ultrasound1 prelim

ultrasound1 prelim - 10.0 The Physics of Ultrasound...

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10.0 The Physics of Ultrasound Ultrasound is sound with frequencies higher than the highest frequency that can be heard by human beings >20KHz). Medical ultrasound operates typically between 1 and 10MHz. The propogation characteristics are defined in the theory of acoustics and medical ultrasound is principally concerned with compression waves rather than shear waves. •Basic acoustic physics, wave equation •Plane, spherical wave solutions of the wave equation •Wave propogation in media, energy, intensity, reflection, refraction •Transmission, reflection coefficients, attenuation in media, scattering •Doppler effect •Beam formation and focusing •Beam patterns, diffraction formulation •Paraxial, Fresnel and Fraunhofer approximations
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Summary of Previous Lecture •Reviewed a number of statistical distributions used to model imaging noise—Binomial, Poisson, Gaussian. •Discussed the implications of the Central Limit Theorem •Discussed SNR as a fundamental image quality parameter and the trade-offs that often must be made between SNR and resolution •Discussed the how the variance gets replaced by the noise-power- spectrum when the noise is correlated (ie, not “white”) and looked at a CT correlated noise example. •Talked about how CNR describes the imaging system’s ability to detect a target. •Distinguished QDE and DQE: QDE=#detected/#incident, DQE=SNR 2 out /SNR 2 in •Covered the practical concept of Contrast-Detail analysis. •Discussed non-random effects: distortion and other artifacts •Reviewed sensitivity, specificity, and the ROC curve
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We model tissue as a fluid, to first order and one definition of a fluid is a medium that doesn’t support shear waves—only compression or longitudinal waves
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Review of basic mechanical properties F F L area A force per unit area is Stress (compressive or tensile)=F/A The response of the object to stress is to change length ΔL, the fractional change in the length of the object is the Strain = ΔL/L strain stress A B C Plotting stress vs strain we find a linear region up to a point A, a non-linear region that still returns to the original shape when the force is removed (up to the elastic limit, B) and then a non-linear region leaving the object permanently stretched up to the fracture point C. The linear region demonstrates Hooke’s Law (F=-kx). The slope in the
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ultrasound1 prelim - 10.0 The Physics of Ultrasound...

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