X-Ray Interaction with
Matter & Human Biology
IMAGE CREATION
 ATOMS
 INTERACTION WITH
“MATTER”
 ATOMIC NUMBER
Patient Interactions
 **Photoelectric**
 Classic Coherent
Scatter
 **Compton
Scattering**
 Pair Production
 Photodisintegration
Interaction in
The body begin
at the atomic
level
Atoms
Molecules
Cells
Tissues
Organ structures
Interactions of X-rays with matter
• No interaction; X-ray passes
completely through tissue and
into the image recording
device.
• Complete absorption; X-ray
energy is completely absorbed
by the tissue. No imaging
information results.
• Partial absorption with
scatter; Scattering involves a
partial transfer of energy to
tissue, with the resulting
scattered X-ray having less
energy and a different
trajectory. Scattered radiation
tends to degrade image quality
and is the primary source of
radiation exposure to operator
and staff.
Coherent Scattering
 Also called: Classical scattering or Thompson
scattering
 Occurs with energies below 10 keV
 Incident x-ray interacts with an atom of
matter, causing it to become excited.
Immediately the atom releases this excess
energy and the scattered x-ray.
Coherent Scattering
 The wavelength is equal to the incident x-ray
or equal energy.
 The only difference is the direction of travel
 Energy in = Energy out - Only changes is
direction
Classical (Coherent) Scattering
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Excitation of the total
complement of atomic
electrons occurs as a result
of interaction with the
incident photon
No ionization takes place
Electrons in shells “vibrate”
Small heat is released
The photon is scattered in
different directions
No loss of E
Compton Effect or Compton
Scattering
 Occurs throughout the diagnostic
imaging range
 The incident x-ray interacts with the
outer electron shell on an atom of
matter, removing it.
 It not only causes ionization but scatters
the incident x-ray causing a reductions
in energy and the change of direction.
Compton scatter
 A fairly high energy (high kVp) x-ray photon ejects an
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outer shell electron.
Though the x-ray photon is deflected with somewhat
reduced energy (modified scatter), it retains most of its
original energy and exits the body as an energetic
scattered photon.
A Compton e- is also released
Since the scattered photon exits the body, it does not
pose a radiation hazard to the patient.
It can, however, contribute to film fog and pose a
radiation hazard to personnel (as in fluoroscopic
procedures).
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Compton scatter
 Both the scattered x-ray and the Compton
electron have enough energy to cause more
ionization before loosing all their energy
 In the end the scattered photon is absorbed
photoelectrically
Compton Effect
 The Compton electron looses all of its kinetic
energy by ionization and excitation and drops
into a vacancy in an electron shell previously
created by some other ionizing event
 The probability of Compton effect increases
as photon energy increases, however the
atomic number does not affect the chances of
the Compton effect
Compton Scatter
 Compton is just as likely to occur with soft
tissue as bone. Compton can occur with any
given photon in any tissue
 Compton is very important in Radiography,
but not in a good way.
 Scattered photons provides no useful
diagnostic information
Compton Effect
 Scattered radiation produces a uniform
optical density on the radiograph that reduces
image contrast
 Scattered radiation from Compton contributes
to the majority of technologists exposure,
especially during fluoroscopy
 STAY AWAY FROM YOUR PATIENT !
Scatter from the Patient
during Fluoroscopy
ISOEXPOSURE CURVES
Photoelectric Effect or Absorption
 Inner-shell ionization
 The photon is not scattered it is totally
absorbed
 The e- removed from the atom of matter is
called a photoelectron, with an energy level
equal to the difference between the incident
photon and the e- binding energy.
Binding Energy is very important
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Table 10-2
PHOTOELECTRIC ABSORBTION
IN THE PATIENT
(CASCADE OF ELECTRONS)
Photoelectric effect
• A relatively low energy (low kVp) x-ray photon
uses all its energy (true absorption) to eject an
inner shell electron,
• leaving an orbital vacancy.
• An electron from the shell above drops down to
fill the vacancy and, in doing so, gives up
energy in the form of a characteristic ray.
• The photoelectric effect is more likely to occur
in absorbers of high atomic number (eg, bone,
positive contrast media)
• and contributes significantly to patient dose,
• as all the photon energy is absorbed by the
patient (and for the latter reason, is responsible
for the production of short-scale contrast).
Electron transitions
 Are accompanied by the emission of more x-
rays – secondary radiation
 Secondary radiation behaves much like
scatter radiation
 Secondary contributes nothing to the image
 The probability that any given photon will
undergo a photoelectric interaction is
dependent on the photon energy and the
atomic number of the atom
CASCADE
Photodisintegration
• PHOTOELECTRIC
ABSORBTION
IS WHAT GIVES US
THE CONTRAST
ON THE FILM
Important X-ray Interactions
 Of the five interactions only two are
important to radiology
 Photoelectric effect or photoelectric
absorption
 Compton scatter
 Which two tube interactions are
important?
Compton scatter
 Contributes to no useful information
 Is independent of the atomic number of
tissue. The probability of Compton is the
same for bone atoms and for soft tissue
atoms
 The probability for Compton is more
dependent on kVp or x-ray energy
Compton Scatter
 Results in image fog by optical densities not
representing diagnostic information
 Photon are Photons
IR is does not know
the difference
Photoelectric Absorption
 Provides information to the IR because
photons do not reach the IR
 This represents anatomic structures
with high x-ray absorption
characteristics; radiopaque structures;
tissue with high atomic number; or
tissue with high mass density
Attenuation – The total reduction in the # of photons
remaining in an x-ray beam after penetration through tissue
 Absorption = x-ray disappears (Photoelectric,
Pair production & Photodisintegration)
 Scattering = partially absorbed, x-ray
emerges from the interaction traveling in a
different direction (sometimes with less
energy)
 Absorption + Scattering = Attenuation
3 Types of x-rays are important for
IMAGE FORMATION
 DIFFERENTIAL ABSORPTION = the
difference between those x-rays absorbed
and those transmitted to the IR
 Compton scatter (no useful information)
 Photoelectric absorption (produces the light
areas on the image)
 Transmitted x-rays (produces the grey/dark
areas on the image)
• The probability of radiation interaction
is a function of tissue electron density/
atomic number, tissue
thickness/density, and x-ray energy
(kVp).
• Dense material like bone and contrast dye
attenuates more X-rays from the beam
than less dense material (muscle, fat, air).
• The differential rate of attenuation
provides the contrast necessary to
form an image.
• Table 10-10 & 12-4
Differential Absorption
 Increases as the kVp is reduced
 Approximately 1% of photons that interact
with the patient (primary beam) reach the IR.
Of that 1% approximately 0.5% interact to
form the image
Differential Absorption
 The difference in x-ray interactions
 Fundamental for image formation
 Occurs because of Compton Scattering,
Photoelectric absorption, and X-ray
transmission
Differential Absorption
Compton vs. Photoelectric
 Below 60 kVp Photoelectric absorption is
predominant above 60 kVp Compton scatter
begins to increase.
 Dependent on the tissue attenuation
properties
 Table 10-13
Differential absorption factors
 High atomic number = larger atoms
 Mass Density = how tightly the atoms of
tissue are packed
 Z # for air and soft tissue are about
the same the OD changes are due to
mass density difference
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Table 12–3 & 12-5
Radiation Protection
 Producing high-quality radiographs require
careful technique selection, reducing kVp
improves differential absorption and image
contrast
 However, patient dose is increased because
more photons are absorbed by the body