Quality Control Testing of Screen Speed

1. Quality Control Testing of Screen Speed

2. 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

3. 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

5. 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.

6. 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.

7. 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.

8. 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,

10. 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.

11. 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

12. 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.

14. 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.

15. 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.

16. 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).

17. 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.

21. Other Methods Of Measuring Spatial Resolution • Point spread function (PSF) • Line spread function (LSF) • Edge spread function (ESF) • Modulation transfer function (MTF)

22. 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.

23. 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.

25. 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.

26. 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.

29. 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.

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

31. film/screen contact test Laboratory

32. 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)

33. 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

34. 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.

35. End