2. BASIC OVERVIEW OF XRAY BEAM
‣ Xray’s belong to a group of radiation known as the ELECTROMAGNETIC
RADIATIONS.
‣ All the electromagnetic radiations have dual characteristics.
‣ WAVE NATURE
‣ PARTICLE NATURE
‣ Wave nature can be understood by :- c= νλ
‣ Particle nature can be understood by :- E=hv
4. WHAT IS SCATTERED RADIATION
‣ When useful X-ray beam interacts with objects then it results in the
production of a secondary beam known as the Scattered Radiation
νν
5. OVERVIEW OF SCATTERING
‣ Scattering is basically of five types:-
‣ COHERENT
‣ COMPTON
‣ PHOTOELECTRIC
‣ PHOTODISINTEGRATION
‣ PAIR PRODUCTION
‣ (of these compton scattering occurs mostly in diagnostic radiology)
6. DISADVANTAGES OF SCATTERED RADIATION
‣ Basically, when x-rays are absorbed then they cease to exist but
when they are scattered then they are deflected from there original
path and hence carry no useful information...
‣ At the end they only add up NOISE (unwanted densities) to the film.
‣ The end result is that the radiographic image is of a poor quality
which makes it difficult to diagnose it. Contrast of the film is also
degraded.
8. SOLUTION TO DECREASE THE SCATTERED
RADIATION
VARIOUS DEVICES TO REDUCE SCATTER
‣ Filters
‣ Aperture diaphragms
‣ Cones and cylinders
‣ Collimators
‣ Beam centering devices
‣ Radiographic Grids
9. PRINCIPLE FACTORS CREATING SCATTER
‣ More scatter is created in bone than in soft tissue.
‣ When the beam is restricted then less scatter occurs
and hence less of it reaches the film.
‣ Technical factors may have to be manipulated to
reduce the amount of scatter.
10. ATTENUATION
‣ The term ATTENUATION refers to the reduction in the
intensity of an X-ray beam as it passes through the
matter or body of the subject.
‣ The reduction in intensity may be caused by either
absorption or deflection of photons from beam.
11. 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.
14. 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
15. LINEAR ATTENUATION COEFFICIENT
‣ Quantitative measurement of the attenuation per cm of the
absorber.
‣ Most important of diagnostic radiology, denoted by μ
‣ 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.
16. 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 tha a beam of
low HVL thus expressing the quality of xray beam.
‣ HVL= 0.693/μ
17. MASS ATTENUATION COEFFICIENT
‣ It is used to quantify 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 1gm of MAC does not depend on
physical state of matter.
18. 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.
19. POLYCHROMATIC RADIATION
‣ It contains the whole spectrum of photons of various energies.
Generally the mean energy of polychromatic radiation is between
1/3rd to ½ of its peak energy.
‣ When polychromatic radiation passes through an absorber, the
transmitted photons undergoes changes in both quality and
quantity.
20. APPLICATIONS TO DIAGNOSTIC RADIOLOGY
‣ The photons of an xray 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 differential attenuation
between the tissues.
21. 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 quantity. 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:-
‣ Kilovoltage(kVp)
‣ Part thickness
‣ Field size
22. SCATTER RADIATION
▸ FACTORS AFFECTING SCATTER RADIATION:-
1. Kilovoltage(kVp)
2. Part thickness
3. Field size
23.
24. 1.FIELD SIZE
▸ It is the most important factor in production of scatter radiation
Narrow beam of radiation
Irradiated small volume of tissues
Generates small volume of scatter radiation
25. 2. KILO VOLTAGE(KVP)
▸ Radiation with low energy range(20-30kev)
Photoelectric effect dominates
Little production of scatter radiation
▸ Radiation with high energy range
Increased percentage of compton reaction
More production of scatter radiation
26. 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. Expect when we use compression band during contrast
procedures.
