1. Fluoroscopic Image Quality Considerations

2. Factors to be considered are: (Quantum Noise or Scintillation)
Image Quality
Because the “imaging chain” of a fluoroscopic system is a complex system, there are more factors that affect image quality as compared to static radiography
Factors to be considered are:
Contrast
Resolution
Distortion
Quantum Mottle
(Quantum Noise or Scintillation)

3. (This affects overall image contrast exactly as in static imaging)
Image intensified fluoroscopic contrast is affected by:
Scattered ionizing radiation (same as static radiography)
Penumbral light scatter from input and output screens
Light scatter within the image intensification tube
Transmitted incident primary beam striking the output phosphor
Image contrast can be increased and/or controlled by electronically increasing the amplitude of the video signal
Background Fog
All of these phenomena contribute to produce a background ‘fog’ that raises the base density of the virtual image
(This affects overall image contrast exactly as in static imaging)

4. Contrast
An inherent decrease in image contrast near outer (or peripheral) edges of fluoroscopic image
Increasing lowest density value decreases total visible contrast
Lowest Density Value
Total Visible Contrast
Due to imperfect intensifier tube and electron beam geometry
Overall effect is of reduced image contrast

5. Image Quality – Resolution
Resolution is the ability of the imaging system to differentiate small objects as separate structures when they are positioned closer together
Measured in line pairs per millimeter (lp/mm)
Object size
Spatial frequency
Inverse Relationship
As object size becomes smaller,
the spatial frequency becomes higher
Overall resolution of an imaging system is expressed in terms of its
“modulation transfer frequency” (MTF) as a function of spatial frequency

6. Resolution Modular Transfer Function (MTF)
“A” is a set of patterns of dark and light bars.
There are 4 sets of bars at increasingly smaller spacing.
“B” is an “image” of what those bars might look like when imaged by a lens. The edges of the dark and light regions will be blurred. The closer the spacing, the more blurred they become.

7. Resolution
Ability to resolve recorded detail will vary depending on geometrical factors: (same as static radiography)
Input and output screen diameter
Minification Gain
Electrostatic Focal Point
Viewing System Resolution
(monitor resolution)
Phosphor crystal size and thickness
OID
Geometrical factors are of a different nature than in static radiography

8. Resolution Capabilities
Zinc-cadmium input phosphor intensifier tubes
1-2 lp/mm
Cesium iodide (CsI) input phosphor image intensifiers
4 lp/mm
Optical mirror systems that permit “indirect” viewing of the fluoro output screen
3 lp/mm
Magnification or multi-field image intensifiers
6 lp/mm
(in mag mode)

9. Size Distortion
Size distortion is caused by the same factors that affect static radiography magnification: OID
Multifield image intensifiers that produce magnification by changing the electrostatic focal point do not significantly affect actual size distortion
Some size distortion is always present in the minified image
Size distortion becomes more visible in a magnified image

10. Is significant when distorting 8-10% of the image area
Shape Distortion
Is significant when distorting 8-10% of the image area
Shape distortion (pincushion) is primarily caused by shape of image intensification tube
Inherent edge distortion at output screen even with concave input screen
Electron stream focusing not uniform across entire field of image intensifier
Electrons at outer edges of image flare outward as they are electrostatically focused
Due to repulsion of like charges
Partially due to divergence of primary x-ray beam

11. Vignetting
Center of output screen brighter than periphery due to unequal magnification of the electron stream causing unequal illumination at output phosphor
In center of image:
Resolution is better
Distortion is minimized
Contrast is improved
periphery
center

12. Veiling Glare
Veiling glare is mainly the consequence of light scatter
in the output window of the image intensifier
Scattered light
(like scattered radiation)
adds to the background signal
Thus, the contrast of the image
is reduced
Measuring the contrast ratio of an image intensifier is a good method to quantify the magnitude of this problem

13. Quantum Mottle, Quantum Noise, Scintillation
Blotchy or grainy appearance caused by insufficient radiation to create a uniform image
Static radiography: mA and time as mAs controls quantity of photons creating density on image receptor
Fluoroscopy: factor of time limited by length of time human eye can accumulate or integrate enough visible light photons from the fluoro imaging chain to be perceived
This time period is 0.2 seconds
Fluoro mA must be high enough to avoid excessive mottle as perceived by the observer

14. Quantum Mottle, Quantum Noise, Scintillation
Creates a special problem as fluoro units are operated with the minimum mA (dose) possible to activate the fluoro screen
Quantum Mottle, Quantum Noise, Scintillation
Inherently present in any electronic video system
as ‘video noise’ or ‘electronic noise’
Creates a special problem as fluoro units are operated with the minimum mA (dose) possible to activate
the fluoro screen
A low SNR would produce an image where the "signal" and noise are more comparable and thus harder to discern from one another
mA too low causes “excessive” quantum mottle / scintillation
The image above has a sufficiently high SNR to clearly separate the image information from background noise
mA increased enough to permit image signal to be visualized

15. Quantum Mottle, Quantum Noise, Scintillation
Factors affecting quantum mottle include:
Solutions
Increasing efficiency of any of these will reduce quantum mottle
Viewing system
(direct, monitor, film, etc.)
Initial Radiation Output
Efficiency
Quantum
Mottle
Conversion efficiency of input screen
Total Brightness Gain
Common solution is to increase fluoro tube mA
Distance of observer from viewing sytem (inverse square law of light)
Beam attenuation by subject matter
(Results in an increased
patient dose rate)

16. Quantum Mottle
Another factor affecting quantum mottle is:
Time
How long the x-ray tube is energized by the fluoroscopist
The fluoro “mA” must be high enough to avoid excessive mottle as perceived by the observer
This is because the human eye needs 0.2 seconds to accumulate or integrate enough visible light photons from the fluoro imaging chain for an image to be percieved
The minimum amount of time needed for a “look” is 0.2 seconds

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What’s Next?
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Presented by
Based on:
Principles of Radiographic Imaging, 4th Ed.
By: R. Carlton & A. Adler
Radiologic Science for Technologists, 8th Ed.
By: S. Bushong
Syllabus on Fluoroscopy Radiation Protection, 6th Rev.
By: Radiologic Health Branch – Certification Unit