X-Ray Production & EmissionBushong Ch. 8 & 9
Objectives: Review x-ray production
requirements
X-ray tube interactions
X-ray emission spectrum
PRODUCTION OF X RAYSRequirements:
a source of fast moving electrons must be a sudden stop of the
electrons’ motion in stopping the electron motion,
kinetic energy (KE) is converted to EMS energies
heat & x-ray energies
How “X-rays” are created Power is sent to x-ray tube via cables mA (milliamperage) is sent to
filament on cathode side. Filament heats up – electrons “boil
off” Negative charge
How “X-rays” are created Positive voltage (kVp) is applied to ANODE Negative electrons = attracted across the
tube to the positive ANODE.
Electrons “slam into” anode – suddenly stopped.
X-RAY PHOTONS ARE CREATED
How “X-rays” are created Electron beam is focused from the cathode to
the anode target by the focusing cup
Electrons interact with the electrons on the tungsten atoms of target material
PHOTONS sent through the window PORT – towards the patient
Radiographic Equipment
X-ray Tube Construction
GF
ED
C
A
B
Principle Parts of the X-ray Imaging System Operating Console
High-voltage generator
X-ray tube
The system is designed to provide a large number of e- with high kinetic energy focused to a small target
E- traveling from cathode to anode
Projectile e- interacts with the orbital e- of the target atom. This interaction results in the conversion of e- _______ energy into ________ energy and ________ energy.
Tube Interactions 3 possible tube interactions
Tube interactions are generated from _____ slamming into ________?
Heat (99%), EM energy as infrared radiation (heat) & x-rays (1%)
X-rays = Characteristic (20%) or Bremsstrahlung (80%)
Heat Most kinetic energy of projectile e- is
converted into heat – 99%
Projectile e- interact with the outer-shell e- of the target atoms but do not transfer enough energy to the outer-shell e- to ionize
Heat is an excitation rather than an ionization
Heat production Production of heat in the anode increases
directly with increasing x-ray tube current & kVp
Doubling the x-ray tube current doubles the heat produced
Increasing kVp will also increase heat production
Characteristic Radiation – 2 steps Projectile e- with high enough energy to
totally remove an inner-shell electron of the tungsten target
Characteristic x-rays are produced when outer-shell e- fills an inner-shell void
All tube interactions result in a loss of kinetic energy from the projectile e-
It is called characteristic because it is characteristic of the target elementin the energy of the photon produced
Only K-characteristic x-rays of tungsten are useful for imaging
Bremsstrahlung Radiation Heat & Characteristic produces EM energy
by e- interacting with tungsten atoms e- of the target material
Bremsstrahlung is produced by e- interacting with the nucleus of a target tungsten atom
Bremsstrahlung Radiation A projectile e- that completely avoids the
orbital e- as it passes through a target atom may pass close enough to the nucleus of the atom to convert some of the projectile e- kinetic energy to EM energy
Because of the electrostatic force?
Bremsstrahlung
is a germanword meaningslowed-downradiation
X-ray energy Characteristic x-rays have very specific
energies. K-characteristic x-rays require a tube potential of a least 70 kVp
Bremsstrahlung x-rays that are produced can have any energy level up to the set kVp value. Brems can be produced at any projectile e- value
Discrete spectrum Contains only specific values
Continuous Spectrum Contains all possible values
Characteristic X-ray Spectrum Characteristic has discrete energies based
on the e- binding energies of tungsten
Characteristic x-ray photons can have 1 of 15 different energies and no others
Characteristic x-ray emission spectrum
Bremsstrahlung X-ray Spectrum Brems x-rays have a range of energies and
form a continuous emission spectrum
Factors Affecting the x-ray emission spectrum Tube current, Tube voltage, Added
filtration, Target material, Voltage waveform
The general shape of an emission spectrum is always the same, but the position along the energy axis can change
Quality The farther to the right the higher the
effective energy or quality
Quantity The more values in the curve, the higher
the x-ray intensity or quantity
mAs A change in mA or s or both results in the
amplitude change of the x-ray emission spectrum at all energies
The shape of the curve will remain the same
mA increase from 200 to 400
kVp A change in voltage peak affects both the
amplitude and the position of the x-ray emission spectrum
Filtration Adding filtration is called hardening the x-
ray beam because of the increase in average energy
Characteristic spectrum is not affected & the maximum energy of x-ray emission is not affected
Filtration Adding filtration to the useful beam
reduces the x-ray beam intensity while increasing the average energy
Added filtration is an increase in the average energy of the x-ray beam (higher quality) with a reduction in x-ray quantity Lowering the amplitude and shifting to the
right
What kVp does this graph indicate?
Target Material The atomic number of the target affects
both the quantity and quality of x-rays
Increasing the target atomic number increases the efficiency of x-ray production and the energy of characteristic and bremsstrahlung x-rays
Target material
Voltage Waveform 5 voltage waveforms: half-wave
rectification, full-wave rectification, 3-phase/6-pulse, 3-phase/12-pulse, and high-frequency.
Maintaining high voltage potential
Voltage generators
X-ray Quantity or Intensity What units of measurement is used for
radiation exposure or exposure in air?
Milliampere-seconds (mAs) – x-ray quantity is proportional to mAs
Kilovolt Peak (kVp) – If kVp were doubled the x-ray intensity would increase by a factor of four or kVp2
X-ray Quantity or Intensity Distance – x-ray intensity varies inversely
with the square of the distance from the x-ray target
When SID is increased, mAs must be increased by SID2 to maintain constant OD
Filtration 1 to 3 mm of aluminum (Al) added to the
primary beam to reduce the number of low-energy x-rays that reach the patient, reducing patient dose
Filtration reduces the quantity of x-rays in the low-energy range
Reducing low-energy photons
X-ray Quality or Penetrability As the energy of an x-ray beam is
increased, the penetrability is also increased
High-energy photons are able to penetrate tissue farther than low-energy photons
High-quality = high-penetrability Low-quality = low-penetrability
HVL = Half-Value Layer What is the HVL
HVL is affected by the kVp and added filtration in the useful beam
Photon quality is also influenced by kVp & filtration
HVL is affected by kVp
HVL In radiography, the quality of the x-rays is
measured by the HVL
The HVL is a characteristic of the useful x-ray beam
A diagnostic x-ray beam usually has an HVL of 3 to 5 mm Al
HVL 3 to 5 mm Al = to 3 to 6 cm of soft tissue
HVL is determined experimentally and a design specification of the equipment
X-ray Quality Kilovolt Peak (kVp) = increasing the kVp
increased photon quality and the HVL
Types of Filtration Diagnostic x-ray beams have two filtration
components – inherent filtration and added filtration
Inherent filtration – The glass enclosure of the tube (the window) – approximately 0.5 mm Al equivalent
Added Filtration 1 or 2 mm sheet of aluminum between the
tube housing and the collimator
The collimator contributes an additional 1mm Al equivalent added filtration
Compensating filter A filter usually made of Al, but plastic can
be used to maintain OD when patient anatomy varies greatly in thickness
Are useful in maintaining image quality. They are not radiation protection devices
Wedge filter
Compensating Filter What is an aspect of the tube design that
works as a compensating filter?
What causes this?
Questions?