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BASIC FLUOROSCOPY REVIEW

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Fluoroscopy: a 'see-through' operation with motion. Conventional Fluoroscopy ... See more in Lecture L20. Digital Image recording ... – PowerPoint PPT presentation

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Title: BASIC FLUOROSCOPY REVIEW


1
BASICFLUOROSCOPYREVIEW
WEEK 4 RTEC 244
2
Topics
  • Example of fluoroscopy systems
  • Image intensifier component and parameters
  • Image intensifier and TV system

3
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

4
Conventional Fluoroscopy
5
Older Fluoroscopic Equipment(still in use in
some countries)
Staff in DIRECT beam Even no protection
6
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
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
Conventional Fluoroscopic Unit
9
Photons used Fluoro vs Radiography
10
Conventional Fluoroscopy
11
Light Levels and Fluoroscopy
12
Modern Image Intensifier based fluoroscopy system
13
Modern fluoroscopic system components
14
Modern Fluoroscopic Unit
15
Image Intensifier
16
Functioning of Image Intensifier
17
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

18
Intensifier Flux Gain
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
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

21
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

22
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

23
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

24
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

25
Conventional Input Phosphor
26
Cesium Iodide (CsI) Phosphor
27
Viewing Fluoroscopic Images
28
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

29
(No Transcript)
30
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.

31
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.

32
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)

33
Fluoroscopic Dose Rates
34
Intensifier Format and Mag Modes
35
Intensifier Format and Modes
36
Vidicon (tube) TV Camera
37
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

38
Photoconductive camera tube
39
Video Field Interlacing
40
Vidicon Target Assembly
41
TV Monitor
42
Synchronization (Sync Signals)
43
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
44
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

45
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)

46
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.

47
Line pair gauges
  • GOOD RESOLUTION POOR RESOLUTION

6 LP/MM AT SPOT CASSETTE 2
LP/MM AT TV
48
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.

49
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.

50
Image (PIN CUSHION)distortion
51
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.

52
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.

53
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
54
Digital radiography principle
55
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.

56
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.

57
RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
Thank you to the presentation by IAEA Training
Material on Radiation Protection in Diagnostic
and Interventional Radiology
58
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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