Lecture 7

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define Luez's Law (Law of Reflection)
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Terms in this set (44)
define Luez's Law (Law of Reflection)
for an oblique incidence, the angle of incidence equals the angle of reflection.

reflection angle = incident angle
What are the two principles that always apply to oblique incidence?
1. conservation of energy
2. Luez's Law (Law of Reflection)
define oblique incidence
angle of incidence is other than 90 degrees.

reflection and transmission are uncertain (unlike normal incidence)
define refraction
change in sound direction as it passes from one medium to another.
What are the two conditions that satisfy refraction?
1. oblique incidence (not normal incidence)
2. different propagation speeds between the media.
What does refraction cause on imaging artifacts?
improper lateral placement on display, edge shadowing.
define Snell's Law
used in calculation of angle of refraction.
Snell's Law equation
[sin (θt)/sin (θi)] = c2/c1
Snell's Law equation (in words)
[sin (transmission angle)/sin (incident angle)] = [propagation speed medium 2/propagation speed medium 1]
Under what conditions will the transmission angle equal the incident angle?
when the speeds of the two media are identical.

speed 2 = speed 1
Under what conditions will the transmission angle be greater than the incident angle?when the speed of medium 2 is greater than medium 1. speed 2 > speed 1Under what conditions will the transmission angle be less than the incident angle?when the speed of medium 2 is less than medium 1. speed 2 < speed 1define sineratio of two sides of a right triangle.sine equation (in words)sine = opposite / hypotenusedefine harmonics imageimage created by processing reflections that are twice the fundamental frequency.How are harmonics generated: linear or nonlinear? Why?Nonlinear. Sound in compressions are faster than sound in rarefactions.define tissue harmonicsa miniscule amount of energy is converted from the fundamental frequency to the harmonic frequency. the fundamental wave is removed, only the harmonic is processed.What level intensity does a harmonics wave begin?low intensity, then increases in a nonlinear fashion. it picks up less noise than a fundamental wave.How does harmonics improve image quality?less distortion and side or grating lobes.Where does much of the distortion in a harmonic wave occur?within the first few centimeters of the surface of skin. tissue harmonics signal do not exist at extremely superficial depths.What are the advantages of harmonics waves? (***fix***)1. Better resolution (higher frequencies processed) 2. Narrower beams (higher frequencies = longer NZL's, more gradual beam tapering) 3. less divergence in the far zone 4. Fewer side lobes 5. Better signal to noise ratioWhat are the disadvantages of harmonics waves? (***fix***)Lower power output (harmonic waves are weaker than fundamental waves)How do harmonic waves generate less side and grating lobes? (***fix***)1. the waves are nonlinear and only generate strong waves, not weak ones. 2. the sound beam passes through the tissue and distorts; it generates side and grating lobes. 3. the main beam is strong to make strong waves which are generated by the non-distorted main beam. 4. few harmonics are generated by the side and grating lobes because they are weak.define Pulse Inversion Harmonicsimaging technique designed to utilize harmonic reflections. It is distortion free, while eliminating distorted fundamental reflections (waves)Describe how the pulse is sent in Pulse Inversion Harmonics.two consecutive pulses are sent down the same scan line. The first pulse is normal. The second pulse is an inverted, out-of-phase copy of the first pulse.What happens to fundamental waves in Pulse Inversion Harmonics? Why?The two pulses are combined in the receiver. Fundamental waves have linear behavior, the out-of-phase pulse cancels out the fundamental waves (destructive interference).How come harmonic waves do not cancel each other out in Pulse Inversion Harmonics?They exhibit nonlinear behavior. The reflections are in-phase and interfere constructively.define Power Modulation Harmonic ImagingDesigned to augment harmonic reflections. It contains less distortion, while eliminating distorted fundamental waves (reflections).What is the difference between Power Modulation Harmonic Imaging and Pulse Inversion Harmonics?In Power Modulation, the second pulse is twice the strength of the first pulse (or the first pulse is half the strength of the second pulse). The second reflection is stronger and contains the harmonic waves. The first reflection is doubled, and subtracted from the second reflection, leaving the harmonics wave with less distortion.What are the disadvantages of using Pulse Inversion Harmonics and Power Modulation Harmonics?it reduces temporal resolution due to the extra pulses sent. two pulses are transmitted down each scan line and twice the number of pulses are required to create each image.define Contrast Mediasubstances that are introduced by injection to patient. injected to the circulation intravenously are used to increase echogenicity.What are contrast media also called?microbubbles that can pass through capillary walls.What are the purposes of contrast media (microbubbles)?1. provide image enhancement in gray scale and Doppler images. 2. improve gray scale differentiation between lesions with similar echogenicity. 3. improvement of weaker Doppler signals.What are the working principles of contrast media?1. Great impedance difference between tissue and gas -- it improves reflection. 2. Microbubbles expand and contract at harmonic frequencies. 3. Microbubbles are constructed of an outer shell and inner gas. 4. The shells and gases change depending on longevity and resonance characteristics that are needed.define Contrast Harmonicscreated during reflection as energy is converted from the fundamental frequency to the harmonic frequency. the microbubbles expand and contract non-uniformly.How do the strengths of tissue and contrast harmonics compare?Contrast harmonics created by microbubbles are much stronger than tissue harmonics. Contrast harmonics yields improvement in spatial resolution and fewer artifacts.What is Constructive Interference of Sound Waves (in phase)?1. waves are at the same place at the same time. 2. Result is cumulative - waves reinforce one another.What is Destructive Interference of Sound Waves (out-of-phase)?1. Waves not in the same place at the same time. 2. Result is negative - cancel each other out.define mechanical indexthe estimation of the amount of contrast harmonics producedmechanical index equation (in words)peak RAREFACTIONAL pressure / (square root of the frequency)What is the relationship between harmonics and mechanical index?nonlinearWhy do low mechanical index beams not generate harmonics?microbubbles expand and contrast in a linear fashion (uniformly).What mechanical index (number) is considered high?greater than 1.0Why do high mechanical index beams generate substantial harmonics?bubbles expand and break apart. bubble disruptions is an example of nonlinear behavior that creates strong harmonic reflections.