Chapter 15 Notes Flashcards
principal factors that affect amount of scatter produced are
kilovoltage and the irradiated material
as kilovoltage increases
scatter increases
as volume of irradiated tissue increases
scatter increases
volume will increase as
the field size increases or as the patient thickness increases
higher Z# materials have
more electrons and a greater chance of absorbing photons
what is the most commonly employed beam restrictor in radiography
the collimator
the collimator permits
an infinite number of field sizes
the exposure field should never
exceed the size of the image receptor
kVp affects
the penetrability of beam
increasing kVp will
decrease photoelectric absorption, increase Compton scatter, and decrease dose
decreasing kVp will
increase photoelectric absorption, decrease Compton scatter, and increase dose
decreasing field size without mAs compensation can
increase image noise
thicker and denser body parts will
produce more scatter
the collimator’s bottom shutters will
reduce penumbra
define pneumbra
geometric unsharpness around periphery of image (also known as edge sharpness)
off-focus radiation occurs from
areas of x-ray tube other than focal spot area
the collimator’s upper shutters will
reduce off-focus radiation reaching IR
the light field needs to be accurate within what percent of SID
+-2%
the mirror used in the light field will
add to system’s inherent filtration
positive beam limitation (PBL) devices
automatically collimates beam to size of image receptor
examples of ancillary devices
lead blockers and lead masks
higher Z# will cause
greater number of photoelectric absorption interactions and less scatter
scatter is produced by what interaction
Compton interactions
Compton interactions occur in
the outer shell
reducing penumbra will
improve sharpness of recorded image
describe the light field set up
a mirror is mounted in the path of the x-ray beam at a 45 degree angle; light source is placed opposite of the mirror
aperture diaphragm
a flat sheet of metal with an opening cut in the center and attached to x-ray tube port
aperture diaphragm is usually made of
lead
cones and cylinders
essentially circular aperture diaphragms withe metal extensions
the cone has an extension that
flares or diverges (upper diameter is smaller than bottom)
ancillary devices are designed to
restrict the beam to a specific shape for a particular examination
lead blocker
a sheet of impregnated rubber that can be cut to any size or shape
lead blocker can be placed where
behind patient during LAT L-Spine; above shoulder during AP
lead blockers are most helpful when
examining large patients
lead mask
usually cut to correspond to particular field size and is secured to end of collimator
