Download RAD 254 Chapter 28 Digital Fluoroscopy

RAD 254 Chapter 27
Digital Fluoroscopy
Terms to remember
DSA digital subtraction angiography
Registration
Interrogation time
Hybrid subtraction
CCD
ROI
PACS
Advantages of DF
• Speed of image acquisition
• Post processing “tweaking”
– Spatial resolution is determined by the matrix size
(usually 1024 X 1024) and the size of the image
intensifier
• DF operates at “conventional mAs” (hundreds
of mA rather than less than 5 mA as
conventional fluoro)
• BUT DF operates in pulsed progressive fluoro
Pulsing terms
• Interrogation time = time to switched on
and reach the mA and kVp level
• Extinction time = time for the tube to be
switched off (usually times less than 1 ms)
Receptor
• The receptor is usually a “charge coupled
device”
– CCD’s are very sensitive to light and have a
much lower level of noise than TV camera
This results in much higher SNR than conventional TV
cameras/systems
Thy also have NO lag time or “blooming” and require NO
maintenance
CCD’s can be “docked” directly to the II’s output phosphor
Advantages of CCD’s
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High spatial resolution
High SNR
High Detective Quantum Efficiency (DQE)
No warm up required
No lag/blooming
No spatial distortion/maintenance
Unlimited life
Unaffected by magnetic fields
Lower dose
DR Subtraction
Two primary types
• Temporal subtraction
– Single kVp setting
– Normal filtration
– Good contrast resolution (1% @ 1mm)
– Simple arithmetic image subtraction used
– Motion artifacts are a problem
(misrepresentation)
– Total subtraction is able to be achieved
– Subtraction limited by number of images
• Energy Subtraction
– Rapid voltage switching is used
– Filter switching is preferred
– Higher x-ray energy used for contrast res.
– Complex image subtraction required
– Motion artifacts (misrepresentation) are
reduced
– Some residual bone is survived (shows)
– More types of subtraction are possible
• IF BOTH ARE COMBINED = HYBRID
DF/D subtraction dose
• DF & D Subtraction usually result in much
higher patient dose and PULSED
imaging is required to lower it!
• Storage and image distribution as already
discussed are utilized
Chapt 28 The Digital Image
Spatial resolution
Contrast resolution
Contrast-detail curve
Pt. Dose considerations
Spatial resolution
• -the ability of an imaging system to
RESOLVE and render on the image a
HIGH CONTRAST object (shades of gray
differences in close objects to one
another)
Spatial FREQUENCY
• Refers to LINE PAIR!
– Line pair per millimeter )lp/mm)
– The higher the spatial frequency, the HIGHER
the spatial resolution!
• In digital imaging, spatial resolution is limited by
PIXEL size.
Modulation Transfer Function
(MTF)
• The ability of an imaging system to render
objects of DIFFERENT sizes onto an
image (ratio of image to object due to
spatial resolution)
Contrast Resolution
• The HIGHEST possible contrast is TWO
shades of gray – Black and White!
Dynamic Range
• -is really the OD shown on the H & D
curve = ranges from base + fog (.2 or
lower) to the solarization point
• The number of shades of GRAY that an
imaging system can produce (remember
the human eye can only see about 30
different shades of gray!)
NOISE
• Image noise limits contrast resolution
(SNR)
• Post processing of the image allows for
manipulation and visualization of shades
of gray (window and leveling)
Patient dose
• Digital imaging should reduce pt. dose (in
reality, it had done just the reverse!) – the
“in in doubt, burn it out” syndrome and
post image manipulation – “dose creep!”
Misc. information
• Focal spot size determines spatial resolution in
film/screen systems
• Post image manipulation = contrast resolution in
digital regardless of pt. dose
• Pt. dose in digital should be LOW because of
DQE (probability of a photon interacting with an
image receptor due to atomic number, etc)
• DQE = measure of x-ray absorption efficiency