Medical physics, OCR- Unit 5, module 4

Medical physics, OCR- Unit 5, module 4

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Sameer Jahabarali Physics
Unit 5, module 4 Medical physics
X-rays are electromagnetic waves with wavelengths between 10-12m-10-9m. it shows
electromagnetic properties, therefore it can be diffracted, if grating id small enough, and
can take part in the photoelectric effect.
X-rays are produced in x-ray tube, which supplies a high voltage and a good vacuum. The
electron in the cathode (negative) are accelerated, across the vacuum (hence no
collision with molecules on the way) - towards the anode (positive). In the anode if the
electrons have the right velocity, the target electrons (inn the anode) will be `knocked' in
to a higher energy level. When the target electron returns to its original energy level, it
releases an x-ray.
X-rays interactions with matter include three phenomena's:
Photoelectric effect- where a surface exposed to x-ray radiation (or UV) absorbs the
x-rays to release photoelectrons. The kinetic energy of the photoelectrons released is
almost equal to the energy of the x-ray photon, this is because the work function has
almost negligible energy in comparison.
Pair production- is when a high-frequency X-ray collides with a particle to produce
and electron and a positron. The energy released is the sum of the energy of the
electrons and the positrons
The Compton effect- is a collision between an X-ray photon and an electron, where
both the X-ray and the electron are deflected. Conservation of momentum could be
applied to this effect. The energy loss of an electron is greater if the angle of
deflection is greater as well.
Intensity of an X-ray is the power per unit cross-sectional area. X-rays can be beamed from a
point source, in which case the inverse square law is applied to find intensity, or X-rays can be collimated, as
in to say it is a straight beam and intensity hardly changes.
X-rays absorption of a collimated beam interacts with the substance it passes through. The interaction varies
with the frequency of the x-ray (energy), where photoelectrons, Compton
effect and pair-production are relative interaction with increasing
frequency of X-ray beam. The intensity of the X-ray as it passes through
the substance decreases exponentially, where the halve value thickness
is the distance required for the intensity to halve.
Image enhancements can be carried out through:
Using photographic film that is more sensitive to X-rays, or putting a fluorescent plate behind the
X-ray film to increase exposure
Using a contrast medium, this absorbs X-rays to improve contrast of image. Barium is given in a
meal to enhance the X-ray of the digestive tract.
Using an image intensifier, such a phosphor which glows under X-rays, this can then be recorded
using a high-quality digital camera.

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Sameer Jahabarali Physics
Unit 5, module 4 Medical physics
Uses of X-rays in internal body structures include:
2D X-rays- where an x-rays is exposed to a broken limb from a point source, to produce a sharp
Angiograms- two digitised X-ray images, with one image with a contrast medium are subtracted
from one another. This produces a final image that only shows the difference.…read more

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Sameer Jahabarali Physics
Unit 5, module 4 Medical physics
Positron emission tomography (PET) detects abnormal metabolic or chemical activity. It uses medical
tracers such as radiolabeled glucose as it can be injected into the bloodstream and be taken up for
respiration in tissues.…read more

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Sameer Jahabarali Physics
Unit 5, module 4 Medical physics
Ultrasound is a high frequency sound-wave, with a frequency beyond the human hearing ability. It can travel
across a medium in a longitudinal wave, this can be used in non-invasive medical imaging.
Piezoelectric effect is ability of some materials (crystals) to create a voltage, when mechanical stress is
applied. This can take inform of separating electrical charges across a crystal lattice.…read more


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