X Ray Film, Characteristics & Film Processing

Rakesh C A

Introduction

• X-ray films are the most important material used to “decode” the information carried by the attenuated x-ray beam, when they are made to pass through the tissue

• They capture the invisible image into visible form.

X-ray Film

• Main part of a X-ray film is a radiation-sensitive, photographically active material made in the form of emulsion quoted on the supporting material called base.

Types of Films

• Basis of coating:– Single Emulsion– Double Emulsion

• Based on use of screens– Non Screen type– Screen type

• Single screen• Double Screen

• Based on sensitivity– Blue sensitive films – Green sensitive films – Panchromatic films

RADIOGRAPHIC FILM

Some History

• Photography began before x-rays were discovered• Certain silver compounds react to light making image

production possible• These images could be made permanent by

treatment with certain chemicals• This phenomenon was applied to x-ray procedures

More History

In 1812, silhouettes were recorded on glass platesIn 1819, the solvent action of sodium thiosulfate on

silver chloride was discoveredIn 1839, the phenomenon of development was

discovered by Louis DaguerreOne year later, it was discovered that treating exposed

silver chloride paper with sodium chloride would make the image permanent

Even More History

• By the time x-rays were discovered, photography was already an art

• Photographic film with a nitrocellulose base was already being marketed by George Eastman

• The first x-rays were recorded on glass plates• These were coated with emulsion on one side only• The exposure dose was quite high• Glass plates were used up until WWI

And, Finally

• During WWI, nitrocelluose based film was found to be a more feasible choice for recording x-rays

• This film was single-emulsion• It was later discovered that double-emulsion

responded to x-rays faster• The flaw with nitrocellulose-based film was its easy

flammability• In 1924, cellulose triacetate replaced the

nitrocellulose based film• In 1960, the first medical radiographic film using a

polyester base was introduced.

Base• Supports the fragile photographic emulsion.

Film Base Requirements

• Clear with low light absorption - should not produce visible pattern on the radiograph

• Strength, thickness and flexibility of the base must allow for ease of developing

• Must have Dimensional Stability: Maintain size and shape during processing, handling and storage.

• Low flammability

Types of Bases1. Glass Plates – Used in past, thin layer of emulsion on

one side.2. Cellulose Nitrate – Used in 1914,

– inflammable – Caused fire accidents.

3. Cellulose Tri acetate – Adopted in 1924 – Non inflammable.

4. Polyester – Adopted in 1960.– Better dimensional stability and colorless.– Dimethyl terepthalate(DMT) and ethylene glycol are

brought together under low pressure and high temperature to form molten polymer, stretched into sheets.• Eg. Cronex

Tinted blue film

• Triacetate and Polyester are clear and colorless.• Adopted in 1933, blue tint was added to the x-

ray film in an effort to produce a film that was “easier” to look at.

• Causes less eye strain.• Blue tint can be added to either to the base or

to the emulsion.• All present x ray films are blue tinted.

Emulsion

• Photosensitive Layer of the film.

• Key ingredients:– Gelatin– Silver Halide

• Thickness not more than 0.5 mils ( 5-25 µm.)

Gelatin

• Produced from Cattle bones.• Advantages:

-Keeps ‘Silver Halide grains’• Well dispersed• Prevents Clumping of grains

-Developing solutions can penetrate Gelatin rapidly without damaging the structure and strength.

- Easily available in large and uniform quantity.

Silver Halide

• Light sensitive material in emulsion.• Composition:

– 90-99% Silver Bromide– 1-10% Silver Iodide - it increases sensitivity

• Silver Iodo-Bromide crystals are precipitated and emulsified in Gelatin

• Precipitation reaction involves addition of Silver Nitrate to Soluble Halide to form soluble silver halide.

AgNO3 + KBr AgBr + KnO3

• The Silver Iodo-Bromide crystals in the emulsion is in the form of crystals suspended in gelatin.

• Crystals are formed by Ions of Ag+, Br-, I- arranged in Cubic Lattice.

Crystal lattice

• Crystal is formed by Ag+ Br- and I-

• Crystal size will vary from 1.0 to 1.5 microns in diameter.

• Each cubic centimeter of Emulsion contains 6.3 x 109 crystals.

• 1 grain averages 1 - 10 million silver ions

Silver Iodo Bromide Crystals

• An Imperfect Crystal (perfect crystal has almost no photographic sensitivity).

• Several types of crystal defects noted.

Crystal Defects

• A Point defect consists of a Silver Ion that has moved out of its normal position in crystal lattice (Interstitial Ions).

Crystal Defects

• A dislocation is a line imperfection in the crystal.

• Cause a strain in the wall structure.

• Iodine ion strains the crystal in this way

Chemical sensitization

• Chemical sensitization of the crystals are produced by adding allythiourea, a sulfur containing compound to the emulsion , which reacts with silver halide to form silver sulfide.

Sensitization

Sensitivity Speck• This Silver Sulphide

is usually located on surface of the crystals and is referred as “sensitivity speck”.

Point defect in cubic lattice.

Sensitivity speck by Silver sulfide

The sensitivity speck traps electron and form latent image.

The Latent Image

• Remnant radiation interacts with the silver halide crystals

• Mainly by the photoelectric interaction• The energy deposited into the film is in the same

pattern as the subject that was exposed to radiation• This invisible image is known as the latent image

• A latent image on photographic (radiographic) film is an invisible image produced by the exposure of the film to light (radiation).

The Manifest Image

• By chemically processing the latent image it is made visible

• Certain chemicals permanently fixate the image onto the film

Latent Image

(invisible)

Manifest Image(visible)

Formation of Latent Image

• Metallic silver is black, so it is this metallic silver that produces black areas on a developed films.

• Exposure of silver-iodo-bromide grains to light photons emitted by screen / direct x-ray exposure initiates the formation of atomic silver to form a visible pattern.

Gurney Mott hypothesis

This process repeats.

• Negative Bromine Ions lose electron to form bromine Atoms leaves the crystal and diffuses into Gelatin.

Latent Image Formation: 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.

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.

Direct X-Ray Exposure of Film

• Photoelectric & Compton interactions in film (silver halide in the emulsion)

• Liberated electrons have long ranges• These electrons strip other electrons from

Bromide ion– Bromine atoms & free electrons produced – Electrons captured at sensitivity speck as before

Direct X-Ray Exposure of Film

• Efficiency– most photon energy lost

• much energy lost in gelatin• only 3 - 10% of photon energy produces silver

– sensitivity varies with• kVp (energy)• processing

• Film as a dosimeter– 20% accuracy– badge include filters of various thicknesses

• allows estimate of x-ray spectrum

Adhesive Layer

• Firm attachment between emulsion layer and film base is achieved .

• Guards integrity during processing and fixing.

Super Coating

• Thin layer of Gelatin• Protects the emulsion

from mechanical damage

• Prevents scratches and pressure marks.

• Makes the film smooth and slick

FILM PROCESSING

Film Processing• Series of events after the film is exposed to X-rays

• There is another stage in the manual processing known as rinsing in between development and fixing.

Development

• It is the first stage in processing of the radiograph.

• Amplifies latent image by 100,000,000!

• The primary purpose: convert the invisible latent image into visible form.

• Processing initiated at latent image speck

Chemistry Of Developer

• Development is a process of chemical reduction. • The reduction is achieved by the developer donating

electrons to silver ions in the exposed silver bromide and iodide grains converting them to atoms of metallic silver.

Ag+ + electron Ag

• The mode of action of developer is not fully understood but the existence of electric charge barriers around the halide grains is thought to be involved.

• Both exposed and unexposed silver bromide grains are surrounded by a negative charge barrier of bromide ions created by the excess of potassium bromide employed in the synthesis of silver bromide during the manufacture of emulsion.

• The charge barrier protects the silver bromide from attack by electrons in the developer solution.

Charge Barriers

UNEXPOSED GRAIN

Charge Barriers

EXPOSED GRAIN• Exposed silver bromide grains

possess a weakness in the charge barrier caused by the presence of neutral silver atoms, which have collected at the sensitivity speck.

• This development center enables electrons from the developer to penetrate the grain and reduce all its silver ions to metallic silver.

Development

• Silver atoms at latent image center act as catalyst

• Grain either develops entirely or not at all

Constituents Of The Developer

• Replenisher solution:– This consists of:

1. Solvent2. Developing agents3. Accelerator4. Buffers5. Restrainer6. Preservative7. Hardener8. Sequestering agents

1. Solvent

Water is the solvent commonly used in replenisher solution.– This also controls developer activity by diluting its effects. – It has a softening effect on the gelatin, thus allowing the

developing chemicals to penetrate the emulsion and act on the silver halides.

– The presence of calcium salts in the water (hard water) may form a chalky deposit or scum on the surface of the film.

– More serious would be contamination of the solvent with dissolved metals like copper and iron. The presence of only a fewer parts per million of copper could cause chemical fogging.

– In practice, such effects are extremely rare.

2. Developing Agents

• These are the reducing agents, which carry out the primary function of supplying the electrons that convert the exposed silver halide grains to silver.

• Characteristics:1. Selectivity2. High activity: Selectivity and activity tend to be

antagonistic properties. An agent with high activity generally has low selectivity and vice versa.3. Should be resistant to bromide ions in the solution.

• No single agent satisfies all these requirements Modern X-ray developers use a combination of 2 developing agents phenidone and hydroquinone known as PQ developer.

• Phenidone is a quick acting reducing agent capable of developing all exposed silver halide grains. However, its selectivity is low and if used alone would result in high fog levels.

• Hydroquinone requires a strong alkaline medium for its action. This is more selective than phenidone but slower in onset of action. Once its action has begun the development proceeds vigorously although lightly exposed grains are not affected by hydroquinone.

– Hydroquinone and Phenidone – High Contrast– Metol – High Speed/Low Contrast/Fine grain

Reaction• Involves donation of Electron by developing agent to form

metallic Silver by Silver Ion (with inactivation of developing agent and liberation of hydrogen ions )

Alkaline

Medium

• Advantages of PQ developers1. Tolerant of increase in bromine ion concentration.2. High selectivity and low chemical fog.3. Adequate activity even in low concentrations.4. Available in liquid concentrate form.5. Fast acting6. Adequate contrast7. Super additive effect

SUPER ADDITIVE EFFECT

3. Accelerators: PQ developers need alkaline medium for their action (10 - 11.5 pH). Includes Sodium Hydroxide, Sodium Carbonate and Borates. – This alkali is known as accelerator since its

effect is to accelerate the developing process.4. Buffers: It has the effect of maintaining the

pH of a solution within close limits. – Normally adequate buffering action is provided

by the carbonates used as accelerators and sulphides used as preservatives. Thus no additional buffers are necessary.

5. Anti-Foggants/Restrainers• Decrease the formation of Fog (fog is the development

of the unexposed silver halide grains that do not contain a latent image ).

• It also decrease the development of the latent image.• Anti-foggants permit rapid development of exposed

grains at higher temp with minimized fog development.• The development process itself produces as a byproduct

potassium bromide which is a very effective restrainer.• So the developer replenisher need not include

potassium bromide among its constituents. However it is usual to provide a powerful restrainer / antifoggant such as benzotriazole.

6. Preservatives• Sodium sulphite • Oxidized products of developing agents form colored material that can stain the emulsion,

sodium sulphite forms colorless soluble products after combining with colored oxidized products of developing agents (sulfonates).

• It acts as a preservative by preventing oxidation of hydroquinone by removing dissolved oxygen in the solution and at the interface.

oxidation Coloured product(stain the emulsion)

Sodium Sulphite

7. Hardeners:

Powerful organic hardeners such as gluteraldehyde which prevent the excessive swelling of gelatin in the emulsion when it absorbs water during development.– If the emulsion is over hardened the speed with

which the developing agents penetrate the silver halide grains is reduced.

8.Sequestering agents: Prevent precipitation of insoluble mineral salts, which tend to occur in hard water areas. Compounds based on EDTA are used for this purpose.

9.Others: Bactericides and fungicides.

Rinsing

• Rinsed for about 30sec in running water.

• Will remove the developer diffused into gelatin.

• Reduces the speed of development.

Replenishment• During development Bromide Ions are released by the

reduction of silver ions to atoms and they pass into solution to increase Br concentration, which limits the life of the developing solution.

• The purpose is to maintain:– developing agent conc. – preservative conc. – Bromide conc. and – pH at constant level

(during the process of development the developing agents, preservatives are consumed and H+, Br+ are increased )

Development reaction (high volume)

2AgBr + H2Q + Na2SO3

2Ag + HBr + HQSO3Na + NaBr

• Bromide and acid are formed (pH is lowered)• Developer is consumed• Replenisher formulas have a higher pH ,contains no bromide.

• Rate of replenishment — 60ml of the developer is replaced with replenisher for each 14 x 17 inch film.

Oxidation Reaction (low volume)

H2Q + Na2SO3 + O2

HQSO3Na + NaOH + Na2SO4

• pH is raised• No bromide is produced.

• Replenisher formulas have a lower pH ,contains bromide and high sulfite conc. to retard oxidation.

• Rate of replenishment — 90ml of the developer is replaced with replenisher for each14 x 17 inch film (rate is higher to increase the developer turn over rate).

Factors affecting development:1. Constitution of developing solution2. Developer temperature3. Development time

1. Constitution of developing solution: For a particular level of exposure image density depends both on the emulsion characteristics and on the developer activity. The developer activity is influenced by1. Choice of developing agents and their relative proportions.2. Concentration of developing agents in solution.3. pH of the developer solution.4. Concentration of restrainer and antifoggant.

Factors affecting development:1. Constitution of developing solution2. Developer temperature3. Development time

2. Developer Temperature: Developer activity increases with temperature as many other chemical reactions.High temperature development: A range of 38-420C is used which enables 90 seconds or even faster cycle times to be operated.Low temperature development: Operated at around 300C and can still produce very rapid results.Some developers are extremely versatile and can be used over a range of temperature requiring different processor cycle times (ex. a 7 minutes cycle at 200C, a 90 seconds cycle at 300C). Medium temperature development:

Between 33-370C.

Effects on the image of increased temperature:

• Slightly raised temperature causes – increased image density for

the same exposure (thus increased film speed).

– Slightly increased chemical fog.– Increased image contrast.

• More severe raise in temperature leads to – gross increase in density.– Un-acceptable increase in

chemical fog.– Reduction in contrast.

Effects on the image of decreased temperature:

• If the temperature raise is left uncorrected, the developer becomes exhausted resulting in– low density and – low contrast.

• A more severe fall in temperature leads to – gross overall reduction of

density and – loss of contrast.

Factors affecting development:1. Constitution of developing solution2. Developer temperature3. Development time

3. Development time:– Defined as the time between the entry of a specified part of the

film into the developing solution and exit from the developing solution of the same part of the film.

• Factors determining development time:(a) Developer activity

(b) Type of emulsion: Developer solution takes longer time to penetrate a thick emulsion than a thin one. Non-screen films require longer time.

(c) Agitation of the developer solution: This is not a problem in automatic processors because of constant motion of the solution caused by the roller mechanism

FIXING

• It has 4 major functions:1. To stop further development

• Making it acidic

2. To clear the image • by removing the remaining silver halide from emulsion

3. To fix the image• no longer sensitive to light

4. To complete the process of hardening of the film emulsion

Constituents of the fixing solution:1. Solvent2. Fixing agent3. Acid4. Hardener5. Buffer6. Preservative7. Anti-sludging agent

1. Solvent

• Water

2. Fixing Agent

• 2 agents:– Cyanides

• Poisonous• Not generally used.

– Thiosulfates - Sodium and Ammonium Salt (more active) – called Hypo.

AgBr + sod. Thiosulfate

Ag thiosulfate complex + NaBr(water soluble)

3. Acid:– Prevents dichoric fog by inhibiting developing agents.– Provides a suitable environment for the hardening agents in

the fixer.– Acetic acid is used usually at a pH of 4-4.5.

4. Hardener:– Reduces drying time and prevents physical damage.– Aluminium chloride and aluminium sulfate (or Chromium

compounds) are used commonly.

5. Buffer:– Prevents sulphurization.– Neutralizes the developer– Optimizes hardener activity– Sodium acetate is commonly used in conjunction with acetic

acid

6. Preservative:– Retards decomposition of thiosulphates– Sodium sulphate is commonly used

7. Antisludging agent:– Boric acid is commonly used and this prevents sludging of

insoluble aluminium compounds in the hardener.

Washing Film must be washed well with water after developing

and fixing. Removes all thiosulfite complexes. Tap water is a satisfactory washing medium. In a 90

seconds cycle about 15 seconds is allowed for the washing stage, while in manual processing a minimum of 10 minutes is advised.

Incomplete wash causes retained hypo to react with silver to form silver sulfide acquiring brown colour.

Hypo + Silver

Silver Sulfide(brown) + Sodium sulfite

DRYING

• All the surface water and most of that retained in the emulsion should be removed.

• The drying medium is dry air of low humidity, which accelerates the evaporation process and reduces drying time.

• Heated air can retain more moisture than cold air and is therefore a more effective drying medium. However, the excessive use of heat may damage the film emulsion.

• Air temperatures between 40-650C are commonly used.

Total process of developing in schematic representation

The Automatic Film Processor

Processor (Top View)

4 Steps of Processing

• Developing – formation of the image

• Fixing – stopping of development, permanent fixing of image on film

• Washing – removal of residual fixer

• Drying – warm air blowing over film

Systems of the Automatic Processor

1. The Film Feed Section2. Transport System3. Temperature Control

System4. Recirculation System5. Replenishment System6. Dryer System7. Electrical System

1. The Film Feed Section• As a film is fed to the processor, so

the cycle of events listed below is initiated:

1. Drive motor energized (to turn the rollers).

2. Safelight above feed tray extinguished.

3. Developer and fixer replenisher pumped into tanks.

4. Drier heater energized.5. Wash water flow rate boosted.6. Film signal delay timer activated

(audible signal which will sound 1-3s after the trailing edge of the film has passed the entry rollers, to let the operator know that the next film can be fed to the processor).

2. The Film Transport Section

• A system of rollers that moves the film through the developer, fixer, washing and drying sections of the processor.

• Also acts as a squeegee action to remove excess chemicals from the film.

Make up of the Transport System

1. Entrance roller or detector roller2. Vertical or Deep racks (transport racks)3. Crossover assembly4. Squeegee assembly

1. Entrance Roller Or Detector Roller

• Entrance rollers grab film and draw it into developer

• Entrance rollers separate slightly, film passes between rollers activating microswitch controlling replenishment of chemicals

• When film is completely in developer tank, bell ring or light flicks on – safe to turn on light

2. Vertical or Deep racks (transport racks)

• Moves film into and through solutions and dryer

• Uses a turn around assembly at the bottom of the tank to turn film direction upward.

3. Crossover Assembly

• Moves film from developer to fixer tank and from the fixer to the wash tank

• Forces solutions from film back into the tank it is coming from.

4. Squeegee Assembly

• Moves the film from the wash tank to the dryer

• Squeegee action removes excess water from the film.

Transport System (Rollers)

Turnaround

Entrance

Deep Racks

Crossover

Squeegee

Dryer

Water System

• 2 functions:– Washing the film– Temperature control

Washing

• Removes the last traces of processing chemicals and prevents fading or discoloration.

• This enables long term storage capability

3. Temperature Control System

• Maintains developer, fixer & dryer temperature

Processing Temperatures

Developer 35° C

Fixer 35 ° C

Wash 32-35 ° C

Dryer 57 ° C

4. Circulation or Recirculation or Filtration System

• Agitates developer solution• Removes reaction particles by the use of a filtration

system• Helps stabilize developer temperature.

– Agitation and circulation– Agitation keeps solutions in contact with a heater element

in the bottom of the tank and prevents layering of chemicals

– Maintains developer temperature– Heating element is controlled by a thermostat

Recirculation System• Controlled by recirculation pumps that agitate solutions to keep

them mixed to maintain constant temperature• Circulation of water required to wash residual fixer (12 litres per

minute)

5. Replenishment System

• Fixer & developer levels drop as films processed

• System replaces lost chemicals• Microswitch of entrance rollers starts

replenishment pump – stops when film exits entrance rollers

Replenishment System

• Typical replenishment rates: 60-70 mls of developer, and 100-110 ml of fixer for every 14 inches of x-ray film (per 35 x 43cm crosswise film)

Types of Replenishment

• Volume Replenishment– A volume of chemicals are replaced for each film

that is processed.• Flood Replenishment

– Periodically replenishes chemicals regardless of the number of films processed.

6. Dryer System

• Dries the film before its removal for viewing• If not dry, difficult to hang on viewing box• Consists of blower, ventilation ducts, vented

dryer tubes & exhaust system• Blower draws in air from room and passes it

over heating coils• Heated air enters ventilation ducts & dryer

tubes & then blows over film• Moist warm air vented

Processing Rates

• Amount of time it takes a film to go through processor – ranges from 45-210 seconds

• Film manufacturers determine temperatures and replenishment rates

Processing Times

Developer 20-25secs

Fixer 20

Wash 20

Dryer 25-30

Travel Time 10

Total Time 90 sec

Daylight Automatic Processors

• Enable film to be processed without need for darkroom

• Special cassettes• Increase in department

efficiency, no need for special darkroom staff

• Disadvantages – Cost– Mechanical breakdowns

Automatic Film Processing: Benefits

• Compact size• Faster• Density and contrast is constant• Time and temperature controlled• Produces dry radiograph immediately

Automatic Film Processing: Disadvantages

• Artifacts caused by rollers.• Expensive and requires maintenance.• Manual processing required as a back up in

case of break down

Maintenance

To maintain quality, attention needed in 3 areas:

1. Quality control2. Processor cleanliness3. Basic operation

DARK ROOM CONSTRUCTION

LOCATION

• Centrally located• Serviced by hatches

from the adjacent imaging room

• Away from damp or hot areas

• Accessible in terms of power and water supply

• Adjoining viewing room

SIZE

• Minimum floor area of 10 sq meter

• Ceiling height of 2.5 - 3 meter

• Size may be reduced depending upon the department needs

RADIATION PROTECTION

• Walls adjacent to the radiographic room should be shielded with correct thickness of the lead all the way to the ceiling

• In the interests of both darkroom staff and film material alike

• 1.6 mm lead is mostly used

FLOORS

• Non-porous flooring• Non-slip flooring• Chemical resistant• Stain proof• Durable & easy to

maintain• Light coloured (low-light

working conditions)

• Asphalt tiles• Porcelain tiles• Clay tiles• Plastic tiles may be used

in the dry dark-rooms

WALLS/CEILING

• Light in colour to reflect as much light as possible onto the working surface

• Easy to wipe or clean• Covered with chemical resistant materials• Special paints, varnish, ceramic or plastic wall

VENTILATION AND HEATING

• Satisfactory working conditions for the staff• Good film handling and storage conditions• Efficient automatic processor performance• Relative humidity is maintained at around 40-60 %• Room temperature maintained between 18-20 degree

celsius• A minimum of 10 air changes per hour• All of these conditions can be achieved by using a good air-

conditioning system• Alternatively, fairly satisfactory ventilation can be achieved

by using an extractor fan sited higher than and diagonally opposite a second fan, the latter being so placed as to obtain fresh and filtered air from outside.

TYPE OF ENTRANCE

• SINGLE DOOR SYSTEM

• DOUBLE DOOR SYSTEM

• MAZE TYPE ENTRANCE

• LABYRINTH

• ROTATING DOOR SYSTEM

DOUBLE DOOR ENTRANCE

MAZE TYPE ENTRANCE

LABYRINTH ENTRANCE

ROTATING DOOR ENTRANCE

Fire Safety

• Ideally, all darkrooms should be provided with an alternative exit, which should be indicated clearly and left unobstructed at all times

DARK ROOM ILLUMINATION

• WHITE LIGHTING

• SAFELIGHTING

WHITE LIGHTING

• For inspection & maintenance of cassettes & screens• Cleaning of work surfaces • Servicing of equipment

• Sited close to the ceiling • Moderate in intensity

– (60w tungsten, 30w fluorescent )• Preferably centrally placed• More than one switch preferable• Identification of respective switches is important

SAFE LIGHTING

• DIRECT SAFE LIGHTING: • Light from safe lamp

directly falls onto the work surface– Eg. Beehive safelamp

• Minimum distance of 1.2 m from the working surface

• Best for loading & unloading areas

INDIRECT SAFE LIGHTING

• Directs the light towards the ceiling which reflects light back into the room

• Is intended to provide general illumination of the dark room

• Suspended atleast 2.1 m above floor level

Safe lamp for both direct & indirect illumination

SAFE LIGHT FILTERS

• Sheet of gelatin dyed to the appropriate colour and sandwiched between two sheets of glass for protection

• Used in conjunction with a 25 W lamp

• Extremes of heat and temperature deteriorates the filter gelatin

• Should be cleaned periodically

How Does A Safelight Work?• When white light is passed

through coloured filters, certain wavelengths (or colours) are absorbed by the filters, whilst those wavelengths, which correspond to the colour of the filters will be transmitted.

• Making the correct selection of safelight filter (matching the filter to the film), means choosing a filter, which will transmit a colour to which the film is relatively unresponsive, whilst stopping all light to which the film is most sensitive.

Spectrum Transmission Graph:

• Manufacturers produce graphs for their safelights called spectral transmission or filter transmission graphs.

• Their purpose is to indicate that part of the visible spectrum, which will be transmitted by the filter, and so aid the radiographer in matching the appropriate filter to the type of film in use.

• Panchromatic film presents special problems, since it will have colour sensitivity extending as far as the red end of the spectrum.

• It is thus advisable to process such film in complete darkness.

SPECTRAL TRANSMISSION GRAPH

MONOCHROMATIC FILM ORTHOCHROMATIC FILM

How Safe Is Safe Lighting?

• No safe lighting is completely safe; all films will become significantly fogged if exposed to safelights for long enough.

• This is because safelight filters are not perfect absorbers of the undesirable wavelengths and, in truth, all films have some sensitivity to all wavelengths.

• Thus, the intensity of illumination and the film-handling time must be kept to a minimum if significant fogging is not to occur.

Effect Of Excessive Safe Light Exposure

Two principal features occur when film is exposed to safelights for too long:

1. An increase in gross fog;2. An overall loss of

contrast.

DARK ROOM EQUIPMENTS

• Automatic processor• Manual process unit• Processing chemicals• Hangers for suspending film• Cassette• Film storage hopper• Loading bench• Cupboards

AN AUTOMATIC PROCESSOR

MANUAL PROCESS UNIT

PROCESSING CHEMICALS

                                       

Hangers used for suspending films during processing

CASSETTES

FILM HOPPER FOR THE STORAGE OF UNEXPOSED FILMS

DARK ROOM DRY BENCH SYSTEM

Layout of a typical dark room : ample storage & work surfaces

HEALTH AND SAFETY IN THE PROCESSING AREA

• ELECTRICAL SAFETY

• GENERAL SAFETY

• CHEMICAL SAFETY

ELECTRICAL SAFETTY

• All electrical equipments to be sited well away from sinks & manual processing units

• Adequate earthing of all electrical appliances• No trailing cables from appliances• Pull-cord switching for lights

GENERAL SAFETY MEASURES

• The maximum level of safe lighting consistent with film sensitivity

• Adequate ventilation• Second exits for fire safety

CHEMICAL HAZARDS

• Processing chemicals contain many toxic substances and must always be handled with care.

• Staff should always be aware of the harmful effects of exposure to the chemistry its fumes through inhalation, ingestion or skin contact.

• All staff involved with the handling of film chemistry should be regularly advised to read product labels and mixing instructions before handling solutions.

• Safety glasses, facemask, rubber gloves and plastic apron should be available within the processing area for each procedure.

Control of substances hazardous to health (COSHH) Regulations 1988

Make it clear that employers have a responsibility, in so far as is reasonably practicable, to prevent or adequately control exposure to fumes and chemicals using measures other than the personal protective equipment.

Photographic Characteristics of X-Ray Film

Photographic Density

• Film’s response to incident radiation• Tissue absorption

FilmAbsorptionVariations

PhotographicDensityVariations

Tissue Absorption Dependencies

• Patient– composition– thickness

• Beam– energy spectrum

• kVp• phase• filtration

FilmAbsorptionVariations

PhotographicDensityVariations

PHOTOGRAHIC OR OPTICAL DENSITYMeasure of film blackness or opacity

PHOTOGRAHIC OR OPTICAL DENSITY

• Opacity is doubled by an increase in density of 0.3

• Useful densities range: 0.3 - 2.0

50% down to 1% of light transmitted

PHOTOGRAHIC OR OPTICAL DENSITYDefinitions

Opacity ability of film to block light = Io/It

Transmittance ability of film to transmit light=It/Io

Higher density value meansdarker film less light transmitted

Base + Fog

Unexposed film has optical density > 0 (min = 0.12)

BaseThe plastic material absorbs small amount of lightblue dyeOD ~ 0.07

Fogdevelopment of unexposed silver halide grainsOD ~ 0.05

Why Logarithms?1. Easily represent large

dynamic ranges– factors of 10

2. Represent physiologic response of eye to differences in light intensity

3. Densities can be added

100,00010,000

1,000100

101

543210

Input Logarithm

Using logarithms the difference between 10,000 and 100,000 is

the same as the difference between 10 and 100

Why Logarithms?1. Easily represent large

dynamic ranges– factors of 10

2. Represent physiologic response of eye to differences in light intensity

3. Densities can be added

O.D.=0.3

O.D.=0.5

O.D. = 0.8+

Plot derived by giving a film a series of exposures, developing the film, and plotting the resulting density against the known exposure.

Sensitometric Curve• Other Names:

– Characteristic Curve– H & D Curve (after

inventors Hurter & Driffield)

• Properties:– Base + fog– Dmax

– Speed– Contrast– Latitude

Characteristic CurveLinear portion in mid

densitiesFlatter portions at

bottom & topchange in exposure

results in little density change

Shoulderflat portion of curve near

tophigh exposure & density

Toeflat portion of curve near

bottomlow exposure & density

Characteristic Curve

• “Straight Line” region– density approximately

proportional to log relative exposure

Radiographic Contrast

• Density difference between image areas

• Depends upon– Subject contrast– Film contrast

SubjectContrast

DensityDifference

FilmContrast

Subject Contrast Dependency

• Thickness• Density• Atomic differences• Energy spectrum (kvp)• Contrast material• Scatter radiation

Film Contrast Dependancy

1. Characteristic curve of the film2. Film density3. Screen or direct x-ray exposure4. Film processing

1. Characteristic curve of the filmFilm Gamma

maximum slope of characteristic curve

D2 - D1

Gamma = -------------------- log E2 - log E1

Slope shows change in film density for given change in exposure

Ranges from 2.0 – 3.5

1. Characteristic curve of the film

• Average gradient– slope between points

with densities at ends of useful range

– usually between 0.25 and 2.0

Average Gradient>1: exaggerates subject

contrasttypical for x-ray film

=1: no change in subject contrast

<1: decreases subject contrast

2. Contrast vs. DensitySubject contrast depends on densitySlope of H & D curve changes with density

log relative exposure

OpticalDensity H & D Curve

Slope of H & D

3. Direct (non-screen) X-Ray Exposure

• Requires Much Higher Exposure (30mR Vs 1mR)

• Lower Contrast• More Scatter Sensitivity (X Ray Film Is More Sensitive To Lower Kvp X Rays,

The Scatter Radiation)

– Rare Earth Screens Less Responsive To Lower Energies Of Scattered Radiation

• Direct X Ray Exposure Will Produce Lower Average Gradient.

• Average Gradient Is Maximum When The Film Is Exposed With Intensifying Screens.

4. Film Processing

• Longer time or higher temperature (up to a point)– increases average gradient (increase contrast)– increases film speed (increases density)– increases fog (decreases contrast)

• Follow manufacturer’s recommendations to optimize processing parameters

1.0

Speed

Definition of speed– reciprocal of exposure (in roentgens) required to

produce density of 1.0 above base + fog

log relative exposure

OpticalDensity

B + F

Speed & Contrast on the curve• Contrast controls slope of characteristic curve

log relative exposure

OpticalDensity

log relative exposure

OpticalDensity

LowerContrast

HigherContrast

Speed & Contrast on the curve• Speed controls left-right location of

characteristic curve

log relative exposure

OpticalDensity

log relative exposure

OpticalDensity

X

X

SlowerSpeed

FasterSpeed

Latitude

• Definition– The range of log relative exposure (mAs)

producing density within acceptable range (usually 0.25 to 2.0)

log rel. exp.

OpticalDensity

.25

2.0

Latitude

Latitude• Inversely related to

contrast– high contrast = low latitude– low contrast = high latitude

• Significance– Variation from optimal technique

less critical– higher range of subject contrasts

imaged on single film (such as chest)

Double-Emulsion Film: Advantages

• Physical advantage– Emulsion shrinks when it dries– Having two emulsions minimizes curling

• Photographic advantage– Faster system

• Two screens used• DE film – 2x contrast• Increase density Increase speed• Each emulsion optimally captures light produced by “its”

screendouble emulsion film screens

Double-Emulsion Film• Why use 2 thin emulsions rather than 1 thicker one?

– Light photons are easily absorbed by the emulsion, however, only the outer layer of the emulsion is affected by light from intensifying screens.

– Light produced closer to emulsion• less light spreadX-Ray X-Ray

Crossover Exposure or Print Through Exposure

• Light from one screen exposes opposite emulsion

TopScreen

BottomScreen

BottomEmulsion

TopEmulsion

Film

X-Ray

Crossover

• caused by incomplete absorption of light by adjacent emulsion

• poorer resolution– light travels further, spreads more

• can account for up to 40% of total exposure

X-Ray

Crossover Reduction

• Increase the light absorption in the silver halide grains of the film emulsion – – use light-absorbing dye on

film base

• can reduce crossover exposure ~ 13%

• also reduces system speed by up to 40%

X-Ray

Crossover Reduction by Increasing Light Absorption

• Match screen light emission to silver halide natural sensitivity

1. Yttrium tantalate phosphor intensifying screens

2. Adding a dye, matched to light emission of the screen, to the emulsion

– reduces crossover without decreasing speed

• use flat film grains– present larger surface to incoming light

– Large surface-area-to-volume ratio absorb more light photons

• Kodak “T-Mat” film

• cuts crossover ~ X2

Transparency vs Print

Why are radiographs viewed as transparencies rather than prints?

– The density of a print is related to the amount of light reflected or absorbed by the paper

– maximum print density between 1.3 & 2.0– maximum usable transparency density up to 3.0– transparencies offer greater density range

Thank You!!!