Revision lecture 1
- 1. INTERACTION OF X-RAY INTERACTION OF X-RAY WITH MATTER:WITH MATTER:• X-rays entering a patient can be absorbed, scattered, or transmitted. • When an X-ray is absorbed in a patient, all of the X-rays energy is transferred into the patient's tissue. • Scattering changes the X-ray's direction and reduces its energy. • Transmitted x-rays pass through the patient without interaction. • Most diagnostic x-rays are absorbed or scattered.
- 4. Ideal Grid Ideal Grid • block all scattered radiation › Reality: lead strips permit some scatter to get through to film Lead Interspace
- 5. 1.The Screen-Film Cassette Cassette Light-tight and ensures screen contact with film Front surface - carbon fiber ID flash card area on back Back lined with lead to reduce back-scatter. 1 or 2 Intensifying Screens Convert x-rays to visible light Mounted on layers of compressed foam (produces force) Ease to open & closed, under low light conditions. Lightweight, no sharp edges or corners which may injure patients or staff. Vinyl covered front providing warmth. Availability in range of film sizes 5
- 6. Cassettes Cassettes • Cassettes are rigid, light-tight devices that hold x-ray film in contact with the intensifying screen
- 8. Structure of the film, intensifying screen and cassette
- 9. Screen Phosphors Screen Phosphors • Screen color differs based on the type of phosphor and dyes imbedded in screen. • Emits blue, ultraviolet or green light that exposes the film. • Decreases exposure by 100 fold compared to non-screen film technology.
- 10. X-ray film and Intensifying screens X-ray film and Intensifying screens Poor film screen contact produce image unsharp (poor detail image)
- 11. X-ray film and Intensifying screens Close contact must be maintained between the film and intensifying screens (in light tight cassettes) to minimize the loss of detail and to protect the film from extraneous light. A B The Fundamentals of Radiography. Kodak
- 12. Noise Effects of Changing CE vs. AE (2) What happens to noise in image when the AE is increased (thicker screen)? If AE is increased, 10% more x-ray photons detected, then reduction of 10% in incident x-ray beam is required to deliver same amount of film darkening (as before increasing AE) Since the fraction of increase in x-ray photon detection and reduction in incident x-ray intensity is same, the total number of detected x-ray photons is the same. No change in noise However, spatial resolution will get worse with thicker screens 12
- 13. Screens Screens • The x-ray photon will strike the phosphor crystal in the screen. • The excited phosphor will emit a specific wavelength of light that exposed the film. • The efficiency of the screen is based on: › Thickness of phosphor layer › The type of phosphor › The size of the phosphor crystal • Trade-off: increased efficiency, decreased spatial resolution (thicker layer, larger crystal size).
- 14. Screens Screens • The thicker the phosphor layer the less detail of the image • Note the spreading of the light in the thicker screen layers
- 15. Screens -- Resolution Screens Resolution • Resolution (detail) can be measured using a line resolution phantom. • Resolution is defined and the smallest number of line pairs than can be seen › High detail screen (50 speed) may resolve 16 LP/mm › Rapid screen (400 speed) may resolve 5 LP/mm but require 1/8th the radiation
- 16. Spectral Matching Spectral Matching • Intensifying screen phosphors contain different elements • The color of light emitted will also differ › Yttrium tantalate - Violet › Barium lead sulfate – Blue › Lanthanum oxybromide – Blue › Gadolinium oxysulfide – Green › Calcium Tungsten - Blue • Activators › Thulium activated - Blue › Terbium activated - Green
- 17. Spectral Matching Spectral Matching • It is important that the wavelength of light emitted by the screen matches the sensitivity of the film. • Specific filters required for safety light for green versus blue sensitive film.
- 18. Intensifying Screen Function and Geometry Modulation Transfer Function (MTF) describes the resolution properties of an imaging system The MTF illustrates the fraction (or %) of an object’s contrast that is recorded by the imaging system as a function of object size (spatial frequency) Frequency (linepairs or cycles/mm) F=1/2∆, ∆ = object size As screen thickness ↑ MTF ↓ 18
- 19. Sensitivity or Speed (3) A general relationship between exposure requirements (sensitivity) and speed values: sensitivity (mR) =128/ speed. For example, a screen with true speed value of 100 requires an exposure of 1.28 mR to produce a 1 unit of density. 100-speed – detail work (thinner screens, slower, better spatial resolution) 600-speed – angiography (thicker screens, decreased spatial resolution) The range of system sensitivity and speed values used in radiography is shown below: Speed (Sensitivity (mR 1200 0.1 800 0.16 400 0.32 200 0.64 100 1.28 50 2.56 25 5.0
- 20. Sensitivity or Speed Sensitivity is expressed in terms of exposure required to produce a film density of 1 unit above the base plus fog level. Speed values such as 100, 200, 400, ect, compare the relative exposure requirement of different screen. Most speed numbers are referenced to a so called par speed system that is assigned a speed value of 100. 20
- 21. Comparison of typical exposure factors in an Comparison of typical exposure factors in an AP projection of the knee(12:1 moving grid( AP projection of the knee(12:1 moving grid( › Type of radiography exposure factors --------------------------------------------------------------- › Ultrahigh speed screens 20mAs(100mAx0.2 sec at 60kvp) › High speed screens 30mAs(100mAx0.3 sec at 60kvp) › Par speed screens 60mAs(100mAx0.6 sec at 60kvp) › Slow speed screen 120mAs(100mAx1.2 sec at 60kvp) › Direct exposure › 4800mAs(100mAx12 sec at 60kvp)
- 22. Comparison screen speed to image details Comparison screen speed to image details Using of fast speed more intensifying screen increase image unsharpess by diffusion of light
- 25. X-Ray Film Construction X-Ray Film Construction • Film base • Adhesive layer – attaches emulsion to base • Emulsion layer • Supercoating Adhesive Layers ”007. ”0005. Supercoating Film Base Emulsion Layers
- 26. Film Base Film Base • • • • structural support for fragile emulsion low light absorption Film no visible pattern Base flexible, thick, & strong – processing – handling – viewbox insertion / removal abuse • dimensional stability – in processing – For archival » varying humidity
- 27. Film Base Materials Film Base Materials • early films used cellulose nitrate – Flammable • “safety” base – cellulose triacetate used until 1960’s – polyester » .007 inches thick – Base color » Clear » Blue dye added » Requested by radiologists • reduces eye strain Film Base ”007.
- 28. Emulsion Emulsion • Most films use two emulsions – each emulsion <=.5 mil thick » thicker emulsion = less light penetration • Gelatin – keeps silver halide grains dispersed / prevents clumping – allows penetration of processing solutions without compromising strength or permanence – made from cattle bones Emulsion Layers
- 29. Supercoating Supercoating • Thin supercoating covers emulsion • protects from mechanical damage • makes film smooth & slick for use with processors Supercoating
- 30. Silver Halide Silver Halide • light sensitive • 90 - 99% silver bromide • 1 - 10% silver iodide – increases sensitivity • small crystals (grains) precipitated and emulsified in gelatin – crystal has lattice structure » Ag+ / Br- / I- – silver nitrate added
- 31. Silver Halide Silver Halide • precipitation determines crystal size & concentration – typical size: 1 - 1.5 microns – 1 grain averages 1 - 10 million silver ions • chemical sensitization of crystal – sulfur-containing compound added to emulsion – silver sulfide formed » usually located on crystal surface » called sensitivity speck » traps electrons to begin formation of latent image centers
- 32. Light Image on Film Light Image on Film • light photon allows escape of electron in bromine ion (Br -) • neutral bromine atoms leave crystal, go into emulsion gelatin • electron travels to, fixed in sensitivity speck • negative sensitivity speck attracts mobile silver (Ag+) ion forming silver atom Ag+ + electron Ag • repeated trapping of electrons results in growth of silver
- 33. Latent Image Centers Latent Image Centers • one light photon produces one silver atom • silver atoms collect at sensitivity speck – no visible change in grain • visible amounts of silver deposited at latent image centers during processing • one or more latent image centers per grain – 3 - 6 centers required for grain to be developable – centers may contain 100’s of silver atoms
- 37. Latent Image Formation: Latent Image Formation: Gurney-Mott Gurney-Mott Light photon absorbed by/ejects Br electron Electron trapped at sensitivity speck Neg electron attracts interstitial Ag+ ion Ag+ and e- combine to form neutral (black) Ag If >6-10 Ag0 accumulate at speck, it becomes a latent image center: ie, .it is developable
- 38. Processing Processing • Amplifies latent image by 100,000,000! • forms visible silver • reduces silver ions into neutral black metallic silver atoms which remain on the film after processing Ag+ + electron Ag • processing initiated at latent image speck – grain either develops entirely or not at all
- 39. Processing Processing • Silver atoms at latent image center act as catalyst • Grains with no latent image also develop much more slowly – Developer time is fundamental in development – processing should stop when maximum difference between exposed & unexposed crystals
- 40. Processing Developing Processing Developing Solution Solution • developing agent – hydroquinone – phenidone or metol – combination yields development rate greater than sum of each • alkali – adjusts pH • preservative (and oxidation preventative) – sodium sulfite • restrainers – antifoggants (reduces development of unexposed grains)
- 41. Developing Time Developing Time • Controlled by – Speed of transport – Film path in develop rack » System of rollers & chains which direct film through developer tank • “90 second” processor – 90 seconds from film in to film out – Time in developer ~ 20 second – Time in fixer & wash tanks controlled by size of fixer & wash racks » Transport speed does not change because at any time films may be in any or all tanks
- 42. Developing Developing • Temperature controlled to ~0.5 degrees 90 - 95 degrees for 90 second processor 100+ degrees for 60 second processor • Replenishment – automatic addition of fresh chemistry to replace chemistry depleted in development – even with replenishment, chemistry must be completely replaced periodically
- 43. Fixing Fixing • Function – removes remaining silver halide / silver ions without damaging metallic silver – hardens gelatin • composition – cyanides (poisonous & not usually used) – thiosulfates » sodium or ammonium salt • hypo – buffers to maintain pH
- 44. Washing Washing • Removes fixer chemicals • Fixing leaves milky appearance on film – unwashed film turns brown with age
- 45. (3)Production (3)Production Permanent Form Permanent Form • This involves the action of a chemical agent to make the hidden image visible. This stage is known as development, and it is followed by further chemical processes which fix the image and make it into permanent record.
- 46. A Film Processor
- 47. The Two Steps in the Formation of a Film Image
- 48. Latent Image Air/soft tissue Many x-rays penetrate and expose many silver halide crystals Bone Amalgam/gold Fewer x-rays penetrate and not as many silver halide crystals are exposed Few, if any, x-rays penetrate; silver halide crystals not exposed Exposure centers=
- 49. Developing Development centers in crystals struck by x-rays are converted into black metallic silver Air/soft tissue Bone Metal
- 50. (Developing (continued Entire crystal converted to black metallic silver Air/soft tissue Bone Metal
- 51. Fixing In the fixing solution, the unexposed silver halide crystals are removed from the film by the fixing . solution Air/soft tissue Bone Metal
- 52. Sequence of Events That Convert a Transparent Film Grain into Black Metallic Silver
- 55. Fog and Scatter Fog and Scatter • Fog and Scatter reduce contrast
- 56. Fog and Scatter Fog and Scatter • Fog and Scatter reduce contrast • Scatter › › › produces unwanted density mostly a result of Compton interactions increases with » kVp » part thickness » field size • collimation reduces scatter
- 57. Fog Fog • Development of film grains not exposed to light or x-rays • produces unwanted density • lowers radiographic contrast
- 58. Determining Fog Determining Fog • run half sheet of film through developer, fixer, wash, dryer • run other half through all but developer • compare densities › difference is fog
- 59. Exposure Fog Exposure Fog • also called “fog” but different from development fog • refers to accidental exposure to radiation Wall Darkroom Wall
- 60. Development Fog Development Fog • Development of unexposed grains › “true” fog • Sources of optical density increase › Storage » high temperature » high humidity › › › chemistry contamination excessive developer time excessive developer temperature
- 61. Fog and Scatter Fog and Scatter • Alter characteristic curve • Reduce contrast at clinical densities • less effect at higher densities
- 62. :Some References