MEDICAL PHYSICS

ContentsPhysics of the Eye and EarBiological Measurement and Imaging

The EyeCiliary muscles relax (lens flattens) distant objectsCiliary muscles contract (lens gets fat) close up objectsRefraction at cornea produces inverted image on retina

The EyeDim 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 EyeNerve ends respond to changes in light intensityEye is constantly scanning so that new nerves are stimulatedPersistence 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 CorrectionConvex Lens: light rays converge

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

Defects of Vision & Their CorrectionConcave 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 CorrectionLens Formula:

1/f = 1/v + 1/uv = image distance, u = object distance

M = v/uM = 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 EarSound: longitudinal waveAcoustic impedance: density multiplied by the speed of sound in the material- low conducts sound well- high insulates sound wellIntensity: power per unit area (decreases with distance from source)Pinna: funnels sound waves into ear canalCanal: increases sound intensity Tympanic membrane vibratesOssicles (three small bones) vibrateStrike oval window of the cochleaNerve cells detect sound & convertit into electrical impulses forprocessing by the brain

Physics of the EarPlace Theory: brain determines pitch by noting the place on the basilar membrane where the message is strongestFrequency Theory: frequency of vibrations of the basilar membrane as a whole is translated into an equivalent frequency of nerve impulsesNeurons, 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 brainIntensity is measured on the decibel scale:

Physics of the EarHearing Loss:- mechanical damage due to a blow on the head- disease (stop ossicles from moving)- exposure to excessive noise (tinnitus)- ageing

Nerve ImpulsesElectrical Signals in the Body: carried by neurones- cells have membrane potential: extra K+ ions inside, Na+ ions outside potential difference = 70mVWhen membrane is stimulated:- becomes permeable to Na+ions whichdiffuse due to the negative chargePotential rises initially to 0 mV (depolarisation)& then to +30 mV (reverse polarisation)Membrane becomes impermeable to Na+ions & they are trapped within nerve cellK+ions diffuse out of the membrane which restores the potential (repolarisation)Process takes about 2 msThen the K+ions are pumped out, process takes about 50 ms

Nerve ImpulsesPumps materials as a fluid quickly around body

Double circulatory system:

Nervous ImpulsesDeoxygenated blood enters through the vena cava into the right atriumIts then pumped through a valve into the right ventricle chamberAnd then up through the pulmonary valve into the pulmonary artery towards the lungsOxygenated blood enters through the pulmonary veins into the left atriumIts then pumped through a valve into the left ventricleAnd then through the aortic valve and out of the aorta to the rest of the body

Nerve ImpulsesArteries carry blood away from the heart at high pressure in thick walled lumenVeins carry low pressure blood back to the heart. Veins have thinner walls and valves to prevent backflow of bloodCapillaries have thin walls to allow glucose and oxygen to diffuse through

Nerve Impulses

Nerve ImpulsesCarry blood at high pressure

Outermost layer is composed of connective tissueMedia smooth muscle cells and elastic tissueIntima in direct contact with flow of bloodLumen cavity in which blood flows

Nerve ImpulsesMost exchange of nutrients and gases takes place hereSmall diameter, large surface area for diffusion

Lungs carbon dioxide is exchanged for oxygenTissues O2, CO2, nutrients and wastes are exchangedKidneys wastes are released to be eliminated from bodyIntestine nutrients are picked up, wastes are released

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

UltrasoundUltrasound: 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

UltrasoundUltrasound is often used to detect functions inside the body:Ultrasound is:longitudinalpassed through a gel on the skin to prevent reflectionreflected as it passes from one tissue to another (velocity change)absorbed by tissue materialabsorbed & reflected by many percentages depending on the tissue type

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

UltrasoundResolution: 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 beamLateral: use narrow pulses; determined by beam width

A-scan: amplitude modulated display

B-scan: brightness modulated display

UltrasoundReal time B-scan: moving images. Up to 100 probes are usedMovement of blood: through Doppler effect

Advantages:low frequency (low energy) beamnon-invasive & no discomfortmore effective than X-ray for images of soft tissueportable equipment

Disadvantages:skilful operator & image requires skilful interpretationimage resolution is very easily reduced

Medical OpticsOptical fibre: glass rod that conducts light by TIRLight 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

Snells Law:n1sin1 = n2sin2Critical Angle:sinc = n2/n1

Medical OpticsIf the fibre is bent sharply significant loss of light- radius of bend must be 20x diameter to prevent such losses

Fibre diameter: 10mEndoscope of 3mm diameter would have approx. 40 000 fibresThe more fibres, the greater the resolution

fibre optic light guidesheathoptical fibre

Medical PhysicsUses of Endoscopy:cut out diseased tissuetake a biopsyseal a site of bleeding with heatremove an obstructive objectkeyhole surgery

Medical PhysicsLASER: 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 amirror that totally reflects the photons,while at the other is a mirror that partiallyreflects the photons

Medical PhysicsProperties 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-RaysUses- 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 targetProduction 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-RaysMaximum energy: all electrons energy is converted into the photons energy- Kinetic energy = photon energy- Kinetic energy = charge of electron voltage

eV = hfh = Plancks constant

andc = f

soeV = hc/

X-RaysGeneration 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-RaysControlling 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 atoms nucleus.Electron & positron emerge, losing energy by ionisation until the positron is annihilated by an electron generates 2 identical photons

X-RaysDangers 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 reduceswith distance

I = I0e-x

X-RaysUses of X-rays:- diagnosis- treatment of cancers (radiotherapy) with high energy X-rays

X-ray workers1) wear a film badge to check the amount of radiation they get2) wear lead aprons while the machine is in use3) verify that the machine is in an enclosed room and the controls are in a separate room4) ensure that that there is no entry into the X-ray room while the machine is in use

SummaryThe EyeDefects of Vision & Their CorrectionPhysics of the EarNervous ImpulsesUltrasoundMedical PhysicsX-Rays