BASIC FLUOROSCOPY REVIEW

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BASICFLUOROSCOPYREVIEW
WEEK 4 RTEC 244
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Topics

• Example of fluoroscopy systems
• Image intensifier component and parameters
• Image intensifier and TV system

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Fluoroscopy a see-through operation with motion

• Used to visualize motion of internal fluid,
  structures
• Operator controls activation of tube and position
  over patient
• Early fluoroscopy gave dim image on fluorescent
  screen
• Physician seared in dark room
• Modern systems include image intensifier with
  television screen display and choice of recording
  devices

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Conventional Fluoroscopy
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Older Fluoroscopic Equipment(still in use in
some countries)
Staff in DIRECT beam Even no protection
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Direct Fluoroscopy obsolete
In older fluoroscopic examinations radiologist
stands behind screen and view the
picture Radiologist receives high exposure
despite protective glass, lead shielding in
stand, apron and perhaps goggles
Main source staff exposure is NOT the patient but
direct beam
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Fluoroscopy

• X-ray transmitted trough patient
• The photographic plate replaced by fluorescent
  screen
• Screen fluoresces under irradiation and gives a
  life picture
• Older systems direct viewing of screen
• Nowadays screen part of an Image Intensifier
  system
• Coupled to a television camera
• Radiologist can watch the images live on
  TV-monitor images can be recorded
• Fluoroscopy often used to observe digestive tract
• Upper GI series, Barium Swallow
• Lower GI series Barium Enema

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Conventional Fluoroscopic Unit
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Photons used Fluoro vs Radiography
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Conventional Fluoroscopy
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Light Levels and Fluoroscopy
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Modern Image Intensifier based fluoroscopy system
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Modern fluoroscopic system components
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Modern Fluoroscopic Unit
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Image Intensifier
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Functioning of Image Intensifier
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Image intensifier component

• Input screen conversion of incident X Rays into
  light photons (CsI)
• 1 X Ray photon creates ? 3,000 light photons
• Photocathode conversion of light photons into
  electrons
• only 10 to 20 of light photons are converted
  into photoelectrons
• Electrodes focalization of electrons onto the
  output screen
• electrodes provide the electronic magnification
• Output screen conversion of accelerated
  electrons into light photons

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Intensifier Flux Gain
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Image intensifier parameters (I)

• Conversion coefficient (Gx) the ratio of the
  output screen brightness to the input screen dose
  rate cd.m-2?Gys-1
• Gx depends on the quality of the incident beam
  (IEC publication 573 recommends HVL of 7 ? 0.2 mm
  Al)
• Gx depends on
• the applied tube potential
• the diameter (?) of the input screen
• I.I. input screen (?) of 22 cm ? Gx 200
• I.I. input screen (?) of 16 cm ? Gx 200 x
  (16/22)2 105
• I.I. input screen (?) of 11 cm ? Gx 200 x
  (11/22)2 50

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Intensifier Performance

• Conversion factor is the ratio of output
  phosphor image luminance (candelas/m2) to x-ray
  exposure rate entering the image intensifie
  (mR/second).
• Very difficult to measure access to output
  phosphor
• No absolute performance criteria

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Intensifier Brightness Gain (BG)

• BG Minification Gain x Flux Gain
• Minification gain (MG) The ratio of the squares
  of the input and output phosphor diameters. This
  corresponds to concentrating the light into a
  smaller area, thus increasing brightness
• MG (Input Diamter )2 / (Output Diameter)2

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Intensifier Brightness Gain (cont)

• Flux Gain (FG) Produced by accelerating the
  photoelectrons across a high voltage (gt20 keV),
  thus allowing each electron to produce many more
  light photons in the output phosphor than was
  required to eject them from the photcathode.
• Summary Combining minification and flux gains

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Intensifier Brightness Gain (cont)

• Example
• Input Phosphor Diameter 9
• Output Phosphor Diamter 1
• Flux Gain 75
• BG FG x MG 75 x (9/1)2 6075
• Typical values a few thousand to gt10,000 for
  modern image intensifiers

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Fluoroscopic Noise (Quantum Mottle)

• Fluoroscopic image noise can only be reduced
  by using more x-ray photons to produce image. Can
  possibly accomplish in 3 ways
• Increase radiation dose (bad for patient dose)
• Frame-averaging
• forms image using a longer effective acquis time
• Can cause image lag (but modern methods good)
• Improve Absorption Efficiency of the input
  phosphor

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Conventional Input Phosphor
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Cesium Iodide (CsI) Phosphor
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Viewing Fluoroscopic Images
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Image intensifier parameters (IV)

• Overall image quality - threshold contrast-detail
  detection
• X Ray, electrons and light scatter process in an
  I.I. can result in a significant loss of contrast
  of radiological detail. The degree of contrast
  exhibited by an I.I. is defined by the design of
  the image tube and coupling optics.
• Spurious sources of contrast loss are
• accumulation of dust and dirt on the various
  optical surfaces
• reduction in the quality of the vacuum
• aging process (destruction of phosphor screen)
• Sources of noise are
• X Ray quantum mottle
• photo-conversion processes, film granularity,
  film processing

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TV camera and video signal (II)

• Older fluoroscopy equipment will have a
  television system using a camera tube.
• The camera tube has a glass envelope containing a
  thin conductive layer coated onto the inside
  surface of the glass envelope.
• In a PLUMBICON tube, this material is made out of
  lead oxide, whereas antimony trisulphide is used
  in a VIDICON tube.

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TV camera and video signal (I)

• The output phosphor of the image intensifier is
  optically coupled to a television camera system.
  A pair of lenses focuses the output image onto
  the input surface of the television camera.
• Often a beam splitting mirror is interposed
  between the two lenses. The purpose of this
  mirror is to reflect part of the light produced
  by the image intensifier onto a 100 mm camera or
  cine camera.
• Typically, the mirror will reflect 90 of the
  incident light and transmit 10 onto the
  television camera.

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Automatic Brightness Control

• Monitoring Image Brightness
• Photocell viewing (portion of) output phosphor
• TV signal (voltage proportional to brightness)
• Brightness Control Generator feedback loop
• kVp variable
• mA variable/kV override
• kVmA variable
• Pulse width variable (cine and pulsed fluoro)

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Fluoroscopic Dose Rates
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Intensifier Format and Mag Modes
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Intensifier Format and Modes
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Vidicon (tube) TV Camera
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Type of TV camera

• VIDICON TV camera
• improvement of contrast
• improvement of signal to noise ratio
• high image lag
• PLUMBICON TV camera (suitable for cardiology)
• lower image lag (follow up of organ motions)
• higher quantum noise level
• CCD TV camera (digital fluoroscopy)
• digital fluoroscopy spot films are limited in
  resolution, since they depend on the TV camera
  (no better than about 2 lp/mm) for a 1000 line TV
  system

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Photoconductive camera tube
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Video Field Interlacing
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Vidicon Target Assembly
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TV Monitor
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Synchronization (Sync Signals)
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Different types of scanning
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INTERLACED SCANNING
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625 lines in 40 ms i.e. 25 frames/s
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PROGRESSIVE SCANNING
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TV RESOLUTION-Vertical

• Conventional TV 525 TV lines to represent entire
  image. Example 9 intensifier (9 FOV)
• 9 229 mm
• 525 TV lines/229 mm 2.3 lines/mm
• Need 2 TV lines per test pattern line-pair
• (2.3 lines/mm) /2 lines/line-pair 1.15 lp/mm
• Actual resolution less because test pattern bars
  dont line up with TV lines. Effective resolution
  obtained by applying a Kell Factor of 0.7.
• Example 1.15 x 0.7 Kell Factor 0.8 lp/mm

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TV RESOLUTION-Horizontal

• Along a TV line, resolution is limited by how
  fast the camera electronic signal and monitors
  electron beam intensity can change from minimum
  to maximum. This is bandwidth. For similar horiz
  and vertical resolution, need 525 changes (262
  full cycles) per line. Example (at 30
  frames/second)
• 262 cycles/line x 525 lines/frame x 30
  frames/second
• 4.2 million cycles/second or 4.2 Megahertz (MHz)

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TV camera and video signal (V)

• On most fluoroscopy units, the resolution of the
  system is governed by the number of lines of the
  television system.
• Thus, it is possible to improve the high contrast
  resolution by increasing the number of television
  lines.
• Some systems have 1,000 lines and prototype
  systems with 2,000 lines are being developed.

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Line pair gauges

• GOOD RESOLUTION POOR RESOLUTION

6 LP/MM AT SPOT CASSETTE 2
LP/MM AT TV
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TV camera and video signal (V)

• On most fluoroscopy units, the resolution of the
  system is governed by the number of lines of the
  television system.
• Thus, it is possible to improve the high contrast
  resolution by increasing the number of television
  lines.
• Some systems have 1,000 lines and prototype
  systems with 2,000 lines are being developed.

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Image intensifier parameters (II)

• Brightness Uniformity the input screen
  brightness may vary from the center of the I.I.
  to the periphery

Uniformity (Brightness(c) - Brightness(p)) x
100 / Brightness(c)

• Geometrical distortion all X Ray image
  intensifiers exhibit some degree of pincushion
  distortion. This is usually caused by either
  magnetic contamination of the image tube or the
  installation of the intensifier in a strong
  magnetic environment.

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Image (PIN CUSHION)distortion
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Different fluoroscopy systems

• Remote control systems
• Not requiring the presence of medical specialists
  inside the X Ray room
• Mobile C-arms
• Mostly used in surgical theatres.

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TV camera and video signal (VII)

• A series of electronic circuits move the scanning
  beams of the TV camera and monitor in
  synchronism. This is achieved by the
  synchronizing voltage pulses. The current, which
  flows down the scanning beam in the TV monitor,
  is related to that in the TV camera.
• Consequently, the brightness of the image on the
  TV monitor is proportional to the amount of light
  falling on the corresponding position on the TV
  camera.

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TV image sampling
IMAGE 512 x 512 PIXELS
HEIGHT 512
WIDTH 512
ONE LINE
VIDEO SIGNAL (1 LINE)
64 µs
52 µs
IMAGE LINE
SYNCHRO
12 µs
DIGITIZED SIGNAL
LIGHT INTENSITY
SAMPLING
SINGLE LINE TIME
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Digital radiography principle
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Digital Image recording

• In newer fluoroscopic systems film recording
  replaced with digital image recording.
• Digital photospots acquired by recording a
  digitized video signal and storing it in computer
  memory.
• Operation fast, convenient.
• Image quality can be enhanced by application of
  various image processing techniques, including
  window-level, frame averaging, and edge
  enhancement.
• But, the spatial resolution of digital photospots
  is less than that of film images.

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TV camera and video signal (VIII)

• It is possible to adjust the brightness and
  contrast settings of the TV monitor to improve
  the quality of the displayed image.
• This can be performed using a suitable test
  object or electronic pattern generator.

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RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
Thank you to the presentation by IAEA Training
Material on Radiation Protection in Diagnostic
and Interventional Radiology
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Where to Get More Information

• Physics of diagnostic radiology, Curry et al, Lea
  Febiger, 1990
• Imaging systems in medical diagnostics, Krestel
  ed., Siemens, 1990
• The physics of diagnostic imaging, Dowsett et al,
  ChapmanHall, 1998