Fluoroscopy (Erin) Flashcards
The entrance surface dose should not exceed ??? mGy/min for any given field of view
100mGy/min
An anti-isowatt curve for automatic brightness control causes the kV and mA to…
increase in proportion with one another to maintain image brightness
What is the equation for brightness gain
Minification gain x flux gain
What is the equation for minification gain
(D input / D output)2 (where D is diameter of input and output screen respectively)
What is the typical number value for flux gain
50
What is the typical value number for minification gain
100
What is the input phosphor made of?
Sodium activated caesium iodide (CsI:Na)
What is the X-ray absorption efficiency of the input phosphor (%)
70-90%
What colour light photons does the input phosphor convert X-rays to?
Blue spectrum light photons
What is the photocathode made of (material)?
Antimony caesium (SbCs3)
What is the role of the photocathode?
Absorption of fluorescent light photons (from inout phosphor) and conversion to electrons
Is the input screen maintained at a positive or negative voltage with respect to the anode (output screen)
Negative voltage
What is the potential different (in kV) across the image intensifier
25kV
What is the role of focusing electrodes in the image intensifier
Metal rings held within the tube that are held at positive voltages with respect to the photocathode. Contains the electrons in the tube to travel along paths that lead them directly to the output screen
What is the output screen made from?
Silver activated zinc cadmium sulphide (ZnCdS:Ag)
What is the typical diameter of the output screen?
25-35mm
Around 2.5cm
Skin doses to patients may be reduced by… (3)
- Increasing the patient to detector distance
- Reduced Minification/magnification
- Lower/less exposures
- Smaller field of view
What are the 3 layers of the input screen in an image intensifier and how do they work?
- Input window (Aluminium)
- Phosphor layer of caesium iodide that converts X-rays to light and channels them
- Photocathode layer of antimony caesium to convert light into electrons
In fluoroscopy at the output stage what happens
The output layer is Zinc cadmium sulphide silver and it converts electrons back into light
Magnification improves image…? But because of this, minification gain …? And therefore dose must…?
Image quality
Decreases
Increase
Fluoroscopy artefact: describe Veiling glare
Spreading of signal across the field of view where it shouldn’t be, due to scattering of light within the image intensifier
DSA: what kind of subtraction is required from the pre contrast compared to post contrast image
Logarithmic subtraction
DSA: the subtraction process has what effect on noise?
Increase image noise but perception of vessels is increased
DSA: uses what kind of tube voltages?
Usually at least 70kVP to maximise attentuation in iodine based contrast
What is the typical diameter of the input screen?
25cm
Magnification equation involving distances?
Magnification = Focus-film distance / (Focus-film distance - Object-film distance)
Increasing the focus-film distance will increase/reduce magnification?
Reduce
Reducing the object to film distance will increase or reduce magnification?
Reduce
What is the effect of a thicker input screen crystal on efficiency and spatial resolution?
Thicker crystal will be more efficient at converting X-rays to light photons
however the spatial resolution is reduced with thicker crystals
How thick are the input screen CSI crystals?
5 micrometers
What is the k edge of caesium iodide doped with sodium in the input screen
33 and 36 keV
What colour of light photons are produced at the output screen
Green (530nm wavelength)
How is the image between the input screen and output screen changed (in terms of orientation)?
Inverted
And
Flipped left to right
Digital fluoroscopy: what happens to light photons after leaving the output screen?
They enter a CCD which converts the light into electrons for readout and digital conversion
Digital fluoroscopy: read out at the CCD happens at what frame rate?
About 30 fps
What is the function of the aluminium backing at the output screen?
Acts as a anode
Prevent light photons from leaking back into the image intensifier
What is flux gain?
When a single electron has enough energy from acceleration to create many light photon in the output phosphor
Dual energy subtraction images: how is this done?
Separate images taken with one being low kv and one being high kv
What is the purpose of dual energy subtraction images?
To bring out bony detail usually.
Can be used the other way round too for chest imaging (remove rib detail)
ABC is achieved by using signal from what part of the image?
Centre
What is the purpose of ABC?
To guide adjustment of kvP and mas to maintain image brightness with different body parts
During angiographic procedure, to maintain brightness which parameter should be increased preferentially?
mAs
Kv should be held at about 60 to take advantage of k edge of iodine
A digital spot image is equivalent to how many seconds of continuous screening?
2 seconds
A low dose high kv curve is useful for which group of imaging?
Paediatric imaging
In a low dose high kv curve, the dose is preserved at the expense of…?
Image quality
Spatial resolution in Fluoro is most limited by…
Light spread in output phosphor
Spatial resolution in fluoroscopy is about…?
5 lp/mm
Fluoroscopy usually uses a focal spot of size…?
0.6mm
DSA: what image is subtracted from what?
The pre contrast mask image is subtracted from subsequent contrast runs
What is pixel shifting?
A technique to reduce motion artefact in Fluoro
The CCD array of a. Fluoroscopy unit has matrix size of…?
1024x1024 pixels
Standard tube current in fluoroscopy is…?
2-6mA
IN fluoroscopy, patient ESD usually increases with image intensifier field size. T/F?
False
T/F- The entrance surface dose rate should ideally be less than 50mGy/min
True
