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Beam Limiting Devices
N/A
31
Physics
Professional
03/19/2013

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Term
purposes of beam restriction
Definition
limit patient exposure
reduce amount of scatter produced in patient
improve image quality
Term
as beam restriction ___, field size and patient dose _____
Definition
increases, decreases
Term
with reduced scatter, radiographic contrast ____
Definition
increases
Term
most important characteristic of image quality
Definition
contrast
Term
two major factors that create scatter radiation
Definition
high kVp
volume of tissure irradiated
Term
types of beam limiting devices
Definition
aperature diaphragm
cones and cylinders
collimator
PBL
Term
bottom shutters of collimator
Definition
reduce penumbra
Term
upper shutters of collimator
Definition
reduce amount of off focus radiation
Term
PBL
Definition
Positive Beam Limiting device
automatic colimators are adjusted by tube head once IR is placed in bucky
can be overridden
Term
as filtration is _____, so is beam quality, but quantity is ___
Definition
increased, decreased
Term
types of filtration
Definition
inherent
added
compensating
Term
inherent filtration provided by
Definition
thickness of glass envelope
dielectric oil coolant
port or glass window of housing
Term
most inherent filtration comes form
Definition
port/glass window of housing
.5-1.0 mm Al/Eq
Term
inherent filtration in mammography tubes
Definition
.1mm Al/Eq
Term
mammography tubes are made of
Definition
beryllium
Term
functions well as a low energy absorber, atomic number of 13
Definition
aluminum
Term
two sources of added filtration
Definition
collimator device
mirror
Term
average thickness of collimator device
Definition
1.0 mm Al/Eq
Term
types of compensating filters
Definition
wedge
trough/bilateral
bow-tie
cone
step-wedge
Term
total filtration
Definition
= inherent filtration (.5mm) + added filteration (added filter of 1.0mm and mirror 1.0mm)
=2.5mm Al
Term
doesnt include any compound or compensating filters
Definition
total filtration
Term
NCRP
Definition
requires minimum total filtration levels
Term
filtration requirement for <50 kVp
Definition
.5mm Al/Eq
Term
filtration requirement for 50-70 kVp
Definition
1.5 mm Al/Eq
Term
filtration requirement for >70 kVp
Definition
2.5 mm Al/Eq
Term
filtration reduces patient dose by
Definition
eliminating low energy photons from primary xray beam
Term
when filtration in increased, ___ must also be increased to maintain density
Definition
technical factors
Term
any significant increase in filtration (1.0 mm Al/Eq) will have an overal _____
Definition
entrance skin exposure reduction effect
Term
point of diminishing returns where reduction in ESE does not warrant tube loading increase
Definition
beyond 3.0 mm Al/Eq of filtration
Term
HVL
Definition
measures quality of xrays
represents thickness of absorbing material to reduce xray intensity to half its original value
Term
result of low energy attenuation shifts xray emission spectrum to high energy side, resulting in
Definition
xray beam with higher energy
greater penetrability
better quality
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