Radiographic Image contrast & image resolution

Radiographic Image contrast & image resolution

1. Radiographic Image Quality & Measurements Nitish Virmani Lecturer SGT University

2. Radiographic Contrast Radiographic contrast  is the degree of density difference between two areas on a radiograph. Contrast makes it easier to distinguish features of interest, such as defects, from the surrounding area. The image to the right shows two radiographs of the same stepwedge . The upper radiograph has a high level of contrast and the lower radiograph has a lower level of contrast. While they are both imaging the same change in thickness, the high contrast image uses a larger change in radiographic density to show this change. In each of the two radiographs, there is a small circle, which is of equal density in both radiographs

3. Radiographic Quality  Radiographic Quality refers to the fidelity with which the anatomic structures being examined are imaged on the film.  Three main factors:  Film Factors  Geometric Factors  Subject Factors  Characteristic of radiographic quality:  Spatial Resolution ( Recorded Detail )  Contrast Resolution ( Visibility of Detail )  Noise (Visibility of Detail)  Artifacts

4. Spatial Resolution  Resolution is the ability to image two separate object s and visually distinguish one from the other.  Spatial Resolution is the ability to image small structures that have high subject contrast such as bone-soft tissue interface.  When all of the factors are correct, conventional radiography has excellent spatial resolution . Contrast Resolution  Contrast resolution is the ability to distinguish structures with similar subject contrast such as liver-spleen, fat-muscle.  Computed tomography and MRI have excellent contrast resolution. Conventional radiography is fair to poor.

5. Noise  Noise is an undesirable fluctuation in optical density of the image.  Lower noise results in a better radiographic image because it improves contrast resolution.  Two major types:  Film Graininess- no control over  Quantum Mottle- some control over Film Graininess  Film graininess refers to the distribution in size and space of the silver halide grains in the film emulsion.  Similar to structure mottle that refers to the size and shape of the phosphors in the intensifying screens.  Inherent in image receptor, and are not under the control of technologist, and they contribute very little to radiographic noise.

6. Quantum Mottle  Quantum mottle refers to the random nature by which x-rays interact with the image receptor.  Principal contributor to radiographic noise.  Image produced with few x rays will have higher QM than image produced with from large number of x rays. use of very fast intensifying screens results in increased QM.  The use of high mAs , low kVp settings and of slow image receptors reduces quantum mottle.  Very fast screens have higher quantum mottle because it takes fewer x-rays to make the image. Spe e d  Resolution and noise are intimately connected with speed.  While the speed of the images receptor is not apparent on the image, it influences both resolution and noise.

7. Radiographic Quality Rules  Fast Image receptors have high noise and low spatial and contrast resolution.  High spatial and contrast resolution require low noise and slow image receptors.  Low noise accompanies slow image receptors with high spatial and contrast resolution. Film Factors of Quality  Characteristic curve  Density  Contrast  Latitude  Processing  Time  Temperature

8. Sensitometry  Sensitometry is the study of the relationship between the intensity of exposure of the film and the blackness after the film is processed.  Unexposed film is clear with a blue tint after processing.  Properly exposed film appears with various shades of gray.  Heavily Exposed film is black after processing.  Two principles involved.  Exposure of the film  Amount of light transmitted through the processed film of optical density.  Used to describe the relationship of radiation exposure and blackness or optical density on the film.

9. Characteristic Curve  This relationship is called the characteristic curve or H & D curve of the film.  H & D stands for Hurter and Driffield. Parts of the Characteristic Curve  Toe and shoulder low and high , exposure levels, where large changes in exposure results in small changes in OD.

10. Parts of the Characteristic Curve  The straight line or intermediate area is where very small changes in exposure results in large changes in density.  This is the important part of the curve in radiography,where properly exposed radiographs appear.

11. Log Relative Exposure (LRE)  X-ray films responds to a wide range of exposure from 1 mR to 1000 mR.  It is not the absolute exposure that is of interest but rather the change in OD over each exposure interval  Exposure is represented on logarithmic manner(log relative exposurer) .

12. Optical Density  It is not enough to say that OD is the degree of blackening of a radiograph,or that a clear area of the radiograph represent low OD and a black area represent high OD.  OD density has a pre size numeric value that can be calculated if the level of light incident on a processed film(Io)and the level of light transmitted through that film(It) are measured.  OD=log 10 Io/It Optical Density Range  The optical density range is from 0.0 to 4.0 corresponding to clear and absolute black repectively.  Useful range in general radiography is from 0.5 to 2.25.  Image range is 0.5 to 1.25 OD

13. Fog density and base density  Most unexposed and processed film has an OD in the range of 0.1 to 0.3,corresponding to 79% and 50% transmission, respectively.these ODs of unexposed film are due to base density and fog density.  Base density is inherent in the base of the film and is due to the composite of the base and s the tint added to the base.  Fog density results from development of silver grains that contain no useful information. Higher fog density reduces the contrast of the image.  The tint of the base of the film and the inadvertent exposure of the during processing.  Range is from 0.1 to 0.3. Should be never above 0.30 most is .21 OD

14. Things may Impact Base Fog  Film storage  Film exposure to wrong spectrum of light or light intensity.  Chemical contamination.  Improper processing.  High Base fog levels reduce contrast. Contr a st  The variations in the OD in the radiograph is called radiographic contrast.  Marked differences in OD----High contrast radiograph.  OD differences are small----Low contrast radiograph  Radiographic Contrast is the combined result of image receptor contrast and subject contrast .  Image receptor contrast refers to the contrast inherent in the film and influenced by the processing of the film.

15. Contr a st  Subject contrast is determined by the size, shape and x-ray attenuating characteristics of the subject being examined and the energy (kVp) of the x- ray beam. Image Receptor Contrast  Inherent to the film and screen combination but is influenced by:  Range of Optical Density  Film Processing Technique  Film type is determined by the type of intensifying screens used.  Film-screen images always have higher contrast compared with direct exposure images.

16. Image Receptor Contrast  The slope of the straight line portion of the H & D curve is the receptor contrast.  The average gradient is a straight line drawn between the densities of 0.25 and 2.00 + base fog. Average Gradient  The average gradient is a straight line drawn between 0.25 OD and 2.0 OD above base plus fog.  This is the useful range of optical density in on most radiographs.

17. Speed  Speed is the ability of the receptor to respond to low x-ray exposure.  The H & D curve is useful in comparing speed when selecting film or screens.  A relative number of 100 given to Par Speed Calcium Tungstate Screens.  High Speed Calcium Tungstate has a speed of 200. Half of the exposure is needed to produce the same image.  Rare earth screen film combinations range is speed from 80 to 1600.

18. LATITUDE  Latitude can be observed on the H & D curve.  Latitude refers to the range of exposure that will produce a diagnostic range OD.  Latitude and Contrast are inversely prop o rt i onal.  Wide latitude has a wide gray scale or low contrast. (B)  Narrow latitude has a short scale or high contrast. (A) Latitude is designed into some screen and film combinations. With wide latitude, the error factor in technique is wider.  Latitude can also be impacted by the technical factors

19. Film Processing  Radiographic Quality is impacted by film processing parameters.  The developer must be at the proper concentration and at the correct temperature. The film must also spend the correct amount of time in the developer.  This is the time & temperature relationship .

20. Numerical Aperture  ( NA ) the  numerical aperture  ( NA ) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one material to another, provided there is no refractive power at the interface.

21. Point Spread Function (PSF) The ideal point spread function (PSF) is the three-dimensional diffraction pattern of light emitted from an infinitely small point source in the specimen and transmitted to the image plane through a high numerical aperture (NA) objective. It is considered to be the fundamental unit of an image in theoretical models of image formation. When light is emitted from such a point object, a fraction of it is collected by the objective and focused at a corresponding point in the image plane. However, the objective lens does not focus the emitted light to an infinitely small point in the image plane. Rather, light waves converge and interfere at the focal point to produce a diffraction pattern of concentric rings of light surrounding a central, bright disk, when viewed in the x-y plane. The radius of disk is determined by the NA, thus the resolving power of an objective lens can be evaluated by measuring the size of the Airy disk (named after George Biddell Airy).

22. The image of the diffraction pattern can be represented as an intensity distribution as shown in Figure. The bright central portion of the Airy disk and concentric rings of light correspond to intensity peaks in the distribution. In Figure, relative intensity is plotted as a function of spatial position for PSFs from objectives having numerical apertures of 0.3 and 1.3. The full-width at half maximum (FWHM) is indicated for the lower NA objective along with the Rayleigh limit

23. Point Spread Function, PSF R esponse of an imaging system to a point source M ost basic measure of resolution properties of an imaging system D escribes the extent of blurring that is introduced by an imaging system T wo-dimensional (2D) function PSF ( x , y ) Rotationally symmetric/ asymmetric D escribes the extent of blurring that is introduced by an imaging system

24. Point Spread Function, PSF R esponse of an imaging system to a point source P oint S ource symmetric response of „imaging system” asymmetric response of „imaging system”

25. Stationary Imaging System - the PSF remains constant over the FOV of the imaging system Nonstationary Imaging System has a different PSF depending on the location in the FOV assymetric system

26. Line Spread Function, LSF Edge Spread Function, ESF

27. How this works With MTF

28. Modular Transfer Function The Modulation Transfer Function (MTF) is a method of determining the response of an imaging system to different spatial frequencies in the images. Spatial frequency is generally expressed as cycles, or line pairs, per millimeter ( lp /mm) in analog environments, but for digital systems, cycles per pixel (c/p) is more appropriate where sensor sizes vary from one detector to another. Two important parameters for determining the image quality in spatial domain are contrast and resolution. MTF is an equivalent term which is used to characterize the image in frequency domain. Generally, MTF for an imaging system is plotted as % amplitude against spatial frequency.

29. At frequencies where the MTF of an imaging system is 100%, the object details are unattenuated and the image retains its full contrast. Similarly, at a MTF of 50%, the contrast of the imaging system is reduced by half of the original value. In imaging science, the MTF is usually normalized to 100% at very low frequencies. The focal spot of the radiographic unit is one of the most important parameters, because it influences the resolution of the image. There is a direct relationship between the focal spot and the MTF curve of a radiography system. For large focal spot x-ray machines, MTF values reach a minimum (i.e. zero) at comparative low spatial frequency and vice versa.

30. Here, d 1  is the source-to-object distance and d 2  is the object-to-film distance. The parameter “f” equals the spatial frequency of the sinusoidal object. The above equation is similar to an optical system because Fourier transformation of a slit with width ‘a’ is (sin πfa / πfa). The value of MTF function will be zero where frequency is such that the argument of sine function is equal to π or its multiples. If frequency is ‘f ’, for the MTF to be zero, The above equation shows that the maximum spatial frequency of the object (f ) (i.e. the finest details of the object) is inversely proportional to the proportional focal spot size of the x-ray machine. MTF has a multiplicative property. In radiography, the effective MTF is the multiplication of the MTF of film and that of the focal spot. Therefore, MTF(effective) = MTF(film) x MTF(focal spot)

31. THE LINE  spread-function (LSF) and modulation transfer function (MTF) of a screen-film system convey important information regarding the light diffusion in the system, One of the parameters influencing overall image quality. The LSF of a screen-film system is the relative illuminance distribution in the image of a narrow slit. Thus, the LSF is a direct measure of light diffusion in screens and film. The MTF is a measure of the ability of the systems to image, frequency for frequency, a radiation pattern in which intensity varies sinusoidally with distance in one dimension in the object plane. The LSF and MTF of screen—film systems are parameters that can be used as a measure of the image deterioration due to light diffusion only. The effect on image quality of other system characteristics, such as film contrast and absorption of x-ray quanta in the screens resulting in quantum mottle, is not measured by the LSF and MTF.  Thus, the LSF and MTF do not, in themselves, provide a measure of the overall image quality attainable with a screen—film system . The LSF and MTF are essentially different means of expressing the effect of light diffusion in the screen—film system, and they are mathematically related (Fourier transform pair) so that when one has been measured, the other can be calculated. Note- This may vary system to system because its dependency on system

32. In Short The resolution and performance of an optical microscope can be characterized by a quantity known as the modulation transfer function (MTF), which is a measurement of the microscope's ability to transfer contrast from the specimen to the intermediate image plane at a specific resolution. Computation of the modulation transfer function is a mechanism that is often utilized by optical manufacturers to incorporate resolution and contrast data into a single specification. The modulation transfer function (MTF) indicates the ability of an optical system to reproduce (transfer) various levels of detail (spatial frequencies) from the object to the image. Its units are the ratio of image contrast over the object contrast as a function of spatial frequency. It is the optical contribution to the contrast sensitivity function (CSF).

33. MTF: Cutoff Frequency . 5 1 1 mm 2 mm 4 mm 6 mm 8 mm modulation transfer 5 10 15 2 2 5 3 spatial frequency (c/deg) cut-off frequency c ut o ff f   a 57.3   Rule of thumb: cutoff frequency increases by ~30 c/d for each mm increase in pupil size

34. Measurements of Image Quality PSF = Point Spread Function LSF = Line Spread Function CTF = Contrast Transfer Function MTF = Modulation Traffic Function

35. Point Spread Function PSF “Point” object imaged as circle due to blurring Causes finite focal spot size finite detector size finite matrix size Finite separation between object and detector Ideally zero Finite distance to focal spot Ideally infinite

36. Quantifying Blurring Object point becomes image circle Difficult to quantify total image circle size – difficult to identify beginning & end of object Intensity ?

37. Quantifying Blurring Full Width at Half Maximum (FWHM) width of point spread function at half its maximum value Maximum value easy to identify Half maximum value easy to identify Easy to quantify width at half maximum Maximum Half M a x i m um FWHM

38. Line Spread Function LSF Line object image blurred Image width larger than object width Intensity ?

39. Contrast Response Function CTF or CRF Measures contrast response of imaging system as function of spatial frequency Lower F re q u e ncy Higher F re q u ency Loss of contrast between light and dark areas as bars & spaces get narrower. Bars & spaces blur into one another.

40. Contrast Response Function CTF or CRF Blurring causes loss of contrast darks get lighter lights get darker Lower F re q u e ncy Higher F re q u ency Higher Con t r ast Lower Con t rast

41. Modulation Transfer Function MTF Recorded Contrast (reduced by blur) f r e q uency M T F Contrast provided to film Fraction of contrast reproduced as a function of frequency Freq. = line pairs / cm 1 50%

42. M T F Can be derived from point spread function line spread function MTF = 1 means all contrast reproduced at this frequency MTF = 0 means no contrast reproduced at this frequency

43. M T F If MTF = 1 – all contrast reproduced at this frequency R e co r d e d Contrast Contrast provided to film

44. M T F If MTF = 0.5 – half of contrast reproduced at this frequency R e c o r ded Contrast Contrast provided to film

45. M T F If MTF = 0 – no contrast reproduced at this frequency R e co r d e d Contrast Contrast provided to film

46. Modulation Transfer Function (MTF) MTF = I max - I min I max + I min -MTF is a measure of the contrast of an aerial pattern, -For well-separated images, MTF ~ 1, -For smaller images, MTF<1 -In general, MTF should be >0.5. Intensity Intensity Displacement D i s p l a c eme n t

47. Component MTF Each component in an imaging system has its own MTF – each component retains a fraction of contrast as function of frequency System MTF is product of MTF’s for each component. Since MTF is between 0 and 1, composite MTF <= MTF of poorest component

48. Modulation Transfer Function MTF is a measure of an imaging system’s ability to recreate the spatial frequency content of scene MTF is the magnitude of the Fourier Transform of the Point Spread Function / Line Spread Function. 1 .0 C u t- o ff Spatial frequency

49. Thank You Nitish Virmani Lecturer Department of Radio-Imaging Technology Faculty of Allied Health Sciences SGT University Email- [email protected]