Transcript Document

Medical Physics at A Level
X-Rays
Contents
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Uses
Production of X-Rays
Brehmsstrahlung radiation
Characteristic radiation
The X-ray tube
Rotating anode
Intensifying screen
Barium Meal
Barium Enema
X-Rays are:• high frequency, high energy
electromagnetic rays.
• undetectable by the human senses
• very penetrating
• low localised ionization.
• generated when high energy electrons
struck a metal target
Uses:
• Imaging for detection of broken bones or tumors:
– simple X-ray,
– CT scanning,
– barium meal scanning.
• High energy rays for treatment of cancer
(destroying cells) - radiotherapy
• As incident beams in 'material characterization
technologies' (ways of finding out the properties
of materials), such as X-ray Diffraction, X-ray
Photoelectron Spectroscopy and Auger Electron
Spectroscopy.
From the syllabus:• Physical principles of the production
of X-rays:
– rotating-anode X-ray tube;
– methods of controlling the beam
intensity,
– the photon energy,
– the image sharpness and contrast and
– the patient dose
Production of X-rays
• X-rays are generated when high energy electrons
struck a metal target .
• The kinetic energy of the electron is transformed
into electromagnetic energy.
• Two kinds of x-rays are generated during this
process.
– Brehmsstrahlung radiation, also called "braking
radiation" or white x-rays is produced due to
electron deceleration.
– Characteristic x-rays are also produced when
electrons in target metal make transitions
between atomic energy levels.
Brehmsstrahlung radiation
• Bremsstrahlung is the
German word for
'slowing down' or
'braking', and here it
is used to describe
the radiation which is
emitted when
electrons are
decelerated or
"braked" when they
are fired at a metal
target.
Brehmsstrahlung radiation
• Accelerated charges
give off
electromagnetic
radiation, and when
the energy of the
bombarding electrons
is high enough, that
radiation is in the xray region of the
electromagnetic
spectrum.
Brehmsstrahlung radiation
• It is characterized by
a continuous
distribution of
radiation called
continuous x-ray
spectrum which
becomes more intense
and shifts toward
higher frequencies
when the energy of
the bombarding
electrons is increased.
Brehmsstrahlung radiation
• A projectile electron can lose any
amount of its kinetic energy in an
interaction with the nucleus of a target
atom.
• So the bremsstrahlung radiation
associated with the loss can take on a
corresponding range of values.
• For example, an electron with kinetic
energy of 70 keV can lose all, none, or
any intermediate level of that kinetic
energy in a bremsstrahlung interaction.
Brehmsstrahlung radiation
• The Bremsstrahlung X-ray produced can
have an energy in the range of 0 to 70
keV.
• Here, 70 keV is the energy that
corresponds to the cut off wavelength
(smallest wavelength - highest
frequency therefore the highest
possible energy - use E=hf=hc/l to
calculate it). This is different from the
production of characteristic x-rays
that have specific energies.
Characteristic X-rays
• Characteristic X-rays are produced by
transitions of orbital electrons from
outer to inner shells.
• Bombarding electrons can release
electrons from inner energy level orbits.
• Higher electrons can then fall into the
vacancy and if the energy gap between
the levels is great enough X-rays will be
produced.
Characteristic X-rays
• Since the electron binding energy for
every element is different, the
characteristic X-rays produced in the
various elements are also different.
• This type of X-radiation is called
characteristic radiation because it has
precisely fixed, or discrete, energies and
that these energies are characteristic of
the differences between electron binding
energies of a particular element.
• The effective energy characteristic Xrays increases with increasing atomic
number of the target element.
Characteristic X-rays
• The two sharp peaks
in the graph are
characteristic X-rays
which occur when
vacancies are
produced in K-shell of
the atom and
electrons drop down
from above to fill the
gap.
Characteristic X-rays
• The X-rays produced by
transitions from L to K
levels are called K-alpha xrays, and those from M to
K transition are called Kbeta x-rays.
• Transitions to the L-shell
are designated as L x-rays.
The graph also shows the
"brehmsstrahlung"
radiation which forms the
base for the two sharp
peaks.
Example to calculate the
emitted x-ray energy:
For tungsten, K electrons have binding energies of
69.5 keV, and L electrons are bound by 12.1 keV.
A K-shell electron is removed from a tungsten
atom and is replaced by an L shell electron.
What is the energy of the characteristic X-ray
that is emitted (in keV)?
ANSWER: 57.4 keV (the difference in the
energies of the shells).
The X-Ray Tube
The X-Ray Tube: Thermionic
Emission
• Electrons are produced by thermionic
emission in the cathode. This is
heated by a relatively low voltage
supply. At a cathode current of 100
mA, for example, 6 x 1017 electrons
travel from the cathode to the anode
of the X-ray tube every second.
The X-Ray Tube: electron
missiles!
• They are accelerated from the
cathode to anode across a high
voltage. As the kinetic energy of the
electrons increases, both the
intensity (number of x-rays) and the
energy (their ability to penetrate) of
the X-rays produced are increased.
Rotating anode
• To increase the output, tubes with a
rotating anode of a diameter up to
200 mm for a better dissipation of
heat are used.
• The anode is accelerated up to 9000
rpm within less than 1 second.
• Heat buildup will rapidly damage the
target unless some provision is made
for its dissipation.
Rotating anode
• The most common solution to the problem
in medical x-ray tubes is to mount the
anode target on the armature of an
electric motor so that the target becomes
a spinning disc which has the capacity to
absorb the heat over a large area even
though the actual focal spot is quite small.
• The size of the focal spot affects the
sharpness of the radiograph. As the focal
spot is enlarged, the radiograph becomes
less sharp… therefore a spinning anode is
better than having a bigger focal spot
The X-Ray Tube: X-Ray
Production
• When these electrons bombard on the
heavy metal atoms of the target, they
interact with these atoms and transfer
their kinetic energy to the target.
• These interactions occur within a very
small depth of penetration into the target.
• As they occur, the electrons slow down
(brake!) and finally come nearly to rest, at
which time they can be conducted through
the x-ray anode assembly and out into the
associated electronic circuitry
The X-Ray Tube: Energy
Changes
• The interactions result in the
conversion of kinetic energy into
thermal energy and electromagnetic
energy in the form of X-rays.
• By far, most of the kinetic energy is
converted into heat.
• The electrons interact with the
outer-shell electrons of the target
atoms but do not transfer sufficient
energy to these outer-shell electrons
to ionize them.
The X-Ray Tube: Energy
Changes
• Rather, the outer-shell electrons are
simply raised to an excited, or higher,
energy level. The outer-shell
electrons immediately drop back to
their normal energy state with the
emission of infrared radiation.
• The constant excitation and
restabilization of outer-shell
electrons is responsible for the heat
generated in the anodes of X-ray
tubes.
The X-Ray Tube: Heat
Production
• Generally, more than 99% of the
kinetic energy of projectile electrons
is converted to thermal energy,
leaving less than 1% available for the
production of X-radiation.
• In this sense, the X-ray machine is a
very inefficient apparatus.
The X-Ray Tube: Heat
Production
• The production of heat in the anode
increases directly with increasing
tube current. Doubling the tube
current doubles the quantity of heat
produced.
• Heat production also varies almost
directly with varying the high tension
voltage too.
The X-Ray Tube:
efficiency
• The efficiency of X-ray production is
independent of the tube current.
• Regardless of what mA is selected, the
efficiency of X-ray production remains
constant.
• The efficiency of X-ray production
increases with increasing projectileelectron energy.
• At 60 kVp, only 0.5% of the electron
kinetic energy is converted to X-rays; at
120 MeV, it is 70%.
The X-Ray Tube: Target
Material
• needs to have a high Z (proton number) so
that transitions of high enough energy to
emit X-ray radiation are possible
• needs to have a high melting point because
so much heat energy is produced.
• tungsten is ideal (Molybdenum for softer
X-rays needed for breast X-rays)
Action
•
Increasing
the tube
voltage
Graph of Intensity against
X-Ray photon energy
Effect
•
Increasing
the high p.d.
that is used
to
accelerate
the
electrons
will give the
average
electron
more energy
when it hits
the target
Clarity of image
•
Shape of spectrum
spreads out to encompass
higher energies
•
Too high an
energy of X-ray
will penetrate
too well to give
good definition if they all get
through - no
shadow - picture!
•
range is increased
•
60-125 KV is
usually employed
- giving energy
of about 30 keV
•
Characteristics in the
same place (natch!!)
•
area under the curve
increases
Action
AC/DC voltage
(AC necessary to get
higher voltages - can use
transformers! DC
acquired by electronic
rectiication and
'smoothing' circuitry)
Effect
Electrons produced by
thermionic emission only
accelerated across half
of the time!
Graph of Intensity
against X-Ray photon
energy
graphs for both are the
same except the DC one
is double the intensity
throughout (only
accelerated across to
target on half of the
wave).
Action
Effect
Graph of
Intensity against
X-Ray photon
energy
Increasing the
tube current (low
voltage one!)
Increases the
rate of thermionic
emission - more
electrons hit the
target - more Xrays produced.
Shape of
spectrum remains
the same
Overall increase
of exposure of
film
range is the same
but bigger dose to
patient!
Characteristics in
the same place
(natch!!)
more heating of
the target
area under the
curve increases
Clarity of image
Action
Increasing
exposure time
Effect
Graph of
Intensity against
X-Ray photon
energy
Clarity of image
Overall increase
of exposure of
film
but bigger dose to
patient!
more heating of
the target
risk of blur due to
movement of
patient - big
problem with
organs that
cannot be
constrained.
Action
Changing Target
Material
Effect
An increase in Z
(proton number)
will increase the
probablity of
electron
interactions of
enough energy to
produce X-rays so more X-rays
will be produced.
Graph of
Intensity against
X-Ray photon
energy
The
Characteristic
peak positions will
change - Ks will
shift towards
higher energies
(these depend on
the target
material!).
Clarity of image
allows choice of
X-ray energies
that give best
difference in
attenuation for
the part to
viewed.
range is the same
area under the
curve increases
soft X-rays are
needed for soft
tissue - harder
ones for bone.
Action
Using a
filter
(material
placed in
the X-ray
beam path)
Effect
Absorbs mainly
lower energy
X-rays - and
produces a
'harder' more
penetrating
beam)
Graph of Intensity
against X-Ray photon
energy
area under the curve is
smaller (as some of the Xrays have been absorbed).
Shape changes as mainly Xrays are reduced from the
lower energy values.
range is smaller - but high
energy the same.
Characteristics in the same
place (natch!!)
Clarity of image
reduces unwanted
X-rays and
therefore the
scatter due to
them - better
contrast
Action
Reducing beam
size
Effect
Graph of
Intensity
against X-Ray
photon energy
Clarity of image
less scatter better contrast especially if a
collimator is used
(lead grid that
only allows Xrays in a
particular
direction to get
through.
Action
Artificial Contrast
Media
Effect
See barium
meal/enema
Graph of
Intensity against
X-Ray photon
energy
Clarity of image
Clearly outlines the
inner surface of
internal bodily
organs by coating
them in a radioopaque material barium sulphate.
Action
Intensifying
Screens
Effect
Decreases the
required exposure
time.
Graph of
Intensity against
X-Ray photon
energy
Clarity of image
Make image
clearer with a
lower X-ray dose
Action
Focal spot size
Effect
Graph of
Intensity
against X-Ray
photon energy
Clarity of image
Small focal spot
produces
sharp images
BUT also intense
heating of
target
Intensifying Screens
• X-ray and other photographic films are
sensitive to the direct action of the x-rays,
but the photographic effect can be increased
very appreciably, and exposure time can be
decreased by the use of an intensifying screen
in contact with each side of the film.
• One form of intensifying screen consists of
lead foil, or a thin layer of a lead compound
evenly coated on a paper backing. Under the
excitation of x-rays of short wavelength and
gamma rays, lead is a good emitter of
electrons, which expose the sensitive film, thus
increasing the total photographic effect.
Intensifying Screens
• Another form of intensifying screen consists
of a powdered fluorescent chemical--for
example, calcium tungstate, mixed with a
suitable binder and coated on cardboard or
plastic. Its action depends on the fact that it
converts some of the x-ray energy into light,
to which the film is very sensitive.
• The decision as to the type of screen to be
used-or whether a screen is to be used at alldepends on a variety of circumstances, and is
made by the radiographer.
Types of film
• Several special types of x-ray film have
been designed for the radiography of
materials. Some types work best with lead
screens, or without screens. Other types
are intended primarily for use with
fluorescent intensifying screens.
Types of film
• X-ray films are commonly coated with
emulsion on both sides of the support (to
double the chance of exposure and
therefore decrease the dose) --the
superposition of the radiographic images
of the two emulsion layers doubles the
density and hence greatly increases the
speed at which the X-Ray image is formed
(halving the dose to the patient).
Types of film
• X-ray films coated on one side only
(single-coated films) are available for use
when the superposed images in two
emulsions might cause confusion - when a
very detailed image of an area is required.
Barium meal
• A barium meal is an x-ray examination
of the stomach and your oesophagus
(gullet). Often pictures of the first
part of the small intestine (the
duodenum) are also taken.
• For the test to be successful the
stomach should be as empty as
possible and so the patient will
probably be asked not to eat or drink
anything for six hours before the
examination.
Barium meal
• The patient will be asked to swallow
some fizzy tablets or granules, with a
little water. These will expand the
stomach with gas which makes it
easier to get a clear view of things. It
is very important that the patient
does not belch once s/he has taken
these. Sometimes s/he if also given an
injection of a drug to relax the
stomach and stop it moving while the
x-rays are taken (this can cause some
blurring of vision for an hour or so and
if this happens it is best not to drive).
Barium meal
• The patient is then given a cup of
'barium' to drink. It is actually barium
sulphate (a radiopaque - contrast
medium) and the mixture used normally
contains defoaming agents and a
mixture of constituents that make it
have excellent coating characteristics.
It is often fruit flavoured and is not at
all unpleasant.
Barium meal
• The barium shows up on the X-rays as
it strongly absorbs X-rays and
therefore outlines the gullet and
stomach in the X-ray picture.
• A number of X-ray pictures will then
be taken. This is completely painless.
• The examination is usually completed
within 30 minutes.
Barium Enema
• A barium enema is an x-ray
examination which involves filling the
large intestine with barium through a
tube inserted into the rectum. It is
similar to the meal - just inserted
into the body the other way round!
Barium Enema
• The patient can eat and drink quite
normally once the test is completed.
The barium will be passed out with
your bowel motions during the next
few days, it may make motions paler
in colour than normal.
• The results of the examination will
usually be available a few days later.