Units, Detection, and Measurements (PART II)

quality factor

an adjustment multiplier that is employed in the calculation of dose equivalence to detail the ability of a dose of any kind of ionizing radiation to cause biologic damage

linear energy transfer (LET)

the amount of energy transferred on average by incident radiation to an object per unit length of track, or passage, through the object

kiloelectron volts per micrometer (keV/µm)

linear energy transfer (LET) is expressed in units of

high, low

radiation with a ___ LET transfers a large amount of energy into a small area and can therefore do more biologic damage than a radiation with a ___ LET

greater

high LET has a quality factor that is ___ than the quality factor for low LET radiation

equivalent dose

it is the product of the average absorbed dose and the radiation weighting factor, depending on the type of radiation

sievert (Sv)

SI unit for equivalent dose:

radiation equivalent man (rem)

conventional unit for equivalent dose:

radiation weighing factors (WR)

a "dimensionless factor" that was chosen to account for differences in biologic impact among various types of ionizing radiation

equivalent dose (EqD)

obtained my multiplying the absorbed dose (D) by the radiation weighting factor (WR)

EqD = D x WR , Sv = Gy x WR

formula for EqD

effective dose

it is the "sum of the weighted equivalent doses for all irradiated tissues or organs"

sievert (Sv)

SI unit for effective dose:

radiation equivalent man (rem)

conventional unit for effective dose:

tissue weighted factor (WT)

specifically takes into account the relative detriment to each specific particular organ and tissue

effective dose (EfD)

absorbed dose (D) is multiplied by a radiation weighting factor (WR) to obtain EqD, and that product is multiplied by a tissue weighting factor (WT)

EfD = D x WR x WT

formula for EfD

sievert or millisievert

EfD is expressed in

collective effective dose (ColEfD)

the sum of the individual doses received in a given period of time by a specified population from exposure to a specified source of radiation

person-sievert

radiation unit for ColEfd:

total effective dose equivalent (TEDE)

a radiation dosimetry quantity that was defined by the Nuclear Regulatory Commission (NRC) to monitor and control human exposure to ionizing radiation

total effective dose equivalent (TEDE)

the sum of effective dose equivalent from external radiation exposures and a quantity called committed effective dose equivalent (CEDE) from internal radiation exposures

total effective dose equivalent (TEDE)

a particularly useful dose monitor for occupationally exposed personnel such as nuclear medicine technologists and interventional radiologists, who are likely to receive significant radiation exposure during the course of a year

0.05 Sv

whole body TEDE regulatory limit for occupationally exposed personnel:

0.001 Sv

whole body TEDE regulatory limit for the general public:

CT dose Index (CTDI)

it is determined by an ionization measurement using a long, pencil-like, cylindrical ionization chamber that has been inserted into a cylindrical acrylic phantom

CTDI - CT Dose Index

is the dose to any point in the patient, including scatter from CT slices in both directions

CTDw

a weighted average of two measured CTDI values

CTDI = 1/3 (CTDIcₑₙₜₑᵣ) + 2/3 (CTDIₚₑᵣᵢₚₕ)

formula for CTDIw:

CTDIᵥₒₗ

it is the average absorbed dose within the scanned volume

pitch

is the relationship between the movement of the couch and the x-ray beam collimator dimension

CTDIᵥₒₗ = CTDIw / pitch

formula for CTDIᵥₒₗ

helical scans

pitch is greater than 1

nonhelical or axial scans

pitch is 1

dose length product (DLP)

it represents the product of CTDIᵥₒₗ and the irradiated scan length

dose length product (DLP)

characterized the volumetric extent along the patient's body that has been irradiated with an average absorbed dose

DLP = CTDIᵥₒₗ x scan length

formula for DLP:

mGy-cm

unit for DLP:

mGy

unit for CTDIᵥₒₗ:

cm

unit for scan length:

scan length

derived from the product of slice width, the number of slices, and pitch

EfD = DLP x EfDLP

formula for effective computed tomography dose:

millisieverts per milligray-centimeter (mSv/mGy-cm)

EfDLP is expressed in

November 8.,1895

German physics professor Wilhelm Conrad Roentgen discovered "x-rays" during an experiment investigating the nature of cathode ray and fluorescent materials

somatic damage

individuals who were exposed to substantial doses of x-rays in the early years after their discovery developed

1900-1930

skin erythema dose was used as the unit for measuring radiation exposure, a tolerance dose was established for occupationally exposed individuals that could be regarded as a threshold dose

1950s

maximum permissible dose (MPD) replaced tolerance dose

1977

dose equivalent or effective dose equivalent replaced the MPD

1991

ICRP replaced effective dose equivalent with the term effective dose, which is still in use today

effective dose

it is based on the energy deposited in biologic tissue by ionizing radiation; takes into account both the type of radiation and the variable sensitivity of the tissues exposed to the radiation

1980

ICRU adopted the SI units for use with ionizing radiation

SI radiation units

preferred for specifying radiation quantities because the traditional system of units does not fit into the metric system that provides "one unified system of units for all physical quantities"

coulomb per kilogram (C/kg)

used for specifying x-ray or gamma ray exposure in air only

air kerma

an SI quantity that is used to express radiation concentration transferred to a point, which may be at the surface of a patient's or radiographer's body

dose area product

the sum total of air kerma over the exposed area of the patient's surface

absorbed dose (D)

the amount of energy per unit mass absorbed by an irradiated object

Gray (Gy)

used for measuring absorbed dose in air or for measuring absorbed dose in tissue

equivalent dose (EqD) and effective dose (Efd)

quantities of choice for measuring biologic effects when all types of radiation must be considered

equivalent dose (EqD)

specifies how the potential for biologic damage from different types and doses of radiation will be equivalent if correct weighting factors are included

effective dose (EfD)

describes the total biologic damage to a human that is caused by equivalent doses received by specific organs

sievert (Sv), millisievert, microsievert

units used to specify EqD and EfD, used for occupational radiation exposure

collective effective dose

represents an attempt to describe the radiation exposure of a population or group from low doses of different sources of ionizing radiation

total effective dose equivalent (TEDE)

designed to account for all possible sources of radiation exposure and used for dose monitoring of occupationally exposed personnel who are likely to receive significant radiation exposure over the course of a year

committed effective dose equivalent (CEDE)

a measure of the probabilistic health effect on an individual resulting from an intake of radioactive material into the body