Published on December 6, 2016
1. IMAGE QUALITY IMAGE Contrast Resolution Noise Artifacts Distortion
2. INTRODUCTION - medical images for medical necessity (anatomical/ functional information) - quality of medical image determined by: * imaging method * equipment characteristics * imaging variables (transducers, coils, kV, gain,TE) (* skills of operator and viewing condition)
3. Iterative reconstruction * different imaging methods * different equipment characteristics
4. * different equipment characteristics
5. * different imaging variables
6. * different imaging variables 80 kV 140 kV
7. Contrast - difference in tissue caracteristics between specific points low contrast medium high contrast - result of a number of different steps during creation, image processing and image displaying
8. Contrast • three main contributors: 1. Subject contrast - intrinsic factors - extrinsic factors 2. Detector (film) contrast 3. Displayed contrast (image reconstruction in digital radiograph, CT, MRI, PET,...)
9. Homogeneous Incident X-ray Beam Patient Detector Profile of X-ray beam emerging from patient prior to reaching detector C= A-B/A A>B; C is 0,0-1,0 (1-100%) A=N0e-mx and B=N0e-m(x+z) C = 1-e-mz m attenuation coefficient z thickness 1. Subject contrast
10. A=N0e-mx and B=N0e-m(x+z) C = 1-e-mz m attenuation coefficient z thickness if z is larger or m is larger C is higher! x-rays of low kV C is higher!
11. 1. Subject contrast - Intrinsic factors = actual anatomical or functional changes in the patient’s tissues, which give rise to contrast Physical properties Physiological properties (f.e. atomic number) (f.e. metabolism) •
12. 1. Subject contrast - Extrinsic factors = optimization of image-acquisition protocol to enhance subject contrast Changing x-ray energy Contrast agent
13. 1. Subject contrast Changing TR i TE
14. 2. Detector (film) contrast
15. 2. Detector (film) contrast
16. 3. Displayed contrast - raw image information is processed into an image that is finally meant for physician viewing - medical images have bit depths ranging from 10, 12,14 bits (1024, 4096 to 16384 shades of gray) - Modern displays capable of displaying 8-bit to 10-bit (256 to 1024 shades of gray) display computer needs to convert the higher bit depth data encoded on the image to the spectrum of gray scale on the monitor
17. lung window (W 1600/ C -600 H.U)
18. mediastinal window (W 350/ C 40 H.U.)
19. bone window (W 1600/ C 300 H.U.)
20. Resolution - Spatial resolution in radiology refers to the ability of the imaging modality to differentiate two objects
21. Point Spread Function, PSF = response of an imaging system to a point source • most basic measure of resolution properties of an imaging system • describes the extent of blurring that is introduced by an imaging system • two-dimensional (2D) function PSF(x,y) • Rotationally symmetric/ asymmetric • describes the extent of blurring that is introduced by an imaging system
22. Point Spread Function, PSF = response of an imaging system to a point source point source symmetric response of „imaging system” asymmetric response of „imaging system”
23. 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
24. Line Spread Function, LSF Edge Spread Function, ESF
25. Slit imaging in projection radiography Imaging a plane in tomographiy sytem Image of Line Profile of line is measured perpendicular to the line
26. gaussian blur original increasing edge enhancement Results of 2D image processing using a variety of different convolution kernels
27. G(x )= H(x ') k (x-x ') dx ‘= H(x ) k(x ) LSF (x )=PSF (x, y) ⊗ LINE( y) LSF x = PSF x y dy
28. Modulation Transfere Function, MTF = spatial frequency response of an imaging system • measures resolution in frequency domain (Fourier Transformation) • measured in line pairs per millimeter (lp/mm) • limited by Nyquist limit
29. Modulation Transfere Function, MTF = spatial frequency response of an imaging system 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Prostorna frekvencija (ciklus/mm) 10% MTF limiting spatial resolution 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 MTF(f) OUTPUTINPUT
30. Noise = irregular granular pattern - degrades image information (render images non-diagnostic) - present in all electronic systems - originates from a number of sources
31. Plain film
32. Plain film Decreasing radiation intensity
33. CT - Noise can be decreased by increasing mAs - Noise can be decreased by changing filters during reconstruction MRI - main source is the patient's body (RF emission due to thermal motion) - Noise can be decreased: - use the correct coil and ensure that it is well tuned - use a large FOV - select thick slices
35. Medical images for medical necessity? Department of Brain and Cognitive Sciences, MIT / Athinoula A. Martinos Imaging Center at the McGovern Institute for Brain Research Neuroscientist Rebecca Saxe captured an incredible image of herself holding her 2-month-old son, Percy, and it may be the first image of its kind. She and her colleagues took the image simply because they wanted to see it, not for any specific diagnosis or study.
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