RADIATION PROTECTION UNIT

2Chapter 3 interactions of x-radiation with matter

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.  

TECHNICAL FACTORS ( EXPOSURE FACTORS) 2 main factors for diagnostic radiographyKvpmAs

Both contribute to dose to patientBoth are controlled by radiographer

KVP

Kilo-Voltage Peak

Peak- the highest energy level of the level of photon

Controls quality of the beam aka – penetrating power

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

INTERACTIONS WITH TISSUE

X-rays can do one of two things: 1. interact with atoms Energy is transferred from x-rays to patientProcess 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

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”

TUBE PARTSAnode• + 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

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

THE BEAMPRIMARY radiation- mainstream radiationSCATTER- photons that are deviated from their path Small angle scatter- changed in direction but not enough to keep it from reaching IR

Degrades imageRadiographic Fog occurs with scatter- overall degrading of image

REMNANT- exit or image forming photons

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)

DIRECT VS INDIRECT TRANSMISSION TO THE IR DIRECT 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

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

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

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 kVpCollimation Use of contrast media- barium or iodine

High atomic numbers

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

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(2 ²) = 8

M=3 2(3²)=19

Binding Energy- energy required to disassemble the atom

e- e-

1. COHERENT SCATTERING- CLASSICAL/UNMODIFIED/ELASTICX-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)

COHERENT CONT’D- RESULTS Rayleigh Scatteringthe net effect of coherent or unmodified scatteringthe change in direction

Thompson Scattering Low-energy photon interacts with one or more free electronsPhoton energy is absorbed and then reradiated in a different direction No change in wavelength occurs

Neither play an important role in radiography !

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

PHOTOELECTRIC CONT’D- RESULTSCharacteristic Photon/Characteristic RayThe released energy AKA fluorescent radiation

Auger effect ( awzhay) – Radiationless effectInstead 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 atomsCauses fluorescent yield to be lower in high atomic number atoms

Fluorescent yield- the number of x-ray emitted during photoelectric interactions

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.

3. COMPTON SCATTERING- INCOHERENT/ MODIFIED SCATTERING/ INELASTIC SCATTERINGX-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

4. PAIR PRODUCTION X-ray photon must have energy of at least 1.022 Mev ( million electron volts) for this to occurX-ray comes into nucleus or ( nuclear field) of atom and disappearsThis 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, 0.511 Mev x-ray photons are given off in opposite directions of each other Used in therapy and PET

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: NeutronProton-neutron combo- deuteronAlpha particle

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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