Medical PhysicsMedical Physics

ContentsContents

Physics of the Eye and EarPhysics of the Eye and Ear Biological Measurement and ImagingBiological Measurement and Imaging

The EyeThe Eye Ciliary muscles relax Ciliary muscles relax

(lens flattens) (lens flattens) distant distant objectsobjects

Ciliary muscles contract Ciliary muscles contract (lens gets fat) (lens gets fat) close close up objectsup objects

Refraction at cornea Refraction at cornea produces inverted produces inverted image on retinaimage on retina

The EyeThe Eye

Dim light: iris dilates to allow light inDim light: iris dilates to allow light in

- rods operative (see in black & white)- rods operative (see in black & white)

Bright light: iris contract to avoid light floodingBright light: iris contract to avoid light flooding

- cones are sensitive to either red, green & blue light- cones are sensitive to either red, green & blue light

Colour seen depends on proportion in which each type Colour seen depends on proportion in which each type of cone is stimulated of cone is stimulated

The EyeThe Eye

Nerve ends respond to changes in light intensityNerve ends respond to changes in light intensity Eye is constantly scanning so that new nerves are Eye is constantly scanning so that new nerves are

stimulatedstimulated Persistence of vision: limited response of eye due to Persistence of vision: limited response of eye due to

delay times in between nerve ending responsesdelay times in between nerve ending responses

If the eye is focused on a near object in bright light, If the eye is focused on a near object in bright light, distant objects are visible reasonably clearly. If the eye distant objects are visible reasonably clearly. If the eye focuses on the same object at the same distance in dim focuses on the same object at the same distance in dim light, distant objects are blurred. The light, distant objects are blurred. The depth of fielddepth of field is is reducedreduced

Defects of Vision & Their Defects of Vision & Their CorrectionCorrection

Convex LensConvex Lens: light rays converge: light rays converge

Power (Dioptres, D) = 1 / focal length (m)Power (Dioptres, D) = 1 / focal length (m)

Defects of Vision & Their Defects of Vision & Their CorrectionCorrection

Concave LensConcave Lens: light rays diverge: light rays diverge

Power (Dioptres, D) = 1 / focal length (m)Power (Dioptres, D) = 1 / focal length (m)

focal length is focal length is negativenegative as focus is virtual (virtual as focus is virtual (virtual image)image)

Defects of Vision & Their Defects of Vision & Their CorrectionCorrection

Lens FormulaLens Formula::

1/f = 1/v + 1/u1/f = 1/v + 1/u v = image distance, u = object distancev = image distance, u = object distance

M = v/uM = v/u M = magnification (no units)M = magnification (no units)

Myopia (short sightedness)Myopia (short sightedness): cornea too curved or lens too : cornea too curved or lens too powerful powerful distant object images form in front of retina distant object images form in front of retina and blurand blur

Hypermetropia (long sightedness)Hypermetropia (long sightedness): eye not powerful : eye not powerful enough enough near object images form behind retina and blur near object images form behind retina and blur

Physics of the EarPhysics of the Ear Sound: longitudinal waveSound: longitudinal wave Acoustic impedance: density multiplied by the speed Acoustic impedance: density multiplied by the speed

of sound in the materialof sound in the material- low - low conducts sound well conducts sound well- high - high insulates sound well insulates sound well

Intensity: power per unit area (decreases with Intensity: power per unit area (decreases with distance from source)distance from source)

PinnaPinna: funnels sound waves into ear canal: funnels sound waves into ear canalCanalCanal: increases sound intensity : increases sound intensity Tympanic membraneTympanic membrane vibrates vibratesOssiclesOssicles (three small bones) vibrate (three small bones) vibrateStrike oval window of the Strike oval window of the cochleacochleaNerve cellsNerve cells detect sound & convert detect sound & convertit into electrical impulses forit into electrical impulses forprocessing by the processing by the brainbrain

Physics of the EarPhysics of the Ear Place TheoryPlace Theory: brain determines pitch by noting the : brain determines pitch by noting the

place on the basilar membrane where the message is place on the basilar membrane where the message is strongeststrongest

Frequency TheoryFrequency Theory: frequency of vibrations of the : frequency of vibrations of the basilar membrane as a whole is translated into an basilar membrane as a whole is translated into an equivalent frequency of nerve impulsesequivalent frequency of nerve impulses

Neurons, however, cannot fire as rapidly as the Neurons, however, cannot fire as rapidly as the frequency of the highest-pitched sound frequency of the highest-pitched sound volley volley principle: nerve cells fire in sequence to send a rapid principle: nerve cells fire in sequence to send a rapid series of impulses to the brainseries of impulses to the brainIntensity is measured on the

decibel scale:

Physics of the EarPhysics of the Ear

Hearing LossHearing Loss::

- mechanical damage due to a blow on the head- mechanical damage due to a blow on the head

- disease (stop ossicles from moving)- disease (stop ossicles from moving)

- exposure to excessive noise (tinnitus)- exposure to excessive noise (tinnitus)

- ageing- ageing

Nerve ImpulsesNerve Impulses Electrical Signals in the BodyElectrical Signals in the Body: carried by neurones: carried by neurones

- cells have membrane potential: extra K- cells have membrane potential: extra K++ ions inside, Na ions inside, Na++ ions ions outside outside potential difference = 70mV potential difference = 70mV

When membrane is stimulated:When membrane is stimulated:

- becomes permeable to Na- becomes permeable to Na++ions whichions which

diffuse due to the negative chargediffuse due to the negative charge Potential rises initially to 0 mV (depolarisation)Potential rises initially to 0 mV (depolarisation)

& then to +30 mV (reverse polarisation)& then to +30 mV (reverse polarisation) Membrane becomes impermeable to NaMembrane becomes impermeable to Na++

ions & they are trapped within nerve cellions & they are trapped within nerve cell KK++ions diffuse out of the membrane which restores the potential ions diffuse out of the membrane which restores the potential

(repolarisation)(repolarisation) Process takes about 2 msProcess takes about 2 ms Then the KThen the K++ions are pumped out, process takes about 50 msions are pumped out, process takes about 50 ms

Nerve ImpulsesNerve Impulses

Pumps materials as a fluid quickly around bodyPumps materials as a fluid quickly around body

Double circulatory system:Double circulatory system:

Nervous ImpulsesNervous ImpulsesDeoxygenated blood enters through the vena cava into the right atrium

It’s then pumped through a valve into the right ventricle chamber

And then up through the pulmonary valve into the pulmonary artery towards the lungs

Oxygenated blood enters through the pulmonary

veins into the left atrium

It’s then pumped

through a valve into the left ventricle

And then through the aortic valve and out of the aorta to the rest of

the body

Nerve ImpulsesNerve ImpulsesArteries carry blood away

from the heart at high pressure in thick walled

lumen

Veins carry low pressure blood back to the heart. Veins have thinner walls and valves

to prevent backflow of blood

Capillaries have thin walls to allow glucose and oxygen to diffuse

through

Nerve ImpulsesNerve Impulses

Nerve ImpulsesNerve Impulses Carry blood at high pressureCarry blood at high pressure

Outermost layer is composed of connective tissueOutermost layer is composed of connective tissue Media – smooth muscle cells and elastic tissueMedia – smooth muscle cells and elastic tissue Intima – in direct contact with flow of bloodIntima – in direct contact with flow of blood Lumen – cavity in which blood flowsLumen – cavity in which blood flows

Nerve ImpulsesNerve Impulses Most exchange of nutrients and gases takes place hereMost exchange of nutrients and gases takes place here Small diameter, large surface area for diffusionSmall diameter, large surface area for diffusion

Lungs Lungs carbon dioxide is exchanged for oxygen carbon dioxide is exchanged for oxygen Tissues Tissues O O22, CO, CO22, nutrients and wastes are exchanged, nutrients and wastes are exchanged Kidneys Kidneys wastes are released to be eliminated from body wastes are released to be eliminated from body Intestine Intestine nutrients are picked up, wastes are released nutrients are picked up, wastes are released

Nerve ImpulsesNerve Impulses Electrocardiogram (ECG)Electrocardiogram (ECG): allows doctors to look at the : allows doctors to look at the

electrical behaviour of the heart to diagnose problemselectrical behaviour of the heart to diagnose problems

- Body fluids transmit some of the electrical activity to - Body fluids transmit some of the electrical activity to the surfacethe surface

- Signals are reduced in size, with amplitudes of about - Signals are reduced in size, with amplitudes of about 1mV1mV

- Suitable ECG output: patient must be relaxed so nerve - Suitable ECG output: patient must be relaxed so nerve cell activity does not disrupt data of electrical activity of cell activity does not disrupt data of electrical activity of heartheart P wave: depolarisation &

contraction of the atriaQRS wave: depolarisation & contraction of the ventriclesT wave: re-polarisation and relaxation of the ventricles

UltrasoundUltrasound UltrasoundUltrasound: sounds above the audible frequency range : sounds above the audible frequency range

for humansfor humans

UsesUses: non-invasive imaging, used to detect distances, : non-invasive imaging, used to detect distances, depths and for medical purposesdepths and for medical purposes

Generation & DetectionGeneration & Detection: ultrasound probe (transducer) : ultrasound probe (transducer) generates & detects ultrasound waves, usually by use of generates & detects ultrasound waves, usually by use of a piezoelectric transducer. A voltage is induced when a a piezoelectric transducer. A voltage is induced when a quartz crystal is stretched or compressed, which can be quartz crystal is stretched or compressed, which can be large enough to create a spark. If a voltage is applied to large enough to create a spark. If a voltage is applied to a piezoelectric material, it changes shape. For an a piezoelectric material, it changes shape. For an alternating voltage, the crystal will vibrate. Maximum alternating voltage, the crystal will vibrate. Maximum energy transfer occurs when the crystal is in resonanceenergy transfer occurs when the crystal is in resonance

UltrasoundUltrasound Ultrasound is often used to detect functions inside the Ultrasound is often used to detect functions inside the

body:body: Ultrasound is:Ultrasound is: longitudinallongitudinal passed through a gel on the skin to prevent reflectionpassed through a gel on the skin to prevent reflection reflected as it passes from one tissue to another reflected as it passes from one tissue to another

(velocity change)(velocity change) absorbed by tissue materialabsorbed by tissue material absorbed & reflected by many percentages depending absorbed & reflected by many percentages depending

on the tissue typeon the tissue type

This determines an overall image when the wave is This determines an overall image when the wave is connected to a receiverconnected to a receiver

UltrasoundUltrasound ResolutionResolution: distinction of image using ultrasound e.g. what : distinction of image using ultrasound e.g. what

distance in the body does a pixel on the monitor show?distance in the body does a pixel on the monitor show?

higher frequency = better resolutionhigher frequency = better resolution

AxialAxial: use short pulses; determined resolution in direction of : use short pulses; determined resolution in direction of beambeam

LateralLateral: use narrow pulses; determined by beam width: use narrow pulses; determined by beam width

A-scanA-scan: amplitude modulated display: amplitude modulated display

B-scanB-scan: brightness modulated display: brightness modulated display

UltrasoundUltrasound Real time B-scanReal time B-scan: moving images. Up to 100 probes : moving images. Up to 100 probes

are usedare used Movement of blood: through Doppler effectMovement of blood: through Doppler effect

AdvantagesAdvantages:: low frequency (low energy) beamlow frequency (low energy) beam non-invasive & no discomfortnon-invasive & no discomfort more effective than X-ray for images of soft tissuemore effective than X-ray for images of soft tissue portable equipmentportable equipment

DisadvantagesDisadvantages:: skilful operator & image requires skilful interpretationskilful operator & image requires skilful interpretation image resolution is very easily reducedimage resolution is very easily reduced

Medical OpticsMedical Optics Optical fibreOptical fibre: glass rod that conducts light by TIR: glass rod that conducts light by TIR Light travels down a core surrounded by glass cladding: Light travels down a core surrounded by glass cladding:

slightly lower refractive index (n) slightly lower refractive index (n) prevents loss of prevents loss of light energy if the core passes into material of a higher light energy if the core passes into material of a higher refractive indexrefractive index

Snell’s LawSnell’s Law::nn11sinsinθθ11 = n = n22sinsinθθ22

Critical AngleCritical Angle:: sinsinθθcc = n = n22/n/n11

Medical OpticsMedical Optics If the fibre is bent sharply If the fibre is bent sharply significant loss of light significant loss of light

- radius of bend must be 20x diameter to prevent such - radius of bend must be 20x diameter to prevent such losseslosses

Fibre diameter: 10Fibre diameter: 10μμmm Endoscope of 3mm diameter would have approx. 40 Endoscope of 3mm diameter would have approx. 40

000 fibres000 fibres The more fibres, the greater the resolutionThe more fibres, the greater the resolution

fibre optic light guide

sheath

optical fibre

Medical PhysicsMedical Physics Uses of EndoscopyUses of Endoscopy:: cut out diseased tissuecut out diseased tissue take a biopsytake a biopsy seal a site of bleeding with heatseal a site of bleeding with heat remove an obstructive objectremove an obstructive object keyhole surgerykeyhole surgery

Medical PhysicsMedical Physics LASERLASER: light amplification by stimulated emission of : light amplification by stimulated emission of

radiationradiation

- When a photon of the right wavelength hits an - When a photon of the right wavelength hits an excited atom of certain materials, it can stimulate the excited atom of certain materials, it can stimulate the emission of a second photon of exactly the same emission of a second photon of exactly the same wavelength and phase as the firstwavelength and phase as the first

- If enough atoms are excited, the photons can - If enough atoms are excited, the photons can stimulate further emissions of further photons, all stimulate further emissions of further photons, all travelling in the same direction.travelling in the same direction.- At one end of the material there is a- At one end of the material there is amirror that totally reflects the photons,mirror that totally reflects the photons,while at the other is a mirror that partiallywhile at the other is a mirror that partiallyreflects the photonsreflects the photons

Medical PhysicsMedical Physics Properties of lasersProperties of lasers::

- monochromatic (single colour)- monochromatic (single colour)

- coherent (in phase)- coherent (in phase)

- produce continuous light (pulses are caused by - produce continuous light (pulses are caused by shutters)shutters)

- absorbed by skin: increased by melanin- absorbed by skin: increased by melanin

- used in fibre optics to help guide light beam- used in fibre optics to help guide light beam

- CO- CO22 lasers lasers cut away delicate tissue e.g. brain cut away delicate tissue e.g. brain

Safety issuesSafety issues::

- severe & deep burns caused by beams- severe & deep burns caused by beams

- beam shined into eye will cause blindness- beam shined into eye will cause blindness

X-RaysX-Rays UsesUses

- identify bone fractures- identify bone fractures

- identify tooth decay- identify tooth decay

- identify tumours & disease in soft tissue- identify tumours & disease in soft tissue

- treatment of tumours by radiotherapy- treatment of tumours by radiotherapy

X-raysX-rays: photons of em radiation produced when a target : photons of em radiation produced when a target of heavy metal is struck by electrons travelling at high of heavy metal is struck by electrons travelling at high speed. Approx. 1% of the electrons produce an X-ray speed. Approx. 1% of the electrons produce an X-ray photon photon the rest is lost in heating up the target the rest is lost in heating up the target

Production of X-raysProduction of X-rays::

- decrease velocity of electron or- decrease velocity of electron or

- remove an inner electron. Electrons replace the inner - remove an inner electron. Electrons replace the inner electron & photons are emitted as the electrons undergo electron & photons are emitted as the electrons undergo transitions from different energy levelstransitions from different energy levels

X-RaysX-Rays Maximum energy: all electron’s energy is converted into Maximum energy: all electron’s energy is converted into

the photon’s energythe photon’s energy

- Kinetic energy = photon energy- Kinetic energy = photon energy

- Kinetic energy = charge of electron × voltage- Kinetic energy = charge of electron × voltage

eV = hfeV = hf h = Planck’s constanth = Planck’s constant

and…and… c = fc = fλλ

so…so… eV = hc/eV = hc/λλ

X-RaysX-Rays Generation of X-raysGeneration of X-rays::

- rotating anode tube - rotating anode tube evacuated glass tube, cooled in evacuated glass tube, cooled in oil with a lead surroundoil with a lead surround

- Hot filament boils off electrons which are accelerated - Hot filament boils off electrons which are accelerated by anode voltageby anode voltage

- hit target; most are released as heat, but few become - hit target; most are released as heat, but few become X-rayX-ray

- to prevent melting, target spins on a motor at - to prevent melting, target spins on a motor at 3000rpm3000rpm

X-RaysX-Rays Controlling the X-raysControlling the X-rays::

- cannot be focused- cannot be focused- sharp images are produced by small sources (point - sharp images are produced by small sources (point source)source)

Absorption of X-raysAbsorption of X-rays: occurs when x-rays pass through : occurs when x-rays pass through materials due to energy loss by…materials due to energy loss by…- - ScatteringScattering: X-ray photons are reradiated as lower : X-ray photons are reradiated as lower energy photonsenergy photons- - EjectionEjection: X-rays are ejected (photoelectric effect). : X-rays are ejected (photoelectric effect). Photons of visible light are emitted as atom comes out of Photons of visible light are emitted as atom comes out of the excited statethe excited state- - Compton ScatteringCompton Scattering: electron & lower energy X-ray : electron & lower energy X-ray photon are emittedphoton are emitted- - Pair ProductionPair Production: V. high energy photon interacts with : V. high energy photon interacts with atom’s nucleus.atom’s nucleus.Electron & positron emerge, losing energy by ionisation Electron & positron emerge, losing energy by ionisation until the positron is annihilated by an electron until the positron is annihilated by an electron generates 2 identical photonsgenerates 2 identical photons

X-RaysX-Rays Dangers of X-raysDangers of X-rays::

- water ionises to produce free radicals which produce - water ionises to produce free radicals which produce HH22OO22

- enzymes & DNA are damaged- enzymes & DNA are damaged

- parts of cells are damaged- parts of cells are damaged

- cell division is damaged (- cell division is damaged ( mutations) mutations)

- tissue & organ damage- tissue & organ damage

- life expectancy shortens- life expectancy shortens

- mutations cause gene alterations in populations- mutations cause gene alterations in populations

Attenuation of X-raysAttenuation of X-rays::

- inverse square law: intensity reduces- inverse square law: intensity reduces

with distancewith distance

I = II = I00ee--μμxx

X-RaysX-Rays Uses of X-raysUses of X-rays::

- diagnosis- diagnosis

- treatment of cancers (radiotherapy) with high energy - treatment of cancers (radiotherapy) with high energy X-raysX-rays

X-ray workersX-ray workers……

1) wear a film badge to check the amount of radiation 1) wear a film badge to check the amount of radiation they getthey get

2) wear lead aprons while the machine is in use2) wear lead aprons while the machine is in use

3) verify that the machine is in an enclosed room and 3) verify that the machine is in an enclosed room and the controls are in a separate roomthe controls are in a separate room

4) ensure that that there is no entry into the X-ray room 4) ensure that that there is no entry into the X-ray room while the machine is in usewhile the machine is in use

SummarySummary The EyeThe Eye Defects of Vision & Their CorrectionDefects of Vision & Their Correction Physics of the EarPhysics of the Ear Nervous ImpulsesNervous Impulses UltrasoundUltrasound Medical PhysicsMedical Physics X-RaysX-Rays