US physics

Question 1

1. What are the mechanical index (MI) and thermal index (TI)?
2. What does MI measure?
3. What is the effect of moving the focal zone closer to the probe?
4. What is the effect of gain on MI?

Answer 1

1. They are estimates of the mechanical & thermal bioeffects of the US beam.
2. MI measures the potential of inducing a cavitation-related bioeffect. It is the peak pressure/SQRT(center frequency of the beam), so if you increase the center frequency, you decrease MI.
3. It also increases MI.
4. It has no effect on MI as it doesn’t affect US energy.

Question 2

1. What do B and M mode stand for?
2. What is the relative strength of these to each other and to pulsed Doppler?

Answer 2

1. B = brightness (grayscale); M = motion.
2. M-mode is 4x greater than B-mode.

• Pulsed Doppler is 20x greater than B-mode.

Question 3

How many shades of gray are displayed in US images?

Answer 3

• 256 shades of gray (Think 50 shades of gray, plus 206, the # of bones in the body.)

Question 4

What is the advantage of spatial compound sonography?

Answer 4

• It improves tissue plane definition.
• It takes sound beams from multiple different angles to form the images.
• The downside is that it can have a smoothing effect & is more likely to produce motion blur b/c frames are compounded to produce images.
• It can also reduce the conspicuity of a foreign body b/c it can reduce the artifact related to the FB.

Question 5

What is the benefit of tissue harmonics (seen here on the R)?

Answer 5

• It improves the evaluation of deep structures.
• Also improvies the visibility of surface joints & tendons.

Question 6

Name the 3 components of spatial resolution in ultrasound?

Answer 6

• Axial, lateral & elevational resolution.
• Axial: separating 2 objects in the direction of the US beam.
• Lateral: separating 2 objects perpendicular to the direction of the US beam.
• Elevational: perpendicular to the image plane.

Question 7

Tissue harmonics, when used, can minimize what type of artifact?

Answer 7

Side lobe

Question 8

Increasing the depth or width of the field of view will have what effect?

Answer 8

• Decrease the frame rate.
• …as the image size will increase.

Question 9

1. Name the artifact.
2. Which anatomic structure is responsible for generating this artifact?

Answer 9

1. Mirror image: liver parenchyma is shown below & above the diaphragm equidistant from but deep to the strongly reflective interface.
2. Lung: it acts as an acoustic mirror as gas is the best acoustic mirror (reflects almost 100%, hence clean shadowing behind it).

• These are generated by the false assumption that an echo returns to the transducer after a single reflection.

Question 10

Which phenomenon is responsible for the following US artifact?

Answer 10

• Artifact = acoustic shadowing.
• Caused by absorption, defined as the loss of sound energy 2dry to conversion to thermal energy.
• Absorption is greatest in bone/calcification.

Question 11

• What is responsible for the artifacts in this image?

Answer 11

• Refraction.
• This is shadowing at the edges of a fluid-filled structure.
• It is caused by the change in direction of the transmitted US beam at a tissue boundary when the beam is not perpendicular to the boundary.
• Is commonly seen at fat-muscle and tissue-fluid interfaces.

Question 12

• Which artifact is shown?
• How is it caused?

Answer 12

• Ring-down from a gas-filled loop of bowel.
  
  • A line or series of parallel bands extending posterior to a gas collection.
• The transmitted US energy causes vibrations within fluid trapped b/w multiple gas bubbles. These vibrations create a continuous sound wave that is transmitted back to the transducer.

Question 13

Which of the following has the highest acoustic impedance?

• Air
• Fat
• Muscle
• Bone

Answer 13

• Bone.
• Acoustic impedance is defined as the product of the density of the medium & the speed of sound in that medium, so the denser the medium, the higher the impedance.
  
  • It is also a measure of tissue stiffness.
  • Metal also has high acoustic impedance.

Question 14

• What is the Fresnel zone?

Answer 14

• The near field of the US beam. (KB mnemonic: Fresno/Fresnel is much closer to you than Germany (Fraunhofer).
  
  • It has a converging beam profile.
• The far/diverging field is called the Fraunhofer zone.
• Fresnel zone length is defined as d^2 / 4 x (wavelength); where d=transducer diameter.
  
  • So the Fresnel zone increases with larger diameter size & higher frequency (lower wavelength).

Question 15

What effect does decreased depth have on frame rate?

Answer 15

Decreased depth reduces the amount of information, so it increases the frame rate.

Question 16

• What can improve axial resolution in an US image?

Answer 16

• Increased transducer frequency.
• Axial resolution is the ability to separate 2 closely spaced objects in the direction of the US beam.
• To resolve 2 objects (avoid overlapping returned echoes), the distance b/w the 2 objects must be at least 1/2 of the spatial pulse length.
  
  • Spatial pulse length = the # of cycles emitted per pulse by the transducer x wavelength.
  • Decreasing the spatial pulse length will improve the axial resolution, which can be achieved by using a higher frequency.

Question 17

Where is the near field/far field interface located, roughly?

Answer 17

At a depth that is about half the transducer diameter.

This is where the effective beam diameter is about half the transducer diameter.

Question 18

What is the clinical significance of side-lobe artifacts?

Answer 18

• They can produce the appearance of debris or tissue w/in fluid-filled structures.
• They are located outside of the primary desired acoustic beam, and may create artifactual low-level echoes, which are best appreciated w/in anechoic cystic & fluid-filled structures, e.g., as gallbladder sludge.

Question 19

1. What’s the difference b/w axial & lateral resolution?
2. How do you increase each?

Answer 19

• Axial:
  
  • The ability to resolve structures located at different depths.
  • Use a higher-frequency probe.
• Lateral:
  
  • The ability to resolve structures located at the same depth.
  • Narrow the beam at the level of interest, i.e., change the focal zone.

Question 20

1. Name this setting (arrow).
2. What does it do?

Answer 20

1. Colour-write priority.
2. This is the gray-scale intensity threshold above which all colour info is suppressed, so it reduces the amount of colour signal that you see.

Question 21

Power Doppler:

1. What does it show?
2. Is there any aliasing?
3. Is the Doppler angle important here?

Answer 21

1. The power of the Doppler signal rather than the mean frequency shift.
2. No, b/c the frequency shift data are not displayed.
3. No, as power of the signal is not affected by angle.

Question 22

Why is CEUS useful for RCC diagnoses?

Answer 22

• It has a high sens/spec for characterizing indeterminate renal masses–especially those that are hypovascular.
• CEUS can identify perfusion that may be too minimal to detect by MRI or CT b/c of blending of enhancement w/surrounding normal tissue.
• So CEUS has higher sensitivity for detecting flow in hypovascular tumours.

Question 23

• Why not angle perpendicular to the vessel when doing Doppler US?

Answer 23

• B/c this can give the impression of no flow or mirror image artifact.
• The echo should be angled between 30-60°.

Question 24

1. In Doppler, when does aliasing artifact occur?
2. Name 4 ways in which aliasing can be reduced/eliminated?

Answer 24

1. When the Doppler shift is greater than the Nyquist freqency.
2. Decrease Doppler shift by either: a) using a lower frequency probe; or b) increasing the Doppler angle closer to 90.
3. Increase the pulse repetition frequency (increasing Nyquist) or increase the scale.

Question 25

What does time gain compensation do?

Answer 25

* Makes the image look better. * Adjusts the gain depending on the depth of the echo. * It can give uniform brightness from top to bottom. * It compensates for loss of echo strength caused by depth of the reflector, so it treats echoes differently depending on the depth from which they are returning.

Question 26

What is the formula relating speed, wavelength and frequency?

Answer 26

speed = wavelength x frequency

Question 27

What speed does the US machine assume that sound travels in all human tissue?

Answer 27

1,540 m/s

Question 28

How does sound travel in hard vs. soft tissues?

Answer 28

* Hard = not compressible = high speed. * Soft = compressible (e.g., gas) = low speed.

Question 29

A loss of 3 dB results in a loss of what % of power?

Answer 29

50%

Question 30

Define impedance.

Answer 30

* density of a material x the speed of sound. * Essentially, this is the stiffness of a material.