Finals | Xray Tube Flashcards
A special type of diode (anode & cathode)
X-ray Tube
What is a diode?
Any electronic tube with two electrodes
An x-ray tube is a special type of a/n —
diode
Electrodes
Cathode and anode
The xray tube is (1) seen by (2)
- rarely
- radiologic technologists
What energy does an xray tube convert?
Converts electrical energy to electromagnetic energy
Xray Tube External Components
- Support Structure
- Ceiling Support System
- Floor-to-Ceiling Support System
- C-Arm Support System
- Protective Housing
- Glass/Metal Enclosure
Xray Tube Internal Components
- Cathode
- Anode
Xray tube is contained in a (1) and therefeore (2)
- protective housing
- inaccessible
Two primary parts of the xray tube
- Cathode
- Anode
Most frequently used support system
Ceiling support system
Consists of two perpendicular sets of ceiling-mounted rails. This allows for both longitudinal and transverse travel of the x-ray tube.
Ceiling support system
Ceiling support system consists of two perpendicular sets of (1). This allows for both (2) and (3) travel of the x-ray tube.
- ceiling-mounted rails
- longitudinal
- transverse
Other positions can be chosen and locked by the radiologic technologist.
Ceiling support system
Some ceiling-supported x-ray tubes have a single control that removes all (1), allowing the tube to (2).
- locks
- “float.”
Some — have a single control that removes all locks, allowing the tube to “float.”
ceiling-supported x-raytubes
Has a single column with rollers at each end, one attached to a ceiling-mounted rail and the other attached to a floor-mounted rail.
Floor-to-Ceiling Support System
Floor-to-Ceiling Support System has a single column with (1) at each end, one attached to a (2) and the other attached to a (3).
- rollers
- ceiling-mounted rail
- floor-mounted rail
A variation of a floor-to-ceiling support system is —
floor support system
Interventional radiology suites often are equipped with these
C-Arm Support System
Where is the IR attached in a C-Arm Support System?
The image receptor is attached to the other end of the C-arm from the x-ray tube
Variations of the C-Arm Support System
L-arm or U-arm support
When x-rays are produced, they are emitted (1), that is, with (2) in (3).
- isotropically
- equal intensity
- all directions
Only x-rays emitted through the special section of the x-ray tube called the — are used
window
X-rays emitted through the window
Useful beam
X-rays that escape through the protective housing
Leakage radiation
Contributes nothing in the way of diagnostic information and result in unnecessary exposure of the patient and the radiologic technologist
Leakage radiation
Leakage radiation result
Unnecessary exposure of patients and radiologic technologist
What does the protective housing provide?
- Mechanical support for the xray tube and protects it from damage caused by rough handling
- Contains diala oil that serves as both insulator against electric shock and as a thermal cushion to dissipate heat
- Has cooling fans to air cool the tube or the oil in which the x-ray tube is immersed
Functions of the diala oil
INSULATOR against electric shock and THERMAL CUSHION to dissipate heat
An x-ray tube is an electronic (1) with components contained within a (2) or (3) enclosure
- vacuum tube
- glass
- metal
An enclosure is relatively large, perhaps (1) long and (2) in diameter
- 30 to 50 cm
- 20 cm
The glass enclosure is made of (1) to enable it to (2) the tremendous heat generated.
- Pyrex glass
- withstand
Metal enclosure tubes maintain a constant (1) between the (2) of the tube current and the enclosure. Therefore, they have a longer life and are less likely to fail. Virtually all (3) x-ray tubes now use metal enclosures
- electric potential
- electrons
- high-capacity
Have a longer life and are less likely to fail.
Metal enclosure tubes
The x-ray tube window is an area of the glass or metal enclosure, approximately (1), that is (2) and through which the (3) of x-rays is emitted
- 5 cm^2
- thin
- useful beam
Disadvantage of a glass enclosure
More likely to fail, lesser tube life
A segment of glass that is thinner than the rest of the glass envelope
Tube window
Contributes to inherent filtration
Glass enclosure
The cathode is the (1) side of the x-ray tube; it has two primary parts, a (2) and a (3)
- negative
- filament
- focusing cup
A coil of wire similar to that in a kitchen toaster, but it is much smaller
Filament
Diameter and length of the filament
approximately 2 mm in diameter
1 or 2 cm long.
Emits electrons when it is
heated
Filament
When the current through the filament is sufficiently high, the (1) of the filament atoms are (2) and (3) from the filament. This phenomenon is known as (4).
- outer-shell electrons
- “boiled off”
- ejected
- thermionic emission
Filament composition
Thoriated tungsten
Reason why the filament is made up of thoriated tungsten
- Higher thermionic emission than other metals
- High melting points of 3410°, therefore is unlikely to burn out like the filament of a light bulb
- Tungsten does not vaporize easily
Tungsten does not vaporize easily. If it did, the tube would become (1) quickly, and its (2) would be coated with tungsten
- gassy
- internal parts
If vaporized, tungsten can cause (1) and can lead to (2)
- arcing
- tube failure
The addition of (1) to the tungsten filament enhances the efficiency of (2) and prolongs (3)
- 1% to 2% thorium
- thermionic emission
- tube life
(1) with (2) on the inside of the glass enclosure is the most common cause of tube failure
- Tungsten vaporization
- deposition
The filament is embedded in a (1) called the (2)
- metal shroud
- focusing cup
Because all of the electrons accelerated from cathode to anode are electrically (1), the electron beam tends to (2) owing to (3).
- negative
- spread out
- electrostatic repulsion
The focusing cup is (1) so that it electrostatically (2) the electron beam to a small area of the (3)
- negatively charged
- confines
- anode
Where does the effectiveness of the focusing cup depend on?
- Focusing cup size and shape
- Focusing cup charge
- Filament size and shape
- Position of the filament in the focusing cup
It serves as a grid (exposure switch) in a grid-controlled x-ray tube
Focusing cup
Focusing cup serves as a (1) (exposure switch) in a (2) x-ray tube
- grid
- grid-controlled
Cloud of electrons
Space charge
A phenomenon of the space charge that makes it difficult for subsequent electrons to be emitted by filament because of electrostatic repulsion
Space charge effect
Space charge effect is a phenomenon of the space charge that makes it difficult for subsequent electrons to be emitted by filament because of (1)
- electrostatic repulsion
Tendency of space charge to limit emission of other electrons from filament
Space charge effect
Thermionic emission at (1) and (2) can be space charge limited
- low kVp
- high mA
The small focal spot is associated with the —
small filament
A/n (1) is directed through the appropriate filament
electric current
Used when better spatial resolution is required
Small focal spot
Limiting factor of the small focal spot
Heating of the target is concentrated onto a smaller area
Used when when other techniques that produce high heat are required
Large focal spot
Important for high-resolution magnification radiography and mammography
Round focal spot
These are possible with the rotating anode
- Higher tube current
- Shorter exposure time
Positive side of x-ray tube
Anode
Two types of anodes
Stationary and rotating
Where are stationary anode x-ray tubes used?
- Dental x-ray imaging systems
- Portable imaging systems
- Other special-purpose units in which high tube current and power are not required
It is used when high tube current & power are NOT required
Stationary anode
Where are rotating anode x-ray tubes used?
General purpose x-ray
Why do general purpose x-ray make use of rotating anode?
Because they must be capable of producing high-intensity x-ray beams in a short time.
3 functions of the anode
- Electrical conductor
- Mechanical support for target
- Thermal dissipator
The anode also must be a good thermal dissipater. When the (1) from the cathode interact with the anode, (2) of their kinetic energy is converted into (3). This (3) must be dissipated quickly.
- projectile electrons
- more than 99%
- heat
Most common anode materials
Copper (Cu-29)
Molybdenum (Mo-42)
Graphite
Major engineering hurdle in designing higher
capacity x-ray tubes
Adequate heat dissipation
Area of the anode struck by the electrons from the cathode.
Target
In STATIONARY anode tubes, the target consists of —
tungsten alloy embedded in the copper anode
In ROTATING anode tubes, the (1) is the target
- entire rotating disc
TARGET
Alloying the tungsten (usually with [1]) gives it added (2) strength to withstand the stresses of (3) and the effects of repetitive (4) and (5).
- rhenium
- mechanical
- high-speed rotation
- thermal expansion
- contraction
Mammographic targets
Molybdenum (Mo-42)
Rhodium (RH-45)
High-capacity x-ray tubes have (1) or (2) layered under the tungsten target.
- molybdenum
- graphite
Characteristics of Mo & Graphite
▪ Lower mass density than tungsten
▪ Make anode lighter & easier to rotate
Tungsten is the material of choice for the target for general radiography for three main reasons
- Atomic number (74)
- Thermal conductivity nearly equal to that of copper
- High melting point (3410°C)
Tungsten has a high melting point (3410°C compared with [1] for copper) and therefore can stand up under high tube current without (2) or (3)
- 1100°C
- pitting
- bubbling
Characteristics of X-ray Targets basis
- Atomic Number
- K X-ray Energy (keV)*
- Melting Temperature (°C)
Tungsten symbol
W
Molybdenum symbol,
Mo
Rhodium symbol
Rh
Tungsten Z
74
Molybdenum Z
42
Rhodium Z
45
Tungsten K X-ray Energy
69 keV
Molybdenum K X-ray Energy
19 keV
Rhodium K X-ray Energy
23 keV
Tungsten Melting Temperature
3400°C/ 3410
Molybdenum Melting Temperature
2600°C
Rhodium Melting Temperature
3200°C
Allows the electron beam to interact with a much larger target area; therefore, the heating of the anode is not confined to one small spot
Rotating anode x-ray tube
The rotating anode x-ray tube allows the electron beam to interact with a much (1) target area; therefore, the heating of the anode is (2) to one small spot, as in a stationary anode tube.
- larger
- not confined
Heat capacity can be further improved by —
increasing the speed of anode rotation
Anode stem is usually made of (1) because it is a poor (2). It is also (3) so as to reduce its (4)
- molybdenum
- heat conductor
- narrow
- thermal conductivity
Most rotating anodes revolve at (1) rpm (revolutions per minute). While the anodes of high-capacity x-ray tubes rotate at (2) rpm.
- 3400
- 10,000
Used to turn the anode
Electromagnetic induction motor
How does the anode rotate inside an enclosure with no mechanical connection to the outside?
An electromagnetic induction motor is used to turn the anode.
An induction motor consists of two principal parts separated from each other by the glass or metal enclosure namely:
- Stator
- Rotor
Part of the induction motor outside the glass or
metal enclosure
Stator
Consists of a series of
electromagnets equally spaced around the neck of the tube
Stator
Stationary coil windings
Stator
A shaft inside the enclosure made of bars of
copper and soft iron fabricated into one mass
Rotor
Rotating part of rotor
Shaft
Rotor composition
Bars of copper and soft iron
