Physics of ultrasound imaging

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Beginning with basic physics of ultrasound, in the presentation how an ultrasound image is constructed is tried to be revealed by investigation of the wave propagation through the tissue.

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Physics of ultrasound imaging

  1. 1. PHYSICS OF ULTRASOUND IMAGING
  2. 2. SOUND • Human : 20 Hz – 20 kHz • Ultrasound : 20 kHz – above • Medical US : 1-10 MHz
  3. 3. ULTRASOUND WAVES • Propagation: wave-like fashion (acoustics), by expansion and compression of material • Can be absorbed, refracted, focused, reflected and scattered • Image: acquired by echoes produced by scattering or reflection of complex tissues
  4. 4. WAVE GENERATION • Transducers, as both generators and receivers • Compressing a small volume of tissue, then releasing it • Due to elastic properties of the material, compressed volume expand to its past equilibrium, making neighboring volumes compress • As this process continues successively through tissues, wave is generated • Longitudinal waves (mostly in US) and shear waves
  5. 5. WAVE EQUATION •
  6. 6. WAVE SOLUTIONS •
  7. 7. ATTENUATION •
  8. 8. SCATTERING • If wavelength >> target in the body, scattering occurs • They vibrate as small spherical bodies, giving rise to spherical waves
  9. 9. SCATTERING •
  10. 10. FIELD PATTERN MODEL •
  11. 11. DIFFRACTION FORMULATION • A series of simplifying acoustical approximations • Especially for far field, it gives very accurate formula
  12. 12. NARROWBAND PULSE •
  13. 13. TRANSMITTED SIGNAL •
  14. 14. RECEIVED SIGNAL •
  15. 15. RECEIVED SIGNAL •
  16. 16. PLANE WAVE APPROXIMATION •
  17. 17. PLANE WAVE APPROXIMATION •
  18. 18. PLANE WAVE APPROXIMATION •
  19. 19. PARAXIAL APPROXIMATION •
  20. 20. FRESNEL APPROXIMATION •
  21. 21. FRESNEL APPROXIMATION •
  22. 22. FRAUNHOFER APPROXIMATION •
  23. 23. FRAUNHOFER APPROXIMATION •
  24. 24. FRAUNHOFER APPROXIMATION •
  25. 25. REVIEW •
  26. 26. PULSE ECHO IMAGING •
  27. 27. PULSE ECHO IMAGING •
  28. 28. PULSE ECHO IMAGING •
  29. 29. TRANSDUCER MOTION •
  30. 30. GEOMETRIC ASSUMPTION •
  31. 31. ACTUAL FIELD PATTERNS •
  32. 32. ACTUAL FIELD PATTERNS •
  33. 33. COMMENTS ON PULSE ECHO IMAGING •
  34. 34. A-MODE Medical Imaging- Systems and Signals Prince, J.L. – Links, J.M. • Envelope detected, gain compensated signal, Amplitude Mode
  35. 35. M-MODE • Time evolution of repetitive A-Mode signals, which are set to images as columns: Motion Mode
  36. 36. B-MODE Medical Imaging- Systems and Signals Prince, J.L. – Links, J.M. • As transducer moves along the x-plane, A-mode signals are keyed to the x-position of the transducer • Then, the keyed signals produce B-Mode image by brightness modulating -- Brightness Mode
  37. 37. TEXTURE (SPECKLE) IN B-MODE IMAGE Image from gehealthcare.com • Granular, with a mottled dot pattern
  38. 38. SPECKLE • Tissues have lots of scatterers, which are:  very numerous  randomly positioned throughout the tissue • Generally scatterers are too close to be resolved on images. • In fact, an echo and the corresponding «dot» on the US image is a combined signal from a group of scatterers. • So interference, constructiveness and destructiveness plays a mojer role in imaging them • Interestingly many characteristics of the pattern, depend also on the imaging system itself. (freq.)
  39. 39. ARTIFACTS • Artifacts: structures and features on the image that we construct, which is not 1-1 with the real object • Assumptions that giving rise to incorrect results in artifacts in imaging- and the reality: Assumptions Reality Reflectors on the beam axis Beam has a certain beamwidth, so sometimes echoes are picked up from structures off the axis c is constant along the tissue Slight changes in c is present, which leads to erronous location Echo strength indicates organ echogenicity Slight deflections and refractions of an US beam might occur, causing to wrong location
  40. 40. ARTIFACTS Reverberation Mirror Image Beamwidth effects radiographics.rsna.org jultrasoundmed.org sciencedirect.com
  41. 41. RESOLUTION •
  42. 42. RESOLUTION •
  43. 43. BIBLIOGRAPHY • Medical Imaging Signals and Systems, J.L. Prince, J.M. Links • Medical Imaging Systems, A. Macovski • Foundations of Biomedical Ultrasound, R.S.C. Cobbold

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