Objectives Review Film Review Intensifying Screens Technique Changes for Screen Speeds
What is the function of radiographic film? What is exit radiation? What is another name for exit radiation? What is an IR? Name some examples
X-Ray Film Film is a media that makes a permanent record of the image. Image recorded on film is caused by exposure to photons
X-ray Film cont’d Radiographic film is/was most common image receptor Two parts: 1. Base 2. Emulsion
FIRST “FILM” GLASS PLATES WW 1 CELLULOSE NITRATE HIGHLY FLAMMABLE EASILY TORN RESPONSIBLE FOR MANY FIRES IN HOSPITAL BASEMENTS CELLULOSE TRIACETATE
Early Film base Cellulose Triacetate Highly Flammable Emulsion would crack & peel away from base when chemicals were too hot RETICULATION
Film Construction - BASE Made of a polyester plastic Must be clear, strong, consistent thickness Tinted pale blue or blue-gray (reduces eye strain) COATED ON 1 OR 2 SIDES WITH EMULSION
Film Construction - EMULSION Film emulsion can be on one side or both sides of base (single emulsion / double emulsion) Protective overcoat layered on top of emulsion Emulsion is a gelatin containing the film crystals Emulsion is the “active” layer of film
FILM COMPOSTION SINGLE OR DOUBLE EMULSION COATED ON A BASE EMULSION : GELATIN WITH SILVER HALIDE CRYSTALS BASE: SUPPORT (POLYESTER)
Film Emulsion Made of mixture of gelatin & silver halide crystals (fluorine, chlorine, bromine, & iodine) Most x-ray film emulsions made of : silver bromide (98%) silver iodide (2%) Photographically active layer – activated by light & radiation to create image
Silver halide crystals Crystals are cubic in structure in structure Crystal lattice containing ions containing ions
Film construction Film is manufactured to have specific characteristics of speed, contrast & resolution Film is also manufactured to be sensitive to specific colors of light = spectral matching Direct-exposure film = thicker emulsion with more silver halide crystals
X-Ray Film Cross Section
FILM CONSTRUCTION BASE WITH EMULSION CAN BE ON 1 (SINGLE EMULSION) OR 2 SIDES (DOUBLE EMULSION) MUST BE MATCHED WITH 1 OR 2 SIDED INTENSIFYING SCREENS
Image formation X-ray photons converted to light photons Image before processing = latent image Made visible by chemical processing After proper chemical processing = manifest image
IMAGE ON FILM SINGLE EMULSION = BETTER DETAIL DOUBLE EMULISON = LESS DETAIL PARALLAX With double emulsion – an image is created on both emulsions – then superimposed – slight blurring of edges With double emulsion – an image is created on both emulsions – then superimposed – slight blurring of edges
PARALLAX – each emulsion has an image single image overlaped – edges less sharp
Radiation interacts releasing e- Silver atoms buildup at the sensitivity center, building the latent image center The group of silver atoms is called the latent image center
LIGHT VS DARK AREAS ON FILM DARK SPOTS – SILVER HALIDE CRYSTALS THAT HAVE BEEN EXPOSED TO PHOTONS – TURN TO BLACK METALLIC SILVER AFTER PROCESSING LIGHT AREAS – NO CRYSTALS EXPOSED – SILVER HALIDE IS WASHED AWAY WITH PROCESSING
Processing The term applied to the chemical reactions that transform the latent image into a manifest image
FILM direct exposure & screen-film or film-screen SIZES 14 X 17 14 X 14 11 X 14 10 X 12 8 X 10
Film Sizes Standard “inches”: 8” x 10” 10” x 12” 11” x 14” 14” x 17” Metric: 18cm x 24cm 24cm x 30cm 30cm x 35cm 35cm x 43cm
Screen-Film Most widely used IR ?? Many Types of Film Used in Medical Imaging Table 11-1 Screen-Film has several characteristics to consider: contrast, speed, spectral matching, anticrossover dyes, and safe light requirements
Contrast Manufactured in multiple levels High-contrast (black-and-white image) High-contrast (black-and-white image) Low-contrast (more gray) Low-contrast (more gray) Exposure LATITUDE = the range of exposure techniques that produce an acceptable image (medium, high or higher)
So what is the difference? Depends on the size and distribution of the silver halide crystals High-contrast = smaller crystals, uniform grain size Low-contrast = larger crystals, wider range of sizes
Film Speed Single vs Double emulsion Size of Crystals Thickness of emulsion Intensifying screen used
Screen Speed Efficiency of a screen in converting x-rays to light is Screen Speed.
Film Speed Greater efficiency = less exposure = faster -Standard screen speed class of 100 -200 screen speed is twice as fast Speeds for routine work: 200 – 800 Speeds for high detail: 50 - 100
Film Characteristics Size of silver halide crystals & emulsion thickness determine speed of film and degree of resolution Speed – the response to photons Resolution – the detail seen
Screen speed vs Spatial resolution
Film Speed / Crystal size Larger crystals or Thicker crystal layer Faster response= less detail, and less exposure (chest x-ray) less exposure (chest x-ray) Finer crystals / thinner crystal layer =Slower response, greater detail, more exposure (extremity)
IMAGE ON FILM Crossover is when the exposure of the emulsion by light is from the opposite side of the intensifying screen Causes increased blur on the image
CROSSOVER Reducing crossover by adding a dye to the base
Reducing Crossover Changing the shape of the crystal improved light absorption and reduced crossover
Loss of Detail
Spectral Sensitivity OR SPECTRAL MATCHING Film is designed to be sensitive to the color of light emitted by the intensifying screens Blue – UV light sensitive film – CALCIUM TUNGSTATE screens Green, Yellow-Green light sensitive film - RARE EARTH screens
Film is either blue-sensitive or green-sensitive Rare earth- green emitting green emitting screens must screens must use a red filter use a red filter
Direct-Exposure Film Thicker emulsion and more crystals Not sensitive to light Not commonly used because of increased patient dose Very detailed images
Laser Film Uses the digital electronic signal The intensity of the laser beam is varied in direct proportion to the strength of the image signal = LASER BEAM MODULATION The more intense the signal the darker the image
Laser printers Provide consistent image quality for multiple film sizes. Most lasers only print on 14 X 17 regardless of initial IR Printers can be linked to multiple users (CT, MRI, US & Computed Radiology)
Laser Film Is silver halide film sensitive to the red light emitted by the laser Laser film is light sensitive Laser film must be handled in the darkroom in total darkness… Why?
Blue or Green filter would work
Duplicating film Single-emulsion film (active layer toward the initial radiograph) Exposure to light reduces OD on duplicating film (short time = dark film) Light is exposed through the initial radiograph
FILM BIN - STORAGE
Film Storage Clean, dry location, light tight location 40 – 60 % Humidity 70 º Fahrenheit Away from chemical fumes Safe from radiation exposure Standing on edge Expiration date clearly visible. Film can be stored for about 45 days, use the first-in first-out rule
Film Handling Do not bend or crease Hands must be clean Film is sensitive to pressure and scratches What happens if any of these happen?
X-ray Film Sensitivity Light X-rays Gamma Rays Gases Fumes Heat Moisture Pressure Static Electricity Age So what happens??
FILM FOG!!!! Unintended uniform optical density on a radiograph because of x-rays, light, or chemical contamination that reduces contrast & affects density
Intensifying Screens Bushong Ch 13
Cassettes Cassettes serve 3 important functions: 1. Protect film from exposure to light 2. Protect film from bending and scratching during use. 3. Contain intensifying screens, keeps film in close contact to screen during exposure.
CASSETTE or FILM HOLDER The CASSETTE is used to hold the film during examinations. It consist of front and back intensifying screens, and has a lead (Pb) backing. The cassette is light tight
Cassette Features - Front Exposure side of cassette is the “front”. Made of radiolucent material – easily penetrated by x-rays, lightweight metal alloy or plastic material made of resin. Intensifying screen mounted to inside of front.
Cassette Features Back Back made of metal or plastic Inside back is a layer of lead foil – prevents backscatter that could fog the film Inside foil layer is a layer of padding – maintains good film/screen contact Back intensifying screen mounted on padding Has the ID blocker (patient identification)
Image creation 1% of xray photons that leave patient Interact with phosphors of intensifying screens 100’s of light photons created to make image on film Light photons expose silver halide crystals in the film emulsion Turn black metallic silver after procession
Intensifying Screens Flat surface coated with fluorescent crystals called phosphors that glow, giving off light when exposed to x-rays.
Intensifying Screens Phosphors RARE EARTH – (emits green light) Developed in 1980’s Most efficient – most common in use today CALCIUM TUNGSTATE (blue light) Not as efficient
Rare Earth Screens Gadolinium Lanthanum Yttrium Found in low abundance in nature
Cardboard Cassettes Direct x-ray exposure to film required 25 to 400 times more radiation to create an image on the film BETTER DETAIL THAN FILM SCREEN (NO BLURRING OF IMAGE FROM LIGHT) ALL EXPOSURE MADE FROM X-RAY PHOTONS BIG DOSE TO THE PATEINT
INTENSIFYING SCREENS DISADVANTAGES: less detail than direct exposure (detail better with rare earth than calcuim tungstate screens) ADVANTAGES: 1. Reduce patient exposure 2. Increase x-ray tube life
Screen Construction Polyester plastic base – support layer Phosphor layer – active layer Reflective layer – increases screen efficiency by redirecting light headed in other directions Protective coating
Phosphor Layer Active layer – x-ray photons converted to light photons *Photoelectric Effect
Screen Speed A relative number that describes how efficiently x-rays are converted into usable light Ranges from 100 (slow) to 1200 (fast)
Screen Speed Greater efficiency = less exposure = faster -Standard screen speed class of 100 -200 screen speed is twice as fast Speeds for routine work: 200 – 800 Speeds for high detail: 50 – 100 Increasing speed also increases image noise
Technique Changes Relative Speed = Film speed & Screen speed mAs 1 = RS 2 mAs 2 RS 1 They are inversely related
Image Noise Speckled background on the image Caused when fast screens and high kVp techniques are used. Noise reduces image contrast The percentage of x-rays absorbed by the screen is the detective quantum efficiency (DQE) The amount of light emitted for each x-ray absorbed is the conversion efficiency (CE)
SCREEN SPEEDS Quantum Mottle causing a grainy, mottled or splotch image Often results of using very fast-speed screen-film systems
The light photons generated in the intensifying screen are emitted by phosphor crystals. These crystals are significantly larger than the silver halide crystals in the film use of a screen reduces image sharpness somewhat Some examinations requiring extremely fine detail use screens with small crystals.
Rare Earth Screens Have higher DQE (detective quantum efficiency). Higher x-ray absorption abilities. Have higher CE (conversion efficiency). More light emitted per x-ray absorbed by the screen.
Spatial resolution The use of intensifying screens lowers spatial resolution compared with direct- exposure radiographs. Spatial resolution expressed by the number of line pairs per millimeter (lp/mm)
The higher the lp/mm the smaller the object that can be imaged Very fast screens = 7 lp/mm Fine-detail screens= 15 lp/mm Direct-exposure screens = 50 lp/mm
Screen speed vs Spatial resolution
Wire mesh test – check for screen- film contact. Good contact
Wire mesh test – check for screen- film contact. Warped cassette – poor contact
Care of Screens Image artifacts can appear if screens are modified Small scratches can leave artifacts Dirty screens can leave artifacts Screens should be cleaned once each month with manufacturer’s cleaner with antistatic compounds