SPI Flashcards
Sound is
A type of wave that carries ENERGY from place to place
Sound is made up of
A series of compressions and rare fractions
Compressions are
Areas of increased pressure and density
Rarefactions are
Areas of decreased pressure and density
Sound must travel thru
A medium
Cannot travel thru a vacuum
A sound wave is
Mechanical and longitudinal
Sound travels in
A straight line
Acoustic propagation properties are
The effects of the medium on the sound wave
Biological effects are
The effects of a sound wave on biologic tissue
Acoustic Variables
Pressure
Density
Distance
(Help identify which are sound waves)
Pressure is
The concentration of force within an area
Units= pascals (Pa)
Density is
Concentration of mass within volume
Units= kg/cm^3
Distance is
Measure of particle motion
Units= cm, ft, miles
Transverse waves
Particles move perpendicular (right angle 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
Describe the features of a particular sound wave
Period Frequency Amplitude Power Intensity Wavelength Propagation speed
Period
Time required to complete a single cycle
Units= microseconds
Determined by source
Not adjustable
Frequency
of events that occur in a particular time frame
Units= Hz
Determined by sound source
Not adjustable
Period and frequency relationship
Reciprocals
Inverse relationship
(Period decreases, frequency increases)
(Shorter period, higher frequency)
(Longer period, lower frequency)
Bigness parameters
Describe beam’s strength
All behave the same way
Amplitude
Power
Intensity
Amplitude
Difference between average and max value of an acoustic variable
Units= Pa, g/cm^3, cm, dB
Can be adjusted
Decreases as sound propagates thru body
Power
Rate of work is preformed
Units Watts
Determined by source (initially)
Can be adjusted
Decreases as sound propagates thru body
Relationship between power and amplitude
Power is proportional to amplitude^2
Examples
If amplitude is tripled the power is increased by a power of 9
If the amplitude is halved the power is decreased by a factor of 4
If the amplitude is doubled the power is increased by a power of 4 (quadrupled)
Intensity
Concentration of energy in a sound beam
Units W/cm^2
Determined by source (initially)
Can be adjusted
Equation for intensity
Intensity (W/cm^2)= power(W)/beam area (cm^2)
Intensity=power
Intensity=amplitude^2
Wavelength
Length or distance of a single cycle
Units= m, mm
Determined by source and medium
Not adjustable
Equation for wavelength
Wavelength(mm)= prop speed/frequency
In soft tissue= 1.54/frequency
Propagation speed
Rate that sound travels thru medium
Aka velocity or speed
Units= m/s, mm/s
Determined by medium
Not adjustable
All sound travels at
The same speed thru any specific medium
Speed and wavelength relationship
Directly related
Sound in a slow medium has a short wavelength
Sound in a fast medium has a long wavelength
General Rule of average speed of all sound in soft tissue
1540 m/s
Lung(air)<
Rules of thumb
Density and stiffness
Stiffness and speed= same direction
Density and speed = opposite direction
Compressibility and elasticity are opposites of?
Stiffness
Bulk modulus is
The same as stiffness
Changes in stiffness have the greatest effect on?
Speed
Stiffness is related to
Change in shape (squishability)
Density is related to
Weight
Audible, Infra, and Ultrasound
Audible =frequencies between 20 Hz - 20,000 Hz (20kHz)
Ultrasound = greater than 20kHz
Infrasound= less than 20 Hz
Phase relationships
Constructive interference = in phase waves
Deconstructive interference = out of phase waves
Pg. 22
Pulsed sound parameters
Pulse duration PRP PRF Duty Dactor Spatial Pulse Length
Pulse Duration
Time from start of pulse to end of that pulse (actual time pulse is “on”)
Units= microseconds
Determined by source
Not adjustable
Equation Pulse duration = # of cycles in pulse x period
PRP
Pulse repetition period
Time from start of one pulse to the start of the next Pulse
Units= msec or any unit of time
Determined by source
Can be adjusted (only changes listening time)
PRP is determined by
Imaging depth
As PRP increases depth increases
As PRP decreases depth decreases
PRF
Pulse Repetition Frequency
of pulses created by system in one second
Units Hz (per sec)
Determined by source
Can be adjusted
PRF and depth have what kind of relationship?
Inverse
Shallow image = higher PRF
Deep image = lower PRF
PRP and PRF have what kind of relationship
Reciprocals
Equation PRP (sec) x PRF (Hz) = 1
Duty Factor
% of time that system transmits sound
Unitless
Determined by source
Can be adjusted when imaging depth is changed
Shallow image higher duty factor
Deep image lower duty factor
Spatial Pulse Length
Length/ distance from start to end of one pulse
Units=mm
Determined by source and medium
Not adjustable
Determines axial resolution
Equation for Spatial Pulse Length
SPL (mm)= #cycles x wavelength (mm)
Intensity
The concentration of the power in a beam
Intensity is a key parameter for
Bioeffects
5 key words for intensity
Peak-max value Average Spatial Temporal (transmit and receive) Pulsed (transmits only)
Units for intensity
W/cm^2 (all intensities)
SPTP
Spatial Peak Temporal Peak
Highest Value
SATA
Spatial Average Temporal Average
Lowest Value
SPTA
Spatial Peak Temporal Average
Most important for thermal bioeffects
Three Commandments if Intensity
- Intensities May be reported in various ways with respect to time and space
- Intensity is the key parameter with regard to bioeffects
- Peak is greater than average so SPTP is the highest intensity and SATA is the lowest
Decibels are
A RELATIVE comparison or ratio between the final of the initial two strengths
(See the word relative think dB)
Decibel is what kind of scale?
Logarithmic
Positive decibels
Means getting bigger or increasing
3dB - two times bigger than original intensity
10 dB - ten times bigger than original
6dB- four times bigger than original
9dB- eight times bigger than original
Pg. 39
Negative decibels
Getting smaller intensity decreasing
- 3dB means one half original value
- 10 dB means one tenth original value
Hint: for negative dB calculate positive dB then invert
Attenuation
Decrease in intensity, power and amplitude of a sound wave as it travels
Hint: unrelated to speed
Units of attenuation
dB
Less attenuation means
Lower frequency and shorter path length
More attenuation means
Higher frequency and longer path length
Three components of attenuation
Absorption
Scattering
Reflection
Absorption
Primary, sound converted to heat
Scattering
Redirection of sound in many directions
Two types
Diffuse and Rayleigh
Diffuse scattering
Backscatter
Rough boundary
Rayleigh scattering
Reflector much smaller than wavelength of sound
Higher frequency undergoes this
RBC is an example of this
Rayleigh scattering is related to
Frequency^4
Attenuation in different media
Air has much more attenuation than in soft tissue (gel used to remove air from path of US)
Lung and Bone have more attenuation than soft tissue. Bone absorbs lung scatter
Water is much less than soft tissue
Attenuation in blood is less than soft tissue
Attenuation and penetration
In soft tissue lower frequency results in less attenuation
Attenuation ultimately limits max imaging depth where meaningful reflections are obtained
Reflection
When propagating sound energy strikes a boundary between 2 media and some returns to the transducer
Specular reflection
Reflections from a smooth reflector (mirror) and return in one direction.
Well seen when sound strikes boundary at 90 degrees
Strongest reflections are produced?
With normal incidence 90 degrees
Attenuation coefficient
Amount of attenuation per cm
Units dB/cm
Attenuation coefficient relationship to frequency
Direct relationship
As frequency increases, attenuation coefficient increases
Attenuation coefficient in soft tissue
0.5dB/cm/MHz
One-half if the transducers frequency
Equation
Attenuation
Total attenuation (dB) =path length (cm) x attenuation coefficient(dB/cm)
For numerical questions on board remember
Always make sure units are perfect from question to answer
Impedance
associated with a medium
Units Rayls (Z)
Reflection depends on
Different acoustic impedances
Equation for impedance
Impedance (rayls Z) = density (kg/m^3) x propagation speed (m/s)
Incidence
Normal = 90 degrees
PORNN
Perpendicular Orthogonal Right Angle Ninety Degrees Normal
Oblique incidence
Anything other than 90 degrees
Acute- less than 90
Obtuse- more than 90
We know nothing
Reflected angle= incident angle
Incident intensity
Intensity of sound wave at instant prior to striking a boundary
W/cm^2
Reflected intensity
The intensity that after striking a boundary changes direction and returns back from where it came
W/cm^2
Transmitted intensity
Intensity that after striking a boundary continues on in the same general direction that it was originally traveling
W/cm^2
Equation for incident intensity
Incident intensity= reflected intensity+transmitted intensity
Conservation of energy exists where?
At a boundary
Intensity Reflection Coefficient (IRC)
The percentage of the US intensity that bounces back when the sound strikes a boundary
Usually less than 1%
Intensity Transmission Coefficient (ITC)
The percentage of the incident intensity that after striking a boundary continues on in the general direction that it was originally traveling
ITC more than 99%
In biologic media
% of reflection
Soft tissue - air = 99% reflection
Soft tissue - bone= 50% reflection
Soft tissue - Soft tissue = <1%
Normal incidence
Reflection occurs only if the two media at the boundary have different acoustic impedances
When impedance is the same no reflection
If no reflection 100% transmission
PORNN
For normal incidence
Perpendicular Orthogonal Right Angle Ninety degrees Normal
Oblique incidence
Anything other than 90%
Acute less than 90
Obtuse more than 90
Refraction
Transmission with a bend
Requires oblique incidence and different speeds
Snell’s Law
Describes physics of refraction
sin(transmission angle)/ sin(incident angle) = prop speed 1 / prop speed 2
Chart bottom of pg 53
Time of flight is what?
Aka go return time, round trip time
Time needed for a pulse to travel to and from the transducer to the reflector
1.54 km/sec in soft tissue
How are time of flight and distance related?
Directly
When time of flight is measured we can determine what?
Reflector depth
13 microsecond rule
Pg 57.
Speed equation
Speed =distance/time
What is a transducer?
Any device that converts one form of energy into another.
Piezioelectric effect
A property of certain materials to create a voltage when pressure is applied or when the material is mechanically deformed
Reverse piezioelectric effect
Materials deform or change shape when a voltage is applied to them.
Piezioelectric materials
Aka Ferroelectric
Lead zicornate titanate
PZT
Ceramic
Active element
Crystal
Curie Point
When PZT is heated above this temp. And loses it’s piezioelectric properties, PZT is depolorized
360°C or 680°F
Sterilization
Complete destruction of all living microorganisms by means of exposure to heat, chemical agents, or radiation.
Most critical objects that require sterilization
Those that penetrate mucous membranes or skin
Disinfection
Application of chemical agent to reduce or eliminate infectious organisms on an object.
Objects that require disinfection
Insturments that only come in contact with skin such as a transducer.
When disinfecting transducers what should you use?
Cold germacides such as gluteradehyde or cidex
DO NOT USE HEAT
Transducer components
Case Electric shield Acoustic insulator PZT ( 1/2 wavelength thick) Wire Matching layer Backing material
Case
Protects internal components from damage and insulates patient from electric shock
Acoustic insulator
Thin barrier of cork/rubber “uncouples” internal components; prevents vibration in case.
Matching Layer
One quarter wavelength thick.
Has an impedance between those of the skin and active element to increase the percentage of transmitted US between the active element and skin.
Gel’s impedance is in between?
Those of the matching layer and skin to further increase the efficiency of sound transmission.
Matching layer impedances
PZT>MATCHING LAYER>GEL>SKIN
Damping element/ backing material
Reduces ringing of PZT
commonly made of epoxy resin impregnated with metal powder
RULE: short pulses create more accurate images.
Damping material imaging transducer characteristics
Damping is effective (backing material) Short pulse length and duration Low sensitivity Wide bandwidth Low Q factor Decreased output power
Bandwidth
Range of frequencies between highest and lowest frequency emitted from transducer
Imaging transducers have what kind of bandwidth?
Wide or broadband because of backing material
Q factor
Unitless number representing extent of damping
Imaging transducers are low Q
Continuous Wave Transducers have what frequencies?
Sound waves frequency equals the frequency of the voltage applied to the PZT by the machine’s electronics
Electrical freq=acoustic freq.
Pulsed transducers have what kind of frequency?
Determined by thickness of PZT and speed of sound in PZT.
High frequency pulsed transducer has what?
Thin crystal
Fast PZT
Low frequency transducer has what?
Thick Crystal and slow PZT
When PZT crystal is half as thick the sound frequency is?
Twice as high
