Study sets, textbooks, questions
Upgrade to remove ads
Chapter 6: The X-ray Tube
Terms in this set (43)
What is the name of the modern day x-ray tube?
An electronic vacuum containing 2 electrodes within a glass or metal enclosure (a diode).
-more efficient X-ray production
-longer tube life
The Cathode (-)
Negative side of the X-ray tube.
2 primary parts:
A coil of wire.
approximately 2mm in diameter and 1 or 2 cm long.
Emits electrons when it is heated.
The outer shell electrons of the filament atoms are "boiled off" and ejected from the filament (thermionic emission).
Filaments are usually made of thoriated tungsten.
Provides a higher thermionic emission than other materials.
melting point is 3410 degrees C.
not likely to burn out like a light bulb filament.
The addition of 1% to 2% thorium to the filament enhances efficiency of thermionic emission and prolongs tube life.
Chemical symbol: W
Atomic number: 74
K shell binding energy: 69 keV
Melting poing: 34000 degrees celsius
What is the most common cause of X-ray tube failure?
vaporization of tungsten inside the envelope does ultimately occur.
occurs with glass envelope X-ray tubes, less likely with metal envelopes.
Malfunction is usually abrupt due to being caused by arcing (sparking).
The Focusing Cup
Filament is embedded in a metal cup called a focusing cup.
electron beam tends to spread out due to electrostatic repulsion.
Focusing cup is negatively charged to electrostatic ally confine and electron beam to a small area of anode.
effectiveness of the focusing cup is determined by:
-size and shape
-filament size and shape
-position of the filament in the focusing cup
most rotating anode tubes have 2.
filaments side by side creating 2 focal spot sizes (small and large)
When X-ray unit is turned on a low current is passed throughout he filament to warm it up.
at this low current, there is no tube current as thermionic emission is not possible.
Once filament current is high enough for thermionic emission, a small rise in filament current results in a large rise in tube current.
xray tube current is adjusted by controlling the filament current.
Relationship of tube current and filament current depends on the tube voltage.
fixed mA stations of 100, 200, and 300 mA usually correspond to discrete connections on the filament transformer or to precision resistor.
When emitted from the filament, electrons are in the vicinity of the filament before they are accelerated to the anode.
Due to negative charges, electrons form a cloud around the filament called a space charge ( a cloud of electrons)
Space charge effect- negative charges of electron buildup begin to oppose emission of additional electrons.
Obstacle in producing X-ray tubes with currents greater than 1000 mA
Thermionic emission at kVP increases mA can be space charge limited.
Most xray tubes have two focal spots:
-large (.3 - 2 mm)
smaller focal spot size is used for better detail (spatial resolution)
Larger focal spot size is used for larger body parts or when techniques that produce a lot of heat are required.
actual X-ray source
where X-rays are born.
Selection is usually made by the technologist with the mA selector.
300 mA and lower can use either focal spot.
400 mA and higher can only use large focal spot due to heat capacity of the anode
The focal spot is an actual area on the anode where the electrons will hit.
The small focal sot is associated with the small filament and the large focal spot is associated with the large filament
an electric current is applied to the elected filament to create heat for thermionic emission.
The Anode (+)
Positive side of the X-ray tube
3 primary parts:
Heat dissipation is the biggest challenge in engineering high capacity X-ray tubes.
Copper, molybdenum, and graphite are the most common anode materials due to their ability to dissipate heat
What are the 2 types of anodes?
Stationary- used in units where high tube current and power are not needed (dental, some portables)
Rotating- used in stationary X-ray rooms
Functions of the Anode
An electrical conductor
provides mechanical support for the target dissipates heat.
99% of electrons kinetic energy is converted to heat
The area on the anode that is struck by the electrons from the cathode.
made of tungsten alloy
alloying the tungsten (usually with rhenium) adds mechanical strength.
High capacity tubes have molybdenum or graphite layered under the tungsten target to make the anode lighter and easier to rotate.
Why is tungsten the material of choice for the target?
3 main reasons:
1. High atomic number (74)
- high energy X-rays
2. Thermal conductivity n ear that of copper
3. High melting point (3400 degree celcius)
-can withstand high tube current
Much larger target area then stationary
heating is not contained to one small spot
nearly 500 times the area to interact with electron beam than that of a stationary.
higher tube currents and shorter exposure times are possible.
Induction motor- used to rotate anode
1. Rotor (made of copper)
-rotates the target rapidly during X-ray production
-located inside the enclosure
- electric motor that turns the rotor at high speed
-located outside the enclosure
can dissipate more heat by increasing the rotation speed of the anode.
most rotating anodes revolve at 3,600 rpm (revolutions per minute)
High capacity anodes rotate at up to 10,000 rpm
The shaft between the anode and the rotor
made of molybdenum because it is a poor heat conductor
occasionally the rotor mechanism of the anode fails and the anode becomes overheated an pits or crake causing tube failure.
the stator consists of a series of electromagnets equally spaced around the neck of the tube
inside the enclosure is the rotor
The rotor is a shaft made of bars of soft copper and soft iron that are fabricated in one mass
the rotating anode is powered by an electromagnet induction motor that consists of the stator and rotor
Electromagnetic Induction Motor
current int he stator windings induces a magnetic field that surrounds the rotor.
stator windings are energized sequentially so that the induce magnetic field rotates on the axis of the stator.
the magnetic field interacts with the ferromagnetic rotor causing it to rotate in synch with the activated stator windings.
Enclosure that contains the cathode and anode.
glass and metal
What are the purposes of the envelope?
1. Allows efficient flow of electrons from cathode to anode ( no air molecules to bump into)
-more efficient X-ray productions
-longer tube life
2. Provides insulation from electric shock
(cathode and anode are electrical charges)
3. Dissipates heat by inducting it to insulating oil that surrounds it.
what is the purposes of tube housing?
1. filled with oil to keep the tube cool.
2. insulated to prevent electric shocks to the radiographer.
3. Lead-lined to limit leakage radiation.
When technologist pushes the exposure button, there is a short delay (rotor prep) before exposure is made.
filament is heating and anode is accelerating to appropriate speed
only then is kVp applied (exposure)
two part exposure button should be depressed in one motion to extend tube life by minimizing time that the filament is heated
Coast time- the time it takes the rotor to stop rotating
For a new tube, the coast time is approximately 60 seconds
Lince Focus Principle
as the size of the focal spot decreases the heat on the target tis concentrated onto a smaller area (limiting factor to focal spot size)
Line focus principle- target designed with an angle to allow for a larger area for heating
results in an effective focal spot size that is smaller than the actual focal spot size.
Effective spot size- area projected onto patient and IR
Diagnostic tubes have target angels form 5 to 20 degrees
limiting factor of target angle is the ability of the cone of X-rays produced to cover the largest field size used (14x17)
when a smaller IR is used, the anode angle can be steeper.
Bi angular targets
Two different target angles
combined with different length filaments
very flexible combination
Anode Heel Effect
result of the line focus principle
causes the radiation intensity on the cathode side of the field to be greater than that on the anode side (can be a 45% difference)
This is due to electrons interacting with target atoms at various depths into the target
the smaller (steeper) the anode angle, the larger the heel effect
important to consider heel effect when imaging structures that differ greatly in thickness or mass density
always place the thickest part under the cathode side of the X-ray tube
EX: AP T-spine mammo
anode heel effects results in?
smaller effective focal spot on anode side of X-ray beam
less radiation intensity on anode side of X-ray beam
X-rays produces outside of the focal spot ( think water splash)
-extends the size of the focal spot
-increases skin dose modestly
-can significantly reduce image contrast
-can image patient anatomy that is intended to be excluded by collimation
reduced by incorporating a fixed diaphragm in the tube housing near the window
metal enclosure extracts electrons that are reflected from the focal spot and conducts them away.
use of a metal envelope reduces this
What can lead to tube failure?
1. A single very high exposure
2. a long exposure time
3. a cold anode
4. vaporization of the filament
1. a single very high exposure can result in pitting of the anode
-pitting of the anode occurs over time with use.
-wear and tear of the anode results in tube failure
3. a cold anode
-a cold anode hit with a high kVp espouser can result in cracking of the anode
- wear and tear of the anode results in tube failure
2. a long exposure time can overheat the anode.
Damages the bearings of the rotor assembly
results in warping and rotational friction of the anode, overheating of the anode and tube failure.
4. The most common type of tube failure:
vaporization of the filament causes tungsten to coat the glass or metal enclosure which results in arcing and tube failure.
Ways the technologist can extend the tube life:
use minim techniques necessary for given exam
-use minimum techniques necessary for a given exam
-faster image receptors
-tube warm up procedures
tubes can last for years if taken proper care of.
Systems must be warmed up with a low technique operation
max radiographic tenchiques should need be applied to a cold anode
Tube Rating Charts
Today protective circuits are built in to prevent use of unsafe techniques and heat overload.
3 types of tube rating charts:
-radiographic rating chart
-anode cooling chart
-housing cooling chart
(still used to check protective circuits)
Radiographic Rating Chart
A chart is supplied with every X-ray tube
for a given mA, any combination of kVp and time that is below the mA curve is safe
allows technologist to gauge whether a specific technique is safe for the tube they are using
Anode cooling charts
anode has a limited capacity for storing heat
thermal capacity of an anode is obtained in an anode cooling chart
demonstrates maximum heat capacity in HU's and cooling time necessary
Heat Unit Formula
in xray, thermal energy is measured in heat units (HU's)
HU= kVp x mA x s x rectification constant
Rectification constants: (modification factor)
Single phase generator -1
3 phase and HF generator- 1.40 (bushong)
Try this problem: How many heat units are generated by an exposure of 70 kVP, 300 mA and .15 seconds of a high frequency rectified?
70 x 300 x .15 x 1.40
Variable kVp tecnique chart
kVp varies with the thickness of anatomical part by 2 kVp/cm
produces high contrast images (low kvp)
enhances the visible
fixed kVp technique chart (arthur fuchs)
for each anatomical part there is an optimum kVp
-poduces low contrast images (high kvp)
decreases patient does (high kvp)
reduces xray tub wear
greater exposure latitude
shorter time settings (less motion)
what are the 4 charatcersits of the X-ray tube used in todays radiographic room?
1. coolidge xray tube
uses a rotating anode
uses a metal
The kVp control is found between what?
The autotransformer and the step-up transformer
Sets with similar terms
Sharon, chapter 6 bushong
Ch. 6 -- The X-ray Tube
Exposures I -- Chapter 6
Sets found in the same folder
X-ray interactions - Ch 12
Chapter 6 The Xray Tube
The x-ray tube chapter 6
Minimizing patient exposure
Other sets by this creator
Chapter 42: Magnetic Resonance Imaging - Adler
Other Quizlet sets
#1 Biology and the Tree of Life
OMM II - Still Technique (Final)
Chapter 4- Cultural, Legal, and Ethical Considerat…
Honors Biology (Lancon) - Chapter 7 Mid-Term - 12/…
what does the smv sinuses projection show?
T/F: Latex enema tips should be used for pediatric barium enemas for children younger than 1 year of age
What is the most unstable odontoid fx?
What is nephrogenic systemic fibrosis (NSF)?