RAD-172 EXPOSURE SYSTEMS & AUTOMATIC EXPOSURE CONTROLS

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what is a device used to maesure body part thickness

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what is apr
- anatomically programmed radiography
- systems are commonly utilized computerized exposure control units that combine an automatic exposure control (AEC) system w/ exposure factors to correspond to specific anatomical procedures (body part button = tech factors for that part)
is AEC an exposure system

no
AEC controls what

the length of the exposure (time)
what happens when CR and DR are underexposed

the image will look grainy
anything tabletop uses manual technique but uses what system

apr no aec because aec uses phototiming and photocells
the success of technique charts depends on what three factors
- consistent equipment function(ma time and kp)
- consistent performance examination (SID screen film combination grid ratio positioning collimation tech factors)
- consistent processing of radiographs
what is the goal of any exposure system
all exposure systems require the use of ____
- is to provide a method of consistency in the quality of image production
- technique charts
List the two types of exposures systems and explain them

variable kvp system (kvp is changed and manipulated) and fixed kvp systems (kvp does not change mas is changeable)
what is a variable kvp system

a system in which the kvp to be used for a particular projection is varied depending on measured body part thickness
what are pros and cons to variable kvp systems
- pros:
- allows for small changes in exposure for small changes in part thickness
- higher contrast (can affect perceived image resolution)
- cons:
- more patient dose
- higher repeart
- tube wear out
in the historical development of radiography what were the measurements of calculating kvp

2 kvp x every cm in tissue thickness + 30 = new kvp
what is a fixed kvp system

the kvp is held constant and uses and optimal kvp for each body part selected for a given range of subject densities and contrasts while the mAs is varied to achieve an appropriate image density
what are the pros and cons to a fixed kvp system
- pros:
- reduce in pt dose
- give more info withing range
- increased consistency in image receptor exposure
- lengthens exposure latitude
- reduces xray tube wear
- allows for shorter exposure times - reduce pt motion
- easier to memorize
- cons:
- higher amt of scatter results in reduced image contrast
a fixed kvp system chooses the ____ kvp for the procedure selected

optimal
what are the two main methods in part measurement
- part thickness
- and measurement of central entering to cr exiting
apr is controlled by what

a integrated circuit or computer chip that has been programmed with exposure factors for different projections and positions of different anatomic parts
in apr when the body part is selected can the radiograpger adjust the exposure factors

yes
what is the most common exposure system

fixed variable system
what are the three characteristics of APR systems
- commonly used computerized exposure control
- control console provides choices of anatomical parts with suggested exposure factors
- factors can be modified for non routine conditions
what are the three characteristics for the apr systems with AEC
- apr systems incorporate AEC devices
- selection of exposure time eliminate (mas)
- must still choose ma kvp and distance
- requires learning when to use and when to avoid
which body parts would would need the aec to be turned off
ex?
- anybody part that requires a a breathing technique of a long exposure time
- sternum ap scapula transthoracic for prox humerus lateral thoracic
what does aec or automatic exposure control do
what device is used to do this
- it stops the exposure at the right time so that a radiograph with good density is produced
- a photocell
AEC controls ___

the time or the length of the exposure
where are the photocells

in bucky or near it and cannot be seen
a photocell is either a _____ or a ______

ionization chamber or photomultiplier
what is exit radiaiton

small angle scatter and penetration photons
list the structure of the photocells in the table
- table top
- grid
- sensors
- then the bucky tray
how does the ir produce a preset OD (optical density)

through an electronic feedback circuit radiation exposure is terminated when a sufficient number of xrays have reached the IR
what is the usual range of the radiographic densities

.25 and 2.5 above base + fog
what is od range
which is lightest and darkest
- 0-4
- 0 lightest
- 4 darkest
what would happen if AEC was used on a large patient

it would take a long time for enough exit radiation to get through the patient and into the photocell to charge the xray timer to the predetermined amt of charge exposure length would be long
what would happen if aec was used on a shorter patient

the patient would cause a shorter length of exposure
what is the most common photocell to activate

the two outer or the center cell
what is the backup timer
what is it used as
- when aec is used the ordinary timer used for an exposure made without aec is the back up
- it is use a safety device for the equipment to save the xray tube life
if a ordinary pa chest exposure time is set for .20 second but the backup time is .15 what will happen to the image
- the amt of radiation receive will stop faster and produce an underexposed image because the appropriate time is .20
- there the backup timer should be set to a time longer than what is needed
what happens if aec fails to work
what happens if the backup time is set to high
- the backup time will kick in n stop the exposure
- then the patient will receive a larger dose of radiation
what do radiographers do to prevent backup timing making their image too light faster backup time?

they set the backup time very high so it does not cut off earlier but this is not good practice because it increase pt dose
are all aec's equipped with backup timers

yes
can backup timers exceed tube limit and what is there percentage of the anticiapated manual exposure mAs
- no
- 150%
according to us public law 90-602 generators must terminate the exposure at ______ for exposures above ____ kvp and ______ mAs for exposures under ____kvp
- 600 mAs; 50
- 2000 mAs;50 kvp
AEC affects only _____ ______ and time controls _____ exposure

exposure time; IR
what happens to the aec once kvp is changed

the aec will compensate woth an adjustment in exposure time to adjust Ir exposure
If kvp is decreases IR exposure ______ so the AEC will _____ time to compensate

decreases; increases (longer time)
If kvp is increases IR exposure ______ so the AEC will _____ time to compensate

increases; decrease (shorter faster time)
an increase or decrease in kvp when using aec changes ______ and _____ _____

contrast; IR exposure
high or increase in ma causes a _____ exposure time
a lower or decrease in ma causes a _____ iexposure time to produce adequate density
which is best to pick
- decrease (faster) shorter
- increase (slower) longer time
- high ma = short exposure time to beat pt motion
when should we use a two photo cell arrangement
and a 1 center cell
- pa chest kub ribs pelvis
- shoulder lat chest femur hip lat spine
what would happen if the radiographer chose the wrong cell for an exam

the body part will be underexposed or overexposed depending which wrong cell was chosen
what would happen if the center cell was chosen for a pa chest instead of a two cell arrangement

the image will be over exposed because the body part centered over the cent4er cell will be exposed appropriately (the thoracic vertebrae) and the lung field will be way too black because the outer cell was not selected
how does a photocell determine the length of the exposure

by averaging the tissue densities over a certain cell
what happens to an image when using a center cell but the body part was not position correctly in the middle of the cell

the photocell will detect some of the body part and some parts that werent over the center (blank spots) so when the photocell averages the length thru tissue thickness it will become underexposed
what happens to your image a pa chest was centered to low or too high
- if it was too low the photocell will average the radiation for the abdomen and the chest will be overexposed
- if it was too high the photocell will average the radiation and thickness for the upper chest (which is thinner) and the lungs will be underexposed
when should collimation be avoided
too close collimation would result in
not enough collimation would result in
- when the collimation is close to the activated chambers
- overexposed image
- in a light image underexposed
when using aec with barium where should the body be positioned

the body part with the barium should be positioned over the photocell
the density of the radiograph with aec is determined by the _____ ____ settings on the timer

predetermined charge
when would we need to adjust density options with aec

when patient is extremely large or patient is really small
what is the only wat to make a radiograph lighter or darker when using aec
what are the density control labels
- density control options
- -2 -1 0 +1 +2
density controls:
-1 =
+1 =
0 =
-2 =
- -1 = 25% decrease in relative exposure
- +1 = 25% increase
- 0 = normal density
- -2 = 50% decrease in dens
- +2 = 50% increase in dens
what is minimum response time
modern scr's may have a minimum reaction tim of less than _____ second
- shortest amt of time it takes ionization chambers or photocells and tube to turn off exposure
- .001 seconds
what are scr's

silicon controlled rectifiers are electronic timers (most used today) capable of accurate exposures short as 0.001 sec with only a millsecond delay

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RAD-172 EXPOSURE SYSTEMS & AUTOMATIC EXPOSURE CONTROLS

Description

xray

Updated

2014-04-20T02:09:17Z

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