Unit 4: Image Brightness/ABC/Mag. and Fluoro + Pt Exposure and AEC Flashcards
what is flux gain?
a ratio of the number of light photons produced at the OP screen to the number of x-rays absorbed by the IP screen
what is the formula for flux gain?
numberof output light photons/number of input x-ray photons
what is minification gain?
the ratio of the square of the IP screen diameter to the square of the OP screen diameter
What is the formula for minification gain?
(Di/Do)^2
OP screen diameter is usually:
IP screen diameter varies from:
1 inch (2.5 cm)
9-25 or 30 cm
what determines how much brighter an image will be at the OP screen?
minification gain
what is brightness gain?
the ability of an II tube to increase the illumination of an image
brightness gain =
minification gain x flux gain
the brightness gain of most II tubes is:
5000 - 30,000
brightness gain is now called _________________ and is measured in _______________.
Conversion Factor
candela per squared meter (cd/m^2)
What is Automatic Brightness Control (ABC)?
brightness controlled by and altered by the machine varying the kVp, mA, or both
ABC is designed to compensate for changes in what?
tissue density
what is the purpose of ABC?
to maintain image brightness on the display monitor
For a fluoro system equipped with ABC mechanism and where the x-ray tube is fixes below the table, moving the image intensifier away from the pt will:
increase pt dose
The function of an ABC mechanism of an image intensified fluoro system is to:
maintain a fixed dose rate to the image intensifier
the greater the voltage supplied to the electrostatic lenses, the greater the ____________ and closer the ______ ______ moves toward the IP screen. This causes ______________.
acceleration; focal point; magnification
image magnification during fluoro is controlled by:
varying the voltage on the electrostatic lenses
Which of the following will occur if voltage to the electrostatic lenses increase?
magnification
viewing the fluoroscopic image in magnification mode increases:
mA
spatial resolution
pt dose
what determines the location of the focal point and the FOV?
IP screen diameter
what are advantages of fluoro mag mode?
better spatial resolution
less image noise
less vignetting
what are disadvantages of fluoro mag mode?
increased pt dose
reduced FOV
what is the weakest link in the fluoro imaging chain?
monitor
what increases during mag?
MTF
image blur caused by moving the tube rapidly during a fluoro exam is due to:
camera tube lag
collimation, intermittent exposure, removal of grid, LIH, dose spreading, beam hardening, pulsed exposure, and cumulative timer are all:
dose reduction techniques in fluoro
last image hold is a:
dose-saving feature
what is dose spread?
method that spreads the max dose over a broader area of the pt’s skin
some reduction of max skin dose can be achieved by rotating the fluoroscope about a ____________ within the anatomy of interest.
centimeter
during dose spread, the angle is changed to:
expose different tissue
increasing kVp will create a:
harder beam
maintaining the highest kVp that will provide acceptable image contrast leads to:
lower skin dose
beam energy primarily depends on the _____ selected and the amount of _________ in the beam
kVp; filtration
for beam hardening, you need to:
increase kVp and lower mA
filtration reduces:
skin dose
how much did an addition of 0.1-0.3 mm of copper reduce skin dose?
30-50%
size distortion is primarily caused by _____ during fluoro.
OID
to have the lowest dose to the pt, have the IR:
right on top of the pt
What is a consequence of using “low dose” during fluoroscopy?
quantum mottle
during fluoro, which will produce the greatest amount of scatter?
patient
in the non-mobile fluoro unit, the x-ray tube moves with the:
image intensifier
The back-up timer is typically set at
5 mins
before continuation of fluoro what should be done if the timer goes off?
timer should be reset
AEC stands for:
automatic exposure control
modern equipment uses ionization chambers introduced by _________________ in _____
Russell H. Morgan; 1942
Where are AEC detectors located?
in between the pt and IR
what is crucial when it comes to AECs and ionization chambers?
positioning
plastic inserts for collimators are only accurate at:
the intended SID
collimation should be avoided when located:
close to activated chambers
Collimating within the field of an AEC receptor will result in?
overexposure
inadequate collimation may result in:
light image
what is the max exposure per US Public Law?
mAs
modern minimum reaction time to terminate exposure in AEC:
.001 seconds
for density controls:
0 =
-1 =
-2 =
+1 =
+2 =
normal
25% decrease in exposure
50% decrease in exposure
25% increase in exposure
50% increase in exposure
poor positioning skills result in increased ___________ when using AEC?
repeat rate
What does “APR” stand for?
anatomically programmed radiography
What is “APR”?
system that allows radiographer to select a button that represents anatomic area with exposure factors for that area
Having the outer cells on during a lateral chest will result in?
underexposure
Having only the center cell of the AEC activated for a PA chest will result in?
overexposure
the safety features of a fluoro unit do not include what?
high grid ratio
