QC Quiz 3

grid ratio

h/w

higher grid ratio

more lead, more xray absorbed, need more mAs

grid cutoff

loss of remant radiation absorbed by lead strips and loss of density on film/receptor exposure caused by grid errors or CR not centered

off level grid

grid supposed to be flat but is slanted or tube angled (parallel or focused grids). cut off across entire image and looks underexposed

off center/lateral decentering grid

grid shifted laterally and CR not centered, loss of density across entire image and loss of receptor exposure

off focus/distance decentering

focused grids need specific SID, cut off severe at edges of image

upside down focused grid

center of image good. complete loss of exposure on lateral aspects of image

uniformity

all lead strips uniform height/width
manufacturing defect or dropping on edges

uniformity test

image a homogenous phantom, process and take density readings at different points
every point should be within ±0.1
CR and DR measure pixel brightness and should be within 20%

density readings #

every point within ±0.1 or 10%

CR/DR pixel brightness

within 20%

alignment

proper centering and distance

why would we xray a grid

to check for barium on it

misaligned grid

misaligned grid lead can cause an increase in attenuation of primary xray beam
loss of image quality and increased pt dose

scatter caused by

high kVp levels, more matter, more FOV, more filtration

reduce scatter production

less kVp, less matter, collimation

reduce scatter reaching image receptor

grids, air gap

grid conversion formula

MAS1/MAS2=GCF1/GCF2

grid conversion factors

None=1, 5:1=2, 6:1=3, 8:1=4, 10/12:1=5, 16:1=6

flouro is

the single most acute dose to pt and personnel

flouro spatial resolution

1-3 lp/mm

flouro tubes have _____ compared to general xray tubes

heat capacity

flouro QC tests should be done

every 6 months

QC tests for flouro

same as diagnostic: focal spot, filtration, grid, generator tests

image intensifier

digital flat panel detector
a vacuum tube with flow of electrons

function of II

brighten image

II increase in brightness due to

increase in xray photons in the xray tube hitting the intensifier
increase in kvp and ma

input phosphor II

cesium iodide - xray to light

aluminum II

placed in front of input to reflect light

photocathode II

cesium and antimony - light to electrons

output phosphor II

zinc cadmium sulfide - electrons to light

what will cause loss of detail/signal in flouro

large FOV, less photons/mAs in beginning, vignetting/curved input phosphor, far pt to II tube

what decreases brightness in II

magnification mode, increase in pt thickness/tissue density

minification gain

input diameter²/output diameter²

image from larger input onto a smaller output causes the image to be brighter

flux gain

high voltage accelerates electrons increasing their kinetic energy from photocathode to output phosphor
causes an increase in the amount of light photons releeasd at output phosphor

brightness gain formula

minification gain x flux gain

brightness gain

amount of brightness increased due to intensifier vs non intensified image

brightness gain in II tube #s

5k-30k candela/m² and decreases 10% a year

brightness in II is affected by

kVp and mA
automatic brightness control (ABC)
automatic exposure rate control (AERC)

magnification

smaller the input phosphor size the more magnification
more detail to see smaller objects

magnification disadvantages

less input phosphor being used= decrease in minification gain causing image to be lighter
decreased FOV
increase mA will increase skin dose

magnification factor

Input phosphor 1/ input phosphor 2

magnification dose increase

input phosphor 1²/ input phosphor 2²

conversion factor

the amount of light produced by the output phosphor per unit of xradiation incident on the input phosphor
output intensity measured in candelas- unit of luminous intensity

CF

50-300 candela/m²
decreases 10% a year, pt dose increases from this

veil glare/flare

light reflects from the window of the output phosphor reducing image contrast
occurs most often when moving from one body part to another
moving from chest to abdomen can cause a sudden increase in brightness

pincushion distortion

caused by projecting an image from a curved surface (input phosphor) onto a flat surface (output phosphor)
distortion and decrease in brightness is greater toward the lateral edges

vignetting

a decrease in image brightness at lateral portions of the image

flouro visual checks happen when

every 6 months
tower and table locks
power assist- tower should be able to move freely over tabletop
bucky slot cover should move to cover opening

bucky slot cover measure radiation

on both sides of cover. amt outside should be 10% of input exposure

deadman switch

must have this
radiation only operates when on switch
tested on acceptance
frequent inspection for sticking or malfunction

timer

must have a 5-min reset timer
timer accuracy done with stopwatch

park

should have a lock for park position
when locked in position should not be able to energize flouro mode

collimation

light field to radiation field should not exceed 3% of SID

table

should move freely and stop at appropriate spot
table table indicator and actual angle should be within 2 degrees

table table indicator and actual angle should be within

2 degrees

AERC and how it works

7.5 mm thick phantom.
dosimeter between phantom and xray source
expose for 10 seconds and record reading
add another 7.5 mm and expose again.
dosimeter reading should be double

radiation exposure limits tabletop

should not exceed 10R/,min or 88 mGy/min

in fixed flouro equipment source to tabletop

15 in (38cm)

in portable flouro equipment source to tabletop

12 in (30cm)

air kerma

can be used instead of exposure or intensity to measure amt of radiation reaching patient
measured in rads or Gy
measured in air at where the center of the useful beam enters the patient
air kerma max should not exceed 44 mGy/min or 5 rad/min

best spot for II and xray tube for pt

II closer, xray tube further

DAP meter

dose area product measures dose to pt in mGy/cm²

exposure should be

constant in a single room each time tested.
when comparing different rooms should be within 25%
tested annually

ways to reduce pt exposure

collimation, last frame/image hold - save, shortest pt to II distance, highest possible kVp, pulsed flouroscopy, using largest input phosphor, longest tube to pt distance

image lag

continuation or persistence of an image, blurring the object, when the II is moved over the pt