Fluroscopy

Fluroscopy

1. Fluoroscopy Rakesh C A

2. 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

3. Purpose To visualize, in real time: organ motion ingested or injected contrast agents insert stents (endless)

4. CONVENTIONAL FLUOROSCOPY INVENTED BY THOMAS EDISON (1896)

5. Early Fluoroscopy Early fluoroscopy = the image was viewed directly – the xray photons struck the fluoroscopic screen – emitting light.

6. Direct Fluoroscopy: obsolete In older fluoroscopic examinations radiologist stands behind screen and view the picture

7. Conventional Fluoroscopic Unit Consisted of: x ray tube x ray table fluoroscopic screen

8. Activated zinc cadmium sulfide Green Screen

9. Conventional Fluoroscopy systems 9 30 min for dark adaptation

10. 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

11. Older Fluoroscopy DISADVANTAGES : ROOM NEED COMPLETE DARKNESS PATIENT (& RADIOLOGIST) DOSE WAS VERY HIGH ONLY ONE PERSON CAN VIEW IMAGE 11

12. Visual Physiology Fluoroscopic Image viewing based on Human Vision Rods Cones There are more than 100000 rods and cones per square millimetre of retina.

13. 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

14. 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

15. 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 scotopic vision 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 )

16. 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)

17. 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

18. Image Intensifier

19. Modern fluoroscopic system components

20. IMAGES ARE VIEWED ON A TV SCREEN/MONITOR

21. Basic Components of “Imaging Chain” Fluoro TUBE Primary Radiation PATIENT EXIT Radiation Image Intensifier ABC Image Recording Devices Fiber Optics OR Photospot CINE Cassette VIDICON Camera Tube CONTROL UNIT TV LENS SPLIT

22. Basic Components of “Imaging Chain” Fluoro TUBE Primary Radiation PATIENT EXIT Radiation Image Intensifier ABC Image Recording Devices Fiber Optics OR Photospot CINE Cassette VIDICON Camera Tube CONTROL UNIT TV LENS SPLIT

23. 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

24. Fluoroscopy mA Low, continuous exposures 0.05 – 5 mA ( usually ave 1 – 2 mA) Radiographic Exposure (for cassette spot films) 100 – 200 mA

25. FLUORO TUBES TUBE ABOVE THE TABLE TUBE UNDER THE TABLE

26. Basic Components of “Imaging Chain” Fluoro TUBE Primary Radiation PATIENT EXIT Radiation Image Intensifier ABC Image Recording Devices Fiber Optics OR Photospot CINE Cassette VIDICON Camera Tube CONTROL UNIT TV LENS SPLIT

27. 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

28. Image intensifier - Components protective vacuum case input window, input phosphor input photocathode electrostatic focussing lens accelerating anode output phosphor

29. 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

30. Image intensifier - Components input window, input phosphor input photocathode electrostatic focussing lens accelerating anode output phosphor protective vacuum case

31. Input Screen

32. Input screen Input 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. 33

34. Cesium Iodide ( CsI ) Phosphor on 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

35. Cesium Iodide ( CsI ) Phosphor on Input Phosphor

36. Conventional Input Phosphor

37. Input Screen Input phosphor and photocathode are kept in close contact so that there is no loss in resolution

38. 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.

39. 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 strength under atmospheric pressure.

40. Thickness of the input phosphor layer Advantages 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

41. I nput 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.

42. 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 ).

43. Why CsI:Na ??

44. 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 dos e .

45. Why CsI:Na ?? 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.

46. Why CsI:Na ?? K-edge energies for CsI is in the diagnsotic range 36keV for Cs and 33 keV for I

47. Why CsI:Na ?? 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 .

48. 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 .

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

50. 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

51. Electrostatic Focussing Lens Photoelectrons are accelerated from the photocathode to the output phosphor by the anode These are positively charged 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 .

52. 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 .

53. 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.

54. 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 photons than the initial photo e - (increase app 50 folds ) Electrons Light photons

55. Output phosphor Anode is a very thin (~0.2 m) coating of aluminum on the vacuum side of the phosphor

56. 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 back into the image intensifier and exposing the photocathode. (prevents retrograde) The film also serves as a reflector to increase the output luminance . Electrons Light photons

57. WE WILL HAVE TO DRAW THIS!!! 58

58. Image Intensifier Performance

59. Image Intensifier Performance Brightness Gain Conversion Factor

60. 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 = 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  

61. 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 = =  

62. Conversion Factor With age  Brightness Gain  Patient Dose  The higher the conversion factor, the more efficient the image intensifier.

63. Minification gain Definition : the ratio of input area to the output area of the image intensifier. Minification Gain = 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 .  

64. Minification gain The minification gain does not improve the statistical quality of the fluoroscopic image . It will not change the contras t of the image , but it will make the image appear brighte r .

65. 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.

66. 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

67. Brightness Gain and Conversion Factor The brightness gain comes from two sources that are completely unrelated: the minification gain the flux gain . Brightness Gain =  

68. Imaging Characteristics

69. 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.

70. 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.

71. 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.

72. 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

73. 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

74. 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

75. 3. Geometric unsharpness Can be avoided by placing the image intensifier as close to the patient body as possible.

76. 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 .

77. 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

78. DRAWBACKS 1. CONTRAST: There is decrease in the contrast of the image formed due to two main reasons- Since there is no complete absorption of the x rays by the input phosphor those which pass through this layer becomes incident on the output layer which decreases the contrast because this adds only to the brightness and does not help in image formation. Those x rays which strike the output phosphor liberate light photons some of which diffuse back towards the input phosphor.These light photons which are incident on the input phosphor in turn releases electrons from this layer causing blurring of the image when it strikes the output phosphor. 2. Geometric unsharpness can be avoided by placing the image intensifier as close to the patient body as possible. 3. Unsharpness due to electron focusing is called vignetting -in this case there is decrease in the intensity of the brightness at the periphery as compared to central area.This is because of the inability of the focusing cups to prevent the flaring up of peripheral beams of x rays as it reaches the output phosphor. 4. Unsharpness due to input phosphor is due to the lateral diffusion of light after being produced by the input phosphor before reaching the photo cathode. 5. Quantum noise is due to inefficient conversion of the x ray photons to light photons or by using x rays of low energy.This can be minimized by using crystals with high quantum detection efficiency. 6. Structure mottle is due to the presence of the crystals, which themselves cast shadows.However the mottling due to this is insignificant. 7. Gas Spots :Due to the presence of gas within the vacuum tube as x rays pass through it ,gets ionized releasing electrons there by causing bright spots in the images when it strikes the output phosphor.

79. 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.

80. Pincushion Distortion Pincushion distortion is a geometric, nonlinear magnification across the image. A ppearance of straight lines curving towards the edges The distortion is easily visualized by imaging a rectangular grid with the fluoroscope.

81. 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

82. 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 .

83. Vignetting A fall-off in brightness at the periphery of an image is called vignetting . As a result, the center of an image intensifier has better resolution , increased brightness , and less distortion .

84. 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.

85. 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.

86. 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

87. Note focal point moves farther from output in mag mode Intensifier Format and Modes

88. 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

89. MAG MODE VS PT DOSE % mag =   Pt dose =  

90. Viewing the Fluroscopic Image

91. Basic “Imaging Chain”

92. Basic Components of “Imaging Chain” Fluoro TUBE Primary Radiation PATIENT EXIT Radiation Image Intensifier ABC Image Recording Devices Fiber Optics OR Photospot CINE Cassette VIDICON Camera Tube CONTROL UNIT TV LENS SPLIT

93. We have stopped at the output phosphor

94. Viewing Fluoroscopic Images

95. 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

96. 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.

97. 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 .

98. 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

99. TV image What’s our final aim?

100. 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

101. 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

102. TV Camera Converts light to coded electrical signal Camera Tube vidicon cheapest / compact / laggy plumbicon enhanced vidicon / less lag CCD Semiconductor not a tube TV Camera Light electrical signal

103. Vidicon TV Pick-up Tube

104. Vidicon (tube) TV Camera

105. 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

106. 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

107. Vidicon Target Assembly The target assembly contains 3 layers - the face plate, signal plate and photo-conductive layer . Vidicon tubes use antimony trisulfide (Sb 2 S 3 ) ( photo-conductive ) while Plumbicon TM use lead oxide ( PbO ) in mica matrix The globules are approx 0.025 mm in diameter Each globule capable of absorbing light photons and releasing electrons equivalent to intensity of the absorbed light

108. Vidicon Target Assembly

109. CCD REPLACED THE CAMERA IN VIDEO SYSTEM 1980 Video Camera Charge Coupled Device

110. 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

111. CCDs have been developed primarily for the domestic video camera market They are: Compact lightweight possess improved camera qualities compared to photoconductive cameras.

112. 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.

113. Basic Components of old fluoroscopic “Imaging Chain” Fluoro TUBE Primary Radiation PATIENT EXIT Radiation Image Intensifier ABC Image Recording Devices Fiber Optics OR Photospot CINE Cassette VIDICON Camera Tube CONTROL UNIT TV LENS SPLIT

114. 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

115. I.I. AND CCD LIGHT SIGNAL

116. FUTURE – CCD REPLACED BY SILICON PIXEL DETECTORS

117. Video Signal Voltage level indicates brightness Blanking during non-video retrace

118. 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.

119. 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.

120. 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.

121. CRT

122. Television Scanning beam scanning for standard TV 525 lines in total image 30 images ( frames ) scanned per second Oscillators Vertical Horizontal Vertical (Slower) Horizontal (Faster)

123. Eye can detect flashes – upto 50 pulses per second TV monitor only displays – 30 frames per second FLICKER

124. Video Field Interlacing

125. Progressive Scanning progressive scanning used on newer systems, lines scanned in order no interlacing

126. Synchronization (Sync Signals)

127. 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

128. 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

129. 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)

130. 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

131. 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

132. 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 $$$$

133. Resolution Summary Vertical resolution depends on Number of scan lines Horizontal resolutio n 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

134. Television Image Quality Depends upon: Resolution Contrast Lag

135. (1) Fluoro Resolution On TV Depends Upon TV resolution total lines Frame rate bandwidth Size of imaged field

136. 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

137. 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

138. (2) Overall System Contrast V idicon 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.

139. ABC FEEDBACK LOOP Generator Exposure Control KVp mA Automatic Brightness Control Sensor Light Intensity

140. 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 .

141. Brightness Control: Generator feedback loop kVp variable mA variable/kV override kV+mA variable Pulse width variable (cine and pulsed fluoro )

142. 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.

143. Image quality Spatial resolution of the II best described by modulation transfer function (MTF) The limiting resolution of an imaging system is where the MTF approaches zero Higher magnification modes (smaller fields of view) are capable of better resolution Video imaging system degrades the MTF substantially

144. The limiting spatial resolution is the size of the smallest object that an imaging system can resolve. The limiting resolution of modern image intensifiers is between 4 and 5 cycles/mm.

145. Image quality (cont.) Contrast resolution of fluoroscopy is low compared with radiography. Excellent temporal resolution of fluoroscopy is its strength and its reason for existence

146. Recording the Fluroscopic Image

147. Types Direct film recording I ndirect recording Recording motion .

148. Direct Film Recording

149. Spot Film Devices

150. 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

151. 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 .

152. One of the major shortcomings of conventional spot film devices is the delay involved 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.

153. Automatic Film Changers

154. 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.

155. Indirect Recording

156. Photofluorography

157. 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.

158. 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 photospot film. 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.