Transcript X-Ray Production & Emission

X-Ray Production &
Emission
Objectives:
 Review
x-ray production
requirements
 X-ray
tube interactions
 X-ray
emission spectrum
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 element
in 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 eof the target material

Bremsstrahlung is produced by einteracting 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 german
word meaning
slowed-down
radiation
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 xray 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, 3phase/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 xray 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?
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