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Ultrasound Physics

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Ultrasound Physics

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    1. Ultrasound Physics 04: Scanner

    2. Resonant Frequency Frequency of Highest Sustained Intensity Transducers 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 minimized by dampening transducer element

    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)

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

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

    8. Bandwidth range of frequencies present in an ultrasound pulse

    9. Quality Factor (Q) Unitless Quantitative Measure of Spectral Purity

    10. Which has a Higher Quality Factor?

    11. 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. An Aside about Reflections Echoes occur at interfaces between 2 media of different acoustic impedances speed of sound X density

    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

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

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

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

    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

    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

    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. Electronic Scanning Transducer Arrays Multiple small transducers Activated in groups

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

    22. 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

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

    27. Linear Phased Array

    28. Linear Phased Array

    29. Linear Phased Array

    30. Linear Phased Array

    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

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