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Chapter 4:
Diagnostic X-Ray Production
Learning Objectives: Chapter 4
• Understand the mechanical components both inside
and outside of an x-ray unit
• List & describe the parts of an x-ray tube and trace the
creation of an x-ray
• Describe the Line Focus Principle and its application in
radiography
• Define the focal spot, and understand problems that
can occur with it
• Define the anode heel effect
Steps in Creating a Radiograph
1. Getting power to the unit and tube
2. Heating the tube & producing energy
3. Converting energy to x-rays
4. Focusing the beam
5. Exposing the film
Getting Power to the Unit
•Power (W) = current (I) x volts (V)
oUnit of power is the watt
•Toady’s x-ray units use a max. of 300 milliamperes
and 125,000 volts
oHow many mA are in 1 ampere?
oHow many watts is this?
*Generators in clinics are rated at 30,000 W min.
How many kilowatts is this?
Equipment Relating to Power
•Energy that is needed travels from a
power plant to clinic generator
• Transformers boost power along the way
• Alternating current travels in 1 positive
and then 1 negative pulse
• This provides low current, with high voltage
• Cheaper and more efficient than direct current
•Power goes to the circuit breaker first
• Accepts the current into the unit (milliamperage)
• Needs to have a ground wire for safety
*When would the circuit be interrupted?
Equipment Relating to Power
•Wall switch- within reach of the x-ray unit
• Required by law
•On-off switch located on x-ray unit too
oBrings power directly to the x-ray unit
•Line voltage compensator
oIncreases/decreases incoming voltage
oWon’t see this on new units
*Radiology uses mA to regulate the number
of electrons used to produce x-rays*
Getting Power to the X-Ray Tube
3 transformers in an X-ray circuit:
1. Autotransformer – increases incoming voltage
• Allows operator to select kV’s to produce a radiograph
2. High-voltage transformer
• Final increase of voltage
• From 220 volts to 125,000 volts (maximum)
3. Filament transformer
• Produces voltage to the x-ray tube filament in the form
of heat
• Filament requires specific temp. for exposure to happen
X-Ray Unit
4 essentials to every unit:
• Control Panel/Generator
• X-Ray tube
• High-tension transformer
• Dedicated power line
How are X-Rays Produced?
•When electrons are slowed or stopped by the
atoms of a target area, x-rays are produced.
•This happens inside the x-ray tube, which is a
vacuum environment.
oVacuum allows electrons to keep their energy
•Once the electrons strike the target area, an x-ray
beam of photons is created.
X-Ray Tube
•Where X-rays are generated
oTransformers supply the power to here
•We need 5 elements to produce x-rays:
1.
2.
3.
4.
5.
Source of electrons
A way to accelerate the electrons
Obstacle free path
Target area
A heat-resistant glass envelope to provide a
vacuum environment
•Produces not only x-rays but a lot of heat
The X-Ray Tube
Cathode
Provides a source of electrons and directs them
towards a target area
oOverall holds a negative charge
• Circuit is activated  filament transformer sends
electricity here  heats up  electrons leave the
filament
oTwo filaments made of tungsten
• Filaments emit electrons when heated electrons are
held less tightly and become excited
• Filaments are found in the focusing cup of the cathode
• Point directly towards the target area on the anode
Rotating Anode
•Contains the target of the x-ray tube
oWhere the electrons are being sent to
• Where are they coming from?
oTarget is mounted on an angled disc
•Acts as an electrical conductor
oTransmits the electrons back to the generator
• Completes the circuit
•Made of tungsten, tungsten molybdenum or
tungsten/rhenium alloy
oAll heat resistant materials: which is best?
•Rotates so photons are not always focused on
the same spot
oDissipates the heat
Rotor Circuit
•Turns the rotating anode
oMore than 3000 revolutions/minute!
•Activated when the filament transformer starts
to heat the cathode
•Prepares the anode to
receive electrons
•Could be the source of loud
noise
oHappens at the bearings
Stationary Anode
•Anode is fixed in place
oWhat is the concern here?
oOperator must follow the warning lights on the
machine!
•Made of copper
oHas a wide stem
•Also composed of tungsten
oTo handle the x-ray conversion
•When are these used?
The Line Focus Principle
The change of direction of the radiation due
to the angle of the bevel on the anode.
•Ideal bevel angle is <15° from vertical
oThis makes the x-ray
beam very narrow
oNarrow beam = high
resolution image
Purpose is to make a small
“effective focal-spot size”.
Possible Focal Spot Issues:
1. Off-Focus Radiation:
• Produced by electrons
without great aim (hit outside
of the target)
• Bounce off the anode and to
the patient outside of the
focal spot
• Collar of lead around tube
normally prevents this
• Can appear as artifact on film
if the lead is not sufficient
*Prevent by having machine
maintenance performed
Off-Focus Radiation
Possible Focal Spot Issues
2. Heat Bloom:
• Target is exposed to radiation
a great amount in a short time
oAnode gets too hot and is not allowed
to cool
• Outer edges of focal spot
expand
• Enlarges effective focal spot
size
• Decrease in image resolution
(Image loses sharpness)
*Can be eliminated by checking
your anode cooling chart
Focal Spot Bloom
20
The Anode Heel Effect
Causes the intensity of radiation to be greater
on the cathode side of the x-ray tube
•Bevel of the angled anode
limits the amount of x-rays
produced on anode side
•Place thicker end of patient
on the cathode side
oHead usually to right
The Exposure Switch
Sets the events of producing an x-ray in motion
•Usually 2-stages
o1st stage activates the rotor, filament circuit, and
transformers
o2nd Stage causes the exposure through the tube
•Can be hand controlled or via foot pedal
oExposure takes two movements
•You must follow the light signals
o“wait”, “ready”, etc
Exposure Concerns:
•You are now generating x-radiation!
oThoughts?
•Be familiar with the noises
oThey may scare your patient – be ready!
• May need to condition the patient
•Sound of boiling liquid is bad – tube could
rupture