Radiographic cassettes

RADIOGRAPHIC CASSETTES

Presented by:Sudil Paudyal

B.Sc.MIT 1st yr.Roll no.51

4-4-2011 RADIOGRAPHIC CASSETTES BY SUDIL 1

Introduction A radiographic film cassette is a

rectangle or square plastic or metallic container used to hold x-ray films (exposed or unexposed) and intensifying screens in close and uniform contact with one another.


Functions

1. Hold intensifying screens and protect them from damage.

2. Exclude all light from entering the cassette and fogging the film.

3. Maintain a close and uniform contact between the film and screens.

4. Exclude dust and dirt from the sensitive screens.

5. Act as a medium from exposure upto further processing of film.


Features of an ideal cassette1. Lightweight to facilitate easy handling and carrying.2. Easy to open and close under low light conditions.3. No sharp edges or corners which might injure patients

or staff.4. Strong and rigid to withstand physical damage from

daily wear and tear.5. The front part must provide minimum beam attenuation,

be of uniform thickness and have no irregularities.6. Internal rear surface must have an adequate layer of

lead foil attached to minimize the risk of back scatter.7. Cassette construction should be so that there is uniform

contact between film and screen by the use of foam sponges.

8. Availability in range of film sizes.

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Construction Consists of a front and a back hinged

at one edge. Thin sheet of lead foil attached on the

back side which is further attached to a plastic foam pressure pad and an i.s.

The front, also referred to as cassette wall contains front i.s. and a short lead blocker used for patient identification.



Contd… A range of locking methods are used,from

spring clips to sliding locking bars,which serve to exclude light and along with foam pads maintain close contact between film and screens.

All the internal metal or plastic surfaces are given black coating to prevent internal light reflections.

To ensure good film-screen contact a slightly curved cassette back is used which minimizes photographic unsharpness.


Materials used in cassette construction Nowadays, for lightweight cassette design

synthetic materials are used.Cassette front: In order to minimize beam

attenuation ,cassette front should confirm to british standards (BS 4304/1968). It states that if metal it should have an Al equivalent of not more than 1.6 mm when used at 60 kvp or if plastic no more than 0.2 mm Al equivalent.


Metal(e.g. aluminium),plastic laminate or carbon fibre are materials commonly used because of the following advantages:

Strength Light weight Low beam absorption The use of carbon fibre cassettes can mean

significant reduction in patient dose because of their lower beam attenuation,esp. at low kv.


Cassette back: May be of metal or plastic construction and

lined with lead foil to protect from back scattered radiotion from bucky tray or other surface.

BS recommendation states that it should have a lead eq. of at least 0.12 mm when used with 150 kv equipment.


Cassette fittings: Clips or fasteners- usually stainless

steel Hinges - metal or plastic Presssure pad - plastic foam sponge


Types of cassettes available1. i) Single screen cassettes: Have a single i.s. Designed to be used with single sided

emulsion film Principal application in mammography.

ii) Double screen cassettes: Have screen on both sides Used with double sided emulsion film Used in general radiography


2. Curved cassettes: Internally constructed as conventional

cassette with double sided i.s. Only shape is curved to provide good

contact between object and cassette.


Mainly available in two types:i. First type used when object- cassette

contact cannot be achieved with conventional flat cassettes

ii. Second type used to obtain views of mandible and maxilla in orthodontic radiography


Fig: Two types of curved cassettes. (a) is used where the necessary close contact between object and

cassette cannot be achieved with a conventional cassette.e.g. intercondylar views of flexed knee.

(b) is used to obtain panoramic views of mandible (orthopantomography).


3. Gridded cassette: Have a secondary radiation grid

between the front i.s. and the front of cassette.

Used for radiography where a conventional bucky system is unavailable e.g. in ward radiography.


4. Multi section cassettes: Designed to hold 3-7 films, with their

respective i.s. and spacers of about 5 or 10 mm in thickness

Advantage of using spacers: Films which are separated by 5mm spacers

will produce images of body sections 5mm apart and like wise with 10 mm spacers


Fig. A multisection cassette for tomography. A selection of multisection cassettes are available with a choice of spacers thickness and number of screen pairs.


Uses of multisection cassettes: Most common use in tomography where

they are used to produce a set of films each bearing an image of different layer height within the body with one exposure

Another use in multiple radiography where by using single exposure different images having varying densities and contrast can be produced


In the multiple radiography procedure, the cassette is loaded with a set of films and varying speed i.s. but no spacers.

Due to variation in i.s. speeds used, one exposure gives a set of films each with different density range and contrast.


Vacuum cassettes

Made from flexible vinyl material Have a valve attached at one end Inside is removable plastic folder

containing single i.s. For use a single sided emulsion film is

inserted inside the folder . The entire folder is placed inside the

cassette and sealed. Vacuum pump is attached to the valve and

air expelled, thus bringing the is and film in close contact


Fig. A vacuum cassette. Once the film has been inserted and the sliding clamp replaced, all air is drawn out via the non return valve thus bringing the film and screen in close contact with one another.


Uses of vaccum cassette:

Primarily developed for use in mammography

In joint radiography, their inherent flexibility can be used as an advantage in positioning, e.g. under a flexed joint


Formatter cassettes Consist of frame designed to hold two single

sided emulsion films by their edges Also consist of two removable slides which

protect the film from light exposure when not in use

Do not contain intensifying screens

USES: Used for imaging from CRT and TV monitors in

ultrasound, nuclear medicine, CT,DSA and MRI Used in conjunction with formatter or video

imager


Loaded under safelight conditions by removing the two slides and sliding the individual films, emulsion side facing outward, ensuring that they are securely held in side channels by their long edges

Finally the slides are inserted When required for use the cassette is

inserted into the multiformat imager and slide nearest to the film to be exposed is removed


Angiography cassettes Can hold upto 30 sheets of films, size 35*35

cm each positioned with metal spacers After loading and sliding the two panels, the

cassette can be inserted into the AOT equipment

Exposed films will have been transferred via exposure area of AOT apparatus to the take-up cassette

A button on the front is pressed closing the sliding lid

The take-up cassette can then be taken to darkroom and processed



Photo fluorography cassette

Photo fluorography or the recording of images from output phosphor of an image intensifier tube is usually carried out on 70 mm or 105 mm roll film or on 100 mm sheet film.

All the above various film formats have their individual cassette system.


Computed radiography (CR) cassettes Use PSP plates in place of film and screens The plates are coated with europium-

activated Barium Fluoro-Halide (BaFX: Eu 2+).

Halide used may be bromide, iodide or a combination of both.

Cassettes are used just like conventional cassettes on normal radiographic equipment and available in similar sizes.

Information is stored in PSP imaging plates as electrons, in semi-stable higher energy states


Contd… Image information is acquired by scanning the

plate by a laser beam Red laser light excites these trapped electrons

during scanning Electrons eject from the higher energy levels and

come down to the base level. They emit a higher energy blue light during this process.

This light is captured by a light guide, converted into electrical signals, amplified, digitised and used to form the image.


The imaging plate is ready for re-use after exposure to white light.

Patient information and cassette ID needs to be linked in a CR system , as there is no direct electrical connection between the CR reader and the cassette. A bar code reader or a chip embedded on the CR cassette is used for this purpose.


Figure 2. Schematic of a CR imaging system, screen, and scanner.

Samei E et al. Radiographics 2004;24:313-334

©2004 by Radiological Society of North America4-4-2011 RADIOGRAPHIC CASSETTES BY SUDIL 32

Loading and unloading a cassette: The life of i.s. depends on the care during

film loading and unloading. Unloading: Under safelight, the cassette is placed face

downwards on the bench and locking clip released. The cassette is then turned over and the front of the cassette is tripped so that the film falls from the cassette well. The film is removed with free hand and the cassette loaded.


Loading: Under safelights, the cassette is placed

face downward on the bench and as before opened from the back. The unexposed film, tightly gripped at its edge is lowered gently into the cassette well. The cassette is closed by bringing over the back and engaging the locking clip.


Care of cassettes1. Should be treated gently2. Number of cassettes carried at a time

should be limited. Should be carried securely between body and arm, with fingers holding their bottom edge.

3. If stored on edge, it should be ensured that they are as near vertical as possible

4. Contact with fluids should be avoided



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Health & Medicine

Radiographic cassettes