Appendix G1: Chapter 12:
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
X-Ray Interaction with Matter
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
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
*
*
*
*
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.
Photodisintegration
* Energy above 10 MeV
* Incident photon heads
into the nucleus, is
absorbed and emits a
nucleon or nuclear
fragment.
Soft tissue
Z=7.4
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
Appendix G2: Chapter 12:
X-Ray Interaction with Matter
Bone
Z=13.8
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)
Barium: Z = 56. For the
the alimentary tract. Air:
(negative contrast) is
being used with barium.
Iodine: Z = 53. For
the vascular system.
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