X-Rays Flashcards
How are X-rays produced?
Thermionic emission: a heated filament acts as the cathode (it heats up and surface electrons gain energy)
There is a voltage (up to 200 kV) between electrodes which accelerates the electrons.
When electrons strike the anode, they lose some KE in the form of X-ray photons emerging in all directions.
There is a thinner window which allow X-rays to produce a collimated beam and this size can be controlled using metal tubes to absorb X-rays
The anode rotates because most of the energy is transferred to the anode so it becomes hot.
What is the characteristic radiation?
An electron may cause a rearrangement of electrons in the anode atom, decreasing its energy level so a single photon is released.
Describe the interaction of X-Rays in the photoelectric effect
Less than 100 keV, photon absorbed by electron so electron escapes from atom.
Describe the interaction of X-Rays in Compton scattering
0.5-5MeV, The incident photon loses some of its energy to eject an electron from atom. The larger the angle of deflection, more energy is lost and longer wavelength.
Describe the interaction of X-Rays in Pair Production
> 1.02MeV, the photon disappears in the electric field of the nucleus and produces an electron-positron pair.
What does the attenuation coefficient (u) show?
The fraction of intensity absorbed for a certain distance. Bigger u means more absorption.
How is u related to Z?
P.E. u prop Z^3
C.S. u independent of Z
P.P. u prop Z^2
How is u related to E (Energy of incident photon)?
P.E. u prop 1/E^3
C.S. u decreases slowly with E
P.P. u increases slowly with E
How do image intensifiers work?
X-Ray photons are absorbed by a phosphor screen and produce lots of light photons.
These hit a photocathode producing electrons from photons (PE effect). The number of electrons are increased.
These are focused by an anode onto another phosphor screen producing light which is the image.
Overall, X-Ray photons are converted into an increased number of visible photons.
Why are image intensifiers used?
The patient is exposed to X-Rays for less time.
It makes a brighter image.
Real time images can be formed.
How do contrast media work?
High u, so absorb a lot of X-rays. High Z used.
This can shows tissues with similar u differently.
Produces good contrast in soft body tissues with similar u & Z.
Injected/swallowed into soft tissue areas.
Scan shows outline of soft tissue.
What is used as a contrast medium?
Barium or iodine.
How does a CAT scanner work?
An X-Ray tube rotates around the patient in a ring of (static) detectors.
A thin beam is used (to only get a thin slice at a time)
Images of slices through the patient are taken.
The X-Ray tube moves along the patient.
The signals from the detector are used by the computer to produce a 3D image.
What is the advantage of CAT scans compared with normal X-Rays?
You get a 3D image
They can distinguish tissues with similar attenuation coefficients.
Show the precise positions and shape of tumours.
