Finals Flashcards
(120 cards)
One of the two photographic properties that comprise visibility of detail; degree of overall blackening of the film
Density/image receptor (IR) exposure
The image is visible to the human eye only because sufficient IR exposure/density and contrast exists to permit the structural details to be percieved
Visibility of detail
Monitor control function that can change the lightness and darkness of the image on a display monitor but is not related to IR exposure; luminous intensity (measured in candelas) of the display monitor’s light emission
Brightness
Digital post-processing that produces changes in density/brightness
Window level
Range of densities of human visibility
OD 0.25-2.50
What is the controlling factor of density/IR exposure (principal method for adjusting for insufficient or excessive exposure)?
Milliamperage-seconds (mAs)
Film density should remain unchanged as long as the intensity and duration of the x-ray exposure (controlled by mAs) remains unchanged; fails for extremely short (less than 0.01 second) or long (more than a few seconds) exposure times
Reciprocity law
Minimum change necessary to cause visible shift in film density
30% of mAs or any other influencing factors that would equal this change
Acceptance limit for density/IR exposure that requires repeating exposure
30% over- or underexposed
General rule of thumb for mAs changes
Make adjustments in increments of doubles or halves
9 influencing factors affecting density/IR exposure
Kilovoltage (kV) Focal spot size Anode heel effect Distance (SID and OID) Filtration Beam restriction Anatomical part Grid construction Image receptors
3 ways kV alters the intensity of the beam reaching the IR
Controls the energy and therefore the strength of the electrons striking the target of the x-ray tube for any given mAs
Controls the average energy of the x-ray photons produced at the anode target therefore a change in kV alters the intensity of the beam when the mAs and other factors remain the same
Affects the production of scatter
Used as a guide to maintain the same IR exposure/density when kV changes
A 15% increase in kV causes a doubling of exposure to the IR; a 15% decrease in kV causes a halving or exposure to the IR
15% rule
Range of kVp the 15% rule is accurate in
60-100 kVp
Large focal spots tend to do this more at higher mA’s (because the incident electron beam is not as easily focused by the focusing cup) and may occasionally reach a point where they alter the IR exposure; rare for this to cause a visible density/IR exposure difference
Blooming
Due to the geometry of the angled anode target, the radiation intensity is greater on the cathode side
Depending on the angle of the anode, this can cause an IR/exposure variation of up to 45%
More pronounced when the collimator is open wide than when it is closed because a greater portion of the peripheral beam and therefore a greater portion of the intensity difference, reaches the IR when the collimator is wide open
More significant when using extremely small angle anodes (12º or less)
Anode heel effect
At what end of the tube is IR exposure always greater?
Cathode
How can the anode heel effect be minimized?
By collimating the beam and eliminating as much of the intensity difference at the periphery as possible; if a given film size is required collimation may be achieved by using a greater SID
The intensity (exposure) varies inversely with the square of the distance
Inverse square law
What formula represents the inverse square law?
I1/I2 = D2^2/D1^2
Direct square law that states mAs must increase when distance increases, and vice versa, in order to maintain IR exposure
To maintain IR exposure, mAs (or an influencer) must be changed to compensate for the exposure change
Only accurate within a moderate acceptance range; the nonlinearity of some components in the imaging system makes it impossible to exactly quantify the relationship between radiation beam intensity and IR exposure
Exposure maintenance formula
What is the exposure maintenance formula?
mAs1/mAs2 = D1^2/D2^2
OID has an effect on density/IR exposure
Uses and increases OID to prevent scatter (which would normally cause a visible increase in density/IR exposure when radiographing large patients) from reaching the IR
By increasing OID, scatter that would normally strike the IR will miss it, causing a decrease in density/IR exposure
However in most instances OID variations are insufficient to cause visible density/IR exposure changes
Air-gap technique
Density/IR exposure _______ when filtration is increased
Decreases
