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ultrasound physics 1
following the edelman book, sound, pulsed sound, intensity
| Term | Definition |
|---|---|
| sound creates images by | sending short bursts into the body |
| acoustic propagation properties | the effects of the medium upon the sound wave |
| biologic effects | the effects of the sound wave upon the biologic tissue |
| sound | a type of wave that carries energy, not matter, from place to place |
| compressions | areas of increased or higher pressure and density |
| rarefactions | areas of decreased or lower pressure and density |
| what type of wave is sound? | mechanical and longitudinal |
| sound travels in | a straight line |
| acoustic variables do what | identify which waves are sound waves |
| three acoustic variables | pressure, density, distance |
| pressure | concentration of force within an area, force/area |
| density | concentration of mass within a volume |
| distance | measure of particle motion |
| pascals (Pa) | units of pressure |
| kg/cm^3 | units of density |
| cm, feet, miles | units of distance |
| transverse wave | particles move in a perpendicular direction, right angles or 90 degrees, to the direction of the wave |
| longitudinal wave | particles move back and forth in the same direction as the wave |
| acoustic parameters | period, frequency, amplitude, power, intensity, wavelength, speed |
| what do acoustic parameters do? | describe the features of a particular sound wave |
| period | the time required to complete a single cycle |
| units of period | any unit of time |
| what is period determined by? | the sound source |
| typical values of period | 0.06 to 0.5 microseconds |
| frequency | number of certain events that occur in a particular time |
| units of frequency | per second, 1/second, hertz (Hz) |
| typical values of frequency | 2 MHz to 15 MHz |
| audible sound | frequency is between 20 Hz and 20,000 Hz and can be heard by man |
| ultrasound | greater than 20,000 Hz and can not be heard by man, the frequency is too high |
| infrasound | less than 20 Hz and can not be heard by man, the frequency is too low |
| frequency and period are | reciprocals |
| what type of relationship do frequency and period have? | inverse relationship, when one goes up, the other goes down |
| equation for period and frequency | frequency (Hz) x period (sec) = 1 |
| what are the three bigness parameters? | amplitude, power, and intensity |
| amplitude | the difference between the average value and the maximum value of an acoustic variable, half the peak-to-peak amplitude |
| units for amplitude | those of the acoustic variables, pressure - pascals, density - grams/cubic cm, particle motion - cm, inches, units of distance |
| peak-to-peak amplitude | the difference between maximum and minimum values of an acoustic variable |
| power | the rate that work is performed, or the rate of energy transfer |
| units for power | watts |
| what is the relationship between power and amplitude? | power is proportional to the wave's amplitude squared |
| intensity | the concentration of energy in a sound beam |
| intensity depends upon | both the power and the cross-sectional area of the beam |
| equation for power | intensity(watts/cm^2) = power(watts) / beam area (cm^2) |
| what is the relationship between intensity and power? | intensity is proportional, or directly related, to power (when one goes up, the other goes up) |
| what is the relationship between amplitude and intensity? | intensity is proportional to the amplitude of the wave squared |
| wavelength | the length of distance of a single cycle, similar to the length of a single boxcar in a long train |
| units for wavelength | meters, mm, or any unit of length |
| what is wavelength is determined by? | both the sound source and the medium |
| what is the equation for wavelength? | wavelength (mm) = propagation speed (mm/us)/frequency (MHz) |
| wavelengths in soft tissue | sound with a frequency of 1 MHz has a wavelength of 1.54mm, sound with a frequency of 2 MHz has a wavelength of 0.77mm |
| what is the equation for wavelength in soft tissue? | wavelength(mm) = 1.54 mm/us / frequency (MHz) |
| propagation speed | the rate that sound travels through a medium, also called velocity or speed |
| what is propagation speed determined by? | the medium only - density and stiffness |
| what is the speed of sound in air? | 330 m/s |
| what is the speed of sound in lung? | 300 - 1,200 m/s |
| what is the speed of sound in fat? | 1,450 m/s |
| what is the speed of sound in soft tissue? | 1,540 m/s |
| what is the speed of sound in tendon? | about 1,850 m/s |
| what is the speed of sound in bone? | 2,000 - 4,000 m/s |
| stiffness and speed | same direction |
| density and speed | opposite directions |
| compressibility and elasticity | opposites of stiffness |
| bulk modulus | the same as stiffness |
| when bulk modulus increases, speed | increases |
| interference | when two waves overlap at the same location and at the same time, they combine into a single new wave |
| constructive interference | occurs when the amplitude of the new, combined wave is greater than the original two waves, in-phase waves interfere constructively |
| destructive interference | the amplitude of the new wave is less than one of the original waves, out-of-phase waves interfere destructively |
| in diagnostic imaging, what kind of pulses are used to create anatomic images? | short pulses |
| pulse | a collection of cycles that travel together |
| what are the two components of pulsed ultrasound? | the cycles (on or transit time) and the dead time (off or receive time) |
| five additional parameters that describe pulsed sound | pulse duration, pulse repetition period, prf, duty factor, spatial pulse length |
| pulse duration | the time from the start of a pulse to the end of that pulse, the actual time that the pulse is on |
| what is the unit of pulse duration? | usec - any unit of time |
| what is pulse duration determined by? | by multiplying the number of cycles in the pulse and the period of each cycle |
| what are the typical values of pulse duration? | in clinical imaging, pulse duration ranges from 0.5 to 3us. |
| equation of pulse duration | pulse duration (us) = # of cycles in pulse x period (us) |
| spatial pulse length | the length or distance that an entire pulse occupies in space, the distance from the start to the end of one pulse |
| what is the unit of spatial pulse length? | mm - any unit of length |
| what are the typical values of spatial pulse length? | 0.1 to 1 mm |
| equation of spatial pulse length | spatial pulse length (mm) = # of cycles x wavelength (mm) |
| pulses that are short in time are ______ in distance | short in distance |
| pulse repetition period | the time from the start of one pulse to the start of the next pulse, includes one pulse duration and one listening time |
| what is the unit of pulse repetition period? | msec or any unit of time |
| what are the typical values of pulse repetition period? | 100 us to 1 ms |
| pulse repetition period is determined by? | the imaging depth |
| as pulse repetition period increases, imaging depth _____ | increases |
| as pulse repetition period decreases, imaging depth _____ | decreases |
| PRF | the number of pulses created by the system in one second, pulse repetition frequency |
| what are the units of PRF? | hertz, Hz, per second |
| what are the typical values of PRF? | from 1,000-10,000 Hz (1-10 kHz) |
| true or false, PRF is related to frequency | false |
| duty factor | the percentage or fraction of time that the system transmits sound |
| what are the units of duty factor? | unitlless |
| what are the typical values for duty factor? | from 0.1% to 1% or 0.001 to 0.01 (little talking, lots of listening) |
| continuous wave sound can or cannot create anatomical images? | cannot |
| terms that have the same meaning to shallow imaging | high pulse repetition frequency, short pulse repetition period, high duty factor |
| terms that have the same meaning to deep imaging | low pulse repetition frequency, long pulse repetition period, low duty factor |
| intensity | the concentration of power in a beam |
| intensity is or is not uniform across a sound beam | is not uniform |
| spatial intensity | an ultrasound beam does not have the same intensity at different locations within the beam |
| temporal intensity | pulsed ultrasound does not have the same intensity at different times |
| peak | the maximum value |
| average | the mean value |
| spatial | referring to distance or space |
| temporal | referring to all time (transmit & receive) |
| pulsed | referring only to the time the pulse exists (transmit only) |
| SPTP | spatial peak, temporal peak - highest value |
| SATP | spatial average, temporal peak |
| SPTA | spatial peak, temporal average - most important for thermal bioeffects |
| SATA | spatial average, temporal average - lowest value |
| SPPA | spatial peak, pulse average |
| SAPA | spatial average, pulsed average |
Created by:
arielenicole
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