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Exposure Unit 1

Grids & AEC

What are three ways to control scatter? Collimate, use appropriate tech factors, use a grid
T/F: the primary purpose of a grid is to improve image contrast True
T/F: Grids improve contrast by reducing scatter production False; grids prevent xrays to strike the IR, not scatter production.
T/F: The use of grids increases patient radiation. True
T/F: Grid ratio is calculated by dividing the height of the lead strip by the thickness of the lead strip False: Grid ratio= height of Pb strip/thickness of the interspace (between Pb strip)
T/F: grids improve recorded detail False: grids have no effect on sharpness
T/F: the most common type of grid is focused linear True
T/F: The central ray can never be angled with a cross grid True
T/F: An unfocused grid works best with a short SID False; unfocused grids work best with long SIDs because there is less beam divergence.
T/F: The interspace material should be radiolucent true
T/F: the number of grid lines per inch determines the grid radius False; this is the definition of grid frequency. Grid radius is the distance between the grid and the convergent point (aka focal distance).
For EXPOSURES ONLY: as scatter radiation increases what happens to film density, contrast, and detail visibility? film density increases, contrast and detail visibility decrease.
Describe the construction of a grid. a series of radiopaque material (grid lines) alternating with radiolucent material (interspace)
What is the purpose of a grid? improve radiographic contrast.
How does a grid achieve its purpose? absorbs the scatter that exits the patient. A grid is placed between the anatomical part and the IR so that it limits the amount of exit photons reaching the IR in an exposure. This improves image quality.
When are grids used? Thicker body parts measuring more than 10cm and exams that require more than 60KVP used. thicker parts means more scatter is produced due to fluid and muscle. Exit scatter increases as well.
What properties should a grid's interspace material have? aluminum/plastic. radiolucent; maintain separation from Pb strips and not able to absorb moisture
What is the purpose of a reciprocating grid? to blur the grid lines while moving
What is another name for the moving grid? Potter-Bucky diaphragm.
What kind of exams use stationary grids? mobile, stretcher and cross table exams
What happens to contrast, noise and image quality when the grid ratio increases? contrast increases, noise decreases, and image quality increases
Explain how grid ratios affect patient dose? If the grid ratio is increased, that means it is more effecient at absorbing the photons which means it needs more mAs when used. Using more mAs increases patient dose.
Explain the difference between focal distance and focal range. focal distance is the distance between the grid and the convergence point. Focal range is the recommended SID to be used within the focused grid. The further away from the focal distance, the more grid cutoff.
Name the 2 types of linear grid line patterns. And what kinds of exams they are used for. Long dimension: grid lines run LW, used in table and wall buckys Short dimension: grid lines run CW, used in portable CXR and decubs
Why are the cross hatch grids not very commonly used? because you cannot angle the tube or it will produce grid cutoff. Angling the tube makes it angling across the grid lines.
What type of grid ratios are used for portables? According to Ms. Brake. low grid ratios (5:1/6:1)
a)70 kVp, 100 mAs, 40 inch SID, 8:1 grid b)70 kVp, 100 mAs, 40 inch SID, 12:1 grid c)70 kVp, 125 mAs, 40 inch SID, 8:1 grid d)70 kVp, 125 mAs, 40 inch SID, 12:1 grid Which one will produce the greatest density? c) will produce the greatest film density. look at the highest mAs but the lowest grid ratio
a)80 kVp, 125 mAs, 40 inch SID, 16:1 grid b)80 kVp, 100 mAs, 40 inch SID, 12:1 grid c)80 kVp, 75 mAs, 40 inch SID, 8:1 grid d)80 kVp, 50 mAs, 40 inch SID, 5:1 grid Which will produce the longest scale of contrast? d) because it has the lowest grid ratio and is the least efficient.
what are the two types of grids? focused and unfocused
what do the grid lines look like in focused grids? strips are canted (slanted) match the CR beam divergence; the SID used must = grid focus/radius; most common type
what do the grid lines look like in an unfocused grid? strips are parallel and don't match the beam divergence; best used at long SIDs where there is less beam divergence.
What is grid ratio? the ratio of height of Pb strips to the distance between them.
T/F: higher grid ratios are more efficient at absorbing scatter true
what is the grid ratio for a grid that is 6mm thick with interspace material that is .5mm wide? 12:1
What happens to IR exposure, density, and patient dose as grid ratio increases with digital? IR exposure will decrease, density will not be effected (digital), image will be mottled due to not enough photons getting to IR, and patient dose will decrease.
How is the grid conversion factor calculated? GCF= mAs with grid/mAs without a grid grid conversion factor = bucky factor
what is the bucky factor for a 8:1 grid? 4
what is the bucky factor for a 5:1 grid? 2
what is the bucky factor for a 12:1 grid? 5
what is the bucky factor for a 16:1 grid? 6
what is the bucky factor for no grid? 1
An AP CXR on an obese patient are 85kV @ 8 mAs with no grid, the tech decides to use a 6:1 grid to improve contrast. what will the new tech factors have to be set for to maintain IR exposure and density? 85kV @ 24 mAs
AP knee is taken at 70 kV @ 20 mAs using a 12:1 bucky grid. A trauma patient comes in and needs an AP knee but remains in the stretcher w/o grid; what will the tech factors be when not using a grid? 12:1 = 5 GCF; divide 20 mAs by 5 since no grid will be used. new mAs = 4. Tech factors = 70kV @ 4 mAs.
A portable Abd. uses 8:1 grid using 25 mAs. the tech repeats it uses a 16:1 grid instead at 85 kV. how much mAs will need to be used on the repeat? mAs1/mAs2 = GCF1/GCF2 25mAs/ x = 4/6 x=37.5 mAs needed for repeat at 85kV
AP CXR uses 75kV and 5 mAs using no grid. Patient is obese and the tech uses a grid but forgot to change the technique. what happens to film density and IR exposure? 6:1 = GCF of 3. the film density and IR exposure are reduced by 1/3. the mAs needs to be at least 3x increased. with no change in technical factors the IR exposure decreases because not enough photons are going to IR, they are getting absorbed instead.
What are two ways grid efficiency can be measured? grid selectivity & contrast improvement factor
What are 3 factors that affect grid efficiency? grid ratio, grid frequency, and Pb/lead content
what is grid frequency? number of grid strips per in or cm
What is the most important factor that affects grid frequency? lead content
T/F: focal range affects the grid efficiency false; has no effect
How is lead content measured? g/cm^2
As lead content increases what happens to scatter removal and contrast? scatter removal increases and contrast increase too
As lead content increases what happens to grid ratio, and grid frequency? grid ratio increases because more lead = more efficient, grid frequency decreases because more lead content = less strips per in/cm.
When two grids have the same ratio, would you pick the grid with low or high frequency? low frequency= more lead content= more efficient
what is grid cutoff? occurs when the CR and grid are not properly aligned
What does grid cutoff do to image quality? loss of detail visibility, density and contrast
T/F: High ratio grids have more positioning latitude than low ratio grids. False; low ratio grids have more positioning latitude than higher ratio grids. That's why low ratio grids are used on portable CXR.
What is off-level grid cutoff? angling the tube across grid lines or when the grid is tilted underneath the patient.
What is off-center grid cutoff? when the CR is directed to a position on the grid other than the center/ centering to a point to the left or right of the convergence point (lateral decentering).
what is off-focus grid cutoff? when the SID the xray is taken at is not within the focal range of the grid. the further away the tube is from the focus of the grid, the more amount grid cutoff will be noticeable.
What is upside-down grid cutoff? the tube side must always face the tube.
Which grid cutoff only occurs with focused grids only? upside-down & off-center
what cutoff displays a narrow strip of exposure in the middle of the radiograph? upside-down cutoff
Which cutoffs display a loss of density over the entire image? off-level and off-center
What is the type of grid cutoff to occur with CR receptors/short dimension grids? It happens when the laser scans the storage phosphor is parallel with the grid strips. Moire pattern
What is another name for Moire pattern? Aliasing artifact
What can be used to prevent the moire pattern from happening? use high frequency grids greater than the sampling frequency
If a grid is not in use, what is an alternative? Air gap technique; uses a large OID to minimize scatter radiation to image receptor
If OID is increased, what happens with magnification and blur? increases both magnification and blur
If SID is increased, what happens to magnification and image detail? decreases magnification and improves sharpness
What grid ratio is required when doing abdomen, pelvis, and L-spine exams? 8:1 or higher
what grid ratio is used when doing portable CXR? 5:1 or 6:1
As a general rule of thumb, what happens to grid efficiency when grid ratio increases? As grid ratio increases, grid efficiency increases as well. Because the higher the grid ratio, more lead strips are present and will absorb more scatter to improve contrast
As a general rule of thumb, what happens to lead strip thickness and lead content when grid frequency increases? As grid frequency increases, the lead content decreases and the strips get thinner. Although they are thinner there are more lead strips per in/cm.
What is positioning latitude? how forgiving the grid is to error when positioning it under the anatomic structure. High ratio grids have less positioning latitude than low ratio grids.
what is lateral decentering? only occurs with focused grids. same thing as off-center cutoff. Occurs when there is not equal amount of space on each side of the grid and the CR is misaligned with the center of the grid (too far left or right from the convergence line)
What type of grid ratio is typically used for portable work? low ratio grid; they require less mAs & have more positioning latitude.
What does AEC stand for? Automatic Exposure Control
What does AEC measure? exit radiation and automatically terminates the exposure when the needed IR exposure is reached.
T/F: With AEC all films should have the same density. True
What are the two types of AEC systems? Phototimers, Ionization Chambers
What is the difference between phototimers and ionization chambers? Phototimers: exit-type devices because they use light paddles as detectors and sit behind the IR to measure the radiation after it strikes the IR. Ionization chambers: entrance-type devices, in front of IR to measure radiation before striking the IR.
How do ionization chambers work? Ionization chambers are filled with air. When radiation hits the chambers it ionizes the air which creates an electrical charge and trips a switch to terminate the exposure.
Can AEC be used for tabletop or stretcher exposures? No, because the AEC system is built into the wall or table bucky. If the tech used AEC on these procedures the patient would be overexposed. Maybe use AEC with a lateral CXR in a stretcher against wall bucky though.
What does APR stand for? Anatomic Programmed Radiography
What does it mean when you select the APR? it selects the anatomy of interest to xray on the control panel and it sets programmed exposure techniques. APR can have AEC or manual setting.
What are concerns with using AEC with digital? Digital IRs don't show if the image was under or overexposed by looking at the image. Techs must then pay attention to the sensitivity numbers. AEC systems must be recalibrated when switching from analog to digital.
T/F: When using AEC, selecting a higher kVp will shorten the exposure time without affecting film density. True
T/F: When using AEC, kVp and mA selection do not affect film density. True; the calibration will effect density
T/F: If the backup timer is set too low, and underexposed image may be produced True
T/F: If the needed exposure time is shorter than the MRT, the image produced will be underexposed. False; overexposed. MRT= minimum response time the exposure time can never be shorter than the MRT, if you need something shorter you will only get MRT and then image will be overexposed.
T/F: Increasing the AEC density setting to +2 will increase the density of a digital image. False; for digital it will be the exposure indicator.
T/F: Using the center detector for a PA or AP chest will produce an overexposed image. True
T/F: Proper positioning and detector selection are very critical for correct exposures when using AEC. True
T/F: a photomultiplier tube or diode converts xrays into electrical charge. False; converts light into electrical charge.
T/F: Grid efficiency is increased when the grid frequency is increased. False: Grid frequency = more lead strips per in/cm = thinner strips = less lead content making the grid efficiency decrease
How can collimation affect AEC? too much collimation can fool the AEC into thinking the tissue is denser resulting in more exposure than is really needed. Too little collimation causes more scatter to be produced shutting the AEC off too early causing underexposure.
Why is it important to properly position the patient when using AEC? it causes under or overexposure. Ex. If an AP/PA chest is done using the center detector the spine is going to be underexposed
If a digital image is under or overexposed due to improper positioning to the detector what does it look like? underexposed = mottle; overexposed will look fine (automatic rescaling) but the sensitivity number will not be appropriate
What is the backup timer? required with AEC; terminates the exposure when the AEC does not due to malfunction or tech error. prevents overexposure to patient and tube.
What might cause the backup timer to stop the exposure? selecting wrong bucky, using AEC for non-bucky studies, orthopedic implants
Why should AEC not be used when there is hardware present? Will cause overexposure because the metal implant will be blocking the detector and AEC will not shut off because the detector is not able to measure the radiation; surrounding tissue will be overexposed.
What is the rule of thumb for a backup timer setting? 2x more than the expected manual time or 150-200% of the anticipated manual technique
What happens when the backup timer is less than the needed exposure? the image will be underexposed.
T/F: the exposure time needed for proper IR exposure must be greater than or equal to the MRT. True
For an AEC unit that are 3 button selection, what can the tech select? kv, mA & s
What are some conditions when the MRT could affect proper exposure? pediatrics and geriatrics because of the decreased in part thickness and bone density. Also for pediatrics sometimes the exposure time needs to be less than the MRT. Higher mA and kV and higher RS#s are used.
Give situations that can cause the backup timer to terminate exposure. choose incorrect bucky, AEC is used for tabletop exams, hardware is present, not enough kV is set or an AEC malfunction or excessive collimation.
What is the major cause of exposure error when using AEC? the incorrect chamber/detector is selected on the control panel or the patient is not properly centered over the correct chamber.
what is true about AEC during digital imaging? a)adjusting the mA value affects image brightness b)adjusting the kVp affects brightness c)adjusting the backup timer affects image brightness d)adjust density controls affects exposure to IR. D
which statement is true? a)ionization chambers are called phototimers b)ionization chambers measure radiation before it interacts with the IR c)phototimers are more modern d)phototimers measure radiation before it interacts with IR. B
Which is true; the purpose of a backup timer is to: a)ensure a diagnostic exposure each time AEC is used b)produce consistent levels of exposure on all exams c)determine the exposure time that is used d)limit unnecessary xray exposure D
what happens if AEC is activated for a stretcher study? a) inappropriate short exposure occurs b)inappropriate long exposure occurs c)appropriate exposure probably occurs d)underexposure of image occurs B
The purpose of APR is to: a)present the tech with a preselected set of exposure factors b)override AEC when the tech has made a mistake c)determine which AEC detectors should be used for a certain exam d)prevent over & underexposure of exams A
When using AEC with digital, assuming all other factors are correct selecting the center chamber on a PA chest results in: a)decreased exp. in lungs b)increased exp. in lungs c)appropriate exp. in lungs d)increased mottle in image B
When using AEC with digital, assuming all other factors are correct selecting the -2 on a PA CXR results in: a)increased mottle noise b)increased brightness in lungs c)appropriate brightness in lungs d) A & C D
Using a film screen system and AEC, a CXR in lateral is imaged using 70kVp instead of 120 this image would have: a)increased density b)decreased density c)increased contrast d)decreased contrast C) lowering kVp = more contrast
AEC devices work by measuring: a)radiation leaving the tube b)radiation that exits the patient c)radiation that is absorbed by the patient d)attenuation of primary radiation by patient B
A purpose of beam-restricting devices is to ___ by changing the size and shape of the primary beam. a)increase pt. dose b)decrease scatter rad. produced c)increase exp. to IR d)decrease image contrast B
Compared with parallel grids, focused grids ___. a)have greater grid frequency and lead content b)can be used with either side facing the tube c)have a wider range of grid ratios and frequencies d)allow more transmitted photons to reach IR D
With which one of the following would a convergent line be formed if imaginary lines from its grid lines were drawn in space above it? a) linear focused b)crossed focused c)linear parallel d)crossed parallel A
If 15 mAs is used to produce an image without a grid, what mAs would be needed to produce the same image quality using a 16:1 grid? a)45 b)60 c)90 d)105 C
Grid cutoff, regardless of the cause, is most recognizable on a film radiograph as reduced what? a)contrast b)recorded detail c)density d)positioning C
Off-focus grid cutoff occurs by using an SID that is not __. a)within the focal range of grid b)equal to the focal distance of grid c)level of convergent line of grid d)level of convergent point of grid A
the type of motion most used for moving grids today is: a)longitudinal b)reciprocating c)circular d)single stroke B
A grid should be used whenever the anatomic part exceeds how many cm? 10
The air gap technique uses an increased ___ instead of a grid. a)kVp b)mAs c)SID d)OID D
Created by: mokapis