chapter 18-19 Flashcards
what are the purposes of field size limitation
- to minimize radiation exposure to patient
- preserve subject contrast in the remnant x-ray beam
who discovered the first x-ray beam filter
william rollins
what is the most effective device for providing a sharp field border which force the x-ray beam to effectively pass through
collimator
what is produced anywhere outside of the focal spot when electrons scatter and the strike the glass envelop
off focus radiation
off focus radiation is produced in ______ of the xray tube
any portion
another name for positive beam limitation
automatic collimators
when can PBL be overridden
when the field needs to be smaller than the IR
the ____ should always be fully included within the xray field
the anatomy of interest
clipping the anatomy can result in…
repeating exposures, doubling patient exposure
for anatomy smaller than the IR allow at least ____ of light beyond each edge
1/2 inch or 1 cm
the amount of scatter radiation is controlled by
-size of the patient
- size of the x-ray field
subject contrast is changed when the ratios when between primary rays, photoelectric interactions and compton scatter interactions are…
altered
good subject contrast within the patient is dependent upon
proportions of different interactions within the patient relative to each other
how does smaller collimation affect subject contrast
subject contrast is improved
what does field size affect
visibility functions of the image
what is the four cm rule
for ever 4 cm change in part thickness, adjust the mAs by a factor of 2 or kVp by 15%
what is the minimum change rule
a 35% increase in the mAs, or 15% decrease in kVp to bring any significant alteration in final exposure
5 types of body habitus
- sthenic
- hyposthenic
- asthenic
- hyperstenic
- large muscular
who should you reduce kVp for
pediatric or geriatric patients
what are the types of tissues used on radiography
- metal
- bone
- soft tissue
- fat
- gas
additive diseases
increase fluid, bone, or metal content within tissue
why does positive contrast absorb more radiation
appears radiopaque
rule of thumb for additive diseases
increase overall technique by 50%
for a pathological condition to require an adjustment in technique it must alter one of the five techniques which are…
- air
- fat
- fluid
- bone
- metal
