Radiographic film & intensifying screens w10
Transcript of Radiographic film & intensifying screens w10
Radiographic Film & Intensifying Screens
Bushong Ch 11 & 13
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 baseCellulose 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 EMULSIONCOATED 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
Crystal lattice
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
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
SIZES14 X 1714 X 1411 X 1410 X 128 X 10
Film Sizes
Standard “inches”:8” x 10”10” x 12”11” x 14”14” x 17”
Metric:18cm x 24cm24cm x 30cm30cm x 35cm35cm 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) 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 CharacteristicsSize of silver halide crystals &
emulsion thickness determine speed of film and degree of resolution
Speed – the response to photonsResolution – 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)Finer crystals / thinner crystal layer
=Slower response, greater detail, more exposure (extremity)
FILM SPEEDS
FASTER SPEED – REDUCES PATIENT EXPOSURE
FASTER SPEED - REDUCES IMAGE DETAIL
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 ORSPECTRAL 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
screens must
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
LightX-raysGamma RaysGasesFumes
HeatMoisturePressureStatic
ElectricityAge
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
GadoliniumLanthanumYttriumFound 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 SCREENSDISADVANTAGES: 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
Intensifying screens
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
SCREEN SPEEDS
FASTER SPEED – REDUCES PATIENT EXPOSURE
FASTER SPEED - REDUCES IMAGE DETAIL INCREASES NOISE (LIGHT BLURING AROUND IMAGE)
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.
Image Quality
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
Questions?