Radiologic Units: What You Need to Know

Transcription

1 Radiologic Units: What You Need to Know TODD VAN AUKEN M.ED. RT (R)(MR) Agenda Greys, Sieverts, Coulombs per kg, & Becquerel's Conventional Units Other Concepts (LET, Q-Factor, Effective Dose, NCRP Report # 116) Crazy Happenings with Radiation Erythema Dose Coulomb per kg or C/kg-Exposure (X) A Coulomb (C) is the basic unit of electric charge Measurement of positive and negative particles created when radiation ionizes atoms in dry air (Radiation intensity in the air) Unit of Exposure for X-rays and Gamma Rays (Restricted to Photons only) Used for x-ray equipment calibration (X-ray tube output) Conventional Unit: Roentgen (R)

2 Air Kerma Kinetic energy is transferred from the primary beam (photons) to the patient (e-) SI quantity that can be used to measure radiation concentration transferred to a point at the surface of a patient s or radiographer s body The energy transfer is called the kinetic energy released in the material or kerma (air kerma) Dose Area Product (DAP)- sum total of Air Kerma over the exposed area of the patient s surface (entire amount of energy delivered to the patient) Gray (Gy)-Absorbed Dose (D) Amount of energy per unit mass absorbed by an irradiated object 1 joule (J) of energy absorbed in 1 g of absorbing material Absorption of energy may result in biological damage As the Z# of the object increases the absorbed dose increases Conventional Unit: RAD (Radiation Absorbed Dose) Linear Energy Transfer Linear Energy Transfer Low LET Low LET High LET High LET LET= Rate at which energy is released as a charged particle travels through matter Dependent upon mass and energy of radiation or particle

3 Linear Energy Transfer Low LET High LET The higher the mass or the lower the energy, the higher the LET Sievert- Equivalent and Effective Dose Dose: The amount of radiation Energy absorbed in the body Various types of radiation/particles produce varying amounts of damage (LET) Developed to measure biologic response of tissue from exposure to different types of photon radiation and particles Unit employed on dosimetry reports Conventional Unit: REM (Roentgen-Equivalent-Man) Quality Factor or Weighting Factor (W R ) Type of Radiation Q Factor Gray Sieverts X-Ray Gamma Ray Beta Particles Thermal Neutrons Fast Neutrons Alpha Particles Sievert = Product of the absorbed dose in Gy and the quality factor (Q) or Weighting Factor (W R )

4 Dose Equivalence (EqD) To calculate dose equivalence, use the following formula: EqD = D x W R Example: What is the total exposure a patient would receive if he/she were exposed to the following doses of ionizing photon and particulate radiation:.5 Gy of x-rays,.1 Gy of fast neutrons,.05 of alpha particles? EqD = D x Q x-rays =.5 x 1 or.5 Sv fast neutrons =.1 x 10 or 1 Sv alpha particles =.05 x 20 or 1 Sv EqD = or 2.5 Sv Effective Dose (EfD) Tissues in the body are not equally affected by ionizing radiation This method is employed to estimate the dose to each individual organ The effective dose (EfD) will allow you to estimate the dose to an organ that is located outside of a protective lead apron Effective Dose (EfD) Effective Dose (EfD) To calculate effective dose, use the following formula: EfD = W R x W T x absorbed dose Example: What would the total exposure to the breasts be if a patient were exposed to a total body dose of.5 Gy of x-rays? EfD = W R x W T x absorbed dose EfD = 1 x 0.05 x.5 EfD =.025 Sv or 25 msv to the breasts

5 Becquerel (Bq)- Radioactivity Review Quantity of radioactive material/number of radioactive atoms decaying per second (not the radiation emitted) (C/kg) Primarily employed in Nuclear Medicine Conventional Unit: Curie (Ci) NCRP Report #116 ALARA and Risk-Benefit Balance when using diagnostic amounts of radiation Occupational Exposure- annual effective dose limit is 50 msv (5 rem) Cumulative effective dose (CEfD) limit = Age (in years) x 10 msv (1 rem) General Public- annual effective dose limit for frequent exposure is 1 msv (.1 rem) General Public- annual effective dose limit for infrequent exposure is 5 msv (.5 rem) Equivalent dose limit for embryo or fetus for all gestation is 5 msv (.5 rem or 500 mrem) Types of Dosimeters: Dosimeters are required if there is a possibility that a HCW will receive more than 1/10 of the recommended dose limit (DL)

6 Photographic Emulsion: Earliest type employed (1940s) Film Badge: Contain metal filters Can be worn up to 3 months which is the maximum allowable time Can measure doses as low as.1 mgy Disadvantages May be affected by heat and humidity Does not provide an immediate reading Cannot be reused Thermoluminescence Dosimeters or TLD: Contain Lithium Fluoride (LiF) rods, chips, disks or powder Thermoluminescence Dosimeters or TLD: Crystals store energy when exposed to ionizing radiation During processing, the LiF crystals are heated Light is emitted in proportion to radiation exposure An electric signal is generated by a photomultiplier tube Can be worn up to 3 months between readings Advantages Reusable and not affected by heat and humidity Can measure doses as low as.05 mgy Primary disadvantage is that it does not provide an immediate reading Lithium Fluoride (LiF)

7 Optically Stimulated Luminescence or OSL: Aluminum Oxide (Al 2 O 3 ) is used as the radiation detector Exposure to ionizing radiation causes some electrons to be moved out of their normal position During processing, a laser causes these electrons to move back to their original position Light is released that is in proportion to the radiation exposure The light is directed toward a photodiode, which generates a signal denoting the individual s level of exposure Can be worn up to 3 months between readings Primary advantage is that an OSL can measure doses as low as.01 mgy Optically Stimulated Luminescence or OSL: Aluminum Oxide (Al 2 O 3 )