PHYSICS
39.4.Yu.pptxVIDEO
Sound = mechanical energy that propagates through a continuous, elastic medium by the compression and rarefaction of the units in that medium. Speed of a sound wave is purely determined by the medium that it travels in. We cannot control speed of soundwave. we can control the frequency of wavelength. Independent of this the wave will travel at a set speed depending on the medium it travels in. Wavelength is the compensation mechanism that links the set frequency and the predetermined speed of sound in a particular tissue.
Compression: high localized pressures
Rarefaction: regions of low localized pressures. Plot with x axis representing normal value of tissue.
Speed (c) = frequency (f) x wavelength (w)
Speed of sound changes depending on the medium that the soundwave is traveling. certain material properties determine how fast sound travels through it.
Frequency of sound does NOT change. This is what we set/determine.
Wavelength: links frequency and speed.
Period (T) = 1 / f
Amount of time for 1 cycle of a wave to pass a specific point; measured in units of time.
What inherent properties of a tissue determine the speed of sound?
c = sqrt (elastic property / inertial property)
Elastic property (bulk modulus): tissue stiffness / resistance to compression / ability to move to transfer energy and return to normal. How readily do units of the material return to their resting state? The stiffer the tissue, the less compressible the tissue is, the higher the bulk modulus and the faster sound travels.
Inertial property (density): amount of force required to move units within tissue. Represented by density of tissue; the more tightly packed the material is the more force/inertia has to propagate energy through the tissue. The less dense a material, the faster sound propagates through. Density is how tightly packed particles are in medium. the tighter they are the slower sound travels.
RUNNING MAN ANALOGY
The runner runs at set cadence/frequency, the number of steps per minute is constant. The speed that he is running depends on the ground being run on and air being run through.
Bulk modulus = ground; the harder the ground/stiffer or less compressible the faster the runner will run. Wavelength or distance taken with each stride will be longer. If hard ground became sand, less compressible and less stiff they would run slower bc bulk module has decreased causing shortening in wavelength.
Density = air; thinner the air the faster the runner will run. If syrup, the increased density the runner will run much more slowly. Wavelength would decrease drastically.
Speed of sound changes depending on the material. Though the materials appear to increase in density though the bulk modulus in relation to the density increases at much higher rate. As density increases, speed decreases as a result of the BM in proportion to density increases at much higher rate. they re independent factors of one another
Tissues that are more dense happen to have higher BM. stiffer tissues more resistant to compression. it is the BM that account's for increase in speed. If all had the same bulk modulus then the increasing density would decreased speed of sound.
PRESSURE & INTENSITY
Pressure (amplitude) is proportional to pressure^2
Pascal (Pa) = local pressure changes in tissue; amplitude of soundwave; important for calculating power of US.
Intensity = amount of power (energy/time) per area
How much work is done in the tissue?
Decibel scale is a relative intensity scale.
Near field & far field; US beam has varying intensities if cut in cross-section.
Deconstructive interference: when waves are out of phase; reduction in intensity
Constructive intereference: waves are in phase; increased intensity. THis is why intensity is highest in the center of the beam.
Continuous: leaves no time to listen to echoes bouncing back
Pulse echo: transmits pulse wave into tissue and wait for reflections from tissue boundaries.
Differences in tissue acoustic impedance determines how much echo is transmitted through to the next boundary and how much is reflected back.
Range equation: T = 2D/c; Tc/2 = D
US fires off sequential lines of US pulses, waits for echoes to return, plots tissue boundaries based on time it took to return to probe. Each line, multiple pulses are sent. Grayscale values will be assigned to the tissue boundaries based on intensity of the echo returning to the machine.
Tissue boundaries with high differences in acoustic impedance will appear bright.
5 parameters to describe pulse echo:
Pulse duration: time taken for entire pulse to be emitted from the machine: # cycles x period or # cycles / f
Fixed according to frequency; We can only change the receive time; we cannot repeat next pulse until echoes return to probe to prevent interference. The deeper we want to scan, the larger the receive time must be. Chance in receive time will change 3 factors (PRP, PRF
Spatial pulse length: # cycles x wavelength of single wave
Pulse repetition period: time between start of one pulse and the start of the next; pulse + receive time. as we image shallower depths we can reduce our receive time and PRP. PRP is inversely proportional to PRF
Pulse repetition frequency: # of pulses that we can fit in within 1 sec.
Duty factor: PD / PRP; % of time the US is transmitting pulse; determines intensity that patient is receiving. DF = PD/PRP; of that minute how long are we transmitting wave