Ultrasound

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Ultrasound

  1. 1. +Basic Ultrasound PhysicsDr. A. Ferguson
  2. 2. + Objectives  Level 1 Knowledge Components  Ultrasound physics, terminology, and safety  Equipment care, ultrasound techniques, and controls
  3. 3. + The transducer Acoustic lens Impedance CABLE matching to skin Damper material Piezoelectric crystal (sends and receives)
  4. 4. + The ultrasound wave Wavelength Amplitude (dB) V=f Velocity m/s 1540 m/s approx Delivered in pulses (bursts) • Length of pulse varies • Frequency of pulse varies 1 second: cycles/second = frequency(Hz) Clinical use varies from 2.5-20MHz
  5. 5. + Velocity in tissue Medium Velocity of US (m/sec) Air 330 Fat 1450 Water 1480 Soft tissue 1540 Kidney 1560 Blood 1570 Muscle 1580 Bone 4080
  6. 6. + The ultrasound beam Beam width Near zone Focal zone Divergence angle Side lobes Unfocused transducer Focused transducer
  7. 7. + Ultrasound beam lobes Feldman M K et al. Radiographics 2009;29:1179-1189
  8. 8. + Resolution Axial (along length of beam) – most precise  Smallest resolvable distance = 2 x  Higher frequency = better resolution  Independent of depth Lateral (across beam)  Varies with depth  Within focal zone may be as good as axial Elevational (within the slice)  Slice might be 3-8mm wide with some probes  Strong reflectors at edges may appear in centre Contrast (shades of gray)
  9. 9. + Ultrasound/tissue interaction Transducer Skin 1 2 Scattering Reflection Tissue interface 3 4 Refraction Attenuation
  10. 10. + Scattering  Structures with radius < wavelength scatter US  e.g. RBCs and micro-structures within tissues  Scattering is multidirectional  Only small portion of incident US gets back to probe  Scattering from RBCs contributes to DOPPLER effect  Tissue scattering results in speckled appearance
  11. 11. + Reflection  Critical to image generation  Depends on:  Angle of beam relative to tissue  Change in acoustic impedance* across boundary  Smooth tissue boundaries act almost as mirrors  Called “specular reflectors” e.g. pleura * Acoustic impedance = tissue density x US velocity in the tissue
  12. 12. + Acoustic impedance Medium Acoustic impedance* Air 0.0004 Lung 0.18 Fat 1.34 Liver 1.65 Blood 1.65 Kidney 1.63 Muscle 1.71 Bone 7.8 * x106 Rayls
  13. 13. + Refraction  Waves deflected passing through interface  Can be useful in focusing US waves  Results in artefacts
  14. 14. + Attenuation  Loss of US energy as it passes through tissue  Depends on  Attenuation coefficient of tissue  Frequency of transducer  Distance from transducer  Intensity of transmitted US  AIR has a very large attenuation coefficient  Lower frequencies penetrate better than high
  15. 15. + Attenuation values Medium Half-power distance (cm) Water 380 Blood 15 Soft-tissue (non-muscle) 1-5 Muscle 0.6-1 Bone 0.2-0.7 Air 0.08 Lung 0.05
  16. 16. + Image artifacts Poor image quality Images of structures that are either  Not there at all  Present in a different location than image suggests Lack of visualisation of structures Images that differ in size or shape from reality Some artifacts are clinically useful
  17. 17. + Image artifacts  Acoustic shadowing  Acoustic enhancement  Refraction artifact  Reverberation artifact  Comet-tail artifact  Mirror-image artifact  Ghosting artifact  Beam-width artifact  Ring-down artifact  Speed-displacement artifact
  18. 18. + Beam-width artifact Adjust focal zone Grey dot assumed to be in main beam area Grey dot outside beam Area of interest outside focal zone Area of interest inside focal zone Feldman M K et al. Radiographics 2009;29:1179-1189
  19. 19. + Side-lobe artifact Black dot signal may return from multiple side-lobes resulting in duplication on screen Feldman M K et al. Radiographics 2009;29:1179-1189
  20. 20. + Reverberation artifact US bounces back and forth between two strong reflectors Feldman M K et al. Radiographics 2009;29:1179-1189
  21. 21. + Ring-down artifactRing of bubbles with fluid trapped centrally. Fluid vibrations detected as strongsignal and displayed as line behind true source. Feldman M K et al. Radiographics 2009;29:1179-1189
  22. 22. + Mirror-image artifactsUS beam bounces between structure and deeper strong reflector e.g. diaphragm.This means probe receives signals as if from same object on other side of reflector. Feldman M K et al. Radiographics 2009;29:1179-1189
  23. 23. + Speed-displacement artifact Discontinuous diaphragm signPart of beam encounters tissue where velocity is much lower than 1540 m/s,e.g. fat. Returning signal appears to come from deeper in body. Feldman M K et al. Radiographics 2009;29:1179-1189
  24. 24. + Refraction artifactRefraction at an interface between two objects makes the deeper objectappear in false location. Feldman M K et al. Radiographics 2009;29:1179-1189
  25. 25. + Acoustic shadowing Strong attenuator means weak beam beyond = shadow Feldman M K et al. Radiographics 2009;29:1179-1189
  26. 26. + Acoustic enhancementSignal behind weak attenuator is stronger than at same level in adjacent tissues.Gives impression of brighter structures deep to low attenuator =enhancement Feldman M K et al. Radiographics 2009;29:1179-1189
  27. 27. + Probe types Sector Linear array Curved array
  28. 28. + Use of Gain Near field Far field Attenuation Original Max Gain Min Processed Time-gain compensation (TGC)

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