Download DMI 261

DMI 261
Radiation Biology
And
Protection
Unit 2 RADIATION
Electromagnetic Spectrum
– The frequency range of electromagnetic radiation and
the photon wavelength associated with them.
Important to Radiography
Visible light
– Smallest segment of the EM spectrum
– Only part of the spectrum that we can directly
sense
– Radiographs viewed on viewboxes (visible
light)
Radio frequency
– Very low energy, very long wavelength
– Used for communication, TV and radio
– Microwaves (shorter radiowaves), cell
phones, highway radar, cooking
– MRI uses radiowaves.
Ionizing radiation – gamma & x-rays
– High energy, short wavelength
– Sources
X-rays – emitted from electron cloud in a man
made tube
Gamma – emitted from nucleus of a radioactive
atom
– Both have similar energy and travel at the
speed of light
Characteristics of Ionizing
Radiation
Electromagnetic radiation
High energy, high frequency, short
wavelength
Travels at the speed of light
X and gamma are identical except for
origin
No mass
Electrically neutral
Characteristics of Ionizing
Radiation
Interaction with matter causes electron
excitation or ionization
Affects photographic film and fluorescent
crystals
Behaves as a wave and a particle
Travels in straight lines
Produces chemical & biological changes
Produces secondary & scatter radiation
Interactions of Radiation with
Matter
Coherent Scattering
– X-ray photon interacts
with a target atom
causing it to vibrate.
– Electrons emit
electromagnetic waves
with the same energy
as the incoming
photon.
Coherent Scattering
(also called Rayleigh Scattering)
Occurs mostly below 30 kVp
Small change in direction occurs
(scattering of x-ray photon)
Diagnostic radiology – some x-ray are
scattered by classical scattering and may
contribute to radiographic fog.
Compton Scattering
Responsible for most
scatter radiation
produced during
radiographic
procedures
–
–
–
–
Forward scatter
Small angle scatter
Backscatter
Side scatter
Compton Scattering
important
Compton Scattering
Probability in Diagnostic Radiology
– As photon energy increases, probability
increases
– 100 kVp beam = significant interactions
– Occurs along with photoelectric absorption
– Can cause large amounts of scatter
Grids to absorb scatter (radiographic fog)
Lead aprons in fluoroscopy
Compton Scattering is responsible for most
of the scatter radiation produced during a
radiologic procedure.
Photoelectric Absorption
X-ray absorption
interaction in which
the photon is not
scattered, but totally
absorbed.
important
Photoelectric Absorption
Most important mode
of interaction between
x-ray photons and the
atoms of the patient’s
body for producing
useful images.
(photons #1 and 2)
Auger Electrons
Outer shell electrons that are released
(instead of characteristic radiation) when
an electron moves from an outer to an
inner shell during the photoelectric
absorption interaction.
Photoelectric Absorption
Probability in Diagnostic Radiology
– Dominant interaction in diagnostic radiology
– Between 30 and 150 kVp, it is the most
important interaction for producing useful
images.
Photoelectric Absorption
Probability in Diagnostic Radiology
– Probability increases as the effective atomic #
increases:
Air 7.6
Compact bone 13.8
Barium 56
Iodine 53
– Because it has a higher effective atomic #,
bone will undergo more PE absorption than
an equal amount of soft tissue or air.
Photoelectric Absorption
Probability in Diagnostic Radiology
– Probability increases as the energy of the xray photon decreases (longer wavelength)
and the effective atomic # of the irradiated
atoms (bone vs. soft tissue) increases.
Attenuation / Absorption / Contrast Media
Pair Production
Incoming photon
interacts with the
nucleus of an atom
and disappears
Only occurs with
photons greater than
1.022 MeV
Pair Production & PET
Annihilation reaction photons are products
of radioactive decay of an unstable
isotope.
Scanner detects this reaction and
produces a cross-sectional image of the
radioactivity
Commonly used isotopes: fluorine-18,
carbon-11, nitrogen-13
PET scan demonstration of epilepsy on the
right side of the brain