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

QUESTIONS ?

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?