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31 Cards in this Set
- Front
- Back
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Decibel - definition and equation
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relative log scale represents ratio of final to initial values.
dB=10log(Final/Initial) |
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Attenuation - definition
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decrease of intensity and amplitude as ultrasound beam travels through tissue. always a negative number, dB
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Total Intensity Loss or Total Attenuation equation
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Btotal = 10 log (Ifinal/Iinitial)
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Log scale of attenuation to linear ratio of final/initial
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-3dB = 1/2
-6dB = 1/4 -9dB = 1/8 etc |
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Total Amplitude Loss equation
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B = 20log (Af/Ai)
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Causes of Attenuation , not part of attenuation and equation
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Absorption, Diffraction and Reflection.
Refraction no contribution Btotal = Ba + Br + Bd |
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Absorption - definition
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when US energy is converted into different type - acoustic to heat. up to 80% of attenuation
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Factors of Absorption
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Frequency
Depth Viscosity - ability of molecules to move over each other Relaxation Time |
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Attenuation due to Absorption equation
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Ba = udf
u=.5 dB/cmMhz attenuation coefficient |
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Half Value Layer definition and factors
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thickness of material that will reduce intensity to 1/2 original value due to attenuation. corresponds to 3dB loss
- depends on frequency and medium |
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Reflection definition and three types
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redirection of sound beam - contributes to attenuation and imaging
Specular, diffuse, and rayleigh |
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Specular Reflection definition
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reflection of wave from very smooth surface, interface of two media. Responsible for image
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Normal Incidence and Oblique
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Perpendicular, orthogoanl and 90 degrees to interface
anything other than normal, measured in respect to normal |
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Law of Reflection
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Angle of Incidence equals Angle of Reflection. normal incidence is the best
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Acoustic Impedance definition and equation
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Z is the amount of resistance of the medium to the sound wave. Z=pv density and sound velocity. characteristic of medium. Rayls = kg/m squared sec
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Difference acoustic impedance does what...
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causes sound waves to be reflected, greater the difference more the sound will be reflected
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Reflection coefficient definition and equation
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fraction of incident intensity that the reflected wave will contain.
aR = [(z2-z1)/(z2+z1)]squared no units |
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Percent reflected
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%R = aR x 100%
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Transmission Coefficient definition and equation
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fraction of incident intensity that the transmitted wave will contain. at = 4z1z2/(z2+z1)squared no units
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Diffuse Scattering - definition
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backscatter occurs when the interface is not smooth. reflected are still returned in general direction but weaker, less intense
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Rayleigh Scattering - definition
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non-specular reflection, when sound waves strikes very small objects, comparable in size to wavelength, begin vibrating own frequency producing sound waves
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Attenuation due to Reflection equation
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Br=10log100/%R
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Overall Attenuation largest to smallest
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Air>>Bone, Lung > Soft Tissue > Water
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Refraction definition and requirements
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change in direction of propagation as when enters a different medium. need different velocity and at oblique angles.
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Snell's Law
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Sin Inc / Sin Trans = V1/V2
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V1>V2 FST
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Angle of transmittance bends towards normal
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V1<V2 SFA
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Angle of transmittance bends away
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Critical Angle - definition and equation
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as angle of incidence increases, angle of transmittance will reach 90 degrees. Angle of Incidence when called Critical Angle. Sin Cr/Sin 90 = v1/v2
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When Inc Angle > Critical Angle
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total internal reflection, does not go into second medium
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V1=V2
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no refraction. angle Incidence = angle transmitted
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Normal Incidence
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No refraction, need oblique angle.
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