M16 Flashcards
X-rays have very short wavelengths, approximately .
10-8
to 10-9 m
The higher the energy of an x-ray,
the shorter is its wavelength.
X-rays interact at these various structural levels through five mechanisms:
coherent scattering, Compton scattering, photoelectric effect, pair production,
and photodisintegration.
Two of these – – are of particular importance to diagnostic radiology.
Compton scattering and photoelectric
effect
X-rays with energies below approximately 10 keV interact with matter by
coherent scattering, sometimes called
classical scattering or Thompson
scattering.
was the first physicist to first describe coherent
scattering.
J. J. Thompson
the incident x-ray interacts with a target atom causing it
to become excited.
In coherent scattering
the incident x-ray interacts with a target atom causing it
to become excited.
In coherent scattering
The _____ atom immediately releases this excess energy
as a scattered x-ray with wavelength equal to that of the incident x-ray and
therefore of equal energy.
target
However, the direction of the scattered x-ray is
different from that of the incident x-ray.
The result of _______ is a change in direction of the x-ray without a change in its energy. There is no energy transfer and therefore no ionization.
coherent scattering
Most coherently scattered x-rays are scattered in the
forward direction.
is of little importance to diagnostic radiology.
Coherent scattering
primarily involves low-energy x-rays, which contribute little to medical image. however, occurs throughout the
diagnostic range. At ___ kVp, a few percent of the x-rays undergo coherent scattering, which contributes slightly to image noise, the general graying of an
image that reduces image contrast.
Coherent scattering
70
X-rays throughout the diagnostic range can undergo an interaction with outer-
shell electrons that not only scatters the x-ray but reduces its energy and
ionizes the atom as well. This interaction is called
Compton scattering.
the incident x-ray interacts with outer-shell electron and
ejects it from the atom, thereby ionizing the atom. The ejected electron is called
a
In Compton scattering,
Compton electron.
The energy of a Compton-scattered x-ray is equal to the difference between the
energy of the incident x-ray and the energy of the ejected electron.
During _____, most of the energy is divided between the scattered
x-ray and the Compton electron. Usually, the scattered x-ray retains most of the energy.
Compton scattering
Both the _____ may have sufficient
energy to undergo additional ionizing interactions before they lose all their
energy.
scattered x-ray and the Compton electron
in tissue can occur with all x-rays and therefore is of
considerable importance in x-ray imaging. However, its importance involves a
negative sense.
Compton scattering
provide no useful information on the
radiograph. Rather, they provide a uniform optical density on the screen-film radiograph and uniform intensity on the digital image receptor that results in
reduced image contrast.
Scattered x-rays
The scattered x-rays from ___ can create a serious radiation
exposure hazard in radiography and particularly in fluoroscopy. A large amount of radiation can be scattered from the patient during fluoroscopy. Such radiation
is the source of most of the occupational radiation exposure that radiographers
receive.
Compton scattering
X-rays in the diagnostic range can also undergo ionizing interactions with inner-
shell electrons. The x-ray is not scattered, but is totally absorbed. This process
is called ____ and earned _____ the 1921 Nobel Prize
in physics.
photoelectric effect
Albert Einstein
The electron removed from the atom, called the ______ , escapes with kinetic energy equal to the difference between the energy of the incident x-ray
and the binding energy of the electron.
photoelectron
Characteristic x-rays are produced after a _____. Ejection of a K-shell photoelectron by the incident x-ray results in a vacancy in the K-shell.
This unnatural state is immediately corrected when an outer-shell electron,
usually from L-shell drops into the vacancy.
photoelectric interaction
If an incident x-ray has sufficient energy, it may escape interaction with
electrons and come close enough to the nucleus of an atom to be influenced by the strong nuclear field. The interaction between the x-ray and the nuclear
field causes the x-ray to disappear, and in its place, two electrons appear, one
positively-charged (positron) and one negatively charged. This process is
called process is called
pair production.
The energy equivalence of the mass of an electron to be0.51 MeV . Because
two electrons are formed in pair production interaction, the incident x-ray photon
must have at least ____ of energy. An x-ray with less than ____ cannot
undergo pair production.
1.02 MeV
Any of the x-ray’s energy in excess of 1.02 MeV is
distributed equally between the two electrons as
kinetic energy.
The electron that results from pair production loses energy through excitation
and ionization and eventually fills a vacancy in an atomic orbital shell. The
positron unites with a free electron, and the mass of both particles is converted
to energy in a process called ____.
Because pair production involves only x-rays with energies greater than 1.02
MeV, it is unimportant in x-ray imaging, but is very important for positron
emission tomography imaging in nuclear medicine
annihilation radiation.
X-rays with energy above approximately 10 MeV can escape interaction with electrons and the nuclear field and be absorbed directly by the nucleus. When this happens, the nucleus is raised to an excited state and instantly emits a
nucleon or other nuclear fragment. This process is called
photodisintegration.
