X-rays Flashcards
Explain the uses of X-rays
- Standard X-rays used for diagnostic imaging (x rays pass easily through skin/soft tissue but not bone)
- Used in CAT scans
- Used to diagnose tooth decay
Explain how x rays are produced
- Electrons build up at the hot cathode filament from the power source.
- Very high potential difference (voltage) is reached and the electrons jump across towards the W target.
- most electrons wasted as heat
- some electrons are slowed down and deflected (give off x rays)
- electrons come into contact with the electrons in the W target, causing transitions for the electrons in the W target (they excite and de-excite)
- The transitions give off a packet with a frequency that we cannot see called an x ray.
The process of x ray production is inefficient and a lot of energy is wasted as heat. What is done to limit this?
- The W target has a copper mounting because it conducts heat.
- It is cooled by circulating oil through the mount.
- Spinning the W target at high speeds helps it to stop overheating.
How does CT scanning work?
- Filtered back projection reconstructs an image by filtering many views (thousands values) from different angles to give a good resolution before back projection.
attenuations are filtered back to construct an image using an algorithm, removing any blurring from simple back projection=mathematically exact reconstruction of the image.
This allows cross sectional images produced to be accurate and of a good resolution for physicians.
Uses of CT scans?
- Used to produce cross sectional images of the body using specialised x ray equipment.
- Best method for detecting cancers like lymphoma and cancers of lung, liver, kidney, ovaries and pancreas.
- Used to detect pulmonary embolisms.
Know how and why voltage can affect x rays
High voltage : Better contrast because highest energy has increased but if it gets too high, a lot of high energy x rays would go straight through so no contrast left (blank dark image).
Low voltage: Less contrast because range of x ray energies is smaller but if not enough x-rays go through, you end up with silhouette (blank white image),
Know how and why current can affect x rays
High current: More intensity (due to larger frequency of high energy x rays going through)
If current too high = too many high energy x rays so image end up too dark because too many x rays went through.
Low current: Less intensity (due to lower frequency of x rays going through)
If current too low = not enough x rays go through so image end up too white (silhouette).
Know how and why filters (metal) can affect x rays
With Filter: Reduces intensity and blocks out weak energy x-rays that would be absorbed too quickly by the patient, leaving the strong energy x-rays behind. This improves quality of image.
Without Filter: Higher intensity but low energy x-rays would unnecessarily be exposed to patient. Low quality x-rays = Poorer image
What is the effect of tube voltage increasing?
- Maximum photon energy increases
- Peak of continuous spectrum moves up and to right
- Total area under curve increases (total intensity)
- More characteristic lines may appear as x ray photon energies increase (characteristics stay same height in same place)
What is the effect of increasing tube current on x rays?
The higher the tube current, more electrons released = more x-rays released
- Maximum photon energy stays same
- Peak of continuous spectrum moves higher
- Total area under curve (total intensity) increases
- Characteristic lines get longer (but at same time energies)
What is the effect of using a filter on x rays?
- Filtration improves quality of x-ray beam
- Filtration reduces intensity of x ray beam
- Increases in average peak energy (continuous and characteristics get shorter)
- Maximum photon energy stays same
