Additional Modes & Capabilities

Question 1

How does compound imaging work?

Answer 1

Multiple images are acquired by steering the beam in different directions. These images are then combined to give a single, compound image.

Question 2

What are the advantages of compound imaging?

Answer 2

Improved imaging of interfaces
Speckle is reduced

Question 3

What are the disadvantages of compound imaging?

Answer 3

Reduced temporal resolution
Loss of artifacts
- Directed parallel to the beam (shadowing, enhancement, edge shadowing, reverberation, comet tail, ring down)
- Given the multiple beam directions, these are reduced or lost
- Beam width and slice thickness are also different for each beam

Question 4

Describe transmit pulse distortion

Answer 4

Tissue is a non-linear medium for ultrasound
As the transmitted pulse travels through the medium, it changes shape
This occurs due to the change in propagation speed that occurs as tissue pressure changes
Peaks move faster, due to increased tissue stiffness
The wave becomes distorted as a result
This only occurs when the intensity is high and therefore only at the centre of the beam

Question 5

Describe transmit pulse distortion

Answer 5

Tissue is a non-linear medium for ultrasound
As the transmitted pulse travels through the medium, it changes shape
This occurs due to the change in propagation speed that occurs as tissue pressure changes
Peaks move faster, due to increased tissue stiffness
The wave becomes distorted as a result
This only occurs when the intensity is high and therefore only at the centre of the beam

Question 6

How does harmonic imaging work?

Answer 6

Distorted echoes contain energy at harmonic frequencies. Undistorted echoes do not.
The machine eliminates the original transmitted frequency using pulse inversion.
Only detects harmonic frquencies.

Question 7

What are the advantages of harmonic frequencies?

Answer 7

The high intensity zone is narrower than the full beam, so the lateral resolution improves
The width in the elevation plane is narrower than the slice thickness, so the slice thickness artifact is reduced
The intensity in the sidelobes is far lower, so sidelobe artifacts are eliminated
Reverberation artifact caused by superficial tissue is eliminated (as the pulse is undistorted in shallow tissue)

Question 8

What are the disadvantages of harmonic imaging?

Answer 8

Halved frame rate
Penetration is reduced as the image is formed at a higher frequency
Does not reduce speckle