MOD 10 - AEC

AEC

a system that automatically terminates the exposure according to a pre-set quantity of photons

AEC Function/Purpose

intend to deliver consistent and reproducible exposures across a wide range of anatomical thicknesses and tube potentials

how an AEC works

xray photons ionize air within the ionization chamber and produce an electrical voltage output signal → once the pre-determined reference voltage is met, the system terminates the exposure

AEC Ionizing Chambers

rectangular pucks made of radiolucent materials, located between the grid and receptor

how is the exposure measured when there are multiple chambers selected

the exposure measurement is averaged between the selected chambers to achieve the termination threshold

what does the AEC control

the time the exposure is running

Benefits of AEC

• takes out “guess work“ of exposure selection

• corrects dose creep

• efficiency

Equipment Benefit when using AEC

low mA and long time improves tube life rather than high mA -short time exposures that lead to vaporization and tube arching over time

Risks and Limitations of AEC

• exposure errors

• incorrect detector selection

• collimation

exposure errors and AEC

occurs due to inaccurate positioning / when the selected chamber is not over the ROI

Backup Timer

• a system where it terminates the exposure in the event of chamber failure or other error to prevent excessive patient exposure

• set by the tech

adequate backup timer

set to terminate the exposure once it exceeds 1.5x the average mAs

collimation errors

• when collimation restricts exposure to an active chamber, or when lead shielding interferes with the chamber exposure

• absence of collimation can allow scatter to reach the chamber and prematurely terminate the exposure

AEC Limitations

patient conditions

• destructive pathologic conditions

• pt’s with recent positive contrast exam (leads to over-exposure)

• implants/prosthesis (leads to over-exposure)

• bowel obstructions with excessive gas accumulations (leads to under-exposure)

Density Step Increases

each 1.0 increase = 25% increase in exposure needed to ionize chambers and turn off AEC

General Rules for Using AEC

• never use on anatomy that is too small or narrow to entirely cover the cell

• positioning and centering must be perfect

• must have adequate collimation (minimize scatter)

• do not use if prosthetic, orthotic or other devices present in the body