Image Characteristics

1. Image Characteristics

2. What is an image? Dictionary meaning • An optical appearance • A form of semblance • A mental representation • An idea or conception

3. The term image may be applied to: • A picture such as a photograph, a painting or a sketch which has a physical existence • An idea or concept which has a mental existence (When we see an object like an apple or a picture of an apple we can imagine the taste of apple)

4. Visual images are of two types • Real images – those having physical existence such as pictures which are accessible to scientific measurements and objective study • Mental images – those generated as mental pictures within our minds and which are accessible to subjective study

5. Real Images • Real images consist of patterns of light intensity and possibly variations of colour • The patterns of light intensity are created in one of three ways • Viewing by reflected light from a surface • Viewing by light transmitted by semitransparent layer • Viewing by light emitted by a fluorescent layer

6. Viewing by reflected light from a surface

7. Viewing by light transmitted by semitransparent layer More light transmitted Less light transmitted

8. Viewing by light Emitted from a fluorescent layer High Light of different intensities X-rays of different intensities Base Pigmented layer Low

9. Image characteristics Real images display four essential characteristics • Noise • Contrast • Sharpness • Resolution

10. Noise Real images consists of two components • A meaningful pattern which carries information about the object - Signal • A spuriouschaotic pattern carrying no information about the object - Noise

11. Effect of noise • The presence of noise limits the amount of information which can be extracted from the image. Especially the finer details of structure may be lost by being swamped by the effect of noise. Example :- fogging on the radiographic image

12. Good image fogged image High signal to noise ratio Low signal to noise ratio

13. Signal –to- noise ratio • When under the optimum conditions the magnitude of the signal is very much greater than the magnitude of the noise, the signal –to-noise ratio is said to be high and much information is gained. • When under the adverse conditions the signal-to-noise ratio is low much information is lost.

14. Contrast • Contrast is the difference of appearance of a feature or a structure in an image from its surrounding. • It may be described as the difference between the shades of gray or degree of luminance on an image Eg. Optical Density (degree of blackening) difference on a radiograph. Luminance (brightness) difference on TV screen

15. High Low OptimumContrast Contrast Contrast

16. Sharpness • Sharpness is concerned with how suddenly blackening changes at the boundary between adjacent parts.

17. Sharpness The boundary between two areas A & B appears very sharp A B Density D2 D1 0 5 10 15 20 Distance (mm)

18. Unsharpness The boundary between two areas A & B appears unsharp A B Density D2 D1 0 .2 .4 .6 .8 Distance (mm) The steeper the slope the more sharp the image appears. The shallower the slope the more blurred the image

19. Sharpness, unsharpness & lack of sharpness • No image is perfectly sharp • Every image has a certain lack of sharpness • Unsharpness is an objective concept which can be measured • Sharpness is our subjective perception of unsharpness, and depends on contrast and unsharpness

20. Contrast & perception of unsharpness • We judge one image boundary to be sharper than another, even though they are both equally unsharp, if the contrast of the first image is greater.

21. Specifying unsharpness • Image unsharpness can be expressed by means of transfer functions such as • Edge spread function • Point spread function • Line spread function OR • As a simple numerical quantity

22. Edge spread function • Using the microdensitometer trace of an image of a perfectly sharp object edge. The density values should be expressed in relative terms. A B Density 1 0 X Y 0 .1 .2 .3 .4 Distance (mm) Unsharpness is expressed as the distance over which the density is changing (XY) = 0.15 mm

23. Point spread function (PSF) • This is the microdensitometer trace of a point structure in an object (e.g. a minute hole in a sheet of lead) Object Relative optical density 1 Image 0.5 0 Distance Measure the density along this line

24. Line spread function (LSF) • This is the microdensitometer trace of a line structure in an object (e.g. a thin slit in a sheet of lead) Object Relative optical density Image 1 0.5 The unsharpness can be expressed as a simple numerical value as the FWHM measurement ( Full Width at Half Maximum) 0 Distance FWHM

26. Perception of unsharpness • In radiography it is said that an image whose unsharpness is <0.1mm will appear to be sharp under normal viewing conditions • The perception of unsharpness is more determined by the density gradient than by the simple measure of the unsharpness • Density gradient depends both on unsharpness and density difference (Contrast) • Thus an image of high contrast appear sharper than one of low contrast, even though both images have the same measured unsharpness

27. Contrast, Density gradient & Unsharpness Image A Image B Density D2 D4 D3 Unsharpness = X D1 Unsharpness = X Distance (mm) Distance (mm) Contrast of A > Contrast of B Image A Appear sharper than image B

28. Resolution • The resolution of a system is its ability to demonstrate closely spaced structures in the subject as separate entities in the image E.g. The trabeculae pattern of bone • The resolution of an image refers to its ability to show small structures separately • The smaller the structures visible the higher the resolution of the image/system

29. Measurement of resolution • The resolution is expressed in terms of the smallest spacing /highest spatial frequency of high and low dense structures visible in the image • This is done subjectively by radiographing a test object consist of a grid of closely spaced lines, alternately radiopaque and radiolucent.

30. Resolution test object • Each radiopaque line and corresponding space is known as a line pair • The spacing of line pairs is expressed as line pairs per millimetre (lp/mm) Line pairs

31. Resolution test object

32. Comparison of resolution • The resolution of different imaging systems may be compared by reference to the Modulation Transfer Function (MTF) of each system * Modulation = frequency of change (of structure , etc.) • MTF is a method of assessing the success with which modulations of structure (detail) in an object are transferred into modulations of density or luminance in the image

33. Modulation in the image • MTF = ------------------------------------ Modulation in the object MTF = 1 means that image reproduces exactly the variations in the object. It is a characteristic of imaging systems that, as the detail in the object becomes finer, the ability of the system to record that detail becomes progressively reduced. As the spatial frequency increases MTF decreases.

34. Modulation transfer function curve of a film-screen system System 1 System 2

35. Factors limiting resolution • The resolution of a radiographic image is influenced at every stage in the process of image production. • If we consider each step in this process as links in a chain, then the quality of the final image can be no better than that of the weakest link . • We must identify the weakest links and try to improve them.

36. But we must realize that in improving one aspect we may weaken another. • There is a complex interrelationship between many of the factors concerned, including all of the image characteristics discussed here. • Only when we have studied all the ramifications of image production will we be able to understand fully how resolution may be optimized.

37. Next • Production of & Characteristics of the invisible x-ray image