appendix h: chapter 40: fluoroscopy. spot film camera (photospot) (105 mm in this case) spot film...
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Appendix H: Chapter 40: Fluoroscopy
Spot film camera (Photospot)(105 mm in this case)
Spot filmdevice
Cine (35 mm)camera
Videocamera
Image intensifier
Cine film. Rolls of 35mm film, hundredsof feet long, typically contained multipleprojections of the coronary arteries andoften the left ventricle.
Though most institutions have gone digital,many will still have cans of film and a cineprojector stashed in an archive.
Image intensifier
X-ray tube
Cine camera
Patient table
Videocameraor CCD
A C-arm fluoroscope
The transfer of energy through the fluoroscopic imaging chain
1. image intensifier, 2. the video camera, and 3. the CRT monitor
The three major pieces of equipment in the fluoroscopic imaging chain are the:
1. X-rays pass through the body or are attenuated, thusforming a pattern (aerial image) of the anatomy they pass through.
Photons from the X-ray tube
2. X-rays excite atoms of a fluorescent screen, light is given off. This is fluoroscopy in its simplest form but the image is very dim.
3. Light stimulates a photoemissive material to liberate electrons. Image pattern is maintained
4. Electrons are accelerated toward a tiny fluorescent screen at other end of image intensifier. Concentration of electrons and added kinetic energy make image tens of thousands of times brighter.
5. Optical lens focuses light
6. Video camera (old) or CCD(microchip, like in homecamcorders)translates image into electrical(video) signal
7. Computer assigns pulses from video signal values in binary code (0s and 1s) and stores them as image data
Video signal
8. Displayedon monitor
Video signal ImageIntensificationtube
How a Fluoroscopic Image Gets Intensified and Digitized
Image Intensifier (II)
Input PhosphorCsI
PhotocathodeCesium & Antimony
Glass envelope
Electrostatic focusinglens
Output PhosphorZinc Cadmium Sulfide Anode
25,000 V
Concave surface so all electrons arrive at the output screen at the same time, but causes vignetting.
The transfer of energy through the fluoroscopic imaging chain
Focal Point
For every one incident X-ray photon
1000 light photonsat the input phosphor
50 Photoelectronsat the photocathode
3000 light photonsat the output phosphor
Flux Gain
Photocathode
Electrostatic focusinglens
Output Phosphor(zinc-cadmium sulfide)
Anode25kV + potential
Electrons accelerated across the tube gain kinetic energy from theattractive force of the anode (conversion efficiency). The collision at the output screen liberates that energy in the form of more light photons
Minification Gain
Input Phosphor(CsI)
Output Phosphor(zinc-cadmium sulfide)
The ratio of the areas of the inputand output screens is expressed asthe minification gain.
1” diameter
9” diameter
92
12 = 81 times
Total Brightness Gain
The product of the flux gainand the minification gain is the total brightness gain.
If the flux gain were 70, andthe brightness gain 81
70 x 81 = 5670 total brightness gain
5000-30,000 is the range
Quantum MottleBecause the image intensifier makes the image on the output screen thousands of times brighter than the image on the input screen,much less radiation is needed.
If too few photons are used, theimage becomes grainy and unacceptable for diagnostic purposes.
Generally speaking a better image is always obtained by using more photons, but the price is paid in patient dose.
Conversion factor
The intensity of illumination at the output phosphor (candela per meter squared) to the radiation intensity that produced it (mR/s)Typical conversion factors of 50 to 300 relate to the 5000-30,000 BG
Veiling Glare
Scatter radiation from x-ray, electrons, and light
Multifield (Duel focus) Electronic Magnification
(As opposed to increased OID magnification)
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7”
9”
11”11” mode 7” mode
By increasing the positive charge on the electrostatic focusing lens,the convergence (focal) pointis changed (furtherfrom out put screen).
Duel focus or Electronic Magnification
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7”
9”
11”11” mode 7” mode
When the convergence point is further from the output screen, (blue) the photoelectrons have further to diverge, and the imagearriving at the output screen is larger.
Electronic magnificationcreates better resolution
But patient dose isincreased.
Automatic Brightness Control (ABC)
While doing fluoroscopy the .5 to 5 mA, and the kVp will be automatically adjusted to compensate for changes in part thickness, composition of the part as the fluoro tube is being moved. ABC adjusts the mA. Lag is evident, especially if the tube is moving fast.
ABC also increases patient dosewhen using electronicmagnification
Resolution Measuring the resolving power of imaging equipment using a line pair test tool
Measured in line pairs. One line and one space is a line pair
1 mm
= 1 line pair per millimeter of spatial resolution
A Line Pair Test Tool(for Testing Spatial Resolution)
The Test Tool provides line pairs of various sizes to measure spatial resolution
1 mm 1 mm 1 mm 1 mm 1 mm
Vidicon and Plumbicon camera tubes
Vidicon Camera
Tube
Window
Signal Plate
Target
Steering and deflecting coils
Globules
Electron gun(cathode)
Control grid
Anode
Video Signal(from signalplate)
The transfer of energy through the fluoroscopic imaging chain
Optical lensto focus light from II
Coupling of the II to the camera
* Fiber optics
* Lens coupling
* Beam splitting mirror allows photospot and camera filming
The transfer of energy through the fluoroscopic imaging chain
Optical lensto focus light from II
Lowintensity
High intensity
No
signal
Modulation
Line 1
Line 524
Interlaced Scanning
262 1/2 Odd Lines scanned first = Field 1
Line 525
262 1/2 Even Lines scanned first = Field 2
Line 2
2 Fields = 1 Frame
Why are densities reversed on the fluoro monitor* Low atomic densities =* Low attenuation =* Input phosphor glows brightly* Camera target highly excited. * Video signal is strong
* Light area on monitor
Radiography (Film or digtal) On Fluoro
* Low atomic densities =* Low attenuation =* Many photons interact with IR =
* Dark area on image
Comparison of the lowdensity (air filled)maxillary sinus as seenon film or digital monitor,and in standard fluoroscopic mode.
The transfer of energy through the fluoroscopic imaging chain
Optical lensto focus light from II
Lowintensity
High intensity
No
signal
Question: How is a conventional fluoroscopic, analog imaging chain converted to digital?
ALU
CUPrimaryMemory
SecondaryMemory
(RAM)
ADC
DAC
1011
* Both are radiographic exposures
* Both are taken during fluoroscopy
* Both are 9 x 9 inch films made especially for this purpose, (though some fluoroscopes use standard size cassettes)
* Both were filmed using the same device, but in different formats.
Spot films recorded on a spot film device
4on1
1on1
Spot film camera (Photospot)(105 mm in this case)
Spot filmdevice
Cine (35 mm)camera
Videocamera
Image intensifier
35 mm cine film. Real time motion, projected on a projector.
* Both were taken during fluoroscopy
* Both are fluoroscopic exposures (i.e. taken off the output phosphor of the II.)
* Both are serial films (not designed to be projected as a moving image, but taken in rapid sequence such as 1, 2, or 4 a second
Spot films (Photospots) recorded on a spot film (photospot) camera
* Both are filmed by the same camera but are different sizes.
105 mm
90 mm (cut film)
105 mmidentifiableby sprocketsthat drives the roll of film that is unique forthis size
Flat Screen Monitors