Quality Control Testing of Screen Speed

Quality Control Testing of Screen Speed

QC testing of screen speed should occur on acceptance and then yearly.

Evaluate first whether similar cassettes marked with the same relative speed are the same using the following procedure

Procedure Make an exposure of a step-wedge or

homogenous phantom onto an image receptor so that the center of the image has an optical density of about 1.5

Expose each image receptor to the same technical factors.

Process, and take optical density readings of the same center area in each. If all have the same relative speed, the optical density should not vary by more than +-0.05

Screen Uniformity Testing

During manufacturing processes, inconsistencies may occur in which the phosphor layer is applied more thickly at one portion of the screen than at another.

Sometimes during cleaning excessive rubbing may remove more of the phosphor layer.

Screen uniformity testing should be performed on acceptance and then yearly.

Procedure

Make an exposure of a step-wedge or homogenous phantom onto an image receptor so that the center of the image has an optical density of about 1.5

Process and take optical density readings in the center and in each of the four quadrants of the image. It should not be more than +-0.05.

Any film/screen image receptors that exceed this limit should be removed from service.

Spectral Matching

The film and intensifying screen should match each other because the film are sensitive to a specific color.

A blue-violet emitting screen phosphor should be used with a monochromatic films.

A green-emitting screen phosphor should use with an orthochromatic film.

Panchromatic

Is a type of black-and-white photographic film that is sensitive to all wavelengths of visible light.

A panchromatic film therefore produces a realistic image of a scene. Almost all modern photographic film is panchromatic,

Orthochromatic

Orthochromatic photography refers to a photographic emulsion that is sensitive to only blue and green light, and thus can be processed with a red safelight.

It is used on all radiographic films.

Screen Resolution

Intensifying screens should be able to demonstrate clear images of patient anatomy so that the proper diagnosis can be obtained.

The ability of an imaging system to accurately display images.

There are two types of resolution Contrast resolution Spatial resolution

Contrast Resolution

Is the ability of an imaging system to distinguish structures with similar x-ray transmission as separate entities (in short separate shades of grays)

It is affected by the sensitivity of the image receptor speed and the amount of radiographic mottle (noise).

If the radiographic mottle is increased, contrast resolution is decreased.

The Relationship Between The Quantum Noise, Contrast Resolution And Screen Speed

Is that the faster the screen speed screens, the lower the mAs values are used, which in turns increases the quantum mottle and lowers the contrast resolution.

Spatial Resolution

Is the ability of an imaging system to create separate images of closely spaced objects.

In other words, do the two object appear sharp and clear, or do they blur together?

This is determined by the amount of light diffusion that occurs between the screen and film.

Spatial Resolution

It is affected by: Screen thickness Phosphor crystal size Film/screen contact.

The most common method of measuring spatial resolution is the spatial frequency.

The unit of spatial frequency is the line pairs per millimeter (lp/mm).

Line Pairs Per Millimeter Resolution Chart.

a line pair is a space and line each being 0.1mm wide.

The greater the line pairs per millimeter value the smaller the object that can be imaged and the better the spatial resolution.

The human eye can read up to 5lp/mm, but most screen system cannot provide this level of spatial resolution.

Other Methods Of Measuring Spatial Resolution

Point spread function (PSF) Line spread function (LSF) Edge spread function (ESF) Modulation transfer function (MTF)

Point Spread Function

Is a graph that is obtained with a pinhole camera and a microdensitometer.

The pinhole camera creates a black dot in the center of a film and microdensitometer is used to take readings.

The values are plotted on a graph versus the distance from the center of the point.

The narrower the peak on the graph, the better spatial resolution and image quality.

Line Spread Function

Is a graph that is more accurate and easier to obtain than the PSF graph.

It requires an aperture with a slit that is 10um wide instead of the pinhole camera.

The density readings are taken of the centerline and plotted.

Edge Spread Function

It requires a sheet of lead to be placed on a cassette and exposed.

Density readings are taken at the border between the black and white areas and plotted on a graph.

Modulation Transfer Function

Is a numeric value that is used to measure the spatial resolution and is obtained from the LSF graph with a mathematical process known as fourier transformation.

Screen Condition

Intensifying screens must be free of dirt, stains and defects to properly function.

A regular schedule at least every 6 months of screen cleaning with an antistatic solution should be a standard department policy.

A UV light may be used to examine the surface of the screen if there are any stains.

Why do we need to have a QA/QC on Just an intensifying screen?

Laboratory

film/screen contact test

film/screen contact test

Frequency of test Yearly As necessary

Equipment required Cassette to be tested Test tools:▪ Box of paper clips or sheet of perforated zinc or

fine wire mesh, large enough to cover a 35 x 43cm (14 x 17” film), with a square hole, about 10cm from one edge, approximately 2 to 2cm square)

Method

Load the cassette to be tested and place it face up on the tabletop

Cover the whole of the cassette with the test tool (if cassette distribute evenly).

Set a FFD (SID) of 150cm[60”] ( the longer FFD (SID) reduces geometric unsharpness)

Collimate to cover whole of cassette Make exposure using a 50kVp and

6mAs Process film

Evaluation

If a densitometer is available measure the image.

Inspect the image, looking for areas that look blurred.

A noticeable area of unsharpness could be caused by: Damaged cassette Screen packing, deterioration An air pocket

When using a close mesh wire test the poor film/screen contact areas may also have a higher density.

End