Chapter 10 Interventional Radiology Flashcards
examples of IR procedures
stenting, vertebroplasty, angioplasty, chemoembolization
what does IR do
use images to direct needles and catheters, avoiding large incisions
flat panel detectors in IR
-similar to digital radiography
-if fluoro, must minimize electronic noise because detected signals are a hundred times lower than in radiography
flat panel detector material
CsI
flat panel detector FOV
smaller FOV = increase in Kair at image receptor to reduce image mottle (no ABC, this is just done to reduce perceived mottle)
-increases in Kair are programmed by vendor
flat panel detector pixel size
170 um
is a TV or CCD required to read out the flat panel detector?
No, they are read out electronically
response of flat panel detectors
linear response over wide dynamic range
flat panel detector vs II FOV adjustements
-halving FOV would quadruple Kair for II but only fouble Kair for FPD
-FPD fluoro performed with larger FOV would likely add four pixels together (binninb) to reduce mottle
-binning also halves resolution
FPD vs II image detectors
both are 400 um CsI
FPDs have carbon cover which transmits slightly more x-rays than the 1.5 mm Al used in II
-ie. FPD detects 90% vs 75%
fluoro of II vs FPD
FPD: 3 lp/mm
II: 1 lp/mm using 500 line TV
distortions for FPD vs II
II have pin cushion, S, vignetting, glare, saturation
-FPD is excellent
-FPDs have linear response that is superior to sigmoidal response of II
what are equalization filters
use Pb/plastic mixtures to reduce transmission of x-rays to peripheral regions used as anatomical landmarks
digital subtraction angiography
-mask image without contrast is subtracted from corresponding image with contrast to show the vasculature
-removes anatomical background
road mapping
permits an image to be captured and displayed on a monitor while a second monitor shows live images
-can also be used to capture images with contrast material and overlay it onto a live fluoro image
tube voltage in angiography and dsa
70 kV, to match k-edge of iodine
tube current, exposure time of angriography and DSA
400 mA
50 ms
200 mAs
Kair at image receptor in angiography and DSA
3 uGy/image, similar to radiographic imaging
frame rates in angiography/DSA
4 frames/s
source to image distance in IR
100 cm
-shorter SIDs increase skin dose and image distortion from variable magnification
-longer SIDs require increased x-ray tube output (IS law)
what does geometric magnification introduce?
focal spot blurring
increased skin doses
air gap (scatter exiting patient irradiates techs)
-minimize by reducing gap between patient and image receptor
minimum focus to skin distance
38 cm for fixed systems
30 cm for mobile units
where are grids optional?
-IR in infants and knees
-removing grids (with AEC) will halve Kair
entrance Kair of lateral vs AP projections
lateral is double
entrance Kair for normal sized patient in angiography and DSA
1-3 mGy/image
what is interventional reference point
imaginary point, 15 cm closer to focal spot than system isocenter
-IR gantries rotate around the isocenter
IRP Kair
measured by vendors in air
-excludes patient backscatter
-NOT patient skin dose, conservative estimate of patient skin dose
how to convert IRP Kair into peak skin dose
-have to account for physical differences between air/tissue, backscatter radiation, x-ray attenuation by the table
-also account for differences in IRP location relative to patient skin and overlap from multiple projections
what is dose index
peak skin dose/ IRP Kair
-usually 0.5-0.8
typical peak skin dose
2 Gy
what procedures have highest peak skin doses
TIPS and embolization procedures
could be > 5 Gy
how to reduce chance of skin overlap?
use collimation to reduce overlap of beams from opposite obliquities
skin exposure policy in IR
< 2 Gy: radiation burns not ecpected
2-5 Gy: inform patient of possible burn
>5 Gy: MP should evaluate assumption of IRP Kair as peak skin dose
unintended skin dose of what Gy is a sentinel event?
15 Gy
-requires root cause analysis and visit from joint commission inspector
how many IR procedures result in serious radiation burn requiring major clinical intervention?
1/10,000
kerma area product values in IR vs radiography, fluoro
mean of 200 Gycm^2
10X higher than fluoro
100X higher than radiography
IR procedures with high KAP
spine arteriovenous malformation eolization > 500 mGycm^2
KAP for nephrostromy obstruction
25 Gycm^2
patient effective dose/ KAP factors for infants vs adults
10X higher in infants
typical patient effectrive doses for IR procedures
10-50 mSv
30 mSv mean
E/KAP for head
0.03 mSv/Gycm^2
E/KAP for body
0.1- 0.25 mSv/Gycm^2
operator dose 1 m from patient
0.1 % of patient dose
principle source of scatter in IR
where x-ray beams enter patient
radiation entering is 100X more intense than radiation entering
operators should stand away from entry points
typical IR operator effective dose
5 uSv for IR procedure with KAP of 200 Gycm^2
annual effective doses ~ 5 mSv for IR fellow, 1-5 mSv for tech
larger patients require more radiation therefore staff get more radiation
effective dose for spine embolization
90 mSv
effective dose for nephrsostomy
6 mSv
effective dose for pulmonary angiography
15 mSv
how are eye lens doses monitored?
place dosimeter close to eye
IR operator eye doses
< 150 mSv/year, but likely more than 20 mSv/year as recommended by ICRP 2011
effect to patient and operator dose if FOV area is halved
patient skin dose + 50 %
patient effective dose - 25 %
operator effective dose - 25%
effect to patient and operator dose if 0.2 mm Cu added
patient skin dose halved
patient effective dose halved
operator effective dose halved
effect to patient and operator dose if switch from AP to lateral projection
patient skin dose doubles
patient effective dose doubles
operator effective dose doubles
double badge dosimeters
-one dosimeter outside lead apron and one inside
fetal dose for pregnant workers
0.5 mSv/month
collimating beam to reduce area by 25 % will reduce patient and operator dose by what amount?
25 %
how does increasing distance from patient reduce dose to operator?
by IS law
how much does 0.5 mm Pb shielding transmit
< 10 %
shielding clothing for body
wrap-around aprons
skirt and vest
thyroid collars
lead impregnated sterile gloves- attenuate 50% of incident radiation
-90% reduction
eye lens protection
glasses with lateral shields (oeprators usually looking at monitors not at patient)
why don’t leaded glasses reduce dose as much as predicted given the low transmission of leaded glass?
x-rays can be backscattered into the lens from areas exposed beyond the leaded glass
-only 65% reduction
what is best way to reduce eye lens dose
“hang down” transparent lead barrier between patient and operator
how much do regular plastic or glass glasses attenuate the radiation?
< 5 %
takes 30 mm of glass or tissue to attenuate beam by 50 %
how much Pb in walls do IR rooms have?
2 mm
ancilliary shielding
lead draoes attached to table to reduce scatter from x-ray tube and patient
leaded plastic transparent drapes between patient and operator
lead barriers in room where staff can stand behind
what % of IR patients are likely to have cumulative IRP Kair > 2 Gy?
30%
1 Gy- 50%
2 Gy- 30%
3 Gy - 20 %
5 Gy - 5%
what % of IR patients would received a peak skin dose > 5 Gy?
2 %
1 Gy- 40 %
2 Gy - 25%
3 Gy- 10%
5 Gy- 2%
why use flat panel detectors instead of image intensifiers?
less artifacts
average kerma area product for IR procedures
200 Gy-cm2
patient scatter radiation results in doses at 1 m that are what % of radiation intensity incident on the patient?
0.1%
by how much do the Pb impregnated gloves reduce extremity dose by?
50%
