1. Dr Khazi Mohammed Athar
PROPERTIES OF X RAYS
INTERACTION BETWEEN X RAY AND
MATTER
ATTENUATION
FILTERS
X RAY BEAM RESTRICTORS
2. Properties of x rays
1. X rays are invisible
2. X rays have no mass
3. Travels at speed of light in vacuum
4. Travels in straight line
5. Have very short wavelength
6. Unaffected by electric and magnetic fields
7. They cannot be refracted
8. They cause ionisation
9. Have high penetrating power
10.Photographic film is blackened by x rays. Fluorescent materials glow
when x rays are directed at them.
3. INTERACTION BETWEEN X RAY AND MATTER
Most important factor is atomic makeup of tissue and not its
molecular structure.
1) Coherent scattering
2) Photoelectric effect
3) Compton scattering
4) Pair production
5) Photodisintegration
4. COHERENT SCATTERING
Interaction in which radiation undergoes change in direction without change
in wavelength.
Also called as unmodified /
classical scattering.
2 types
1) Thomson
2) Rayleigh
5. PHOTOELECTRIC EFFECT
Incident photon must have sufficient
energy to overcome the binding energy
of electron.
Most likely to occur when photon energy
and electron binding energy nearly same
Tighter an electron bound in its orbit,
more likely is to be involved in photo
electric reaction.
6. Applications
Photoelectric effect produces radio graphic images of excellent quality.
Because it doesn’t produce scatter radiation and also enhances natural
tissue contrast.
Because number of reactions depends on third power of atomic number,
photoelectric effect magnifies difference in tissues composed of
different elements such as bone and soft tissue.
7. COMPTON SCATTERING
The energy of incident photon is
distributed in two ways, part of it goes
to recoil electron as kinetic energy and
rest retained by deflected photon.
Probability of occurrence depends on
total number of electrons in absorber
which in turn depends on density and
number of electrons per gram.
And also depends on energy of
radiation
8. In diagnostic energy range, fewer photons scatter forward and more
scatter back at an angle of 180°, photons retain most of their original
energy at narrow angle of deflection.
Scatter radia from patients during fluoroscopic examin is almost as
energetic as primary beam. Creates real safet hazard for personnel in
exposure rooms..
9. PAIR PRODUCTION AND PHOTODISINTEGRATION
High energy photon interacts with nucleus, photon disappears and energy
is converted into matter in the form of 2 particles.
One is ordinary electron and other is positron.
Interaction cannot take place with photon energy less than 1.02MeV
In Photodisintegration part of nucleus is ejected by high energy photon in
the form of proton, neutron and alpha particle or clusters of particles.
Photon must have energy to overcome nuclear binding of order 7- 15MeV
Neither of two has any importance in diagnostic radiology where we use
energies <150keV.
10.
11. ATTENUATION
DEFINITION:
Attenuation is reduction in intensity of x ray beam as it traverses through
the matter.
Caused by either by absorption or deflection of photons from the beam.
It’s a measure of change in x ray intensity.
14. Attenuation coefficients
It’s a measure of quantity of radiation attenuated by given thickness
of absorber.
LINEAR ATTENUATION COEFFICIENT:( mu)
Quantitative measurement of attenuation per centimetre of absorber.
Unit: per centimetre
It is for monochromatic radiation
Specific for both energy of x ray beam and type of absorber.
15. When the energy of radiation is increased number of x rays that are
attenuated decreases and so does the linear coefficient.
HALF VALUE LAYER:
It’s the absorber thickness required to reduce the intensity of original
beam by one half
HVL = 0.693/mu
Beam with high HVL is more penetrating beam than one with low HVL
16. MASS ATTENUATION COEFFICIENT
Used to quantitate attenuation of materials independent of their physical
state.
Obtained by linear attenuation coefficient / density (mu/ rho)
Unit of x ray absorber is grams / square centimetre.
Ex: Water, ice and water vapour have same mass attenuation coefficient.
Because 1g of all three has exactly same amount of mass. Thickness of
1g/sq cm of water is 1cm, ice 1.09cm and water vapor 1670cm. These
thickness will attenuate same amount of x rays( ie 20%).
19. Polychromatic radiation
It contain spectrum of photons of various energies, most energetic
being determined by peak kilovoltage used to generate.
In general mean
energy of poly
chromatic radiation
is between one third
and one half of its peak
energy.
20. As polychromatic radiation passes
through absorber, transmitted
photons undergoes change in both
quality and quantity.
21. Application in Diagnostic radiology
Image formation depends on differential attenuation between tissues. Size
of this differential determines the amount of contrast in x ray image.
At low photon energies: Most of difference in x ray attenuation between
bone and soft tissue results from difference in number of photoelectric
reactions.
At high photon energies: The difference in X-ray attenuation between
bone and soft tissues is result of difference in number of Compton
reactions.
22. SCATTER RADIATION
Secondary radiation comes from Compton scattering
Factors affecting scatter radiation
1) kilovoltage
2) Part thickness
3) Field size
23. FILTERS
Filtration is process of shaping x ray beam to increase ratio of photons
useful for imaging to those photons that increase patient dose or decrease
image contrast.
Levels of filtration
1) Inherent filtration
2) Added filtration
3) The patient
24. Inherent filtration
Filtration resulting from absorption of x rays as they pass through x ray
tube and it’s housing
Materials involved are Glass envelope, insulating oil surrounding tube and
window in tube housing.
It’s measured in Aluminium equivalents, which represents thickness of
aluminium that would produce same degree of attenuation as thickness of
material in question.
It varies between 0.5 to 1.0 and glass envelope is responsible for most of
it.
25. Added filtration
Results from absorbers placed in path of x ray beam.
Ideally filter material should absorb all low energy photons and transmit
all high energy photons.
The energy of radiation filtered from beam can be regulated by selecting
material with appropriate atomic number.
Aluminium(13) excellent filter for low energy radiation.
Copper(29) filter for high energy radiation.
Filter thickness: 2mm Aluminium filter absorb nearly all photons with
energies less than 20keV.
26. Filtration selectively reduces
low energy photons in x ray beam
(area under curve). Intensity on
low energy side is reduced more
than high energy side of curve.
Effect on exposure factors:
Intensity of x ray beam is reduced
because filter absorb some photons
at all energy levels. To compensate
for loss of energy we must increase exposure factors(mAs).
27. Wedge filters
To obtain films of more uniform density when part being examined
diminishes greatly in thickness from one side of field to other.
When one side of patient is considerably
thicker than other, wedge compensates for
difference. Less radiation is absorbed by
thinner part of filter, so more is available
to penetrate thicker part of patient.
Often used in lower limb angiography.
28. HEAVY METAL FILTERS
K-edge filters: When percentage transmission of monochromatic
radiation through 1mm of lead is considered, sudden change in
transmission occurs at 88keV, which is the binding energy of K shell
electron. This is called K edge.
This filters make use of k edge of elements with atomic number greater
than 60, may offer advantage when imaging barium or iodine.
The maximum contrast is obtained when photon energy of x ray beam is
close to but slightly above the k absorption edge of absorber in question.
29. The purpose of heavy metal filters is to produce x ray beam that has high
number of photons in specific energy range.
K edge of aluminium is 1.6 keV,
too low to be useful as an energy
selective filter for diagnostic
radiology.
Heavy metal filters transmit signific
antly narrower spectrum of energies
than aluminium.
30. Molybdenum Filters
Molybdenum target x ray tube is used for film screen mammography to
take advantage of 17.5keV K alpha and 19.6keV K beta characteristic
radiation.
When operated at 30-40keV it will produce Brehmsstrahlung radiation
with energies higher than 20keV. This higher energy radiation will reduce
contrast in breast soft tissues. To reduce higher energy radiation,
molybdenum filter of 0.030 mm thickness is used.
Filter will attenuate x rays just above 20 keV(K edge of Mo) but transmit
57% of K alpha(17.5keV) and 67% of K beta(19.6keV) radiation of
Molybdenum.
31. X ray beam restrictors
X ray beam restrictor is a device that is attached to opening in x ray tube
housing to regulate size and shape of x ray beam.
Classified into 3 categories 1) Aperture diaphragm
2) Cones and cylinders
3) Collimators.
Aperture diaphragm: Consists of sheet of lead with hole in centre. Size
and shape of hole determines the size and shape of x ray beam.
Lead is soft so aperture can be altered in any desired size or shape but
disadvantage is it produces fairly large penumbra at periphery of beam.
32. The centre of x ray field
exposed by entire focal
spot but periphery of field
sees only portion of focal
spot, This partially
exposed area is called
penumbra.
Width of penumbra can
be reduced by positioning aperture diaphragm as far away from x ray
target as possible.
33. Cones and Cylinders
Flared shape of cone would seem to be ideal geometric configuration for
x ray beam restrictor but flare of cone is usually greater than flare of x
ray beam, in which case base plate that attaches device to tube housing is
only part that restricts the x ray beam.
Beam restriction with cylinder takes place at far end of barrel so there is
less penumbra.
Major disadvantage of aperture diaphragms, comes and cylinders is
severe limitation they place on number of available filed sizes.
34. Collimators
Collimators are best all around x ray beam restrictor.
2 Advantages 1) It provides an infinite variety of rectangular x ray fields
2) Light beam shows centre and exact configuration of x ray field.
Two shutters (S1andS2) control
beam dimensions. They move
to get as unit so that second
shutter aligns with first to clean
up its penumbra.
Shutters functions as two adjust
able aperture diaphragms.
35. Each shutter has four or more lead plates.
These plates move in independent pairs.
One Pai can be adjusted without moving
other which permits infinite variety of
square or rectangular field. When shutters
are closed they meet at centre of x ray
field.
Collimator can identify centre of field by painting cross line on thin
sheet of plexiglas mounted on end of collimator.
X ray field size in case for various target film distances is indicated by
calibrated scale on front of collimator.
36. X ray field illuminated by light beam
using bulb in collimator, which is
deflected by mirror mounted in path
of x ray beam at angle of 45°.
Target of x ray tube and light bulb
should be exactly same distance from
centre of mirror.
As light beam passes through second
shutter opening it is collimated to
coincide with x ray beam.
37. Positive beam limiting devices
PBL devices uses automatic collimators in which shutters are motor driven
Perfectly aligned collimator will leave unexposed border on all sides of
developed film.
PBL devices must be accurate to within 2% of source to image distance.
FUNCTIONS OF RESTRICTORS
1) Protect the patient
2) Decrease scatter radiation
