Standard Units of Measurement - Fundamentals of Radiologic Science

This set of vocabulary flashcards covers the fundamental base and special quantities, laws of motion, units of work and power, heat transfer mechanisms, and radiologic science measures such as exposure, dose, and radioactivity.

Length (Pre-1960)

The distance between two lines engraved on a platinum-iridium bar kept at the International Bureau of Weights and Measures in Paris, France.

Meter ($m$)

A base unit based on the speed of light, redefined in 1960 in terms of the wavelength of an orange light emitted from krypton-86 ($1/299,792,468\,s$).

Kilogram ($kg$)

Originally defined as the mass of $1000\,cm^3$ of water at $4^\circ C$. It is a unit of mass.

Second ($s$)

A base unit of time based on the vibration of cesium atoms.

Velocity ($v$)

Also called speed, it is a measure of how fast something is moving and the rate of change of its position with time ($v = d/t$). The SI unit is $m/s$.

Acceleration ($a$)

The rate of change of velocity with time, calculated as $a = (v_f - v_o)/t$. The unit is $m/s^2$.

Newton’s First Law: Inertia

A body will remain at rest or will continue to move with constant velocity in a straight line unless acted on by an external force.

Newton’s Second Law: Force

The force ($F$) that acts on an object is equal to the mass ($m$) of the object multiplied by the acceleration ($a$) produced ($F = ma$). The unit is the Newton ($N$).

Newton’s Third Law: Action/Reaction

For every action there is an equal and opposite reaction.

Weight ($Wt$)

A force on a body caused by the pull of gravity, calculated as $Wt = mg$. On Earth, gravity ($g$) is $9.8\,m/s^2$, while on the moon it is $1.6\,m/s^2$.

Momentum ($p$)

The product of mass and velocity ($p = mv$).

Work ($W$)

The product of force and distance ($W = Fd = mad$). The unit is the Joule ($J$).

Power ($P$)

The rate of doing work, expressed as the quotient of work by time ($P = W/t$). The unit is $J/s$ or watt ($W$).

Radiation (Heat)

The transfer of heat by the emission of infrared radiation.

Conduction

The transfer of energy from one area of an object to another.

Convection

The transfer of heat by the movement of the heated substance from one place to another.

Exposure

A measure of ionization produced in air, measured in Roentgen ($R$) or $C/kg$, where $1\,C/kg = 3876\,R$.

Absorbed Dose

A measure of the energy deposited in any medium by any type of radiation, measured in rad or Gray ($Gy$).

Equivalent Dose ($H$)

A measure of the biological effect of a particular type of radiation on organs or tissues, calculated by multiplying absorbed dose by the radiation weighting factor ($w_R$). Units include rem and Sievert ($Sv$).

Radiation Weighting Factor ($w_R$)

A specific value that accounts for the ability of different types of ionizing radiation to cause varying degrees of biological damage (e.g., X-rays = 1, Alpha = 20).

Effective Dose ($E$)

A quantity that takes into consideration the different radiosensitivities of different organs and tissues using a tissue weighting factor. The unit is the Sievert ($Sv$).

Radioactivity

The rate of decay or disintegration of a radioactive material, measured in Curie ($Ci$) or Becquerel ($Bq$).