chapter 7- AEC

What is automatic exposure control (AEC)

If enabled, it eliminates the need for the radiographer to manually select mAs

_____ is very important for proper use of AECs

Patient positioning

Can AECS be used on all projections?

NO

Do all Imaging systems include AEC circuits

NO. Portable X-ray units typically don’t

The term “——“ originates from early AEC systems

Phototimer

1. Phototimers (orignaited from early AEC systems) used _____ screens that converted X-ray energy into emitted light photons

2. This light was then absorbed by a _____ tube that converted its energy into electrical current

Fluorescent

Photomultiplier

Modern AEC systems use a ___ ___ chamber

Gas ionization

What are these

AEC units

AEC unit modern systems: 1

Ionization chamber

AEC modern unit: 2

IR

Gas ionization chamber

• what happens upon exposure?

X-rays ionize air molecules within the ionization chamber, which ionizes the air molecules and frees electrons

Gas ionization chamber

• step 2

Freed electrons are attracted to a positively charged anode wire where they form an electrical current that can be measured

Gas ionization chamber

• step 3

The greater the measurement of electrical current, the greater the amount of X-ray exposure the ionization chamber has received

Gas ionization chamber

• step 4

When a predetermined amount of exposure has been received and detected by the AEC circuit, the exposure is automatically terminated

AEC intensity control is also called what

Master density or density control

What does the AEC intensity control do

Increases or decreases the present sensitivity of the AEC/ density by specific percentages, such that the resulting exposure time is extended or shortened

What is a common format for AEC intensity control (Or master density or density control)

-3, -2, -1, 0, +1, +2, and +3

AEC intensity control common format

• what does N or 0 mean

Normal exposure

AEC intensity control

• Genuinely each setting represents a ____% Additive increase or decrease

25%

• -1 = 25% decrease in relative exposure

• -2 = 50% decrease in relative exposure

• +1 = 25% increase in relative exposure

• +2 = 50% increase in relative exposure

When adjusting the AEC intensity control, the _____ of exposure will be adjusted (Which impacts the ____ Used)

Length, mAs

The AEC intensity control should only need to be used _____ - The exception rather than the rule

Infrequently

Minimum response time

• the AEC is not _____

Instantaneous

Minimum response time (MRT):

Defined as the minimum amount of time it takes for the electric current to detect and react radiation

Minimum response time (MRT) Is typically measured in the _____ Range

Thousandths (0.001)

What is a potential problem for the minimum response time (MRT)?

This can be a problem if the mAs Is very small

• (If a very small mAs (mA x time) Calls for a time smaller than the minimum response time the exposure is incapable of occurring that quickly)

What is the solution to the potential problem for the minimum response time (MRT)?

The technologies could maintain the same mAs But use a small smaller mA And a large larger time to ensure the exposure is longer than the minimum response time

MRT practice:

• assume a room has a MRT of 0.005 seconds

• Suppose the ideal technique for a pediatric trust projection is 65KVP and 0.8 mAs. If the 300 mA Setting is used, In over exposure of almost twice too much would be delivered. Why?