Ultrasound Physics

Ultrasound Physics PowerPoint PPT Presentation

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. Perpendicular Incidence. Sound beam travels perpendicular to boundary between two media. . . . . . . . 90oIncidentAngle. 1. 2. . . Boundarybetweenmedia. . Oblique Incidence. Sound beam travel not perpendicular to boundary. . . . . . . . ObliqueIncidentAngle(not equal to 90o). 1. 2. . . Bou

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Ultrasound Physics

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1. Ultrasound Physics Reflections & Attenuation

2. Perpendicular Incidence Sound beam travels perpendicular to boundary between two media

3. Oblique Incidence Sound beam travel not perpendicular to boundary

4. Perpendicular Incidence What happens to sound at boundary? reflected sound returns toward source transmitted sound continues in same direction

5. Perpendicular Incidence Fraction of intensity reflected depends on acoustic impedances of two media

6. Intensity Reflection Coefficient (IRC) & Intensity Transmission Coefficient (ITC) IRC Fraction of sound intensity reflected at interface <1 ITC Fraction of sound intensity transmitted through interface <1

7. IRC Equation Z1 is acoustic impedance of medium #1 Z2 is acoustic impedance of medium #2

8. Reflections Impedances equal no reflection Impedances similar little reflected Impedances very different virtually all reflected

9. Why Use Gel? Acoustic Impedance of air & soft tissue very different Without gel virtually no sound penetrates skin

10. Rayleigh Scattering redirection of sound in many directions caused by rough surface with respect to wavelength of sound

11. Diffuse Scattering & Rough Surfaces heterogeneous media cellular tissue particle suspension blood, for example

12. Scattering Occurs if boundary not smooth Roughness related to frequency frequency changes wavelength higher frequency shortens wavelength shorter wavelength “roughens” surface

13. Specular Reflections Un-scattered sound occurs with smooth boundaries similar to light reflection from mirror opposite of scatter from rough surface wall is example of rough surface

14. Backscatter sound scattered back in the direction of source

15. Backscatter Comments Caused by rough surfaces heterogeneous media Depends on scatterer’s size roughness shape orientation Depends on sound frequency affects wavelength

16. Backscatter Intensity normally << than specular reflections angle dependance specular reflection very angle dependent backscatter not angle dependent echo reception not dependent on incident angle increasing frequency effectively roughens surface higher frequency results in more backscatter

17. PZT is Most Common Piezoelectric Material Lead Zirconate Titanate Advantages Efficient More electrical energy transferred to sound & vice-versa High natural resonance frequency Repeatable characteristics Stable design Disadvantages High acoustic impedance Can cause poor acoustic coupling Requires matching layer to compensate

18. Resonant Frequency Frequency of Highest Sustained Intensity Transducer’s “preferred” or resonant frequency Examples Guitar String Bell

19. Operating Frequency Determined by propagation speed of transducer material typically 4-6 mm/msec thickness of element

20. Pulse Mode Ultrasound transducer driven by short voltage pulses short sound pulses produced Like plucking guitar string Pulse repetition frequency same as frequency of applied voltage pulses determined by the instrument (scanner)

21. Pulse Duration Review typically 2-3 cycles per pulse Transducer tends to continue ringing minimized by dampening transducer element

22. Damping Material Goal: reduce cycles / pulse Method: dampen out vibrations after voltage pulse Construction mixture of powder & plastic or epoxy attached to near face of piezoelectric element (away from patient)

23. Disadvantages of Damping reduces beam intensity produces less pure frequency (tone)

24. Bandwidth Damping shortens pulses the shorter the pulse, the higher the range of frequencies Range of frequencies produced called bandwidth

25. Bandwidth range of frequencies present in an ultrasound pulse

26. Quality Factor (“Q”) Unitless Quantitative Measure of “Spectral Purity”

27. Damping More damping results in shorter pulses more frequencies higher bandwidth lower quality factor lower intensity Rule of thumb for short pulses (2 - 3 cycles) quality factor ~ number of cycles per pulse

28. Transducer Matching Layer Transducer element has different acoustic impedance than skin Matching layer reduces reflections at surface of piezoelectric element Increases sound energy transmitted into body

29. Transducer Matching Layer placed on face of transducer impedance between that of transducer & tissue reduces reflections at surface of piezoelectric element Creates several small transitions in acoustic impedance rather than one large one

30. Transducer Arrays Virtually all commercial transducers are arrays Multiple small elements in single housing Allows sound beam to be electronically Focused Steered Shaped

31. Electronic Scanning Transducer Arrays Multiple small transducers Activated in groups

32. Electrical Scanning Performed with transducer arrays multiple elements inside transducer assembly arranged in either a line (linear array) concentric circles (annular array)

33. Linear Array Scanning Two techniques for activating groups of linear transducers Switched Arrays activate all elements in group at same time Phased Arrays Activate group elements at slightly different times impose timing delays between activations of elements in group

34. Linear Switched Arrays Elements energized as groups group acts like one large transducer Groups moved up & down through elements same effect as manually translating very fast scanning possible (several times per second) results in real time image

35. Linear Switched Arrays

36. Linear Phased Array Groups of elements energized same as with switched arrays voltage pulse applied to all elements of a group BUT elements not all pulsed at same time

37. Linear Phased Array timing variations allow beam to be shaped steered focused

38. Linear Phased Array

39. Linear Phased Array

40. Linear Phased Array

41. Linear Phased Array

42. Listening Mode Listening direction can be steered & focused similarly to beam generation appropriate timing variations applied to echoes received by various elements of a group Dynamic Focusing listening focus depth can be changed electronically between pulses by applying timing variations as above

43. 1.5 Transducer ~3 elements in elevation direction All 3 elements can be combined for thick slice 1 element can be selected for thin slice

44. 1.5 & 2D Transducers Multiple elements in 2 directions Can be steered & focused anywhere in 3D volume

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