Fundamentals of x ray imaging i

Fundamentals of X-ray Imaging
Simbarashe.T Gashirai
MSc,BScHons Radiography
National University of Science and Technology, Zimbabwe23/01/2019

Introduction: The Electromagnetic Spectrum
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EM Spectrum Cont’d
• EM spectrum is basically the range of all EM
radiations.
• EM Radiation - stream of photons travelling in
a wave like pattern carrying energy and
moving at the speed of light.
• Only difference between radiowaves, visible
light and gamma rays is the energy of the
photons.
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EM Spectrum Cont’d
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Discovery of X-rays: 8 November 1895
• Wilhelm Roentgen,
University of Wurzburg,
Germany.
• Applied a potential
difference across a partially
evacuated glass tube.
• Observed emission of light
from a fluorescent material
some distance away.
• Had to be due to radiation
produced by experiments –
x-radiation.
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Discovery of X-rays
• Series of experiments
showed:
– New radiation could penetrate
various materials.
– Could be recorded on
photographic plates.
• Within a month of discovery,
x-rays were being explored as
medical tools in Germany,
France, UK and USA23/01/2019

Medical Imaging
• Marked the genesis of medical imaging
• Prior to this, physicians were limited in ability to
obtain information about illness and injuries of
patients.
• Essentially relied on the five senses
• What they could not see, hear, feel, taste or smell
often went undetected.
• Medical imaging provided a window into the
body without having to cut through it.
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Medical Imaging
It ALL begins with imaging, if you can’t SEE it,
then you can’t DIAGNOSE it or TREAT it !

The Medical Image
A medical image is a Pictorial Representation of a
measurement of an object or function of the body
Many different ways exist to acquire medical image data

Principles of X-ray Production
• X-rays are produced by bombarding metal
targets with high speed electrons.
• Two types of interactions with the target
produce radiation:
– An interaction with electron shells
produces characteristic x-ray photons;
– Interactions with the atomic nucleus
produce Bremsstrahlung x-ray photons.
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Principles of X-ray Production
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Principles of X-ray Production - Bremsstralung
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The interaction process that produces the most photons is the
Bremsstralung process

Principles of X-ray Production - Bremsstralung
• Electrons that pass close to a nucleus are
deflected and slowed down by the attractive
force from the nucleus.
• The energy lost by the electron during this
encounter appears in the form of an x-ray
photon.
• All electrons do not produce photons of the
same energy.
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X-ray Photon Energy Spectrum
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X-ray Photon Energy Spectrum
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In reality, the bremsstralung spectrum Looks more like this due to
effects of x-ray tube filtration

X-ray Production – Characteristic Radiation
• Interaction occurs only if incoming electron has a Ek
> EB of orbital electron within the atom.
• Electron is dislodged from the atom and leaves a
vacancy that is filled by an electron from a higher
energy level.
• As filling electron moves down to fill the vacancy, it
gives up energy emitted in the form of an x-ray
photon.
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•Electron is
dislodged from the
atom and leaves a
vacancy
•Interaction occurs
only if incoming
electron has a Ek >
EB of orbital
electron within the
atom.

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Vacancy is then
filled by an
electron from a
higher energy
level.
As electron
moves down to
fill the vacancy, it
gives up energy
emitted in the
form of an x-ray
photon.

• X-ray photons produced in this manner are referred to
as characteristic radiation
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• The energy of the photon is
characteristic of the chemical element
that serves as the anode material.

Example – Characteristic Radiation (Tungsten Target)
• Electron dislodges a tungsten K-shell electron (EB =
69.5 keV).
• Vacancy is filled by an electron from the L shell (EB =
10.2keV).
• What is the energy of the characteristic x-ray photon
released from this transition?
• Characteristic x-ray photon energy equals energy
difference between these two levels - 59.3 keV.
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A subscript alpha (α) denotes filling with an L-shell electron, and beta (β) indicates filling
from either the M or N shell.

• Characteristic radiation
produces a line
spectrum with several
discrete energies, whereas;
• Bremsstrahlung produces
a continuous spectrum of
photon energies over a
specific range
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Bremsstrahlung vs Characteristic Radiation
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The typical spectrum of X-ray photons produced by
bombarding metal target with electrons
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Recap
• X-ray photons are produced when a metal target
is bombarded with high speed electrons
• Two interaction processes between electrons and
target material occur simultaneously:
– Interaction with atomic nucleus – Bremsstrahlung
radiation.
– Interaction with orbital electrons – Characteristic
radiation.
• X-ray photon spectrum produced is a continuum
of photon energies up to a maximum value and is
also characterised by sharp characteristic peaks.
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The X-ray Tube and Its
Components
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X-ray Production – The X-ray tube
• To produce Medical images with X-rays, a
source is required that:
– Produces enough x-rays in a short time
– Allows the user to vary the x-ray energy
– Provides x-rays in a reproducible fashion
– Meets standards of safety and economy of
operation
• X-ray tubes are specially designed in order to
achieve these requirements
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X-ray Tube
• Composed of two
principle elements –
anode and cathode.
• Electrons are emitted
from a heated cathode –
thermionic emission
• Accellerated through a
large potential difference
(20kV – 120kV.
• Strike anode – x-rays are
produced.
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X-ray Tube
• Anode – component where x-radiation is
produced.
• Has two primary functions:
– Convert electronic energy into x-rays
– Dissipate heat created in the process – x-ray
production
• Anode material is selected to enhance these
functions.
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Target material
• Most x-ray tubes use tungsten, which has an
atomic number of 74, as the anode material.
• Tungsten has a high melting point (3422֯C).
• Tungsten is almost unique in its ability to
maintain its strength at high temperatures,
and it has a high melting point and a relatively
low rate of evaporation.
• Rotates in order to aid with heat dissipation
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Filament/Cathode
• Expels electrons from the electrical circuit
(thermionically).
• Focusses them into a well-defined beam aimed at the
anode.
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Envelope
• Anode and cathode are contained in an airtight
enclosure, or envelope.
• Provides support and electrical insulation for
anode and cathode assemblies
• Maintains a vacuum in the tube.
• Presence of gases in x-ray tube would allow
electricity to flow through the tube freely.
• This would interfere with x-ray production and
possibly damage the circuit.
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X-ray Tube Housing
• Encloses and supports other tube components.
• Functions as a shield and absorbs radiation,
except for radiation that passes through the
window as the useful x-ray beam.
• Dissipates most of the heat created within the
tube.
• The space between the housing and insert is
filled with oil.
• Provides electrical insulation and transfers heat
from the insert to the housing surface.
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The X-ray Circuit
• Energy used by the x-ray tube to produce x-
radiation is supplied by an electrical circuit.
• Connects the tube to source of electrical energy -
the generator.
• The generator receives the electrical energy from
the electrical power system
• Converts it into the appropriate form (DC, direct
current) to apply to the x-ray tube.
• The generator also provides the ability to adjust
certain electrical quantities that control the x-ray
production process.
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The X-ray Circuit
• The three principle electrical quantities that
can be adjusted are the:
– KV (the voltage or electrical potential applied to
the tube)
– MA (the electrical current that flows through the
tube)
– S (duration of the exposure or exposure time,
generally a fraction of a second)
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Effect of kV in X-ray Production
• KV establishes the energy of the electrons as
they reach the anode.
• No x-ray photon can be created with an
energy greater than that of the electrons.
• Max photon energy, therefore, in keV is
numerically equal to the max applied
potential in kV (kilovolts).
• In most equipment max photon energy
coincides with kVp
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What would be the effect of increasing kVp
on the x-ray spectrum?
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kV controls the hardness of the beam

Effect of mA
• Tube current is controlled by the
filament/heater current.
• The greater the filament current, the greater
the amount of thermo-electrons available for
acceleration.
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What would be the effect of increasing mA
on the X-ray spectrum?
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Increases the intensity - quantity

X-ray Beam filtration
• Some x-ray photons are of such low energy –
they would not be able to penetrate the
subject.
• Contributes to ‘radiation dose’ to the patient
without any useful purpose.
• Need to block off such photons from x-ray
spectrum before they reach patient.
• Aluminium filter is frequently fitted accross x-
ray window so as to block off ‘soft radiaton’.
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X-ray Beam filtration
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Recap
• X-ray tube required to:
– Produce enough x-rays in a short time
– Allow the user to vary the x-ray energy
– Provide x-rays in a reproducible fashion
– Meet standards of safety and economy of
operation
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Recap
• X-ray tube has different components for
various purposes:
– Anode
– Cathode
– Glass envelope
– Tube housing and insulation
– Electricity supply
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Recap
• kVp, mA and s are parameters that may be
used to control the:
– Quality (Hardness/penetrating power) of x-ray
beam.
– Quantity (Intensity) of x-ray photons.
• Beam filtration is necessary to remove low
energy photons that are not beneficial to the
patient.
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Use of X-rays in Imaging of
Internal Body Structures
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The concept of Differential Attenuation
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X-radiation affects photographic plates
In a way similar to visible light
Shadows of internal body structures can therefore be recorded

Differential Attenuation
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Which Chest appears to have
pathology/infected?
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Pathology modifies the attenuation properties of normal tissue
This can be detected on x-ray

Image Quality
• Quality of the image obtained depends on the
sharpness and contrast of the image.
• Sharpness – ease with which edges of
structures can be determined from image.
• Contrast – differences in optical
density/degree of blackening in image.
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Factors Affecting Unsharpness
• Movement of subject during data acquisition. -
Movement unsharpness.
• Nature of x-ray source (not a point source but a
finite source) – Geometric unsharpness
• Distance of object from image receptor.
Geometric unsharpness.
• Nature of Image receptor – Photographic
unsharpness.
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Geometric Unsharpness – Finite
source size
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Geometric Unsharpness – effect of
object – receptor distance
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Which Image Has Better Contrast?
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Image contrast is important in differentiating normal from diseased tissue

Factors affecting Image Contrast
1. Energy of the x-ray beam (kV)
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Factors affecting contrast
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2. Scattered Radiation

Reducing scatter – collimation and
antisatter grid
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Factors affecting contrast
3. Body part
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Use of contrast agents
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Use of Contrast Agents
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Recap
• X-radiation affects photographic plates in a
way similar to visible light.
• Different body structures attenuate x-rays
differently.
• Shadows of different body structures can
therefore be captured on photographic plate.
• Pathology modifies the attenuation properties
of normal tissue - this can be detected on x-
ray
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Recap
• Radiographic Image quality can be described by image
contrast and sharpness
• Sharpness affect by:
– Movement of suject
– Finite size of x-ray source
– Distance of object from image receptor
• Contrast affected by
– X-ray photon energy (kV)
– Scatter
– Body part characteristics
• Contrast agents can be used to enhance contrast in
body parts of low inherent contrast.
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Attenuation of X-rays in Matter
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X-radiation in a vacuum
• When x-ray photons radiate from a source in a
vacuum, the intensity decreases n proportion
to the inverse of the square of the distance
from the source.
• Approximately the same behaviour occurs in
air.
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X-radiation in a medium
• In a medium where absorption processes are
occuring -
• Intensity of a parallel beam decreases by a
constant fraction when passing through equal
small thickness of the medium
• Gives rise to an exponential decrease in
transmitted beam intensity.
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Beer’s Law
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I – transmitted intensity
I0 – Initial intensity
µ - Linear attenuation coefficient (cm-1)
X – Thickness of material (cm)

• If initial intensity and final
intensity are known –
then the nature of the
attenuator can be
deduced.
• Conversely if nature of
attenuator and initial
intensity is known – then
transmitted intensity can
be calculated
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Recap
• X-rays traversing in a vacuum or in air follow
the inverse square law.
• X-rays traversing in matter follow beer’s law
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Computed Tomography
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Limitations of General X-ray imaging
• In your opinion what are some of the
limitations of general x-ray imaging?
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Problem of Superimpostion of
Structures
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Computed Tomography
• In 1972 – Sir Godfrey Hounsfield developed
the first CT machine that could image slices.
• Slices were free of superimposition from over
and underlying structures.
• This was achieved by obtaining various
measurements at different angular positions.
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Based on Hounsfield’s Scanner
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CT IMAGE RECONSTRUCTION
 A CT image is composed of a matrix of pixels representing the
average linear attenuation co-efficient in the associated
volume elements (voxels).
 Effect of rotation is criss-crossing of x-ray beam, this creates
small discrete areas (voxels)
Diagnostic Radiology Physics: a Handbook for Teachers and Students – chapter 11, 95

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• Available techniques are:
• Simple Back Projection
• Simultaneous Equation Techniques
• Filtered Back projection (FBP)
• Fourier Reconstruction
• Iterative Techniques

Simultaneous Equations
• Illustration on board. - 4 Voxel.
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Recall
• What reconstruction
does is giving you and
intensity value.
• Intensity can be related
to µ
• µ is actually characteristic
of a particular type of
material. (i.e bone, fat,
muscle, fluid etc)
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CT Image Presentation
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A shade of grey indicating the amount of attenuation in
each voxel is assigned to the corresponding pixel

Fundamentals of x ray imaging i

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