CH 15 BEAM RESTRICTIONS/ physics exam 3

Scatter radiation is produced during ______

Compton interaction,

How to minimize the amount of scatter?

By restricting x-ray beam and using a grid

The principle factors that affect the amount of scatter produced RE?

1. Kilovoltage

2. 2. The irradiated material

Kilovoltage affects _____

Affects the penetrability of the beam as

As KVP increases the percentage of x-rays that undergo Compton _______, at the same time, the percentage of photoelectric affect_________ reducing

Increases, decreases, reducing pt dose

Compton interactions create_______

Create scatter as kill voltage increases.

When you decrease KVP you increase

You increase photoelectric absorptionm

When you decrease KVP you increase_______

Increase patient dose

Affect on image quality:

–Shorter scale of contrast

Volume of Irradiated Material

CONTROLLED BY:

*FS (field size)

*Patient thickness

Thicker body parts produce more ——-

Scatter

Denser body parts produce more

Scatter

in order to decrease Decreasing Part Thickness you can use ?

Compression devices used to improve spatial resolution and contrast

–Decreases patient thickness

–Results in lower patient dose

Higher atomic # materials have greater number of electrons within each atom therefore increasing the odds of striking an electron So, the Higher the atomic number of a material, the greater will be the __________

number of photoelectric absorption interactions and less scatter

Ex Bone, iodine, barium, and lead absorb more radiation(high contrast seen)

Some beam restrictors include?

•Aperture diaphragms, cones, and cylinders

•Collimators

•Ancillary devices

Collimator consists of

Consist of sets of lead shutters at right angles to another, and move in opposite pairs

• permit infinite number of field sizes

The bottom shutters of the collimator reduce

Penumbra along periphery.

Also known as edge un-sharpness

•Reducing Penumbra improves sharpness of recorded image edge

Upper shutters reduces _______

off-focus radiation reaching IR

•Off-focus radiation produces images beyond exposed field of radiation

•Similar to shadows of the patient

Some collimators are equipped with PBL what is PBL

positive beam limitation device automatically collate the beam to the size of the image receptor.

•Exposure field should NEVER exceed the size of the IR

•Possible to override PBL

–Can reduce beam to smaller field than cassette size

Aperture diaphragm- refers to

flat sheet of metal usually lead with an opening in the center and attached to x-ray tube port

Ancillary Devices Include

Lead blockers

–Shields

•Lead masks

–Attach to collimator

Cones and Cylinders Are ?

Cones and Cylinders are circular aperture diaphragms with metal extensions

•Cones has extension that diverges

•Cylinder does not