Radiography Essentials Ch11 scatter radiation and its control

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Scatter Radiation and Its Control

backscatter

Scatter radiation that is directed back toward the x-ray tube.

Bucky

A moving grid.

coherent scattering

One of three interactions of radiation with the human body tissues. It occurs at low energy levels with energy transferred to the atom. Only about 5% of exposures is caused by this interaction. Also known as Thompson scatter.

Compton effect

One of three interactions of radiation with the human body tissues. In this effect, the x-ray photon is scattered from the body rather than going directly through to expose the film. This effect is undesirable in radiographic imaging.

coned-down image

A radiography of a very small area of the subject; used when fog compromises the ability to see specific details in a large body part.

crosshatch grid

A composite of two grids with the lead strips at right angles to each other; also called a crossed grid.

focal range

The range of source-image distances (SID) at which the grid will not absorb significant amounts of primary radiation.

focused grid

Grid for general-purpose use; the lead strips are aligned to the direction of the diverging primary x-ray beam.

grid

A device placed between the patient and the IR to absorb scatter radiation.

grid cassette

A special cassette with a grid built into the front side.

grid cutoff

Excessive absorption of useful radiation by the grid.

grid frequency

The number of lead strips per inch.

grid lines

An image of the grid itself in radiographic images; also called grid striping.

grid radius

The distance at which the primary x-ray beam is parallel to the focused lead strips of the grid.

grid ratio

The relationship between the height of the lead strips and the width of the spaces between them; determines the effectiveness of the grid.

parallel grid

A grid with strips that are parallel to each other, rather than focused. A

photoelectric effect

One of three interactions of radiation with the human body tissues. In this effect, the x-ray photon is totally absorbed in the body tissues.

scatter radiation

Result of either coherent scattering or the Compton effect.

secondary radiation

Radiation produced by the photoelectric effect.

stationary grid

A high-frequency grid that does not move during the exposure.

characteristic photon

Produced during photoelectric interactions and is considered secondary radiation.

contrast

Scatter radiation creates fog that reduces _______ and reduces the visibility of detail.

higher kVp

Results in more scatter radiation fog.

principal source

The patient is the _______ of scatter radiation in radiography.

true absorption

Photoelectric interactions.

In the diagnostic range of kVp used (50 to 100) the majority of radiation interactions with the body are ____ interactions.

Compton.

fog

Unwanted exposure to the film.

The fog produces an overall increase in radiographic ____.

Density.

Greater ____ of tissue, the more scatter radiation.

Volume.

More scatter radiation when the ____ of the subject is increased.

Thickness.

____ radiation is better able to escape the subject and cause fog on the radiograph.

High-energy.

The more ____ the absorbing matter is, the more scatter radiation that is produced because more of the primary beam is absorbed.

Dense.

factors affecting scatter radiation fog

Tissue thickness, field size, milliampere-seconds, and kilovoltage.

radiographic grid

The principal method for reducing scatter radiation fog.

A grid is used when a body part becomes greater than ____ in thickness.

10 to 12 cm.

other strategies for limiting scatter fog

Use of an air gap, limitation of the field size, and reduction of kVp.

____ x-rays not moving in the same direction as the primary x-ray beam are absorbed by the grid.

Scattered.

A ____ has the form of a thin plate and is covered by a protective aluminum coating.

Grid.

A grid is constructed of tiny, tissue-thin ____ strips, placed on edge separated by radiolucent interspacing material.

Lead.

lead strips

Absorb scatter radiation while permitting remnant radiation to pass through.

The effectiveness of a grid is determined by the ____.

Grid ratio.

The ____ the grid ratio, the less variation is permitted and the greater is the efficiency of the grid.

Higher.

the range of typical grid ratios

5:1 to 16:1.

grid cassettes/mobile radiography - what ratio?

5:1 and 6:1.

general purpose - what ratio?

8:1.

general purpose/chest radiography - what ratio?

12:1.

high-kilovoltage radiography only - what ratio?

16:1.

Grid frequencies range between ____.

60 and 196 lines/inch.

two methods employed to prevent objectionable grid lines

Using a moving grid or using a grid with very high frequency.

Moving the grid during the exposure ____ the image of the grid lines so that the grid image is not visible on the film.

Blurs.

Bucky grids typically have a ratio of ____.

12:1 to 16:1.

Bucky grids typically have a frequency of ____.

85 to 103 lines/inch.

grid cabinet

Upright cassette holders.

Stationary grids typically have a ratio of ____.

8:1 or 12:1.

Stationary grids typically have a frequency of ____.

At least 103 lines/inch.

common grid cassette ratios

5:1, 6:1 or 8:1.

Grid cutoff appears as ____ radiographic density on the side of the image.

Decreased.

No ____ occurs when the x-ray beam is correctly aligned to the grid.

Grid cutoff.

Grid cutoff occurs when the x-ray tube is centered to one side of the grid rather than to the ____.

Focal center line.

causes of cutoff

Lateral decentering, source-image receptor distance outside focal range, lateral angulation or grid level off, and grid reversed.

The higher the grid ratio, the more ____ the alignment must be.

Precise.

The ____ of a parallel grid is infinity.

Radius.

A crosshatch grid is desirable because it has an effective ratio ____ than the ratios of two grids combined.

Greater.

The x-ray tube cannot be ____ when using a crosshatch grid.

Angled.

OID

Object-image receptor distance.

Increasing OID will ____ the intensity of scatter radiation at the film.

Decrease.

the lateral projection of the cervical spine

One procedure in which the air gap is used by default.

for coned-down images what size IR and square field is used

8 x 10 IR and a 5- or 6-inch square field

"nine-penny test" or collimator tool test

Used to check the collimator.

standard control limit for collimator is that the x-ray light field and the radiation field must be within ____.

± 2% of the SID.

standard control limit for the tube's beam alignment is that the tube must be mounted so that the x-ray beam is within ____.

1 degree of perpendicular.

When kVp is decreased this requires an increase in ____ to compensate.

Milliampere-seconds.

the direction the majority of photons that are scattered will go

Back toward the x-ray tube.

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