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Lecture 1 Notes

1fz2kcft backward wavelength determines the resolution

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Unformatted text preview: wave are widely used in ultrasound imaging   Forward traveling wave Wavelength is the spacing between peak or valleys of the wave at any time Given ft, the wavelength depends on c, which depends on tissue properties! –  λ=1/fz=2π/k=c/ft Backward –  Wavelength determines the resolution of ultrasound imaging –  Ex: ft=3.5 MHz, c=1540m/s (most tissue), λ=0.44mm Reflection/refraction: Snell’s Law   Reflection / refraction at interfaces follows Snell’s law: sin(θi) c1 = sin(θt) c2 θi θr I c1   Acoustic Impedance, Z [MRayl = 106 kg/m2/s] Z = density [kg/m3] x sound speed [m/s] c2 –  Determines the amplitude of the reflected / transmitted waves at interface –  Complex scattering properties of tissues are due to acoustic impedance interfaces in microstructure of tissues θt R Acoustic Impedance T θi = θr Reflection at Interfaces: Example Reflection at Interfaces Reflection Coefficients: (Normal Incidence) Reflection: R = |(Z2-Z1)/(Z2+Z1)| Transmission: T = 1-R = 2Z2/(Z2+Z1) Z1 Z2 I R Reflected Sound Pressure: Muscle Zm~1.7 MRayl T Fat Zf~1.4 MRayl I R R/I = | (Zf-Zm)/(Zf+Zm) | = 0.3/3.1 ~ 0.1 (10% = -20dB) T Attenuation of Ultrasound Attenuation of Ultrasound   Attenuation = Energy lost through interactions between ultrasound waves and soft tissues:   Attenuation is frequency dependent: a(f) = ao fn - ao is the attenuation coefficient at 1 MHz –  n ~1 for most soft tissues –  Absorption: Power deposited in tissue (Heat) –  Scattering   Atten...
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