Applied Physics Flashcards
What are the three types of SI units
Number of each
Fundamental 7
Supplementary 2
Derived unlimited
What are the 7 fundamental SI units
Length m
Mass kg
Time s
Electric current A
Temperature K
Amount of substance mol
Light intensity cd (candela)
What are the 2 supplementary SI units
Angle , radian , rad
Solid angle , steradian, sr
Define the derived units of velocity and acceleration
Velocity m.s^-1
Acceleration m.s^-2
Define force and pressure by SI units with name, derivation and description
Force, newton, kg.m.s-2, acceleration of mass of 1kg at 1m.s^-2
Pressure, pascal, kg.m^-1.s^-2, pressure which exerts a force of 1 new to per square meter of surface area
Define frequency by SI units with name, derivation and description
Frequency, hertz, 1.s^-1, number of cycles per second
Define energy, power and charge by SI units with name, derivation and description
Energy - joule, kg.m^2.s^-2, energy expanded moving a resistive force of 1 newton a distance of 1m
Power - watt, kg.m^2.s^-3, rate of energy expenditure of 1 joule per second
Charge - coulomb, A.s, electric charge passing a fixed point in a conductor when a current of 1 ampere flows for 1 second
Positive multiplying factors to power ten
Deca 1
Hecto 2
Kilo 3
Mega 6
Giga 9
Tera 12
Peta 15
Exa 18
Negative multiplying factors to power 10
deci 1
Centi 2
Mili 3
Micro 6
Nano 9
Pico 12
Femto 15
Atto 18
1kPa of pressure is the equivalent of how many:
Bar
Atmospheres
mmHg
cmH2O
Bar 0.01
Atmospheres 0.01013
mmHg 7.5
cmH2O 10.2
How many kPa in 1 Bar
100
100kPa equals how many atm
1.013
What is 158mmHg in SI units
7.5mmHg = 1kPa
1mmHg = 0.133kPa
158mmHg = 21kPa
Differentiate mass and weight
Mass is the amount of matter present measured in kg, it is constant no matter where the object is
Weight is the gravitational force acting on the object in newtons. Weight = mass x acceleration due to gravity
What is the relationship between force and pressure
Pressure = force/area
What is the quantity of a distance moved
SI unit
Displacement
m
How is work done calculated
Work done = force x distance moved
What is power
The rate of doing work
Energy expended per unit time
Power = work / time
How does a barometer work
Has a measuring column of fluid (mercury) CLOSED to the atmosphere in a open reservoir, this is balanced against atmospheric pressure acting on the reservoir. As atmospheric pressure increases it pushes down on the reservoir pushing fluid up the tube by hydrostatic pressure.
How does a manometer work
Has an open column of fluid (mercury or alcohol/water at lower pressures), the base attached to a closed limb with the unknown pressure on the end.
When the unknown pressure is applied a gauge pressure is obtained looking at the hight of the fluid (comparing unknown pressure against atmospheric)
How does absolute pressure differ from gauge pressure
Absolute pressure includes the effect of atmospheric pressure (atmospheric pressure + gauge pressure)
What sort of measurement is barometric pressure
Absolute pressure
1 atmosphere in mmHg and kPa
760mmH101.4kPa
On a pressure volume graph what represents work done
Area under the curve
What would a lung pressure volume curve look like if there were no frictional losses
What actually happens
They would overlie as energy in would equal energy out
Curve exhibits hysteresis, at any given pressure on the inspiratory loop volume is lower with the area between the two loops representing energy wasted overcoming friction.
How much energy does breathing require
Work of breathing takes around 6J/min, a power requirement of around 0.1w
However, it’s only about 10% efficient so overall energy requirement around 1W or 1% of total BMR
Calculate stroke work done by measurable readings
Stroke work done = (MAP-PCWP) x SV
Power requirement of heart to provide 5.6L/min flow
1W at 10% efficiency thus 10W - around 10% BMR
What is heat
What is temperature
Heat Energy that can be transferred from hot to cooler objects
Temp Measure of an objects thermal state (how hot or cold it is)
What is heat measured in
J
What is a calorie
The amount of heat required to raise the temperature of 1g of water by 1oC
How many J in 1cal
4.16
What is 0 celcius in farenheight and kelvin
32 f
273.15 k
What is the specific heat capacity
The amount of heat energy required to raise the temp of 1kg of a substance by 1oC
What is heat capacity
The amount of heat required to raise the temp of an object by 1oC (no weight)
Thus heat capacity = mass (kg) x specific heat capacity (kJ/kg/oC)
Rough specific heat capacity of water and human tissue
Water 4.16kJ/kg/oC
Human tissue 3-4
What alters freezing and boiling point of a substance
Pressure
What can change the state of a substance
Temperature and pressure
What is the critical temperature
The temperature above which a gas cannot be liquified by increasing pressure
What is the critical temperature for oxygen
What is the critical temperature of nitrous oxide
Implication
-119oC oxygen
36.5oC nitrous oxide
Oxygen cannot be liquified at room temperature
Nitrous oxide can be liquified at room temperature
What is pseudo critical temperature
Implication for entanox
When gases mix alters critical temp
In entanox nitrous oxide mix with oxygen lowers critical temp from 36.5 to -6oC, thus at room temp entanox is all gas, however, if cold the nitrous oxide liquifies thus first half of cylinder all oxygen then becomes a hypoxic mix near end.
On an isotherm graph of volume vs pressure how do substances behave at temps higher than critical temp
Always a gas, as pressure increases volume decreases hyperbolically (fast at first then slowing)
On an isotherm graph of volume vs pressure how do substances behave at temps equal to critical temp
At low pressures exists as a vapour, volume decreasing as pressure increases. When pressure exceeds a critical threshold becomes liquid (curve suddenly shoots up, further increases in pressure make very little difference to volume)
On an isotherm graph of volume vs pressure how do substances behave at temps lower than critical temp
Exist as a liquid at high pressure, as pressure lowers volume constant until reaching boiling point (this is the saturated vapour pressure) - producing a mixture of liquid and vapour, volume then varies at a constant pressure based on degree of vapourisation (increasing as it vapourises), then once all vapourised volume again starts to expand as pressure decreases.
Define critical pressure
The minimum pressure, at critical temperature, to liquify a gas
Define critical volume
The volume occupied by 1 mol of a gas at critical temp and critical pressure
What is the triple point of water, definition and value
The point at which water can exist in all three phases, liquid, solid and vapour
Pressure 0.006 atm, temp 0.01oC
Define gas
What is its critical temp
Substance which is normally in gaseous state at room temp and atmospheric pressure
It’s critical temp is below room temp so cannot be converted to a liquid by increasing pressure at room temp.
Define vapour
What is its critical pressure
A gaseous substance which is I the gas phase at a temp lower than it’s critical temperature
Critical temp above room temp thus can be liquified at room temp by increasing pressure.
How is a vapour formed
Evaporation of a liquid or escape of molecules from the liquids surface
Very slightly from the surface of solids (sublimation)
What is the saturated vapour pressure
The pressure exerted by a vapour when in contact and equilibrium with its liquid phase
The vapour concentration increases until in equilibrium - amount leaving is equal to amount returning, and no further increase in vapour concentration occurs
What is the boiling point with respect to saturated vapour pressure
The temp at which SVP equals atmospheric pressure
How does saturated vapour pressure change with temperature
Increases
What are the latent heat of fusion and vaporisation
The phenomenon of no temperature change occurring during change of state. The heat energy added to change from solid to liquid or liquid to gas is used to change state not increase temperature. (LHoFusion = energy to change 1kg from solid to liquid without change of temp, LHoVaporisation = same but for liquid to vapour)
What happens to a substances latent heat of vaporisation as temp changes
Explain
Decreases - though latent heat is more marked at freezing and boiling points it occurs throughout and as temp drops more energy is needed for molecules to make distance and break away.
What features of a inhaled gas would lead to heat loss
Cold, dry (due to evaporation into it)
What are absolute and relative humidities
Which varies with temperature
Absolute - mass of water vapour in given volume of air kg/m3
Relative - ratio of mass of water in a given volume of air at a given temp to the mass of water required to saturate that given volume at the same temperature
Relative varies with temp, absolute does not.
If you humidify air to 100% saturated at 20oC what is its relative humidity at body temp
Around 40%
What is conduction in heat transfer
Atoms maintain a fixed position but energy is transferred by direct contact, as heated atoms vibrate the vibration is passed on to neighbours increasing their energy
What is convection in heat transfer
A region of a substance becomes heated, density decreases and it rises resulting in cooler denser fluid or gas replacing it - causes a convection current transferring heat away
What is radiation in heat transfer
Release of infra red radiation, allows heat transfer across a vacuum
Order of heat loss mechanisms under anaesthetisa
Radiation
Convection
Evaporation
Respiratory
Conduction
What are the three key gas laws
Boyles - pressure x volume = constant - pressure is inversely proportional to volume
Charles - volume / temperature = constant - volume is proportional to temperature
Gay Lussac’s - pressure / temperature = constant - pressure is proportional to temperature
What is the ideal gas equation
Constant = PV/T
Can be expanded to
PV = nRT (where r is the universal gas constant and n is number of moles of gas)
Value of universal gas constant.
8.32J/oC
What does pressure relate to in a cylinder of gas? What about a cylinder of vapour?
Gas = amount of gas left (PV = nRT - assuming v, r and t are constant P is directly proportional to V)
Vapour = the saturated vapour pressure - thus not amount of vapour remaining as more may be produced from the liquid as it is removed and used.
What is avogardros hypothesis
Equal volumes of gas under the same temp and pressure contain equal numbers of molecules
What is Avogadro’s number
What volume does 1 mole of gas occupy at normal pressure and temp
6.022x10^23
22.4L
What is Dalton’s law
Implication
If a mixture of gases are placed in a container the pressure exerted by each gas is equal to that which it would exert if it alone occupied the container
Thus in a gas mixture the pp exerted by each gas is proportional to its fractional concentration
What is adiabatic compression or expansion and implication
Heat energy is not added or removed when a change in pressure or volume occur in a gas, thus when a gas expands temperature will drop and extra warming may need to be added.
Does gravity effect gasses
Yes. But to a lesser extent than liquids as lower density
How is flow produced in liquids and gasses, what influences its behaviour
Applied by a pressure gradient
Influenced by density and viscosity
Do fluid dynamics apply to liquids and gasses
Yes
What is viscosity
Effect on flow
Implications for gasses
The stickiness of a fluid
More viscous the fluid slower the flow
Only becomes apparent at much higher velocities in gases as much less viscous
How is viscosity quantified
What are the components
Coefficient of viscosity = shear stress / shear rate
Shear stress - drag force of one layer of the fluid against the neighbouring layer (which reduce in velocity to 0 next to a fixed surface
Shear rate - the velocity gradient perpendicular to the direction of flow (ie the gradient from 0 next to a fixed surface to maximum)
What are the units of viscosity
Poises (from poiseuille)
What are the effects of temp on viscosity
Liquids reduce viscosity with higher temps whilst gases increase
What are Newtonian fluids
Fluids where the viscoicty is constant regardless of the velocity gradient produced during flow (ie if shear stress is higher shear rate also increases keeping viscosity constant)
Is blood a Newtonian fluid
No
It is a sheer thinning fluid - viscosity falls as shear rate between layers increase
How can viscosity be measured
Measure rate of flow of fluid down a tube
Spin a drum containing a sample and a suspended needle - needle moves as fluid does due to torque thus is displaced giving a measurement on a scale
What is released due to the shearing action of fluids
What is the implication of this
Heat
Analogous to friction between two solid substances rubbing
Implication is it causes loss of energy and thus damping of a system
What is Hagen poiseuille law
Flow = pie.pressure gradient.radius^4 / 8.viscosity.length
What is kinematic viscosity
The ratio of viscosity to density
If high (high viscosity low density) then turbulent flow will be suppressed but if low then eddies and disturbances may persist for a long time
What is Reynolds number
How is it calculated
Determins whether flow will be laminar or turbulent, <2000 tends to be laminar >4000 tends to be turbulent
Re = vL/mu = velocity.length / kinematic viscosity
What characteristics predispose to laminar flow
Viscous fluids
Narrow tubes
Low velocities
What characteristics predispose to turbulent flow
Thin dense fluids, wide bore irregular tubes, high velocity
What happens to pressure drop along a tube as flow of fluid within it increases
Initially laminar flow linear drop in pressure as flow velocity increases, crosses a tipping point into turbulent flow as Reynolds number increases with velocity and pressure drop rate increases
What sort of flow is found in what airways
Trachea and main bronchi - turbulent
Small airways - laminar
What happens to fluid flow when a tube narrows
As flow related to area and velocity then as area decreases velocity must increase to keep flow rate constant
What law suggests that pressure drops as flow velocity increases? Why
What caveats apply to this
Bernoulli equation
Fluid has potential energy provided by the pressure gradient and kinetic energy because it is moving
When the fluid speeds up, eg due to a narrowing, then it must have gained kinetic energy, as a result it’s potential energy must have decreased indicating the pressure must have dropped
Fluid is non Compressable
Effect of gravity is negligible Ie a gas, or a fluid in a horizontal tube.
What is the Venturi effect
What are its uses
Gas passes through a narrowing producing a lower pressure due to the Bernoulli effect.
Flow driven nebulisers
Estimation of flow velocity by measuring pressure drop (though not linear)
How do jet ventilators work
High pressure stream injected - creates area of low pressure dragging more air in behind it
How do venturi masks work
High velocity stream which entrains in surrounding air by viscous drag, NOT the Venturi effect
What is electric charge
What is it measured in
An accumulation of excess or a deficit in electrons in an object.
Measured in coulombs.
What is a coulomb
The electric charge equal to the charge possessed by 6.24x10^18 electrons
Or
The charge which passes any point in a circuit in a second when a current of 1 amp is flowing
What is electrical current
The movement of electrical charge
What is an ampere
If two conducting wires are close to each other they produce a force between then due to their magnetic fields that depends on the size of their currents.
1 amp is the current, if flowing in two parallel wires of infinite length placed 1m apart in a vacuum with produce a force on each wire of 2x10^7newtons per meter.
What is electrical potential
What is it measured in
If anywhere has a positive electrical potential it has potential energy and the charge will move away from it to a point of lower potential
Volts
What is the electrical potential of earth
Taken as a reference point for 0
What is potential difference or voltage
When a potential difference is applied across a conductor an electric current is produced with flow of positive charge from higher to lower potential.
This is measured in volts and 1 volt is the potential difference producing a change in energy of 1 joule when 1 coulomb is moved across it.
What are the moving components of electrical charge in solution and in a wire
In solution positive or negative ions
In a wire only electrons (thus flow of electrons in opposite direction to direction of current flow which is from positive charge to negative)
What is resistance
The electrical property of a conductor which opposes the flow of current through it
What is ohms law
The current flowing through a resistance is proportional to the potential difference across it
V=IR
What is power in an electrical circuit
Energy expended or work done per second (watts or joules per second). Dissipated as heat flowing through a resistor. P
P=VI
V or I can be substituted by ohms law giving P=I^2.R or P=V^2/R
What is direct current
Sources
Current flowing only in one direction
Usually a battery or power adaptor
What is alternating current
Sources
Supply in which current reverses direction cyclically
Voltage vs time gives a sinusoidal curve
Mains power supply
How is ac voltage described in terms of numbers
Eg mains voltage
By peak voltage and frequency
340v 50Hz
Note mains voltage usually quotes as 240v which is the root mean square - this equals the DC voltage which would have the same heating effect
What can points on an electrial sine wave be define as
Point in time
Phase angle
What is the phase difference of electrical sine waves
Comparison between two sine waves
If phase difference 0 they are in phase, if phase difference 180o they are fully out of phase (negative peak coincides with positive peak of other wave)
What is the difference between resistance and reactance
Resistance - a devices ability to resist DC current - R, measured in ohms
Reactance - a devices ability to resist AC current - X, measured in ohms, varies with the frequency of the AC applied
What is the impedance of an electrical device
A combination of resistance and reactance
Impedance (Z) = sq. root R^2 + X^2
What is a resistor
A conductor that opposes AC and DC flow used to reduce currents of voltages in a circuit
What is a Wheatstone bridge circuit
A ring of four resistors supplied by a dc voltage across diagonally opposite corners. Galvnometer in the middle dividing to two sides.
One resistor is unknown, one on the other side is variable.
Variable resistor adjusted until voltage balanced. At this point unknown equals variable. (Or as variable changes reading on galvanometer changes thus can be measured).
What is a capacitor
Structure
A device that exhibits capacitance - the ability to store electric charge.
Two conducting plates separated by a thin layer of insulating material.
How does a capacitor work
When a voltage is applied across the plates there is a surge in current. Once plates are charged equally no more current flows
What is capacitance measured in
What is stored
Farads (F)
Usually in micro or pico farads for most things.
Charge (Q) = capacitance (C) x voltage (V)
How do capacitors behave differently with ac and dc?
Significance
In dc act as a full block (very high resistance) as plates separate so current cannot pass
In ac as current alternates back and forth resistance overall is low
This allows capacitors to act as a way of diverting off unwanted ac signals to earth whilst preserving dc.
How do capacitors sum
In parallel simply add capacitance
In series 1/total = 1/C1 + 1/C2 etc
Ie opposite of resistors
What is an inductor
Effect
A circuit component consisting of a conductor wrapped in a spiral around a ferrous core.
When voltage applied it creates a surrounding magnetic field. when the voltage is applied there is a lag between application and current flowing - it builds up with time (and conversely drops off with time) as the magnetic field is created (and collapses)
Units of inductance
Henry (H)
How does an inductor behave differently with ac and dc
With dc resistance is just equal to the resistance of the coil
With ac has much higher reactance, which increases with frequency - this helps to smooth out spikes in power supply or to filter out ac
How does a defibrillator work
Two phases
Charging - dc power charges capacitor to joules set by operator
Discharging - switch connects capacitor to patient circuit with inductor, second switch (shock button) releases it to patient - inductor slows spread of energy giving a more effective broader waveform
What is a transformer
Two inducers wound around same ferrous core. As they are close current changes in one circuit induce a current in the outer due to coupling of the magnetic field
What do the voltages in a transformer correspond to? Effect
Input voltage to coil one
Ratio of coils coil 2 : coil 1
V2 = V1 (N2/N1)
Effect is to allow step up or step down of voltage in a circuit
What is a diode
Uses
A semiconductor that only allows current to flow through it in one direction thus converts ac to dc. - used in defibrillators to convert mains to dc to charge capacitor. Used in mains acting as a battery and chargers. Are also used as protection in circuits.
What is a transistor
Use and structure
A semiconductor used to amplify small current signals
Three connections - base signal, collector and emitter
Small input from base produces an amplified signal in the collector circuit
Hazards of electrical equipment
Macro shocks
Micro shocks
Diathermy hazards
Electrical burns
Fires and explosion
How can electric shocks occur
Body forming a connection from live mains line to earth (either directly or via object)
Body forming a connection from live mains line back to neutral mains line whilst isolated form earth (e.g. in rubber shoes)
What is the difference between the live and neutral line?
Live line carries current from generator to device, neutral back
They have the same current flow!
What is the effect of AC or DC source on the outcome of an electric shock
AC or DC - dc produces a single spasm throwing victim clear, ac causes tetanic muscle spasm at 50Hz potentially causing uncontrollable grip of shocking object prolonging duration of shock. also AC more likely than DC to cause arrhythmia and ac causes sweat release causing lowering of skin resistance increasing tissue current!
What factors may influence effect of an electric shock on patient
Ac or dc source
Magnitude of current
Tissue current passes through
Current density
Duration of current passage
Pre existing disease
What frequency of ac current is less likely to cause arrhythmias? Application
> 1mHz
Diathermy has frequency >1kHz thus very unlikely to cause arrhythmia
How many times more risky is an ac shock than a dc shock to a patient at the same current
About 3x
What are the varying effects of an ac shock based on current magnitude
0-5mA tingling
5-10 pain
10-50 severe pain and muscle spasm
50-100 respiratory spasm, vf, myocardial failure
>100 disrupted epithelium and cell membranes
What factors determine magnitude of a current during electrocution
Voltage applied
Impedance (AC resistance of skin) eg skin thickness, sweating
Impedance of earth connection
Tissue impedance
Rough values of skin impedance and tissue impedance
Resistive shoes
Skin 1000-200,000 ohms
Tissue <500 ohms
Resistive shoes 200,000 ohms
Current experienced by a ac 240v shock if:
Skin resistance 2000 ohms, tissue 300 ohms and shoes 200,000 ohms
What if not wearing shoes
Total resistance 202.3kOhms thus I = 240/202300 = 1.2mA (tingling)
Without shoes 4300Ohms thus I = 240/4300 = 55.8mA = (possible VF)
What is current density relating to electrocution
Example in clinical medicine
Total current flow / cross sectional area the current flows through
Diathermy tip has very high current density, rapidly spreads out and much lower current density over diathermy pad - thus tissue burnt at probe but not pad despite actual current being the same
How can electricity burn
Direct heating effect
Chemical burn from an electrolytic effect
How can macroshocks be prevented
Suitable equipment to specification
Earth circuites
Isolated patient circuits
Isolating transformer
Circuit breaker
Suitable footwear
How can electrial equipment be classified for safety specifications
Class 1 earthed
Class 2 double insulated not earthed
Class 3 low voltage battery powered
What are earth circuits in relation to macro shock s
Keep exposed metal work at 0 potential thus such metalwork cannot deliver an electric shock
Disadvantages of earthed circuits
Can act as a conduit if touched simultaneously with touching a voltage source
Can produce leakage currents acting as a source of micro shocks (especially if multiple earths are used at slightly different potentials)
What are leakage currents
Small electric currents (<500microamps)
Produced unintentionally eg by fault
Leak down earths or may cause micro shocks
What is a coaxial cable
Earthed outer conductor surrounds an inner wire
What is an isolated patient circuit
No earth connection to the patient
What is an isolating transformer
All equipment attached to patient supplied via transformer so no equipment touching patient directly connected to mains
What is a circuit breaker
Sensitive switch that disconnects when abnormal currents detected
What must footwear do to be electrically safe
Impedance high enough to stop patient earthing
Allow leakage currents to stop static build up
What is a micro shock
Current delivered internally to the myocardium causing arrhythmias
How might a patient connected to two earths generate a micro shock
Eg. One earth connected to an ecg, the other to a pulmonary artery catheter
If there is a slight difference of potential between them then a current may be generated causing a micro shock.
How can microshocks be prevented
Appropriate equipment
Suitable footwear
Anti static flooring
Isolated patient circuit (no earth to patient)
Optimally designed earthing (multiple equipment to a single earth)
Correct humidity in theatre
What are the classifications of equipment to prevent microshocks
Cardiac floating - leakage of <50microamps
Body floating - leakage of <500microamps
How does diathermy work
Uses high frequency currents to generate heat, focused dense current at site of diathermy and low density at pad
Risks of diathermy
Unwanted burning
Electric shock
Interference with monitoring
Interference with pacemakers
How can diathermy hazards be prevented
Use a isolated patient circuit
Use an isolating capacitor (short circuits the high frequency current to earth preventing unwanted burns, but keeps patient isolated from low frequencies reducing risk of microshocks)
Ensure diathermy pad properly applied
Avoid inadvertent patient contact with earthed metalwork
Use of bipolar diathermy (restricts current to small area)
How can electrical burns occur
Flash burns - high voltage arcing from body to earth
External burns - ignition of surroundings (eg clothing or vapours)
Tissue burns - burn at point of contact with high voltage
What elements are needed for an fire? Examples from an operating theatre
Inflammable agent - eg alcohol, cyclopropane, ethyl chloride
Oxidising gas - eg oxygen, air, nitrous oxide
A source or ignition (e.g. diathermy, sparks, lasers)
Inflammable concentrations - fire needs dense enough concentration of inflammable agent in the oxidising gas.
What is a stoichiometric concentration in respect to fire
When the ratio of the two above points are in proportion required for the chemical reaction may cause an explosion rather than a fire
How can fire and explosion be prevented in the operating theatre
Use of non flammable agents
Keep flamable agents at a radius away from likely sources of ignition
Use anti static clothing and footwear
Adaquate air changes
Scavenging systems
Awareness of major risks like laser and use of appropriate equipment
What do the symbols APG and AP mean on equipment
Anaesthetic proof (can be within 25cm of flammable anaesthetic agents)
Anaesthetic proof G (can be within 5cm of flammable anaesthetic agents)
What is a sound wave
A longitudinal compression wave which causes variation in position of the medium it is travelling in creating alternating regions of compression and rarefaction
What is the range of frequency of human hearing
What is the range of frequency of ultrasound scanners
Adult Hearing 15Hz - 20kHz
Child Hearing 15Hz - 40kHz
Ultrasound scanner 2-20MHz
What alters wavelength of sound
The medium it is travelling in
What is the significance of wavelength in ultrasound
Detectable reflection only occurs from objects with dimensions greater than the wavelength being used, thus more detail from shorter wavelength scans
Equation for sound velocity
Implication for ultrasound
Velocity = frequency x wavelength
As more definition with shorter wavelengths, these are produced by using a higher frequency (wavelength = velocity / frequency
Roughly what size of objects will be visible on ultrasound with a frequency of 10 MHz
1.5mm
Speed of sound in air and water
In the body?
Air 330m/s
Water 1500m/s
Variable in the body - wet stuff all around 1500m/s, bone 4000m/s, gas 330m/s
What determins velocity of sound in a substance
Density, pressure, temperature
What is acustic impedance
Z (acoustic impedance) = acustic pressure / particle velocity
Or
Z = density of material x speed of sound
Attenuation of sound wave as oscillations in medium diminish and dissipate
How do sound waves reflect
What sort of reflection is it
When they hit a boundary between two media with different acoustic impedances
Specular reflection - angle of incidence = angle of reflection
How is the amount of sound reflected at a boundary calculated
AI = acoustic impedance
Percentage reflection = (AI1-AI2) / (AI1+AI2) x100
Why is gel used in ultrasound scanning
To match acoustic impedance so all of the sound isn’t reflected back from the high contrast between probe and skin
What is the implication of acoustic impedance for ultrasound
Neighbouring Objects with greatest variation in acoustic impedance show up more brightly due to higher reflection
When such a large reflection occurs much of the sound is reflected thus this leaves a shadow behind such a transition
Which creates a greater shadows on ultrasound, soft tissue to bone or soft tissue to air interface
Soft tissue to air, the differences in impedances are much greater
What happens when ultrasound waves hit a object or surface irregularity that is smaller than their wavelength?
Scatter (non specular reflection
As well as specular reflection what occurs to ultrasound waves as they cross a boundary
How is this calculated
Effect
Refraction (deviation from original course)
Snells law:
Sini/Sinr = velocity in medium 1/velocity in medium
Can lead to focusing effect behind fluid filled objects
Why do ultrasound signals become attenuated
Absorption of energy by tissues (most of it, dependant on frequency)
Reflection of the beam
Scatter
Divergence of the beam (as not perfectly parallel)
How is ultrasound exposure measured
Peak acoustic intensity (mW/mm^2) averaged over time and space
Thus exposure depends on intensity, area and time exposed
How can ultrasound cause damage?
Heating - temp rise due to absorption of ultrasound energy, can cause damage especially to early fetus. Effects bone most due to higher acoustic impedance (more energy absorbed)
Cavitation - the production and excitation of gas bubbles in tissue exposed to ultrasound due to negative pressure production, more an issue with high intensity ultrasound eg in physiotherapy
Mechanical effect - object experiences a force from the USS on one side but less so on other (as attenuated) thus distorted and damaged
What is a clinical effect of ultrasound mediated mechanical damage
Common cause
Red cell agglomeration
Doppler USS
Safe duration of exposure of focused and unfocused ultrasound from a heating perspective
50 and 500s respectively
Risk factors for heat exposure ultrasound
Doppler mode
B mode scanning
How does ultrasound work
Emits pulses of sound wave (each 2-3 cycles) which are reflected.
Probe acts as both transducer (emmitor) and reciever
How is an ultrasound wave produced in a scanner
How is a recieved wave recorded
Piezoelectric effect
Application of voltage over certain crystalline materials causes oscillating change in width of crystal producing a compression wave
Also used in reception, compression of the crystal produces an electrical signal.
What materials can be used to produce piezoelectric effects
Ceramic lead zirconate titanate, pzt
Different modes of ultrasound
A, B, M, Doppler
What is A mode ultrasound
Detects tissue boundaries and distance (based on time for reflection) from the transducer
What is b mode ultrasound
Takes a mode into 2 dimensions producing a slice image on the screen
Portrays high amplitude reflections portrayed as whiter
What is m mode ultrasound
Used to detect movement of reflecting surfaces along a single scan line
Often used along with b mode (used to select the best single line for m mode)
Plots distance from transceiver of the boundary of interest on the y axis against time on the x
Clinically used in cardiac ultrasound
What is Doppler mode ultrasound?
Can detect velocity of moving objects portraying it as colour scale on top of m mode scan
What colours does Doppler cause to objects moving towards and away from it
Towards probe - red
Away from probe - blue
Fucking stupid as shorter wavelengths are blue and longer red!
Why does the Doppler effect occur?
Reflected waves have different wavelength to incident because objects moving in relation to probe
Velocity of wave in the medium is constant thus a moving target causes a wavelength and frequency shift
Thickness of typical field used in b mode USS
1mm
Relationship between frequency and depth of penetration in bmode ultrasound
Generally increasing frequency improves resolution but lowers depth
Where does a fast scan assess for blood
Pelvic
Perihepatic
Perisplenic
Pericardial
Why does TOE produce better images than TTE
Lower attenuation so higher frequency can be used identifying smaller objects with less scatter
How does a MRI work
Magnet applied
Aligns all dipoles
Radiofrequecy pulse applied - usually set specifically to hydrogen non
Causes hydrogen nucli to acquire wobble (precession) to original spin, giving a larmor frequency
Each nuclei emits electromagnetic radiation which is received by the MR scanner and converted into an image
How can the image of an MRI scan show differences between tissues
Different tissues release different radiation when relaxing
Different radio frequency pulses can enhance differences between tissues
What are T1 and 2 constants in MRI
T2 - relaxation of transverse component with the decay of the precessional motion to 0
T1 - relaxation of the longitudinal component with restoration of longitudinal magnetic field
What are t1 and t2 weighted mri. Images
T1 - tissues with long t1 time appear dark (eg csf) and short t1 time light (fat)
Usually shows good contrast between boundaries, fluids dark and fats bright, contrasts grey and white matter well
t2 - tissues with long t2 time appear bright (eg csf) and short t2 time dark (eg muscle)
Fluids appear bright and other tissues varying greys, shows oedema well (appearing brighter) and abnormal collections of fluids
What are the different rf pulse sequences that can be used in mri
Spin echo - best quality images but take longer
Gradient echo - faster
What sort of magnets can be used in an mri scanner
Permanent - low strength for weight and non uniform, no power needed
Resistive - good weight to power ratio but uses loads of heat
Superconducting - good weight to power ratio
What is magnetic field strength measured in
Tesla and gauss
1 Tesla is 10000 gauss
What is the earths magnetic field
What about mri scanner
50 micro Tesla (0.5 gauss)
Mri scanner 0.2 to 2 Tesla
What coils are in mri scanners and what do they do
Gradient coils - superimpose magnetic graidents on field gi bing a spatial framework so scanner can pick out a slice
Transmitter coils - transmit the rf pulse
Reciever coils - receive the larmour waves
Shield coils - reduce fringe magnetic filed due to opposing field produciton
Biohazards from MRI
Displacement of metal
Rf exposure can cause heating of tissues and conductors (eg metalwork or monitoring)
Peripheral nerve stimulation with discomfort
Sensory discomfort including nausea and vertigo
Light types from short to long wavelengths
Gamma
X-ray
Uv
Visible
IR
Microwave
Radio wave
What produces the energy of light
The transverse vibration of the photons emitted from the light source
Which direction do particles move in compared to the direction of the travel of light
At 90o (like particles of water moving up and down whilst the wave travels forward towards the beach)
What are the wavelengths of visible light
Which colour at which end?
400-700nm
Blue at 400, red at 700
How is speed of light altered by medium it is travelling through
Slows with density
Speed of light in a vacuum
299,792,458 ms
What is the consequence of the change in speed of light between different media
When light crosses into a different media speed changes and it is bent (refracted)
When passing from less dense to dense it bends towards normal
When passing form dense to less dense it bends away from normal
What law governs refraction of light
Snells law
Sini/sinr=c1/c2
Sin of angle of incidence over angle of refraction = spend of light in media 1 over 2
What is the refractive index of a medium
Speed of light in vacuum/speed of light in medium
When does snells law stop applying
Why
When moving from a dense to less dense medium And When the angle of incidence is greater than the critical angle
Because when moving from dense to less dense angle of refraction is greater than angle of incidence but it cannot become greater than 90o
What are unpolarsied and polarised light
Unpolarised - photon vibration at 90o to direction of travel of light ray, but at random different directions within this plane
Polarised - particles only oscillating in a single direction
How can optically active compounds be assessed
Place between a vertical and horizontal polarisor. Ordinerally no light can pass through both, but if opitically active compound between it rotates the light allowing it to pass
Examples of levorotatory substances
L bupivicaine
L atracurium
Adrenalin
Nor adrenaline
Example of dextrorotatory substances
D tubocurarine
What happens to light energy when it passes through a substance? What lores governs this?
Some is absorbed
Lambert-Bouguer law:
Transmitted light = incident light x e^-absorbance (product of thickness and quantity)
Beers law
Solution absorbance is a linear function of molar concentration
Combined Lambert Beer law
Absorbance = molar extinction coefficient.molar concentration.thickness
How can light be used to measure concentration of a compound in a mixture
Transilluminate with monochromatic light at wavelength where absorbance is maximal then use lambert beer law
What does laser stand for
Light amplification by stimulated emission of radiation
Characteristics of laser light
Monochromatic
In phase
Doesn’t diverge - remains a narrow beam
High light intensity can be produced with low power source
What is the physics behind the stimulated emission part of laser
When a high energy atom is struck by a photon it looses energy by giving out two light particles with the same frequency - these can either strike other atoms or be projected as light
Parts of a laser
Source of energy to raise electrons to excites state
A laser substance capable of stimulated emission
System of mirrors to repeatedly reflect light back and forward to amplify it
Characteristics of laser mirrors
One is partially transparent to allow light to escape in a highly parallel coherent beam which is then focussed
Laser safety classification
Where do surgical lasers fall
Level 1-4 (least to most dangerous) based on wavelength and power
Surgical lasers are class 4 - they are specifically designed to damage tissue
Safety precautions for laser use
Training for all staff
Matt surfaces in operating theatre and on instruments
No inflammable material in vicinity
Eye protection for staff and patients
Well ventilated and smoke extraction
Blinds
Water
Types of laser material and uses
Ruby - early use in eye surgery
Argon - eye surgery, removal of birthmarks
CO2 - most surgery, superficial, removal of this layers of tissue
Neodymium yttrium aluminium garnet - coagulation and cutting, endoscopic
What makes argon lasers suitable for eye surgery
Pass through water and aqueous humour but absorbed by haemoglobin and pigment
What makes CO2 lasers suitable for removing thin layers (and thus most surgery)
Well absorbed by water so low penetration (<200mm)
What makes neodymium yttrium aluminium garnet lasers good for coagulation and cutting
Not absorbed by water thus good tissue penetration
Which lasers can be used endoscopically
Argon and neodymium etc.
not co2
Wavelength of X-ray
10^-10 to 10^-13
How are X-rays generated
Bombardign a high temp anode in a high voltage electric field with electrons generated at cathode - these bombard a tungsten target on the anode and scatter of - directed by use of an aperture
What are atomic mass number and atomic number, which determines type of element
Mass number = protons + neutrons
Atomic number = protons
Types of element determined by atomic number (eg carbon has atomic number 6, but can have mass number 10,12 or 14 depending on number of neutrons)
Product of uranium decay
Lead
Types of radioactive decay
What are they
Alpha particle - 2 protons and 2 neutrons (a helium nucleus)
Beta particle - an electron created by splitting of a neutron into a proton and an electron
Gamma radiation - electomagentic radiation with wavelength less than 10^-12m
Which form of radiation emission will cause a change in the type of element?
Beta radiation as a neutron changes to a proton thus atomic number changes
What is the half life of a radioactive element
The time taken for the mass of the element to decline by half
What is the relationship between t1/2 and time constant for radioactive decay
T1/2 = 0.693K
Where K is time constant
Units of radioactivity
Becquerel (Bq) - 1 disintegration per second
What is the SI unit of absorbed radiation dose?
Grey
1 gray = dose of radiation giving an absorbed energy of 1J/kg soft tissue
Why is the grey perhaps not the ideal measure for radiation exposure? What can be used instead
Damage done by ionising radiation doesn’t just vary with energy transferred but the type of energy e.g. a gamma or beta particle will do much more damage than an X-ray of equivalent grays.
Instead use sievert. (sv) - the radiation dose equivalent to 1 Gy of absorbed ration from a 200kV X-ray in terms of biological damage
Maximum permitted dose of radiation per year
5mSv
Background dose of radiation per year
1.25 mSV
Dose of radiation that would cause acute nausea
Dose that would kill in a few days
1sV over a few hours
10sV over a few hours
