Interaction X-Rays / Object
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KARTik vasuInteraction X-Rays / Object
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KARTik vasuHealthCar
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Interaction X-rays / object
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Basic concept of radiography
• X-rays are emitted from a source toward (part of) the object
to be imaged
• Depending upon the characteristics of the parts of the
object met on their path through the object, the X- rays
– will pass through undeviated, but may be attenuated
– or will be scattered by (parts of) the object
passing through, little attenuation
passing through, heavy attenuation
scattered ray
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Basic concept of radiography
• What gets through the object in an undeviated way, and
eventually is directed to the recording device, will form the
intended image
• Scattered radiation
– creates non representative image information
– and therefore should be controled to avoid such non useful image
information appearing on the film
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Interactions in the object
• Basically two types of interactions between the incoming X-
rays and the object need to be considered.
– the photoelectric effect
– the compton effect
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Photoelectric effect
• Occurs when an incoming X-ray encounters an electron from the inner
shell
– the electron is ejected, using most of the incoming X-ray energy
– an electron from an outer shell fills the hole
• a low energy X-ray is emitted
• the low energy X-ray will quickly be absorbed in the surrounding matter
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Photoelectric effect
X-ray
photoelectron
low energy X-ray,
absorbed quickly
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Photoelectric effect
• Wanted and of major importance in radiography
– results in complete attenuation of the incoming X-ray
– responsible for most of the contrast in radiology
• But … the patient’s exposure is high as all the energy of the
incoming photon is absorbed by the patient !
• Occurs more often
– with low energy X-rays
– materials with high atomic number
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Compton effect
• Occurs when an incoming x-ray encounters an electron
from an outer shell (with little binding energy)
– the electron is ejected
– the remaining X-ray energy is emitted again as X-ray
• in a direction different from the direction in which the X-ray
entered the object
• with only slightly less energy
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Compton effect
photoelectron
scattered X-ray
X-ray
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Compton effect
• The compton effect is unwanted in radiography as it results in
– scattered radiation
– loss of contrast
• The Compton effect is more likely with higher energy X-rays,
i.e. with high kV settings.
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Attenuation of non-deviated X-rays
• How much of the incoming X-rays is absorbed by the object (in
general) depends upon
– the physical density of the (distinctive parts of the) object
– the atomic number of the parts met
– the thickness of the object
E.g. the thicker the body through which the X-rays must pass,
the more the X-rays will be attenuated.
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Attenuation of non-deviated X-rays
• The attenuation of non-deviated X-rays in the human body
– In general radiographic imaging, there are basically 5 perceivable
object densities
– These perceivable densities are, sorted in increasing attenuation of
X-rays
• air
• fat
• water, organs and tissue (soft tissue, blood) (*) (**)
• bone
• metal
(*) The density of an organ can be changed by administering
radio-contrast agents
(**) Density varies depending upon organ: e.g. heart > lungs
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The HVL (Half Value Layer)
• The HVL is defined in order to be able to compare materials
and objects for what concerns their attenuation capabilities.
• The Half Value Layer is defined as
the thickness of the material required to reduce, under
narrow beam geometry, the intensity of an incoming X-ray
beam to half of its value before entering the material
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The HVL (Half Value Layer)
• Narrow beam geometry / HVL test set-up
collimator
attenuator
detector
X-ray source
scattered photons are
not detected
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The HVL (Half Value Layer)
• The HVL value for a material is dependent on the energy of the
photons (i.e. the kV value of the source)
• Some HVL values (in mm)
Tissue Al Pb
70 keV X-rays 37 11 0,2
140 keV X-rays 44 18 0,3
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Interactions X-rays/object: a summary
Basic concept of radiography
Interaction X-rays/object
Interactions in the object
Photoelectric effect
Attenuation of
non-deviated X-rays Half Value
Layer
Compton effect
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