Fluoroscopic Equipment Flashcards
Image intensifier: structure/function of glass or metal envelope
- Contains all components of image intensifier tube
- Maintains vacuum
Image intensifier: structure/function of input phosphor
- Coated with CsI
- Converts x-rays into light
- Bonded to photocathode
I.I.: structure/function of photocathode
- Made of antimony and cesium compounds
- Receives light photons from input phosphor and emits photoelectrons (photoemission)
- Many light photons are required to produce one photoelectron
- Electrons contain the fluoroscopic image
I.I.: structure/function of electrostatic focusing lenses
- Negatively charged electrodes/plates that line the inside of the II tube
- Negative charge accelerates and focuses the electron stream
- Focuses wide stream of electrons into narrow one so it can fit through the hole in the anode – charge on plates can affect the optical focus/focal point
I.I.: structure/function of anode
- Positively charged circular plate with a hole in the middle
- Attracts photoelectrons from photocathode
I.I.: structure/function of output phosphor
- Receives photoelectrons and converts them into light photons
- Each photoelectron emits is converted into 50-75x more light than what was originally received by the photocathode
I.I.: What is the focal point?
When the electrostatic focusing lenses narrow the electron beam, they reach a point where it inverts itself, this is the optical focus/focal point.
Which two components of the image intensifier produce light?
Input phosphor and output phosphor
Describe the path of x-rays from when they enter the input phosphor to when they exit the output phosphor, including the conversions.
X-rays →Input phosphor → {light photons} → photocathode → {electrons}→ electrostatic focusing lenses → Anode → Output phosphor → {light photons}
An II is struck with 250 x-ray photons and emits 500 000 light photons. What is the flux gain?
Flux gain = Output photons/Input photons
500,000/250 = 2000
What is the minification gain for an II with an input phosphor diameter of 25 cm and an output phosphor diameter of 2.5 cm?
Minification gain = (Di/Do)2
252 = 625 2.52 = 6.25
625/6.25 = 100
A 23cm II has an output phosphor size of 5cm and a flux gain of 500. What is its brightness gain?
Minification gain: 232 = 529 52 = 25 529/25 = 21.16
Brightness gain: 21.16 x 500 = 10580
what is the flux gain for an II that emits 550 000 light photons when struck with 125 x-ray photons?
4400
True or false: The primary benefit of fluoroscopy is the ability to assess structures.
False- assessment of structure and function
How is magnification accomplished in the II?
By adjusting the voltage on the electrostatic lenses
◦Focal point position moves closer to the input screen
◦Image is magnified because it still fills the entire screen on monitor
What are common sizes of magnification levels in II?
25cm/17cm
or
25cm/17cm/12cm
How is patient dose impacted by magnification? Why?
Increased patient dose, because decreasing # of photoelectrons reaching output phosphor means mA is increased to maintain adequate brightness
flux gain formula
(# of output light photons) ÷ (# of input x-ray photons)
minification gain formula
(di ÷ do)2
di = diameter input phosphor
do = diameter output phosphor
brightness gain formula
minification gain × flux gain
benefits of magnification mode
- Better spatial resolution
- Better contrast resolution
Magnification factor formula
(full field diameter) ÷ (magnification field)
patient dose formula for magnification:
dose = (full field diameter)2 ÷ (magnification field)2
what is veiling glare in fluoroscopy?
◦This is when internal scatter radiation can reduce the contrast of image intensifiers
◦Light from bright image areas spreads into dark image areas, reducing contrast
◦Scatter radiation can include x-rays, electrons, light photons
◦Modern equipment reduces veiling glare by modifying aspects of the output phosphor
