Rakesh C A

Fluoroscopy: a “see-through” operation with motion Used to visualize motion of internal

fluid, structures

Operator controls activation of tube and position over patient

Modern systems include image intensifier with television screen display and choice of recording devices

Purpose

To visualize, in real time: – organ motion

– ingested or injected contrast agents

– insert stents

–

–

–

–

–

– (endless)

CONVENTIONAL FLUOROSCOPYINVENTED BY THOMAS EDISON (1896)

Early Fluoroscopy

Early fluoroscopy = the image was viewed directly – the xrayphotons struck the fluoroscopic screen –emitting light.

Direct Fluoroscopy: obsolete

In older fluoroscopic examinations radiologist stands behind screen and view the picture

Conventional Fluoroscopic Unit

Consisted of:

x ray tube

x ray table

fluoroscopic screen

Activated zinc cadmium sulfide

Conventional Fluoroscopy systems9

30 min for dark adaptation

Photons used: Fluoro vs Radiography

Spotfilm Fluoroscopy

kVp: 85 85

mA: 200 3

Time (sec): 0.3 0.2*

mAs: 60 0.6

Ratio: 100 1

Older Fluoroscopy

• DISADVANTAGES:

– ROOM NEED COMPLETE DARKNESS

– PATIENT (& RADIOLOGIST) DOSE WAS VERY HIGH

– ONLY ONE PERSON CAN VIEW IMAGE

11

Visual Physiology

Fluoroscopic Image viewing based on

Human VisionRods

Cones

There are more than 100000 rods and cones per square millimetre of retina.

Cones = Photopic (daylight) Vision

• cones are less sensitive to light

• concentrated on the center of the retina in an area called fovea centralis

• capable of responding to intense light levels

• threshold is about 5x10-1 mL

• Cones are better at visualizing small detail than rods

• ability to perceive fine detail is called visual acuity

• cones are better at detecting differences in brightness levels than rods (contrast perception)

• cones are sensitive to a wide range of wavelengths but rods are essentially colour blind

Rods = Scotopic (night) Vision

• sensitive to light and are used during dim light situations

• located on the periphery of the retina

• No rods in fovea; so scotopicvision is entirely peripheral vision

• The density of rods is less over the remainder of the retina than the density of cones in fovea.

• threshold for rod vision is 10-6 mL (milliLambert)

• Scotopic (rod) vision is less acute than photopic(cone) vision

• Rods are most sensitive to blue-green light –daylight levels reduce the sensitivity to low illumination levels –hence the need for dark-adaptation with red goggles (to filter out blue green wavelengths)

• The dim fluroscopic vision required use of rod vision, with its poor visual acuity and poor ability to detect shades of gray (contrast).

• What was needed:

– Image bright enough to allow cone vision

– Without excess radiation exposure

Image Intensifier

Modern fluoroscopic system components

IMAGES ARE VIEWED ON A TV SCREEN/MONITOR

Basic Components of “Imaging Chain”

FluoroTUBE

Primary

Radiation PATIENT

EXIT Radiation

Image Intensifier

ABC Image Recording Devices

Fiber Optics OR

Photospot

CINE

Cas

sett

e

VIDICON

Camera Tube

CONTROL

UNITTV

LENS

SPLIT

Basic Components of “Imaging Chain”

FluoroTUBE

Primary

Radiation PATIENT

EXIT Radiation

Image Intensifier

ABC Image Recording Devices

Fiber Optics OR

Photospot

CINE

Cas

sett

e

VIDICON

Camera Tube

CONTROL

UNITTV

LENS

SPLIT

X-ray tube

• Similar to diagnostic 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

– Fluoroscopic tube can operate by foot switch

– Equipped with electrically controlled shutter

Fluoroscopy mA

Low, continuous exposures

0.05 – 5 mA

(usually ave 1 – 2 mA)

Radiographic Exposure (for cassette spot films)

100 – 200 mA

FLUORO TUBES

TUBE ABOVE THE TABLE TUBE UNDER THE TABLE

Basic Components of “Imaging Chain”

FluoroTUBE

Primary

Radiation PATIENT

EXIT Radiation

Image Intensifier

ABC Image Recording Devices

Fiber Optics OR

Photospot

CINE

Cas

sett

e

VIDICON

Camera Tube

CONTROL

UNITTV

LENS

SPLIT

Image Intensification Tubes

• Developed in 1948

• Is designed to amplify the brightness of an image

• New II are capable of increasing image brightness 500-8000 times

Image intensifier - Components

• protective vacuum case

• input window,

• input phosphor

• input photocathode

• electrostatic focussinglens

• accelerating anode

• output phosphor

Vacuum Case

• When the image intensifier was first introduced, it had a small input size and a glass vacuum case.

• Modern image intensifiers have input field sizes up to 57 cm in diameter with little image distortion, and the vacuum cases are usually made of metal.

• Encased in Lead housing = 2mm Pb

Input Screen

Input screenInput screen consists of four layers:

• The vacuum window (thin Al window that is part of the vacuum bottle)

• A support layer (also thin Al), curved for accurate electron focusing

• The input phosphor (CsI in thin, needle-like crystals)

• The photocathode (a thin layer of antimony and alkali metals, such as Sb2S3) that emits electrons when struck by visible light

33

Cesium Iodide (CsI) Phosphoron Input Phosphor

• CsI crystals grown linear and packed closely together

• The column shaped “pipes” helps to direct the Light with less blurring

• Converts x-ray photons to visible light

Cesium Iodide (CsI) Phosphoron Input Phosphor

Conventional Input Phosphor

Input Screen

Input phosphor andphotocathode are kept inclose contact so that thereis no loss in resolution

For undistorted focussing, all photoelectrons must travel the same distance.

The input phosphor is curved to ensure that electrons emitted at the peripheral regions of the photocathode travel the same distance as those emitted from the central region.

• The input phosphor is curved to ensure that electrons emitted at the peripheral regions of the photocathode travel the same distance as those emitted from the central region.

• It also gives the image intensifier better mechanical strengthunder atmospheric pressure.

Thickness of the input phosphor layerAdvantages

• higher x-ray absorption efficiency more x-ray photons can be absorbed and converted to light photons in the phosphor layer.

• requires fewer x-ray photons to generate the same amount of light photons at the image intensifier output window, thus reducing patient dose.

Disadvantages

• light photons are scattered laterally within the phosphor layer, thus reducing the spatial resolution.

• Currently, the thickness of an input phosphor layer is a compromise between spatial resolution and x-ray absorption efficiency and typically measures between 300 and 450 mm

Input phosphor material

• To maximize the conversion efficiency from x ray

photons to photoelectrons, the mass attenuation coefficient of the input phosphor material should be matched with the spectrum of the x rays emerging from the patient.

• Ideally, the light spectrum of the input phosphor should also match the sensitivity profile of the photocathode.

Input phosphor material

• The initial phosphor used in early image intensifiers was zinc-cadmium sulfide (ZnCdS),

• The current phosphor of choice is cesium iodide (CsI:Na).

Why CsI:Na??

1. The mass attenuation peaks in CsI:Na, compared with those of ZnCdS,are more closely matched to the transmitted xray spectrum, thus increasing the absorption of the transmitted x-ray photons. Increasing the absorption efficiency decreases the patient’s dose.

Why CsI:Na??

2. It has a high atomic number from Cs (Z = 55) and I (Z = 53),which also results in higher x-ray absorption.

• CsI screens absorbs 2/3 rd of the incident beam as compared to less than 1/3 rd for zinc cadmium sulfide.

Why CsI:Na??

3. K-edge energies for CsI is in the diagnsoticrange 36keV for Cs and 33 keV for I

Why CsI:Na??

4. CsI:Na can be evaporated onto the substrate in crystal needle form.

These needles act like light pipes, in a manner similar to the light propagation in a fiber-optic faceplate, thus reducing cross scatter inside the phosphor screen and yielding better spatial resolution.

Photocathode material

• The photocathode layer is made of antimony cesium (SbCs3).

• To maximize the conversion efficiency from light photon to photoelectron, light emitted from the input phosphor should match the sensitivity spectrum of the photocathode.

CsI:Na has a better spectral match to the antimony-cesium compound (SbCs3).

Image Intensifier

• The input phosphor converts x-ray to light

• Photocathode turns light into electrons (called photoemission)

• Now we have electrons that need to get to the anode……….. this is done by the electrostatic lenses

Electrostatic Focussing Lens

• Photoelectrons are accelerated from the photocathode to the output phosphor by the anode

• These are positivelycharged electrodes that are placed inside the glass envelope.

• These lenses help in preventing the diverging of the x-ray beams as they travel from cathode to anode.

• Electron focussing inverts and reverse the image ,this is called as point inversion, because all electrons pass through a common focal point .

Accelerating Anode

• Located in the neck of the II tube

• The potential applied at the anode is +25 to +35 kv more as compared to the cathode.

• This results in gain of kinetic energy by the electrons .

When the resulting high energy electrons strike the output phosphor produces more number of light photons and hence there is increase in the brightness of the image.

Output Phosphor

• Typically is called P20,• Materials used: ZnS:CdS: Ag

activated• converts electrons into visible

light• smaller than the input

phosphors (to 1 inch)• Crystal size and layer thickness

are reduced to maintain resolution in minified image.

• photo e- have much higher energies than when they were emitted from input screen

• can produce more light photonsthan the initial photo e- (increase app 50 folds)

ElectronsLight

photons

Output phosphor

• Anode is a very thin (~0.2 m) coating of aluminum on the vacuum side of the phosphor

Output phosphor• On the vacuum side of the

output phosphor surface, the anode of the electron optics system has a thin aluminum film coating.

• This aluminum film allows electrons to pass through, but it is opaque to light photons generated on the fluorescent screen.

• It stops these photons from being scattered backinto the image intensifier and exposing the photocathode. (prevents retrograde)

• The film also serves as a reflector to increase the output luminance.

Electrons Light photons

WE WILL HAVE TO DRAW THIS!!!

58

IMAGE INTENSIFIER PERFORMANCE

Image Intensifier Performance

Brightness Gain Conversion Factor

Brightness gain or Intensification factor

• Definition:

– output luminance level (or brightness) of an image intensifier divided by the output luminance level of a Patterson B-2 fluoroscopic screen when both are exposed to the same quantity of radiation.

Brightness Gain = 𝑰𝒏𝒕𝒆𝒏𝒔𝒊𝒇𝒊𝒆𝒓 𝒍𝒖𝒎𝒊𝒏𝒂𝒏𝒄𝒆

𝑷𝒂𝒕𝒕𝒆𝒓𝒔𝒐𝒏 𝑩−𝟐 𝒍𝒖𝒎𝒊𝒏𝒂𝒏𝒄𝒆

• The Patterson B-2 fluoroscopic screen was typically used for fluoroscopy before image intensifiers intensifiers were introduced.

• Drawback: lack of reproducibility

• Typical values: a few thousand to >10,000 for modern image intensifiers

Conversion Factor (ICRU)

• Definition:

– the output luminance level of an image intensifier divided by its entrance exposure rate.

• It is a measure of how efficiently an image intensifier converts the x rays to light.

Conversion Factor = 𝑳𝒖𝒎𝒊𝒏𝒂𝒏𝒄𝒆 𝒐𝒇 𝒐𝒖𝒕𝒑𝒖𝒕 𝒑𝒉𝒐𝒔𝒑𝒉𝒐𝒓

𝑰𝒏𝒑𝒖𝒕 𝑬𝒙𝒑𝒐𝒖𝒓𝒆 𝑹𝒂𝒕𝒆

= 𝑐𝑑/𝑚2

𝑚𝑅/𝑠𝑒𝑐

Conversion Factor

• With age Brightness Gain

Patient Dose

• The higher the conversion factor, the more efficient the image intensifier.

Minification gain

• Definition:– the ratio of input area to the output area of the image

intensifier.

Minification Gain = 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑖𝑛𝑝𝑢𝑡 𝑠𝑐𝑟𝑒𝑒𝑛

𝐴𝑟𝑒𝑎 𝑜𝑓 𝑜𝑢𝑡𝑝𝑢𝑡 𝑠𝑐𝑟𝑒𝑒𝑛=

𝑑𝑖2

𝑑𝑜2

• A smaller output window size will just compress more photons into a smaller area, producing a smaller but brighter image.

• Because the number of photoelectrons leaving the photocathode is equal to the number striking the output phosphor, the number of photoelectrons per unit area at the output phosphor increases.

Minification gain

• The minification gain does not improve the statistical quality of the fluoroscopic image.

• It will not change the contrast of the image, but it will make the image appear brighter.

Flux gain

• Definition:– The ratio of the number of light photons striking

the output screen to the ratio of the number of x-ray photons striking the input screen.

• The flux gain results from the acceleration of photoelectrons to a higher energy so that they generate more fluorescent photons at the output phosphor.

FLUX GAIN

• 1000 light photons at the photocathode from 1 x-ray photon

• photocathode decreased the number of electrons so that they could fit through the anode

• Output phosphor = 3000 light photons (3 X more than at the input phosphor!)

• This increase is called the flux gain

• Flux gain is almost always 50

Brightness Gainand Conversion Factor

• The brightness gain comes from two sources that are completely unrelated:

– the minification gain

– the flux gain.

• Brightness Gain = 𝑀𝑖𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝐺𝑎𝑖𝑛 × 𝐹𝑙𝑢𝑥 𝐺𝑎𝑖𝑛

IMAGING CHARACTERISTICS

1. Contrast

The contrast ratio of an image intensifier is defined as

• the brightness ratio of the periphery to the center of the output window when the center portion of an image intensifier entrance is totally blocked by a lead disk.

• The contrast ratio is typically specified in two ways: large area and small detail area.

• The large area or 10% area contrast ratio is measured by putting a lead disk, which has a surface area equal to 10% of the useful entrance area of the image intensifier, at the center of the input surface of the image intensifier.

• The small detail, or 10-mm area contrast, is measured by putting a 10-mm lead disk at the center of the input surface of the image intensifier.

• Measurements are made at 50 kVp without additional filtration.

• Currently, new image intensifiers have contrast ratios in the range of – 10:1 to 30:1 for the 10% area contrast ratios.– 15:1 to 35:1 for the 10-mm area contrast ratios.

Two factors diminish contrast

First:

• input screen does not absorb all the incident photons

• some of the transmitted ones can be absorbed by the output phosphor

• photons increase the brightness at the output phosphor but does not contribute to image formation

Two factors diminish contrast

Second:

• light flow from the output phosphor to the photocathode (retrograde)

• light flow generates more photo e- and also increases the brightness but does not contribute to the real image

• Contrast deteriorate as intensifier ages.

• Both mechanisms result in a brighter fog, thus reducing contrast

2. Sideways Light Scattering

Unsharpness due to the lateral diffusion of light after being produced by the input phosphor before reaching the photo cathode.

So keep both as close as possible

3. Geometric unsharpness

Can be avoided by placing the image intensifier as close to the patient body as possible.

4. Lag

• Persistence of luminescence after x-ray stimulation has been terminated.

• Lag degrades the temporal resolution of the dynamic image.

• usually of short duration-older tubes(30-40 ms) with CsI tubes-1ms.

• lag in modern fluoroscopic systems is more likely caused by the closed-circuit television system than the image intensifier.

example:

ZnS:CdS:Ag fluorescent screen 1% of the image luminance remains after 0.1 s and about 0.1% remains after 0.5 s

Artifacts

• Image intensifiers come with a variety of imperfections or artifacts

– pincushion distortion

– S distortion

– vignetting

– veiling glare

• Some of these artifacts are caused by improper calibration and can usually be corrected.

Pincushion Distortion

• Pincushion distortion is a geometric, nonlinear magnification across the image.

• Appearance of straight lines curving towards the edges

• The distortion is easily visualized by imaging a rectangular grid with the fluoroscope.

S Distortion

• Electrons within the image intensifier move in paths along designated lines of flux.

• External electromagnetic sources affect electron paths at the periphery of the image intensifier more, than those nearer the center.

• This characteristic causes the image in a fluoroscopic system to distort with an S shape

• Larger image intensifiers are more sensitive to the electromagnetic fields that cause this distortion.

• Manufacturers include a highly conductive mu-metal shield that lines the case in which the vacuum bottle is positioned to reduce the effect of S distortion.

Vignetting

• A fall-off in brightness at the periphery of an image is called vignetting.

• As a result, the centerof an image intensifier has better resolution, increased brightness, and less distortion.

Veiling Glare

• Scattering of light and the defocusing of photoelectrons within the image intensifier are called veiling glare.

• Veiling glare degrades object contrast at the output phosphor of the image intensifier.

• X-ray, electron, and light scatter all contribute to veiling glare.

MULTI FIELD IMAGE INTENSIFIERS

• In this type either the central part of the image can be viewed or the whole image.

• This can be brought about by increasing the charge of the focusing lens.

Magnification Tubes

• Greater voltage to electrostatic lenses

– Increases acceleration of electrons

– Shifts focal point away from anode

• Dual focus

– 23/15 cm 9/6 inches

• Tri focus

– 12/9/6 inches

Note focal point moves farther from output in mag mode

Intensifier Format and Modes

MAG MODE VS PT DOSE

• MAG USED TO ENLARGE SMALL STRUCTURE OR TO PENETRATE THROUGH LARGER PARTS

• PATIENT DOSE IS INCREASED IN THE MAG MODE

DEPENDANT ON SIZE OF INPUT PHOSPHOR

MAG MODE VS PT DOSE

% mag = 𝐼𝑃 𝑜𝑙𝑑 𝑠𝑖𝑧𝑒

𝐼𝑃 𝑛𝑒𝑤 𝑠𝑖𝑧𝑒Pt dose =

𝐼𝑃 𝑜𝑙𝑑 𝑠𝑖𝑧𝑒2

𝐼𝑃 𝑛𝑒𝑤 𝑠𝑖𝑧𝑒2

Viewing the Fluroscopic Image

Basic “Imaging Chain”

Basic Components of “Imaging Chain”

FluoroTUBE

Primary

Radiation PATIENT

EXIT Radiation

Image Intensifier

ABC Image Recording Devices

Fiber Optics OR

Photospot

CINE

Cas

sett

e

VIDICON

Camera Tube

CONTROL

UNITTV

LENS

SPLIT

We have stopped at the output phosphor

Viewing Fluoroscopic Images

Fluoroscopic Image monitoring

• Optical Coupling:The light output from the II needs to directed to a video camera and then to a television screen.

There are two ways of coupling the output window to the input of a video camera;

- Lens coupling- Fibre optic coupling

Lens coupling

- uses a pair of optical lens and a “beam splitting mirror” (to enable other accessories like spot film camera or cine camera) and an aperture.

- loss of image brightness due to lens system and beam splitting.

- Aperture controls the amount of light passes through to the TV camera.

Lens coupling

- A wide aperture will allow most light on to the video camera, thus reducing patient dose but the image will have high noise.

- A narrow aperture will allow only a fraction of the light on to the video camera, thus increasing patient dose but reducing the image noise.

Fibre optic coupling

• Uses fibre optic cables thus reducing light loss from the II to video camera

• Prevents any additional accessories being used.

• Preserves better spatial resolution

TV image

What’s our final aim?

TV Image

• Composed of discrete horizontal scan lines

• No of lines independent of monitor size• broadcast TV standard

– 525 lines

• High definition– 1025 lines– becoming more popular– more expensive

Viewing system

• It is development of the image from output screen to the viewer these include video, cine and spot film systems

• Most commonly used is video as closed circuit through cables to avoid broadcast interference

TV Camera

• Converts light to coded electrical signal

• Camera Tube

– vidicon

• cheapest / compact / laggy

– plumbicon

• enhanced vidicon / less lag

– CCD

• Semiconductor

• not a tube

TVCamera

Light

electricalsignal

Vidicon TV Pick-up Tube

Vidicon (tube) TV Camera

Video camera Tubes

• Video camera;

– is a cylindrical glass tube of 15 mm diameter and 25 cm long

– contains a target assembly, a cathode & electron gun, electrostatic grids and electromagnetic coils for steering and focusing of electron beams

Cathode• Is an electron gun which

emits electrons by heat (thermoionical) and shaped by the grid

• Electron accelerated toward the target

• Focusing coil bring the electron to a point to maintain resolution

• Pair of deflecting coils serve to cause the electron beam to scan the target in a path as a raster pattern

Vidicon Target Assembly

The target assembly contains 3 layers - the face plate, signal plate and photo-conductive layer.Vidicon tubes use antimony trisulfide (Sb2S3) (photo-conductive) while PlumbiconTM use lead oxide (PbO) in mica matrixThe globules are approx 0.025 mm in diameterEach globule capable of absorbing light photons and releasing electrons equivalent to intensity of the absorbed light

Vidicon Target Assembly

CCD REPLACED THE CAMERA IN VIDEO SYSTEM

1980

Video Camera Charge Coupled Device

Semiconductor Video Cameras

• These cameras are based on the charged coupled device (CCD) technology

• CCDs consist of a semiconductor chip which is sensitive to light – not vacuum tubes

• The chip contains many thousands of electronic sensors which react to light and generate a signal that varies depending on the amount of light each receives.

• When the light photon strikes the photoelectric cathode of CCD electrons are released

CCDs have been developed primarily for the domestic video camera market

They are:• Compact• lightweight • possess improved camera qualities compared to

photoconductive cameras.

CCD SYSTEM ADVANTAGE OVER CAMERA SYSTEM

• LOW LEVEL OF ELECTRONIC NOISE• HIGH SPATIAL RESOLUTION• NO LAG OR BLOOMING• NO MAINTENANCE• UNLIMITED LIFE• UNAFFECTED BY MAGNETIC FIELD• LINEAR RESPONSE• LOWER DOSE• A scanning electron beam in an evacuated environment

is not required,The image is read by electronic means.

Basic Components of old fluoroscopic “Imaging Chain”

FluoroTUBE

Primary

Radiation PATIENT

EXIT Radiation

Image Intensifier

ABC Image Recording Devices

Fiber Optics OR

Photospot

CINE

Cas

sett

e

VIDICON

Camera Tube

CONTROL

UNITTV

LENS

SPLIT

Basic Componets of “NEW DIGITAL” Fluoro“Imaging Chain”

Fluoro TUBE

Primary

Radiation PATIENT

EXIT Radiation

Image Intensifier

ABC CCD

Analog to

Digital

Converter

ADC

TV

I.I. AND CCD

LIGHT

SIGNAL

FUTURE – CCD REPLACED BY SILICON PIXEL DETECTORS

Video Signal

• Voltage level indicates brightness• Blanking during non-video

– retrace

Video Monitor

• A video monitor is used to display images acquired by the video camera of a fluoroscopy system.

- The image is described as a “softcopy”

- The video monitor is similar to an oscilloscope, ie, a scanning of the electron beam but in a raster fashion.

Video Monitor

• It is an evacuated glass tube which contains an electron gun, a number of focussing & steering electrodes and a phosphor screen.

• The electron gun forms the cathode and the electrons are accelerated by a high voltage towards the phosphor screen.

• The impact of the electrons on the screen causes it to fluoresce and the resulting light forms the image.

Video Monitor• Video monitors generally have two

viewer adjustable controls;

contrast - controlled by the number of electrons in the electron beam

brightness - controlled by the acceleration of the electrons in the tube

These have a strong influence on the quality of displayed images.

CRT

Television Scanning

• beam scanning for standard TV– 525 lines in total image

– 30 images (frames) scanned per second

• Oscillators– Vertical

– HorizontalVertical(Slower)

Horizontal(Faster)

• Eye can detect flashes – upto 50 pulses per second

• TV monitor only displays – 30 frames per second

FLICKER

Video Field Interlacing

Progressive Scanning

• progressive scanning

– used on newer systems, lines scanned in order

– no interlacing

Synchronization (Sync Signals)

Synchronization• TV Camera & Monitor must be

synchronized– In phase with each other

• Camera Control Unit adds special sync pulses sent at end of each horizontal line & vertical field – Horizontal and Vertical Syncronization Pulses

• Generated during retrace– horizontal retrace

• beam returned to left side of screen– vertical retrace

• beam returned to the top of screen– Turns off video during retrace

Vertical Retrace

Horizontal Retrace

Vertical Resolution

• proportional to number of vertical scan lines

• theoretic maximum– half number of visible scan lines

– black lines alternate with white

• max. line pairs = video lines / 2

Vertical Resolution

• actual limit lower than theoretical

~ 10% of lines occur during retrace

• returning beam from bottom to top of image

– scan lines may not perfectly synchronize to high resolution object

• typically 525 lines yield ~ 370 lines (185 line pairs)

Bandwidth (Bandpass)

• Varying frequency varying video signal

• The frequency range that the electronic components of the video system must be designed to transmit.

• sound (16Hz to 30,000Hz)

• no sharp frequency cutoff – not all frequencies transmitted or displayed with

same quality

– Gradual degrading

Bandwidth (Bandpass)

• What it means for video

– camera

• how fast camera can turn electrical signal on & off

– monitor

• how rapid a change in incoming electrical signal monitor can display

Horizontal Resolution

Bandwidth = [Horizontal Resolution] X [Video Lines] X [Frame Rate]

cycles------------scan line

lines---------frame

frames---------

sec

cycles----------

sec

Bandwidth[Horizontal Resolution] = -------------------------------------------

[Video Lines] X [Frame Rate]

= X X

Frequency of video signal

525 30

$$$$

Resolution Summary

• Vertical resolution depends onNumber of scan lines

• Horizontal resolution depends on– bandwidth– number of scan lines– frame rate

• Systems designed to yield approx. equal horizontal & vertical resolution~ 4.5 MHz typical bandwidth for 525 line system

Television Image Quality

• Depends upon:

– Resolution

– Contrast

– Lag

(1)Fluoro Resolution On TV Depends Upon

• TV resolution– total lines

– Frame rate

– bandwidth

• Size of imaged field

Overall TV Resolution (Example)

• typical 9” image tube• typical 185 line pairs for 525 line TV system

185 line pairs 1 inch------------------- X -------------- = .8 line pair / mm

9 inches 25.4 mm

• Higher number is better

Conventional TV Systems

• Fluoro Resolution–9 inch mode => 0.8 line pairs / mm–6 inch mode => 1.2 line pairs / mm–4 inch mode => 1.6 line pairs / mm

(2) Overall System Contrast

• Vidicon reduces contrast by about 20%

• monitor enhances contrast by up to 2X

– adjustable by operator

– brightness & contrast controls

• Plumbicon does not cause any decrease in image contrast.

ABC FEEDBACK LOOP

Generator

Exposure

Control

KVp

mA

Automatic

Brightness

Control Sensor

Light

Intensity

ABC• When the ABC mode is selected, the ABC

circuitry controls the X-ray intensity measured at the Image-Intensifier so that a proper image can be displayed on the monitor.

• ABC mode was developed to provide a consistent image quality during dynamic imaging

• The ABC compensates brightness loss caused by decreased I-I radiation reception by generating more X-rays (increasing mA) and/or producing more penetrating X-rays (increasing kVp).

• Conversely, when the image is too bright, the ABC compensates by reducing mA and decreasing kVp.

• Brightness Control: Generator feedback loop

– kVp variable

– mA variable/kV override

– kV+mA variable

– Pulse width variable (cine and pulsed fluoro)

The top curve increases mA more rapidly than kV as a function of patient thickness, and preserves subject contrast at the expense of higher dose.The bottom curve increases kV more rapidly than mA with increasing patient thickness, and results in lower dose, but lower contrast as well.

Recording the Fluroscopic Image

Types

• Direct film recording

• Indirect recording

• Recording motion.

Direct Film Recording

Spot Film Devices

• This rather familiar system, located in front of the image intensifier, accepts the screen-film cassette and “parks” it out of the way during fluoroscopy (Fig 1).

• One major limitation is the range of film sizes available for spot film imaging.

• Spot film devices usually allow more than one image to be obtained on a single film.

• Slightly more magnification

• Source to skin distance is shorter – skin entrance exposure higher

• The field size in spot film imaging is generally smaller than that used in general radiography. - reduces scatter - tends to reduce dose.

• Grids used in fluoroscopy generally have a lower grid ratio and therefore a smaller Bucky factor, which also leads to lower dose.

• One of the major shortcomings of conventional spot film devices is the delayinvolved in moving the cassette into position.

• In gastrointestinal imaging, this delay can be overcome by using photofluorography.

• In vascular imaging, more rapid film movement is achieved with automatic film changers.

Automatic Film Changers

Automatic Film Changers

• used in vascular imaging

• The number of films and filming rates must be preprogrammed for proper operation.

• limits the automatic changer to one film size, usually 35 x 35 cm.

• The typical film changer holds up to 30 films in the receiving magazine.

Indirect Recording

Photofluorography

Photofluorography

• More rapid filming - as many as 200 films• The film is cheaper and needs less storage space

than radiographic film. There is less delay between fluoroscopy and filming.

• Higher frame rates and longer runs are possible.• It is possible to view the images on the TV

monitor as they are being produced. Doses can be reduced.

• The disadvantages are poorer resolution and viewing a less than full-size image.

Digital Fluorography

• Digital charge coupled device (CCD) TV cameras are rapidly replacing conventional TV cameras in fluoroscopic systems.

• This result is about half the resolution of a photospotfilm. This resolution loss is made up for by the ability to digitally increase display contrast, reduce noise, and enhance the edges of digital images.

• Digital CCD cameras offer a compromise between radiation dose and image quality, with the added advantages of digital image manipulation and storage.

Recording Motion

Cine FluorographyVideotape RecordingMagnetic Disc RecordersOptical Discs

Cine Fluorography