LECTURE 7
Prepared by:-
KAMARUL AMIN BIN ABDULLAH @ ABU BAKAR
UiTM Faculty of Health Sciences Medical Imaging Department LESSON OBJECTIVES At the end of the session, the students should be able to: Briefly explain the purpose, construction and principles of fluoroscopy. Briefly explain the image intensifier, the principles and its construction. List the components in II and explain the function of each components. Briefly explain its viewing and recording system.
10/18/2012 Kamarul Amin (c) 2 INTRODUCTION
Fluoro: is dynamic radiographic examination. Fluoroscopy is primarily domain of the radiologist. However, the role of radiographer to assist and routine post-fluoroscopic radiography. Fluoroscopy was discover 1896.
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1. X-ray Tube.
2. Image Receptor (Image Intensification).
3. Viewing Systems
4. Recording Systems
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X-ray Tube and Image Intensification are mounted to a C-arm to maintain their alignment at all times.
C-arm permits the image receptor to be raised and lower to vary the beam geometry for maximum resolution while x- ray tube remains in position.
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C-arm can move all direction.
2 types of C-arm: under couch, and over couch
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Carriage is the arm supports the equipment suspended over the table include I.I., x-ray tube, control power drive, spot film selection, tube shutters, spot filming, cine camera, video input tube etc.
Exposure cannot commence until the carriage is return to a full beam intercept position.
10/18/2012 Kamarul Amin (c) 10 1) X-ray Tube
Similar to General X-ray Tubes except: Designed to operate for longer periods of time at much lower mA i.e. fluoroscopic range 0.5-5 mA Tube target must be fixed to prevent an SOD of less than 15 inch (~ 40 cm). Fluoroscopic tube can operate by Foot Switch Equipped with electrically controlled shutter.
10/18/2012 Kamarul Amin (c) 11 2) Image Intensification (II)
Was developed 1948.
Is designed to amplify the brightness of an image.
New II are capable of increasing image brightness 500-8000 times.
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Major components of an II are:-
i. Input Window ii. Input Phosphor iii. Electrostatic Lenses iv. Output Phosphor v. Output Window vi. Envelope
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The primary x-ray beam exits the patient and strikes the input screen of the II, which is a vacuum tube with a cathode and an anode.
Fluorescent screen is built into the image intensifier as input screen, which absorbs the x-ray photons and emits light photons.
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Photocathode is 2nd layer which prevent divergence of the light.
The photocathode absorb the light and emits electrons.
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Then electrons accelerated from the cathode toward the anode and the output screen by 25 kV potential difference.
Electrostatic lenses is used to accelerate and focus the electron beam.
The output screen absorbs the electrons and emits light photons.
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II is encased in a lead lined housing that effectively absorbs the primary beam.
A getter is ion pump is used to remove ions during operation and maintain the vacuum within the tube.
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i) Input Window
Older IIs used glass ==> x-ray scattering and absorption effects in this material. Now use thin sheet ( 0.25 - 0.5 mm) of aluminium or titanium==> strength to sustain vacuum & minimal x-ray attenuation.
10/18/2012 Kamarul Amin (c) 22 ii) Input Phosphor
Uses CsI doped with Na, deposited on aluminium substrate. CsI:Na is grown in a structure of monocrystalline needles, each ~ 0.005 mm in diameter < 0.5 mm long. ==>Uses total internal reflexionto transmit as much light as poss'. The aluminium substrate ~ 0.5 mm thick The input phosphor is typically 15 to 40 cm in diameter.
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Cs and I are good absorbers of x-photons at diagnostic energies: K-edges at 36 and 33 keV, respectively. The CsI:Na phosphor gives emitted blue visible light, directed to photocathode. Intermediate layer (e.g. indium oxide) ==>high optical transmission. Also chemically isolates the phosphor and photocathode. The photocathode usually an alloy of antimony and caesium SbCs3. Photons interact mainly via photoelectric events ==> they disappear and produce recoil electrons.
10/18/2012 Kamarul Amin (c) 25 CUT OFF VIEW OF II
10/18/2012 Kamarul Amin (c) 26 iii) Electrostatic Lenses
A vacuum enables the electrons to travel without interacting with anything A voltage ~ 25 to 35 kV accelerates the electrons. Electrodes are used in places (electron optics) to guide ( focus) the electrons onto the output phosphor.
10/18/2012 Kamarul Amin (c) 27 Cont’d..
Electron current of ~10-8 to 10-7 A is produced. Both (i) acceleration and (ii) focusing of the electrons enables image intensification.
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The image at the output phosphor is inverted relative to the input image at the input phosphor (see the focal point due to the e-optics). Input phosphor and photocathode are in truth curved This means the electron path lengths are equalized and reduces image distortion.
10/18/2012 Kamarul Amin (c) 29 iv) Output Phosphor
ZnCdS: Ag is deposited on the ouput window ~ 0.005 mm thick and 25 to 35 mm in diameter. Emits a green light upon absorption of the photo-electrons from the photocathode.
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A thin aluminium film on the inner surface of the phosphor (i) electrical connection for ANODE (ii) to reflect light back towards the output window – attempts to maximize the output luminance and to prevent these light photons from `going back' into the II and interacting with the photocathode.
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Various designs exist and have intention of enhancing the 'straight thru' transmission of photons and preventing back reflections into the II. Examples are: (i) a glass window (e.g. 15 mm thick) with external anti-reflection layers, (ii) tinted glass window and (ii) fibre-optic window The output window image is transmitted to an optical system to be viewed by a cine- camera, photographic camera, video camera or combinations of these.
10/18/2012 Kamarul Amin (c) 33 10/18/2012 Kamarul Amin (c) 34 vi) II Envelope
II envelope is made from glass or non- magnetic stainless steel Input window is welded to the envelope. Entire assembly is housed inside a metal container which contains lead, for radiation shielding, and mu-metal, to shield the electron optics from external magnetic fields.
10/18/2012 Kamarul Amin (c) 35 Cont’d..
The input window is typically protected by an aluminium faceplate (e.g. 0.5 mm thick) and is also a safety device in case of implosion of the II. Some IIs also have an anti-scatter grid mounted at the faceplate.
10/18/2012 Kamarul Amin (c) 36 Fluoroscopic Generators
Same as those used for static/conventional radiography.
10/18/2012 Kamarul Amin (c) 37 Brightness Control
Automatic Brightness Control Automatic adjustments made to exposure factors by equipment. Automatic Gain Control Amplifies video signal rather than adjusting exposure factors.
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Image Quality
Contrast
Resolution
Distortion
Quantum Mottle
10/18/2012 Kamarul Amin (c) 40 Contrast
Controlled by amplitude of the video signal. It is effected by penumbral light scatter in the input and output screens. Affected by scatter radiation. Back scatter effect from the output to the input screen→ background fog. Edge of the image decreases image contrast.
10/18/2012 Kamarul Amin (c) 41 Resolution
The primary limitation is 525-line raster pattern of the video camera monitor. Spot film or direct optical viewing depend on geometrical factors, includes minification gain, electrostatic focal point, input and output screen diameter, viewing system resolution i.e. TV, OID, phosphor size and thickness. CsI II capable of 4 lp/mm, magnification or multifield image intensifiers capable of up to 6 lp/mm.
10/18/2012 Kamarul Amin (c) 42 Distortion
Size distortion is caused the same factors affect by static radiographic e.g. OID.
Shape distortion is caused by geometric problems.
Edge distortion problem (vignetting).
10/18/2012 Kamarul Amin (c) 43 Quantum Mottle
Insufficient radiation which cause grainy appearance. Should be control by high mA and time setting. Can be also from video noise. Factors influence mottle are, total no. of photons arriving retina which include radiation output, beam attenuation, conversion efficiency, minification gain, flux gain, total brightness gain, viewing system, distance of the eye from the viewing system.
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Viewing Systems • 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.
10/18/2012 Kamarul Amin (c) 46 10/18/2012 Kamarul Amin (c) 47 10/18/2012 Kamarul Amin (c) 48 • 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.
10/18/2012 Kamarul Amin (c) 49 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.
10/18/2012 Kamarul Amin (c) 50 Viewing Systems
1. Video Viewing System
2. Video Camera Tubes A. Cathode B. Anode
3. Video camera charge-coupled device (CCD)
4. Video monitor
5. Digital
10/18/2012 Kamarul Amin (c) 51 1. Video Viewing Systems
Closed circuit television Video camera coupled to output screen and monitor Video cameras Vidicon or Plumbicon tube CCD
10/18/2012 Kamarul Amin (c) 52 2. Video Camera Tubes
Plumbicon and vidicon tubes similar Different target materials. Plumbicon has faster response time than vidicon.
10/18/2012 Kamarul Amin (c) 53 Cont’d..
Components of Video Camera Tubes:- 1. Cathode Control grid 2. Electromagnetic focusing coils 3. Electrostatic deflecting coils 4. Anode Face plate Signal plate Target
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1) Cathode
Heating assembly Electron gun thermionic emission
Control grid Shapes electron beam
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2) Electromagnetic Focusing coils Shape electron beam into single point 3) Deflecting coils Cause electron stream to scan target in raster pattern
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4) Anode Face plate Signal plate Positively charged thin film of graphite. Target Changes light pattern to electronic signal sent to video system.
10/18/2012 Kamarul Amin (c) 57 Video Camera Charged Coupled Devices (CCD)
Semiconducting device.
Emits electrons in proportion to amount of light striking photoelectric cathode.
Fast discharge eliminates lag.
10/18/2012 Kamarul Amin (c) 58 3. Video Camera Charged Coupled Devices (CCD)
Operate at lower voltages than video tubes.
More durable than video tubes.
10/18/2012 Kamarul Amin (c) 59 4. Video Monitor
10/18/2012 Kamarul Amin (c) 60 5. Digital Fluoroscopy
Image intensifier output screen coupled to TFTs.
TFT photodiodes are connected to each pixel element.
Resolution limited in favor of radiation exposure concerns.
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Recording the Fluoroscopic Image
1. Dynamic Systems Cine Film Systems Videotape Recording
2. Static Spot Filming Systems Cassettes 105 mm Chip Film
3. Digital Fluoroscopy
10/18/2012 Kamarul Amin (c) 63 Dynamic Systems
1. Cine Film System
2. Videotape Recording
10/18/2012 Kamarul Amin (c) 64 1. Cine Film Systems
Consist of cine camera positioned behind output screen.
Required 90% of image intensity for proper exposure.
16 mm and 35 mm formats are currently use.
More pt dose.
Record series of static image at high speed.
Shutter and pulses of radiation should synchronize for the exposure.
Generator and fluoro x-ray tube must able to handle large heat loads.
10/18/2012 Kamarul Amin (c) 65 2. Videotape Recording
VHS-S system required.
High resolution camera.
Recorders tape and monitors.
Operate same as home video systems.
10/18/2012 Kamarul Amin (c) 66 Static Spot Filming Systems
1. Cassettes
2. 105 mm Chip Film
10/18/2012 Kamarul Amin (c) 67 1. Cassettes
Standard size - 9” x 9”.
Stored in lead-lined compartment until ready for exposure.
When exposure is made, mA is raised to radiographic level.
Multiple image formats.
10/18/2012 Kamarul Amin (c) 68 2. 105 mm Chip Film
12 frames per second.
Beam splitting mirror.
10/18/2012 Kamarul Amin (c) 69 Digital Fluoroscopy
Use CCD to generate electronic signal.
Signal is sent to ADC.
Allows for post processing and electronic storage and distribution.
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