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Emily Imaging > Ultrasound! > Flashcards

Ultrasound! Flashcards

1
Q

Name the most common use for ultrasound in ROP? Why is ultrasound useful for this?

A

Brachy!

Useful because you can see in real-time, no dose delivered.

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2
Q

What frequencies are used in ultrasound?

A

2-10 MHz… for higher resolution imaging can be up to 50MHz

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3
Q

What does an ultrasound actually measure?

A

Changes in pressure as a function of time

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4
Q

Sound waves are longitudinal or transverse?

A

longitudinal. Compression and Rarefaction

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5
Q

What is an elastic media?

A

One that obeys Hooke’s law F=-kx.

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6
Q

What is compressibility?

A

measure of the relative volume change of a medium as a response to isotropic pressure. Written as beta.

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7
Q

What is the Bulk Modulus?

A

inverse of compressibility, k

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8
Q

define the speed of sound in terms of bulk modulus

A

c=sqrt(k/density)

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9
Q

what is the speed of sound in human tissue?

A

1540 m/s

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10
Q

Why do we generally assume speed of sounds increases with increasing density, even though they are inverse?

A

because the bulk modulus also generally increases accordingly

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11
Q

define decibels in terms on intensities and in terms of pressures

A

dB = 10log(I2/I1) = 10log(P2^2/P1^2)=20log(P2/P1)

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12
Q

What is a half value thickness in terms of decibels?

A

medium thickness required to reduce the intensity of the beam by half (-3dB)

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13
Q

What is acoustic Impedence?

A

can be thought of as the stiffness of a compressible medium. How much of an US wave is transmitted or reflected. Z=c*denity

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14
Q

What determines reflection and transmission at an interface?

A

the idfference in the two acoustic impedences. larges difference, more reflection

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15
Q

When do we see specular relfection or nonspecular?

A

specular occurs at a smooth surface. this happens when the wavelength of the US wave is much greater than the structural variations in the tissue.. Non-spec occures when the imperfection are about a wavelength

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16
Q

reflection coefficient?

A

describes what fraction of intensity incident on an interface that is reflected
Ri = Ir/Ii = (Z2cosxi - Z1cosxt / Z2cosx1 + Z1cosxt )^2

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17
Q

Pressure amplitude transmission coef

A

Rp=Pr/Pi = sqrt (Reflection coef)

18
Q

Intentisy transmission coef

A

Ti = 1 - Ri

19
Q

What percentages of US pressure in reflected at:
Bone-Muscle?
Fat-Muscle?

A

64%

12%

20
Q

What is speckle?

A

deterministic interference pattern formed by a medium containing sub-resolution scattering objects at fixed locations. Does not ocrrespond to structure but to interference patterns as recieved by the transducer.

21
Q

When a beam is not incident perpendicular, what dictates how w wave will refract?

A

snells law:

c1sinx1=c2sinx2

22
Q

AT when angle of incidence does max reflection occur?

A

Perpendicular

23
Q

What is the attenuation coef? unit?

A

relative intensity loss per distance travelled for a given medium. dB/cm.

24
Q

attenuation pressure formula?

A

p = p0exp(-alphaz), alpha is attenuation coef.

25
Q

Rule of thumb for attn coef

A

1dB per cm travelled per MHz

26
Q

Do higher frequencies attenuate more rapidly?

A

yes

27
Q

how is the focal region defined?

A

region over which the beam width is no more than two times the width at focal spot.

28
Q

Near field length=?

A

d^2/(4*wavelength), d is the transducer diameter

29
Q

Far field angle of divergence?

A

sinx=1.22*lambda/d

30
Q

how waould you produce a shallower focal zone?

A

by activating the outer elements in the array before the inner ones.

31
Q

What are “side lobes”?

A

extra constructive interference away from the central axis.

32
Q

what is axial resolution?

A

half the spatial pulse length. spatial pulse length = N*wavelength

33
Q

at what distance from the transducer is lateral resolution optimized?

A

at focal region

34
Q

define elevation resolution?

A

slice thickness perpendicular to imaging plane

35
Q

what is pulse repetition frequency (PRF) and pulse rep period (PRP)?

A

number of pulses per second, and time between sequential pulses

36
Q

What is Duty Factor?

A

Pulse Duration/PRP. uaualy around 1%

37
Q

What is A-Mode Imaging and why do we use it?

A

Amplitude Mode Imagin! just displays pulse recieved. used to locate interfaces, cracks

38
Q

What is B-Mode imaging?

A

typical ultrasouns

39
Q

What is the frame rate in B-Mode?

A

number of line scans to complete a single B-Mode image multiplied by the lines per second.

40
Q

what is M-mode imaging?

A

Motion mode is the echo from a single beam passing through moving anatomy and displayed as a function of itme.

41
Q

Which assumptions cause artifacts in US?

A
  • speed of sound is constant
  • no scatter
  • reflection only comes from along central axis
42
Q

Name 4 types of US aritfacts

A
  • reverberation
  • shadowing/attenuation
  • enhancement
  • mirroring

Emily Imaging (6 decks)

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