Fluoroscopy – Viewing Systems TV Monitors Based on: Principles of Radiographic Imaging, 3rd 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 PPT created by Jed Miles, BSRS, RT(R), CRT-CA

Video CRT Monitor A video monitor (television tube) is a cathode ray tube (CRT) Consists of: A vacuum tube Electron gun as part of cathode at back of tube External coils for focusing and steering an electron beam Fluorescent phosphor coating on the inside of the front screen Anode is plated onto the front screen Video signal is amplified and transmitted by cable to the television monitor, where it is transformed back into a visible image

Video CRT Monitor – How it Works An image is created as the electron gun produces a stream or beam of electrons from the camera’s video signal onto the TV screens phosphor The electron beam follows the same raster pattern used by the video camera The video signal received by the picture tube is modulated Modulation means the magnitude of the video signal received from the camera tube is directly proportional to the light intensity received by the camera tube (vidicon, plumbicon, or orthicon) Thus, unlike the camera tube, the electron beam of the CRT monitor varies in amplitude or intensity in accordance to the modulation of the video signal

Video CRT Monitor – How it Works The intensity of the electron beam is modulated by the control grid, which is attached to the electron gun The electron beam is focused onto the output fluorescent screen by external electrostatic coils Electron beam scans across the output screen using the exact same raster pattern as the camera tube Phosphor crystals emit light when struck by electrons and transmit it as a visual image through the glass of the screen to the viewer Phosphor crystals are composed of linear crystals aligned perpendicular to the glass envelope to reduce lateral light dispersion Phosphor layer is usually backed by a thin layer of aluminum, which transmits the electron beam but reflects the light

Black and White - Video CRT Monitor

Analog TV Monitor

Color - Video CRT Monitor Close up of color CRT Tube

Video CRT Monitor – Image Quality Video CRT Monitors (television) quality is affected by the number of scan lines and the bandpass of the TV camera system The video monitor is the most restrictive element in the fluoro imaging chain resolution 525 line monitor is capable of 1-2 lp/mm 1000 line system doubles the spatial resolution

TV Camera & TV Monitor – Image Quality Overall Image quality is affected by Horizontal resolution Vertical resolution Contrast Brightness Lag In the television-camera tube, as the electron beam reads the optical signal, the signal is erased In the television-picture tube, as the electron beam creates the television optical signal, it immediately fades Hence the term ‘fluorescent screen’

Image Quality - Contrast Contrast levels of a TV monitor can be adjusted on the TV monitor itself Contrast should be set as follows The darkest object in the scene is just below the black level on the monitor The brightest objects of interest do not completely saturate or ‘white out’ details of the image It is appropriate to adjust the contrast and brightness control to maximize the visibility of the object even at the expense of increased noise

Image Quality - Brightness Changes in brightness will affect image quality When the fluoroscope is moved from the abdomen to the chest, a sudden surge of brightness will flood the system The image will become chalky white and detail is lost Brightness levels are controlled by the automatic brightness control (ABC) Usually the ABC will stabilize the image brightness and the x-ray exposure factors The brightness level can be manually increased, but will not improve image quality Usually brightness and contrast are adjusted in combination – contrast is brought to near maximum and brightness is adjusted for satisfactory luminance

Video CRT Monitor – Viewing Conditions Viewing Conditions: viewing conditions change with viewing distance This allows the raster pattern to blend from the viewers perspective 525 line pattern on a 9” monitor has a minimum viewing distance of 37” 525 line pattern on a 17” monitor has a minimum viewing distance of 70” High resolution monitors (over 1,000 lines) of the same sizes may be viewed at closer distances Viewer can adjust the brightness and contrast of the image at the monitor itself – not the “imaging chain”

Image Quality – Horizontal Resolution Bandwidth or Bandpass refers to the total number of cycles per second available for display by the television camera and monitor electronics This number will set and overall limit the resolving power (capability) of the TV camera It is a product of scan lines, frame rate, and frequency rate Horizontal resolution is the ability to resolve the image dots on each scan line Frequency bandwidth is the maximum number of samples per line per unit time Increasing bandwidth will allow the camera to sample more often per second Example using dots: . . . . . . . .Versus …………… Increased bandwidth = increased horizontal resolution

Image Quality – Vertical Resolution Vertical resolving power is the ability of a TV system to resolve objects spaced apart in the vertical direction (to resolve horizontal lines) More lines = better resolution Vertical resolution varies with the size of the object, as well as the diameter dimension of the input phosphor It is essentially, the vertical reproduction of the image as seen from the output phosphor by the pick up tube Vertical resolution can be measured by the following formula Vertical resolution (lp/mm) = number of horizontal lines across the object 2 x diameter of object in millimeters

Image Quality – Vertical Resolution 512 scan lines on target 1024 scan lines on target Vertical resolving power is the ability of a TV system to resolve objects spaced apart in the vertical direction (to resolve horizontal lines) More lines = better resolution

Vertical Resolution - Kell Factor The Kell factor is a component of vertical resolution It is defined as the ratio between the vertical resolution and the number of scan lines in the TV system The ratio between the actual vertical resolution of a TV monitor (as specified in TV lines) and the number of horizontal scan lines is called the Kell factor Calculated by the following formula Kell factor = vertical resolution number of scan lines

Vertical Resolution - Kell Factor Imaged phantom on left side has an increased Kell Factor and consequently better resolution

Image Quality - Lag Screen lag is an undesirable yet useful property of vidicon tubes Lag is a blurring of the TV image when the fluoro tower is moved rapidly from one area to another Lag occurs because it takes certain amount of time for the image to build up and decay on the vidicon target globules Visible lag is not caused by the image intensifier

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