Physics I - lecture 11.5 Midterm Review Answers.docx

**[DMS 104 Midterm Review]** 1. What is attenuation? *Attenuation is the weakening of sound as it propagates through the tissues*. 2. How does attenuation limit the image? *Attenuation limits the penetration therefore the depth* 3. 4. What is the dominant contributor to attenuation? *The...

**[DMS 104 Midterm Review]** 1. What is attenuation? *Attenuation is the weakening of sound as it propagates through the tissues*. 2. How does attenuation limit the image? *Attenuation limits the penetration therefore the depth* 3. 4. What is the dominant contributor to attenuation? *The dominant contributor to attenuation is absorption.* 5. What is absorption? *Absorption is the conversion of sound into heat.* 6. What is the unit of attenuation? *Decibel = dB* 7. What is the decibel? What are 3 dB and 10 dB *It's the unit used to quantify attenuation* *3db = 50% attenuation reduction 10db = 90% attenuation reduction* 8. The initial intensity for a transducer is 100mW/cm^2^. The sound beam at a distance of 1cm is attenuated by -3db. What is the intensity value at a distance of 1cm? 10. What is the attenuation in soft tissues? 12. What is HVLT? 13. *The attenuation coefficient is the attenuation per \_\_\_Centimeter\_\_\_\_ of sound travel.* 14. For soft tissue, there is approximately \_\_0.5\_ dB of attenuation per centimetre for each MHZ of frequency. 15. The attenuation coefficient in soft tissue \_\_\_*[Increases]*\_\_\_ as frequency increases. 16. For soft tissue, if frequency doubles, attenuation is \_\_*doubled*\_\_\_\_\_\_\_\_\_. If path length is doubled, attenuation is \_\_\_\_*doubled*\_\_\_\_. If both frequency and path length are doubled, attenuation is *\_\_\_quadrupled\_\_\_\_\_.* 17. Absorption is the conversion of \_\_*[sound]*\_\_\_\_\_\_\_ into \_\_\_\_*[heat]*\_\_\_\_\_\_. 18. Is attenuation in bone higher or lower than in soft tissue? *Higher* 19. If an echo returns 130 µs after a pulse was emitted by a transducer in soft tissue, at what depth is the echo producing structure located? 20. A sound wave travels 26 µs to a reflector in soft tissue. How far away is the reflector, assuming the medium is soft tissue? 21. A reflector is 10 mm away from the transducer. How long does it take for sound to get back to the transducer? 22. A reflector is 20 mm away from the transducer. How long does it take for sound to get back to the transducer? 23. What is the reflected sound beam known as? When the perpendicular beam is reflected, what happens? Echo. When the beam hits an interface and is reflected, it goes back to the receiver and is picked up to form our images on the screen. 24. When the perpendicular beam is transmitted, what happens? The beam moves through the 2^nd^ medium in the same direction as the incident sound beam. 25. What is acoustic impedance? It's symbol? It's unit? Equation with units? Acoustic Impedance is a measure of the resistance of a medium to the transmission of sound. Its symbol is "z", its unit is Rayls and equation 𝑧 = c (m/s) × ρ(kg/m3). 26. What does it mean if there is no reflection at a boundary? All is transmission, no echo -- the medium impedances are equal 27. What must the reflective/transmission coefficients add up to? 1 or 100 % 28. How is oblique incidence different from perpendicular incidence? The oblique is any angle other than perpendicular 29. What is refraction? What determines it? Why is it important? Refraction is a change in the direction of sound as it crosses a boundary. It is determined by the different acoustic velocity/impedance in the different mediums. It contributes a great deal to the formation of our image. 30. What does Snell's law tell us about? The law tells us the relationship between angle of incidence/transmission and the velocity of sound of 2 media 31. What is a specular reflector? What anatomy would produce one? It is a flat smooth boundary that reflects well ("mirror-like"). It is produce at the artery walls or the diaphragm. 32. What is scattering? What does it depend on? Scattering is the formation of echo in many directions by rough surfaces or by heterogeneous media or very small objects. It depends on the surface of the interface ( ie: rough) and the size of the object compared to the size of the wavelength 33. Describe why TGC is needed? 34. What is the TGC slope? 35. What does the operator need to keep in mind when setting the TGC? 36. What is the difference between TGC and overall gain? 37. What are the factors affecting TGC setting? The same things that affect attenuation! - *Tissue composition* - *Attenuation coefficient* - *Transducer frequency* 38. If a medium has an attenuation coefficient of 2db/cm, what would be the best choice for the slope of the TGC? 39. Express the ratio of 1 volt to 1 millivolt in dB? 40. What is speckle? *Speckle if a form of acoustic noise and shows as the graininess of tissues within the image* *.* 41. How is speckle produced? *It is produced by constructive/destructive interference patterns within the returning echoes producing darker and brighter dots on the screen.* 42. What is CEUS? *Contrast enhanced ultrasound* 43. What are contrast agents? *Microbubble polymers introduced intravenously into the bloodstream to highlight particular isoechoic abnormal tissues (eg. Lesions)* 44. What are the characteristics of contrast agents? *Stable, small enough to perfuse, different z to surrounding medium, non-toxic, easy administration* 45. How does the contrast agent function? 46. How are contrast agents used to show perfusion? *Increased US amplitude can pop the bubbles and we can see the tissues and the rate at which they fill up again with the agent.* 47. What is the units for amplitude? *Pascal, N/cm^2^, kg/cm^3^, volts*