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Introduction to Medical Imaging > Ultrasound 3 > Flashcards

Ultrasound 3 Flashcards

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USMLE® Step 1 flashcards
1
Q

What are the assumptions about formation of ultrasound images?

A
  1. the medium has a constant speed of sound
  2. medium has a constant attenuation
  3. the beam axis is straight
  4. sound travels only along the beam axis and scatters only from target on axis
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2
Q

How is the position of the target calculated?

A

d = c * t/2

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3
Q

How are focusing delays calculated?

A

assuming that the spede of sound is constant

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4
Q

What happens for waves which pass through paths with higher sound speed?

A

The waves arrive earlier

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5
Q

What happens to paths passing through areas of lower sound speed?

A

The waves are delayed

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6
Q

What happens when certain waves arrive earlier and others arrive delayed?

A

Defocusing of the beam
Poorer resolution

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7
Q

What happens as a result of an incident beam arriving on a boundary at an angle?

A

It will be refracted
the objects can be displaced, or displayed 2ice

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8
Q

What is acoustic enhancement?

A

If sound passes through regions of lower attenuation, the following region appears brighter and brighter echoes appear below

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9
Q

What happens opposite to acoustic enhancement?

A

Acoustic shadowing

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10
Q

What is acoustic shadowing?

A

If sound passes through a region of more highly attenuating tissue, the following region can appear darker

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11
Q

What is a reflection artefact?

A

If the angle of reflection of a beam incident at an angle at the surface is large enough
Returning reflected beam can miss the transducer

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12
Q

What is a reverberation artefact?

A

At 90 degrees of incidence, multiple reflections etween a boundary and a transducer can occur, appearing as a repetition of boundary at increasing image depth

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13
Q

What is the mirror image artefact?

A

when there are specular reflection + reflecting/scattering features
the resulting mirror image is created on the far side of the boundary

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14
Q

What is the Doppler effect?

A

a change in observed frequency compared to emitted frequency, due to relative motion between observer and source

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15
Q

What is the equation of received frequency?

A

received frequency = transmitted frequency * (1 + blood flow velocity / sound of speed in overlying tissue)

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16
Q

With a stationary source: what would the transmitted frequency be equal to for a stationary source?

A

transmitted frequency = received frequency

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17
Q

What would the received frequency be equal to for a source moving towards the receiver?

A

received frequency > emitted frequency

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18
Q

What would the received frequency be equal to for a source moving away from the receiver?

A

The frequency of the received frequency < emitted frequency

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19
Q

What does the magnitude of the Doppler shift depend on?

A

The relative velocity between source and observer

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20
Q

What is the shift in frequency in the Doppler shift?

A

it is equal to the difference betwen the transmit and receive frequencies = 2 * transmit frequency * velocity of the blood relative to the transducer / speed of sound

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21
Q

To measure the blood flow what must there be?

A

Some movement towards or away from the transducer

22
Q

What does the doppler shift depend on?

A

The angle between the direction of blood flow and the beam axis

23
Q

What is the edited shift in frequency considering the fact that there is an angle between the direction of blood flow and the beam axis?

A

2* transmission frequency * velocity of blood flow * cos(theta)/ speed of sound

the angle is the angle between the direction of movement of blood flow and the beam axis

24
Q

What is one application of the Doppler effect?

A

The measurement of blood flow velocity to diagnose arterial stenosis (narrowing of the arteries)

25
Q

What is the relationship between blood flow velocity and narrowing of the vessel?

A

velocity of blood flow increases where the vessel narrows

26
Q

What is the velocity of blood flow equation?

A

v = speed of sound * frequency difference / 2* transmitted frequency * cos(angle between the direction of blood flow and the beam)

27
Q

What is the volume flow rate?

A

The volume of blood flowing through the vessel lumen in a given period of time

28
Q

What is the mathematical definition of volume flow rate?

A

Q = A* average flow velocity across the vessel (as it varies with position within the vessel)

29
Q

What is Continuous Wave Doppler

A

Rather than using only short pulses (required to obtain depth information), Doppler ultrasound can be
performed with continuous wave ultrasound. Probe has 2 elements to transmit and receive continuously. Used
for listening to e.g fetal blood flow. Portable handheld units -> Doppler frequency converted to sounds, no
image.

30
Q

What is the Pulsed wave Doppler

A

Short pulses are used to obtain Doppler signals from a specific region. Depth of sample volume and range gate
length (length of sample volume) can be chosen by the operator. The beam angle can be adjusted and angle
correction cursor lines up with the direction of flow.

31
Q

Doppler signal processing

A

demodulated
comparison against a continuous reference signal
the received pulses are multiplied with the reference signal then low pass filtered to retrieve the Doppler shift frequency.

32
Q

Spectral Doppler mode

A

Real time display of frequency or calculated velocity with time
Brightness -> amplitude of frequency component. Range of frequencies due to range of blood flow velocity
within vessel. Velocity varies across vessel, during cardiac cycle, between vessels.

33
Q

Pulse repetition frequency

A

Rate of transmission of pulses: pulse repetition frequency
Doppler modes require higher PRFs to adequately sample the Doppler frequencies

34
Q

To sample the Doppler shift properly, what must the Pulse repetition frequency be greater than or equal to ?

A

THe Nyquist limit = 2f_d

35
Q

What happens if the samplying frequency (pulse repetition frequency) is lower than the nyquist limit?

A

aliasing occurs: the signal will be misrepresented

36
Q

WHat does aliasing cause?

A

high frequencies to be registered as low frequencies

37
Q

What is colour flow doppler?

A

The mean flow velocity is colour coded/

38
Q

What is the power doppler?

A

Another type of colour flow Doppler, which displayes power of Doppler signal

39
Q

What artefacts does colour flow suffer from?

A

Aliasing if PRF is too low compared to the nyquist limit
noise and bleeding of colour from vessel if gain is too high

40
Q

What are the main applications of ultrasound?

A

IMaging many areas of the body for diagnosis
imaging for guidance of interventional prodedures

41
Q

how are scans performed for obstetrics and gynaecology?

A

12 weeks
20 weeks

42
Q

What happens in the 12 week scan?

A

Dating scan to determine fetal gestational age by measuring length

43
Q

What is the 20 weeks scan?

A

Fetal anomaly scan to examine the fetus and placenta and measure growth
Includes early pregnancy scanning and souvenir scanning

44
Q

what type of scanner is used for obstetrics and gynaecology?

A

Curvilinear probe, 2D matrix/volume probe for 3D/4D imaging, frequencies 2-9 MHz

45
Q

What is the abdominal ultrasound for?

A

imaging of the liver, kidneys, spleen, etc.
investigate for tumours, cysts, stones, blood flow and other pathologies
use a curvilinear array with frequencies 2-9 MHz

46
Q

What is echocardiography?

A

Using B-mode, M-mode and colour Doppler imaging to image heart valve motion, blood flow through the heart, tumours, congenital defects
A small footprint phased array probe

47
Q

Peripheral vascular ultrasound

A

Image arteries and veins in the neck are imaged
Colour flow and spectral Doppler are used to examine blood flow

48
Q

What does an increased flow velocity and turbulence indicate?

A

Narrowing and disease

49
Q

What is musculoskeletal ultrasound used for?

A

imaging muscles, tendons, ligaments, joints, etc.
a linear array probe is used

50
Q

Breast ultrasound

A

in addition to mammography, it is used to diagnose tumours, cysts, and masses
a linear array is used

51
Q

What are other applications of ultrasound?

A

imaging small parts using a small footprint phased array, linear array 4-18 MHz
thyroid- tumours, gland size, needle biopspy
testicles
hands
ophthalmic
neonatal
lung ultrasound

52
Q
A

Introduction to Medical Imaging (14 decks)

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