Download X-ray generation, interaction and detection

Radiation physics basics
Clémence Driol
IQ/system engineer
GE Healthcare
Gerhard Brunst
Clinical Leader X-ray Europe
GE Healthcare
Agenda
X-ray Basics
X-ray generation, interaction and detection
Image Quality and Dose
Radiation dose definitions, ALARA
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Radiation Physics Basics
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X-ray Basics
Introduction
Nature of X-rays
• X-Rays are electro-magnetic waves,
like light and radio frequency waves;
• Like all other waves, X-rays are also
Photons; this aspect is predominant
• Energy of X-Rays is usually measured
in eV and mostly its multiple: keV
• For medical imaging, energy of
photons ranges from ~10 keV to less
than 150 keV. (visible light ~2eV)
Fig: electro-magnetic spectrum
• X-rays are ionizing radiations
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X-ray dose and image quality
Inseparable
•
Images are created by the interaction of xrays with materials
•
During this interaction x-rays leave some
energy in the patient
•
dose at the Image Detector is about 100 to 1000
less than dose entering the patient
•
X-ray production, interactions with matter,
and detection are random in nature, so there
is statistical variation which we see as image
“noise”.
•
Object size and radiographic attenuation
affect our ability to visualize them against the
noisy background.
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X-ray Basics
X-ray generation
X-ray Tube
• Electrons are accelerated in a high electrical field (kV)
• When they hit the anode they have an energy of keVp,
• most electrons (99%) just cause heat in the anode
• 1% will create a photon of high energy, that‘s an x-ray photon of
x-ray energy keV < keV p (the electron energy)
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X-ray beam generation: atomic view
Bremsstrahlung
1.0
X-ray
X ray are generated by sudden
deceleration of energetic electrons on
the anode: Bremsstrahlung.
Normalized Counts
0.8
Characteristic Radiation:
Spectral lines characteristic of the
anode material
but not important for radiography
and interventional procedures
0.6
0.4
Continuous spectrum
e*kVp
(energy of the
electron)
0.2
mA = # photons
0
0
20
40
kV Contrast
60
80
Energy (keV)
100
120
kV  Penetration
Fig: X-ray Spectrum
Distributed spectrum = multi-energetic photons
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X-ray beam generation: in X-ray tube
Vacuum
Power
applied to
the tube
FILAMENT
CATHODE
1%
X-ray beam
99 %
HEAT
• Rotating anode to distribute the heat
• Size of the anode to accumulate the heat for some time (Anode heat capacity)
• Cooling system to maintain continuous fluoro (heat dissipation)
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Which X-ray Spectrum to Use?
soft x-rays (low energy, low keV) are
absorbed in a short distance into the body
- dose to patient but no use for imaging
medium x-rays are partially absorbed,
differently in different tissue material
- they provide information about the body
hard x-rays penetrate all material,
- don‘t provide image information
X-ray absorption vs photon energy
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Which X-ray Spectrum to Use?
soft x-ray photons can be
blocked by filtration
(Cu of various thicknesses)
avoid very x-ray photons
by using moderate kV
X-ray absorption vs photon energy
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Shaping the X-ray beam: Filtration
Spectrum after 25 cm Water
Spectrum at patient entrance
4.00E+02
7.00E+04
3.50E+02
3.00E+02
5.00E+04
Photon Nr
Photon Nr
6.00E+04
4.00E+04
3.00E+04
2.00E+04
2.50E+02
2.00E+02
1.50E+02
1.00E+02
1.00E+04
5.00E+01
0.00E+00
0.00E+00
15
25
35
45
55
65
75
85
15
25
35
Energy (keV)
regular filtration
0.2 mm Cu additional filtration
45
55
65
75
85
Energy (keV)
regular filtration
0.2 mmCu additional filtration
Softer radiation does not contribute significantly to image
Spectral filtration (Cu) eliminates this radiation before it reaches the patient
BUT – Potentially reduces image contrast because harder spectrum
BUT - Requires higher X-Ray Tube power to be effective
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Tube Take Away
• 99% of the electrical power is converted into heat (tube must be cooled,
specs for heat dissipation)
• The X-ray beam is multi-energetic (continuous spectrum)
• kV controls mainly the maximum x-ray energy
• mA controls the intensity of the beam
• filter removes low energy x-rays, shifts peak to higher energy
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X-ray Basics
X-ray absorption in the body
Interaction of X-rays with matter
• In vacuum, photons move along a straight line, no attenuation
• In materials, photons can be
-
Transmitted, no interaction
-
Absorbed, they disappear, transferring their energy to the material
-
Scattered, they are deviated, and keep the same energy (elastic scatter), or
transfer a fraction of their energy to the material (inelastic scatter)
• Interactions can be multiple
• Images are created by the interaction of X-rays with materials
• During this interaction, X-rays leave some energy in the material:
energy at the image receptor is 100 to 1000 times less than energy
entering the object (patient)
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Interaction of X-rays with matter (patient)
Absorption:
Scatter:
Incident photon
Incident photon
E0
In the photoelectric effect, an X-ray is
absorbed by an atomic electron, which is
ejected from the atom causing ionization.
Photoelectric effect is the most
contributor for the image formation
Scattered
Photon
Eω
In Compton scattering, an incident X-ray interacts
with an outer-shell electron. The electron is ejected
from the atom, causing ionization. A scattered x-ray
photon emerges at an angle ω relative to the
incident photon’s trajectory (scatter) or it gets
absorbed in further interactions.
Compton effect contributes to the image
but also cause fog on the image (reduced
contrast) and dose to the operator.
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Interaction of X-rays with matter
X-ray imaging principle
Low density tissue (lung with lots
of air): low absorption, high
transmission
medium density tissue (soft
tissue): medium absorption,
medium transmission
Dense tissue (bone): high
absorption, low
transmission
Contrast is the difference in signal due to either different attenuating thickness of
different attenuating material.
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Absorption
On their path more and more of the x-ray photons are absorbed
Dose within the patient similar to an exponential curve
but stretched due to beam hardening
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Attenuation of X-rays with AEC
1
10.0
2
20.0
AEC brings dose at patient exit to the
required level for good image quality
10.0
40.0
By doubling the object thickness
for same exit dose entrance dose is
doubled
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Attenuation of X-rays
3
10.0
3
80.0
Adding another doubling layer cause
another doubling of dose when exit dose
is maintained
10.0
160.0
4x the same thickness required 8x the
entrance dose than 1x the thickness
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Interaction of X-rays with matter
Effect of scattered radiation in image
object to
detect
• Contrast reduction due to added
offset in the image
• How to minimize scattered radiation?
 Minimize generation of scatter
•
Collimate to the smallest viewing area
that is required (less area that contributes
to scatter)
•
primary
radiation
scattered
radiation
image receptor
Minimize patient thickness by avoiding
steep angles when possible
 Reduce scatter effect
•
•
Add air-gap
Use anti-scatter grid
antiscatter grid
Anti-scatter grid reduce contrast loss but slightly increase the dose.
Remove-it for pediatric patients which cause little scatter.
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Quantum noise
“rain” of photons
Quantum noise is due to the quantified nature of X-rays
(made of a finite quantity of photons)
10 impacts
100 impacts
500 impacts
1000 impacts
To get a clear image, we need enough x-ray photons:
• sufficient dose per image
in general higher dose results in better image quality
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X-ray Basics
Image capture
CsI Detector Technology
1 photon X
(~0.15-0.3Å)
X ray
absorption
3000 light
photons
(~5000Å)
Light
emission
Scintillator:
CsI-Needles
X-ray Photon
Light
absorption
~1450 e-
electron – hole
generation
Conversion to
digital signal
Visible Light
Pixel Size
Amorphous Silicon Detector
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Image Quality
DQE = Detective
Quantum Efficiency
System must
be optimized
for spatial
resolution,
contrast, and
noise at the
same time
Contrast
Noise
Spatial
resolution
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Detector Efficiency DQE
DQE = Detective Quantum Efficiency
Detector
DQE =
Information Out
Information In
Patient
DQE measures how much of the latent X-ray
image that reaches the detector is accurately
captured to form a viewable image.
sensitivity: transfer of signal and of contrast
into the image
XRT
DQE combines the measures of spatial resolution, sensitivity and noise.
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Radiation Physics Basics
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Image Generation Take away
• Images are created by the interaction of X-rays with materials in the
body
• During this interaction X-rays leave some energy in the body
• Scatter radiation reduce relative contrast and need to be reduced.
• X-ray production, interactions with matter, and detection are random in
nature, so there is statistical variation which we see as image “quantum
noise”.
• DQE is the best scientifically accepted standard for assessing X-ray
detector dose efficiency.
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Image Quality and Dose
ALARA concept, Radiation risks
X-ray dose and image quality:
Relations
For each examination
• dose must be sufficient to provide the required
image quality for the diagnostic purpose
• but dose should not be higher than necessary
ALARA
As low as reasonably achievable
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GE Healthcare
Radiation dose
Physics
Definitions
PATIENT DOSE
ABSORBED DOSE:
Energy per unit mass in this
location
(J/kg=Gy)
Gray
Nature of
Radiation
DOSE EQUIVALENT:
Dose of x-rays that would
have same Biological Effect
(Sv)
Sievert
Localization vs. anatomy
measured
ORGAN DOSE:
Dose Equivalent integrated
over the organ
(Sv)
xxx
weighting by risk
PATIENT ENTRANCE DOSE (measurable)
•
Free in Air Skin Exposure (or Air KERMA) (Gy, R)
•
generally without backscatter
+ Integration
EFFECTIVE DOSE:
Sum of Organ Doses,
weighted by risk specific to
the organ
(Sv)
Biological effect
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GE Healthcare
effective dose cannot be measured, but is estimated in model calculations
Displayed Dose parameters
Cumulated Air Kerma
Focal Spot
67cm on
DiscoveryTM
Reference
Isocenter
Point
15cm
•
Live Monitor
Air Kerma is cumulated over
the course of exam and
displayed
•
•
Reference Monitor
Air Kerma is a local
dose property
By standard, it is determined
for a fixed distance to the x-ray
tube for isocentric C-arm
(Interventional Reference Point
or Patient Entrance Reference
Point)
The cumulated air kerma is
related to the patient skin
dose to some extent:
•
The reference point is only an
“average” representation of the
patient skin location
•
The accumulation is performed
over all gantry angulations and
table positions
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GE Healthcare confidential & proprietory
Displayed Dose parameters
Cumulated Dose Area Product (DAP)
•
Focal
Spot
Area2
Area1
AirKerma1
DAP is the product of Air
Kerma and exposed area and
is cumulated over the course
of exam and displayed
•
it is independent of distances
to the focal spot
•
DAP should be more
representative of long-term risk
(stochastic risk increases with
dose and with exposed area,
both are captured in DAP)
•
The accumulation is performed
over all gantry angulations and
table positions
•
Several guidance levels based
on DAP
AirKerma2
DAP = AirKerma1 * Area1 = AirKerma2 * Area2
Live Monitor
Reference Monitor
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GE Healthcare confidential & proprietory
Dose vs Dose-Area Product (DAP)
ESAK (Entrance Surface Air Kerma) vs AP (Dose Area Product):
Quantity
Dose (ESAK)
Pro
• Estimates deterministic risk of
skin burn, >> particularly
important for interventional
procedures
• Required reporting by JCAHO1 in
some regions.
DAP
• Coarse estimate of lifetime cancer
risk (but does not take into
account the different sensitivity of
the various exposed organs)
Con
Subject to inaccuracies:
• Source-skin distance
• Peak skin dose when using
multiple gantry positions and
FOVs
less useful to estimate skin burn
risk' (a small area exposed to a high dose
is reported the same as a large area with
a smaller dose)
• Easy to measure
Both Dose (ESAK) and DAP measures have their use.
Dose (ESAK) is best to assess deterministic skin injury risk.
DAP is a coarse estimate of (stochastic) lifetime cancer risk
Joint Commission on Accreditation of Healthcare Organizations
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GE Healthcare confidential & proprietory
Whose responsibility?
Clinical imaging optimization:
Benefit
Cost
=
Clinical Objective
~
=
Risk
Clinical Image Quality
Imaging Dose
Efficiency
=
Dose
Rewrite the Equation:
Clinical Image Quality

Imaging Dose Efficiency
Manufacturer’s Responsibilities:
Clinician’s Responsibilities:
To decide his/her thresholds
of acceptable image quality,
by procedure type.
• Maximize Imaging Dose Efficiency
• Provide wide range of Dose/IQ
selections to operator
• Provide Dose Feedbacks to clinician
for awareness and decision making.
• Automate the process as much as
possible.
x
Dose
Clinician’s
Responsibilities to
• Manage dose controls
during procedures.
• Monitor dose during
procedure
Clinicians AND Manufacturer have key roles in Patient Dose Optimization
Maximization of Imaging Dose Efficiency is primarily the responsibility of the
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manufacturer
GE Healthcare confidential & proprietory
Auto Exposure Control AutoEx
Automatic Dose Setting: Principles of Operation
Detector
Average Signal is read from detector
Patient
Auto Ex
XRT
AutoEx™ Calculates Effective Patient
Thickness (EPT), based on:
• Average Detector Signal
• kVp, mA, Pulse Width
• Spectral Filter
• FOV, SID
Generator
kVp
mA
Pulse width
Spectral Filter
Focal spot
Using new EPT, AutoEx Determines
Technique & Spectral Filter for next
exposure from trajectory table,
and sends to generator.
High level of automation, optimization and flexibility.
Adapts to a wide range of imaging situations.
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Radiation Physics Basics
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Dose Take Away
• X-ray is Electro-magnetic Radiation, with a spectrum far beyond UV, much more
energetic, making it able to penetrate matter
• An X-ray image reflects the difference in transmission through the different tissues
• X-ray is an ionizing radiation
– This property is used to detect and measure it
– It is also responsible for biological damage
• The absorbed dose is expressed in Gray
• Effective Dose (biological effect) is measured in Sievert (SI units)
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Radiation Physics Basics
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Radiation physics basics
Clémence Driol
IQ/system engineer
GE Healthcare
Gerhard Brunst
Clinical Leader X-ray Europe
GE Healthcare