DMI 261DMI 261

Radiation BiologyRadiation Biology

AndAnd

ProtectionProtection

Unit 2 RADIATIONUnit 2 RADIATION

Electromagnetic SpectrumElectromagnetic Spectrum– The frequency range of electromagnetic radiation and The frequency range of electromagnetic radiation and

the photon wavelength associated with them.the photon wavelength associated with them.

Important to RadiographyImportant to Radiography

Visible lightVisible light– Smallest segment of the EM spectrumSmallest segment of the EM spectrum– Only part of the spectrum that we can directly Only part of the spectrum that we can directly

sensesense– Radiographs viewed on viewboxes (visible Radiographs viewed on viewboxes (visible

light)light)

Radio frequencyRadio frequency– Very low energy, very long wavelengthVery low energy, very long wavelength– Used for communication, TV and radioUsed for communication, TV and radio– Microwaves (shorter radiowaves), cell Microwaves (shorter radiowaves), cell

phones, highway radar, cookingphones, highway radar, cooking– MRI uses radiowaves.MRI uses radiowaves.

Ionizing radiation – gamma & x-raysIonizing radiation – gamma & x-rays– High energy, short wavelengthHigh energy, short wavelength– SourcesSources

X-rays – emitted from electron cloud in a man X-rays – emitted from electron cloud in a man made tubemade tube

Gamma – emitted from nucleus of a radioactive Gamma – emitted from nucleus of a radioactive atomatom

– Both have similar energy and travel at the Both have similar energy and travel at the speed of lightspeed of light

Characteristics of Ionizing Characteristics of Ionizing RadiationRadiation

Electromagnetic radiationElectromagnetic radiation

High energy, high frequency, short High energy, high frequency, short wavelengthwavelength

Travels at the speed of lightTravels at the speed of light

X and gamma are identical except for X and gamma are identical except for originorigin

No massNo mass

Electrically neutralElectrically neutral

Characteristics of Ionizing Characteristics of Ionizing RadiationRadiation

Interaction with matter causes electron Interaction with matter causes electron excitation or ionizationexcitation or ionization

Affects photographic film and fluorescent Affects photographic film and fluorescent crystalscrystals

Behaves as a wave and a particleBehaves as a wave and a particle

Travels in straight linesTravels in straight lines

Produces chemical & biological changesProduces chemical & biological changes

Produces secondary & scatter radiationProduces secondary & scatter radiation

Interactions of Radiation with Interactions of Radiation with MatterMatter

Coherent ScatteringCoherent Scattering– X-ray photon interacts X-ray photon interacts

with a target with a target atomatom causing it to vibrate.causing it to vibrate.

– Electrons emit Electrons emit electromagnetic waves electromagnetic waves with the same energy with the same energy as the incoming as the incoming photon.photon.

Coherent ScatteringCoherent Scattering(also called Rayleigh Scattering)(also called Rayleigh Scattering)

Occurs mostly below 30 kVpOccurs mostly below 30 kVp

Small change in direction occurs Small change in direction occurs (scattering of x-ray photon)(scattering of x-ray photon)

Diagnostic radiology – some x-ray are Diagnostic radiology – some x-ray are scattered by classical scattering and may scattered by classical scattering and may contribute to radiographic fog.contribute to radiographic fog.

Compton ScatteringCompton Scattering

Responsible for most Responsible for most scatter radiation scatter radiation produced during produced during radiographic radiographic proceduresprocedures– Forward scatterForward scatter– Small angle scatterSmall angle scatter– BackscatterBackscatter– Side scatterSide scatter

Compton ScatteringCompton Scattering

important

Compton ScatteringCompton Scattering

Probability in Diagnostic RadiologyProbability in Diagnostic Radiology– As photon energy increases, probability As photon energy increases, probability

increasesincreases– 100 kVp beam = significant interactions100 kVp beam = significant interactions– Occurs along with photoelectric absorptionOccurs along with photoelectric absorption– Can cause large amounts of scatterCan cause large amounts of scatter

Grids to absorb scatter (radiographic fog)Grids to absorb scatter (radiographic fog)

Lead aprons in fluoroscopyLead aprons in fluoroscopy

Compton Scattering is responsible for most Compton Scattering is responsible for most of the scatter radiation produced during a of the scatter radiation produced during a

radiologic procedureradiologic procedure..

Photoelectric AbsorptionPhotoelectric Absorption

X-ray absorption X-ray absorption interaction in which interaction in which the the photonphoton is not is not scattered, scattered, but totally but totally absorbed.absorbed.

important

Photoelectric AbsorptionPhotoelectric Absorption

Most important mode Most important mode of interaction between of interaction between x-ray photons and the x-ray photons and the atoms of the patient’s atoms of the patient’s body for producing body for producing useful images.useful images.

(photons #1 and 2)(photons #1 and 2)

Auger ElectronsAuger Electrons

Outer shell electronsOuter shell electrons that are released that are released (instead of characteristic radiation) when (instead of characteristic radiation) when an electron moves from an outer to an an electron moves from an outer to an inner shell during the photoelectric inner shell during the photoelectric absorption interaction.absorption interaction.

Photoelectric AbsorptionPhotoelectric Absorption

Probability in Diagnostic RadiologyProbability in Diagnostic Radiology

– Dominant interaction in diagnostic radiologyDominant interaction in diagnostic radiology

– Between Between 30 and 150 kVp30 and 150 kVp, it is the most , it is the most important interaction for producing useful important interaction for producing useful images.images.

Photoelectric AbsorptionPhotoelectric Absorption

Probability in Diagnostic RadiologyProbability in Diagnostic Radiology– Probability increases as the Probability increases as the effective atomic #effective atomic #

increases:increases:Air 7.6Air 7.6Compact bone 13.8Compact bone 13.8Barium 56Barium 56Iodine 53Iodine 53

– Because it has a higher Because it has a higher effective atomic #,effective atomic #, bone will undergo more PE absorption than bone will undergo more PE absorption than an equal amount of soft tissue or air.an equal amount of soft tissue or air.

Photoelectric AbsorptionPhotoelectric Absorption

Probability in Diagnostic RadiologyProbability in Diagnostic Radiology

– Probability increases as the energy of the x-Probability increases as the energy of the x-ray photon decreases (longer wavelength) ray photon decreases (longer wavelength) and the effective atomic # of the irradiated and the effective atomic # of the irradiated atoms (bone vs. soft tissue) increases.atoms (bone vs. soft tissue) increases.

Attenuation / Absorption / Contrast MediaAttenuation / Absorption / Contrast Media

Pair ProductionPair Production

Incoming photon Incoming photon interacts with the interacts with the nucleusnucleus of an atom of an atom and disappearsand disappears

Only occurs with Only occurs with photons greater than photons greater than 1.022 MeV1.022 MeV

Pair Production & PETPair Production & PET

Annihilation reaction photons are products Annihilation reaction photons are products of radioactive decay of an unstable of radioactive decay of an unstable isotope.isotope.

Scanner detects this reaction and Scanner detects this reaction and produces a cross-sectional image of the produces a cross-sectional image of the radioactivityradioactivity

Commonly used isotopes: fluorine-18, Commonly used isotopes: fluorine-18, carbon-11, nitrogen-13carbon-11, nitrogen-13

PET scan demonstration of epilepsy on the PET scan demonstration of epilepsy on the right side of the brainright side of the brain