chapter 11 Flashcards
radiation generated in the x-ray tube that does not exit form the collimator opening but rather penetrates the protective tube house and to some degree the sides of the collimator
leakage radiation
radiation that emerges directly from the x-ray tube collimator and moves without deflection toward a wall, door, viewing window and so on. also called direct radiation or useful beam
primary beam
Tube housing is constructed so that leakage radiation measured at a distance of 1m from the x-ray source does no exceed _____ when the tube is operated at its highest voltage at the highest current that allows continuous
1mGya/hr (100mR/hr)
2 other functions of the tube housing
- confines the high voltage entering the tube to prevent electric shock
- makes cooling of the x-ray tube possible
what is the radiographic table made of
carbon fiber material
For the SID the distance and centering indicators must be accurate within what percent of the SID respectively
2% and 1%
collimation is accomplishes primarily through the use of what?
light localized, variable aperture collimator
what are other types of collimators
aperature diaphragms and cones and cylinders
what does the 2 sets of collimators do?
1st set- reduces the amount of off focus radiation
2nd set- consists of 2 pairs that can be adjusted independently
to minimize skin exposure to electrons produced by photon interaction with the collimator, the patients skin surface should be how far below the collimator
15cm
what do some collimators have to prevent collimators from being closer than 15 cm to the patient
spacer bars
what is luminance
refers to the brightness and quantifies the intensity of the light source
what is luminance measured in
nit or candela per square meter
the x-ray beam and the light beam must coincide within ___% of the SID
2%
consists of electronic sensors that sends signals to collimator housing. when activated collimators are automatically adjusted to radiation field matches the size of the IR
Positive beam limitation (PBL)
State regulatory standards require accuracy of __% of the SID with PBL.
2%, some states may require an accuracy of only 35
simplest of all beam limits devices, consists of a flat piece of lead with a hole of designated size and shape that is attached to the x-ray tube head
aperture diaphragm
what is the purpose of filtration
reduces exposure to patients skin and superficial tissue by absorbing most of the lower energy photons (long wave) from the heterogeneous beam, it increases the mean energy, or quality of the x-ray beam, hardening of the beam
the absorbed dose of the patient ____ when the correct amount of types of filtration are placed in the path of the radiographic beam
decrease, it absorbs some of the photons and decreased the quantity of incident radiation. the remaining photons in the beam as a whole, are more likely to be absorbed in body tissue
Two types of filtration
inherent filtration and added filtration
what are the parts of the inherent filtration
glass envelope, insulating oil, and glass window
what glass window inherent material amounts to ___ of aluminum equivalent
0.5mm Al
What can they use to added filtration
sheets of aluminum (or equivalent) of appropriate thickness.
the regulatory standard for total filtration of ____ is required for fixed units operating above 70kVp
2.5mm Al Eq
what determines the amount of filtration required
kVp
stationary fixed radiographic equipment operating at 50-70kVp require a total filtration of?
1.5mm Al Eq
stationary fixed radiographic equipment operating below 50 kVp requires a total filtration of?
0.5mm Al Eq
mobile diagnostic units and fluoro equipment require a minimum of?
2.5mm Al Eq
what are the commonly used filters in mammo
molybdenum and rhodium
when the x-ray target is made of molybdenum each following filters may be used?
0.3 mm molybdenum filter and 0.025mm rhodium filter
when the x-ray target is made of rhodium what filter may be used
0.025mm rhodium filter
what is the most widely selected material as a filter
Aluminum (Z=13)
what is efficient in removing low energy x-ray through the photoelectric effect, it is readily available, inexpensive and easily shaped
aluminum
As filtration is increased so is beam quality and quantity is ___
decreased
what must be measured to verify adequate filtration
half value layer (HVL)
the thickness of a designated absorbed necessary to reduce an x-ray beam to half its original value
half value layer
when and who checks the HVL
physicist at least once and year or when x-ray tube has been replaces or repairs have been made on housing collimating system
what is the best method for specifying x-ray quality or effective energy of the x-ray beam
half value layer
HVL is measure in what
mm of aluminum
devices that partially attenuate x-rays directed toward a thinner or less dense, area while permitting more x-rays to strike the thicker, or denser area
compensating filters
compensating filter are made out of
aluminum, lead acrylic or other suitable materials
do compensating filters increase or decrease dose?
decrease by reducing repeat exposure
as consistency in output radiation intensity for identical generator settings from one individual exposure to subsequent exposures
reproducibility
sequential radiation exposures should be within ___%?
+/- 5 %
included in the code of diagnostic x-ray equipment, it is the ability of radiographic unit to produce a constant radiation output for various combinations of mA and exposure time
exposure linearity
exposure linearity must be within ___% for adjacent mA
10%
what do intensifying screens do
convert x-ray energy into visible light to produce radiographic density on the film
what film speed are standard for general radiography
400 speed systems
film screen combinations range from
200-1200
as kV increased, effective screen speed____.
increases
why do they use carbon fiber as a material for front of cassette
less dose, carbon fiber absorbs about half as much radiation as previously used cassette materials
as grid ratio increased, patient dose___
increases
the ratio of the height of lead strips in the grid to the distance between them. High ratio grids reduce scatter more effectively than do low ratio grids. however, high ratio grids require more radiation exposure
grid ratio
A source skin distance (SSD) of at least ___ must be used in mobile radiography for patient safety
30cm (12 inches)
for CR And DR it is still customary to used a grid for anatomy measuring over___ for techniques that exceed 70 kVp
10 cm
Dr systems have the following advantages over both CR and conventional SFS..
lower dose
ease of use
immediate imaging results
manipulation of the image
benefits of image intensification fluoro include
- increased image brightness
- saving of time of the radiologist
- patient dose reduction
product of dose and volume of tissue irradiated
integral dose
the source to skin distance must not be less than ___ for stationary fixes fluoroscopes
38cm (15 in)
SSD for mobile fluoroscopes?
12 inches
fluoroscopic technical factors for children necessitate a decrease in kVp by as much as __%
25%
patient dose decreases by how much during fluoro procedures when aluminum filtration increased from 1-3mm of aluminum
one fourth
in standard image intensification fluoroscopy, an x-ray beam HVL of what is considered acceptable when kVp ranges from 80-100
3-4.5mm aluminum
current federal standards limit entrance exposure rates of general purpose intensified fluoroscopic units to a maximum ___ mGya per min
100mGy or 10R/min
entrance exposure rates for fluoroscopy equipment equipped with high level control may produce a skin entrance exposure rates as high as ____ per minute
200 mGya per min (20R/min)
a primary protective barrier of __mm of lead equivalent is required for fluoroscopic units
2mm
a spacer or collimator is usually installed to prevent any part of the patient from coming closer than __ to the tube target
30cm
Cine or movie camera uses how big of film to recoded the image from the output phosphor
16-35mm
what is the most common cine size used and why
35mm and because image quality is better
which procedure can have that highest patient dose ?
cine procedures
what are the cine cameras have film frame rates.
7.5, 15, 30, 60
switching from 23cm to a 15cm FOV and 30-60 frames per second can do what to the tabletop exposure rate
approx. doubles the tabletop exposure rate, if both are doubles the exposure rate goes up by factor of 4
Factors that play a role in determining dose levels
- image intensification input phosphor exposure level set by vendor
- grid factor
- SSD
Collimating (increases or decreases) the integral dose while (increases or decreases) image quality
decreases, increases
In 1994, the FDA limited the tabletop exposure rate of fluoroscopic equipment to ___ mGy/min unless HLC mode was present, in which case routine fluoro was limited to ___ mGy.min when the system was not in HLC mode and unlimited when it was in HLC mode
100mGy
50mGy
FDA has recommended a notation be placed in patients record if a skin dose of ___Gy is received
1-2 Gy
