Overview on the topic of x-ray attenuation

1. X-ray Attenuation & law
of exponential attenuation
RAVI CHRISTIAN
M.SC MRIT
DMIMS(DU)

3. Introduction
 In a clinical environment, all X-ray beams (although not all X-ray photons) will
interact with the medium through which they pass and therefore, to a greater or lesser
extent, the beam will be attenuated
 Attenuation is the reduction in the intensity of an x-ray beam as it traverses
matter, by either the absorption or deflection of photons from the beam
 Attenuation = absorption + scatter.
 Primarily affected by the atomic number of the medium and the energy of the X-rays
photons

5. ATTENUATION COEFFICIENTS
 An attenuation coefficient is a measure of the quantity of radiation attenuated by a given thickness
of an absorber.
 The name of the coefficient is determined by the units used to measure the thickness of the
absorber.

6. ATTENUATION COEFFICIENTS
 The linear attenuation coefficients (cm-1)
 The mass attenuation coefficients (cm2/g)
 The electronic attenuation coefficients(cm2/electron)
 The atomic attenuation coefficients(cm2/atom)

7. Linear Attenuation Coefficient
 The linear attenuation coefficient (µ) describes the fraction of a beam of x-rays
or gamma rays that is absorbed or scattered per unit thickness of the absorber.
 The linear attenuation coefficient is the most important coefficient for diagnostic
radiology.
In Simple word
 It is a quantitative measurement of attenuation per centimeter of absorber, so it
tells us how much attenuation we can expect from a certain thickness of tissue.

8.  Because we measure our patients in centimeters, it is a practical and useful
attenuation coefficient.
 The unit of the linear attenuation coefficient is per centimeter, and thus the name
linear attenuation coefficient, because a centimeter is a linear measurement.
 The expression per Cm is the same as 1/cm, and is usually written cm-1.
 Its symbol is the Greek letter µ.

10.  Water, fat, bone, and air all have different linear attenuation coefficients,
 Linear Attenuation Coefficient is for monochromatic radiation
 The size of the coefficient changes as the energy of the x-ray beam changes.
 When the energy of the radiation is increased, the number of x rays that are
attenuated decreases, and so does the linear attenuation coefficient.
exponential equation for
x-ray attenuation

11. Linear attenuation coefficient- problem
 The main problem with using linear attenuation coefficient as the guide to how
much attenuation will occur in a given thickness of material is that as the
physical conditions of the material change (a material could be a solid, a liquid
or a gas), then the accuracy of that coefficient can change.
 If, for example, a material was heated and expanded, it is extremely likely that
its thickness would change (i.e. its volume would increase, its density would
decrease and the number of atoms per unit volume would decrease
proportionately). This would mean that the linear attenuation coefficient would
also change

12. Mass attenuation coefficient
 However, if the linear attenuation coefficient were divided by the density of the
medium, the numerical value would remain proportionally consistent. In this
context, density is represented by ρ (the Greek letter rho): (total) mass attenuation
coefficient (μ/ρ).
 The mass attenuation coefficient is, therefore, the linear attenuation coefficient
(represented by μ) divided by the density of the medium (represented by ρ) and
this ratio removes the anomalies caused by any physical changes which may
occur as a result of environmental conditions

13. Mass attenuation coefficient
 The mass attenuation coefficient is independent of density.

15. Attenuation Coefficient and Beam Energy
 The attenuation coefficient varies with photon energy. The linear attenuation
coefficient is defined for monochromatic beams.
 There is difficulty in the application of this coefficient to polychromatic beam.
Basically X-rays are polychromatic radiation beams.
 The low energy components are removed, while it is passing through the matter.
Hence, the effective energy of the beam increases, resulting beam hardening
effect. In addition, X-ray tube and filter also hardens the beam.
 Hence, diagnostic X-rays are heavily filtered beam that can be approximated to
monochromatic X-rays.

16. Law of exponential attenuation
 If a beam passes through an absorber of thickness x,
both absorption and scattering takes place.
 As a result, the transmitted beam will have less
number of photons and it is given by the relation
 where I is the number of transmitted photons, I0 is the
number of incident photons, e is the base of natural
logarithm and µ is the linear attenuation coefficient of
the absorber material.

17. FACTORS AFFECTING
ATTENUATION
 Increasing the radiation energy increases the number of
transmitted photons (and decreases attenuation), while
increasing the density, atomic number, or electrons per
gram of the absorber decreases the number of transmitted
photons

18.  Thickness of attenuator:
 The greater the thickness of attenuating
material, the greater is the attenuation. X-rays
are attenuated exponentially
 Density and attenuator:
 As the density of the material increases, the
attenuation produced by a given thickness
increases
 Thus different material such as water, fat, bone
and air have different linear attenuation
coefficient, as do the different physical state or
densities of a material such as water vapor, ice
and water

19.  Atomic number of attenuator: Materials with higher
atomic number (Z) have higher attenuation values
for a given thickness , The higher atomic number
indicates there are more atomic particles for
interaction with the x-ray photons

20. Attenuation of Monochromatic radiation
A beam of 1000 photons is directed at a water phantom. With each
succeeding centimeter of water, 20% of the remaining photons are removed
from the beam. The quality of monochromatic radiation does not change as
it passes through an absorber. (Change in the number, or quantity, of
photons in the beam.)

21. Attenuation of polychromatic x-rays
 Polychromatic beam contain a
spectrum of photon energies
 Because of the spectrum of photon
energies, the transmission of a
polychromatic beam through an
absorber does not strictly follow
exponential law
 Photons of low energy are attenuated
more rapidly than the high energy
photons
 The number of transmitted photon and
quality of beam changes with
increasing absorber thickness

22.  Half-value layer (HVL):
The half value layer (HVL) is defined as the thickness of material required to reduced
the intensity of x or gamma ray beam to one half of its initial value
 Tenth-Value Layer (TVL):
The tenth-value layer (TVL) is the thickness of the material necessary to reduce the
intensity of the beam to a tenth of its initial value

23.  The half-value layer is always stated together with the value of the applied voltage and
the filtration.
 Aluminum and copper are the materials commonly used to specify HVL.
 HVL is measured by thickness of aluminium in diagnostic X-rays and it is given for its
effective energy.
 For diagnostic X-ray beam energies, the HVL for soft tissue ranges from 2.5 to 3.0 cm.
 The half value layer (HVL) of the mammographic beam is about 0.3–0.4 mm Al. This
depends upon the kVp range and type of target/filter used in the tube.

24. THANK
YOU….