Medical Applications of Physics

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  • Created by: Ella
  • Created on: 10-10-13 19:28

EM spectrum

  • Vacuum - 3x10^8 m/s
  • EM waves travel slower through matter
  • Absorbed causing heating, cancerous changes & damage to cells & tissue
  • Create an alternating current with same freguency as the radiation itself
  • Electromagnetic spectrum: Radio, Micro, Infra-red, Visible light, Ultraviolet, Xrays & Gamma
  • Gamma: Short wavelenght, High frequency, High energy - dangerous, detect malfunction in organs, kill body tissue
  • X-rays: dangerous, pass through flesh but absorbed by bone
  • UV: Fluorescent lights, sunburn & skin cancer, darker skin aborbs more so less reaches deeper body tissue 
  • Visibe light: Wavelenght - 300nm and 650nm, Communication - fibre optic cables, photography 
  • IR: cause heating when absorbed by object, night time photography 
  • Microwaves: heating in water, burns to body tissue, global communications - satellite, pass through Earths atmosphere, narrow beams
  • Radio : Low frequency, Low energy, Long wave length, diffracted, broadcasting
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X-rays

  • Short wavelength - diameter of an atom
  • Pass through skin & soft tissue 
  • Absorbed by bone or metal 
  • Photography of bone to check for damage
  • Dental problems
  • Industry - metal components & welds for cracks
  • Ionising - cancerous, precautions are taken in hospitals to limit dose
  • Treat cancer

CT Scanners

  • Computerised tomography 
  • Create 2d-pictures of the inside body 
  • Some sophistcated software can reassemble them into 3d image 

Charge-coupled Devices

  • Form electronic images of X-rays
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Ultrasound

  • Sound-like wave 
  • Frequency higher than human hearing range (20KHz)
  • Partially reflected when they meet a boundary between two different materials
  • Distance (m) = speed (m/s) x time (s)
  • Non-ionising, safer than X-rays
  • Unborn babies and shattering kidney stones

Creating Ultrasound Images

  • Oscilloscope
  • Time x-axis & Amplitude y-axis
  • Example: 

Each square on the x-axis is 0.02ms (0.00002s) we can calculate the time it takes for the sent pulse to return

4 squares = 4x 0.00002 = 0.00008s Therefore it takes 0.00008s for signal to return. If the speed of the ultrasound in the tissue is 1500m/s

Distance = speed x time = 1500 x 0.00008 = 0.12m

As this is there and back, divide by 2 to get the distance on way = 0.06 = 6cm 

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Refraction

  • Change of direction of light as it passes from one transparent material to another
  • Speed of waves will change when they cross a boundary - change direction
  • Air to glass - wave slow down & angle of refraction smaller than angle of incidence
  • Glass to air - wave speed up & angle of refraction greater than angle of incidence
  • Cause a vritual image to appear (an image that isn't really there)
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Refractive Index

  • Sin i/Sin r = n (refractive index)
  • Glass - 1.6, Water - 1.33, Diamond - 2.4
  • n is always greater than 1
  • i < c; r < 90 : i is less than the critical angle, most of light refracted, small amount reflected back into denser medium
  • i = c; r = 90 : i is equal to the critical angle, angle of refraction is 90, slightly more light reflected back into denser medium 
  • i > c; TIR : i is greater tahn the critical angle, all light reflected back into denser medium, boundary behaves like a mirror  
  • Sin i/Sin r = 1/n
  • If i = c and r = 90, sin 90 =1 therefore Sin c = 1/n or n = 1/Sin c 
  • The critical angle is the angle of incidence for which the angle of refraction is 90
  • Total internal reflection occurs when angle of incidence is greater than the critical angle & all light is reflected back into denser medium
  • TIR - sending visible light along optical fibres
  • Endoscopes - see inside body without cutting open
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Lenses

  • Curved piece glass - form an image
  • Two types Covex - converging & Concave - diverging 
  • Only convex can form real image

 (http://d1jqu7g1y74ds1.cloudfront.net/wp-content/uploads/2008/08/concave_convex.gif)

Diverging (concave) lens: 

  • Parallel rays get refracted away from a point called priniciple focus - always virtual 
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Convex Lens

  • Parallel rays of light brought to a focus at the principal focus (focal point)
  • Focal length - distance from the lens to the principle focus 
  • Strong lens - short focal length 
  • Light refracts on both sides as it passes through the lens 

Converging (convex) lenses

  • Ray diagrams 
  • Object in front of lens & image opposite side of lens
  • Real or virtual, upright or inverted
  • Object is placed at different distances from the lens different image will be formed
  • Object placed at a distance less than the focal length, magnified image will form, upright, bigger & virtual
  • Magnification = image height/ object height 
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The Eye

  • Light enters through cornea
  • Cornea & eye lens focus light into retina
  • Iris adjusts the size of pupil, controlling how much light enters
  • Ciliary muscles alter thickness, to control focusing, attached by the suspensory ligaments
  • Retina equivalent to film in camera or CCD in digital camera
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Range of vision

  • Near point of 25cm & far point of infinity
  • Long-sightedness - eyeball being too short, unable to focus, can't see near objects clearly 
  • Short-sightedness - eyeball being too long, unable to focus, can't see distant objects clearly (http://www.millingtondawson.com/images/long_short_diagram.jpg)
  •  Long-sightedness - converging lens, Short-sightedness - diverging lens 
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Correcting Vision Problems

  • Power of lens - P = 1/f, f = focal length (m)
  • lens power measured in Dioptre
  • Converging len - positive power
  • Diverging lens - negative power
  • Focal lenght is determined by refractive index of material from which the lens is made & curvature of the two surfaces of the lens
  • The greater the refractive index, the flatter the lens - lens can be manufactured thinner
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