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Radiation Protection in Digital Radiology

Radiation Protection in Digital Radiology. Digital Radiographic Image Processing L05. Educational Objectives. List three main purposes of digital image processing Explain the term “greyscale histogram” Show how radiographic technique factors affect the greyscale histogram

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Radiation Protection in Digital Radiology

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  1. Radiation Protection in Digital Radiology Digital Radiographic Image Processing L05

  2. Educational Objectives • List three main purposes of digital image processing • Explain the term “greyscale histogram” • Show how radiographic technique factors affect the greyscale histogram • Suggest how errors in digital image processing can contribute to unnecessary radiation exposure to patients Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  3. The quality of any monochrome image can be described in conventional terms. • Density (darkness) • Contrast • Sharpness • Noise • Artefacts Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  4. Advantages of DR images vs. analogue images • Densitycan be modified. • Contrastcan be modified. • Sharpness can be modified. • Noise can be modified. Result of 20 years of development! Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  5. Advantages of analogue images vs. DR images • Densityfunction inherently nice. • Contrastinherently higher. • Sharpnessinherently higher. • Noiseinherently lower. Result of 110 years of development! Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  6. DR densityis adjustable and arbitrary • Acquisition is independent from display • Code values in the raw DR image can be translated to any display level • This allows DR to compensate for over- and under-exposure, producing a consistent appearance Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  7. Incorrect exposure factor selection in screen-film radiography Overexposed Underexposed Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  8. DR compensates for incorrect exposure factor selection Overexposed Underexposed Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  9. The raw DR image has low contrast • DR has an extremely wide latitude, which implies low contrast for an imaging system that is “display limited” (limited by the latitude of the display). • DR code values can be remapped to generate high contrast for “values of interest” (VOI), while sacrificing contrastfor other values. • This is the primary purpose of DR image processing Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  10. Idealised Greyscale Histogram • Goal is to represent anatomy, A, with good contrast • B is air • C is scatter contribution - only outside collimators • D is scatter contribution -only image of anatomy outside collimated area, barium, or lead # pixels A B C D Exposure (or greyscale) Region of clinical interest Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  11. # pixels Increased mAs shifts the Greyscale Histogram • Changing mAs does not affect subject contrast, as long as the dynamic range is not exceeded A B C D Exposure (or greyscale) Region of clinical interest Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  12. Increased kVp squeezes the Greyscale Histogram • Higher kVp => less subjectcontrast # pixels B A C D Exposure (or greyscale) Region of clinical interest Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  13. Primary job of image processing: identify values of interest and maximize their contrast. • Detection of collimator boundaries or anatomy, “exposure recognition” • Window width and window level are adjusted relative to greyscale histogram • Density is thus also adjusted • This is “acquisition processing” Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  14. Did we skip a step? • How did we assure that the response of the detector was uniform over the entire field of view? • “Pre-acquisition processing”, or “preprocessing”, corrects for detector imperfections and variable response. • Some include auto-ranging in preprocessing. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  15. Detector characteristic function for CR Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  16. Raw data Raw, ranged data Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  17. “Matched latitude” is another feature of histogram re-scaling Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  18. Code values can be remapped in more complex ways to modify contrast • Modifying contrast is the secondary role for image processing • Contrastis compromised for some values of interest in order to enhance contrastin others • “post-acquisition processing” Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  19. There are many brand-names for post-acquisition processing • Look-up-table (LUT, Gradation Processing) • Unsharp Mask (Frequency Processing) • Multi-frequency Processing (Musica®) • Multi-Objective Frequency Processing • Dynamic Range Control • Tomographic Artifact Suppression • Energy Subtraction (Dual Energy) Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  20. Code values can be remapped to a non-linear function • This function might have lower contrast for lighter and darker features with higher contrastfor values in the middle range, to achieve a film-like appearance. • Code values within the values of interest are translated by means of a Look-up Table (LUT). • This is “Gradation Processing”, “Sensitometry”, and “grey-scale rendition” Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  21. Raw, ranged data Gradation-processed data Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  22. The DR image has limited sharpness • Sharpness is limited by pixel dimensions • The smallest feature that can be resolved by CR is a “line pair” represented by one “dark” pixel next to a “light” pixel. • Maximum spatial resolution is the sampling rate (pixels per mm) divided by 2 (pixels per line pair) • This is also called the “Nyquist Frequency” or “Nyquist Limit” • A large format cassette with 2000 pixels along the 35 cm dimension would have about 6 pixels per mm and a maximum spatial resolution of 3 lp/mm Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  23. Practical resolution is less than the Nyquist frequency • Factors besides sampling compromise sharpness • X-ray focal spot dimensions • Blur in Indirect DR and CR • Optical and mechanical imprecision in IDR and CR • Afterglow in fast-scan dimension in CR • Limit of resolution is where Modulation Transfer Function(MTF) has decreased to 10% Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  24. Enhancing sharpness: a secondary purpose of image processing • If one can selectively increase the contrast of features in the image that represent large changes in code value over a few pixels, one can increase sharpness. • Two methods • Unsharp Mask (Frequency Processing) • Musica® Edge Contrast • “edge restoration” Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  25. Unsharp mask process • Start with original image • Create a blurred version of the original image by averaging all pixels within a small region called a “kernel”. • A large kernel blurs large features • A small kernel blurs small features • Subtract the blurred image from the original image to make a difference image or mask • The mask contains features that were NOTblurred • Add the mask back to the original image • Resulting image has enhanced features that were NOTblurred • Enhancement controlled by a “boost” factor Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  26. Unsharp mask process blurred Original image Blurred image Orig – blurred Orig + Diff Difference image Sharpened image Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  27. Musica® Edge Contrast • Raw image is decomposed into sub-bands, each representing an octave of spatial frequencies. • Adding all sub-bands together would reconstitute the original image • The contrast of features in each sub-band is modified according to a function. • The degree of enhancement is controlled by the value of a single parameter. • Differential enhancement is controlled by a second parameter value. • The modified sub-bands are added back together to create a modified image. • Extra enhancement of high frequency sub-bands emphasizes edges Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  28. Gradation-processed data Raw, ranged data Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  29. Gradation-processed data Musica® Processed data Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  30. blurred Original image Blurred image f(blurred) Orig + f(blurred) DRC image Dynamic Range Control (actually a form of “contrast enhancement”) Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  31. Dual Energy Subtraction Imaging: Uses low energy image and high energy image … • Two images are acquired • Weighted combination and subtraction of these images produces “bone only” and “soft tissue only “ images • Quality of images depends on energy separation Bone only image Soft tissue only image Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  32. Conventional vs. DES images Where is the lesion? Is it calcified? Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  33. Sufficient x-rays must reach the detector to produce the radiographic image. • At the same dose, the smaller the pixel size, the fewer x-rays in each pixel, and the worse the noise. • Larger the pixel size, worse the sharpness. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  34. 9 µGy Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  35. 3 µGy Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  36. 0.9 µGy Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  37. Post-acquisition processing can reduce noise. (generic term is “noise reduction”) • Because noiseis considered as high frequency variation, attenuating high spatial frequencies can reduce noise. • This is effectively a “high pass filter” • Unsharp Mask can do this • Musica® Noise Reduction can do this • This also attenuates small clinical features! • Corollary is that, enhancing small features also enhances noise! Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  38. Optimization in DR imaging cannot ignore patient dose! • In order to make a diagnostic radiographic image, a sufficient number of x-rays must reach the detector. • Unfortunately, the x-rays must pass through the patient to reach the detector. • The ALARAPrinciple dictates that the examination should be performed with the lowest reasonable dose to the patient. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  39. Acquisition processing involves assumptions: • Radiographic technique • Composition of anatomic region imaged • Use of collimation Images with very different greyscale histograms Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  40. Post-acquisition processing is controlled by exam-specific parameters • There are literally thousandsof permutations of allowable parameter settings. • Extreme values can have dramatic effects on the image. • There is no general agreement on the optimum values for the parameters. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  41. Auxiliary purpose of image processing: improve usability • Imprint demographic overlays • Add annotations • Apply borders or shadow masks • Flip and rotate • Increase magnification • Conjoin images • Scoliosis • Full leg • Modify sequence of views Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  42. Conjoined images: early vs. modern Note: different contrast Better contrast matching Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  43. Inappropriate roles for image processing • Compensate for inappropriate radiographic technique • Compensate for poor calibration of acquisition and display • Deletion of non-diagnostic images Recovery of non-diagnostic images to prevent re-exposure is last resort, not routine activity! Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  44. Bad practice still translates into bad images. Automation has not been invented to correct for • patient motion • poor inspiration • bad positioning • improper collimation • incorrect alignment of x-ray beam and grid • wrong exam performed • wrong patient examined • double-exposure. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  45. Radiographers need to recognize image artefacts ... • … and take appropriate action when artefacts occur. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  46. Conclusions: • DR image quality can be described in conventional terms. • DR image processing has three purposes: • Identify values of interest and maximize their contrast • Modify contrast within values of interest • Improve usability of the image • Image quality cannot be optimized without considering patient dose. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  47. Answer True or False • Acquired images from DR system are independent of display • DR has wide latitude • Spatial resolution of DR images are limited by pixel dimensions Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  48. Answer True or False • True. Acquisition is independent from display, code values in the raw DR image can be translated to any display level • True. DR has extremely wide latitude, ie., it has low contrast and is limited by the latitude of the display • True. The factors involved are focal spot thickness, blur in indirect DR (IDR) and CR, after glow and optical and mechanical imprecision in IDR and CR. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  49. References • Flynn MJ Processing digital radiographs of specific body parts. Advances in Digital Radiography: RSNA Categorical Course in Diagnostic Radiology Physics. Samei E and Flynn MJ eds (2003) 71-78. • Seibert JA Digital radiographic image presentation: preprocessing methods. Advances in Digital Radiography: RSNA Categorical Course in Diagnostic Radiology Physics. Samei E and Flynn MJ eds (2003),63-70. • Chotas HG, Ravin CE. Digital radiography with photostimulable storage phosphor: control of detector latitude in chest imaging. Invest Radiol27 (1992),822-828. • Huda W, Slone RM, Arreola M, Hoyle BA, Jing Z. Significance of exposure data recognizer modes in computed radiography. SPIE2708 (1996),609-616. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

  50. References (continued) • Freedman M, Pe E, Mun SK, Lo SCB, Nelson M. The potential for unnecessary patient exposure from the use of storage phosphor imaging systems. SPIE 1897(1993) 472-479. • Gur D, Fuhman CR, Feist JH, Slifko R, Peace B. Natural migration to a higher dose in CR imaging. Proc Eighth European Congress of Radiology. Vienna Sep 12-17(1993)154. • Huda W, Slone RM, Belden CJ, Williams JL, Cumming WA, Palmer CK. Mottle on computed radiographs of the chest in pediatric patients. Radiology199 (1996) 249-252. Radiation Protection in Digital Radiology L05 Digital Radiographic Image Processing

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