Download Lecture 2 - X-ray tube

FRCR: Physics Lectures
Diagnostic Radiology
Lecture 2
The X-ray tube, the physics of X-ray
production and ‘exposure factors’
Dr Tim Wood
Clinical Scientist
Overview
• The X-ray tube
• Controlling the X-ray spectrum
– Exposure factors
– Filtration
• X-ray beam uniformity
– The anode-heel effect
From last time…
X-ray Properties
•
•
•
•
•
•
•
•
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
Planar or three-dimensional?
• Planar imaging is the most common technique
used in diagnostic radiology
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–
–
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General radiography e.g. PA chest
Mammography screening
Intra-oral dental radiography
Fluoroscopy (but some modern ones can do 3D)
• The anatomy that is in the path of the beam is all
projected onto a single image plane
– Tissues will overlap and may not be clearly visible –
contrast is generally poorer than in 3D imaging
techniques
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
Interaction Processes
• Elastic scattering
• Photoelectric effect
• Compton effect
Photoelectric Effect
Compton Effect
The Mass Attenuation
Interaction Coefficient
The Mass Attenuation
Interaction Coefficient
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
The X-ray tube
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%)
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
Bremsstrahlung
Characteristic X-rays
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
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
Back to tube design…
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
Geometric unsharpness and the
focal spot
• Spatial resolution is dependent upon (more on
this next time…):
– Geometrical unsharpness
– Motion unsharpness
– Absorption unsharpness
• Geometric unsharpness is related to the fact that
we cannot (and in fact do not want to) produce
an ideal point source of X-rays
– The focal spot of the X-ray tube has a finite size that
results in blurring across the edge of structures
– Can be reduced by using a smaller focal spot,
decreasing the object-film distance (OFD) or using a
longer focus-to-film distance (FFD)
Geometric unsharpness –
The ideal point source
Ideal point source
of X-rays
FFD
Object
OFD
Film/detector
Geometric unsharpness –
A ‘real’ focal spot
Focal spot of
finite size, f
FFD
Object
OFD
Film/detector
Penumbra
Geometric unsharpness and the
focal spot
• So, 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
The focal spot
• Apparent focal spot size will vary across the film
– Elongated on cathode side, contracted on anode side
• Target angles vary between 7 and 20°
– Steeper angles allow greater tube loading
– BUT, the useful X-ray field is smaller due to the anodeheel effect (more on this later)
– Hence, steep target angles suitable for applications with
limited fields of view e.g. mammography, cardiology
• Typical focal spot sizes are
–
–
–
–
0.15-0.3 mm for mammography
0.6-1.2 mm for general radiography
0.6 mm for fluoroscopy
0.6-1.0 mm for CT
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
The diagnostic X-ray tube
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
Filtration
Output
0.5 mm Al
2.5 mm Al
7.5 mm Al
0
20
40
60
keV
80
100
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 beam uniformity
• The X-ray tube emits Xrays in all directions
• A collimator system is used
to adjust the beam to the
required size
– Two pairs of parallel blades
of high attenuation material
that can be adjusted to
define the required
rectangular field size (has a
light beam system for
visualisation)
The anode heel effect
• Ideally, the X-ray beam defined by the jaws
would be uniform across the whole image
• However, this is not the case due to the anode
heel effect, which results from the combination
of the angled anode and the depth at which Xrays are generated
– The steeper the target angle, the worse the effect
– Filtration differences at the edge of the field, the
inverse square law and apparent focal spot size also
influence beam uniformity, but to a lesser extent
The anode heel effect
The anode heel effect
• The electrons penetrate a few micrometres
below the surface of the anode before
generating X-rays
• Hence, the X-rays that are generated in the
target may be attenuated on their way out
• X-rays travelling towards the anode edge of the
field (A) will have to pass through more of the
target before exiting the tube
– Hence, attenuation will be greater on this edge, and
beam intensity will be lower than on the Cathode side
of the field (B)
– Roughening of the anode surface as the tube ages
make this effect worse
The anode heel effect
• Generally not noticeable on most films
– This effect is corrected for in digital imaging through
‘flat-fielding’ the detector
• The anode heel effect is actively exploited in
some modalities e.g. mammography (more on
this later)
• Can be minimised by using greater focus-to-film
distances, smaller fields and shallower target
angles