X-ray Cassettes & Screens-Introduction

Functions

• Sensitivity of an X-Ray film to direct X-Ray photons is very low. The efficiency of X-Ray film to absorb X-Ray photons is only 1%. 99% of X-Ray photons are wasted.
• Intensifying screen has a greater efficiency to absorb X-Ray photons and convert them into ultraviolet light or visible light.
• Screen reduces exposure time and X-Ray dose.
• Screen reduces kinetic unsharpness.
• Cassette provides lightproof enclosure for an X-Ray film, which is sandwiched between a pair of screen.

Cassette provides contact between film and screen required for a sharp image by expelling out air trapped between film and screen.
Absorption, Conversion & Emission

Technology

Construction Elements: Cassettes

• An aeronautical grade aluminum alloy is formed to give a curved profile. The curvature is optimized for displacement of air trapped inside the cassette body.
• A lead sheet is affixed inside the cassette shell to prevent backscatter.
• An open cell structured polyurethane foam in the tray allows the trapped air to escape and facilitates good film-screen contact.
• The front plate of the cassette is made from an aluminum alloy which resists bending pressure, exerted on it when the cassette is closed.
• Each construction element has a controlled tolerance of 0.1 mm and is engineered to eliminate gaps and leakages.

Construction Elements: Cassettes	Technical Parameters of Screens
Construction Elements: Screens	Types of Phosphor

Radiation Protection Apparel

Maintenance & Care

Storage

• Store in dust-free environment.
• Avoid exposure to extreme environmental conditions.
• Storage area should be free of X-Ray radiation and chemical fumes.
• Do not keep any weight on cassette or screen.

Cleaning

• Periodical cleaning of screen & cassette ensures high image quality.
• Moisten a cellulose cloth (non-fluffy) with antistatic cleaning agent.
• Wipe the cleaner softly and evenly over the whole surface of the screen.
• Remove dust from corners of cassette.
• Leave the cassette open for approximately 10 minutes to enable the solvent to evaporate completely.
• Close the cassette after screen surface has dried completely.
• Never put excess cleaner on screen surface.
• Never put the cassette on end for drying. This may lead to distortion of screen.

Mounting Screen in Cassette

• Clean cassette to remove dust particles.
• Ensure that the cassette is not warped or bent and that it closes perfectly.
• If the cassette is warped or bent, replace it.
• Peel off the protective liners of adhesive tape from the front screen.
• Place front screen in the center in the front tray of the cassette, tape side down, and gently press along the edges.
• Peel off the protective liners of the back screen and place it over the front Screen, tape side up, making sure that the edges of both screens coincide exactly.
• Gently close and lock the cassette and wait for half an hour for perfect adhesion.
• Enter installation date on the enclosed cassette label and affix it to the back of the cassette.

Standards & Testing

Standards
Numerous standards have been developed in various countries for cassettes and screens

• These include DIN (Germany), ANSI, IEC, JS (Japan), MIL (U.S. Military), ISI (India) and others.
• With emergence of ISO standards in Europe, various global standards are veering towards ISO standard 4090.
• The ISO 4090 and ISO 4090-1 standards for cassette and screen define dimension, dimensional tolerances, geometrical accuracy, absorption standards, design parameters as well as stipulate testing methods.

Testing
Key areas for performance measurement of cassettes and screen, on the basis of ISO 4090 are:
Film-Screen Contact Test

• Contact is the key measurement of radiological image quality of the cassette.
• Good contact leads to accurate diagnosis.
• Contact test should be ideally done once a year.

Film-Screen Contact Test

Film-screen contact has a significant influence on radiographic image quality. Film-screen contact leads to accurate diagnosis. The cassette should be tested for contact at least once in a year.
Procedure

• Clean the screen and cassette with screen cleaning solution recommended by manufacturer.
• Allow screen to dry completely.
• Load cassette with film and wait for approximately 3 minutes for trapped air to escape out of the cassette.
• Place a wire grid (3.15 mm mesh width for conventional radiography) on top of the cassette.
• Expose the wire grid at 70 kV, so that exposed film has density of approx. 2.8.
• Process the film and view an illuminator from a distance of at least 1.5 meters.

Observation

• Appearance of overall uniform density indicates uniform film-screen contact.
• Areas, which appear dark, indicate areas of poor or lack of contact. Such dark areas in diagnostically important parts of the image are not acceptable.
• Please view good contact and poor contact images.

Good Contact

Poor Contact

Speed & Resolution Test

• Quantum of X-Ray dosage required to raise the density of the X-Ray film to the desired level depends on the Screen speed.
• A screen speed of 400 together with the use of a regular speed film addresses the requirements of most of the general radiological examinations.
• Resolution of screen is the key to clear film reading.

Speed & Resolution Test

Procedure

• Load the pair of test screen and reference screen in the standard cassette.
• Load the X-Ray film.
• Use pre-exposed step wedge as an object.
• Expose step-wedge at 110 FFD, 70 kV, 8 mAs.
• Develop the exposed film in automatic processor. Measure density by using densitometer.
• Measure density for 3 steps of the step-wedge for test screen and reference screen exposed. Record the readings.
• Density to measure at marked center step and step above and below it for 3 step readings. The density of reference screen center step should be between 1.2 to 1.5 D. Adjust mAs to get correct density.
• Plot a graph of Density vs. Log IT (I is current & T is time), considering that each step corresponds to 0.05 IT. Use a scale of 15 mm = 0.05 IT on X-axis, and suitable scale on Y-axis as per density values.
• Draw a line perpendicular to X-axis at density value corresponding to center step for the reference screen.
• Draw a line perpendicular to X-axis from point of intersection of line drawn from center point of reference screen and line graph of test screen.
• Calculate difference of log IT on X-axis. It is the difference of the reading of test screen and reference screen.

Calculate speed as shown below:

A = (Difference x 0.05)/15
B = Log constant for reference screen
Speed = Antilog of B x 100.

Measurement of Resolution by X-Ray Exposure

• Use same pair of screen for resolution test as used for speed testing.
• Expose test screen & reference screen with resolution test kit as an object.
• Develop the exposed film in automatic processor. Measure density of exposed film at area 1 inch below the resolution image. The density should be between 2.2 to 2.6 D.
• Measure resolution by viewing the film illuminator using 10x lens. Lines clearly visible in maximum resolution step is considered as resolution and recorded for both reference & test screens.

Cassette Light-Proofness Test

• Light leakage in diagnostically important areas, on the edges of a film leads to loss in portions of an X-Ray image.
• The cassette has to be appropriately designed, engineered and must use optimum construction elements to result in light-proofness.

Cassette Light-proofness Test

Light-proofness of cassette is a basic requirement to protect from loss of data of a radiological examination.
Procedure

• Cassette is fitted with a pair of screens and loaded with X-Ray film.
• It is exposed for 10 minutes to a 100 watt frosted tungsten filament lamp located at a distance of 1 metre (39 inches) from the cassette.

Evaluation

• Increase of density of more than 0.1 on the edge indicates light leakage.
• Diagnostically important areas for light-proofness are all 4 edges to a maximum extent of 3 mm (1/8th inch)

Acceptable Light Proofness

Non-Acceptable Light Proofness

Cassette Durability Test

• Durability in cassette is essential for maximizing the return on investment in cassettes.
• A durable cassette should be able to withstand periodical accidental drops during usage.

Cassette Durability Test

Durability of a cassette is tested by carrying out a drop test.
Procedure

• Carry out a film-screen contact test and a light-proofness test on the cassette to be tested for durability.
• Drop the cassette from a height of 1 metre (39 inches) on a concrete floor.
• Cassette should be dropped in such a way that it strikes each corner and each edge, for a total of 8 drops.
• A film-screen contact test and a light-proofness test should be done after each of the 8 drops.

Evaluation

• In case of a durable cassette, there is no deterioration in the extent of film-screen contact and light-proofness.