Image quality Flashcards
recap: define density
- degree of blackening of image
recap: density influenced by (5)
- mAs
- kV
- distance
- filtration
- scatter radiation
recap: define contrast
- degree of diff in density btw 2 areas
recap: contrast influenced by (3)
- kV
- filtration
- scatter radiation
recap: define detail (sharpness)
- degree of sharpness of recorded lines on image
recap: detail influenced by (4)
- distance
- focal spot size
- motion
- magnification
recap: define distortion
- deviation of image of the true shape of object
recap: distortion influenced by (2)
- positioning
- magnification
recap: define artefacts
- appearance on film that is not normally present on film
- prod by artificial means
recap: artefacts influenced by (3)
- external objects
- equipment
- processing
xray beam: ways of xrays entering patient (3)
- absorbed
- scattered
- transmitted
xray beam: exit radiation
- combo of transmitted + scattered radiation
xray beam: type of radiation leaving patient in og trajectory? and prod?
- transmitted radiation
- prod diagnostic image
scatter radiation: define
- secondary radiation emitted from interaction of xrays w matter
scatter radiation: general features
- generally lower in energy
- changes direction
- no useful info for radiographic image
scatter radiation: reflected scatter will (2)
- may be reflected off surrounding objects
- source of exposure to personnel
scatter radiation: effect
- lead to reduction in radiation contrast
- increased density
scatter radiation: best time to remove scatter
- before reaching image receptor
- greatly improve image contrast
scatter radiation: solutions (3) before hitting receptor
- collimators
- grids
- air gap technique
scatter radiation: list factors increasing scatter (3)
- size of object
- xray beam energy
- field size
scatter radiation: features- size of object
- increase in tissue thickness increase amount of scatter = more possible interactions
scatter radiation: features- xray beam energy
- the higher the energy (higher kV) = more scatter
- lower energy beams more likely to be absorbed vs. scattered
scatter radiation: features- field size
- larger field sizes = more scatter
- more possible interactions
scatter radiation: name (3) beam restricting devices
- collimators
- diaphragms
- cones
scatter radiation: features- beam restricting devices (3)
- restrict field size to area of interest
- limit patient exposure in unnecessary areas
- improve image contrast by decreasing scatter
scatter radiation: collimators
- present on all xray machines
- defines size, shape of 1˚ xray beam
- light beam outlines area of exposure
scatter radiation: diaphragms
- added to collimator: further limit exposed area
- diaphragm changes rectangular shape -> circle (at point of exit of xray beam)
scatter radiation: cones
- added to collimator: further limit exposed area
- cone brings point of exit of xray beam closer to patient (limit to circular shape= more effective at limiting scatter)
scatter radiation: pros of diaphragms/cones
- good for spot views where improved image contrast is necessary
grids: general features and comprised of
- dev by Bucky 1913 for greater image contrast
- modern grid: series of lead strips (grid material: radiopaque) alternated w plastic/Al strips (interspace material: radiolucent)
- reduces amount scatter radiation within exit radiation reaching film
grids: designed for
- allowing passage of transmitted radiation (xrays in straight direction from source)
- scattered xrays absorbed by grid matter
grids: improves?
- radiographic contrast
- reduce scatter radiation
grids: cons- (3)
- absorbs portion of transmitted (‘useful’) radiation, reduced xray quantity and density
- grid lines prod in film, lessens diagnostic value of image
- may be prone to grid cut off
grids: cons- absorbed transmitted radiation solution
- maintain optical density: mAs increased = higher patient dose
grids: cons- grid lines prod in film lessening diagnostic value solution
- moving grid overcomes this
grids: cons- grid cut off solution
- needs to be correctly positioned w respect to image receptor
- centre of xray should be centre of grid
grids: grid cut off features
- undesirable absorption of 1˚ rays by grid
- misalignment of transmitted beam, lead strips increases progressively toward edges of grid
bucky: features
- grid placed in front of bucky, in front of image receptor
grid ratio: determines? and depends on
- determines amount of scatter radiation removed by grid
- depends on height of grid strip and distance btw grid strips
grid ratio: the higher the grid ratio? effectiveness
- more effective cleaning up scatter
- angle of scatter allowed less than lower ratios
grid ratio: higher grid ratio- gap size?
- smaller gap btw strips/ increased height of strips
grid ratio: needs increased/decreased mAs? to maintain density
- need increased mAs (higher patient dose)
grid ratio: define
- ratio of height of lead strips to dist btw lead strips (thickness of interspace)
grid ratio: formula
GR = h/D
height, dist
grid ratio: typical range
from 5:1 - 16:1
grid ratio: cons
- while removing scatter is difficult to align properly
grid freq: define
- no. of lead strips per cm
grid freq: higher grid freq means? (2)
- thinner strips
- less visible on radiographic image
grid freq: assoc w higher/lower grid ratio and better at?
- assoc higher grid ratio
- higher patient dose
- better reduction of scatter radiation
grid freq: typical range
24 - 80 lines/cm
bucky (grid) factor: features
- how much scatter removed by grid
- technical factors adjusted to prod same optical density
bucky (grid) factor: scatter?
- accounts for sig portion of density of final radiograph
bucky (grid) factor: compensation
- increase exposure when removing scatter
- compensates for decreased xrays hitting image receptor
bucky (grid) factor: B?
- ratio of mAs required w grid to mAs w/o grid
= prod same optical density
bucky (grid) factor: B formula
mAs (grid) ÷ mAs (without grid)
bucky (grid) factor: always greater than?
1
bucky (grid) factor: typical range
3 - 5
bucky (grid) factor: the higher bucky factor, the higher?
higher grid ratio and freq
linear parallel grids: consists of
- parallel strips
linear parallel grids: effect
- high ˚ of grid cutoff (esp outer edges due to diverging xrays)
linear parallel grids: define grid cut off
- undesirable absorption of transmitted rays by grid
linear parallel grids: cut off features
- misalignment of transmitted beam + lead strips increases progressively towards edges of grid
- cut off at periphery (both sides)
linear parallel grids: cut off reduced by?
- increasing SID
- less divergence of xray beam
linear parallel grids: tilted cut off
- xray beam not perpendicular to grid
- poor tube positioning
- cut of one half of image
linear focus grids: features
- grid material aligned to coincide w diverging beam
- allows less grid cut off, more expensive to prod
- take care when positioning grid compared to xray tube (due to geometric limitations)
- intended dist and side must face xray tube marked on grid (when backwards = extreme grid cutoff)
linear focus grids: cut off
- when correctly positioned: no cut off
- incorrect SID: falls outside focal range of grid
cross parallel grids: features
- as parallel only clean up scatter one direction, crossed overcomes this
- 2 linear grids perpendicularly opposed
cross parallel grids: pros
- efficient scatter cleanup
- improves contrast
- positioning critical
cross parallel grids: central beam?
- central beam must coincide w centre of grid
cross parallel grids: can u use angle techniques?
- no
- due to grid cutoff
crossed focused grids: features
- 2 linear focused grids opposed
crossed focused grids: need? (3)
- central beam positioned correctly
- correct dist
- correct placement of grid in bucky
moving/reciprocating grids: features
- reduces grid lines on image (grid vibrated during exposure
- heard as audible ‘rattle’
- grid moved by bucky
air gap technique: features (2)
- alt method for reducing scatter grid
- uses increased OID
air gap technique: approx gap used
10 - 15cm
air gap technique: used in eg.
- lateral cervical spine radiographs
- shape of normal anatomy
air gap technique: cons
- increased magnification, focal spot blur
air gap technique: to maintain same optical density?
- increased mAs
air gap technique: increased mAs same as using what grid?
8:1 grid
image detail: define
- degree of sharpness of recorded lines on image
- smallest separation of 2 lines/edges that can be recognised as separate objects on image
image detail: define spatial resolution
- quantification of image detail
- measured: line pairs/mm
image detail: decreased image detail assoc w high/low contrast
- low contrast
image detail: features penumbra aka
- area of decreased image detail
- aka geometric unsharpness
image detail: affected by (2)
- blur
- magnification
image detail: blur factors (3)
- focal spot size
- distance
- motion
image detail: magnification factors (1)
- distance
focal spot: features
- xrays not emitted from point, rather from rectangular-shaped focal spot
- most xray tubes have 2 focal spot sizes
focal spot: changing spot size (w constant kV and mAs) does not change (2)
- xray quality or quantity
focal spot: does limit?
- limits max possible xray quantity
focal spot: sml/lrg spot used for normal imaging? features (2)
- lrg spot
- allows sufficient mAs for denser body parts
- assoc w decreased image detail
focal spot: sml/lrg spot used for fine detail imaging? and features (3)
- sml spot
- areas don’t need v high mAs to penetrate
- able to get improved image detail
- extremity radiography
focal spot blur: combo
- combo of xray beam, rectangular size of focal spot = focal spot blur
focal spot blur: larger the focal spot, more/less focal spot blur?
- more spot blur
focal spot blur: most vital factor for?
- spatial resolution
focal spot blur: eg. poor resolution (less detail) w smaller/larger focal spot
- larger focal spot
anode heel effect: features
- angles anode prod xray beam w more intensity on cathode side
anode heel effect: why less intense on anode side?
- xrays running parallel to anode angulation absorbed by anode, giving decreased intensity
anode heel effect: focal spot blur greater on anode/cathode side xray tube?
- cathode
- decreased xrays on anode side -> smaller effective focal spot, less blur
focal spot blur: divergence of beam (interacting w object) will increase/decrease when increase SOD (source to image dist)?
- divergence decreases w increased SOD
focal spot blur: increasing SOD increase/decrease focal spot blur
- will decrease spot blur
focal spot blur: SOD increased/decreased by increasing SID?
- increased SID (source to image dist)
focal spot blur: divergence of transmitted radiation (exiting from an object) will increase/decrease when increase OID (object to image dist)?
- divergence increases w increased OID
focal spot blur: decreasing OID will increase/decrease focal spot blur?
- decrease spot blur
focal spot blur: increased/decreased SOD by increasing SID?
- increased SOD (?)
focal spot blur: decreased by- list (4)
- smaller focal spot
- larger SID (larger SOD): if change SID need to consider image density affected
- smaller OID (larger SOD)
- anode side of tube
motion blur: define and is major cause of?
- blur caused by patient moving during xray exposure
- one of major causes: repeat radiographs
reducing motion blur: list (2)
- time
- stabilisation
reducing motion blur: time and controls what movement?
- shortest possible exposure times
- controls for involuntary movement of heart and lungs
reducing motion blur: stabilisation and controls what movement?
- restrict patient motion by instrument, restraining device
- controls for voluntary movement
linear tomography: features
- xray tube deliberately moved during exposure time while image receptor is moved in opp direction
- used to blur structures on either side of plane of interest (see slice of subject vs. structures superimposed)
- position of fulcrum determines object plane
linear tomography: greater angle = the thicker/thinner slice
- thinner the slice
magnification: define
- enlargement of object on resultant image
magnification: due to
- divergence of xray beam
magnification: measured by?
- magnification factor (MF)
- ratio of image size to object size
magnification: formula
MF = i ÷ O
image size : object size
magnification: magnification of 1 means?
- same size as object
magnification: MF is also proportional to? (when object size is unknown helpful)
- SID to OID
magnification: MF formula using SID, OID
MF = SID ÷ SID-OID
magnification: how to minimise
- min divergence of beam:
- larger SID
- smaller OID
magnification: at 100cm MF approx?
1.1
magnification: at 180cm MF approx?
1.05
magnification: formula to find size of og object (O) using image (i), SID, OID
O = i(SID-OID) ÷ SID
decreased image detail assoc w: list (7)
- decreased SID
- decreased SOD
- increased OID
- increased focal spot size
- cathode side of xray tube
- increased motion
- increased magnification
distortion: define
- unequal magnification of diff portions of same object
distortion: depends on (3)
object:
- thickness
- position
- shape
distortion: object thickness
- thicker = more distortion
distortion: object position- depends on
- depends on relative position of object, image receptor, central xray beam
distortion: object position- (2) eg.
- object plane vs image plane not parallel (increased OID in some areas)
- objects positioned off central ray beam (more xray divergence)
distortion: object position- shape distortion? (2)
- elongation
- foreshortening
distortion: object position- patient positioning
- important to prevent distortion and magnification as possible
distortion: object shape
- irregular objects produce irregular distortion
image assessment: check (5)
- contrast
- density
- detail
- distortion
- artefacts
