Dynamic Imaging: Fluoroscopy Ch 10 Flashcards
ADC
analog-to-digital converter
- Device that take the video (analog) signal and divides into a number of bits ( 1s and 0s) that the computer “understands.”
- number of bits into which the signal is divided determines the contrast resolution (number of shades of gray) of the system
**output of the ADC is then transferred to main memory and is manipulated so that a digital image in matrix form is stored
ABC
automatic brightness control
automatically adjusts the kVp, mA (or both) to maintain overall appearance of the fluoroscopic image (contrast and density)
Tissue thickness
ABS = automatic brightness stabilizer
brightness gain
A brighter image is the result of high-energy e- striking a small output phosphor = Image Intensification
an expression of the luminance at the output phosphor divided by the input exposure rate
Acceleration & focusing of the electron beam in an II
Example: 150 x-rays strike the input phosphor and 150,000 light photons exit the output phosphor…what is the brightness gain?
BG = 1,000
camera tube
vacuum tube 6” in length that enclosed an electron gun and a photoconductive target assembly.
TV camera tube
Vidicon/Plumbicon
newer CCD
CCD
charge-coupled device
- light sensitive semiconducting device that generates a electrical charge when stimulated with light and stores this charge in a CAPACITOR.
continuous fluoroscopy
X-ray exposure continues without interruption while the exposure pedal/button is activated.
- 30 frames of fluoroscopy per second
- patient received a higher dose in continuous mode
electrostatic focusing lenses
negatively charged plates found along the length of the image intensifier tube.
repel the e- stream and focus on the small output phosphor
fluoroscopy
used to view the dynamic structures anatomy using x-rays.
flux gain
accelerating e- increase the light intensities at the output phosphor
brightness gain = minification gain x flux gain
image intensification
process in which the exit radiation from a patient interacts with the input phosphor for conversion to visible light – converted to e- by photocathode and e- are focused by lens – accelerated toward anode of the output phosphor and change to light to create a brighter image.
input phosphor
Made of cesium iodide and bonded to curved surface of the tube
- cesium iodide absorbs exit radiation and emits light in response
magnification mode
Improves spatial resolution
-increases patient dose
photocathode
Made of cesium and antimony compounds, these metals emit e- in response to light
(process called photoemission).
Photocathode is bonded to input phosphor using a very thin adhesive layer
Layers are curved = so the light/e- all travel the same distance to the output phosphor
output phosphor
coated with light-emitting crystals
- zine cadmium sulfide
light intensities from out phosphor are converted to an electrical video signal and sent to TV monitor to view.
pulsed fluoroscopy
Fluoroscopy mode
- x-ray exposure is NOT continuous
- has gaps of NO exposure
- can choose 2 - 15 frames per second
- patient dose can be reduced
spatial resolution
smallest structure that can be detected in an image
measured in lp/mm
- fluoroscopy has 4-6 lp/mm
Digital Fluoroscopy
Early digital units employ CCD attached to II
Newer models have FPD instead of II & use mA similar to radiographic mode (100 - 1000 mA)
Indirect = cesium iodide amorphous silicon detector (current most common)
Direct = amorphous selenium direct capture detector
Capable of 60 frames per second
Radiation Safety in Fluoroscopy
Table 10-1
Methods to Reducing Patient and Personnel Dose During Fluoroscopy
Key points:
- use lowest pulse mode/lower rate
- use last image hold
- keep fluoroscopy time to a minimum
- increase collimation
- remove grid
- minimize use of magnification mode
- document fluoroscopy time
- ABC units table exposure should not exceed 10R/min (5/R w/o ABC)
- SSD 15 “ for stationary fluoroscopy
- SSD 12” on mobile C-arm units
- Time - Distance - Shielding
- Bucky slot cover 0.25 mm of lead-eqiv
- Lead drape (curtain) 0.25 mm lead
Radiation Safety in Fluoroscopy
Table 10-1
Methods to Reducing Patient and Personnel Dose During Fluoroscopy
Key points:
- use lowest pulse mode/lower rate
- use last image hold
- keep fluoroscopy time to a minimum
- increase collimation
- remove grid
- minimize use of magnification mode
- document fluoroscopy time
- ABC units table exposure should not exceed 10R/min (5/R w/o ABC)
- SSD 15 “ for stationary fluoroscopy
- SSD 12” on mobile C-arm units
- Time - Distance - Shielding
- Bucky slot cover 0.25 mm of lead-equid
- Lead drape (curtain) 0.25 mm lead
- Personnel aprons 0.5 mm lead equivalent (when kVp is 100 or higher)
Conversion Factor
an expression of the luminance at the output phosphor divided by the input exposure rate, and its unit of measure is the candela per square meter per milliroentgen per second (cd/m2/mR/s)
Newer term used to express brightness
CF is approximately 1% of the brightness gain
Magnification Mode
Trifocus Image Intensifiers 30/23/15
When operated in magnification mode, the voltage to the electrostatic focusing lenses is increased. This increase tightens the diameter of the electron stream, and the focal point is shifted farther from the output phosphor
Beam collimation is adjusted = more radiation dose to the patient due to the smaller collimated field
Increased spatial resolution - ability to visualize smaller structures
Example: MF = 30/15 = 2 x mag