Test 4 - CH 12, 13 Flashcards
What is the primary result of Compton interactions?
scatter
two major factors that affects the amount of scatter radiation produced and exiting the patient
volume of tissue irradiated
kVp
the volume of tissue depends on what two things?
part thickness
x-ray beam field size
what decreases the x-ray field size and the amount of tissue irradiated?
beam-restricting devices
what is used to improve image quality by absorbing scatter that exits the patient
grids
(T/F) grids reduce scatter in the patient
false
- only reduces the scatter that reaches the IR
beam restriction limits what and reduces what?
limits patient exposure
- reduces amount of scatter radiation produced w/i patient
what changes the shape and size of the primary beam?
beam-restricting devices
increasing collimation does what to the field size?
decreases field size
beam restriction does what to contrast?
increase
for collimation, quantum noise increases so what has to be adjusted?
mAs
what happens to quantum noise when field size increases?
decreases
when going from a 14x17 IR to a 8x10, what should be done to mAs
double
a flat piece of lead (diaphragm) that has a hole (aperture) in it and is placed directly below the x-ray tube
aperture diaphragm
where is an aperture diaphragm placed?
under collimator
what is caused from the aperture diaphragm because of its close proximity to the radiation source
large area of unsharpness on the edges
an aperture diaphragm with an extended flange attached to it
cone or cylinder
cones and cylinders are slid onto the tube where?
directly below the window
cones and cylinders does what to sharpness around the image
limit
which is more useful, cones or cylinders
cylinders
(T/F) cones and cylinders are almost always made to produce a circular projected field
true
what is considered the best beam-restricting device
collimators
where is a collimator located? and consists of what?
below tube window
-lead shutters
what are collimators equipped with?
- white light source
- mirror
- measurement
- crosshairs
what automatically limits the size and shape of the primary beams to the size and shape of the IR
automatic collimators
- positive beam-limiting (PBL)
(T/F) with a PBL you are allowed to go larger than the IR
false
devices that have very thin lead strips with radiolucent interspaces, intended to absorb scatter emitted from the patient
radiographic grids
in grids, lead lines and interspace material is covered by what?
aluminum front and back panel
Grid ration formula
height / width of interspace
grid patterns
linear: lead lines run in 1 direction
cross-hatched: lines run at right angles
lead line orientation:
nonfocused and focused grid
nonfocused: lead lines run parallel
focused: lead lines are angled to match divergence of beam
(T/F) nonfocused grids allow more transmitted photons to reach IR
false
focused
the convergent point/line determines what?
the focal distance of a focused grid
distance between the grid and the convergent line or point
focal distance (grid radius)
what is the focal range
recommended range of SID that can be used with a focused grid
stationary are what type of grids and examples (2)
nonmoving
-cassette : grid attached to IR permanently
-cap: permanently mounted grid and allows the IR to slide behind it
(T/F) when grids are stationary it’s possible to see grid lines on the radiographic image
true
example of moving/reciprocating grids
potter-bucky diaphragm: below table-top and above the tray that holds IR
grid motion is controlled by the what?
x-ray exposure switch
grids move slightly back and forth in a ______ direction over the IR during the ____ exposure
lateral
entire
short-dimension grids run ___ to the long axis, long lead strips run ___ to the long axis
perpendicular
parallel
a decrease in the number of transmitted photons that reach the IR because of some misalignment of the grid
grid cutoff
when does an upside-down focused grid cutoff occur?
when a grid is placed upside down on the IR
-results in grid line going opposite the angle of divergence
off-level grid cutoff occurs when
when the x-ray beam is angled across the lead
off center grid cutoff occurs when
when the CR of the beam is not aligned with the center of a grid
off-focus grid cutoff occurs when
when using an SID outside of the recommended focal range
(T/F) off focus grid cutoff radiographically look different, whether it’s being too close or too far away
false
look the same
what is the Moiré effect, how is it caused?
zebra pattern artifact that can occur when a stationary grid is used during CR imaging, caused by the grid frequency being similar to the laser scanning frequency
grid selection involves consideration of what 3 things?
contrast improvement
patient dose
likelihood of grid cutoff
air gap technique is based on what?
increased OID
(T/F) placing a lead shield on the x-ray table absorbs scatter radiation
true
Does using a lead blocker decrease pt dose? why?
No, only absorbing scatter that already left the pt
what is AEC?
automatic exposure control
-used to control the amount of radiation reaching the IR by terminating the length of the exposure
what factors does the tech set when using AEC?
kVp, mA, IR, grid
AEC is excellent at producing what when used properly?
consistent levels of exposure
How does AEC system know when to terminate the exposure?
uses radiation detectors that takes the radiation transmitted through the patient and converts it into an electrical signal
two types of AEC
phototimers: 1st gen
ionization chambers: more common
How many radiation detectors are there and what is the techs role with them?
3
-tech chooses which ones to activate
Phototimer AEC devices uses what to convert light to electricity?
fluorescent screen and a device
a photomultiplier (PM) tube is a device that converts what?
visible light energy into electrical energy
a solid-state device that performs the same function
photodiode
with exit-type devices, what serves as the detectors?
light paddles coated with fluorescent material
with exit-type devices, how is the timer tripped?
when radiation interacts with the paddles it produces light, which is then converted to electricity
which is used more, exit or entrance type devices? examples of both?
entrance more used
exit = phototimer
entrance = ionization chamber
with entrance-type devices, how is the timer tripped?
when exposed to radiation, the air in the chambers becomes ionized, creating an electrical charge, it travels along the wire to the timer circuit
mAs readout / second readout is what?
the actual amount of mAs used for that image
the kVp level selected must be at least what?
at least the minimum kVp to penetrate the part
using higher kVp does what to exposure time and overall mAs needed?
decreases
decreases
increasing mA on the control panel does what to exposure time?
decreases/shortens
(opposite is true)
(T/F) minimum response time refers to the longest time that the system can produce?
false
shortest time
minimum response time with modern AEC is what?
1 ms
(T/F) minimum response time is usally longer with AEC systems than with other types of radiographic timers
true
what refers to the maximum length of time the x-ray exposure will continue when using an AEC system?
Backup time
(T/F) backup time is a safety mechanism when an AEC fails or the equipment isn’t used properly
true
what allows the radiographer to adjust the amount of preset radiation detection values?
exposure adjustment (density controls)
regularly using _______ settings to produce an acceptable image indicates that a problem exists
plus or minus exposure adjustment
what chambers/detectors do you choose?
ones that will be superimposed by the anatomic structures of greatest interest
failure to use the proper detectors could result in what?
underexposure or overexposure to the IR
(T/F) failure to select the correct bucky can result in the pt and IR being exposed to excessive radiation
true
improper centering may result in what?
underexposure/overexposure to the IR
(T/F) AEC size can be adjusted
false
- fixed and can’t be adjusted
what should you do if the detector combination is larger than the area of interest?
use a manual exposure technique
how does the AEC compensate for patient thickness?
if thicker, the exposure time will lengthen to reach the preset exposure
what may require greater technical consideration when using AEC?
excessive bowel gas
destructive pathologic conditions
postive contrast media
how is collimation a factor in AEC systems
no collimation vs close collimation
- can cause the exposure to be terminated
- could overexpose the pt
what does the AEC do when anatomically programmed techniques are used in conjunction with AEC?
- selects and displays manual expo factors
- selects and displays AEC detectors to be used
what are essential procedures to maintain the proper functioning of the AEC
calibration and QC
how can AEC performance be monitored?
by imaging a homogenous pt equiv. phantom with additional thickness plates
what are preestablished guidelines used to select standardized manual / AEC exposure factors for each type of exam?
exposure technique charts
what 3 things do exposure technique charts do
produce consistent quality images
reduces repeat radiograph studies
reduce patient exposure
for technique charts to be effective, what should be established and what must operate properly?
quality standards
radiographic system
devices that measure part thickness
caliper
where can you find the exact location for measuring part thickness?
technique chart
(T/F) the primary expo. factors of kVp and mAs and Ir type, grid ration are included regardless of the type of technique chart used?
true
two types of technique charts
variable - fixed mAs
fixed kVp - variable mAs
chart that is based on the concept that kVp can be increased as the anatomic part size increases
variable kVp - fixed mAs
what is critical to the efficacy of the variable kVp chart?
accurate measurement of part thickness
(T/F) determination of the baseline kVp for each anatomic area has been standardized
false
not been standardized
chart that uses the concept of selecting an optimal kVp that is required for the exam and adjusting the mAs for variations in part thickness
fixed kVp - variable mAs
what is kVp that is high enough to ensure penetration of the part but not too high to diminish radiographic contrast?
optimal kVp
(T/F) optimal kVp required for each anatomic area has not been standardized
true
once optimal kVp is established fixed kVp - variable mAs charts alter what?
the mAs for variations in part thickness
(T/F) using higher kVp ranges with DR systems can reduce the variability among exposure techniques for the same / similar regions
true
primary tools needed to develop a exposure technique chart
caliper
phantoms
calculator
this concept states that different parts of the same size can be imaged by use of the same expo factors, provided that the kVp needed to penetrate the part is used
comparative anatomy
(T/F) pediatric patients require high kVp and low mAs compared to adults
false
low kVp
lower bone density may require what in exposure?
reduction
(T/F) geriatric patients may need an increase in exposure techniques
false
(T/F) geriatric patient may need an increase in kVp and mAs, grids, and imaged in quadrants
true
(T/F) diff projections and positions of the same part don’t often require modification of exposure factors
false
do require modification
fiberglass and plaster casts require what?
fiberglass = no change
plaster = increase
inflatable, fiberglass, wood, aluminum, and solid plastic splints require what?
inflatable and fiberglass = no change
everything else may be increased if it’s in the path of the primary beam
diseases or conditions
- that increases the absorption of the part
- that decreases the absorption of the part
additive
destructive
contrast media is used when anatomic tissues have what subject contrast?
low
contrast agent that
- produces more brightness
- produces less brightness
positive
negative
When going from a 11x14 field size to a small 4in cone, what must be increased and why?
Increase exposure
Need to compensate for the decrease in the number of X-ray photons that otherwise occur
Simplest type of beam restricting device
Aperture diaphragm
Can you adjust the field size with an aperture diaphragm?
No it’s a set size
Which beam restricting device has the least amount of area of unsharpness
Cones and cylinders
Grids are usually used when part thickness is what and kVp is higher than what?
Party thickness is greater than 10cm
KVp greater than 60
Increasing grid ratio for the same grid frequency will increase what two things
Increase the amount of lead content
Increase scatter absorption
If you increase grid frequency and use the same grid ratio, there is less what because why?
Less lead content
- width of the interspace or thickness of the lead have been decreased
Decreasing the overall lead content will result in what?
Decreased scatter absorption
Which grid pattern absorbs more scatter and why
Crossed grids
- contains more lead strips that are in two directions
Can you angle the X-ray tube when using a crossed grid
No
Grid cutoff
Which type of grid (focused/non focused) allows more transmitted photons?
Focused
What grid matches the size of the cassette and is used by placing it on top of the IR
Wafer grid
How do the grid lines get blurred when using a stationary grid
Its slightly moved laterally (back and forth) during the entire X-ray exposure
GCF formula
mAs with grid / mAs without grid
What grid cutoff occurs with both focused and parallel grids
Off level
- X-ray beam is angled
What can help eliminate the moire effect from happening
Higher grid frequency or moving grid
What describes the number of lead lines per unit length
Grid frequency
(T/F) difference in AEC systems lies in the type of device used to convert radiation into electricity
True
When the detectors are positioned behind the IR vs in front of the IR
Exit type device
Entrance type device
Setting the backup time _________ of the expected exposure time is appropriate
150-200%
Is the detector able to distinguish transmitted radiation from scattered?
No
(T/F) IRs cannot be interchanged easily since the AEC is calibrated to terminate exposure levels at a preset level
True
What ensures the consistent and appropriate exposures to the IR are produced
Calibration
An X-ray generator should always have the same level of radiation exposure no matter what technical factors are set
Max variable variation
Reproducibility of the exposure
+- 5%
Variations between the state kVp and the X-ray beam quality must be within
+- 5%
QC checks used to determine kVp accuracy
Wisconsin test tool: holes in the IR
Digital kVp meter
Exposure time directly affects the total quantity of radiation emitted from tube
Timer accuracy QC
+- 5% for over 10ms
+- 20% for less 10ms
The same mAs using different mA and time combinations should produce the same radiation output
Reciprocity law
+- 10%
Sequential increases in mAs should produce the same sequential increases in exposure
Linearity
+- 10%
Focal spot size variation
Percent of blooming allowable for:
0.8 mm or less, 0.8mm to 1.5mm, 1.6mm or greater
50%
40%
30%
How well the collimator regulates the field size and area that is illuminated on the IR
Light field radiation fells congruence QC
+- 2%
Center of the bucky tray must be centered to the X-ray beam
X-ray beam bucky alignment QC
Within 1% of the SID
All medical X-ray units must be equipped with a SID indicator
SID indicator QC
+- 2%
