Chapter 27 Medical Imaging Flashcards
How were X-rays discovered?
A voltage was passed between two electrodes through a discharge tube with a gas at a low pressure. As the voltage was increased he could see the shadow of his bones on a plate.
How are X rays produced?
High speed electrons are fired at a target metal and come to a very sudden stop, releasing an X-ray.
What is the function of the heater in the X ray tube?
It produces electrons by thermionic emission
What is the function of the vacuum?
It removes all air particles to stop collisions.
What is the function of the Anode?
To attract the electrons and stop them rapidly, it can get very hot so needs a high melting point.
What is the function of the window?
To allow the X rays to pass out of the tube.
What is the function of the Oil cooling?
To cool down the anode.
What is the function of the Lead shielding?
To stop the radiation escaping and causing harm to undesired areas.
Why are X ray machines not that efficient?
About 99% of the electrons go into heating up the anode.
What is the typical value for the wavelength of X rays?
10^-11m
What are the two peaks in the current-wavelength graph characteristic of?
The identity of the target metal.
What happens during simple scattering?
An X ray photon interacts with an electron in the atom but doesn’t have enough energy to remove the electron, so is simply scattered without losing any of its energy.
What happens during the photoelectric effect?
The X ray photon is absorbed by one of the electrons in the atom and the electron uses the energy to escape from the atom. This is the most dominant form of attenuation in hospital x rays.
What happens during Compton scattering?
An X ray photon interacts with an electron, ejecting the electron and the X ray is scattered with lowered energy.
What happens during Pair production?
An X ray photon interacts with the nucleus of the atom. The photon dissapears and a postitron and electron are produced.
