RAD350 Chapter 1

radiation =

energy

radiation travels at

speed of light (3 ×10⁸ m/s) in a vacuum

What surrounds the traveling x-ray photon?

electrical + magnetic fields that are at right angles to each other

Fundamental properties of x-rays

• travel in straight lines

• heterogenous wavelengths

• cannot be focused by a lens

• polyenergetic photons

three forms of radiation have energy to interact with matter (ionizing radiation)

• ultraviolet

• x-ray

• gamma ray

another form of ionizing radiation

radioactive decay

consequences of ionization in human cells

• production of free electrons (free radicals)

• injury to cell that may manifest itself as abnormal function or loss of function

technical factors that can limit energy deposited in living tissue

• kVp

• mAs

Radiation protection

effective measures employed by radiation workers to safeguard patients, personnel, and the general public from unnecessary exposure to ionizing radiation

Cardinal rules of protection

1. increase distance

2. limit time

3. use shielding

(Units of Measurement)

• Air Kerma (exposure in air)

• Absorbed dose (exposure in tissue)

Gray (Gy)

(Units of Measurement)

Exposure (in Air)

[side note: old term replaced by Air Kerma]

Coulomb/Kg

(Units of Measurement)

Equivalent Dose (exposure in tissue)

Effective Dose (exposure in tissue)

Sievert (SV)

(Units of Measurement)

“A’s have ____”

“E’s have ____”

Grays, V’s

Exposure

Direct measurement of the ionization of air molecules

• Number of ionizations in air

• calculated in Coulomb/kg

An increase in mA or intensity

• increase # of photons

• increase ionizations

• increase pt exposure

Best unit for measurement in air

Air Kerma

Air Kerma

• measures the energy of ionization in air - related to exposure

• expressed as Gy_a

Kerma

kinetic energy released in matter (matter or per unit mass)

Absorbed Dose

• energy absorbed in matter per unit mass

  • includes photoelectric absorption and compton scattering

• expressed as Gy_t

• often described in smaller doses as milliGray (mGy_t)

Effective Dose

• calculated dose that takes into account type of radiation patient exposed to (equivalent dose) as well as what part of body was irradiated

• expressed as Sieverts (SV)

• has 3 factors

3 factors that make up effective dose formula

1. product of absorbed dose (Gy)

2. Radiation weighting factor (W_R)

3. tissue weighting factor (W_T)

effective dose formula

Gy x W_R x W_T

2 factors that make up equivalent dose formula

1. Absorbed Dose (AD)

2. Radiation weighting factor (W_R)

Equivalent Dose formula

AD x W_R

Who carries responsibility for determining medical necessity for the patient vs risk?

physican

Non-ionizing alternatives

• MRI

• Ultrasound

Diagnostic Efficacy

degree to which the diagnostic study accurately reveals the presence or absence of disease in the patient while adhering to radiation safety guidelines

Diagnostic Efficacy is maximized when

essential images are produces with the least radiation exposure to the patient

ALARA

as low as reasonably achievable

• Time: minimize time of radiation exposure

• Distance: Doubling distance between body and radiation source will divide radiation exposure by a factor of 4 (Inverse Square Law)

• Shielding: Using absorber materials such as lead for x-rays is an effective way to reduce radiation exposures

Cardinal Rules

Primary Cardinal rule

Distance

(try to increase your distance from source (patient) as much as possible

In a hospital setting, RSO is responsible for

• Execution

• Enforcement

• Maintenance of the ALARA program

RSO

Radiation Safety Officer

_______ helps to ensure the highest quality of service

Educating patients about imaging procedures

BERT

Background Equivalent Radiation Time

“The radiation received from having a chest x-ray is equivalent to

spending approx. 10 days in your natural surroundings”

Annual exposure

3.0 mSv

PA CXR

0.1 mSv 10 days in nature (BERT)

(CT Scans)

CXR

0.08 mSV

(CT Scans)

CT Abd

10 mSv

CT exposure dose is

100x greater than 1 CXR

Alliance for Radiation Safety in Pediatric Imaging

• founded in 2007

• Goal is to raise awareness among non-radiology users of the high exposure from CT

Image Wisely Campaign

• ACR and RSNA

• Goal: decrease amount of radiation used in imaging and eliminate unnecessary procedures

• Dec 2012: Minnesota became 1st state to endorse Image Wisely and Image Gently campaigns