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X RAYS
What are x rays?
Electromagnetic waves with a wavelength
of between 10-8 and 10-13m
Found between UV and gamma radiation
How are X-rays produced?
X-rays are produced when fast moving
electrons are rapidly decelerated as a result of
striking a heavy metal target.
A very high voltage (100 kV) is placed across electrodes
in an evacuated tube
Electrons are produced at the cathode which is heated
by an electric current passing through the filament.
The high p.d. accelerates the electrons across to the
anode.
Most of the EK of the incident electrons is converted to
thermal energy of the tungsten atoms, about 1% is
turned into X-rays
The target gets very hot and must be cooled
by passing oil or water through it.
The tube has to contain a vacuum
to reduce the energy loss of the
electrons colliding with gas molecules
Recall from
quantum mechanics
E = hf
E = hc
λ
If the accelerating p.d increases
then the E of the X-rays increases.
If E increases, f increases and
lambda decreases.
Recall from
quantum mechanics.
E = eV
fmax = eV/h
Characteristic X-rays
These are produced
when an incident electron
knocks an inner shell
electron out of an
(tungsten) atom. Other
electrons move levels to
fill the gap. These
transitions emit X-rays
These X-rays have
definite energies.
Bremsstrahlung Radiation
As the incident electrons decelerate when they hit
the tungsten target energy is released as X-rays
Bremsen "to brake" and Strahlung "radiation"
Bremsstrahlung = braking radiation
Bremsstrahlung radiation produces
continuous spectrum X-rays
1. Calculate the maximum frequency and
minimum wavelength for X-rays produced
from an X-ray tube in which the accelerating
voltage is
A) 80 kV
B) 120 kV
How do X-rays interact
with matter?
X-rays can be considered as particles or waves
If you pass a beam of X-rays through matter its
intensity will be reduced. This is because the Xrays are absorbed or scattered.
The intensity of the beam is attenuated.
There are three mechanisms
1. The photoelectric effect
Incident X-ray photon ejects one of the orbital
electrons.
An electron from a higher energy level may
drop to fill the vacancy, emitting a
characteristic x-ray photon
2. Compton Scattering
This occurs for higher incident X-ray energies
The incident photon is scattered by an orbital
electron in an atom. Some of the photons energy is
given to the electron which then goes off in a different
direction from that of the scattered photon. The
ejected electron may have an energy from 0 to 2/3 of
the incident photon energy.
The greater the deflection of the photon, the greater
its energy loss.
3.Pair-pair Annihilation
A high energy X-ray photon interacts with the
nucleus of an atom of the absorbing material.
An electron-positron pair is produced and when
the positron is annihilated by an electron, two
identical low energy X-rays photons are emitted.
Intensity
W
Wm-2
m-2
If the X-ray beam is spreading out from a
point source the inverse square law applies
Collimation
In a collimated beam intensity is constant
When a collimated beam passes through a substance
its intensity decreases
The amount of absorption varies considerably with
the frequency of the X-rays
The absorption of low frequency X-rays is mainly due
to the photoelectric effect.
For higher frequency X-rays Compton scattering is
the dominant absorption mechanism and for very high
frequency X-rays it is pair production.
2 In an X-ray tube, electrons are accelerated from rest
through a pd of 72.4 kV before they hit the target anode.
2 (a) Calculate the kinetic energy of an electron as it
reaches the anode. Give your answer to an appropriate
number of significant figures.
(2 marks)
2 (b) Assuming that the electron gives up all this energy
to form an X-ray photon, calculate the wavelength of the
photon.
(2 marks)
The intensity of the x–ray beam decreases
exponentially with the thickness (x) of the
substance.
Write down the equation of intensity as the beam
passes through a material.
𝐼 = 𝐼0 𝑒 −𝜇𝑥
Attenuation coefficient = μ (units m-1)
Typical values for μ are:
For a vacuum: 0
Flesh: 100 m-1
Bone: 300 m-1
Lead: 600 m-1
Half-Value Thickness
•
This is the distance through a material through
which the X-rays must pass that halves the
intensity.
Using the Equation
•
Calculate the percentage of the intensity of X-rays not
absorbed after passing through 1 cm of flesh, bone
and lead.
•
Calculate the half value thickness of bone.
Describe the use of X-rays in imaging
internal body structures including the use of
image intensifiers and of contrast media
•
•
•
Photographic Film: Requires considerable exposure,
produces only a still image.
• Quality can be improved using a film that is more
sensitive to X-rays
• Put a fluorescent plate behind the film.
Use an X-ray absorbing substance as a contrast medium.
• Giving patients barium meal (barium sulphate) improves
contrast.
Use an image intensifier
• Using digital methods instead of film. This includes dots
that respond to X-rays.
• Can be recorded to give a moving image, or printed.
X-ray detectors
1.
Black and white photographic film; requires a beam of
high-intensity.
2.
Intensifying screen, which contains a material that
absorbs energy from the X-rays and re-emits it as light by
fluorescence. This light then produces an image.
3.
Fluoroscopic image intensifier. X-rays cause electrons
to be emitted from a photocathode. They produce a bright
image on a fluorescent screen. This is used when a
moving image is needed.
e) and f) Questions
•
Suggest a typical value for the intensity of an X-ray beam.
•
Use values from previous examples.
•
90,000 V
•
24 mA
•
Efficiency of X-ray tube 60%
•
X-ray tube output efficiency: 0.5%
•
General area of cover: 20 cm2 – 200 cm2.
e) and f) Questions
•
Calculate the intensity on an X-ray plate beneath 3 cm of bone, 15
cm of muscle and 2 cm of lead.
•
Initial intensity: 4.8 x 103 W m-2
•
Attenuation constants:
•
Lead 90
•
Bone 53
•
Muscle 6.9
January 2011
•
8a) Describe the use of image intensifiers and contrast media when X-rays are
used to produce the images of internal body structures.
•
b) A student suggests an image intensifier uses the photoelectric effect.
Explain why this is incorrect.
•
c)i) Explains how the production of a CAT scan image differs from that of a
simple X-ray image.
•
c)ii) Describe the advantages of a CAT scan compared to an X-ray image.
•
Jun
2010
10)a) State and describe one way in which X-ray photons
interact with matter.
•
B) Intensity of an initial beam of X-rays is reduced to 10%
of its initial value after passing through 3.00 mm of soft
tissue. Calculate the thickness of the soft tissue that
reduces the intensity to 50%.
•
C)i) Explain how imaging intensifiers are used to improve
the quality of the X-ray image. (explain how the image is
made brighter)
•
C)ii) Explain how contrast media are used to improve the
quality of the X-ray image
Jun 2011
•
No exam questions on material to date.
Jun 2012
•
7)c) What does Io represent?
•
c)ii) Bone attentuation: 3.3 cm-1. Calculate half value
thickness.
•
Describe the use of X-rays
in imaging internal body
structures.
2Dof the
X-rays
Traditional
X-rays show a shadow
part of the body
being imaged. This can be used to show bone breakages.
•
The shadow will be reasonably sharp provided the X-rays
come from a point.
•
An extended light source will give a fuzzy image.
•
Draw a diagram to show the difference!
•
2D imaging can be difficult in certain situations!
•
Tibia blocking the Fibula
•
Ulna blocking the radius
•
X-ray of the chest cavity
•
Radiographer not positioning person correctly
•
These can cause overlap and block out the desired area.
Angiograms
•
These are obtained by a method
called subtraction technique.
•
X-ray is taken and then digitised.
•
A contrast medium is then injected,
and another X-ray is taken and
digitised.
•
The first image is “taken away” from
the first, so that only differences are
shown.
•
Eliminates all the detail that is not
required.
•
Computer can be programmed to get
rid of patient movement.
Computerised Axial
Tomography
Computers along with X-rays can
•
be used to give 3D images of the
body.
•
Images must be taken from
different viewpoints.
•
Final image is generally a slice of
the body taken horizontally.
CAT Scan
•
The X-ray source is shielded.
•
X-ray beam emerges from a point and spreads
out through the patient.
•
Fan shaped, very thin beam.
•
Irradiates only very small sections at a time.
•
X-rays are detected by a ring of up to 1000
detectors.
•
Once it completes one revloution, it has moved
up 1 cm.
•
This can take slices, or compose a 3D image of
an organ.
CAT Scans
•
There is a dose of radiation, but this is less than it
used to be because of increased sensitivity of the
sensors.
•
Advantage: Can be taken quickly, so can have
many in one day.
Jan 2011
•
8)c)i) Explain how the production of a CAT scan image
differs from that of a simple X-ray image.
•
ii) Describe the advantages of a CAT scan compared
to an X-ray image.
Jan 2013
•
6)d) Describe the operation of a computerised axial
tomography scanner. State one of the advantages
of a CAT scan image over a conventional X-ray
image.