PHYSICS - Fluoro Flashcards
Typical grid ratio (fluoro)
10:1 (same as general radiography)
Grid indicated when tissue thickness is…
> 12 cm (e.g. not an extremity or peds)
ABC monitors…
light eminating from output phosphor (I.I. systems)
ADC monitors…
electronic signal detected by FPD
Conversion gain
decreases with increasing image intensifier age
Solutions for decreased conversion gain
use larger aperture, allow ABC to use more radiation, replace system
Large F#
smaller aperture (and vice-versa for small F#)
Effect of a larger F# on dose and noise
larger F# => less noise, more dose
Effect of binning on SNR and spatial resolution
increased SNR, decreased spatial resolution
Effect of binning on dose
can decrease dose while maintaining SNR, at the cost of spatial resolution
Combining 4 DELs reduces dose by…
50%
Pitch (fluoro)
size of a DEL; measured from center to center
Effect of geometric mag on AK and KAP
increased AK and KAP (from ABC via inverse square law)
Effect of electronic mag (I.I.) on AK and KAP
increased AK (from ABC), same KAP (smaller area exposed)
Effect of electronic mag (FPD) on AK and KAP
slightly increased AK, same KAP (smaller area exposed)
Each electronic magnification setting increases dose by…
1.4 to 2 times
Which increases dose more, geometric or electronic mag?
geometric magnification
Do I.I. or FPD systems have better dynamic range?
FPD systems
Effect of electronic mag (I.I.) on resolution
increased spatial resolution (because less minification)
Effect of electronic mag (FPD) on resolution
no change in spatial resolution (just zooming in)
Resolution-limiting component for I.I. systems
quality of display TV
Resolution-limiting component for FPD systems
DEL size
More or less scatter with a large FOV (less collimation)?
more scatter with larger FOVs (relative to smaller FOVs)
Dose reduction techniques
position patient away from source, use smallest possible FOV (collimation), avoid magnification (either kind), pulsed fluoro
Effect of collimation
smaller FOV => less scatter, less dose
Optimal position of source and detector relative to patient
maximize SOD and minimize SID; taking advantage of inverse square law to decrease patient dose while maximizing x-rays reaching detector
Pulsed fluoro
can maintain image quality at decreased dose or improve image quality at same dose; less susceptible to motion
Ideal kVp for DSA
70 kVp (take advantage of k-edge of iodine)
Digital spot modifications (relative to regular fluoro)
increased mA, larger focal spot; also no binning => higher resolution
Focal spot in fluoro (relative to general radiography)
smaller focal spot utilized to limit geometric blurring; 0.3-0.6 mm
Effect of frame averaging
decreased quantum mottle => increased SNR; risk of lag/ghosting/persistence
Vertical resolution of FPD display is determined by…
number of raster lines; line pair = 2 raster lines
Horizontal resolution of FPD display is determind by…
number of dots per raster line; line pair = 2 dots
Ideal place for operator to stand
same side of patient as detector
Lateral and oblique views require higher or lower dose?
higher dose
50% of beam is attenuated in what distance
initial 3-5 cm of tissue
Effect of lead apron
0.5 mm Pb stops 95% of radiation (or 99% per Ram)
Lead should be worn if within…
6 feet of patient/source
Federal regulation for entrance dose rate (fluoro)
cannot exceed 88 mGy/min (measured at 30 cm from detector surface)
Federal regulation for high level control entrance dose rate (fluoro)
cannot exceed 176 mGy/min (measured at 30 cm from detector surface); must have a visual or audible alarm
Personnel dose standing 1 meter from patient
1/1000 of patient dose
Most influential factor on patient dose in fluoro
fluoro time
Joint commission sentinel event
cumulative dose of 15 Gy or more to a single field; requires root-cause analysis and visit by JC inspector
Entrance air kerma limit for DSA
no limit for DSA
Absorbed dose
energy deposited per unit mass; expressed in Gy (J/kg)
Equivalent dose
absorbed dose times a radiation weighting factor; expressed in Sv
Effective dose
equivalent dose times a tissue weighting factor; expressed in Sv
Air kerma relates to which risk type?
deterministic risk (CAK does too)
Kerma area product relates to which risk type?
stochastic risk
Lower pulse rate (or fps)
decreased dose (despite an increase in mA to maintain perceived image quality), decreased temporal resolution
Replace I.I. when conversion gain falls to…
50% or less
KERMA (meaning)
Kinetic Energy Release per unit MAss
Effect of collimation on dose (fluoro)
unchanged AK, decreased KAP; ABC is turned of with collimation (per Huda)
Artifact: periphery is dark than center (I.I.)
vignetting artifact; shorter path to center of detector than to edge of detector
Geometric magnification in fluoro is typically performed by…
increasing SID (unchanged SOD); result is increased dose by ABC => increased AK and KAP
Binning is useful with…
large FOVs; usually turned off for electronic magnification and digital spot => higher resolution
Artifacts that FPD systems do not get
pincushion, S-distortion, vignetting, glare, saturation
Artifacts that FPD systems do get
bad pixel, lag/ghosting
50% reduction in frame rate (pulsed fluoro) reduces dose by…
30% (don’t worry about why, just memorize 30%)
Spatial resolution is tested for with…
lead bar pattern
Image distortion is tested for with…
mesh screen or plate
Typical entrance dose rate
30 mGy/min
Road map
similar idea to DSA (subtraction of normal anatomy); uses lower dose => more noise; vessels appear white
DSA
higher dose with a lower frame rate; less noise
Benefits of minimizing ODD
decreased dose (inverse square law), decreased geometric blurring, detector blocks scatter to personnel
AK measurement
ionization tube at the international reference point (15 cm toward x-ray tube from machine’s isocenter)
Relationship of AK and peak skin dose (PSD)
AK is an appromixation of the PSD; PSD is not easily measured
Effect of mag 1, mag 2, and mag 3 on AK
2x, 4x, and 8x, respectively
