Chapter 13 PowerPoint

3 interactions can happen in matter

• Photon in transmitted through the patient

• X-ray photon is absorbed/attenuated in patient (patient dose)

• Photon scatters

Attenuation

reduction in the number of x-ray photons in the beam, and subsequent loss of energy, as the beam passes through matter

Allowed dosage for tech

1mGy/hr @ 1 meter

Factors that effect attenuation

• Type of tissue and atomic number (higher atomic # materials attenuate more of the beam)

• Density of tissue

Attenuation means

reduction in intensity of x-ray beam as a result of of absorption, scatter and divergence

Increase of kVp

• decrease attenuation

• decrease absorption

• decrease subject contrast

Absorption of the x-ray photons happens through

photoelectric effect

Differential Absorption

different objects (tissues) absorb radiation differently

(Absorption)

Increase thickness/density =

increase absorption

(Absorption)

Air =

not dense (black) decreased absorption

(Absorption)

Bone =

more dense (white) increased absorption

Absorption full stop

Bone, Metal, lead, barium

Attenuation slow down

soft tissue, fat, or fluid

Transmission pass on through

Air

Differential Absorption

The difference between the x-rays absorbed and the x-rays transmitted

Two main x-ray interactions in patient (within diagnostic range)

• Compton effect

• photoelectric effect

Photoelectric effect

most important in diagnostic radiology

most important for contrast

Compton scatter

important in diagnostic radiology (bad, don’t want it)

“Fog”

Coherent scatter is also known as

Classical

(Coherent Scatter)

Occurs at

low energy - 10 kEv (filtered out)

(Coherent Scatter)

Photon in

scatter photon out

(Coherent Scatter)

Incident x-ray photon interacts with

atom, causing it to become excited

(Coherent Scatter)

Atom releases this excess energy as a scattered x-ray with wavelength

equal to that of the incident x-ray

(Coherent Scatter)

Scattered x-ray exits atom in

different direction from incident x-ray

(Photoelectric Effect)

Photon in (absorbed)

Electron out (results in cascade effect)

(Photoelectric Effect)

Inner shell electron knocked out of orbit

Ionization

(Photoelectric Effect)

ejected electron is termed

photoelectron

(Photoelectric Effect)

Main source

radiation dose to the patient

(Compton Scatter)

Photon in

Photon out (with less energy)

(Compton Scatter)

Energy partially absorbed by

loosely bound outer shell electron

(Compton Scatter)

ionization

knocks electron out of orbit

(Compton Scatter)

Produces

scatter photon (low energy) going in different direction

&

compton electron

(Compton Scatter)

Source of most

Occupational (tech) exposure

(Compton Scatter)

Source of the most _____ seen on IR

fog

• decreases image contrast

(Probability)

As atomic # and tissue density rises

Photoelectric interactions rises

(Probability)

Not dependent on atomic #

As # of targets rises

Compton interactions rises

• As volume of tissue rises, scatter rises, fog rises

• As kilovoltage rises, scatter rises, fog rises

• As matter density rises, scatter rises, fog decreases

• As field size rises, scatter rises, fog rises

Factors affecting quantity of scatter radiation fog

Contrast media is used to

increase contrast

• Higher atomic number

• allows visualization of structures

• Iodine (56) or barium (53)