1. George David Associate Professor Ultrasound Physics 04: Scanner ‘97

2. Resonant Frequency Frequency of Highest Sustained Intensity resonantTransducer’s “preferred” or resonant frequency Examples  Guitar String  Bell

3. 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)

4. Pulse Duration Review typically 2-3 cycles per pulse Transducer tends to continue ringing dampening  minimized by dampening transducer element Pulse Duration = Period X Cycles / Pulse

5. 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) Damping Material Piezoelectric Element

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

7. George David Associate Professor Bandwidth Damping shortens pulses  the shorter the pulse, the higher the range of frequencies bandwidthRange of frequencies produced called bandwidth

8. Bandwidth range of frequencies present in an ultrasound pulse Frequency Intensity Ideal Frequency Intensity Actual Bandwidth Operating Frequency

9. operating frequency Quality Factor = ----------------------------- bandwidth Quality Factor (“Q”) Unitless Quantitative Measure of “Spectral Purity” Frequency Intensity Actual Bandwidth

10. Which has a Higher Quality Factor? Frequency Intensity A Frequency Intensity B operating frequency Quality Factor = ----------------------------- bandwidth Same Operating Frequency!

11. George David Associate Professor 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

12. George David Associate Professor An Aside about Reflections Echoes occur at interfaces between 2 media of different acoustic impedances  speed of sound X density Medium 1 Medium 2

13. 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 Medium 1 Medium 2 IRC + ITC = 1

14. IRC Equation Z 1 is acoustic impedance of medium #1 Z 2 is acoustic impedance of medium #2 2 reflected intensity z 2 - z 1 IRC = ------------------------ = ---------- incident intensity z 2 + z 1 For perpendicular incidence Medium 1 Medium 2

15. George David Associate Professor Reflections Impedances equal  no reflection Impedances similar  little reflected Impedances very different  virtually all reflected 2 reflected intensity z 2 - z 1 Fraction Reflected = ------------------------ = ---------- incident intensity z 2 + z 1

16. Why Use Gel? Acoustic Impedance of air & soft tissue very different Without gel virtually no sound penetrates skin 2 reflected intensity z 2 - z 1 IRC = ------------------------ = ---------- incident intensity z 2 + z 1 Acoustic Impedance (rayls) Air400 Soft Tissue1,630,000 Fraction Reflected: 0.9995

17. 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 Transducer – skin interface

18. 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 reflected intensity z 2 - z 1 IRC = ------------------------ = ---------- incident intensity z 2 + z 1 ( ) 2 Matching Layer

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

20. George David Associate Professor Electronic Scanning Transducer Arrays  Multiple small transducers  Activated in groups

21. George David Associate Professor Electrical Scanning arraysPerformed with transducer arrays  multiple elements inside transducer assembly arranged in either »a line (linear array) »concentric circles (annular array)

22. George David Associate Professor 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

23. 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

24. Linear Switched Arrays

25. 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 1 2

26. Linear Phased Array timing variations allow beam to be  shaped  steered  focused Above arrows indicate timing variations. By activating bottom element first & top last, beam directed upward Beam steered upward

27. Linear Phased Array Above arrows indicate timing variations. By activating top element first & bottom last, beam directed downward Beam steered downward By changing timing variations between pulses, beam can be scanned from top to bottom

28. Linear Phased Array Above arrows indicate timing variations. By activating top & bottom elements earlier than center ones, beam is focused Beam is focused Focus

29. Linear Phased Array Focus Focal point can be moved toward or away from transducer by altering timing variations between outer elements & center

30. Linear Phased Array Focus Multiple focal zones accomplished by changing timing variations between pulses Multiple pulses required slows frame rate

31. 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 2