US: Basic Ultrasound Physics Flashcards Preview

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Flashcards in US: Basic Ultrasound Physics Deck (22)
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1

What is the speed of sound in tissue assumed by most commercial ultrasound scanners?

1540m/s

2

How is the speed of sound in a medium related to the bulk modulus (K) and the density (ρ)?

c = (K/ρ)^0.5

3

How is the acoustic impedance related to the speed of sound and the density?

Z = pc

4

What is the equation for the depth of the far field?

D = (a^2)/λ
a is the aperture (1/2 diameter of the transducer array)
λ is the wavelength

5

What is the equation for the angle of divergence in the far-field?

Θ = arcsin(0.61λ/a)

a is the aperture (1/2 diameter of the transducer array)
λ is the wavelength

6

What is another name for the near-field?

Fresnel zone

7

What is another name for the far-field?

Fraunhofer zone

8

For a focused lens (with speed of sound c1) with a curvature of R, what is the equation for the length of the near field (F)?

F = R /[(c2/c1)-1]
c2 = speed of sound in tissue.

9

What is the equation for the beamwidth at the focus?

w = λF/a

λ = wavelength
F = focal depth
a = aperture radius

10

What is the equation for the amplitude reflection coefficient?

R = (Z2-Z1)/(Z2+Z1)
Z2 and Z1 are the acoustic impedances of the two media.

11

What is the equation for the intensity reflection coefficient?

I = [(Z2-Z1)/(Z2+Z1)]^2
Z2 and Z1 are the acoustic impedances of the two media.

12

Under what conditions is specular reflection observed?

When the diameter of the object is much larger than the wavelength.

13

Under what conditions is Rayleigh scattering observed?

When the diameter of the object is much smaller than the wavelength.

14

What is the scattered power proportional to?

(d^6) x (f^4)

15

What causes ultrasonic scattering in tissue?

Microscopic variations in acoustic properties within tissue parenchyma.

16

How does scatter appear on ultrasound images, and what is the reason for this appearances?

Scatter is low-power and non-directional, leading to a dark uniform speckled appearance.

17

What is the equation for attenuation of an ultrasound beam through tissue?

I=I(0)*Exp(-α*z)


α is the frequency-dependent ultrasound coefficient (dB/cm).
z is the depth in tissue.

18

What is the equation for the frequency dependent ultrasound coefficient?

alpha = af^b
a is the frequency independent ultrasound coefficient (dB/cm/MHz)
b~1.1.

19

What is the value of b for liver and muscle?

1.1

20

What is the value of b for blood and bone?

1.2

21

What is the value of b for breast tissue?

1.5

22

An intensity reduction of 3dB is equivalent to what fractional change in intensity?

1/2.