US physics Flashcards
1
Q
- What are the mechanical index (MI) and thermal index (TI)?
- What does MI measure?
- What is the effect of moving the focal zone closer to the probe?
- What is the effect of gain on MI?
A
- They are estimates of the mechanical & thermal bioeffects of the US beam.
- 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.
- It also increases MI.
- It has no effect on MI as it doesn’t affect US energy.
2
Q
- What do B and M mode stand for?
- What is the relative strength of these to each other and to pulsed Doppler?
A
- B = brightness (grayscale); M = motion.
- M-mode is 4x greater than B-mode.
- Pulsed Doppler is 20x greater than B-mode.
3
Q
How many shades of gray are displayed in US images?
A
- 256 shades of gray (Think 50 shades of gray, plus 206, the # of bones in the body.)
4
Q
What is the advantage of spatial compound sonography?
A
- 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.
5
Q
What is the benefit of tissue harmonics (seen here on the R)?
A
- It improves the evaluation of deep structures.
- Also improvies the visibility of surface joints & tendons.
6
Q
Name the 3 components of spatial resolution in ultrasound?
A
- 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.
7
Q
Tissue harmonics, when used, can minimize what type of artifact?
A
Side lobe
8
Q
Increasing the depth or width of the field of view will have what effect?
A
- Decrease the frame rate.
- …as the image size will increase.
9
Q
- Name the artifact.
- Which anatomic structure is responsible for generating this artifact?
A
- Mirror image: liver parenchyma is shown below & above the diaphragm equidistant from but deep to the strongly reflective interface.
- 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.
10
Q
Which phenomenon is responsible for the following US artifact?
A
- 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.
11
Q
- What is responsible for the artifacts in this image?
A
- 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.
12
Q
- Which artifact is shown?
- How is it caused?
A
- 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.
13
Q
Which of the following has the highest acoustic impedance?
- Air
- Fat
- Muscle
- Bone
A
- 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.
14
Q
- What is the Fresnel zone?
A
- 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).
15
Q
What effect does decreased depth have on frame rate?
A
Decreased depth reduces the amount of information, so it increases the frame rate.
20
Q
- Name this setting (arrow).
- What does it do?
A
- Colour-write priority.
- 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.
21
Q
Power Doppler:
- What does it show?
- Is there any aliasing?
- Is the Doppler angle important here?
A
- The power of the Doppler signal rather than the mean frequency shift.
- No, b/c the frequency shift data are not displayed.
- No, as power of the signal is not affected by angle.
22
Q
Why is CEUS useful for RCC diagnoses?
A
- 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.
25
Q
What does time gain compensation do?
A
- 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.
