chapter 11 EXAM 5 Flashcards
safety requirements are required for:
tube housing xray control panel radiographic exam table source to image distance indicator other devices and accessories
xray tube housing requirements
lead lined metal tube housing to protect patient and tech from leakage, scatter and off focus radiation
leakage radiation
radiation that does not exit form the collimator opening but penetrates the protective tube housing and the sides of the collimator
primary beam (direct radiation, useful beam)
radiation that emerges directly from the xray tube and moves without deflection toward a wall, door, etc
the tube housing is constructed so that leakage radiation measured at a distance of what from the xray source does not exceed what when the tube is at it’s highest mAs/Kv
1m from the xray source does not exceed 1 mGya/hr
the tube housing also does what
it confines high voltage to prevent electric shock
makes cooling of xray tube possible
control panel/console requirements
must indicate the conditions of exposure and indicate when xray tube is energized
usually satisfied with use of kVp and mAs
radiographic exam table requirements
must be strong and support the pt.
thickness must be uniform and radiolucent to absorb only a minimal amount of radiation
what is the xray table typically made of
carbon fiber material
source to image receptor distance indicator requirements
can be a tape measure or a laser
distance and centering indicators
distance and centering indicators must be accurate to within what of the SID
within 2% and 1% of the SID
t or f
under no circumstances should the xray beam exceed the size of the image receptor
true
collimation is accomplished primarily through the use of
a light localized, variable aperture collimator
other types of collimators include
aperture diaphragms
cones and cylinders
all collimation devices reduce what and improve what
scattered radiation and improves image quality
light localizing variable aperture rectangular collimators have 2 sets of adjustable lead shutters they do what
- reduces the amount of off focus radiaiton
2. consists of 2 pairs that can be adjusted independently
light localizing variable aperture rectangular collimators have
a light source and a mirror
skin sparing :
to minimize skin exposure to electrons produced by photon interaction with the collimator, the patient’s skin surface should be at least
15 cm below the collimator
space bars
some collimators contain space bars to prevent collimators from being closer than 15 cm to the pt.
what is luminance
the brightness and quantifies the intensity of a light source
brightness is measured in
nit or candela per square meter
a reading of 15 foot candles corresponds to a collimator light source with a lumincance of about
161 nit or 161 candela per square meter
the xray beam and the light beam must coincide to within what of the SID
within 2% of the SID
Positive Beam Limitation PBL
ensures that the radiographic beam is no larger than the IR
PBL was required on all xray systems in the US between the years of
1974 and 1994
not required anymore
state regulartoy standards require accuracy of
2% of the SID with PBL
some state may require only 3%
aperture diaphragm
simplest of all beam limiting devices
rectangular shape is most common and can be used in many systems
cones and cylinders
modifications of the aperture diaphragm
restricts the useful beam to the required size
reduce scatter and improve contrast resolution
t or f
sharper size restriction is achieved when the cone or cylinder is longer
true
filtration (also called hardening of the xray beam)
reduces exposure to patient’s skin and superfical tissue by absorbing most of the lower energy photons
increases the quality of the beam
2 types of filtration
- inherent filtration
2. added filtration
examples of inherent filtration
glass envelope
insulating oil
glass window
the inherent filtration material amounts to about what aluminum equivalent
0.5mm aluminum equivalent
the collimator provides an additional inherent filtration of what aluminum equivalent
0.1mm aluminum equivalent
added filtration
sheets of aluminum or the equivalent that are added to the tube
the inherent filtration and added filtration combine to equal
the required amount necessary to filter the useful beam
does the kVp of an xray unit determine the amount of filtration required?
yes
the regulatory standard for total filtration of 2.5 mm Aluminum equivalent is required for fixed xray unit operating above
70 kVp
because each xray tube and collimator has a total inherent filtration of 1.5mm aluminum equivalent, the manufacturer place and additional
1mm Al Eq filter between the tube housing and collimator to meet the minimum regulatory requirement
stationary radiographic equipment operating at 50-70 kVp require a total filtration of
1.5 mm Al Eq
stationary radiographic equipment operating below 50 kVp requires a total filtration of
0.5mm Al Eq
mobile diagnostic units and fluoroscopic equipment requre a minimum filtration of
2.5 mm Al Eq
what are commonly used as filters in mammography
molybdenum and rhodium filters
when the xray target is made of molybdenum each of the following filters may be used
a 0.3mm molybdenum filter
a 0.025 mm rhodium filter
when the xray target is made of rhodium what filter can be used
a 0.025 mm rhodium filter
as filtration is increased, so is what
but what is decreased
the beam quality increases but quantity is decreased
Half value layer HVL
the thickness of a designated absorber necessary to reduce an xray beam to half of its original intensity
for diagnostic beams the HVL is expressed in
millimeters of aluminum
HVL is the best method for
specifying xray quality or effective energy of the xray beam
compensating filters
devices that partially attenuate xrays directed toward a thinner or less dense area while permitting more xrays to strike the thicker or denser area
2 kinds of compensating filters
- wedge
2. trough
exposure reproducibility
consistency in output radiation intensity for identical generator settings from one individual exposure to subsequent exposures
sequential radiation exposures should be reproducible within
+ or - 5%
exposure linearity
the ability of a radiographic unit to produce a constant radiation output for various combos of mA and time
exposure linearity must be within what for adjacent mA stations
10%
intensifying screens are made up of
predominantly rare earth screens
intensifying screens
convert xray energy into visible light to produce radiographic density on the film
significantly reduces exposure time
enhances film exposure process
effect of faster screen film systems on patient dose:
when the speed of screen film systems doubles the radiation exposure is reduced by
50%
as kV increases, effective screen speed
increases
increased kv with less mAs
rare earth intensifying screens absorb about how much more energy than their predecessors
5x
radiographic grids
remove scattered radiation before reaching the IR and improves contrast and visibility of detail
when are grids used
when the pt. body part exceeds 10cm
grids always increase what but improve what
increase patient dose
improves quality of recorded image
an increase in what exposure factor is required with the use of a grid
increased mAs
as grid ratio increases, patient dose
increases
grid ratio
the ratio of the height of the lead strips in the grid to the distance between them
t or f
high ratio grids reduce scatter more effectively than low ratio grids, however, high ratio grids require more radiation exposure increasing patient dose
true
a source to skin distance of at least what must be used in mobile radiography for patient safety
30cm
when the source to skin distance is short, the patient’s entrance exposure is
significantly greater than the exit exposure
decrease in SID increases the
source to skin dose
the sensitivity of the phosphor used in CR is about equal to a what speed screen film combination
200 speed screen film combination
kv controls
radiographic contrast
what are the advantages that DR systems have over both CR and conventional screen film systems
lower dose
ease of use
immediate imaging results
manipulation of the image
what is a disadvantage to DR systems
they don’t allow the user to change the grid or have a preinstalled grid that is not easily accessible to the user
fluoroscopic systems inculde
xray tube
image intensifier
c arm
what are the 3 advantages of image intensification
- increased image brightness
- saving of time for the radiologist
- patient dose reduction
image intensification requires less milliamperage than old fashioned fluoro. how much mA with image intensification is used
1.5-2 mA is used in image intensification
3-5 mA was used before
in fluoro the increase in magnification results in a decrease in
image clarity and the resulting image is dimmer
intermittent of pulsed fluoro significantly decreases
patient dose, especially in long procedures
integral dose
product of dose and volume of tissue irradiated
the source to skin distance for stationary fluoro must not be less than
and for mobile fluoroscopes
38 cm 30 cm (mobile)
fluoroscopic technical factors for children need a decrease in kVp by as much as
25%
a minimum of how much filtration must be permanently installed in the path of the useful beam of the fluoroscopic unit
2.5 mm Al Eq
with image intensification a total of how much filtration should be used
a total of 3 mm Al Eq or greater is preferred
patient dose decreases by how much during fluoro procedures when aluminum filtration increase from 1 to 3 mm of Al
dose decreases by 1/4th
when kVp ranges from 80-100 in standard image intensification fluoroscopy, an xray beam HVL of how much is considered acceptable
3-4.5
cumulative timing device
a resettable device that times the dray beam on time that will sound an audible alarm or temporarily interrupt the exposure after it has been activated for 5 mins
current federal standards limit entrance exposure rates of general purpose intensified fluoroscopic units to a max of
100 mGya per minute
entrance exposure rates for fluoroscopy equipment equipped with high level control may produce a skin entrance exposure rate as high as
200 mGya per minute
high level fluoro is used in what procedures
interventional procedures
a primary protective barrier of how much is required for a fluoroscopic unit
a primary protective barrier of 2mm lead equivalent
t or f
the exposure control switch/foot pedal on fluoro units must be the dead-man type AKA only continuous pressure applied by the operator will keep it activated
true
t or f
image intensifier distance should be as short as possible to reduce patient entrance dose
true
t or f
cine procedures can result in the highest patient dose of all diagnostic procedures
true
in cine procedures, when film rates are high so is
patient dose
when a smaller viewing mode or a lower speed cine film is used, patient exposure
increases
pulse progressive systems
xray beam is turned off while image is being scanned, decreasing dose and pulsed back on for the next image.
High level control fluoro HLCF
an operating mode in which exposure rates are substantially higher than those normally allowed in routine procedures
the higher exposure rate of the HLCF allows
visualization of smaller and lower contrast objects that don’t usually appear during routine fluoro
in 1994 the FDA limited tabletop exposure rate of fluoroscopic equipment to what?
unless HLC mode was present in which case routine fluoro was limited to what when the system was not in HLC mode and what when it was in HLC mode
100 mGya/min
50 mGya/min
unlimited
FDA has recommended a notation be placed in the patient’s record if a skin dose in the range of what is received
the location of the area of the pts. skin should also be recorded
a dose of 1-2 Gyt
since 2000, alarmed state regulatory agencies have imposed a restricition on high level radiation exposure rates with the image intensifier at a distance of 30 cm above the tabletop, the max continous fluoro entrance exposure rate for HLCF is
200 mGya / min
