RADIATION PROTECTION UNIT

2Chapter 4 radiation quantities and units

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.  

HISTORY

November 1895- Roentgen discovered x-rays

December 1895 Reported findings- discovery

1896- 23 cases of radiodermatitis was reported

1898- Thomas Edison invented fluoroscope

1904- 1st x-ray fatality Clarence Dally- Edison’s friend and assistant- radiation induced CA Suffered with severe x-ray burns and amputation of both arms

1870’s and 1880’s- Sir William Crooks – invented the 1st x-ray tube- partial vacuum

1913- William D. Coolidge- invented the vacuum tube

INJURIES Between 1911 and 1914- at least 54 cancer deaths and 198 cases of radiation induce malignancy

Earliest unit of measurement – skin erythema dose- amount of radiation required to cause redness to the skin after irradiation – Gray- Not accurate

1928- Roentgen was accepted as unit of exposure

1931- 1stdoes limiting recommendation made by Advisory Committee on x-ray and radium protection. .2R/day Now called National Council on Radiation Protection and Measurement

1937 “Roentgen” became internationally accepted as unit of exposure since it had be defined

DOSE OVER THE AGES

Annual Maximum Permissible Dose ( MPD)- expressed in Roentgen

1931- 50

1936-30

1948- 15

1958- 5

INJURY TYPESOccupational exposure- exposure to radiation works – early as 1910

Seen in radiologists and non-radiologists

Somatic damage-biologic damage to the body- reported as early as 1896

Early deterministic somatic effects- appear within minutes, hours, days or weeks- dose dependent

organic damage- will most likely occur once a threshold has been met

Late deterministic somatic effects / late stochastic effects-recognized in 1936- months or years after exposure and genetic effectsLate Deterministic- increases the chances of occurring Late Stochastic- do not have a threshold – arbitrary

Tolerance dose- measurement of how much a person could be exposed to radiation without apparent acute effectsThreshold dose- a dose lower than the tolerance dose-

Radiodermatitis- reddening of the skin

Blood disorders- aplastic anemia from bone marrow failure

Leukemia- over production of white blood cells

RADIATION QUANTITIES - MAIN UNITS 1. Exposure (X) Roentgen

2. Air kerma (J/kg)2 or Gya or Gyt

3. Absorbed Dose ( D) Rad or Gray

4. Equivalent Dose ( EqD) Rem or Sievert

5. Effective Dose ( EfD) Sievert or Milli-Sievert

1. EXPOSURE- ROENTGEN Amount of radiation that may strike an object – used for x-ray equipment calibration

R=traditional unit C/kg = SI unit

Roentgen ( R) – a unit of radiation exposure that will liberate a charge of 2.58 x 10-4 coulombs per kg of air

Amount of charge released by x-rays as they pass thru dry air

conversions : Roentgen to Coulomb 1 R = 2.58 x 10 -4 C/Kg

Coulomb to Roentgen C/Kg 1 C/kg = 3876 ≈ 3880 R

2.58 x 10 -4 C/Kg

1 coulomb = 1 ampere-second basic unit of electrical charge ( how much electricity is transferred by a current of 1 A in 1 second)

Unit of exposure in air

What comes out of the machine

Refers to energies of less than 3 MeV

2. AIR KERMA SI unit- measurement of radiation to an object- in air –radiation intensity in air Used for x-ray tube output and inputs to image receptors Gradually replacing the traditional quantity, exposure

J/Kg for Air Gray (Gy)Gya ( air) or Gyt ( tissue)

Expressed as a the kinetic energy( Joules) released in a unit of mass (kilogram)

DAP- Dose Area Product- modern measurement to determine entire amount of energy delivered to the patient by the beam- Used with radiographic and Fluoro units

Kinetic energy released in matter Kinetic energy released in material Kinetic energy released per unit mass

May start seeing use in clinical settings

3. ABSORBED DOSE- RAD OR GRAY

RAD- traditional unit Gray – SI Unit ( usually expressed as mGy or cGy)

Gyt

Unit of quantity, absorbed dose- amount absorbed by the patient

RAD= Radiation absorbed dose or ( D )or absorbed dose

How much energy is transferred to an irradiated object by ionizing radiation

Proportional to the degree of attenuation : the more attenuation , the higher the dose received, the more biological damage

Traditional unit: 1 Rad is = to the radiation necessary to deposit energy of 100 ergs in 1 gram of material

Erg- unit of energy transfer

1 RAD = 100 erg/g

1 RAD = 1/100 Gray so 5000 Rad = 50 Gy

500 rad= 5 Gyt or 500 rem = 5 Sv

SURFACE INTEGRAL DOSE (SID) Exposure area product or amount of radiant energy transferred by radiation to the body during an x-ray

Traditional unit R-cm2 SI unit Gy-m2

LET (LINEAR ENERGY TRANSFER) The amount of energy transferred on average by incident radiation to an object per unit length of travel through the object

Radiation that has a high LET transfers a large amount of energy into a small area- more damage

Low LET – travels farther but less damage to one specific area

X-ray are considered low LET radiation; they don’t deposit as much energy in a given space as opposed to other radiations

High LET= high QF= higher biological damage

QUALITY FACTOR – (QF)

Factor used to determine the ability of a dose of radiation to cause biological damage

Different for different types of radiation: example :alpha more damaging than x-ray so alpha gets a higher rating

Number assignment of radiation types

QF is based on LET

3. EQUIVALENT DOSE (EqD)– REM OR SIEVERT Rem= traditional unit Sievert ( Sv) – SI Unit

REM= Radiation Equivalent Man

Used in rad protection for occupationally exposed individuals

Measures the biological effectiveness of radiation

The absorbed does equivalent of any type of ionizing radiation that produces the same biological effect as one rad of x-radiation

Different types of radiation produce different amounts of biological damage Use radiation weighting factor (WR ) factor takes into account the type and energy of the radiation

Same numeric value as the QF

EqD CONT’D - DETERMINE ABSORBED DOSE Equivalent Dose= absorbed dose x radiation weighting factor

SI Eqd= D x WR

which is Sv = Gy x WR

TRADITIONAL REM= RAD x QF

1 Sv= 1000 mSv

5. EFFECTIVE DOSE ( EfD)

used to measure the overall risk of exposure to humans from radiation

“sum of the weighted equivalent doses for all irradiated tissues or organs “ NRCP report 116

SI Sieverts or milli-Sieverts Traditional Rem

EfD= D x WR x WT

WT – tissue weighting factor- risk associated with irradiation of different body tissues- risk of developing CA when all tissues are given the same amount of radiation

Utilizes BERT – for comparisons for environment sources

COLLECTIVE EFFECTIVE DOSE ( ColEfD) used to describe the radiation exposure of a population or group from low doses of radiation

Product of the exposure of the persons exposed

SI Unit = person-Sievert traditional = man-rem

If 200 people received an avg EfD of .25 Sv. The ColEfd = 200 x .25 = 50 person sieverts

TOTAL EFFECTIVE DOSE EQUIVALENT (TEDE) Used in radiation dosimetry

Defined by the Nuclear Regulatory Commission ( NRC) to monitor and control human radiation exposure – all sources of radiation

Sum of effective dose equivalent from external radiation and CEDE ( committed effective dose equivalent) internal exposure

“committed dose”- probability of health effects from intake of radioactive material into the body

Nuc Med techs and IR radiologists- use to monitor high dose occupational workers

TEDE whole body limit 0.05 Sv (worker) or 0.001 Sv for general population

REVIEW

Copyright © 2014 by Mosby, an imprint of Elsevier Inc. 20

SI UNITS PRESENTLY IN USE

Radiation quantities Exposure Air kerma Absorbed dose (D) EqD EfD

Radiation units Coulombs per kilogram (C/kg) Gray (Gy) Gy Sievert (Sv) Sv

Copyright © 2014 by Mosby, an imprint of Elsevier Inc. 21

TRADITIONAL NONMETRIC UNITS GRADUALLY BECOMING OBSOLETE

Radiation quantities Exposure Absorbed dose (D) Equivalent dose (EqD) Effective dose (EfD)

Units of measure R Rad Rem Rem

1. A pt receives 3000rad: what is the dose in SI?

a. 6000Gy b. 3000 cGy c. 300rad d. 30R rad and cGy are equal so B

2. 10 Sieverts equal ___________ rem. 1 Sv = 100 rem so 10 x 100= 1000

3. 1000 rem =____________Sieverts . rem / 100 1000/100 = 10 Sv 4. 1 Rem = __________ mSv .

100 rem = 1000 mSv so 1000 / 100 = 10 mSv

5. Convert 8000 rad to Gy:___________8000/ 100 = 80 Gy

6. Convert 7 Gy to rad: _____________7 x 100= 700 rad

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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