lec4

absorption of photons by tissue
most ionize tissue atoms and then dissapear
some will SCATTER, move into other directions

9% - no interaction (e- very high E)
7% - coherent scattering
27% photoelectric absorption
57% compton scattering

happens with low energy photons
tissue atom excited, returns to ground state, emit 2ndary photon with similar energy from original
does not contribute to imaging, some exposure to patient

most common interaction
primary contributor of image formation
photon interacts with tissue atom inner shell e-
photon gives off all energy, disappears

the tissue inner shell e- lodged out, then there is a vacancy, later filled by the outer ring moving to inner ring

emits characteristic radiation

3rd power of atomic number of absorber

bone

when xray photon interact with OUTER shell e-
tissue atom deflects the xray photon and scatter to new direction

contribute to image form (not more than photoelectric abs), will degrade img quality
probability more likely where electron density is greater (bone>tissue)

more xrays absorbed = fewer xrays hit detector = brighter (RO)
why enamel is brighter on xray bc it absorbs more xray

abs dependent on: abs coeff of object and thickness of absorber

contentration, total radiation
3Rs: roentgen, rad, rem

concentration of radiation delivered to specific point

amt energy absorbed by an object or person

used to compare biologic effects of diff radation types on tissue; tells you what type of radiation was utilized

because different kinds of radiations can be more damaging even at the same dose due to differing energy

particulate type; has high LET (Linear energy transfer)

used to estimate risk in humans
considers relative biologic effects of diff types of radiation, as well as radiosensitivity of diff tissues in terms of cancer risk