AEC- Automatic Exposure Control

How to select cells.

The left, right, and center cell are connected to each other. When more then one cell is selected the exposure is averaged between them.

What is the relationship between AEC and screen speed?

The machine is set up to function with a certain screen speed. If a faster screen is used overexposure can be a result. If a slower screen is used underexposure can be a result.

Factors that influence AEC accuracy.

Part position, detector selection, density setting, minimum response time, back up timer.

What are 2 types of AEC?

Photo-Timer and Ionization chamber.

What are come common AEC errors?

Selecting the wrong bucky, Selecting the wrong chamber. Overcollimation. Undercollimation. Too low of a MA station. Too high of an MA station.

What determines detector selection?

The correct cells must be selected for the anatomical location of the part of interest.

What is the recommended exposure setting for a backup timer?

150% of expected MAS needed for exposure.

What is a backup timer?

A safety device that prevents the accidental overexposure of a patient. Maximum exposure allowed by law is 600MAS.

What is Automatic Exposure Control?

A tool that allows us to consistently reproduce images of uniform density.

What is a photo-timer?

A true photo timer is an exposure timed due to visible light emission.

What is a capacitor?

An electrical component that temporarily stores a charge.

Where is the photo timer located?

Behind the bucky.

What is the basic design of a photo timer AEC?

Fluorescent screen, Lucite tubing, photo multiplier tube, an electrical circuit to terminate exposure.

Why is collimation important when using AEC?

If the image is not properly collimated this may cause the exposure to terminate prematurely resulting in underexposure.

What type do modern x-ray rooms use?

Ionization chamber type.

What is a limitation of AEC?

It does not compensate for all pathological conditions.

What is the main function of AEC?

It will deliver the exact exposure required to penetrate the body part positioned over the selected chamber.

What is a fluorescent screen?

Its like an intensifying screen, converts x-ray photons to light photons.

What is minimum response time?

The shortest exposure possible for AEC to function.

What determines density settings?

This setting should be left alone. AEC compensates for part thickness.

Describe Ionization chamber AEC.

Uses 2 positively charged electrodes with a thin layer of air in between them. Located between the bucky and the image receptor. Chambers are housed in an aluminum shell. Photons ionize air inside the chambers.

Patient positioning

When using AEC your positioning must be perfect.