Principles of Diagnostic
Imaging 1
Lecture 7 - X-ray Interaction with Matter
Pair Production
nIf the incident x-ray has
sufficient energy, it may
escape the electron
cloud and comes close
enough to the nucleus to
come under the
influence of the strong
electrostatic field of the
nucleus.
Pair Production
• The strong electrostatic field of the nucleus causes the
photon to disappear and in its place appear two
electrons.
• One is positively charged and called a positron while
the other remains negatively charged. This is called
Pair Production.
Pair Production
nIt takes a photon with 1.02 MeV to undergo Pair
Production.
nThe resultant positron and electron has an energy equal
to 0.51 MeV.
nAny x-ray with less than 1.02 MeV can’t undergo this
interaction.
nAny energy in excess of 1.02 MeV is distributed equally
between the two electrons as kinetic energy.
nIt is not important to diagnostic x-ray.
Photodisintegration
• High energy X-ray photons with
energies above 10 MeV can
escape interaction with both the
electrons and nucleus
electrostatic fields.
• It is absorbed into the nucleus
that excites the nucleus resulting
in the release of a nucleon or
other nuclear material.
Photodisintegration
nLike pair production,
the high energy
needed to cause this
makes it unimportant
to diagnostic
radiography.
Differential Absorption
nOnly Compton and Photoelectric Effects are important interactions
that the x-ray may have with matter in the diagnostic spectrum.
nMore important than the x-rays resulting from these effects are a
third type, those transmitted through the body without interacting.
Differential Absorption
nThose that make it through the body contribute to the radiograph.
nIt should be clear that Compton Scatter X-rays contribute no useful
information.
nThe film does not recognize the scattered x-rays as representing an
interaction of the straight line from the target.
Differential Absorption
nThese scattered x-rays result in film fog, a generalized dulling of the
image on the radiograph by film densities not representing diagnostic
information.
nTo reduce this type of fog, we use techniques and apparatus to
reduce the amount of scatter reaching the film.
Differential Absorption
• X-rays that undergo photoelectric interaction provide diagnostic
information to the image receptor.
• Since they do not reach the film, these x-rays are representative of
anatomic structures with high x-ray absorption characteristics. These
structures are said to be Radiopaque
Differential Absorption
• The other x-rays that penetrate the body and are transmitted without
interaction are said to be Radiolucent.
n Radiopaque n Appears Bright
n Radiolucent n Appears Dark
Differential Absorption
• The radiographic image is the result of the difference between
those x-rays absorbed photoelectrically and those not absorbed at
all.
• This characteristic is called differential absorption.
Differential Absorption
nExcept at very low kVp, most x-rays that interact do so by the
Compton effect
nAs a rule of thumb, less than 5% of x-rays incident on the patient
reaches the film and less than one half of these interact with the film.
Differential Absorption
nThe radiographic image results from less than 1% of the x-rays
emitted from the tube.
nTherefore careful control of the x-ray beam is necessary to produce
high quality radiographs!
Differential Absorption
nDifferential Absorption increases as the kVp is lowered but lower
kVp results in a higher patient radiation exposure.
nA compromise is needed for each examination.
Differential Absorption
nNotice how much of the
x-rays are absorbed
photoelectrically in bone
compared to the soft
tissue.
Differential Absorption
nThe photoelectric absorption of
bone is about 7 times greater than
in soft tissue regardless of the
energy.
Differential Absorption
nAs kVp is increased fewer interaction occur so more
x-rays are transmitted without interaction.
nAt low energies the majority of the x-rays
interactions are photoelectric, whereas at high
energies, Compton scattering predominates.
nAs kVp is increased, more x-rays reach the film so
lower output (lower mAs) is required.
• At 20 KeV the probability of photoelectric effect = the probability of
Compton scattering in soft tissue.
• Below this most of the x-rays goes to photoelectric interaction
• Above à Compton scattering
Differential Absorption
nTo image small differences in
soft tissue, one must use low-
kVp in order to get maximum
differential absorption.
nThis is the principle for
mammography.
Differential Absorption
nHigh kVp can be used for
examinations of bony
structures since the
crossover for
photoelectric and
Compton scattering is
about 40 keV.
n This lowers patient
exposure.
Dependence on Mass Density
nWe know that we could image bone even if the differential
absorption were not atomic number related because bone has a
higher mass density than soft tissue.
nDensity is the quantity of matter per unit volume
nThe interaction between x-rays and soft tissue is proportional to the
mass density of the tissue.
Mass Densities of Materials Important in
Radiography
Human Tissues Mass Density
•Muscle •1.00
•Fat •0.91
•Bone •1.85
•Lung •0.32
Mass Densities of Materials Important in
Radiography
Contrast Media Mass Density
•Barium •3.5
•Iodine •4.93
•Air •0.001293
Other
•Concrete •2.35
•Lead •11.35
Contrast Examinations
nIn Medical radiography. To better image soft tissue structures such as
internal organs, contrast media are used.
nThe primary items are Barium with an atomic number of 56 and
iodine which has an atomic number of 53.
nAir can be combined with the contrast.
Exponential Attenuation
nAn interaction such as photoelectric effect is called an absorbing
process because x-rays disappear.
nAll interactions in which the x-ray photon is only partially absorbed
such as the Compton effect is called a scattering process. Pair
production, Photodisintegration and Classic scatter are scattering
processes.
Exponential Attenuation
• The total reduction in the number of x-rays
remaining in an x-ray beam following penetration
through a given thickness of matter is called
attenuation.
• X-rays are attenuated exponentially which means
they do not have a fixed range of matter.
• They are reduced in number by a given percentage
for each incremental thickness of the absorber.
Exponential Attenuation
References
• Radiologic Science for Technologists - : Physics, Biology, and Protection.
By Stewart C. Bushong
