Transcript Tech Training Interactions and Equip-comp

X-ray Physics and Technology
Radiation Protection for X-ray Technologists
Dr Tim Wood
Clinical Scientist
Overview
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Atoms, electrons & radiation
X-ray production (‘low’ kVp X-ray tubes)
X-ray interactions with matter
Factors affecting patient dose
X-ray detectors
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Film
CR
DR
Image intensifiers
Atoms and Electrons:
The basics
Outer electrons
(negatively charged)
orbiting the nucleus
An inner nucleus made up of
protons (positively charged)
and neutrons (zero charge),
jointly known as nucleons
Atomic Structure
• Neutral Atom
• Number of electrons = number of protons
• +ve & -ve charges cancel out; the atom is said to be
in its Ground State
• Number of protons (Z = Atomic Number) and
electrons determines the chemical nature of
the atom
• If the Atomic Number (Z) changes, so does
chemical nature and behaviour of the atom
Atomic Structure
• Electrons carry a single negative charge and
orbit around the nucleus at precise distances or
levels
• The electrons are held in place by the attractive
forces generated by the positive protons in the
nucleus
• These outer levels are known as Orbits, Shells
or Energy Levels
Atomic Structure
Properties of Electrons:
Energy Level Rules
• Electrons can move from shell to shell, but
cannot exist in the Forbidden Zone between
shells
• The energy holding each electron in orbit in its
shell is known as the Binding Energy (BE)
and will vary from shell to shell
• To remove an electron from its shell, it needs to
receive an amount of energy greater than the
BE
Properties of Electrons:
Ionisation
-
• If an electron is
removed from a
neutral atom, it is no
= +1
0 charge
charge
longer neutral but
+
positively charged
• A positively charged
atom is known as a
Positive Ion
• The size of the positive charge equals the number
of electrons removed
• This process is known as Ionisation
Properties of Electrons:
Excitation
• Where an electron in a
neutral atom is raised to
a higher level, by
Excitation
applying energy (less
than the BE), the atom
remains neutral but is in
an Excited State
• This process is known as
Excitation
• After a short time, the electron
falls back to its original
position and releases energy
in the form of radiation
Energy, The Electromagnetic
Spectrum and X-rays
Wave Properties
• Energy may be transported (propagated) by
waves, described as electromagnetic
radiation
• Waves are oscillations
• Each wave as it moves generates an
electrical and magnetic field; hence
electromagnetic radiation
The Electromagnetic Spectrum
Visible
Ionising Radiation
Non-ionising Radiation
Infrared
Ultraviolet
Near
Radar
Far
X Rays
FM
TV
Gamma Rays
Short wave
Cosmic Rays
10-14
Power
Transmission
Broadcast
10-12
10-10
10-8
10-6
10-4
10-2
1
102
104
106
108
Wavelength in Metres
1010
High
108
106
104
102
1
10-2
10-4
10-6
Energy – Electron Volts
10-8
10-10
10-12
10-14
Low
Waves or Particles?
• EM radiations spread like waves,
over space. However, the way
they give up their energy is
distinctly not wavelike
• Absorption of energy occurs in
well-defined chunks of energy,
known as wave packets or more
correctly photons
Radiation
• Non-ionising
• Electromagnetic – Ultraviolet, visible, infrared,
microwaves, radio & TV, power transmission
• Ionising
• Capable for producing ions when interacting with
matter
• Natural & man-made
• Electromagnetic – X-, gamma and cosmic rays
• Particles – alpha, beta, neutrons, positrons,
electrons
Alpha particle () – Helium nucleus
0
+
+
0
Types of Ionising Radiation
Stopped by a piece of paper
Beta particle ()/electrons
Stopped by a layer of
clothing or a few mm of a
relatively low density
material such as aluminium.
Gamma rays ()/X-rays
Stopped by cm thicknesses
of lead or feet of concrete
The Physics of X-ray Production
X-ray tube design - basic principles
• Electrons generated by thermionic emission
from a heated filament (cathode)
• Accelerating voltage (kVp) displaces space
charge towards a metal target (anode)
• X-rays are produced when fast-moving electrons
are suddenly stopped by impact on the metal
target
• The kinetic energy is converted into X-rays
(~1%) and heat (~99%)
X-ray tube design
Stationary anode – dental X-ray tube
Rotating anode – general X-ray tube
X-ray tube design
• Evacuated glass envelope (allow electrons to
reach the target)
• Filament (cathode) is source of electrons, with a
focussing cup around it to generate a narrow
beam of electrons
– Often dual focus to offer finer resolution on diagnostic
sets
Thermionic emission
• Applying a current to the
filament causes it to heat up to
~2200°C (‘white hot’ like a light
bulb)
• ‘Free’ electrons in the metal
gain enough energy to
overcome the binding potential
– Can overcome the forces holding
them in the metal and escape
from the surface
• Tungsten metal is ideal
material
Thermionic emission
• Require two sources of
electrical energy to generate
X-rays
– Filament heating current (~10
V, ~10 A)
– Accelerating voltage of
between 30-150 kV (30,000150,000 V); this results in a
current of electrons between
the anode and cathode (0.51000 mA)
Electron production in the X-ray tube
kV
Applied voltage chosen to give
correct velocity to the electrons
mA
-
Filament
(heats up on prep.)
+
Target
The physics of X-ray production
• Electron reaches the anode with kinetic energy
equivalent to the accelerating potential (kVp)
• Electrons penetrate several micrometres below
the surface of the target and lose energy by a
combination of processes
– Large number of small energy losses to outer
electrons of the atoms = heat
– Relatively few, but large energy loss X-ray producing
interactions with inner shell electrons or the nucleus
Heat generating processes
• When an electron (e-) strikes the target, most
likely interaction is with loosely bound e-s that
surround nuclei
• Relatively weak interactions – slight deflection,
ionisation or excitation
• Small amount of energy transfer (per interaction)
– observed as heat
• However, accounts for ~99% of all energy
dissipated from e- beam in the diagnostic range
Bremsstrahlung
Bremsstrahlung
• If e- passes close to nucleus, strong electromagnetic
interaction – decelerates, and deflected
• Radiates energy in all directions as X-ray photons,
up to a maximum equivalent to kVp = continuous
spectrum
• High energy cut-off (≡ kVp) due to release of all
energy in head on collision with heavy nucleus
• Low energy cut-off due to self-attenuation by target,
X-ray window and additional filtration
• >80% of X-rays produced are Bremsstrahlung
(except for mammography)
Bremsstrahlung
Characteristic X-rays
Characteristic X-rays
• Interactions with tightly bound e- (typically K-shell)
• If energy of e- exceeds binding energy (BE) of
bound state → ionisation
• Vacancy leaves atom unstable
• e- from higher state drops down (most often from
L- or M-shell), releasing X-ray photon
(energy = difference in BE)
• Gives characteristic peaks on X-ray spectrum that
are specific to the target material (BE  Z2)
• For Tungsten target, Kα = 58 keV and Kβ = 68 keV
– Not observed below 70 kVp
The X-ray spectrum
• Combination of these yields characteristic spectrum.
4.00E+05
60 kVp
80 kVp
120 kVp
3.50E+05
3.00E+05
Intensity
2.50E+05
2.00E+05
1.50E+05
1.00E+05
5.00E+04
0.00E+00
0
20
40
60
80
Energy (keV)
100
120
140
The X-ray spectrum
• The peak of the continuous spectrum is
typically one third to one half of the maximum kV
• The average (or effective) energy is between
50% and 60% of the maximum
– e.g. a 90 kVp beam can be thought of as effectively
emitting 45 keV X-rays (NOT 90 keV)
• Area of the spectrum = total output of tube
– As kVp increases, width and height of spectrum
increases
– For 60-120 kVp, intensity is approximately
proportional to kVp2 x mA
Controlling the X-ray spectrum Exposure factors
• Increasing kVp shifts the spectrum up and to the
right
– Both maximum and effective energy increases, along
with the total number of photons
• Increasing mAs (the tube current multiplied by
the exposure time) does not affect the shape of
the spectrum, but increases the output of the
tube proportionately
• kV waveform – three-phase or high frequency
generators will have more high energy photons
than single phase. Hence, output and effective
energy are higher
The X-ray spectrum
4.00E+05
60 kVp
80 kVp
120 kVp
3.50E+05
3.00E+05
Intensity
2.50E+05
2.00E+05
1.50E+05
1.00E+05
5.00E+04
0.00E+00
0
20
40
60
80
Energy (keV)
100
120
140
Quality & Intensity
Definitions:
• Quality = the energy carried by the X-ray
photons (a description of the penetrating
power)
• Intensity = the quantity of x-ray photons in the
beam
• An x-ray beam may vary in both its intensity and
quality
Quality
• Describes the penetrating power of the X-ray
beam, and is governed by the kilo-voltage (kVp)
• Usually described by the Half-Value Thickness
– i.e. the thickness (in mm) of Al required to half the
beam intensity for a given kVp
• Typically >2.5 mm Al for general radiography
• Changing the quality of the beam will change the
contrast between different types of tissue.
• A highly penetrating beam is referred to as
‘Hard’ and a poorly penetrating beam as ‘Soft’
Intensity
• Intensity - is the quantity of energy flow onto a
given area over a given time; the ‘brightness’ of
an x-ray beam
• The tube current (mA) is a measure of X-ray
beam intensity
• Intensity is directly proportional to mA.
– i.e. Double the mA, double the dose (quality not
affected)
• Intensity is also affected by kVp
X-ray tube design
• Heat generation is a significant problem for Xray tubes, and is generally the limiting factor
upon their use
• Hence, it is necessary to:
– Ensure efficient cooling mechanisms – take the heat
away so it doesn’t build up with multiple exposures
– Have mechanisms to prevent over-heating – should it
get too hot, have mechanisms in place to stop further
exposures
– Minimise heat generation on a single point of the
anode (stop it melting!)
X-ray tube cooling
• Generally, the tungsten target is mounted on a
copper block/rotor (either directly or indirectly) that
extends out of the evacuated glass envelope
• Heat is transferred from the target to the
surrounding coolant (most often oil, but very
occasionally water) via conduction and/or radiation,
which in turn gives up its heat to the atmosphere
(possibly through a heat exchanger)
• Expansion bellows can detect when the coolant is
getting too hot (or by other means) and prevent
further exposures
• BUT, what about spreading the heat generating
processes over a larger area?...
The rotating anode
• Heat can be spread over a large area by rotating
the anode during exposure
• Tungsten annulus set in a Molybdenum disk
attached to a copper rotor
• The assembly is rotated via an induction motor
• Full rotation ~20 ms
• Takes about 1 s to get up to speed
– The prep phase (push the exposure switch down to
the first stop until you can hear it whirring) before
pushing down all the way to expose
X-ray tube design –
The rotating anode
The focal spot
• To minimise geometric unsharpness, the
smallest focal spot should be used…
• BUT, this would be at the expense of excess
heating and reduced tube life
• The solution is to use an angled target as the
source of X-rays
– Angle allows broad beam of electrons to give a
smaller apparent focal spot
– Have multiple filaments for focal spot size selection –
large focal spot for general use (tube lasts longer),
and small focal spot where better resolution is
required
X-ray tube design – dual focus
The focal spot
Actual focal spot size
Heat rating
• kV, mA and exposure time should be such that
the temperature of the anode does not exceed
its safe limit
– The control system is designed to prevent exposures
that exceed the tube rating
• Require much higher tube ratings for CT and
interventional fluoroscopy units
Shielding
• X-rays are emitted from the target in all
directions, not just towards the patient
• Hence, Lead shielding is used in the tube
housing to absorb X-rays not required for imaging
of the patient
• Legal requirements on how much ‘leakage’
radiation is emitted from the tube during
operation
– Medical Physics testing checks this during the Critical
Examination of new installations
The diagnostic X-ray tube
X-ray Interactions with Matter
X-ray Properties
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•
•
•
•
•
Electromagnetic photons of radiation
Emitted with various energies & wavelengths
not detectable to the human senses
Travel radially from their source (in straight
lines) at the speed of light
Can travel in a vacuum
Display differential attenuation by matter
The shorter the wavelength, the higher the
energy and hence, more penetrating
Can cause ionisation in matter
Produce a ‘latent’ image on film/detector
X-ray interactions with matter
• Contrast is generated by differential attenuation
of the primary X-ray beam
• Attenuation is the result of both absorption and
scatter interactions
• Scatter occurs in all directions, so conveys no
information about where it originated – can
degrade image quality, if it reaches film/detector
• Scatter increases with beam energy, and area
irradiated
Pass through
Absorption
Attenuation
Scatter
Attenuation
• For a mono-energetic photon beam:
where, I = final intensity, I0 = incident intensity, µ
= attenuation coefficient, x = thickness
• Equal thicknesses of material reduce the
intensity by the same fraction (half-value
thickness).
Attenuation
• Attenuation coefficient, µ, decreases with
increasing photon energy (except for absorption
edges)
• Increases with atomic number of material, Z
• Increases with density of material, ρ
• Transmission of radiation @ 70 kVp;
– 1 cm of soft tissue  66% transmitted
– 1 cm bone  17% transmitted
– 1 cm tooth  6% transmitted
Forward vs. Back-scatter
• Forward scatter is most likely, but ...
• Forward scatter is attenuated by the patient, and
• Deeper layers receive a smaller intensity, so there
are fewer scattering events
• Overall, see more back scatter.
• Advantage for image quality (less scatter, but more
attenuation at the detector), but may pose a risk in
terms of radiation protection
Forward vs. Back-scatter
Interaction Processes
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•
•
•
Elastic scattering
Photoelectric effect
Compton effect
Pair production
Elastic Scatter
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•
•
•
Photon energy smaller than BE
Causes e- to vibrate – re-radiates energy
No absorption, only scatter
< 10% of total interactions in diagnostic range
i.e. not significant
2
Z
Probabilit y 
E
Photoelectric Effect
• Process of complete absorption
• ~30% of interactions in diagnostic range
• Energy is transferred to bound e-, which is
ejected at a velocity determined by difference in
photon and BE
• e- dissipates energy locally, and is responsible for
biological damage
3
Z
Probabilit y  3
E
• Hence, main source of radiographic contrast
(and dose), and why Lead is used in protection
Photoelectric Effect
Photoelectric Effect
• Leaves atom in unstable state – electronic
reconfiguration results in emission of X-ray or
Auger electron
• Auger emission more probable for low Z material –
short range in tissue (= more biological damage)
• Low energy X-rays reabsorbed locally
• Rapid fall-off with increasing energy
Compton Effect
• Process of scatter and partial absorption –
inelastic scattering
• Photon collides with a free electron (photon
energy >> BE)
• Loses small proportion of its energy and changes
direction
• Energy loss depends on scattering angle and
initial photon energy
• Photon free to undergo further interactions until
completely absorbed (Photoelectric)
Compton Effect
Compton Effect
• Compton scatter mass attenuation coefficient
almost independent of energy over diagnostic
range
Z
Probability 
A
• Ratio of Z/A similar for most elements of
biological interest (~0.5) – offers little in terms
of radiographic contrast
Pair Production
• A high energy photon, in the vicinity of a
nucleus, is converted to a positron-electron pair
• The positron annihilates with an electron to give
two 0.51 MeV gamma photons
• Promoted by heavy nuclei
• Rapid increase with energy above 1.02 MeV
(not important in diagnostic range)
Probabilit y  Z
Pair Production
The Mass Attenuation
Interaction Coefficient
• Each process is independent – can add the
interaction coefficients to give the total mass
attenuation coefficient
• Z dependence is the source of contrast in
radiographic imaging
The Mass Attenuation
Interaction Coefficient
K-edge
Photoelectric
Total
Elastic
Compton
Pair production
The Mass Attenuation
Interaction Coefficient
Maximising Radiographic Contrast
• Maximise contrast due to Photoelectric absorption
– use lower energy photon beams (note, it is the
mean energy of the beam, not kVp that is
important)
• Use scatter rejection techniques such as scatter
grids and air gaps
• Limit beam to smallest area consistent with
diagnostic task to minimise amount of scatter
generated
• BUT…
Maximising Radiographic Contrast
• More Photoelectric absorption means higher
patient dose
• Scatter rejection techniques attenuate the primary
beam, so a higher patient dose is required for
acceptable image receptor dose
• NEED TO BALANCE IMAGE QUALITY WITH
PATIENT DOSE!!!
• Hence, the principle of ALARA (As Low As
Reasonably Achievable)
– Use the highest energy beam that gives acceptable
contrast, consistent with the clinical requirements
Factors affecting patient dose
Factors Affecting Patient Dose
• Tube Current (mA)/Exposure Factor (mAs)
– Double the mA/mAs, double the intensity
– Beam quality not affected
• Tube Voltage (kVp)
– Intensity α kVp2
– Penetrating power increases with kVp
– Higher kVp reduces skin dose
• Filtration (mm Al)
• Focus-to-skin distance
Patient dose reduction
Filtration and beam hardening
• ‘Soft x-rays’ contribute to patient dose without
contributing to image production
• Placing Al filters in the beam will increase beam
quality – this is known as ‘Beam Hardening’
– Alternative materials may be used for filtration in
specialised applications e.g. mammography (Mo, Rh,
Ag) and fluoroscopy (Cu)
• Lowest energy photons are most readily
absorbed as photoelectric absorption dominates
(proportional to the E3)
• As the beam passes the Al, the proportion of low
energy photons is reduced, and the average photon
energy increases
Filtration
Patient dose reduction
Filtration and beam hardening
• Hence, Patient dose is reduced with little affect on
the radiation reaching the detector
• However;
• Radiographic contrast is reduced due to the higher
mean energy of the beam
• Greater exposure factors required to yield satisfactory
dose at film/detector (have to drive the tube harder,
and hence tube life may be reduced)
• The X-ray beam is also filtered by the target that
they are produced in, the coolant oil and the
window of the housing
• ‘Inherent filtration’ equivalent to about 1 mm Al
Focus-to-skin Distance:
The Inverse Square Law
• For a point source,
and in the absence
of attenuation,
intensity decreases
as the inverse of the
square of the
distance
• This is a statement of
the conservation of
energy
2
2
2
1
D1 r

D2 r
The inverse square law
• Patient dose can be
significantly reduced by
increasing the distance to
the X-ray tube
– FSD < 45 cm should not be
used (<60 cm for chests –
180 cm used in practice)
X-ray Detectors
Film-Screen Imaging
• Traditionally, all X-ray image capture has been
through X-ray film
Emulsion
Adhesive
layer
Film base
Emulsion
Protective
layer
Film
• Film is actually much more sensitive to visible
light and UV than it is to X-rays
– Hence, use a fluorescent screen to convert X-ray
photons to light photons
– Enables lower patient dose!
• A latent image is formed upon exposure, which
cannot be seen unless the film undergoes
chemical processing
– Mobile silver ions are attracted to electrons liberated
by light photons, forming a speck of silver metal on
the surface
Processing
• The invisible latent image is
made visible by processing
• There are three stages to this
process;
– Development
– Fixing
– Washing
Processing
• First stage is development:
– Film is immersed in an alkaline solution of a reducing
agent (electron donor)
– Reduces positive silver ions to metallic grain of silver
(black specks)
– Unexposed crystals are unaffected by the developer –
bromide ions repel the electron donor molecules
– However, given sufficient time, the developer will
penetrate the unexposed crystals
– The amount of background fog is dependent upon the
time, strength and temperature of the developer
Processing
• Second stage is fixing:
– If the film is exposed to light after the first stage, the
whole film becomes black
– To ‘fix’ the film, unaffected grains are dissolved by an
acid solution, leaving the X-ray image in the form of
black silver specks
• Final stage is washing:
– The film is washed in water and dried with hot air
– Inadequate washing would result in a brown/yellow
film over time (from excess acid) and smell
Optical Density
• Optical Density: the
amount of blackening in
the film
• Defined as the log of the
ratio of the intensities of
the incident and
transmitted light
– log is used as the eyes
response is logarithmic
The characteristic curve
Optical
density
Linear region,
gradient = gamma
• Plotting OD against
log exposure gives
Saturation
the Characteristic
Curve of the X-ray
film
Solarisation
• Different types of film
– subtle differences
but all basically the
Fog
same
Log exposure
The characteristic curve
• Depends on type of film, processing and storage
• Fog: Background blackening due to
manufacture and storage (undesirable)
– Generally in the range 0.15-0.2
• Linear portion: useful part of the curve in which
optical density (blackening) is proportional to the
log of X-ray exposure
• The gradient of the linear portion determines
contrast in an image and patient exposures
must lie within this region
– Need to match this to the clinical task!
• Hence, film suffers from a limited and fixed
dynamic range
Automatic Exposure Control (AEC)
• Limited latitude of film makes it difficult to
choose correct mAs – skill and experience of
radiographer
• Alternative is to use an AEC to terminate the
exposure when enough dose has been delivered
to the film
• AEC is a thin radiation detector (ionisation
chamber) behind the grid, but in front of the film
(though in mammo it is behind to avoid imaging
the chamber on the film)
• Usually three chambers that can be operated
together or individually
Automatic Exposure Control (AEC)
• When a predetermined level of radiation is
detected, the exposure terminates
• Choice of chambers determined by clinical task
– e.g. left and right for lungs in PA chest, but central if
looking at spine
• Also has a density control that can increase or
decrease exposure where necessary
• AEC limited to exposures in the Bucky system
Digital imaging
What is a digital image?
• A digital image can be thought of as an array of
pixels (or voxels in 3D imaging) that each take a
discrete value
• The value assigned is dependent on the X-ray
intensity striking it
• Depending on its value, each pixel is assigned a
shade of grey
• Pixel size may determine the limiting spatial
resolution of the system
What is a digital image?
Why bother with digital?
• Film has been used since the beginning, so why
are we changing to digital techniques?
– Increased latitude and dynamic range
– Images can be accessed simultaneously at multiple
workstations
– Viewing stations can be set up in any location
– Uses digital archives rather than film libraries
– Images quicker to retrieve and less likely to be lost
– Post processing
– Softcopy reporting – lower cost if do not print
– No need for dangerous processing chemicals
Disadvantages of digital
• Initial cost
• Problems with interconnectivity
• Lack of information and set up of automatic
exposure control (AEC)
• Lack of link between exposure and brightness
– Potential for dose creep (see following slides)
• Human error in choosing exam type and speed
class
• Generally poorer limiting spatial resolution when
compared with film
Dynamic Range - Film
• With conventional film,
too low a dose will
results in a ‘thin’ film
• Too high a dose results
in a very dark film
• Fixed and limited
dynamic range – must
match exposure
parameters to the film
being used
– Gives a measure of
control over patient dose!
Dynamic Range - Digital
• With digital, too low a dose will
still produce a recognisable
image (just a bit noisy!)
• Similarly, too high a dose will
produce a recognisable image
(but with very little noise!)
• Consequences:
– Less retakes = GOOD
– Dose creep = BAD – must pay
special attention to digital imaging
to ensure doses are optimised
Computed Radiography
Computed Radiography (CR)
• Was the first digital technique available
commercially
• Exploits storage phosphors which emit light that
is proportional to the intensity of the X-rays that
hit it, when they are stimulated by a laser
beam
• The most common digital technique as it is the
cheapest (at least in the short term)
– Old X-ray sets used for film-screen radiography can
be used, provided exposure factors and AECs are
adjusted for the new type of detector
CR Components
Physical Principles of Computed
Radiography (CR)
• Fluorescence describes the immediate release
of low energy light photons after the absorption
of X-ray photons (exploited in traditional filmscreen radiography)
• Phosphorescence describes the delayed
release of light photon energy. This is the
principle of CR
CR Stage 1: Image Capture
• Image receptor is a laser stimulable phosphor,
known as an image plate (IP)
• Capture image by irradiating an IP in the same
way as conventional film
– Does not need a new X-ray system when replacing
film-screen (just make sure automatic exposure
controls are re-calibrated)
• Typically ~40% of X ray photons are absorbed
• IPs retain majority of absorbed X-ray energy as
a pattern of electrons in meta-stable energy
states
– The spatial distribution of stored electrons is
equivalent to the pattern of absorbed x rays – latent
image
CR Stage 2: Image Read Out
• A red Laser is used as this matches the energy
gap between Colour Centre and conduction
band
• Light in the blue end of the visible spectrum is
emitted
• Hence, optical separation of input and output
light photons
– Means a colour filter can be used to prevent laser
photons contaminating the output signal
• Blue light photons are collected via a
photomultiplier tube and digital image is
produced
Dynamic Range
• Film dynamic
range defined by
characteristic
curve  10:1
• CR dynamic range
> 10,000:1
• Linear relationship
between
log(signal) and
log(dose)
Körner M et al. Radiographics 2007;27:675-686
Detector Dose Indicators (DDI)
• The DDI has been introduced for digital imaging as an
indication of the level exposure on a broad region of the
detector
• Analogous to the OD of film
• The definition of DDI is manufacturer specific!
–
–
–
–
Some manufacturers have high DDI = underexposure
Some the other way round
Some are a function of the log of dose
Some are linear…
• Manufacturers will provide an indication of acceptable
range of DDI, but local departments must validate these
– DRLs and OPTIMISATION
• Operators should monitor DDI of patient exposures to
ensure doses remain acceptable
Digital Radiography
Digital Radiography
• Directly acquire the data in digital format (no
separate read-out phase like with CR)
– Improves throughput of X-ray systems – could be
important in chest clinic, mammo, etc
• Most expensive method, as it requires complete
dedicated X-ray system
• Main technologies:
– Phosphor coupled to a read out device – Indirect
conversion
– a-Se/TFT array – Direct conversion flat panels
Indirect conversion
• Indirect conversion involves converting the X-rays
into visible light (in a phosphor), and detecting the
resulting light photons (akin to film-screen
radiography!)
• Either amorphous silicon (a-Si) photodiode TFT
array, or CCD for readout
• Sharpness limited by both pixel pitch of readout
array, and spread of light in phosphor
– Usually CsI(Tl) needle phosphors to focus light down to
the detector (like mini-fibre optics to minimise spread)
– Needle phosphors can be thicker (more efficient)
Indirect flat panels
• Phosphor => X-ray to light photons
• Light photons detected in photodiode array =>
light photons to electrical charge
• Read out by the amorphous silicon TFT array
(discussed after direct conversion)
• Can be manufactured as a single panel up 45 x
45 cm2, but in practice tend to be made up of
four smaller detectors ‘stitched’ together
– Tiled detectors
– Requires image processing and interpolation to cover
the join between panels
CCD detectors
• CCD light detectors (like in a camera) can only
be manufactured in relatively small sizes
– Usually need multiple CCDs to cover image area
(‘tiled detector’), or slot scanning technique
– Also thicker than flat panels due to the optics between
the phosphor and detector
Direct conversion flat panels
• Amorphous Selenium (aSe) is a photoconductor
– Converts X-rays directly to
electrons
• Deposited directly onto
amorphous silicon TFT
array
• No phosphor, hence no
light spread
• Resolution governed by
effective pixel pitch
The TFT array
• Amorphous Silicon thin-film transistor array
• Transistors amplify electrical signals
• Electrical charge is stored in the TFT array until
release by applying a high potential
• Each row of detectors is connected to the same
activating potential (gate-line control), and each
column to a charge measuring device (read-out
electronics)
• The activating potential is applied row-by-row, so
the timing of the detected signal determines the
position of the pixel from which it originates
• Each pixel ~100 μm
The TFT array
Modern flat panels
Advantages of DR over CR
• Image displayed immediately to operator in room
• Faster
– Greater throughput of patients as no
intermediate read-out phase
• Slightly better resolution (CR limited by laser spot
size and scatter)
• Harder wearing imaging device (as long as you
don’t drop it!)
– Or at least that’s the theory…
Disadvantages of DR over CR
• Much more expensive
• Need to refurbish X-ray room
The Image Intensifier
The image intensifier
• The X-ray image intensifier (II) captures the time
varying image
• Today, images are viewed via a remote display
(image capture via CCTV (old units) or CCD)
• Computerised image processing is used to
enhance presentation
The image intensifier
• Three main components:
– The input screen
• The input window
• The input phosphor
• The photocathode
– The electron-optics
– The output screen
Gain
• Extent to which the II has intensified the light
output from the system
• Ratio of the brightness of the output phosphor to
that of the input phosphor
• Generally, 1 light photon from the input = 1
electron from the photocathode
• After acceleration to 25 keV, 1 electron = many
light photons in output phosphor
• This is the flux gain
– Typically ~50
Gain
• Minification gain is intensification from reducing
the image size at the output
• Equal to the ratio of the areas of the two screens
– For a 300 mm input and 30 mm output, minification
gain would be (300/30)2 = 100
• Overall gain is the product of the two
– ~5000 for this case!
• Gain is not measurable – use the conversion
factor as more practical descriptor
– Ratio of brightness out to dose rate at the input
(typically 25-30 Cd m-2 (μGys-1)-1
• Conversion factor deteriorates with time and
usage (loss of detection efficiency in the
phosphor)
The ABC
• In fluoroscopy, manual control of exposure
factors is not practical
• ABC takes a measurement of light intensity of
the output, or signal from the camera (most
common on modern systems) and feeds back
changes required to the X-ray generator to
maintain adequate light intensity
• Generally, only the central region of the image is
used by the ABC
• Can increase kV and/or mA
• The way it changes exposure factors can be
quite complex, and will depend on the clinical
application
The ABC
Brightness controlled by
simultaneous adjustment
of kVp and mA
Brightness controlled
mainly by adjustment of
kVp
The tube current is maximised
at 3 mA
Tube heating  kV x mA
(99% energy goes to heat)
X-ray intensity  kV2 x mA
Penetrating power with kV
Contrast with kV
Flat Panel Fluoroscopy
Flat Panel Fluoroscopy
• As with general radiography, flat panels are now used
on modern fluoroscopy systems
– Phosphor coupled to a TFT – Indirect conversion
– a-Se/TFT array – Direct conversion flat panels
• High quality dynamic and static image capture
• No distortions like the II (as long as set up correctly)
• Not able to do genuine magnification
– Resolution fixed by pixel pitch
• Use Automatic Exposure Control (AEC) – set level of
exposure required for adequate image
– Detector used to control the system
– Doses generally (but not always) lower on modern flat panel
systems compared with II
Flat Panel Fluoroscopy
• Increased dynamic range
– II has limited contrast ratio ~30:1
– Flat panel may use full 14-bit depth
• Superior limiting spatial resolution to II
– 3 lp mm-1 compared with 1-1.2 lp mm-1 for II (largest
field size)
• Directly acquire the data in digital format
• But, more expensive
• Can be used for Cone Beam CT (essentially a
very wide multi-slice CT)
– Poorer images compared with a ‘proper’ CT scanner
CT
How do we get the images?
• Tube and detector rotate smoothly around
the patient
• X-rays are produced continuously and the
detectors sample the X-ray beam approx
1000 times during one rotation
How do we get the picture?
• Back Projection
– Reverse the process of measurement of
projection data to reconstruct image
– Each projection if smeared back across the
reconstructed image