DENT 506 Radiation Dosimetry & Protection

Created by KTay1127 Plus

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Radiation Doses

Two basic systems
-Traditional and SI (international system)
Four basic types of measurements
1. Exposure
2. Absorbed dose
3. Equivalent dose
4. Effective dose

How much radiation am I getting?

It depends on:
Settings on x-ray machine, film type, collimator size, processing, etc.
Type of measurement made
-Skin dose
-Critical organ dose
-Effective dose

Exposure

Air kerma (kinetic energy released in matter)
-Capacity of radiation to ionize air
-Measures kinetic energy transferred from photons to electrons, expressed in Grays (Gy), where 1 Gy = 1 joule/kg
-Replaced the roentgen (R) - traditional unit of exposure in air (but many dose meters still record R or mR)

Measuring Exposure

-Ionization chamber filled with air
-Exposed to x rays
-Converted to measurement of surface exposure

Absorbed Dose

Amount of radiation actually absorbed per unit mass
SI units
-Gray (Gy), where 1 Gy = 1 joule/kg
Traditional units
-Rad (radiation absorbed dose) (100 ergs/g)
Conversion
-1 Gy = 100 rad, 1 rad = 0.01 Gy = 1 cGy
-1 mrad = 10µGy (frequently used conversion)

Measuring Absorbed Dose

-TLDs* inserted into special phantom
-Exposed to x rays
-Dose measured when TLDs "read out"
-Can be converted to effective dose

Equivalent Dose

Used to compare biologic effects of different types of radiation to a tissue or organ
-Sum of products of absorbed dose (DT) averaged over tissue and radiation weighting factor (WR)
-Weighting factor depends on type and energy of radiation (WR photons = 1; 5 keV neutrons = 5; alpha particles = 20)
-Allows for comparison of multiple types of radiation affecting same tissue or organ (e.g., background radiation)

Measuring Equivalent Dose

SI units
-Sievert (Sv)
-For diagnostic radiology, 1 Sv = 1 Gy
Traditional units
-Rem (roentgen-equivalent-man)
Conversion
-1 Sv = 100 rem, 1 rem = 0.01 Sv = 1 cSv
-1 mrem = 10 µSv

Effective Dose

Most useful
Used to estimate risk in humans
-Sum of products of equivalent dose to each organ or tissue and tissue weighting factor

-Estimate of dose to whole body that has same risk as dose to localized area; makes comparisons between x-ray techniques easier

Measuring Effective Dose

SI units
-Sievert (Sv)
Traditional units
-Rem
Conversion
-1 Sv = 100 rem, 1 rem = 0.01 Sv
-10 µSv = 1 mrem

Typical Skin Doses

Skin dose
-Easy to measure
-Varies with film type, collimator, kVp
Typical skin doses
-D-speed: 300 mR
-E-speed: 150 mR
-F-speed: 120 mR
-Digital: 60 mR+
-Chest: 14 mR
-Lumbar spine: 290 mR

Skin Dose

Skin dose: easy to measure BUT
-Doesn't account for area and tissue exposed
-Panoramic is larger than bitewing but has lower skin dose due to use of intensifying screens in cassette
-Some tissues more sensitive to x rays than others
-Problem: just add up total number of films to determine overall risk? (Not all hit the same area)
-This is what we record on "green sheet" in patient's chart

Background Radiation

Natural radiation that we are exposed to every day
Varies with location, altitude (higher in mountains)
Average values for USA given below
Cosmic 0.27 mSv
Terrestrial 0.28 mSv
Radon 2.00 mSv
Other 0.40 mSv

Rounded total 3.00 mSv/year (3000 µSv/yr) (~8 µSv/day)

Note: White & Pharoah, 6th ed, shows average world background doses. Doses in USA are higher due to higher radon levels.

Radiation Risk

About 1-11/million FMX
Depends on film type, collimation
Benefits to patients of needed radiographs far outweigh the risks
FMX=full mouth x-ray

ADA Guidelines

Child, adolescent, high risk: 6-12 months
Adult, high risk: 6-18 months
Child, low risk: 12-24 months
Adolescent, low risk: 18-36 months
Adult, low risk: 24-36 months

Patient Protection: Kilovoltage

Kilovoltage
-Low kVp x rays don't penetrate to film
-Minimum 60 kVp
-Most modern x-ray machines operate at 70 kVp or higher
Filtration of beam
-Required by law

Patient Protection: Collimation

Maximum 7 cm diameter with round cone
Long cone preferred over short cone (less spread of beam)
Pointed plastic cone NOT recommended and illegal in some states
Rectangular collimation preferred
Reduces area of exposure
Reduces patient radiation risk by 50-90%

NCRP Dose Limits: Occupational Limits

-Stochastic effects: 50 mSv annual, 10 mSv x age cumulative limit
-Deterministic effects: 150 mSv annual to lens of eye, 500 mSv annual to skin and extremities

NCRP Dose Limits: Nonoccupational Limits

-Stochastic: 5 mSv annual for infrequent exposure, 1 mSv annual for continuous exposure
-Deterministic: 50 mSv annual to lens of eye, skin, extremities
-Embryo-fetus: 0.5 mSv/month after pregnancy is known
-No dose limit for patient

Operator Protection

-Measuring radiation exposure
-Personnel badge: Wear only during work, not when a patient
-No one in the office should receive a dose even close to MPD

NOMAD

NOMAD (Aribex Corp) recently approved for use in Michigan
-Limited to situations where a standard x-ray machine is not feasible
-Can only use in restricted situations

Radiation protection: summary

High kVp
Filtration
Rectangular collimator
Long cone
Film holder/beam guiding devices
F-speed film or digital
Leaded apron with thyroid collar
Proper processing

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