Fundaments of Fluoroscopic Imaging

Fluoroscopy

The production of dynamic (moving) radiographic images in real time (as they occur)

Invented by Thomas Edison in 1896

• one year after Roentgen discovered x-ray 

The original Fluoroscope

Consisted of a metal cone with a fluorescent screen in the bottom and a viewing window in the top

Open Fluoroscopy

a large intensifying screen was positioned behind patient in a completely darkened room - the radiologist had to dark-adapt his eyes to the very dim image

*exposure to both the patient and the radiologist (whose head was directly in the remnant x-ray beam) was high.

Where should x-ray tube be…

underneath patient is best

Where should image intensifier be…

Above patient, as close as possible 

Mobile Image Intensification (C-Arm)

Less scatter radiation is delivered to personnel by placing the image intensifier above the patient, with the x-ray tube under the table.

Carbon Fiber tables allow significant reductions in patient exposure 

Note 

tube be 12 inches away from the patient… NEED to bring i.i. down during procedures. 

Bringing the i.i. closer also improves spatial resolution (reduces magnification)

Reminder …

The patient is our source of dose - where is their scatter direction

RF System…

capable of both radiography and fluoro imaging 

Fluoro detector may be under the patient (so tube on top) 

Very common fluoro room of today

can do static and dynamic imaging 

Development of Improving Dose 

Image intensifier Tube:

invented by John Coltman in 1948

Converted the x-ray image into an electron bean that could be accelerated and focused for intensification 

Dramatically reduced the mA required, saving patient dose 

The image Intensifier tube

Purpose - convert the remnant radiation beam into an amplified light image

• change one form of energy into another 

A large evacuated glass envelope, or vacuum tube, containing 5 electronic components 

• the vacuum adds its efficiency

Available in diameters of 15, 23, 30, or 40 cm (6, 9, 12 or 16 inches) at the input phosphor

The output phosphor is always 2.5 cm in diameter (always 1 inch)

Concentration of the beam from the input to the output phosphor increases brightness by thousands of times

Increasing the physical diameter of the input phosphor results in increased image resolution, since a larger number of pixels are focused onto the same output size, forcing the output pixel size to be reduced.

Mag mode increases spatial resolution too = reduced FOV

The Input Phosphor 

Primary Barrier 

A layer of fluorescent crystals of cesium iodide (beneficial) 

Absorbs x-rays and re-emit energy as yellow-green light 

• changes x-ray photons to light photons 

Needle shaped crystals helps prevent lateral dispersion of light

The Photocathode

A thin photoemissive metal layer, made of a combination of antimony and cesium compounds 

Upon absorbing light energy, emits electrons 

• utilizes the photoelectric effect to do so

Thus, a beam of millions of electrons is produced

(x-rays go into photocathode, electrons come out of photocathodes)

The Electrostatic Focusing Lens 

A series of positively - charged metal rings that become narrower toward the anode end of the tube.

Stronger positive charge on each narrower ring focuses the electron beam onto the output phosphor.

Concentrating the electrons increases output intensity 

• This process is call minification

Minification Gain =

Input phosphor diameter ² / Output phosphor diameter ²

The Accelerating Anode

A small ring of metal in the “Neck” of the tube with 25,000-30,000 volts of positive charge applied to it.

Resulting acceleration of electrons imparts 50-75 times the kinetic energy they start with 

This gain in “flow” or flux gain also increases output intensity 

Flux - Flow of light 

• The number of light photons emitted by the output phosphor to the number of x-rays that struck the input phosphor 
  

Flux Gain =

# of output light photons / # of input x-ray photons

The Output Phosphor

ALWAYS 2.5 cm; about 1 inch in diameter

Made of cesium iodide or zinc cadmium sulfide 

Converts the kinetic energy of the electrons striking it into light 

This light image is then picked up by a CCD or CMOS camera