US physics Unit 6

Harmonic imaging

• image created by reflections at 2x fundamental/transmitted frequency

• image created using harmonic frequencies

transmitted/fundamental Frequency

• what comes out of the transducer

• filtered out of the received F

• gets rid of lower frequencies and makes an image with higher frequencies

fundamental image

image created by the fundamental frequency

harmonic frequency

• twice fundamental frequency

• 2x the fundamental frequency

When is harmonic imaging helpful?

• When there is noise in the image, or fuzzy

• uses the frequencies that are 2x the fundamental image that come back from the transducer to create the image

• makes things more clear and crisp

Why don’t we always use harmonic imaging?

• it can decrease the frame rate

• doubles the pulses

• wouldn’t be great for moving structures

Distortion

• reduced, improves poor images

What happens to sound waves as they travel through the body?

• They change shape and get a little deformed

• the deeper it gets the peaks start to lean

• as it comes out of the transducer the sound wave starts even but starts to lean forward

• like a runner

If the fundamental F goes in at 2 MHz, what will be the harmonic f be when it comes back?

4 MHz

linear behavior

symmetrical or proportional

non-linear behavior

• irregular, disproportionate, asymmetrical

• needed to create harmonics

tissue harmonics

• using harmonic frequencies

• taking double the fundamental frequencies to create images with harmonic frequencies

• non-linear behavior

• sound travels faster through compressions and slower through rarefactions

• has an uneven speed

• causes change in sound wave shape

With tissue harmonics does the strength of harmonic waves increase or decrese as sound travels through tissue?

increase

Do we get more or less harmonics as we increase the depth the sound waves travels and why?

• more

• the sound wave changes it shape/ develope more the deeper you travel

• eventually the peak will be traveling faster than the valley

Are there harmonics at superficial depths?

no

Which image has the harmonics on?

left

Beam strength when creating harmonics

• weak beams= no harmonics

• moderately strong beams= tiny amount

• strong beams= significant harmonics

What behavior relationship is tissue harmonics and beam strength?

non-linear behavior relationship

Where are harmonics produced?

• Along the main axis where beam is stronger

• harmonics get rid of side lobes or grating lobes because there is no harmonics in them

• less likely to create artifacts

Tissue harmonic imaging

• created during transmission of sound, along beam axis

• produced by Patient , NOT the TRX

• possible bc non-linear behavior in the speed

2 US technologies for harmonics

pulse inversion harmonics and power modulation harmonic imaging

pulse inversion harmonics

• sends out a sound wave and a second one that is complete opposite of the first

• destructively interfere

• fundamental signals cancel and what comes back to transducer is harmonic signals

• 2 pulses per scan line

• separates distorted fundamental f from harmonic f in the receiver

power modulation harmonic imaging

• send out two pulses per scan line, one with more power

• doubled during reception

• it takes out/extracts harmonic signals

• eventually cancels out fundamental frequencies

contrast agents

• use microbubbles

• enhance image quality

• strong echogenicity

• the contrast helps light up areas of interest

• helpful in cardiac wall motion

• shows more detail in the mass

microbubbles

• gas bubbles in a shell

• either injected via iv or ingested

• designed to create strong reflection

• uses contrast to light it up

microbubbles requirements

• safe

• metabolically inert (can not reproduce or replicate)

• long lasting

• strong reflector

• small in size (pass through capillaries)

Contrast harmonics

• non-linear behavior from microbubbles and sound interacting with it

• bubbles oscillate

resonance

• non-linear behavior with contrast harmonics

• shrink during high pressure point in sound waves

• expand during low pressure point in the sound wave

• eventually pop

• created during reflection

When are contrast harmonics created

reflection

when are tissue harmonics created

transmission

Mechanical index

• direct relationship with harmonic production

• M depends upon the frequency of transmitted wave and P of the wave

• MI = Peak rarefaction pressure/ square root of frequency