DMI 261 Radiation Biology AndProtection

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 x- ray 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 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