RAD 206 Radiographic Noise/ AEC

Term used to describe deterioration of radiographic image

Radiographic noise

Random background info that is detected but does not contribute to image quality

Radiographic noise Radio

Radiographic Noise is caused by what?

# of x-rays used to expose the PT or mAs

The limited absorption efficiency of x-rays

The randomness of x-ray to light conversion

Is the term given to nonuniform or nonhomogeneous appearance of the recorded image more commonly associated w/ high speed recording

Radiographic Mottle

Attributed to random absorption of the x-ray quanta absorbed by the IR

Quantum mottle

What is the function of AEC ?

Eliminate the need for radiographers to set exposure time

What is the only factor that AEC controls?

Time

Radiographers must know the exact position of the ionization chambers for what reason?

Positioning purposesZthe

The x-ray must pass through the PT and tabletop before or after striking the ionization chamber?

Before

Using poor positioning will result in what when using AEC?

Increase repeat rate

Exposure controls should not be used to compensate for PT part thickness by doing what?

Changing kVp

Why should tight collimation be avoided near ionization chambers in AEC?

B/c if primary beam is collimated from an active chamber, the chamber operates as if the tissue is extremely dense.

Wider collimation will produce scatter radiation that will do what ?

Undercut the PT

Undercutting will cause AEC to terminate exposure early which will have what cause on the image?

Cause some areas of the image to be too light

Is the length of time necessary for the AEC to respond to the ionization and signals to terminate exposure

Minimum response time

What it the minimum response time for modern AEC?

0.001 seconds

Back-up times should be set… of the expected manual technique

150%

What is the function of the back-up timer?

Protect the unit

US public law 90-602 requires that generators automatically terminate AEC exposures at?

600 mAs or 60 kilowatt seconds

This operate by spring action similarly to a hand-wound alarm clock. These timers are not very accurate, and they are not used on modern equipment

Mechanical Timers

These timers use a synchronous motor that operates at 60 rps. Their shortest possible exposure is 1/60 s

synchronous timer

This type of timer operates on an electronic resistive-capacitive circuit based on the time required to charge a capacitor. These timers are accurate, allow exposures as short as 1 millisecond and can be used for serial radiography

Electronic timers

These timers are electronic timers that monitor the product of mA and time, and terminate the exposure when proper mAs is reached

mAs timer

What test is used to check timers and rectifiers?

Spinning top test

These devices w/ very accurate internal clock based on a quartz-crystal oscillator are used to check modern day timing equipment

Solid state radiation detector

Sums the voltages received from each cell and divides by the number of cells that have been activated. When the appropriate voltage for a diagnostic quality exposure is reached, it terminates the exposure

Operations amplifier

When should the density controls be used?

Should be used when the configuration of the ionization chamber cells cannot be adapted to the necessary positioning

How can changes in subject density result in AEC problems?

AEC problems with subject density occur whenever an unexpected density is present or when an expected density is lacking.

What do ionization chambers measure the exposure to?

IR

The automatic exposure control (AEC) will produce a diagnostic exposure of tissue that is placed where on the chamber?

Over the chamber

What does the radiographer need in order for the automatic exposure control (AEC) to produce quality results?

Good positioning Skills

When the correct voltage for a diagnostic quality exposure is reached it is terminated by what?

Operation amplifier

What automatic exposure device is rarely used in present-day radiography?

Phototimer

When there is a problem with density and contrast in an automatic exposure system what is usually the cause?

unexpected density