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Lecture 5 Fluoroscopy and the Systems

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Lecture 5 Fluoroscopy and the Systems 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. 10/18/2012 Kamarul Amin (c) 3 10/18/2012 Kamarul Amin (c) 4 10/18/2012 Kamarul Amin (c) 5 10/18/2012 Kamarul Amin (c) 6 Fluoroscopic Equipment 1. X-ray Tube. 2. Image Receptor (Image Intensification). 3. Viewing Systems 4. Recording Systems 10/18/2012 Kamarul Amin (c) 7 Cont’d.. 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. 10/18/2012 Kamarul Amin (c) 8 Cont’d.. C-arm can move all direction. 2 types of C-arm: under couch, and over couch 10/18/2012 Kamarul Amin (c) 9 Cont’d.. 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. 10/18/2012 Kamarul Amin (c) 12 Cont’d.. Major components of an II are:- i. Input Window ii. Input Phosphor iii. Electrostatic Lenses iv. Output Phosphor v. Output Window vi. Envelope 10/18/2012 Kamarul Amin (c) 13 10/18/2012 Kamarul Amin (c) 14 10/18/2012 Kamarul Amin (c) 15 10/18/2012 Kamarul Amin (c) 16 Principle Operation 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. 10/18/2012 Kamarul Amin (c) 17 Cont’d.. Photocathode is 2nd layer which prevent divergence of the light. The photocathode absorb the light and emits electrons. 10/18/2012 Kamarul Amin (c) 18 Cont’d.. 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. 10/18/2012 Kamarul Amin (c) 19 Cont’d.. 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. 10/18/2012 Kamarul Amin (c) 20 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. 10/18/2012 Kamarul Amin (c) 23 10/18/2012 Kamarul Amin (c) 24 Cont’d.. 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. 10/18/2012 Kamarul Amin (c) 28 Cont’d.. 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. 10/18/2012 Kamarul Amin (c) 30 Cont’d.. 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. 10/18/2012 Kamarul Amin (c) 31 10/18/2012 Kamarul Amin (c) 32 v) Output Window 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. 10/18/2012 Kamarul Amin (c) 38 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. 10/18/2012 Kamarul Amin (c) 44 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.
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