Medical physics

Medical physics

1. x-rays

   • electromagnetic wave
     • short wavelength (diameter of an atom)
   • produced in x-ray tube when fast moving electrons hit a target
   • uses
     • diagnose broken bones + position / condition of teeth
     • x-ray therapy
       • destroys cancerous tissue
         • shorter wavelength than normal x-rays used
       • thick plates between x-ray + body, stop x-rays reaching healthy tissue
         • a gap allows x-ray to reach tumour
     • contrast x-rays
       • tissue filled with contrast medium
         • medium absorbs x-rays easily
           • internal surface easily seen (contrasted)
     • CT Scans
       • creates 3D image
       • x-ray tube moves round
         • x-rays pass through patient at each stage
           • electronic signals go to computer + image displayed
   • performing x-ray
     • 1. light proof cassette containing CCD placed under patient
       • 2. CCD sensors covered by substance that coverts x-rays to light
         • 3. light creates electronic signal that is sent to a computer that displays x-ray image
   • absorbed by bones, teeth + metal
   • transmitted through soft tissue

2. Ultrasound

   • our hearing range = 20 - 20,000Hz
     • ultrasound = range above 20,000Hz
   • uses
     • prenatal scan
       • some of sound wave reflected back to detector at each boundary
         • time delay between initial wave and reflected pulse is used to build image of child
     • A-scan
       • transducer at front of eye sends ultrasounds pulses into eye
         • reflected pulses detected by transducer + displayed on oscilloscope
           • ocilloscope used to measure 'transmit time' of each pulse
       • determines length of eyeball
     • kidney stone removal
       • shatters kidney stone by high frequency vibrations
         • in smaller pieces so less painful to pass

3. Refraction

   • change of direction of light as it passes from one transparent substance to another
   • waves change speed when crossing boundaries
     • change causes change in direction (unless travelling along normal)
   • refractive index = how much substance can refract light ray
   • sine (angle in air) = n x sine (angle in material)

4. endoscopes

   • if angle of incidence is bigger than critical angle, causes total internal refraction
     • at TIR, angle of incidence = angle of refraction
   • uses
     • surgeon (see inside body)
       • contains bundles of optical fibres
         • thin, flexible glass fibres
         • light sent by TIR
     • laser light
       • cut, burn, cauterise
       • colour matched to type of tissue to maximise absorption
       • used in eye surgery. passes through cornea and absorbed by retina