US Basic Physics Principles Flashcards
Each object the sound travels through or
(an intervening substance, as air, through which a force acts or an effect is produced)
MEDIUM
The molecules vibrate in the same direction as the sound travels (e.x. Ultrasound)
LONGITUDINAL WAVES
The molecules vibrate at 90 degrees to the direction of energy travel (e.x. water waves)
TRANSVERSE WAVES
Concentration of force
(units: lb/sq inch, Pascals Pa)
PRESSURE
The concentration of matter (mass per unit volume)
(units: kg/cubic cm)
As density increases, speed decreases (inversely related)
Because the material is heavier and has more inertia. Its more difficult to start and stop the sound moving.
DENSITY
Measure of particle motion
(units: cm, feet, miles)
DISTANCE
The distance of one cycle
Determined by the source and the medium
Wavelength = propag speed (mm/ms)
frequency (MHz)
Units: mm or meter (any unit of length)
WAVELENGTH
The length of time it takes to complete one single cycle of sound or to one complete single wavelength
( Units: seconds, msec, hours – all units of time;
Determined by: sound source)
PERIOD
The number of wave crests passing a point in a single second
(Unit: Hertz = cycles per second)
FREQUENCY
The speed at which sound wave moves through the medium
C = frequency x wavelength
Units: meters/sec or mm/sec
Determined by the medium only
Average speed of sound through the body is 1540 m/s (NEED TO KNOW FOR REGISTRY)
PROPAGATION SPEED OR VELOCITY
The resistance of material to compression
The main factor in determining propagation speed
The harder the material, the less compressible it is
Harder media have higher sound speeds
STIFFNESS
(Compressibility is the opposite of stiffness)
Stiffness overrides density in determining sound speed
EXAMPLES:
Bones are very stiff but also very porous so have a low density
So has a high speed
Solids - highest speed
More molecules per area
Stiffer than liquids or gasses
Liquids - medium speed
More molecules than gas per area
Stiffer than gasses
Fat has lowest speed of the solids
Gasses lowest speed
Smallest number of molecules
(NEED TO KNOW FOR REGISTRY)
Contains additional frequencies that are even and odd multiples of the original frequency
HARMONICS
As the waveform becomes less sinusoidal, the harmonics get stronger
This is function programmed into the machine
Harmonics improve sonographic image quality
Harmonics do not affect the transmit frequency
Received frequency is twice that of the transmit frequency
A pulse is a collection of cycles that travel together
Continuous wave US cannot form images
Must have listening time to form images
The only thing we can change is the listening time between the pulsed cycles
We do this when we change “depth”
Deeper depth, more listening time (have to wait for original pulses to return before sending out new ones!)
Shallower depth, shorter listening times
The pulses themselves never change
PULSED ULTRASOUND
Time from the start of one pulse to the end of the pulse
ONLY the time that the pulses are on or transmitting
The time it takes for one pulse to occur
= # of cycles in the pulse x period Or # of cycles in pulse / frequency
Units: seconds; microseconds
Determined by: the source (transducer) only
Usually 2-3 cycles for imaging; 5-20 cycles for Doppler
PULSE DURATION
Shorter pulses generally create higher quality images
Short pulses have: Few cycles (less sending out)
Each individual cycle has a short period (time)
Percentage of time that the system is transmitting a pulse (or ‘on’ / not listening)
Units – No units (unitless)
The words: factor, coefficient, Index – tell you that there are no units
= Pulse duration (sec) / PRP (sec) x 100
Determined by the sound source
Continuous wave sends out 100% of the time so duty factor for continuous wave is 1.0 (always on!)
Can be changed by the sonographer ……..How?
By changing depth or listening time
DUTY FACTOR
High PRF have high duty factor because they are pulsing often
Number of pulses that occur in a single second
Similar to frequency (number of events occurring per second)
Units: Hz or per second
US transducers usually emit a few thousand pulses per second
Determined by: Sound source
Can be changed by the sonographer
PULSE REPETITION FREQUENCY
As imaging depth increases, PRF decreases (inverse relationship)
As sonographer adjusts depth setting, they change the PRF
Shallow image – high PRF
Deep image – low PRF (takes longer for pulse to travel back to the transducer)
Time from the start of one pulse to the start of the next
Includes both the time that the pulse is on and the time that it is off (dead time)
Determined by: sound source
Units: Seconds, microseconds
Can be changed by the sonographer
We change only the listening time, never the pulse duration itself
PULSE REPETITION PERIOD
PRP decreases as PRF increases because more pulses occur in one second
PRP and PRF are reciprocals
Inverse (one goes up; other goes down)
PRP(sec) X PRF(Hz) = 1
PRP = 1/PRF
PRF = 1/PRP
The distance a pulse occupies in space
DISTANCE from the start of the pulse to the end of the pulse
Units: meter, mm, any unit of distance
= # of cycles in the pulse x wavelength
Determined by the sound source and the medium
Cannot be changed by the sonographer
Decreases with increasing frequency (not with increasing PRF, but increasing frequency)
Shorter pulse lengths give better sonographic image quality
SPATIAL PULSE LENGTH
These fundamental properties never change, regardless if using pulsed wave or continuous wave US
FREQUENCY, PERIOD, WAVELENGTH, PROPAGATION SPEED
Weakening of a sound wave as it travels through media
Determined by frequency of sound and the distance it travels (as each increases, attenuation increases – directly related)
Consists of – Absorption, Reflection, Scattering, (mostly absorption - 80%)
Decrease in amplitude and intensity as sound beam travels
Units: Decibels (dB)
Named after Alexander Graham Bell (telephone)
Bel is the logarithmic ratio of the relative power in two acoustic beams
Decibel is 1/10th of a Bel
ATTENUATION
The farther the sound beam travels, the more attenuation that occurs
In soft tissue, the greater the frequency used, the greater the attenuation
Relative, not absolute units
Two intensities or amplitudes are required for computation of decibels
DECIBELS
Decibels involve the use of mathematical logarithms
The 2nd intensity is smaller than the original intensity
Example: the intensity of the sound that returns to the probe after it has traveled through the body is lower than the sound we originally sent out
NEGATIVE DECIBELS
The 2nd value measured is larger than the original value measured
Example: When we turn up the gain, there is a several decibel increase in the measured amplification.
POSITIVE DECIBELS
ATTENUATION COEFFICIENT
Attenuation is reduction in intensity as wave travels through a medium
Attenuation coefficients are numerical values that express how different materials will attenuate the sound beam per unit path length
Attenuation that occurs with each centimeter the sound travels
Expressed in dB/cm
Total attenuation (dB) =Atten coeff (dB/cm) x distance in cm
Multiply the coeff x the distance traveled
The coefficient itself does not change as path length changes (the coefficient is dB/cm)
However total attenuation changes with path length
Overall attenuation increases when frequency, path length, or attenuation coefficient increases
(Coefficient = ½ transducer frequency)
ATTENUATION COEFFICIENT IN SOFT TISSUE
In soft tissue, the atten coeff and frequency are directly related.
The average attenuation coefficient in soft tissue is ½ the frequency in megahertz
As frequency increases, attenuation increases
The distance the sound beam penetrates into the tissue when its intensity is reduced to half the original value
Represents a 3 dB decrease in intensity (- 3 dB intensity has decreased by ½ )
The higher the frequency, the shallower the depth it occurs – why?
HALF-VALUE LAYER
The fraction of the original intensity remaining at the end of sound travel/after attenuation
INTENSITY RATIO
Conversion of sound to heat
Transfer of energy from the sound beam to the tissue
Directly proportional to frequency
Higher frequency; higher absorption, more attenuation
ABSORPTION
Just like you need a reflection from a mirror to see yourself, we need reflections from the body to make our images.
REFLECTION
Reflections occur at boundaries between two different media.
These are also called acoustic interfaces.
REFLECTION occurs when sound energy strikes a boundary and some gets sent back to transducer. For reflection to occur, we MUST HAVE…..
- An acoustic interface between two media……and…
- The media must have different impedances for reflection to occur
The greater the impedance difference, the greater the reflection that occurs
Material’s resistance to sound traveling through it (unit is Rayl)
A characteristic of the medium only
Each tissue has its own acoustic impedance
It is the acoustic resistance to sound traveling through the medium (represented by Z)
IMPEDANCE
Impedance (Z) = Density (kg/m3) x propagation speed (m/s)
If density or propagation speed increases, impedance increases
-When an US beam strikes a specular reflector (large flat smooth boundary [larger than wavelength]) perpendicularly……
The intensity of the reflected wave depends on the difference in acoustic impedance between the 2 media that form the interface
-The larger the difference in acoustic impedance at the interface…..
The greater the intensity of the sound reflected back toward the transducer
-Sound will not reflect off the interface between 2 media if their acoustic impedance is exactly the same
Very smooth surfaced, mirror-like reflector
Reflector is very large compared to sound beam’s wavelength
Reflection is well-defined and regular
SPECULAR REFLECTOR
TYPES OF REFLECTORS
Backscatter – scatter returning in the same general direction as the transducer
But sound is disorganized and random
Occurs when boundary has irregularities about the same size as the sound’s wavelength
Scattering Non-specular – the random redirection of sound waves in multiple directions
Produced when sound beam strikes a rough surface
Where surface irregularities are same size or smaller than the wavelength
Attenuation process in which the beam interacts with interfaces smaller than the wavelength of the beam
Causes the US energy to be dispersed in multiple directions
Can also be caused by rough or irregular surfaces (called non-specular reflector)
Directly related to frequency; higher frequency sound scatters more (shorter wavelengths)
SCATTERING
Shows that scattering allows US imaging of tissue boundaries that are not perpendicular to the incident sound
Scattering also allows imaging of tissue parenchyma in addition to organ boundaries
Sound scatters symmetrically in all directions
Not related to incidence angle
Directly proportional to the frequency
RAYLEIGH SCATTERING
If frequency is doubled, Rayleigh scattering is 16 times greater
The angle between the incident sound beam and the interface between the 2 tissues
Perpendicular, Orthogonal, Right Angle, Ninety Degrees (PORN) – Occurs when the sound beam strikes a boundary between 2 media at exactly 90 degrees
ANGLE OF INCIDENCE
Incoming sound strikes the boundary at 90 degrees
NORMAL INCIDENCE
sound beam strikes at any angle other than 90 degree
OBLIQUE INCIDENCE
Uncertain if Reflection will Occur!
Must Say we Don’t Know!
Some sound is transmitted; some reflected
REFLECTION WITH OBLIQUE INCIDENCE
Reflection occurs if the 2 media at the boundary have different acoustic impedance
REFLECTION WITH NORMAL INCIDENCE
A bending from the straight line path or a change in direction of a sound wave traveling from one medium to another (goes in same general direction
ASSOCIATED WITH TRANSMISSION
Must have:
1. Oblique Incidence
2. Media with different propagation
speeds
The amount of deflection off the straight line path is directly related to the change in acoustic velocity from one tissue to the other (the difference between the 2)
Slightly different sound speeds creates a small deviation
Greatly different sound speeds creates a substantial deviation
REFRACTION
SNELL’S LAW
The physics of refraction are described by Snell’s Law
For two given media, the sine of the angle of incidence bears a constant relation to the sine of the angle of refraction
Equation states:
Sine (transmission angle) = Propagation Speed 2
Sine (incidence angle) Propagation Speed 1
2 Sound waves encounter / or interfere with one another
The net result is adding the 2 waves together
May be Constructive or Destructive
WAVE INTERFERENCE
Peaks encounter peaks
Troughs encounter troughs
Stronger Wave Results
CONSTRUCTIVE INTERFERENCE
Peaks encounter Troughs
Troughs encounter Peaks
Smaller Wave Results
DESTRUCTIVE INTERFERENCE
To display echoes anatomically correctly on the US screen the US unit uses the time delay between pulse transmission and reception to determine the distance to each interface
Relationship between round trip pulse-travel time (sound to interface and echo coming back from it), propagation speed, and distance to a reflector
Distance to boundary =
go-return time (microsec) x speed
2
D = ct or ½ CT (c = prop
2 speed)
D = distance to reflector
V = velocity or propagation speed
(1540 m/s)
T = round trip travel time (time of flight)
The longer the round trip travel time, the further the distance from the transducer
RANGE EQUATION
In soft tissue, every 13 microsec of go-return time means the reflector is 1 cm deeper in the body
13MICROSECOND RULE
All three are measures of the strength of the sound wave
All are related to one another
All can be changed by the sonographer
All decrease as the sound wave travels through the body
AMPLITUDE-POWER-DENSITY
Strength of the beam
The maximum variation in acoustic variable from its baseline state (when sound is moving through the area)
Units depend on the acoustic variable being affected
Can be expressed in dB units (decibels)
Can be changed by the operator (with intensity and power)
When considering attenuation, the new intensity being compared is always less than the initial intensity
AMPLITUDE
Amplitude is the difference between the average or equilibrium value and the maximum or
The amplitude of US waves emitted from the transducer depends on the electric stimulation sent to the transducer.
The larger the electric pulse the more intense the beam produced and the more particle displacement occurs in the medium
Positive dB are used to express amplifier gain of receiver
Rate that work is performed; rate of energy transfer (WATTS)
Force that causes displacement
Units – Watts
IT is divided by a beam’s cross sectional area
Therefore IT is directly proportional to a wave’s amplitude squared
In a beam IT’s expressed in watts
Directly proportional to the wave amplitude squared
Whatever value or power amplitude increases, square IT and IT goes up that much more
POWER
Concentration of energy in certain areas of the sound beam (used for bioeffects) (WATTS/CM2
IT depends on the power of the beam and the cross-sectional area of the beam
Concentration of energy in a sound beam
Equals the beam’s power divided by the beam’s cross sectional area
Directly proportional to power
If power is doubled, IT is doubled
If power is quartered, IT is quartered
Directly proportional to amplitude squared
If amplitude is doubled, IT goes up 4 times
If amplitude is quartered, IT reduces to 1/16
INTENSITY
The highest intensity area or time of the sound beam (maximum value)
PEAK
take peak, low, and medium intensities and average them together to get average intensity (mean value)
AVERAGE
Space related variations
Related to space/areas of the beam
TERMINOLOGYSpatial intensity is greatest at the center of the beam and at the focus of the beam
Intensity diminishes as you near the edges of the beam
Maximum spatial intensity is called spatial peak (SP)
Spatial average (SA) is the AVERAGE intensity across the US beam (considerably less than spatial peak)
SPATIAL
Time related variations
Temporal non-uniformity
Occurs with time (temporal)
In continuous wave, temporal peak and average are the same
In pulsed US, temporal peak occurs while the pulse is on
Temporal average includes the time that the pulse is off. Low in pulsed US.
TEMPORAL
TERMINOLOGY
SP = Spatial Peak SA = Spatial Average TP = Temporal Peak TA = Temporal Average
BEAM UNIFORMITY
In order of lowest to highest intensities or values
SATA Lowest intensities
SAPA
SATP
SPTA
SPPA
SPTP Highest intensities
Average intensity calculated during the most intense half cycle
Similar in intensity to SPTP
Im
Describes the distribution of the intensity of an ultrasound beam in space
= SP / SA (spatial peak divided by spatial average)
Unitless
1.0 is the minimum value
BUC (Beam Uniformity Coefficient)
