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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Important points from Ultrasound 1 Lecture
•
Bulk compressibility,
•
Ultrasound in tissue is principally a longitudinal (compressive) wave
•
Wave equation in various geometries (plane wave, spherical wave)
•
Speed of sound and density of various biological materials
•
Acoustic Impedance
Z=
ρ
c
•
Acoustic Energy, Power Intensity.
•
Transmission, Reflection for wave Amplitude and Intensity
•
Quarter wave (
λ
/4) matching layer concept
dp
dV
V
1
1
c
Animation courtesy of Dr. Dan Russell, Kettering University
2
1
2
1
2
Z
Z
Z
Z
I
I
R
i
r
I
2
1
2
2
1
4
Z
Z
Z
Z
I
I
T
i
t
I
T
I
+ R
I
=1
(pressure is different!)

http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm
L. Braile, Purdue University
)
2
cos(
0
ft
kx
u
u
)
2
cos(
0
0
0
ft
kx
cu
cu
p
)
2
sin(
2
2
0
ft
kx
f
u
f
iu

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