Chapters 3 & 4 Flashcards
2 reasons why rare earth element screens are preferred over the calcium tungstate
- less radiation exposure
- faster
elements intensifying screens emit blue light
calcium tungstate screens
elements in the intensifying screens emit green light
rare earth screens
why is a short object film used in oral radiography
reduce penumbra and magnification
relationship between the central ray of the xray beam and the object and film must be attained in order to prevent distortion of the image
perpendicular
3 factors that effect film contrast in radiology
- exposure
- processing
- film type
slight vibration of the tube head during film exposure will result in an increase in the size of what structure in the x-ray tube
focal spot
substance that has ability of “converting to light” has what property
florescent
the three basic elements of an x-ray tube needed to produce xrays are
- available source of free electrons
- high voltage to impact speed to the electrons
- target that is capable of stopping/slowing the electrons
what distance factor directly influences magnification of images on the film
target-object distance
object-film distance
a radiograph that is light in density is most likely caused by which exposure problem
too low kVp
too low mA
if a change from the long scale contrast technique to a short scale contrast technique and maintain same densitity what should be done
decrease kVp and increase mA
oral structures of greater density may require increased penetration by xray photons this can be done how?
increase kVp
Intensifying screens used in extraoral radiography
Less radiation exposure to patient
But can also
Decrease sharpness of radiographic image
increasing kVp causes the resultant radiograph to have
longer scale of contrast
what will result in a radiographic image that has many gradiations of gray from totally white to completely black
long scale contrast (low contrast)
what does ALARA stand for
as low as reasonably achievable
increasing kVp results in
low contrast (long scale contrast)
when the mA is increased what must be adjusted to compensate for this increase in order to make a film with the same contrast
decrease exposure time
when voltage is increased
electrons move from cathode to anode with more speed
exposure time is measured in
impulses
term that describes how dark and light areas are differentiated on a film
contrast
overall blackness or darkness of a film is termed
density
if kilovoltage is decreased with no other variations in exposure factors, the result will
appear lighter
a radiograph that has few dark and light areas with many shades of gray is said to have
low contrast
shadow cast rules are often referred to as
geometric factors
contribute to the quality of the radiographic image. factors that related to relationship of angles, points, lines or surfaces
geometric factors
highest voltage to which the current in the tube rises during an exposure is called
kilovoltage peak
when contrasted with the use of 85 to 100 kV for dental x-rays, the use of 65 to 75 kV produces
less penetrating dental x-rays with the longer wavelength
when kilovoltage peak settings are used (65 to 70kVp) a film with _________ will result
high contrast
a film with high contrast
is useful for the detection and progression of dental carries
according to the inverse square law, the intensity of radiation is______proportional to the ________from the source of radiation
inversely
square of distance
according to the inverse square law if the length of the target film distance is cut in half the resultant beam is _______ intense
four times
according to the inverse square law, what factor must be adjusted if the target film distance is doubled
object film distance
the paralleling technique is recommended over the bisecting angle technique because
it gives a less distorted picture of root length
the intensity of x-radiation at any given distance from the source of radiation varies
inversely with the square of the distance
when an 8 inch target film distance is changed to a 16 inch target film distnace (keeping kVp and mA constant) the exposure time should be
quadrupled
to increase penetrability of the xrays their wavelength should be
shortened by increasing kVp
the size of the focal spot in the xray tube influences the radiographic
definition
the optical or overall density of an intraoral film indicates the
degree of darkness
optical density is a function of
kVp, mA, and exposure time
subject contrast is primarily a function of
kVp
how do you change from a low contrast to a high contrast image and still maintain density
decrease kVp and increase mA
image magnification may be minimized by
long cone
image sharpness on a radiograph is increased by
using a small focal spot size
small focal spot size
increases sharpness
short scale contrast (high contrast)
fewer shades of gray and more black against white
gray tones indicate
differences in absorption of the xray photons by the various tissues of the oral cavity
kVp of short scale contrast
low kVp (60-70)
long scale contrast (low contrast)
low and gradual because there are many shades of gray
kvp of long scale contrast
higher kVp (80-100)
structures that permit the passage of xrays with little or no resistance
radiolucent
structures that are dense and absorb or resist passage of xrays
radiopaque
high kVp
lower contrast
low kVp
higher contrast
increased scatter radiation
lower contrast
decreased scatter radiation
higher contrast
large focal spot
decrease sharpness
long target-image receptor distance
increase sharpness
short target-image receptor distance
decrease sharpness
short object-image receptor distance
increase sharpness
long object image receptor distance
decrease sharpness
focal spot
small area on the target where bombarding electrons are converted to xrays
“dead-man” exposure switch
automatically terminates the exposure when operators finger stops pressing on the timer button
what absorbs heat during xray production in tube head
air or gas
