Chapter 3 Intensifying screen

Chapter 3 Intensifying screen

RADIOGRAPHIC IMAGING SCIENCES

INTENSIFYING SCREENS

• An intensifying screen; is a special sheet used to convert xrays into light.
• When x-rays alone are used to create radiographic images,
• most pass straight through the films without causing any film
• blackening (radiographic density).
• This is because x-rays are so penetrative. Only 1% of x-rays
• are absorbed by emulsion layers.
• In order to create adequate film blackening (radiographic
• density), comparatively large exposures have to be made.
• This is overcome by the use of an intensifying screen which
• works to convert small x-ray radiation into large amount of
• light for image formation.
• About 30% of the x-rays striking the screen interact with the
• screen to produce a large number of visible light photons.
• The use of intensifying screen result in considerable lower
• radiation dose to the patient but has the disadvantage of
• causing a slight blurring of the image.
• Important terminologies
• Luminescence; refers to the emission of light from a
• substance bombarded by radiation.
• Luminescence includes two effects; fluorescence
• and phosphorescence
• Fluorescence; refers to the emission of light from a
• substance bombarded by radiation lasting as soon as

radiation exposure is terminated.

• intensifying screens undergo this phenomena. Phosphorescence; refers to the emission of light from
• a substance bombarded by radiation which continues
• for some time (afterglow) even after radiation
• exposure is terminated.
• NB; Phosphorescence is an undesirable phenomenon
• in radiography due to afterglow.
• Intrinsic/photographic unsharpness; refers to radiographic
• unsharpness which arises as a consequence of using film
• and screen material.
• There are three causes of intrinsic/photographic
• unsharpness.
• Divergence of light
• Poor screen/film contact
• Cross-over effect Divergence of light; this refers to outward spread
• of light in all directions from its source.
• Any point of light arising from the intensifying
• screen, will no longer be a point by the time it
• reaches the film emulsion.
• Poor film/screen contact; loss of close and uniform
• contact between the screen and film.
• Cross-over effect; it is the exposure of the
• emulsion by the light from the opposite intensifying

screen.

Screen Construction

• A magnified cross section through an intensifying screen is
• shown below.
• (i)Base; is made from paper, cardboard, or more usually a
• clear plastic such as polyester.
• Its function is to provide a strong, smooth but flexible
• support for the fluorescent layer.
• Thickness of the base is about 0.18mm.Desirable characteristics of base
• Strong but flexible.
• Chemically inert/inactive
• Uniformly radiolucent
• Moisture resistant
• Not discolour with age or on exposure to x-rays
• (ii) Substratum layer; this is the bonding layer between the
• base and the phosphor layer.
• It may be reflective, absorptive or simply transparent in
• nature depending on manufacturer’s intended
• characteristics for the screen.
• Reflective substratum layer; it maximizes the
• effect of the screen by reflecting the light which
• would be lost through the film, back towards the
• film emulsion.
• Absorptive substratum layer; this layer absorbs
• any light travelling backwards towards the screen
• base.
• (iii) Phosphor layer (fluorescent layer); this is the active layer
• of the screen where fluorescence occurs.
• It consists of fluorescent crystals (phosphor crystals)
• which emit light when struck with x-radiation.
• Phosphor crystals are suspended in a transparent
• binder such as polyurethane.
• The binder may also contain carbon granules or
• acutance dye (coloured pigments) whose function is to
• absorb any laterally scattered light within the fluorescent
• layer. →The use of carbon granules minimizes
• photographic unsharpness but reduces speed of the screen.
• NB; Coating weight; refers to the quantity of phosphor
• grains incorporated in a phosphor layer.
• Factors affecting coating weight;
• Grain size; it is inversely proportional to
• coating weight.
• Coating thickness; it is directly
• proportional to coating weight.
• (iv)Supercoat; this is a protective layer and is made up from
• acetate. It helps to assist surface abrasion.
• Types of Phosphor
• Phosphors; are materials which emit visible light when
• exposed to radiation.
• Out of many phosphors available, only a few qualify to be
• used in radiography.
• The two common qualities of phosphors used in
• radiography;
• They are very efficient at x-ray absorption
• They fluoresce strongly, with little afterglow.
• There are two main types of phosphors.
• Conventional phosphors; were common in old
• screens; e.g. calcium tungstate, barium
• fluorochloride and barium strontium sulphate.
• Rare earth phosphors; are common in modern day

screens; e.g. gadolinium, lanthanum and yttrium.Advantages of rare earth phosphors over conventional phosphors;

• They are more efficient at absorbing x-ray photons
• (absorption efficiency or quantum detection
• efficiency)
• They are more efficient at converting x-ray
• photons to light (conversion efficiency)
• Quantum Detection Efficiency (QDE)
• QDE; refers to the ability to absorb incident radiation.
• Rare earth screens are more efficient in this factor compared
• to conventional screens of calcium tungstate. Conversion Efficiency
• This refers to the ability to convert x-ray photon into light.
• Rare earths are more efficient in this factor as well, e.g. 15-
• 20% light conversion efficiency compared to calcium
• tungstate 3-5%
• Activators
• Activators; these are small quantities of some foreign
• materials added to phosphor during manufacture.
• The phosphor-activator combination determines the intensity
• of luminescence and colour of light emitted from the screen.
• Matching film to intensifying screen
• This involves matching films to the colour of intensifying
• screen emission.
• This helps to obtain optimum speed for the film-screen
• system i.e. maximum film blackening for the least
• radiographic exposure.e.g. →a screen phosphor emitting light towards the green
• end of the spectrum is best matched with an orthochromatic
• film also calcium tungstate (blue emitter) and a
• monochromatic film.
• In case of mismatch, the light emission from the screen
• would have a much diminished effect on the film.
• Types of screen and their application
• Screen manufacturers are able to produce a variety of
• screen speeds by the choice of phosphor and size of the
• phosphor grain, the addition/exclusion of absorptive/
• reflective layers and by varying the amount of
• reflective/absorptive material used in screen construction.
• High resolution screen; are manufactured
• substratum layer absorptive.
• Regular screen; is a medium speed screen which aims
• to give the radiographer the best of both, adequate
• speed and sharpness.Fast screen; these screens produce greater film
• blackening for a given radiographic exposure than do
• ‘high resolution’ or ‘regular’ screens.
• Intensification Factor (IF)
• This refers to the ratio of intensity of exposure without
• screens to the intensity of exposure with screens, on the
• same type of film to produce similar density.
• 𝐼𝑛𝑡𝑒𝑠𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 =
• Exposure without screen

Exposure with screens

Quantum Mottle

• This refers to the grainy or mottled appearance on the
• image due to significant gaps on the x-ray photons
• striking the film.
• Also known as image noise.NB; Image produced with few x-rays will have higher

QM with large number of x-rays.