Instrumentation CA1 Flashcards
Transducer +3 clinical examples
Anything that converts one electrical source to another electrical source
Electrodes
Piezoelectric crystal
Spirometer
Filters
Mitigate any unwanted frequencies
Amplifiers
Electric bio signals are very small, so amplifiers increase the signal but is also used for filtration
Signal digitisation
electric bio signals are analogue and will need to be converted into digital form using an analogue to digital converter
When assessing the usefullness of a device, what two peramiters do we take into account
Specificity and Sensitivity
In what cases to we want a high specificity and in what cases do we want a high sensitivity
Want high sensitivity when screening for a disease
High specificity when confirming a disease
When assessing the quality of the medical devices the performance if determine by what 4 values
accuracy, resolution, precision, reproducibility
ROC curve
- what does it plot
- where on the graph would a device be deemed an acceptable classifier
ROC curve asseses the ability of the binary classifier. Plots sensitivity vs specificity
Anything above the positive slanted line is acceptable
What are the 4 rules in classifying non-invasive devices
- either do not touch the patient or contact only intact skin (I)
- intended for channelling or storing blood, body liquids, cells or tissues, liquids or gases for the purpose of eventual infusion into the body (I)
- unless using with blood/body fluids (II1)
- unless connected to a active device class IIa or higher (IIa)
- unless bloodbags
- modifying the biological or chemical composition of human tissues, or cells, blood, other body liquids, or other liquids intended for implantation or administration into the body (IIb)
- unless only filtration, centrifugation or exchange of gas or heart (IIa)
- consisting of a substance intended to be used in vitro in direct contact with human cells (embryo) (III)
- In contact with injured skin or mucous membrane (or invasive device) (IIa)
- unless as a mechanical barrier, compression or absorption (I)
- unless intended for wounds which breach dermis and heal only by secondary intent (IIb)
Qualities of class 1 device
mostly non-sterile and non-invasive
e.g blood pressure machine, stethoscope, reusable surgical instruments
qualities of class IIa devices
short term, non-invasive devices intended for channelling or storing blood, body liquids for eventual administration into the body
If they are principally intended to manage the microenvironment of the cell
e.g ultrasound, diagnostic software
Qualities of class IIb devices
long term, intended to be used principally for injuring the skkin which can breach the dermis
e.g equipment used for intraoperative monitoring, ionizing radiation
intended to administer medicinal product by means of a delivery system
qualities of class 3 device
critical for patients life
e.g pacemaker, stent
What is not the responsibility of the regulatory bodies in medical device standardisation
designing specific safety tests that medical devices must pass
What are the two independent testing agencies
ISO and IEC
What are the 4 levels of risk severity
- Minor - recoverable
- Major - very slow to recover or permanent
- Critical - immediate or prolonged treatment
- Fatal - may result in death
What two components is risk made up of
Probability and consequence
What is requirement for hardware failures
A medical device must be safe in the case of a single fault
2 independent failures should not harm the patient
Therefore equipment must be designed so that the combination of the first and second failures cannot cause a hazard
What are the two primary principles behind software verification
- describe the exact function the software is supposed to perform
- device a very specific test to verify that the software works s designed
Resolution
The smallest differential value that can be measured
An electric shock of 80-600uA is likely to cause what to happen in a body
Ventricular Fibrillation
What is the difference in the travel path between macro and micro electric shocks
Macro - only a small amount of the current flows through the heart
Micro - introduced directly into the body via a high conducting avenue with a direct conduction path to the heart
What two parameters depend how bad an electric shock is
skin resistance and spatial distribution
How do you protect someone after having a macro shock
isolate patient from any grounded surface and all current sources.
electric field force eququation
F = qE
What does Coulombs law state
As you move away from the electric field, the strength gets weaker
Electric force is inversely proportional to R (electric field gets really weak, really quickly as yo move away from it)
What does Gauss Law explain
Explains how there are negative and positive charges that interact with each other without touching through electric fields
What will the net force of the magnetic field be in a magnet
Always 0
What is the role of a battery
A battery chemically rips apart charge allowing them to then snap together again (creating energy)
How do we measure the strength of a battery
put a single charge into the electric field of the batter and measure the electric potential of that single charge using (W=qEd)
What direction is conventional current
Positives moving towards negative
What makes a good conductor (theory and examples)
Good conductors font allow many collision between charges
Gold, platinum, copper
Capacitor + equation
A device with the ability to store electrical charge
C=Q/V
What determines the quality of conductivity of a material
The number of electrons on the outer shell and how bounded they are to the atom
What is the frequency of AC and DC circuit
AC - 50Hz
DC - 0
Are medical devices AC or DC
Medical devices can often use a combination of bothAC and DC circuits and need to change to either between one another. An inverter converts DC to AC while a rectifier converts AC to DC
What do electrodes do
Conduct electricity through non-metallic mediums
What is reactance
The opposing force to current in AC circuit
What is a semiconductor and an example of one
A semiconductor has properties of a conductor and insulator
e.g silicone
What is dopping
This is the process of introducing foreign material to pure crystal form semiconductors that improves their conductivity
n-type vs p-type dopping
n-type: created by doping in extrinsic semiconductor with an element with an additional election
p-type: created by doping in extrinsic semiconductor with an element with notable fewer electrons
How does a semiconductor work
The free electrons from the n-type migrate to the p-type to donate their electrons. There is a region close to where the two sides meet called the depletion layer, where there is no charge as there are no free flowing electrons.
This depletion layer allows of current flow only in one direction (anode>cathode)
What is a photodiode and how does it effect a semiconductor
A photodiode is light sensitive conductor that produces current when it absorbs photons
the direction of the electric current in the diode forces the electrons to move towards the n-side and consequently the holes move towards the p-side. This results in an increase in the number of electrons on the n-side and hold on the p-side, a rise in the electromotive force is observed
What is an LED and how does it effect a semiconductor
A LED is a semiconductor that emits light when a voltage is applied
The electrons from the n-side gain enough energy to cross the junction and recombine with the p-side. The energy released when the free electrons from the n-side combine with the p-side is released in the form of light.
What sort of sensor measure blood oxygen saturation
Optical sensor
What does beer lamberts law explain
The measured absorbance for a single compound is directional proportional to the concentration of the compound and the length of the light path through the sample
What is a thermistor
Semiconductor whose resistance varies with temperature
What is the purpose of a bridge circuit
Used to convert a change in the impedance of a senor into a change in an output voltage
What is Poissons ratio (v)
explains the relationship between longitudinal and lateral strain in material
What does it mean if the Poissons ratio is positive
when the material is stretched it contracts in the perpendicular direction and when compressed it expands laterally
Explain the movement of particles in a sound wave
Sound waves are longitudinal waves - the particles aren’t actually moving the whole length of the wave, they are instead oscillating left and right and passing their energy onto the particle next to it
What is the net movement of particles in sound waves
0
The speed at which a sound wave can travel at is dependant on what two factors
Temperature and medium
How quick does sound waves travel though biological tissue
1540m/s
What frequencies are used in ultrasound for imaging and therapeutic
Imaging - 2-18MHz
Thereputic - 0.8-2MHz
What 3 things can occur to an ultrasound wave when it is emittied from the probe
- Continue through the tissue
- Reflect back to the probe
- Get abosorbed in the tissue
How are high energy ultrasound waves made and what material is commonly used
Piezoelectric effect changes the shape of a piezoelectric material using an electrical potential or a voltage. This is usually ceramic.
Direct vs Converse piezoelectric ultrasound waves
Direct - forcing piezoelectric material to compress there is a voltage coming out of it (converting mechanical input into electrical)
Converse - a voltage input into the piezoelectric material is converted into mechanical energy
Explain transmission in ultrasound
Probe delivers high voltage pulse to the tip of the probe where piezoelectric material is stored, causing this material to change shape quickly. This fast change in shape when held against the tissue causes the tissue to vibrate and transfer sound waves though the tissue
Explain reception in ultrasound
the sound waves come back from the tissue to the peizoelelctric material, causing the material to vibrate causing a small voltage signal to come out of the
What electrical device is an ultrasound probe considered
A transducer
Describe A mode ultrasound and what it primarily meaures
This only measures the amplitude of the sound waves as they return back to the probe.
This gives us information on the distance between tissue planes but not much else.
In B-mode ultrasound what determines the vertical and horizontal position of the image
Vertical - related to the time for each echo to return back to the transducer. Further down the screen, deeper the surface.
Horizontal - based on the position of the transducer probe
In B-mode ultrasound what determines the brightness of the image
The brighter the image the higher the amplitude of the returning sound wave. This means the image tends to get darker as the depth of the location increases
With a lower frequency B-mode ultrasound what resolution would we expect and to what depth can these waves travel
Lower frequency sound waves will give a lover resolution but the energy can travel deeper and can image deeper tissues
Describe M-mode ultrasound and what it shows
This plots a perpendicular image to that of B-mode ultrasound.
Shows the movement in a particular slice over time
What do the different colours in Doppler ultrasound represent
Red - fluid travelling towards the probe
Blue - fluid travelling away from prob
In Ultrasound, posterior shadowing usually looks like what, and what can cause it
Represented as a really dark region in the picture
Caused my material close to the probe that is very reflective so all the energy is reflecting back and not reaching the shadow area.
Same would occur is material close to the probe what extremally absorptive and there was no energy left to reach the deeper regions
In Ultrasound, Posterior enhancements/amplification usually looks like what, and what can cause it
Look like bright spots
Caused by tissue not absorbing the expected amount of energy at a particular depth (e.g if there was a fluid filled cyst the energy would flow right through it and the tissue behind the cyst would be bright)
In Ultrasound, reverbertations usually looks like what, and what can cause it
Look like lots of little white horizontal lines
These are usually due to reflections inside the piece of material inside the body (e.g a metal pin).
Can also be due to different tissue densities throughout the body
What is the thermal index in ultrasound
It tells us the amount of time the ultrasound can be used at a particular energy before it s dangerous
What does it mean if a tissue has a low Wdeg
It is very easily heated up and damaged
What do biosensors do
Convert biological signals into an output signal (usually voltage)
What do electrodes measure
potential difference between two points
What causes the body to have an electrical potential difference
active transport of ions across the cellular membrane during major changes in potential difference of cells
What is the difference from polerised to non-polerized elelctrodes
Polerized elelctordes - have current flowing through them that will charge the electrode-electrode interface causing polerisation
Non-polerized - the potential will not change from its equilibirum through the elelctrode
The potential that the elelctrode develops is dependent on what
The Cl- concentration in the electrode
Explain the conductivity of the different layers of the skin
Dermis/subcutaneous layer - resistor
Epidermis - semipermeable to ionic current
What are two biopotentials in the heart
Pace maker potentials and myocardial action potentials
What are pacemaker potentials in the heart and when do they occur
During diastole, pacemaker cells exhibit slow spontaneous depolarisation known as pacemaker potential
What channels drive pacemaker potentials and myocardial action potentials reciprocally
Pacemaker potentials - voltage gates Ca2+
Myocardial Action Potentials - voltage gates Na+ channels
Explain the direction of propagation of action potentials in the brain and which direction EEG is able to measure better
In the sulcus there is vertical propagation
In the gyrus there is horizontal propagation
EEG measure perpendicular AP’s best, so these tend to be those in the sulcus
What are 3 brain biopotentials
Post synaptic potentials, axonal action potentials, saltatory conduction
Where do post synaptic potentials occur
In the dendrites and cell bodies of the postsynaptic neuron
What is the role of EPSP’s and what is their threshold for activation
EPSP’s depolarise the membrane towards threshold.
They have no individual threshold as they summate to
What is saltatory conduction
The travel of an AP down an axon with myelin sheths
What do EEG measure
electrical potential between dipole space (extracellular space)
What 4 prerequisites are there to record energy of any network of neurons in EEG
- Neurons must be perpendicular to the scalp
- Dendrites must be parallel so their field potentials summate
- Activity must have synchronous structure
- Electrical activity produced by each neuron needs to have the same electrical sign
What 4 things does the amplitude of a recorded potential in an EEG depend on
- Intensity of the electrical source
- Distance from the recording electrode
- Spatial orientation
- Electrical properties on the structure between the source and recording electrode
What three factors would contribute to a EEG signal being very large
- Occur near the recording electrode
- Perpendicular to the recording electrode
- Generated by lots of neurons at once (highly synchronous)
What does surface EMG provide information on
Global muscle function, particularly onset of muscle contraction
What is the corneoretinal potential
biopotential located across the eye
What bio potentials are found in the GI tract
Endogenous pacemaker cells of the intestine crease autonomic contraction of the intestinal smooth muscle
Explain how pulse oximetry works
Deoxygenated HB absorbed more red light then oxygenated Hb.
LED emits red light and the photodiode on the other side detects how much red light passes through and converts this into voltage
What is one factor that causes variation in blood oxygen saturation and how is this accounted for
On a heartbeat basis, small amounts of arterial blood is pumped into the veins, therefore the amount of light passing through the blood varies as a function of time.
This is accounted for by taking the ratio of the maximum and minimum detected intensities over the duration of a heartbeat
What determines the sensitivity of the pulse oximeter
Sensitivity of the photodiode
What does a piezorestrictive pressure sensor measure and how does it work
Measures physical displacement using a strain gauge. The strain gauge can be attached to a diaphragm which detects changes in pressure
What 3 separate effects contribute to the change in resistance of a conductor
- the resistance of a conductor is proportional to it’s length (stretching increases resistance)
- As the conductor is stretched, its cross sectional area is reduced, which also increase the resistance
- The inherent resistivity of some material increases with stretched
What is a gauge factor
Relative resistance change divided by the strain
Venous occlusion plethysmography is an example of what sort of pressure sensor
Piezoretrictive pressure sensors
What do piezoelectric sensors/transducers do
Generate voltage when a mechanical force is applied to one face of the sensor
What are two examples of piexoelectric materials
PVDF and quartz
What is an example of a piezoelectric sensor/tranducer used by a CMP
blood pressure monitors
What do capacitive sensor/transducers measure and how
Measure pressure by detecting changes in electrical capacitance caused by movement of a diaphragm
How does inductive transducers work
Connects a primary and two secondary coils to an input voltage source. The output voltage is the difference between the 2 secondary coils.
When the primary coil moves in one direction, the voltage in that direction increases and the voltage in the other direction decreases
What equation is used to define a AC wave
Asin(2~ft)
What is the term used for a capacitors ability to resist charge
Impendence
What two factors can decrease the impendence of a capacitor
increase in frequency or capacitance
What are the three passive components of a AC circuit
Resistor, capacitors, inductors
What circuit attenuation mean
When the Vout is smaller then the Vin
What is a passive circuit
Don’t contain components with a separate power supply and thus are not capable of providing gain in power
In which direction does a silicone based diode have to be facing to be a ‘forward bias’ and ‘reversed bias’, respectfully
Forward bias - facing in away from the positive source
Reverse bias - facing towards the positive source (thick depletion layer, no flow)
How does a NPN transistor work
If biased correctly, a small amount of current between the base and emitter aligns with a larger current flowing in the path of collector to emitter
