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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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Title: RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY


1
RADIATION PROTECTION IN DIAGNOSTIC
AND INTERVENTIONAL RADIOLOGY
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology
  • L16.1 Optimization of protection in fluoroscopy

2
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 an
    equipment.

3
Topics
  • Example of fluoroscopy systems
  • Image intensifier component and parameters
  • Image intensifier and TV system

4
Overview
  • To become familiar with the component of the
    fluoroscopy system (design, technical parameters
    that affect the fluoroscopic image quality and
    Quality Control).

5
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

6
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

7
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

8
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
9
Older Fluoroscopic Equipment (still in use in
some countries)
Staff in DIRECT beam Even no protection
10
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
    App.II.25
  • performance of diagnostic radiography and
    fluoroscopy equipment and of nuclear medicine
    equipment should be assessed on the basis of
    comparison with the guidance levels

11
Modern Image Intensifier based fluoroscopy system
12
Modern fluoroscopic system components
13
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.

14
Different fluoroscopy systems
  • Interventional radiology systems
  • Requiring specific safety considerations.
  • In interventional radiology the surgeon can be
    near the patient during the procedure.
  • Multipurpose fluoroscopy systems
  • They can be used as a remote control system or as
    a system to perform simple interventional
    procedures

15
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

16
The image intensifier (I.I.)
I.I. Input Screen
Electrode E1
Electrode E2
Electrode E3
Electrons Path
I.I.Output Screen
Photocathode

17
Image intensifier systems
18
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

19
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

20
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.

21
Image distortion
22
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 by the Hüttner resolution
    pattern which should contain several cycles at
    each frequency in order to simulate the
    periodicity

23
Line pair gauges
24
Line pair gauges
  • GOOD RESOLUTION POOR RESOLUTION

25
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

26
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 and/or small details.

27
Overall image quality
28
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

29
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
  • cinema
  • 35 mm film format from 25 to 150 images/s
  • photography
  • rolled film of 105 mm max 6 images/s
  • film of 100 mm x 100 mm

30
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31
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32
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

33
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.

34
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.

35
Photoconductive camera tube
36
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.

37
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.

38
Different types of scanning
11
1
INTERLACED SCANNING
12
13
2
3
15
14
5
625 lines in 40 ms i.e. 25 frames/s
4
17
16
7
6
19
18
8
9
20
21
10
1
2
3
4
5
6
7
PROGRESSIVE SCANNING
8
9
10
11
12
13
14
15
16
17
18
39
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.

40
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.

41
Schematic structure of a charged couple device
(CCD)
42
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.

43
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
44
Digital radiography principle
ANALOGUE SIGNAL
I
t
ADC
Memory
DIGITAL SIGNAL
Iris
Clock
t
See more in Lecture L20
45
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.

46
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.

47
Summary
  • The main components of the fluoroscopy imaging
    chain and their role are explained
  • Image Intensifier
  • Associated image TV system

48
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
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