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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY. L15.1: Optimization of protection in radiography: technical aspects. Topics. Intensifying screen structure and characteristics

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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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  1. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY L15.1: Optimization of protection in radiography: technical aspects

  2. Topics • Intensifying screen structure and characteristics • Screen-film combination • Radiographic film structure and characteristics • Antiscatter grid • Film processor • Darkroom and View Box • Image parameters 15.1: Optimization of protection in radiography: technical aspects

  3. Overview • To become familiar with basic knowledge of the components that form the radiographic chain. 15.1: Optimization of protection in radiography: technical aspects

  4. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 15.1: Optimization of protection in radiography Topic 1: Intensifying screen structure and characteristics

  5. Primary beam attenuation and latent image Film, fluorescent screen or image intensifier Scattered radiation « Latent » radiological image Bone X Soft tissue Air Primary collimation Antiscatter Grid Beam intensity at detector level 15.1: Optimization of protection in radiography: technical aspects

  6. Intensifying screen Layer of material placed immediately adjacent to film in conventional radiography to: • Convert the incident X Rays into radiation more suitable for the light-sensitive emulsion of the radiographic film (X Ray light photons) • Reduce the patient dose needed to achieve a given level of film quality • Reduce the exposure time as well as the power required from the X Ray generator (cost savings) • Increase photoelectric effect  better use of the beam energy (image formation) 15.1: Optimization of protection in radiography: technical aspects

  7. Intensifying screen structure (I) • Supporting Base (mainly polyester material) • chemically neutral, resistant to X Ray exposure, flexible • Reflecting layer (Titanium dioxide - TiO2) • a crystalline compound reflecting photons toward sensitive emulsion • Fluorescent layer (polymer) • crystals dispersed in a suspension of plastic material • Protective overcoat • colourless thin overcoat to help avoid abrasions of fluorescent layer due to the use of screen 15.1: Optimization of protection in radiography: technical aspects

  8. (Incident X Ray beam) Supporting Base (240 m) Screen Reflecting layer (25 m) Fluorescent layer (100 to 400 m) Protective overcoat (20 m) (Light-sensitive film) Intensifying screen structure (II) 15.1: Optimization of protection in radiography: technical aspects

  9. Intensifying screen structure (III) • The fluorescent layer • should: • be able to absorb the maximum quantity of X Rays • convert the X Ray energy into light energy • match its fluorescence with the film sensitivity (color of emitted light) • Type of material: • Calcium tungstate CaWO4 till 1972 • Rare earth since 1970 LaOBr:Tb and Gd2O2S:Tb more sensitive and effective than CaWO4 15.1: Optimization of protection in radiography: technical aspects

  10. Intensifying screen characteristics (I) • IF (Intensification Factor): ratio of exposures giving the same film optical density, with and without screen • 50 < IF < 150 (depending on screen material and X Ray beam energy) • QDE (Quantum Detection Efficiency): fraction of photons absorbed by the screen • 40% for CaWO4 < QDE < 75% for rare earth (depending on crystal material, thickness of fluorescent layer and X Ray spectrum) 15.1: Optimization of protection in radiography: technical aspects

  11. Intensifying screen characteristics (II) •  Conversion efficiency— ratio of light energy emitted to X Ray energy absorbed (%) • 3% for CaWO4 < < 20% for rare earth • C (Detection Coefficient): ratio of energy captured and used by the film to energy emitted by the crystal (%) • C is maximum for screens emitting in UV wave length  90% 15.1: Optimization of protection in radiography: technical aspects

  12. Intensifying screen characteristics (III) Sensitivity of a Conventional Film BaSO4:Eu,Sr YTaO4:Nb Relative Sensitivity of Film BaSO4:Pb CaWO4 250 300 350 400 450 500 550 600 UV Blue Green 15.1: Optimization of protection in radiography: technical aspects

  13. Intensifying screen characteristics (IV) • Intensifying factor: ratio of exposures giving the same film optical density, with and without screen 175 150 125 100 75 50 25 0 Gd2O2S LaOBr Intensifying factor CaWO4 kV 50 60 70 80 90 100 110 120 15.1: Optimization of protection in radiography: technical aspects

  14. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 15.1: Optimization of protection in radiography Topic 2: Screen film combination

  15. Screen film combination • Sensitivity (screen film): The quotient K0/Ka, where K0 = 1 mGy and Ka is the air kerma free-in-air for the net density D = 1.0, measured in the film plane • Screen film system: A particular intensifying screen used with a particular type of film • Sensitivity class: Defined range of sensitivity values of a screen film system • Single emulsion film: One coated film used with one intensifying screen • Double emulsion film: A double coated film used with a couple of intensifying screens 15.1: Optimization of protection in radiography: technical aspects

  16. Screen film combination performance • Spatial Resolution: capability of a screen film combination to record and display a test pattern specified in cycles/mm. Modulation Transfer Function (MTF): description of how sinusoidal fluctuations in X Ray transmission through the screen film combination are reproduced in the image • Noise spectrum: Noise as a function of frequency • Quantum Detection Efficiency (QDE): Measure of combined effect of signal and noise performance as a function of frequency 15.1: Optimization of protection in radiography: technical aspects

  17. Screen film combination performance • Assure that screen emission spectrum matches sensitivity of film being used • Screen film contact • loss of spatial resolution • blurred image • Cleanliness • Inter cassette sensitivity 15.1: Optimization of protection in radiography: technical aspects

  18. Effect of screen on resolution • Screen resolution is dependent on the crystal size and thickness of screen • Direct exposure radiography has better resolution than screen-film (but requires around 40 times the radiation exposure) • Direct exposure film ~ 30 c/mm; 200 speed screen-film system ~ 10 c/mm; 400 screen-film system ~ 6 c/mm; mammography system ~ 15 c/mm 15.1: Optimization of protection in radiography: technical aspects

  19. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 15.1: Optimization of protection in radiography Topic 3: Radiographic film structure, image formation and processing characteristics

  20. Radiographic film(structure and characteristics) • Protective layer (outer surface) • Sensitive layer (~20µ) • Base material (transparency and mechanical resistance) (~170µ) • Binding (base-sensitive layer) or anti cross-over layer • Filtering layer • Sensitivity class 15.1: Optimization of protection in radiography: technical aspects

  21. Radiographic film structure Supercoat Emulsion (~5-20 µm thick) Adhesive layer Base (~200 µm thick) Anti-curl, anti-halation layer Single Emulsion Film 15.1: Optimization of protection in radiography: technical aspects

  22. Film construction • Supercoat - prevents scratching • Base • provides relatively thick, semi-rigid structure to film, but still allowing flexibility • almost (but not completely) transparent • Emulsion • image layer, composed of gelatine and silver halide (Br, I) crystals in ionic form • speed,contrast, resolution varied in emulsion 15.1: Optimization of protection in radiography: technical aspects

  23. Radiographic film structure Supercoat Emulsion Adhesive layer Base Adhesive layer Emulsion Supercoat Double Emulsion Film 15.1: Optimization of protection in radiography: technical aspects

  24. Silver halide reaction • Latent image (invisible) formed by interaction of a light photon from screen, with a halide ion within the crystals, which: • releases an electron, • which in turn reacts with silver ion, • forming atomic silver within the crystal 15.1: Optimization of protection in radiography: technical aspects

  25. Processing • Development • Converts latent image to metallic silver • Fixing • Dissolves unexposed silver halide crystals, leaving only metallic silver, creating a permanent image 15.1: Optimization of protection in radiography: technical aspects

  26. Steps in image formation 15.1: Optimization of protection in radiography: technical aspects

  27. Spectral response and spectral matching • The variation in film sensitivity to the various colours of light • Film is usually blue or blue-green sensitive (orthochromatic) • Screens emit blue (e.g., calcium tungstate) or green (rare earth screens) light • Safelights must not affect film 15.1: Optimization of protection in radiography: technical aspects

  28. Spectral response of film 15.1: Optimization of protection in radiography: technical aspects

  29. Crossover • In double emulsion film, light emitted by one screen can cross over through the adjacent emulsion, and the base and expose the second emulsion • This will reduce the resolution of the image • Is prevented with a light-absorbing dye layer 15.1: Optimization of protection in radiography: technical aspects

  30. Crossover 15.1: Optimization of protection in radiography: technical aspects

  31. Optical density Transmitted light intensity Incident light intensity It I0 Optical Density = log10 I0 / It Film e.g. 10% transmission = 1.00 1% transmission = 2.00 15.1: Optimization of protection in radiography: technical aspects

  32. Characteristic curve of a radiographic film Optical Density (OD) Saturation OD2 Visually evaluable range of densities  = (OD2 -OD1) / (log E2 - log E1)  The  of a film: the gradient of the «straight line» portion of the characteristic curve OD1 Normal range of exposures Base + fog Log Exposure (mR) E1 E2 15.1: Optimization of protection in radiography: technical aspects

  33. Average gradient • The straight line portion of the characteristic curve is difficult to determine (and there may not be one), so the average gradient is measured between optical densities of 0.25 and 2.00 15.1: Optimization of protection in radiography: technical aspects

  34. Film sensitometry parameters • Base + fog: The optical density of a film due to its base density plus any action of the developer on the unexposed silver halide crystals usually 0.15 -0.30. • Sensitivity (speed): The reciprocal of the exposure value needed to achieve a film net optical density of 1.00 • Gamma (contrast): The average gradient of the characteristic curve • Latitude: The range of exposures that can be recorded and visualized on the film. 15.1: Optimization of protection in radiography: technical aspects

  35. Comparison of characteristic curves OD OD Film A Film A Film B Film B 1+B+Fog Log Exposure (mR) Log Exposure (mR) Film A is faster than Film B Film A and B have the same sensitivity but different contrast Film A and B have the same contrast 15.1: Optimization of protection in radiography: technical aspects

  36. 11 12 13 16 17 18 20 21 10 14 15 19 1 2 3 4 5 6 7 8 9 Sensitometric strip Sensitometry: A method of exposing a film by means of a light sensitometer and assessing its response to exposure and development 15.1: Optimization of protection in radiography: technical aspects

  37. Sensitometric strip 15.1: Optimization of protection in radiography: technical aspects

  38. Latitude Film B has higher latitude (range of useful exposures) than film A, but has lower contrast (slope of the curve) 15.1: Optimization of protection in radiography: technical aspects

  39. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 15.1: Optimization of protection in radiography Topic 4: Anti-scatter grid and grid performance parameters

  40. Anti-scatter grid (I) • Radiation emerging from the patient • primary beam: contributing to the image formation • scattered radiation: reduces contrast • the grid (between patient and film) eliminates most of scattered radiation • stationary grid • moving grid (better performance) • focused grid • Potter-Bucky system 15.1: Optimization of protection in radiography: technical aspects

  41. Anti-scatter grid (II) Source of X Rays Patient Scattered X Rays Lead strip Film and cassette Useful X Rays 15.1: Optimization of protection in radiography: technical aspects

  42. Grid performance parameters (I) • Grid ratio • Ratio of the height of the strips to the width of the gaps at the central line • Contrast improvement ratio • Ratio of the transmission of primary radiation to the transmission of total radiation • Grid exposure factor • Ratio of the total radiation without the anti-scatter grid in a specified radiation beam to that with the anti-scatter grid placed in the beam 15.1: Optimization of protection in radiography: technical aspects

  43. Grid performance parameters (II) • Strip number • The number of attenuating lead strips per cm • Grid focusing distance • Distance between the front of a focused grid and the line formed by the converging attenuating lead strips of the grid 15.1: Optimization of protection in radiography: technical aspects

  44. Example of anti-scatter grids (grid ratio) Grid: C Grid: A Grid: B D h    1 h Grid ratio: r = = 5 < r < 16 tg  D • Grid A and B have the same strip number • Grid B and C have the same interspace between the lamella 15.1: Optimization of protection in radiography: technical aspects

  45. Grid selectivity(I) Grid: C Grid: A Grid: B    15.1: Optimization of protection in radiography: technical aspects

  46. Grid selectivity (II) 100 90 80 70 60 55 50 45 40 35 30 25 20 15 10 5 0 • A grid with r = 12 transmits 5% • of scattered radiation • A grid with r = 16 transmits 3.8% • 30% difference in patient dose % of scattered beam transmitted 5% 3.8% r 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 15.1: Optimization of protection in radiography: technical aspects

  47. Grid focusing error(virtual increasing of grid shadow) X Ray source (too far) X Ray source (too close) Grid Film and cassette grid shadow deformation (applicable to both cases) 15.1: Optimization of protection in radiography: technical aspects

  48. G R I D S ho r t es t Long es t CHARACT E R IS T I C S d is t a n ce d is t a n ce Fo c a liz a t i o n R a t i o ( cm ) ( cm ) ( cm ) r 8 0 7 6 8 9 6 8 0 1 0 7 2 9 1 1 0 0 1 0 8 7 1 1 6 1 0 0 1 4 9 1 1 1 0 1 3 0 1 8 0 1 5 0 1 3 Grid focusing error(leading to 25% of beam loss) 15.1: Optimization of protection in radiography: technical aspects

  49. Grid out of center(virtual deformation of grid shadow) Lateral shift X Ray source Film and cassette Grid Grid shadow 15.1: Optimization of protection in radiography: technical aspects

  50. G R I D MA XI MUM CHARACT E R IS T I C S LAT E RAL S H I FT Fo c a liz a t i o n R a t i o ( cm ) ( cm ) r 8 0 7 2 .8 8 0 1 0 2 1 0 0 1 0 2 .5 1 0 0 1 4 1 .8 1 5 0 1 3 2 .9 Grid focusing error due to lateral shift(leading to 25% loss of X Ray beam) 15.1: Optimization of protection in radiography: technical aspects

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