This document discusses the principles of x-ray attenuation and how it determines the appearance of different tissues on radiographs. It explains that the attenuation of x-rays is dependent on the atomic number, density, and thickness of tissues. Tissues with higher atomic number and density, like bone and metal, appear white on radiographs as they absorb more x-rays. In contrast, low density tissues like fat and air appear grey and black as they absorb fewer x-rays. This determines the contrast seen between different tissues that helps with medical diagnosis using radiography.

1. WHITE ,GREY, BLACK- WHY AND
WHEREFORES??????
RADIOGRAPHY

2. • X RAYS USED IN MEDICAL DIAGNOSIS ARE
PRODUCED WITHIN THE XRAY TUBE WHEN
EXPOSURE IS MADE
• DIFFERENT TISSUES IN OUR BODY
ATTENUATES XRAYS AT DIFFERENT EXTENTS
• 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

3. • EXPONENTIAL ATTENUATION : When the number
of photons remaining in the beam decreases by
the same percentage with each increment of
absorber, as with monochromatic radiation
• Attenuation coefficient : Measure of the quantity
of radiation attenuated by the given thickness of
an absorber

4. • Linear attenuation coefficient: most
important for diagnostic radiology
• Quantitative measure of attenuation per
centimeter of the absorber
• Is for monochromatic radiation and is specific
for both the energy of the x-ray and type of
absorber
• When the energy of the radiation is increased
the number of xrays that are attenuated
decreases,and does the linear attenuation
coefficient

5. HALF VALUE LAYER
• The Absorber thickness required to reduce the
intensity of orginal beam by one half
• Common method for expressiong the quality of
an xray beam
• A beam with high HVL is a more penetrating
beam than one with low Hvl
• Hvl of a typical diagnostic beam is
• 30mm : tissue
• 12mm :bone
• 0.15mm : lead

6. Mass attenuation coefficient
• Quantitates the attenuation of materials
independent of their physical state
• Obtained by dividing the linear attenuation
coefficient by the density
• Mass attenuation coefficient is independent of
the density of the absorber

7. Factors affecting Attenuation
• ENERGY
• DENSITY
• ATOMIC NUMBER
• ELECTRONS PER GRAM

8. Density and atomic number
• Elements with higher atomic number are
denser than elements with lower atomic
number
• No relationship between atomic number and
density when different physical states of
matter are involved
• Water – effective atomic number -7.4 (but
density varies in ice,liquid,vapour )

9. Density and electrons per gram
• Density depends on volume ,hence there is no
relationship between density and electrons
per gram

10. Effect of energy and atomic number
• With extremely low energy radiation(20kev)
photoelectric attenuation predominates
irrespective of the atomic number of the
absorber
• Attenuation is always greater when the
photoelectric effect predominates
• As the radiation energy increases Compton
scattering predominates and replace
photoelectric effect

11. • ABSORPTION OF XRAYS IS BASED ON THE
1.ATOMIC NUMBER
2.DENSITY
3.THICKNESS OF THE TISSUES
• TISSUES WITH HIGHER ATOMIC NUMBER WILL
ABSORB MORE RADIATION THAN TISSUE WITH
LOWER ATOMIC NUMBER

13. • DENSITY– AIR AND SOFT TISSUE HAVE SAME
ATOMIC NUMBER BUT DENSITY IS LESS FOR
AIR SO LESS ABSORPTION
• THICKER TISSUES WILL ABSORB MORE XRAYS
THAN THINNER TISSUE OF SIMILAR
COMPOSITION
• GREATER THE AMOUNT OF TISSUE
ABSORPTION FEWER XRAYS REACH THE FILM
AND WHITER THE IMAGE ON THE FILM
• RADIOPAQUE TISSUES APPEAR MORE WHITE
AND RADIOLUCENT TISSUES APPEAR MORE
BLACK

14. BASIC TISSUE RADIOGRAPHIC
OPACITIES
1. GAS - BLACK
• MOST RADIOLUCENT MATERIAL VISIBLE ON
XRAY

15. FAT -GREY
• MORE LUCENT THAN BONE AND SOFT TISSUES
BUT IS MORE OPAQUE THAN AIR

16. SOFT TISSUE /FLUID- LIGHT WHITE
• BOTH SOFT TISSUE AND FLUID HAVE SAME
RADIOLUCENCY. VARIATION IN VOLUME ,
THICKNESS AND DEGREE OF COMPACTNESS
OF SOFT TISSUE CREATES A PATTERN OF
VARIOUS DENSITIES ON THE RADIOGRAPH

18. BONES & MINERAL- WHITE
• BONE IS COMPOSED PRIMARLY OF CALCIUM
AND PHOSPHOROUS
• NORMAL VARIATION IN THE RADIOPACITY
WITHN THE BONE AND BETWEEN BONES
BECOZ OF DIFFERENCE IN RADIOPACITY OF
COMPACT V/S SPONGY BONE ,CORTICAL V/S
MEDULLARY CANAL

20. METAL –BRIGHT WHITE
• MOST OPAQUE SHADOW SEEN ON
RADIOGRAPH