Download Chapter 3: X-RAY PRODUCTION

Chapter 3: X-RAY PRODUCTION
Ionization and Excitation
 Ionization is “knocking” an electron out of an atom
– Creates a free electron + ion (an atom with +1 charge)
– Occurs when radiated with energy above the electron binding
energy
 Excitation is “knocking” an electron to a higher orbit
– When the radiation energy is lower than the binding energy
 After either ionization or excitation, an atom has higher energy
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Chapter 3: X-RAY PRODUCTION
X-Ray Beam Quality and Intensity
 The x-ray beam has two main properties you need to understand.
Beam QUALITY is the ability of the beam to penetrate an object,
its all about the penetrating power of the x-ray photons.
Beam INTENSITY this is the number of x-ray photons in the beam.
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Chapter 3: X-RAY PRODUCTION
Factors Affecting X-ray Quality
1. Filtration.
 The primary purpose of adding filtration to the x-ray beam is to
remove low energy x-rays that have no chance of getting to the film.
 As filtration is increased, so is the beam quality, but quantity is
decreased.
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Chapter 3: X-RAY PRODUCTION
There are three types of filtration:
i.
Inherent Filtration: The glass envelope window equals about 0.5
mm Al
ii.
Added Filtration: Added in collimator
iii. Compensating Filtration: Used to improve image quality or
radiation reduction
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Chapter 3: X-RAY PRODUCTION
i.
Inherent Filtration
 The glass envelope of the tube filters the emerging beam. In
diagnostic x-ray tubes the glass is equal to about 0.5 mm Al.
 As tube ages and more tungsten is vaporized, tungsten will build up on
the inside of the tube that will add more filtration.
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Chapter 3: X-RAY PRODUCTION
ii.
Added Filtration
 One or two mm of aluminum is added filtration placed in the
collimator. This filtration is generally placed on the mirror of the
collimator.
 This filtration attenuates x-rays of all energies emitted from the tube.
This shifts the spectrum to the high side.
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Chapter 3: X-RAY PRODUCTION
iii.
Compensating Filters
 Compensating filters are added to the beam by the operator to
compensate for differences in subject tissue density or type.
 In areas of the body where there are great differences in tissue
density, compensating filters are used to reduce exposure in the area
of less density.
 This reduced patient exposure and improves image quality. The
thoracic spine and full spine x-rays need filtration.
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Chapter 3: X-RAY PRODUCTION
Compensating Filters
This is the 40” Cervicothoracic Compensating Filter.
It may be called the thyroid filter as it reduces the exposure to the upper thorax.
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Chapter 3: X-RAY PRODUCTION
Compensating Filters
This heart shaped filter is used to reduce exposure to the ovaries of females of child bearing age.
It reduces exposure by about 85%.
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Chapter 3: X-RAY PRODUCTION
2.
Kilovoltage
 As the kVp is increased, so is beam quality and therefore HVL.
 An increase in kVp results in a shift of the
x-ray emission spectrum
towards the higher energy side.
 This increases the effective energy of the beam, making it more
penetrating.
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Chapter 3: X-RAY PRODUCTION
Factors Affecting X-ray Intensity
 A number of factors affect X-ray Intensity @ Quantity. These same
factors also control radiographic film density:a)
Milliamperage- Seconds (mAs)
b)
kVp
c)
Distance
d)
Filtration
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Chapter 3: X-RAY PRODUCTION
1. mA x time (s) = miliamperage seconds = mAs
 The X-ray quantity is directly proportional to the mAs. If we double
the mAs, the number of electrons striking the target is doubled.
 300 mA @ 1/30 second = 10 mAs
 200 mA @ 1/20 second = 10 mAs
 100 mA @ 1/10 second = 10 mAs
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Chapter 3: X-RAY PRODUCTION
2. Kilovoltage
 X-ray intensity varies rapidly with changes in kVp.
 The change in quantity is proportional to the square of the ratio of the
change.
 If the kVp is doubled, the intensity would increase by a factor of four.
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Chapter 3: X-RAY PRODUCTION
 What really happens when the kVp is increased?
 When kVp is increased, the penetrability of the x-rays is increased and
relatively fewer x-rays are absorbed in the patient.
 More rays pass through the patients to interact with the film.
 To maintain a constant exposure of the film, an increase of 15% in
kVp should be accompanied by a reduction of one half the mAs.
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Chapter 3: X-RAY PRODUCTION
3. Distance
 Radiation intensity from an x-ray tube varies inversely with the square
of the distance from the target. This is referred to as the inverse
square law.
 It is the same for any type of electromagnetic energy.
 We will explores distances in the Lab.
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Chapter 3: X-RAY PRODUCTION
4. Filtration
 X-ray machines have metal filters inserted into the useful beam.
 The primary purpose is the remove the low energy beam that reach
the patient and are absorbed superficially.
 These low energy photons contribute nothing to the formation of the
radiographic image.
 Filters therefore reduce patient exposure.
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Chapter 3: X-RAY PRODUCTION
Primary and Secondary Radiation
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Chapter 3: X-RAY PRODUCTION
Primary Radiation
 The radiation that coming directly from a source, such as a radioactive
substance or an x-ray tube, without interactions with matter.
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Chapter 3: X-RAY PRODUCTION
Secondary Radiation
 This type of radiation is produced by a scattering of the primary x-ray
beam.
 The x-ray photons and photo-electrons in the beam undergo a change
of direction after interaction with atoms and molecules as they pass
through a substance.
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Chapter 3: X-RAY PRODUCTION
X-ray Tube
 The major components of the modern X-ray tube are: -
a)
cathode (electron source)
b)
anode (acceleration potential)
c)
rotor/stator (target device)
d)
glass/metal envelope (vacuum tube)
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Chapter 3: X-RAY PRODUCTION
a) Cathode
The cathode consists of:
 consists of the filament,
focusing cup, and associated
wiring.
 a spiral of heated low
resistance tungsten wire
(filament) is for electron
emission. Wire is heated by
filament current.
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Chapter 3: X-RAY PRODUCTION
b) Anode
 Consists of the anode, stator,
and rotor.
 Positively charged so that the
electrons from the filament
(cathode) are attracted to it
to produce x-rays.
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Chapter 3: X-RAY PRODUCTION
c) Rotor and Stator
 The rotor is the nonstationary part of a rotary
electric motor or alternator,
which rotates because the
wires and magnetic field of
the motor are arranged so
that a torque is developed
about the rotor's axis.
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Chapter 3: X-RAY PRODUCTION
 The stator is the stationary
part of an electric motor or
alternator. Depending on the
configuration of the motor the
stator may act as the field
magnet, interacting with the
armature to create motion. The
stator may be either a per
magnet or an electromagnet.
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Chapter 3: X-RAY PRODUCTION
Rotating and Stationary Anode
 Stationary Anode: An anode assembly that is immobile.
 Rotating Anode: An anode assembly that turns during exposure.
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Chapter 3: X-RAY PRODUCTION
d) Glass and Metal Envelope
 The envelope is the glass
housing that protects the
tube. It is also used to help
protect from excessive
exposure to x-rays. The
envelope is the first part of
the filtration system.
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Chapter 3: X-RAY PRODUCTION
Basic X-ray Circuit
 There are two main parts of the circuit, one is the main circuit and
the second is the filament circuit.
A. Main part of X-Ray Circuit: supplies power to the x-ray tube so that
x-rays are produced.
B. Filament Circuit: supplies power to the filament of the x-ray tube so
that the filament supplies enough electrons by thermionic emission.
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Chapter 3: X-RAY PRODUCTION
Basic X-ray Circuit
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Chapter 3: X-RAY PRODUCTION
a) Main X-ray Circuit
 Main Switch: The switch that generates the power to the x-ray tube.
 Exposure Switch: A remote control device that permits current to
flow through the circuit.
 Timer: Device used to end the exposure at an accurately measured
preset time.
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Chapter 3: X-RAY PRODUCTION
 In the diagram below are the important parts of the circuit. The blue
part is the main x-ray circuit and the tan part is the filament circuit.
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Chapter 3: X-RAY PRODUCTION
Components
1. main breaker - this is where the alternating current comes from to power the circuit.
2. exposure switch - when you push the button to start an exposure this switch closes to
start the exposure.
3. autotransformer - this is where you adjust the kVp for the exposure.
4. timer circuit - this part of the circuit stops the exposure.
5. high-voltage step-up transformer - this transformer bumps the voltage up so that the xray tube has very high voltage to make the electrons have enough energy to form x-rays.
6. four-diode rectification circuit - this makes the current only go in one direction through
the x-ray tube.
7. filament circuit variable resistor - this variable resistor adjusts the current going to the
filament.
8. filament step-down transformer - this transformer steps the voltage down and therefore
the current up.
9. x-ray tube - this is where the x-rays are created.
10. rotor stator - this rotates the anode.
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Chapter 3: X-RAY PRODUCTION
b) Filament Circuit
Control
Factor
Electrical Device and Location in Circuit
kVp Selection
kVp Level
Autotransformer (between incoming line and
exposure switch)
mA Selection
Filament Current
Variable resistor (in filament circuit between
incoming line and step-down transformer)
Time Selection
Length of exposure
Timer circuit (between exposure switch and
step-up transformer)
Rotor Switch
Speed of rotating anode
Stator (separate circuit from stator of anode
motor)
Exposure Switch Moment of exposure
Switch (between autotransformer and timer
circuit)
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Chapter 3: X-RAY PRODUCTION
References
 John Ball, Adrian D. Moore, Steve Turner (2008) Essential Physics for
Radiographers, Blackwell
 Richard R. Carlton, Arlene McKenna Adler (2005) Principles of
Radiographic Imaging, Delmar
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