17. Fluoroscopy and dynamic imaging Flashcards
in fluoroscopy images the denser tissues appear ___
what kind of image does fluoroscopy produce
darker
reverse contrast type image
for radiography how would you describe the xray burst and how many images are taken
when is the image viewed
single x-ray burst
single image taken with one exposure
viewed after acquisition
for radiography what is the current and is image intensification required
high current (mA)
image intensity sufficient, dont need intensification
for fluoroscopy how would you describe the xray burst and how many images are taken
when is the image viewed
continuous x-ray beam
multiple images
real time image viewing
for fluoroscopy what is the current and is image intensification required
low current (mA)
image intensification needed
what are the 4 components of the fluoroscopy machine in the arm portion
CCD
ABC sensor
Image intensifier
Antiscatter grid
what are the 4 components of the fluoroscopy machine in the portion under the patient
KAP meter
collimator
filters
x-ray tube
what does the image intensifier in the fluoroscopy machine do
intensifies the exposure and increases the number of photons
what are the 5 components of the image intensifier
input phosphor
photocathode
electrostatic focusing lenses (focusing electrodes)
anode
output phosphor
in the image intensifier what interacts with the input phosphor and what occurs
exit radiation from the anatomic area of interest interacts with the input phosphor for conversion to visible light
what occurs at the photocathode in the image intensifier
light intensities from the input phosphor are equal to the intensities of the exit radiation and are converted to e- by the photocathode
what is the name of the process at the photocathode in the image intensifier
photoemission
what happens at the electrostatic focusing lenses in the image intensifier
e- are focused and accelerated towards an anode
what occurs at the output phosphor in the image intensifier
e- are accelerated towards an anode to strike the output phosphor to create a brighter image
what is the input phosphor made of
Cesium iodide
CsI
what is the structure of the materials in the input phosphor
how does the structure relate to its function
CsI has column structure and means incoming x-rays are directed towards cathode and produces e- at specific locations minimizing scatter, improves image quality
what is the purpose of optical coupling
focuses the light from the output phosphor onto the CCD
what is the CCD
charge coupled device camera
what is the purpose of the CCD
receives the light photons from the output phosphor
incident light photons cause e- to be released and
the number of e- produced by the CCD is proportional to what
the number of e- is proportional to the intensity of light
what is flux gain
accelerating e- increases the light intensity at the output
what is the equation for flux gain
flux gain = number of output light photons/number of input x-ray photons
what is minification gain
e- incident on a smaller area increases light intensity
what is the equation for minification gain
area of input phosphor/area of output phosphor = di^2/do^2
what is the equation for the brightness (overall) gain
brightness gain = minification gain x flux gain
what is magnification
voltage to focusing electrodes is increased
only e- from the central area of the input phosphor reaches the output
what is the equation for magnification factor
mag factor = full input diameter/selected input diameter
for a larger FOV describe the
minification gain
spatial resolution
image distortion
dose
larger FOV
higher minification gain
lower spatial disortion
decreased dose
for a smaller FOV describe the
minification gain
spatial resolution
image distortion
dose
smaller FOV
lower minification gain
decreased spatial disortion
increased dose
what is pincushion distortion
result of curved photocathode and inaccurate focusing of e- from the periphery
what is vignetting
loss in intensity between the centre of the image and the edge
fewer light photons are collected at the edge
what is the cause of pincushion distortion
Image intensifiers are round
Test phantom with straight gridlines but gets curves towards the edges = pin cushion distortion
Photocathode is curve and reflects on flat output phosphor and gets worse towards the edges
what is the cause of vignetting
what consideration must be made in terms of vignetting
Result of fact that edges have fewer light photons on the edges
make sure tissues of interest are close to the centre rather than the periphery
what is S distortion
distortions arising from external electromagnetic sources
what is the cause of S distortion
e- follow fields and results in curved path
what is veiling glare and what causes it
scattering of light and defocusing of e- around the image intensifier
what component of the machine is the digital detector similar to
image intensifier
what is an advantage of using digital flat detectors instead of image intensifiers
Single flat detector so can keep these much more compact rather than bulky IIs
what are the 5 advantages of flat panel detector in terms of image quality
improved detection efficiency (except at low exposure)
improved uniformity
no veiling glare
no vignetting
no image distortion
what are the 2 advantages of flat panel detector in terms of the detectors physical characteristics
physically smaller and lighter
improved patient accessibility
what are the 2 types of fluoroscopy in terms of the frames
pulsed and continuous
how is dose reduced in fluoroscopy
lower frame rate
frame averaging
what can frame averaging result in
temporal blurring
what is image hold and what does it do to the exposure
reduce exposure by displaying the last image
what is continuous fluoroscopy
x-ray exposure continues without interruption while the exposure is activated
how many frames of images are taken each second in continuous fluoroscopy
30fps
what happens to patient dose in continuous fluoroscopy
high dose recieved
what is pulse fluoroscopy
x-ray exposure isnt continuous and has gaps of no exposure between each image frame
what does pulse fluoroscopy do in terms of patient dose and patient motion
decreases dose
reduce visibility of patient motion
for graphing the fps what is on the x and y axis in terms of pulsed/continuous fluoroscopy
mA on y axis and time on x axis
what is the time and mAs like for pulsed fluro
Higher mA and shorter time = pulsed
can pick which images we want
is the area between pulsed and continuous fluro the same
Area between pulsed and continuous are the same
what happens to the dose if you reduce the number of frames per sec
reduces dose
what is digital subtraction angiography in terms of what it does
increases visibility of contrast enhanced vessels by removing other anatomy from the image
Take image before contrast agent is applied (mask) just image of anatomy. Inject contrast agent that changes the opacity of vessels. Using baseline mask can subtract away anatomy to just see contrast anatomy
what are the 3 components involved in digital subtraction angiography
mask image
post contrast image
subtraction image
what is road mapping in DSA
use the image at peak opacification as the mask for subtraction in subsequent frames
vasculature appears as a map to guide the procedure
Take initial image to take info about vascular anatomy. Inject contrast agent that changes opacity of vessels so end up with bottom right image where vessels are clear. After a while the contrast washes through and wont hold for very long so want to use this as the reference as shows where the vessels are. End up seeing where vasculature is and is useful for guidelines in vessels
Peak opacification = vessels most bright
what is pixel shifted in DSA
shift an existing mask in post-processing to account for translational movement relative to the acquired mask
minimises the need for an additional exposure to acquire a new mask
Rather than taking map again and giving more dose you could use digital images to shift a few pixels to get better alignment so you can continue w/out another mask exposure
what is the cumulative air kerma measured in and the units
mGy
what is the cumulative air kerma
what is it analogous to
what effects does it involve
kinetic energy released in matter/air
like exposure
deterministic effects (skin)
How much energy is transferred to charged particle in tissue, measure of deterministic effect, exposed too long may damage skin
what is the kerma area product (dose area product)
what is it proportional to
AK x Area
proportional to stochastic (long term) risk
what is the unit of the kerma area product (dose area product)
mGy.cm^2
mGy.m^2
what is the cumulative fluoro time in terms of its unit and what its an indicator of
min
very poor indicator of dose
what are 8 ways to effectively manage dose in fluor examinations
use ABC
minimise use of magnification modes
collimate to region of interest
minimise use of spot flurographs
spread out dose to skin
use lowest appropriate frame rate
use lowest appropriate frame rate
exploit system geometry
reduce dose to personnel
how does using automatic brightness control manage dose in fluro exams
what is ABC
automatic adjustment of kV and mAs to maintain image brightness
can be controlled to prioritise higher contrast or lower dose
time is controlled by what in fluoroscopy
pulse rate
mAs is controlled by what in fluoroscopy
AERC
how does minimising the use of magnification modes manage dose in fluro exams
avoiding geometric magnification as the dose rate is proportional to (mag factor)^2.
use digital zoom instead
Magnification increases dose and as we change the magnification the patient may get moved closer to the tube to achieve magnification so increases dose
how does collimating to the region of interest manage dose in fluro exams
minimises scatter to the tissues of interest
decreases dose
how does minimising the use of spot fluorographs manage dose in fluro exams
acquiring the same pulse rates then get lots of dose to that particular tissue
how does spreading out the dose to skin manage dose in fluro exams
Xray tube in one location, run risk of deterministic effects
Minimise long exposure to single site, move x ray tube
Changing the tube angle and tube position spreads out dose to skin
how does using the lowest appropriate frame rate manage dose in fluro exams
minimise the beam-on time
use fluoro only to observe motion or positioning and use shorter pulse lengths to reduce blur
Reduce pulse length either the mAs needs to increase or the image quality per image decreases
how does using lowest appropriate frame rate manage dose in fluro exams
what is the one downside that might be present
shifting the window of averaged frames reduces noise
Rather than using each single image, combine images together and average them to get higher image quality. Appearance of larger pulse rate image
Improve image quality at expense of lag and temporal resolution
how does using exploiting system geometry manage dose in fluro exams
keep detector close to patient, keep tube away
operator needs to stand beside the detector, not by the x-ray tube
how does using reducing the dose to personnel manage dose in fluro exams
wear appropriate protective equipment and clothing and maximise distance from the x-ray source
monitor cumulative radiation dose
