Download Lecture 1 - Intro

FRCR: Physics Lectures
Diagnostic Radiology
Lecture 1
An introduction to radiography with X-rays
and the X-ray tube
Dr Tim Wood
Clinical Scientist
Learning Objectives
5.2: Distinguish between different types of
diagnostic medical image and understand
how such images are created, reconstructed,
processed, transmitted, stored and displayed
5.3: Describe the construction and function of
medical imaging equipment including the
radiation or ultrasound source, imageforming components and image or signal
receptor
5.4: Indicate how imaging equipment is operated
and describe the imaging techniques that are
performed with such equipment
Learning Objectives
5.5: Identify the type of information contained in
images from different modalities
5.6: Distinguish between different indices or
image quality, explain how they are interrelated and indicate how they are affected by
changing the operating factors of imaging
equipment
5.7: Identify agents that are used to enhance
image contrast and explain their action
5.8: Explain how the performance of imaging
equipment is measured and expressed
Learning Objectives
5.9: Describe the principles of quality assurance
and outline how quality control tests of
imaging equipment are performed and
interpreted
A little bit of history…
• Wilhelm Röntgen discovered X-rays
on 8th Nov 1895
• Took first medical X-ray of wife’s
hand (22nd Dec 1895)
• Used to diagnose Eddie McCarthy’s
fractured left wrist on 3rd Feb 1896
(20 min exposure)
• Awarded first Nobel Prize in Physics
in 1901 for his discovery of ‘Röntgen
rays’
A little bit of history…
Thankfully, things improved!…
What is diagnostic radiology?
ra·di·ol·o·gy
The science dealing with X-rays and other
high-energy radiation, especially the use of
such radiation for the diagnosis and
treatment of disease
Origin:
1895–1900; radio- + -logy
Related forms:
ra·di·ol·o·gist, noun
What is diagnostic radiology?
• The underlying principle of the majority of
diagnostic radiological techniques is that X-rays
display differential attenuation in matter
– When the X-ray beam is targeted at a patient, the
different tissues in the body will remove a different
number of X-rays from the beam
• The resulting modified X-ray flux can then be
‘captured’ by some form of detector to produce a
latent image or radiation measurement
– Detection may be through film, phosphor screens,
digital detectors, etc
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
–
–
–
–
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
Planar or three-dimensional?
Planar or three-dimensional?
1 1 1
1 7 1
1 1 1
Subject contrast
7:1
3 9 3
2D image contrast
3:1
2D detector
Planar or three-dimensional?
• 3D imaging offers superior contrast to 2D
• More techniques are becoming available
– Computed Tomography (CT), Cone beam CT,
Tomosynthesis, etc
• Compromise is that doses tend to be much
higher than the planar image
– e.g. CT chest = 6.6 mSv c.f. PA chest = 0.02 mSv (a
factor of 330 difference!)
• Hence, despite being less common, they
account for a significant proportion of the UK
populations exposure to medical radiation
– CT accounts for 11% of examinations, but 68% of
dose (HPA 2008 review)
X-ray interactions with matter
• It is the physics of the interactions with matter
that determine how each imaging technique
works, and how it is used in clinical practice
• So, a bit of revision…
X-ray interactions with matter
(revision)
• 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
• Elastic scattering
• Photoelectric effect
• Compton effect
Elastic Scatter
•
•
•
•
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
Probability 
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
Probability  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
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
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
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%)
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