Fluoroscopy Flashcards
how is mA altered in fluoro?
mA is significantly decrease to increase exposure times.
If mA is too high, could melt the focal spot or your patient
- Fluoro mA: 0-5
- Gen rads mA: 200-800
parameters for kVp in fluoro compared to general radiography
kVp is the same 50-120 kVp
fluoro focal spot
0.3 -0.6 mm
describe the basic set up of an image intensifier
tube under the table, collimation occurs prior to reaching the patient –> patient gets the juice which generates a bunch of scatter –> scatter gets rejected by the grid –> xrays to image intensifier
In image intensifier:
input phosphor turns xrays into light –> then photo cathode converts light to e- –> e- are accelerated to output phosphor and e- are converted to light
tubes are generally under the table, but when they are ABOVE the table what is the trivia to remember?
the OPERATOR lens dose is higher
will the operator lens dose be higher when the tube is above or under the table?
above
What are focusing electrodes?
A series of elecrodes work to cous the beam and speed up the flow of e-
they are metal plates that are positively charged
https://www.upstate.edu/radiology/education/rsna/fluoro/iisize.php
what is the first component in the image intensifier
the input phosphor
First, x-rays incident on and absorbed by a cesium iodide (CsI) structured phosphor produce a large number of light photons resulting from the energy difference of x-rays (30-50 keV average)
how much is the increase in brightness after minification?
The combination of electronic and minification gain results on the order of 5000X increase in brightness.
brightness gain=flux gain×minification gain
How does II FOV affect minification?
minification gain = input area/output area
Therefore, larger FOV –> large minification gain
smaller FOV –> smaller minification gain
Why is the II convex?
The convex shape not only minimizes the patient distance thus maximizing the useful entrance field size, but it also gives the image intensifier better mechanical strength under atmospheric pressure.
https://pubs.rsna.org/doi/pdf/10.1148/radiographics.20.5.g00se181471
orientation of the image as it hits the output phosphor
inverted and reversed. This occurs as a result of “point inversion” as all the e- pass through tthe same focal spoint in their journey from input to output phosphor
Why does magnification in fluoro result in increase in kVp?
Use of magnification modes in fluoroscopy is usually associated with an increase in the choice of x-ray tube voltage for two reasons:
(a) higher voltages will reduce the entrance skin air kerma which needs to be kept below 90 mGy/minute (10 R/min) for regulatory purposes. Adjusting the x-ray tube voltage with increasing magnification resulted in only relatively modest increases in the entrance air kerma rate (35 mGy/minute -> 50 mGy/minute -> 61 mGy/minute).
(b) the tube current needs to be kept below ~5 mA to minimize the power input into the x-ray tube anode permit continuous fluoroscopy operation without overheating the x-ray tube. The increased in power input to the anode (power is kV x mA watt) was also relatively modest (190 watt -> 230 W -> 260 W).
what is “flux gain”?
increase in magnitude of light coming from the output phosphor relative to the input
how is increased magnitude from input and output phosphor acheived?
accomplished with a high voltage difference between the photocathode and the output phosphor. electrons are accelerated
Accelerating electrons = increased brightness
s distortion artifact
due to earth’s magnetic field
does dose goes go up or down when decreasing the input FOV?
goes UP!
dose goes up because halving the input field of view while keeping all the other factors the same, would cut the image brightness. The compensate for the decrease in image brightness the amount of radiation at the input of the II must be increased to compenstate for the reduction in juice
More radiation, more dose
what happens to dose as the II gets older?
dose increases
more and more dose is required to produce the same level of output brightness as the II ages
when are II replaced?
when the conversion gain falls to 50%
what are three things you can do with an old II?
1) use an aperture with a large hole in it. The downside to this is it increases the image noise
2) Let the automatic brightness control system do its job and crank up the juice (use more radiation)
3) Replace it if the conversion gain falls to 50%
Which increases dose, electronic or geometric magnification?
BOTH!
geometric mag increases dose 2/2 inverse square law: closer to the tube dose is squared
electronic mag (zoom) - if you decrease the input field of view by half then only one fourth of the input phosphor will be irradiated. If all other parameters are held constant the brightness will drop one quarter. the automatic exposure control will ramp up the juice
for IIs, what improves spatial resolution?
electronic magnification
how is the focal zone changed in an II?
changed by applying voltage across focusing electrodes
AERC dose limit (normal person)
87 mGy/min (10 R/min)
“high level mode” control dose limit
176 m Gy/min (20 R/min)
high level mode is for obese patients
what causes pincushion distortion?
with a large FOV you can sometimes get the appearance of bent lines at the periphery (pincushion)
What is S disortion?
due to interfernce of the earth’s magnetic field on the flow of electrons heading towards the II
artifact caused by interferenc of the earth’s magnetic field with the flow of electrons heading toward the II
S-distortion
How do you improve S distortion?
using “mu metal” (a soft magnetic alloy)
vignette artifact on an II
the distances from the focusing point to the outer phosphor tend to vary, with the closest path in the center and the farthest path at the edge –> dark periphery and light center
flare/glare artifact on II
transition from heavy to minimal attenuation - you can see b right white “Glare” at the periphery near the decreased attenuation. This is from overproduction of xrays in this thin area to compensate for the nearby thick area
Saturation artifact on an II
if the dose is cranked up to try and penetrate a very dense object (classically metal) you can end up with regions around the metal appearing very bright
fill factor equation
fill factor = sensitive area/ pitch2
equation for pixel size:
pixel = FOV/matrix
convert FOV from cm to mm before sovling
What is the purpose of binning?
improve SNR
Pixel binning. One additional - and easy - method to improve noise is by pixel binning. We can combine several adjacent pixels in an electronic (flat-panel) detector into one pixel we display on the screen - combine them into one ‘bin’. That way, we have more photons - and less noise.
How does binning affect the spatial resolution?
larger detector element pitch, decreased spatial resolution
maximum spatial resolution = 1 ÷ (2 × pitch).
what are the dowsides to binning?
increases susceptibility to motion artifact and ghosting
is binning typically used for large or small FOV?
large field of view when spatial resolution doesn’t matter as much
what determines spatial resolution of an II fluoro system?
spatial resolution of a TV, which depends on the raster lines
does the display limit the spatial resolution of an FPD system?
no
FPDs usually have displays with the same matrix as the image receptor
better pure spatial resolution FPD vs II?
II sytems have better spatial resolution, which changes with FOV
when does Pulse fluoro reduce dose (when compared to continuous fluoro)?
if you drop the rate BELOW 30 pulse/seconds
if you keep the rate at 30 pulses/s your dose is UNCHANGED compared to continuous fluoro
what is the difference in mAs between regular fluoro and pulse fluoro?
reg cont fluoro: continuous low mA with higher “S”
pulse fluoro: pulsed high mA with lower “S”
S = time, i.e. mAs
is continuous or pulse fluoro better for moving patients?
pulse fluoro good for wriggly patients (like babies) - less motion blur
when does pulsed fluoro reduce dose?
when it is below 30 frame/s
50% reduction in pulse rate results in ______ % reduction in dose
30% reduction in dose.
Not a direct 1:1 relationship (the math behind this concept is not important to know)
what is the difference between mA and mAs as pulse rate decreases
mA will increase and mAs will increase
spatial resolution limited by what for each modality?
FPD system:
I.I. system:
FPD system: spatital res limted by detector element size
I.I. system: spatital res limted by the TV system
what is frame averaging?
image process feature that adds several images together with different weight factors
it improves the SNR, but increases suspectibility to motion artifact and ghosting
how is spatial resolution tested in fluoro?
lead bar pattern
how is distortion tested in I.I, systems?
use a mesh screen or plate. Look for straight lines
how does kVp change for a barium study?
kVp for a barium study is >100 kVp to max out penetration.
kVp for iodine
70 kVp, chosen to max out K-edge
anode angle in fluoro compared to gen rads
smaller anode angle in fluoro
heel effect is negligible because you use a small field of view and small image detector (only the center portion of the xray beam is imaged)
what is the purpose of equalization filters?
reduce intensity of the beam but do NOT completely block the beam. Used to taper radiation profile and often employed when imaging the leg, arm, or pediatric patients
https://www.globehealth.net/fluoroscopy/density-equalization-filters.html
DSA is performed at what kVp?
kVp of 70 to take advantage of the k-edge of iodine
the superficial 3-5cm of skin/fat receives what % of dose?
50%
what alteration to technique needs to be done when thickness of the body part imaged is <10cm?
remove the grid
does a thicker or thinner patient get more skin dose?
thicker! the automatic brightness control sees less penetration and then cranks up the dose
how is dose affected with a lateral view as opposed to a frontal view?
patient and operator dose doubles with a lateral view
how do you calculate total fluoro dose?
total dose = (dose per frame) x (frame rate) x (duration x number of runs)
what should short angiographers do to reduce patient dose?
stand on a block or platform to ensure the source to skin distance (SSD) is at least 100 cm or more
best place to stand for an operator in the fluoro suite
stand/work on the image receptor side of the patient
in long fluoro cases what technique should be employed
dose spreading
the idea is to change the angle of the gantry to spread the skin dose over a broader area decreasing tthe skin dose to any specific location
what is dose area product?
radiation dose to air in mGy multiplied by the collimator area
measurement is INDEPENDENT of beam location
how does dose area product (kerma area product) change with beam location
DAP is independent of beam location
as you move the beam away from the patient the intensity decreases, but the beam spreads out more - these two things occur in equal amounts so the DAP is NOT dependent on location
how does magnification affect KAP and air kerma?
does NOT affect KAP
increases air kerma
What is Interventional Reference Point (IRP)
Interventional Reference Point (IRP)
IRP is a point in space along the central x-ray beam, 15 cm back from the isocenter toward the x-ray tube. This is the point where the reference air kerma is reported. Because of variability in patient size, operator height, and angle of the C-arm, the IRP does not precisely correspond to the skin surface. It is worth noting that no meter is positioned at the IRP. Rather, air kerma is measured close to the source in the center of the beam. Then, using the inverse square law, the value is calculated for the IRP and displayed.
what does kerma stand for?
Kinetic Energy Released in Matter
This is the energy released from an x-ray beam per unit mass of a specified material in a small irradiated volume of matter (eg, air, soft tissue). For x-rays in tissue, this is numerically equivalent to the absorbed dose.
The unit of measurement is the gray (Gy). One Gy is equivalent to one joule (J) of energy per kilogram of matter. For x-rays in air, the kerma is somewhat higher than the absorbed dose, as some of the released energy will be carried out of the small test volume as electron kinetic energy and will not contribute to the local dose.
the dose standing 1 m from the patient is about:
1/1000 of patient dose (0.1%)
what type of raw data processing results in the least dose to the patient?
iterative reconstruction
a thin dampening block results in a broad or narrow bandwidth?
narrow bandwidth
Which of the following happens when the optical aperture size of an image intensifier increases?
A. Entrance skin air kerma decreases
B. Brightness decreases
C. Brightness increases
D. Spatial resolution increases
A. Entrance skin air kerma decreases
Light going through the optical aperture will increase if the size increases. The AERC will redue the quanity of Xrays exiting the tube, lower the entrance skin air kerma
Entrane skin air kerma and E deposition in the xray tube anode are approximately INVERSELY proportional to the exposed area of the input phosphor of the I.I.
(inverse square law) If the FOV is halved –> entrance air kerma is QUADRUPLED
Halving FOV –> doubles mag AND doubles spatial resolution
what does a photocathode initially absorb and subsequently emit?
xrays and electrons
what is the most likely grightness gain of an II compared with a simple scintillator
5000x
how many “lines” are most likely used to generate a standard fluoroscopy frame
500 lines
how many raster lines are most likely used to generate a photospot image
1000 lines
what TV mode in fluoro is likely to have the least motion artifact
progessive lines read out sequentially to generate a whole frame in approx 33ms
interlaced mode, odd lines are read out followed by even lines
how will increased colimation during fluoro afect KAP and CNR?
decrease KAP, increase CNR
how much of an incident xray beam is most likely attenuated by a 2 mm Al fluro table
30 %
for lateral projections, the patient is subject o the full displayed radiation intensities
what is the minimum disatance from the patient to the focal spot on a mobile C arm fluoro system
30 cm
what mAs is required to generate a single fluoro frame
0.1 mAs per single frame
typical tube current is between 1 and 5 mA and images at 30 frames per second with takes 33 ms to generate each image
xray tube output is therefore approximately 3 mA x 0.03 ms or 0.1mAs
what is the prinicpal benefit of perfomring pulsed fluoro compared with continuous fluoro
sharper images
how is the minification gain changed in electronic magnification (mag 1) when the exposed scintillator diameter is reduced from 10 to 7 inches?
1/2
minification gain is directly proportional to the area of the input phosphor that is directly exposed to xrays
compared to normal mode, the exposed area in mag 1 mode is 1/2 and in mag 2 mode is 1/4
who requires fluoro systems sold in the US to have last image hold LIH capability
FDA
why are fluoro imgaes not diagnositc
quantum mottle
radiation intensity used to create a fluoro frame is approximately 0.1 mAs, which is 100 times less than in conventional radiography. As a result, the mottle is appxomiately 10x higher than in radiography (noise is inversely proportional to the square root of the radiation intensity)
the dose increase with increasing magnification for fluro with a digital detector without binning is due to
compensate for increased perceived noise
can you change the fill factor in a digital fluoro detector?
no, this is a fixed thing. cannot change this parameter
Which step in the II does not amplify signal?
conversion from light to photon to electron in the photocathode. you actually lose signal here
how does moving the tower away from the patient affect dose?
increases the dose
creating a short xray pulse in fluoro is accomplished by
with high biasing voltage at a focusing cup
