Lecture 1 Flashcards
- How does EM radiation travel? – essential for?
- What is velocity constant?
- Relationship between energy and wavelength?
- In straight lines. – Essential for image formation.
- Speed of light.
- Inversely proportional
- How are x rays produced?
- In what type of imaging are x rays used?
- How are gamma rays produced?
- In what area of medicine and type of imaging are gamma rays used?
- By the interaction of fast moving electrons with a metal target.
- Radiography.
- By decay of radioactive elements.
- Nuclear medicine and scintigraphy.
Specifications of an x ray machine.
Exposures
Anode type
Focal spot
Filtration
What can an x ray machine be likened to and why?
Electrical circuit – cathode at one end and anode at the other.
- What is the cathode made up of?
- What does the electric current do?
– Name this process. - What does the negatively charged cup containing the cathode do?
- A tungsten wire filament heated by a small electric current.
- Boils off electrons from the cathode.
– Thermionic emission. - Keeps the electrons there until ready to make the exposure where it is focused into a beam.
- What does mA stand for?
- What is mA?
- What does increasing mA do?
- What happens when the button is pressed to take a radiograph? – What is the typical voltage?
- milliamps
- The current that runs through the cathode.
- Increases the current, increasing the temperature, increasing the electrons produced, increasing the x rays subsequently produced.
- Electrons are accelerated across the tube towards the anode because a very high potential difference is being applied between the electrodes. – 50-100kV.
- What does the kV control do?
- What is the anode?
- What is the anode made of? – Why?
- What happens when the electrons hit the anode?
- Alters the potential difference across the tube between the cathode and anode.
- The target for the electrons released by the cathode.
- Made of solid tungsten – High atomic number so is good at producing x rays. – High melting point to withstand heat.
- They are rapidly decelerated as they interact with the atoms of the anode, producing x rays.
What does the kV represent?
The maximum energy of x ray emitted. There is a spectrum of x ray energies produced in the beam that are less than this.
- What % energy produced by machine is heat?
- What % energy produced by the machine is x rays?
- 99%
- 1%
1 What is the Focal Spot?
2. How is heat removed from an anode in a lower output machine?
3. How is heat removed from an anode in a higher output machine? – What is the disc generally made of? – What happens in the prep phase when the button is halfway down?
- The small area of the anode where the electrons from the cathode are focused to impinge.
- Stationary anode. The tungsten target is embedded in a block of copper that helps conduct heat away from the target.
- Anode rotates. The target is a tungsten track around the edge of it. The disc is mounted to a molybdenum rod and is rotated by electric motors. Heat is lost mainly by convection from the surface of the disc. – Graphite and rod made of molybdenum (poor conductor) – the anode starts to rotate in prep for the x ray being taken.
Describe the x ray tube.
The anode and the cathode are placed in an evacuated Pyrex tube so electrons do not interact with any gas particles on the way across. The tube is immersed in oil to help with heat conduction and electrical insulation. The entire tube is surrounded by lead except for a small window where the useful x ray beam emerges.
What can be found on the control panel of the x ray machine?
On/off button
kV control (potential difference across tube – energy of x rays).
mA control (current to cathode and how many electrons we boil off).
Timer
