Unit 4 Radiographic ExposuresChapters: 32,19,20&22

Objectives • Compare various exposure systems and their advantages and disadvantages and

how they work to obtain radiographic images.

• Describe how AEC systems operate.

• Describe how to develop a technique chart and how to use them in the clinical setting.

• List and label on a diagram the components of radiographic film and intensifying screens.

• Discuss film and screen handling, storage and artifacts.

• Explain the steps in the processing procedure.

• Describe two types of luminescence.

CCHAPTER 32Exposure Systems

Goals/ Purpose of Exposure Systems

• Maintain a method of consistency for quality image production

• Preprogramming of control panel based on exposure factors

• Suggestions of starting points

• Reduce repeat rates

• Work as part of departmental quality assurance program

How they work• Based on part thickness

• Radiographer must measure part thickness for complete accuracy• Calipers – device used to measure body parts from central ray entrance to exit points

Types of exposure systems1. Fixed kVp

2. Variable kVp

3. Anatomically programmed radiography systems

1. Fixed Kilovoltage Systems • Developed by Fuchs in 1943 during WWII

• Advantages: • Decrease patient dose• Provide more information on the image• Increase consistency of IR exposure• Lengthen exposure latitude• Reduce tube wear• Decrease time settings

ANATOMY MEASURE EXAM KVP SID GRID SM MAS MD MAS LG MAS

Skull 18 - 21 PA or AP 80 40 YES 12 20 3014 - 17 Lat 80 40 YES 6 12 1818 - 21 Townes/Waters 85 40 YES 12 24 40

Facial Bones 14 - 17 Lat Bucky 80 40 YES 6 10 1214 - 17 Lat Non 60 40 NO 2 4 6

Nasal Bones 18-21 Non Bucky 60 40 NO 2 3 4Cervical Spine 11 - 14 AP/Obl 80 40 YES 6 12 18

11 - 14 Lat 80 72 YES 18 28 4511 - 14 Odontoid 80 40 YES 8 14 2021 - 25 Swimmers 85 40 YES 40 80 160

Thoracic Spine 20 - 24 AP/Obl 80 40 YES 18 28 4028 - 32 Lat 85 40 YES 25 35 50

Lumbar Spine 18 - 22 AP/Obl 80 40 YES 24 40 6427 - 32 Lat 85 40 YES 30 64 100

27 - 32 L5/S1 90 40 YES 30 64 100Chest 20 - 25 PA 117 72 YES 4 8 16

37 - 32 Lat 117 72 YES 8 16 4020 - 25 Port Grid 100 YES20 - 25 Port Non <85 NO

Thorax 20 - 25 Sternum RAO 80 40 YES 20 30 4027 - 32 Sternum Lat 85 40 YES 30 40 50

TECHNIQUE CHART -

2. Variable Kilovoltage Systems• Proposed by Jerman in 1925

• Advantages: • Permit small changes in exposure to compensate for variable body part thickness

• 2 kVp for every 1 cm tissue thickness

• Produce higher contrast images• Enhance visibility of fine detail and perception of resolution

3. Anatomically Programmed Radiography Systems• Completely computerized control units that combine AEC ( automatic exposure

control) with anatomical procedures

• Advantages:• Provides choice of anatomical procedures and suggested exposures• Can be modified but professional judgment must be used• mAs selection is eliminated

Creating an exposure control chart• Exposures of phantoms with optimal center of acceptance curve

• Show images to radiologist to determine which images they prefer

• Create technique chart for selected system used• Variable or fixed

• Perform phantom testing

• Perform clinical trials

• Fine tune

• Continue fine tuning as needed

AEC Systems AKA: Phototiming• Permit various combinations of ionization chambers

• Usually 3 chambers- 2 outer , 1 center• Ionization Chamber- serve to measure the exposure to the receptor• Permit user to select mA and kVp- chambers eliminate TIME only

• Permit density controls- -3 , -2, -1, 0, 1, 2, 3 • Atomically add or subtract density• Used when part can’t be positioned properly over chamber• Should not be used to compensate for part thickness

Disadvantages

• Part must be aligned perfectly with chamber

• Can cause under or over exposure if part not centered correctly

Back-up timers • Back up time ( max exposure) should be set to prevent overexposure

• Should be set at 150% of the anticipated manual exposure

• Laws regulate AEC shut off at 600mAs above 50 kVp and 2,000 mAs below 50kVp

• When backup time is to short , underexposure will happen

Chapter 19Radiographic Film

Terms• Analog- image recorded on radiographic film

• Aerial Image -x-ray beam that exits the patient that carries information about tissues and densities. Aerial image will be put on film to record the information

• Latent image- the invisible image that exists on the film after the film has been exposed but before it has been processed

• Manifest- the observable image that is formed when the latent image undergoes proper chemical processing- the visible image

Components of Film: Base• originally consisted of glass plate ex: flat plate abdomen

• Must be flexible for handling- but tough enough and rigid to put on a viewbox and go through a processor

• Must be uniformly lucent ( light can be transmitted without any additional artifacts being added)

• 1960’s- now made of polyester plastic

• Blue dye added to tint film to reduce eyestrain for the radiologist

• Magenta dye- to improve detail

Components of Film: Adhesive• Applied to the base material before emulsion

• Acts as a “glue” to put emulsion and base together to prevent bubbles or other distortion when film is handled, processed, wet and heated

Components of Film: Emulsion• Consists of a gelatin, in which silver halide crystals are suspended

• Gelatin purpose: • Neutral material that allows chemicals to react with silver halide• Distributes silver crystal evenly over film- no clumping• Gelatin must be clear to allow light to go through it• Must be flexible to allow bending without distorting the image

• Silver halide crystals ( grains) • Silver bromide 95-98% of crystals• Silver iodide• Silver chloride

• Removed during processing so light can get through film for viewing

• Has an antihalation coating applied to back of film ( light absorbing coating)

• Absorbs light coming from emulsion and prevents back scatter and visible light from degrading the image

Emulsion – cont’d• 1. Double Emulsion ( double coated)

• AKA: Duplitized • All film utilized in the Bucky- most film for diagnostic radiology

• 2. Single Emulsion ( coating on one side) • Duplicating film ( copy film) • Fine detail film ( extremity) • Mammography

Components of Film: Supercoat• Layer of hard, protective gelatin

• Prevents soft emulsion from being damaged by scratches, oil form hands, stacking the film

• Makes it very hard to tear a radiograph

• Staples and paperclips can damage the supercoat( scratches) • Reports go in separate file• Don’t paperclip requests to radiographs

Construction of Silver Bromide Crystals• All manufacturing of film takes place in total darkness

• During manufacturing, silver nitrate and potassium bromide are combined in gelatin

• Crystals are flat and somewhat triangular in shape

• 1 cubic mm in film emulsion contains over half billion crystals

Sensitivity Specks ( sensitization specks) • Silver sulfide or gold silver sulfide is added to each crystal

• Development center of crystal

• “Gurney and Mott theory” states that sensitivity specks are essential to the image formation process

• Crystal usually contain : • 1 or more sensitivity specks• Silver atoms• Bromide ions• Iodide ions

• During exposure, photoelectrons and silver ions are attracted to these sensitivity specs, where they combine to form a latent image center of metallic silver

Latent Image Formation• “photographic effect” “ Gurney & Mott theory”

• Latent image- image produced by the interaction of x-rays with the emulsion of a radiographic film prior to development

• The process of ionizing silver and bromide ions having the ( +) cluster around the (-) speck to acquire an image• 1. x-ray or light photon form intensifying screens interacts with a bromide or iodine ion in a

crystal• 2. An electron from the ion is ejected and is attracted to and trapped by a sensitivity speck• 3. Speck becomes negatively charged• 4. Negative charged speck attracts a positive silver ion inside the crystal and speck

becomes neutralized• 5. Process is repeated until clump of silver atoms becomes deposited at the speck

• In the processor crystals convert to black metallic silver

Types of Film • Direct exposure film ( nonscreen film )

• Used in cardboard cassette- no screens• Thicker emulsion with more silver content ( 2-3 x thicker that screen film) • Rarely used due to high dose

• Screen film ( intensifying screen film)• Used in a cassette with an intensifying screen• Thin emulsion- very sensitive to light of screens• Less radiation- less dose• Allows for shorter exposure times

Film Storage and Handling• If not performed properly, can be huge expense to dept

• Age of film• has expiration date

• Can be fogged over time

• Use film with oldest exp date first

• Heat• Store in cool/dry place (Moisture can cause water artifacts)

• 680 or lower

• Humidity• Between 30-60 %

• Too low-dry- static marks

• Too high- wet – water marks

• Light• Light and moisture proof package

• Film is twice as sensitive after exposure

• Low intensity (7-15 watt) dark red light or orange-brown filter

• Safe lights should be installed no closer than 4 feet from working are and feed tray

Film Storage and Handling-cont’d• Ionization radiation

• Film storage should be lead-lined

• Handling• Store on end- not flat ( pressure marks) • Don’t remove film quickly from box- abrasions , static • Careful handling when loading and unloading cassettes• Don’t slide onto feed tray

• Chemical fumes:• Can cause film fog• Should not store film near cleaning solutions , formaldehyde, or other strong chemicals

Film FlasherUsed to put identification on film prior to processing

CCHAPTER 20 Film Processing

Four Primary Steps1. Developing

2. Fixing

3. Washing

4. Drying

Developing• 1st step in wet process

• Black Silver Halide is deposited at the latent image sites and image become visible

• 92-96 0 F for 90 sec

• Primary agents• Two Reducing agents

• Activator

• Restrainer

• Preservative

• Hardener

• and water as a solvent

• Only solution that is effected by contamination• Can be caused by evaporation, splashing, improper cleaning and filling

Fixing • Removes undeveloped silver halides from emulsion

• Permanently “fixes” them image before exposure to light for viewing

• Primary Agent• Clearing agent

• Activator

• Preservative

• Hardener

• Water as a solvent

Archiving- Washing and Drying• Protects from deterioration by chemicals, fading and physical forces

• Washing• removes as much of the developer and fixer as possible- warm water is best but 3-5 lower

than other chemicals• Chemicals can cause damage over time

• Silver stains and yellowing

• Drying• Final stage• 120-150 0 F• Shrinks and seals emulsion

Silver Recovery• Silver halide crystals that are removed during fixer time is recycled

• Silver is more valuable after processing than before- 10 % of the purchase price can be recovered

Artifacts• Guide shoe marks

• occur when the guide shoes in the turnaround assembly of the processor are sprung or improperly positioned

• Entrance roller marks-• occur if roller is wet / damp from cleaning

• Pi lines • occur at intervals because of dirt or a chemical stain on a roller.

• Roller marks • Dirty or warped rollers can cause emulsion pick-off & gelatin buildup. Artifacts appear as

sharp areas of ↑ or ↓OD

• Chemical fog • looks like light or radiation fog & is usually a uniform dull gray

• Wet Pressure Sensitization • produced in the developer tank. Irregular or dirty rollers cause pressure during

development & produce small circular patterns of ↑ OD.

Chapter 22Intensifying Screens and Film/Screen Combination

Cassettes • Film holders use to protect film from light before, during and after the exam

• It creates a light-proof case for the film to utilize the intensifying screens to their best advantage

• Construction • Aluminum and stainless steel frame or plastic frame• Front- uniformly radiolucent to eliminate artifacts• Back is usually colored side with a hinge opening- latch

• Have lead foil in back to absorb excess radiation to prevent back scattering and fogging

• Intensifying screens will be mounted inside the front and back of the cassette. • Should be mounted on a foam pressure pad to provide good contact between screens and film

Intensifying Screens• Location- inside the cassette, one on each side fro double emulsion film

• Purpose- when making an exposure, screens convert x-ray energy to light. • Screen will emit a Luminescence

• Formation of the latent image• From 98-99% of the light of the intensifying screens• 1-2 % formed directly from x-rays striking the film

• Inventor- Thomas Edison 1896

BASE • Made of polyester plastic 1 mm thick- previously made of cardboard and metal

• Acts as a support for the active layer of the screen

• Characteristics: • Rugged, tough and flexible• Moisture resistant• Not damaged by radiation • Will not discolor with age or exposure to air• Chemically inert so that it will not interfere with the phosphor layer and interfere with the

conversion of x-rays to light• Cannot obtain impurities that would be imaged by x-rays : uniformly radiolucent

Reflective Layer• Located between the phosphor and base layers

• Made of magnesium oxide or titanium dioxide

• 25 micrometers thick

• Purpose is to reflect light towards the film, which increases the efficiency of the screen

• Can help to reflect twice as much light towards the film- helps to create latent image and reduce dose to pt.

• Layer may have a dye incorporated into it, which absorbs longer wavelength light that would decrease detail on the film

Phosphor layer• The active layer of the screen

• 150-300 micrometers thick, depending on the speed of the screen

• Contains phosphors which are materials that absorb x-ray energy and emit ligh photons

Protective coating • Protective coating for screen

• Made of protective plastic or cellulose

• 25 micrometers thick

• Purpose is to protect the phosphor layer from abrasions and stains during the loading and unloading of films

• repeated trauma to screens, such as scratches, can remove this coating• Removal of phosphors in next layer can cause white line artifacts from no exposure

• Coating must be transparent to light, so the phosphor light reaches the film to make an image

• Provides a surface that is durable enough to tolerate cleaning

• Helps prevent production of static electricity

Phosphor Characteristics1. High atomic number

• Will increase the probability of an x-ray interaction, called quantum detection efficiency ( DQE)

• High atomic number is needed to permit photoelectric and Compton interactions

2. Conversion efficiency• The ability of the phosphor to emit as much light per x-ray photon interaction as possible• As conversion efficiency increases, pt dose decrease- fewer x-rays are needed to make

image

Phosphor Characteristics3. Spectral Emission

• An indication of the precise wavelength of light emitted by the phosphor • Spectral emission should match the sensitivity of the film to obtain the maximum latent

image formation

4. Luminescence• The ability of a material to emit light • Screen image will represent the aerial image ( exit radiation from pt) in the form of light• When x-rays hit phosphor in screen:

• Parts that are better penetrated in body will be brighter on screen

• Results in darker areas; more exposure in these areas on radiograph

Types of Luminescence

• Fluorescence: light emitted promptly after x-rays strike the crystal• Instantaneous emission• Occurs within 10 -8 seconds

• Phosphorescence• Delayed emission of light• After x-ray has dissipated, phosphor continues to emit light for a period of time• Known as “ screen lag” or “ afterglow” • Occurs after 10 -8 seconds • if this occurs, the film may be removed before the latent image is completely formed• If another film is loaded immediately in the cassette, it may be faintly exposed to a previous

image• This type of luminescence is more common in older screens and in fluoroscopic screens

Luminescence – Cont’d• Maximum fluorescence and minimal phosphorescence is ideal for screens

• Normal life of a screen is 5-7 years

• As screens age their phosphors decrease in activity

• Recommended that all screens in a department get replaced simultaneously so all phosphor activity level will be the same.

Calcium Tungstate vs Rare Earth Screens1. Calcium tungstate

• used by Thomas Edison & considered one of the best phosphors until the late 1970s• conversion efficiency of approximately 5 % (5% of x-ray energy is converted to light

energy)• emits blue light - need blue-sensitive film to match

Calcium Tungstate vs Rare Earth Screens

2. Rare earth• infrequently found in nature and these elements are more difficult to

separate from the earth & most have atomic numbers between 57-71• conversion efficiency of approximately 20 % (4 times more efficient

than calcium tungstate phosphors)• have the advantage of having fast speed but retaining good detail• more expensive than calcium tungstate• require less exposure to obtain a given density; therefore, pt dose is

decreased• less exposure will lengthen the life of the x-ray tube• mostly emits green light, although some rare earths are blue-emitters• manufactured @ different speed levels like calcium tungstate• Each level is approximately twice as fast as a similar calcium

tungstate screen but detail is not lost

Screen Speed• Definition - the ratio of the exposure needed to produce a density on a film

using screens compared to the exposure required to produce the density without screens

• also known as the relative speed, intensifying factor or intensification factor

Factors determining screen speed• Intrinsic factors

• factors built into the screen; under the control of the manufacturer (inherent)

1. Phosphor conversion efficiency• how well the phosphor can absorb photons

• how well the phosphor converts the absorbed energy to light

• rare earth (20%), calcium tungstate (5%) rare earth more efficient

• screen efficiency

• The ability of light emitted by the phosphor to escape the screen & expose a film (a typical screen will absorb 50 % of the light; the other 50 % will expose the film)

Factors determining screen speed• 2. Thickness of active layer (layer of crystals)

• the thicker the layer, the faster the screen• more crystals are available in a thicker layer; therefore, more light is emitted• thicker layers produce more blur on the image due to light spread

• 3. Size of phosphor crystal (just like film)• the larger the crystals, the faster the screen• more light is emitted due to the crystal receiving more photons• larger crystals increase screen efficiency (will scatter less light within the active layer)

Factors determining screen speed• 4 Reflectance of backing of screen

• the more light that is reflected from the backing to the active layer, the faster the screen

• 5. Presence of dyes in reflecting layer• dyes help reduce the amount of scattering long wavelength light photons• dyes will increase detail• dyes will decrease speed

• 6. Concentration of phosphor crystals• the higher the concentration of crystals in an area, the higher the screen speed

Factors determining screen speed• Extrinsic factors

• conditions under which screens are used; under the control (somewhat) of the technologist

• 1. Temperature• as room temperature increases (over 1000 F), screen speed decreases and film speed

increases (technique should be increased)• screens emit more light at lower temperatures, so technique would need to be

decreased• only of concern in extreme climates or outdoor radiography

Factors determining screen speed• 2. Kilovoltage

• an increase in kVp will cause an increase in some screen speeds• phosphors have high atomic numbers, so higher kVps can increase the probability of light

production from the phosphors

Screen Speed Classifications1. Intensification factor system

• exposure without screens compared to exposure with screens

2. Descriptive rating system• “Slow” speed

• Fine detail, high resolution

• Requires more rad’n to get a given density

• Best detail next to using a cardboard cassette holder

• “Medium” speed• Par speed, average or universal speed

• “High” speed• Fast speed, most blur

• Small amount of rad’n required to get a given density on a film

Relative speed number system (RS)

• Fine detail RS 20-80 (less than 100)

• Medium detail RS 100

• High speed RS 200-1200 (more than 100)

Screen Contact• If film-screen contact is not perfect, blurring of structures will occur. This is due to

light diverging out instead of being projected straight down to the film.

• test used is the “wire mesh test”

• film-screen contact is usually better in smaller cassettes, where the screens are more uniformly compressed compared to larger cassettes

Causes of poor film-screen contact:1. Warped cassette front

2. Cracked/twisted cassette frame

3. Loose, bent or broken hinges or latches

4. Presence of a foreign body under a screen

5. Worn contact felt

6. Warped screens due to excessive moisture

References sheet for all documents 59

Care of Screens1. Keep cassette closed except when loading/unloading and cleaning.

2. Use screen cleaner with soft gauze pads; cleaner is also “antistatic”.

3. Some manufacturers recommend soap and tap water because cleaners can wear out the screens.

4. Do not dig the film out with finger nails.

5. Clean screens often depending on use; dusty environments, such as construction areas will require more cleaning, whereas 1-3 months may suffice for regular use.

6. Let screens dry thoroughly before loading with film; film could leave a permanent stain on the screen due to purple dye in film.

7. Fingernail polish, including clear coats, should not be worn when handling film or screens because permanent marks can be left on the screens; if screen cleaner and isopropyl alcohol do not remove the stain, the screen is ruined; will show up as a white streak on image.

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