Section 5: Medical Physics Flashcards
Positive vs negative torque
Positive: aims to produce a counterclockwise rotation
Negative: aims to produce a clockwise rotation
Turning effect of a force on a lever depends on…
Magnitude of F applied
Position relative to axis of rotation at which F is applied
Angle at which F is being applied relative to lever
Conditions for equilibrium
Net external F on system is zero
There is a balance in torque or rotational effect of forces
Rigid body - conditions for equilibrium
Zero translational acceleration
Zero angular acceleration
Centre of gravity - adult vs young child
Adult: around hips - lower COM –> good stability
Young child: near shoulders - high COM –> poor stability
Increasing stability
Standing with legs apart - increases size of base and lowers centre of gravity
Walking frames - increase size of base
Deformation
A change in shape caused by applied forces
Elasticity
Ability of materials to deform under load in a recoverable way
Plasticity
Permanent deformations caused by applied loads
Stress
Force per unit area
Like pressure, but can be diff in diff directions
Strain
A measure of the change in shape
Dimensionless
Compressive strain = -ve
Types of stress
Tensile stress = +ve
Compressive stress = -ve
Pressure vs stress - state
Pressure often used with fluids, gases
Stress often used with solids
Pressure vs stress - direction
Pressure acts perpendicular to surface
Stress can be perpendicular or parallel to surface
Shear stress
Stress parallel to a surface
Young’s modulus
E
AKA modulus of elasticity
Bigger E = stiff
Smaller E = compliant
Constitutive law
The stress-strain relationship for soft tissues
Non-linear
Tendons are composed of…
Collagen and elastin, which combine to give an apparent increase in stiffness with increasing strain
Collagen
For small strains, doesn’t contribute resistance as strings straighten
At larger strains, collagen is straight and resists extension –> increases apparent stiffness
Sound
A longitudinal pressure wave
Purpose of ear
To convert small changes in pressure of sound (compressions and rarefactions) into electrical signals that are interpreted by the brain
A single-frequency periodic waveform can be described by its…
Frequency
Amplitude
Phase
High-frequency vs low-frequency sound
High f: High no of cycles per second –> high-pitched
Low f: Low no of cycles per second –> low-pitched
High vs low intensity sound
High intensity: Large A
Low intensity: Low A
Fourier’s theorem
States that any complex signal can be synthesised by the addition of single-frequency waves of various amplitudes and phases
Similarly, any complex periodic waveform can be decomposed into its frequency components
Sawtooth wave
Many frequency components added tgt
Gives rise to a sharp wave
Human ear - main components
Outer, middle, inner ear
Outer ear - purpose
Collects sound and directs it into ear canal
Middle ear - purpose
Transfers sound from eardrum (tympanic membrane) into oval window via a lever system
Inner ear - purpose
Sorts sound into its frequency components and sends info to brain for processing
Outer ear - pinna
Outermost part of ear
Collects sound and directs them into ear canal
Middle ear - ossicles
3 tiny bones that are elastically collected tgt:
Hammer (malleus)
Anvil (incus)
Stirrup (stapes)
Middle ear - sound is _____
Amplified
Inner ear - cochlea
Snail-shaped structure
Contains tiny hair cells that convert vibration into electrical potentials
Inner ear - high vs low frequencies
High: cross over near windows
Low: cross over at far end
Intensity level - logarithmic scale
Small changes in intensity level can have significant consequences in noise dose
Loudness
The perception of intensity
What is loudness of a sound dependent on
Frequency of sound
Loudness - unit
Phon
Fletcher-Munson curves
Shows lines of equal loudness in phones
Threshold of hearing
The min intensity the human ear can detect at a given frequency
0 phones
Threshold of pain
120 phon
Sound below 0 phon line
Any sound represented by a point below the 0 phon line won’t be heard by a ‘normal’ ear
Hearing loss is often ______ specific
Frequency
What can hearing loss be caused by
Excessive exposure to noise Illness An accident An inherited disorder Age
Presbycusis
Progressive hearing loss due to age - particularly in high-frequency range
Types of hearing loss
Conductive HL
Sensorineural HL
3 dB exchange rate
For every increase in 3 dB of intensity level, the max allowable duration of exposure is halved
Factor daily dose (FDD)
The amount of noise dose a person receives relative to the max allowable level
Audiometer
Tests a person’s hearing
Generates sound signals of various frequencies at diff intensities so the threshold of hearing can be detected
Conductive hearing loss
When the passage of sound is blocked either in the ear canal or in the middle ear
Sensorineural hearing loss
Due to a disorder in the cochlea or in the auditory nerve
Ideal flow - conditions
Incompressible fluids
Non-viscous flow
Steady flow
Streamline flow
Viscosity
Internal resistance of liquid
Viscosity - real fluid
Liquid at surface of pipe is stationary and v is max at centre
Velocity profile is parabolic
Referred to as Poiseuille flow
Coefficient of viscosity (η)
Very runny fluid = low viscosity
Sticky fluid = high viscosity
Poiseuille’s law
Takes into account viscosity
Only valid if flow remains laminar
Reynolds number (Re)
Dimensionless
Gives indication of likely state of flow (laminar or turbulent)
Pathway of light to eye
Cornea Aqueous humour Lens Vitreous humour Retina
Where does most bending of light / refraction occur in the eye
Cornea
Due to curved surface and big difference of index of refraction
Where are images formed in the eye
Retina
Retina
Consists of photoreceptors which convert images into neural signals –> sent to brain via optic nerve
Optical power (P) - units
Diopters (D)
Eye: Lens
Focal length of lens (and thus power of lens) must be able to change / accommodate for objects to be seen clearly at a range of distances
Normal vision
Near point = 25cm
Far point = infinity
Range of vision = 25cm to infinity
Near point
The distance to the closest point a person can see clearly
Far point
The distance to the most distant point a person can see clearly
Range of vision
Near point to far point
Lens of curvature - close vs near objects
For close objects: lens must increase in curvature
For distant objects: lens must flatten
Normal eye relaxes when viewing distant objects
Near-sightedness (myopia) - caused by?
Lens being too strong or eye being too long
Near-sightedness (myopia) - corrected by?
Diverging corrective lens - reduces effective power of a lens that is too powerful to achieve clear vision
Creates a close image at far point of eye –> clearly visible
Far-sightedness (hyperopia) - caused by
Lens being too weak (flat)
Eye being too short
From a deterioration in ability of eye to accommodate- often due to age
Far-sightedness (hyperopia) - corrected by
Converging corrective lens - supplies additional power to eye system
Creates a distant image at near point of eye –> clearly visible
Glasses and contact lenses - assumptions
Assume glasses sit 2cm from front of eye
Assume contact lenses sit directly on eye
Medical imaging - non-invasive
Allows medical professionals to visualise internal anatomy without the need for invasive surgery
How are X-rays produced
Using an x-ray tube
Heated filament produces e- which goes to the metal target, which produces X-rays
All of this is contained with a vacuum / evacuated chamber
Energy of x-ray photons is usually quoted in…
eV
2 processes that x-rays are produced
Characteristic x-rays
Bremsstrahlung
X-rays: Characteristic x-rays
If e- are travelling with sufficient E, they can knock e- out of inner shells of atoms in the target –> vacancy is filled by an e- from outer shells of atom
As the e- moves to a lower-energy shell, they emit the excess energy as x-rays
X-rays: Bremsstrahlung
Incident e- slow down when they hit the target and lose some KE as they’re decelerated
Radiation resulting from deceleration is known as bremsstrahlung
Distribution of these energies is continuous
X-ray spectrum
The distribution of the x-ray intensity per unit wavelength vs photon wavelength
X-rays: Attenuation
The process that causes the no of photons to decrease as an x-ray beam passes through the body
X-rays: What does the degree of attenuation depend on
Composition of tissue - more dense and higher atomic no = higher amount of attenuation = bright, e.g. bone
X-rays: Dark vs bright
If more x-rays pass through patient and hit detector = dark
X-rays: What is the image that is produced called
Projection image
X-rays: Trade-off
Increased image size = decreased sharpness
X-rays: SID vs SOD
SID = source-to-image distance SOD = source-to-object distance
X-rays: Where is the image projected
X-ray detector
Computed tomography (CT)
X-ray tube and detector rotate around the patient at high speeds to quickly obtain many projection images
