Fluoroscopy: Viewing Systems Optical Mirrors TV Camera & TV Camera Tubes Charge Coupled Devices (CCD)

Video Viewing System System includes: Closed circuit video with TV monitor is the most commonly used fluoroscopic viewing system Contained system, eliminates outside and background broadcast interference System includes: Video camera attached to the output side of the II Display Monitor Cathode ray Tube (CRT) or LCD Camera Control Unit Amplifies the video signal Synchonizes video signal between the TV camera and TV monitor Associated cabling

Video (TV) Camera Tubes There are several types of camera tubes. The main difference is the phosphor used as the target material. VIDICON TUBE Most often used in fluoroscopic imaging chains Relatively slow response time, produces image lag Less quantum mottle due to lag producing signal averaging PLUMBICON TUBE Less often used (mainly used in cardiac cath labs) Faster response time and low lag Produces better contrast More quantum mottle (Low lag does not provide signal averaging from previous frame) IMAGE-ORTHICON Very expensive, rarely used CHARGED COUPLED DEVICE (CCD) Solid-state semiconductor (not a vacuum tube)

Charge-Coupled Devices (CCD) Semiconducting device capable of storing a charge from light photons striking a photosensitive surface Photoelectric cathode When light strikes the photoelectric cathode of the CCD… Electrons are released proportionally to the intensity of the incident light Extremely fast discharge time with no image lag Useful for high speed imaging applications More sensitive than video tubes that operate at lower voltages which prolongs useful life Acceptable resolution Not prone to damage from rough handling Advantages

Video (TV) Camera Tubes Camera tubes contain: Series of electromagnetic focusing coils Series of electromagnetic deflecting coils Cathode with a control grid Face Plate Signal Plate Target Anode

Video Camera Tubes – Cathode End Consists of heated filament assembly that supplies a constant electron current across the tube via thermionic emission (the flow of electrons from a metal or metal oxide surface, caused by thermal energy) Electron Gun Shapes the electrons into a stream or beam Helps accelerate electrons to anode end of tube Control Grid Further accelerate and focus the electron beam toward anode end Electrostatic Grids Focus the electron beam into a pencil point stream Focusing Coils Deflect pencil beam to scan target in path known as a raster pattern Deflecting Coils (in pairs)

Video Camera Tubes – Anode End Beam is decelerated by wire mesh-like structure in front of the target assembly This arrangement brings the electron beam to a near standstill where it is straightened to strike the target in a perpendicular fashion

Video Camera Tubes – Target Assembly Video camera tubes consist of three layers sandwiched together: Face Plate (or window) The thin part of the glass envelope closest to the output phosphor of the image intensifier Signal Plate Consists of a thin film of metal or graphite charged with a positive voltage Is thin enough to transmit light from intensifier Is thick enough to conduct an eletronic signal Target Layer (or photoconductive layer) A photoconductive layer coated on the inside surface of the signal plate

Target Assembly - Construction Light photons interact with this layer: When illuminated, this layer becomes photoconductive and releases electrons When dark, this layer acts as an insulator Target Layer (or photoconductive layer) Consists of a thin insulating mica coating in which globules of a light-sensitive photoconductive material are suspended in a matrix mica coating Globules Approximately 0.001 inch (0.025mm) in diameter Size determines resolving power of camera tube (smaller = increased resolution) Vidicon tubes use antimony trisulfide (Sb2S2) Plumbicon tubes use lead oxide (PbO)

Target Assembly – How it Works Light from output phosphor of image intensifier tube arrives Light is transmitted through window and signal plate to target material Antimony trisulfide (or lead oxide) globules absorb light photons from II output screen and emit electrons Electrons are released to anode and removed from tube (equivalent to intensity of absorbed light) Loss of electrons creates positive charge at globule Light is absorbed by globules Electron gun’s beam scans target and discharges globules causing current flow in signal plate Sequential charging and discharging of globules results in modulated current flow in signal plate This creates varying amplitudes of video signal

Target Assembly Magnitude of video signal is proportional to intensity of light from the intensifier As electron beam is incident on a globule some of its electrons will be conducted through target to signal plate and conducted out of tube as video signal If area (globule) of target is dark (i.e. bone attenuated the x-ray beam) there will be less video signal

Target Assembly Click for full explanation Sequential discharging of the globules releases signal plate’s charge as a pulsed signal This pulsing is the varying amplitude of the video signal Pulsed signal Click for full explanation

Scanned Raster Pattern A closed circuit television systems picture is a series of scanned lines of the video camera’s target which are projected fast enough on a TV monitor to create movie-like fluid motion ecx.images-amazon.com 1 frame = 525 scanned lines, each scan line consists of thousands of dots To avoid flicker, each frame is divided into two halves or fields: 1st half scans even-numbered lines 2nd half scans odd-numbered lines

Interlaced Scans Electron beam scans diagonal lines as active traces while horizontal lines are inactive retraces to position for next active trace Thus, 2 interlaced scans (or fields) produce 1 complete frame 262.5 lines scanned every 1/60 second, Entire 525-line raster pattern scanned every 1/30 second 30 frames per second = 15,7000 scanned lines per second

Progressive Scan Analog commercial television uses a 525 horizontal raster line pattern per frame (2 x 262½ fields) Progressive raster pattern scans sequentially from top to bottom (no interlacing) High resolution video systems now offer over 1,000 lines per frame Provides increased resolution

Please close this PowerPoint presentation, and continue the lesson. What’s Next? Please close this PowerPoint presentation, and continue the lesson. Presented by Based on: Principles of Radiographic Imaging, 4th Ed. By: R. Carlton & A. Adler Radiologic Science for Technologists, 8th Ed. By: S. Bushong Syllabus on Fluoroscopy Radiation Protection, 6th Rev. By: Radiologic Health Branch – Certification Unit