Fluoroscopy Flashcards
1
Q
Fluoroscopy
A
- Definition: Radiography in motion. Use of continuous x ray beam to visualize anatomy
- dynamic imaging of dynamic systems
- invented by Thomas Edison in 1896
2
Q
Fluoroscopy: Old System
A
- Old System : developed fluorescent screen with phospor crystals (ZnSO3).
- Screen emitted light when exposed to x rays (fluoresence)
- This old system was used until 1947. Not a good system!
- Why? The light was not bright at all. Increased dose, inefficient, poor spatial and contrast resolution
- Exams had to be done in very dark rooms
- Scotopic (night vision) required = not good
- ↓ visual acuity (recorded detail)
- ↓ contrast resolution
- ↑ dose
- use of rods in our retina to see
- dark adaptation time or integration time - sit in dark room for 20-30 mins prior to exam
3
Q
Fluoroscopy: New System: II tube
A
- invented 1948 - II tube invented
- Image Intensification tube - converts attenuated beam into a very bright image
- 10,000x brighter than the light image from the old conventional screens - no more dark rooms
- ↑ visual acuity - 10x
- ↑ contrast resolution
- ↓ dose
4
Q
Fluoro Image Chain
A
-
Fluoro Tower
- II Tube
- Fiber optics or ODS (Optical Distribution System)
- CCD (video camera)
- ADC (Analog Digital Convertor)
- DAC (Digital to Analog Convertor)
-
TV/Video Monitor
- where image is seen
- communicates with the fluoro tower via cables
-
Fluoro Unit Configuration
- U-arm
- C-arm
5
Q
Image Intensification tube structure
A
- Vacuum glass or metal tube (encased in fluoro tower)
- How does it work?
- exit xrays strike the input screen
- light is released
- light hits photocathode
- which then releases photoelectrons
- photoelectrons accelerated across the tube towards anode (avg KVP of II tube =25)
- photoelectrons are focused together by electrostatic lenses as they travel towards anode and output screen
- photoelectrons strike output screen, and their K.E. is converted into very bright light.
- Light is brighter than that of the input screen
- the light from the output screen is called the optical image (represents pattern of the attenuated x ray beam)
6
Q
Input Screen
A
- absorbs x rays coming from patient
- emits light (fluorescence)
- light pattern = attenuated x ray pattern = anatomical pattern of patient
- screen coated with a thin layer of Cesium Iodide (CsI)
- its thin because it increases spatial resolution
- screen diameter : 6 - 16 inches
- all or portion can be used during fluoro
7
Q
Photocathode
A
- absorbs light image from input screen
- electrons are released - electron pattern = light pattern = exit beam/pattern = anatomical/pattern in pt
- cathode coated with Cesium Antimony (CsSb)
- CsSb reacts with the light from the input screen by releasing electrons
- quantity of electrons at any point on photocathode is equal to the amount of light striking it
- spatial pattern of released electrons = patient image
- input screen and photocathode separated by a thin transparent sheet to prevent the two chemical crystals from interacting
- Photocathode is curved to increase spatial resolution and decrease distortion
8
Q
Electrostatic Lenses
A
- Focus electrons released by photocathode as they travel across the II tube (neg charged)
- cause electrons to converge to a single point (focal point)
- electrons then re-expand as they travel towards output screen
- once they re-expand the image, it is now reversed
9
Q
Anode
A
- Positive charge
- attracts electrons from photocathode causing them to speed across the II tube (25 KEV)
10
Q
Output Screen
A
- struck by electrons (pt image) from photocathode
- converts their KE into light (light image)
- light image = optical image
- this light image is much brighter than the light image that came off the input screen
- output screen coated with thin layer of CsI
- very thin aluminum screen just in front of the output screen to prevent back scatter of light. This helps the image, if not present the back scatter of the light would degrade the image
11
Q
Opitcal Distribution Screen (ODS)
A
- It’s job is to caputre, focus and direct the optical image off the output screen to the CCD (video camera)
- Old ODS - series of fully or partially silvered mirrors that have beam splitters.
- This takes light image off output screen, and sends image to two places at the same time
12
Q
How does the II tube take fairly low quantity amounts of x ray photons and make such a bright light?
Minification Gain
A
- Minification Gain
- Occurs when the same nuber of light photons produced at fairly lg diameter input screen (6-16”) are smooshed into the output screen which has diameter of less than 1 inch
- electrons smooshed together by electrostatic lenses
- electron pattern matches the light pattern off the input screen
- minification of image makes it brighter
- M.G = [Dia Input Screen/Dia Output Screen]^2
- ex M.G. = [12 in/1in]^2 = 144 x brightness gain
13
Q
How does the II tube take fairly low quantity amounts of x ray photons and make such a bright light?
Flux Gain
A
- A measurement of the efficiency of the CsI crystals of the output screen
- how good the CsI crystals are at converting the electrons from photocathode (their KE) into light
- Ex - If one electron strikes the output screen and 50 light photons are released, the flux gain would be 50
14
Q
II tube efficiency measurement
A
- How is it measured? 2 ways
- TBG, Conversion Efficiency
15
Q
Total Brightness Gain (TBG)
A
- TBG = (minification gain) x (flux gain)
- 144 x 50 = 7,200 x
- TBG = 7,200
- This means the light off the output screen is 7200 x brighter than that of the input screen.
- The level of brightness of light has a unit of measurement = candela (cd)
