Imaging II Test 2
What are the factors to consider in using technique charts?
(BADGFS) 1. CR vs. DR 2. Grid vs. Non-Grid 3. SID 4. AEC factors 5. Patient habitus 6. Focal spot
What is the tolerance for collimator light field accuracy?
+/- 1% of the SID employed
What is the tolerance for collimator field size accuracy?
+/- 2% of the SID employed
What is the tolerance for CR alignment accuracy?
+/- 2° of perfect vertical alignment
How are fixed mAs and Variable kVp charts created?
1. A standard technique is determined by doubling the patient thickness in cm and adding 40 2. Two kVp are then added or subtracted for each change in patient thickness
What are the types of beam restriction devices?
1. Collimators 2. Positive Beam Limitation (PBL) 3. Extension Cylinders
Why should collimation be done?
1. Decrease direct tissue dose 2. Decrease scatter from reaching the IR Effect on receptor exposure Effect on spatial resolution and distortion 3. Reduce off-focus radiation
What are the factors to consider whether a single exposure is safe or unsafe?
1. Focal Spot Size 2. Anode Rotation Speed 3. Type of Rectification (type of generator)
Limiting factors for Types of Exposures in regards to heat dissipation?
1. For exposures <1 (s), the target is the limiting factor 2. For exposures >1 (s), the anode is the limiting factor 3. The tube housing is the limiting factor for fluoroscopy
Anode cooling chart is used to determine what?
1. How much total heat is produced during an exposure 2. How long you must wait for the tube to cool
What is a collimator field light composed of?
1. LED bulb 2. Mirror
What are the types of Quality Assurances tests for collimator accuracy?
1. Light Field Accuracy 2. Lead Shutter Accuracy 3. Central Ray Alignment Accuracy
Types of Grids
1. Linear a. Parallel b. Focused 2. Crosshatch
Housing Cooling Chart
1. Look and are used the same as Anode Cooling Charts 2. HU are in millions 3. Fans are sometimes employed to decrease cooling time
What are the attributes of fixed kVp, Variable mAs chart?
1. Most widely employed 2. There is an optimum kVp for all exams 3. Provides consistent penetration
How do you test for collimator light field?
1. Place a template on the IR and set a 24" SID 2. Center to the crosshairs on the template and collimate to the dimensions of the rectangle 3. Expose and process 4. Each dot represents a 1% collimator discrepancy
How is the test conducted for CR alignment?
1. Place the plexiglass cylinder in the middle of the template and set a 24" SID 2. Center to the crosshairs on the template 3. Expose and process 4. Each circle on the IR represents a 1° discrepancy 5. The lead balls must fall within the boundaries of the second circle for proper accuracy
What are the types of Rating Charts?
1. Radiographic Rating Chart 2. Anode Cooling Chart 3. Fluoroscopy Anode Cooling Chart 4. Housing Cooling Chart
How is the test conducted for collimator field size?
1. The collimation it set using the collimator knobs to a predetermined size 2. Radiographic markers or pennies are placed at the edges of the light field 3. Process the IR and determine if the collimation is accurate by measuring the amount of error.
Why is it important to use technique charts?
1. To standardize exposure variables 2. Reduce repeats 3. Decrease patient exposure
Grids with fewer but thicker lead strips will result in what?
1. more primary beam absorption - because of more lead content 2. less visibility of detail - the grid is in the way when creating the image. 3. higher patient dose - because higher x-ray production is needed to go through the grid
Grid frequency of digital image receptors are more sensitive and require what range?
103 to 200 lines/inch (140 on average)
Example of a unsafe exposure per the radiographic rating chart
200 mA 100 kVp 3.0 seconds
Example of 15% rule with radiographic rating chart
200 mA, 100 kVp, 1.0 (s): Safe 400 mA, 85 kVp, 1.0 (s): Unsafe!
Example of Reciprocity law with radiographic rating chart
200 mA, 80 kVp, 2.0 (s): Safe 400 mA, 80 kVp, 1.0 (s): Unsafe!
How much do you increase mAs to compensate when using extension cylinders?
30% to compensate for loss of beam intensity
Example of a safe exposure per the radiographic rating chart
350 mA 80 kVp 0.2 seconds
What amount of OID should be employed using the Air-Gap Technique?
5" to 10" OID
Which will remove more scatter? 6:1 crossed grid or 12:1 linear grid?
6:1 crossed grid
Air-Gap Technique is as effective as using what ratio grid?
8:1 GR
How is off-level grid cutoff caused?
Can be caused by a tilted grid or an improper tube angle
Field Size vs. Scatter Production
Changes in field size are more important when the x-ray field size is small then when it is large.
What is the radiographic rating chart used for?
Determines whether a single exposure is safe or unsafe
Anode cooling chart example
Don't forget to switch the numbers on the bottom!
What is the other technique chart besides fixed mAs, variable kVp chart?
Fixed kVp, Variable mAs chart
What was the first technique chart created?
Fixed mAs, variable kVp Chart
Grid frequency varies between how many lead strips per inch?
GF varies between 50 and 200 lines per inch
Grid Construction
Grids consist of alternating lead strips and interspace material.
What units are used to measure anode heat?
Heat Units (HU) - not BTU
In what way do higher GRs affect accuracy and patient dose?
Higher GRs require more accuracy in their use and result in a higher patient dose
How is off-center grid cutoff caused?
If the tube is not centered directly over the grid.
What is the application for a crosshatch grid?
Interventional Radiography
What affect will the 15% rule have on HU production in the Anode cooling chart?
It does affect the Anode cooling chart examples: 400 mA (x) 100 kVp (x) 0.2 (s) = 8,000 HU 400 mA (x) 115 kVp (x) 0.1 (s) = 4,600 HU 400 mA (x) 85 kVp (x) 0.4 (s) = 13,600 HU
What is the most important factor to consider in grids that is often overlooked?
Lead content
What serves as an indicator of grid performance?
Lead content
Focused linear grid definition
Lead strips are canted to match the divergent CR with the center strips being perpendicular to the IR.
Parallel Linear Grids definition
Lead strips are perpendicular to the IR and parallel to each other
What are the most common type of grids?
Linear grids
What kind of receptor exposure loss is experienced in off-center grid cutoff?
Loss of receptor exposure across the entire image (i.e. even loss of receptor exposure across the IR)
What kind of receptor exposure loss is experienced in off-level grid cutoff?
Loss of receptor exposure across the entire image (i.e. even loss of receptor exposure across the IR)
Fluoroscopy Anode Cooling Chart
Measured in HU/sec. Example: 5 mA and 85 kVp = 425 HU/sec.
Where is the mirror placed in the field light?
Mirror is placed in primary beam
With a greater angle of incident scatter is the radiation more or less likely to be absorbed?
More likely to be absorbed
OID has what type of effect on shape distortion?
None
What affect will the Reciprocity Law have on HU production in the Anode cooling chart?
None examples: 200 mA (x) 75 kVp (x) 0.5 (s) = 7,500 HU 100 mA (x) 75 kVp (x) 1.0 (s) = 7,500 HU
Off-focus Grid Cutoff (aka Off-distance Grid Cutoff) occurs when?
Occurs when the focal range has been breached
Do grids raise or lower patient dose?
Raise
What is the purpose of a crosshatch grid?
Removes scatter in both directions
How does a Positive Beam Limitation (PBL) device work?
Sensors measure IR size and adjust the lead shutters accordingly with an optional override for the shutters.
What is a collimator?
Series of Pb shutters at different levels
How is receptor exposure affected with Upside-down Grid Cutoff?
Severe loss of receptor exposure on the lateral edges of the image
Heat Unit Calculations
Single Phase: mA (x) Time (x) kVp Three Phase 6-Pulse: mA (x) Time (x) kVp (x) 1.35 Three Phase 12-Pulse: mA (x) Time (x) kVp (x) 1.41 High Frequency: mA (x) Time (x) kVp (x) 1.45
Flow of heat dissipation in the tube
Target...Anode...Dielectric Oil...Tube Housing...Air
Comparing two grids of the same GR but different frequency, which would provide the best visibility of detail?
The grid with the higher grid frequency because the lead strips are thinner and won't show up on the image.
Comparing two grids of the same GR but different frequency, which would provide the highest dose?
The grid with the lower grid frequency because the thicker lead strips require more x-ray production to get through the large amount of lead.
Comparing two grids of the same GR but different frequency, which would provide the best efficiency?
The grid with the lower grid frequency because the thicker lead strips will clean up more scatter.
Grid Ratio definition
The height of the lead strips divided by the distance between them. GR = h/D
Grid Frequency definition
The number of lead strips per inch
What is focal range or grid radius?
The range of SIDs that works best with the grid.
What can not be done with the tube with a crosshatch grid?
The tube can not be angled.
Why do parallel linear grids work best with a long SID?
There will be less grid cut off due to the focal range being infinity.
How is GR and the thickness of the lead strips related?
They are not related and are independent of each other.
Extension cylinders add how many levels of apertures?
Two additional
How is a crosshatch (crossed) grids constructed
Two linear grids placed on top of one another and rotated 90 degrees
Upside-down Grid Cutoff occurs when?
When the grid is turned upside down but rarely occurs due to modern grid cap construction
When you use fixed mAs, variable kVp chart what happens to optimum kVp?
You lose the optimum kVp
What is the Air-Gap Technique?
a method used to reduce scatter fog.
Off-focus Grid Cutoff has loss of receptor exposure where on the image?
along the lateral edges of the image
What is another name for a Positive Beam Limitation (PBL) device?
automatic collimator
What effect does collimation have on receptor exposure?
collimation will decrease receptor exposure by reducing the scatter coming back and hitting the IR.
Extension cylinders create what kind of edges on the image?
crisp edges of collimation on image
What happens when using fixed mAs, variable kVp chart during iodine exams?
if the kVp raises too high then the x-ray will penetrate through the iodine which has a low Z#.
What effect does collimation have on spatial resolution and distortion?
it does not affect them.
What is a caveat for using the Air-Gap Technique?
loss of spatial resolution
What technical factors are plotted on the radiographic rating chart?
mA time kVp
Collimator field size quality control is used to ascertain the alignment of what?
measure of the lead shutter alignment
Collimator light field quality control is used to ascertain the alignment of what?
proper mirror alignment
All other grid construction factors constant, with higher GR what happens to scatter?
the greater the scatter clean-up.
For grids with equal GRs, which will be more efficient at removing scatter by virtue of a higher lead content
the one with fewer lead strips because the strips are thicker (have more lead)
Should the tube be cool or warmed up when using radiographic rating chart?
warmed up
