X-ray beam restrictors

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3 factors that determine the quantity of scattered radiation produced within the patient

kilovoltage (kVp_

part thikness

x-ray field size

how to decrease scattered radiation production within the patient

lower kVp

decrease part thickness

decrease x-ray field size

lower kVp

patient radiation dose increases exponentially as lower kVp techniques are substituted

decrease part thickness

not easily done

recumbent radiography

compression bands

recumbent radiography

less crays get through better

decrease x-ray field size

most effective at small x-ray field sizes

could hide necessary diagnostic information

x-ray beam restrictor

device attached to the opening of the x-ray tube housing to regulate the size and shape of the x-ray beam

types of beam restricting devices

aperture diaphragms

cones & cylinders

light beam collimator

why beam restriction decreases scattered radiation to the film

decrease x-ray field size which increases the angle of escape for scattered radiation

as angle of escape increases the angle of capture (to hit the film)

gets smaller and scattered photon is less likely to strike the smaller image

average scatter photon deflects about

50E

only few x-ray photons are able to strike/fog the image

which type of x-ray beam restricting devices are in common usage today

light beam collimators (majority)

extension cylinders (for spot views)

an x-ray system not equipped with a collimator is

difficult to use and should be avoided

why does x-ray beam restriction reduce the amount of scattered radiation on film

increases the angle of escape for scatterred radiation

advantages of a light beam collimator

infinite variety of x-ray field sizes

light beam to accurately position the central ray (CR)

back-up calibrated knobs to use when the collimator light bulb burns out

when film is loaded into Bucky tray (positive beam limiting devices)

sensors in the tray detect the size and position of the film then signal motors in the collimator to position the shutters accordingly

positive beam limiting devices (PBL) must be accurate to within

2% of the FFD being used

positive beam limiting devices (PBL) properly exposed film shows

evidence of collimation on three sides

effectively reduces scatter fog only with (collimator special considerations)

small x-ray fields

to see significant improvement in film quality, x-ray field sizes must be restricted as much as diagnostically possible

beam restriction decreases (collimator special considerations)

film density

decrease film density

small x-ray fields produces less scatter fog causing less optical density (OD) or underexposed film

to maintain constant film density (collimator special considerations)

an increase in mAs is necesssary

increase collimation causes

decrease scatter fog which decreases OD requiring more mAs

when shooting a tightly-collimated spot film, you need to increase the

mAs 50 to 100% or it will be underexposed

backscatter is produced when

x-ray beam penetrates the patient, bucky & strikes the metal cassette tray & tray cabinet

backscatter scatters

backwards towards patient to fog film

what helps to control backscatter and primary beam leak

collimatino

extrafocal radiation test checks for

presence of excessive extrafocal radiation

entrance shutters at the top of good collimators remove up to (extrafocal radiation)

90% of the extrafocal radiation in the x-ray beam

noticeably improving image sharpness and resolution

x-ray/light beam alignment test

how well the collimator light beam lines up with the x-ray beam

should be performed annually

2 reasons for restricting (collimating) beam size

decreased scattered radiation results in increased image resolution (quality)

decreased ionizing radiation dose to the patient

early radiographers restricted the x-ray beam to

improve image quality

what are the reasons to collimate your films properly

increase image quality and decrease radiation dose

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