| Term | Definition |
|---|
| Attenuation | (TA) decrease in amp. and Intensity |
| Amp. decreases exp. & F stays constant | as u/s penetrates through the tissue |
| Attenuation results in: | absorption, reflection and scattering |
| TA | the measure or weakening of sound beam as it goes thru tissue |
| What is TA | attenuation |
| Greater in bone: | absorption and reflection |
| Greater in lung: | scattering |
| Absorption | sound dissipation/conversion into heat |
| Directly related to F | Absorption (Inc. F, absorption and TA also increase |
| viscosity | type of tissue being insonated |
| also know as distance or depth | absorption (farther u travel, more attenuation) |
| Reflection | bounce back/portion of sound beam redirected from a boundary, portion of sound beam used for imaging. |
| Two types of reflection: | Specular and diffuse |
| Specular reflection | boundary is smooth and lg. w/ respect to wavelength, organized, reflected in one direction |
| Specular refelection is dependent on: | the angle of incidence is equal to the angle of reflection |
| Diffuse reflection | aka: backscatter. Lg. rough surface (not smooth) has irregularities, reflected in more than one direction |
| Scattering | Non-specular reflection, reflects sound beam in a number of directions |
| Smallest loss in attenuation: | scattering |
| largest loss in attenuation | absorption |
| Raleigh Scattering | interaction of u/s with rbc's |
| Increases dramatically w/F and is proportional to F to the 4th power | raleigh scattering |
| Attenuation (TA)= | Attenuation coefficient (Ac) x Path length (Pl) |
| Path Length | the distance (depth) to the reflectors sound has traveled. |
| Attenuation Coefficient (Ac) | the amt of attenuation per cm of sound propagated (Ac=1/2 x F) |
| (Half value layer) HVL | penetration depth, the thickness or depth that reduce I to 1/2 its original value |
| HVL | dependent on medium and F (pen. depth (cm) = 3/Ac |
| Thick HVL = | Decreased F and TA |
| Thin HVL = | Increased F and TA |
| Decibles (dB) | a way to expressthe ration of 2 amp. and Intensities, based on logarithmic scale |
| 3 dB rule | 3 dB increase in I = the original I x 2 or 3 dB decrease in I = the original / 2 |
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