Harmonics and contrast agents ch. 17

Tissue harmonics are created in the tissue during when?

Transmission

What happens during pulse inversion harmonics?

Pulse inversion, a positive and negative pulse is transmitted down each scan line then combined, leaving only the harmonic part.

What is power modulation harmonic imaging?

The harmonics are extracted by sending one strong pulse and one weak pulse down each scan line, remaining harmonics forms an image. Frame rate is halved and temporal resolution is reduced.

What nonlinear behavior of a microbubble creates contrast harmonics?

Bubbles expand to a greater extent than they shrink. This uneven behavior is resonance.

When are contrast harmonics created?

Contrast harmonics are created during reflection and MI greater than 0.1.

What are harmonics in ultrasound imaging?

Frequencies that are integer multiples of the fundamental transmitted frequency.

What is the fundamental frequency?

The frequency originally transmitted by the transducer.

What is the most commonly used harmonic in diagnostic imaging?

The second harmonic.

How are tissue harmonics generated?

By nonlinear propagation of ultrasound through tissue.

At what depth do harmonic signals begin to form?

At increasing depths as sound propagates through tissue.

What is tissue harmonic imaging (THI)?

Imaging that receives harmonic frequencies generated within tissue rather than the transmitted frequency.

Why does THI improve image quality?

Harmonic signals originate deeper in tissue and reduce superficial artifacts.

Which artifacts are reduced by tissue harmonic imaging?

Side lobes, reverberation, and near-field clutter.

What happens to axial resolution in harmonic imaging?

It improves due to higher frequency reception.

What happens to penetration when harmonic imaging is used?

Penetration decreases.

Why does harmonic imaging reduce penetration?

Harmonic frequencies are higher and attenuate more rapidly.

Harmonic imaging improves which resolution most?

Contrast resolution.

What must be increased to compensate for reduced penetration in THI?

Output power or gain (within safety limits).

Harmonic imaging is most useful in imaging which patients?

Obese patients or those with difficult acoustic windows.

What are ultrasound contrast agents?

Gas-filled microbubbles injected intravenously to enhance blood flow signals.

Why do contrast agents produce strong harmonic signals?

Microbubbles oscillate nonlinearly when exposed to ultrasound.

Contrast agents primarily enhance which imaging mode?

Doppler and harmonic imaging.

Contrast agents are confined to which space?

The vascular space.

What is the mechanical index (MI)?

A measure of the likelihood of nonthermal bioeffects, including cavitation.

What happens to contrast microbubbles at high MI?

They rupture or are destroyed.

What MI setting is typically used for contrast imaging?

Low MI.

Why is low MI preferred in contrast imaging?

To preserve microbubble and prolong enhancement.

What is contrast-specific harmonic imaging?

Imaging that isolates harmonic signals produced by contrast microbubbles.

Why do microbubbles produce stronger harmonics than tissue?

Due to extreme nonlinear oscillation.

Contrast imaging improves visualization of what structures?

Blood flow and organ perfusion.

Tissue harmonics vs contrast harmonics—key difference?

Tissue harmonics arise from tissue; contrast harmonics arise from microbubbles.

Which harmonic method requires IV injection?

Contrast harmonic imaging.

Which harmonic method reduces near-field artifacts?

Tissue harmonic imaging.

Which imaging parameter is most improved by harmonics?

Contrast resolution.

Tissue harmonic imaging improves image quality primarily because harmonic signals

Originate deeper in tissue

The primary disadvantage of tissue harmonic imaging is

Decreased penetration

Ultrasound contrast agents enhance imaging by:

Producing strong nonlinear harmonic signals

Which setting is most appropriate when imaging with contrast agents?

Low MI

Harmonic imaging primarily reduces which artifact?

Side lobes