RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• L16.1 Optimization of protection in fluoroscopy

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Introduction

• Subject matter fluoroscopy equipment and
  accessories
• Different electronic component contribute to the
  image formation in fluoroscopy.
• Good knowledge of their respective role and
  consistent Quality Control policy are the
  essential tools for an appropriate use of such
  equipment.

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Topics

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

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Overview

• To become familiar with the components of the
  fluoroscopy system (design, technical parameters
  that affect the fluoroscopic image quality and
  Quality Control).

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Part 16.1 Optimization of protection in
fluoroscopy
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 1 Example of fluoroscopy systems

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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
• Modern systems include image intensifier with
  television screen display and choice of recording
  devices

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Fluoroscopy

• X-ray transmitted trough patient
• The photographic plate replaced by fluorescent
  screen
• Screen fluoresces under irradiation and gives a
  live image
• Older systems direct viewing of screen
• 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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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 of staff exposure is NOT the patient
but direct beam
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Older Fluoroscopic Equipment(still in use in
some countries)
Staff in DIRECT beam
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Direct fluoroscopy

• AVOID USE OF DIRECT FLUOROSCOPY
• Directive 97/43Euratom Art 8.4.
• In the case of fluoroscopy, examinations without
  an image intensification or equivalent techniques
  are not justified and shall therefore be
  prohibited.
• Direct fluoroscopy will not comply with BSS
• performance of diagnostic radiography and
  fluoroscopy equipment and of nuclear medicine
  equipment should be assessed on the basis of
  comparison with the diagnostic reference levels

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Modern Image Intensifier based fluoroscopy system
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Modern fluoroscopic system components
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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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Different fluoroscopy systems

• Interventional radiology systems
• Requires specific safety considerations.
• In interventional radiology the physician can be
  near the patient during the procedure.
• Multipurpose fluoroscopy systems
• Can be used as a remote control system or as a
  system to perform simple interventional procedures

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Part 16.1 Optimization of protection in
fluoroscopy
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 2 Image Intensifier component and
  parameters

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The image intensifier (I.I.)
I.I. Input Screen
Electrode E1
Electrode E2
Electrode E3
Electrons Path
I.I.Output Screen
Photocathode

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Image intensifier systems
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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 focusing of electrons onto the
  output screen
• electrodes provide the electronic
  de-magnification
• Output screen conversion of accelerated
  electrons into light photons

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Image intensifier parameters (I)

• Conversion coefficient (Gx) the ratio of the
  output screen brightness to the input screen dose
  rate
• Gx cd.m-2?Gys-1
• 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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Image intensifier parameters (II)

• Brightness Uniformity the input screen
  brightness may vary from the center of the image
  intensifier 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 a local
  magnetic field.

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Image distortion
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Image intensifier parameters (III)

• Spatial resolution limit the value of the
  highest spatial frequency that can be visually
  detected
• it provides a sensitive measure of the state of
  focusing of a system
• it is quoted by manufacturer and usually measured
  optically and under fully optimized conditions.
  This value correlates well with the high
  frequency limit of the Modulation Transfer
  Function (MTF)
• it can be assessed with a resolution pattern
  which contains several sets of patterns at
  various frequency

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Line pair gauges
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Line pair gauges

• GOOD RESOLUTION POOR RESOLUTION

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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 (deterioration of phosphor screen)
• Sources of noise are
• X Ray quantum mottle
• photon-conversion processes, film granularity,
  film processing

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Image intensifier parameters (V)

• Overall image quality can be assessed using a
  suitable threshold contrast-detail detectability
  test object which comprises an array of
  disc-shaped metal details and gives a range of
  diameters and X Ray transmission
• Sources of image degradation such as contrast
  loss, noise and unsharpness limit the number of
  details that are visible.
• If performance is regularly monitored using this
  test, any sudden or gradual deterioration in
  image quality can be detected as a reduction in
  the number of low contrast or small details.

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Overall image quality
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Part 16.1 Optimization of protection in
fluoroscopy
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 3 Image Intensifier and TV system

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Image intensifier - TV system

• Output screen image can be transferred to
  different optical displaying systems
• conventional TV
• 262,5 odd lines and 262,5 even lines generating a
  full frame of 525 lines (in USA)
• 625 lines and 25 full frames/s up to 1000 lines
  (in Europe)
• interlaced mode is used to prevent flickering
• Cine film
• 35 mm film format from 25 to 150 images/s
• photography
• roll film,105 mm max 6 images/s
• film of 100 mm x 100 mm

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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 motion)
• 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 c/mm) for a 1000 line TV
  system

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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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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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Photoconductive camera tube
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TV camera and video signal (III)

• The surface of the photoconductor is scanned with
  an electron beam and the amount of current
  flowing is related to the amount of light falling
  on the television camera input surface.
• The scanning electron beam is produced by a
  heated photocathode. Electrons are emitted into
  the vacuum and accelerated across the television
  camera tube by applying a voltage. The electron
  beam is focussed by a set of focussing coils.

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TV camera and video signal (IV)

• This scanning electron beam moves across the
  surface of the TV camera tube in a series of
  lines.
• This is achieved by a series of external coils,
  which are placed on the outside of the camera
  tube. In a typical television system, the image
  is formed from a set of 625 lines. On the first
  pass the set of odd numbered lines are scanned
  followed by the even numbers. This type of image
  is called interlaced.
• The purpose of interlacing is to prevent
  flickering of the television image on the
  monitor, by increasing the apparent frequency of
  frames (50 half frames/second).
• In Europe, 25 frames are updated every second.

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Different types of scanning
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INTERLACED SCANNING
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13
2
3
15
14
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625 lines in 40 ms i.e. 25 frames/s
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17
16
7
6
19
18
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9
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21
10
1
2
3
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5
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PROGRESSIVE SCANNING
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12
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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 or 2,000

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TV camera and video signal (VI)

• Many modern fluoroscopy systems used CCD (charge
  coupled devices) TV cameras.
• The front surface is a mosaic of detectors from
  which a signal is derived.
• The video signal comprises a set of repetitive
  synchronizing pulses. In between there is a
  signal that is produced by the light falling on
  the camera surface. The synchronizing voltage is
  used to trigger the TV system to begin sweeping
  across a raster line.
• Another voltage pulse is used to trigger the
  system to start rescanning the television field.
  

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Schematic structure of a charged couple device
(CCD)
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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
  video display 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
ANALOGUE SIGNAL
I
t
ADC
Memory
DIGITAL SIGNAL
Iris
Clock
t
See more in Lecture L20
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Digital Image recording

• In newer fluoroscopic systems film recording is
  replaced with digital image recording.
• Digital photospots are 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.

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Summary

• The main components of the fluoroscopy imaging
  chain and their role are explained
• Image Intensifier
• Associated image TV system

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Where to Get More Information

• The Essential Physics of Medical Imaging. JT
  Bushberg, JA Seibert, EM Leidholdt, JM Boone.
  Lippincott Williams Wilkins, Philadelphia, 2011
• The physics of diagnostic imaging, Dowsett et al,
  Hodder Arnold, 2006
• Interventional Fluoroscopy Physics, Technology,
  Safety, S. Balter, Wiley-Liss, 2001