Fluoroscopy/Heat Units Flashcards
Fluoroscopy
Real time dynamic imaging produced on a glass plate covered by a layer of phosphor material (emit light when stimulated). Require dark environment to view dim images
Fluoroscopy development
Thomas Edison - 1896
Image intensification
Few X-ray photons converted to any visible light photons.
- flux gain
Image intensifier
Like an X-ray tube containing: Glass envelope Input phosphor Photocathode Focusing lense Anode Output phosphor
Glass envelope (image intensifier)
Maintains the vacuum within
Input phosphor
Cesium iodide
- efficient at converting X-ray energy to visible light
- needle like crystals
- X-ray is “coming in” to the fluoroscopy tube
Photocathode
Antimony
Light to electron
Focusing lenses
Electrostatic - mutual repulsion
Anode
Positive charge, attracts electrons (shaped like a donut)
Cathode
Negative charge
- emits electrons
Output phosphor
Zinc cadmium sulfide
- efficient at converting electron energy to visible light
- last stage of the fluoroscopy tube
Conversion of energies across the image intensifier
X-ray to light (input phosphor)
Light to electron (photocathode)
Electron to light (output phosphor)
Resulting in 1 X-ray photon amplified to many light photons (the image is intensified)
Flux gain
The increase in light photons due to the conversion efficiency of the output phosphor.
Measures the electron to visible light conversion by the zinc cadmium sulfide.
Good news - flux gain
One electron will produce dozens of visible light photons
Bad news - flux gain
Visible light in the image formation process always results in decreased resolution due to light divergence.
As flux gain increases
Visible light image becomes brighter
Mutual repulsion
Using something negative to push (repel) something negative away
How many light photons are created by each photoelectron that strikes the output phosphor
50-75 (flux gain)
Electrons are accelerated from cathode to anode in the fluoro tube by
Kilovoltage (25-30kv)
Higher KV=higher electron energy=light photons
Minification gain
The result of many electrons leaving the relatively large input phosphor/photocathode impacting the relatively small output phosphor
Standard input phosphor/photocathode sizes are
6, 9, or 12 inches
Standard output phosphor size
1 inch
Mini fixation gain formula
Minification gain =
input phosphor diameter2/output phosphor diameter2
As minification gain increases the visible image becomes brighter
Total brightness gain
He much the image has been intensified by the image intensifier tube (output phosphor intensity measured in candela)
Brightness gain
= minification gain x flux gain
ABC
Automatic brightness control
- maintains a preset brightness level by automatically adjusting the exposure factors to compensate for varying subject
Fluoro imaging techniques
Very low mA (.5-5mA)
Higher KVP utilized
SSD (source to skin distance) in fluoro
Must be at least 15 inches for fixed fluoro
- minimize skin exposure/dose
Must be at least 12 inches for mobile fluoro
- c-arms for surgery/mobile have higher patient does
Quantum mottle - fluoroscopy
Can be a problem caused by not enough X-rays (mAs); image not fully formed.
Not enough photons hitting the input phosphor
Increasing KVP makes it MUCH worse
Magnification tubes
Visible voltage to electrostatic focusing lense
- as voltage increases the electrons are pushed closer to the input phosphor causing the image to be magnified to output phospho
Magnification formula
=total input phosphor diameter/diameter of input phosphor used
TV camera
Placed adjacent to the output phosphor I order to capture and transmit the output intensified image. Radiologist no longer has to view image directly from phosphor screen or be in the path of the primary beam
Two basic types of cameras in use
Vidicon
Plumbicon
Vidicon
Camera used with general fluoro
Plumbicon
Camera used with interventional fluoro
Splitter
Able to split signal from output phosphor to multiple components:
- monitor
- video
- digital video
- hard film
Changes in voltage o the electrostatic focusing lenses causes
The electrons to narrow of widen their stream
Common field sizes for angio
35/25/15 cm
The smaller the mode the more magnified the image
Mag modes increases scatter
Common field sizes for general
25/17 cm
The smaller the mode the more magnified the image
Mag modes increases scatter
Multiple imaging devices on fluoro
TV camera, spot films, cine camera, cassette
- all use the image as displayed on the output phosphor
Fluoro patient radiation protection
Exposure dose should not exceed 10R/min - typical range is from 1-3R/min
5 minute timer
Units must alarm at 5 minute intervals to alert radiologist/surgeon
Magnification fluoro
Causes increased dose.
As you magnify the image, patient does goes up
Occupational exposure
Fluoro is a techs #1 source of exposure
- scatter from patient
Lead aprons must be worn
Lead aprons fluoro
At least .5mm lead equivelant
Bucky slot cover/lead drapes
Must be at least .25mm lead equivelant
3 things for X-ray production
Source of free electrons
Acceleration of free electrons
Abrupt halting of high speed electrons
As high speed electrons are abruptly halted by dense anode target, their kinetic energy converts to:
Heat as thermal energy (99%)
Xray as electromagnetic energy (1%)
Heat
The kinetic energy of molecules (rapid motion = heat)
The enemy of electrical components which make them wear out
Heat
Calorie
The unit of heat.
The amount if heat required to raise the temperature of 1 gram of water 1 degree Celsius
Heat is transferred by 3 means
Conduction
Convection
Radiation
Conduction
The transfer of heat through a material by touching of ohysical contact of solid objects
Convection
The transfer of heat by the mixing of molecules in a liquid or gas
Radiation
The transfer of heat by the emission of infrared radiation
X-ray tubes are cooled primarily by radiation
X-ray tubes are cooled primarily by
Radiation
Modern X-ray machines are controlled to not allow overheating by
Computerized control
Older X-ray machines require a
Manual heat calculation
Heat units are found by the formula
KVP x MA x Time x rectification constant
Rectification constant
Voltage waveform
A more efficient voltage waveform produces what
More heat
Actual focal spot
Aka true focal spot
Area that high speed electrons strike the anode
Effective focal spot
Actual focal spot projected towards patient
Line focus principle
Effective focal spot will always be smaller than true
The modern standard anode angle is
12 degrees
As focal spot increases
Penumbra increases
Penumbra
Loss of detail
Small focal spot
Maximum image detail
Large focal spot
Decreased image detail
Focal spots are measured as
Effective focal spot
Approximate sizes are .5mm up to 2.0mm
Blooming of focal spot
Enlargement of the electron stream as it travels from cathode to anode
Due to mutual repulsion of electrons
Blooming increases as
mAs increases and vice versa
Why not always use a small focal spot
They correspond to the size of cathode filaments
Small filaments are mA limited
Sometimes we need a higher mA station
Tests to determine focal spot size
Line pair resolution
Star pattern test
Pinhole camera test
Effective focal spots up to .8mm
+/- 50%
Effective focal spots between .8 - .15mm
+/- 40%
Effective focal spots greater than 1.5mm
+/- 30%
Anode heel effect
Disparity of X-ray intensity
- more X-rays to cathode side, larger effective focal spot
- fewer X-rays to anode side, smaller effective focal spot
Maximum image detail is found where
At the anode side of the cray tube - smaller effective focal spot
Once heat units are calculated the tube rating chart tells you
How much time for tube to cool down
How long you have to wait before you can safely take the next exposure
Heat units
= KVP x mAs x rectification factor
