Equipment II- Lecture 15 (Fluoroscopy)
How is minification gain determined?
(Input screen diameter)^2 / (Output screen diameter)^2 OR Ratio of area of electrons emitted from the input screen to area of electrons received at output screen.
Describe the input screen of fluoro image intensifier.
.1-.2mm layer of sodium activated caesium iodide (Csl)
What is typical the mA range for fluoroscopic x-ray tubes?
.5-5.0 mA
What is the minimum source-to-skin distance?
12in (mobile equipment) and 15in (stationary equipment)
What is the minimum SOD in fixed fluoro equipment?
15 in
What are the options for a dual focus magnification tube?
23/15; (9in or 23cm; 6in or 15cm)
At what time must an audible alarm sound?
5 minutes
How does fluoro video work?
A camera attached to output phosphor, using CCD, then a display monitor for viewing.
Describe the output screen in fluoroscopy.
A glass fluorescent screen, made of zinc-cadmium sulfide. Emits light when struck by electrons.
Describe electrostatic lenses.
A series of negatively charged electrodes.
Describe fluoroscopic imaging.
A specialized x-ray tube with an IR called a fluoro screen that's viewed during the procedure, leading to an active diagnosis.
What do Electrostatic Lenses do?
Accelerate and focus electron pattern across tube to the anode.
What is used to record the Fluoroscopic Image?
Any recording media with adequate memory can be used, digital device, or DVDs.
Where is the highest energy scatter during fluoro?
At a 90 degree angle to incident beam, and at same level as tech and radiologist's gonads.
There is a wide variety of systems (and names) for maintaining Fluoroscopic Image Density and Contrast. What are some of them?
Automatic brightness control (ABC) Automatic brightness stabilization (ABS) Automatic dose control (ADS) Automatic gain control (AGS)
What is used to generate electronic signal? Where does it go after being generated?
CCD, then to analog to digital converter (ADC), which allows for post processing and electronic storage and distribution.
What impacts image quality in fluoro?
Contrast Resolution Distortion Quantum mottle
What does the Input Screen do?
Converts intercepted beam to light.
What kind of resolution can be produced by CSL image intensifiers and digital systems?
CsI image intensifiers capable of less than 2 lp/mm. Digital fluoroscopic systems achieve 2 lp/mm.
What reduces patient dose in fluoro?
Dynamic and static image recording reduces patient exposure.
What is a raster pattern?
Electron beam accelerated across TV camera tube to anode, slowed at anode by wire mesh in front of target. Discharged into pulses into conductor to a photosensitive surface to view.
What is the primary source of brightness gain?
Electrostatic Lenses
What does the photocathode do?
Emits electrons when struck by light emitted by input screen.
TRUE or FALSE: There are slightly different collimation rules for fluoro.
FALSE. They are the same.
What causes shape distortion in fluoro?
Geometric problems in shape of image intensifier input screen.
What are some other characteristics of digital fluoro imaging?
Good separation of acquisition and display, Last image hold function, Post exposure image processing.
What do Magnification Tubes do?
Greater voltage to electrostatic lenses. Increases acceleration of electrons. Shifts focal point away from anode. Causes image to be magnified at output screen.
What were original fluoroscopes like?
Hand held, or mounted to the table over the tube and patient, causing physicians to view in the primary beam, leading to major protection problems. Mirrors were eventually introduced to take them out of primary beam.
Describe a video monitor for fluoro.
High resolution output screen. Flat screen monitors require regular calibration & QA testing.
What eventually replaced the mirror viewer?
Image intensification items, such as a video camera and monitor.
List image intensification tube components:
Input screen and photocathode, electrostatic lenses, magnification tubes, anode and output screen, total brightness gain, minification gain x flux gain
What are some significant dates/people in the development of the fluoroscope?
Invented in 1896 by Edison, Image intensification tubes developed in 1948, digital predominated by 2005.
What is interrogation time?
Length of time required for generator to come on and achieve necessary kVp and mAs.
What is the tabletop exposure rate (patient exposure)?
Maximum= 10Rems/min; Typical= 1-3Rems/min
How is flux gain determined?
Measurement of increase of light photons due to conversion efficiency of output screen. So, how many light photons are emitted for each electron that strikes the output screen.
What is total brightness gain?
Measurement of the increase in image intensity achieved by an image intensification tube.
Describe C-arm fluoroscopic units used for Mobile Fluoroscopy.
Popular for surgery, operates the same as stationary units.
Describe the anode in fluoroscopy.
Positively charged, 25 kVp, hold in center to allow electrons to pass through to output screen.
What does image intensification do?
Provides higher visual acuity, using night vision. Electronically amplifies brightness of an image.
What affects size distortion in fluoro?
Same parameters as static radiography, primarily OID.
What is a Video Camera CCD?
Semiconducting device, Emits electrons in proportion to amount of light striking it, Fast discharge eliminates lag, Operate at lower voltages than video tubes, More durable than video tubes.
How to protect the radiographer and radiologist?
Single step away from table decreases exposure exponentially Bucky slot cover Lead rubber drape Radiologist as shielding
Shape distortion is not a problem with which type of detectors, and why?
TFT matrices, because uniform resolution across detector array.
In digital fluoro, what is the image intensifier output screen coupled with?
TFTs
Describe a C-arm.
These are not just in surgery. In the fluoro room, there is a mount for the unit, under/over table.
What is extinction time?
Time required to shut down generator in preparation for next pulse.
What should fluoro equipment not be used for?
To preview patient position. This extra dose is unethical.
Describe digital fluoroscopy.
Uses high-power generator, has pulse progressive fluoroscopy mode that pulses the x-ray production from the tube in synchronization with detect signal.
What affects resolution in fluoro?
Varies depending on geometric factors
What is the most common fluoro viewing system?
Video
What is Vignetting or pincushion effect (shape distortion)?
Where electrons being focused tightly together start to repel each other due to their common negative charges. Causes greater image intensity in center of image and less intensity on outside edge.
What other item do typical fluoro machines have?
a foot switch
How is contrast controlled in fluoro?
amplitude of video signal, and Digital systems use post-processing (window width and filtering algorithms).
There is a wide variety of systems (and names) for maintaining Fluoroscopic Image Density and Contrast. However, they all maintain the brightness of the image by:
automatically adjusting exposure factors as necessary according to subject density and contrast
What does quantum mottle look like in fluoro, and how is it remedied?
blotchy, grainy appearance, and increasing mA.
What is the name of the equipment that supports the C-arm?
carriage
What helps to reduce shape distortion?
concave shape
Why is resolution not as high in fluoro?
due to patient exposure concerns
Input screen diameter is the diameter used ______________________________.
during the exam, with collimation
In digital fluoro, what are the photodiodes connected to?
each pixel
Total brightness gain is determined by what 2 factors?
minification gain and flux gain
Brightness gain is equal to:
minification gain x flux gain
A smaller input screen causes more/less magnification.
more
What is used to lower noise?
pixel binning
What is used to cut down on quantum mottle in fluoro?
pulse progressive fluoroscopy mode
What can the carriage do to ensure accuracy?
raise/lower IR
What affects contrast in fluoro?
scattering ionizing radiation Penumbrial light scatter within intensification tube itself