math unjt 4

Two types of AEC

Entrance type and exit type

What is the old name for AEC

Phototimer - must be in Bucky to use

What are pickup devices

They make the phototimer work

What are the two types of pick up devices

1. Photomultiplier tube

2. Ionization chamber

Photomultiplier tube info

“old way”

• a vacuum is located under the IR

• If the pt. is large, the capacitor is charged at a slower rate, creating a large exposure

• May not work well at low kvp

Ionization chamber info

Entrance type

• located between the table and grid

Advantages of AEC

• achieves an ideal, constant receptor exposure (density)

• Consistent

• Speed

Disadvantages of AEC

• photomultiplier can be slow

• Ionization chamber is subject to temperature changes and can be easily damaged

• The more complex the system, the more costly to repair

• You must be dead on with your positioning over the photocells

Requirements for AEC

• positioning must be perfect

• Collimation must be appropriate

• back-up timer must be set

• Technique must utilize appropriate kvp

• Patient size adjustments to receptor exposure if necessary

What are radiographic technique charts

• written tables that provide a means for determining the specific technical factors to be used for a given exam

• It is a guide!

Kvp types

• variable kvp

• Fixed kvp

• High kvp

Variable kvp

• uses a fixed mAs and a kvp that varies according to part thickness

• kv varies 2kvp per cm thickness

  • shorter contrast scale

  • Higher pt. dose

Fixed KVP

• select the optimum kvp and adjust the mAs to the thickness to the thickness of the part

• Usually grouped as small, medium, and large pts

• For small anatomy reduce the mAs by ?%

• For large anatomy increase the mAs by ?%

• For swelling due to trauma increase the mAs by approximately ?%

High KVP

• the kvp selected is generally over 100

• Used for chest radiography and fluoroscopy

Technique charts

• proved a method of consistency in the quality of image production

• Need to be provided for each radio graphic unit

• Monitoring and revising these charts are part of the quality control program