fluoroscopy: viewing systems

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Fluoroscopy: Viewing Systems TV Monitors

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Fluoroscopy: Viewing Systems. TV Monitors. Video CRT Monitor. A video monitor (television tube) is a cathode ray tube (CRT). Consists of:. TV Monitor. Cable. A vacuum tube. Electron gun as part of cathode at back of tube. www.kentmusictechnology.co.uk. External coils - PowerPoint PPT Presentation

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Page 1: Fluoroscopy: Viewing Systems

Fluoroscopy: Viewing SystemsTV Monitors

Page 2: Fluoroscopy: Viewing Systems

Video CRT Monitor

A video monitor (television tube) is a cathode ray tube (CRT)

Consists of:

External coilsfor focusing and steering

electron beam

Fluorescent phosphor coating

inside front screen

Anode plated onto front

screen

A vacuum tubeElectron gun

as part of cathode at back of tube

TV Monitor

Video signal

Video signal is amplified and transmitted by cable to the television

monitor

It is transformed back into a visible

image

Cable

www.kentmusictechnology.co.uk

Page 3: Fluoroscopy: Viewing Systems

Electron Beam

• Image created as electron gun produces stream or beam of electrons from camera’s video signal onto TV screen’s phosphor– Intensity of electron beam

modulated by control grid attached to electron gun

• Electron beam focused onto output fluorescent screen by external electrostatic coils – Scans across the output screen

using the exact same raster pattern as the camera tube

Page 4: Fluoroscopy: Viewing Systems

4

Phosphor Crystals

• Composed of linear crystals aligned perpendicular to glass envelope to reduce lateral light dispersion

• Phosphor layer usually backed by thin layer of aluminum, which transmits electron beam but reflects light

Phosphor crystals

Phosphor crystals emit light when struck by electrons and transmit it as visual image through glass of screen to viewer

Page 5: Fluoroscopy: Viewing Systems

Modulation of Signal

Video signal received by picture tube is modulated

Magnitude of video signal received from the camera tube is directly proportional to light

intensity received by the camera tube(vidicon, plumbicon, or orthicon)

Video signal magnitude

Light intensity received by camera tube

Unlike camera tube, electron beam of CRT monitor varies in amplitude or intensity in

accordance with modulation of video signal

Video signal modultion

Electron beam of CRT monitor

Page 6: Fluoroscopy: Viewing Systems

Monitor Image Quality

Monitor quality is affected by the number of scan lines and the bandpass of the TV camera system. Raster pattern from camera is presented on the

monitor.

Video monitor is the most restrictive element inthe fluoro imaging chain resolution

525 line monitor is capable of 1-2 lp/mm

525 line monitor is capable of 1-2 lp/mm 1000-line system doubles

the spatial resolution

1000-line system doubles the spatial resolution

Page 7: Fluoroscopy: Viewing Systems

In television-camera tube:

As electron beam reads optical signal, signal is

erased.

TV Camera & TV Monitor Image Quality

Overall image quality is affected by:

Horizontal resolution

Vertical resolution

Contrast Brightness Lag

In television-picture tube:

As electron beam creates television optical signal, it immediately fades (hence

the term fluorescent screen).

Page 8: Fluoroscopy: Viewing Systems

Image Quality - Contrast

Contrast levels of a TV monitor can be adjusted on the monitor itself

Contrast should be set as follows:

Darkest object in the scene just below black level on the

monitor

Brightest objects of interest do not completely saturate or “white out” details of the

image

It is appropriate to adjust contrast and brightness control to maximize the visibility of object even at the expense of increased

noise

Page 9: Fluoroscopy: Viewing Systems

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

Page 10: Fluoroscopy: Viewing Systems

Image Quality - Brightness

Brightness levels controlled by:

Usually ABC will stabilize image brightness and x-ray exposure factors

• 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

Brightness is adjusted for satisfactory luminance

automatic brightness

control (ABC)

Page 11: Fluoroscopy: Viewing Systems

Viewing Conditions

Viewing conditions change with viewing distance allowing the raster pattern to blend from the viewer’s perspective

525-line pattern on 9” monitor has

min. viewing distance

of 37”

525-line pattern on 17” monitor has min.

viewing distance of 70”

High resolution monitors (over 1,000 lines) of the

same sizes may be viewed at closer distances

Viewer can adjust brightness and contrast of image at the monitor itself (not the imaging chain)

Page 12: Fluoroscopy: Viewing Systems

Image Quality – Horizontal Resolution

• This number will set and limit the resolving power (capability) of the TV camera• Product of scan lines, frame rate, and frequency rate

Bandwidth or bandpass refers to total number of cycles per second available for display by television camera and monitor

electronics

Page 13: Fluoroscopy: Viewing Systems

Image Quality – Horizontal Resolution

• Frequency bandwidth is maximum number of samples per line per unit time

• Increasing bandwidth will allow the camera to sample more often per second

Horizontal resolution is ability to resolve image dots on each scan line

Increased bandwidth = increased horizontal resolution

versus

Page 14: Fluoroscopy: Viewing Systems

Image Quality – Vertical Resolution

512 scan lines on target

1024 scan lines on target

Vertical resolving power is ability of a TV system to resolve objects spaced apart in the vertical direction (to resolve horizontal lines)

More lines meansbetter

resolution

Page 15: Fluoroscopy: Viewing Systems

Vertical resolution is, essentially, the vertical reproduction of the image as seen from the output phosphor by the pick up tube

Image Quality – Vertical Resolution

Vertical resolution lp/mm = number of horizontal lines across object

2 x diameter of object (mm)

Size of objectSize of object

Diameter of input phosphorDiameter of input phosphor

Vertical resolution varies with:

Page 16: Fluoroscopy: Viewing Systems

Vertical Resolution - Kell Factor

Imaged phantom on left side has increased

Kell Factor and consequently better

resolution

Kell Factor = vertical resolution

number of scan lines

The Kell Factor is the ratio between actual vertical resolution of TV monitor (as specified in TV lines) and the number of horizontal scan lines

A component of vertical resolution

Page 17: Fluoroscopy: Viewing Systems

Image Quality - Lag

Screen lag is an undesirable yet Screen lag is an undesirable yet useful property of vidicon tubesuseful property of vidicon tubes

Fluoro

Tower

Blurring of TV image when fluoro tower is moved rapidly Blurring of TV image when fluoro tower is moved rapidly from one area to anotherfrom one area to another

Lag occurs because it Lag occurs because it takes time for the image takes time for the image to build up and decay on to build up and decay on vidicon target globulesvidicon target globules

Page 18: Fluoroscopy: Viewing Systems

Charge Coupled Device (CCD)

• Mounted at output phosphor of image intensifier tube and coupled by fiber optics or lens system

• Early 1980s – first CCD replaced the TV camera in a video system

CCD is a semiconducting device capable of storing a charge from light photons striking a photosensitive surface

Sensitive component is a layer of crystalline

silicone

Page 19: Fluoroscopy: Viewing Systems

CCD – How it works

Light strikes the crystalline silicone

(photoelectric cathode) of the CCD

Electrons are released proportionally to the

intensity of the incident light

Silicon is illuminated and an electrical charge is

generated

CCD can then be sampled pixel by

pixel (raster format)

Video signal is emitted in raster scanning pattern by moving stored charges along P&N holes to edge of CCD

where they are discharged into a conductor

Semiconductors store this charge in P&N holes, thus storing

charges in a latent form

Computers can then manipulate the digital image

Page 20: Fluoroscopy: Viewing Systems

CCD Spatial Resolution

Systems incorporating a 1024 matrix can produce

images with 10 lp/mm

Spatial resolution of CCD determined by its:

Physical SizePhysical Size

Pixel CountPixel Count

Page 21: Fluoroscopy: Viewing Systems

CCD Resolution

Has linear response curve as compared to other image receptors which have

sigmoid shaped curves

Enables imaging with low light levels (less dose) possible with retained

contrast resolution

• Results in higher signal-to-noise (SNR) ratio and better contrast resolution

• Able to use a lower patient dose per image

Higher sensitivity to light (detective quantum efficiency or DQE) and lower

level of electronic noise than TV camera

Page 22: Fluoroscopy: Viewing Systems

CCD Advantages

HIGH: Spatial resolution Signal-to-noise ratio (SNR) Detective quantum efficiency (DQE)

Extremely fast discharge time (no image lag or blooming)• Useful for high speed imaging applications

Unlimited life span Unaffected by magnetic fields Linear response Lower dose rates needed

NO: Warm up time required Spatial distortion Maintenance

Page 23: Fluoroscopy: Viewing Systems

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