Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
37 Cards in this Set
- Front
- Back
|
What is the filament?
|
Made of Tungsten, Source of electrons, heated to incandescence, From low-voltage circuit.
Emits electrons proportions to temp of filament. |
|
|
What is the focusing cap?
|
Made of molybdenum
Negatively charged concave reflector Focuses electrons emitted by filament into a narrow beam aimed at focal spot on anode. |
|
|
Why do e- move?
|
Repelled by - charged cathode
attracted to + charged anode. Power supply heats cathode filament (low voltage) and establish potential between anode and cathode to accelerate electrons to anode (High voltage) |
|
|
Why tungsten for target?
|
High atomic number=efficient xray production
High melting point=heat is produced abundantly High thermal conductivity=helps dissipate heat. |
|
|
What size is the ideal focal spot?
|
Very small. Heat is generated more though. You get around this with geometry!
|
|
|
What is the focal spot?
|
Area on target (anode) that focusing directs the electrons it produces.
|
|
|
Rotating anodes help do what?
|
Dissipate heat. It makes an increased longevity of teh tungsten target.
|
|
|
What are the three main electrical circuits?
|
1. Voltage circuit -provides the potential difference between the cathode and anode, usually 60-90kV (step-up transformer). The kV determines ENERGY and NUMBER of X-ray photons produces.
2. Cathode current: Controls the current through the cathode, therefore the number of electrons released from the cathode. This in turn determines the NUMBER of Xray photons produced. 3. Timing circuit (exposure duration): Controls the duration of cathode current flow and hence it also determines the NUMBER of X-ray photons produced. |
|
|
Most electrons make heat, but others produce x ray photons by:
|
Bremsstrahlung Radiation and Characteristic Radiation.
|
|
|
What is Bremsstrahlung radiation?
|
Primary source of radiation from x-ray tube, direct hit or near miss of nucleus by an high energy electron. Creates a DEFLECTED or DECELERATED electron.
|
|
|
What is Characteristic Radiation?
|
Contributes a small fraction of photons
Occurs when an incident electron ejects an electron from the tungsten target. Outer electron fills void When void is filled, an x-ray photon is emitted with energy characteristic of 2 binding energies. |
|
|
What is scatter radiation?
|
Fogs the film
REsults from photons interacting with subject: Lower kVp, Collimate the beam to size of the film. Majority of scatter comes from compton scatter. |
|
|
What is Compton Scatter?
|
see slides.
|
|
|
What increases scatter?
|
Subject thickness
Field Size kVp (x-ray beam energy) Scatter produces FOG on the plate/sensor/film. reduces contrast. |
|
|
Why do we filter the beam?
|
X-ray beam initially is a spectrum. Only photons with sufficient energy to go through everything will work. Low energy don't reach the receptor and so they are filtered with an aluminum filter.
|
|
|
Why use aluminum filter?
|
Reduces patient exposure.
|
|
|
What is inherent filtration?
|
Material of beam passes through to outside. Glass of x-ray tube, insulating oil, etc.
|
|
|
What is total filtration?
|
Inherent filtration + external filtration.
Inherent is .5 to 2 mm of aluminum. Government regs require total filtration to be Beam energy up to 70 kVp=1.5 mm aluminum Beam energy higher than 70 kVp= 2.5 mm Al. |
|
|
Why 2 power supplies in the tube head?
|
Charge gradient in the anode, filiment and cup in teh cathode
|
|
|
Why tungsten target?
|
high energy electrons, heat dissapation.
|
|
|
Anode has:
|
Positive
|
|
|
Cathode has:
|
Negative
|
|
|
What causes "electron cloud?
|
Electrical resistance, heat applied to filament
|
|
|
What moves "electron cloud"
|
The gradient from the anode to the cathode.
|
|
|
Why small focal spot?
|
Makes image sharper, but increases the heat though.
Smaller penumbra |
|
|
Describe geometry behind apparent focal spot.
|
20 degrees. If it was 45 degrees, it would change the focal spot. It would be the actual. They use 20 degrees because that diminishes it.
|
|
|
Tube voltage is?
|
don't worry bout it.
|
|
|
What does step-down transformers do?
Why? |
Do not need to know: Produces the heat for the filament to make electron cloud
|
|
|
What does step-up transformers do?
Why? |
Do not need to do: Charge Gradient in anode. to move the cloud to the focal point.
|
|
|
What is most common reaction produced by x-ray tube?
|
Heat
|
|
|
What radiation is stopped by a piece of paper?
|
Alpha
|
|
|
What is focusing cup for?
|
focus electrons onto tungsten target?
|
|
|
Two reasons electron moves?
|
1. Heat produced by e- cloud
2. Charge between anode and cathode. |
|
|
Most common type of radiation in x-ray?
|
Bramstahlung
|
|
|
Define Inherent filtration?
|
Glass, Oil
|
|
|
Total Filtration definition
|
Inherent + Aluminum
|
|
|
Gov regulations
|
<70 kVp 1.5 mm
>70 kVp 2.5 mm |
