1. Appendix G1: Chapter 12: X-Ray Interaction with Matter
Coherent
(Classical:
Unmodified:
Thompson)
scattering
* Coherent
interaction
excites the
whole atom.
* Most interactions occur < 10 keV
* Small % are higher and contribute to fog
* Of little importance to diagnostic x-ray
* Scattered & incident photon energies the same
* Of no value
Compton Effect
* Interaction with outer shell electrons.
* Energies in diagnostic range
* Scatters incident photon which gives up energy
* The greater the angle of deflection of the scattered
x-ray, the more energy is given up (180o gives up 1/3)
* Ionizes the atom
* The energy given up by the incident photon = the
binding energy of the target electron plus the kinetic
energy gained by the newly created Compton electron
* Compton electron & scattered photon may undergo many more interactions
* Of negative value to x-ray imaging
* Source of most occupational exposure
Pair Production
* Energy of at
least 1.02 MeV
* Incident photon
heads into
nucleus,
disappears, and
reemerges as
two electons, one of which is a positron
* The energy equivalence of the mass of an
electron is 51 MeV, which is the minimum
the electrons possess.
* The electron fills a vacant hole n a the
closest atom needing one. The positron
combines with another free electron and the
mass of the two are converted to energy in
an event called annihilation radiation.
Photoelectric interaction
* With inner shell
electrons.
* Total absorption of
the incident photon
* Photoelectron is ejected
with kinetic energy in
excess of binding energy
* Photoelectrons from low
atomic # atoms & low
binding energies (soft tissue) absorb most of the incident
energy
* An outer shell electron takes its place, producing a
secondary photon equal to the difference in the binding
energies
* Secondary photons have no diagnostic value
Photodisintegration
* Energy above 10 MeV
* Incident photon heads
into the nucleus, is
absorbed and emits a
nucleon or nuclear
fragment.

2. X-Ray Interaction with Matter
Compton Effect: This graph demonstrates
that there is little to no effect on Compton
interactions from changes in atomic density,
and that as x-ray energy increases Compton
interactions decrease
Bone
Z=13.8
Soft tissue
Z=7.4
The Z value of soft tissue is
7.4 and the Z of air is 7.2.
Nevertheless, the differential
absorption of soft tissue is
770 times that of air (3 x that
of lungs)
Appendix G2: Chapter 12:
X-Ray Interaction with Matter
Barium: Z = 56. For the Iodine: Z = 53. For the alimentary tract. Air: the vascular system.
(negative contrast) is
being used with barium.
Left: Photoelectric and Compton effects:
* The probability that a photoelectric interaction will occur in bone is about 7 times greater
than in soft tissue.
* At low energies the photoelectric effect predominates
* At higher energies Compton scattering predominates.
* The effect of Compton scatter on the image is negligible
* And, at higher energies, the probability of any interaction decreases.
In Summary
* Two types are x-ray interaction are important: Compton and Photoelectric
* But only one interaction is useful: Photoelectric
* Compton scatter fogs the image and leads to occupational exposure
* Photoelectric absorption is true absorption which provides differential absorption