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Medical Imaging > Ultrasound > Flashcards

Ultrasound Flashcards

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USMLE® Step 1 flashcards
1
Q

What is the name of the part of the ultrasound transducer which absorbs ultrasound from the back face?

It also limits crystal vibration to allow short pulse duration.

A

Backing block

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2
Q

Which one of the following about transducers are not true?

  1. Uses piezoelectric effect to produce ultrasounds
  2. Made of PZT
  3. Sole function is to convert electricals signals into ultrasound
  4. Transducers are heat sensitive
A
  1. Sole function is to convert electricals signals into ultrasound
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3
Q

What is the frequency of sound waves produced by clinical ultrasound machines?

  1. 2Hz-20Hz
  2. 2Hz-20kHz
  3. 2-20MHz
  4. 2-200Mhz
A
  1. 2-20MHz
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4
Q

During an ultrasound scan, the time taken for the echo is 12.2 us and average propogation speed in soft tissue is 1.54mm/us.

What is the distance of the object from the probe?

  1. 9.3 mm
  2. 18.79mm
  3. 9.3m
  4. Not possible to calculate
A
  1. 9.3 mm

Must divide by 2, as the distance is the time to go and return

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5
Q

The clinical imaging of which of the following is very difficult with ultrasound scanning?

  1. Fibroids in uterus
  2. Adult brain
  3. Prostate gland
  4. Foetal growth
A
  1. Adult Brain
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6
Q

Which of the following ultrasound transducers is useful for scanning deep abdomen?

  1. High frequency
  2. Low frequency
  3. Transvaginal transducers
  4. It is not possible to do this
A
  1. Low frequency
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7
Q

Define acoustic impedance

A

It describes how much resistance an ultrasound beam encounters as it passes through a tissue.

Depends on: the density of the tissue (d, in kg/m3) the speed of the sound wave (c, in m/s)

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8
Q

hypoechoic

A

low echogenicity appears dark in the image and is called hypoechoic

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9
Q

Define echogeneity

A

Echogeneity is the ability to bounce an echo, e.g. return the signal in ultrasound examinations

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10
Q

Define hyperechoic

A

high echogenicity appears light in the image and is called hyperechoic

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11
Q

Define accoustic shadow

A

is characterized by a signal void behind structures that strongly absorb or reflect ultrasonic waves.

This happens most frequently with solid structures, as sound conducts most rapidly in areas where molecules are closely packed, such as in bone or stones.

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12
Q

What is colour flow doppler

A

visualization is achieved by color-encoding Doppler information and displaying the colors as an overlay on the 2D image of the heart.

The colors represent the speed and direction of blood flow within a certain area of the image

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13
Q

What type of wave does ultrasound use

A

Uses high frequency sound waves and their echoes

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14
Q

Outline how an ultrasound produces an image

A
  • Sound waves travel into your body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone)
  • Some are get reflected back to the probe, while some travel on further until they reach another boundary and get reflected.
  • The reflected waves are picked up by the probe and relayed to the machine to produce images
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15
Q

2 clinical applications of ultrasound

A
  • Pregnancy
  • Abnormalities in the heart and blood vessels
  • Organs in the pelvis and abdomen
  • Symptoms of pain, swelling and infection
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16
Q

How many dimmensions can US be done in

A
  • 2 D
  • 3 D
  • 4 D (time) – done via post-processing
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17
Q
  • Rarefaction – —– pressure and density
A

Low

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18
Q

Compression – —– pressure and density

A

High

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19
Q

What does sound need to propagate

A
  • Propagates through a medium (not in a vacuum such as space)
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20
Q

Define frequency

A

Frequency: cycles per second (Hz)

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21
Q

What is the clinical frequency of sound used in US

A

2 – 20 MHz

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22
Q

What is the audible range of sound frequencies

A
  • Audible range 20 Hz – 20 kHz
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23
Q

Define wavelength

A

Length of space over which one cycle occurs

24
Q

What is propogation speed

A
  • Speed at which a wave moves through a medium
25
Q

What is the wave equation

A

Propagation speed = frequency x wavelength

Wavelength (mm) = propagation speed (mm/μs) / frequency (Mhz)

26
Q

What happens to wavelength if frequency increases?

A

Wavelength decreases

27
Q

What happens to wavelength if frequency decreases?

A

Wavelength increases

28
Q

What is the average propogation speed of soft tissues

A

Average 1540 m s ¯¹

29
Q

What is this a depiction of

A

Pulsed ultrasound

30
Q

Why is pulsed ultrasound i.e. with gaps important?

A

Gaps are important as consistently putting sound energy into tissues = heat!!

This is the principle behind ultrasound physiotherapy

31
Q

What is the basic principle of US

A

Measures time taken for ultrasound echo to return to ultrasound probe/transducer

32
Q

What causes US reflection

A
  • Tissue interface between organ and tissue interfaces
  • Due to differences in impedance between tissues
33
Q

What US pattern do the following give:

  • Similar impedance
  • Identical Impedance
  • Different impedances
A
  • Similar impedances = weak echo (dark grey dot)
  • Identical impedance = no echo
  • Very different impedances = strong echo* can make it difficult- prevent soundwave travelling beyond i.e. bone
34
Q

What is impedance

A
  • Relates acoustic pressure and speed of particle vibration
  • Impedance = density of medium x propagation speed
35
Q

Describe the US probe

A
  • Converts electrical signal to sound wave, and vice versa:
    • Produces the ultrasound pulse
    • Detects the returning ultrasound echo
  • Uses the piezoelectric effect in order to do this
36
Q

what is this

A

US Probe

37
Q

What is the piezoelectric effect

A
38
Q

What are The piezoelectric crystals

A
  • Lead zirconate titanate (PZT)
  • Electrodes (conducting film) on opposite faces
  • Contains electric dipoles
    • dipole =closely spaced pair of electric charges
  • Dipoles aligned obliquely during manufacturing process
39
Q

What is the effect of electric field on PZT & effect of physical distortion

A
  1. Apply charge to electrode
  2. Dipoles change alignment depending on charge of the electrode
  3. Normal polarity = compression
  4. Reverse polarity = expansion
  5. Changes pressure around the crystal
  6. =sound waves from vibration of crystal
40
Q

Describe the transmission process of US

A
41
Q

Describe the reciever process of US

A
42
Q

Purpose of Coaxial cable

A
  • delivers voltage pulses which drive the crystal
43
Q

Purpose of the backing block

A
  • Absorbs ultrasound from back face
  • Prevents image of the practitioner’s hand etc.
  • Limits crystal vibration to allow short pulse duration
44
Q

Purpose of acoustic insulator

A
  • Cork/rubber/synthetic
  • Safety net- prevents sound energy going into the practitioner’s hand
  • Prevents ultrasound propagating into the plastic housing and operator’s hand
    • u/s emitted only by front surface
45
Q

Purpose of the couplant medium

A
  • To eradicate air gap
  • an impedance mismatch hinders the transmission of sound (impedance = ρc)
46
Q

Give 3 specialities which use US

A
47
Q

What is a Low frequency curvilinear US used for

A

able to penetrate deep into tissues to image internal organs.

Sector shaped image suitable for imaging large areas but still not enough to image whole organ such as liver. Reduced resolution due to frequency.

48
Q

What is the High frequency linear probe used for

A

suitable for imaging surface structures only.

Rectangular shaped image so limited field of view. High frequency improves resolution but limits depth of penetration.

i.e. testicular scan/thyroid/arterial or venous doppler

49
Q

What is the Endo-cavitary/transvaginal used for

A

high frequency, inserted into body cavity e.g. rectum, vagina.

Close to area of interest so can use high frequency to improve resolution. Invasive, uncomfortable, infection control.

50
Q

What are some basic requirements of a US image

A
  • Anatomy
  • Grey scale and contrast sufficient to differentiate between different tissue types
  • Sufficient spatial resolution for application
  • Make spatial measurements (0.1mm)
  • Sufficient temporal resolution for application
    • Key for dynamic techniques
  • Detect and make measurements of blood flow
51
Q

True or False: US can give detailed anatomical information

A

True to an extent- US can highlight pathology but its not extremely detailed

52
Q

What does image resolution depend on for US

A
  • Most high-end ultrasound machines capable of measurement in 0.1mm increments
  • Frequency dependent and therefore resolution depends on patient size.
  • Intra-cavity probes for maximum resolution
53
Q

Additional features of Doppler US?

A

Doppler- change of frequency of sound ways as they are reflected off a moving object

  • Colour flow;
    • Presence of flow
    • Morphology of vasculature
    • Identification of flow anomalies; turbulence, stenotic jets etc
  • Pulsed wave
    • Quantitative flow information; velocity, pulsatility index, resistance index, acceleration index etc

Ultrasound placed 60* to the blood vessel and can measure blood flow forwards and backwards.

54
Q

What can the presence of blood supply/vessels indicate

A

Malignancy

55
Q

Are changes in flow direction/turbulence good or bad?

A

Bad

56
Q

What are microbubbles

A

– (CO2)

  • Administered intravenously
  • Highly echogenic so reflections are greater from areas with microbubbles than without (due to gas)
  • Very delicate so must reduce power output from scanner

Medical Imaging (13 decks)

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