Lesson 13. Viewing and Recording the Fluoroscopic Image. Core Notes:
CCTV:
Figure 13-1: Components of a television system.
· Camera,
· Cable, Fluoroscopic Television systems are always closed-circuit systems, ie they use cables and not wireless.
Ø Camera Control Unit, Signal is amplified here . Reassembling the video signal pulses into a viable image
Ø Monitor, converts signal back to an image for direct viewing. Reassembling the video signal pulses into a viable image done by a camera control unit and television monitor.
o Horizontal Scan Lines screen pixels containing image, contrast, and colour arranged horizontally
o TV (is obsolete as you all know) had 525 scan lines in the US.
§ HD 1920 x 1080 for home LCD, for work we use 2048 x 1080 using Plasma Display Panels or LCD’s
v TV Camera – not NB - Page 176
¨ Remember an electron beam must travel in a vacuum
¨ Obviously electrons move up and down, or side to side. (Vertical and Horizontal)
v Video Signal. Not NB.
Ø Fluoroscopic image from the output phosphor of the image intensifier to the video camera, where it is converted into a series of electrical pulses is the “video signal”
Ø Formation of a video signal
§ Figure 13.5: Not NB
· Globule absorbs light, photoelectrons are emitted
· Positive tube. Electrons are attracted to the anode and removed from the tube.
· The insulated globule now acts as a tiny capacitor, drawing a current to the signal plate
· Mosaic of charged globules replicates the light image focussed onto target.
· Discharge. Electron beam scans the electric image, and restores the target, by filling the mosaic image with electrons. Electrons in the scan beam neutralise the positive charge in the globules; the electrons on the signal plate no longer have an electrostatic force to hold them on the plate. Moving electrons forma current flowing through a resistor, and therefore a voltage through the resistor.
· Voltage collected for each neutralised globule.
· One pixel at a time. Revealing the location and timing of each incident photon of light on the target, to the screen.
Ø Reassembling the video signal pulses into a viable image done by a camera control unit and television monitor.
v CCD: Charged-Coupled Device TV Camera. Not NB
· (Same story as Videocon but stores negative charge instead of positive charge. The readout happens by the charge in the charge buckets from one bucket to the next, forming an electric signal at the edge of the line. Note that the amount of charge in each bucket is different.)
v Television Monitor. (Small summary)
Ø Bushburg p88. Flat panel monitors. Briefly Explain
§ Flat Panel Monitors use LCD’s (Liquid Crystal Display). LCD’s don’t produce light but change the light produced by another source. Most LCD’s are backlit. The LCD layer is usually between two polarising filters.
§ The first filter horizontal polarized light. LCD Layer twists the light (Voltage On or Voltage Off – no twist), The now vertically polarized light is passed through a second polarised filter. The untwisted light gets blocked (Dark) and the twisted light passes the second filter (Bright). You would need RED, GREEN and BLUE colour filter sets for colour LCD’s.
Ø Simple Block Diagram. Explain CCD diagram. Explain how it works using Figure 11-5 p297 Bushburg
· Form images from visible light
· CCD chip = integrated circuit
· Silicon surface of CCD chip is photosensitive
· Electrons are liberated and build up in pixel
TELEVISION SCANNING
v Interlaced horizontal scanning. Eyes 50 Hz, Screen refresh rate 30 Hz. Flicker is detected. Interlaced horizontal scanning is employed to avoid flicker. Only the even lines are scanned in the first frame, and the odd lines are scanned in the second frame. Perception is 60 Hz, and flicker disappears. Figure 13-7
v Video Signal Frequency (Bandpass or “bandwidth”). The wide range of frequencies covered by the video signal. (Analogy using sound Figure 13-8). The Video signal from one scan line with 4 lines or 2 line pairs Fig 13-9.
v Vertical Resolution. More scan lines the better the resolution. 10% of scan time of signal is lost to retrace one line to another.
Ø Remember, vertical resolution depends on the number of vertical lines (such as 525), whereas horizontal resolution is determined by the bandpass /bandwidth of 5Mhz with a 525 line system.
v Synchronization. TV camera and Monitor must be synchronised. Beam at the same place on the image. Special sync pulses are sent at the end of each horizontal and vertical field.
Ø Horizontal and Vertical Retrace (beam returned to the right and top of screen). Video signal is stopped during retrace.
TELEVISION IMAGE QUALITY (Background Only)
v Resolution. Proportional to the number of vertical scan lines. Theoretical maximum is half the visible scan lines. When black lines alternate with white the maximum line pairs is half the video lines. The actual limit is lower than theoretical as 10% of the time is used for retrace, and scan lines need time to be synchronised to high resolution objects. 525 lines yield 185 line pairs. (30% loss for those that are still thinking).
v Contrast
v Lag. More lag, less quantum mottle. Lag is a problem in cardiovascular fluoroscopy (fast movements). Vidicons more lag, Plumbicons more quantum mottle (less lag).
v Automatic Brightness control (LEAVE OUT)
AUTOMATIC GAIN CONTROL (LEAVE OUT)
v Automatic Brightness Control
Ø kVp Variability
Ø mA Variability
Ø Combined Control
Ø Pulse Width Variability
FLUROSCOPIC IMAGE RECORDERS
v Light Image Recorders (Framing is NB)
Ø Spot Film Recorder
Ø Spot Film Cameras
Ø Framing with Spot Film Cameras NB. Figure 13-10. The output of the image intensifier is round, but the shape of film used in spot film camera is square. How do you match the two? Framing refers to the utilisation of the available area on the film.
§ Exact Framing. The entire circular intensifier image is included in the usable square film frame. No part of the image is lost but 21% of the film is wasted (100% Available Film -79% Used Film =21% Lost Film)
· The word “usable” refers to the fact that part of the film is lost to the transportation frame around edge of a frame.
· Not recommended
§ Equal Area Framing. The area of the intensifier image is equal to the usable square of the film area. Approximately 9 % of both the image and the film are lost.
· Recommended
§ Mean Diameter Framing. The diameter of the intensifier image and equal to the mean of the transverse and diagonal dimensions of the useful square film area; 16% of the image is lost but only 4% of the film is unused.
· Recommended
§ Total Overframing. The diameter of the intensifier image is equal to the diagonal dimensions of the useful square film area. The entire film is used, but 36% of the image in unused.
· Not recommended unless maximum magnification is desired
The optimum frame format depends on:
§ The size of the image intensifier
§ The size of the film, and
§ The clinical application for which the image is intended.
Remember the image size of the recorded image enlarges as the diameter of the framing format increases. Overframe small films then larger films. Quantum mottle originates in the fluorescent screen, so it is not affected by the size of the recorded image.
Ø Cinefluorography
Ø Cine Camera
Ø Framing
Ø X-Ray Exposure
§ Synchronisation
TV IMAGE RECORDERS
v Tape Recorders
v Magnetic Disc Recorders
Ø The most important one is random access
v Optic Discs
Once the image is formed by the x-ray beam it is necessary to get the information to the radiologist. With fluoroscopy, this is done by getting the image on a TV monitor. To make such a display the image is processed by a television camera tube. The tube may be a standard vidicon, a plumbicon, or a CCD. The image is displayed on a TV monitor. Standard X-Ray Closed Circuit Television uses a 525 x 525 format with a 5Mhz bandpass. Vertical resolution is limited by the scan line format (number of vertical lines), whereas horizontal resolution is a function of the bandpass. Display formats using more vertical and lines and higher bandpass are available and desirable. (We have better displays on our cell phones today).
It is often necessary to record the fluoroscopic image (or you may be sued). Either the light image from the II output phosphor or the electric signal from the TV camera may be recorded. The light image is recorded by a photospot camera or a cine camera. (you will probably never see these in use). The electronic signal from TV camera may be recorded in analogue or digital format on magnetic tape, magnetic disc or optical disc.
Thanks
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