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