xray interactions with matter

attenuation

the reduction in the number of xray photons, resulting in the loss of energy.

gradual decrease in exposure

attenuation includes both _______ and ________

absorption and scatter

attenuation is influenced by

1. tissue type/atomic number

2. subject thickness

3. kv/photon quality

greater atomic number

greater attenuation

greater volume

greater attenuation

higher kv

less attenuation

x ray photons can either

change direction and scatter (compton), or be absorbed (photoelectric)

when kv increases…

greater chance of compton scatter, less photoelectric

LOW energies/kv

most interactions with tissue are photoelectric

compton interactions

• not dependent on atomic number

• happen when high energy photon rejects a loosely bound outer shell electron

• ionization occurs

• Ei = Es + Eb + Eke

• incident photon is Deflected

• PARTIAL energy trasnfer

• most common interaction in the human body!

compton scatter

impacts image quality by decreasing contrast

k shell

highest binding energy

farther from nucleus

greater total energy

scatter radiation is the primary source of __________ exposure

occupational

Photoelectric effect

• more likely to occur in bone than soft tissue

• contributes mostly to pt dose

• electron vacancy/hole causes a characteristic cascade

• responsible for subject contrast and contributes mostly to PATIENT dose

• occurs with absorbers with a HIGH atomic number (ex contrast media)

coherent scattering

• insignificant in diagnostic radiology

• below 10kev, very low energy photons

Pair production

• occurs between high energy, 1.02 mev and higher

• comes close to strong nuclear field and loses nearly all its energy

• energy creates a pair of electrons (megatron and positron)

• after, each photon posesses .51 mev

not for diagnostic xray. maybe nuc med.

subject density

how tightly packed at the atomic level

__ times more likely photoelectric interaction will happen in bone v tissue

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