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