Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 13
• Viewing and Recording the Fluoroscopic Image

George David Associate Professor Department of
Radiology Medical College of Georgia
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Fluoroscopic TV

• Closed circuit TV
• signal from camera to monitor carried by
  electrical cables
• Components
• camera
• camera control unit (may be part of camera)
• monitor (TV without tuner or sound)
• interconnecting cables
• Image Tube - TV Coupling
• lens
• fiber optic

TV Monitor
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TV Image

• Composed of discrete horizontal scan lines
• lines independent of monitor size
• broadcast TV standard
• 525 lines
• High definition
• 1025 lines
• becoming more popular
• more expensive

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TV Camera

• converts light to coded electrical signal
• Camera Tube
• vidicon
• cheapest / compact / laggy
• plumbicon
• enhanced vidicon / less lag
• CCD
• Semiconductor
• not a tube

electrical signal
Light
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Vidicon TV Pick-up Tube
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Charge Coupled Device (CCD)

• semiconductor device
• not vacuum tube
• stores charge in small discrete wells
• transfers charge to readout device one line at a
  time
• no readout beam, focusing or deflection coils
• shorter than vidicons
• resolution limited by number of scan lines
  (element array)

7
Monochrome Television Monitor

• Another vacuum tube
• electron beam scanned along fluorescent screen

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Monochrome TV Monitor Elements

• electron gun
• control grid
• controls beam intensity at each location
• anode
• fluorescent screen
• external focusing coil
• external deflection coils (steer electron beam)

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CRT
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Television Scanning

• beam scanning for standard TV
• 525 lines in total image
• 30 images (frames) scanned per second
• Oscillators
• Vertical
• Horizontal

Vertical (Slower)
Horizontal (Faster)
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TV Interlacing

• frame scanned in two passes, first the odd lines
  then the even
• One frame takes 1/30th second
• avoids flicker
• each pass called a field
• takes 1/60th second

1st Field (Odd lines)
2nd Field (Even lines)
One Frame


1/30th sec.
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Progressive Scanning

• progressive scanning
• used on newer systems, lines scanned in order
• no interlacing

13
Synchronization

• TV Camera Monitor must be synchronized
• beam at same place on image
• special sync pulses sent at end of each
  horizontal line vertical field
• retrace
• horizontal retrace
• beam returned to left side of screen
• vertical retrace
• beam returned to the top of screen
• Turns off video during retrace

Horizontal Retrace
Vertical Retrace
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Video Signal

• Voltage level indicates brightness
• Blanking during non-video
• retrace

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Vertical Resolution

• proportional to of vertical scan lines
• theoretic maximum
• half of visible scan lines
• black lines alternate with white
• max. line pairs video lines / 2

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Vertical Resolution

• actual limit lower than theoretical
• 10 of lines occur during retrace
• returning beam from bottom to top of image
• scan lines may not perfectly synchronize to high
  resolution object
• typically 525 lines yield 185 line pairs

Vertical Retrace
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Bandwidth (Bandpass)

• Highest frequency that video components are
  designed to transmit or display
• no sharp frequency cutoff
• not all frequencies transmitted or displayed with
  same quality
• Gradual degrading

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Bandwidth (Bandpass)

• What it means for video
• camera
• how fast camera can turn electrical signal on
  off
• monitor
• how rapid a change in incoming electrical signal
  monitor can display
• determines of pairs which can be displayed on
  one horizontal display line
• horizontal resolution

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Bandwidth

• cycles per scan line line pairs per scan line
• higher bandwidth

X
X
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Horizontal Resolution
Bandwidth Horizontal Resolution X Video
Lines X Frame Rate
cycles ------------ scan line
lines --------- frame
frames --------- sec
cycles ---------- sec

X
X

Bandwidth Horizontal Resolution
-------------------------------------------
Video
Lines X Frame Rate
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Resolution Summary

• Vertical resolution depends on
• of scan lines
• Horizontal resolution depends on
• bandwidth
• of scan lines
• frame rate
• Systems designed to yield approx. equal
  horizontal vertical resolution
• 4.5 MHz typical bandwidth for 525 line system
• higher bandwidth required for higher line rates

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Fluoro Resolution On TV Depends Upon

• TV resolution
• total lines
• Frame rate
• bandwidth
• Size of imaged field

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Overall TV Resolution Example

• typical 9 image tube
• typical 185 line pairs for 525 line TV system
• 185 line pairs 1 inch-----------------
  -- X -------------- .8 line pair / mm 9
  inches 25.4 mm
• Higher number is better

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Conventional TV Systems

• Fluoro Resolution
• 9 inch mode gt 0.8 line pairs / mm
• 6 inch mode gt 1.2 line pairs / mm
• 4 inch mode gt 1.6 line pairs / mm
• Conventional TV vs. Image tube
• TV resolution generally poorer than image tube
  (3-5 line pairs / mm typical for II)
• to view image tube at full resolution requires
  photographing output phosphor
• photospot camera (100 mm, 105 mm)
• cine camera

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Overall System Contrast

• vidicon reduces contrast by about 20
• monitor enhances contrast by up to 2X
• adjustable by operator
• brightness contrast controls

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Lag

• Description
• stickiness in image
• apparent during tower motion in fluoro
• not totally undesirable
• smoothes quantum noise
• Cause
• time required for image to build up decay on TV
  tube target
• Etc.
• vidicons exhibit more lag
• plumbicons display more quantum noise

More Lag Less Quantum Noise
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TV Gain

• Output electrical signal amplitude---------------
  -----------------------------light input
  amplitude

electrical signal
Light
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LCD Displays

• Backlight
• Two polarizers oriented in opposite directions
• If no liquid crystals or no voltage applied to
  liquid crystals, no light passes through 2nd
  polarizer

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LCD Displays

• Individual transistors control light transmission
  for each pixel
• Voltage applied to LCD causes it to
• twist light
• alters polarization to orient with 2nd polarizer

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Automatic Brightness Control

• automatic variation of fluoro technique based
  upon image brightness
• Technique depends on absorber

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Automatic Brightness Control

• brightness sensing
• photomultiplier tube sensing light from image
  tube output phosphor
  OR
• intensity of electrical signal from TV camera
• brightness signal sent to generator
• Generator adjusts technique to achieve desired
  brightness

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Automatic Brightness Implementations

• operator sets kVp / generator sets mA
• operator sets mA / generator sets kVp
• system sets both mA kVp
• system adjusts pulse width
• only on some cath lab equipment (expensive)

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Fluoro Recording

• Spot film (obsolete?)
• Record directly on x-ray film
• Upside down radiograph
• Light photography of II output (obsolete?)
• 100, 105 mm photospot camera
• cine
• Record TV image
• VCR
• Digital

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Spot Film

• Upside-down radiograph
• phototimed
• delay before exposure
• switch from fluoro to radiographicmode
• heat up filament
• speed up rotor (sometimes)
• move cassette into position

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Cine

• Viewed as motion picture
• records light image from output phosphor
• selectable frame rate
• 7.5/ 15, 30,60,120 frames/second
• requires secondary switching generator
• rates greater than 30 f/sec. only used in
  pediatrics
• patient exposure proportional to frame rate

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Recording TV Signal

• VCR Magnetic Tape
• Digital

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VCR Magnetic Tape

• VHS or SVHS
• limited tape shelf life (2 years)
• long access times
• limited bandwidth