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What does the protective housing do?
- Protects, provides & reduces what?
- How much mR/hr at one meter/ yard?
1) It protects the tube from damage
2) Provides mechanical support
- 3) Reduces the level of leakage radiation
- - leakage radiation (x-rays that escape through the protective housing)
- Housing must be lined with lead (radiation cannot go through lead)
- - Must reduce leakage radiation to less than 100mR/hr at one meter
- (R= roentgen)
-1 meter = 1 yard
What are Collimators?
- How many set of lead shutters and what does it do to the x-ray beam?
- A light filled with cross-hairs is provided by 2 things?
-2 sets of lead shutters that restrict size & shape of the x-ray beam
-A light filled with cross-hairs is provided by a mirror at a 45 degree angle and a light bulb
What are two main elements/ components in the x-ray tube?
- MO = Molybdenum (42)
- W = Tungsten (74)
- What is it made of?
- What happens when air is removed?
- How do you know there's air in the tube?
- -Made out of pyrex that can withstand tremendous amount of heat
- -Air removed from tube to form a vacuum
- - Which results in more efficient production of x-rays
- - Prevents collision w/ gas molecules
- - Prevents oxidation
-Rust will occur when there's air in the tube
What are the drawbacks from glass enclosure?
- What happens to tungsten plating and what will it effect?
-Tungsten plating; filament vaporizes over time and some tungsten adheres to walls
-Causes arcing; bright discharge of light when current jumps the gap between cathode and anode
-Two benefits of metal vs. glass?
-Longer life than glass enclosure
-Less likely to fail; does not allow the vaporized tungsten to adhere to walls of enclosure.
-Most purchased tubes today use the metal enclosure
What is a Tube window?
-Area where useful beams (x-rays) is emitted
What is a Cathode?
-What does it contain?
- W melting point (in C and F)?
-Negative terminal of the x-ray tube
-Contains focus cup
- -Contains a tungsten wire/ filament wound in the form of a coil
- - High melting point
- - 3410 C - 6170 F
What is Therminonic Emission?
-When filament heated 4 to 6 amps
-Outer shell e- of filament are boiled off & ejected
What is Space charge effect?
Electrons repel each other and form a cloud around the filament
What are 2 funtions of focusing cup?
- What is it made out of?
- -Holds the filament and focuses the electrons
- -Made of Molybdenum (Mo) - which has a negative charge
- 3 funtions?
- -Positive terminal of the x-ray tube
- - Has 3 funtions:
- 1) Acts as electrical conductor
- 2) Acts as thermal conductor
- 3) Provides mechanical support
-Anode has target to provide support.
-What is anode made of?
-Rotates @ ____ rpm/min?
-3 benefits for rotation?
-Copper (Cu, atomic# 29), molybdenum (Mo) & graphite
-rotates at 3400 rpm/min (revolutions per min)
- Benefits for rotation:
- -tube can withstand larger exposure
- -heat dissipates quicker
- -beam interacts w/ larger area of target
What is a Target?
-What is it made of?
-Set into the face of the anode; is the area struck by electrons
- -Has a high atomic# or z number (74)
- - The higher the z number, the more efficient producer of x-rays.
- -High melting point, 3140C & 6170F
What is a Focal Spot?
-Area on target bombarded by e- from filament
-F.S is selected at the mA station or at a separate selector.
Describe small focal spot and its downside.
- Small focal spot:
- -0.1mm - 0.5 mm
- -provides best detail
- -Used on small body part
- -Used with 300mA or less
- -Smaller area is bombarded by e- so more heat is built up
- -Unit used for electric current
- -milli: 1/1000
- -75 mA = 75/1000 = 0.075
Describe large focal spot.
- Large focal spot:
- -0.4mm - 1.2mm
- -Used when x-raying large body parts
- -Used with 300-400 mA or higher
What is an Actual focal spot?
Area on target bombarded by e- from filament
Actual is larger than the effective focal spot.
What is an Effective focal spot?
-Foreshortened size of the beam as it is projected down to the patient or IR.
-Influenced by anode bevel angle
- 2 benefits of angled anode?
- 3 things heat capacity of the anode related to?
- 1) Anode is angled, thus effective area of target is smaller
- - better detail
- - Not as much heat build-up
- 2) Heat capacity of the anode is related to the:
- -Speed of the anode
- -Diameter of the anode
- -size of the focal spot
What focal spot is used to get the best detail w/ the least amount of heat build-up results when using?
A large actual focal spot and a small effective focal spot.
- How do we get that combination?
- Target angle at 7-10 degrees.
Which of the following combination will present the best detail & the greatest heat-loading capacity?
A. 17 degree target angle, 1.2 mm focal spot
B. 17, 0.6 mm
C. 10, 0.6 mm
D. 10, 1.2 mm
D. 10 degree target angle, 1.2 mm focal spot
1 m = 1000 mm
(this multiple choice question has been scrambled)
Extrafocal Radiation (Off Focus radiation)
-Composed of photons that were not produced at the f.s.
-Occurs when e- bounces away from the f.s. and hit another part of the anode
-Causes "ghosting" of structures adjacent to the primary beam
-Diaphragm helps eliminate off focus radiation
- -Intensity of x-rays emitted thru heel of target is reduced
- - Some x-rays get absorbed
- - Cathode side more intense
- - If heel effect occurs, place cathode side of tube at thicker part of body
What area of the body might the effects of the heel effect be seen?
- Major contributor to the heel effect
- - Small target angle
-An electromagnetic device that turns the anode
- -A shaft made of copper & iron
- -Is surrounded by ball bearings
- - 3400 rpm's
- - High capacity tube: 10,000 rmp's
-Consists of a series of electromagnets equally spaced around the neck of the tube.