Fluoro Flashcards
Fluro vs conventional xray
mA
kVp
exposure time
focal spot
Way less mA (0-5 vs 200-800)
Same 50-120
Longer exposure times
Smaller focal spot (0.3-0.6mm) Xray = 1mm
How many ‘spots’ = 1 minute of fluoro
5-10 spots = 1 minute of fluoro
i i setup
ii
———-grid
O patient
collimated beam
Tube
Parts of i i
input phosophor - CsI, Xrays to light
photo cathode - light to electrons
voltage diff
output phosphor - electrons to light
PCP Phosphor Cathode Phosphor
Flux gain
increased magnitiude of light between input and output phosphors 2/2 voltage (25-35 kV) accelerating electrons
Minification gain
output phosphor smaller than input
more electrons/energy per unit area
“Mag” = less minification
Brightness gain
old term due to combined effects of minification gain and flux gain
BG = flux gain x minification gain
Conversion gain
old machines
efficiency
how good i i is at turning electrons back into light
brightness gain and conversion gain both get worse with age of machine = more dose
older machine = more dose
conversion gain level to just replace i i at?
replaced when conversion gain falls to 50%
electronic mag (zoom)
decrease input field of view
smaller input, same output
1.4-2x dose per setting increase in dose (AEC kicks in)
Fluoro abc vs xray
xray abc ups the mA
fluoro abc adjusts mA, kVp or both
Fluoro and spatial res
what improves it?
main limiter?
Improved with magnification (less minification)
limited by resolution of display TV
Dose and…
field of view
Dose increases with decreased field of view in both
i i and FPD systems
Order of mA and kVp increase effect on dose
mA increased before kVp, dose gets higher
kVp increased first, dose GOES UP LESS
Dose and…
filtration
more filtration = fewer low energy Xrays = less dose
Dose and…
adding an aperture
Smaller hole, larger focal length (F#), blocks more light from hitting output phosphor
Greater Dose
Dose and…
kVp
Higher kVp = more penetrating xrays
Lower *skin dose
% blocked by Pb
when to wear
1mm of Pb stops 90%
Wear within 6 feet
Steps to reduce patient dose
Position away from the source
collimating, small FOV (also improves resolution)
Avoiding mag
KERMA
Kinetic Energy Release per unit MAss
TOTAL amount of energy deposited from ionizing radiation divided by a unit of mass (more quantifiable)
TOTAL because kinetic energy of particle movement but also HEAT
AIR KERMA
best way to measure heat and kinetic is to measure it just prior to transfer
AIR KERMA = estimation of how many photons are in a unit of air prior to energy striking skin
estimating peak skin dose based on potential transfer of energy
Kerma Air (area) Product
KAP
Amount of kerma (potential dose) multiplied by cross-sectional area of the xray beam
Total radiation potentially incident on patient/ total radiation used in exam moreso than actual dose to patient
more juice used in a smaller area, change in KAP?
potentially none
Electronic mag, air kerma and KAP
Electronic mag increases Air Kerma and therefore skin dose
DOES NOT increase KAP (decreased cross sectional area)
US Gov Air Kerma rate limit?
87mGy/min
High Level Control
US Gov limit for high limit
176 mGy/min
must have audible or visual alarms
Pin cushion due to?
Large FOV
S distortion
cause
make it better?
Large FOV
Earth’s mag field affecting flow of electrons
mu metal
Flair or Glare artifact
Transition from heavy to minimal attenuation
glare at periphery near decreased attenuation
overproduction of xrays in this area to compensate for nearby high attenuation area
Vignetting artifact
dark periphery, light center
furthest path to outer phosphor at the edges
vignette = short at beginning and end
Saturation artifact
dose cranked up to penetrate a dense object (metal), regions around metal look very bright
Flat panel detectors (FPD) more modern system
Xrays
—————-Grid
——————–carbon fiber protects components
——————-CsI needles (phosphor) xray photons to light
photodiode array———————- light to electrons
read out element, transistors and gates (constructs image from charges in photodiode array
Pitch (in fluoro)?
fill factor?
linear dimension of a detector element
percentage of each detector element actually sensitive to light
Fill factor = sensitive area
Pitch squared
Detector element size and fill factor and spatial resolution
Smaller elements have better spacial res but worse fill factor
ie better spatial res but need more radiation
1 detector element = 1 pixel
that is all
Pixel, matrix, FOV equation
Pixel = FOV(mm)/matrix
1100 x 1100 matrix
25cm FOV
pixel = 250/1100 = 0.23mm
Binning
quantum mottle
several detector elements made into a large detector element
Reduced amount of data (less mottle) less variation in xray photons from pixel to pixel
Binning and
noise
spatial resolution
Less mottle, can reduce radiation and keep same noise
WORSE spatial res
Frame averaging
image processing, combines images together
better SNR
more ghosting and motion
FPD artifacts
NO cushion, S distortion, vignette, glare or saturation
Bad pixel = white or black spots, fixed by interpolating
Lag, Ghosting
Spatial resolution
in i i limited by?
TV. (scan lines, bandwidth, FOV)
not a problem in FPD (displays have same matrix as image receptor)
Vertical resolution (on TV or display) formula
Raster lines x Kell Factor
2 x FOV
so smaller FOV = better spatial resolution
Better pure spatial res FPD or ii?
ii, and change with FOV
pulsed vs continuous fluoro and mAs
when are they equal
pulsed = periodic higher mA
continuous = always on low mA
30 pulses/second dose is unchanged/equal
lower fps has higher mA per pulse, but overall decreased mAs
pulse fluoro good for ?
moving patients (less motion blur)
Reduced dose going from 30 to 15 fps?
30%
Factors affecting spatial res
FOV
smaller better
Factors affecting spatial res
focal spot size
usually doesn’t matter unless anatomy further from receptor
Factors affecting spatial res
binning
worse res, better SNR
Fluoro QA
spatial res?
distortion?
Lead bar pattern for spatial res
Mesh screen or plate (looking for straight lines, not pin cush or S distortion)
kVp and contrast. Iodine vs Barium
Iodine - 70 kV to max out k edge (33keV for I)
Barium 100 kV - use more barium than I, need higher kV to PENETRATE
kVp and dose
higher = less skin dose, slightly more organ dose
if you drop mA (15% kV up, half mA), dose decreases
kV always increases dose in CT
Fluoro in IR
focal spot?
anode angle?
FOV and detector size?
ALL SMALL
small focal spot- looking at small things
smaller anode angle- increased heel effect but doesn’t matter because…
SMALL FOV and detector size (only imaging central portion of beam)
Fluoro in IR
kVp
60-80
Iodine k edge 33
Fluoro in IR
Filter?
soft filter or equalization filter
leg, arm, peds
Fluoro in IR
Grid?
not in extremities or peds
DSA
moving parts don’t get subtracted
image the moving contrast
Dose in IR
?pulsed
% skin dose?
skin dose in a fatty?
lateral and oblique views?
Run dose per frame?
Total dose?
Most IR pulsed
50% of dose delivered to 3-5cm most superficial
Higher in fatties 2/2 abc
more with lat and oblique
0.5 mGy per frame at entrance skin position (10-20x more than fluoro per image)
dose per frame x frame rate x duration of run
SSD in IR
determined by table height
small SSD = high dose
Interventional reference point IRP
ionization chamber with a set ref point, to measure radiation emitted from source
ignores geometry, table attenuation, and back scatter
over or underestimates skin dose but best measure?
Dose at 1 meter from patient relative to patient dose?
1/1000
with no lead
Regulatory doses
HLC
during image recording
87mGy/min 10R/min
176 mGy/min 20R/min
NO limit during image recording IF PULSED
Skin doses
Below 2Gy?
Do nothing
Skin doses
2-5 Gy?
Advise pt to look for burns (10 days post procedure)
Skin doses
above 5Gy
Procedure and dose should be reviewed by physics
Skin doses
Early transient erythema?
2Gy
Skin doses
Temporary epilation?
(hair loss)
3Gy
Skin doses
Chronic/main erythema
6Gy
Skin doses
permanent epilation?
(hair loss)
7Gy
Skin doses
telangiectasia?
10Gy
TEn = TElaNgiectasia
Skin doses
Dry desquamation
13Gy
Skin doses
Moist desquamation/ulceration
18Gy
Skin doses
Secondary ulceration
24Gy
Operator doses
One usually gets in a year?
5mSv
Operator doses
regulatory limit per year
50mSv
Operator doses
fetus dose limit
0.5mSv PER MONTH
