M1U2: Speed of Sound and Ranging for Location

What 2 factors affect the propagation speed?

Density and bulk modulus (stiffness/elasticity of the intermolecular bonds in a medium)

What is density?

Mass per unit volume

TRUE or FALSE: More dense mediums have faster propagation speeds.

FALSE: When just looking at density (not w/ bulk modulus), more dense mediums have SLOWER propagation speeds. Propagation speed and density have an INVERSELY proportional relationship.

TRUE or FALSE: More stiff mediums have faster propagation speeds.

TRUE: Propagation speed and compressibility have an INVERSE relationship. As a medium becomes less compressible (stiff), propagation speed increases.

What is the bulk modulus?

Stiffness of a medium

(stress/strain) or (ΔPressure/(ΔVolume/Volume))

TRUE or FALSE: Having a low bulk modulus means that a medium is more stiff.

FALSE:

- HIGH bulk modulus = STIFF

- LOW bulk modulus = ELASTIC/COMPRESSIBLE

TRUE or FALSE: Propagation velocity has a proportional relationship to bulk modulus.

TRUE: As a medium becomes more stiff (increased bulk modulus), the propagation velocity increases.

More dense mediums have slower propagation speeds. Bone has the highest density but the fastest propagation speed compared to other body mediums. What explains this paradox?

An increase in density usually results in an increase in stiffness (bulk modulus), resulting in higher propagation speed.

What is the range?

Distance from the transducer to an echo-generating structure

What 2 things are needed to position an echo along a scan line?

1. Direction of where echo came from

2. Distance b/w transducer and object causing the reflected echoes

What is the roundtrip effect? How long is the roundtrip time for 1cm of soft tissue? How long is the roundtrip time for 1mm of soft tissue?

- Roundtrip effect accounts for the distance of the sound beam from the transducer --> medium and the reflected echo from the medium --> transducer

- 13 microseconds to travel 1 cm through soft tissue and return to the transducer

- 1.3 microseconds for 1mm of roundtrip travel

What is the range/distance equation?

Distance = 1/2 (propagation velocity x time)

- The value of the propagation speed x time is halved to account for only one direction of travel (just the transducer --> medium, or vice versa)

- Without halving it, we would be accounting for the roundtrip of the sound beam, going from the transducer --> medium --> back to transducer

What is a speed error artifact?

U/S system assumes a propagation speed of 1540m/sec (soft tissue). Therefore, when the propagation speed of a medium is different than 1540m/sec, structures are drawn at incorrect depths

What happens to a structure in an U/S image if it has a higher propagation speed than 1540m/s? (HINT: speed error artifact)

- Structure is drawn closer to the transducer

- As the echo return time is faster, the machine thinks that the structure is closer than it actually is

What happens to a structure in an U/S image if it has a lower propagation speed than 1540m/s? (HINT: speed error artifact)

- Structure is drawn farther from the transducer

- Since the echo return time is slower (increase roundtrip travel time), the machine thinks the object is much farther than it actually is

When is the speed error artifact advantageous?

- Fine needle aspiration (FNA) or biopsies needle location (seeing bayonet sign confirms that needle is within the cyst/mass)

- Tissue composition (could signal there is a structure above that is slowing down/quickening the signal)

What does a bayonet sign result from? (HINT: Fine needle aspiration)

- Speed error artifact from a needle entering a tumour

- Sound travels faster through the tumour, displacing part of the needle to be higher than it actually is

- This causes a bent shape of the needle, resembling a bayonet attached to the end of a rifle