DMSO 1302: Ultrasound Physics: EXAM 2 REVIEW Questions And Answers Graded A+

- greatest contributor to attenuation in tissues - Potential source of bio-effects (discussed later) acoustic speckle - noise resulting from the constructive and destructive interference of small sound wavelets -grainy appearance Amplitude - -For a wave or vibration, the maximum displacement on either side of the equilibrium (baseline) position. - positive amplitudes (above baseline) and negative amplitudes (below baseline) -units that are appropriate to the acoustic variable considered EX: pressure amplitude = pascals (Pa) Attenuation (a) - -is the weakening of sound while it propagates - limits the imaging depth (high frequencies are attenuated faster therefor are shallower than lower frequencies) -its weakening effect must be compensated by the transducer - a = 1/2(f x l) Units = decibels (dB)-3 mechanisms of attenuation: absorption, reflection and scattering -DIRECTLY RELATED TO DISTANCE -DIRECTLY RELATED TO FREQUENCY Absorption - - conversion of sound energy to heat Attenuation Coefficient - - the number of decibels of attenuation that occurs when sound travels one centimeter - Unit: dB/cm - 0.5 dB of attenuation per centimeter for each megahertz of frequency (in soft tissues) - the farther the sound travels, the more attenuation - equal to 1/2 of the operating frequency - f/2 constructive interference of scatter - - The interference that occurs when two waves combine to make a wave with a larger amplitude - In phase - reinforce each other Contrast Agents - -Micro-bubbles of gas stabilized by a protein, lipid or polymer shell create stronger harmonics than tissue therefore detecting these harmonic frequencies increases the contrast between the micro bubbles and the surrounding tissues.- Liquid suspension intravenously administered into the venous circulatory system to increase echogenicity. - Can also be used to administer therapeutic medications for a more localized treatment. - Mainly used for cardiac contrast imaging. Contrast Agents legal in the US? - 1. Definity - octafluoropropane containing liposomes 2. Imagent - dimyristoyl lecithin 3. Optison - perfluoropropane filled albumin Decibles (dB) - - involve the logarithm of two powers or intensities. - quantifies attenuation - dB = initial intensity/final intensity - 3 dB = 50% reduction to 1/2 of the original value - 10 dB = 90% reduction to 1/10 of the original value Density - mass/volume Helps determine propagation speed - as materials become more dense, the speed of sound decreases.- Sound travels faster in media with low density. -Materials that are STIFF but not dense will have the fastest speed EX: bone is stiff but not dense (because of the tiny holes for blood vessels) so sound travels faster in bone than in soft tissues Density and Stiffness -the words start with different letters so they go in DIFFERENT directions -when density increases, speed decreases destructive interference - -occurs when two or more waves in different phases combine to produce a wave with a smaller displacement - out of phase - cancel each other out diffuse reflection (non-specular) - - Reflection that occurs when parallel rays of light hit a rough surface and all reflect at different angles