Lecture 1 Notes

Lecture 1 Notes

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Unformatted text preview: Pulse 20 kHz Just as there are infrared, visible, and ultraviolet ranges in the EM spectrum, so there are infrasound (“infra” = “below,” “beneath”), audible (i.e., sound) and ultrasound (“ultra” = “beyond,” “above”) ranges of acoustic wave frequencies Note that the ratio of the highest to the lowest audible frequencies is 103, while the ratio of the highest to the lowest frequencies of visible light is a bit less than 2! Excitation Pulse RF Amplitude Envelope Amplitude 330 m/s 1500 1600 1480 3000 Ultrasonic Waves   Ultrasound imaging relies on the propagation of sound within tissue   Mechanical (pressure) wave   Pressure distribution (Reflections) (Backscatter)1 2 1 2 1 line of image data Image frame rate is determined by sound speed   Air Water Muscle Fat Bone Scan Beam to build up image Time = Depth Speed of Sound Each medium has a characteristic speed –  c [m/s] = λ[µm] x f [MHz] = wavelength x frequency   Approximate ultrasound speeds Object Transducer     Sound speed = 1540 m/s = 1.54 mm/µs 256 line image / Depth = 10 mm Propagation length = 20 mm (2-way) Time per line = 20/1.54 ~ 13 µs Time per image = 13 x 256 = 3300 µs = 3.3 ms Frame rate = 1/3.3 ms ~ 300 images/s 3D Wave Equation Harmonic Waves Plane Wave       Harmonic plane wave: p(z,t) = cos(k[z-ct]) Viewed at a fixed particle, the pressure changes in time with frequency ft=kc/2π (cycles/s) Viewed at a fixed time, the pressure changes in z with frequency fz=k/2π –  k is called wavenumber     (approximately) Harmonic...
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This document was uploaded on 03/12/2014 for the course EL 5823 at NYU Poly.

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