Transcript Vertical Resolution

Fluoroscopy: Viewing Systems
TV Monitors
Video CRT Monitor
A video monitor (television tube) is a
cathode ray tube (CRT)
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Consists of:
A vacuum tube
Cable
TV Monitor
Electron gun
as part of cathode at back of
tube
Video signal
External coils
Fluorescent
coating
Video signal
is amplified phosphor
and transmitted
by
for focusing and steering
inside frontmonitor
screen
cable to the television
electron beam
Anode
It
is transformed back
plated onto
front
screen image
into
a visible
Electron Beam
• Image created as electron gun
produces stream or beam of
electrons from camera’s video
signal onto TV screen’s phosphor
– Intensity of electron beam modulated
by control grid attached to electron gun
• Electron beam focused onto output
fluorescent screen by external
electrostatic coils
– Scans across the output screen using
the exact same raster pattern as the
camera tube
Phosphor Crystals
Phosphor crystals emit light when struck by electrons and
transmit it as visual image through glass of screen to viewer
Phosphor crystals
•
Composed of linear crystals aligned
perpendicular to glass envelope to reduce
lateral light dispersion
•
Phosphor layer usually backed by thin layer of
aluminum, which transmits electron beam but
reflects light
4
Modulation of Signal
Video signal received by picture tube is modulated
Light intensity received
by camera tube
Video signal magnitude
Magnitude of video signal received from the
camera tube is directly proportional to light
intensity received by the camera tube
(vidicon, plumbicon, or orthicon)
Electron beam of CRT
monitor
Video signal modultion
Unlike camera tube, electron beam of CRT monitor
varies in amplitude or intensity in accordance with
modulation of video signal
Monitor Image Quality
Monitor quality is affected by the number of scan lines and the bandpass
of the TV camera system. Raster pattern from camera is presented on the monitor.
Video monitor is the most restrictive element in
the fluoro imaging chain resolution
525 line monitor is
capable of 1-2 lp/mm
1000-line system doubles
the spatial resolution
TV Camera & TV Monitor Image Quality
Overall image quality is affected by:
Horizontal
resolution
Vertical
resolution
Contrast
Brightness
In television-camera tube:
In television-picture tube:
As electron beam reads optical
signal, signal is erased.
As electron beam creates
television optical signal, it
immediately fades (hence the
term fluorescent screen).
Lag
Image Quality - Contrast
Contrast levels of a TV monitor can be
adjusted on the monitor itself
Contrast should be set as follows:
Darkest object in the scene just
below black level on the
monitor
Brightest objects of interest do
not completely saturate or
“white out” details of the
image
It is appropriate to adjust contrast and brightness control to maximize the
visibility of object even at the expense of increased noise
Image Quality - Brightness
Changes in brightness will affect image quality
The image will
become chalky white
and detail is lost
When the fluoroscope is
moved from the abdomen
to the chest, a sudden
surge of brightness will
flood the system
Image Quality - Brightness
• Brightness level can be manually increased
but will not improve image quality
Brightness levels controlled by:
• Usually brightness and contrast are adjusted
in combination:
automatic
brightness
control
(ABC)
Contrast is brought to
near maximum
Usually ABC will stabilize image brightness
and x-ray exposure factors
Brightness is adjusted
for satisfactory
luminance
Viewing Conditions
Viewing conditions change with viewing distance allowing the raster pattern
to blend from the viewer’s perspective
525-line pattern on
9” monitor has min.
viewing distance
of 37”
High resolution monitors
(over 1,000 lines) of the same
sizes may be viewed at closer
distances
525-line pattern on 17”
monitor has min.
viewing distance
of 70”
Viewer can adjust brightness and contrast of image at the
monitor itself (not the imaging chain)
Image Quality – Horizontal Resolution
Bandwidth or bandpass refers to total number of cycles per second
available for display by television camera and monitor electronics
•
•
This number will set and limit the resolving power (capability) of the TV camera
Product of scan lines, frame rate, and frequency rate
Image Quality – Horizontal Resolution
Horizontal resolution is ability to resolve image dots on each scan line
•
Frequency bandwidth is maximum number of samples per line per unit time
•
Increasing bandwidth will allow the camera to sample more often per second
versus
Increased bandwidth = increased horizontal resolution
Image Quality – Vertical Resolution
Vertical resolving power is ability of a TV system to resolve objects spaced
apart in the vertical direction (to resolve horizontal lines)
More lines
means
better resolution
512 scan lines on target
1024 scan lines on target
Image Quality – Vertical Resolution
Vertical resolution varies with:
Size of object
Diameter of input phosphor
Vertical resolution is, essentially, the vertical reproduction
of the image as seen from the output phosphor by the pick up tube
Vertical resolution lp/mm = number of horizontal lines across object
2 x diameter of object (mm)
Vertical Resolution - Kell Factor
The Kell Factor is the ratio between actual vertical resolution of TV monitor
(as specified in TV lines) and the number of horizontal scan lines
Kell Factor = vertical resolution
A component of
vertical resolution
Imaged phantom on left
side has increased Kell
Factor and consequently
better resolution
number of scan lines
Image Quality - Lag
Screen
lag is
an undesirable
yet from
Blurring of TV image when
fluoro
tower
is moved rapidly
useful
of vidicon tubes
one
areaproperty
to another
Lag occurs because it takes
time for the image to build
Fluoro
up and decay on vidicon
Tower
target globules
Charge Coupled Device (CCD)
CCD is a semiconducting device capable of storing a charge
from light photons striking a photosensitive surface
Sensitive component is a
layer of crystalline silicone
•
•
Mounted at output phosphor of image
intensifier tube and coupled by fiber
optics or lens system
Early 1980s – first CCD replaced the
TV camera in a video system
CCD – How it works
Light strikes the
crystalline silicone
(photoelectric
cathode) of the CCD
Electrons are released
proportionally to the
intensity of the incident light
Video signal is emitted in raster scanning pattern by
moving stored charges along P&N holes to edge of CCD
where they are discharged into a conductor
Semiconductors store this
charge in P&N holes, thus storing
charges in a latent form
Silicon is illuminated
and an electrical charge is
generated
CCD can then be
sampled pixel by
pixel (raster format)
Computers can then manipulate
the digital image
CCD Spatial Resolution
Spatial resolution of CCD determined by its:
Physical Size
Systems incorporating a
1024 matrix can produce
images with 10 lp/mm
Pixel Count
CCD Resolution
Higher sensitivity to light (detective
quantum efficiency or DQE) and lower level
of electronic noise than TV camera
• Results in higher signal-to-noise (SNR) ratio
and better contrast resolution
• Able to use a lower patient dose per image
Has linear response curve as compared to
other image receptors which have sigmoid
shaped curves
Enables imaging with low light levels
(less dose) possible with retained
contrast resolution
CCD Advantages





Extremely fast discharge time (no image lag or blooming)
• Useful for high speed imaging applications
Unlimited life span
Unaffected by magnetic fields
Linear response
Lower dose rates needed
HIGH:
 Spatial resolution
 Signal-to-noise ratio (SNR)
 Detective quantum efficiency (DQE)
NO:
 Warm up time required
 Spatial distortion
 Maintenance
What’s Next?
Please close this PowerPoint presentation,
and continue the lesson.
Presented by
Based on:
Principles of Radiographic Imaging, 4th Ed.
By: R. Carlton & A. Adler
Radiologic Science for Technologists, 8th Ed.
By: S. Bushong
Syllabus on Fluoroscopy Radiation Protection, 6th Rev.
By: Radiologic Health Branch – Certification Unit