X-ray beam restrictors
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Created by:
elifnavarrete on June 29, 2012
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33 terms
Terms | Definitions |
|---|---|
 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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