…
pressure
What type of sensor is a glucose monitor?
Chemical
What type of sensor is a hearing aid?
Acoustic (microphone converts an acoustic signal (sound wave) into a voltage)
What is AC voltage supply?
Time varying voltage
Source varies continuously, flow of electric charge constantly reversing direction
What is 1 degree in radians?
pi/180
360 degrees is how many radians?
2pi - one full cycle
Capacitor
Electrical device with the ability to store charge, two plates very close together, stores charge when voltage applied to its terminals
Impedance
AC Resistance
A capacitor’s ability to resist electrical current
Why are capacitors used in circuits? (2)
- To filter electrical frequencies to remove higher frequencies and noise
- To block DC signals and only allow AC signals to pass through
List 3 passive components of circuits.
What type of circuits have all 3?
resistors, capacitors, inductors
LCR circuits
Inductors
Oppose change in the flow of current
Signal Attenuation
output signal is smaller than the input signal
When is decibel calculation negative?
When signal attenuates
Are passive or active circuits capable of providing gain power? Why?
Active
- Passive circuits do not contain components with a separate power supply
- Active circuits contain components powered from a separate source to the input
Are passive or active circuits better for measuring biopotentials?
Active circuits
- biopotentials are very low in power
- gain can be used
Give Examples of Non-Invasive Medical Devices
ECG
EEG
Ultrasound
Pulmonary Function Test
Give Examples of Invasive Medical Devices
Pacemaker
Loop recorder
Intraoperative Monitoring
Deep Brain Electrodes
Obstacles to CMPs
Small signals
Artefact
Un-cooperative patients
Resistance
The force which oppose the current flow
Impedance
The force which oppose the current flow in AC
Voltage
The electric potential between point
List 3 types of transducers
Electrodes
Piezoelectric crystals
Spirometer
Transducer: piezoelectric crystals
Compression and the crystal converts the energy from sound to the electric
Transducer: spirometer
Uses a turbine to generate a signal
How is instrument resolution calculated
when analogue signal is converted to digital the difference between these discrete values is the resolution of the instrument
Class I medical device subcategories
Is: sterile
Im: measuring
Ir: reusable
If you are looking to screen for a disease, you want a device with…
High sensitivity
If you are looking to confirm a diagnosis, you want a device with…
High specificity
Many device that require sterilisation are now made of plastic. How can this be achieved without heat?
Ethylene oxide
hydrogen peroxide gas plasma, peracetic acid
ozone
Sterility Assurance Level (SAL)
probability of a single viable microorganism occurring on a product after sterilisation
Forward Biased P-N Junction
Thickness of the depletion region reduces, and the diode acts like a short circuit allowing full circuit current to flow.
Reverse Biased P-N Junction
Thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow
Explain the formation of the P-N junction
Piece of N-doped Si brought into contact with piece of P-doped Si.
A junction forms with no charge carriers.
This PN junction is called the depletion layer and has a thickness that varies with voltage
What is a common stain gauge measurement(G) for metals?
2 - 5
What is a common application of a capacitor sensor?
As an accelerometer
What is let-go current?
the amount current that will still allow an individual to release the source, even though muscle contraction is induced
(i.e. cramps from current passing through body)
Name the three performance measures for op-amps.
Signal-to-noise Ratio (SNR)
Common Mode Rejection Ratio (CMRR)
Power Supply Rejection Ratio (PSRR)
What type of feedback would be produced if the output of an op-amp were connected back to V-?
Negative Feedback
What type of feedback would be produced if the output of an op-amp were connected back to V+?
Uncontrollable Gain
In what type of environment would a differential amplifier be used? Explain why this would be preferable over other op-amps in this situation.
Electrically Noisy Environment
single-ended amplifier would also amplify the unwanted noise signal as well as the desired input signal
Name 3 features of an ideal op-amp.
- Infinite open-loop gain (𝐴𝑜𝑎 )
- Infinite input impedance Rin, and so zero input current
- Zero input offset voltage
- Infinite output voltage range
- Infinite bandwidth with zero phase shift and infinite slew rate
- Zero output impedance Rout, and so infinite output current range
- Zero noise
- Infinite common-mode rejection ratio (CMRR)
- Infinite power supply rejection ratio (PSRR)
Name the 2 rules of Op-Amps
- V- and V+ should always have same V
- No input current
Purpose of filters
Removal of noise of sources / parts of signal that mask an important component
- remove source of interference
- smoothen out signal
- DC offset
High Pass Filter
Allows frequencies above corner frequency to pass through, blocks any frequencies below corner frequency
Stop Band
Signal that is blocked from passing through filter
Pass Band
Signal that passes through filter
Corner Frequency
Frequency between the pass band and filter roll-off (transition region) Attenuation = 3 dB (gain of -3dB)
White noise
Signal that contains all the frequencies
Low Pass Filter
Allows frequencies below corner frequency to pass through, blocks any frequencies above corner frequency
Band Pass Filter
2 corner frequencies
Pass band specified by a range
Combination of low and high pass filters
Notch Filter
Sharp signal attentuation at specified frequency
Stop band specified by a range or single frequency
Name 2 methods which can be used to filter digital signal in real time
Embedded processors
Digital Signal Processor
Phase Response
Amount by which signal components of different frequencies are shifted in phase at the output of the filter compared to the input
Linear response
filter in which in the pass band all frequency components are shifted in time by the same amount such that the phase relationships that exited across frequencies at the input of the filter remain at the output of the filter
Is phase response or linear response desirable?
Linear response
- phase response can lead to signal distortion
Group delay
Slope of phase
linear phase
the same fixed delay across all pass band frequencies
Pass band ripple
measure of how flat the amplitude frequency response is in the pass band.
Sampling. Discrete in: (2)
Time
Amplitude
(Amplitude of signal at a point in time)
Sampling. Discrete in: (2)
Time
Amplitude
(Amplitude of signal at a point in time)
Higher or lower sampling rate results in higher accuracy?
Higher
Sampling frequency
1/sampling time (seconds)
Analog signals
Continuous
Infinite range of values
Digital signals
Binary system
Finite resolution
3 bit ADC in terms of resolution
3 bits of resolution
Resolution in ADC
Minimum voltage change in ADC
Nyquist Rate
Fs > 2B
B = bandwidth of signal, ref max f
ADC without aliasing error
