Chapter 9 Fluoroscopy Flashcards
power and currents for fluoro
3 mA
< 0.5 kW
focal spot size
0.6 mm
beam quality
80 kV, 3 mm Al HVL
use 3 mm Al filtration
pediatric fluoro will have additional 0.1-0.2 mm Cu filtration to reduce pediatric dose
grid ratio
10:1
what do image intensifiers do?
conert incident x-rays into bright light images
parts of II
evacuated envelope made of glass or aluminum
-input phosphor
-photocathode
-electrostatic focusing electrodes
-output phosphor
what does input phosphor do
absorbs x-ray photons and re-emits part of them as light photons
what does photocathode do?
absorbs light photons from input phosphor and emits photo-electrons
what does the elctrostatic lens do
accelerates the electrons to high energies and focuses them onto output phosphor
size of input phosphor vs output phosphor
output = 2.5 cm
input = 25 cm
what does output phosphor do?
-absorb electrons and emit light photons
flux gain
of light photons emitted at output phosphor for each photon emitted at input phosphor
-usually ~ 50
what is minification gain
increase in image brightness that results from reduction in image size from input phosphor to output phosphor
-100 for input phosphor of 25 cm vs output phosphor of 2.5 cm (area is 100 X smaller)
what is brightness gain
product of flux gain and minification gain
~ 5000
what does reducing area of input phosphor do?
-reduces minification gain and brightness gain
how do TV systems build up images?
-series of horizontal lines (raster scanning)
-500 or 1000 lines
progressive vs interlaced raster scanning
progressive: each line is read sequentially
interlaced: odd lines read first, then even lines
standard TV display frequency
30 frames/s
how many TV lines does fluoro use
500
CCD vs TV in fluoro
CCD is cheaper
-similar levels of mottle because fluoro imaging is quantum mottle limited
how do we get digital fluoro?
analog voltage signal from TV or CCD is digitized using analog to digital converter
number of pixels in 500 line TV frame
1/4 pf a million pixels
-2 bytes/pixel
-0.5 MB/frame
storage space required for 5 minutes of fluoro at 30 frames/s
4.5 Gb
9000 images
last image hold
lets you look at last acquired image when the x-ray is switched off
what is temporal filtering
-frame averaging
-occurs in real time
-reduces random noise
-only available for digital fluoro
-can introduce lag of some objects
definition of fluoro
view dynamic images in real time
fluoro acquisition rate vs image display rate
acquire at 15 frames/s but display at 30 frames/s
display each frame twice to avoid a flicker
why are fluoro images not of diagnostic quality?
of photons used to create image is a hundred times lower than in radiographic imaging
uses of fluoro
GI study- see barium given to patients
-GU exams- see iodine given to patients
pros and cons of overhead x-ray tubes
-minimizes magnification of kidneys but gives more operator doses
fluoroscopy table FDA requirement
< 2 mm Al equivalence
-the table attenuates about 1/3 of x-ray beam
portable fluoro
C-arm devices
tube voltage when iodine contrast is used
70 kV, so that avg x-ray photon energy is close to iodine k-edge (33 kV)
-maximizes absorption of iodine so you can see the vasculature
tube voltage for GI studies (barrium)
high voltage > 100 kV is used to make sure there is some penetration of the barium in the GI
-collimation is used to reduce scatter, improve contrast, reduce patient dose
exposure times in fluoro
minutes, vs < 0.1 s in radiography
fluoro is _ whereas radiography is _
dynamic
static
how many images does a fluoro test usually take?
tens of thousands
mottle in radiography vs fluuro
10 X less mottle in radiography because tube currents are 100 times higher
spot films
conventional radiographs obtained by introducing an overhead x-ray tube to expose a digital detector
photospot images
diagnostic quality images that are obtained through the fluoroscopic imaging chain
-tube currents increase from a few mA to several hundred mA
focal spot for photospot imaging
1.2 mm, to minimize exposure time
TV lines for photospot imaging
1000 lines, double the resolution of fluoro image with 500 lines
number of pixels in photospot
a million pixels
2 bytes/pixel
2 MB
electronic magnification
irradiates smaller diameter of input phosphor but maintains same output size
-reduces minification gain
automatic brightness control
-used in magnification mode
-increases radiation incident on input to maintain constant brightness at output
Mag I halves the exposed area, doubles Kair incident on II
-Kair quadruples in Mag II and increases by 8 in Mag III
is kerma- area product affected with magnification?
No, because area is reduced but Kair is increased due to automatic brightness control
what does electronic magnification do to peak skin dose?
-increases it (ex. doubles in Mag 1)
patient stochastic risk with magnification mode
not increased, because total radiation incident on patient isn’t changed
pulsed fluoro
-in normal fluoro, tube current flows continuously
-using pulsed reduces motion blur for same patient dose
-
-tube current is on for only a short time, but increased to maintain the same mAs for each frame as in continous fluoro
does pulse fluoro reduce patient dose?
-only when acquiring frames at < 30 frames/s (ex. 15 frames/s)
-when number of acquired images is 15 frames/s, each image is displayed twice so there is no flicker
how much does switching pulse fluoro from 30 fps to 15 fps reduce patient dose by?
35 % not 50%
because 15 fps uses a higher dose per frame to reduce the perceived level of random noise
-increases radiation per frame by ~ 30%
modes of radiation intensity in fluoro
low, normal, high
Kair at low ~ 50% of normal, 50% higher than normal at high
use low mode when looking at high contrast region (mottle less important) and high mode when looking at low contrast lesions (mottle must be reduced)
what happens to brightness with collimation?
it is reduced because areas outside of collimation will be dark
-collimation doesn’t activate ABC
-Kerma-area product is reduced
why use collimation?
-reduces patient stochastic dose
-improves image quality because of reduced scatter
-does not affect patient peak skin dose
what does AEC do
-AEC adjusts radiation to keep constant value of Kair at image receptor
difference between AEC and ABC
AEC keeps Kair at image receptor constant irrespective of x-ray beam area whereas ABC alters Kair only when eletronic magnification is changed
compare fluoro with electronic magnification vs collimation
electronic magnification:
-ABC is on
-Kair is increased
-kerma area product stays same
-resolution improves
collimation:
-ABC is off
-Kair is unchanged
-kerma-area product is reduced
-resolution is unchanged
2 ways to get more radiation on a larger patient
increase mA:
-doesn’t affect HVL and maintains contrast
increase kV:
-reduce contrast
-patients get lower doses from increase in kV vs increase in mA
-systems have several AEC curves to help optimize dose and contrast for larger patients
-usually increase both mA and kV
-when mA is increased more than kV, patient dose and lesion contrast are higher than when kV is increased more than mA
US limit on fluoro entrance Kair rate
100 mGy/min
-this may be inadequate image quality on very large patients
-high level control allows max rate to be increased to 200 mGy/min
-limit doesn’t apply to photospot or cardiac cine images
contrast agents during fluoro
iodine
barium
air
air is negative constrast agent (appears black)
what is veiling glare
light scatter in output phosphor
reduces contrast
Kair at image receptor
0.01- 0.03 uGy/frame
-varies with magnification and changes in frame rate
-diagnostic imaging (spot, photospot) is 1-3 uGy/image
what is used to assess fluoro performance
contrast detail phantoms
deep holes and shallow holes
deep holes are typically seen even when small, shallow holes are harder to see even when large
lesion contrast in photospot image vs fluoro
IDENTICAL
because contrast is independent of mA
HOWEVER, CNR improves because there is less noise
limiting resolution of II
5 lp/mm
-removing TV yields full resolution
what determines II resolution?
input phosphor CsI thickness, which can be made of thin columns to limit the spread of light
resolution of II when viewed through TV camera
reduced from 5 lp/mm to 1 lp/mm with 500 line TV and 2 lp/mm with 1000 line TV
how to improve fluoro resolution
use electronic magnification
halving field of view doubles spatial resolution
resolution in photospot imaging with II
2 lp/mm
1000 line TV image is digitized
resolution in spot imaging (conventional radiography)
-dtermined by detector pixel size
-3 lp/mm
why is the input window curved?
-permits window to be thin, minimizing absorption of incident x-rays
geometric distortion
-input window is cruved
-projecting this surface onto a flat output phosphor results in geometric distortions
pincushion distortion
straight lines appear curved
-less concern with small field size
vignetting artifact
fall-off in brightness at periphery of II field
-less concern with small field size
S-distortion
-due to electron paths being affected by earth’s magnetic fields- distortions vary as the II rotates
Glare
-occurs when there is significant increase in x-ray transmission
-happens at tissue interfaces
-visibility in higher transmission area is reduces because of light scattering in the output phosphor (veiling glare)
saturation
occurs when II has reached its peak value and appears as uniform white intensity in image
lag
moving object appears smeared out
entrance Kair in normal sized adult
10 mGy/min
entrance Kair in normal sized adult for photospot or spot image
1-3 mGy
how does increasing patient thickness by 3 cm affect entrance Kair rate
doubles it
-reducing patient thickness by 3 cm would halve entrance Kair rate when quality kept constant
Kerma area product (Gy-cm^2) for exams in
eophagus
upper GI
Ba enema
cystogram
esophagus 10
upper GI 15
Ba enema 40
cystogram 15
average kerma area product
20 Gy-cm^2, with all exams within factor of 2 of this benchmark
-fluoro studies use KAP 20X higher than correspondin g values in radiography
typical patient effective dose in fluoro-guided exam
4 mSv
-Ba swallows, upper GI studies have lower doses
-Barium enemas have higher doses
-GI and urological studies are moderate dose
size of one frame of digital fluoro
0.5 MB
for fluoro units at the floor, what % of incident beam is attenuated by the table?
30%
% of image intensifier phosphor area exposed for each mode
normal: 100%
Mag 1: 50%
Mag 2: 25%
max allowed Kair in standard fluoro
100 mGy/min
rule of thumb for dose from photospot images
1 min of fluoro (10 mGy/min) delivers same patient dose as 10 photospot images (1 mGy/image)
