ATTENUATION AND FILTERS

1. ATTENUATION AND FILTERS
BY:
PRADEEP PATIL
D Y PATIL MEDICAL COLLEGE AND HOSPITAL
KOLPHAPUR

2. INTRODUCTION
 The term “Attenuation” refers to the reduction in
intensity of an x-ray beam as it passes through the matter
or body of the subject.
 This reduction in intensity may be caused by
either Absorption or Deflection of photons from beam.

3. MONOCHROMATIC RADIATION
When we consider attenuation only in terms of quantity , then
reduction in intensity would merely be reduction in the number
of photons
Attenuation of monochromatic radiation is such that only the
quantity of the radiation changes, while quality remains the
same as it passes through the absorber.

5. Attenuation of monochromatic radiation is exponenetial.

6. ATTENUATION COEFFICIENTS
 Measures the quantity of radiation attenuated by a
given thickness of an absorber.
• Two Types :-
• 1. Linear attenuation coefficient
2. Mass attenuation coefficient

7. LINEAR ATTENUATION COEFFICIENT
 Quantitative measurement of the attenuation per cm of the absorber.
 Most impt. for diagnostic radiology . Denoted by mu.
 It is specific for the type of absorber and energy of the x-ray beam
used.
 It is inversely proportional to the energy of the x-ray beam used.

8. HALF VALUE LAYER (HVL)
 It is the thickness of the absorber which is required to attenuate the intensity
of original beam by one half.
 A beam of high HVL have more penetrating power than a beam of low HVL.
Thus expressing the quality of x-ray beam.
 HVL = 0.693/mu

9. MASS ATTENUATION COEFFICIENT
 It is used to quantitate the attenuation of material independent of their physical state.
 In contrast to Linear attenuation coefficient, Mass attenuation coefficient does not depend on
density.
 Considering an example of water:-
The linear attenuation coefficient of water-vapour is much less than LAC of ice, because the
molecules are more spread out in vapours (low density).
Whereas mass attenuation coefficient of water, ice and water vapour is same because 1 gm of
all three has exactly the same amt. of mass . So
MAC does not depend on physical state of matter.

10. FACTORS AFFECTING ATTENUATION
 RADIATION:-
1.Energy – As radiation energy increases attenuation
decreases
 MATTER:-
1. Density- Linear relationship
2. Atomic Number- Elements with higher Z have higher density .
So higher attenuation.
3. Electrons per gram- Lesser role to play. Elements with low Z
have more electrons per gram.

11. POLYCHROMATIC RADIATION
• It contains the whole spectrum of photons of various
energies. Generally the mean energy of polychromatic
radiation is between 1/3rd to 1/2 of its peak energy.
• When polychromatic radiation passes through an
absorber, the transmitted photons undergoes changes in
both quality and quantity.

13. APPLICATIONS TO DIAGNOSTIC RADIOLOGY
• The photons of an x-ray beam enters the body of the
patient in uniform distribution and emerge in a specific
pattern of distribution.
• Image formation on a film depends on the differential
attenuation of various tissues of the body. And the amount
of contrast of the image is directly proportional to the size
of diferential attenuation between the tissues.

14. SCATTER RADIATION
 X-ray beam that is responsible for the image formation is also called
Primary radiation. But it also contains radiation that does not contribute to
image formation rather it detracts from film quality. That is called scatter
radiation.
 Scatter radiation refers to all the undesirable radiation responsible for
decreasing the contrast of the image.
 FACTORS AFFECTING SCATTER RADIATION:-
1.Kilovoltage(kVp)
2. Part thickness
3. Field size

16. 1. Field size
It is the most important factor in production of scatter
radiation.
Narrow beam of radiation
Irradiates small volume of tissues
Generates small volume of scatter radiation

17. 2. Kilo voltage (kVp)
 Radiation with low energy range (20-30 keV)
Photoelectric effect dominates
Little production of scatter radiation
 Radiation with high energy range
Increased percentage of compton reaction
More production of scatter radiation

18. 3.PART THICKNESS
 Total number of scattered photons keeps on increasing
as part thickness increases.
 But the photons originating in upper layer of body of
patient do not have sufficient energy to reach the film.
 Practically little clinical significance as we can’t decrease
thickness of patient. Except when we use compression
band during contrast procdures.

20.  Filters are sheets of metals placed in the path of x-ray beam near the x-
ray tube housing to absorb low energy radiation before it reaches the
patient.
 Diagnostic x-ray beams are polychromatic radiations which are
composed of photons having whole spectrum of energy.
 High energy photons :- only high energy photons penetrate through
the pt. and form the radiographic image.
INTRODUCTION

21. • Lower energy photons – These are absorbed in first few cms of the pt. body
and doesn’t contribute to the image formation. Also first few cm of the tissue
receive much more radiation than rest of the pt. And contribute to the pt. total
radiation dose.
So, to decrease the radiation load to the pt. lower energy photons should be
filtered before it reaches the pt. by interposing a filter material b/w pt. and x-ray
tube. That is called Filtration

22. LEVELS OF FILTRATION
• X-ray beam from its source is filtered by absorber at
following three diff. levels :-
1. Inherent Filtration
2. Added Filtration
3. The Patient

23. INHERENT FILTRATION
• It results from absorption of x-rays as they pass
through x-ray tube.
• Parts of the tube responsible for this filtration are:- 1.
Glass envelope (maximum)
2. Insulating oil
3. Window in the tube

24. ADDED FILTRATION
 When we place absorber in the pathway of x-ray
beam , that is called – Added filtration.
 The filter material should be such that it should
absorb all low energy photons and transmit all high
energy photons But Such ideal filter material doesn’t
exist.
 Al and Cu are usually taken for diagnostic radiology.

25. • Aluminium filter - used for low energy radiation.
• Copper filter – used for high energy radiation. But
never used alone. It is always used in combination with
Al.
• Photoelectric attenuation in copper produces
characteristic radiation which is energetic enough to
reach the patient and increases the skin dose.

26. COMPOUD FILTER
• Consist of two or more layers of two different materials
usually Cu and Al.
• Arranged in such a manner Cu faces the x-ray tube and
Al faces the patient. Because Al absorbs all the
characteristic radiation produced by the Cu.

27. WEDGE FILTERS
• Special type of wedge shaped filter is used In diagnostic radiology when the
density of the part to be examined is not uniform in thickness.
• Less radiation is absorbed by the thinner part of the filter so more radiation is
available to penetrate the thicker part of the patient.
• Application of wedge filter is helpful in lower limb angiography.

29. MOLYBDENUM FILTER
• Molybdenum target x-ray tubes have molybdenum filters which are used in
mammography.
• Mammography at 30-40 keV, molybdenum tube will produce high energy
radiation that will reduce the contrast of the image.
• So we use molybdenum filter of 0.03mm thickness which will attenuate the
x-rays above 20 keV and subsequently increases the contrast.

30. THANK YOU