1. Radiation Protection Unit 2
Chapter 3 interactions of x-radiation with matter

2. Objectives Differentiate between kVp and mAs as technical factors
Describe absorption verses attenuation
Differentiate between primary, exit, image –forming, and scattered radiation.
List and discuss 2 types of photon transmission.
List the events that occur when x-radiation passes through matter.
Identify the x-ray photon interactions with matter which are important in diagnostic radiology.
Describe the effect of kVp on image quality and patient absorbed dose.
Discuss the historical evolution of radiation quantities and units
Explain the concepts of skin erythema dose, tolerance dose and threshold dose.
List examples of early somatic effect, late somatic effects and late stochastic effects.
Differentiate between somatic and genetic effects.
Differentiate among the radiation quantities exposure dose and effective dose and identify the appropriate symbol for each quantity.
List and explain the International System (SI) units for radiation exposure, air kerma, absorbed dose, equivalent dose and effective dose.
Define or describe: DAP, tissue weighting factor, LET, and effective dose.

3. Technical factors ( exposure factors)
2 main factors for diagnostic radiography
Kvp
mAs
Both contribute to dose to patient
Both are controlled by radiographer

4. kVp
Kilo-Voltage Peak
Peak- the highest energy level of the level of photon
Controls quality of the beam aka – penetrating power

5. mas milliampere-seconds mA x time(seconds) = mAs
Quantity of photons or number of photons delivered
mA= tube current
S= length of time the x-ray tube is activated
Both controlled by radiographer

6. Interactions with tissue
X-rays can do one of two things:
1. interact with atoms
Energy is transferred from x-rays to patient
Process is called absorption – contributes to absorbed dose
Absorption, absorbed dose and potential for biological effects are directly proportional
MUST happen for image to be useful
2. pass through without interaction

7. Beam production Step in production ( simplified!!)
1. Filament is heated and boils off electrons ( negative charge)
2. Electrons travel at high speeds from filament to target through a vacuum to the anode (+) target
3. Electrons hit target and leave tube at speed of light ( PRIMARY RADIATION)
4. Travel through glass window- window acts as filter –
5. Travel through Al filter to get “ hardened”

8. Tube Parts Anode + end Can be stationary or rotating
Metal tungsten or tungsten rhenium
High melting points ( 6191 deg F )
High atomic numbers
Cathode
- end
Filament- responsible for heating ( thermionic emission) and boiling off of electron
Focusing Cup-negative charge behind the filament – responsible for confining & focusing electrons
Filtration
Built into tube- glass window – Al in filter -Permanent inherent filtration
Glass envelope
Made of Pyrex glass
Maintains vacuum- allows for more efficient x-ray production and longer tube life

9. Photon energy
All photons do not have same energy – fluctuates
Photon Energy is </= to the energy of the electrons that hit the target
Energy is expressed in Volts - KiloVolts in X-ray –
Energy fluctuates – Kilo-Volt Peak ( kVp)
1 V aprox = 1 eV ( acquired energy)
100 kVp = 100 keV potential highest – most photons will be 1/3 the energy 33 keV
Penetrating ability of x-rays is increased by increasing kV

10. The beam PRIMARY radiation- mainstream radiation
SCATTER- photons that are deviated from their path
Small angle scatter- changed in direction but not enough to keep it from reaching IR
Degrades image
Radiographic Fog occurs with scatter- overall degrading of image
REMNANT- exit or image forming photons

11. Attenuation of the x-ray beam
Reduction in the number of primary photons as the x-ray beam passes thru the body
Absorbed and scattered x-rays that do not hit film
Caused by:
absorption ( loss of radiation energy) and
scatter ( change in direction)

12. Direct vs indirect transmission to the IR
X-ray photons pass thru the body without interaction
Reach the IR
Create the optimal image
INDIRECT
X-ray photons that reach IR BUT have lost energy because of a interaction
Degrade image quality
Can be reduced with techniques such as : Air Gap or Grids

13. Mass density and its effects
Body structure or mass influences attenuation
Higher atomic number=higher mass
Greater Body part thickness = more attenuation or absorption
Higher mass number= more attenuation or absorption
ALL EFFECT RADIOGRAPHIC DENSITY

14. WHAT IS Radiographic Density?
The amount of overall blackening on a film
Must be “optimal” or sufficient to see the part of interest
High atomic mass numbers will appear white
Low atomic mass numbers will appear black
Controlled by “brightness” level on CR/DR monitor – referred to as lightness or darkness
window level- sets midpoint of the range
“windowing” on monitor increases or decreases
Which is more dense ?
Air, water , soft tissue , bone , Barium

15. Radiographic contrast
Difference in black and white on a film or between adjacent structures
High atomic number = higher absorption = higher biological effect
Controlling factor is kVp
Ways to increase contrast-
Low kVp
Collimation
Use of contrast media- barium or iodine
High atomic numbers

16. Photon interactions
Interactions occur at different energy levels and can be categorized by type:
1. Coherent
2. Photoelectric
3. Compton
4. Pair Production
5. Photodisintegration

17. Structure of atom
Neutral Atom- number of electron in the shells must equal the number of protons in the nucleus
Shells are lettered K,L,M, N,… etc
Number of electrons that can exist in each shell, increase with distance of the shell from the nucleus
Formula= 2n² n= shell number
K=1 so 2(1²)= 2
L= (2 ²) = 8
M= (3²)=19
Binding Energy- energy required to disassemble the atom e- e-

18. 1. Coherent scattering- Classical/unmodified/elastic
X-rays come in contact with atom of body
X-ray is absorbed by the atom
Causes the atom to vibrate
Atom gives off energy by producing a scattered x-ray but does not loose energy
Scattered x-ray has SAME energy, wavelength and frequency as incident x-ray BUT travels in a different direction <20 degrees difference
Atom will not be ionized ( no electrons are ejected)
Occurs mostly below 30 kVp but some throughout all diagnostic ranges (1-50kVp range)

19. Coherent cont’d- results
Rayleigh Scattering
the net effect of coherent or unmodified scattering
the change in direction
Thompson Scattering
Low-energy photon interacts with one or more free electrons
Photon energy is absorbed and then reradiated in a different direction
No change in wavelength occurs
Neither play an important role in radiography !

20. 2. Photoelectric ( true absorption)
Most important interaction for producing a useful image
Incoming x-ray knocks out an inner shell electron K or L ) , ejected atom is called photoelectron ( characteristic photon)
Hole is filled by an electron from an outer shell
When filled, a new x-ray is produced ( secondary x-ray or fluorescent radiation)
Characteristic cascade-when electron holes are filled from outer shell electrons until atom is stable.
New x-ray is lower energy and is usually absorbed by the body
Photoelectric add to patient dose- as the % of photoelectric interactions increase so does the absorption of radiation by the patient
Gives good contrast on image because of the absorption of x-rays
1-50 kVp range

21. Photoelectric cont’d- results
Characteristic Photon/Characteristic Ray
The released energy
AKA fluorescent radiation
Auger effect ( awzhay) – Radiationless effect
Instead of the electron being ejected it transfers energy to another electron within the atom and forces that electron out instead of producing florescent radiation –
More common in higher atomic number atoms
Causes fluorescent yield to be lower in high atomic number atoms
Fluorescent yield- the number of x-ray emitted during photoelectric interactions

22. What effects photoelectric effect ?
Photoelectric effect INCREASES with mass density and high atomic number
Photoelectric absorption will increase when incident photon energy decreases
Photoelectric absorption will increase when atomic number increases
SOOOO……..
The more dense , the more interactions, and more absorption, results in less density on film – more whiteness on film
The less a given structure attenuates radiation, the greater will be its radiographic density on a radiographic film- the more blackness on film
AND…
The greater the difference in the amount of photoelectric absorption, the greater the contrast in the radiographic image will be between adjacent structures of differing atomic numbers.

23. 3. Compton scattering- incoherent/ modified scattering/ INELASTIC SCATTERING
X-ray comes in and knocks out outer shell electrons ( called a recoil electron/ secondary or Compton electron)
Same x-ray will leave atom; now called a scattered x-ray
Scattered x-ray has less energy and goes in a different direction that when it came in
X-ray will become scatter to film or us or interact photoelectrically or interact by Compton again
Large amount of radiation can be “scattered” from patient, type of scatter most responsible for tech dose and fog
Outer shell electrons are free electrons less binding energy , easier to remove
Fogs film, no useful information
60-90 kVp

24. 4. Pair production
X-ray photon must have energy of at least Mev ( million electron volts) for this to occur
X-ray comes into nucleus or ( nuclear field) of atom and disappears
This energy becomes 2 new particles
Negatron (negative electron)- eventually captured by another atom
Positron ( positive electron) - interacts with an electron and they destroy each other
From this destruction, Mev x-ray photons are given off in opposite directions of each other
Used in therapy and PET

25. 5. photodisintegration X-ray must have energy above 10 Mev to occur
X-ray comes in and is absorbed by nucleus of atom
Nucleus gets into an excited state and emits a nuclear fragment
Seen in radiation therapy
3possible releases:
Neutron
Proton-neutron combo- deuteron
Alpha particle

26. Interactions Video http://youtu.be/4p47RBPiOCo Resources:
Radiation Protection in Medical Radiography by Mary Alice Statkeiwicz Sherrer, Paula Visconti, E. Russell Ritenour and Kelli Welch Haynes. 6th and 7th Edition. Elsevier online.
Essentials of Radiographic Physics and Imaging. James N. Johnston and Terri L Fauber. 1st Edition. Elsevier Online.

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