Inside the Body

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  • μ = the attenuation coefficient.
  • Half-value thickness = ln2/μ
  • X-rays are scattered, so a lead grid (columnator) is used so that only X-rays travelling perpendicular to the body hit the photographic plate.
  • A contrast medium can be used to distinguish between organs and the surrounding tissues.
  • A 'barium meal' allows the digestive tract to be seen as barium has a higher attenuation coefficient than the other tissues,
  • X-ray images are enhanced electronically to improve the quality of the image without exposing the patient to excess ionising radiation.
  • CT scanners take X-rays while rotating 360° around the body to produce a 3D image.
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Producing X-rays

  • Electrons are produced by thermionic emittion.
  • The potential difference between the filament and the anode produces an electric field which accelerates the electrons.
  • The electrons hit the heavy metal target and decelerate, causing them to emit an X-ray photon (Brehmsstrahlung- braking radiation).
  • The resulting X-ray beam has photons with a range of energies, producing a continuous spectrum.
  • The target rotates to prevent the anode from overheating and melting.
  • Sometimes, the electrons ionise the atom by knocking an inner electron out of the atom. An outer electron drops into the lower energy level to fill the gap, releasing a high energy photon of a specific frequency and wavelength. This produces the characteristic X-rays on the spectrum.
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Interactions of X-rays with matter

  • The photon may be scattered if its energy is too low for it to be absorbed.
  • The photon may ionise an atom. This leads to lower energy photons being emitted when the outer electrons fall to lower energy levels. The low energy photons are usually easily absorbed.
  • The photon can excite an electron, which produces a similar effect to ionisation.
  • The photon can collide with an electron and give it kinetic energy (Compton scattering). The scattered photon has a lower enegy due to the transfer of energy to the electron.
  • Occasionally, the photon may collide with the nucleus and- if it has a high enough energy- produce an electron-positron pair. The electron and the positron lose energy and eventually annihilate.
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  • High frequency longitudinal waves.
  • Produced using a potential difference across a ceramic crystal, which causes it to oscillate in resonance with the voltage. This creates a high amplitude.
  • A gel is used on the skin to reduce reflection of the waves from the transmitter-skin boundary.
  • The waves are reflected from the various tissue boundaries inside the body. The time taken for the pulse to arrive at the detector is measured and, as the speed of the waves is known, the distance between the skin and the tissue boundary can be calculated (it must be divided by 2 because the time is the time taken for the pulse to travel from the skin to the boundary and back again), The distances can be entered into a computer and then made into an image.
  • Blood flow in a vessel can be calculated using the Doppler effect- the frequency of the waves in the reflected pulse will be difference to that of the original pulse. 
  • 2vcosθ/c = Δf/f
  • vcosθ = the component of the blood's velocity towards the transducer.
  • c = the speed of the ultrasound in tissue.
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  • Protons in the body are aligned with a strong magnetic field.
  • A gradient field is produced across the body.
  • rf waves are transmitted across the body. If this is the Lamor frequency of the protons then they will precess and change their spin state.
  • When the rf waves are switched off, the change in spin state releases radiation from the protons in the form of a secondary rf field.
  • The amplitude of the detected signal depends on the density of protons in the area of the body being scanned.
  • The gradient field changes direction so that protons in different 'slices' of the body resonate.
  • The computer assimilates the information from the different slices to generate the final image.
  • Expensive
  • Time consuming
  • Noisy
  • Superconductors required to produce the strong magnetic fields.
  • Unsuitable for patients with pacemakers or metal implants.
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  • Direct observations inside the body using visible light.
  • High definition, true colour and real time images.
  • Core is surrounded by a medium with a lower refractive index to allow total internal reflection and to reduce dispersion of the light.
  • Incoherant bundle- the arrangement of the fibres does not matter. Used to provide illumination. 
  • Coherent bundle- the fibres remain in the same spatial arrangement relative to all the other fibres. Required to transmit the image; otherwise the image would be useless.
  • Light can take many paths down the fibre, causing multipath dispersion (the light arrives at different times). This is reduced by using thin fibres.
  • Fibres may become misaligned so that the bundle is no longer fully coherent.
  • Light can leak from one fibre to another. This is reduced by coating each fibre with a dark glass.
  • Light intensity decreases along the fibre. The image is enhanced electronically to counteract this.
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  • Charge Coupled Devices.
  • More sensitive to light than photographic film. They can also produce moving images.
  • Light falls on metal oxide semiconductors, which are photodiodes/photosites.
  • Each photosite produces one pixel.
  • Photons liberate electrons from atoms.
  • The photosites are insulated from one another, so charge builds up on each side.
  • Light is proportional to electron distribution.
  • Quantum efficiency- the percentage of photons that are used to generate the image.
  • The charge on each photosite is shifted downwards to the photosite below it.
  • The bottoms row feeds a shift register- the charge is measured and then the row is emptied by shifting each charge horizontally.
  • The shift register produces a seried of pulses which represent the charge level in each photosite.
  • The process is repeated until all sites have been measured.
  • The system identifies which stream of information goes with each pixel.
  • For coloured images, 3 CCDs produce 3 different images- one for red, one for blue and one for green. Mirrors and lenses are used to direct the different colours of light.
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