RT 52A quiz 2

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

-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

• MO = Molybdenum (42)
• W = Tungsten  (74)

• -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

-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

-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

-Area where useful beams (x-rays) is emitted

-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

-When filament heated 4 to 6 amps 

-Outer shell e- of filament are boiled off & ejected

Electrons repel each other and form a cloud around the filament

• -Holds the filament and focuses the electrons
• -Made of Molybdenum (Mo) - which has a negative charge

• -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.

-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

-Set into the face of the anode; is the area struck by electrons

• -Tungsten
•    -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
•   

-Area on target bombarded by e- from filament

-F.S is selected at the mA station or at a separate selector.

• Small focal spot: 
• -0.1mm - 0.5 mm 
• -provides best detail 
• -Used on small body part
• -Used with 300mA or less 

• Downside:
• -Smaller area is bombarded by e- so more heat is built up

• Note:
• mA:
• -Unit used for electric current
• -milli: 1/1000
• -75 mA = 75/1000 = 0.075

• Large focal spot:
• -0.4mm - 1.2mm
• -Used when x-raying large body parts
• -Used with 300-400 mA or higher

Area on target bombarded by e- from filament

Actual is larger than the effective focal spot.

-Foreshortened size of the beam as it is projected down to the patient or IR.

-Influenced by anode bevel angle

• 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

A large actual focal spot and a small effective focal spot.

• How do we get that combination?
• Target angle at 7-10 degrees.

C. 10 degree target angle, 1.2 mm focal spot


1 m = 1000 mm

-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

T-spine

• 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
• -Speed
•    - 3400 rpm's
•    - High capacity tube: 10,000 rmp's

-Consists of a series of electromagnets equally spaced around the neck of the tube.