Download Medical Physics - Revision World

Medical Physics
Contents


Physics of the Eye and Ear
Biological Measurement and Imaging
The Eye



Ciliary muscles relax (lens
flattens)  distant objects
Ciliary muscles contract
(lens gets fat)  close up
objects
Refraction at cornea
produces inverted image
on retina
The Eye

Dim light: iris dilates to allow light in
- rods operative (see in black & white)

Bright light: iris contract to avoid light flooding
- cones are sensitive to either red, green & blue light

Colour seen depends on proportion in which each type of cone
is stimulated
The Eye
Nerve ends respond to changes in light intensity
 Eye is constantly scanning so that new nerves are stimulated
 Persistence of vision: limited response of eye due to delay times
in between nerve ending responses


If the eye is focused on a near object in bright light, distant
objects are visible reasonably clearly. If the eye focuses on the
same object at the same distance in dim light, distant objects
are blurred. The depth of field is reduced
Defects of Vision & Their Correction

Convex Lens: light rays converge
Power (Dioptres, D) = 1 / focal length (m)
Defects of Vision & Their Correction

Concave Lens: light rays diverge
Power (Dioptres, D) = 1 / focal length (m)
focal length is negative as focus is virtual (virtual image)
Defects of Vision & Their Correction

Lens Formula:
1/f = 1/v + 1/u
v = image distance, u = object distance
M = v/u
M = magnification (no units)

Myopia (short sightedness): cornea too curved or lens too
powerful  distant object images form in front of retina and
blur

Hypermetropia (long sightedness): eye not powerful enough 
near object images form behind retina and blur
Physics of the Ear
Sound: longitudinal wave
Acoustic impedance: density multiplied by the speed of sound
in the material
- low  conducts sound well
- high  insulates sound well
 Intensity: power per unit area (decreases with distance from
source)
Pinna: funnels sound waves into ear canal
Canal: increases sound intensity
Tympanic membrane vibrates
Ossicles (three small bones) vibrate
Strike oval window of the cochlea
Nerve cells detect sound & convert
it into electrical impulses for
processing by the brain


Physics of the Ear



Place Theory: brain determines pitch by noting the place on
the basilar membrane where the message is strongest
Frequency Theory: frequency of vibrations of the basilar
membrane as a whole is translated into an equivalent
frequency of nerve impulses
Neurons, however, cannot fire as rapidly as the frequency of
the highest-pitched sound  volley principle: nerve cells fire in
sequence to send a rapid series of impulses to the brain
Intensity is measured on the
decibel scale:
Physics of the Ear

Hearing Loss:
- mechanical damage due to a blow on the head
- disease (stop ossicles from moving)
- exposure to excessive noise (tinnitus)
- ageing
Nerve Impulses







Electrical Signals in the Body: carried by neurones
- cells have membrane potential: extra K+ ions inside, Na+ ions
outside  potential difference = 70mV
When membrane is stimulated:
- becomes permeable to Na+ions which
diffuse due to the negative charge
Potential rises initially to 0 mV (depolarisation)
& then to +30 mV (reverse polarisation)
Membrane becomes impermeable to Na+
ions & they are trapped within nerve cell
K+ions diffuse out of the membrane which restores the potential
(repolarisation)
Process takes about 2 ms
Then the K+ions are pumped out, process takes about 50 ms
Nerve Impulses

Pumps materials as a fluid quickly around body

Double circulatory system:
Nervous Impulses
Deoxygenated
blood enters
through the
vena cava into
the right atrium
Oxygenated
blood enters
through the
pulmonary veins
into the left
atrium
It’s then
pumped through
a valve into the
right ventricle
chamber
It’s then pumped
through a valve
into the left
ventricle
And then up through the pulmonary
valve into the pulmonary artery
towards the lungs
And then through the aortic
valve and out of the aorta to
the rest of the body
Nerve Impulses
Arteries carry blood away from
the heart at high pressure in
thick walled lumen
Capillaries have thin walls to
allow glucose and oxygen to
diffuse through
Veins carry low pressure blood back to the heart.
Veins have thinner walls and valves to prevent
backflow of blood
Nerve Impulses
Nerve Impulses

Carry blood at high pressure

Outermost layer is composed of connective tissue
Media – smooth muscle cells and elastic tissue
Intima – in direct contact with flow of blood
Lumen – cavity in which blood flows



Nerve Impulses






Most exchange of nutrients and gases takes place here
Small diameter, large surface area for diffusion
Lungs  carbon dioxide is exchanged for oxygen
Tissues  O2, CO2, nutrients and wastes are exchanged
Kidneys  wastes are released to be eliminated from body
Intestine  nutrients are picked up, wastes are released
Nerve Impulses

Electrocardiogram (ECG): allows doctors to look at the electrical
behaviour of the heart to diagnose problems
- Body fluids transmit some of the electrical activity to the
surface
- Signals are reduced in size, with amplitudes of about 1mV
- Suitable ECG output: patient must be relaxed so nerve cell
activity does not disrupt data of electrical activity of heart
P wave: depolarisation & contraction
of the atria
QRS wave: depolarisation &
contraction of the ventricles
T wave: re-polarisation and
relaxation of the ventricles
Ultrasound

Ultrasound: sounds above the audible frequency range for
humans

Uses: non-invasive imaging, used to detect distances, depths
and for medical purposes

Generation & Detection: ultrasound probe (transducer)
generates & detects ultrasound waves, usually by use of a
piezoelectric transducer. A voltage is induced when a quartz
crystal is stretched or compressed, which can be large enough
to create a spark. If a voltage is applied to a piezoelectric
material, it changes shape. For an alternating voltage, the
crystal will vibrate. Maximum energy transfer occurs when the
crystal is in resonance
Ultrasound








Ultrasound is often used to detect functions inside the body:
Ultrasound is:
longitudinal
passed through a gel on the skin to prevent reflection
reflected as it passes from one tissue to another (velocity
change)
absorbed by tissue material
absorbed & reflected by many percentages depending on the
tissue type
This determines an overall image when the wave is connected
to a receiver
Ultrasound

Resolution: distinction of image using ultrasound e.g. what
distance in the body does a pixel on the monitor show?

higher frequency = better resolution


Axial: use short pulses; determined resolution in direction of
beam
Lateral: use narrow pulses; determined by beam width

A-scan: amplitude modulated display

B-scan: brightness modulated display
Ultrasound
Real time B-scan: moving images. Up to 100 probes are used
 Movement of blood: through Doppler effect

Advantages:
 low frequency (low energy) beam
 non-invasive & no discomfort
 more effective than X-ray for images of soft tissue
 portable equipment
Disadvantages:
 skilful operator & image requires skilful interpretation
 image resolution is very easily reduced
Medical Optics
Optical fibre: glass rod that conducts light by TIR
 Light travels down a core surrounded by glass cladding: slightly
lower refractive index (n)  prevents loss of light energy if the
core passes into material of a higher refractive index



Snell’s Law:
Critical Angle:
n1sinθ1 = n2sinθ2
sinθc = n2/n1
Medical Optics

If the fibre is bent sharply  significant loss of light
- radius of bend must be 20x diameter to prevent such losses
sheath
optical fibre
fibre optic light guide



Fibre diameter: 10μm
Endoscope of 3mm diameter would have approx. 40 000 fibres
The more fibres, the greater the resolution
Medical Physics
Uses of Endoscopy:
 cut out diseased tissue
 take a biopsy
 seal a site of bleeding with heat
 remove an obstructive object
 keyhole surgery

Medical Physics

LASER: light amplification by stimulated emission of radiation
- When a photon of the right wavelength hits an excited atom
of certain materials, it can stimulate the emission of a second
photon of exactly the same wavelength and phase as the first
- If enough atoms are excited, the photons can stimulate
further emissions of further photons, all travelling in the same
direction.
- At one end of the material there is a
mirror that totally reflects the photons,
while at the other is a mirror that partially
reflects the photons
Medical Physics

Properties of lasers:
- monochromatic (single colour)
- coherent (in phase)
- produce continuous light (pulses are caused by shutters)
- absorbed by skin: increased by melanin
- used in fibre optics to help guide light beam
- CO2 lasers  cut away delicate tissue e.g. brain

Safety issues:
- severe & deep burns caused by beams
- beam shined into eye will cause blindness
X-Rays

Uses
- identify bone fractures
- identify tooth decay
- identify tumours & disease in soft tissue
- treatment of tumours by radiotherapy

X-rays: photons of em radiation produced when a target of
heavy metal is struck by electrons travelling at high speed.
Approx. 1% of the electrons produce an X-ray photon  the
rest is lost in heating up the target
Production of X-rays:
- decrease velocity of electron or
- remove an inner electron. Electrons replace the inner electron
& photons are emitted as the electrons undergo transitions from
different energy levels

X-Rays

Maximum energy: all electron’s energy is converted into the
photon’s energy
- Kinetic energy = photon energy
- Kinetic energy = charge of electron × voltage
eV = hf
and…
c = fλ
so…
eV = hc/λ
h = Planck’s constant
X-Rays

Generation of X-rays:
- rotating anode tube  evacuated glass tube, cooled in oil with
a lead surround
- Hot filament boils off electrons which are accelerated by anode
voltage
- hit target; most are released as heat, but few become X-ray
- to prevent melting, target spins on a motor at 3000rpm
X-Rays

Controlling the X-rays:
- cannot be focused
- sharp images are produced by small sources (point source)

Absorption of X-rays: occurs when x-rays pass through materials
due to energy loss by…
- Scattering: X-ray photons are reradiated as lower energy
photons
- Ejection: X-rays are ejected (photoelectric effect). Photons of
visible light are emitted as atom comes out of the excited state
- Compton Scattering: electron & lower energy X-ray photon are
emitted
- Pair Production: V. high energy photon interacts with atom’s
nucleus.
Electron & positron emerge, losing energy by ionisation until the
positron is annihilated by an electron  generates 2 identical
photons
X-Rays

Dangers of X-rays:
- water ionises to produce free radicals which produce H2O2
- enzymes & DNA are damaged
- parts of cells are damaged
- cell division is damaged ( mutations)
- tissue & organ damage
- life expectancy shortens
- mutations cause gene alterations in populations

Attenuation of X-rays:
- inverse square law: intensity reduces
with distance
I = I0e-μx
X-Rays

Uses of X-rays:
- diagnosis
- treatment of cancers (radiotherapy) with high energy X-rays

X-ray workers…
1) wear a film badge to check the amount of radiation they get
2) wear lead aprons while the machine is in use
3) verify that the machine is in an enclosed room and the
controls are in a separate room
4) ensure that that there is no entry into the X-ray room while
the machine is in use
Summary
The Eye
 Defects of Vision & Their Correction
 Physics of the Ear
 Nervous Impulses
 Ultrasound
 Medical Physics
 X-Rays
